AU2010256864B2

AU2010256864B2 - Hydraulic oilfield lift pump

Info

Publication number: AU2010256864B2
Application number: AU2010256864A
Authority: AU
Inventors: Denis Blaquiere; Gerry L. Lorimer
Original assignee: National Oilwell Varco LP
Current assignee: National Oilwell Varco LP
Priority date: 2009-06-02
Filing date: 2010-06-01
Publication date: 2015-01-22
Anticipated expiration: 2030-06-01
Also published as: US20100300679A1; WO2010141405A2; EP2438264A2; WO2010141405A4; AU2010256864A1; MX2011012819A; WO2010141405A3; CA2763162C; CA2763162A1

Abstract

A vertically orientated hydraulically driven oilfield lift pump unit having a simplified hydraulic system. The system uses a pair of single acting hydraulic cylinders in tandem with a pair of double acting hydraulic cylinders interacting with hydraulic accumulators to provide an energy efficient, robust hydraulic system design with a minimum number of components.

Description

HYDRAULIC OILFIELD LIFT PUMP This application claims priority from United States provisional patent application 5 Serial No. 61/183,264, filed on June 2, 2009, which is incorporated herein by reference. BACKGROUND OF THE INVENTION 1. Field of the Invention. 0 This invention relates to lift pumps suitable for producing hydrocarbons and other liquids from the earth. In particular, this invention is drawn to a very long stroke hydraulically operated lift pump using a combination of single and double acting hydraulic cylinders that are actuated with an energy efficient hydraulic fluid accumulator system. 5 2. Description of the Related Art. Hydraulically operated Oilfield lift pumping units are well known and have been used in the industry for many years. Typically, these units mimic the non-hydraulic mechanical .0 'walking beam' design similar to the pumping unit 10 illustrated in Figure 1. These conventional units are more commonly driven directly by electric motors through gear boxes, but many are also hydraulically driven as shown for example in U.S. Patents 4,201,115; 4,198,820; 3,405,605. These hydraulic units are typically configured similarly to the motor driven units, but have hydraulic cylinders fitted in place of the linkages and 25 gearing of the mechanical system. Limitations of the 'walking beam' design include limited stroke variability, the need for a rotating counter weight of roughly the same weight as the sucker rods and other devices suspended in the borehole, and a stroke limited by the length of the 'walking beam' and/or 30 the height above the ground of the mast upon which the beam is mounted. Nonetheless, these 'walking beam' pumps are ubiquitous in the industry and are readily visible from many major thoroughfares, particularly in Texas and western Canada. 6022806_1 (GHMatters) P97376.AU MCAMP Vertical pumping units, as shown for example in U.S. Patents 4,761,120; 4,512,149; 4,698,968; 4,762,473, typically use hydraulic cylinders aligned vertically which typically connect directly to the sucker rod string and are controlled by complex hydraulic systems. These vertical systems have attempted, and often times succeeded in overcoming many of 5 the above mentioned limitations. However, these units tend to have very complex hydraulic systems because the mechanical design requires the hydraulic system to perform many different functions. Furthermore, these units also tend to have very uneven power cycles, causing a very cyclic loading on the prime mover, and requiring it to have a much higher horsepower rating to power through the peak loads, than the overall average 0 horsepower consumed. BRIEF SUMMARY OF THE INVENTION Described herein is a new type of hydraulically driven oilfield lift pump unit which features 5 a simplified hydraulic system. This system comprises a hydraulic power section and a wellhead mounted lifting section. The hydraulic power section is typically, but not limited to, a flow controlled, pressure compensated hydraulic pump system that can readily be optimally sized to meet the desired lifting speed and weight parameters of the lift section and an electrically controlled 4-way type variable displacement control valve, or other .0 types of suitable control valves. These types of 'power units' are well known and are in common use in the industry. The lifting section comprises a plurality (typically a pair) of single acting hydraulic cylinders, and a plurality (typically a pair) of double acting hydraulic cylinders. The 25 cylinders are mechanically mounted on common end plates such that they all must extend and retract simultaneously. The cap ends of the double acting cylinders are 'closed-coupled' to one or more hydraulic accumulators so that in normal operation all the flow into and out of the cap ends of the 30 cylinders is into or out of the hydraulic accumulators. The accumulators are pre-charged with gas at a pressure such that when the cylinders are fully extended and the accumulators nearly depleted of hydraulic fluid, the double acting cylinders are nonetheless able to statically hold a high percentage, of the lifting load. -2 6022806_1 (GHMatters) P97376.AU MCAMP This may also effectively balance the pump motor's load between the upstroke and the downstroke cycles, and may permit use of a prime mover motor or engine to run under a relatively constant load (compared to conventional pumps) regardless of whether in both the upstroke mode and the downstroke mode. Typical load variations may be in the range 5 of 70% to 130% of the average load, but as will be described, these variations may be reduced even more than this, if desired. When compared to other pump units of this type, the load balancing effect of the accumulators allows the use of a motor with a significantly less horsepower rating (or capacity) than the prior art units. Furthermore, the lower peak power consumption may allow for a lower effective electric rate, as in many locations the 0 electric rate is based on, or at least affected, by the peak load instead of the average load. During the "up" stroke, pressurized fluid from the hydraulic power unit is applied only to the 'cap' ends of the single acting cylinders, causing all of the cylinders to extend together. The pair of single acting hydraulic cylinders can only be powered in the extended direction. 5 The pressure required to operate the single acting cylinders will steadily increase as the cylinders extend because the lifting capability of the double acting cylinders decreases as the accumulators gas volume expands. In the preferred embodiment, the load on the down stroke may typically be about two-thirds of the load on the up stroke. This difference is caused by a transfer of fluid column weight onto the down hole lifting valves. .0 During the 'down' stroke, the pressurized fluid from the hydraulic power unit is applied only to the 'rod' ends of the double acting hydraulic cylinders. The combination of the weight of the load and this hydraulic pressure on the top end of the cylinder causes all the cylinders to retract. This in turn forces the hydraulic fluid on the cap ends of the double 25 acting cylinders back into the accumulators - and therefore filling them. The relative horsepower required for each of the 'extend' and 'retract' cycles may be changed and/or more nearly equalized by adjusting the pre-charge pressure of the accumulators to account for variations in the average lifting load, permitting usage of the smallest possible engine or motor for the hydraulics. Furthermore, this may also help reduce large load fluctuations, 30 reducing stress on the system. However, because in some instances the actual loading may vary over time, an intermediate pre-charge pressure may be chosen. In one aspect there is disclosed a reciprocating oil well pumping unit comprising: a base plate; -3 6022806_1 (GHMatters) P97376.AU MCAMP an end plate spaced apart from the base plate; a single acting hydraulic piston-cylinder assembly extending from the base plate to the end plate; wherein the single acting hydraulic piston-cylinder assembly includes a first hydraulic cylinder, a first piston movably disposed in the first 5 hydraulic cylinder, and a first chamber in the first hydraulic cylinder; a double acting hydraulic piston-cylinder assembly extending from the base plate to the end plate; wherein the double acting hydraulic piston-cylinder assembly includes a second hydraulic cylinder and a second piston movably disposed in the second hydraulic cylinder; wherein the second piston defines a second 0 chamber and a third chamber in the second hydraulic cylinder; a hydraulic accumulator in closed-coupled fluid communication with the second chamber of the second hydraulic cylinder; a hydraulic pump; and a hydraulic control valve coupled to the hydraulic pump, the hydraulic control valve 5 having a first position placing the hydraulic pump in fluid communication with the first chamber and having a second position placing the hydraulic pump in fluid communication with the third chamber; wherein the double acting hydraulic piston-cylinder assembly and the single acting hydraulic piston-cylinder assembly are configured to extend and retract .0 simultaneously to operate the pumping unit. In a second aspect there is disclosed a vertically oriented, hydraulically driven oil well pumping unit comprising: a base plate; 25 an end plate positioned above the base plate; a single acting hydraulic piston-cylinder assembly having: a first hydraulic cylinder, a first piston movably disposed in the first hydraulic cylinder, a first rod coupled to the first piston and extending through a first rod end of 30 the first hydraulic cylinder, and a first chamber at a first cap end of the first hydraulic cylinder; wherein the single acting hydraulic piston-cylinder assembly couples to the base plate at the first cap end and couples to the end plate by the first rod; -4 6022806_1 (GHMatters) P97376.AU MCAMP a double acting hydraulic piston-cylinder assembly having a second hydraulic cylinder, a second piston movably disposed in the second hydraulic cylinder, and a second rod coupled to the second piston and extending through a second rod 5 end of the second hydraulic cylinder; wherein the second piston separates a second chamber at a second cap end of the second cylinder from a third chamber at the second rod end of the second cylinder; wherein the double acting hydraulic piston-cylinder assembly couples to the 0 base plate at the second cap end and couples to the end plate by the second rod; a hydraulic accumulator in closed-coupled fluid communication with the second chamber of the second hydraulic cylinder; a source of pressurized hydraulic fluid; 5 a hydraulic control valve coupled to the source of pressurized hydraulic fluid, the valve having a first position placing the source of pressurized hydraulic fluid in fluid communication with the first chamber and having a second position placing the source of pressurized hydraulic fluid in fluid communication with the third chamber; and 0 wherein the double acting hydraulic piston-cylinder assembly and the single acting hydraulic piston-cylinder assembly are configured to extend and retract simultaneously to operate the pumping unit. In a third aspect there is disclosed a method for operating an oil well pumping unit, the 25 method comprising, (a) supplying pressurized fluid from an accumulator to a first chamber in a cylinder of a double acting hydraulic piston-cylinder assembly; wherein the cylinder also includes a second chamber separated from the first chamber by a piston movably disposed in the cylinder; 30 wherein the double acting hydraulic piston-cylinder assembly extends from a base plate to an end plate; (b) supplying pressurized fluid to a chamber of a cylinder of a single acting hydraulic piston-cylinder assembly; -5 6022806_1 (GHMatters) P97376.AU MCAMP wherein the single acting hydraulic piston-cylinder assembly extends from the base plate to the end plate; (c) moving the piston in the cylinder of the double acting hydraulic piston-cylinder assembly; 5 (d) moving a piston in the cylinder of the single acting hydraulic piston-cylinder assembly; and (e) lifting a sucker rod and fluid in a borehole during the method steps of (c) and (d). BRIEF DESCRIPTION OF THE DRAWINGS 0 Figure 1 is side view of a typical 'walking beam' oilfield lift pump of the prior art. Figure 2 is an overall perspective view of the oilfield lift pump of the present invention. 5 Figure 3 is a perspective view of just the mast section of lifting section of the oilfield lift pump of the present invention. Figure 4 is a hydraulic schematic diagram of the hydraulic power unit of the present invention. 0 Figure 5 is a portion of the hydraulic diagram of the lifting portion of the lifting portion of the oilfield pump of the present invention showing the hydraulic flow arrangement for the "up stroke" portion of the pumping cycle. 25 Figure 6 is a portion of the hydraulic diagram of the lifting portion of the oilfield pump of the present invention showing the hydraulic flow arrangement for the "down stroke" portion of the pumping cycle. 30 -6 6022806_1 (GHMatters) P97376.AU MCAMP DETAILED DESCRIPTION OF THE INVENTION Referring now to figures 2 and 3; the hydraulically driven surface oil well pumping unit 12 of the present invention comprises a base portion 14, including a hydraulic power unit (not 5 shown) and a mast portion 16 comprising a pair of single acting hydraulic cylinders 18 and a pair of double acting hydraulic cylinders 20. Figure 2 shows the hydraulic cylinders 18, 20 of the hydraulically driven oil field pumping unit 12 fully extended, and figure 3 shows the mast portion 16 separated from the rest of the oilfield pumping unit 12 of figure 2 with the hydraulic cylinders 18, 20 fully retracted. The new oil well pumping unit 12 of the 0 present invention is useful in reciprocating a "load" in the wellbore; particularly a load which is substantially higher as the load is being raised, than when the load is being lowered - as is typically the case when pumping oil in an oil well. The cap ends 22a and 22b of the hydraulic cylinders 18, 20 are shown mounted to a 5 common rigid base plate 24. The rod ends 26a, 26b extending from the hydraulic cylinders 18, 20 are shown mounted to a common rigid end plate 28. It is anticipated that in operation the hydraulic cylinders 18, 20 will typically oriented with their cap ends 22a and 22b secured to the base plate 24, but it is also possible to mount them such that the cap ends 22a and 22b are mounted to the end plate 28, and use tension members to 'suspend' .0 the load. The lifting mechanism is then suspended in a framework (not shown) using an intermediate support structure. It is preferred, however, to mount the cylinders 18, 20 with the cap ends 22a and 22b secured to the base plate 24 as shown, as this allows the forces from the 'load' to pass in compression directly from the cap ends 22a and 22b to the base plate 24. A frame with sufficient strength - or a separate base - may be needed to support 25 the mast. Referring now to figure 4, the hydraulic schematic 40 of the hydraulic power unit (shown generally as 30) of the hydraulically driven oil field pumping unit 12 is shown in figure 4 and is typical for pressure compensated power units. The hydraulic power unit 30 typically 30 comprises but is not limited to one or more pumps 42, 44 powered by one or more engine 46 (or alternatively, motors). Hydraulic oil contained in a reservoir 48 is pumped into a control valve 50 which may be a proportional-volume controlled pressure balanced valve as illustrated, or may be a solenoid valve that is essentially open or closed. -7 6022806_1 (GHMatters) P97376.AU MCAMP The valve 50 is configured to reversibly provide pressurized hydraulic fluid to either of line 52a or 52b with the other line being the return. The operation of this valve 50 controls the extension and retraction of the hydraulic cylinders 18, 20. This hydraulic configuration may be comprised of, but not limited to a closed loop, solenoid operated hydraulic 5 pumping unit. The mast 16 lifting section comprises a pair of single acting hydraulic cylinders 18, and a pair of double acting hydraulic cylinders 20. As shown in Figs. 5 and 6, the cylinders 18, 20 are mechanically mounted on common end plates 24, 28 such that they all must extend 0 and retract simultaneously. The cap ends 54, 56 of the double acting cylinders 20 are 'closed-coupled' to one or more hydraulic accumulators 58, 60 so that in normal operation all the flow into and out of the cap ends 54, 56 of the cylinders flows into or out of the hydraulic accumulators 58, 60. 5 The accumulators 58, 60 are pre-charged with an inert gas to a pressure depending upon the weight of the load such that when the double acting cylinders 20 are fully extended they are able to hold a high percentage of the lifting load. As illustrated in Figure 5, during the "up" stroke, pressurized fluid from the hydraulic .0 power unit 30 is applied only to the cap ends 66, 68 of the single acting cylinders 18 through line 52a, causing all four of the cylinders 18, 20 to extend. The pair of single acting hydraulic cylinders 18 can only be powered in the extended direction. The pressure required to operate the single acting cylinders 18 will steadily increase as the 25 cylinders continue to extend because the load capability of the double acting cylinders 20 decreases as pressure drops in the accumulators 58, 60 as they empty their hydraulic fluid. The hydraulic pressure required to move the load is typically higher than the pressure required to 'hold' the load. So, even at the end of their strokes, the reduced pressure available from the accumulators 58, 60 maybe adequate to keep the cylinders firmly and 30 fully extended as the cycle reverses and the cylinders 18, 20 begin to retract. This 'overcharging' of the accumulators solves a known problem in the prior art hydraulic lift pumps, which have been known to 'drop' occasionally as the cylinders reverse and begin to retract. 'Drops' like this, if repeated frequently, may cause fatigue, and perhaps even failure within the members comprising the "load". -8 6022806_1 (GHMatters) P97376.AU MCAMP As illustrated in Figure 6, during the 'down' stroke, the pressurized fluid from the hydraulic power unit 30 is applied only to the 'rod' ends 62, 64 of the double acting hydraulic cylinders 20 through line 52b. The combination of the weight of the load and 5 this hydraulic pressure on the top end of the cylinder causes the cylinders to retract. This in turn forces the hydraulic fluid on the cap ends 54, 56 of the double acting cylinders 20 back into the accumulators - and therefore recharging them, and completing the cycle. In operation, the pre-charge pressures of the accumulators 58, 60 are 'tuned' so that in 0 conjunction with the weight of the load, the horsepower to raise and lower the load is substantially the same throughout the entire extend/retract cycle. This allows optimal sizing of the engine(s) 46 (or motors) driving the hydraulic pumps 42, 44; and may improve the overall useful lifetime of the oil well pumping unit 12, by reducing the accompanying cyclic fatigue. In some selected embodiments the lowering load may be set 5 within a range of about 60% to 95% of the raising load. In other selected embodiments it may be preferable to limit the lowering load to no lower than about 70% of the raising load. In still other selected embodiments, it may be desirable to maintain the lowering load in a range of 75% to 8 5 % of the raising load. 0 In some selected embodiments, this ratio of lowering load to raising load may be further adjusted by varying the pre-charge pressures among the accumulators 58, 60. In this embodiment three or four or more than four accumulations with varying pre-charge pressures may be useful. 25 In other selected embodiments the ratio of lowering load to raising load may be adjusted by varying the distribution of swept volumes among the accumulators. In still other selected embodiments the ratio of lowering load to raising load may be adjusted by using different volume capacities among the accumulators. 30 For those instances where precise tuning and 'leveling' of the horsepower is important, accumulators 58, 60 may be arranged with both different pre-charge pressures, and with different volume capacities, which may 'tuning' even more precise. -9 6022806_1 (GHMatters) P97376.AU MCAMP Although the system described herein is disclosed as having two single acting hydraulic cylinders 18 and two double acting hydraulic cylinders 20, it would be apparent to those skilled in the art that the system only requires one or more of each of the single 18 and/or double 20 acting cylinders to operate in the manner described. The second cylinders 18, 20 5 allow for a more compact 'footprint' and provides for greater mechanical stability to the structure. Whereas the present invention has been described in particular relation to the drawings attached hereto, it should be understood that other and further modifications apart from 0 those shown or suggested herein, may be made within the scope and spirit of the present invention. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word 5 "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. It is to be understood that, if any prior art publication is referred to herein, such reference 0 does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. - 10 6022806_1 (GHMatters) P97376.AU MCAMP

Claims

2. The reciprocating oil well pumping unit of claim 1 wherein the single acting hydraulic piston-cylinder assembly and the double acting hydraulic piston-cylinder assembly are 30 vertically oriented; and wherein the hydraulic accumulator includes a fluid having a pre-charged pressure configured to supply a pressure that develops an upward force exceeding the downward force of a load that includes a sucker rod string when the double - 11 6022806_1 (GHMatters) P97376.AU MCAMP acting hydraulic piston-cylinder assembly is fully extended in use and under the load.
3. The reciprocating oil well pumping unit of claim 1 or 2 further comprising an 5 intermediate plate disposed between the base plate and the end plate; wherein the intermediate plate is secured to the first hydraulic cylinder and the second hydraulic cylinder.
4. The reciprocating oil well pumping unit of any one of the preceding claims wherein the pumping unit further comprises a pressure compensated hydraulic pump. 0
5. The reciprocating oil well pumping unit of any one of the preceding claims wherein the first chamber is positioned between a cap end of the first hydraulic cylinder and the first piston; wherein the second chamber is positioned between a cap end of the second hydraulic 5 cylinder and the second piston; and wherein the cap end of first hydraulic cylinder and cap end of the second hydraulic cylinder are mounted to the base plate.
6. The reciprocating oil well pumping unit of any one of the preceding claims wherein a 0 first rod of the single acting hydraulic piston-cylinder assembly has a piston end and a coupling end, and a second rod of the double acting hydraulic piston-cylinder assembly has a piston end and a coupling end; and wherein the coupling end of the first rod and the coupling end of the second rod are mounted to the end plate. 25
7. The reciprocating oil well pumping unit of any one of the preceding claims wherein the hydraulic accumulator is pre-charged to a pressure configured to hold the load in place with the hydraulic pump not operating. 30 8. The reciprocating oil well pumping unit of any one of the preceding claims wherein when the hydraulic control valve is in the first position and the hydraulic pump is configured to apply hydraulic pressure to the first chamber to extend the single acting hydraulic piston cylinder assembly, the accumulator is configured to supply pressurized fluid to the second - 12 6022806_1 (GHMatters) P97376.AU MCAMP chamber of the double acting cylinder assembly to extend the double acting hydraulic piston cylinder assembly.
9. A vertically oriented, hydraulically driven oil well pumping unit comprising: 5 a base plate; an end plate positioned above the base plate; a single acting hydraulic piston-cylinder assembly having: a first hydraulic cylinder, a first piston movably disposed in the first hydraulic cylinder, 0 a first rod coupled to the first piston and extending through a first rod end of the first hydraulic cylinder, and a first chamber at a first cap end of the first hydraulic cylinder; wherein the single acting hydraulic piston-cylinder assembly couples to the base plate at the first cap end and couples to the end plate by the first 5 rod; a double acting hydraulic piston-cylinder assembly having a second hydraulic cylinder, a second piston movably disposed in the second hydraulic cylinder, and a second rod coupled to the second piston and extending through a second rod .0 end of the second hydraulic cylinder; wherein the second piston separates a second chamber at a second cap end of the second cylinder from a third chamber at the second rod end of the second cylinder; wherein the double acting hydraulic piston-cylinder assembly couples to the 25 base plate at the second cap end and couples to the end plate by the second rod; a hydraulic accumulator in closed-coupled fluid communication with the second chamber of the second hydraulic cylinder; a source of pressurized hydraulic fluid; 30 a hydraulic control valve coupled to the source of pressurized hydraulic fluid, the valve having a first position placing the source of pressurized hydraulic fluid in fluid communication with the first chamber and having a second position placing the source of pressurized hydraulic fluid in fluid communication with the third chamber; and - 13 6022806_1 (GHMatters) P97376.AU MCAMP wherein the double acting hydraulic piston-cylinder assembly and the single acting hydraulic piston-cylinder assembly are configured to extend and retract simultaneously to operate the pumping unit. 5 10. The vertically oriented, hydraulically driven oil well pumping unit of claim 9 wherein the hydraulic accumulator includes a gas having an adjustable pre-charge pressure capable of inducing an upward force on the second piston exceeding the downward force of the load when the double acting hydraulic piston-cylinder assembly is fully extended. 0 11. The vertically oriented, hydraulically driven oil well pumping unit of claim 9 further comprising: a plurality of double acting hydraulic piston-cylinder assemblies and a plurality of single acting hydraulic piston-cylinder assemblies; wherein each double acting hydraulic piston-cylinder assembly and each single acting 5 hydraulic piston-cylinder assembly is directly adjacent two other hydraulic piston-cylinder assemblies; wherein the two other hydraulic piston-cylinder assemblies are selected from: the plurality of double acting hydraulic piston-cylinder assemblies and the plurality of single acting hydraulic piston-cylinder assemblies. .0
12. The vertically oriented, hydraulically driven oil well pumping unit of claim 9 further comprising a plurality of hydraulic accumulators in closed-coupled fluid communication with the second chamber; wherein at least a first hydraulic accumulator has a different 25 pre-charge pressure than at least a second hydraulic accumulator.
13. The vertically oriented, hydraulically driven oil well pumping unit of claim 9 further comprising a plurality of hydraulic accumulators in closed-coupled fluid communication with the 30 second chamber; wherein a first hydraulic accumulator has a different swept volume than a second hydraulic accumulator.
14. The vertically oriented, hydraulically driven oil well pumping unit of any one of claims 9 - 13 further comprising an intermediate plate vertically positioned between the base - 14 6022806_1 (GHMatters) P97376.AU MCAMP plate and the end plate, wherein the intermediate plate is fixably coupled to the first hydraulic cylinder at the first rod end and fixably coupled to the second hydraulic cylinder at the second rod end; wherein the end plate comprises a coupling for a sucker rod string. 5
15. The vertically oriented, hydraulically driven oil well pumping unit of any one of claims 9 - 14 wherein the hydraulic control valve is a proportional-volume controlled pressure balanced valve. 0 16. The vertically oriented, hydraulically driven oil well pumping unit of any one of claims 9 - 15 wherein the source of pressurized hydraulic fluid comprises a pressure compensated hydraulic power unit having a pump.
17. The vertically oriented, hydraulically driven oil well pumping unit of claim 9 wherein 5 when the hydraulic control valve is in the first position and the source of pressurized hydraulic fluid is configured to apply hydraulic pressure to the first chamber to extend the single acting hydraulic piston-cylinder assembly, the accumulator is configured to supply pressurized fluid to the second chamber of the double acting cylinder assembly to extend the double acting hydraulic piston-cylinder assembly. .0
18. The vertically oriented, hydraulically driven oil well pumping unit of any one of claims 9 - 17 wherein a lowering load is 60% to 95% of a raising load.
19. A method for operating an oil well pumping unit, the method comprising, 25 (a) supplying pressurized fluid from an accumulator to a first chamber in a cylinder of a double acting hydraulic piston-cylinder assembly; wherein the cylinder also includes a second chamber separated from the first chamber by a piston movably disposed in the cylinder; wherein the double acting hydraulic piston-cylinder assembly extends from a 30 base plate to an end plate; (b) supplying pressurized fluid to a chamber of a cylinder of a single acting hydraulic piston-cylinder assembly; wherein the single acting hydraulic piston-cylinder assembly extends from the base plate to the end plate; - 15 6022806_1 (GHMatters) P97376.AU MCAMP (c) moving the piston in the cylinder of the double acting hydraulic piston-cylinder assembly; (d) moving a piston in the cylinder of the single acting hydraulic piston-cylinder assembly; and 5 (e) lifting a sucker rod and fluid in a borehole during the method steps of (c) and (d).
20. The method of claim 19 further including (f) supplying pressurized fluid to the second chamber of the double acting hydraulic piston-cylinder assembly; and 0 (g) Returning fluid from the first chamber of the double acting hydraulic piston cylinder assembly to the accumulator, thereby re-pressurizing the accumulator, during the method step of (f).
21. The method of claim 20 further including 5 (h) Pre-charging the accumulator to a pressure that generates an upward force on the piston of the double acting hydraulic piston-cylinder assembly exceeding the downward force of the load when the double acting hydraulic cylinder is fully extended in use and under load. 0 22. The method of any one of claims 19 - 21 wherein (b) and (f) comprise supplying the pressurized fluid with a pressure compensated hydraulic power unit having a pump. - 16 6022806_1 (GHMatters) P97376.AU MCAMP