Abstract:
A centrifugal pump has a gas accumulation reduction system to reduce the risk of gas locking caused by the accumulation of gas at the inlet of the impeller. The gas accumulation reduction system includes: (i) one or more diffuser ports extending through the hub of a diffuser; and (ii) one or more recirculation passages extending through the hub of an impeller. The recirculation passages are in fluid communication with the one or more diffuser ports to permit the recirculation of a portion of pumped fluid through the stage. Additionally, a centrifugal pump that includes at least one turbomachinery stage. The stage includes a rotatable impeller that has an impeller hub with a centrally disposed eye and a plurality of impeller vanes. The impeller is variously configured to encourage mixing of two-phase fluids at the eye of the impeller hub.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of downhole turbomachines, and more particularly to downhole turbomachines optimized for reducing phase separation of pumped fluids. 
     BACKGROUND 
     Submersible pumping systems are often deployed into wells to recover petroleum fluids from subterranean reservoirs. Typically, a submersible pumping system includes a number of components, including an electric motor coupled to one or more high performance pump assemblies. Production tubing is connected to the pump assemblies to deliver the petroleum fluids from the subterranean reservoir to a storage facility on the surface. The pump assemblies often employ axially and centrifugally oriented multi-stage turbomachines. 
     Most downhole turbomachines include one or more impeller and diffuser combinations, commonly referred to as “stages.” The impellers rotate within adjacent stationary diffusers. A shaft keyed only to the impellers transfers mechanical energy from the motor. During use, the rotating impeller imparts kinetic energy to the fluid. A portion of the kinetic energy is converted to pressure as the fluid passes through the downstream diffuser. 
     Although widely used, conventional downhole turbomachinery is vulnerable to “gas locking,” which occurs in locations where petroleum fluids include a significant gas to liquid ratio. Gas locking often causes the inefficient operation or complete failure of downhole turbomachinery. The gas-locking phenomenon can be explained by the dynamics of fluid flow through the impeller and diffuser. As gas and liquid pass through the channels of a diffuser, its flow directions are guided by curved vanes. The change of flow directions usually generates relatively high and low pressure zones in the flow channels. The streamwise and transverse pressure gradients, streamline curvature and slip between different phases contribute to the segregation of the phases. Gas bubbles tend to move into low pressure zones because of the hydrodynamic behavior of bubbles in liquids. When the two-phase mixtures exit the diffuser, there tend to be more bubbles in the low pressure zones than in the high pressures zones. In severe cases, phase separation can occur in the flow. Upon separation, the gas phase tends to accumulate in certain regions of the flow passage, causing head degradation and gas locking. 
     In particular, fluid exiting the diffuser and entering the impeller eye often experiences a pressure drop that is usually higher on the shroud side of the vane at the time of entrance to the vanes of the impeller. This pressure drop increases the separation of gas components from liquid components within the fluid. Centrifugal force tends to carry the heavier liquid components to the outer regions of the impeller while the lighter portions concentrate toward the interior of the impeller eye. Gas locking typically begins at the inlet suction side of the vane and extends the accumulation of the increased bubble size to the hub end of the impeller to complete the gas locking of the pumping system. 
     There is therefore a continued need for an improved pump assembly that effectively and efficiently produces two-phase fluids from subterranean reservoirs. It is to these and other deficiencies in the prior art that the present invention is directed. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment, the present invention includes a centrifugal pump having a rotatable shaft and at least one stage. The at least one stage includes a stationary diffuser and a rotatable impeller connected to the shaft. The at least one stage further comprises a gas accumulation reduction system to reduce the risk of gas locking caused by the accumulation of gas at the inlet of the impeller. The gas accumulation reduction system includes: (i) one or more diffuser ports extending through the diffuser hub; and (ii) one or more recirculation passages extending through the impeller hub. The recirculation passages are in fluid communication with the one or more diffuser ports to permit the recirculation of a portion of pumped fluid through the stage. 
     In other embodiments, the present invention provides a centrifugal pump that includes at least one turbomachinery stage. The turbomachinery stage includes a stationary diffuser that includes a diffuser hub and a plurality of diffuser vanes. The stage further includes a rotatable impeller that has an impeller hub with a centrally disposed eye and a plurality of impeller vanes. The impeller is variously configured to encourage mixing of two-phase fluids at the eye of the impeller hub. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front perspective view of a downhole pumping system in a non-vertical installation. 
         FIG. 2  is a side cross-sectional view of the pump of the submersible pumping system of  FIG. 1 . 
         FIG. 3  is a perspective view of a first preferred embodiment of a multiphase homogenizer. 
         FIG. 4  is a perspective view of a second preferred embodiment of a multiphase homogenizer. 
         FIG. 5  is a cross-sectional view of a diffuser constructed in accordance with a first preferred embodiment. 
         FIG. 6  is an upstream view of the diffuser of  FIG. 5 . 
         FIG. 7  is an upstream view of an impeller constructed in accordance with a first preferred embodiment. 
         FIG. 8  is a perspective view of the impeller of  FIG. 7 . 
         FIG. 9  is an upstream view of an impeller constructed in accordance with a second preferred embodiment. 
         FIG. 10  is a perspective view of the impeller of  FIG. 9 . 
         FIG. 11  is an upstream view of an impeller constructed in accordance with a third preferred embodiment. 
         FIG. 12  is an upstream view of an impeller constructed in accordance with a fourth preferred embodiment. 
         FIG. 13  is a perspective view of an impeller constructed in accordance with a fifth preferred embodiment. 
         FIG. 14  is a side cross-sectional view of an impeller constructed in accordance with a sixth preferred embodiment. 
         FIG. 15  is an upstream view of the impeller of  FIG. 14 . 
         FIG. 16  is a side cross-sectional view of a diffuser constructed in accordance with a second preferred embodiment. 
         FIG. 17  is an upstream view of the diffuser of  FIG. 16 . 
         FIG. 18  is a cross-sectional view of a stage constructed in accordance with a presently preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In accordance with a preferred embodiment of the present invention,  FIG. 1  shows a front perspective view of a downhole pumping system  100  attached to production tubing  102 . The downhole pumping system  100  and production tubing  102  are disposed in a wellbore  104 , which is drilled for the production of a fluid such as water or petroleum. The downhole pumping system  100  is shown in a non-vertical well. This type of well is often referred to as a “horizontal” well. 
     As used herein, the term “petroleum” refers broadly to all mineral hydrocarbons, such as crude oil, gas and combinations of oil and gas. The production tubing  102  connects the pumping system  100  to a wellhead  106  located on the surface. Although the pumping system  100  is primarily designed to pump petroleum products, it will be understood that the present invention can also be used to move other fluids. It will also be understood that, although each of the components of the pumping system  100  are primarily disclosed in a submersible application, some or all of these components can also be used in surface pumping operations, which may include, for example, the transfer of fluids between storage facilities, the removal of liquid on surface drainage jobs, the withdrawal of liquids from subterranean formations and the injection of fluids into subterranean wells. 
     The pumping system  100  preferably includes some combination of a pump assembly  108 , a pump intake  108   a , a motor assembly  110  and a seal section  112 . In a preferred embodiment, the motor assembly  110  is an electrical motor that receives its power from a surface-based supply. The motor assembly  110  converts the electrical energy into mechanical energy, which is transmitted to the pump assembly  108  by one or more shafts. The pump assembly  108  then transfers a portion of this mechanical energy to fluids within the wellbore, causing the wellbore fluids to move through the production tubing to the surface. In a particularly preferred embodiment, the pump assembly  108  is a turbomachine that uses one or more impellers and diffusers to convert mechanical energy into pressure head. 
     The seal section  112  shields the motor assembly  110  from mechanical thrust produced by the pump assembly  108 . The seal section  112  is also preferably configured to prevent the introduction of contaminants from the wellbore  104  into the motor assembly  110 . Although only one pump assembly  108 , pump intake  108   a , seal section  112  and motor assembly  110  are shown, it will be understood that the downhole pumping system  100  could include additional pumps assemblies  108 , seals sections  112  or motor assemblies  110 . 
     Referring to  FIG. 2 , shown therein is a cross-sectional view of the pump  108 . The pump  108  preferably includes a head  114 , a base  116  and a housing  118 . In most applications, the head  114  is connected to a pump discharge or directly to the production tubing  102 . The base  116  assembled with pump intake  108   a  is typically connected to the seal section  112  or another component with the pumping system  100 . However, other arrangements of the pump  108 , seal section  112  and motor assembly  110  are contemplated. 
     The pump  108  further includes one or more turbomachinery stages  120  and a centrally disposed shaft  126  that is configured to rotate about the longitudinal axis of the pump  108 . The shaft  126  transfers the mechanical energy from the motor  110  to the working components of the pump  108 . The housing  118  and shaft  126  are preferably substantially cylindrical and fabricated from a durable, corrosion-resistant material, such as steel or steel alloy. Unless otherwise specified, each of the components described in the downhole pumping system  100  is constructed from steel, aluminum or other suitable metal alloy or material. 
     Each stage  120  preferably includes a rotating impeller  122  fixed to the shaft  126  and a stationary diffuser  124  fixed to the housing  118 . The impeller  122  and diffuser  124  are preferably fixed to the shaft  126  and housing  118 , respectively, with keyed or press-fit connections, although a variety of alternative methods are also acceptable. As addressed herein, novel modifications to the impellers  122  and diffusers  124  have resulted in stages  120  that are well-suited for handling pumped fluids with high gas-to-liquid ratios. 
     Continuing with  FIG. 2 , the pump assembly  108  also includes a fluid homogenizer  132 . Turning to  FIGS. 3 and 4 , shown therein are perspective views of first and second embodiments, respectively, of the homogenizer  132 . The homogenizer  132  includes a homogenizer hub  134  and a plurality of homogenizer blades  136  attached to the homogenizer hub  134 . The homogenizer hub  134  is preferably keyed to the shaft  126  so that the homogenizer  132  rotates with the shaft  126 . In the embodiment depicted in  FIG. 3 , the homogenizer blades  136  are straight and include a homogenizer blade hole  138 . In contrast, the homogenizer blades  136  of the homogenizer  132  depicted in  FIG. 4  are curved. 
     The homogenizer  132  includes a minimum of two homogenizer blades  136  and more preferably includes between three and eight homogenizer blades  136 . The homogenizer blades  136  are preferably set at a minimum pitch (vane angle) of 10 degrees and a maximum vane angle of 90 degrees. The homogenizer  132  is configured so that the homogenizer blades  136  cause the pumped fluid to rotate in the same direction of rotation as the impellers  122 . The homogenizer  132  optionally includes one or more blade holes  138  within the homogenizer blades  136 . Each blade hole  138  is used to further increase mixing and prevents the rotation of the entire fluid mass at the entrance of the downstream impeller  122 . In a particularly preferred embodiment, the optimum width of the vanes should be limited to the length to radius ratio (L/R) of less than one. 
     In the presently preferred embodiment depicted in  FIG. 2 , the homogenizer  132  is positioned at the inlet of the pump assembly  108 . The homogenizer  132  is used to break up and disperse any large gas bubbles or slugs before the fluid passes into the first stage  120 . It will be appreciated, however, that additional homogenizers may be used throughout the pump assembly  108  to further homogenize and blend the gas and liquid components of a multiphase fluid. 
     Turning to  FIGS. 5 and 6 , shown therein are cross-sectional and upstream views of a diffuser  124 A constructed in accordance with a first preferred embodiment. The diffuser  124 A includes a diffuser hub  140 , a diffuser shroud  142  and a plurality of diffuser vanes  144 . The diffuser shroud  142  is configured to fit within the inner surface of the housing  118  (not shown in  FIGS. 5-6 ). As one of ordinary skill in the art will recognize, the number and design of the plurality of diffuser vanes  144  is based on application-specific requirements and not limited by the present invention. In preferred embodiments, each diffuser  124  includes between 3 to 10 diffuser vanes  144 . 
     The profile of the outer diameter of the diffuser hub  140  and the inner diameter of the diffuser shroud  142  are formed by the revolution of at least one line segment that is inclined at an angle to the longitudinal axis of the diffuser  124 A. As best illustrated in the cross-sectional view of  FIG. 5 , in the preferred embodiment, the profile of the diffuser hub  140  resembles a truncated conical form with a linearly decreasing outer diameter in the downstream direction. The inner diameter of the diffuser shroud  142  follows the profile of the diffuser hub  140  to create a downstream throat  146 . Fluid passing through the throat  146  tends to decelerate as it exits the diffuser  124 . 
     Referring generally to  FIGS. 7-15 , shown therein are various depictions of impellers  122  constructed in accordance with preferred embodiments. Each of the impellers  122  includes one or more features that are designed to improve resistance to gas locking in the presence of high gas-to-liquid ratios. Each impeller  122  generally includes an impeller hub  148  and a plurality of impeller vanes  150 . In the presently preferred embodiment, the impeller vanes  150  are configured as spiraled, overlapping flights. Vane angles can vary from a minimum of about 5 degrees to about 25 degrees at the inlet of the impeller  122 , with a maximum of about 50 degrees at the discharge of the impeller  122 . Although not so required, each impeller  122  preferably includes between 2 and 8 vanes. Each impeller  122  includes an impeller eye  149  that constitutes the space between the impeller hub  148  and the leading edge of the impeller vanes  150 . 
     In the particularly preferred embodiment depicted in  FIGS. 7-8 , the impeller  122 A includes a series of primary impeller vanes  150 A and series of secondary impeller vanes  150 B. The primary impeller vanes  150 A extend from the eye  149  of the impeller hub  148  to the edge of the impeller  122 . The secondary vanes  150 B are shorter and positioned outside the primary impeller vanes  150 A. The secondary vanes  150 B extend radially outward from a middle portion of the impeller hub  148 . The use of the primary and secondary impeller vanes  150 A,  150 B decreases the number of vanes near the eye  149  of the impeller hub  148  which in turn reduces the risk of trapping gas bubbles near the eye of the impeller hub  148 . 
     Notably, the embodiment of the impeller  122 A depicted in  FIGS. 7 and 8  is an “open impeller” that does not include a shroud on the upstream side of the vanes  150 . By removing the shroud that is typically found on impellers, the impeller  122 A is capable of running in close tolerance with the downstream side of the diffuser  124 . Minimizing the distance between the diffuser  124  and impeller  122 A further reduces the risk of gas locking by removing an area of low pressure between the discharge of the diffuser  124  and the suction inlet of the impeller  122 A. The smaller space between the diffuser  124  and impeller  122 A also reduces the area in which gas bubbles may accumulate. 
     Turning to  FIGS. 9 and 10 , shown therein are front and perspective upstream views of an impeller  122 B constructed in accordance with a second preferred embodiment. In the second preferred embodiment depicted in  FIGS. 9 and 10 , a series of primary vanes  150 A extends from the eye  149  of the impeller hub  148  to the discharge edge of the impeller  122 B. In the embodiment depicted in  FIGS. 9 and 10 , the secondary impeller vanes  150 B have been eliminated. The impeller  122 B is an open impeller that does not include a shroud on the upstream edge of the primary vanes  150 A. 
     The primary vanes  150 A include two or more vane slots  152 . The vane slots  152  contribute to mixing by allowing a portion of the pumped fluid to pass through the vane  150 A. The mixing provided by the vane slots  152  helps to maintain a homogenous gas-liquid mixture as the fluid passes through the impeller  122 B. Although two vane slots  152  are shown on each vane  150 A in  FIGS. 9 and 10 , it will be appreciated that fewer or additional vane slots  152  may be incorporated within each vane  150 A. Alternatively, it may be desirable to include the vane slots  152  on less than all of the vanes  150 A. 
     Turning to  FIG. 11 , shown therein is a front upstream view of an impeller  122 C constructed in accordance with a third preferred embodiment. The impeller  122 C includes a series of splitter vanes  150 C disposed in a radial array around the eye  149  of the impeller hub  148 . Each splitter vane  150 C includes a central portion  154  and a radial portion  156  extending along the exterior of the impeller  122 C. The impeller  122 C is an open impeller that does not include a shroud along the upstream side of the central portion  154  or the radial portion  156  of the splitter vanes  150 C. The splitter vanes  150 C were designed by taking a conventional vane and removing the middle portion of the vane to create two, spaced-apart sections. The spaced apart central portion  154  and radial portion  156  of the splitter vanes  150 C provides homogenization of the gas and liquid components of the fluid passing through the impeller  122 C. 
     Turning to  FIG. 12 , shown therein is a front upstream view of an impeller  122 D constructed in accordance with a fourth preferred embodiment. In the fourth preferred embodiment, the impeller  122 D is an open (shroudless) impeller that includes a series of primary vanes  150 A and secondary vanes  150 B. The impeller  122 D includes one or more recirculation passages  158  (four are shown in  FIG. 12 ). The recirculation passages  158  pass through the impeller  122 D so that a portion of the pumped fluid from the downstream, high-pressure portion of the impeller  122 D returns to the upstream, lower pressure face of the impeller  122 D. This recirculation of a portion of the pumped fluid further increases homogenization of the fluid. Significantly, the returned fluid is injected near the eye  149  of the hub  148 , where gas bubbles tend to accumulate. The injection of recirculated fluid at the eye  149  of the hub  148  further reduces the risk of gas bubble accumulation at the inlet of the impeller  122 D. 
     Turning to  FIG. 13 , shown therein is an upstream perspective view of an impeller  122 E constructed in accordance with a fifth preferred embodiment. In the fifth preferred embodiment, the impeller  122 E includes a partial shroud  160  extending across the primary vanes  150 A. The impeller  122 E provides the combination of the shroud  160  with the recirculation passages  158 . 
     Turning to  FIGS. 14-15 , shown therein are cross-sectional and front views, respectively, of an impeller  122 F constructed in accordance with a sixth preferred embodiment. In the sixth preferred embodiment, the impeller  122 F includes a shroud partial  160  and recirculation passages  158 . In addition to the recirculation passages  158 , the impeller  122 F further includes shroud apertures  162 . Like the recirculation passages  158 , the shroud apertures  162  provide a bypass through which a portion of the pumped fluid returns to the face of the impeller  122 F. The recirculation of fluid to the inlet of the impeller  122 F increases turbulence and discourages the accumulation of the gas at the suction side of the impeller  122 F. 
     Turning to  FIGS. 16-17 , shown therein are cross-sectional and upstream views, respectively, of a diffuser  124 B constructed in accordance with a second preferred embodiment of the present invention. The diffuser  124 B includes diffuser ports  164  that extend through the diffuser hub  140 . The diffuser ports  164  allow a portion of the pumped fluid to bypass the diffuser vanes  144 , thereby increasing the turbulence and mixing at the downstream side of the diffuser  124 B. The increased turbulence further counteracts the accumulation and separation of large gas pockets within the stage  120 . 
     Turning to  FIG. 18 , shown therein is a cross-sectional view of a stage  120  constructed in accordance with a preferred embodiment. The stage  120  includes two diffusers  124 B surrounding an impeller  122 F. The impeller  122 F and diffuser  124 B are each provided with bypass routes that allow fluid to be recirculated through the stage  120  to improve homogenization. As noted in  FIG. 18 , a portion of the fluid entering the upstream side of the diffuser  124 B passes through the diffuser ports  164  directly into the suction side of the impeller  122 F. As the impeller  122 F imparts kinetic energy into the fluid, a portion is passed around the outside of the impeller  122 F through the shroud apertures  162 . Additionally, higher pressure fluid from the downstream diffuser  124 B is passed back through the diffuser ports  164  to the suction inlet of the impeller  122 F through the recirculation passages  158 . The bypass and recirculation at the interface between the impeller  122 F and diffuser  124 B significantly reduces the accumulation of gas pockets and increases the resistance of the pump  108  to gas locking. 
     Although various features of the preferred embodiments have been depicted separately, it will be understood that it is contemplated that any number of combinations of these features is encompassed within the scope of the present invention. For example, it may be desirable to employ a shroudless impeller  122 A in combination with the diffuser  122 B that includes diffuser ports  164 . Similarly, it may be desirable to mix-and-match different features within a single pump assembly. In a presently preferred embodiment, the stages  120  positioned near the base  116  of the pump  108  are provided with impellers  122  that include vane slots  152  in combination with diffusers  124  that incorporate diffuser ports  164 . The collection of these features collectively comprises an improved solution for reliably pumping fluids with a high or variable gas-to-liquid ratio. 
     It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and functions of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. It will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other systems without departing from the scope and spirit of the present invention.