Abstract:
A submersible pumping system for use downhole, wherein the system includes a first pump, a second pump, a recirculation coupling between the first and second pumps, and a recirculation line for directing cooling flow across the pump motor. The pumps and coupling are independent modular items connected together. Optionally, a multi-stage pump may be retrofitted with the coupling for creating a cooling flow.

Description:
BACKGROUND 
     1. Field of Invention 
     The present disclosure relates to downhole pumping systems submersible in well bore fluids. More specifically, the present disclosure concerns recirculating a portion of the flow pumped by a submersible pump of a downhole pumping system to the intake of the pumping system. 
     2. Description of Prior Art 
     Submersible pumping systems are often used in hydrocarbon producing wells for pumping fluids from within the wellbore to the surface. These fluids are generally liquids and include produced liquid hydrocarbon as well as water. One type of system used in this application employs an electrical submersible pump (ESP). ESPs are typically disposed at the end of a length of production tubing and have an electrically powered motor. Often, electrical power may be supplied to the pump motor via wireline. Typically, the pumping unit is disposed within the well bore just above where perforations are made into a hydrocarbon producing zone. This placement thereby allows the produced fluids to flow past the outer surface of the pumping motor and provide a cooling effect. 
     In some situations the submersible pumping systems are disposed in a wellbore where the pump intale is below the perforations. In this situation, fluid flowing from the producing zone reaches the pump inlet before passing by the motor. As such the produced fluid is pumped to the surface without first cooling the motor. To provide cooling to the pump motor, an ESP system may comprise multiple pumps and a recirculation line that directs flow from the discharge of a lower pump to below the motor. 
     SUMMARY OF INVENTION 
     The present disclosure includes a downhole submersible pumping system disposable in a cased wellbore. The system comprises a lower pump an upper pump, a pump motor in cooperation with the lower pump and upper pump, a seal section, a recirculation coupling connected on one end to the lower pump discharge and on the other end to the upper pump intake. The system also includes a recirculation line having an intake in fluid communication with the recirculation coupling and an exit configured to discharge fluid from the recirculation line onto the pump motor. The recirculation coupling is formed first as a modular independent component and then connected to the lower pump and upper pump. The cooperation between the pump motor and pumps may comprise a shaft extending from the pump motor to both pumps and configured to rotate impellers disposed within the pumps. The recirculation coupling is configured to receive fluid discharged from the lower pump and to direct a portion of the received fluid to the upper pump intake and the remaining portion of the received flow to the recirculation line. Optionally, the lower pump and upper pump originally comprise a part of a multi-stage pumping system and wherein the multi-stage pumping system is retrofitted to include the recirculation coupling between the lower pump and the upper pump. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a side view of a downhole submersible system in accordance with the present disclosure. 
         FIG. 2  shows an enlarged cross-sectional view of the pumping system in  FIG. 1  in a well bore. 
         FIGS. 3A-3C  show detailed cross-sectional views of a second embodiment of  FIG. 1  pumping system. 
     
    
    
     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF INVENTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be through and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     The present disclosure provides embodiments of a downhole submersible pumping system for producing fluids from within a wellbore up to the surface. More specifically, the downhole submersible pumping system described herein includes a system for recirculating flow from the pump discharge to below the pump motor. The recirculating fluid flows across the pump motor and absorbs heat therefrom as the fluid is drawn to the pump inlet. 
     Referring now to  FIG. 1 , one example of an electrical submersible pumping system is shown in side view disposed in a wellbore  5 . The electrical submersible pumping system  20  comprises a pump section  26 . The pump section  26  includes an upper pump  28 , a lower pump  29 , with a recirculation coupling  31  disposed between these two pumps ( 28 ,  29 ). The pumps ( 28 ,  29 ) are centrifugal pumps, each having multiple stages of diffusers and impellers. The electrical submersible pumping system  20  also includes an equalizer section  24  and a motor section  22 ; where the motor section  22  is disposed just below the equalizer section  24 . The equalizer section  24  provides pressure equalization between lubricant in the motor section  22  and the ambient well fluid. Bolts  36  are shown coupling the upper end of the equalizer section  24  to the bottom end  34  of the pumping section  26 . 
     In one embodiment, both the upper and lower pumps ( 28 ,  29 ) comprise independent stand alone pumps that are coaxially connected by the coupling  31  as shown. For the purposes of this disclosure, the term “independent stand alone pumps” refers to standard submersible pumps used for pumping fluids from within a wellbore. Thus, each the upper and lower pump ( 28 ,  29 ), although combined into a single unit, are capable of pumping from within a wellbore without the need for an additional pump. Similarly, in one embodiment the recirculation coupling  31  is also a modular self standing unit formed independent of either the upper or lower pump ( 28 ,  29 ) and later affixed to these pumps as illustrated in  FIG. 1 . 
     In one mode of operation of the electrical submersible pumping system  20  of  FIG. 1  comprises disposing the pumping system  20  within a wellbore  5 . In this embodiment the wellbore  5  includes casing  7  lining the substantial length of the wellbore  5 . The wellbore  5  includes perforations  10  that extend through the casing  7  and into an adjoining subterranean producing zone  8  that surrounds a portion of the wellbore  5 . Production fluid, in the form of liquid hydrocarbons, flows from the zone  8  through the perforations  10  and into the wellbore  5 . 
     The motor  22  provides a rotational motive force on the pumps ( 28 ,  29 ) for rotating impellers disposed therein thereby urging production fluid into the pumping system  20 . In this embodiment a single shaft (not shown in  FIG. 1 ) extends from pump  28  to pump  29 . Using a single shaft instead of dedicated shafts significantly reduces machining time and cost. A pump inlet  32  is provided on the lower side of the pumping system  20  for allowing production fluid into the system  20 . As shown the pump motor  22  is disposed below the perforations  10  and below the pump intake  32 . Accordingly, the production fluid makes its way from the formation  8  and perforations  10  into the pump intake  32  without contacting the pump motor  22  surface. Thus the production fluid flowing straight to the intake  32  from the perforations  10  cannot cool the pump motor  22 . 
     The embodiment of  FIG. 1  also includes a recirculation system comprising the recirculation coupling  31  in fluid communication with a recirculation line (or tube)  38 . A recirculation fluid tap  30  provides fluid communication from the recirculation coupling  31  to the recirculation line  38 . The entrance to the recirculation line  38  is at the wall of the recirculation coupling  31 . The fluid tap  30  includes a port (shown in  FIGS. 2 and 3B  as port  72 ) formed through the recirculation coupling  31 . Included with the recirculation system is a recirculation line exit  39  configured to discharge production fluid below the pump motor  22 . Due to the localized low pressure produced at the pump inlet or intake  32 , any recirculating production fluid inserted into the wellbore by the recirculation line  38  (via the line exit  39 ) will be drawn up the wellbore  5 . The recirculating production fluid flows up the wellbore annulus  40  between the pumping system  20  and the inner circumference of the casing  7  and across outer surface of the pump motor  22 . Since the production fluid that passes over the pump motor  22  cools the motor, providing fluid communication between the recirculation coupling  31  and downhole of the pump motor  22  provides the required cooling needed to operate the pump motor  22  within the subterranean wellbore  5 . Optionally, a clamp  42  may be used to connect the lower end of the recirculation line  38  to an extension tube  44 ; where the extension tube  44  extends downward in the wellbore  5  from the bottom end of the motor section  22 . 
     The portion of the produced fluid that flows into the pump intake  32  is urged upwards from the lower pump  29  through the exit of the recirculation coupling  31  into the intake of the upper pump  28 . The upper pump  28  further pressurizes the production fluid where it is discharged from the upper pump into associated production tubing  18  for delivery to the Earth&#39;s surface. Thus the pump intake  32  serves as a pump system fluid inlet for allowing fluid flow to the intake of both the lower pump  29  and the upper pump  28 . 
       FIG. 2  provides an enlarged cutaway view of an embodiment of an electrical submersible pumping system  20  having an upper pump, recirculation coupling, and lower pump. In this embodiment, upper pump  28  has internal threads  33  on its lower end that engage mating threads on the upper portion of a recirculation coupling  31 . Seals may be provided in this threaded coupling between these two elements. Lower pump  29  has internal threads  35  coupled to the lower portion of the recirculation coupling  31 . Thus, in this cutaway embodiment, the exit of the recirculation coupling  31  is illustrated communicating with the upper pump  28  intake. Similarly, the recirculation coupling  31  intake communicates with the of the lower pump  29  discharge. 
     A single integral shaft  27  is shown coaxially disposed within the upper pump  28  and lower pump  29 . The shaft  27  is coupled to impellers  37  disposed within the upper pump  28  and optionally a shaft bearing  84  supports and centers the shaft  27  within the upper pump  28 . The lower portion of the shaft  27  resides within the lower pump  29  also optionally centered within the lower pump  29  by a corresponding shaft bearing  87 . A converging conical plenum  86  describes the space where the lower pump discharge meets the recirculation coupling  31  intake. The recirculation tube  38  is shown connected on its first end to a port  41  formed through the wall of the recirculation coupling  31 . An optional orifice  47  may be included for regulating the recirculation fluid flow rate. As shown in the recirculation tube  38  is disposed in the recirculation tubing  38 , however it can also be positioned within the port  41 . Establishing the orifice size and type varies the pump design and application, however sizing the orifice is within the scope of those skilled in the art. Alternatively, a threaded fitting may be employed for attaching the tubing  38  to the port  41 . In such an embodiment, an orifice may be mounted into the fitting. The orifice  47  may comprise a “ferulle” type fitting having a sloping reduced inner diameter. The orifice  47  may also comprise a plate with a reduced diameter opening within the plate for restricting and regulating fluid flow. 
     With reference now to  FIG. 3A , a cutaway view of the upper pump section  52  of an alternative embodiment of the electrical submersible pumping system  50  is provided in more detail. As shown in this view, the upper shaft  64  is connected to impellers  58  that rotate within spaces formed in the diffusers  60 . The impellers  58  rotate with rotation of the shaft  64 . The upper pump section  52  discharges into a discharge head  71 . An annulus  61  is formed within the discharge head  71 , and is shown tapering inwards as it extends away from the upper portion of the upper pump section  52 . The discharge head  71  is shown connected to the upper terminal portion of the upper pump section  52  by a threaded connection  59 . However other forms of coupling may be included, such as a flanged bolted fitting. Optional seals are shown for a pressure and fluid seal protecting the inner portions of the pumping system  50  from the wellbore fluid. The upper pump section  52  further comprises a housing  53 , where the diffusers  60  are coaxially located along its inner circumference. The housing  53  further includes threads to mate with corresponding threads on the discharge head  71  to form the threaded fitting  59 . 
     Referring now to  FIG. 3B , a cross sectional view of the recirculation coupling  54  is shown in an enlarged illustration. As shown, the upper end of the recirculation coupling  54  is attached to the lower end of the upper pump section  52  by a threaded connection  67 . The shaft  64  extends downward from the upper pump section  52  to an optional shaft coupling  68  formed within the inner annulus of the recirculation coupling  54 . A housing  55 , forming the outer confines of the recirculation coupling has a generally annular configuration leaving a generally hollow space along the axis of the recirculation coupling  54 . The annular space  70  also includes a support and bearings  76  formed to receive the upper shaft  64  therein. 
     In this view, a port  72  is shown formed through the wall of the housing  55  thereby providing for fluid communication between the annular space  70  and the inner circumference of the recirculation tube  74 . Accordingly, the port  72  may be configured as a constriction to regulate flow therethrough to supply a requisite amount of cooling fluid from within the annular space to the outer surface of the pump motor  22 . The constriction dimensions would depend on the discharge flow of the lower pump  56  and the cooling requirements of the pump motor  22 . It is believed it is well within the capabilities of those skilled in the art to create an appropriately sized port to meet these parameters. Optionally, an orifice  75  may be included within the tube  74  for regulating the recirculation flow. Referring now to the lower end of the recirculation coupling  54 , the upper end of the lower pump section is shown threadingly coupled thereto. 
       FIG. 3C  provides an enlarged cutaway view of an embodiment of the lower pump section  56  of the electrical submersible pump system  50 . In this embodiment, the shaft  65 , which extends downward from the shaft coupling  68 , is shown passing through the lower pump section connecting to each of the impellers  78 . The corresponding diffusers  80  are shown residing within the housing  57  of the lower pump section  56 . As is known, the combination of the impellers  78  rotating within the diffusers  80  imparts a pressurizing force onto the fluid for urging it into the region above the lower pump section  56 . An inlet  82  formed through the structure of a lower head fitting  83  provides a fluid inlet for production fluids to enter the pumping system  50  from the wellbore  5 . 
     One of the many advantages of the pumping system disclosed herein is the modular ability to create the pumping system from independent stand alone elements. Previously known pumping systems having a recirculation element or recirculation function required a dedicated discharge head in a corresponding recirculation pump that directed recirculation flow upstream of the pump motor. The modular configuration disclosed herein comprises independent stand alone elements that do not require the dedicated machining and design of the recirculation discharge head. The recirculating pumping system described herein can easily be produced by using off the shelf components that do not require specific machining. 
     In the embodiments discussed, stage compression of the lower pump may be achieved by use of a compressible member, i.e., a wave washer that would be compressed to apply a force to a diffuser stack and would accommodate differences in diffuser stack and/or housing lengths due to manufacturing tolerances. Also, a bearing spider may be installed for compressing the diffuser stack in the lower pump. 
     In one optional embodiment, a recirculation system of the present disclosure is formed by retrofitting a multi-stage pumping system. A multi-stage pumping system includes two or more dedicated individual pumps coaxially disposed at different locations along the axis of the pumping system. A recirculation coupling in accordance with that disclosed herein may be inserted in the space between the severed pumps. In this embodiment the circulation coupling will have its intake and exit coupled with the respective severed ends of the multistage pumping system. By coupling the recirculation coupling with the severed ends, an integrated recirculation pumping system may be formed for insertion into and operation within a wellbore. A retrofit kit could be developed that includes all of the components needed to convert an on the shelf standard pump for recirculation applications. 
     It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the invention is therefore to be limited only by the scope of the appended claims.