Patent Publication Number: US-2015071794-A1

Title: Self-aligning, fluid-driven pumping unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. Provisional Application No. 61/875,561, filed Sep. 9, 2013, which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present disclosure relates to an improved type of oilfield pumping unit used to reciprocate a down-hole rod pump. Historically, conventional oilfield pumping units used a “walking beam” pivoting on a ground-based support frame. The beam is rocked back and forth by a crank arm connected to a rotary gear-drive. The beam is typically connected to the rod string via a wire rope, and the reciprocating motion of the beam provides the reciprocal lifting and lowering of the rod string. These pumping units were relatively large, massive structures, necessitated by the overhung lifting loads. 
     More recently, hydraulically powered pumping units have become increasingly popular. In one configuration, a linear motion system employs a single hydraulic cylinder centered above or below the polished rod. Often, a mast is used to suspend a single hydraulic cylinder above the polished rod. The polished rod is directly coupled to the cylinder, and hydraulic pressure applied to the ring side of the piston causes the cylinder to retract, thus lifting the rods. The mast can be attached to the ground, the casing, or the tubing. Other configurations have employed an offset cylinder with one or more pulley wheels are attached to the rod-end of the cylinder. A wire rope passes across the pulley wheel such that one rope end is attached to the rod string and the other end is dead-lined. As the cylinder extends, the wire rope transmits a lifting motion to the rod string. 
     SUMMARY 
     Described herein is a compact artificial lift hydraulic pumping system providing simplified installation, longer life and ease of maintenance. A beam is pivotally mounted to a wellhead, ground, or other anchor structure, and one or more hydraulic cylinders are asymmetrically mounted with respect to the beam. The mounting is designed to pivot and slide about the pivot so as to provide self-aligning lift forces to the down-hole pump rod string. The self-aligning nature of the pump system eliminates side loads on both the hydraulic cylinder and polished rod. 
     In some embodiments, the system comprises a beam asymmetrical and parallel to the polished rod. The base of the beam is pivotally or rigidly attached to the wellhead, tubinghead or casing and one or more hydraulic cylinders are mounted parallel but axially offset from the center of the pivoting beam. In some embodiments, the end of the hydraulic cylinder is attached to a traveling assembly that comprises a traveling head that rolls along a track formed by the profile of the beam. The polished rod is attached to the traveling head such that the reciprocating movement of the hydraulic cylinders is thus coupled to cause a similar movement of the rod string. In one embodiment, the traveling assembly comprises a rotating member, such as a sprocket or sheave, connected to the rod end of the hydraulic cylinder. The traveling assembly further includes a flexible linkage, such as a chain, cable, wire, or rope passing over the rotating member and providing a lifting force to the polished rod. 
     Some embodiments require no elevated mast to suspend a hydraulic cylinder above the polished rod. The entire pumping unit may be assembled at ground-level, and then pivotally erected into place. This can be done without the use of overhead lifting equipment such as a crane or boom truck. Some embodiments may be configured to lift the rods when hydraulic pressure is applied to cap end of the hydraulic cylinder and the cylinder is extending. In the prior art utilizing a single cylinder suspended above the well, the ring side of the cylinder must be used to lift the rods on the up-stroke. Since the ring side of the cylinder piston has less surface area than the cap side, this configuration requires higher hydraulic pressure to develop the force required to lift the rods. 
     In some embodiments, it may be desirable to suspend the hydraulic cylinder above the polished rod, using the retracting cylinder force to lift the rods from the well. Here again, the unit may be assembled at ground level and pivotally erected into place. 
     The embodiments described herein also have a number of inherent advantages over traditional hydraulic pumping units configured with wire rope sheaves. In the prior art, the wire rope often become a high wear item due to the physical limitations of the size of the sheave. While conventional “walking beam” pumping units are configured with a wire-rope bending radius of 70 inches or more, the prior art hydraulic units are typically configured with rope sheaves with a bending radius of 12 inches or less. This small diameter bending radius severely reduces the life of the wire rope, thus increasing maintenance cost, and increasing HSE risk due to frequency of wire rope failures. In contrast, to the rigidly mounted cylinders provided in the prior art, the embodiments described herein allow use of larger radius sheaves due to the freely pivoting nature of the cylinder attachment to the well structure. In such a case, the angle of the cylinder with respect to the polished rod would infinitely change as the cylinder passes between the retracted and extended position. 
     Other advantages are present with respect to multiple symmetrically arranged hydraulic cylinders. Some hydraulic pumping units require two or more specially designed hydraulic cylinders rigidly attached between upper and lower mounting plates, the illustrative embodiments may utilize a single cylinder configuration. Fewer cylinders would inherently reflect lower initial cost, as well as lower future maintenance cost. Further still, regardless of the number of hydraulic cylinders, these embodiments may utilize low-cost, commodity type hydraulic cylinder configured with pin and clevis end connections. These cylinders are less expensive than custom-manufactured cylinders with a ridged mounting base arrangement required in the prior art. In some embodiments, the cylinders may be rigidly attached to a vertical mounting beam at multiple points. Thus able to are better able to handle column loading than the base-mounted cylinders found in the prior art. 
     Still other advantages are present with respect to rigid mounted cylinder configurations, whether that be single or multiple cylinder configurations. Rigid mounting of the cylinders with respect to the polished rod requires a high degree of accuracy in manufacturing and installation so as to cause the lifting force to be aligned parallel and congruent with the polished rod. Yet even with such care, there may still be some slight misalignment. Such misalignment of rigidly mounted cylinders will inherently cause side loads and wear between the polished rod and the stuffing box, and between the hydraulic cylinder rod and the cylinder rod bushings. The gimbaled mounting presented in this illustrative embodiment overcome these deficiencies by providing pivoting and sliding degrees of freedom with respect to the wellhead mounting pins, thus allowing the lifting force to be always transmitted parallel to the polished rod. 
     The illustrative embodiments also overcome problems associated with low pressure wellhead fixtures. In contrast to the bolted flanged face tubing holder utilized in high pressure wellheads, many low pressure wellheads utilize hammer union pack-off assemblies, thus lacking any flat faced surfaces to which a base plate or cylinder mounting assembly could be bolted or fastened. The illustrative embodiments overcome that problem by utilizing existing, symmetrically opposed wellhead piping ports as a means of attachment. 
     Hydraulic pumping units require a directional shifting valve to cyclically change the flow of oil and the directional motion of the hydraulic cylinders. An electric limit switch that senses the end of each stroke of the hydraulic cylinder may be used to shift a solenoid operated hydraulic valve from one position to another. In an improved system, mechanical controls linked to the movement of the hydraulic cylinder travel may be utilized to shift a valve spool from either the “up” or “down” position to the other. A spring detent mechanism is required to snap the valve from one position to the other so as to prevent the hydraulic directional valve from being stuck in a center position where no oil flows to the cylinder, preventing completion of the stroke. Unfortunately, sudden reversal of direction of the cylinder caused by the snap action of a spring detent mechanism on the valve can cause tensile or buckling fatigue and failure of the down-hole rod string, in addition to accelerated wear from the shock loads on the hydraulic system. Soft shifting valve configurations can be used to reduce the acceleration and deceleration forces at the beginning and end of each stroke. 
     Hydraulic systems typically have an overall energy efficiency of between 75%-85%. The loss of motive energy is translated into heating of the hydraulic fluid. This heat must be removed in order to prolong the life of the hydraulic components. Fan-powered heat exchangers may be used to cool the hydraulic fluid. In certain applications, particularly in shallow well pumping applications, the volume of fluid pumped from the well may be sufficient to dissipate this excess heat. In this situation, a tube and shell heat exchanger may use the fluid pumped from the well as a medium to dissipate the heat from the hydraulic fluid. A temperature switch in the production fluid downstream of the heat exchanger could be used as a “pump-off” controller. Production fluid pumped from the well is normally at a constant temperature. As such, a higher than normal temperature reading after the heat exchanger would signal that the volume rate of the production fluid from the well is decreasing, thus indicating the well is approaching a pumped-off condition. This may trigger a control circuit to set the pumping unit to an idle or off state. Alternatively, a similar temperature monitoring circuit could monitor the hydraulic fluid temperature for the same purpose. In either case, a reset timer would be used to re-start the pump cycle at a pre-determined interval. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following figures are included to illustrate certain aspects of the present disclosure, and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, without departing from the scope of this disclosure. 
         FIG. 1  illustrates a profile view of a hydraulic pumping unit in a vertical operating position according to an illustrative embodiment, a traveling head assembly of the pump unit being near a top of the stroke; 
         FIG. 2A  illustrates a profile view of the pumping unit of  FIG. 1  in an assembly or well maintenance position; 
         FIG. 2B  illustrates a top view of a mounting base attached to a wellhead according to an illustrative embodiment; 
         FIGS. 3A ,  3 B and  3 C illustrate front, partial front and enlarged, and top views of a traveling head assembly according to an illustrative embodiment; 
         FIGS. 4A and 4B  illustrate profile and side views of a pumping unit utilizing a roller chain to lift rods according to an illustrative embodiments; 
         FIGS. 5A and 5B  illustrate a profile view of a pumping unit utilizing a mechanical spring and counter balance arrangements to reduce the supplied energy to lift the rods according to an illustrative embodiment; 
         FIGS. 6A and 6B  illustrate profile views of a hydraulic pumping unit in an operating position according to an illustrative embodiment, the hydraulic pumping unit of  FIG. 6A  having a larger diameter sheave or sprocket than that of  FIG. 6B ; 
         FIGS. 7A and 7B  illustrate profile and top views a shifting system for a hydraulic pumping unit; 
         FIG. 8  illustrates an enlarged exploded view of the shifting system of  FIG. 7A ; 
         FIG. 9A  illustrates an enlarge profile view of a plurality of stationary components associated with the shifting system of  FIG. 7A ; 
         FIG. 9B  illustrates a top view of a mounting frame used to mount the shifting system of  FIG. 7A  to a beam; 
         FIG. 9C  illustrates an enlarged profile view of the shifting system of  FIG. 7A ; and 
         FIG. 10  illustrates a schematic of a hydraulic circuit associated with the shifting system of  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     In the following detailed description of the illustrative embodiments, reference is made to the accompanying drawings that form a part hereof. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is understood that other embodiments may be utilized and that logical structural, mechanical, electrical, and chemical changes may be made without departing from the spirit or scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the embodiments described herein, the description may omit certain information known to those skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the illustrative embodiments is defined only by the appended claims. 
     Unless otherwise specified, any use of any form of the terms “connect,” “engage,” “couple,” “attach,” or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the elements and may also include indirect interaction between the elements described. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to”. Unless otherwise indicated, as used throughout this document, “or” does not require mutual exclusivity. 
     As used herein, the phrases “hydraulically coupled,” “hydraulically connected,” “in hydraulic communication,” “fluidly coupled,” “fluidly connected,” and “in fluid communication” refer to a form of coupling, connection, or communication related to fluids, and the corresponding flows or pressures associated with these fluids. In some embodiments, a hydraulic coupling, connection, or communication between two components describes components that are associated in such a way that fluid pressure may be transmitted between or among the components. Reference to a fluid coupling, connection, or communication between two components describes components that are associated in such a way that a fluid can flow between or among the components. Hydraulically coupled, connected, or communicating components may include certain arrangements where fluid does not flow between the components, but fluid pressure may nonetheless be transmitted such as via a diaphragm or piston. 
       FIG. 1  shows one embodiment of the hydraulic pumping unit  10  configured in the operating position. The base  11  is pivotally attached to a rigid part of the well structure  12 . Shown here, the base is pivoting on lugs  13  that are installed in unused ports of the wellhead  14 . These lugs  13  typically screw into wellheads  14  with 2 inch pipe threads. These threads can easily support the reactive force generated by the hydraulic pumping unit in most shallow well applications. Symmetrically opposed ports in the tubing head  9  may be used as well. The hydraulic pumping unit  10  may also be supported by symmetrically opposed pipe nipples carrying flow products from the well. For added strength, these nipples can be thick-wall Schedule  80  or stronger pipe. Alternatively, similar types of lugs can be welded or fastened to the well casing or a ground mounted structure so as to provide an alternate support to the pivoting base. The mounting lugs may be positioned close to the ground so as to facilitate easy access for installation and maintenance. 
     An upright beam  15  is rigidly attached to the base  11 . In the operating position, the beam is horizontally offset, but approximately parallel with the polished rod  16 . Typically, a standard 6 or 8 inch wide-flange I-beam would be used for most shallow well applications. In some embodiments, the beam  15  merely acts as a guide for the traveling head assembly  17  and is not subject to column loading. Light strength material such as aluminum or fiberglass composite can be used instead of steel. 
     One or more hydraulic cylinders  18  are connected between the base  11  and the head assembly  17 . The length of the hydraulic cylinder  18  determines the stroke of the pumping unit. The polished rod load determines the required diameter of the hydraulic cylinder and the system pressure. In some long stroke applications, limitations on the buckling strength of the hydraulic cylinder rod  19  may be design criteria. Typically, a single 4 inch diameter hydraulic cylinder would meet the design criteria for most shallow well applications. Alternately, multiple cylinders can be symmetrically arranged about the beam  15  to provide additional lift capacity. 
       FIG. 2A  shows a configuration of the hydraulic pumping unit in the out-of-service position. When well-work is required, the head assembly  17  may be detached from the polished rod  16 , and the pumping unit  10  allowed to pivot out of the way of any down-hole operations. Initial assembly and future periodic maintenance of the pumping unit would similarly occur in this lay-down position as well. The unit would then be rotated back into the operating position using either man-power, or a simple winching device. 
       FIG. 2B  shows the top view of the base  11  pivoting on lugs  13 . Pivot bushing  19  can be of any suitable bearing material, such as brass or steel. Alternatively, other configurations may employ roller or ball bearings to reduce wear. While the base  11  is designed to pivot about one axis, a small amount of lateral movement perpendicular to the pivot axis may be provided so as to also accommodate small misalignment in that axis as well. The movement of the base  11  about the non-pivoting axis may be through sliding of the pivot bushing  19  along the pipe lugs  13 , or through a gimbaled or linear bearing assembly. As such, the entire pumping unit has multiple degrees of freedom. 
       FIG. 3A  shows the profile view of the head assembly of one configuration of the pumping unit. This head assembly  17  is used to transmit the force of the hydraulic cylinder rod  19  to the polished rod  16 . The head  17  assembly travels along a track formed by the beam  15 . Typically, rollers  20  would be used to reduce friction as the head moves up and down along the beam  15 . The rollers  20  can be flanged so as to keep aligned with the edge of the beam  15 . Alternate configurations may include rollers that roll against both front and back flanges of beam  15 . The rollers  20  can be steel, aluminum, polyurethane, nylon, or any other suitable material. To minimize maintenance cost, replaceable rollers are preferably designed to wear ahead of the beam  15 . 
       FIG. 3B  shows the detail of the attachment of the polished rod  16  to the head assembly  17 . In one configuration, a spherical bearing  21  is used between the rod clamp  22  and the base plate  23  to allow for any misalignment in the travel of the head assembly  17 . The base plate  23  is configured with a bearing holder  24  such that bearings  21  may be easily replaced if necessary. Alternate head bearing assembly configurations may also be employed to similarly provide freedom of movement in either one or two axis. The use of this head bearing assembly, in combination with the pivot and slid bearing of the base, provide the geometry necessary to eliminate side loads while lifting the polished rod  16 . 
       FIG. 3C  illustrates the top view of the head assembly  17 . In the illustrated configuration, normally the weight of the down-hole rod string acting on the polished rod  16  keeps the rollers in contact with the beam  15 . At times when downward force is not present, such as during installation or when the down hole pump becomes “stacked-out”, guide  25  and back-plate  26  serve to keep the head assembly  17  from rotating beyond the flange face of the rollers  27 . Other roller or guide configurations could provide similar function to keep the head assembly  17  aligned with the beam  15  at all times. 
       FIG. 4A  shows an alternate configuration to the track mounted traveling head arrangement previously described. Here, one or more hydraulic cylinders  28  are rigidly attached to the beam  15 . Freely turning sprockets  29  or sheaves are attached to the traveling portion of the cylinder  28 . Because the entire hydraulic pumping unit  30  can pivot in relation to the well structure, the diameter of the sheave or sprocket  29  has no bearing on the angle of force applied to the polished rod  16 . In one embodiment, roller chain  31  is connected to the bridle  32  to lift the polished rod  16 . The other end of the roller chain is “dead-lined”  33  and fastened to the beam  15 . 
       FIG. 4B  reflects the front view of sprocket or sheave head assembly. The vertical spacing of the rod clamp  34  to the polished rod  16  is achieved through adjusting the point of attachment of the cylinder mounting bracket  8  to the beam  15 , and through changing the length of the roller chain  31  attached to the bridle  32 . In the event of a failure of a single roller chain or wire rope, load sensing circuits can be used to shut off the supply of hydraulic fluid so as to prevent asymmetrical forces to the pumping unit. 
       FIG. 5A  illustrates the counter-balancing of the off-center mass of the pumping unit. For safety reasons, it is important to provide some restraint of the pumping unit  34 , in the event the polished rod  16  fails, or some other instance that could allow the pumping unit to unexpectedly fall away from the upright position. A spring  35  may be used to counteract and balance the overhung load of the pumping unit about the pivot point  37 . This counter-balance force may also be in the form of a counter weight  36  on the opposite side of the pivot point  37 . Alternately, if minimal pivot movement is desired, a ridged or semi ridged attachment may be employed in place of the spring  35  to restrain the hydraulic pumping unit  34  to the well structure  12 . 
     Energy efficient hydraulic circuits may be employed with the hydraulic pumping units described herein. A charge of compressed gas such as nitrogen may be used in the form of an accumulator to balance the dead weight of the rod string. Another form of energy savings applicable in this and other hydraulic pumping units is to use one or more coil springs  38  to serve the same balancing function as the hydraulic accumulator. In one embodiment, the springs would be sized such that the average hydraulic energy necessary to lift the polished rod  16  on the upstroke is identical, but opposite that required to retract the pump on the down stroke. For example, if the polished rod load is 10,000 lbs. during the upstroke, and the dead weight of the rod string in the well is 5,000 lbs., the springs  38  would be sized to provide an average lifting force of 7,500 lbs. This simple example ignores the difference in force developed on the ring side and cap side of the hydraulic cylinder. In practice, the ideal mechanical spring configuration would attempt to make the hydraulic horsepower equal on both the up stroke and the down stroke. 
       FIG. 5B  illustrates how multiple springs  38  could be installed to provide a lifting force to the head assembly  17  in order to more evenly distribute the duty cycle of the hydraulic system. Springs may be stacked, or concentrically arranged to provide the desired force. Center core  39  provides lateral support for the springs. Die springs are able to provide the millions of cycles necessary for this type of application. 
       FIG. 6A  illustrates the significant pivot motion of a hydraulic pumping unit throughout the stroke cycle when using a large diameter sheave or sprocket. Large diameter sheaves provide longer life for wire rope.  FIG. 6B  illustrates a hydraulic pumping unit with a sheave or sprocket sized to minimize pivot motion. 
     In contrast to the prior art, a simplified shifting and control system is now presented. As illustrated in  FIG. 7A , a control rod  40  is attached through linkage  50  to the head assembly  47  so as to be indexed with the travel of hydraulic cylinder  28 . The control rod  40  is used to transmit the mechanical motion necessary to shift the directional control valve  55  from one position to the other. Adjustable position stops  44  attached to control rod  40 , acting on shift springs  43 , are used to set the actual point of directional change in relation to the stroke of cylinder  28 . In the up-stroke valve position, the pressurized hydraulic line  57  delivering fluid from the hydraulic pump  58  and is fluidly coupled to the lower end of the hydraulic cylinder  28 , causing the cylinder to extend. In the down-stroke valve position, the lower end of the hydraulic cylinder  28  is fluidly connected to the hydraulic fluid return path  59 , thus returning hydraulic fluid to the reservoir tank and allowing the cylinder to retract. The speed of the up-stroke cylinder movement is dependent on the volume rate of the hydraulic pump  58 . Independently, the speed of the falling cylinder may be controlled with a needle valve  60 , an over-balance valve, or other hydraulic control mechanisms. 
       FIG. 7B  illustrates a top view through cross section AA of  FIG. 7A . As illustrated here, control rod  40  is connected to head assembly  47  through linkage  50 . The control rod  40  may be positioned to run along any point of beam  15 , here illustrated as traveling within the flanges of an “I” beam. 
       FIG. 8  illustrates an exploded view of one embodiment showing various components of a spring detent snap-action shift mechanism. As illustrated, control rod  40  travels up and down through shift tube  62 , which in turn is free to travel within guide tube  63 . As the travel of control rod  40  begins to approach the point representing the end of the hydraulic cylinder stroke, control rod stop  44  begins to force the shift spring  43  against the shift tube  62 . Shift tube  62  in turn applies force, either directly or indirectly to valve spool  41 . To minimize wear on the valve spool resulting from the millions of cycles of repeated shifting, and in recognition of potential slight miss-alignment of the shifting components, a non-ridged coupling of the shifting tube  62  to the valve spool  41  may be preferred. 
     Valve spool  41  is held in place by spring detent assembly  42 , illustrated here in prospective view showing only one half of the symmetrically arranged roller bearings that will seat in either the upper detent seat or lower detent seat, each seat corresponding to a respective valve position. The travel of control rod  40  causes shift spring  43  to be compressed until such time there is sufficient force to suddenly unseat detent mechanism. Once unseated, compressed shift spring  43  begins to force valve spool  41  from the present position to the other. Instead of snapping quickly to the alternate valve position, velocity control devices  45 , coupled to valve spool  41 , dampens the stored energy of the shifting spring  43 , thus allowing the valve spool to travel slowly and smoothly from one position to the other. Velocity control device  45  may be an adjustable, variable orifice shock absorber, commercially available to control the velocity of an object being acted upon with an applied force. In a preferred embodiment, the amount of time involved in the shifting process is between 1-5 seconds. 
       FIG. 9A  illustrates one embodiment of a preferred mounting arrangement of the various components of the hydraulic shifting system. In this arrangement, stationary components including directional control valve  55 , guide tube  63 , and velocity control device  45  are rigidly attached to mounting frame  49 , which in turn mounts to a ridged structure of the hydraulic pump beam. As illustrated, hydraulic directional control valve  55  is mounted to the front face of mounting plate  52 , while guide tube  63  is mounted behind.  FIG. 9B  illustrates the mounting frame  49  attached to the beam  15 . Set screws  53  may be used to provide adjustable attachment of mounting frame  49 . As illustrated, mounting frame  49  is shown mounted within the flanges of an “I”-beam. Control rod  40  is illustrated passing behind mounting frame  49 .  FIG. 9C  illustrates the operation of the shift control mechanism with the addition of a control lever  54  for manual activation of directional control valve  55 . 
     While one embodiment of “soft” mechanical shifting of a hydraulic directional control valve has been described herein, it should be noted that other types of mechanical hydraulic control could provide additional beneficial results. For instance, a valve spool profile can configured so as to provide taper so as to create a variable Cv factor as it travels from one position to the next. In such a case, both valve spool position within the valve body and valve spool geometry can provide non-linear velocity control as the spool changes from one flow path to the other. 
     In yet another embodiment, two separate valves can be linked to the movement of the hydraulic cylinder stroke such that one valve provides variable flow control, and another separate valve provides directional control. For instance, as the hydraulic cylinder approaches the end of a stroke, a flow control valve begins to progressively reduce the volume rate of fluid flowing to the directional shifting valve. Based on the design of the control valve, the reduction in flow can be in direct or variable relationship to the position of the cylinder with respect to the desired reversal point. An adjustable stop on the flow control valve is used to set a minimum fluid volume rate so as to allow the cylinder to “creep” slowly to the point where direction is reversed. Similar to the function as previously described, a detent used in combination with shifting springs would be used to shift a separate directional control valve. Upon shifting direction, the control valve would progressively open from “creep” for full open position. 
       FIG. 10  illustrates one embodiment of a simplified hydraulic circuit using a minimal number of hydraulic component. Electric motor or other prime mover  69  is connected to drive hydraulic pump  58 . Relief valve  71  is provided, but provides no operational function other than for over-pressure safety issues. Two position directional control valve  55  is positioned for either extending the cylinder, or allowing the cylinder to retract. The valve is illustrated in the extend-cylinder position. In this configuration, fluid from reservoir tank  80  flows into the suction of the hydraulic pump  58 . It is noted that the reservoir tank  80  can be either open to atmosphere, or a nitrogen charged accumulator so as to assist with lifting the dead weight of the downhole rod string. In either case, hydraulic fluid is discharged from the pump  58  and flows through the by-pass of needle valve  74  into the base of hydraulic cylinder  75 , causing the cylinder to extend. For the retract-cylinder phase of the cycle, directional control valve  55  shifts to a second position allowing hydraulic fluid from the extended cylinder to return through needle valve  74 , through valve  55 , then ultimately to reservoir tank or accumulator  80 . While directional control valve  55  is in this retract position, hydraulic fluid from the pump is similarly directed to the same common return path as the fluid from the retracting hydraulic cylinder. As such, the motor  69  and pump  58  are unloaded when hydraulic pressure isn&#39;t needed, thus eliminating the need for expensive pressure compensated, variable displacement pumps, as well as eliminating repeated starting and stopping the motor pump combination. 
     The heat exchanger  76 , necessary to maintain oil temperature within operating limits, may be of tube and shell construction. In contract to conventional air cooled-radiator type oil coolers, tube-and shell oil coolers can be designed to handle the high operating pressure of the hydraulic system. In one embodiment, fluid pumped from the well may be used as the coolant. As illustrated, heat exchanger  76  is located just prior to the accumulator or tank  80 . It is noted that the heat exchanger can be located in most any flow path in the system. 
     While the embodiments described herein refer to the term “hydraulic” when describing the motive fluid used to raise and lower the cylinders, it should be noted that any type of fluid or mechanical energy could be similarly employed to achieve the same results, including pneumatic sources of energy. For example, an internal combustion engine may be located at or near the location of the pumping unit. In such a case, waste heat from the engine may be converted into steam, either alone or combined with additional input energy. This steam could be used to provide the fluid power necessary to raise and lower the pumping unit. Alternate forms of mechanical linear actuators may also be used to provide the lifting force described herein as that produced by a hydraulic cylinder. 
     It should be apparent from the foregoing that an invention having significant advantages has been provided. While the invention is shown in only a few of its forms, it is not limited to only these embodiments but is susceptible to various changes and modifications without departing from the spirit thereof.