Abstract:
A dual displacement pumping system is disclosed. In one embodiment, the system includes a subsurface pump, a tubing string, and a surface pumping unit connected to the subsurface pump by a reciprocating member. The subsurface pump includes a pump barrel mounted to the end of the tubing string, and a plunger mounted to the end of the reciprocating member. Valves cause downward motion of the plunger in the pump barrel to force fluid from the lower end of the pump barrel into the reciprocating member, and fill the upper end of the pump barrel with well bore fluid. They also cause upward motion of the plunger to force fluid from the upper pump barrel into the tubing string, and fill the lower pump barrel with well bore fluid. Thus, both pumping movements are exploited, nearly doubling the volume of fluid pumped with a conventional surface configuration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application relates to U.S. patent application Ser. No. 09/775,246, filed Feb. 1, 2001, which is a continuation of issued U.S. Pat. No. 6,220,358. The present application further relates to Disclosure Document Nos.  487891 ;  488489 ; and  496525 ; respectively filed on Jan. 25, 2001, Feb. 6, 2001, and Jul. 2, 2001, with the U.S. Patent and Trademark Office under the Disclosure Document program. All of these references are hereby incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to a system for pumping fluid from a well. More specifically, this invention relates to a system in which a dual-displacement, subsurface pump is driven by reciprocating motion of a sucker rod or tubing string, thereby producing fluid with both halves of the stroke cycle.  
           [0004]    2. Description of Related Art  
           [0005]    To extract fluids such as water or hydrocarbons from the earth, people traditionally drill a hole through overlying formations to the fluid-containing reservoir. If the fluid pressure in the reservoir is sufficient, the fluids will fill the hole and flow to the surface of their own accord. More commonly, however, fluids will enter the hole and remain pooled near the bottom. These fluids must be pumped to the surface.  
           [0006]    Of particular interest to this disclosure are wells with high water/oil ratios and high fluid volumes. These may occur, for example, in secondary recovery oil wells where water is injected to “sweep” the last traces of hydrocarbons from a reservoir.  
           [0007]    A popular pumping system for these wells includes an electric submersible pump. In this system, the pump is typically attached to the lower end of production tubing and submerged in the fluid. An electrical cable is typically attached to production tubing to supply power for the pump. However, for deeper wells, the installation of pumping system becomes cumbersome, requiring manual strapping of the cable to the production tubing, and careful insertion to avoid accidental severing of the cable downhole. Once in place, the power dissipation in the cable may become a significant portion of operational costs.  
           [0008]    For most wells of this type, the traditional pumping system includes a single-displacement reciprocating pump. The pump is typically attached to the lower end of production tubing and submerged in the fluid. A sucker rod string extends through the production tubing between the pump and a surface pump unit on the surface. The surface pump unit reciprocates the sucker rod string to drive the single-displacement pump. Although reliable, this pumping system generally requires a large surface pumping unit, and it productively utilizes only one half of the pumping cycle.  
           [0009]    An alternative pumping system that is sometimes employed for these wells is a progressive-cavity pumping system. In this system, a progressive-cavity pump is attached to the lower end of a sucker rod string and inserted through production tubing to be submerged in the fluid. The sucker rod string connects the pump to a surface pump unit. The surface pump unit rotates the sucker rod string to drive the progressive cavity pump. Although these pumps can be run at high speed, such operation commonly causes failure in the sucker rod string. This failure is normally attributed to improper installation and/or inertial torque stresses. These systems are also subject to depth limitations.  
           [0010]    Accordingly, a need exists for a pumping system that can operate reliably and more economically than existing pumping systems.  
         SUMMARY OF THE INVENTION  
         [0011]    The problems outlined above are addressed by a dual-displacement pumping system. In one embodiment, the system includes a dual-displacement pump, a tubing string, and a surface pumping unit connected to the dual displacement pump by a reciprocating member. The reciprocating member is preferably a continuous tubing string, but a threaded tubing string or a sucker rod string with a hollow portion at its terminal end may alternatively be used. The dual displacement pump includes a pump barrel mounted to the end of the first tubing string, and a plunger mounted to the end of the reciprocating member. A valve configuration is provided so that downward motion of the plunger in the pump barrel forces fluid from the lower end of the pump barrel to enter the reciprocating member and from there, to travel to the surface. Downward motion of the plunger also fills the upper end of the pump barrel with fluid from the well bore. The valve configuration also causes upward motion of the plunger to force fluid from the upper end of the pump barrel to enter the tubing string (and travel thence to the surface), and causes the lower end of the pump barrel to fill with fluid from the well bore. In this fashion, both movements of the pumping cycle are fully exploited to nearly double the volume of fluid pumped with a conventional surface configuration.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    A better understanding of the present invention can be obtained when the following detailed description of the preferred embodiment is considered in conjunction with the following drawings, in which:  
         [0013]    [0013]FIG. 1 is an overall view of a preferred pumping system embodiment using a reciprocated continuous tubing string;  
         [0014]    [0014]FIG. 2 is an overall view of a preferred pumping system embodiment using a reciprocated sucker rod string;  
         [0015]    [0015]FIG. 3 is a cross-sectional side view of a preferred embodiment a subsurface pump; and  
         [0016]    [0016]FIG. 4 is a cross-sectional side view of an alternative embodiment of a subsurface pump. 
     
    
       [0017]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]    Turning now to the figures, FIG. 1 shows a first pumping system embodiment. A well has been drilled through the earth to intersect a fluid reservoir  102 . The well is generally lined with casing  104  that extends from the well head  106  to below the fluid reservoir  102 . The casing  104  is perforated  108  where it intersects the reservoir to allow fluid to flow into the interior of casing  104 . A blow-out preventer  110  is attached to the well head  106  for controlling fluid and gas flows from the well.  
         [0019]    A pump body  112  is affixed to the lower end of a production tubing string  114  and lowered through the blow-out preventer  110  to be submerged in the fluid pooling at the bottom of the well. The production tubing is secured to the well head  106 . Also, the pump body  112  is preferably set downhole using standard well servicing techniques. A pump plunger  116  is affixed to the bottom of a continuous tubing string  118  and lowered through the interior of the production tubing string until it is properly seated in pump body  112 . A packing unit (not specifically shown) in blow out preventer  110  seals the gap between the continuous tubing  118  and the blow out preventer  110 , but allows for vertical movement of the tubing  118 . A surface pump unit  120  reciprocates (cyclically raises and lowers) the continuous tubing string  118 , thereby reciprocating the plunger  116  in the pump body  112 . As discussed in greater detail below, the reciprocation of the plunger  116  forces fluid upward through the continuous tubing string  118  and/or the production tubing string  114  to the surface.  
         [0020]    The surface pump unit  120  shown in FIG. 1 employs a “walking beam” pump configuration to reciprocate continuous tubing string  118 . It is recognized that other alternative pump configurations may be suitable for imparting reciprocative motion to a subsurface pump plunger (e.g., hydraulic pumping units), and these alternative configurations may be employed without departing from the underlying principles of the present invention.  
         [0021]    Surface outflow from the continuous tubing string  118  is preferably conveyed to a fixed outflow passage  126  via a flexible high-pressure hose  124 . A U-shaped tube  122  is preferably connected between the continuous tubing string  118  and the flexible hose  124  to minimize wear and fatigue in flexible hose  124 . Surface outflow from the production tubing  114  exits through outflow passage  130 . Outflow passages  126  and  130  may convey the fluid outflows to an aboveground storage tank  132 . In a preferred embodiment, the pumping system produces fluid outflows through outflow passage  126  during downward motion of plunger  116 , and produces fluid outflows through outflow passage  130  during upward motion of plunger  116 . However, as explained in greater detail below, the fluid outflows may be entirely produced through the continuous tubing outflow  126  or entirely through the production tubing outflow  130 . In either of these cases, both the upward and downward motions of the plunger  116  contribute to the overall fluid outflow.  
         [0022]    Also shown in FIG. 1 is a ball valve  134  that controls surface flow to and from the casing interior. This may be used to open the casing interior to the ambient air during the initial “priming” of the well (i.e., the initial fluid fill of the tubing) to prevent an excessive pressure differential from being built up across the subsurface pump, as this could prevent the “prime” from being established.  
         [0023]    [0023]FIG. 2 shows an alternate pumping system embodiment that replaces the continuous tubing string  118  with a solid sucker rod string  218  that reciprocates in the same manner. A similar subsurface pump configuration is used. The pump plunger  116  is coupled to the sucker rod string  218  by a short tubing section  215 . Downward motion of the plunger  116  forces fluid from a chamber defined by the pump body  112  into the short tubing section  215 . The short tubing section  215  is preferably perforated above the pump body  112  to allow fluid from the tubing section  215  to flow to the surface through the production tubing  114 .  
         [0024]    [0024]FIG. 3 shows a preferred subsurface pump configuration for use with either of the pumping systems shown in FIGS. 1 and 2. For clarity, however, the ensuing discussion will focus solely on the embodiment that employs a continuous tubing string, but it is recognized that the continuous tubing string may be replaced by a sucker rod string with a hollow terminal portion.  
         [0025]    A coupler  302  connects the pump body  112  to the production tubing  114 . The pump body  112  includes an outer shell  304 , a pump barrel  306 , and an end cap  310 . A seal  312  prevents fluid leakage between the pump barrel  306  and end cap  310 . Outer shell  304  is preferably a threaded cylinder concentric with the pump barrel  306 .  
         [0026]    The complete pump configuration includes a pump plunger  318  coupled to the lower end of continuous tubing  118  (or to the lower end of short tubing section  215 ). A check valve  322  is movably mounted on the continuous tubing  118  above the plunger  318 . Between the check valve  322  and the continuous tubing string  118  is a sealing layer that allows axial motion but prevents fluids from passing between the valve and continuous tubing string. When the plunger  318  is lowered into the pump barrel  306 , the check valve  322  preferably rests on a valve seat  323  formed by coupler  302 . The contact surfaces of the check valve  322  and coupler  302  may be conical or spherical sections.  
         [0027]    Loosely mounted on the continuous tubing  118  above the check valve  322  is a centralizer  324 . The centralizer  324  preferably has three or more fins that fit within a landing nipple  325  attached to the bottom of production tubing  114 . (In an alternative embodiment, the fins may simply provide a tight frictional fit against the inside of production tubing  114 .) A coupling  326  (or fins, latches or other projections) is located on the continuous tubing  118  above the centralizer  324 . As the continuous tubing is through the production tubing  114 , the coupling  326  forces the centralizer  324  along before it.  
         [0028]    During the installation of the subsurface pump, the pump body  112  is lowered into the well on the end of the production tubing  114 . After the pump body  112  has been placed at the desired depth, the plunger  318 , check valve  322 , centralizer  326 , and coupling  326 , are lowered on the end of continuous tubing  118  through the interior of the production tubing  114  until the plunger  118  enters the pump barrel  306 . Once the plunger  318  enters the pump barrel  306 , the check valve  322  rests on the valve seat  323 . The continuous tubing  118  is lowered until the centralizer  324  is forced into place just above the check valve  322 . This position of the continuous tubing  118  represents the lowest allowable stroke position. Thereafter, as the continuous tubing  118  is reciprocated, the fit between the centralizer  324  fins and the landing nipple  325  holds the centralizer  324  in place.  
         [0029]    Once the plunger  318  is in place in the pump barrel  306 , two chambers are defined. The first chamber is defined in the pump barrel  306  below the plunger  318 . An inlet check valve  314  is provided in end cap  310  to fill the first chamber with fluid from the well bore as the plunger  318  is raised. An outlet check valve  320  is provided in plunger  318  to transfer the fluid from the first chamber to the interior of the continuous tubing string  118  as the plunger  318  is lowered. The fluid transferred to the continuous tubing string forces a similar quantity of fluid from the top of the continuous tubing string  118  at the surface.  
         [0030]    The second chamber is defined in the annulus between the pump barrel  306  and the continuous tubing  118 . Check valve  322  operates as an outlet check valve to transfer fluid from the second chamber to the interior of the production tubing string  114  as the plunger  318  is raised. The transferred fluid forces a similar quantity of fluid from the top of the production tubing string  114  at the surface. Note that centralizer  324  operates to “hold down” the check valve  322  as the plunger  318  is raised. This keeps the check valve  322  near the seat  323  so that the check valve  322  closes quickly at the beginning of the down stroke.  
         [0031]    A set of one or more inlet check valves  316  is provided in the end cap  316  to fill the second chamber with fluid from the well bore as the plunger  318  is lowered. The second chamber is filled via an annular passage between the pump shell  304  and the pump barrel  306  that connects the set of inlet check valves  316  to perforations  308  at the upper end of pump body  306 . The set of inlet check valves  316  are preferably evenly spaced about the circumference of the end cap  310 .  
         [0032]    In the embodiment of FIG. 3, the check valve  322  shown is of the traveling-valve type, with a hold down provided by the centralizer  324 . One of ordinary skill in the art will recognize that alternative configurations are possible, including without limitation, a set of flapper valves, or a set of ball-and-seat valves. Each of these valve types opens in response to differential pressure in one direction, and closes in response to differential pressure in the opposite direction. In the same vein, the check valves  314 ,  316 , and  320 , are shown as ball-and-seat valves. One of ordinary skill in the art will recognize that one or more of these valves can be replaced with alternate check valve configurations such as, e.g., flapper valves.  
         [0033]    Accordingly, the subsurface pump configuration described above is a dual-displacement pump. That is, fluid is forced to the surface on both the upward and downward movements of the pump stroke. Depending on the chosen dimensions of the described dual-displacement pump, this configuration advantageously pumps about 1.8 times the fluid volume per stroke as a single-displacement pumping system configuration, without a commensurate increase in effort. As an added advantage, existing wells can be modified by simply replacing the existing single-displacement pump with the described double displacement pump.  
         [0034]    Various contemplated dimensions for the dual-displacement pump are now provided, but these dimensions may of course be altered without departing from the underlying principles of the invention. The casing  104  may be of any standard size, although it is preferred that the minimum inner diameter be no less than five inches. The production tubing string  114  is preferably 2⅞ or 3½ inch tubing. The continuous tubing string  118  is preferably between about one- and two-inch tubing. The pump  306  barrel preferably has an interior diameter of more than 1.5 inches, and a length of more than about seventy-four inches. The pump shell  304  preferably has an exterior diameter of more than about three inches.  
         [0035]    Of course, the dual-displacement pump configuration shown in FIG. 3 is only one of many variant configurations which may be used without departing from the scope of the attached claims. Other valve locations and configurations may be used. For example, the pump shell  304  may be eliminated and inlet check valves  316  located in coupler  302 . Additionally or alternatively, the outlet check valve  322  may be replaced with a locking pump lid, and outlet check valves placed in plunger  318  to transfer fluid from the second chamber to the interior of the continuous tubing string  118  when the plunger  318  is raised.  
         [0036]    Numerous advantages may be obtained by using the disclosed pumping system. For example, existing well head and short stroke pumping units may be used, thereby eliminating any retrofitting requirements for a different artificial lift system such as electric submersible pumps, progressive cavity pumps, or even large capacity, long stroke pumping units.  
         [0037]    Another advantage which may be obtained from the disclosed pumping system is the ability to pump fluid from a multilayered reservoir without losing the opportunity to avoid gas lock by unloading or venting undesired gas through the annular space. Fluids from the multiple layers are allowed to flow down the annulus between the casing and the tubing string and to submerge the pump. Gasses flow up the annulus and may be removed from the wellhead at the surface.  
         [0038]    Advantageously, the disclosed pumping system is compatible with existing surface installations and equipment including well heads, production manifolds, prime movers and flow lines. The inclusion of the hydraulic hose assembly is considered to be a minor adaptation to any existing surface installation.  
         [0039]    The availability of coiled tubing in different diameters, wall thickness and grades of steel, allows the disclosed pumping system to be adapted for various pump depths, various well fluids, and various pumping volumes.  
         [0040]    Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, threaded tubing may be used in place of coiled tubing. The tubing may be made of steel or composite materials (composite tubing). In fact, for highly corrosive environments, composite tubing may be preferred.  
         [0041]    Additionally, this pumping system may be powered by means other than a beam pumping unit. For example, a hydraulic pumping unit may replace the beam pumping unit. It is intended that the following claims be interpreted to embrace all such variations and modifications.