Variable speed pump control for maintaining fluid level below full barrel level

A variable speed pump control system and method which senses operational parameters during the first one half of the down stroke to control pump speed to maximize production. The method and equipment maintains ths fluid level of a well as low as possible while avoiding the pump-off condition. A variable speed motor drives a pump jack and a control means varies the pump speed. Means are provided for simultaneously sensing the pump speed, load on the rod, and the position of the rod in the pump stroke. These measurements are utilized to calculate the power transferred between the rod string and the beam during a portion of the downstroke. Before the pump is continuously operated, a series of measurements are made in the full barrel pumping condition to determine the power transferred between the rod and beam at various speeds. These are utilized to establish a relationship between pump speed and power during a portion of the downstroke. The well is operated and the measured values obtained during pumping are compared to the established relationship between pump speed and power. The pump speed is varied according the established relationship to power to optimize the fluid level in the well.

BACKGROUND OF THE INVENTION 
The most commonly used method for production by artificial lift is use of a 
pump jack--rod pumping system. In rod pumping, a pump jack is used to 
vertically reciprocate a rod extending down to the production zone of the 
well. The rod is connected to a subsurface pumping unit which consists of 
a piston in a pump barrel connected to the rod to reciprocate within the 
barrel and lift the fluid. The dependability and economy of these pump 
jack systems makes these systems highly desirable and generally used. 
In the design of these systems, it is generally accepted that the capacity 
of the pumping system will exceed the maximum production of the oil well 
as the production rate declines. As a result, if the systems are operated 
at maximum capacity, the system will become what is known in the industry 
as "pumped-off," reducing the efficiency of this system due to a partially 
filled condition in the barrel. The partially filled pump barrel is caused 
by the pump removing liquid faster than the well produces. In addition, 
the pumped-off condition can result in damage to the rod string and pump. 
As a consequence, control systems are currently available for detecting the 
pumped-off condition and for controlling the operation of the pump in 
response to detection of this condition. The history of the development of 
various of control systems is outlined in the 1977 Society of Petroleum 
Engineers of AIME Paper entitled "Successful Application of Pump-Off 
Controllers SPE No. 6853." As is pointed out therein, a development of 
generally applicable pump-off control methods was complicated by pumping 
abnormalities not associated with pump-off such as gas interference, 
harmonic pumping speeds, down-hole friction, equipment vibrations, 
corrosion, changes in the reservoir performance and the like. 
Historically, attempts to solve the varied problems of an efficient 
pump-off control has taken on many forms. 
The initial efforts to control pump-off are basically an attempt by a pump 
operator to match the pumping speed to the production rate of the well or 
reservoir. However, in order to obtain maximum production from the well, 
it is generally necessary to maintain the lowest possible fluid level in 
the well, and therefore the lowest possible back pressure on the 
formation. In order to assure a low average fluid level, it is necessary 
to provide a pumping system with a capacity in excess of the productive 
capacity of the well. The excess pumping capacity required to maintain the 
low fluid level ensures that pump-off will occur unless the pump is 
controlled in some manner. The first effort made to deal with the pump-off 
problem was to manually start and stop operation of the pumping system. In 
this approach the lease operator would estimate the amount of pumping time 
required to obtain maximum production from the well and maintain the fluid 
level as low as possible. This approach required a pumper periodically to 
turn the well pump off and on to regulate the pumping operation. This 
method suffered from the disadvantage of being less than exact and labor 
intense. 
The first attempts to automatically control the operation of the pumping 
system were to install timers which automatically stopped and started the 
operation of the pumping system. For example, the time clocks would 
automatically operate the pump for a period of time every hour. Again, 
these systems suffered from the disadvantage of being inexact in that the 
operator was required to estimate the amount of pump operation which would 
maximize production. The tendency, of course, in these time systems was to 
over pump the well to assure not missing any fluids, thereby causing the 
inherent production maintenance problems. 
As a result of the inaccuracies inherent in a system which estimate fluid 
level, methods have been developed for analyzing the loading on the pump 
rod to determine when the pump-off condition occurs. Since rod loading is 
directly affected by the pump loading, a number of characteristics of the 
rod loading can be used to detect pump-off of a well. Various portions of 
the rod load versus position relationship of a well has been utilized to 
sense pump-off. One example of such a system is found in the U.S. Pat. No. 
3,951,209 to Gibbs issued Apr. 20, 1976 entitled "Method For Determining 
The Pump-Off Of A Well." In this method, a dynamometer is used to monitor 
the total power input to the rod string to sense when power input 
decreases to determine when the well pump-off occurs. This system 
determines the power input to the well pump by integrating the rod load as 
a function of displacement. When the actual horse power input to the top 
of the rod string falls below a set minimum, a computer can be utilized to 
transmit a signal which stops the pumping unit for a period of time. This 
system is sometimes called an on-off pump-off control in that the system 
senses the pump-off condition and terminates the pumping operation for a 
period of time. However, in this type of system, the pumping rate has to 
be set to exceed the production rate of the well. The system operates by 
pumping until the pump-off condition is reached and shutting pumping 
operations down until fluid re-accumulates in the well. However, as was 
previously pointed out for maximizing production, the fluid level in the 
well needs to be maintained as possible without reaching a pump-off 
condition. And thus during that period of time, when the pump is not 
operated and fluid is flowing from the formation into the well, production 
will be lost because the fluid level is at too high a level. Even though 
the Gibbs patent suggests that a computer can be used to constantly 
monitor and adjust the shut-in periods to minimize the loss of production, 
the system does not provide a means for maintaining the most efficient 
fluid level for purposes of production. 
Two later patents provide variations of the on-off system taught in the 
Gibbs patent. The first is U.S. Pat. No. 4,015,469 to Womack, issued Apr. 
5, 1977 entitled "Pump-Off Monitor For Rod Pump Wells." In the Womack 
patent the same off-on method is used, however, the method of determining 
when pump-off has occurred is somewhat refined. In Womack. instead of 
integrating the power over the entire stroke, only the power input during 
a portion of the stroke is considered. In this patent, Womack suggests 
that a considerable difference in energy input between the pumped-off and 
normal pumping condition can be found in the last quarter of the upstroke 
and the first quarter of the downstroke. As Womack points out, the 
difference between the energy input for the pumping condition and the 
energy for the pumped-off condition is usually only five to fifteen 
percent of the total power input and that errors in the measurement of the 
load of the rod string or displacement of the rod string can produce an 
error in the final results which may prevent sensing of the pumped-off 
condition by measuring only a portion of the stroke. Womack attempts to 
overcome problems present in an on-off system which compares against a set 
point to determine pump-off. 
The second variation of Gibbs is found in the U.S. Pat. to Patterson No. 
4,034,808 issued July 12, 1977 entitled "Method For Pump-Off Detection." 
Patterson likewise uses an on-off system and utilizes rod performance 
during only a portion of the pump's cycle to sense the pumped-off 
condition. Patterson suggests using only the first quarter of the 
downstroke of the differences in energy between the pumped-off condition 
and the pumping condition are substantial. Patterson utilizes this portion 
of the pump stroke to determine whether or not the pumped-off condition is 
present to shut the system off. 
These on-off systems suffer from the disadvantage of inhibiting well 
production during the shut-down period and also require that the system 
reach the inherently damaging pump-off condition before the pump operation 
is controlled. 
One attempt has been made to dynamically control the fluid level in the 
well and maximize production. That system is described in the U.S. Patent 
to David Skinner. No. 4,145,161, issued in 1977 entitled "Speed Control." 
This system utilizes a variable speed controller and electric motor to 
continuously control the rate of removal of fluid from the well. The 
Skinner system measures the total electrical power supplied to the pump 
motor and regulates the pump motor speed based upon the fact that as fluid 
level decreases the total power increases. To implement the system, 
Skinner pumps the well down at a predetermined speed and monitors the 
total electrical power supplied as the well pumps down. When the well 
becomes pumped-off, the proportionality between the power and speed can be 
determined and set for a point before pump-off establishing a 
proportionally constant to be used in operating that particular well. This 
method leaves three major shortcomings when in actual use. The first is 
that it has been found that the so-called proportionality constant is not 
in fact a constant over the pumping rates and is rather a relationship 
whose proportion varies with speed. When Skinner assumes that the 
relationship is a constant, error is inescapable. Skinner recognizes this 
problem and suggests avoiding selecting a point too close to pump-off 
without informing a person of skill how to avoid being too close or even 
how to tell when one is too close. Second, Skinner controls directly 
proportional to fluid height above full barrel. Skinner is incapable of 
controlling in the more effective range of fluid height between pump-off 
and full barrel. Finally, Skinner's system is subject to errors induced by 
changes in system supply voltage. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention improves the method and equipment for maintaining the 
fluid level of a well as low as possible while avoiding pump-off. The 
invention utilizes a variable speed motor to drive a pump jack and control 
means for varying the speed of the pump. Means are provided for 
simultaneously sensing the pump speed, load on the rod and the position of 
the rod in the pump stroke. During operation, these measurements are 
utilized to calculate the power transferred between the rod string and the 
beam during a portion of the down stroke. Calculating the power only 
during the downstroke is performed because during the downstroke the 
inflowing fluid column is separated from the pump and the rod string by 
the standing valve at the bottom of the pump and in this portion of the 
stroke, the differences between a full pump and the pumped-off condition 
are the largest. Before the pump is continuously operated, a series of 
power measurements are made in the full barrel pumping states to determine 
the power transferred between the rod and beam at various speeds. These 
are utilized to establish a relationship (not necessarily linear) which is 
later used to control the well. The well is then operated and the values 
obtained during pumping are compared to the relationship to correct the 
well during operation. In this manner, variations in the proportionality 
constant as a function of speed are taken into consideration to accurately 
control the well to operate at an effective fluid height over a range of 
fluid production rates.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings wherein like reference characters will be 
used throughout the several views to refer to like or corresponding parts 
there is shown in FIG. 1 the improved oil well pumping control system of 
the present invention which, for purposes of description, is identified by 
reference numeral 10. System 10 uses a pumping unit 12 which is driven by 
an electric motor 14. A conventional variable speed motor controller 16 is 
connected to the electric motor 14 whereby the speed of the motor 14 and 
pumping rate of the pumping assembly 12 can be varied by the motor 
controller 16. A master controller 18 is coupled to the variable speed 
motor controller and pump assembly 12. As will be described in more 
detail, a master controller 18 receives data relating to the load on the 
pumping rod as a function of the position of the beam in the pumping 
stroke and the master controller, in turn, sends control signals to the 
variable speed motor controller to vary the pumping rate to maximize 
production. 
Data relating to the load on the pump rod string 20 can be obtained through 
use of a conventional load transducer 22 such as a strain gauge or the 
like. Data relating to position of the beam 24 can be obtained through a 
position transducer 26 such as a potentiometer or the like connected to 
the beam 24. Data relating to the speed of the pump stroke can be obtained 
through use of a conventional pump stroke speed sensor 28 which could be 
connected, for example, to the beam 24. In addition, the data can be 
coupled to the motor controller 16 and transducers 22 and 26 by a cable 
link in which case the central controller can be remotely located and used 
to control the operation of more than one pump at a time. 
Power transferred between the rod 20 and beam 24 during a portion of the 
pump cycle can be calculated from the measurements taken by the 
transducers. The variable speed motor controller 16 is of a conventional 
construction and operates to vary motor 14 speed by varying the line 
frequency of the power supplied to the motor 14 as a function of data 
received from master controller 18. Master controller 18 also contains a 
conventional on-off control which likewise operates to start and stop 
motor 14 as a function of data received from master controller 18. 
Variable speed motor controller of this type are conventional in 
construction and readily available from numerous manufacturers. 
Master controller 18 comprises a microprocessor based controller using STD 
BUS construction, manufactured by Pro-Log Corporation, 2560 Garden Road, 
Monterrey, Calif. 93940, Cards Part #7890-07, 7717-02, 7714A-01, 7715A-03, 
7507, 7316-04, 7907A and Analog Devices, One Technology Way, Norwood, 
Mass., Card Part #RTI-1281 are present and connected in a conventional 
manner to receive analog data from transducers 22 and 26 and supply an 
analog control signal to motor controller 16 as will be described in 
detail. Microprocessor based controller can also be obtained form other 
sources such as WinSystems, Inc., Arlington, Tex., and their assembly and 
connection to receive analog data and provide analog output is well known 
to persons who are skilled in the art. 
In FIG. 2, a down-hole oil well pump 30 is illustrated schematically in a 
perforated casing 32 positioned in a producing formation 34. Positioned 
inside the casing 32 is a vertical reciprocal pump piston 36 in sliding 
sealing engagement with the walls of a pump barrel 38. Piston 36 is 
illustrated at the upper extent of its travel or top dead center and is 
connected to rod 20. Piston 36 is reciprocated vertically between levels 
"A" and "B." A standing check valve 40 permits flow only from the casing 
32 into the pump barrel 38. A second check valve 42 permits flow only from 
below to above the piston 36. In operation on the down stroke of piston 36 
from position "A" to "B," fluid trapped in the barrel below the piston 
will be pumped above the piston through valve 42. On the up stroke from 
position "B" to "A" fluid above, the piston is lifted while fluid flows 
into the barrel through check valve 40. 
The pumped off state occurs when the pump operates at a rate so that the 
fluid entering the pump barrel during the up stroke reaches approximately 
only to level "B." In this condition, on the down stroke the piston 
undesirably will be forced downward by the weight of the liquid supported 
above the piston and no pumping will occur. 
If the pump is operating at a rate whereby the fluid removal rate is less 
than the rate fluid is flowing into the case from the formation, fluid 
will undesirably accumulate in the case above the full barrel level "A." 
Fluid buildup of this type increases pressure on the formation and retards 
production. Ideally. for maximum production fluid buildup in the casing 
should be minimized. 
It has been found that the pump can be best controlled and the fluid 
buildup can be minimized if the fluid flows into the barrel at a rate such 
that the fluid level approaches but does not exceed the full barrel height 
"A." Level "C" illustrates this ideal level for production and control, 
with the pump piston shown with a small gaseous volume 44 present below 
the piston at top dead center position. This optimization is believed to 
be partially due to the fact that because the pump is operated in a 
slightly starved condition, fluid buildup is minimized (and partially) 
because production of the well pump is more accurate when operated below 
full barrel. 
It has been found when the fluid level in the pump barrel is below the full 
barrel level "A" but above the pump off level "B" total power transferred 
between the rod 20 and beam 24 varies more during the down stroke. This 
difference is even greater during the first half of the down stroke. As 
the fluid level falls below full barrel, the absolute value of the total 
power transferred between the beam and rod during this portion of the down 
stroke increases. Other measurable parameters of the degree of pump off 
(such as load on the rod or beam, work performed by the rod or beam, motor 
power. etc.) vary similarly during this portion of the stroke. It is to be 
understood that using power measurement is preferred, however, other 
parameters could be used to control pump down in accordance with the 
teaching of the present invention. 
FIG. 3 illustrates a sample graph for a well showing the relationship 
between total power transferred between rod and beam during a portion of 
the down stroke as a function of speed. In the graph, the Y axis 
represents the power transferred and the X axis represents pump speed. The 
relationship of these variables for a given well at full barrel fluid 
levels, i.e., those at or above "A" in FIG. 2, is shown as plot A'. It is 
readily apparent that the relationship shown as A' is not linear. The 
power values were determined by totaling the power during the portion of 
the pump cycle from 190.degree. to 240.degree. past bottom dead center. 
Plot C' estimates the relationship for a fluid level C of FIG. 2 below 
full barrel (A in FIG. 2) but above pump off (B in FIG. 3). As can be seen 
by comparing the plots A' and C' at a given speed, the power transferred 
between the rod and beam during a portion of the down stroke increases as 
the fluid level drops from level A to C. As will be described in detail, 
the non linear relationship of speed versus power of plot C' can be used 
to control the well pump speed to maximize production. 
For an existing producing well determining the relationship shown by plot 
C' is premature. However, plot A' can be easily determined by varying the 
pumping speed in a full bore condition and calculating the corresponding 
power transferred. From this relationship, plot C' can be calculated by 
increasing the power values by a uniform percentage, for example, ten 
percent over the range of motor speeds. As will be described in detail, 
the relationship represented by plot C' of FIG. 3 can be used as a basis 
for varying the motor speed (and pumping cycle speed) to maximize 
production by maintaining the fluid level in the barrel below full barrel, 
such as shown as level C in FIG. 2. 
To accomplish the method of the present invention, the power to speed 
relationship must first be determined for a given well. The method steps 
of start up are shown in FIG. 4. 
Referring to FIG. 4, the method steps of setting up a well for use with the 
improved pumping system of the present invention are shown. Set up method 
is utilized to determine the characteristic relationship of a given well 
full bore power to speed. Before beginning, the improved pumping system of 
the present invention is assembled as shown in FIG. 1. 
In the first step shown in FIG. 4, pumping of the well is temporarily 
stopped so that the well can be shut down a sufficient time to allow fluid 
to flow from the formation into the annulus and to accumulate to a level 
above full bore. It is best to allow the fluid to accumulate in this first 
step to a sufficient height so that the fluid level will remain above full 
barrel during the performance of the steps of the set up method. 
Once fluid has sufficiently accumulated in the pump, the pump is operated 
at a set speed and the system is allowed to stabilize for a short period 
of time. While operating the pump in the stable condition of Step 2, the 
load on the rod is measured and the beam position is simultaneously 
measured by use of the transducers 22 and 26 shown in FIG. 1. This data is 
transmitted to the master controller and the master controller is suitably 
programmed to calculate and store the total power transferred between the 
rod and the beam during only a portion of the first half of the down 
stroke. Preferably, the total power is calculated for a portion of the 
stroke between 190.degree. and 240.degree. following top dead center. 
According to a method of the present invention, an average can be 
determined and stored corresponding to the pump speed. 
In Step 5, Steps 1 through 4 are repeated while operating the pump at 
various speeds to obtain a relationship of pump speed to power in the full 
barrel condition. 
In Step 6, the values obtained in Steps 1 through 5 are utilized to 
calculate a relationship of speed to power for an optimum fluid level 
below full bore by increasing the power values by a uniform percentage. 
For example, the power values obtained in Steps 1 through 5 may be 
increased by ten percent over the range of motor speeds. As the fluid 
level falls below full barrel, the absolute value of the total power 
transferred between the beam and rod during the down stroke increases. 
Therefore, operating the pump at a speed that results in total power 
values slightly greater than those obtained in Steps 1 through 5 for a 
full barrel will result in the pump being operated in a slightly starved 
condition. Production from the well is maximized when the pump is operated 
at a speed to control the rate of fluid flow into the barrel such that the 
fluid level approaches but does not exceed full barrel. 
In Step 7, this relationship for an optimum fluid level is stored in memory 
in the master controller 18. Once the set up method, illustrated in FIG. 
4, is completed, operation of the improved pumping system of the present 
invention can begin. 
In FIG. 5, the method steps of the control method for operating the 
improved pumping system of the present invention is schematically 
illustrated. In operation, variable speed motor controller 16 starts the 
electric motor 14, actuating the pumping assembly 12 at a preselected 
speed. While operating the well at the preselected speed, transducers 22 
and 26 continuously measure the force transfer between the beam 24 and rod 
20 in the position of the beam 24. 
In FIG. 5, Step 1 is shown as measuring the force on the rod 20 and 
position of the beam 24. [These measurements can be selective or 
continuous depending on whether or not the operator desires to use these 
measurements for additional control functions other than controlling the 
optimum production speed of the well pump.] 
In Step 2, the master controller 18 has been programmed to calculate the 
absolute value of the power transferred between the pump and the rod 
during a portion of the down stroke. The portion of the down stroke 
selected should be identical to that selected during the setup method and 
in the illustrated example is from 190.degree. to 240.degree. after bottom 
dead center. 
In Step 3, the power value is obtained from Step 2 and is used to obtain a 
moving average value of the power transferred during a set number of 
previous pump cycles. For example, if the operator desires the system to 
be quickly responsive, the average could be determined over only one of 
the previous strokes and if the operator wishes the system to respond more 
slowly, the average could be determined over a larger number of cycles. 
In Step 4, the average determined in Step 3 is compared to the power value 
at that motor speed in the stored relationship determined during the 
startup method If the power for that speed differs from the average more 
than a set percentage--say, for example, two percent--than the speed will 
be adjusted according to a formula. If the power value does not differ 
more than two percent, the system would return to Step 1 and begin the 
process anew. 
The formula for determining the new speed is as follows: 
New speed=Current speed-(Current Speed *(Ave. calculated Power-Power Curve 
Value)/Average Calculated Power)* Gain/100. 
Once the new speed is calculated, a control signal is sent to the motor 
controller 16 which, in turn, adjusts the motor speed accordingly. 
In Step 5, a delay can be taken before returning to Step 1 if the speed has 
been has been adjusted whereby the system is allowed to reach a steady 
state condition. After the delay, the system would return to Step 1 and 
begin the system analysis again. 
Although not illustrated in FIG. 5, it is to be understood of course that 
load and position measurements could also be sensed to determine whether 
or not various malfunctions have occurred in the system. For example, if 
during the pumping cycle, the peak load on the rod becomes less than a 
desired minimum load on the rod, then the master controller will send a 
signal to the motor controller 16 to disengage motor operation and set an 
alarm indicating that a broken pump rod is present. In addition, the motor 
can be stopped if a stuck traveling valve is sensed by determining that 
the difference between the minimum and the maximum rod load is smaller 
than a preset minimum, or the system can be disabled to protect a pump rod 
from damage if the load on the rod exceeds a maximum of a preset time 
limit. The system can even be used to determine the pump off condition and 
act as a pump off controller.