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Title:
ROD-PUMPING SYSTEM
Document Type and Number:
WIPO Patent Application WO/2014/124525
Kind Code:
A1
Abstract:
A rod-pumping system is provided. In one embodiment, the system includes a downhole pump (24) positioned in a well (14) and coupled to a well string, such as a sucker- rod string (22). The system also includes a first hydraulic actuator (26) arranged with respect to the well string so as to enable the first hydraulic actuator to move the well string within the well. The first hydraulic actuator is connected in fluid communication with a second hydraulic actuator (28). A control pump (32) is connected to both the first and second hydraulic actuators to enable the control pump to alternate between pumping control fluid to the first hydraulic actuator to cause the well string to move in a first direction within the well and pumping control fluid to the second hydraulic actuator to cause the well string to move in an opposite, second direction within the well. Additional systems, devices, and methods are also disclosed.

Inventors:
DONOHOE, Nicholas (8235 Wagner Road, Edmonton, Alberta T6E 4N6, CA)
BASSET, Lee D. (P.O. Box, Lloydminster, Alberta T9V 2S1, 21027, CA)
Application Number:
CA2014/000116
Publication Date:
August 21, 2014
Filing Date:
February 14, 2014
Export Citation:
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Assignee:
ICI ARTIFICIAL LIFT INC. (8235 Wagner Road, Edmonton, Alberta T6E 4N6, CA)
International Classes:
F04B47/08; E21B43/12; F04B49/00; F15B15/00
Foreign References:
GB905062A1962-09-05
US2560441A1951-07-10
Attorney, Agent or Firm:
TOMKINS, Donald V. (740 - 100 Stree, Edmonton Alberta T5J 0P6, 10150, CA)
Download PDF:
Claims:
CLAIMS

1. A rod-pumping system comprising:

a downhole pump (24) positioned in a well (14);

a well string (22) coupled to the downhole pump;

a first hydraulic actuator (26) arranged with the well string so as to enable the first hydraulic actuator to move the well string within the well;

a second hydraulic actuator (28) connected in fluid communication with the first hydraulic actuator; and

a control pump (32) connected to the first hydraulic actuator and to the second hydraulic actuator in a manner that enables the control pump, during operation, to alternate between pumping control fluid to the first hydraulic actuator to cause the well string to move in a first direction within the well, and pumping control fluid to the second hydraulic actuator to cause the well string to move in an opposite, second direction within the well.

2. The rod-pumping system of Claim 1, wherein the control pump is connected to a rod end of the first hydraulic actuator.

3. The rod-pumping system of Claim 1, wherein the second hydraulic actuator is connected to at least one accumulator (136) set to counterbalance the full load of the first hydraulic actuator.

4. The rod-pumping system of Claim 3, wherein the connection of the control pump to the first and second hydraulic actuators enables the control pump to offset hydraulic balance between the first and second hydraulic actuators.

5. The rod-pumping system of Claim 1, comprising sensors (110, 112, 138) to facilitate detection of piston positions in the first and second hydraulic actuators.

6. The rod-pumping system of Claim 5, comprising a controller (42) configured to receive input from the sensors that facilitate detection of the piston positions and to synchronize the first and second hydraulic cylinders.

7. The rod-pumping system of Claim 1, comprising an auxiliary pump (34) that enables the introduction of fresh control fluid to a hydraulic circuit that includes the first hydraulic actuator and the second hydraulic actuator.

8. The rod-pumping system of Claim 1, wherein the first hydraulic actuator is a double-acting hydraulic cylinder.

9. The rod-pumping system of Claim 1, wherein the first hydraulic actuator is coupled to a wellhead (16) installed at the well.

10. The rod-pumping system of Claim 1, wherein the well string includes a sucker-rod string that is coupled to a rod clamp (72) that engages the first hydraulic actuator.

11. A method comprising:

lowering a well string (22) that is positioned in a well and is coupled to a downhole pump (24) within the well, by pumping control fluid to a first linear actuator (26); and

raising the well string by pumping control fluid to a second linear actuator (28) that is connected to the first linear actuator.

12. The method of Claim 11 , wherein pumping control fluid to the first linear actuator includes pumping control fluid to a rod end of the first linear actuator.

13. The method of Claim 11, comprising:

determining the positions of pistons within the first linear actuator and the second linear actuator; and

synchronizing the first linear actuator and the second linear actuator based on the determined positions of the pistons.

14. The method of Claim 11, comprising alternating flow of control fluid between the first linear actuator and the second linear actuator by reversing flow of a bi-directional pump (32) connected to the first linear actuator and the second linear actuator.

15. The method of Claim 11, comprising:

continually flushing control fluid from a hydraulic or pneumatic circuit including the first and second linear actuators and replacing the flushed control fluid with fresh control fluid during operation of the first and second linear actuators; and maintaining synchronization between the first and second linear actuators during operation.

Description:
ROD-PUMPING SYSTEM

BACKGROUND

[0001] This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the presently described embodiments. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present embodiments. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

[0002] In order to meet consumer and industrial demand for natural resources, companies often invest significant amounts of time and money in finding and extracting oil, natural gas, and other subterranean resources from the earth. Particularly, once a desired subterranean resource such as oil or natural gas is discovered, drilling and production systems are often employed to access and extract the resource. These systems may be located onshore or offshore depending on the location of a desired resource. Further, such systems generally include a wellhead assembly mounted on a well through which the resource is accessed or extracted. These wellhead assemblies may include a wide variety of components, such as various casings, valves, pumps, fluid conduits, and the like, that control drilling or extraction operations.

[0003] In some instances, resources accessed via wells are able to flow to the surface by themselves. This is typically the case with gas wells, as the accessed gas has a lower density than air. This can also be the case for oil wells if the pressure of the oil is sufficiently high to overcome gravity. But often accessed oil does not have sufficient pressure to flow to the surface and the oil must be lifted to the surface through one of various methods known as artificial lift. Artificial lift can also be used to raise other resources through wells to the surface, or for removing water or other liquids from gas wells. One form of artificial lift uses a pump that is placed downhole in the well and is operated by a reciprocating rod string extending through the well from the downhole pump to the surface. Such systems are commonly referred to as rod-pumping or sucker-rod systems. SUMMARY

[0004] Certain aspects of some embodiments disclosed herein are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below.

[0005] Embodiments of the present disclosure generally relate to a rod-pumping system for lifting fluids from a well. The rod-pumping system of one embodiment includes a pair of hydraulic actuators, such as hydraulic cylinders; a rod string moved by one of the hydraulic actuators and coupled to a downhole pump; and a control pump. The hydraulic actuators are connected in series, and the control pump is connected to pump fluid to one end of each hydraulic actuator such that alternating the flow direction from the control pump controls operation of the hydraulic actuators and operates the downhole pump via the rod string. In some embodiments, sensors are used to detect the positions of pistons in the actuators and a controller synchronizes the pistons to facilitate proper operation.

[0006] Various refinements of the features noted above may exist in relation to various aspects of the present embodiments. Further features may also be incorporated in these various aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. Again, the brief summary presented above is intended only to familiarize the reader with certain aspects and contexts of some embodiments without limitation to the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] These and other features, aspects, and advantages of certain embodiments will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0008] FIG. 1 generally depicts a production system having an artificial lift system to draw fluid from a well to the surface in accordance with one embodiment of the present disclosure;

[0009] FIG. 2 is a block diagram of various components of the artificial lift system of FIG. 1 in accordance with one embodiment;

[0010] FIG. 3 generally depicts a hydraulic actuator of the artificial lift system of FIG. 2 mounted on wellhead equipment in accordance with one embodiment;

[0011] FIG. 4 is a cross-section generally depicting certain components of the hydraulic actuator of FIG. 3 in accordance with one embodiment;

[0012] FIG. 5 generally depicts the hydraulic actuator of FIG. 3 with a piston rod of the actuator extended from its housing; and

[0013] FIG. 6 schematically depicts certain hydraulic components and control components of the production system of FIG. 1 in accordance with one embodiment of the present disclosure.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0014] One or more specific embodiments of the present disclosure will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

[0015] When introducing elements of various embodiments, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. The terms "comprising," "including " and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements. Moreover, any use of "top," "bottom," "above," "below," other directional terms, and variations of these terms is made for convenience, but does not require any particular orientation of the components.

[0016] Turning now to the present figures, a system 10 is illustrated in FIG. 1 in accordance with one embodiment. Notably, the system 10 is a production system that facilitates extraction of a resource, such as oil, from a reservoir 12 through a well 14. Wellhead equipment 16 is installed on the well (e.g., attached to the top of casing and tubing strings in the well). In one embodiment, the wellhead equipment 16 includes a casing head, a tubing head, and a stuffing box. But the components of the wellhead equipment 16 can differ between different applications, and such equipment could include various casing heads, tubing heads, stuffing boxes, pumping tees, and pressure gauges, to name only a few possibilities. [0017] The system 10 also includes an artificial lift system 18. In one embodiment generally depicted in FIG. 2, the artificial lift system 18 is a rod-pumping system including a sucker-rod string 22 coupled to a downhole pump 24 in the well 14. The downhole pump 24 can be of any suitable design, such as one in which the downhole pump 24 includes a stationary valve and a traveling valve (connected to the sucker-rod string 22) that cooperate to lift fluid from a reservoir to the surface. The sucker-rod string 22 extends through the well 14 and is also coupled to a linear actuator, such as slave cylinder 26 in FIG. 2. This linear actuator enables movement of the sucker-rod string 22 back-and-forth within the well 14 to operate the downhole pump 24 and raise fluid (e.g., oil, gas condensate, or water) from the reservoir 12 to the surface. In other embodiments, the sucker-rod string 22 could be replaced by some other structure (e.g., a coiled tubing string) connected to the downhole pump 24 and moved by the linear actuator to lift fluids from the reservoir 12. As used herein, the term "well string" means any device or structure that extends through a well and enables a linear actuator (e.g., slave cylinder 26) to operate a downhole pump. The term encompasses, but is not limited to, both sucker-rod string and coiled tubing.

[0018] As depicted in FIG. 2, the artificial lift system 18 includes an additional linear actuator in the form of master cylinder 28. The master cylinder 28 cooperates with the slave cylinder 26 in controlling movement of the sucker-rod string 22, as discussed in greater detail below. Although here described as cylinders 26 and 28, it will be appreciated that the linear actuators could be provided in any suitable form (including non-cylindrical shapes, for instance) and may also be referred to as rams, jacks, or the like. In some embodiments, the artificial lift system 18 is a hydraulic system with hydraulic linear actuators. But while the examples provided below refer to such a hydraulic system, it is noted that in other embodiments the artificial lift system 18 can instead be a pneumatic system with pneumatic linear actuators that are operated with a gaseous control fluid, such as compressed air.

[0019] The artificial lift system 18 depicted in FIG. 2 also includes a pumping system 30. This pumping system 30 includes a primary pump 32 and an auxiliary pump 34. As will be described in greater detail below, the primary pump 32 is connected in fluid communication with both of the hydraulic actuators (i.e., slave cylinder 26 and master cylinder 28 in the depicted embodiment) to control movement of pistons inside these actuators. Thus, primary pump 32 is also referred to herein as control pump 32. In one embodiment, the control pump 32 is a bidirectional pump capable of pumping hydraulic control fluid (e.g., hydraulic oil) back-and-forth between the slave cylinder 26 and the master cylinder 28 by reversing the flow direction. If additional control fluid is needed, the control pump 32 can draw such fluid from a fluid tank 38. The control pump 32 is driven by a prime mover 36, such as a diesel engine. But any suitable prime mover 36 could be used, such as a propane engine, a natural gas engine (which could include an engine run on casing head gas produced at the well 14), or an electric motor.

[0020] The auxiliary pump 34 is connected to provide fresh control fluid (i.e., new or reconditioned control fluid) to at least some portions of the hydraulic circuit that includes the slave cylinder 26 and the master cylinder 28. The auxiliary pump 34 can also, but need not, draw control fluid from the fluid tank 38 and be driven by the prime mover 36 independent of the control pump 32. (It is noted that taking a slip stream off of the control pump circuit to feed the auxiliary pump 34, while possible, would reduce system speed.) In one embodiment, control fluid is flushed from various portions of the hydraulic circuit (e.g., with one or more flushing valves) and replaced by fresh control fluid pumped from the fluid tank 38 by the control pump 32 and the auxiliary pump 34. The flushed control fluid can be routed through a conditioning system 40, such as through one or more filters to remove any contaminants in the control fluid and through a cooler that lowers the temperature of the control fluid to reduce wear on components of the artificial lift system 18. Control fluid reconditioned by the conditioning system 40 can be returned to the fluid tank 38 to be reused in the pumping system 30.

[0021] The pumping system 30 also includes a controller 42. The controller 42 processes inputs from various sensors 44 to control operation of other components of the pumping system 30 (e.g., the control pump 32, the auxiliary pump 34, the prime mover 36, any flushing valves, and the cooler of the conditioning system 40) and, by extension, operation of the slave cylinder 26 and the master cylinder 28. In some embodiments, the sensors 44 include proximity switches and a linear position transducer that allow the determination of positions of pistons within the slave cylinder 26 and the master cylinder 28. The sensors could also, for example, include a temperature sensor to monitor temperatures of control fluid or components in the artificial lift system 18, pressure sensors to detect hydraulic pressures at various locations within the system 18, or a level sensor to detect the amount of control fluid available in the fluid tank 38. The controller 42 in one embodiment is a programmable logic controller that is programmed to provide the control functionality described herein. But in other embodiments, the controller 42 could be any circuit-based device (with or without software) suitable for controlling operation of the artificial lift system 8, such as a processor-based device that executes instructions (firmware or software) stored in a suitable memory of the device.

[0022] The depicted pumping system 30 also includes a skid 46 to facilitate transport of various system components. For example, in one embodiment the primary pump 32, the auxiliary pump 34, the prime mover 36, the fluid tank 38, the conditioning system 40, the controller 42, and some sensors 44 are provided on the skid 46, allowing an operator to more easily move all of these components to a desired location. Other components, such as the master cylinder 28, can also be mounted on the skid 46.

[0023] As noted above, the slave cylinder 26 engages the sucker-rod string 22 to operate the downhole pump 24 and lift fluid up the well 14 to the surface. The slave cylinder 26 can be attached to wellhead equipment to receive the sucker-rod string 22 and operate the downhole pump 24. One example of such an arrangement is depicted in FIGS. 3-5. In this embodiment, the slave cylinder 26 is positioned over a polished rod 50 of the sucker-rod string 22 and is connected, via a pedestal stand 52, to a tubing head 54 of the wellhead equipment 16. Although not depicted in the present figure, the tubing head 54 can be coupled to other wellhead equipment 16, such as a casing head, above the well 14. The tubing head 54 includes a flow conduit 56 for conveying fluid lifted from the well 14, as well as a test conduit 58. While the slave cylinder 26 is depicted as being connected to the tubing head 54 by the pedestal stand 52 in FIG. 3, the slave cylinder 26 can be connected to the wellhead equipment 16 in any suitable manner (e.g., connected in other ways or to other components of the wellhead equipment 16). Indeed, in some embodiments the slave cylinder 26 is not connected to the wellhead equipment 16 at all, and is instead positioned adjacent the wellhead equipment 16. In one example of such an embodiment, a slave cylinder 26 is mounted adjacent to, but separated from, the wellhead equipment 16 and interacts with the sucker-rod string 22 via cables and a bridle.

[0024] The wellhead equipment 16 also includes a stuffing box 60 attached to the tubing head 54. As will be recognized by those knowledgeable in the art, the stuffing box 60 includes packing that allows the polished rod 50 to move up-and-down through the stuffing box 60 while inhibiting leaking. The polished rod 50 is connected to a series of sucker rods to form the sucker-rod string 22 that extends through the well 14 to the downhole pump 24. Movement of the polished rod 50 causes corresponding movement of the sucker rods to operate the downhole pump 24. In other embodiments, the stuffing box 60 is omitted and the slave cylinder 26 itself isolates wellbore fluids from the external environment.

[0025] The slave cylinder 26 includes a housing 66 in which a piston rod 68 is disposed. The piston rod 68 of the depicted embodiment is hollow (see FIG. 4) and a connecting rod 70 is disposed within the bore of the piston rod 68. The connecting rod 70 is coupled to the polished rod 50 as part of the sucker-rod string 22. A rod clamp 72 is coupled to the connecting rod 70 and is positioned at the end of the piston rod 68 to allow the sucker-rod string 22 to be suspended in the well 14. In other embodiments, the connecting rod 70 is omitted and the polished rod 50 of the sucker- rod string 22 engages the slave cylinder 26 (e.g., via a rod clamp 72 attached to the polished rod 50 itself). By applying appropriate pressure, the piston rod 68 can be extended or retracted with respect to the housing 66. For instance, in the present embodiment hydraulic control fluid can be provided into the housing 66 through a connection 78 at the cap end of slave cylinder 26 to cause the piston rod 68 to extend from the housing 66, and through a connection 80 at the rod end of the slave cylinder 26 to cause the piston rod 68 to retract into the housing 66. For convenience, the slave cylinder 26 can include a fluid conduit 82 and a connection 84 coupled to the connection 80, allowing a fluid supply hose or pipe to be connected to the more- accessible (i.e., closer to the ground) connection 84. Although not depicted in the present figure, in one embodiment the primary pump 32 is connected directly to the connection 84 via a hose or pipe and the master cylinder 28 is connected to the slave cylinder 26 via the connection 78. Also, the rod clamp 72 in some embodiments rests on the piston rod 68 such that the sucker-rod string 22 is lifted by extension of the piston rod 68 from the housing 66 but is lowered by gravity when the piston rod 68 retracts. In other embodiments, the rod clamp 72 can be attached to the piston rod 68 such that the sucker-rod string 22 is driven by the piston rod 68 in both directions. In either of these instances, movement of the piston rod 68 can be said to cause reciprocal movement of the sucker-rod string 22.

[0026] As depicted in FIG. 4, the hollow piston rod 68 may be mounted about a hollow tube 90 in the housing 66. The tube 90 provides a bearing surface for the hollow piston rod 68 and isolates the working pressures in the slave cylinder 26 from the connecting rod 70 and other components in the well 14. The piston rod 68 may be extended or retracted by manipulating pressure against the piston head 92. In the embodiment depicted in FIGS. 3—5, the slave cylinder 26 is a double-acting cylinder. Specifically, pressurized hydraulic control fluid can be directed into the chamber 96 (e.g., via connection 78) to cause the piston head 92 to move and the piston rod 68 to extend from the housing 66, as depicted in FIG. 5. Conversely, such control fluid can be directed into the chamber 98 (e.g., via connection 80) to move the piston head 92 in an opposite direction and to retract the piston rod 68 to the position depicted in FIG. 3. Although not depicted in the present figures for the sake of clarity, it will be appreciated that the slave cylinder 26 will generally include additional components, such as various seals that inhibit leakage from the housing 66 or between the chambers 96 and 98.

[0027] As noted above, the connecting rod 70 is positioned with respect to the piston rod 68 and is coupled to the sucker-rod string 22 to enable the movement of the piston rod 68 to operate the downhole pump 24. In the embodiment depicted in FIG. 4, the connecting rod 70 is coupled to the polished rod 50 of the sucker-rod string 22 with a connector 102. But the connecting rod 70 (or, in the absence of the connecting rod 70, the slave cylinder 26) could engage the polished rod 50 in any suitable manner that allows the slave cylinder 26 to operate the downhole pump 24. [0028] In at least some embodiments, the slave cylinder 26 includes one or more sensors to detect the position of the piston head 92 within the housing 66. Any suitable sensor could be used, such as a proximity switch or a linear transducer. In one embodiment depicted in FIGS. 3 and 5, sensors are provided on an external guide 108 and include proximity sensors or switches 110 and 112. As one example, the external guide 108 can be a length of pipe (e.g., PVC pipe) with proximity switches 110 and 112 mounted on its exterior. A trigger 114, such as a piece of metal in the case of inductive proximity switches, is connected to a support 116. The support 116 moves with the piston rod 68, causing the trigger 114 to move between the proximity switches 110 and 112. As the piston rod 68 extends from the housing 66 toward the position depicted in FIG. 5, the trigger 114 is drawn upwardly through the guide 108 toward proximity switch 110. Once the proximity switch 110 is activated by the trigger 114, an output signal is sent to the controller 42 to cause the piston rod 68 to begin to retract back into the housing 66 (toward the position depicted in FIG. 3). While the piston rod 68 retracts, the trigger 114 travels through the guide 108 toward the proximity switch 112. Once this switch 112 is activated, an output signal is sent to the controller 42 to cause the piston rod 68 to reverse direction again. In this manner, the proximity switches 110 and 112 facilitate reciprocal motion of the piston rod 68 between the extended and retracted positions and of the sucker-rod string 22 within the well 14. In another embodiment, the position of the master cylinder 28 (e.g., as determined by a linear position transducer 138 (FIG. 6)) is used to control operation of the system and output from the switches 110 and 112 can be compared to the detected position of the master cylinder 28 to verify proper operation of the system.

[0029] Certain operational aspects of the rod-pumping system described above may be better understood with reference to FIG. 6, which is a schematic diagram generally depicting hydraulic connections between certain hydraulic components of the system 18 and control of these hydraulic components by the controller 42 in accordance with one embodiment. In this diagram, the master cylinder 28 includes a floating piston 120 having a piston rod between piston heads 122 and 124. As noted above with respect to the slave cylinder 26, it will be appreciated that the master cylinder 28 will generally include other components that are not depicted in the present figure, including seals that inhibit leaking from, or between different portions of, the cylinder 28. The piston rod of piston 120 is positioned through a wall or bulkhead 126 of the master cylinder 28 with the piston heads 122 and 124 disposed on opposite sides of the bulkhead. This generally divides the interior of the master cylinder 28 into four isolated chambers, referred to herein as chambers 128, 130, 132, and 134.

[0030] The master cylinder 28 is arranged in series with the slave cylinder 26, with the chamber 128 connected in fluid communication with the chamber 96 on the cap end of the slave cylinder 26. Primary pump 32 is connected in fluid communication with both the chamber 130 and the chamber 98 on the rod end of the slave cylinder 26. Pressuri2ed hydraulic fluid in the chambers 128 and 130 may be manipulated to act on the piston head 122 and move the piston 120 within the master cylinder 28. The master cylinder 28 in FIG. 6 is assisted by one or more accumulators 136 connected to the chamber 134. As will be appreciated, the accumulators 136 are energy storage devices that apply pressure to hydraulic fluid in the accumulators. Any suitable accumulators 136 may be used, but in at least some embodiments the accumulators 136 are compressed gas accumulators (e.g., nitrogen accumulators). Pressure stored in the accumulators 136 is transmitted via hydraulic fluid in the chamber 134 to the piston head 124. Chamber 132 in FIG. 6 is connected in fluid communication with the fluid tank 38 so as to not inhibit movement of the piston 120 in response to hydraulic pressures within chambers 128, 130, and 134. Specifically, chamber 132 is connected to draw fluid from the fluid tank 38 when chamber 132 expands and to expel the fluid back to the fluid tank 38 when the chamber 132 contracts.

[0031] The system depicted in FIG. 6 is a proportional, intelligent, closed-loop hydraulic system in which feedback from various sensors (e.g., proximity switches 110 and 112, a position transducer 138, and other sensors 144) is used by the controller 42 to adjust operation of the system. During operation, primary pump 32 alternates pumping of pressurized control fluid between the chamber 130 of the master cylinder 28 and chamber 98 at the rod end of the slave cylinder 26. Particularly, pumping of the control fluid into the chamber 130 by the primary pump 32 causes the piston 120 to move to the left in FIG. 6, and movement in this direction is assisted by the pressure stored in the accumulators 136 (transmitted via fluid in chamber 134). Movement of the piston 120 in this manner reduces the volume of chamber 128, causing pressurized control fluid to flow from chamber 128 toward chamber 96 at the cap end of the slave cylinder 26. This in turn causes the piston head 92 to move toward the rod end of the slave cylinder 26 and extends the piston rod 68 from the housing 66 of the slave cylinder 26 to raise the sucker-rod string 22 in the well. Such motion can be generally referred to as an upstroke. Conversely, the primary pump 32 can pump fluid into the chamber 98 of the rod end of the slave cylinder 26 to move the piston head 92 away from the rod end and retract the piston rod 68. This movement causes the sucker- rod string to be lowered further into the well and may be referred to as a downstroke. The movement of the piston head 92 during the downstroke causes control fluid to flow from the chamber 96 toward the chamber 128, causing the master cylinder piston 120 to move to the right in FIG. 6. This also causes pressurized fluid from chamber 134 to be pushed into the accumulators 136, thereby storing energy for the next upstroke.

[0032] In one embodiment, the one or more accumulators 136 are set (e.g., pre- charged) to counterbalance the full load of the slave cylinder 26 during operation. That is, the force on the master cylinder piston 120 caused by the one or more accumulators 136 meets or exceeds the load on the slave cylinder piston due to gravity (which includes loading by the components borne by the piston rod 68, such as the sucker-rod string 22 and any portion of the downhole pump 24 connected to move with the sucker-rod string 22). This fully counterbalanced arrangement is in contrast to systems in which the load on the slave cylinder is only partially counterbalanced. In such partially counterbalanced systems, upstrokes rely on pressure from control pumps to provide sufficient force to overcome gravitational loading on pistons of slave cylinders coupled to a rod string and the systems rely on gravity to retract the slave cylinder piston and push the master cylinder piston back toward accumulators. But having the primary pump 32 control one side of the slave cylinder 26, as described above, reduces or eliminates the reliance on gravity to retract the slave cylinder piston, allowing the counterbalance pressure from the accumulators 136 to be set at or above the load on the slave cylinder piston. [0033] In one embodiment, the biasing force from the accumulators 136 balances the full load on the piston of slave cylinder 26, and the primary pump 32 is used to offset this hydraulic balance between the cylinders to provide directional control and speed control of the piston rod 68 (and, by extension, of the sucker-rod string 22). As one line from the primary pump 32 is connected to the rod end of the slave cylinder 26 (rather than the pump having direct control over the reciprocating of the master cylinder 28 by being direcdy connected to both sides of its piston), the pistons of the two cylinders move in consort based on differential pressures. Such an arrangement allows the use of a differential cylinder (e.g., slave cylinder 26) in a closed-loop system without venting to atmosphere. This is in contrast to other arrangements that rely on gravity to reset the piston of a slave cylinder and in which the rod end of the slave cylinder is vented to atmosphere so as to not inhibit movement of the piston. By not venting the rod end of the slave cylinder 26 to atmosphere, the present arrangement avoids large pressure differentials across seals provided on piston head 92 to isolate the chambers 96 and 98 (such pressure differentials could contribute to premature seal failure) and reduces the likelihood of rust and contamination of the rod end components of the slave cylinder 26, like piston rod 68.

[0034] In at least some embodiments, the master cylinder 28 is constructed proportionally to the slave cylinder 26 to have larger piston heads and a shorter stroke. The connection of the primary pump 32 to both the slave cylinder 26 and the master cylinder 28 in the manner depicted in FIG. 6 also allows the cylinder pistons to be controlled by applying pressure to less surface area on the piston heads of the cylinder pistons. Particularly, the piston rod 68 may be retracted and the master cylinder piston 120 pushed toward the accumulators by pumping fluid into chamber 98 to act on the effective area of the piston head 92 about the piston rod 68 rather than pumping fluid into chamber 128 to act on the greater surface area of the piston head 122. By controlling operation via a smaller piston head area, the present arrangement increases system efficiency and allows the use of a smaller control pump 32, smaller flow from the control pump 32, and less horsepower to achieve a given amount of lift. [0035] During normal operation of the hydraulic system depicted in FIG. 6, the pistons of the slave cylinder 26 and the master cylinder 28 are synchronized. That is, movement of the piston 120 to the left in FIG. 6 causes the piston head 92 (and its piston rod 68) to move up in the slave cylinder 26 with a desired stroke length, and movement of the piston head 92 down causes the piston 120 to return to the right. But this synchronous movement relies on proper amounts of control fluid in the system, and particularly within the portion of the hydraulic circuit between the piston heads 92 and 122 (including chambers 96 and 128). If too little control fluid is provided in this portion of the hydraulic circuit compared to the rest of the circuit, pressure from the chambers 98 and 130 (as well as the slave cylinder load and the accumulator pressure) will cause the piston heads 92 and 122 to be too close together. And if too much control fluid is provided in this portion, the pressure in the chambers 96 and 128 will cause the piston heads 92 and 122 to be too far apart. In either of these cases, one of the piston heads 92 or 122 would bottom out (i.e., the piston would reach the end of its stroke) before the other has moved a desired amount. In such a condition, the slave cylinder 26 and the master cylinder 28 can be considered to be out-of-sync.

[0036] To facilitate proper operation, the controller 42 receives inputs from various sensors and controls the pumping system components to synchronize the cylinders 26 and 28. Sensors (e.g., proximity switches 110 and 112, and linear position transducer 138) facilitate determination of the positions of the cylinder pistons. Based on this information, the controller 42 can synchronize the cylinders 26 and 28. More specifically, at startup, the controller 42 determines whether the cylinders are synchronized. If they are not, the controller 42 automatically synchronizes the cylinders before starting normal operation of the system. That is, the controller 42 operates the pumps 32 and 34, the flushing valves, or both to vary the amount of fluid in the various chambers and, consequently, to adjust the distance between the piston heads 92 and 122 in the circuit such that the pistons of both cylinders 26 and 28 can travel their intended stroke lengths during operation. As one example, such synchronization can be achieved by pumping control fluid with the primary pump 32 into chamber 98 to retract the piston rod 68 and then varying the amount of fluid in chamber 128 (to properly space the piston head 122 with respect to the piston head 92) by either pumping additional fluid into the chamber 128 with the auxiliary pump 34 or by flushing fluid from chamber 128. Such synchronization of the cylinders 26 and 28 can also be based on other inputs, such as feedback from other sensors 144 (e.g., pressure sensors in the hydraulic circuit, temperature sensors, and a level sensor in the fluid tank 38 (FIG. 2)). The controller 42 could also control other components of the system, such as the prime mover 36.

[0037] In some embodiments, the controller 42 provides additional control functionality. For instance, the controller 42 can vary operation of the hydraulic system based on temperature detected by one or more temperature sensors. On a cold startup, the controller 42 may operate the system at a reduced speed (e.g., operate the primary pump at a set percentage of maximum flow) until a desired system temperature is achieved. This may reduce cavitation of the pumps and damage to seals and filter elements in the system. And during operation, the controller 42 can cause operation of the system to be slowed if the detected temperature exceeds a first threshold temperature and stopped if the detected temperature exceeds a second, higher threshold. It is noted that continued operation at a reduced speed allows hydraulic fluid to be passed through a cooler of the conditioning system 40, which may allow the system to cool faster than if the system were simply stopped.

[0038] Additionally, the controller 42 may provide an emergency alarm or shut-off function to stop undesirable operation of the system, which may include motion of the piston 120 outside of a desired range. By way of example, the master cylinder 28 and the slave cylinder 26 may be configured such that the maximum volume of the chamber 128 exceeds that of chamber 96 to which it is hydraulically connected. Movement of the piston head 122 to the left in FIG. 6 causes control fluid to flow from the chamber 128 to the chamber 96. But once chamber 96 is filled to its maximum volume (which may be determined by the controller 42 based on proximity switch 110 detecting, via trigger 114 (FIG. 5), that piston head 92 is at the end of the slave cylinder 26), additional movement of the piston head 122 in the same direction could damage the cylinders (e.g., by causing pressures within these chambers to exceed the rated maximum of the cylinders). In at least some embodiments, the controller 42 can operate a valve to flush fluid from the portion of the hydraulic circuit including chambers 96 and 128 if it detects (e.g., via a pressure sensor) that the pressure in that portion of the hydraulic circuit is too high.

[0039] Further, in some embodiments the controller 42 monitors the position of the piston 120 (via transducer 138) to facilitate synchronization and ensure the piston 120 is not traveling too close to the end of the master cylinder 28 at chamber 128. The controller 42 can be programmed with one or more threshold distances based on the allowable maximum travel of the piston 120. For example, if the distance from the end of the master cylinder 28 and the piston head 122 falls below a first, set threshold the controller 42 can trigger an alarm to alert an operator that the piston 120 is moving to a distance from the end of the cylinder that is near a minimum allowable distance. In addition to or instead of triggering an alarm, the controller 42 can automatically activate the auxiliary pump 34 to pump additional control fluid into the chamber 128 to try to push the piston 120 to the right in FIG. 6 and synchronize the system such that the detected distance between the piston head 122 and the end of the master cylinder 28 is maintained above the first threshold. In the event the controller 42 is unable to synchronize the system and the distance from the end of the master cylinder 28 and the piston head 122 falls below a second, set threshold (e.g., set to the minimum allowable distance), the controller 42 can deactivate the system to allow servicing.

[0040] Closed-loop hydraulic systems typically have sections of dead fluid (i.e., control fluid that cannot be removed from a hydraulic circuit for cooling or filtration). Additionally, synchronization between master and slave cylinders is maintained by trying to minimize fluid loss across seals of the system, and dead fluid resulting from trying to minimize fluid losses can lead to damage as the fluid degrades or is contaminated. But in some embodiments of the present technique, and as generally noted above, control fluid can be continually flushed from the different portions of the hydraulic circuit and replaced by fresh control fluid. The flushed fluid can be conditioned (e.g., filtered and cooled) via conditioning system 40 and returned to the hydraulic circuit or to the fluid tank 38. During such flushing and refilling, the controller 42 monitors the positions of the pistons of the cylinders 26 and 28 and can automatically keep or put the system back in synchronization by controlling the operation of the primary pump 32 and the auxiliary pump 34 to replace the flushed control fluid with a corresponding amount of fresh control fluid. In some embodiments, the auxiliary pump 34 operates independently from the primary pump 32 and pulls control fluid directly from the fluid tank 38, which has fluid that will be generally cooler and cleaner than fluid that could be drawn from the primary pump 32. For instance, in the embodiment depicted in FIG. 6, control fluid pumped between chamber 98 and chamber 130 to operate the cylinders 26 and 28 can be flushed from the hydraulic circuit and replaced with fluid drawn from the fluid tank 38 by the primary pump 32. Further, control fluid used in the portion of the circuit including chambers 96 and 128 can be flushed and replaced with fluid drawn from the tank 38 by the auxiliary pump 34. Control fluid in chamber 134 and the accumulators 136 can also be flushed and replaced with fluid drawn from the tank 38 by the auxiliary pump 34.

[0041] While the aspects of the present disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. But it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.