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A pump jack system for vertically reciprocating a downhole pump in an oil and gas well. The pump jack system includes a cylinder assembly having a drive cylinder and two balance cylinders. A piston in the drive cylinder may be used to provide an upstroke of a sucker rod string of a downhole pump. Th

A pump jack system for vertically reciprocating a downhole pump in an oil and gas well. The pump jack system includes a cylinder assembly having a drive cylinder and two balance cylinders. A piston in the drive cylinder may be used to provide an upstroke of a sucker rod string of a downhole pump. The pistons in the balance cylinders may be used to provide a downstroke of the sucker rod string. Lower chambers in the balance cylinders counterbalance the lifting and lowering of the piston in the drive cylinder. The pump jack system also includes an accumulator for maintaining a relative constant fluid pressure in the lower chambers of the balance cylinders.

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1. A pump jack system for vertically reciprocating a downhole pump, the system comprising: a drive cylinder including a drive barrel, a drive piston disposed within the drive barrel, and a drive rod with a lower end affixed to the drive piston, wherein the drive piston includes a fluid passage provi

1. A pump jack system for vertically reciprocating a downhole pump, the system comprising: a drive cylinder including a drive barrel, a drive piston disposed within the drive barrel, and a drive rod with a lower end affixed to the drive piston, wherein the drive piston includes a fluid passage providing fluid communication across the drive piston within the drive barrel to create a single drive chamber within the drive barrel, wherein the drive cylinder includes only a single fluid port in fluid communication with the single drive chamber;two balance cylinders positioned on either side of the drive cylinder, each balance cylinder including a balance barrel, a balance piston disposed within the balance barrel, and a balance rod with a lower end affixed to the balance piston, wherein an upper balance chamber and a lower balance chamber within each balance barrel are separated by each balance piston, wherein the lower balance chambers are in fluid communication;an upper cross member operatively connecting an upper end of the drive barrel and an upper end of each of the balance barrels;a lower cross member operatively connecting a lower end of the drive barrel and a lower end of each of the balance barrels; anda rod cross member operatively connecting an upper end of the drive rod and an upper end of each of the balance rods in a fixed configuration, wherein the rod cross member is configured for lifting the balance pistons upon upward movement of the drive piston when a fluid is pumped into the drive chamber through the single fluid port and for lowering the drive piston upon downward movement of the balance pistons when a fluid is pumped into the upper balance chambers. 2. The pump jack system of claim 1, wherein the rod cross member is configured for lowering the drive piston upon downward movement of the balance pistons. 3. The pump jack system of claim 1, wherein each of the two balance pistons includes an upper surface area, and wherein a lower surface area of the drive piston is approximately equal to the sum of the upper surface areas of the two balance pistons. 4. The pump jack system of claim 1, wherein the upper balance chambers are in fluid communication. 5. The pump jack system of claim 1, further comprising a drive seal and two balance seals, wherein the drive seal is disposed within an upper end of the drive barrel and around the drive rod for sealing the drive chamber, and wherein each balance seal is disposed within an upper end of the balance barrel and around the balance rod for sealing the balance chamber. 6. The pump jack system of claim 1, further comprising an axial bore through the drive piston and through the entire length of the drive rod, wherein the lower cross member includes an aperture in fluid communication with the axial bore, and wherein the axial bore and the aperture are dimensioned to receive a sucker rod string of a downhole pump. 7. The pump jack system of claim 6, further comprising a sealing tubular disposed through the axial bore, wherein a lower end of the sealing tubular is affixed to the lower cross member and the upper end of the sealing tubular is disposed within the axial bore in all positions of the drive piston to provide a fluid seal separating the axial bore from the drive chamber. 8. The pump jack system of claim 1, further comprising a sheave assembly including a sheave rotatably connected to the rod cross member, a wire rope disposed around a portion of a circumferential surface of the sheave with a first end of the wire rope anchored to the lower cross member, and a carrier member attached to a second end of the wire rope, wherein the carrier member is configured to engage a sucker rod string of the downhole pump. 9. The pump jack system of claim 8, further comprising a frame assembly for positioning the lower cross member a horizontal distance from a wellhead, the frame assembly including two or more vertical support members and a horizontal support member operatively interconnecting the vertical support members and the wellhead. 10. The pump jack system of claim 9, wherein the horizontal support member includes a transport assembly having a crank and a screw member for moving the lower cross member toward or away from the wellhead. 11. The pump jack system of claim 1, further comprising an accumulator in fluid communication with the lower balance chambers, wherein the accumulator is configured to maintain a relative constant fluid pressure in the lower balance chambers as the balance pistons are lifted and lowered. 12. The pump jack system of claim 11, wherein the accumulator includes a bladder accumulator. 13. The pump jack system of claim 11, wherein the accumulator includes an accumulator cylinder having an accumulator barrel, an accumulator piston disposed within the accumulator barrel, wherein an upper balance chamber and a lower balance chamber within the accumulator barrel are separated by the accumulator piston, wherein the lower accumulator chamber is in fluid communication with the lower balance chambers, and wherein the upper accumulator chamber is in fluid communication with a supply unit. 14. The pump jack system of claim 13, wherein an upper end of the accumulator barrel is operatively connected to the upper cross member and a lower end of the accumulator barrel is operatively connected to the lower cross member. 15. The pump jack system of claim 13, wherein the supply unit includes an N2 gas bottle. 16. The pump jack system of claim 13, further comprising a leak detector operatively connected to the lower accumulator chamber, wherein the leak detector is configured to detect leaks from the lower accumulator chamber or the lower balance chambers. 17. The pump jack system of claim 16, wherein the leak detector includes a ram member biased by a spring member, the ram member configured to generate an alert when the accumulator piston contacts an upper end of the ram member. 18. The pump jack system of claim 17, wherein the leak detector further includes: a base member disposed within the lower accumulator chamber along the lower cross member, the base member having an aperture and an upper radial shoulder, the aperture housing a portion of the ram member and the spring member, wherein the spring member biases a radial extension of the ram member in an upward direction;wherein in a neutral position the radial extension of the ram member engages the upper radial shoulder of the base member, an upper end of the ram member extends beyond the upper radial shoulder into the lower accumulator chamber, and a lower end of the ram member is disposed within an aperture through the lower cross member; andwherein in an alert position the lower end of the ram member extends beyond a lower surface of the lower cross member. 19. The pump jack system of claim 1, further comprising one or more tie rods interconnecting the upper cross member and the lower cross member. 20. A method of reciprocating a downhole pump in a wellbore, comprising the steps of: a) providing a pump jack system comprising: a drive cylinder including a drive barrel, a drive piston disposed within the drive barrel, and a drive rod with a lower end affixed to the drive piston, wherein the drive piston includes a fluid passage providing fluid communication across the drive piston within the drive barrel to create a single drive chamber within the drive barrel, and wherein an axial bore extends through the drive piston and through the entire length of the drive rod; two balance cylinders positioned on either side of the drive cylinder, each balance cylinder including a balance barrel, a balance piston disposed within the balance barrel, and a balance rod with a lower end affixed to the balance piston, wherein an upper balance chamber and a lower balance chamber within each balance barrel are separated by each balance piston, wherein the lower balance chambers are in fluid communication; an upper cross member operatively connecting an upper end of the drive barrel and an upper end of each of the balance barrels; a lower cross member operatively connecting a lower end of the drive barrel and a lower end of each of the balance barrels, wherein the lower cross member includes an aperture in fluid communication with the axial bore through the drive rod and drive piston; a rod cross member operatively connecting an upper end of the drive rod and an upper end of each of the balance rods in a fixed configuration, wherein the rod cross member is configured for lifting the balance pistons upon upward movement of the drive piston, and wherein the rod cross member is configured for lowering the drive piston upon downward movement of the balance pistons;b) positioning the pump jack system at a wellbore site such that the drive cylinder is disposed above the wellbore with the axial bore of the drive piston and drive rod in fluid communication with a wellhead;c) running a sucker rod string of a downhole pump through the wellhead and through the aperture of the lower cross member and the axial bore of the drive piston and drive rod, and attaching an upper end of the sucker rod string to the rod cross member with a clamp;d) lifting the sucker rod string by pumping a first fluid into the drive chamber thereby lifting the drive piston, the rod cross member, and the balance pistons;e) counterbalancing the lifting of the drive piston by drawing a second fluid into the lower balance chambers as the balance pistons are lifted;f) lowering the sucker rod string by pumping a third fluid into the upper balance chambers thereby lowering the balance pistons, the rod cross member, and the drive piston. 21. The method of claim 20, wherein the pump jack system further comprises an accumulator in fluid communication with the lower balance chambers, and wherein step (e) further comprises drawing the second fluid from the accumulator into the lower balance chambers to maintain a relative constant fluid pressure in the lower balance chambers as the balance pistons are lifted. 22. The method of claim 21, further comprising the steps of: g) counterbalancing the lowering of the rod cross member by collecting the second fluid from the lower balance chambers in the accumulator as the balance pistons are lowered. 23. The method of claim 20, wherein the first fluid, the second fluid, and the third fluid consist of a hydraulic fluid. 24. A method of reciprocating a downhole pump in a wellbore, comprising the steps of: a) providing a pump jack system comprising: a drive cylinder including a drive barrel, a drive piston disposed within the drive barrel, and a drive rod with a lower end affixed to the drive piston, wherein the drive piston includes a fluid passage providing fluid communication across the drive piston within the drive barrel to create a single drive chamber within the drive barrel; two balance cylinders positioned on either side of the drive cylinder, each balance cylinder including a balance barrel, a balance piston disposed within the balance barrel, and a balance rod with a lower end affixed to the balance piston, wherein an upper balance chamber and a lower balance chamber within each balance barrel are separated by each balance piston, wherein the lower balance chambers are in fluid communication; an upper cross member operatively connecting an upper end of the drive barrel and an upper end of each of the balance barrels; a lower cross member operatively connecting a lower end of the drive barrel and a lower end of each of the balance barrels; a rod cross member operatively connecting an upper end of the drive rod and an upper end of each of the balance rods in a fixed configuration, wherein the rod cross member is configured for lifting the balance pistons upon upward movement of the drive piston, and wherein the rod cross member is configured for lowering the drive piston upon downward movement of the balance pistons; a sheave assembly including a sheave rotatably connected to the rod cross member, a wire rope disposed around a portion of a circumferential surface of the sheave with a first end of the wire rope anchored to the lower cross member, and a carrier member attached to a second end of the wire rope;b) positioning the pump jack system at a wellbore site such that the carrier member is disposed above a wellhead of the wellborec) attaching an upper end of a sucker rod string of a downhole pump to the carrier member;d) lifting the sucker rod string by pumping a first fluid into the drive chamber thereby lifting the drive piston, the rod cross member, the balance pistons, the sheave, and the carrier member;e) counterbalancing the lifting of the drive piston by drawing a second fluid into the lower balance chambers as the balance pistons are lifted;f) lowering the sucker rod string by pumping a third fluid into the upper balance chambers thereby lowering the balance pistons, the rod cross member, the drive piston, the sheave, and the carrier member. 25. The method of claim 24, wherein the pump jack system further comprises an accumulator in fluid communication with the lower balance chambers, and wherein step (e) further comprises drawing the second fluid from the accumulator into the lower balance chambers to maintain a relative constant fluid pressure in the lower balance chambers as the balance pistons are lifted. 26. The method of claim 25, further comprising the steps of: g) counterbalancing the lowering of the rod cross member by collecting the second fluid from the lower balance chambers in the accumulator as the balance pistons are lowered. 27. The method of claim 24, wherein the first fluid, the second fluid, and the third fluid consist of a hydraulic fluid.