Abstract:
A tubing pump includes a seal housing insertable into a production string of tubing with a seal assembly mounted in an interior of the seal housing. The assembly includes at least an upper seal and seal holder and a lower seal and seal holder, each seal energizable by fluid to provide a fluid-tight seal around a reciprocating tubular member. The seals and holders are isolated from each other in a manner whereby the lower seal remains un-energized until the seal above it fails.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the invention relate to a wiper and seal assembly that prevents debris, such as sand particles, from entering an operating region of a pump. 
         [0003]    2. Description of the Related Art 
         [0004]    To obtain fluids from an earth formation, a wellbore is drilled into the earth to intersect an area of interest within the formation. Upon reaching the area of interest, artificial lift means is often necessary to carry production fluid (e.g. hydrocarbon fluid) from the area of interest within the wellbore to the surface. Some artificially lifted wells are equipped with sucker rod lifting systems. 
         [0005]    Sucker rod lifting systems generally include a surface drive unit, a sucker rod string, and a downhole pump. The pump generally includes an outer barrel and an operating member, such as a plunger, axially movable within the barrel to lift fluid to the surface. The sucker rod string generally comprises several rods connected together but may be one continuous rod, and is the primary link between the drive unit at the surface and the pump plunger. In one instance, reciprocating pumping action moves a traveling valve on the pump plunger, collecting fluid on the down-stroke and lifting the fluid to the surface on the up-stroke. 
         [0006]    Sucker rod-type lifting systems include insert pumps, where the entire assembly is run into the well with its own string of tubulars attached to the sucker rod string. These pumps are easy to get in and out of the well, but result in a smaller diameter and fluid path for the collection of hydrocarbons. Tubing pumps, on the other hand, have barrels that are actually screwed into and become part of the production string. The result is a greater capacity but difficulty in removing and repairing the barrel due to its location in the production string. 
         [0007]    One problem associated with sucker rod lifting systems is wear within the annular region between the plunger and the barrel due to wellbore debris, such as sand. Since the annular region is typically about 0.002 inches to about 0.005 inches (per side), sand particles of various size enter the region and act as an abrasive, which quickly forms “grooves” in both the barrel and the plunger sliding surfaces. Such wear significantly diminishes the life of the barrel and the plunger, and can lead to costly repair and frequent maintenance. With tubing pumps, wear is a particular problem due to the difficulty of recovering and replacing the barrel portion of the pump. 
         [0008]    Therefore, there is a need for an improved assembly to prevent debris from entering an operating region of a pump. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention generally relates to downhole pumps. In one embodiment, a tubing pump includes a seal housing insertable into a production string of tubing with a seal assembly mounted in an interior of the seal housing. The assembly includes at least an upper seal and seal holder and a lower seal and seal holder, each seal energizable by fluid to provide a fluid-tight seal around a reciprocating tubular member. The seals and holders are isolated from each other in a manner whereby the lower seal remains un-energized until the seal above it fails. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0011]      FIG. 1  is a section view of a seal housing, a seal assembly and a centralizer within the seal housing and a plunger inside the seal assembly. 
           [0012]      FIG. 2  is an isometric view showing a seal holder and seal. 
           [0013]      FIG. 3  is an enlarged section view showing a section of the seal assembly and the centralizer of  FIG. 1 . 
           [0014]      FIG. 4  is a section view showing an embodiment of the invention that includes a bypass. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  is a section view of certain parts of a tubing pump. Parts shown include a seal housing  100 , a seal assembly  110  within the housing and a plunger  120  inside the seal assembly. Typically, the seal housing is installed as a “sub” in a tubing string at a location above a barrel (not shown). Threads at an upper  125  and lower  130  end of the seal housing permit its installation in the string. Tubing pumps are well known in the art and their operation is disclosed in U.S. Pat. No. 5,765,639 which is incorporated by reference herein in its entirety. In operation, the plunger  120  is connected at an upper end to a rod string and is reciprocated as shown by directional arrows  135 ,  140  while the other parts of the pump, including the barrel and seal assembly, remain stationary. 
         [0016]    The seal assembly described herein is primarily intended for use in a tubing pump having a plunger that is relatively long and a barrel that is relatively short. For example, in the preferred embodiment, a seal housing is installed above the barrel and the plunger is of a sufficient length to reciprocate without exiting the barrel or the seal housing as it moves between its upper and lower strokes. As shown in  FIG. 1 , the seal assembly in one embodiment includes four separate seals  150  that are stacked in an inner wall of the seal housing  100  in a manner whereby the seals seal an annular area created between themselves and the plunger  120 . Each seal includes a seal holder  155 . O-rings  160  seal an area between the seal holder, and seal holder wall and O-rings  165  seal an area between the seal holder and the seal. In the embodiment shown, the seal assembly is held at a lower end by a shoulder  170  formed in the inside wall of the seal housing  100  and at an upper end by a snap ring  175  that seats in a groove  176  ( FIG. 3 ). The seals  150  are intended to be redundant with only the uppermost seal being energized at any one time. Each lower seal becomes energized and operates only as the seal above it fails. However, the seals all operate together with each providing a wiping function as the plunger moves past them. The embodiment shown uses four seals  150 , but it will be understood that the invention can be practiced with as many or as few redundant seals as is desired, limited only by space and expected wear on the seals. By stacking seals in a manner that permits them to act in series, the sealing function lasts longer, and expensive and time-consuming removal of the tubing string due to failed seals or damaged pump components is delayed or avoided. 
         [0017]      FIG. 2  is an isometric view of a seal holder  155  and seal  150 , and  FIG. 3  is an enlarged view of the seal/seal holder and a centralizer  180 . Considering both figures, the seal holder  155  includes an inner surface  156  which mates to an outer surface  151  of the seal  150 . When assembled, the cylindrical seal  150  is lowered into the cylindrical seal holder  155  until interference between the surfaces  156 ,  151  causes it to “snap” into place and be retained by a formation  157  formed on surface  156 . A gap  185  remains between an upper part of the seal  150  and the holder  155  to receive pressurized fluid (primarily when the plunger  120  moves upwards  135  in the housing  100 ) and help energize the seal. In one embodiment, the seal  150  is constructed of a robust Teflon-like material. In another embodiment the seal is constructed of a more pliable material whereby the seal is deformable due to the mating  45  degree angles between the seal and the lower surface of the seal holder  155  ( FIG. 3 ). By providing an assembly in which each seal  150  has its own holder  155 , the seal elements  150  are isolated from each other and the energizing and operation of each is separate from the others. In this way the seals do not seal in unison but rather, each remains essentially unused until the one above it fails. 
         [0018]    Each seal  150  is installed in its seal holder  155  in a manner that permits fluid (arrow  186 ) to enter gap  185  and act on the rear surface  151  of the seal  150 , thereby “energizing” the seal in the direction of the reciprocating plunger  120 . In this manner, a fluid column above the seal  150  acts to assure its sealing action against the surface of the plunger  120 . As the uppermost seal becomes worn and/or damaged, its integrity fails and the fluid is permitted to contact the seal therebelow, energizing it against the plunger. In this manner, multiple redundant seals are available to be used in series to avoid having to pull the tubing string (and with it the seal housing  100  and seal assembly  110 ) from the well. In every case, the reciprocating plunger is wiped by each seal, even those that are not operating to seal the plunger. 
         [0019]    Also shown in  FIG. 3  is the centralizer  180  which is mounted above the seal assembly  110 . In the embodiment shown, the centralizer&#39;s role is to ensure that the plunger  120  is centered relative to the seals  150  as it is initially inserted into the seal housing  100 . The centralizer  180  has a slightly smaller inside diameter than the outside diameter of the plunger  120 . In this manner, the inside diameter of the centralizer deforms slightly as the plunger initially passes through, ensuring an entry in the center of the housing  100  and also wiping an outer surface of the plunger  120  as it reciprocates during use. A “cutout”  181  allows the centralizer to act like a hinge, thus reducing the amount of force required for the plunger to pass through. The centralizer  180  is retained in the housing by the snap ring  175  at a lower end and by a retaining member  190  at an upper end. The retaining member may be a spring that acts to keep the centralizer biased towards the plunger  120 . 
         [0020]    In one example of operation, the seal assembly  110  described herein is assembled by stacking a predetermined number of seal holders  155  and seals  150  in the seal housing  100 . As disclosed, each seal and seal holder mates together, in the housing and the assembly is held in place at an upper and lower end. A centralizer  180  is installed and in turn held in place by a spring member  190  at an upper end thereof. 
         [0021]    Thereafter, the seal assembly  110  within the seal housing  100  is installed as a sub in a production tubing string above a barrel of a tubing pump, ensuring that as the pump operates, its plunger will reciprocate across the surface of the seals  150 . As the pump components are inserted into the well at the lower end of a sucker rod string, the plunger  120  encounters an upper end of the seal housing  100  where the centralizer  180  encounters a lower end of the plunger and guides it into the center of the seal housing, thereby avoiding damage to the seal assembly  110 . 
         [0022]    Once the pump is operating and the plunger  120  is reciprocating in the seal assembly  110 , fluid enters a gap  185  between the uppermost seal  150  and its seal holder  155 . The pressurized fluid acts on an O-ring  165  between the holder and seal element, especially during an upstroke of the plunger  120  when a column of production fluid is being raised toward the surface of the well. If and when the first seal becomes inoperable due to wear or damage, the process will be repeated utilizing the second seal and its holder. In this manner, four seals can operate, fail and all can act as wipers before the barrel and seal portion of the pump require removal. 
         [0023]      FIG. 4  is a section view showing an embodiment of the invention that includes a bypass  400 . As shown the bypass permits fluid to avoid the seal assembly as the pump operates. In one embodiment, the bypass is provided to ensure fluid “slippage” and increase the amount of fluid that passes from an upper to a lower end of the pump. The bypass consists of a housing  402  that houses a fluid path  404  of the bypass. The fluid path travels the length of the seal assembly from an upper port  406  to a lower port  408  where it is re-introduced into the pump. As illustrated by directional arrow  135 , the bypass is designed to operate on the upstroke of the plunger  120 . In the embodiment shown, a filter or screen  414  is installed at an upper end of the seal assembly to ensure that fluid bypassing the seals (with their wiping action) is filtered prior to contacting the pump components it will contact. A circumferential recess  410  is provided for fluid communication around the filter  414  and the filter is retained and sealed in the housing with a mounting plate  412 . It is notable that even when the bypass is provided, the seals  150  act to wipe the surface of the plunger  120  as it moves past them. 
         [0024]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. For example, while the invention has been described with the seals on a stationary member, the invention can be used with the seals on the reciprocating member. Additionally, while the invention has been described for use in a tubing pump, it could also be used with other downhole pumps, like insert pumps.