Note: Descriptions are shown in the official language in which they were submitted.
CA 02717720 2010-10-15
CALCULATION OF DOWNHOLE PUMP FILLAGE AND CONTROL
OF PUMP BASED ON SAID FILLAGE
BACKGROUND OF THE INVENTION
Field of the Invention
Embodiments of the present invention generally relate to estimating efficiency
and controlling the operation of a downhole pump disposed in a wellbore. More
particularly, embodiments of the present invention generally relate to
accurately
determining a transfer point of a pump stroke and controlling the pump and
fluid
production of the well based on the transfer point.
Description of the Related Art
The production of oil with a sucker-rod pump is common practice in the oil and
gas industry. Typically, the pumping system is designed with the capacity to
remove
liquid from the wellbore faster than the reservoir may be able to supply
liquid into the
wellbore. In a fluid-producing well that is aided by artificial lift in the
form of a rod
pumping system, a condition may arise where the pump is not completely filled
with
fluid on each pump stroke. The well is said to be "pumped off," and the
condition is
known as "pounding."
It is desired to know the quantity of fluid entering the pump on each stroke
(the
pump "fillage") for a number of purposes including, e.g., to stop the pumping
system
periodically to allow more fluid to enter the wellbore or to control the speed
of the
pumping system so that it does not pump more fluid than enters the wellbore.
Knowing the pump fillage also allows the total amount of fluid produced by the
well to
be calculated.
Other methods have previously relied on the shape of the graphical
representation of the downhole card to compute the pump fillage. For example,
U.S.
Patent No. 5,252,031 to Gibbs, entitled "Monitoring and Pump-Off Control with
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Downhole Pump Cards," teaches a method for monitoring a rod pumped well to
detect various pump problems by utilizing measurements made at the surface to
generate a downhole pump card. The graphically represented downhole pump card
may then be used to detect the various pump problems and control the pumping
unit.
Owing to the diversity of card shapes, however, it can be difficult to make a
diagnosis
of downhole conditions solely on the basis of the shape of the graphical
representation. Furthermore, in some instances, such graphical techniques may
lead
to inaccurate determinations of the pump fillage such that fluid production
calculated
therefrom may be incorrect.
Accordingly, techniques and systems that rely less on human interpretation in
determining the pump fillage would be desirable.
SUMMARY OF THE INVENTION
Embodiments of the present invention generally provide methods and
apparatus for determining the pump fillage using data arrays of pump position
with
respect to time and pump load with respect to time.
One embodiment of the present invention is a method. The method generally
includes determining a transfer point of a stroke of a pump, the pump
typically having
a pump barrel, a plunger, a standing valve, and a traveling valve; and
calculating a
pump fillage based on the transfer point, wherein the pump fillage is a volume
of the
pump barrel between a bottom of stroke of the plunger and the transfer point.
For
some embodiments, the method further comprises controlling the pump based on
the
pump fillage.
Another embodiment of the present invention is a method. The method
generally includes determining position versus time and load versus time data
of a
pump, determining a data set comprising the load versus time data ordered
according to load values, dividing the data set into a bottom portion and a
top portion,
computing an average bottom value based on the bottom portion, an average top
value based on the top portion, and a half value of the data set, computing a
first
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pump fillage value comprising a ratio of a position value corresponding to the
average bottom value to a position value corresponding to the average top
value,
computing a second pump fillage value comprising a ratio of a position value
corresponding to the half value to the position value corresponding to the
average
top value, and determining a pump fillage based on a combination of the first
and
second pump fillage values.
Yet another embodiment of the present invention is a method. The method
generally includes determining position versus time and load versus time data
of a
pump; dividing a load data range of the load data into a number of increments;
determining a position value, from the position data, corresponding to each of
the
increments; computing a data set comprising an estimated pump fillage for each
of
the increments based on a ratio of the position value corresponding to each
increment to the position value of a top of stroke (TOS); dividing a pump
fillage range
of the data set into a number of intervals; for each interval, determining a
number of
occurrences of each estimated pump fillage having a value within that
interval; and
determining a probable pump fillage interval, wherein the probable pump
fillage
interval is the interval where the number of occurrences is at a maximum. For
some
embodiments, the method further comprises selecting one of a plurality of pump
fillage methods having a calculated pump fillage corresponding to the probable
pump
fillage interval.
Yet another embodiment of the present invention provides a system. The
system generally includes a pump and a control unit for controlling the pump.
The
pump typically has a pump barrel, a plunger, a standing valve and a traveling
valve.
The control unit is typically configured to determine a transfer point of a
stroke of the
pump and to calculate a pump fillage based on the transfer point, wherein the
pump
fillage is a volume of the pump barrel between a bottom of stroke of the
plunger and
the transfer point.
Yet another embodiment of the present invention provides a system. The
system generally includes a pump and a control unit for controlling the pump.
The
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control unit is typically configured to determine position versus time and
load versus
time data of the pump, to determine a data set comprising the load versus time
data
ordered according to load values, to divide the data set into a bottom portion
and a
top portion, to compute an average bottom value based on the bottom portion,
an
average top value based on the top portion, and a half value of the data set,
to
compute a first pump fillage value comprising a ratio of a position value
corresponding to the average bottom value to a position value corresponding to
the
average top value, to compute a second pump fillage value comprising a ratio
of a
position value corresponding to the half value to the position value
corresponding to
the average top value, and to determine a pump fillage based on a combination
of
the first and second pump fillage values.
Yet another embodiment of the present invention provides a system. The
system generally includes a pump and a control unit for controlling the pump.
The
control unit is generally configured to determine position versus time and
load versus
time data of the pump; to divide a load data range of the load data into a
number of
increments; to determine a position value, from the position data,
corresponding to
each of the increments; to compute a data set comprising an estimated pump
fillage
for each of the increments based on a ratio of the position value
corresponding to
each increment to the position value of a top of stroke (TOS); to divide a
pump fillage
range of the data set into a number of intervals; for each interval, to
determine a
number of occurrences of each estimated pump fillage having a value within
that
interval; and to determine a probable pump fillage interval, wherein the
probable
pump fillage interval is the interval where the number of occurrences is at a
maximum.
Yet another embodiment of the present invention provides a computer-
readable medium containing a program which, when executed by a processor,
performs operations. The operations generally include determining a transfer
point of
a stroke of a pump, the pump typically having a pump barrel, a plunger, a
standing
valve, and a traveling valve; and calculating a pump fillage based on the
transfer
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point, wherein the pump fillage is a volume of the pump barrel between a
bottom of
stroke of the plunger and the transfer point.
BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the above-recited features of the present
invention can be understood in detail, a more particular description of the
invention,
briefly summarized above, may be had by reference to embodiments, some of
which
are illustrated in the appended drawings. It is to be noted, however, that the
appended drawings illustrate only typical embodiments of this invention and
are
therefore not to be considered limiting of its scope, for the invention may
admit to
other equally effective embodiments.
FIG. 1 is a schematic depiction of an illustrative sucker-rod pumping unit
with a
control unit for controlling the pump in an effort to extract fluid from a
well;
FIGs. 2A and 2B illustrate position versus time data for a completely filled
or
nearly filled well and a pumped off well, respectively;
FIG. 3 is a flow diagram of exemplary operations for controlling a pump based
on a transfer point of a pump stroke;
FIG. 4 is a flow diagram of exemplary operations for determining a transfer
point of a pump stroke using a Method of Ratios according to an embodiment of
the
invention;
FIGs. 5A-C graphically illustrate data sets used in determining a transfer
point
of a pump stroke according to the operations of FIG. 4;
FIG. 6 is a flow diagram of exemplary operations for determining a transfer
point of a pump stroke using a Method of Positions, according to an embodiment
of
the invention;
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FIGs. 7A-C graphically illustrate data sets used in determining a transfer
point
of a stroke according to the operations of FIG. 6;
FIG. 8 is a flow diagram of exemplary operations for determining a transfer
point of a stroke using a Method of Areas, according to an embodiment of the
invention;
FIG. 9 is a flow diagram of exemplary operations for determining a transfer
point of a pump stroke using a modified Method of Positions, according to an
embodiment of the invention;
FIG. 10 illustrates a downhole card leaning to the right;
FIG. 11 is a flow diagram of exemplary operations for determining a transfer
point of a pump stroke using a Method of Loads, according to an embodiment of
the
invention;
FIG. 12 graphically illustrates a data set used in determining a transfer
point of
a stroke according to the operations of FIG. 11;
FIG. 13 is a flow diagram of exemplary operations for determining a pump
fillage of a pump stroke using a Method of Ordering, according to an
embodiment of
the invention;
FIGs. 14-15 graphically illustrate data sets used in determining a pump
fillage
of a stroke according to the operations of FIG. 13;
FIG. 16 is a flow diagram of exemplary operations for verifying a pump fillage
of a pump stroke using a Method of Multiple Pump Fillage, according to an
embodiment of the invention; and
FIGs. 17-18 graphically illustrate data sets used in verifying a pump fillage
of a
stroke according to the operations of FIG. 16.
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DETAILED DESCRIPTION
Embodiments of the present invention provide numerical methods for
determining pump fillage using data arrays of pump plunger position with
respect to
time and/or pump plunger load with respect to time. This may allow well
operators to
accurately monitor the pump fillage and control the pump accordingly.
The production of oil with a sucker-rod pump system 100 such as that
depicted in FIG. 1 is common practice in the oil and gas industry. In the pump
system 100, a rod pump 104 consists of a tubular barrel 106 with a valve 114
(the
"standing valve") located at the bottom that allows fluid to enter from the
wellbore, but
does not allow the fluid to leave. Inside the pump barrel 106 is a close-
fitting hollow
plunger 110 with another valve 112 (the "traveling valve") located at the top.
This
allows fluid to move from below the plunger 110 to the production tubing 108
above
and does not allow fluid to return from the tubing 108 to the pump barrel 106
below
the plunger 110. The plunger 110 may be moved up and down cyclically by a
horsehead 101 at the surface via the rod string 102, wherein the motion of the
pump
plunger 110 comprises an "upstroke" and a "downstroke," jointly referred to as
a
"stroke."
During the part of the pump cycle where the plunger 110 is moving upward
(the upstroke), the traveling valve 112 is closed, and any fluid above the
plunger 110
in the production tubing 108 may be lifted towards the surface. Meanwhile, the
standing valve 114 opens and allows fluid to enter the pump barrel 106 from
the
wellbore.
The highest point of the pump plunger motion may be referred to as the "top of
stroke" or TOS, while the lowest point of the pump plunger motion may be
referred to
as the "bottom of stroke" or BOS. At the TOS, the weight of the fluid in the
production tubing 108 may be supported by the traveling valve 112 in the
plunger
110 and, therefore, also by the rod string 102. This load causes the rod
string 102 to
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be stretched. At this point, the standing valve 114 closes and holds in the
fluid that
has entered the pump barrel 106.
During the part of the pump cycle where the plunger 110 is moving downward
(the "downstroke"), the traveling valve 112 initially remains closed until the
plunger
110 reaches the surface of the fluid in the barrel. Sufficient pressure may be
built up
in the fluid below the traveling valve 112 to balance the pressure due to the
column
of fluid to the surface in the production tubing 108. The build-up of pressure
in the
pump barrel 106 reduces the load on the rod string 102; this causes the
stretching of
the rod string 102 that occurred during the upstroke to relax. This process
takes
place during a finite amount of time when the plunger 110 rests on the fluid,
and the
horsehead 101 at the surface allows the top of the rod string 102 to move
downward.
The position of the pump plunger 110 at this time is known as the "transfer
point" as the load of the fluid column in the production tubing 108 is
transferred from
the traveling valve 112 to the standing valve 114. This results in a rapid
decrease in
load on the rod string 102 during the transfer. After the pressure below the
traveling
valve 112 balances the one above, the valve 112 opens and the plunger 110
continues to move downward to its lowest position (the BOS). The movement of
the
plunger 110 from the transfer point to the BOS is known as the "fluid stroke"
and is a
measure of the amount of fluid lifted by the pump 104 on each stroke. In other
words, the portion of the pump stroke below the transfer point may be
interpreted as
the percentage of the pump stroke containing fluid. This percentage is the
pump
fillage.
Typically, there are no sensors to measure conditions at the pump 104, which
may be located thousands of feet underground. However, there exist numerical
methods to calculate the position of the pump plunger 110 and the load acting
on the
plunger from measurements of the position of and load in the rod string 102 at
the
pumping unit located at the surface. These measurements are typically made at
the
top of the polished rod 118, which is a portion of the rod string 102 passing
through a
stuffing box 103.
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If there is sufficient fluid in the wellbore, the pump barrel 106 may be
completely filled during an upstroke. FIG. 2A illustrates position versus time
data
obtained downhole where the pump 104 may be completely or nearly filled. This
represents ideal pump operation where the transfer point 202 may be at the TOS
of
the plunger 110 for a completely or nearly filled pump barrel 106.
A condition may arise where the pump 104 is not completely filled with fluid
on
an upstroke. If there is not sufficient fluid in the wellbore, the barrel 106
may be only
partially filled, and there may be a void left between the fluid and the
plunger 110 as
it continues to rise. Operating the pump system 100 with only a partially
filled pump
barrel is inefficient and, therefore, undesirable. The well is said to be
"pumped off,"
and the condition is known as "pounding." For a pumped off well, in contrast
with the
completely filled pump of FIG. 2A, a plot of the position versus time data may
contain
a plateau 200, as shown in FIG. 2B. The plateau 200 may most likely contain
the
transfer point 201. For a pumped off well, the transfer point may most likely
occur
after the TOS of the plunger 110. Thus, well operators may wish to accurately
determine the transfer point in an effort to monitor the pump fillage and to
control the
pump accordingly, thereby preventing damage to the rod string 102 and other
components of the pump system 100.
FIG. 3 illustrates operations 300 for controlling a pump system 100 based on a
transfer point of a stroke, according to embodiments of the present invention.
The
operations may begin at 310 by determining the transfer point, wherein a load
is
transferred from the pump's traveling valve 112 to the pump's standing valve
114.
For some embodiments, the transfer point may be continuously determined for
each
and every pump stroke. For other embodiments, the transfer point may be
determined periodically or, in some cases, less frequently when the transfer
point is
at or near the TOS, and more frequently, when the transfer point is determined
to be
substantially less than the TOS.
At 320, the pump fillage (a fluid volume) may then be calculated based on the
transfer point. In other words, this fluid volume may be calculated by
determining the
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volume of the pump barrel 106 between the transfer point and the BOS. If the
transfer point is accurately determined, the calculated pump fillage may most
likely
be correct. A control unit 116, which may be located at the surface, may
control the
pump system 100 and, thus, the motion of the pump 104 based on the pump
fillage
at 330. For example, the control unit 116 may control the pump cycle
frequency, the
pump interval and the delay between pump intervals (i.e., the variable pump
duty
cycle). The pump fillage may also be used to compute pump efficiency, the
produced
volume and/or the average production rate.
DETERMINING THE TRANSFER POINT USING A METHOD OF RATIOS
Various numerical methods for accurately determining the transfer point may
exist. For example, FIG. 4 illustrates operations 400 for determining the
transfer
point at 310 using a Method of Ratios, according to embodiments of the present
invention. Data sets comprising position and load of the pump 104 with respect
to
time may be determined from measurements made at the surface and/or downhole
at
410 by any of various suitable sensors. For some embodiments, the data sets
may
correspond to measurements made at the pump 104. For some embodiments, the
measurements may be transmitted to the control unit 116 for data collection
and
analysis.
FIG. 5A displays a plot of example position versus time data, while FIG. 5B
depicts a plot of example load versus time data. In FIG. 5A, a plateau 500 may
indicate that the pump 104 is not completely filled during the part of the
pump cycle
where the plunger 110 is moving downward. The plateau 500 may most likely
contain the transfer point 501. When the pump 104 is not completely filled,
the
transfer point may most likely occur after the TOS of the plunger 110.
At 420, a data set may be computed, comprising a ratio of the first derivative
of the load versus time data to the first derivative of the position versus
time data, as
displayed in FIG. 5C. The ratio may display a series of peaks after the TOS.
For
some embodiments, the position and load data may be normalized before
calculating
CA 02717720 2010-10-15
the derivatives. At the transfer point, the first derivative of position with
respect to
time (i.e., velocity) exhibits a local maximum, and the first derivative of
load with
respect to time exhibits an absolute minimum. Individually, these phenomena
are not
always unique; however, the ratio of the first derivatives typically exhibits
a distinct
maximum at the transfer point. Therefore, the transfer point may be determined
at
430 based on a maximum (e.g., the first maximum) of the ratio data set that
exceeds
a threshold after the TOS.
DETERMINING THE TRANSFER POINT USING A METHOD OF POSITIONS
FIG. 6 illustrates operations 600 for determining the transfer point at 310
using
a Method of Positions, according to embodiments of the present invention.
Position
of the pump 104 with respect to time may be determined from measurements made
at the surface and/or down hole at 610 by any of various suitable sensors. For
some
embodiments, the position versus time data may correspond to measurements made
at the pump 104. A plot of example position versus time data is displayed in
FIG. 7A.
In FIG. 7A, a plateau 702 may indicate that the pump 104 is not completely
filled
during the part of the pump cycle where the plunger 110 is moving downward.
The
plateau 702 may most likely contain the transfer point 701 at the inflection
point.
When the pump 104 is not completely filled, the transfer point may occur after
the
TOS of the plunger 110.
At 620, a first data set may be computed comprising the first derivative of
the
position versus time data (i.e., velocity). At 625, the TOS of the pump, and
more
specifically of the plunger, may be determined. For some embodiments,
determining
the TOS of the plunger may comprise finding a critical value 704 of the first
data set,
as displayed in FIG. 7B. For other embodiments, the TOS may be simply
determined
to occur at the maximum position in the position versus time data.
A second data set may be computed at 630 comprising the second derivative
of the position versus time data (i.e., acceleration), and at 640, an absolute
minimum
706 of the second data set occurring after the position corresponding to the
TOS may
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be determined, as displayed in FIG. 7C. At 650, the transfer point may be
determined based on a maximum 708 of the second data set that occurs between
the TOS and the absolute minimum 706.
Data below a minimum pump fillage threshold (e.g., about 10-15%, or more
preferably about 5%) may not be used in determining the transfer point because
the
calculation of the pump fillage at 320 may not be accurate. If the maximum 708
is
not above the minimum pump fillage threshold, then this position may not be
considered as the transfer point because an accurate pump fillage cannot be
guaranteed.
DETERMINING THE TRANSFER POINT USING A METHOD OF AREAS
FIG. 8 illustrates operations 800 for determining the transfer point at 310
using
a Method of Areas, according to embodiments of the present invention. The
Method
of Areas may be used, for example, when there is sufficient fluid in the
wellbore to
cause the pump barrel 106 to be completely filled or nearly filled, as
illustrated in FIG.
2A, such that the Method of Ratios of FIG. 4 or the Method of Positions of
FIG. 6 fail
to determine the transfer point, which may be at or near the TOS of the
plunger 110.
In addition, the Method of Areas may be used when the well is pumped off
enough
that the pump fillage is less than the minimum pump fillage threshold (e.g.,
about 10-
15%, or more preferably about 5%).
In the Method of Areas, the position versus time and load versus time data
may be determined at 810 from measurements made at the surface and/or downhole
using any of various suitable sensors. For some embodiments, the data may
correspond to measurements made at the pump 104. At 820, the area of an
estimated ideal rectangular downhole card may be computed, based on the ranges
of the position versus time and load versus time data. In other words, the
maximum
and the minimum load and the maximum and the minimum position may be
determined. Then, the difference between the maximum and minimum positions may
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be multiplied by the difference between the maximum and minimum loads to
compute
the estimated downhole card area.
At 830, the area of an actual downhole card corresponding to the position
versus time and load versus time data may be computed. For some embodiments,
this actual downhole card area may be computed using Riemann sums. At 840, a
ratio may be computed of the actual downhole card area, which represents the
energy expended at the pump, to the estimated downhole card area.
At 850, if the ratio is greater than a threshold (e.g., around 80%, or more
preferably at least 60%), then the transfer point may be determined as being
at the
TOS of the plunger at 860. If the ratio is less than or equal to the threshold
at 850,
then, at 870, the pump fillage may be determined as being less than the
minimum
pump fillage threshold, or the pump fillage calculation may be considered as
inconclusive (i.e., bad data). Any time the pump fillage calculation (PFC) is
considered inconclusive, the transfer point may be determined on the next or
any
subsequent pump stroke.
DETERMINING THE TRANSFER POINT USING A MODIFIED METHOD OF
POSITIONS
FIG. 9 illustrates operations 900 for determining the transfer point at 310
using
a modified Method of Positions, according to embodiments of the present
invention.
The modified Method of Positions may be used, for example, to catch an event
occurring when the bottom right of a downhole card may lean to the right. FIG.
10
illustrates an example downhole card leaning to the right at 1002. This method
may
compare a position value 1004 at the top of the card with a position value
1006 at the
bottom of the card. If the position value 1006 at the bottom of the card is
greater
than the position value 1004 at the top of the card, the pump fillage may be
set
automatically to 100%. For some embodiments, the pump fillage may only be set
to
100% if the position value 1006 is greater than the position value 1004 by a
predetermined threshold.
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Position of the pump 104 with respect to time may be determined from
measurements made at the surface and/or downhole at 910 by any of various
suitable sensors. For some embodiments, the position versus time data may
correspond to measurements made at the pump 104. A plot of example position
versus time data is displayed in FIG. 7A. In FIG. 7A, a plateau 702 may
indicate that
the pump 104 is not completely filled during the part of the pump cycle where
the
plunger 110 is moving downward. The plateau 702 may most likely contain the
transfer point 701 at the inflection point. When the pump 104 is not
completely filled,
the transfer point may occur after the TOS of the plunger 110.
At 920, a first data set may be computed, comprising the first derivative of
the
position versus time data (i.e., velocity). At 930, the TOS of the pump, and
more
specifically of the plunger, may be determined. For some embodiments,
determining
the TOS of the plunger may comprise finding a critical value 704 of the first
data set,
as displayed in FIG. 7B. For other embodiments, the TOS may be simply
determined
to occur at the maximum position in the position versus time data.
At 940, a top position value 1004 and a bottom position value 1006 of the
position versus time data may be determined. At 950, if the bottom position
value
1006 is greater than the top position value 1004 (i.e., the bottom right of a
downhole
card may be leaning to the right), then the transfer point may be determined
to be at
the TOS of the pump at 960.
If the bottom position value 1006 is less than or equal to the top position
value
1004, a second data set may be computed at 970 comprising the second
derivative
of the position versus time data (i.e., acceleration). At 980, an absolute
minimum 706
of the second data set occurring after the position corresponding to the TOS
may be
determined, as displayed in FIG. 7C. At 990, the transfer point may be
determined
based on a maximum 708 of the second data set that occurs between the TOS and
the absolute minimum 706.
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Data below a minimum pump fillage threshold (e.g., about 10-15%, or more
preferably about 5%) may not be used in determining the transfer point because
the
calculation of the pump fillage at 320 may not be accurate. If the maximum 708
is
not above the minimum pump fillage threshold, then this position may not be
considered as the transfer point because an accurate pump fillage cannot be
guaranteed.
DETERMINING THE TRANSFER POINT USING A METHOD OF LOADS
FIG. 11 illustrates operations 1100 for determining the transfer point at 310
using a Method of Loads, according to embodiments of the present invention.
Load
of the pump 104 with respect to time may be determined from measurements made
at the surface and/or downhole at 1110 by any of various suitable sensors. For
some
embodiments, the load versus time data may correspond to measurements made at
the pump 104. For some embodiments, the measurements may be transmitted to
the control unit 116 for data collection and analysis. An exemplary plot of
load versus
time is displayed in FIG. 5B.
At 1120, a data set may be computed, comprising the first derivative of the
load versus time data, as displayed in FIG. 12. During the downstroke, wherein
the
load on the rod string 102 may be reduced, the first derivative of the load
versus time
data may indicate a negative slope 1202. The transfer point may be determined
at
the end of the downstroke at 1130 based on an absolute minimum 1204 of the
data
set occurring after a top of stroke (TOS). For some embodiments, the Method of
Loads may be modified similar to the Method of Positions of FIG. 9 to catch
the event
occurring when the bottom right of a downhole card may lean to the right,
wherein the
pump fillage may be set automatically to 100%.
DETERMINING PUMP FILLAGE USING A METHOD OF ORDERING
FIG. 13 illustrates operations 1300 for determining pump fillage using a
Method of Ordering, according to embodiments of the present invention. The
Method
CA 02717720 2010-10-15
of Ordering relies on ordering downhole data according to load values. The
method
may separate the data in load sections, wherein the pump fillage may be found.
Data sets comprising position and load of the pump 104 with respect to time
may be determined from measurements made at the surface and/or downhole at
1310 by any of various suitable sensors. For some embodiments, the data sets
may
correspond to measurements made at the pump 104. For some embodiments, the
measurements may be transmitted to the control unit 116 for data collection
and
analysis.
FIG. 5A displays a plot of example position versus time data, while FIG. 5B
depicts a plot of example load versus time data. In FIG. 5A, a plateau 500 may
indicate that the pump 104 is not completely filled during the part of the
pump cycle
where the plunger 110 is moving downward. The plateau 500 may most likely
contain the transfer point 501. When the pump 104 is not completely filled,
the
transfer point may most likely occur after the TOS of the plunger 110.
At 1320, a data set may be determined comprising the load versus time data
ordered according to load values. The data set may be ordered according to
increasing or decreasing load values. FIG. 14 illustrates a data set ordered
by
decreasing load values. At 1330, the data set may be divided into a top
portion 1402
and a bottom portion 1404. It is to be understood that the top and bottom
portions
1402, 1404 need not be equal portions. For some embodiments, the top portion
1402 and the bottom portion 1404 may comprise a top half and a bottom half,
respectively.
At 1340, the top portion 1402 and the bottom portion 1404 of the data set may
be used to approximate a position value 1502 corresponding to an average top
value
and a position value 1504 corresponding to an average bottom value,
respectively. A
position value 1506 corresponding to a half value may also be computed at 1340
representing the 50% point of the downhole data, as displayed in FIG. 15. At
1350, a
first pump fillage value may be computed, comprising a ratio of the position
value
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CA 02717720 2010-10-15
1504 (corresponding to the average bottom value) to the position value 1502
(corresponding to the average top value). At 1360, a second pump fillage value
may
be computed, comprising a ratio of the position value 1506 (corresponding to
the half
value) to the position value 1502 (corresponding to the average top value). At
1370,
a pump fillage may be determined based on a combination (e.g., an average) of
the
first and second pump fillage values.
DETERMINING PUMP FILLAGE USING A METHOD OF MULTIPLE PUMP
FILLAGE
FIG. 16 illustrates operations 1600 for verifying a pump fillage using a
Method
of Multiple Pump Fillage, according to embodiments of the present invention.
The
Method of Multiple Pump Fillage may be used, for example, as a check for the
correct pump fillage, which may be selected from one of a plurality of pump
fillage
methods described above, having a calculated pump fillage. The method may
compute an estimated pump fillage at several places on a downhole card and use
statistical analysis to verify the correct value for the pump fillage. This
method may
be used as a check in the event that the above-described methods yield
different
pump fillages.
Data sets comprising position and load of the pump 104 with respect to time
may be determined from measurements made at the surface and/or downhole at
1610 by any of various suitable sensors. For some embodiments, the data sets
may
correspond to measurements made at the pump 104. At 1620, a load data range of
the load data may be divided into a number of increments N (e.g., 20). For
some
embodiments, the load value at each increment may be determined by first
computing a ratio of the load span to the number of increments N. Then with
the first
increment having a load value of zero, the ratio may be added at each
increment to
determine respective load values.
At 1630, a position value from the position data, corresponding to each of the
increments, may be determined. For example, FIG. 17 illustrates position
values
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CA 02717720 2010-10-15
(x1...x13) for 13 increments (i.e., N = 13). At 1640, a data set comprising an
estimated pump fillage for each of the increments may be computed based on a
ratio
of the position value corresponding to each increment (x1...x13) to the
position value
of a top of stroke (TOS). At 1650, a pump fillage range of the data set may be
divided into a number of intervals. At 1660, for each interval, a number of
occurrences of each estimated pump fillage having a value within that interval
may
be determined. For some embodiments, a probability density function may be
calculated for the intervals.
At 1670, a probable pump fillage interval may be determined, wherein the
probable pump fillage interval is the interval where the number of occurrences
is at a
maximum. For some embodiments, a value in the probable pump fillage interval
may
be considered as the pump fillage (e.g., the lowest value, the median value,
or the
highest value). For other embodiments, at 1680, one of a plurality of pump
fillage
methods described above, having a calculated pump fillage corresponding to the
probable pump fillage interval, may be selected. This selected, calculated
pump
fillage may be considered as the pump fillage. For example, FIG. 18
illustrates the
data set divided into 34 intervals, showing a probability density function of
the pump
fillage distribution, wherein a peak 1802 may indicate the pump fillage
interval where
the pump fillage may lie. The number of increments and intervals may be
increased
to produce more accurate results, but exceeding a certain number of increments
may
pick up fluctuations.
Using any of the numerical techniques described herein may provide a more
accurate determination of pump fillage than other methods, such as those based
on
graphical representations of a downhole card, wherein it can be difficult to
make a
diagnosis of downhole conditions due to the diversity of card shapes. More
accurate
pump fillage calculations may provide better pump control, increased pump
efficiency
and more accurate well-production calculations.
Any of the operations described above, such as the operations 300, may be
included as instructions in a computer-readable medium for execution by the
control
18
CA 02717720 2010-10-15
unit 116 or any other processor. The computer-readable medium may comprise any
suitable memory for storing instructions, such as read-only memory (ROM),
random
access memory (RAM), flash memory, an electrically erasable programmable ROM
(EEPROM), a compact disc ROM (CD-ROM), or a floppy disk.
While the foregoing is directed to embodiments of the present invention, other
and further embodiments of the invention may be devised without departing from
the
basic scope thereof, and the scope thereof is determined by the claims that
follow.
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