US3376826A - Sucker rod compensator for subsurface well pumps - Google Patents

Description

3 Sheets-Sheet 2 136.5%.

SUCKER ROD COMPENSATOR FOR SUBSURFACE WELL PUMPS April 9, 1968 Filed April 29, 1966 mvsmozz TZPLMHDGE L. Ceoms United States Patent 0 3,376,826 SUCKER ROD COMPENSATOR FOR SUBSURFACE WELL PUMPS Talmadge L. Crowe, Houston, Tex., assignor to Baker Oil Tools, Inc., Commerce, Calif., a corporation of California Filed Apr. 29, 1966, Ser. No. 546,351 22 Claims. (Cl. 103-202) ABSTRACT OF THE DISCLOSURE A compensator located in a sucker rod string adjacent to a deep well pump and embodying upper and lower telescopically arranged members, in which the upper member progressively shifts step-by-step relatively upwardly along the lower member in response to reciprocation of the sucker rod string to increase the overall length of the members, until the travelling valve of the pump taps against the standing valve of the pump.

The present invention relates to subsurface Well pumps, and more particularly to devices for insuring the proper length of stroke or travel of a pump plunger in its companion barrel.

In a deep well pump operated by a string of reciprocating sucker rods, the etficiency of the pump is oftentimes comparatively low because of the inability of the travelling valve of the pump to shift downwardly toward the standing valve of the pump to a sufficient extent. Preferably, the travelling valve should move on its downstroke to an extent in which it is disposed closely adjacent to the standing valve of the pump. Such close spacing is not obtained in many cases for a variety of reasons, including improper initial spacing between the travelling and standing valves upon assembly of the pumping apparatus, or variations in the spacing between the travelling valve and standing valve caused by changes in fluid levels in the well, the temperature of the fluids in the well, and in the density of the fluids being pumped. The incomplete or insurficient lowering of the travelling valve on its downstroke results in reduction in efliciency, ranging from complete inability to pump the fluid caused by trapping of gas between the travelling and standing valves (a gas lock condition) to no loss in efficiency whatsoever, which would occur where the pump barrel fills completely with an incompressible fluid of consistent density, a condition occurring rarely in oil and gas Well operations.

When pumping a compressible fluid, a portion of the downstroke is consumed in compressing a fluid below the travelling valve to a pressure somewhat greater than the hydrostatic pressure of the column of fluid in the tubing above the travelling valve. At this point, the travelling valve will open and fluid previously trapped or contained between the travelling valve and standing valve will be transferred relatively upwardly through the travelling valve for the remainder of the downstroke. It is evident that the amount of downstroke consumed before the travelling v-alve opens can vary considerably, since it depends on the hydrostatic pressure, the pump intake pressure, flow restrictions, and the compressibility of the fluids entering the pump. It is only after the travelling valve opens that eflFective work can be performed. Accordingly, to obtain maximum possible pump volumetric efliciency, it is necessary that the unswept volume of the pump be held to a minimum; that is, at the bottom of its stroke the travelling valve should be as close to contacting or bottoming out on the standing valve cage as is practical.

To avoid the above-noted difficulties, the sucker rod string can be adjusted in length to cause the travelling 3,376,826 Patented Apr. 9, 1968 valve or pump plunger to actually pound against the barrel at the bottom of each stroke. This procedure may produce a high volumetric efliciency, but it normally occasions premature fatigue failure of the pumping system in general and of the rod string in particular, producing high and sometimes prohibitive operating expenses.

Accordingly, it is an object of the present invention to provide an apparatus that insures the positioning of a travelling valve closely adjacent to the standing valve of a pump at the bottom of the travelling valve stroke, regardless of the prior absence of such close positioning between the parts.

Another object of the invention is to provide an apparatus for automatically changing the length of a sucker rod string to the extent required to insure positioning of the travelling valve of a pump plunger closely adjacent to the standing valve of the pump at the bottom of the travelling valve stroke. In fact, the effective length of the sucker rod string may be changed to an extent producing light contact or tapping of the travelling valve or part connected to it against a stationary pump part at the bottom of each stroke of the travelling valve.

A further object of the invention is to provide a sucker roo' compensator that automatically adjusts and corrects the length of the overall sucker rod string in response to movement of the rod string to secure maximum efliciency of the pump in the well bore reciprocated by the rod string. A full pump stroke is assured for maximum volumetric efiiciency and the elimination of gas lock, despite changes in well conditions that tend to alter the effective length of the rod string.

This invention possesses many other advantages, and has other objects which may be made more clearly apparent from a consideration of a form in which it may be embodied. This form is shown in the drawings accompanying and forming part of the present specification. It will now be described in detail, for the purpose of illus- ,trating the general principles of the invention; but it is to be understood that such detailed description is not to be taken in a limiting sense, since the scope of the invention is best defined by the appended claims.

Referring to the drawings:

FlGURE l is a diagrammatic view of a sucker rod operated pumping system for pumping fluids from a well bore;

FIGS. 2 and 2a together constitute a longitudinal section through a sucker rod compensator embodied in the system shown in FIG. 1, parts being shown in elevation, FIG. 2:: being a lower continuation of FIG. 2;

FIGS. 3 and 3a are a diagrammatic section through the rod compensator of FIGS. 2 and 2a in combination with a pump, with the parts of the combination in one relative position, FIG. 3a being a lower continuation of FIG. 3;

FIGS. 4 and 4a are views corresponding to FIGS. 3 and 3a, with the parts in another relative position.

As disclosed in the drawings, a string of tubing A is disposed in a well bore B, and has a known type of deep well pump C therewithin that includes a barrel 10 suitably secured or anchored in the tubing string, and a plunger 11 which is reciprocable in the barrel through upward and downward movement of a sucker rod string 12 connected thereto. As is well known, this sucker rod string may be connected to a suitable mechanism for effecting its reciprocation, such as a walking beam mechanism D, hydraulic pumping mechanism, or the like.

The outer barrel 10 of the pump C has a standing valve including a valve seat 13 through which well bore fluid can flow upwardly into the barrel, downward flow of the fluid from the barrel being prevented by a standing valve element 14 adapted to move downwardly into engagement with the seat. The pump plunger 11 is movable upwardly and downwardly in the barrel, containing a travelling valve device in the form of a ball valve element adapted to shift downwardly into engagement with its companion seat 16 surrounding the inlet 17 of the plunger. During the downstroke of the plunger 11, its travelling valve 15, 16 moves downwardly toward the standing valve 13, 14, the latter closing and the fluid thereabove passing through the seat 16 of the travelling valve, unseating its valve element 15 in an upward direction, and then passing upwardly through the openings 20 in the plunger partition 21 to a position thereabove.

When the plunger 11 is moved upwardly, its valve element 15 engages its seat 16, causing the plunger to lift the entire fluid column in the tubing string A and discharge a portion of it at the top of the well bore. The fluid in the plunger itself can pass through its upper ports 22 into the barrel 10 thereabove and through the upper ports 23 in the guide portion 24 of the barrel through which the plunger rod 25 is reciprocable. During the upward movement of the plunger 11, its travelling valve 15, 16 moves upwardly away from the standing valve 13, 14, the valve element 14 of the latter moving off its seat and allowing well bore fluid to flow upwardly through the seat 13 and through openings 18 in a barrel partition 19 into the barrel.

For maximum volumetric efliciency, it is desirable for the plunger 11 to move on its downstroke to a position in which its travelling valve 15, 16 is closely adjacent to the standing valve 13, 14 of the barrel. Accordingly, on the upstroke the plunger 11 will partake of a maximum effective travel in the barrel 10 and will lift a maximum quantity of fluid that may have passed through the travelling valve seat 16 into the plunger and outer barrel thereabove. If the fluids contain a substantial quantity of gas,

assurance is had that the gas between the travelling and standing valves will be compressed suflficiently on the downstroke of the plunger to overcome the hydrostatic head of fluid in the tubing string A, for the purpose of unseating the travelling valve element 15 and causing the fluid above the standing valve 13, 14, including the gas therein, to travel upwardly into and through the plunger 11. If the travelling valve 15, 16 does not move downwardly to its maximum extent, then gas locking might occur under some conditions in which the travelling valve does not open on its downstroke, the plunger 11 on its upstroke merely allowing the trapped gas to expand, the standing valve element 14 also remaining in its closed condition with no additional fluid from the well bore flowing past it and into the pump barrel 10.

To offer assurance that the plunger 11 and its travelling valve 15, 16 will shift downwardly to the desired maximum extent, an automatically adjustable compensator 27 is included in the sucker rod string 12, as between the plunger rod 25 and a sucker rod section 28. As disclosed, the plunger rod 25 is preferably secured to a suitable length of polished rod 29, which, for example, may be 30 feet long, and this polished rod is attached directly or through a short pony rod (not shown) to the threaded pin 30 of a lower sub 31, constituting a portion of the apparatus. The threaded pin 32 of an upper sub or cylinder head 33 of the compensator apparatus is secured to the sucker rod section 28, the sucker rods extending to the top of the hole. The top sub or upper cylinder head 33 is threadedly secured to the upper end of the upper sleeve section 34 of a cylinder 35, the lower end of the sleeve, in turn, being threadedly secured to a lower cylinder head 36 threadedly attached to a lower cylinder sleeve 37, the lower end of which is connected to a short sleeve or piston retainer 38 that may have =axial lugs or clutch elements 39 formed therein, for the purpose of meshing with companion clutch elements 40 projecting upwardly from the lower sub 31. If rotation is not to be transmitted from the cylinder to the sub 31 and polished rod 29, the clutch elements may be omitted.

The lower sub 31 is threadedly secured to the lower end of a piston rod structure 41, including a lower rod section 42, the upper end of which is threadedly attached to a coupling 43, which, in turn, is threadedly secured to the lower end of an upper piston rod section 44, the upper end of which is threadedly attached to a piston slidably mounted in the upper cylinder sleeve 34. A suitable side seal ring 46 is carried by the piston and slidably seals against the inner wall of the upper sleeve, the upper piston rod 44 being laterally spaced from the cylinder sleeve 34 to provide an annular cylinder 47 in which a load transmitting liquid, such as oil, is disposed. Leakage of the oil in a downward direction from the cylinder space 47 is prevented by a suitable seal ring 48 mounted in the cylinder head 36, slidably sealing against the periphery of the upper piston rod section 44.

Upward movement of the cylinder 35, as on the upstroke of the sucker rods, will effect upward movement of the piston 45 and the piston rod 41 connected thereto, since the oil in the annular cylinder 47 is essentially tnapped between the piston 45 and the lower cylinder head 36. However, a very small quantity of the oil is allowed to escape through a bleeder passage 50 in the piston and through a choke orifice 51 which opens into an upper cylinder space 52 between the upper end of the piston 45 and the upper cylinder head 33. As disclosed in FIG. 2, this choke orifice 51 is constituted by a set screw 53 threaded into a companion threaded longitudinal bore 54 in the piston communicating with the bleeder passage 50, the choke orifice being presented by the helical path between the external and internal screw threads of the screw 53 and the companion piston bore 54, the fluid discharging from the threads at the upper end of the piston and through radial slots 55 in a nut 56 threaded on the set screw 53 and bearing against the upper end of the piston 45, locking the set screw in place.

Fluid in the cylinder space 52 above the piston 45 can pass downwardly through an axial passage 57 in the piston communicating with a central by-pass passage 58 in the upper piston rod section 44, that communicates with one or a plurality of side ports 59 in the piston rod coupling 43 that opens into an annular cylinder storage space or reservoir 60 between the piston rod structure 41 and the cylinder 35 below the lower cylinder head seal 48. The lower end of this cylinder space 60 is closed by a floating annular piston 61 slidable along the inner wall of the lower cylinder sleeve 37, and also along the periphery of the lower solid piston rod section 42. Leakage of fluid downwardly from the annular cylinder space 60 is prevented by an outer seal ring 62 mounted on the floating piston or floating cylinder head 61 and sealing against the inner wall of the lower cylinder sleeve 37, and by an inner seal ring 63 slidably and sealingly engaging the periphery of the rod 42, this inner seal being held in place by a retainer ring 64 held upwardly in place against the inner seal 63 by a split retaining ring 65 underlying the ring 64 and disposed in an internal groove 66 in the floating piston or cylinder head.

The floating piston 61 transmits the hydrostatic head of fluid in the tubing string A to the oil in the reservoir 47, and also in the rod passage 58 and the piston passage 57, as well as the cylinder space 52 above the piston 45. It allows for expansion and contraction of the parts and of the oil due to temperature changes in the well bore, and insures the retention of all of the oil in the system which can move only between the reservoir 60 and the load transmitting annular space 47 between the piston 45 and the lower cylinder head 36. The oil above the piston 45 can, under conditions described hereinbelow, also pass downwardly through a check or one-way valve 68 in the piston that constitutes part of a return passage 69, the lower portion of which opens into the upper annular cylinder space 47. This check valve includes a seat 70 threaded in the piston and engaged by a ball valve element 71, which is movable upwardly thereagainst by a helical compression spring 72. When the pressure in the cylinder space 52 above the piston 45 exceeds the fiuid pressure in the power cylinder space 47, the ball valve element 71 is shifted downwardly from its seat 70, allowing fluid in the cylinder above the piston 45 to pass downwardly through the return passage 69 into the power cylinder space 47. However, the check valve68 prevents fiow of fluid in a reverse direction from the power cylinder space 47 into the cylinder space 52 above the piston 45. Fluid can only flow from the power cylinder space 47 into the cylinder above the piston through the choke orifice screw device 51.

The pump assembly C is connected to the polished rod 29, the latter being connected tothe lower sub 31 of the compensator apparatus, the upper sub 33 being connected to the sucker rods 28, 12 thereabove. A sufficient length of polished rod 29 is provided to maintain tension on the rod compensator 27 and to overcome friction of the pump plunger 11 on its downstroke. This assembly is run in the hole on the sucker rod string 12, the pump C being pumped into its seat 90 in the tubing string upon reaching bottom. After the pump has been seated, the rod string 12 is spaced out and the usual polish rod 91 at the top of the hole is appropriately hung on the power mechanism D at the top of the hole, so that when pumping is started, the pump will be tapping bottom very lightly; that is to say, the plunger 11 will be tapping the barrel lightly, or a coupling 92 on the plunger rod 25 will be tapping the barrel guide 24 lightly. At this time, it may be assumed that the compensator 27 is fully telescoped, as disclosed in FIGS. 2, 2a, the clutch elements 39, 40 being engaged and permitting torque to be transmitted through the compensator, if required.

' Thepumping mechanism D at the top of the hole is then caused to reciprocate, effecting reciprocation of the sucker rods 12 to shift the plunger 11 in the barrel 10. When the pumping operation starts, the tubing string A will elongate, as a result of increased hydrostatic pressure therewithin due to the rising column of liquid, and also due to a decreased hydrostatic pressure or head in the well annulus surrounding the tubing string, reducing the buoyant effect of the liquid in the well casing on the tubing string. In addition, the initiation of the pumping operation will increase the average temperature of the tubing string A, causing it to elongate, and such increased temperature will also tend to cause the sucker rod string 12 to elongate. However, the rod string 12 may also shorten, as a result of the buoyancy of the liquid within the tubing string. The overall net result is that the initial relative spacing of the travelling valve 15, 16 and standing valve 13, 14 of the pump will start changing with the first stroke of the pump, and will continue changing throughout the life of the well, the changes also being due to changes in density of the fluid in the well bore passing through the pump. In a typical case, the travelling valve. may be spaced above the standing valve at the bottom of each plunger stroke.

The rod compensator 27, within the limits of the relative movement that can take place between its cylinder and piston structures 35, 41, 45, automatically compensates for the changing well conditions. On the upstroke of the sucker rods 12, the upward force required to lift the fluid in the tubing string A is being transmitted from the cylinder 35 through the load transmitting oil or liquid in the upper annular chamber 47 to the piston 45, the cylinder 35, piston 45, and piston rod structure 41 moving essentially upwardly as a unit to elevate the travelling valve 15, 16 with respect to the standing valve 13, 14. The pressure developed in the trapped oil within the upper cylinder space 47 will cause some of the oil to bleed through the small space or orifice 51 between the socket set screw 53 and its tapped hole 54 in the upper control piston, this oil or fluid transferring into the chamber 52 above the piston 45. The rate of fluid transfer is very low. As a result, the upward shifting of the outer cylinder 35 relative to the piston 45 is very small during each upward stroke of the mechanism. As an example, only sufi'icient oil or fluid will be able to transfer through the set screw orifice 51 during each complete upward stroke of the outer cylinder 35 as to result in an elevation of the outer cylinder along the piston by approximately 0.05 inch. In other words, the rod compensator 27 will 1 extend or elongate a comparatively short distance on each upward stroke of the sucker rods 12, and of the rod compensator 27 and pump C connected thereto. On the downstroke of the sucker rods, rod compensator and pump, no transfer of fluid will take place, the trapped oil or liquid in the cylinder space 47 retaining the piston 45 in its relative position above the lower cylinder head 36. The elongation of the rod compensator 27 will continue on each upstroke, fluid transfer taking place to a small degree, allowing the overall length of the rod compensator 27 to increase by small increments, which, in the example given above, may be 0.05 inch per upstroke. The progressive incremental elongation of the rod compensator will actually result in the plunger 11 and the travelling valve 15, 16 occupying a lower position in the barrel 10 on each subsequent downstroke of the pump mechanism, until the pump valve rod coupling 92 engages or taps the guide 24, or the plunger 11 taps a shoulder 95 in the pump barrel, as shown in FIGS. 4, 4a.

When the tapping commences at the bottom of each stroke of the pump mechanism, the cylinder will shift downwardly relative to the piston by a short distance because of the fact that the string of sucker rods 12 thereabove can still move downwardly by a short distance, which, in the example given above, will be 0.05 inch. This will result in the upper cylinder head 33 moving downwardly toward the piston 45 by an incremental distance (0.05 inch), forcing the fluid in the cylinder space 52 above the piston downwardly past the check valve 68 and through the return passage 69 into the annular power cylinder 47, thereby effecting a return of the liquid to such cylinder. As a result of the downward shifting of the cylinder 35 along the piston structure 45, 41 by the short distance, the overall length of the rod compensator 27 is shortened, and this shortening is equivalent to the amount of its elongation on each upstroke. On the next upstroke of the sucker rod string 12, including the compensator 27 and the travelling valve 15, 16 of the pump, the rod compensator will again elongate by virtue of the transfer of a small amount of liquid from the upper annular cylinder 47 through the orifice 51 into the cylinder space 52 above the piston 45; but on the downstroke, a light tapping of the coupling 92 against the valve rod guide 24, or of the plunger 11 against the barrel 10, will again occur, the compensator 27 shortening again by the fact of increasing the pressure of the oil in the cylinder 52 above the piston 45 and causing such increased pressure to open the check valve 68 and return the excess oil through the passage 69 to the annual power cylinder 47.

As a result, the rod compensator 27 will elongate and contract, depending on the variable conditions in the well. Any excess stroke that will bring the valve rod coupling 92 into contact with the valve rod guide 24 will arrest further downward movement of the piston structure 45, 41 and will result in a transfer of fluid from above the piston 45 downwardly through the check valve 68 into the power cylinder 47 to automatically shorten the compensator 27 and the effective length of the entire sucker rod string 12. Assurance is had that the plunger 11 will be lowered within its companion barrel 10 to the maximum desired extent on each downstroke of the sucker rods, insuring a complete upward sweep of the pump barrel by the plunger on its upstroke.

Not only is assurance had that a maximum upstroke of the plunger 11 in the barrel 10 is secured, but the fact that the plunger is shifted downwardly to its lowermost position on each downstroke insures against gas locking, since the bringing of the travelling valve 15, 16 closely adjacent to the standing valve 13, 14 will insure sufficient compression of. the gas between the two valves as to overcome the hydrostatic head of the fluid in the tubing string A above the travelling valve, effecting opening of the latter and upward passage of gas into the plunger 11. Accordingly, upon the upstroke of the plunger 11 and the upward shifting of the travelling valve 15, 16 away from the standing valve 13, 14, the latter will be opened and additional fluids, including gas, if present, will unseat the travelling valve element 14 and pass into the plunger 11 and barrel 10 thereabove.

The compensator 27 automatically adjusts the plunger 11 so that it will merely lightly tap the barrel 10. In the example given above, it is only necessary to effect a downward travel of the cylinder 35 along the piston structure 45, 41 of about 0.05 inch to relieve the engageing force of the plunger on the barrel, and such light tapping is insufficient to effect any fatiguing of the pumping system and of the rod string. This light tapping is to be distinguished from the pounding action that has occurred heretofore, when rod lengths were used as to cause striking of the plunger mechanism against the barrel.

I claim:

1. In apparatus wherein a pump is disposed in a well bore and is reciprocated by a string of sucker rods for elevating the well production to the top of the well bore, the combination therewith of a compensator forming a part of the sucker rod string and comprising an upper device connected to an adjacent sucker rod member and a lower device connected to the pump, said upper and lower devices being shiftable relative to each other, means for transmitting longitudinal motion between said devices to effect operation of said pump, and means operable in response to movement of the sucker rod string in pumping the well bore for effecting shifting of said devices longitudinally with respect to each other to increase the overall length of said upper and lower devices and to retain said upper and lower devices at such increased length.

2. In apparatus as defined in claim 1; wherein said means for effecting shifting of said devices produces progressive shifting of said upper device relatively upwardly along said lower device to produce step-by-step increase in the overall length of said upper and lower devices.

3. In apparatus as defined in claim 1, wherein said upper and lower devices are telescoped one within the other, said motion transmitting means comprising a liquid said means for effecting shifting comprising an orifice for allowing escape of a portion of said liquid from its motion transmitting location.

4. In apparatus as defined in claim 1; wherein said upper and lower devices are telescoped one within the other, said motion transmitting means comprising a liquid, said means for effecting shifting comprising an orifice for allowing escape of a portion of said liquid from its motion transmitting location, and means for returning said liquid to its motion transmitting location.

5. In apparatus as defined in claim 1; wherein said upper and lower devices are telescoped one within the other, said motion transmitting means comprising a space provided between said devices filled with a liquid through which longitudinal motion is transmitted between said devices, said devices providing a liquid storage reservoir therebetween, said means for effecting shifting comprising a choke orifice through which liquid from said space can flow to said reservoir.

6. In apparatus as defined in claim 1; wherein said upper and lower devices are telescoped one within the other, said motion transmitting means comprising a space provided between said devices filled with a liquid through which longitudinal motion is transmitted between said devices, said devices providing a liquid storage reservoir therebetween, said means for effecting shifting comprising a choke orifice through which liquid from said space can flow to said reservoir, and means for transmitting the 8 pressure of the fluid in the well bore externally of said apparatus to the liquid in said reservoir.

7. In apparatus as defined in claim 1', wherein said upper and lower devices are telescoped one within the other, said motion transmitting means comprising a space provided between said devices filled with a liquid through which longitudinal motion is transmitted between said devices, said devices providing a liquid storage reservoir therebetween, said means for effecting shifting comprising a choke orifice through which liquid from said space can flow to said reservoir, and check valve control means for returning liquid from said reservoir to said space.

8. In apparatus wherein a pump is disposed in a well bore, said pump including a standing valve mechanism and a travelling valve mechanism thereabove reciprocated by a string of sucker rods for elevating the well production to the top of the well bore, the combination therewith of a compensator in the sucker rod string comprising an upper device connected to an adjacent sucker rod member and a lower device connected to the travelling valve mechanism, said upper and lower devices being shiftable relative to each other, means for transmitting longitudinal motion between said devices to effect reciprocation of said travelling valve mechanism toward and from said standing valve mechanism, means operable in response to movement of the sucker rod string in pumping the well bore for effecting relative shifting of said upper device upwardly along said lower device to increase the overall length of said upper and lower devices and bring the travelling valve mechanism to a bottom position closer to said standing valve mechanism, and means for retaining the overall length of said upper and lower devices at such increased value.

9. In apparatus as defined in claim 8; wherein said shifting means effects progressive incremental shifting of said upper device relatively upwardly along said lower device to produce step-by-step increase in the overall length of said upper and lower devices and step-by-step lowering of the bottom position of said travelling valve mechanism.

10. In apparatus as defined in claim 8; wherein said shifting means effects progressive incremental shifting of said upper device relatively upwardly along said lower device to produce step-by-step increase in the overall length of said upper and lower devices and step-by-step lowering of the bottom position of said travelling valve mechanism, and means for effecting decrease in the overall length of said upper and lower devices upon lowering of said travelling valve mechanism into contact with said standing valve mechanism.

11. In apparatus as defined in claim 8; and means for effecting shifting of said lower device upwardly of said upper device upon lowering of said travelling valve mechanism into contact with said standing valve mechanism.

12. In apparatus as defined in claim 8', wherein said upper and lower devices are telescoped one within the other, said motion transmitting means comprising a space provided between said devices filled with a liquid through which said longitudinal motion is transmitted between said devices, said devices providing a liquid storage reservoir therebetween, said means for effecting shifting comprising a choke orifice through which liquid from said space can flow to said reservoir.

13. In apparatus as defined in claim 8; wherein said upper and lower devices are telescoped one within the other, said motion transmitting means comprising a space provided between said devices filled with a liquid through which said longitudinal motion is transmitted between said devices, said devices providing a liquid storage reservoir therebetween, said means for effecting shifting comprising a choke orifice through which liquid from said space can flow to said reservoir, and check valve control means for returning liquid from said reservoir to said space.

14. In a compensator for varying the effective length of a rod string in a well bore: inner and outer devices telescoped one within the other, means for transmitting longitudinal motion between said devices, and means operable in response to reciprocation of the rod string in the well bore for effecting progressive incremental shifting of one of said devices upwardly along said other of said devices to produce step-by-step increase in the overall length of said inner and outer devices, and means for retaining the overall length of said upper and lower devices at such increased value.

15. In a compensator as defined in claim 14; and means operable in response to reciprocation of the rod string in the well bore for effecting decrease in the overall length of said inner and outer devices.

in. In a compensator as defined in claim 14; wherein said motion transmitting means comprises a liquid, said means for effecting incremental shitting comprising an orifice for allowing escape of a portion of said liquid from its motion transmitting location.

17. In a compensator as defined in claim 14; wherein said motion transmitting means comprises a liquid, said means for effecting incremental shifting comprising an orifice for allowing escape of a portion of said liquid from its motion transmitting location, and means for returning said liquid to its motion transmitting location.

18. In a compensator as defined in claim 14; said motion transmitting means comprising a space provided between said devices filled with a liquid through which longitudinal motion is transmitted between said devices, said devices providing a liquid storage reservoir therebetween, said means for ettecting shifting comprising a choke orifice through which liquid from said space can flow to said reservoir.

19. In a compensator as defined in claim 14; said motion transmitting means comprising a space provided between said devices filled with a liquid through which longitudinal motion is transmitted between said devices, said devices providing a liquid storage reservoir therebetween, said means for effecting shifting comprising a choke orifice through which liquid from said space can flow to said reservoir, and means for transmitting the pressure of fluid externally of said apparatus to the liquid in said reservoir.

20. In a compensator as defined in claim 14; said motion transmitting means comprising a space provided between said devices filled with a liquid through which longitudinal motion is transmitted between said devices, said devices providing a liquid storage reservoir therebetween, said means for effecting shifting comprising a choke orifice through which liquid from said space can flow to said reservoir, and check valve control means for returning liquid from said reservoir to said space.

21. In apparatus wherein a pump is disposed in a well bore and is reciprocated by a string of sucker rods for elevating the well production to the top of the well bore, the combination therewith of a subsurface compensator forming a part of the sucker rod string and comprising an upper device connected to an adjacent sucker rod member and a lower device adjacent to and connected to the pump, said upper and lower devices being shiftable relative to each other, means for transmitting longitudinal motion between said devices to efifect operation of said pump, and means operable in response to movement of the sucker rod string in pumping the well bore for effecting shifting of said devices longitudinally with respect to each other to increase the overall length of said upper and lower devices and to retain said upper and lower devices at such increased length.

22. In apparatus wherein a pump is disposed in a well bore, said pump including a standing valve mechanism and a travelling valve mechanism thereabove reciprocated by a string of sucker rods for elevating the well production to the top of the well bore, the combination therewith of a subsurface compensator in the sucker rod string comprising an upper device connected to an adjacent sucker rod member and a lower device adjacent to and connected to the travelling valve mechanism, said upper and lower devices being shiftable relative to each other, means for transmitting longitudinal motion between said devices to eiiect reciprocation of said travelling valve mechanism toward and from said standing valve mechanism, means operable in response to movement of the sucker rod string in pumping the well bore for eflecting relative shifting of said upper device upwardly along said lower device to increase the overall length of said upper and lower devices and bring the travelling valve mechanism to a bottom position closer to said standing valve mechanism, and means for retaining the overall length of said upper and lower devices at such increased value.

References Cited UNITED STATES PATENTS 1,623,928 4/1927 Lewellen et al. 74586 2,259,019 10/1941 Carr et al. 74-586 2,259,020 10/ 1941 Wineman 74586 ROBERT M. WALKER, Primary Examiner. DONLEY J. STOCKING, Examiner.

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