Note: Descriptions are shown in the official language in which they were submitted.
--` 1326297
T 8137
METHO~ AND APPARATUS FOR MEASURING
DIFFERENTIAL FLUID POWER OUTPUT OF A PUMP
The use of soft starters for electrical sub-
; mersible pumps (ESP's) within the past few years had
led to an opportunity to increase production by pumping
ESP's off, i.e., pumping well fluid levels down to the
pump intake in order to obtain maximum production from
a well by lowering its bottomhole pressure. Operating
an ESP in this mode means that the ESP is continually
cycled on and off as the unit is pumped off, shut down
for a short period of time to allow the well to par-
tially fill, and then restarted. This could not have
been done prior to the use of soft starters since ESP
~ failure was common on restart.
" Besides minimizing restarting failures, a pump off
'~ operation also requires reliable pump off detection and
, 15 control to not allow an ESP to operate after it has
;~ become gas-locked. Failure to shut down a gas locked
, ESP will result in premature failure due to over-
, heating. Gas locking occurs when an ESP ingests suffi-
, cient gas so as to no longer be able to pump fluid to
the surface, the result of either large gas bubbles
, being present in the well fluid or of the pump intake
~1' being uncovered at pump off. In accordance with the
,~ present invention an ESP pump off controller has been
developed to meet the needs of reliably detecting and
shutting down an ESP when gas locked or pumped off
since existing ESP motor controllers have been proven
to inadequately control under these critical condi-
tions.
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Existing ESP motor controllers have been adapted
from surface motor control packages where motor
operation is more stable and motor control is less
critical. For example, it is not critical for a motor
controller to prevent a surface centrifugal pump from
running dry since this will not damage the pump or its
motor, but a downhole ESP will fail rapidly if it is
run after losing fluid flow to the surface. These motor
controllers monitor the running current (or power
consumption) of the motor and compare it to a manually
adjustable, fixed setpoint. When the current drops
below this underload setpoint for a prescribed length
of time, the motor is shut down.
Experience has shown that this existing method of
motor control is unreliable since pumping ESP's are
seen to be prematurely shutting down in underload or
not at all. The reason for this unreliability is that
- the manually entered setpoints are often guessed, or at
best, based on varying rules of thumb which may have no
correlation to what is going on downhole. As a result,
setpoints are frequently set too high causing premature
- shutdown and loss of production or set too low failing
to shut the ESP down, causing failure of the ESP and
` loss of production.
The primary purpose of the present invention is to
provide a method and apparatus for shutting down a pump
motor when a motor underload has occurred, for example
due to pump off or gas lock. Gas locking and pump off
have been found, as discovered in accordance with the
present invention, to be characterized by a sudden drop
in motor load and fluid power output when gas enters
the pump.
The method according to the invention comprises:
(a) measuring pump power consumption at timed inter-
vals;
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(b) determining a recent pump motor power consumption
from the measurements:
(c) determining a previous pump motor power consump-
tion from the measurements;
(d) deriving the difference between steps (b) and (c);
and
(e) denoting the difference as differential pump fluid
power output.
The apparatus according to the invention
, 10 comprises:
(a) means for measuring pump power consumption at
~ timed intervals;
,~ (b) means for determining a recent pump motor power
consumption from the measurements;
(c) means for determining a previous pump motor power
consumption from the measurements;
(d) means for deriving the difference between steps
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(b) and (c); and
" (e) means for denoting the difference as indicative of
~'- 20 differential pump fluid power output when said
difference exceeds a predetermined quantity.
, Several alternative methods and apparatus embody-
~, ing the invention may be used to reliably determine if
-, gas locking or pump off has occurred by measuring pump
operating parameters at the surface. A preferred method
i~ uses the following logic: (1) computations are per-
, formed on measured motor load to determine if a drop in
motor load has occurred. (2) For non-gassy pumping
~, application, the ESP is &hut down on the first indica-
tion of a drop in motor load. This will occur at pump
off; gas will not enter the pump and cause motor load
to drop until the fluid level in the well is pumped
`- down and the pump intake is uncovered. (3) For gassy
pumping application, the ESP is shut down only when it
gas-locks. Motor load will drop each time gas enters
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the pump, but will recover when the gas exits with the
pump fluid. When a large amount of gas enters the pump
and the pump becomes gas-locked, motor load will drop
- but will not recover since the gas is trapped in the
pump. In this application, the ESP is shut down if
motor load drops and does not recover within an
adequate length of time.
Preferably, the method (and apparatus for
conduction the method) includes shutting the pump (or
pump motor) down when the differential pump fluid power
output exceeds a predetermined amount for a predeter-
mined length of time (or when an underload is detected
or alternatively, shutting the pump motor down when an
underload is detected which exceeds a predetermined
length of time).
Other purposes, distinctions over the art, advan-
tages and features of the invention will be apparent to
one skilled in the art upon review of the following
description with reference to the accompanying drawing,
` 20 in which: Figure l shows a flow diagram of a preferred
controller sequence of steps developed in accordance
with the present invention.
In accordance with the present invention several
parameters are monitored independently, or in any
combination to determine if a change in electrical
- submersible pump (ESP) motor load has occurred. These
are apparent power, actual power, reactive power, power
, factor, and current (since voltage is generally
~`- constant). Each one of these parameters will drop when
` 30 the ESP motor load drops. Ratios of combinations of
these parameters may also be monitored since the ratios
will change when motor load drops. If current is used
to monitor motor load, voltage may also be measured as
a secondary parameter to ensure current fluctuations
; 35 are not the result of voltage spikes or sags, i.e.,
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current can fluctuate due to other reasons than motor
load. For example, the power company may not supply
uniform voltages, or storms may cause variations, etc.
Motor load parameters can be measured with a
variety of techniques. Parameters can be measured
~ directly or can be first subjected to filtering or
"~ smoothing with a root mean square or averaging tech-
nique before being measured. Measurements can be taken
in an analog fashion with mathematical calculations
performed with analog circuitry. Alternatively, analog
measurements can be converted to digital and mathe-
matical computations performed with either digital
hardware or with software such as in a microprocessor.
Digital sampling rates, the length of time evaluated in
computations, and data storage requirements are inter-
y related but can vary widely. In a preferred embodiment,
~ the present invention used an analog-to-digital
-~ sampling rate of 4 Hz, although sampling rates less
than once every 15 minutes may be used at different
, 20 time period, or used in the computations as described
hereinafter.
, Unlike existing motor control technology that
compares motor power or current to a setpoint, the
control methods of the present invention utilize
various techniques to determine if pump off has
occurred as described hereinbelow. To determine if
motor load has dropped, a comparison of the most recent
motor load measurements may be made to any previous
average motor load. Thus, the most recent motor load
measurement can be a single data point or the average
of many data points that last occurred. There is no
-~ limit to the number of data points or the length of
time over which the average can be calculated. Testing
of the present invention was successful when using the
- 35 average of the most recent one second of current and
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also using just the last single point reading of
current. The previous motor load average is preferably
computed over any time interval from ESP start to the
first data point used in the most recent average. One
method of determining the previous motor load is to
continually recalculate the moving average of the motor
load for any set length of time prior to the most
recent average data. There is no limit to the number of
data points or the length of time over which the
average is calculated. Testing of the present invention
has successfully used a moving 5-second average of
current. The required degree of drop in the motor load
parameter must be established in order to identify a
potential gas lock or pump off condition. This degree
~ 15 is dependent upon which parameter is being monitored,
J but is still quite flexible. In accordance with the
present invention, a criteria that a current drop must
be greater than 5 per cent was successfully shown but
, testing has also indicated that success may be obtained
with a criteria anywhere from l per cent to 20 per cent
and a wider range of one-quarter per cent to 30 per
i cent is possible although errors are more prone to
^~ occur in the wider range. In a gassy pumping condition,
motor load must drop and remain down for a period of
time before the ESP is shut down for gas locking. This
required time the motor load must remain down is
dependent on the length of time used in the averages
in the above steps. In accordance with the present
! invention, there was successfully tested l0 seconds as
- 30 one time criteria, but it was easily feasible to use
anything greater than 2 seconds. However, this time
limit could be cut to zero if longer time periods were
used to calculate the two averages of motor loading.
~ The maximum time limit is only a function of how much
; 35 risk of damage it is possible to take with the ESP
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-' before shutting down (for example, an hour or more
would be extreme).
Computation of the differential of m~tor load with
respect to time is another way to determine if motor
load has dropped.
Motor load is sampled at regular intervals as
stated above and the differential motor load is cal-
culated by subtracting the previous motor load from the
most recent motor load, and dividing the difference by
time between the two measurements. The previous and
most recent motor load values can each be single point
^ measurements or the averages of several measurements. A
significant negative result indicates a drop in motor
load such as when an ESP is gas locked or pumped off.
` 15 The degree to which the differential must be negative
depends on sampling rates and the time interval over
which the differential is calculated. The ESP is pumped
-` off if the differential becomes significantly negative
and does not then become significantly positive. The
differential method of controller calculation can be
performed digitally or in an analog fashion.
- Integration of the motor load for a given period
of time is yet another way to calculate average motor
~' loads.
Motor load is sampled at regular intervals as
! stated above and stored in a data array. The area under
`1 the motor load versus time curve is calculated for the
`~ most recent time period by using an integration tech-
nique. The most recent time period can be any length
specified by the user depending on the sensitivity
required (the shorter the length, the more sensitive
the calculation to changes in motor load).
The integration controller calculated method can
; also be performed digitally or in an analog fashion.
Gas locking or pump off is indicated if the most recent
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integration of motor load is less than the previous
integration be a predetermined amount. The length of
time over which the integrations are performed, whether
the reference integration is calculated at a fixed
point in time or on a moving basis, the degree of drop
required to be significant, and the time required for
the drop to remain down in gassy applications will all
- vary similarly as in the first method described above.
Finally, another way to determine if motor load
has dropped is by performing a statistical analysis of
- motor load. As in the other methods, motor load is
sampled at regular intervals and stored in a data
array. The sample distribution statistics are cal-
culated from the previous motor load samples taken for
a given length of time and the most recent motor load
.;3 sample is compared to it. Drops in motor load that fall
outside of control limits calculated from the previous
motor load sample distribution and the desired sample
confidence interval indicate a significant drop in
motor load has occurred. The confidence interval that
is used is dependent on the probability of error that
it is possible to accept and can vary accordingly. If
the most recent sample of motor load falls below the
1 calculated lower control limit for a predetermined
`~ 25 length of time, then the ESP has gas locked or pumped
off and is shut down. Additionally, a statistical
; calculation in motor load variance or standard
i deviation indicates that motor load has become more
variable, another indication that gas has entered the
pump or pump off has occurred.
~ The pump off/gas locking controlling developed in
:~ accordance with the present invention can be utilized
in several ways. Thus, it is possible to be used as a
. controller subassembly. The developed controller is
: 35 wired in series with an existing motor controller to
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augment/replace the underload functions of the existing
controller. Alternatively, the apparatus can be inte-
grated into a single motor controller package. Thus, it
is necessary to replace the underload functions in a
motor controller with the gas lock/pump off controller
~ functions. Also alternatively, the apparatus of the
- present invention can be integrat~d into an intelligent
remote terminal unit. This unit exercises motor control
- functions to include pump off/gas lock control and has
the additional capabilities of monitoring ESP
operation, storing operation data, and communication
with a ~central computer.
; Having thus generally described the apparatus and
method of the present invention, as well as its
numerous advantages over the art, the following is a
more detailed description of a preferred embodiment
thereof given in accordance with specific reference to
- the drawings.
Step 1: sample current continuously (analog to
digital conversion) every 25 second. Step 2: start
controller when current exceeds ~ amp (occurs when ESP
is started). Step 3: start controller functions when
the current spike on ESP start is over. Step 4: take
the most recent sample of current and store it in the
` first position of the data array for use later in the
controller computations. Step 5: after the ESP is
; started, begin calculations only after the date array
is full (6 seconds of current samples. Step 6:
calculate the most recent one second average of current
by averaging the first four values in the data array.
Step 7: if the pump off counter is not equal to zero,
then the last calculated difference was more than a 5%
drop, indicating that the ESP already has gas in it and
may be pumped off. Step 8: if the ESP is not already at
potential pump off, calculate the normal previous
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- 10 -
~ 5-second average of current by averaging the last 20
i~ samples in the date array. Step 9: if the ESP is
already at potential pump off, do not recalculate the
previous 5-second average of current. Use the previous
average of current calculated when current first
dropped in order to compare the most recent current to
its original level. Step lO: subtract the most recent
l-second average from the previous 5-second average of
current. Step 11: if the calculated difference is
greater than a S% drop, then gas has entered the pump
and the ESP is potentially pumped off. Step 12: shut
the ESP off in non-gassy pumping conditions since a
current drop greater than 5% will only occur at pump
off. Step 13: count the length of time the ESP is in a
~ 15 potential pump off condition (one count equals 25
; seconds). Step 14: in gassy pumping conditions, the ESP
` has pumped off and is shut down if current does not
return to its original 5-second average (before current
drop occurred) in lO seconds or less. Step 15: prepare
- 20 the date array for the next current sample by bumping
the data in the array down one. This effectively erases
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the oldest current sample and makes room for the next
current sample to be added to the top of the array
(first position).
While the above description is primarily directed
to detecting electrical pump motor underload which is
indicative of gas lock or pump off, the basic invention
is more broadly drawn to methods and related apparatus
for monitoring and controlling pump operation by
measuring changes in pump input power. This is done (a)
in pumps with electric motor drives by measuring motor
load and comparing present motor loads to previous
motor loads, (b) in pumps with hydraulic motor drives
by measuring hydraulic power consumed (input pressure
and flow rate - output pressure and flow rate) and, as
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1326297
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above, comparing present motor loads to previous motor
loads. Potential pump/motor combinations include (1)
centrifugal pump with electric motor drive, (2) centri-
fugal pump with hydraulic motor drive, (3) positive
displacement pump with electric motor drive, (4)
positive displacement pump with hydraulic motor drive.
The foregoing description of the invention is
merely intended to be explanatory thereof, and various
changes in the details of the described method and
apparatus may be made within the scope of the appended
claims without departing from the spirit of the
invention.
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