WO1983000203A1

WO1983000203A1 - Moving mass pump jack and method of operation

Info

Publication number: WO1983000203A1
Application number: PCT/US1982/000859
Authority: WO
Inventors: Domenith Clarence Basolo; Stuart Mccornack Oliver; Sanford Jennings Parsons
Original assignee: Domenith Clarence Basolo; Stuart Mccornack Oliver; Sanford Jennings Parsons
Priority date: 1981-06-26
Filing date: 1982-06-25
Publication date: 1983-01-20
Also published as: EP0083366A1; EP0083366A4

Abstract

A well pump includes a Sampson post (2) supporting a walking beam (1) which oscillates to cause a horsehead (3) to operate one or more sucker rods in a well bore. The invention avoids need for a flywheel, gearbox or other mechanical means connected to the base to cause or control oscillating motion of the walking beam. A carriage (5) movably supports a balance mass (4) on the walking beam and is positioned by a hydraulic cylinder (6) or other positioning means to change the relative position of the balance mass, thus causing oscillation of the walking beam. A moving mass control system (11-21) regulates the positioner means and the stroke timing thereof so that a wide range of variable operating conditions can be instituted. One embodiment includes a walking beam (1') adapted for connection at its ends with sucker rods (53a, 53b) in different well bores.

Description

MOVING MASS PUMP JACK AND METHOD OF OPERATION Background of the Invention

The present invention relates to pumps used to recover subterranean oil as well as other liquids or fluids and particularly to pumps of the type in which pumping motion is originated at the surface and transmitted through a reciprocating string of rods to a pump located within the well bore. Well pumping apparatus and devices have been used for many years, especially in oil field operations. These existing devices generally involve complex and expensive mechanical systems to produce oscillating motion in a walking beam and provide the pumping action. Most of these devices are mechanically adjustable to regulate the depth of stroke and to deliver a certain weight or quantity of fluid such as oil or water, for example, for each stroke as well as to regulate the overall delivery rate for the fluid.

Problems have arisen in the prior art in adjusting the length and/or rate of the pumping stroke since most mechanical controls are rigidly set for particular conditions of operation. Thus, it is difficult and expensive to change the mode of operation for a given pump in order to fit instant requirements of different operating conditions.

Other problems have arisen in the connections and adjustment mechanisms which are complex and require intensive labor to achieve precise mechanical settings. Furthermore, most pumps in the prior art have been oriented to one well bore and are not easily adaptable to operation for other well bores.

Typical problems encountered in deep well pumps employing sucker rods have involved failure of the reduction gearing and associated pit ans, bearings, etc. due to varying load conditions encountered within the pumping cycle. Over the years, efforts to more effectively counterbalance imposed loads has resulted in more complex and expensive pumping units without achieving substantial advances over relatively primitive pump jacks.

In some instances in the prior art, counterweights have been directly mounted on the walking beam to help offset the very substantial unbalanced loads placed on the gear reducers by the pumping of subterranean fluids from great depths. For example, in Downing ϋ. S. Patent 2,432,735, counterweights were moved in timed fashion by a hydraulic cylinder. However, additional means such as a gear reducer, pitman connection or second motor means was required in order to control oscillatory motion of the walking beam. In Scoggins D. S. Patent 3,209,605, a prime mover as well as an associated gearbox were movably located on the walking beam. However, operation of the walking beam was here again regulated by additional linkage to either the Sampson post or the base of the pumping unit in order to control the desired oscillatory motion.

Accordingly, there has been found to remain a need for an improved method and apparatus for pumping oil and other fluids from substantial subterranean depths.

Summary of the Invention

The present invention provides a pumping method and apparatus for use in any depth well without the need for gearboxes, pit ans and associated accoutrements while maintaining complete balance in the pumping method and apparatus at all times. Furthermore, far less energy is required to accomplish pumping because of the balanced system.

Generally, the invention comprises a reciprocating pump of the type having reciprocating pumping motion originating at the surface of the ground and including an oscillating walking beam, a string of rods depending from one or both ends of the walking beam for reciprocation with the bore of one or more wells for operating a pump unit disposed therein. A balance mass is suitably located on the walking beam with motor means moving the balance mass in reciprocable fashion to produce oscillatory motion of the beam.

One form of the invention specifically comprises a conventional reciprocating pump of the walking beam and sucker rod type in which a balance mass is mounted on the walking beam for reciprocating movement thereupon and in which the balance mass is driven in linear reciprocating fashion by control means, preferably a servo-system operated by a microprocessor which may be of either the digital or analog type, for example.

In one embodiment of the invention, the walking beam .is equipped with horseheads at each end for interconnection with sucker rods of adjacent well bores, the balance mass being reciprocated over a central portion of the walking beam. Reciprocating motion of the balance mass is thus sufficient to cause oscillation of the walking beam without the need for cranks or other devices interconnecting the walking beam with either the Sampson post or the pump base. Operation of the balance mass in this manner also obviates the need for any counterbalancing device.

Although doublehead pump jacks have been known in the past, prior art emobodi ents generally used a gearbox and pitman type of operation, for example, and did not work from a state of balance at any time in their stroke cycle. The moving mass pump jack is in balance in the middle portion of each oscillatory cycle while the double horsehead moving mass pump jack is substantially in inherent balance at all times, thus requiring very small amounts of operating energy to ____

_____ . ^v/I move the balance mass and produce oscillation of the beam resulting in pumping operation.

Accordingly, it is an object of the present invention to conserve energy and to more efficiently pump oil and other subterranean fluids compared to conventional harmonic motion pumping units.

Another object of the present invention is to entirely eliminate the need for counterbalancing induced loads during the pumping cycle by virtue of not allowing such loads to be introduced into the system.

Another object of the invention is to allow the stroke frequency and/or length to be varied while the pumping cycle is being carried out without the need for interrupting pump operations and to be able to introduce predetermined pause at either the top and/or bottom of any stroke cycle through a programming of the processor in order to prevent premature pumpoff and other phenomena common to harmonic motion pumping devices.

It is also an object of the invention to provide an acceleration pattern for the sucker rod system consistent with minimum stress in the system and particularly on the sucker rod in order to develop a dynamometer readout of a substantially circular shape as opposed to a readout having large spikes and long straight lines as was prevalent in the prior art.

It is also an object of the invention to provide a moving mass pump jack including controls permitting adjustment in stroke length as well as stroke timing, the pump assembly being further adapted for achieving a substantial pump stroke by means of a mechanical multiplier interconnected between the pump assembly and sucker rod means in the well bore.

In summary, the present invention obviates problems presented by prior art pumps through the following important features:

1. Oscillating motion of the walking beam is

O Pl produced without a direct mechanical coupling or connection between the walking beam and the Sampson post or pump base.

2. The invention avoids the need for a flywheel or gearbox.

3. The invention includes a moving mass control system, preferably in the form of a servo-unit, which may be adapted to permit instant changing of the length and/or timing of the pump stroke. 4. The .satire pump unit of the invention is capable of being moved to and adapted for use in a different well bore, having a substantially different depth and different operating conditions, the pump unit being capable of adjustment by simple changing of the balance mass and control program, for example.

5. The pump assembly of the invention is adaptable for use with a variety of power sources which may include electrical, hydraulic or other power means. For example, hydraulic cylinders, electric motor and ball screw combinations, linear actuators, etc. may all be used as suitable drive means for the moving mass pump of the present invention. However, it is generally preferred that the drive means comprise either a hydraulic motor driven rack and pinion or a drum and cable system.

The processor control for the method and apparatus of the invention is of particular importance in view of the problems of pumping a fluid such as oil subject to changes in viscosity, specific gravity and frictional resistance to the pump components. In the past, these variations have been overcome in conventional pump units through the use of "brute force" by selecting and fitting the unit with the largest motor and gearbox combination foreseeably required by a particular well. The various embodiments of the moving mass pump of the present invention address these problems preferably through the use of a multiplicity^of sensors to gather data on changing conditions on a stroke-by-stroke basis during the pumping cycle. This date is transmitted to a microprocessor, an analog or digital computer, for example, for analysis and subsequent output command to the moving mass in order to maintain smooth, efficient pumping action. Sensor functions employed in this regard may include but are not necessarily limited to carriage position, carriage travel direction, walking beam angle, walking beam position, sucker rod load and/or electrical load.

Additional objects and further advantages of the invention will become clear from the following detailed description taken in conjunction with the associated drawings.

Brief Description of the Drawings

FIGURE 1 is a schematic drawing of the moving mass pump jack showing the operative elements.

FIGURE 2 is a drawing of the movable carriage and replaceable counterweights which are movably positioned along the walking beam.

FIGURES 3A and 3B are schematic diagrams respectively showing digital and analog versions of the servo-control mechanism which regulates the carriage motion to provide an adjustable pumping stroke and oscillation frequency for the walking beam.

FIGURE 4 is a fragmentary view of another embodiment of the moving mass pump jack including horseheads at either end of a single walking beam for operation of the pump in connection with two adjacent well bores.

FIGURE 5 schematically represents a moving mass pump jack modified by means of a mechanical multiplier or stroke extender as contemplated by the present invention.

Description of the Preferred Embodiment 7 Referring to FIGURE 1, there is seen the movin mass pump jack of the present invention. A walking bea 1 is supported via pivot bearing l_a on Sampson post 2. The Sampson post 2 is supported by a suitable support structure 2_a.

At one end of the walking beam 1, a connected horsehead 3 is aligned approximately over a well bore (not shown) . As is conventional in the art, the horsehead is connected by wire lines to sucker rods which extend down into the well bore.

At the other extremity of the walking beam 1, there is positioned a movable carriage 5. The lower en of carriage 5 supports an axle 5_a to which are mounted cylindrical balance masses 4_a and 4_D. Mounted on, within or above the walking beam 1 is a hydraulic cylinder 6 or other positioning means which may be but is not limited to an electric motor, hydraulic motor, electric linear actuator, or electric motor with ballscrew combination having a fixed connection 6_a to the walking beam 1. The hydraulic cylinder 6 has an extendible shaft 6_j on which the shaft-end is fixedly connected at 6_C to carriage 5.

A power source means 7 is shown in FIGURE 1 which may be an electric motor and^' hydraulic pump combination, internal combustion engine powered hydraulic pump or other suitable power source means. Connection means 12 connects to cyliner 6.

Adjacently mounted is servo means 8 which may be either a digital or analog control mechanism shown in FIGURES 3A, 3B.

A frontal view of the carriage assembly 5 is seen in FIGURE 2. Carriage 5 is seen supporting axle 5_a which extends outward on either side to support the counterweights 4_a and 4_>. Within the carriage assembly 5, there is encompassed the walking beam 1 on top of which reside rollers 5_r which permit the carriage to move relative to the walking beam.

Referring again to FIGURE 1, a linear shaft position indicator 10 is mounted on hydraulic cylinder 6 and is used to indicate the position of the shaft 6_a at any given moment.

An angular beam-position indicator 9 is located adjacent the walking beam bearing l_a to provide a readout of the beam orientation from the horizontal position. As seen in FIGURE 1, the distance between the pivot bearing l_a and the center of axle 5_a is designated as R5. The value in length of this distance R5 varies accordingly as the hydraulic cylinder 6 and shaft 6_ regulates the position of carriage 5 and its associated balance mass 4.

The distance R5 from the bearing centerline l_a to the balance mass centerline 5_a is controlled as a function of the walking beam angle and angular velocity. One preferred algorithm which may be used is: R5 ■ Cl (C2 + C3 cos 2 α ) where

Cl is a constant dependent upon the individual system. A typical value for Cl could be 1.0. C2 is a constant dependent upon the individual system. A typical value could be 4.9 feet. C3 is a constant dependent upon the partiuclar system and operating mode. It could typically be 0.1 feet. α is a function of the walking beam angle from the horizontal and has a typical range of plus or minus 30°. The above terms can be adjusted to provide for oil weight and fluid effects on the polished load rod. The terms α and C3 cos 2ct can be generated mechanically or electrically. The adjustments are normally in terms 9 of shift of effective average center of mass and of balance mass velocities. ^~ can be generated by direct shaft pickoff and scaling; 2« can be generated by eithe mechanical or electrical multiplication. C3 can be generated by mechanical or electrical scaling and multiplication.

Referring to FIGURE 3A, the servo-control apparatus is shown for a digital embodiment. The counterweight mass position is sensed by sensor 10 via loops 22 and 23. The walking beam angle (α) is sensed by sensor 9. the load on the sucker rod may be detecte by a sensor 10A.

The outputs of sensors 9, 10 and 10A are fed through analog/digital converters 11, 12 ands 12A to a central processing unit 13. The CPU 13 uses program data and parameter data in memories 15 and 16. A remot input means 14 is used to feed parametric data reflective of desired operating conditions to memory 16 for use by the CPU 13. ^«rhe output control signals from CPU 13 go to digital/analog converter 17 which feeds servo-amplifier 18. Amplifier 18 regulates servo valve 19 to control servo cylinder 20 and displacement pump 21 which operates activator 6 to position balance mass 4. The analog servo-control apparatus equivalent is schematically shown in FIGURE 3B. The beam angle (α) sensor output is amplified by amplifier 29 and fed to range sample circuit 33 and standoff sample circuit 34. The range sample output is adjusted by circuit 35 and integrated by circuit 37 to form the y input to multiplier 39. The x input to multiplier 39 is derived from driving function generator 31 and buffer amplifier 32.

The output of multiplier 39 provides one input to difference amplifier 41. The second input to 41 comes from summing point circuit 40 which takes (a) the position sensor 10 output signal (adjusted by gain

OMP circuit 10_a) as one input and (b) the standoff signal from 34 after integration by integrator 38. The output of difference amplifier 41 is power-amplified by 42 to provide power to activator 6 which positions the counterweight mass 4.

The servo-control means includes, but is not limited to, electronic digital control means, electronic analog control means, and mechanical analog control means. The preferred embodiments for current applications are electronic digital and electronic analog control means. A typical electronic digital servo-control will include: input sensors from the walking beam and from the moving carriage (these sensors may be digital or analog with analog/digital conversion means applied as needed) . The sensors provide angular data with regard to walking beam position and also linear position data with regard to carriage movement. The central processing unit (CPU) compares this data with desired tabular, historical and program data and provides an output digital servo-signal to a digital/analog converter. In turn, the digital/analog converter drives a servo amplifier which then drives the actuation means causing movement of the counterweight mass. The actual position of the counterweight is fed back directly through a weight position sensor to the CPU. The effect of this position movement is fed back indirectly (via a beam angle sensor) as a beam angle signal to the CPU. The servo-control means drives the pump jack into motion by positioning the balance mass on the walking beam either inboard or outboard of the balance point, depending on the desired direction of motion of the walking beam. Inertial and gravitational forces on the walking beam are the summation of the forces on the beam, the balance mass and the appended pump string. Changes in the vector velocity and position of the balance mass will change the summed forces on the entire pumping system and are sufficient to control stroke length and oscillation rate of the pump. In addition, control by this system means permits a finer control of stresses in and between the elements of the pumping system that would be permitted by use of a uniform harmonic motion. A typical analog servo-control means functions in a similar fashion, but uses an analog function generator providing a sinusoidal or other driving analog signal which is processed by a combination of analog elements such as integrators, multipliers, amplifiers, and summing point circuitry to form a totally analog servo loop.

The physical principle of the pumping machine is that of a force-balance beam as exemplified by the mechanism often called a "teeter-totter." In order to apply this principle to pumping, it has been necessary to develop control algorithms which will generate a "polished rod" load curve consistent with maximum rod life. This means that a smooth change and minimal change in polished rod stress (over the pumping cycle) is the preferred condition.

The control function must be such that this can be accomplished for varying well and bottom hole conditions which can give natural rod resonance. The entire control function is performed by positioning the balance mass at various positions over the pumping cycle. This has been found to require a modified sinusoidal positioning of the center of mass of the balance mass with time. The positioning with time is done by use of the servo-control and positioning means. In practice, repositioning of the balance mass will normally have reversed the direction of its motion before the stroke motion reverses. The exact timing and positioning is dependent upon individual well and bottom hole conditions.

The above described invention will be seen to provide a liquid pumping mechanism wherein adjustable servo-control of the motion-inducing Balance mass is made easily adaptable to any variety of pumping requirements while facilitating minimal stress on the sucker rod, all the while with minimal parts and with easily replceable balance masses.

Referring now to FIGURE 4, the double horsehead moving mass pump jack generally indicated at 51 includes components corresponding to those described above in connection with FIGURES 1 and 2. Accordingly, similar primed numerals are employed for components of the double horsehead moving mass pump jack 51. The horseheads at opposite ends of the beam 1' are indicated respectively at 3_a and 3fc. Each horsehead is connected through a cable 52_a or 53_D with sucker rods 53_a and 53_D representative of different well bores (not otherwise shown) . The cables 52_a and 52_, may be interconnected with the sucker rods 53_a and 53_D through sucker rod load sensors as generally indicated respectively at 54_a and 54_b. AS noted above, it is particularly important in connection with the double horsehead moving mass pump of FIGURE 4 that the well bore load supported by the respective horsehead 3_a and3_D tend to balance each other. However, it will be obvious that certain imbalances may occur during operation. Accordingly, operation of the double horsehead pump is generally similar to that described above for the single horsehead pump embodiment of FIGURES 1 and 2. However, within the double horsehead pump embodiment, imbalances in the loads for the two wells may be sensed during each operating cycle and adjusted for by movement of the

OMPI balance mass 4.

Preferably, the carriage 5' for the double horsehead pump jack 51 is positioned by a rotary hydraulic motor and drum assembly 55 interconnected with the carriage by means of a cable 56 trained about an idler pulley 57. Thus, the hydraulic motor 55 may be operated in opposite directions for producing reciprocating movement of the balance mass 4' .

FIGURE 5 illustrates a pump assembly 110 generally of the type disclosed by the above noted references. In summary, the pump assembly 110 includes a walking beam 111 pivotably mounted at 112 on a Sampson post 113 to permit oscillation of the walking beam in a vertical plane. A horsehead 114 is mounted on one end of the walking beam 111 for interconnection with pump means in the well in a manner described below.

As with the embodiment of FIGURES 1 and 2, Various operating characteristics of the pump assembly are detected or sensed during each stroke or cycle by means (not shown) associated with a control unit 121 which thereupon operates the positioner 118 to regulate movement of the balance mass 116 and establish the pumping stroke and frequency for the reciprocating pump assembly. The control unit 121 is capable of varying both the timing and length of the pumping stroke within limits established by the length of the walking beam 111.

The pumping assembly achieves a substantially increased length of pumping stroke within the well by means of a mechanical multiplier or stroke extender generally indicated at 122.

The stroke extender 122 may be one of a variety of mechanical combinations known in the prior art, but is preferably of a type employing a cable together with one movable pulley and at least one idler or fixed pulley in order to achieve the mechanical advantage or stroke amplification referred to above. Accordingly, the stroke extender 122 preferably comprises a bridle 123 in the form of a flexible line interconnecting the horsehead 114 with a traveler or movable pulley 124. A cable 126 secured at one end 127, preferably to a base member 128 of the pump assembly, is trained over the traveler or movable pulley 124 and one or more idlers or fixed pulleys 129 and 131 for interconnection with a sucker rod 132 representing a pump means in the well (not otherwise shown). The use of two idlers, as shown by the fixed pulleys 129 and 131, permits the stroke extender 122 to provide proper interconnection between the horsehead 114 and the sucker rod 132.

The idler pulley 129 is connected to the same base member 128 of the pump assembly while the second idler pulley 131 is mounted on a tower or. vertical extension 133 also supported by the base member 128 of the pump assembly. The other end 134 of the cable 126 is interconnected with the sucker rod.132 through a conventional spreader bar 136. Thus, the stroke extender 122 provides an effective stroke length for the sucker rod 132 twice the effective stroke length for the horsehead 114. At the same time, the stroke extender 122 acts as a force reducer in that force applied by the horsehead 114 through the bridle 123 is twice the force applied to the sucker rod 132 by the cable 126.

The method of operation for the present invention is believed apparent from the preceding description. However, operation of the invention is summarized below in order to assure a complete understanding of the invention.

Initially, the pump assembly 110 is operated by the control unit 121 which shifts the balance mass 116 in reciprocating fashion upon the walking beam 111 in order to achieve oscillating motion of the walking beam. Resulting pumping action is applied to the sucker rod 132 by the horsehead 114 through the stroke extender 122. In operation, both the length and frequency of the pumping stroke may be adjusted by the control unit 121 as described in detail within the above noted reference. In addition, the effective stroke of the sucker rod 132 is doubled relative to the effective stroke of the horsehead 114 by means of the stroke extender 122.

Various modifications and alternatives are believed apparent from the preceding description. For example, it would be possible in the embodiment of FIGURE 5 to mount the horsehead 114 on the same end of the walking beam 111 as the balance mass 116 and rearrange the stroke extender to permit a different arrangement of movable and idler pulleys between the horsehead and sucker rod. Accordingly, the scope of the present invention is defined only by the following appended claims.

OMPI

^R Ύ

Claims

CLAIMS:

1. A reciprocating pump assembly for lifting fluid in a well bore comprising; a pump structure, a walking beam mounted on a pivot axis of the pump structure for oscillation in a generally vertical plane, one end of the beam being interconnected with sucker rod means in the well bore, sensor means for monitoring the angular position of the walking beam, a balance mass mounted on the walking beam for movement between the pivot axis and the other end of the walking beam, motor means for moving the balance mass on the walking beam, and control means responsively coupled with the sensor means for causing the motor means to move the balance mass back and forth on the walking beam and establish the pumping stroke and frequency for the reciprocating pump assembly.

2. The reciprocating pump assembly of Claim 1 wherein the sensor means includes means for instantly sensing the angular relation of the walking beam and a portion of the pump structure.

3. The reciprocating pump assembly of Claim 1 wherein the motor means is mounted on the walking beam.

4. The reciprocating pump assembly of Claim 3 further comprising a separate power source for operating the motor means, the power source being mounted apart from the walking beam.

5. The reciprocating pump assembly of Claim 1 wherein the control means is programmable for regulating the pumping stroke and/or frequency.

6. The reciprocating pump assembly of Claim 1 further comprising a carriage assembly for movably supporting the balance mass on the walking beam.

7. The reciprocating pump assembly of Claim 1 wherein the control means includes means for minimizing stress on the sucker rod means during operation of the pump assembly.

8. The reciprocating pump assembly of Claim 1 further comprising additional sensor means for monitoring the position of the balance mass on the walking beam, the control means being responsive both to the sensor means for monitoring the angular position of the walking beam and the additional sensor means for monitoring the position of the balance mass.

9. The reciprocating pump assembly of Claim 1 wherein the control means comprises a servo-control apparatus for regulating the action of said drive means during each oscillatory motion cycle of said walking beam.

10. The system of Claim 9 wherein said servo-control apparatus includes means for controlling the rate of oscillatory motion of said walking beam, means for controlling the stroke length of the said sucker rod, and means for minimizing the stress on said sucker rod during the pumping cycle.

11. The system of Claim 9 wherein said servo-control apparatus includes sensing means to develop signals representative of the walking beams angle, and the position of said balance mass means, memory means to store data, programs and parameters useful to the control of pumping operations, input means for feeding data to said memory means and to a processor, and a processor for processing said sensing means signals and said memory program and parameter data to develop output control signals for said drive means.

12. The system of Claim 11 which includes means for converting said output control signals into mechanical motion of said drive means to regulate the position of said balance mass means.

13. The reciprocating pump assembly of Claim 1 further including a stroke extender assembly interconnecting one end of the walking beam with sucker rod means in the well bore, the stroke extender

- iPO including multiplier means for varying the effective stroke length of the sucker rod means relative to the one end of the walking beam.

14. The pump assembly of Claim 13 wherein the mechanical multiplier means of the stroke extender assembly includes cable means trained over a movable pulley and fixed idler means or operatively interconnecting the one end of the walking beam with the sucker rod means.

10

15. The pump assembly of Claim 14 wherein a horsehead is mounted on the one end of the walking beam and is interconnected with the movable pulley by suitable bridle means, a cable being fixed at one end and trained over the movable pulley and first and second

15 ixed idler pulleys for interconnection with the sucker rod.

16. A reciprocating pump assembly for lifting fluid in two adjacent well bores, comprising a pump structure, a walking beam mounted on a pivot axis of the

20 pump structure for oscillation in a generally vertical plane, one end of the beam being interconnected with sucker rod means in one of the well bores, a second end of the beam being interconnected with sucker rod means in the second well bore, sensor means for monitoring the

25 angular position of the walking beam, a balance mass mounted on the walking beam adjacent its pivot axis for movement along the walking beam, motor means for moving the balance mass on the walking beam, and control means responsively coupled with the sensor means for causing

30 -the motor means to move the balance mass back and forth on the walking beam and thereby establish the pumping stroke and frequency for the reciprocating pump assembly in each of the well bores.

-_

17. The reciprocating pump assembly of Claim

16 wherein the control means includes means for minimizing stress on the sucker rod means in each of the si well bores during operation of the pump assembly.

18. The reciprocating pump assembly of Claim 16 wherein the balance mass is adapted to offset load imbalances between the two well bores while establishing the pumping stroke and frequency for the reciprocating pump assembly.

19. In a method for operating a reciprocating pump assembly to lift fluid in a well bore, the steps comprising pivotably mounting a walking beam for oscillation in a generally vertical plane, interconnecting one end of the walking beam with sucker rod means in the well bore, mounting a balance mass for movement on the walking beam, operating motor means for moving the balance mass back and forth on the beam, monitoring the angular position of the beam, and controlling operation of the motor means in response to the angular position of the beam for establishing the pumping stroke and frequency of the reciprocating pump assembly.

20. The method of Claim 19 wherein the angular position of the beam is monitored by sensing its angular relation to a supporting structure for the pivotably mounted beam.

21. The method of Claim 19 wherein the motor means is mounted on the beam.

22. The method of Claim 19 wherein operation of the motor means is controlled by minimizing stress on the sucker rod means during operation of the pump assembly.

23. The method of Claim 22 wherein stress on the sucker rod is minimized by means of a delay interval when the direction of the sucker rod means changes.

24. The method of Claim 19 further comprising the step of monitoring the position of the balance mass on the walking beam and controlling operation of the motor means in response both to the angular position of the beam and the position of the balance mass on the beam.

25. The method of Claim 19 further comprising the step of interconnecting the opposite end of the walking beam with sucker rod mens in a second well bore, the balance mass being adapted to offset load imbalances between the two well bores while establishing the pumping stroke and frequency.

26. The method of Claim 19 wherein the one end of the walking beam is interconnected with sucker rod means in the well bore through a stroke extender assembly including multiplier means for adjusting the effective stroke length of the sucker rod relative to the effective stroke length of the one end of the walking beam.

27. The method of Claim 26 wherein the mechanical multiplier means of the stroke extender assembly includes cable means trained over a movable pulley and fixed idler means for operatively interconnecting the one end of the walking beam with the sucker rod means.