Abstract:
A lift and method for reciprocating a downhole pump includes an linear actuator, such as a piston-cylinder arrangement, located coaxially above the stuffing box of a well. The linear actuator includes a connecting rod that passes through and seals against the stuffing box, thereby taking the place of the traditional polished rod and allowing the linear actuator to be positioned lower and closer to the well. The connecting rod may be connected to the traditional polished rod below the stuffing box. A stopper may be selectively removed to allow the connecting rod to be pulled clear of the stuffing box while using the polished rod to seal the well, thereby allowing the linear actuator to be removed for maintenance.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to oilfield equipment, and in particular to wellhead-mounted reciprocating sucker rod pumping units, commonly referred to as pump jacks. 
       BACKGROUND 
       [0002]    Hydrocarbons are often produced from well bores by reciprocating downhole pumps that are driven from the surface by pumping units. A pumping unit is connected to its downhole pump by a rod string, which is assembled from a number of sucker rods. Often, the downhole pump is referred to as a sucker rod pump and the surface pumping unit that reciprocates the rod string is referred to as a pump jack, although other terms may also be used by routineers in the art. Several types of pumping units are known in the art, including the prolific walking beam style pumps, and pumps that employ a piston-cylinder arrangement. 
         [0003]    The uppermost rod in the rod string is called a polished rod, so named because of its smooth finish. The pump jack carries the polished rod, typically via a wire rope bridal and carrier bar. The polished rod extends through a packing gland or stuffing box at the wellhead for providing a dynamic well seal as the polished rod strokes up and down. A rod string of sucker rods hangs from the polished rod within a tubing string located within the well casing. The rod string is connected to the plunger of the subsurface pump. In a reciprocating cycle of the pump jack, formation fluids flow into the well and pump housing during the downstroke, and well fluids are lifted within the tubing string during the rod string upstroke. 
         [0004]    One type of pump jack is a wellhead-mounted linear lift system. The linear lift system has an elongate frame vertically oriented and mounted atop the wellhead. The frame may be supported with guy wires. The top of the frame carries a ram assembly oriented to be in line with the wellhead. The ram assembly may be hydraulic or pneumatic, for example. A piston rod extends downwardly from the ram assembly and connects to the polished rod of the sucker rod pump above the wellhead. Application of a pressurized fluid to the ram assembly lifts the piston rod, the polished rod, the rod string, and the plunger of the downhole pump to lift well fluids within the tubing string. A subsequent release of pressure to the ram assembly allows the piston rod, the polished rod, the rod string, and the plunger of the downhole pump to descend, thereby completing one pump cycle. 
         [0005]    Such linear lift systems are typically characterized by a small footprint, low weight, high load capacity, an easily adjustable ultra-long stroke, and a simple, low-maintenance actuator that uses a single-acting ram. Because of these advantages, there are circumstances in which it is desirable to use a wellhead-mounted linear lift system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Embodiments are described in detail hereinafter with reference to the accompanying figures, in which: 
           [0007]      FIG. 1  is a perspective view of an improved linear lift system according to a present embodiment showing a lower wellhead mount and an upper linear actuator, which may be a piston-cylinder assembly as shown; 
           [0008]      FIG. 2  is an enlarged perspective longitudinal cross section of the upper end of the piston-cylinder assembly of  FIG. 1 , showing a stopper mounted in the top end of the cylinder for limiting upward piston travel; 
           [0009]      FIG. 3  is an enlarged perspective longitudinal cross section of the lower end of the piston-cylinder assembly of  FIG. 1 , showing details of the piston, connecting rod, gland, and seals; 
           [0010]      FIG. 4  is a perspective view of the lower end of the piston-cylinder assembly and upper end of the wellhead mount of  FIG. 1 , showing a position adjustment mechanism for precisely aligning the cylinder with a wellhead; 
           [0011]      FIG. 5  is an elevation in partial cross section of the linear lift system of  FIG. 1  mounted at a wellhead and connected to the polished rod of the sucker rod pump during initial installation or maintenance of the lift, for example; and 
           [0012]      FIG. 6  is an elevation in partial cross section of the linear lift system of  FIG. 5 , showing the lift during normal pumping operation with the piston located at the top of its operational stroke. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates a linear lift system  10  according to a present embodiment that provides a reduced overall height for the same stroke capability as compared to a conventional wellhead linear lift system of prior art, thereby minimizing weight and maximizing transportability. 
         [0014]    Lift  10  includes a linear actuator assembly  12  carried atop a wellhead mount  14 . If necessary, linear actuator assembly  12  may include a frame assembly  16 , guy wires (not illustrated), or the like, for laterally support. In the embodiment shown, the linear actuator assembly frame  16  includes a series of tie rods  17  and support plates  18 , although other suitable arrangements may be used as appropriate. 
         [0015]    In one embodiment, linear actuator assembly  12  includes a piston-cylinder assembly  20 , which may be hydraulic or pneumatic for example. Other linear actuators, such as mechanical or electromagnetic, may be used as appropriate. 
         [0016]    Wellhead mount  14  can take any number of forms provided it can carry linear actuator assembly  12  at an appropriate distance above a wellhead and allow access to the wellhead for operation and maintenance. For example, as illustrated, wellhead mount  14  includes a baseplate  22  for mounting to the ground with a wellhead opening  24  formed therethrough. Linear actuator assembly  12  is mounted to a pedestal  27 , which is carried above baseplate  22  by a table  26  and legs  28 . 
         [0017]      FIG. 2  shows the top end of the piston-cylinder assembly  20  as it is configured for pumping operation. Referring to  FIG. 2 , piston-cylinder assembly  20  includes a cylinder  30 , a piston  32  that fits closely and slides within cylinder  30 , and a connecting rod  34  for transmitting the movement of piston  32 . Connecting rod  34  is connected to piston  32  via a yoke fitting  36  and pin or bolt  38  to allow for some minor misalignment between connecting rod  34  and piston  32 . 
         [0018]    The top of cylinder  30  is terminated by an upper plug  40 , which may be threaded into cylinder  30 . Upper plug  40  includes a port  48  through which actuation fluids may enter and exit the upper portion of cylinder  30 . In a preferred embodiment, upper plug  40  has a central bore formed therethrough, into which an elongate, rod-shaped stopper  42  is received. The top end of stopper  42  has an enlarged knob  44  with a circumferential sealing element  45 , such as an o-ring. The bore within upper plug  40  has a profile that defines a seat  46  into which knob  44  is received so as to suspend stopper  42  from upper plug  40  and that forms a seal against sealing element  45 . Stopper  42  is removably secured in place within upper plug  40  by one or more conventional fastening techniques, such as by threading, retaining ring, pins, etc. (not illustrated). 
         [0019]    As illustrated in  FIG. 2 , stopper  42  limits the upward travel of piston  32 . During installation and maintenance, stopper  42  can be removed so that piston  32  can travel to s higher elevation, as described in greater detail below with respect to  FIGS. 5 and 6 . Stopper  42  may include a conduit  43 , which can be used for locating a proximity sensor, switch, pressure sensor, or the like and the bottom of the stopper and routing the corresponding electrical conduits out through the top of the cylinder. Packing, stuffing, or a similar gland seal (not illustrated) is disposed within conduit  43  to provide a pressure-tight seal, as is known in the art. 
         [0020]      FIG. 3  shows the bottom end of the piston-cylinder assembly  20 , with piston  32  located at the bottom of its stroke. The bottom of cylinder  30  is terminated by a lower plug  50 , which may be threaded into cylinder  30 . Lower plug  50  includes a port  58  through which actuation fluids may enter and exit the lower portion of cylinder  30 . Lower plug assembly  50  has a central bore through which connecting rod  34  passes. A gland  52  mounts to the bottom of lower plug  50 . Gland  52  includes various wipers and seals  54  that engage and provide a dynamic seal against the outer surface of connecting rod  34 . 
         [0021]    Referring to  FIG. 4 , the lateral position of linear actuator assembly  12  is ideally adjustable with respect to wellhead mount  14  so that linear actuator assembly  12  can more easily be brought into coaxial alignment with a wellhead above which it is mounted. In one embodiment, linear actuator assembly  12  is mounted to pedestal  27 , which is slideably carried atop table  26 . Table  26  includes raised blocks or nut plates  60  having threaded holes formed therethrough. Bolts  62  are threaded through nut plates  60  and engage pedestal  27 . Tightening or loosening of bolts  62  translates pedestal  27  in lateral and transverse directions. Once pedestal  27  is properly aligned, it its position is fixed with respect to table  26  by clamping bars  64 . 
         [0022]      FIGS. 5 and 6  illustrate the operation and novel features of linear lift system  10 . Referring to  FIG. 5 , linear lift system  10  is initially positioned over a conventional wellhead  70 , which may include a Christmas tree  72  and a stuffing box  74 . Stopper  42  is removed from cylinder  30 , and piston  32  is positioned at its upmost position, abutting upper plug  40 . In this position, the bottom end  35  of connecting rod  34  is located above stuffing box  74 . The bottom end  35  of connecting rod  34  is connected to the conventional polished rod  76  of the sucker rod pump. 
         [0023]    Unlike a typical pump jack, which uses a carrier bar or other connector to connect to the polished rod that is larger than the outer diameter of the polished rod, according to a preferred embodiment, connecting rod  34  has the same outer diameter as polished rod  76 , and it connects to polished rod  76  using a connection that is the same size or smaller than the polished rod outer diameter. Because of this feature, as shown in  FIG. 6 , polished rod  76  can be lowered by connecting rod through stuffing box  74  so as to completely clear the stuffing box packing, and connecting rod  34 , which also has a polished surface, is used to create the well seal with stuffing box  74 . 
         [0024]      FIG. 6  shows linear lift system  10  configured for normal pumping operation, with the sucker rod pump at the top of its stroke. Stopper  42  is installed in upper plug  40  as described above with respect to  FIG. 2 . The longitudinal length of stopper  42  is selected so that when piston  32  abuts the bottom end of stopper  42 , the sucker rod plunger is at the top of its allowed travel. At this position, connecting rod  34  takes on the traditional sealing function of polished rod  76 , and polished rod  76 , located entirely below stuffing box  74 , functions as an ordinary sucker rod. Accordingly, in order to accommodate linear lift system  10 , a length of sucker rod must be removed from the rod string. 
         [0025]    By using a connecting rod  34  that doubles as a polished rod, a distance essentially just shy of an entire stroke length can be eliminated from the overall height of the linear lift system  10  as compared to a linear lift system of prior art. That is, with a prior art lift system, at the bottom of the stroke the connecting rod is located just above the stuffing box, but with linear lift system  10 , at the top of the stroke the connecting rod is located just below the stuffing box. 
         [0026]    By connecting connecting rod  34  to polished rod  76  rather than to a sucker rod (and removing the polished rod  76 ), one can remove linear lift system  10  from the well without having the disassemble or remove stuffing box  74 , thereby maintaining positive control over the well. 
         [0027]    The Abstract of the disclosure is solely for providing the United States Patent and Trademark Office and the public at large with a way by which to determine quickly from a cursory reading the nature and gist of technical disclosure, and it represents solely one or more embodiments. 
         [0028]    While various embodiments have been illustrated in detail, the disclosure is not limited to the embodiments shown. Modifications and adaptations of the above embodiments may occur to those skilled in the art. Such modifications and adaptations are in the spirit and scope of the disclosure.