Note: Descriptions are shown in the official language in which they were submitted.
~ 333i~?
The invention relates generally to the control of
mechanical pumps used to transfer liquids from any fluid
reservoir, and more particularly toward methods and systems
for optimizing the overall production efficiency of any
pumping well, based upon accurate measurement of the time-
averaged rate that incompressibl~ liquids exit the pump
discharge. The invention also relates to the design of
electromechanical sensors that accurately measure the
instantaneous rate of both pulsating and steady-state fluid
flow, and to methods and apparatus for processing measured
flow-rate information to detect liquid "pump-off" and to
ascertain the performance of both pumping equipment and fluid
reservoir. Such information may be utilized to identify
production degradation, and to solicit servicing of the
reservoir and equipment as re~uired to maintain optimum
production efficiency.
Since the first commercial oil well was drilled in
Pennsylvania by Colonel Drake in 1859, more than two million
wells have been completed in the United States for the
production of crude oil and natural gas. While most of these
wells have now been abandoned, American Petroleum Institute
records currently indicate that by the end of 1985 there were
approximately 880,000 producing hydrocarbon wells still
operating within the territorial limits of our nation.
Unfortunately, most of these wells are now marginal producers
- due to their natural production decline, and will soon be
abandoned as they become unprofitable to operate. Thus, to
satisfy its increasing demand for energy, America has no
choice but to locate and develop additional petroleum
reserves each year. Since most readily accessible reserves
have previously been developed, however, new production can
now only be obtained at great risk and expense to the
operator. This same general trend of declining production
and escalating expense prevails throughout the free-world
today.
With these facts in mind, the importance of obtaining
maximum productlon efficiency from every available well site
becomes increasingly more apparent with the passage of time.
Since hydrocarbons are essentially a non-renewable resource,
the world's total supply of available energy is greatly
-- 1 --
1 333962
maintain a positive income stream from each existing well
site. Once a well has been completed, its economic life will
thereafter be determined by its ability to produce
hydrocarbons at a profit. When operating expenses,exceed
production revenues, most wells will be plugged and abandoned
even though they are perfectly capable of producing
additional reserves under pump. By increasing the efficiency
of such pumping operations, the commercial life of a typical
well can usually be extended for many years to economically
extract additional reserves from the ground. In many
situations the additional reserves that may be obtained by
optimization of the pumping process will comprise a
substantial share of the ultimate production potential of a
well. Such optimization is especially important for stripper
wells that, by definition, produce less than 10 barrels of
oil per day, since the expense of operating such wells
typically offsets a substantial share of the resulting
production revenue.
Most wells are currently drilled by high-speed rotary
methods that utilize special drilling fluids to lubricate and
cool the drill bit, circulate cuttings out of the hole and
control naturally occurring formation pressures. During the
course of drilling, one or more tests are typically conducted
to measure the fluid content, pressure, temperature and/or
productivity of each zone of interest. Open hole logs and
drill-stem tests are frequently run, and cores may be taken
of some intervals, to determine matrix composition, porosity,
permeability and hydrocarbon saturation.
Once a well has been drilled and tested, the well-bore
is typically lined with one or more strings of heavy steel
casing to prevent the hole from collapsing under pressure. A
section of casing is then cemented in place by pumping a
high-strength cement slurry down its interior and circulating
it back towards the surface through cementing ports to fill a
portion of the annulus between the well-bore and the liner.
Various known methods, including cementing packers and staged
cementing, are frequently used to keep the cementing
materials from contacting and infiltrating the most
productive reservoirs. By completing a well in this manner,
the casing and cement also serve to shut-off the flow of
unwanted water into the well from porous formations that lie
above or below the productive zones of interest.
After the well has been cased and cemented, the liner is
perforated at selected locations to allow for the entry of
desired formation fluids. This operation is typically
accomplished by means of explosive charges. Abrasive jets of
pressurized sand and liquid are sometimes used to establish
-- 2 --
1 333q62
communication with the formation, and open-hole completion
techniques eliminate the need for such operations by keeping
both casing and cement away from the formation altogether.
Following perforation of the casing, artificial
stimulation of each productive interval is typically required
to enhance the rate of fluid entry into the well-bore If
the formation is composed of sandstone, stimulation is
usually accomplished by pumping large volumes of viscous
fluids into the reservoir under pressure to hydraulically
fracture the formation matrix. Such an operation typically
creates a large vertical fracture that extends outward from
the casing, although in some situations this fracture will be
horizontal, depending on the weight of overburden. To
prevent the flow channel from closing once the treating
pressure has been removed, a propant (usuaIly coarse sand or
spherical ceramic bails) is pumped into the formation during
this process to hold the fractured formation walls apart.
Limestone formations, unlike sandstone, are typically
stimulated by pumping large volumes of acid into the matrix
under pressure to create a maze of permeable flow channels
that extend outwardly from the casing for a considerable
distance into the formation.
Once artificially stimulated, a well is ready to be
completed into a tank or pipeline. This is done by equipping
the well with the necessary downhole and surface equipment
for the removal of formation liquids from the casing.
Although many wells have sufficient reservoir pressure to
flow naturally to the surface, most require the use of a
downhole pump to mechanically lift both water and oil above
ground. Several basic types of pumps are employed for this
purpose, including positive displacement reciprocating pumps,
electrically operated downhole submersible pumps, rotary
screw pumps, and gas or hydraulically operated plunger lift
or jet velocity systems. Because conventional surface
mounted pumping units are of simple and rugged design, most
wells are currently equipped with this type of equipment that
converts the rotating motion of an electric motor or
gas/diesel engine into a reciprocating up and down
motion. This motion is used to activate a piston pump that is
located downhole near the end of a string of production
tubing. The downhole piston pump typically has a single
acting ball check valve known as the "standing valve" located
within the lower inlet side of a polished steel or brass
cylinder called the "barrel". Contained within the upper
portion of this barrel is a moving check valve known as the
"traveling valve", which is actuated from the surface by a
string of "sucker rods" that connect the valve to the pumping
-- 3
1 333962
unit. To prevent fluid from leaklng back to its suction
side, the traveling valve is often equipped with a plurality
Of "valve cups" which seal the clearance betwee~n the
traveling valve and the working barrel. These cups ar'e made
out of nylon, leather or other pliable composition materials,
and require periodic replacement together wi~h the polished
balls and seats when they become worn or corroded. Metal-to-
metal piston pumps operate essentially the same, but do not
make use of valve cups; instead, they rely on a very small
clearance between the polished metal plunger and cylinder to
restrict the bypass of liquid.
A second type of downhole pump which is currently used
on a small percentage of U.S. and foreign wells is the
"electric submersible pump". This pump consists of a multi-
stage centrifugal pump assembly in combination with a high-
efficiency electric motor that is attached to the end of the
string of production tubing. The only surface equipment
required for this type of installation is a motor control
panel that regulates power applied to the downhole motor by
means of electric wires that are run downhole with the tubing
string and pump. These pumps a ~ used for high volume
applications, and are quite expensive to install and operate.
In such installations all downhole electric equipment is
cooled by the fluids that are pumped.
Gas and hydraulic plunger lift systems require the use
of high-pressure pumping equipment located above ground, and
a free traveling plunger located within the tubing string
that is periodically pumped to the surface to purge the
tubing of formation liquids. Once the plunger reaches the
wellhead, it is then allowed to free-fall back to bottom in
preparation for the next operating cycle. Rotary screw pumps,
on the other hand, utilize the rotating motion of an
aboveground motor that drives the sucker rod string to turn a
polished steel mandrel within a rubber stator fixed to the
bottom of the tubing. This rotary screw motion "squeezes"
liquid to the surface, and is quite efficient when used at
depths of less than 2000 feet. Other pumping means utilize
the lifting action of a high-velocity stream of pressurized
gas or liquid injected into the tubing at formation depth to
cause fluid to flow continuously to the surface by means of a
pressure or density gradient.
Turning now to the dynamics of well performance, it is
important to realize that a producing well is essentially a
low pressure region that has been artificially introduced
into a naturally occurring geologic reservoir for the purpose
of removing resident formation fluids such as water, oil and
natural gas. By maintaining the well-bore at a hydrostatic
- 4 -
1 333962
pressure lower than the prevailing reservoir pressure,
formation fluids will continuously flow into the bore hole at
a rate that is essentially proportional to the established
pressure differential between formation and casing~ For
production to be sustained, casing fluids must be continually
removed and transported to either surface tanks or pipelines
by natural or artificial means to prevent the bore hole
pressure from returning to equilibrium with the reservoir.
Initially, many wells have sufficient bottom hole
lG pressure to flow naturally to the surface without the
assistance of mechanical pumping means; these wells are said
to exhibit "artesian flow". As reservoir pressures become
depleted with time, however, all wells eventually require
mechanical pumping means to lift formation liquids to the
surface. Since the reciprocating piston pump is the type of
equipment most commonly used for this purpose, the discussion
that follows is primarily directed towards those applications
that make use of this class of hardware. The ensuing
comments should be considered generic in nature unless
otherwise stated, however, since the same operating
characteristics and problem areas will typically be observed
with any other type of mechanical pumping equipment.
Most wells produce a combination of water, oil and
natural gas, together with a small ~ount of solid particular
contaminants that are transported into the well-bore by the
stream of flowing fluids. Such materials will only flow into
the casing when the hydrostatic pressure of liquid and gas
contained there is reduced below the naturally occurring or
artificially enhanced formation pressure. For the purpose of
this discussion it will be assumed that all transported solid
contaminants remain in suspension within the column of
produced liquids, and that the total volume of such
contaminants is small relative to the total volume of flowing
liquids. It will also be assumed that this mixture of solids
and liquids behaves exactly the same as a column of pure
water and oil, from a fluid mechanics standpoint, and that
all completed zones are commingled and serviced by a common
downhole pump.
Whenever a well is completed to simultaneously produce
from more than one production interval, the total rate of
fluid entry into the casing is governed by the individual
rates of fluid entry from each completed reservoir. From a
theoretical standpoint, the instantaneous rate of fluid entry
into the casing from any one reservoir is a function of many
variables such as formation pressure IIPf", casing pressure
~Pcll~ reservoir permeability "H", fluid viscosity "V" and
flowing surface area "A" of the stimulated formation. For
-- 5 --
` 1 333q62
compressible fluids such as natural gas and condensate, the
equatiOn which relateS these variables to describe the daily
fluid entry rate can be quite complicated depending on the
actual pressures and temperatures involved. For relatively
incompressible liquids such as water and oil, however, the
combined fluid entry rate "QF" of both liquids may be
described with reasonable accuracy over a wide range of
operating conditions by the following mathematical expression
that is derived from the Darcey Equation for laminar flow:
QF = (kA)(H/V)(Pf-PC) (1)
Since the total instantaneous rate of incompressible
fluid entry from any one reservoir is equal to the combined
entry rates of water and oil, the correct fluid production
factor (H/V) to use in this equation is a function of the
absolute viscosities and relative permeabilities of both
water and oil contained within the formation. This factor
depends on the current saturation level of each liquid, and
may be expressed mathematically as (H/V) = (H/V)W + (H/V)o~
Although the actual value of (H/V) will change slowly with
time as fluid is extracted from the reservoir, its prevailing
magnitude is essentially constant at any particular time
regardless of the pressure drive established between
formation and casing. Likewise, the constant "k" depends
only on the units of flow desired, such as gallons per minute
(GPM) or barrels of fluid per day (sFPD), and the constant
"A" depends only on the naturaLly occurring reservoir
porosity and stimulation techniques utilized. Thus, once a
reservoir has been completed, the only factor in equation (1)
over which the operator has any day-to-day control is the
pressure drive (Pf-PC). Since the remaining factors
(kA)(H/V) are essentially constant and independent of
pressure drive, on a daily basis, equation (1) may be
rewritten as follows:
QF = (K) * (Pf ~ Pc) ~2)
When a well is first drilled, its naturally occurring
reservoir pressure is typically on the order of 350 psi to
450 psi for every 1000 feet of depth below ground level,
although significantly greater pressure gradients may
frequently be encountered. If several productive zones are
encountered, each zone usually has its own reservoir pressure
which depends only on the depth and content of that
particular formation. During the initial period of "Primary
Recovery", the natural pressure of each producing interval
-- 6 --
1 333~62
declines exponentially with time as fluids are extracted by
the natural pressure drive (Pf-PC). This means that the
fluid entry rate "QF" into the casing from each zone also
declines exponentially with time. Following the hatural
depletion of any reservoir, its remaining formation pressure
may then be artificially enhanced by the introduction of
repressuring agents such as water, carbon dioxide or nitrogen
to allow for the continued production of hydrocarbons during
a period of "Secondary ~ecovery".
From the above discussion it should be obvious that the
total rate of fluid entry into a well is equal to the
summation of the individual fluid entry rates "QF" from each
zone completed. Although each formation may have its own
reservoir pressure IIPf", production factor (H/V) and flowing
surface area "A", their individual fluid entry rates are all
governed by the same basic equation (1) presented above.
This equation indicates that the total fluid productian rate
"QF" obtained from each producing interval is proportional to
the pressure drive (Pf-PC) established across that
formation. Thus, to achieve the greatest total rate of fluid
entry into the casing for any given set of reservoir
conditions, it is only necessary to reduce the hydrostatic
pressure within the casing to the lowest value possible.
This may be accomplished by pumping all of the liquid from
the casing, and by keeping the casing gas pressure as low as
possible.
It is important to note that the casing pressure "Pc"
which affects fluid entry rate "QF" is equal to the
arithmetic sum of the casing gas pressure at wellhead plus
the hydrostatic pressure of contained liquids at formation
depth. Since casing gas is either vented to atmosphere or
delivered into the pipeline, the required wellhead gas
pressure is usually fixed by marketing considerations over
which the operator has very little control. Thus, by
removing all liquids from the casing, the greatest production
is achieved for any specified gas delivery pressure.
Whenever water and oil are allowed to accumulate above the
productive interval, the actual rate of fluid entry into the
casing is less than optimum since the pressure drive (Pf-PC)
is reduced by the combined hydrostatic head of these
liquids. Since the ratio of oil and gas production to total
fluid production (i.e. "oil cut" and "gas/oil ratio") remains
essentially constant, the total daily production of
hydrocarbons will also be less than optimum whenever liquids
are allowed to accumulate within the casing.
1 333962
Except in instances of an artesian well, the maximum
rate that fluid can be removed from the casing is controlled
by the capacity of the pumping equipment installed.~ This
capacity "Qp" may be computed as the theoretical displacement
S of the downhole pump multiplied by the overall volumetric
efficiency of all associated downhole equipment. Thus, if a
particular downhole pump has a displacement of 200 BEPD, and
if it operates at 80% volumetric efficiency as observed on
the surface, then its actual pumping rate "Qp" into the tank
or pipeline will be 160 BFPD. This rate is the combined
pumping rate for all incompressible fluids being transported,
and assumes that a full head of liquid is available to the
suction inlet on each successive stroke or revolution of the
pump. The actual pumping capacity of any centrifugal, rotary
screw or piston pump may be computed as fol~ows:
Qp = (Displacement) * (Volumetric Efficiency) (3)
For purposes of this discussion, the physical
displacement of any mechanical pump installation is
considered to be a function only of its geometry and speed of
operation, and is not dependent on such factors as rod
stretch or internal fluid leakage. These inefficiencies,
together with all other factors which affect the net
production efficiency of a well, are conveniently grouped
together and accounted for under the general heading of
"overall volumetric efficiency". This efficiency is defined
as "The ratio of actual fluid delivery rate to the surface,
divided by the theoretical volumetric displacement of the
downhole pump", and has nothing to do with the overall
thermodynamic efficiency of surface equipment from a
mechanical or electrical standpoint.
Whenever fluid is sucked into a downhole pump, its
volumetric efficiency is first reduced by the effects of
viscosity, friction and inertLa that combine to restrict the
entry of fluid into the suction chamber. Typically this
"suction efficiency" is near 100% for mechanical pumps
operating at slow pumping speeds, and decreases as the
pumping speed is increased. As the fluid level within the
casing is lowered, suction efficiency continuously declines
since there is progressively less hydrostatic pressure at the
pump inlet to drive liquid past the standing valve and into
the pumping chamber. This decline typically is on the order
of a few percentage points, and is essentially linear with
time. When all stored water is finally depleted from within
the casing, the suction efficiency will further decline by a
few additional percentage points as the pump begins to ingest
8 --
1 ~3~2
the pad of high visCosity oil that floats on top of the
water. This last change is rather abrupt since the water/oil
interface within the casing is quite well defined. The
importance of these two slight but perceptible changes' in the
overall volumetric efficiency of downhole pumping equipment
will be more fully described hereinafter.
Once in the chamber of a piston pump, liquid must first
pass through the traveling valve on its downstroke before it
can be lifted towards the surface on the following upstroke.
During this fluid charging period, the hydrostatic pressure
of liquids within the tubing string will be supported by the
standing valve, which typically leaks some fluid back into
the casing due to an imperfect seal between its ball and
seat. Throughout the following upstroke, the weight of
liquid transfers to the traveling valve, and some fluid will
then leak past the cups or metal plunger and the seated
traveling ball to return to the suction side of the valve.
Rod stretch reduces piston travel to less than the input
stroke of surface equipment, and small leaks in the tubing
joints allow pressurized liquid to return to the casing
rather than being pumped to the surface. All told, the
combination of these various factors work together to reduce
the overall volumetric efficiency of all downhole pumping
equipment below the theoretical limit of 100%.
Based on the above definition of volumetric efficiency,
the theoretical capacity of any reciprocating piston pump may
be readily calculated since its mechanical displacement then
becomes a simple function of pump diameter, stroke and
frequency of operation. Initially, the volumetric efficiency
of this type of equipment is typically on the order of 80-95%
depending on the particular application and equipment
configuration involved. With time, this efficiency declines
significantly as the various mechanical components wear with
use. At times, this degradation can be quite rapid due to
the effect of sand or other contaminants flowing through the
pump, and sucker rod failure or large tubing leaks will
usually result in the immediate cessation of fluid being
transported to the surface. The co~tinuous operation of such
equipment without a full head of liquid available to its
inlet also causes a rapid degradation of performance since
the metal plunger or traveling valve cups are then not
properly lubricated. Most of these same factors also affect
the performance of centrifugal or rotary screw pumps, which
have a theoretical capacity that is similarly determined by
their physical geometry and speed of operation. Because of
these considerations, the actual volumetric efficiency of a
1 333962
_
downhole pump is rarelY known with any degree of accuracy
once such equipment has been operated for any length of time.
It is a common misconception t~at a downhole pisto~n pump
will only move fluid to the surface on the upstroke. This
S assumption is not always correct, as confirmed by strip-chart
recordings (made with the assistance of the herein disclosed
invention) of the instantaneous fluid exit rates from many
pumping wells that have ranged in depth from 600 to 7600
feet. It is of particular interest to note that this
erroneous assumption actually provided the design basis for
some prior art motor control devices that reportedly operate
based upon the detection of fluid "pump-off".
In order to understand why a piston pump can displace
fluid to the surface on both the upstroke and the downstroke,
it is only necessary to study the geometry of the working
barrel and tubing string when the polish rod, sucker rods and
traveling valve are at their maximum and minimum ve~tical
limits of travel. It will first be noted that when the
polish rod is at the upper limit of its stroke, there exists
within the working barrel a volume of liquid that will soon
be displaced through the traveling valve as it makes its
downward stroke. Assuming that the well is not "pumped-off",
this volume of fluid is very nearly equal to the cross-
sectional area of the working barrel multiplied by the length
of the pumping stroke. Once on top of the traveling valve,
however, this same volume of liquid must occupy a greater
height within the working barrel since the cylinder volume
above this valve is now reduced by the volume of the sucker
rods which actuate said valve. The net effect of this change
in geometry is that fluid is usually displaced upward within
the tubing string by the downstroke of the traveling valve.
With regard to the capacity of the tubing string in the
vicinity of the wellhead, it can be seen that at the top of
the upstroke there exists a section of tubing whose liquid
volume may be calculated as the volume of tubing less the
volume of sucker rods based upon their respective cross
sectional areas multiplied by the length of the pumping
stroke. On the downstroke, the volume of sucker rods within
this upper section of tubing is replaced by the greater
volume of the polish rod, which typically has a larger
diameter than the rod string. Thus, on the downstroke of the
pump, the polish rod acts to displace an additional volume of
liquid to the surface. In similar fashion, this displacement
acts in reverse on the upstroke to reduce the net volume of
fluid exiting the wellhead.
-- 10 --
1 333962
The net effect of both displacements mentioned above is
additive~ and is offset somewhat by the fact that as fluid
exits the working barrel into the tubing string at d~wnhole
pump elevation, there exists a slight reduction in the
average upward velocity of liquid within the tubing since it
is typically of larger diameter than the working barrel. of
further influence are the effects of leakage past the
traveling and standing valves during the up and down strokes
respectively, and the effects of possible leakage through a
plurality of tubing joints When all such displacements and
inefficiencies are taken into account, it is frequently found
that the typical downhole piston pump installation moves a
considerable portion of its total pump capacity to the
surface on the downstroke. Many wells, in fact, actually
move more fluid on the downstroke than on the upstroke,
depending on the physical dimensions and efficiencies of the
particular equipment involved.
Whenever formation fluids ente~ the casing under optimum
production conditions, the hydrostatic pressure acting upon
these liquids is greatly reduced below the reservoir pressure
"Pf". Because of this, gaseous hydrocarbons originally
dissolved within the water and oil come out of solution and
physically separate from the other constituents in accordance
with their natural order of densities. Water, being the
heaviest, falls immediately to the bottom of the well where
it accumulates and eventually enters the pump first. Oil,
being lighter, rises to float on top of the water and gas,
being the lightest, rises to fill the remainder of the casing
between liquid interface and wellhead.
Once inside the casing, the amount of gas that remains
in liquid solution is dependent only upon the absolute
pressure and temperature of the casing fluids at formation
depth. If the wellhead gas pressure is not very high, then
the gas pressure acting upon the fluid interface at the
bottom of the hole will be essentially the same as the gas
pressure measured at the surface. Due to the greater
densities of water and oil, however, the hydrostatic pressure
within each column of liquid increases linearly with depth
below the gas/liquid interface. Thus, the amount of gas in
solution within the combined liquid column also increases
significantly with increasing depth of iiquid accumulation.
If, for example, casing gas is maintained at a pressure of
100 psig at the wellhead in order to deliver regulated gas
into the pipeline, and if liquid is allowed to build within
the casing to a height of 500 feet above the pump inlet
before such equipment is actuated, then the initial
hydrostatic pressure acting upon this column of liquid
-- 11 --
1 333962
increases uniformly from 100 psig at the liquld surface to
300 psig at the pump lnlet, assumlng an average llquld
pressure gradient of 0.40 psig per foot of depth, In this
case the first liquid ingested into the pump will contain
natural gas in solution at a pressure of 300 psig, and the
last liquid ingested into the pump just prior to "pump-off"
will contain natural gas in solution at a pressure of 100
psig.
Throughout the pumping cycle, liquid is sucked into the
pump and discharged on top of the traveling valve, where the
hydrostatic pressure within the tubing string is directly
related to its setting depth below ground level. If the pump
is located 5000 feet below the surface, for instance, then
hydrostatic pressure within the tubing is approximately 2000
psig at pump elevation. At this pressure, the gas contained
within the llquid column can not possibly come out of
solution since it has previously out-gassed to a saturation
pressure of between 100 and 300 psig as previously
described. As this liquid is pumped to the surface, however,
the hydrostatic pressure within the tubing string decreases
by approximately 40 psig for every ~00 feet of vertical rise;
thus, when the first liquid ingested by the pump comes to
within 700 feet of the surface, its hydrostatic pressure will
have decreased to 300 psig assuming that the wellhead
discharge pressure is 20 psig. As the liquid continues to
rise above this depth, its hydrostatic pressure further
decreases and gas begins to expand out of the super-saturated
liquid. This escaping gas continues to expand as it
approaches surface elevation, causing the liquid to "flow in
head" or surge into the lead line. A similar out-gassing of
all additional liquid ingested by the pump likewise occurs in
this example at depths ranging from 700 to 200 feet below
ground level, where the hydrostatic tubing pressure declines
below the minimum casing saturation pressure of lO0 psig.
This normal escapement and expansion of dissolved gas
within the tubing string chills the liquid and increases its
volume as it approaches and finally exits the wellhead. Such
expansion causes paraffin to congeal within the tubing, and
also causes the apparent volumetric efficiency of the
downhole pump to increase since the final volume of separated
gas and liquid exiting the wellhead is much greater than the
original volume of gas-saturated liquid ingested at the pump
inlet. By using a conventional fluid back-pressure valve in
the liquid discharge line at the wellhead, as hereinafter
disclosed, the hydrostatic liquid discharge pressure can be
maintained greater than the greatest possible pump inlet
pressure to avoid such problems.
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1 333962
When a well first starts to pump after being shut-down
for a certain length of time, there is us~ally an excess
reserve of liquid contained within its casing. Sinice the
pump initially has plenty of liquid available to its'inlet,
fluid first exits the wellhead at an average rate that is
identically equal to the pumping rate "Qp" of downhole
equipment. As the fluid level within the casing is reduced
by pumping, additional liquids enter from the formation at an
increasing rate that is determined solely by the changing
pressure drive (Pf-PC). Should the available fluid entry
rate "QF" be greater than the established pumping rate "Qp",
the hydrostatic casing pressure will eventually decline
sufficiently to cause new liquids to enter at a rate that is
identically equal to the pumping rate ~i.e. QF = Qp). Once
equilibrium has been established, no further change in the
average casing fluid level will occur except as dictated by a
gradually changing reservoir pressure, or by a change in the
actual pumping rate due to a degradation of the overall
pumping efficiency. If the established pumping rate "Qp" is
greater than the maximum available fluid entry rate "QF",
however, then the well will event~ally "pump-off" when the
pump's initial reserve of liquid is depleted from the
casing. Following such event, the average rate of liquid
exiting the wellhead can thereafter be no greater than the
average rate of new fluids entering the casing from the
formation. Accordingly, the energy expended by the prime
mover will be inefficiently utilized by the downhole pump if
it continues to operate after fluid "pump-off".
Regardless of the type of mechanical pumping equipment
used, the downhole pump can be severely damaged if it is
operated for any appreciable length of time without a
substantial head of liquid available to its inlet. If a
piston pump depletes all of the liquid from the casing, for
instance, it will thereafter operate in a condition referred
to as "fluid pounding" wherein there is insufficient liquid
available to the pump on its suction stroke to completely
fill the pump barrel with liquid. Under such conditions the
pump barrel fills partially with gas, and heavy shock loads
are then developed on each successive downstroke as the
traveling valve abruptly slams into the liquid interface.
These shock loads tend to unscrew the sucker rods which are
typically screwed together in 25 feet lengths, thereby
causing rod separation that requires a time consuming and
expensive "fishing job" to repair. Also, without a
substantial charge of liquid passing through the pump on each
stroke, wear on the traveling valve cups or metal plunger is
accelerated due to insufficient lubrication and the tendency
- 13 -
1 333962
for sand and other solids to preclpitate out of the fluid
stream. The resulting shock loads due to fluid pounding are
also very detrimental to the structural integrity oflsurface
pumping equipment.
In similar fashion, when a downhole submersible pump
depletes all of the liquid from the casing, it will
thereafter operate at reduced efficiency due to the effects
of cavitation induced by the ingested gas. Not only does the
pump motor receive insufficient cooling, but the centrifugal
pump vanes can be severely damaged by shock loads induced by
the collapse of gas bubbles as they travel through the pump
The rubber stator and polished metal mandrel of a rotary
screw pump can also suffer similar damage if not operated
with a full head of liquid available to its inlet. Sustained
fluid pounding also tends to prematurely wear out the
stuffing box seals as a result of improper lubrication. This
situation will frequently result in a loss of considerable
fluid through these worn seals, thereby threatening the
adjacent environment and necessitating shut-down of equipment
while repairs and clean-up are effected. For these reasons,
it is imperative that no type of mechanical downhole pump be
operated for any sustained period of time in a severe
"pumped-off" condition.
Whenever a downhole mechanical pump is allowed to
operate for any length of time in a "pumped-off" condition,
the degree of severity of fluid pounding or cavitation is
determined by the dimensionless ratio of fluid entry rate
"QF" divided by the pumping rate "Qp". By definition, the
fluid entry rate "QF" that is used throughout this disclosure
shall include any volume of solid particular contaminants
that might be suspended within, and transported with, the
volume of produced liquids. If the established ratio of
"QF/QP" is just slightly less than 1.0, then the pump
receives essentially a full charge of liquid on each suction
stroke or revolution, and the effects of fluid pounding or
cavitation are almost imperceptible. If the ratio "QF/Qp" is
near 0, however, then the pump receives very little liquid in
relation to its capacity, and the effects of fluid pounding
or cavitation are quite severe. Between these two extremes
is a transition zone wherein the detrimental effects of fluid
pounding or cavitation become more severe as the ratio
"QF/Qp" approaches zero. By contrast, whenever the ratio
QF/Qp is greater than 1.0, the well will never "pump-off"
inasmuch as fluid can continuously enter the casing at a rate
greater than the actual pumping rate of the downhole
equipment. Accordingly, in this situation, the production
potential of the well will be limited by the capacity of the
- 14 -
1 333962
pumping equipment installed, rather than by the ability ofthe formation to deliver fluids.
From the above discussion, it should be obvious that the
greatest production of oil and gas is obtained at the least
operating expense by equipping a well with a downhole pump
that has a capacity "Qp" which is identically equal to the
maximum available fluid entry rate "QF". Unfortunately, this
result is practically impossible to achieve (and even harder
to maintain) in actual practice since both the pumping rate
and fluid entry rate of any given well completion will vary
considerably from day-to-day due to the effects of changing
pump efficiency, reservoir pressure and average fluid
viscosity. For this reason, most operators elect to install
pumping equipment whose actual volumetric capacity is greater
than the maximum available fluid entry rate of the well, and
then attempt to control the operating cycle of their prime
mover (i.e. electric motor or gas/diesel engine) by the use
of a timing device that is manually set to provide for the
periodic operation of such equipment. By so doing, the
effective pumping capacity of downhole equipment is reduced
by the "Duty Cycle" of the prime mover, which is easily
controlled from the surface by selecting the desired
relationship between "Run Time" and "Rest Time" as follows:
Cycle Time = Rest Time + Run Time (4)
Duty Cycle = Run Time/Cycle Time (5)
From a theoretical standpoint, the required Duty Cycle
of both downhole and surface pumping equipment is equal to
the computed value of the dimensionless ratio "QF/Qp". To
derive this relationship, it is convenient to assume that
each repetitive operating cycle of the pump will begin at the
start of the "rest period" and will end at the conclusion of
the following "run period". Under these conditions, the
start of each operating cycle is marked by the onset of
"fluid pounding" or "cavitation", which begins when the
casing liquid level has been reduced to the pump inlet.
Since fluid is neither created nor destroyed by the pumping
process, and since the inventory of liquids within the casing
is always the same at each instant of time when "pump-off" is
first reached, "cycle time" and "run time" are closely
related to the average values Of "QF" and ~QP~ as follows:
(QF) * (CYC1e Time) = (Qp) * (Run Time) (6)
- 15 -
1 3339~2
~ This continuity equation assumes that "QF" is
essentially constant throughout the entire operating cycle,
and further assumes that fluid only exits the casinglduring
periods of actual pump operation. Both of these assu~ptions
5 are fairly realistic for a properly run well that utilizes a
fluid back-pressure valve to minimize the effects of gas
expansion in the tubing string, as previously discussed, and
that utilizes short rest times to prevent fluid from building
excessively within the casing during the rest period. This
equation also assumes that fluid exits the wellhead at a
constant average rate l~QP~ whenever the downhole pump is
actuated by the prime mover, even though such equipment
rarely performs in this ideal fashion for reasons hereinafter
discussed. By making such an assumption, however, the
limiting value of the required duty cycle for both downhole
and surface equipment can be readily calculated by combining
equations (5) and (6) to yield:
Duty Cycle = QF/Qp
Unfortunately, the actual values of "QF" and ~IQPI~ are
rarely known by the operator to any degree of accuracy.
Thus, the operator has little choice but to guess at the
correct setting for "run time" and "rest time" when
programming a conventional timing device, unless he is
willing to pay the price to conduct frequent and expensive
production tests to measure the average value f "QF" and
"QP" based on actual fluid delivery into a calibrated tank.
Also, conventional timing devices are generally programmable
only in discrete increments of fifteen minutes or more, which
means that accurate selection of the desired duty cycle is
not possible in most situations with such equipment.
Even when the correct values of ~QF~ and IIQPII are
accurately known, total fluid production into a tank or
pipellne is less than optimum when pumping equipment is
controlled by a conventional timing device that is programmed
according to the dimensionless ration "QF/Qp". Such devices,
being passive in nature, make no allowance for the transients
of initial start-up, or for the fact that selected "rest
times" may be inadvertently lengthened, or "run-times"
improperly shortened, by unforseen power interruptions. Such
devices additionally make no allo~ance for the fact that
fluid will frequently "fall-back" into the casing during
periods of equipment "rest" as the result of leaks in the
tubing string or downhole pumping valves, and make no
allowance for the transient effects of sand and/or gas that
- 16 -
1 333962
~ frequently interrupt normal pump operation as they pass
through the suction chamber together with formation fluids.
Because of these considerations, the proper ~election
and regulation of the required duty cycle for any pa~rticular
well completion is quite difficult to achieve using
conventional timing equipment that must be manually
programmed by the operator. Accordingly, most wells are
either under-pumped or over-pumped to some degree, with an
attendant reduction in either fluid production or operating
efficiency respectively.
If optimum production is to be maintained by a
mechanical pump without the adverse effects of fluid pounding
or cavitation, then it is essential that a proper "rest time"
be selected for programming into the motor control device
that is used to regulate the duty cycle of downhole
equipment. This may be clearly understood by considering the
fact that the rate of fluid entry (QF) into the casing
decreases exponentially with time as the available pressure
drive (Pf-PC) diminishes with increasing fluid height. Since
the greatest fluid buildup occurs during the first few
minutes of liquid accumulation, the average daily fluid entry
rate into the casing will be severely affected by the "rest
time" selected for its pumping equipment. A well that
requires five hours, (i.e., 300 minutes) to accumulate 500
feet of liquid during the "rest period", for instance, will
require only 6.2% of this time (i.e., 19 minutes) to
accumulate 25% of this volume, and will require only 15% of
such time (i.e., 45 minutes) to accumulate 50% of this
volume. For this reason, it is imperative that the total
daily "rest time" of any pump be limited in duration and
uniformly distributed throughout each 24 hour operating
period.
The optimum "rest time" for any well is a function of
its casing size, tubing size, fluid entry rate, bottom hole
pressure, oil cut, gas/oil ratio, fixed overhead expense,
energy cost, maintenance expense, pumping rate and certain
other factors such as the water disposal cost and prevailing
market price for oil and gas production. In general, long
"rest times" result in lost production whereas short "rest
times" result in excessive maintenance problems due to the
frequent cycling of surface and downhole equipment. With few
exceptions the optimum "rest time" for any particular well
results in a slight but almost imperceptible trade-off of
production revenue for a greatly reduced expense of energy
consumption and equipment maintena~ce. "Rest times" on the
order of a few minutes to several hours are usually
appropriate for most wells, depending on the established
- 17 -
1 333962
value of QF/Qp, although greater intervals may safely be used
whenever fluid entry rates are extremely low and/or formation
pressures extremely high.
Various types of "pump-off detectors" have been devised
over the years to control the operating cycle of a producing
well. Some of the most common "pump-off" detection systems
utilize a vibration sensor mounted on the Sampson post or
gear box of the pumping unit to detect the slight change in
system oscillation that normally c-curs at the onset of fluid
pounding or cavitation. Other systems utilized a strain-
gauge mounted on the polish rod, walking beam or pitman arm
to detect the change in time-averaged rod loading which
results from less fluid being moved to the surface after
"pump-off". Solid-state motor current sensors have recently
been used to detect the slight reduction-in average power
output of the prime mover that normally occurs at the onset
of fluid pounding or cavitation, and fluid flow switches have
been utilized to indirectly detect the change in pumping rate
of downhole equipment which occurs when the reserve of liquid
is first depleted from within the casing. Certain other
devices attempt to avoid "pump-off" altogether by measuring
the actual fluid level within the casing; these systems
typically operate by means of a downhole float switch mounted
on the tubing string immediately above the pump inlet, or by
means of a surface generated -acoustic signal that is
reflected off of the liquid/gas interface within the casing.
Unfortunately, all of the above methods for detecting
"pump-off" require that a sensing circuit be accurately
- calibrated for the specific installation at hand. _ Fluid
switches, for instance, typically operate by detecting a
change in the average or peak flow line pressure at the
wellhead, or by detecting a change in the average or peak
pressure differential across an orifice plate installed in
said line. When the average fluid exit pressure (or pressure
differential across the orifice plate) decreases below a
preselected trigger point, or when the peak pulsating
pressure amplitude or pressure differential ceases to rise
above this preselected reference point, then the system
automatically assumes that "pump-off" has occurred.
Selection of the correct trigger point for each application
requires that the operator have a detailed knowledge of the
pumping characteristics of his well, since the typical
"before" and "after" fluid exit pressures (or pressure
differentials across the orifice plate) must be known with
reasonable accuracy for proper calibration of equipment at
time of installation. Similar considerations will also apply
to "pump-off" detection systems that operate on the basis of
- 18 -
changing rod load, equipment vibration or prime mover power
output. Thus, the correct trigger point for each well
installation can only be determined by trained engineers or
technicians in the field, where conventional "pu~p-off"
detection equipment must be accurately calibrated for each
particular set of operating conditions.
Perhaps the greatest deficiency of conventional "pump-
off" detection equipment concerns their inability to
automatically respond to normal changes in both reservoir and
downhole equipment performance. Once a conventional sensing
circuit has been calibrated to a specific set of operating
conditions, it can thereafter only respond to changes in the
measured parameter (i.e. pressure, oad, vibration or power)
that occur relative to the selected point of reference. Most
of these parameters change on a daily basis throughout the
operating life of a well, however, and thus frequent
recalibration of conventional "pump-off" detection equipment
is required for dependable operation.
Still another problem with conventional "pump-off"
detection equipment concerns their inability to operate with
great sensitivity in situations where the well is operating
at a high ratio of "QF/Qp". As previously discussed, the
effects of fluid pounding or cavitation decreases with
increasing values f "QF/QP", and disappear completely when
the well is operated at a ratio of 1.0 or higher. Also,
slight changes in the pumping rate of downhole equipment
normally occur prior to the initiation of "pump-off" due to
the changing level and viscosity of fluids within the
casing. Unfortunately, the operator rarely knows the actual
operating conditions of his well, and thus he can not depend
on conventional equipment to perform properly under all
situations. This limitation severely restricts the wide-
spread use and application of conventional "pump-off"
detection equipment, regardless of their construction or mode
of operation.
From the foregoing discussion it should be readily
apparent that a new and improved method and apparatus for
detecting the onset of fluid pounding or cavitation at "pump-
off" would be quite beneficial to the efficient operation of
most producing wells. The present invention is directed
toward providing that method and apparatus.
An embodiment of the present invention measures,
computes and displays all important reservoir and equipment
performance parameters, and automatically alerts the operator
if the production potential of either well or pumping
- - 19 -
1 333962
~ equipment falls below a minimum acceptable level of
performance. The system accurately detects the onset of
fluid pounding or cavitation for any ratio of "QF/Qp" ~reater
than 0.0 and less than a reasonable upper li~it of
approximately 0.95, which is only slightly less than the
upper limiting value of QF/Qp=1.0 below which fluid "pump-
off" will always occur. A manual override circuit is
provided to bypass automatic operation of the well should the
operator so desire.
10The system accurately monitors the performance of both
fluid reservoir and downhole pumping equipment, and
automatically regulates the Duty Cycle of all pumping
equipment based upon the established value of "QF/Qp", so as
to optimize total fluid production and minimize operating
expense by limiting downhole pump operation to times when a
full head of liquid is available to its inlet. Provision is
made to automatically compensate for the transient effects of
gas or sand passing through the pump, and to compensate for
the detrimental effects of supply-line power interruptions
and fluid fall-back in the tubing string.
The system accurately measures the established duty
cycle of both surface and downhole pumping equipment,
together with total production time, total run time, and
total number of operating cycles for any specified production
period that a well is under its control. These parameters
are displayed in digital format with frequent automatic
update for benefit of the operator, regardless of whether the
well is automatically or manually controlled.
In addition, the system accurately measure the average
rate "QF" that incompressible solids and liquids are entering
the casing from the formation, and displays this performance
information in digital format with frequent automatic update
for benefit of the operator. The system additionally
measures the current average incompressible fluid pumping
rate "Qp" of all downhole equipment associated with the well,
without regard to whether the resulting flow is steady-state
or pulsating (i.e. highly transient) in nature. This
information is used by the system to automatically compute
and display the resulting overall volumetric efficiency of
all downhole pumping equipment.
In order to provide for accurate and reliable control of
the well under situations where the dimensionless ratio
"QF/Qp" is quite high (i.e. near the upper limiting value of
1.0 for "pump-off"), the system automatically adjusts its
control of a well to compensate for the slight but
perceptible change in the average incompressible pumping rate
"Qp" of downhole equipment that typically occurs as the
- 20 -
1 333q62
result of changing fluid levels and viscosities within the
casing during pump operation- Additional compensation is
made on an automatic basis to adjust for the sradual;change
in pumping rate that normally occurs as the average oil cut
and gas saturation of produced liquids changes throughout the
operating life of a well.
In order to correctly document the production history of
a well, the system accurately measures and records the
incompressible volume of all liquids exiting the wellhead
during a specified production period, and displays this
important performance information in digital format with
frequent automatic update for the benefit of the operator.
System accuracy is essentially independent of average fluid
viscosity, density, temperature, gas saturation, oil cut and
ambient weather conditions, without regard to whether such
flow is steady-state or pulsating (i.e. highly transient) in
nature.
All system hardware is mounted above ground for economy
of installation and maintenance, and is designed for fast and
simple connection to either new or existing wells. Such
equipment is designed to operate safely and reliably at any
supply-line voltage normally encountered in the field. All
electronic circuits are protected against transient power
surges and voltage spikes caused by lightning discharge near
the well-site, and the entire system is capable of accurate
and reliable operation over the entire range of ambient
temperatures and weather conditions that might normally be
encountered in the oil patch.
All system apparatus is self-calibrating to any well
regardless of the type of mechanical equipment installed
(i.e. reciprocating piston, centrifugal or rotary screw
pump), and regardless of the theoretical displacement and
volumetric efficiency of such equipment. No special
programming skills or prior knowledge of well performance or
downhole pump conditions is required of the operator in order
to achieve efficient and automatic control of any well, and
the fluid sensor is self-cleansing of all contaminants
normally associated with production formation liquids.
All elements of the invention are designed to function
automatically, in direct response to the measured rate that
produced liquids are extracted from the casing. This rate is
determined by a fluid sensor that is mounted in the tubing
discharge line of the wellhead to constantly monitor the flow
characteristics of such production. By accurately measuring
the true instantaneous rate that all incompressible liquids
exit the wellhead at each instant of time, and then
integrating this rate over a reasonable production interval
- 21 -
1 333~62
- that is sufficiently large to dampen out the transient
charaCteristics of pulsating or variable flow, the time-
averaged rate of fluid discharge may be accurately determined
for any selected production interval.
Primary control- of all pumping equipment is
automatically established by means of ~nique "pump-off"
detection apparatus that requires no special calibration at
time of installation, and that automatically adjusts for
normal changes in the operating characteristics of both well
and equipment throughout the production life of the
reservoir. This novel system accurately determines the onset
of fluid pounding or cavitation by sensing the rather abrupt
decrease in average downhole pumping rate that typically
occurs when the excess reserve of stored formation liquids is
first depleted from within the casing at time of "pump-off".
During periods of normal pump operation, incompressible
fluids exit the wellhead at an average rate "Qp" that is
precisely determined by the mechanical displacement and
volumetric efficiency of downhole pumping equipment. Once
the stored reserve of excess liquids has been removed from
the casing, however, fluids thereafter exit the wellhead at
an average rate that is solely determined by the established
fluid entry rate "QF" of new production. It is this
extremely predictable behavior that allows for the accurate
determination of fluid "pump-off" for any well regardless of
the flow characteristics of its reservoir, and regardless of
the condition or configuration of downhole pumping equipment
installed.
For efficient regulation of the well under all normal
production circumstances, automatic control of the prime
mover is divided into four distinct control intervals that
are sequentially advanced by the system during each complete
operating cycle of the pump. These control intervals are
referred to as the 1) Rest Period, 2) Prime Period, 3)
Production Period and 4) Verification Period. These four
sequencing intervals will be defined in greater detail
hereinafter in connection with the description of the
preferred embodiments of the present invention.
For purposes of the present discussion, the "Rest
Period" of normal pump operation begins when the prime mover
is automatically shut-down by the "pump-off" detector
following confirmed identification of this event. This
period is considered to be the initial phase of each
repetitive operating cycle, and is included in the control
sequence to provide sufficient time for a new reserve of
liquid to build within the casing prior to activation of
pumping equipment. The duration of this interval is
- 22 -
1 333962
controlled by a timing circuit that is manually programmed by
the operator based on his general knowledge of production
characteristics for the area surrounding his lease.i The
actual "Rest" time selected for programming is not critical
S as long as it falls within the general guidelines set forth
in the preceding discussion. Following termination of this
"Rest Period", a signal is automatically sent to the motor
control circuit of the system to initiate operation of all
pumping equipment.
The "Prime Period" of normal pump operation begins
immediately upon termination of the "Rest Period", when
pumping equipment first starts to operate, and continues
until such time as a steady (though perhaps pulsating) stream
of liquids emerge from the wellhead at an average stabilized
rate that is solely regulated by the average pumping rate
"Qp" of all downhole equipment. This "Prime Period" is
included in the control sequence of pump operation to allow
for the fact that liquids will frequently "fall back" into
the tubing string during the "Rest Period", and to compensate
for the fact that pumping equipment may be initially "gas-
locked" when first activated due to the prior ingestion of
casing gas at the conclusion of the previous operating cycle.
Transient effects within the tubing string such as fluid
separation or gas expansion near the wellhead are also
compensated for during this second important phase of
automatic pump control.
The "Production Period" of normal equipment operation
begins immediately upon termination of the "Prime Period",
following automatic system determination that the average
fluid exit rate has stabilized at the wellhead. Once this
operating sequence begins, the system automatically measures
the actual pumping rate "Qp" of all downhole equipment in
order to establish a meaningful baseline of reference for the
"pump-off" detection circuit previously described. This
rate, which is a function of the operating characteristics
and physical condition of downhole equipment, is also used to
automatically compute the overall volumetric efficiency of
all downhole equipment based on the known value of mechanical
pump displacement that is programmed into the system by the
operator at time of installation. The resulting value of
"Pump Efficiency", which is computed only once during each
operating cycle, is then displayed in digital format for
benefit of the operator.
Throughout the "Production Period" the system will
continuously upgrade its stored baseline of reference to
allow for the progressive decrease in average pumping rate
that normally occurs as the fluid l~vel within the casing is
- 23 -
1 333q62
reduced, and to compenSate for the abrupt decrease in pump
efficiency that normally occurs when the pump finally removes
all stored water from the casing and begins to ingest the pad
of oil that floats on top. This baseline rate is an ~verage
composite of all pumping rates measured during the previous
few minutes of pump operation, and thus does not immediately
reflect the abrupt change in pumping rate that typically
results when the downhole pump finally removes all liquids
from the casing.
During all periods of normal pump operation, the system
continuously monitors the current average rate of fluid exit
from the wellhead and compares this average rate with the
baseline rate in order to determine the onset of fluid
pounding or cavitation. Before "pump-off" the current rate
and baseline rate will be essentially the same; after "pump-
off" the current rate will be less than the baseline rate by
an amount that is linearly related to the dimensionless ratio
"QF/Qp" previously discussed. By sensing this change and
allowing for normal transients caused by the passage of gas
or other contaminants through the pump, the advent of fluid
pounding or cavitation will be quickly and accurately
detected for any well regardless of its reservoir and
equipment characteristics.
Following any preliminary indication that "pump-off"
has occurred, the system automatically enters a short
"Verification Period" of controlled pump operation in order
to properly confirm that all excess liquids have indeed been
removed from the casing. This last sequential phase of each
pumping cycle is required to compensate for any non-typical
transient effects within the tubing string that might
temporarily reduce the average fluid discharge rate at the
wellhead. Such transients might be caused by the passage of
sand, gas or other contaminants through the downhole pump, or
by the momentary surge of liqulds due to gas expansion at the
wellhead. During this "Verification Period" the automatic
termination of pump operation is delayed to provide
sufficient time for such transients to stabilize. Should the
measured pumping rate return to normal before the conclusion
of this "Verification Period", then the control sequence is
immediately reversed to reenter and extend the preceding
"Production Period"; in this case it is properly assumed that
a transient was responsible for the false indication of
"pump-off", and thus the erroneous signal is ignored. If, on
the other hand, the average fluid discharge rate does not
return to the previously measured baseline rate within the
"Verification Period" allowed, then the initial indication of
"pump-off" is assumed to be correct and the present operating
- 24 -
~ ~33 ~2
~ cycle is terminated. In this case the pump is immediately
de-energize so that the well can enter its next sequential
"Re~t Period" as herein described.
In order to minimize expensive production down-t'ime that
frequently results from the unexpected malfunction of pump or
control equipment, the invention is provided with an
automatic warning system that alerts the operator whenever 1)
the volumetric efficiency of all downhole pumping equipment
falls below a minimum acceptable value, 2) the fluid flow-
sensing element of the control circuit ceases to operateproperly, or is improperly sized for the particular
installation, or 3~ normal control-system power is
interrupted. Provision is also made for the more rapid
sequencing of each pump cycle so that prime mover operation
is limited in duration and eventually terminated in
situations where an adequate flow of liquids can not be
properly established or maintained from the wellhead. This
last feature restricts the operation of pumping equipment in
situations that might otherwise cause damage to the downhole
pump or stuffing box rubbers, or in situations where an
excessive amount of power is being wasted by inefficient
pumping.
Since it is a primary object of the invention to present
the operator with a complete set of meaningful performance
information that can be used to assist him with the efficient
control of his well, the present invention automatically
records the total number of operating cycles that are
initiated by the control circuit during any specified
production period. The total dur.~tion of this production
interval is also recorded, as is the total time of prime
mover operation and the total volume of liquids removed from
the casing. By measuring the net change in total fluid
production and prime mover operating time on a frequent basis
throughout the specified production period, the current
average fluid entry rate "QF" and duty cycle of pump
operation are also calculated automatically. All of this
performance information, together with the current pump
efficiency, is then displayed in digital format with frequent
updates.
In accordance with another aspect of the present
invention, the fluid sensing assembly includes a housing that
contains an internal flow passage separated into inlet and
discharge chambers by a rigid barrier wall that contains a
fixed-arèa orifice for controlling and directing the passage
of any acceptable homogeneous mixture of solids, liquids and
gases from one chamber to the other. A clapper plate
assembly mounts within the discharge chamber of the housing,
- 25 -
1 ~39~
in close proximity with, and parallel to, the discharge plane
of the orifice. This clapper assembly pivots on its integral
shaft in linear angular response to the instantaneous
volumetric discharge rate of such mixture as it passes
through the orifice to strike the clapper plate. By
definition, an acceptable homogenous mixture is one that
imparts the same angular response to the clapper plate as
would be imparted by a stream of pure incompressible liquid
having the same average mass-density and viscosity as the
stream of said homogeneous mixture. Thus, small amounts of
undissolved gases and relatively small particles (i.e., small
relative to the orifice size and clapper mass) may be
included within the homogeneous mixture without affecting the
accuracy of the clapper response to any noticeable extent,
lS provided that the average mass-density and viscosity of such
mixture is known for calibration purposes.
A permanent magnet, rigidly attached to the pivot shaft
of the clapper assembly, is contained within a third chamber
of the housing into which the clapper shaft extends. A
20- linear Hall-effect sensing element mounts within a fourth
chamber of the housing, near the magnet but separated
therefrom by a thin non-magnetic pressure barrier that
isolates the sensing element from fluid contact. The sensing
element and magnet sense the instantaneous angular position
of the pivot shaft and its attached clapper plate.
Electronic circuitry contained within the fourth chamber, or
any other dry chamber of the housing, amplifies and
compensates the output signal of the Hall-effect sensor to
obtain a calibrated output voltage signal that is linearly
related to the instantaneous volumetric flow-rate of the
known homogeneous mixture as it passes through the orifice,
without regard to the ambient temperature acting upon the
outside of said housing, or to the temperature of the mixture
passing therethrough.
It is to be noted that such a device, when properly
constructed and calibrated for a mixture of known pressure
and viscosity, produces an output signal ~Vsl~ that is
accurately related to the instantaneous volumetric flow-rate
"Q", orifice area "A", average fluid density "DF" and clapper
density "Dc" by a constant of proportionality "k" as follows:
Vs = (k*Z/A)* ~DF/(DC ~ DF~ (8)
Thus, for any given fluid density, clapper density and
orifice configuration, the calibrated output voltage ~Vsll of
any such device is linearly related to the volumetric flow-
rate "Q" of the known mixture passing through it, provided
- 25 -
1 333962
that the flow-rate "Q" is less than some maximum limiting
value whiCh typically corresponds to a clapper displacement
of between 25 and 30. ~he actual range of linearity for
any particular clapper/orifice geometry may be readily
determined by laboratory testing with the homogeneous mixture
in question. Such testing will also determine the correct
value of the constant of proportionality "k", which is
primarily related to the internal geometry of the sensor
assembly, and to its physical orientation relative to the
Earth's gravitational field. This factor also includes the
variable effects of pressure an~ viscosity upon clapper
response, which are of secondary importance when the sensor
is used to monitor the volumetric flow-rate of a known
homogeneous mixture of incompressible solids and liquids.
The calibrated sensor assembly described above may also
be utilized to accurately monitor the volumetric flow-rate of
any other homogeneous mixture of solids, liquids and gases
having a different pressure, density and/or viscosity than
the mixture used for sensor calibration. Properly
constructed, the response of the clapper plate will be
essentially independent of the average viscosity of the
homogeneous mixture that strikes it, since the moment arm of
frictional forces acting upon the clapper will be negligibly
small about the pivot shaft. If such mixture is comprises
entirely of solids and incompressible liquids, then the
factor "k" will also be essentially independent of the
internal static pressure of the flowing mixture. Whenever
the mixture includes large quantities of undissolved gas
bubbles, however, then it will cease to behave as an
incompressible mixture. In such situations the constant of
proportionality '!k" must be evaluated to include the effects
of fluid compressibility, which are related to the internal
geometry of the sensor and to the static pressure of the
flowing mixture. Such effects may be readily determined at
time of sensor calibration, when the instantaneous output
voltage signal is determined based upon a known standard of
reference. Once such calibration is achieved, the sensor may
then be used with other homogeneous mixtures of known
pressure, density and viscosity in order to accurately
monitor the instantaneous volumetric flow-rate of such
mixtures as they pass through the sensor housing. In such
situations the correct volumetric flow-rate "Q" may be
accurately determined at each instance of time by adjusting
the instantaneous output signal of the sensor for the known
effects of pressure, density and viscosity as hereinafter
described.
- 27 -
~ Still other objects and advantage~s~ present
inventiOn will become readily apparent to those skilled in
this art from the following detailed description, wherein we
have shown and described only the preferred embodimentiof the
invention, simply by way of illustration of the best mode
contemplated by us of carrying out our invention. As will be
realized, the invention is capable of other and different
embodiments, and its several details are capable of
modifications in various obvious respects, all without
departing from the invention. Accordingly, the drawings and
description are to be regarded as illustrative in nature, and
not as restrictive.
Figures lA and lB comprise a schematic elevation showing
how the invention is used in a typical oil well installation.
Figure 2 is a schematic elevation of the invention
showing some of the circuit elements found in thé data
processing and control unit.
Figure 3 is an external perspective view of a preferred
embodiment of the flow sensor, constructed in accordance with
the invention.
Figure 4 is a cross-sectional view of the fluid sensor
assembly taken along line 4-4 of Figure 3.
Figure 5 is an exploded perspective view of the sensor
assembly of Figure 3.
Figure 6 is a cross-sectional view of the fluid sensor
assembly taken along line 6-6 of Figure 3.
Figures 7A through 7D comprise a block diagram of the
electronic circuits of the subject invention.
Figures 8A through 8D comprise a schematic diagram of
detailed electronic circuitry of the subject invention.
Figure 9 is a graphic depiction of the various control
signal responses of the preferred embodiment of the
invention.
Figures 10 and 11 are graphs depicting the sequence of
events of the pump-off detector control signals, in
accordance with the invention.
Figure 12 is a block diagram of the electronic circuits
of a microprocessor controlled embodiment of the subject
invention.
- 28 -
1 333962
With reference to Figure 2, the digital well-control
system (DWCS) of the present invention is comprised ~f four
basic hardware assemblies that are referred to herein,as the
fluid sensor 48, cable 8, data processing and control unit
(DPCU or control unit) 2, and back-pressure valve 50. Each
of these components is surface mounted near the wellhead or
existing motor control panel, and each works in conjunction
with the other to monitor and control the performance of
both downhole and surface mounted pumping equipment, as
hereinafter described.
As depicted on Figures 1 and 2, a typical well
installation has a string of production casing 64 that
extends downward from the surface of the earth 54 to some
completion depth 78 that lies below a producing fluid
reservoir 84. The annulus between the open bore-hole 72 and
casing 64 is filled with a cement slurry 80 from the bottom
of the completion interval 76 to some point 74 well above
the fluid reservoir 84 in order to consolidate the hole and
keep unwanted formation fluids from communicating with the
producing reservoir 84. Cement 80 and casing 64 are both
selectively perforated at multiple location 82 to provide
permeable flow-channels through which desired fluids may
enter the casing 64 from the reservoir 84. If necessary,
the reservoir 84 may be stimulated by acid or hydraulic
fracture 86 to enhance the rate of fluid entry into said
casing 64.
Contained within casing 64 is a string of production
tubing 66 that hangs from wellhead 62 and extends downward
to a depth 88 that is near the producing reservoir 84.
Attached to the bottom of this tubing string 66 is a piston
pump assembly 98 that is comprised of a barrel 92, a
traveling ball valve 90, a standing ball valve 94 and a pump
inlet 96, The pump 98 is actuated by a string of sucker
rods 68 that attach to traveling valve 90 and extend upward
within the tubing string 66 to connect with a polish rod
assembly 42 near the surface 54. The polish rod assembly 42
passes through production tee 46 and stuffing box 44 of the
tubing assembly 66 to connect with horsehead 36 of the
pumping unit assembly 34 by means of bridle assembly 38 and
polish rod clamp 40 that supports the entire rod assembly 42
and 68 and fluid column 70 within the tubing string 66.
Horsehead 36 of the pumping unit assembly 34 is
connected to rocking arm 32 that rests upon and pivots about
the top of sampson post 30. This assembly rests upon the
base structure 24 of the pumping ur.lt assembly 34, to which
is also mounted the prime mover 14 and speed-reduction
- 29 -
1 333q62
~-- gearbox 20. The input shaft of gearbox 20 is driven by
prime mover 14 that delivers power to sheave 18 by means of
flexible power transfer belts 16. The rotating output~shaft
of gearbox 20 is connected to crank arms 26 that im~art a
reciprocating motion to rocking beam 32 by means of pitman
arms 22. Attached to crank arms 26 are balance weights 28
that serve to balance the combined static load of rod string
68 and fluid column 70 as such fluids are pumped from depth
88 to surface 54. All pumped fluids exit production tee 46
and pass through fluid sensor 48 and fluid back-pressure
valve 50 before entering a fluid transfer line 52 that
transports both liquids and their dissolved gas constituents
to either tank or pipeline (not shown).
The average static discharge pressure of all fluids
passing through sensor 48 is established by means of back-
pressure valve 50, and is measured by means of pressure
gauge 58. Casing gas flows directly into gas pipeline 56 at
an average static discharge pressure that is measured at
wellhead 62 by pressure gauge 60. Gasses and liquids are
later separated and measured by equipment not shown.
Stuffing box 44 serves as a packing gland to prevent
pressurized tubing fluids from leaking out of production tee
46 as pumping unit 34 imparts a reciprocating up-and-down
motion to polish rod 42, rod string 68 and traveling valve
90 which in turn lifts fluid column 70 to the surface 54.
Mounted at surface elevation 54 near prime mover 14 is
the Data Processing and Control Unit (DPCU) 2. ~his unit
receives unregulated AC line power by means of cable 6, and
delivers highly regulated DC power to fluid sensor 48 by
means of wire harness 8. Fluid sensor 48 measures the
instantaneous volumetric flow-rate of all incompressible
liquids exiting production tee 46, and sends this
information back to the DPCU 2 by means of wire harness 8
for fluid density correction and further processing. DPCU
2 uses measured flow-rate information to establish efficient
automatic control of prime mover 14 by means of control line
12 and magnetic starter 10. Measured flow-rate information
is also used by DPCU 2 to evaluate the production
performance of fluid reservoir 84 and all downhole pumping
equipment, including downhole pump 98, tubing string 66 and
sucker rods 68. All meaningful performance parameters are
automatically computed and displayed in digital format by
DPCU 2 with frequent update for benefit of the operator as
previously described. Should the performance of either
reservoir 84 or downhole equipment (66, 68 or 98) fall below
certain reasonable limits, then DPCU 2 will automatically
terminate the resulting inefficient operation of prime mover
- 30 -
1 333962
14, and will simultaneouslY actuate a horn and/or strobe
light 4 to advise the operator of his need to perform
maintenance on the well. I
The fluid sensor 48 mounts in the liquid discharge~ line
5Z of the wellhead, immediately downstream of production tee
46, and basically comprises housing 156 (Figure 3) that
controls the flow of fluids as they exit the tubing string
66. A sensing element 158 responds to the instantaneous
volumetric flow-rate of the fluids as they pass through the
housing. Electronic amplification and referencing circuitry
120 (Figure 4) contained on PCB 106 converts the measured
flow-rate response into a temperature compensated output
voltage signal that is linearly related to the absolute
magnitude of the highly variable fluid discharge rate.
The cable 8 is used to interconnect the fluid sensor 48
with the control unit 2. The cable includes a conventional
wire harness 154 that contains four insulated electrical
conductors which are surrounded by braided metal shielding.
The shielding is encapsulated within an oil-proof vinyl
covering. Each end of the cable terminates with a polarized
weatherproof electrical connector 152 that quickly and
easily interfaces with the fluid sensor and DPCU in the
field. The four shielded conductors of the cable are used
to provide the sensor with: 1) regulated "B+" power of
approximately 15 vdc; 2) a temperature compensated precision
voltage reference "Vtc" of approximately 12 vdc; 3) a common
0 vdc earth ground buss; and 4) an output channel over which
the analog flow-rate signal "Vf" is çontinuously transmitted
to the DPCU for further amplification and evaluation
processing.
The control unit 2 (DPCU) provides regulated DC power
to all system components; monitors, computes and displays
the downhole performance of the fluid reservoir and all
pumping equipment based upon measured flow-rate information;
and controls the operating cycle of the prime mover to
optimize the production efficiency of the well.
The backpressure valve 50 is of conventional design and
construction, being comprised of a spring loaded ball or
plunger (not shown) that automatically regulates the fluid
exit area of a fixed discharge orifice contained within the
valve's housing assembly. This valve is mounted in the
liquid discharge line of the wellhead, downstream of the
fluid sensor 48, and is manually adjusted at installation to
keep all formation gasses in complete liquid solution within
the tubing string 66 at all times. By so doing, the total
incompressible volume of all produced liquids may be
accurately computed using flow-rate information measured by
- 31 -
1 333q62
sen50r 4R without the need of signal adjustment for the
effects of compressibil~tY~ To achi-ve thi~ re~ult, the
wellhead dischar~e pressure mu-t be ma~ntained at or~above
the ~reate~t bottom hole pre8sure that wlll act upon the
downhole pump inlet at any time du~ing the operatin~ cycle.
This pres~ure $8 equal to the summation of the measur-d
caslng ga~ pre~ure at the wellhead, p1us the hydrostat$c
pre~ure of fluid bulldup within the casing immediately
following each successive ~est per$od. Thls last component
~ay be readllSy computed know~ng the casing volume factor,
flu~d entry rate, rest time and average fluid denslty of
produced formation llquid~. All of these factors are either
known by the operator with sufficient accuracy at time of
installat$on, or can be accùrately measured during the first
few days of actual pump operation.
With reference to Figures 3-6, the fluid sensor hou~ng
156 may be constructed of bron2e, stainle~s steel~,
f$berglass, ceramic or any other hi~h-strongth and
dimenslonally stàble material that is non-magnetic and
corroslon resi~tant, The hou~ing i8 configured similar to
that of a conventional Y-pattern check valve, with th- inlet
chamber 101 and di-charge flow chamber 103 being sQparated
by a rigid barrier wall 105 that conta1ns a fixed area
orifice 107 through wh~ch all produced formation liqulds
must pass. Machined into the barrier wall is a smooth
annular seat~ng surface 109 that surrounds the dlscharge
edge of the orifice to provlde a tight seal with the mating
surface of a pivoting clapper di~ 1;8. The di~k and its
attached clapper arm 160 should rotate as an integral unlt
a~out a pivot ax~s 111 that is ~ocated above and
perpendicular to the longitudinal flow axis 113 of the
housing, and which ls parallel to the plane of the orifLce
seat 109. In order that gravitati~nal forces might always
act to ~eep the clapper di~k in clos- proxim~ty with the
orifice discharge plane, the seating surface and barrier
wall should both be inclined by approximately 45 degree6
from the horizontal.
The clapper d1sk lS8 and lt~ lntegral p$vot arm 160 are
rlgidly attached to a smooth, round pivot pin 13~ tha~
mount~ wlthin a bor~-hole llS that is machined cro~wl~e
through the housing body 156. Thi6 precision bearing
surface 115 i8 drilled and reamed concentric with the
de~ired plvot axi~ 111 ln order to accurately position the
clapper a6sembly relative to it~ orifice seat ln order to
minimize the effect of viscous drag upon the rotational
re~ponte of the clapper the axis of the pivot shaft 138 ig
located a8 clo8e as po~sible to the plane of the orifice
- 32 -
1 333~62
~eat 109. One cide of the bearin~ surface 115 extendsthrough the external housing wall to provlde easy access to
the pivot shaft 138 during as8embly and calibration
operatlons~ the hole i5 plugged by a cap 168 and ga~Ket 166
when the operations are completed. The other siae 117 of
bore-hole llS extends through the opposite wall of the
housing lnto a third pressure chamber 146 that contains a
small cylindrical U-shaped magnet 124 which i- porm~nently
attached to the end of the pivot ~haft 138 at ~lme of
assembly. The chamber 146 i8 ma¢hlned into a solid bo~s 148
that extends ~n a hor~zontal direction from the ~ide of
hous~ng 156, and which i~ ca~t or for~ed a~ an integral part
of this support$ng member.
Located ad~acent to thi- inner pres~ure chamber 1~6 18
a fourth outer chamber 144 that serves to`contain a small
printed circuit board IPCB) 106 upon w~$ch are mounted
various electronic components 120. ~oth of these chamber~
are preferably cylindrlcal ln shape, and machined ooncentrlc
with the plvot axis of the clapper assembly ln order to
provide for the proper fit and operation of all components
that wlll be mounted therein.
Prior to ~ormin~ bearing ~urface 115, the clapper
member 158 15 positioned and restrained within the housing
after both the or~fice seat 109 and clapper seat have been
machined smooth and ~lat. ~y l~ne drilling both mat~ng
parts toqether, a good metal-to-metal seal i~ readily
achieved at the clapper/orifice ~nterface. Following
completion of this operation, chambers 144 ant 146 can then
be machined to their proper dimensions by u~ing the
resulting shaft bore-hole as a pllot for the required
cutter~. To facilitate the lnstallation of a cylindrical
baffle-plate 116 and O-Ring 134 that serves as a pre~sure
barrier between both compartments 144 and 146, the inner
magnet chamber 146 should be of smaller d~ameter than the
outer PC~ chamber 144. In thi~ manner the ba~fle-plate
assemb~y 116 can be readily mounted at tne bottom of the PCB
chamber 144 by a plurality of small cap screws 110 that
engage the seating surface 125 which then surrounds the
inner magnet chamber. Thls con~truction also m~nimizes the
preQ~ure forces that act upon the baffle-plate mounting
screw~ 110, while still providlng ample room for t~e PCB 106
and its electrical compon~nts 120. For obvious rea60n~ ~ the
de~ired orlfice 107 diameter should be machined into the
flow-chamber barr~er wall at the sam~ time t~at the orifice
seating surface 109 is cut and finished.
1 333962
In assembling the sensor assembly, the magnet 124 is
bonded with epoxy or other acceptable adhesive material to
one end of the clapper shaft 138. A thin low frliction
thrust washer 132 is positioned around the shafit 138
immediately adjacent to the rear edge of the magnet, and
this entire assembly is inserted through the housing bore-
hole 117 and 115 to engage the clapper arm 160 which holds
this member in position. The baffle-plate 116 is installed
at the bottom of the PCB chamber 144 using an O-Ring 134 and
cap screws 110 to provide a secure barrier between both
chambers. The lower baffle-plate protrusion 118 extends
into the interior of the hollow cylindrical magnet 124 to
engage the end of its pivot shaft to limit the axial play of
this assembly.
Once the pivot-pin 138 and baffle-plate 116 assemblies
have been installed, the completed PCB 106 with all
electronic co~ponents is mounted on three small standoffs
114 with screws 104 that serve to position this assembly
within the outer PCB chamber. Due to space limitations, all
solid-state components with the exception of the linear
Hall-effect sensor 112 and its adjacent temperature
compensating zener diode 108 are mounted on the top surface
of the PCB, away from the baffle-plate 116 and magnet 124.
One such Hall-effect sensor is made by Texas Instruments
under product No. TL-173. This construction provides for
easy access to several trim pots during calibration
operations.
By contrast, the Hall-effect sensor 112 and zener diode
108 are mounted on the lower surface of PCB 106 so they are
contained within the hollow baffle-plate protrusion 118 that
extends between the poles of the magnet 124. In this manner
the Hall-effect sensor 112 can readily sense the angular
position of the magnet 124, and both Zener diode 108 and
Hall-effect sensor 112 are exposed to the same operating
temperature at all times. A weatherproof electrical
connector 150 is permanently installed within the lower
wall 127 of the PCB chamber 144 to provide for proper
input/output of the four electrical channels previously
referenced. All pins of connector 150 are connected with
the proper PCB terminals by means of short jumper wires 129
and solder connections.
Once the PCB assembly has been installed and interfaced
with its electrical connector, final assembly and
calibration of the sensor assembly can begin. The first
adjustment that must be made concerns proper phasing of the
magnet 124 and shaft 138 relative to the Hall-effect sensor
112 and orifice seat 109. By removing cap 168 and reaching
- 34 -
1 3~62
through the open end of the pivot-pin bore-hole, shaft 138
and its attached magnet may be easily rotated by means of
screwdriver slot 136 to properly orient both components so
that the output voltage signal of the Hall-effect ~ensor
will be at its average null position when the clapper is
resting upon seat 109. Properly phased, the output voltage
of the sensor increases with increasing pivotal lift of the
clapper. For reasons later discussed, the angular position
of shaft 138 is then adjusted by a negative rotation of
approximately 12 degrees in order to obtain the desired
phasing for a zero flow condition. Once this phasing has
been accomplished, the clapper member is permanently
attached to the pivot shaft by a set screw 162 and adhesive
material introduced into the clearance between shaft and
clapper boss. After such bonding, the housing access port
170 and shaft bore-hole 115 are the~ plugged with removable
caps 164 and 168, respectively, using either thread
compound, O-Rings or gaskets as desired.
Before continuing with a detailed discussion of final
sensor calibration, it is first necessary that the general
operating characteristics of the mechanical and electrical
flow-rate sensing elements used in this invention be
described in sufficient detail to provide a basic
understanding of the response that is to be derived from
these components. With reference to Figure 4, the fluid
sensor contains the linear Hall-effect sensor 112 that
detects the angular orientation of the permanent magnet 124
which is rigidly attached to the pivoting clapper shaft 138.
Theoretical considerations, confirmed by actual
laboratory tests, indicate that the instantaneous angular
displacement "c" of the clapper assembly relative to its
orifice seat is linearly related to the instantaneous
volumetric flow-rate "Q" of any homogenous fluid mixture
that passes through the orifice to strike the clapper plate,
provided that such mixture behaves within the sensor as an
incompressible fluid from a fluid mechanics standpoint.
Theory also indicates that this deflection is related to the
orifice area "A", average fluid density "DF", and clapper
density ''Dc'' by a constant or proportionality "k" that
relates all of the above parameters as follows:
c = (k*Q/A)* ~DF/(Dc ~ DF) (9)
As previously disclosed, the constant of
proportionality "k" may be readily determined in the
laboratory at time of sensor calibration by using a
homogeneous incompressible liquid of known average mass-
- 35 -
1 333962
densitY to establish a meaningful standard of reference for
the particular sensor in question. If the sensor is
properly constructed, the measured value of "k" will jbe a
primary function of sensor geometry only, and will npt be
greatly affected by the actual value of fluid pressure or
viscosity selected for the calibration liquid. Once
calibrated, the rotational response of the clapper plate and
its attached pivot pin will thereafter be accurately
described by the above equation (9) whenever the sensor is
used to monitor the instantaneous volumetric flow-rate of
any other incompressible homogeneous liquid of known average
mass-density, the instantaneous angular response of the
clapper assembly is linearly related at all times to the
instantaneous volumetric flow-rate of any such liquid
passing through the sensor, provided that the linear
deflection range of the assembly is not exceeded.
Due to the effects of the rotating magnetic field, the
output signal of the Hall-effect sensor 112 is sinusoidal in
nature, being a primary function of the magnetic flux angle
"c" of the pivot shaft. Because of trigonometric
considerations, however, the output of this sensing device
is essentially linear with angular rotation of the clapper
assembly for any reasonable positive or negative
displacement about the "O" degree null position. This
linear relationship is maintained with conslderable accuracy
for relatively large angular displacements in either
direction, such accuracy gradually decreasing from 100% at a
displacement of "O" degrees to approximately 99% at a
displacement of + 14. ~y phasing the calibrated "no-flow"
position of the clapper/magnet assembly to correspond with
the negative 12 degree angular position of sensor 112, and
then restricting the operation of this assembly to flow-
rates that cause an angular rotation of no more than 24
degrees, the output voltage of the Hall-effect sensor 112 is
then linearly related to the actual volumetric flow-rate of
all incompressible fluids measured with a high degree of
accuracy. Thus, for any specific orifice size "A", fluid
density "DF" and clapper density ''Dc'', the instantaneous
output signal "Vf" of such temperature compensated circuitry
is linearly related to the instantaneous volumetric flow-
rate "Q" of any incompressible homogeneous liquid by a new
constant of proportionality "K" that is essentially
independent of fluid pressure and viscosity as follows:
Vf = (K*Q/A~ ~DF/(Dc~DF) (10)
- 36 -
1333q62
- The electronic circuitry 120 contained on the Sensor
PCB 106 of Figure 5 is designed to provide an accurate
linear output response over the entire range of calibrated
flow-rates, from a "no-flow" condition of 0.0 gpm to some
limiting value that can be readily determlned on the flow-
bench for each specific orifice size, based upon a known
orifice area "A", clapper density ''Dc'', and calibrating
fluid density DF .
Accurate temperature compensation of Hall-effect sensor
112 is achieved by means of an electronic circuit that
matches the linear temperature drift of the zener diode 108
to the temperature characteristics of the Hall-effect sensor
112. Because no two devices are exactly alike, compensation
is accomplished by an adjustable resistor network that trims
the greater positive temperature coefficient of the selected
diode with the lesser positive temperature coefficient of
the actual Hall-effect sensing device 112 used in this
assembly. Properly calibrated, the adjusted zener voltage
has the same temperature response (+B*dT) as the Hall-effect
sensing element. Both output signals are then applied to
one stage of a voltage differencing amplifier 202, which
continuously subtracts the trimmed reference voltage (Vr +
B*dT) from the sensor output voltage (Vs + B*dT) to derive a
new output voltage "Vo" that is non-temperature dependent as
follows:
Vo = (Vs + B*dT) - (Vr + B*dT~ = (Vs - Vr) (11)
In order that both input signals to amplifier 202
always change together with changing operating temperatures,
the Hall-effect sensor 112 and zener diode 108 are mounted
immediately adjacent to one another in the same hollow
protrusion 118 previously described. Calibration of the
temperature compensating circuit is achieved by adjusting a
trim pot 204 on the zener voltage division network 206 so
that the reference voltage applied to the input resistor 208
of the negative input of op-amp 202 has the same temperature
characteristic as the sensor voltage applied to the input
resistor 211 of the positive input. Since the operating
characteristics of the op-amp 202 chip must also be
stabilized for variable ambient temperatures, and for any
variations in fluid temperature that act upon the housing
and its contained electrical circuit, the op-amp is located
within a small oven enclosure 212 that maintains a constant
chip temperature of approximately 150 F at all times.
1 333962
With reference to Figure 8A, the output signal "Vo" of
the first voltage differencing amplifier 202 is next be
applied to the input of a second op-amp 215 in order to
amplify the temperature compensated signal and reference it
to ground potential. Basic amplification of the input
voltage is accomplished by means of the various fixed
resistances 217 utilized on the input and feedback loops of
this second op-amp, and final calibration of signal gain is
achieved by means of a trim pot 216 on the output of op-amp
215. Proper ground reference is achieved by adjusting the
voltage tap 218 on the negative il,put bias circuit of Op-amp
215 so that the second stage output voltage is exactly 0.0
vdc at a measured flow-rate of 0.0 gpm. Following this
operation, a known flow-rate "Q" is then passed through the
sensor housing so that the output signal of the second op-
amp can be correctly adjusted by potentiometer 216 for the
particular flow-rate , orifice size and fluid density in
question. Properly calibrated, the sensor output voltage
"Vf" will be exactly 0.000 vdc at 0.0 gpm and 10.000 vdc at
the maximum linear flow-rate specified for that orifice
size. For any given flow-rate, this output signal remains
constant with changing fluid temperatures and ambient
conditions. In order for these objectives to be met, it i5
necessary that the Hall-e~ffect sensor and zener diode be
driven by a highly stabilized precision reference voltage
"Vtc", which is supplied together with B+ voltage by the
control unit 2 through cable 8. All input leads are
protected against power surges and lightning strikes by
transient voltage suppressor 222 as shown, and the entire
PCB assembly is then fully encapsulated in epoxy following
final calibration. After encapsulation, a cover plate 102
is installed over the PCB chamber 144 to provide additional
protection and aesthetic appeal to the entire assembly.
Proper selection of the correct orifice size for each
particular well installation is determined by the average
pumping rate of all downhole equipment, since the maximum
instantaneous rate that fluid flows through the sensor 99
should never exceed the maximum linear rate specified for
the selected orifice size. In order to allow for the
variable effects of fluid density and pump efficiency, and
for the quasi-sinusoidal characteristics of pulsating flow,
actual sensor capacity should always be selected at least
10% greater than the theoretical capacity of any installed
centrifugal or rotary screw pump, and at least 85~ greater
than the theoretical displacement of any piston pump. Four
different sensor sizes ~A through D) have been selected for
efficient coverage of practically all stripper well
- 38 -
1 333962
installations; these relative sizes, together with their
rated capacity for the accurate measurement of both
pulsating and steady-state flow, are as follows: ~
Sensor Size Pulsating Capacity Steady-State Capacity
A 75 BFPD 125 BFPD
B 150 BFPD 250 BFPD
C 300 BFPD 500 BFPD
D 600 3FPD 1000 BFPD
The regulated "B+" power supply 200 (Figure 8A)
contained within DPCU 2 basically comprises an AC step-down
power transformer 224 with 115-230-460 vac primary input
voltage taps that provide a nominal secondary output of
approximately 22 vac with 90% regulation at a steady current
delivery of 3.0 amps Dc A full-bridge diode rectifier 226
converts AC power to DC. A regulating DC filter capacitor
228 of approximately 6800 micro-farad capacity is connected
across rectifying circuit 226 to dampen-out the voltage
transients imposed by the AC charger. A "first-pass" NPN
power transistor 230 with controlling zener diode 232
provides a regulated output of approximately 19 vdc. A
manual DPDT switch 234 is connected to the emitter of
25 transistor 230. A "second-passl' NPN power transistor 214
is controlled by an voltage sensing op-amp 250 with feedback
loop and voltage regulating zener diode 240 to provide for a
highly regulated "B+" output voltage of approximately 15.0
vdc.
The emergency "Ve" power supply 210 of the DPCU 2
regulates the automatic shutdown of all critical system
components whenever total interruption of normal operating
power is warranted. This system, which connects to the 19
volt power buss of the previously described "B+" power
supply, serves as both a latching relay and crowbar circuit
to sequentially apply emergency ''Ve'l power to a malfunction
indicator control circuit, and to remove normal B+ power
from all other pumping and control system components,
following positive activation of either the four-cycle
40 Shutdown 554 or the Fxcess B+ current detector 553. By so
doing, this protective system guards against wasteful power
consumption and equipment damage that might otherwise occur
due to the unforseen failure of mechanical or electrical
equipment, or due to operator negligence.
- 39 -
1 333962
`~ With reference to Figure 8A, the emergency "Vel' Power
supply 210 basically comprises a regulated NPN power
transistor 242 with controlling zener diode 244 that
provides emergency l'Vel' power when activated by voltage
sensing Op-amp 238. This Op-amp has a reference voltage of
approximately 6 vdc applied to its negative input pin by
resistive network 249, and the two previously referenced
triggering signals applied to its positive input terminal.
A time-delaying RC circuit 246 with blocking output diode
248 interrupts normal B+ power by driving the negative input
of regulating op-amp 250 high.
The excess "B+" current detector shown in Figure 8A
includes a l/lOth ohmn dropping power resistor 437 that is
placed in series within the 19 volt power buss of the B+
power supply to provide for an instantaneous voltage
response that is proportionately related to the amount of DC
current flowing through this buss. A voltage sensing op-amp
253 switches "high" when the DC current passing through
resistor 437 exceeds a certain limiting value of
approximately 3.5 amps. A voltage dividing trim
potentiometer 439 is adjusted to apply a calibrating
reference voltage to the positive input of the voltage
comparator 253, and a time-delaying RC circuit 252 is used
to dampen the output response of the control circuit by
approximately one (1) second in order to provide for the
normal passage of reasonable translents without false
triggering. The output of the current detection is
connected to the crowbar latch of the Emergency 'lVe~ power
supply by way of the non-volatile CMOS memory chip 553
(Figure 8B) that is used to drive the LED indicating light
for this circuit. Once this circuit has been activated,
normal operation of all system components can thereafter
only be reinstated by a manual reset of this memory chip
followed by a momentary interruption of DC control power by
switch 234 lFigure 8A).
The "Vtc" precision voltage reference 260 shown in
Figure 8A provides a precisely calibrated reference voltage
for use by the temperature stabilizing oven thermostat, and
by the flow-rate and low pump efficiency monitors herein
described. The voltage reference 260 includes precision
voltage reference chip 255 bearing the product designation
No. LM3999 and made by National Semiconductor. Voltage
reference 255 controls the output of an NPN power transistor
254 by means of a switching op-amp 256 with voltage dividing
feed-back loop 258. This feed-back loop is used to amplify
the nominal 7 VDC signal supplied by the voltage reference
255, and to impart greater current output capability to the
- 40 -
1 333962
~ resulting reference voltage. The required "Vtc" reference
voltage is determined by selection of the voltage dividing
resiStors 258 used to construct the regulating feed-back
loop of the op-amp. Accurate temperature stabilization of
this circuit is achieved by means of compensating circuitry
located within the voltage reference 255 itself, and by the
physical mounting of all electrical components within a
temperature stabilized oven enclosure 262.
The motor controller and performance monitors of
Figures 8C and 8D are sequenced by a digital time clock that
delivers a precisely regulated s~uare wave output which
oscillates at a constant frequency of 6.666667 Hz whenever
DC power is applied. This pulse is generated by a 3.579545
MHz XTAL Quartz oscillator 266 that drives pulse shaping
circuitry located within the oven enclosure 262 and by
external circuitry 268 that digitally divides the resulting
square wave pulses by a constant value of approximately
536,931 to deliver the 0.15 second pulse referenced above.
The stabilized signal then passes through various digital
dividers, rotary switches and electronic gates to establish
the proper sequencing for all control and performance
measuring circuits to be described.
The temperature stabilizing oven 262 accurately
regulates the operating characteristics of certain system
components. These components include the digital quartz
oscillator 266 (Figure 8D), precision voltage reference 260
(Figure 8A) and the two voltage controlled oscillator (VCO)
535 and 403 (Figure 8C) that are required for the accurate
measurement of pump efficiency and total produced fluid
volume, respectively. A network of internally mounted
heating resistors 276 (Figure 8A) receive electrical energy
from an externally mounted NPN power transistor 278 to
maintain a constant operating temperature within the oven
262.
As depicted in Figure 8A, the governing oven controller
530 includes a voltage sensing amplifier 280 that drives the
base of power transistor 278 through a current limiting
resistor 282, and a voltage dividing resistance network 284
that contains a negative temperature coefficient thermistor
286 which serves as the temperature sensing element. The
voltage which is applied to the pLus input of the
controlling op-amp 280 decreases with increasing oven
temperature due to the decreasing resistance of the
thermistor 286. Thus, by selecting the proper resistance
network 284, the op-amp can be calibrated to interrupt power
to all heating elements at an internal oven temperature of
approximately 150F. This temperature may typically be held
- 41 -
1 333962
to within plus or minuS 2 for any ambient temperature within
the anticipated range of -40 to +120F. In order to
maintain such calibrated accuracy during actual field
operation, the resistance network 284 must be powerediby the
stabilized "Vtc" reference voltage 257.
The power-on delayed-pulse generator 290 shown in
Figure 8D assures the proper sequencing of all motor control
and performance measuring circuits following initial
application of DC power. The generator 290 is controlled by
a dual programmable timer 292 with supporting resistors,
capacitors and diodes that function together as one unit to
deliver a "positive-going" output pulse after a reasonable
delay of several seconds. This delay provides sufficient
time for the B+ power supply 200 and all system components
to power-up and achieve their normal operating state before
initial sequencing is effected. Following this delay, an
initializing pulse is automatically transmitted to the
various electronic circuits that control the pump efficiency
monitor 548, the duty cycle monitor 520, the fluid entry
20 rate monitor 510, the prime period controller 350, the
production sequence controller, and the four-cycle shutdown
500 in order that each might begin their operation in proper
sequence. Pulse generator 290 is similar in design to a
second pulse-delaying circuit 506 that is included to reset
the digital counters every 1440 minutes for the periodic
measurement and digital display of average duty cycle and
fluid entry rate every 24 hours.
As previously noted, the OUtpl1t voltage signal l'Vf" of
the fluid sensor is linearly related to the instantaneous
flowing velocity "Q/A" of all produced liquids that pass
through its fixed-area orifice, and to the square-root of
the density ratio ''DF/(DC - DF)" that controls the acting
clapper response mechanism. Because of this dependency on
both the orifice area "A" and the average fluid density
"DF", the incoming flow-rate signal "Vf" must be adjusted by
the system for each of these controlling parameters in order
to obtain an accurate measure of the true volumetric flow-
rate that exits the wellhead at each instant of time.
Similar adjustments may also be required for the effects of
fluid pressure and viscosity, depending on the internal
geometry of the sensor assembly and the degree of
compressibility of the flowing homogeneous mixture. The
required steps for processing this signal may be easily
visualized by rearranging equation (10) as follows:
Q = Vf * ~(Dc - DF)/DF * (A/k) (12)
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1 333962
For simplicity of design and operation, both analog and
digital compensating means are utilized within the DPCU 2 to
correctly adjust the resulting flow-rate signal for the
controlling orifice function (A/k). Such compensat'ion is
performed on a selective basis within each performance
measuring circuit as required, and is initiated by means of
a 3-pole four-position rotary switch 294 (Figure 8C) that
selects the correct processing channels for each of the
previously referenced orifice sizes A through D. The
specific means utilized within each particular circuit for
such flow-area compensation will be discussed in greater
detail hereinafter.
Compensation for average fluid density "DF" is
accomplished at the same time for all circuits by analog
fluid density amplifier 300 that adjusts the gain of the
incoming flow-rate signal IIVf" before this signal is
buffered and distributed for further processing. With
reference to Figure 8A, this amplifier is comprised of a
voltage differencing op-amp 302 with a variable resistance
voltage tap 308 connected to its positive input and a fixed
resistance voltage divided feed-back loop 306 connected to
is negative input. The particular resistance values
selected for construction of this amplification circuit are
based on a curve fit of the required signal gain for fluids
having an average specific gravity (ASG) of between 0.80 and
1.10 relative to fresh water. For simplicity of operation,
the input control knob of the variable resistance
potentiometer 308 used in this circuit is also calibrated in
units of specific gravity. This input parameter must be
computed by the operator using the proper oil cut (OC), oil
specific gravity (OSG) and water specific gravity (WSG) for
the well as follows:
ASG = (OC)*(OSG) + (1-OC)*(WSG) (13)
Fortunately, the average oil cut and specific
gravities of produced formation fluids are typically known
with sufficient accuracy to allow for the accurate
determination of all affected performance parameters. Fluid
densities, for instance, may be readily measured by the use
of a calibrated hydrometer, and average oil cut may be
easily computed by dividing the known oil production rate of
the well by the total fluid production rate measured by the
DPCU. The construction and operation of both the Fluid
45 Pressure Amplifier 301 and Fluid Viscosity Amplifier 303 of
Figures 7A and 8A are similar ~o the construction and
operation of Fluid Density Amplifier 300 described above.
- 43 -
1 333962
All three of these circuits may be incorporated within the'
electronic circuitry 120 of the sensor PCB 106 if desired,
and means may also be incorporated within such circuit~y for
automatically adjusting the required inputs to, each
amplifier based upon the continuous measurement of pressure,
density and viscosity by conventional means.
Prior to further processing by the various performance
measuring and control circuits, the amplified flow-rate
signal must first be buffered to strengthen its ability to
reference many additional circuits without loss of
accuracy. Such buffering is accomplished by means of a
voltage sensing op-amp 312 that drives the current limiting
base resistor 314 of an NPN power transistor 316 whose
output voltage is connected by way of a feed-back loop 310
to the negative input of the op-amp. In this manner the op-
amp and transistor function together as a voltage following
circuit that supplies a buffered output signal "Vb" from the
B+ power supply of Figure 8A.
The sensor size confirmation circuit 320 depicted in
Figure 8A provides an automatic visual warning whenever the
fluid sensor 48 is operated at an instantaneous flow-rate
that exceeds the maximum linear rate specified for the
selected orifice size. A fixed resistance voltage dividing
network 322 applies a constant reference voltage of
approximately 10.0 vdc to the negative input of a voltage
sensing op-amp 324 that drives an NPN power transistor 326
with current limiting base resistor 328. The transistor is
used to power an LED warning light 327 with current limiting
resistor 329. The buffered flow-rate signal "Vb" is
continuously applied to the positive input of op-amp 324 so
that the advisory LED is illuminated whenever this buffered
voltage exceeds its linear limit of approximately 10 vdc.
The clapper motion detector 330 (Figure 8B) limits the
operation of both downhole and surface mounted pumping
equipment should the fluid sensor 48 cease to function
properly during any production period, as hereinafter
described. A fixed-resistance voltage dividing network 332
applies an input control signal of approximately 99~ of Vb
to the positive input of a voltage sensing op-amp 334 that
has a time-averaged reference voltage signal applied to its
negative input from the buffered output "V22" of a "short-
term" pumping rate integrator 502. An RC circuit 331 with
decaying time-constant of approximately 20 seconds is
quickly charged by the output of op-amp 334. A second
45 voltage comparing op-amp 333 has output of RC circuit 331
applied directly to its negative input pin. A fixed
resistance voltage divider 335 applies a constant reference
- 44 -
1 333962
voltage of approximately 0.650 vdc to the poSitiVe input of
op-amp 333. A blocking AND gate 337 passes the output
signal of the second op-amp only during the Prod~ction
Sequence. Two inverters 341, 343 deliver either a "hi~h" or
"low" output signal whenever their input signal is driven
"low" or "high" by the AND gate 337.
The output of the first op-amp 334 switches "high"
whenever the instantaneous pumping-rate signal "Vb" exceeds
the "time-averaged" pumping-rate signal by 1% or more, as
determined by integrator 502. Thus, if the clapper moves by
more than 1% from its average deflected position, the RC
circuit 331 will quickly charge to saturation voltage, and
the output of the second op-amp 333 thereafter remains
normally "low". Should the clapper cease to move from its
average position for any reason, however, then the first op-
amp 334 immediately switches "low" to prevent the capacitor
from being recharged to saturation voltage. This action
causes the output of the second op-amp 333 to switch ~high"
following a fixed decay period of approximately 60 seconds,
as determined by the saturation voltage, cutoff reference
voltage and RC time constant of the controlling circuit
elements.
The resulting time-delay allows for the variable nature
of pulsating flow, and compensates for any transient
mechanical problems. Once the second op-amp 333 has been
switched "high", this positive indication of a "stuck
clapper" is allowed to pass through AND gate 337 during
periods of normal pump operation to drive the "stuck
clapper" control buss 338 "high". This buss then
distributes the resulting control signal to the various
other circuits in order to block the additional counting of
flow-rate pulses normally delivered by the "Total Fluid
Production" measuring circuit 430 (Figure 8C), prevent
automatic reset of the "4-cycle Shutdown" sequencer 500
(Figure 8B), clock the "stuck clapper malfunction indicator"
LED memory chip 551, and collapse the integrated
"Verification Sequence" control signal 371 by way of
transistor 433 so as to automatically terminate the
established "Production Period" of pump operation.
Each operating cycle of the pump is divided into four
sequential controlling modes of surface and downhole
equipment operation that are referred to herein as 1) the
Rest Period, 2) the Prime Period, 3) the Production Period
and 4) the "Pump-Off" Verification Period. The Rest Period
is required to provide formation fluids with sufficient time
to build a new reserve of liquids within the casing prior to
reactivation of the pumping equipment. This period, which
- 45 -
1 333962
followS the "Pump-Off" Verification Period of the last
operating cycle, is controlled by a digital timing circuit
342 that is programmed by the operator using a single pole,
eight-position rotary switch 344 to select the desired rest
interval, as shown in Figure 8B. Rest times of 2, 4, 8, 16,
32, 64, and 128 minutes are available from a binary ripple
counter 346 that receives a digital clock pulse at its input
every 15 seconds from circuit 270. This pulse is obtained
by passing the 0.15 second clock pulse on llne 347 through
two separate digital dividers 349 that each deliver one
output pulse for every 10 input pulses received. The
counter 346 is automatically reset to "0" and its output
disabled by the Pump Relay Power Buss 436 during prime mover
operation. Clocking of counter 346 can therefore only occur
during the Rest Period when pump power is "off". The output
pulse of the counter is then supplied to the "half-
monostable" pulse generator 351 of the Prime Sequence
control circuit 350, hereafter described, by way of the
selected rotary switch pole 441. The resulting monostable
pulse thereby initiates operation of both surface and
downhole pumping equipment at the conclusion of each
sequential rest period.
Under normal operating conditions the Prime Sequence
requires approximately one minute to complete once a
consistent stream of liquids ex t the wellhead. An
additional two minutes of steady pump operation thereafter
are required to assure the proper evaluation of downhole
equipment and fluid reservoir performance. For this reason,
it is necessary that a sufficient reserve of liquid be
allowed to accumulate within the casing during the Rest
Sequence to provide for at least three minutes of
uninterrupted pump operation at the time-averaged pumping
rate "Qp" of fluids being transported to the surface. The
minimum rest time required to assure proper evaluation of
all performance parameters on a continuing basis may
therefore be computed for any given fluid entry rate "QF" by
using conservation of mass considerations as follows:
Minimum Rest Time = 3*(Qp/QF - 1) (14)
The equation holds true for any value of the dimensionless
ratio "Qp/QF" greater than unity (i.e., Qp/QF > 1). It is to
be noted that this ratio "Qp/QF" is the reciprocal of the
ratio "QF/Qp" previously referenced. When this reciprocal
ratio is less than one, the well never "pumps-off" since new
fluid enters the casing at a greater rate than the pumping
capacity of installed downhole equipment. In such
- 46 -
1 333962
situations the fluid level within the casing stabilizes at
some intermediate depth that restricts the entry of new
liquids so that the time-averaged "QF" is equal to "Qpj'.
If the programmed rest time is excessively ~long,
however, then fluid production will be severely restricted
by the unnecessary buildup of liquids within the casing. It
is rec~ ended, therefore, that rest times on the order of
three to twelve times the minimum acceptable value computed
by means of equation (14) be stored in the control unit 2 to
provide for some margin of error. Such selection should
result in pumping times of from 9 to 36 minutes per
operating cycle, assuming that all gas is quickly purged
from the downhole pump at the start of the Prime Period.
The Prime Period controller 350 (Figure 8B) regulates
the initial operation of surface and downhole pumping
equipment during each pumping cycle until a consistent time-
averaged stream of liquids exits the wellhead. This
controller compensates for the transient effects of-fluid
fall-back and gas separation that may have occurred within
the tubing string during the preceding Rest Period, and
additionally compensates for the compressible effects of
casing gas ingested by the downhole pump during the previous
"Pump-Off" Verification Period. Such transients affect the
accuracy of fluid measurements made at the wellhead by fluid
sensor 48, and must therefore be stabilized before the next
performance measuring and evaluation sequence of pump
operation can begin.
With reference to Figure 8B, a programmable timer 352
initiates pump operation at the start of each Prime Period,
and limits the duration of pump operation to approximately
16 minutes if fluid can not be made to exit the wellhead in
consistent amounts within this reasonable priming interval.
A "half-monostable" pulse generator 351 triggers timer 352
at the start of each Prime Period. An NPN power transistor
35 356 with current limiting base resistor 358 and controlling
signal invertor 362 supplies power to the prime power buss
435 in order to activate the prime mover relay control
circuit. A voltage sensing op-amp 443 with verified control
signal 371 applied to its positive input, and constant
reference voltage of approximately 10 vdc applied to its
negative input, initiates termination of the Prime Sequence
upon conformatiQn by the control signal integrator 370 that
a consistent stream of liquids is exiting the wellhead. A
time-based sequencing circuit 455 controls both the final
termination of the Prime Period, and the start of the
Production Period, so that the two-minute measure of
1 333~62
downhole pump efficiency is properly regulated by node 349
of the digital clock circuit.
The amount of pumping time required to completely fill
the tubing string with liquid, and thus establi'sh a
consistent time-averaged fluid exit rate at the wellhead,
depends on many factors including the time to purge the
downhole pump chamber 92 of any ingested casing gas, the
pumping rate "Qp" after such purge, the level of fluid
within the tubing string 66 at the start of such pump
operation, and the annular liquid storage area of the tubing
string 66. Under normal production circumstances this
transient pumping time interval is measured in terms of
minutes or seconds, rather than hours. Following a
prolonged shut-down of the well, however, the initial Prime
Period could require many hours to complete; under these
circumstances such priming is best accomplished by placing
the controller 2 in its "manual" mode of operation by means
of switch 234 so that the four-cycle shutdown circuit 500
will not automatically limit pump operation to four
successive Prime Intervals of 16 minutes each. After
completion of this initial Priming Period, the controller
should then be placed in its "automatic" control mode to
provide for the continued automatic regulation of the prime
mover relay 445.
Upon direct application of DC control power at the
start of system operation, the Prime Period controller 350
receives its first sequencing pulse from the power-on
delayed-pulse generator circuit 290. Following completion
of the initial operating cycle, controller 350 thereafter
receives all further sequencing pulses from the Rest Period
Time controller 342. Each sequencing pulse immediately
triggers the timer 350 output "low" to drive the Prime Power
Buss voltage "high" by way of invertor 362 and transistor
356, thereby initiating pump operation. Once activated,
rest timer 342 continues to regulate operation of the prime
mover relay 445 until timer 352 is reset and disabled by
either its own 16 minute timing pulse, or by voltage
comparator 443 as hereafter described. This op-amp is
controlled by the verification control signal integrator
370, which receives its input signal from the "pump-off"
detector 380. Once the integrated control signal 371
exceeds a negative-pin bias voltage of approximately 10 vdc,
op-amp 443 immediately switches "high" to apply a steady
Prime Sequence termination signal to one input of the AND
45 gate 447 that interfaces with the timer 352.
- 48 -
1 333962
The other input of AND gate 447 is connected to a
"half-monostable" pulse generator 451 that, together with
the AND gate 447, jointly comprise the time-based sequencing
circuit 455. This circuit is periodically activatediby a
1.5 second digital clock pulse on line 453. When sequencing
circuit 455 receives its next "high" input pulse, the Prime
Sequence termination signal generated by the voltage
comparator 443 passes through AND gate 447 to reset and
disable timer 352. This action causes the inverted output
of the timer to go "low", thereby turning off transistor 356
that drives the Prime Period Power Buss 435. In this
fashion the Prime Period is terminated in proper phase with
the 0.15 second digital clock pulse to assure an accurate
two-minute measure of downhole pump efficiency at the start
of each Production Period.
The production period control circuit 360 depicted in
Figure 8B includes a voltage sensing op-amp 426 that
regulates the continued operation of the prime mover power
buss 436 following conclusion of each Prime Period. A fixed
resistance voltage dividing network 357 applies a constant
reference voltage of approximately 2 vdc to the negative
input of op-amp 426. A digital timer 4S9 (Figure 8D) limits
duration of this basic production interval to 256 minutes of
continuous pumping should fluid "pump-off" not be detected
within this reasonable period of time. Since this circuit
momentarily shares joint control of the motor relay power
buss with the prime period controller 350, the output signal
of each regulating circuit must be connected to the input
pin of relay control op-amp 422 by means of a blocking
diodes 355 as shown.
As with the Prime Period controller, normal operation
of the Production Period controller is directly related to
the performance of the Verification Control signal
integrator 370. Following the initial prime of downhole
equipment, the output voltage 371 of this integrator slowly
increases from an initial value of 0 vdc towards a
saturation level of approximately 12 vdc. When this signal
371 exceeds the 2 vdc reference voltage level that is
applied to the negative input of the Production Period
voltage comparator 426, the output of this op-amp switches
"high" to jointly share control of the pump relay power buss
with the Prime Period controller. Following a normal prime
verification period of approximately 30 seconds, the
integrated control signal 371 rises above the 10 vdc
reference level applied to the negative input pin of Op-amp
443 to switch "off" this Prime Period voltage comparator.
Once such switching has occurred, relay control circuit 390
- 49 -
1 333962
.~
is thereafter regulated solely by the production period
voltage comparator in the manner described hereinafter.
Once initiated, the Production Period continuesluntiL
it is terminated by either the 256 minute timer 459 ~r the
voltage comparator 422. If the established pumping rate
"Qp" is greater than the maximum rate "QF" that new fluid
can enter the casing from the fprmation, the well eventually
"pumps-off" when all excess liquid has been removed from the
casing. At this point in time the average fluid exit rate
at the wellhead abruptly declines, and will thereafter be
controlled by the average fluid entry rate "QF" rather than
by the available pump capacity "Qp". When "pump-off"
detector circuit 380 (Figure 8B) detects this abrupt change,
it quickly terminates its "high" output signal to the
verification integrator 370. Following such termination,
the integrated control signal 371 begins to decline from its
12 vdc saturation level to an "at rest" value of 0 vdc. If
the previously measured pumping rate "Qp" does not
reestablish itself within an allowed Verification Period of
20 approximately 30 seconds, the integrated control signal 371
continues its decline through the 2 vdc reference voltage
level of comparator 426 to terminate the output of
comparator 422. When the output of comparator 426 goes
"low", the relay controller 390 is "switched off" to
interrupt power to the prime mover relay 445 by the way of
transistor 432 and the well then enters into its next
sequential Rest Period.
Should normal pump-off detection not occur within 256
minutes from the start of the Prime Period, the Production
Period is automatically terminated by a digital timing
circuit 459 that artificially collapses the integrated
control signal 371 by means of an NPN power transistor 433
that is connected to the negative input pin of the control
signal integrator 382 as shown in Figure 8B. This Digital
Timer 459 is reset at the start of each Prime Period by the
Pump Relay Power Buss 436, and pulses "high" after receiving
a total of 1024 input pulses from the 15 second digital
clock 270. Such control is included to guard against the
possible loss of the "baseline" pumping rate that is
required for the "Pump-Off" detector 380 to perform
properly.
Control of the "pump-off" ~erification Period is
regulated by a linear integrator 370 of Figure 8B. A
voltage differencing op-amp 382 with capacitive feed-back
45 loop 384 has fixed resistance voltage taps 386 and 388 on
both the positive and negative inputs, respectively. These
two resistive networks control the different integrating
-- 50 --
1 333962
time constants of capacitors 384 during periods of positive
and negative integratiOn as hereinafter described. The
positive input of op-amp 382 is driven by the output signal
on line 381 of the "pump-off" detector 380. Whenever'fluid
exits the wellhead at a time-averaged rate that remains
essentially constant at some value other than "0", or that
increases with time, this output signal switches from "low"
to "high" to activate integrator 370. Following activation,
integrator 370 immediately begins to increase its output
voltage 371 from an "at rest" level of 0 vdc towards a
saturation level of approximately 12 vdc. Once the
integrated control signal 371 exceeds a base reference level
of approximately 2 vdc, the production period voltage
comparator 360 activates to assume joint control of the
motor relay power buss 436 with the prime period control
circuit. If the output control signal of the "Pump-Off"
detector 380 remains "high" without interruption, then the
integrator 370 requires approximately 30 seconds to increase
its output voltage from 2 vdc to 10 vdc to "turn off" the
Prime Period controller 350. Should this input signal be
interrupted for any reason before such switching occurs,
however, then verification integrator 370 reverses its
direction of integration to reduce its voltage output so as
not to terminate the established Prime Period. In this
event, it is assumed that the downhole pump 98 and/or tubing
string 66 of Figure 1 is not properly primed, and that the
output signal of the "Pump-Off" detector 380 is simply
responding to the transient effects of gas or debris as they
pass through the system.
Once the Prime Period has been properly terminated by a
Verified control signal of approximately 10 vdc, operation
of the prime mover relay 445 (Figure 8D) is regulated only
by the "high" or "low" state of the production period op-amp
426 (Figure 8B) and the 256 minute timer 459 (Figure 8D).
Should the "pump-off" detector 380 (Figure 8B) sense an
abrupt decrease of more than approximately 4.4% in the
average fluid exit rate at the wellhead at any time within
the 256 minute operating period, then this primary motor
controller immediately assumes that "pump-off" has occurred
and switches its output signal from "high" to "low"
accordingly. This response causes the verification
integrator 370 to immediately begin to integrate its output
voltage 371 "down" from approximately 12 vdc towards 0
vdc in order to terminate the Production Period at a
reference voltage level of approximately 2 vdc. Once this
switching occurs, the prime mover is turned off and the next
Rest Period immediately begins. Should the average pumping
- 51 -
1 333962
rate return to its previously measured level before this
~ series of events occurs, however, or should it stabilize at
a new level that is at least 95.6~ of the previous rate,
then the Verification control signal quickly integrateS back
up to its previous saturation level of approximately 12 vdc
to extend the length of the established Production Period.
The "pump-off" detector 380 includes an input signal
buffer 383 that serves to impart a high reverse current sirk
impedance to the processed flow-rate signal, as viewed from
10 the input nodes of the two analog integrators 402 and 502
discussed below. A primary analog signal integrator 502
delivers a buffered output voltage "V22" that is linearly
related to the average "short-term" pumping rate of all
downhole equipment. A secondary analog signal integrator
15 402 delivers a buffered output voltage "V100" that is
linearly related to the long-term "baseline" pumping rate of
all downhole equipment. A pumping-rate signal comparator
391 delivers a "high" output voltage signal whenever the
average short-term pumping rate exceeds approximately 98~ of
the baseline pumping rate. A voltage comparator 393 with
coupling diodes 397 and 401 improve the transient response
time of the slower "baseline" pumping rate integrator 402
during the Prime and Verification Periods. This "pump-off"
detector 380, which is responsible for primary control of
the prime mover power relay 445 during all transient and
steady-state pumping operations, connects directly to the
verification control circuit integrator 370. Input signal
buffer 389 of Figure 8B is constructed using a voltage
sensing op-amp 383 with direct feedback loop 385 between its
output and negative input terminals. Connected to the
positive input of op-amp 383 is the previously buffered "Vb"
flow-rate signal on line 387. By constructing this input
buffer as a voltage follower, its instantaneous output
voltage "Vbo" will be equal to the input flow-rate signal at
all times, and yet reverse-current will be blocked. This
buffered voltage is applied directly to the input side of
both the primary and secondary pumping-rate signal
integrators 402 and 502.
The primary integrator (lower RC current path 502) is
40 constructed using a 220K precision metal film resistor 392
and 100 micro-farad low-leakage electroiytic capacitor 394
to provide for an integrating time-constant of approximately
22 seconds. Due to the high impedance of this circuit, the
integrated capacitor voltage is connected directly to the
positive input of a voltage sensing op-amp 396 in order to
provide a buffered output signal "V22" that is essentially
identical to the time-averaged capacitor flow-rate signal.
- 52 -
I 3339~
In order to compensate for the voltage drop of leakage
current flowing through resistor 3 ~ into capacitor 394, the
feedbaCk loop of the buffering op-amp 396 should be provided
with an identical 220K current limiting resistor 398 to
balance the circuit response.
The secondary "baseline" pumping-rate integrator (upper
RC current path 402) is similarly constructed, except that
it utilizes a 1.0 meg precision metal film resistor 404 in
series with a 100 micro-farad low-leakage electrolytic
capacitor 406 to provide for an integrating time-constant of
approximately 100 seconds. This rather large time-constant
is required to establish and maintain an accurate measure of
the "baseline" pumping rate in a manner that is relatively
insensitive to any abrupt change that might occur in the
"short-term" pumping rate. The input to this second RC
integrating circuit 402 is obtained from a voltage tap 408
that is constructed using a 220 ohm precision metal film
resistor in series with a lOK precision metal film resistor
so that approximately 97.85% of the buffered "Vbo" flow-rate
signal from op-amp 383 will always be applied to the input
side of the 1 Meg resistor 404. For reasons previously
discussed, the feedback loop of the "baseline" signal buffer
incorporates a resistor 412 of approximately 1.2 Meg to
balance the voltage response of this circuit. The exact
value of this resistor should be trimmed at time of
manufacture to assure that the output voltage difference
between both buffering op-amps is the same percentage of any
steady input signal, to a high degree of accuracy, over the
entire operating range of 0-10 vdc through which the output
voltage of op-amp 383 will typically operate.
Following integration and buffering of the "short-term"
and "baseline" pumping rate signals, the resulting output
voltages "V22" and "V100" are then compared to one another
by means of a voltage sensing op-amp 391 in order to obtain
a unified control signal that is directly related to the
pumping status of the specific well in question. Since the
"baseline" voltage signal "VlO0" is connected to the
negative input of op-amp 391, and the "short-term" signal
"V22" to the positive input, the output voltage of this
signal comparator is "high" whenever the "short-term"
pumping rate exceeds 97.85% of the average "long-term" rate
that fluid exits the wellhead. Should the "short-term" rate
decrease abruptly below some limiting percentage of the
"baseline" rate at any time after both capacitors 394 and
406 have been fully charged, then the output of comparator
391 immediately switches "low" to indicate that a change in
the established pumping rate has been detected. Such change
- 53 -
1 333962
is always associated with the onset of fluid pounding or
pump cavitation at time of liquid "pump-off". Due to the
tran5ient voltage response of both circuits 402 and 502, and
the need for a reasonable verification period of
approximately 30 seconds to confirm that "pump-off" has
indeed occurred, such determination can be accurately made
for any situation that might be encountered as long as the
dimensionless ratio "QF/Qp" is less than an upper limiting
value of approximately 95.6~.
As previously noted, both integrating capacitors 406
and 394 of the signal-averaging circuits 402 and 502 are
charged by the simultaneous application of the some buffered
flow-rate signal 'IVboll to their respective inputs. When the
output IIVbo" of op-amp 383 declines towards "0" from some
instantaneous peak value, however, the high reverse current
sink impedance of signal buffer 389 prevents the discharge
of these two capacitors back into the source circuit. Thus,
both capacitors are required to discharge their average
voltage signals to the Ground Buss of the B+ Power Supply
20 through the 10,220 ohm resistance network 408 of the
"baseline" voltage tap. Since this resistance is much less
than the 220K and 1.0M resistors 392 and 402 through which
the capacitor current must also flow, the respective time-
constants of discharge are essentially the same as the time-
constants for charging both circuits. The 22 second time-
constant specified for the "short-term" integrator is
selected because reciprocating piston pumps may frequently
be operated at speeds as low as 4 or 5 strokes per minute.
Thus, the "short-term" integrating capacitor 394 always
carries a voltage across it that is effectively related to
the average pumping rate measured during more than one
pumping stroke of any typical equipment installation.
In order for the "Pump-Off" detector 380 to function
properly at high values of "QF/Qp", the "baseline" capacitor
406 must be fully charged before all stored liquid is
depleted from the casing. Unfortunately, charging this
capacitor normally requires more than 8 minutes of
continuous steady-state pumping to reach 99% of its desired
operating voltage, since its controlling RC Time Constant
must be selected reasonably high as previously noted for
proper system performance under all anticipated operating
conditions. Following "pump-off", a similar period of time
is required to fully discharge the capacitor 406 before the
next operating cycle of the pump can begin. Since it is not
possible to guarantee such long integrating periods under
all operating conditions, however, the transient voltage
response of this "baseline" integrating circuit 402 must be
- 54 -
1 333962
`~ artificially enhanced during the first few minutes of each
start-up and shut-down sequence of the pump. Voltage
coupler 393 limits the instantaneous voltage diff~erence
between "short-term" and "baseline" capacitors 394 and 406
during periods of significant positive and negative signal
integration. To achieve the desired result, two separate
current paths must be utilized within this special
compensating circuit.
During periods of significant positive integration, the
10 voltage spread between capacitors 394 and 406 is limited by
a voltage sensing op-amp 395 that has its negative input
connected directly to the output signal of the "baseline"
voltage buffer 424 previously described. The positive input
of this op-amp is connected to a fixed-resistance voltage
divider 457 that receives its source signal from the output
of the "short-term" integrator buffer 396. ~his voltage tap
457 is constructed using a lK resistor in series with a l5K
resistor so that 15/16ths of the "short-term" integrated
signal is applied to the positive Lnput of op-amp 395.
Whenever the buffered "baseline" voltage "V100" is less than
93.75% of the buffered "short-term" voltage "V22", op-amp
395 switches "on" to quickly charge the "baseline" capacitor
406 through a blocking diode 397 and lOK current limiting
resistor 399. In this manner the "baseline" capacitor 406
receives a rapid initial charge during each start-up
sequence, before it is then allowed to stabilize by its
normal response at 97.85% of the time-averaged pumping rate
signal "Vbo". In similar fashion, the "baseline'i capacitor
voltage can never exceed the "short-term" capacitor voltage
by more than 0.6 vdc during periods of significant negative
integration since it is rapidly discharged into the more
responsive "short-term" circuit by means of an
interconnecting diode 401 and its associated current-
limiting resistor 399. By constructing this circuit as
shown in Figure 8B, the transient response of the "baseline"
integrator 402 will be greatly enhanced during the start-up
and shut-down sequence, without sacrificing its novel
ability to assist with the sensitive detection of "pump-off"
at high values of QF/Qp -
The transient response characteristics of both the
"short-term" and "baseline" integrators 402 and 502 and
graphically presented in Figure 10 for a typical operating
situation that is based upon an arbitrarily selected total
cycle time of 260 seconds. For purposes of illustration,
this cycle is divided into a rest period of 60 seconds,
prime period of 40 seconds, production period of 130 seconds
and "pump-off" verification period of 30 seconds. Also
- 55 -
1 333962
presented on this graphic display is a curve for the
integrated control signal 371 that is driven by the output
of the "pump-off" detector 380. It should be noted that in
this example a rest time of one minute is used for p~rpose
of illustration, even through the shortest rest time
available from the timing circuit 344 shown in Figure 8D is
two minutes.
It will be noted from Figure 10 that the Rest Period
begins at the end of the previous "pump-off" Verification
Period, at a time "O" of the illustrated pumping cycle.
Upon removal of power from the prime mover relay buss 436,
the integrated control signal 371 artificially collapses to
0 vdc for reasons previously discussed. During the Rest
Period, the buffered "short-term" capacitor voltage "V22"
quickly decays due to its relatively short 22 second time-
constant, and the buffered "baseline" capacitor voltage
"V100" also decays quickly towards an "at rest" value of 0
vdc due to the beneficial effects of circuit coupling. At
the end of the 60 second Rest Period, the timing circuit 344
initializes pump operation once again by means of the Prime
Period controller 350.
In the example of Figure 10 it is assumed that the pump
operates for 10 seconds before fluid begins to exit the
wellhead in consistent amounts. Once a consistent pumping
rate has been established, the "short-term" capacitor 394
quickly integrates upward towards its illustrated steady-
state value of Emax = 8 vdc. Such integration requires
approximately 110 seconds to reach 99~ of this level, at a
cycle time of approximately 180 seconds. When the buffered
"short-term" capacitor voltage "V22" exceeds the decayed
buffered "baseline" voltage "V100", the output of the "Pump
Off" detector 380 switches "high" to activate the control
signal integrator 370. Due to the beneficial effects of
circuit coupling, such switching occurs almost as soon as
fluid first exits the wellhead, at a cycle time of
approximately 70 seconds, and from that point on the
integrated control signal 371 begins to increase linearly
towards its illustrated saturation level of 12 vdc.
After a 30 second Prime Verification Period, the system
enters into its normal Production Period of pump operation
at an illustrated cycle time of 100 seconds. It should be
noted that a sufficient reserve of liquid is now available
to the pump to assure continuous pump operation during the
two-minute performance measuring period that follows Prime
Verification. During this period of time the buffered
"baseline" capacitor voltage "V100" is quickly charged to
approximately 96% of its ultimate level by voltage coupler
- 56 -
1 333962
393. This circuit ceases to function after approximately 97
seconds of continuous operation, at an illustrated cycle
time of approximately 167 seconds, when the normal rate of
"baseline" voltage increase finally exceeds the couple~ rate
5 of increase. Two-hundred and twenty-six (226) seconds into
this operating cycle, "pump-off" is achieved when the casing
is finally depleted of all excess liquids. At this point in
time the pumping rate abruptly drops according to the ratio
"QF/Qp", and the integrated voltages "V22" and "V100" of
10 both "short-term" and "baseline" capacitors immediately
start to decay exponentially towards their new steady-state
vaIues of:
V22 = (QF/Qp) * (Emax) and V100 = (V22) * (97.85%).
Since the buffered "short-term" capacitor voltage "V22"
is initially 5% greater than the buffered "baseline"
capacitor voltage "V100" in this example, a short period of
time is required for the more responsive "short-term"
20 voltage to decay below the falling "baseline" voltage. This
"pump-off detection time" has been computed by iterative
methods to be approximately 4 seconds for the example
illustrated in Figure 10. Once the "short-term" voltage
decays below the "baseline" voltage, the output of the
25 "Pump-Off" detector 380 then switches "low" to begin the
"Pump-Off" Verification Period. During this 30 second
interval of time, the integrated control signal 371 steadily
declines towards a termination level of 2 vdc, as determined
by voltage comparator circuit 360, and each buffere~l
30 capacitor voltage declines towards the new steady-state
value previously given. If the initial pumping rate is not
reestablished within the 30 second Verification Period, the
prime mover shuts down to begin the next sequential
operating cycle as illustrated at a cycle time of 260
35 seconds.
The relationship that exists between fluid exit time,
"Pump-Off" time, fluid entry rate ~QF~ and pumping rate IIQPI~
is clearly illustrated by the graphic presentation of Figure
10. During the preceding "Pump-Off" Verification Period new
40 liquids were removed from the casing at the same time-
averaged rate "QF" that they entered from the formation.
Because of this, there can be no excess reserve of liquids
within the casing at the start of the illustrated operating
cycle. During the first 226 seconds of this cycle, new
45 liquids continue to enter from the formation at the same
average rate "QF" as before, assuming that the Rest Sequence
is not excessively long so as to allow an inordinate amount
-- 57 --
1 333962
of liquid to build within the casing to restrict such
entry. This fluid must then be removed by 156 seconds of
continuous pump operation as shown, at an average rate "Qp",
in order to achieve "pump-off" once again. Since no'fluid
will exit the wellhead during the assumed 10 second initial
Prime Period, continuity considerations indicate that 226 *
QF = 156 * QP, which yields the illustrated value of QF/Qp =
69%.
With reference to Figure 11, the total amount of time
required for the motor controller to respond to fluid "pump-
off" may be computed as the summation of an initial "Pump-
Off" detection time interval (T1-To~ and a final
verification time interval (T2-T1). Since the verification
time interval always remains fixed by circuit design at
approximately 30 seconds, the detection times for the two
illustrated examples of Figures 10 and 11 must be 4 seconds
and 30 seconds respectively. The basic relationship that
controls circuit response immediately following fluid "pump-
off" is:
Response Time = Detection Time + Verification Time (15)
The required detection time for any operating situation
is a function only of the dimensionless ratio "QF/Qp", since
this ratio controls the shape of the two capacitor decay
curves "V22" and "V100" for the "short-term" and "baseline"
pumping rate integrators 402 and 502. The actual detection
time required for any specific value of "QF/Qp" may be
computed by noting that the "sho~t-term" and "baseline"
voltages are always equal to each other at the start of each
Verification Period. Thus, control circuit switching is
initiated whenever "V22" = "V100". By using conventional
iterative methods to solve the two exponential equations
that describe the "short-term" and "baselines" capacitor
voltage curves, the required detection time interval (T1-To)
may be accurately computed for any selected value of
QF / Qp .
It will be noted from Figure 11 that whenever the
buffered capacitor voltages "V22" and "V100" are allowed to
decay for a sufficiently long period of time, the "short-
term" voltage "V22" quickly stabilizes at a new level that
is once again greater than the decreasing "baseline" voltage
"V100". Thus, the exponential expression for V22 = V100
actually has two solutions "Tl" and "T2" for each value of
"QF/Qp" below an upper limiting value of unity (i.e. 1).
For proper control system response to be initiated, the
switching interval (T2 - Tl) must be greater than the
- 58 -
1 333962
constant specified "Pump-Off" Verification Period of 30
seconds. The length of time that transpires between initial
switching at time "T1" and final switching at time "T2" can
be shown to decrease as the controlling value of~"QF/Qp"
increases. Should this time interval be less than the
required 30 second Verification Period, then the integrated
control signal 371 will reverse its course of direction
before the Verification Period can be terminated, thereby
preventing the incorrect shutdown of the prime mover. The
limiting value of "QF/Qp" for proper control system response
is therefore determined by switching times "T1" and "T2"
that differ by the exact duration of the Verification
Period. This value, as previously reported, has been
computed to be 0.956 using the iterative methods and time-
constants set forth above. Whenever the established pumpingratio is above this limiting value, the system can not
respond adequately to fluid "pump-off". At this upper
limiting value of 0.956, system response time is computed to
be approximately 60.3 seconds. Below this limiting value,
system response increases rapidly with decreasing "QF/Qp" to
a lower limit of approximately 30.6 seconds. Such
limitation is of no serious consequence, however, since the
pump will be receiving essentially a full charge of liquid
on each stroke when operated at a ratio of QF/Qp = 0.956 or
greater.
The prime mover power relay controller 390 of the DPCU
2 includes a voltage sensing op-amp 422 that receives its
positive input signal from either the prime power buss 435,
Production Sequence op-amp 426, or from the second pole of
DPDT manual override switch 234 as shown in Figures 8A and
8B. Op-amp 422 has a constant reference voltage of
approximately 6 vdc applied to its negative input by a
fixed-resistance voltage dividing network 428. Op-amp 422
drives an NPN power transistor 432 by means of a current-
limiting base resistor 434 to supply DC power to the motorcontrol relay power buss 436 whenever it is desired that the
prime mover 14 be turned on to lift fluid to the surface.
The total production time monitor 400 (Figure 8D) is
designed to count 1 minute clock pulses whenever normal
operating power is provided by the B+ Power Supply of the
DPCU. A non-resettable binary counter 438 divides the 15
second digital clock pulse previously referenced by a
constant ratio of 4 to deliver an accurate 1 minute output
pulse to one input of AND gate 442. The other input of this
AND gate is connected directly to the B+ Power Buss so that
the input clock pulse passes through this device whenever B+
power is "high". AND sate 442 drives a circuit-grounding
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1 333962
NPN transistor 444 by way of current-limiting base resistor
446 to trigger a manually resettable six dlglt dlsplay
counter 448. Thls counter is of conventlonal design,ibelng
provided with an lnternally mounted battery ' that
continuously drlves lts CMOS memory and LCD dlsplay even
when power is removed from the system. Due to the
construction of this circuit, each 1 minute clock pulse is
registered by the counter whenever normal system operation
is in effect, regardless of the position of the DC control
switch. Should B+ power be interrupted for any reason by
the emergency l'Ve" power circuit 210 however, then the
automatic counting of such pulses immediately ceases in
order to provide the operator with meaningful information
concerning the time of such power interruption.
The total pumping time monitor 410 (Figure 8D) is
designed to count 1 minute cloc~ pulses whenever DC power is
supplied to the prime mover relay controller 445. AND gate
452 has one input connected to the 1 minute clock pulse from
divider 438, and the other input connected to the pump relay
20 power buss 436. The output of AND gate 452 drives NPN
transistor 454 by way of resistor 456 to trigger a manually
resettable display counter 458 that is similar to counter
448. Each 1 minute clock pulse is registered by counter 458
only when DC power is supplied to the prime mover relay
control buss 436.
The total operating cycle monitor 420 (Figure 8C~
includes a pulse-shaping AND gate 462 that has one input
connected to the B+ power buss 201 and the other input
connected to the prime power buss 435. The output of this
device drives a circuit-grounding NPN Transistor 466 by
means of current-limiting resistor 468 to trigger a manually
resettable counter 472 that is s'imilar to counter 448.
Counter 472 is indexed by one digit whenever power is first
applied to the prime power buss 435 at the start of each
pumping cycle, regardless of the position of the DC control
switch.
' The total fluid production monitor 430 (Figure 8C)
computes and records the total cumulative volume of all
liquids that exit wellhead 62 and pass through fluid sensor
48 during any selected production interval. This circuit
includes a temperature stabilized voltage controlled
oscillator (VCO) 403 that accurately converts the previously
buffered "Vb" analog flow-rate signal 387 into a pulse-
shaped digital output signal, the frequency of which is
linearly related at all times to the exact instantaneous
magnitude of the density corrected flow-rate signal "Vb".
The output frequency of this VCO is calibrated at time of
- 60 -
1 333962
-
manufacture to 2489 HZ for an input voltage signal of
exactly 10 VDC, and O HZ for an input voltage signal of
exactly O VDC. Accuracy of this calibration is maintained
under all operating conditions by enclosing VCO 403 within
the temperature stabilized oven enclosure 262.
AND gate 474 allows the output frequency signal of VCO
403 to pass only when proper operation of the fluid sensor
48 is confirmed by clapper motion detector 330 (Figure 8B).
Binary ripple counter 476 ~Figure 8C), interconnected with
one pole of rotary switch 294, serves to reduce the VCO
output frequency by a constant division of either 4096,
2048, 1024, or 512 in order to properly compensate for the
installed orifice size A through D of fluid sensor 48. A
second division circuit 478 controlled by DPDT switch 488
divides the fluid volume frequency signal by a constant
factor of 42 whenever units of barrels rather than gallons
are desired.
Circuit grounding NPN transistor 482 with current-
limiting resistor 484 triggers resettable counter 486 to
totalize all resulting fluid-volume pulses. This counter,
which is similar to counter 448, is indexed by one digit
whenever l/lOth of a gallon or l/lOth of a barrel of liquid
passes through fluid sensor 48, depending on the position of
switch 488. This DPDT switch also serves to automatically
reset counter 486 whenever the operator elects to change the
recorded units of volume from barrels to gallons, or visa
versa, or whenever the operator elects to begin a new
production interval of record.
The fluid entry rate monitor 440 of Figure 8D computes
and displays the average daily rate, in barrels of fluid per
day (BFPD), that produced formation liquids are exiting the
wellhead. Since matter will neither be created nor
destroyed by the pumping process, this exit rate will be
essentially the same as the rate of new fluid entry into
casing 64 from reservoir 84. In order to compensate for
minor fluctuations in the instantaneous fluid entry rate
that normally occur during each operating cycle of the pump,
this computation is made using flow-rate measurements that
are averaged over a 24 hour production interval of 1440
minutes. The accuracy of this computation will be quite
high in situations where the stored reserve of liquids
within the casing does not change appreciably during this 24
hour measuring period, or in situations where any net change
in downhole fluid inventory is a small percentage of the
total volume of liquids that are produced during such period
of time. The greatest potential error associated with this
method of fluid entry rate computation is a function only of
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1 333962
the "Rest Time" selected for programming by the operator, as
follows: !
Max Potential Error = + (100~) * (Rest Time/1440) ~(16)
With reference to Figure 8D, it will be noted that the
fluid entry rate monitor 440 includes a divider 492 that
reduces the fluid volume pulse frequency obtained from line
428 by a factor of 10 in order to deliver a single input
clocking pulse to counter 494 for each barrel of liquid that
exits the wellhead. Resettable BCD counter 494 (Motorola
#MC14553) totalizes all such fluid volume pulses thus
received during each 24 hour counting period, and upon
receipt of a latching pulse from NOR gate 504, stores the
resulting BCD count in its internal memory for further
processing by the BCD-to-seven segment decoder/driver 508.
This decoder/driver (Motorola #MC14511) powers a three digit
common-cathode LED display 510 to present the results of the
previous 24 hour pulse count to the operator while the
current fluid entry rate is being registered by counter 494.
This new count will subsequently be displayed during the
next 24 hour production interval, and will be updated every
24 hours thereafter in similar fashion.
It will be noted from Figure 8D that counter 494 is
- 25 reset to "0" at the start of each 24 hour counting period by
the output of delayed-pulse generator 506, which is similar
in construction to previously described delayed-pulse
generator 290. This second pulse generator 506 receives its
triggering input pulse from NOR gate 504, which also latches
30 counter 494. Pulse generator 506 serves to delay the reset
of counter 494 by a few milliseconds whenever NOR gate 504
issues its sequencing output pulse, in order that counter
494 might first latch its existing pulse count in memory
before resetting to start a new fluid entry rate
measurement.
As previously noted, NOR gate 504 receives its first
triggering pulse from delayed pulse generator 290 shortly
after DC power is applied to the control circuit of the DPCU
2. This same initializing pulse resets dividers 498, which
thereafter pulses "high" every 24 hours to trigger half-
monostable pulse generator 499. The resulting output
sequencing signal of pulse generator 499 is then applied to
NOR gate 504 in order to latch and reset the BCD counter 494
every 24 hours as previously described.
1 333962
The Duty Cycle Monitor 450 (Figure 8D) computes and
displays the average percentage of total production time
that the downhole pump 98 must be operated in ord~er to
transport all produced formation liquids to the suiface.
This circuit is similar in construction and operation to
fluid entry rate monitor 440, and shares all of the same
sequencing components for latch and reset of counters 514 as
previously described for counter 494. Divider 518 reduces
the 0.15 second input clock frequency from line 347 by a
10 constant factor of 576 in order to provide exactly lO00
output pulses to ANC gate 516 for every 1440 minutes of
continuing operation. AND gate 516 passes these pulses to
the input clock pin of resettable BCD counter 514 (Motorola
#MC14553) only during periods of prime mover operation, when
15 buss 436 is switched "high". Counter 514 totalizes and
stores the resulting pulse count, which is updated every 24
hours by the sequencing circuit previously described. While
a new pulse count is being recorded, BCD-to-seven segment
decoder/driver 522 (Motorola #MC14511) drives a three-digit
common cathode LED display 520 to provide the operator with
an accurate presentation of the average duty cycle (%)
measured during the previous 24 hour operating period.
The Pump Efficiency Monitor 460 (Figure 8C) computes
and displays the total volumetric efficlency of all downhole
25 pumping equipment (i.e. rods 68, tubing 66 and pump 98 of
Figure l) based on the theoretical displacement of such
equipment as observed at the wellhead. This displacement,
expressed in units of BFPD, must be programmed into the data
processing and control unit (DPCU) 2 of the invention by the
operator at time of field installation using control knob
534 of mechanical display 467. The theoretical displacement
in barrels of fluid per day (BFPD) of any reciprocating
piston pump may be easily computed from the known piston
diameter (inches), stroke (inches) and frequency of cyclic
operation (cps) as follows:
Displacement = (0.117)(D2)(stroke)(frequency) (17)
Similar commutations may be made for centrifugal and
rotary screw pumps, based on their theoretical displacement
at 100% volumetric efficiency. It is important to note that
the recognized effects of rod elasticity may be included in
the above calculation if desired, although such allowance is
not necessary for the accurate measure of pump efficiency
relative to the programmed pump displacement. For excellent
accuracy to be achieved with any type of mechanical pump, it
is only necessary that the "Rest Time" selected for
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1 333962
programming into the DPCU 2 be sufficiently long to provide
for at least three minutes of uninterrupted pump operation
once fluid begins to exit the wellhead in cons stent amounts
following each sequential rest period.
With reference to Figure 8C, performance monitor 460
includes a fixed-resistance analog voltage division network
556 with associated center-pole of the four-position rotary
switch 294 that serves to divide the buffered "Vb" flow-rate
signal 387 by a constant factor that is proportional to the
programmed sensor size (A, B, C or D) currently in use. A
variable-resistance analog voltage division network 526,
with associated signal buffer 528, serves to calibrate the
operating characteristics of this circuit at time of
manufacture. A second variable-resistance analog voltage
division network 532 with calibrated mechanical input dial
467, potentiometer 413, fixed resistor 465 and amplifier
463, serves to divide the buffered input flow-rate signal by
a variable denominator that is proportional to the pump
displacement programmed in the field by means of knob 534.
A temperature-stabilized voltage controlled oscillator (VCO)
535 converts the resulting analog voltage signal into a
pulse-shaped digital output signal, the frequency of which
is linearly related at all times to the instantaneous value
of the density corrected flow-rate signal "Vb" divided by
the programmed pump displacement. The output frequency of
this VCO circuit is calibrated at time of manufacture to
2133 HZ for an input voltage signal of exactly 10 vdc, and 0
HZ for an input voltage signal of exactly 0 vdc. Accuracy
of this calibration is maintained under all operating
conditions by enclosing VCO 535 within the temperature-
stabilized oven enclosure 262.
Divider 536 delivers one output pulse to AND gate 538
for every 256 digital input pulses that it receives from VCO
535. AND gate 538 passes all such clocking pulses to
resettable BCD counter 542 only when activated by pump relay
power buss 436 of motor control circuit 390. Counter 542
(Motorola #MC14553) totalizes all such normalized flow-rate
pulses received during the first 120 seconds of pump
operation immediately following proper termination of each
verified prime period, and upon receipt of a latching pulse
from half-monostable pulse generator 461, stores the
resulting BCD count in its internal memory for further
processing by the BCD-to-seven segment decoder/driver 546.
This decoder/driver ~Motorola #MC14511) powers a three-
digit common-cathode LED display 548 to present the
resulting pump efficiency measurement to the operator. This
measurement, which is expressed as a percent ~%) of the
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1 333962
~ programmed pump displacement, is upgraded during each
operating cycle.
It will be noted from Figure 8C that pump efficiency
counter 542 is disabled by prime power buss 435 duri~g each
sequential priming period of cyclic pump operation.
Following each verified prime period, power buss 435
switches "low" to enable the register of counter 542 and to
trigger half-monostable pulse generator 495. The resulting
"high" output pulse of this sequencing circuit
simultaneously resets the output of divider 555, NOR latch
409, counter 542, and divider 536 "low". Upon receipt of
the next 800 input pulses from the 0.15 second digital clock
buss 347, following two minutes of steady pump operation,
the output of divider 555 pulses "high" to trigger half-
monostable pulse generator 411. The resulting "high" outputsequencing pulse of this circuit triggers NOR latch 409 at
the end of the two minute pump efficiency measuring period
thus defined. Once the output of latch 409 triggers "high",
it will thereafter remain "high" until reset "low" by pulse
generator 495 at the start of the next two-minute measuring
period for the following pump cycle. The resulting output
pulse of NOR latch 409 triggers half-monostable pulse
generator 461, which then issues a single "high" output
pulse to latch counter 542 at the end of each two-minute
measuring period as previously described. This entire
sequencing circuit is initialized by power-on delayed pulse
generator 290, upon application of initial DC power to the
control circuit of the DPCU 2.
Operation of pump monitor 460 is best understood by
considering the total number of pulses that are recorded by
counter 542 during each two-minute measuring period,
whenever fluid passes through sensor 48 at a steady rate
that is exactly equal to the rated steady-flow capacity of
the controlling sensor orifice. To simplify this
illustration, assume that the theoretical capacity of the
~ downhole pump is identically equal~to the rated capacity of
the sensor, and that all downhole equipment is operating at
100% volumetric efficiency. Under these conditions, the
buffered output voltage of the sensor is exactly 10.0 vdc,
following proper correction for fluid density. This analog
voltage signal 387 is applied directly to the second pole of
rotary switch 294 as shown in Figure 8C, and is thereafter
divided by the appropriate resistance network 556 and 526
that adjusts the applied flow-rate signal "Vb" for the
selected orifice size, as follows:
Vs24 = V~ ~ ~steady-state sensor capacity) (18)
( 1000 BFPD
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1 333962
Once flow-rate signal "Vb" has been adjusted for the
rated sensor capacity and trimmed by factory calibration
potentiometer 526, it is then buffered by voltage follower
528 so that further processing of this signal will not
affect the accuracy of analog division networks 556 and 526.
The resulting buffered signal "V528" is then applied to the
input of variable-resistance divider 532 that serves to
normalize the signal for the programmed pump displacement.
Analog divider 532 is comprised of a precision 100k ten-turn
- 10 linear potentiometer 413 that has one end of its resistance
element left open-circuit as shown in Figure 8C, and that
has its wiper element connected to the DC ground buss of the
control circuit by means of a 2.Ok resistor 465. Input knob
534 and its mechanically linked counter 467 are phased with
potentiometer 413 at time of factory calibration so that a
numerical reading of 20 sFPD on counter 467 corresponds to a
wiper resistance of 0 ohms, and a reading of 1000 BFPD
corresponds to a wiper resistance of 98k ohms. By
constructing this circuit as described, the resulting output
voltage signal "V465" of the wiper is always equal to the
input signal "V528" multiplied by a displacement
amplification ratio of (20 BFPD/displacement). Thus, the
output voltage "V465" of potentiometer 413 will at all times
be defined as follows:
V46s = (0.02 * VB) (sensor capacity/displacement) (19)
Under the assumed operating conditions of this
particular illustration, the output voltage "V465" of
30 potentiometer 413 will be a constant 0.20 vdc. This signal
is then amplified by a constant gain of 50, by means of op-
amp 463, before being applied to the input of VCO 535. It
may be seen, therefore, that VCO 535 will always be driven
by an input signal of 10.0 vdc whenever the downhole pump is
operating at 100% volumetric efficiency relative to the
programmed pump displacement. This fact, which holds true
regardless of the selected sensor size and actual pumping
rate "Qp", may be readily confirmed by similar mathematical
analysis of other steady-state ex~mples. The resulting
40 signal (V465 = 10.0 vdc at 100% pump efficiency) causes VCO
535 to deliver a steady output frequency of 2133 hz, which
is then divided by a constant factor of 256 by means of
divider 536 in order to apply a steady frequency (in this
steady-state example) of 8.333 hz to the input of counter
45 542. Such frequency causes counter 542 to register a total
of (8.333 x 120) = 1000 digital pulses during the two-minute
data acquisition period that begins at the start of each
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normal production period of cyclic pump operation. Since
each pulse corresponds to 1/lOth of a percentage point, LED
display 548 will correctly indicate a pump efficie~cy of
100.0% under these assumed operating conditions.
The response of monitor 460 may be further illustrated
by assuming that the steady-state pumping rate "Qp" of all
downhole equipment is reduced to 50% of the programmed pump
displacement, which for this second example should once
again remain equal to the rated capacity of the installed
sensor. Since the average fluid discharge rate has been cut
in half, the output of sensor 48 is now 5.0 vdc rather than
10.0 vdc as previously assumed. This means that the output
voltage signals of resistance networks 556 and 526, buffer
528, potentiometer 413 and amplifier 463 will also be
15 reduced by 50%. Likewise, the output frequency of VCO 535
will be reduced by 50% in this example, since its output
signal always varies linearly with the applied input
voltage. The output of VCO 535 will therefore be (50%)(2133
hz) = 1067 hz in this particular situation. This frequency,
20 when divided by a factor of 256, results in only 500 digital
pulses being recorded by BCD counter 542 during each 120
second pump efficiency measuring period. At the conclusion
of each such computation, this pulse count is correctly
displayed to the operator as a pump efficiency of 50.0%
which is identical to the assumed volumetric efficiency of
downhole equipment in this second illustration. In general,
monitor 460 always records a digital count that is equal to
the average measured pumping rate "Qp" divided by the
programmed pump displacement entered into counter 467 by the
operator by means of knob 534. This result holds true even
when the flow is of a pulsating nature, since BCD counter
542 integrates the resulting instantaneous digital frequency
over its 120 second counting period to arrive at a true
average value of the normalized "Vb" flow-rate signal upon
which the measure of downhole pump efficiency is based.
The Low Pump Efficiency Monitor 470 (Figure 8C) is
designed to automatically terminate the established
production period of normal pump operation whenever the
measured pumping rate "Qp" of all downhole equipment is
determined to be less than an arbitrarily assigned value of
25% of the programmed pump displacement. With reference to
Figure 8D, it will be noted that monitor 470 includes a
conventional digital-to-analog (D/A) converter 361 that
receives its input clocking pulse~from the divide-by-1024
digital output node of total fluid production frequency
divider 476, by way of AND gate 419. D/A converter 361 is
configured as a linear stair-step generator, being comprised
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of a resettable binary counter 479 and associated "R-2R"
resistive ladder network 481. Ladder network 481 includes a
calibrating potentiometer 483, with output wiper voltage
"V483" being app]ied directly to the positive input te,rminal
5 of amplifier 429. This amplifier, which imparts a constant
gain of approximately 140~ to the input voltage signal
"V483", is comprised of voltage sensing op-amp 485 with
resistive feed-back network 487 connecting its output and
negative input terminals to ground. D/A converter 361 is
10 calibrated at time of manufacture by means of potentiometer
483 so that the output voltage "V429" of op-amp 485 becomes
exactly 10.000 vdc whenever counter 479 is indexed by 486
clocking pulses following reset of its input register.
The reset of D/A counter 479 is automatically
15 accomplished during periods of cyclic pump operation by
means of voltage invertor 415 that receives its input
control signal from the pump relay power buss 436. Su~h
reset will be periodically achieved following termination of
each "pump-off" verification period, and upon initial
20 application of DC control power to the various electronic
circuits of the DPCU by means of switch 234 (Figure 8A).
Following such reset, AND gate 419 is sequentially
enabled/disabled by NOR latch 409 and voltage invertor 417
so that clocking pulses from frequency divider 476 are
25 registered by counter 479 only during the two-minute pump
efficiency measuring period that immediately follows proper
termination of each verified prime period. The resulting
output voltage "V429" of op-amp 485 is applied directly to
the positive input of voltage comparator 473 as shown in
30 Figure 8C. This voltage is proportional to the established
pumping rate "Qp" of all downhole equipment, divided ~y the
rated steady-state flow capacity of the installed fluid
sensor 48.
Connected to the negative input terminal of voltage
35 comparator 473 is a temperature-stabilized precision
reference voltage "V471" that is at all times proportional
to the programmed volumetric displ~cement of downhole pump
98, divided by the rated steady-state flow capacity of the
installed fluid sensor 48 IFigure 1). Reference voltage
40 "V471" is obtained by means of an analog voltage division
network that is programmed by the operator in the field
using sensor size selector switch 294 and knob 534 of pump
displacement counter 467. This analog division network is
comprised of a fixed-resistance network 524 and grounding
45 potentiometer 471 (Figure 8D) that are connected to the
left-hand pole of four-position rotary switch 294 as shown
in Figure 8C. Rotary switch 294 and its associated voltage
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1 333962
dropping resisters are used to divide the applied 12.0 vdc
precision reference voltage "vtc" by factors of 1, 2, 4, and
8 for sensor sizes A through D respectively. Thus, the
input voltage to potentiometer 471 will be either 12.d vdc,
6.0 vdc, 3.0 vdc or 1.5 vdc depending on the position of
rotary switch 294 for the selected senscr size. Since the
wiper arm of potentiometer 471 (Figure 8D) is mechanically
linked to the input programming knob 534 of counter 467
(Figure 8C), the output wiper voltage "V471" of this analog
division circuit will always be proportional to the
programmed pump displacement divided by the rated steady-
state flow capacity of the installed fluid sensor. This
fact may be readily confirmed by mathematical analysis of
several different examples for each selected sensor size.
Due to proper selection of the D/~` converter input
clocking frequency division ratio and output voltage signal
amplification ratio at time of manufacture, by means of
divider 476 (Figure 8C) and amplifier 429 (Figure 8D),
respectively, as previously described, both input voltages
"V429" and "V471" of voltage comparator 473 will be exactly
equal to each other at the conclusion of each two-minute
pump efficiency measuring period whenever the established
pumping rate "Qp" is exactly 25% of the programmed pump
displacement. Any pump efficiency greater than 25% will
result in "V429" being greater than "V471", and any
efficiency less than 25% will result in "V429" being less
than "V471", at the conclusion of the two-minute pulse
counting period. Thus, the output of voltage comparator 473
will be switched and maintained "low" throughout the rest
and prime periods of each pump operating cycle by the
combined action of sequencing inverters 415 and 417, and
will only switch "high" during the two-minute pump
efficiency measuring period of that operating cycle, if the
volumetric efficiency of all downhole pumping equipment is
determined to be greater than the value of 25% arbitrarily
selected for the control circuit of the preferred
embodiment. Once the output of comparator 473 switches
"high", however, it will thereafter remain "high" until
reset at the start of the next operating cycle at the
conclusion of the "pump-off" verifieation period.
With reference to Figures 8B and 8D, it will be noted
that the output signal of voltage comparator 473 is applied
to the reset terminal of four-cycle shutdown counter 569 by
way of AND gate 571. This signal serves to reset the four-
cycle shutdown 500 hereinafter described whenever pump
efficiency is determined to be greater than 25%, provided
that proper fluid sensor operation is confirmed by clapper
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1 333962
motion detector 330 (Figure 8B) that enables AND gate 571 by
way of invertor 343. The output of comparator 473 (Figure
8C) is also used to terminate the established operating
cycle at the end of each two-minute pump efficiency
measuring period whenever pump efficiency is determined to
be less than the minimum acceptable value of 25%. This is
accomplished by means of invertor 427 and AND gate 425 that
apply a "high" sequencing signal to the control buss of
transistor 433 (Figure 8B) by way of diode 431 in order to
collapse the integrated control signal 371 that activates
op-amp 426. Such actions can only take place after
termination of the two-minute pump efficiency measuring
period, since AND gate 425 is disabled until that time by
the actions of AND gate 421, in response to the output of
15 NOR latch 409 and pump mode discriminator 513,
Operation of the above described circuit is best
understood by considering the following example for a
typical well installation. Assume that the displacement of
the downhole pump is 150 BFPD, and that such equipment is
operating at 25.2~ volumetric efficiency. Further, assume
that sensor size "B" is properly installed in the fluid
discharge line of the wellhead, together with a properly
adjusted fluid back pressure valve 50, and that rotary
switch 294, and pump displacement counter 467 are properly
set for the maximum rated capacity of such equipment. In
this situation, voltage comparator 473 (Figure 8D) of the
low pump efficiency monitor 470 is supplied with a constant
reference voltage of 0.900 vdc, computed as follows:
30 Reference voltage = (12.0)(1/2)(150/1000) = 0.900 vdc (20)
At the start of each rest period, the output voltage
of D/A converter 361 will be reset to "0" by the actions of
invertor 415, in response to the "low" voltage state of pump
relay power buss 436. Such action causes the output of
voltage comparator 473 to switch "low", and its inverted
output to switch "high". This inverted output signal is
blocked by AND gate 425, however, which remains disabled
throughout the rest, prime and pump efficiency measuring
periods by the controlling actions of AND gate 421. During
the rest and prime periods, the output of comparator 473
remains "low" since the input clock register of counter 479
is disabled by AND gate 419. At the start of the production
period, the output of NOR latch 409 switches "low" to
45 disable AND gate 421 and enable AND gate 419. During the
next 120 seconds, the clock register of counter 479 will be
pulsed 44 times in this example by the combined actions of
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1 333962
VCO 403 and divider 476 in response to the average output
voltage of fluid sensor 48 as follows:
Pulses = (25.2% * 150 BFPD)(2489 hz)(120 seconds) = 44~.1 (21)
( 250 BFPD )( 1024
The register of 44 pulses by D/A converter 361 during the
two-minute pump efficiency measuring period causes the output
voltage of amplifier 429 to rise from its initial value of 0
vdc to a final value of ~44/486)~10.0 vdc) = 0.905 vdc. Since
this amplified output voltage is ~reater than the 0.900 vdc
reference voltage signal that is applied to the negative input
terminal of comparator 473, the output of comparator 473 will
switch "high" before the end of the two-minute pump efficiency
measuring period. This action causes the reset of four-cycle
shutdown 500 ~Figure 8B) and, additionally, causes the output
of invertor 427 to switch "low" to prevent the early
termination of the production period when AND gate 425 ~Figure
8D) is enabled at the end of this measuring period. Any pump
efficiency in excess of 25% will cause the same system
response, since the total number of pulses recorded by the D/A
converter 361 during its two-minute counting period will
increase as the average pumping rate "Qp" increases. Should
pump efficiency fall below the limiting design value of 25%,
however, then the output of comparator 473 will remain "low"
for the entire operating cycle. Such action will initiate
early termination of the production sequence by way of AND
gate 425, and will prevent the reset of four-cycle shutdown
500 ~Figure 8B), for reasons previously described.
The control sequence light circuit of the DPCU is
provided to apprise the operator of the current status of pump
operation. A rest period LED display 501 with current-
limiting input resistor 503 and driving NPN transistor 505 is
actuated by a signal invertor 507 that receives its input
signal from the output of op-amp 422 as shown in Figure 8B. A
prime period LED display 509 with current-limiting input
resister 511 and blocking diode 523 receives its input signal
from prime control buss 435. A production period LED display
517 with current-limiting input resister 519 and blocking
40 diode 521 receives its input signal from the output buss 423
of a pump mode discriminator 513. This discriminator is
comprised of a signal invertor 527, AND gate 529, and NPN
transistor 531 with current-limiting base input resister 533.
Pump mode discriminator 513 receives its two input control
45 signal from prime control buss 435 and op-amp 422. This
discriminator delivers a "high" output signal to buss 423 only
during the normal production period of pump operation. All
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1 333962
signal to buss 423 only during the normal production period
Of pump operation. All three LED lights referenced above
may be checked for proper operation by activation of
momentary lamp test switch 525 that delivers DC power to
these lights by way of three blocking diodes shown but not
numbered on Figure 8B.
The malfunction indicator light circuit of the control
unit 2 has been designed to provide the operator with a
positive visual indication of the most recent motor control
sequencing action taken by each of the four error-detection
circuits herein referenced. As shown in Figure 8B,
individual circuits 541 through 544 are provided to indicate
the current output status of the clapper motion detector
330, the low pump efficiency monitor 470 (Figure 8D), the
excess B+ current detector 252 (Figure 8A) and the four-
cycle shutdown 500 (Figure 8B), respectively. Each of these
individual circuits is controlled by its assigned flip-flop
memory device 551-554 that delivers a "high" output signal
whenever its input clock register is pulsed by the output
signal of the corresponding error-detection circuit. Due to
the internal operating characteristics of each flip-flop and
the non-volatile nature of its CMOS memory, a "high" output
signal from any circuit will be permanently maintained until
such time as the controlling flip-flop is reset "low". This
feature enables the operator to determine which malfunction
has caused the shut-down of system operation, upon
reapplication of s+ control power.
With reference to Figure 8s, the malfunction indicator
light circuit referenced above includes two Dual-D flip-
flops with memory that are connected to a common DC power
buss 515 that receives continuous DC power from either the
normal "B+" power supply 200 or the emergency "Ve" power
supply 210 d~pending on which supply is currently
activated. Regulated power is supplied to the common buss
35 515 by way of two blocking diodes 563 and 565 that prevent
the direct interaction of one power supply with the other.
Each memory chip contains two electrically isolated dual D-
Type flip-flops that have their signal controlling "D" input
pins connected to the common power buss 515 and their
individual "set" pins connected to the ground buss of the
DPCU 2. The "Q" output of each flip-flop is connected to an
NPN power transistor with current-limiting base resistor
that also receives DC supply powel- from power buss 515 in
order to drive an LED indicating lamp by way of its current-
limiting input resistor.
1 3339~2
Inasmuch as the detection of improper clapper motion
and/or low pump efficienCY will only be used by the motor
controller to terminate the established ProductLon `Period
early, and since such measure will not be directly used
within the control unit 2 to cause the immediate and
permanent cessation of all further pumping operations except
by way of the four-cycle shutdown 500, the corresponding
flip-flop circuits 551 and 552 for these two control
parameters are always reset during each successive Prime
Period by the "positive going" output signal of the
production sequence controller 426. By contrast, the two
flip-flop circuits 553 and 554 that are respectively
activated by a "high" output signal from the excess B+
current detector 252 (Pigure 8A) and the four-cycle shutdown
500 (Figure 8B), must be manually reset by the operator as
shown using the manual reset switch 561 prior to continued
pump operation. Since the output from each of these
circuits 553 and 554 is applied directly to the input
switching buss of the emergency l'Ve" power supply by way of
20 two blocking diodes 563 and 565, this design assures that
the cause of unscheduled equipment shutdown will be brought
to the operator's attention before further operation of the
pump is attempted.
The four-cycle shutdown 500 (Figure 8B) of the DPCU 2
is provided to terminate the automatic operation of all
downhole and surface mounted pumping equipment whenever the
measured per~ormance of either the fluid sensor or downhole
pump is determined to be unacceptable during each of four
consecutive operating cycles. A pulse-shaping AND gate 567
has both of its inputs connected to the prime control buss
435, and its output connected to the input clock register of
decade counter 569. The reset of this counter is connected
to a signal blocking AND gate 571 that has one input
connected to the inverted output of the clapper motion
detector 330, and the other input connected to the non-
inverted output of the low pump efficiency evaluator 470
(Figure 8D). The reset terminal of counter 569 (Figure 8B)
is also connected by way of a signal blocking diode 573 to
the output node of the power-on delayed pulse generator 290
~Figure 8D). The fifth sequential output node of decade
counter 569 (Figure 8B) is connected to the input clock
register of the four-cycle flip-flop 554.
Upon initial application of B+ power, the power-on
pulse generator 290 (Figure 8D) resets all outputs of
45 counter 569 (Figure 8B) to their initial output state of 0
vdc in order to initialize the four-cycle shutdown S00
herein described. Thereafter, decade counter 569 is indexed
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1 333962
forward by one count at the start of each successive Prime
Period by the pulse-shaPing AND gate 567 that is connected
to its input clock register as shown in Figure 8B. Whenever
clapper motion and pump efficiency are both deemedlto be
within their acceptable limits following their respective
data acquisition periods, counter 569 i~ reset by AND gate
571 so that all counter outputs once again return to their
initial "low" state before the start of the next operating
cycle. Should either clapper motion or pump efficiency be
judged unacceptable by their respective evaluation circuits,
however, then such reset will not occur; in this situation
the next sequential output of counter 569 will be indexed
"high" at the start of the next Prime Period.
If the measured performance problem does not correct
itself within four consecutive operating cycles, then the
fifth output of counter 569 eventually pulses "high" at the
start of the fifth sequential Prime Period. This response
actuates the input switching buss of the emergency ''Ve'l
power supply in order to terminate all further operation of
the pump. This response also actuates the input clock
register of four-cycle flip-flop 554 in order to activate
the input switching buss of the emergency ''Vel' power supply
to advise the operator of such action. Upon such actuation,
the voltage level of the emergency l'vell power supply buss is
latched "permanently high" by the non-volatile memory of
flip-flop 554. Such latching also occurs whenever the "Q"
output of the excess B+ current detection flip-flop is
switched "high". Once such actuation has occurred, both
flip-flops must then be manually reset by the operator using
switch 561 before operation of the prime mover can be
resumed.
Although the functions set forth above are described as
being implemented using hard wired circuitry and discrete
electronic components, which is preferred in the
electrically noisy environment within which the system is
designed to operate, it is to be understood that functions
could alternatively be carried out by computer
implementation. Thus, it is contemplated that a standard
microprocessor such as a type Z-80 could be programmed by
firmware in a read-only memory (ROM) and be connected to a
random access memory (RAM) 456 for temporary storage of
data, in a conventional manner. An output of the
microprocessor could control the well pump and the various
displays and alarm-strobes described above. Control inputs,
such as toggle switches, keyboards, etc., to tailor the
operation of the device woul~- be applied to the
microprocessor which could also regulate the serial
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1 333962
transmission of stored data by conventional microwave ortelephone systems as depicted in Figure 13.
Although the present invention has been shown and
described in terms of a specific preferred embodime~t, it
will be appreciated by those skilled in the art that changes
or modifications are possible which do not depart from the
inventive concepts described and taught herein. Such
changes and modifications are deemed to fall within the
purview of these inventive concepts.
