Monitoring and pump-off control with downhole pump cards

A method for monitoring a rod pumped well to detect various problems, i.e., fluid pound, rod parts, pump problems, high fluid levels, imbalance of the pumping unit, rate of production and others. The method utilizes measurements made at the surface to calculate a downhole pump card. The method utilizes the downhole pump card to detect the various pump problems and control the pumping unit.

BACKGROUND OF THE INVENTION 
The present invention relates to oil wells and particularly to wells that 
are produced by rod pumping. The term `rod pumping` refers to a pumping 
system in which a reciprocating pump located at the bottom of the well is 
actuated by a string of rods. The rods are reciprocated by a pumping unit 
located at the surface. The unit may be of the predominant beam type or 
any other type that reciprocates the rod string. A beam pumping unit 
utilizes a walking beam pivotally mounted on a Samson post with one end of 
the beam being attached to the rods and with the beam being reciprocated 
by a drive unit. The drive unit consists of a prime mover connected to a 
reduction unit that drives a crank to reciprocate the walking beam. 
The downhole pump consists of a barrel attached to (or part of) the 
production tubing string that is anchored to the well casing. A plunger 
reciprocates in the barrel which is attached to the end of the rod string. 
The barrel is provided with a standing valve, and the plunger is provided 
with a traveling valve. On the down stroke, the traveling valve opens and 
the standing valve closes, allowing the fluid in the barrel to pass 
through the plunger. On the up stroke the traveling valve closes allowing 
the plunger to lift fluid to the surface while the standing valve opens 
and the plunger draws more fluid from the well into the barrel. 
Pumping systems are normally sized so that they can produce essentially all 
of the fluid from the well using controllers which alternately pump the 
well or shut it down when necessary to allow more fluid to enter the 
casing. The controllers can be simple clock timers that start and stop the 
pumping unit in response to a set program or controllers that control the 
pumping unit in response to some measured characteristics of the pumping 
system. 
Controllers that control the pumping unit in response to measured pumping 
characteristics are designed to shut the pumping unit down when the well 
has pumped off. This saves energy and prevents damage to the pumping 
system. The term pumped-off is used to describe the condition where the 
fluid level in the well is not sufficient to completely fill the pump 
barrel on the upstroke. On the next downstroke the plunger will impact the 
fluid in the incompletely filled barrel and send shock waves through the 
rod string and other components of the pumping system. This can cause harm 
to the pumping system such as broken rods or damage to the drive unit or 
downhole pump. All pump-off controllers are designed to detect when a well 
pumps off and to shut the well down. 
In the Applicant's prior U.S. Pat. No. 3,951,209, there is described a 
controller that measures at the surface both the load on the rod string 
and the displacement of the rod string. From these measurements, one can 
obtain a surface dynamometer card and the area of the card will be the 
power input to the rod string. Since the pumping system will be lifting 
less fluid when the well pumps off, the power input to the rod string will 
also decrease. The decrease in power will result in a decrease in the area 
of the surface dynamometer card. This decrease in area is used as an 
indication of a pump-off condition and the pumping unit is shut down. U.S. 
Pat. No. 4,015,469 describes an improvement of the '209 patent in which 
only a portion of the area of the surface card is considered. In 
particular, the '469 patent utilizes only the last part of the upstroke 
and the first part of the downstroke to detect pump-off. This is the 
portion of the surface card in which pump-off is usually shown. 
Other methods have also been developed for detecting pump off. For example, 
U.S. Pat. No. 3,306,210 discloses a pump-off controller that monitors the 
load on the polished rod at a set position in the downstroke. Pump-off is 
detected when the load exceeds a preset level at that set position. U.S. 
Pat. No. 4,583,915 discloses a pump-off controller that monitors an area 
outside the surface dynamometer card. More particularly, the patent 
discloses monitoring an area between the minimum load line and the load 
line at the top of the stroke. Other pump-off controllers have monitored 
the electrical current drawn by the drive motor to detect pump-off. 
The Applicant's U.S. Pat. No. 4,490,094 discloses a pump-off controller 
that monitors the instantaneous speed of revolution of the drive motor 
during a complete or portion of the cycle of the pumping unit. Pump off is 
sensed by calculating a motor power from measured speed which is less than 
motor power corresponding to a completely filled pump barrel. Both the 
surface load and position of the rod string can also be determined from 
the monitored instantaneous speed of the drive motor. 
SUMMARY OF THE INVENTION 
The present invention determines pump-off by monitoring the down hole pump 
card instead of the surface card as described in the prior art. The use of 
the downhole pump card eliminates errors caused by ambiguities in the 
surface card and obscuring effects of downhole friction along the rods. 
The use of the downhole pump card, in addition, permits the controller to 
detect additional malfunctions of the pumping unit that are difficult to 
detect when surface cards are used. For example, the fluid production of 
the well can be calculated from the pump card and when compared to the 
actual production will detect a leak in the production tubing string. The 
downhole card will also allow the controller to monitor for possible 
slipping of the tubing anchor. In addition, the use of the downhole card 
will provide more accurate sensing of high fluid levels and gas 
interference. 
In addition to providing for conventional starting and stopping of the 
pumping unit to control the well, the invention can also control the well 
by varying the pumping speed. The pumping speed is varied in response to 
the change in a selected parameter of the downhole pump card. The 
parameter may be the area or portion of the area inside or outside of a 
downhole card. Likewise, the parameter may be the change in the net liquid 
stroke of the pump. 
The invention utilizes the surface measurements of load and displacement of 
the rod string to calculate the downhole card. These measurements can be 
direct measurements using load and position transducers or indirect 
measurements as described in the Applicant's prior U.S. Pat. No. 
4,490,094. The invention provides a method for correcting and converting 
the measurements described in patent '094 into rod position measurements 
that correlate with load measurements. This provides a series of 
load-displacement measurements from which the downhole card can be 
calculated. 
The downhole pump card can be obtained using several methods including the 
method described in Applicant's U.S. Pat. No. 3,343,409. This method 
utilizes surface measurements of load and position of the rod string to 
construct a downhole pump card. The downhole card is obtained by the use 
of a computer to solve a mathematical expression described in the patent. 
An alternative is to construct an analog circuit of the pumping system. It 
will be appreciated that while an analog circuit provides an instantaneous 
downhole card, it is unique to the particular pumping system. Thus, it 
must be changed for each pumping system. 
The invention preferably uses a special purpose microprocessor-based 
controller or a general purpose remote terminal unit (RTU) that can be 
programmed to incorporate the present invention. These units are offered 
for sale by various manufacturers and can be made functional by installing 
a properly programmed EPROM.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is shown a surface pumping unit 10 used for 
producing the oil well 13. While a conventional beam pumping unit is 
shown, the method is applicable to any system that reciprocates a rod 
string including tower type units which involve cables, belts, chains and 
hydraulic or pneumatic power systems. Pumping unit 10 has a walking beam 
11 which reciprocates a rod string 12 for actuating the downhole pump 
disposed at the bottom of the well. The pump is a reciprocating type 
having a plunger attached to the end of the rod string and a barrel which 
is attached to the end of (or is part of) the production tubing in the 
well. The plunger has a traveling valve and a standing valve is positioned 
at the bottom of the barrel. On the upstroke of the pump, the traveling 
valve closes and lifts the fluid above the plunger to the top of the well 
and the standing valve opens and allows additional fluid from the 
reservoir to flow into the pump barrel. On the downstroke, the traveling 
valve opens while the standing valve closes allowing the fluid in the pump 
to pass upward through the plunger into the production tubing. The well is 
said to be pumped-off when the pump barrel does not completely fill with 
fluid on the upstroke of the plunger. On the next downstroke, the plunger 
will contact the fluid in the incompletely filled barrel at which point 
the traveling valve will open. The impact between plunger and fluid will 
cause a sudden shock to travel through the rod string and the pumping 
unit. This mechanical shock can, of course, cause damage to the rod string 
and other pumping equipment. Thus, an effort is made to shut down the well 
when it reaches a pumped-off condition to prevent damage to the equipment 
as well as save power. 
The walking beam is reciprocated by a crank arm 14 which is attached to the 
walking beam. The crank arm is provided with a counterweight 15 that 
serves to balance the rod string that is also suspended from the walking 
beam. The crank arm 14 is driven by an electric motor 20 connected to a 
gear reduction 21. The present invention can utilize the instantaneous 
motor speed which is indicated as a signal 22 and a monitored position of 
the pumping unit to help determine when the well is pumped off. The 
position of the pumping unit can be detected by a sensor 23 which detects 
the passage of the crank 14 of the pumping unit. This sensing unit can be 
either magnetic or Hall effect type unit, or it could be a switch which is 
closed by the passage of the crank or counterweight. The invention can 
also be implemented with direct measuring position transducers. 
The load and motor speed and crank sensor signals are supplied to a special 
controller or remote terminal unit 24 that comprises a microprocessor and 
associated circuitry. The microprocessor can be programmed directly by 
using a keyboard which is attached to the microprocessor or by using a 
laptop computer which is temporarily attached to the microprocessor or by 
using a radio system for remote programming. The controller is coupled to 
a start-stop circuit 25 which starts and stops the motor 20 in response to 
signals received from the controller. 
The data collected from the motor speed and the position of the pumping 
unit can be converted to load on the rod string and position of the rod 
string following the method described in Applicant's U.S. Pat. No. 
4,490,094. Once the data is converted it will form a series of load and 
position data pairs that can be used to calculate a surface card. The 
downhole pump card can be calculated following the method described in the 
Applicant's prior U.S. Pat. No. 3,343,409. However, load from motor speed 
is usually not accurate enough to calculate a pump card. Load from a load 
cell at the polished rod is preferred. Both the conversion of the data and 
the calculating of the downhole pump card can be accomplished by the 
controller 24. The controller can be programmed as described above, either 
by using an EPROM which provides the proper instructions for the 
microprocessor unit or by programming a memory circuit in the controller 
by means of a keyboard temporarily attached to the controller. The 
controller comprises a small computer which has sufficient memory capacity 
to store data and contain the computational algorithms. 
While the method described in the Applicant's prior U.S. Pat. No. 4,490,094 
can be used for relating the motor speed to polished rod load, the method 
can also be used to determine polished rod position. To determine a 
starting point a signaling device is used to signal a particular position 
of the pumping unit. This is obtained by the signaling device 23 shown in 
FIG. 1. Preferably, this signal is obtained at a known position, for 
example, the bottom of the stroke of the pumping unit. Using motor 
revolutions and unit geometry the position of the polished rod for various 
positions of the crank (or some other movable member of the pumping unit, 
i.e., the beam or the pitman) is calculated starting at the known point. 
Thus, one will obtain a table of values in which the crank position in 
degrees will be related to the polished rod position in inches starting 
from the known position. If this is the bottom of the stroke, then zero 
crank angle will equal zero rod position and at 180 degrees, the polished 
rod will be near the top of the stroke for normal pumping unit geometry. 
Having these values, one can then determine the crank position for various 
revolutions of the drive motor using the expression: 
##EQU1## 
wherein .theta.=crank position starting at known position N=number of 
revolutions of the motor per stroke of pumping unit. 
K=motor revolutions since beginning at known position. The position of the 
polished rod can be readily calculated by determining the crank angle for 
any known number of motor revolutions and referring to the precalculated 
values to obtain the surface rod position. 
While the above description relates to the use of motor revolutions and 
pumping unit geometry for determining both the surface rod position and 
load on the polished rod at various positions, other methods may be used. 
For example, the method using a position transducer and load transducer 
described in the Applicant's U.S. Pat. No. 3,951,209 may be used. 
Obviously, if the rod position and load are measured at a series of 
points, it will not be necessary to convert the data since the 
measurements will provide the load and position data points required for 
computing the downhole card. Inferring surface position from motor 
revolutions and unit geometry has the practical advantage of eliminating 
the initial and maintenance cost of a direct measuring transducer. 
Referring now to FIG. 2A, there is shown a downhole pump card for a full 
pump and a pump that is partially filled and pumped off in FIG. 2B. 
Referring to the Figures, the line 30 represents the load on the pump rod 
plotted against the displacement of the pump. Line 30 is called the 
downhole pump card. The single cross-hatched area represents the power or 
energy required for lifting fluid by the pump. The pumping off can be 
determined by various methods. For example, one could monitor the area of 
the pump card to the right of the position line 31 and when this area has 
been reduced by a certain percentage, the well will be deemed pumped off. 
Likewise, one could measure the downstroke area outside the pump card 
above a load line, for example, line 33. In this case, pump off would be 
determined when the area increases by a preset amount (see the double 
cross-hatched areas). Referring to FIG. 2B, pump off could also be sensed 
when net liquid stroke NS becomes less than gross stroke GS by a preset 
amount. Still another way to detect pump off using FIGS. 2A and 2B is to 
compare the inside areas to the right and left of an arbitrarily selected 
line 31. The unit would be shut down when the areas differed by a preset 
amount. Areas beneath the downstroke load trace and above an arbitrarily 
selected load line to the right and left of line 31 can also be used to 
cause shut down (refer to the double cross-hatched areas on FIGS. 2A and 
2B). Similarly, pump off could be determined by monitoring the load at a 
fixed or predetermined position in the downstroke to determine when the 
load exceeds a preset load. This is illustrated by the point 32 in the two 
pump cards. When the load on the downstroke exceeds the preset load at the 
position 32, the well will be deemed to have pumped off. The pumping off 
of the well could also be determined by comparing the total area of the 
pump card and monitoring it to detect pump off. As can be seen from FIGS. 
2A and 2B, the determining of pump off by measuring the area to the right 
of the position line 31 is much more sensitive than utilizing the total 
area of the pump card for determining pump off. 
The use of downhole pump cards to determine when a well has pumped off also 
provides the additional advantage of determining the actual quantity of 
fluid being lifted by the pump. This is important since it will allow one 
to determine if the production tubing string in the well has a leak or if 
the production test is correct. By comparing the calculated fluid lifted 
by the pump with the measured production from the well, one can determine 
if fluid is being lost through leaks in the production tubing. The fluid 
lifted by the pump can be determined by utilizing the net stroke of the 
pump as indicated by the dimension NS in FIGS. 2A and 2B using the 
following formula 
EQU P=0.1166 (NS) (SPM).alpha.D.sup.z) 
In the above expression, P is the instantaneous production rate in barrels 
per day, SPM is the pumping speed in strokes per minute and D is the 
diameter of the downhole pump in inches. The daily production rate can be 
determined by considering the pump fillage (determined from NS) and the 
amount of time pumped with various pump fillages. 
Referring to FIG. 3, there is shown a series of pump cards that are 
reproduced as a result of the zero load offset of the load cell changing 
due to temperature changes or other factors which affect the load cell. If 
the speed of the motor is used as described with reference to FIG. 1 for 
determining the load and position of the rod string, the following method 
will not be required for correcting the data. Likewise, if polished rod 
mounted load cells are used as described in the Applicant's previously 
issued U.S. Pat. No. 3,951,209, no correction will usually be required. 
Correction is often required for the use of beam mounted load cells in 
which the zero load offset changes as the temperature of the beam changes. 
As shown in FIG. 3, there are three separate pump cards, each of which 
have a minimum load point L.sub.c (correct loads), L.sub.2 (loads too 
high) and L.sub.2 (loads too low). The new zero load offset for the load 
cell is determined by calculating the change in the load offset L by 
subtracting the minimum load L.sub.2, or L.sub.2 from the reference 
minimum load L.sub.c. The reference minimum load on the pump card can be 
obtained by temporarily inserting in the rod string a calibrated polished 
rod mounted load cell to determine a pump card with the correct reference 
minimum load, L.sub.c. Once the reference load L.sub.c is determined, it 
is retained in the controller. The zero load offset of the beam mounted 
load cell can be corrected by algebraically adding L to all loads. It is 
preferable that a correction is made only when the change in the offset L 
exceeds a preset amount. This will prevent trivial changes in the zero 
offset of the load cell. Likewise, it is preferable to limit the maximum 
amount by which the zero load offset can be changed for each stroke of the 
pump. This will prevent the zero load offset from being changed in 
response to a minimum load that is a violation of a preset minimum load in 
the pump off controller. 
Referring to FIG. 4, there is shown a downhole pump card in which the pump 
has considerable gas in the fluid filling the pump. High pressure gas in 
the well fluid, called gas interference, is normally not a reason for 
shutting down the pumping unit. Under these conditions no fluid pound will 
occur and there is no need to shut down the pumping unit although it must 
be monitored to detect the occurrence of pump off. As shown by the curves 
40, 41 and 42 the gas is compressed in the initial portion of the 
downstroke until the pressure equals the fluid pressure at the foot of the 
well's tubing. The curve 41 is taken as the compression curve (pump load 
release line) for a pumped off well at a selected pump intake pressure as 
follows: 
EQU PL=A (Pa-Pb) 
where 
A=area of pump, sq inches 
PL=pump load, lbs 
Pa=pressure above plunger at foot of tubing, psi 
Pb=pressure in pump below plunger, psi 
Pb=C/(A (GS-NS-X)).sup.n, psi 
X=distance measured downward from top of stroke, inches 
C=PIP(A (GS-NS)).sup.n 
n=polytropic exponent for gas compression (say 1.25) 
PIP=preset pump intake pressure, psi 
GS=gross stroke from pump card, inches 
NS=net liquid stroke from pump card, inches 
Under normal operating conditions low pressure gas will be removed from the 
well fluid and the well can be pumped off and should be shut down. Under 
some conditions, high pressure gas in the fluid will not be removed as the 
pump operates (a condition called gas interference) and it is not possible 
for the well to pump off. In this case the well should not be shut down 
because production would be lost. The magnitude of pump intake pressure 
affects the curvature of the load release curve shown on the pump card. 
This is illustrated by the curves 40, 41, 42 of FIG. 4. Pump intake 
pressure along curve 40 exceeds pump intake pressure along curve 41 which 
exceeds pump intake pressure along curve 42. The present invention thus 
has the ability to discern between pump off which calls for shut down and 
gas interference which calls for continuous pumping. A reference load 
release curve 41 is established by selecting a desired pump intake 
pressure and liquid fillage at shut down. Then monitoring for pump off is 
done by continually comparing the load release traces to the reference 
trace 41. If the release trace 42 is above the reference trace, the well 
is said to have pumped off and is shut down. If the release trace 40 shown 
by the pump card is below the reference trace, gas interference is known 
to be occurring and pumping is continued. The microprocessor used in the 
pump off controller can be programmed to make the above calculation for 
the reference load release curve 41. 
Another condition that occurs in wells is the condition called high fluid 
level. This condition normally occurs when the well has been shut down for 
an extended period of time and more formation fluid builds up in the well 
bore than would normally build up during the normal shut down periods of 
the pumping unit. Under these conditions less work is required to lift the 
fluid to the surface since the distance which the fluid must be lifted is 
decreased. This condition is illustrated in FIG. 5 where the curve 50 
corresponds to a full downhole pump with a normal low fluid level in the 
well and the curve 51 indicates the downhole pump card with a higher than 
normal fluid level in the well. The area inside the pump card represents 
pump work or power and is less in the high fluid level condition. The 
double cross-hatched area outside of the pump card between the downstroke 
load line and a load line passing through the minimum load point on the 
downhole pump card will remain substantially constant regardless of the 
fluid level in the well. Compare this area with the larger double cross 
hatched area shown in FIG. 2B for a pumped off well with a low fluid 
level. This also shows that it is possible to determine a pumped off 
condition by measuring the area outside of the pump card as described 
above. 
A second method for determining when a high fluid level exists uses the 
computed fluid load FL on the downhole pump. Using the pump card the fluid 
load is determined by subtracting the minimum load from the maximum load. 
The minimum load is calculated as the average pump load over a selected 
portion of the down stroke. Similarly the maximum load is calculated as 
the average pump load over a selected portion of the up stroke. As fluid 
level rises, fluid load decreases as shown in FIG. 5. The fluid load on 
the pump is calculated for normal operating conditions and stored in 
memory. Upon succeeding startups of the pumping unit after a shutdown 
period, the fluid load can be calculated and compared to the stored 
reference fluid load. If the calculated fluid load is substantially less 
than the stored reference value of the fluid load, the well has a high 
fluid level and is not pumped off and pumping should be continued. When 
the calculated fluid load approaches the stored fluid load reference 
value, one should monitor the well for a pumped off condition using any of 
the methods described above. 
Referring now to FIG. 6 there is shown the logic for using a downhole pump 
card to control the speed of the pumping unit so that the pumping rate 
matches the rate at which fluid flows into the well. Using today's 
technology, it is possible to control the speed of the drive motor of a 
pumping unit using methods such as eddy current drives, variable frequency 
drives or variable sheave devices. By using the downhole pump card the 
desired speed of the pump can be determined to maintain near complete pump 
fillage. 
As shown in FIG. 6, the downhole pump card is first calculated from data 
collected at the surface using the method described in the Applicant's 
prior patent or any other suitable method. Selected parameters are 
identified such as total area A within the card, net liquid stroke NS, 
present pumping speed SPMp and fluid load FL. Then the existence of high 
fluid level is checked using a remembered fluid load on the verge of 
pump-off FLf or by using the area below the down stroke trace as 
previously described. If a high fluid level is found, pumping speed is 
increased by a selected amount not to exceed the preset maximum speed SPMx 
and the process is continued by calculating another pump card. If fluid 
level is not high, an adjusted speed SPMa is calculated using any of the 
methods described herein including 
EQU SPMa=A SPMp/Af 
where Af is the remembered card area when the pump was full but on the 
verge of pump off. An alternate formula for adjusting pumping speed is 
EQU SPMa=NS SPMp/GS 
where GS is the remembered gross stroke when the pump was full. The 
adjusted speed is not allowed to exceed maximum allowed speed SPMx or to 
be less than minimum allowed speed SPMn. The adjusted speed is also 
compared to previous speed in a dead band comparator to eliminate trivial 
changes. A signal is then sent to the prime mover to change speed to the 
adjusted value SPMa. The selected parameters are updated to allow for 
changing conditions. The adjusted speed becomes the present speed. If pump 
card area exceeds the remembered value then the remembered value becomes 
the newly calculated pump card area. If the newly determined fluid load 
exceeds the remembered value, the remembered value becomes the newly 
computed fluid load. If the newly calculated net stroke exceeds the 
remembered gross stroke, the remembered gross stroke becomes the newly 
computed net stroke. Then another pump card is calculated and the process 
is repeated. In using the above logic, it is obvious that the maximum 
speed of the pumping unit will be controlled by mechanical parameters and 
the maximum speed capability of the drive motor. Likewise, the minimum 
speed should be set at some level which will allow sufficient range of 
adjustment to match the pumping speed to the rate at which fluid is 
flowing into the well. This is easily accomplished with present motors 
which allow adjustment of speed near zero to the maximum attainable by the 
motor. 
Referring now to FIG. 7, a method is revealed as to how data is collected 
for computing pump cards using unit geometry and revolutions of selected 
drive train components. The microprocessor is continually waiting for 
interrupt signals from transducers mounted on the motor and pumping unit 
crank. When a signal from the motor is sensed, the processor knows that 
the motor has made a revolution from which motor speed can be determined 
from the time required to make a revolution. This motor speed and 
revolution time are remembered. As soon as possible after a motor 
revolution is completed, surface rod load is measured and remembered. The 
process is continued by measuring and remembering motor speed, revolution 
time and load for successive revolutions until an interrupt from the crank 
transducer signals that a complete stroke of the unit has occurred. Then 
as revealed in this invention, motor revolutions and pumping unit geometry 
are used to compute surface rod position. The computational process for 
pump cards usually requires that surface rod and position data be gathered 
at equal time increments. If so required, the data gathered revolution by 
revolution (not at equal time increments because of variations in motor 
speed) is adjusted to an equal time basis by interpolation. Then as FIG. 7 
further shows, a pump card is computed and an operational decision based 
on this invention is made to stop the unit, continue pumping as is or 
alter pumping speed. The process is thereby continued. 
FIG. 8 shows a process for gathering data and computing pump cards on a 
real time basis using unit geometry and sensors on rotating components of 
the drive train. As previously described, the transducer on the motor 
signals completion of a motor revolution at which time load is measured 
and position is inferred from unit geometry. Then, a load-position point 
on the downhole pump card is computed. This requires a fast pump card 
algorithm which can produce a computed load-position pair before the motor 
completes another revolution. At 1200 motor revolutions per minute, this 
allows less than 0.050 seconds for all of the computations. The process is 
continued revolution after revolution until a crank transducer interrupt 
is received which indicates afull cycle of the unit has been completed and 
a complete pump card has been constructed. At this time, operational 
decisions are made according to this invention. The advantage of the real 
time calculation is that distortion of the pump card due to non-steady 
conditions does not occur.