Abstract:
A technique for facilitating the movement of multi-phase fluids. The technique utilizes a compressor pump and a production pump. The compressor pump compresses a fluid to remove vapor phase and then discharges the pressurized fluid to a production pump. The production pump produces the pressurized fluid to a desired location with greater efficiency due to reduction of the vapor phase.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to movement of fluid, such as a high gas-to-liquid ratio fluid, and particularly to the use of multiple pumps, in which at least one pump pressurizes the fluid and delivers the pressurized fluid to a production pump.  
         BACKGROUND OF THE INVENTION  
         [0002]    Certain types of pumps, such as centrifugal pumps, can lose efficiency or even be damaged when pumping multi-phase fluids having a relatively high gas content. For example, such pumps often are used in the production of subterranean fluids, such as oil, where the fluid can exist in a multi-phase form within the reservoir. In one type of application, a wellbore is drilled into the reservoir of desired fluid, and a pumping system is deployed in the wellbore to raise the desired fluid. The pumping system may comprise an electric submersible pumping system that utilizes a submersible motor to power a production pump, such as a centrifugal pump. When the produced fluid is a multi-phase fluid comprising oil and gas, performance of the pumping system can be substantially limited.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention relates generally to a technique for moving fluids having a relatively high gas-to-liquid ratio, such as certain fluids produced from subterranean reservoirs. The technique can be utilized with, for example, an electric submersible pumping system used within a wellbore for the production of oil. Of course, the technique may have applications in other environments and with other types of fluid.  
           [0004]    In this technique, a compressor pump is employed to compress the vapor phase in a multi-phase fluid. This pressurized fluid is then delivered to a production pump that moves the fluid to a desired location. By delivering fluid to the production pump with reduced or eliminated vapor phase, the efficiency and longevity of various types of production pumps can be improved.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and:  
         [0006]    [0006]FIG. 1 is a front elevational view of an exemplary electric submersible pumping system disposed within a wellbore;  
         [0007]    [0007]FIG. 2 is a front elevational view of an exemplary electric submersible pumping system utilizing the present technique;  
         [0008]    [0008]FIG. 3 is a partial cross-sectional view taken generally along the axis of a production pump and a compressor pump, according to one aspect of the present invention;  
         [0009]    [0009]FIG. 4 is a cross-sectional view of the compressor pump illustrated in FIG. 3 taken generally along the axis of the pump;  
         [0010]    [0010]FIG. 5 is an enlarged view of a portion of a stage similar to those illustrated in FIG. 4; and  
         [0011]    [0011]FIG. 6 is a cross-sectional view similar to that of FIG. 4 but showing an alternate embodiment of the pump.  
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0012]    Referring generally to FIG. 1, an exemplary application of the inventive technique is illustrated. Although this is one embodiment of the invention, a variety of other applications and environments may benefit from the inventive technique disclosed herein. In this embodiment, an electric submersible pumping system  10  is illustrated. Submersible pumping system  10  comprises a variety of components depending on the particular application in which it is used. Typically, system  10  comprises at least a production pump  12  which, in this application, is a centrifugal pump. The system also comprises a submersible motor  14  that powers production pump  12 . Typically, a motor protector  16  is coupled to motor  14  to isolate internal motor fluids from wellbore fluids. Furthermore, submersible pumping system  10  comprises a fluid intake  18  and a vapor phase reduction or compressor pump  20 . (See also FIG. 2)  
         [0013]    In the illustrated example, submersible pumping system  10  is designed for deployment in a well  22  within a geological formation  24  containing desirable production fluids, such as petroleum. In this application, a wellbore  26  is drilled and lined with a wellbore casing  28 . Wellbore casing  28  typically has a plurality of openings  30 , e.g. perforations, through which production fluids flow into wellbore  26 .  
         [0014]    Submersible pumping system  10  is deployed in wellbore  26  by a deployment system  32  that also may have a variety of forms and configurations. For example, deployment system  32  may comprise tubing  34  connected to electric submersible pumping system by a connector  36 . Power is provided to submersible motor  14  via a power cable  38 . Submersible motor  14 , in turn, powers production pump  12  and compressor pump  20  which draws production fluid in through pump intake  18  and pumps the production fluid to production pump  12 . Production pump  12  then pumps or produces the fluid to a collection location  40 , e.g. at the surface of the earth. In this embodiment, production pump  12  produces fluid through tubing  34 .  
         [0015]    It should be noted that the illustrated electric submersible pumping system  10  is an exemplary embodiment. Other components can be added to this system and other deployment systems may implemented. Additionally, the production fluids may be pumped to the surface through tubing  34  or through the annulus formed between deployment system  32  and wellbore casing  28 . These and other modifications, changes or substitutions may be made to the illustrated system.  
         [0016]    As illustrated best in FIG. 2, the various components of electric submersible pumping system  10  are coupled together at appropriate mounting ends. For example, production pump  12  typically includes an outer housing  42  having an upper mounting end  44  and a lower mounting end  46 . Similarly, compressor pump  20  comprises an outer housing  48  having an upper mounting end  50  and a lower mounting end  52 . Intake  18  also has an upper mounting end  54  and a lower mounting end  56 ; motor protector  16  has an upper mounting end  58  and a lower mounting end  60 ; and submersible motor  14  has at least an upper mounting end  62 .  
         [0017]    The various mounting ends permit each of the components to be selectively coupled to the next adjacent components for assembly of a desired electric submersible pumping system  10 . This modular approach permits individual components to be substituted, removed, repaired and/or rearranged. In the embodiment illustrated, adjacent mounting ends are held together by appropriate fasteners, such as bolts  64 .  
         [0018]    The illustrated production pump  12  and compressor pump  20  are separate or independent units that may be selectively and independently coupled into electric submersible pumping system  10  at a variety of locations. In the present embodiment, compressor pump  20  is coupled to production pump  12  at a location upstream from production pump  12 . In this manner, compressor pump  20  receives wellbore fluid through intake  18  and sufficiently compresses the wellbore fluid to remove undesired pockets of vapor phase in the wellbore fluid. The pressurized fluid is discharged directly to production pump  12 , e.g. a centrifugal pump. With the vapor phase removed or substantially reduced, production pump  12  is able to efficiently produce fluid to desired location  40 .  
         [0019]    As illustrated in FIG. 3, a desirable compressor pump  20  comprises a helicoaxial pump contained within its own separate housing  48 . As described above, housing  48  has an upper mounting end  50  that may be selectively coupled to the next adjacent component which, in this case, is production pump  12  and specifically lower mounting end  46  of production pump  12 . The mounting ends may be standard mounting ends used with components of electric submersible pumping systems. To aid explanation, compressor pump  20  will hereinafter be referred to as helico-axial pump  20 .  
         [0020]    Helico-axial pump  20  comprises a central or axial shaft  66  that is rotated or powered by submersible motor  14 . Shaft  66  is rotatably mounted within housing  48  by appropriate bearing structures  68 . Typically, shaft  66  comprises a splined lower end  70  and a splined upper end  72  to facilitate coupling to corresponding shaft segments in adjacent components. Furthermore, shaft  66  typically extends through a plurality of stages  74 . The number of stages will vary according to the level of pressurization desired for a given environment or application. However, the embodiment illustrated in FIG. 3 shows eight stages  74 .  
         [0021]    Each stage  74  comprises a helical impeller  76  rotationally affixed to shaft  66 . The helical impeller  76  may be rotationally affixed to shaft  66  in a variety of ways known to those of ordinary skill in the art, such as through the use of a key and keyway (not shown). As illustrated best in FIGS. 4 and 5, each helical impeller  76  comprises a central hub portion  78  and a fin  80  helically wrapped about central hub portion  78 .  
         [0022]    Each stage  74  also comprises a diffuser  82  designed to direct fluid discharged from the corresponding helical impeller  76 . An exemplary diffuser  82  is rotationally affixed with respect to housing  48  and comprises a central opening  84  to rotatably receive shaft  66  therethrough. Each diffuser  82  further comprises a flow channel  86  through which fluid is directed upwardly upon discharge from helical fin  80  of the subsequent, lower helical impeller  76 . In this design, a bearing assembly or bearing unit  89  is combined with at least some and often all of the diffusers  82  to promote longevity of the pump.  
         [0023]    When shaft  66  and helical impellers  76  are rotated, fluid is drawn through a housing inlet  88  from intake  18  and directed upwardly through each stage until discharged through a housing outlet  90  to production pump  12 . In the embodiment illustrated, shaft  66  is coupled to a shaft  92  of production pump  12  by an appropriate coupling device  94 . Thus, rotation of shaft  66  causes rotation of shaft  92  in production pump  12 . Generally shaft segments  66  and  92 , as well as other shaft segments for additional components, each have a single diameter. It should be noted that the production pump  12  illustrated in FIG. 3 is a centrifugal pump as is commonly used in electric submersible pumping systems for the production of wellbore fluids. However, other types of production pumps also may be utilized in some applications.  
         [0024]    The helico-axial pump  20  is designed to generate a lower head than centrifugal pump  12 . Also, the efficiency of the helico-axial pump  20  may be lower than that of the production pump provided it is able to compress the vapor phase in the fluid to a level the centrifugal pump  12  is able to handle without substantial, detrimental head degradation. The use of a helico-axial pump to remove vapor phase is particularly beneficial and, in combination with a centrifugal pump, has resulted in substantially improved production parameters. Additionally, the modular design of the system with separate pump housings and separate shafts connected by coupling device  94  permit ease of assembly, disassembly, servicing, replacement, etc. of either or both pumps.  
         [0025]    Furthermore, bearing assemblies  89  promote longevity and reliability of pump  20 . In the embodiment illustrated in FIG. 5, the bearing assemblies  89  are combined with individual diffusers  82  to provide a combined diffuser/bearing unit. The exemplary bearing assembly  89  comprises a radial bearing  96  mounted in a bearing seat or receiving area  98  of diffuser  82 . An annular bushing  100  is mounted to shaft  66  and deployed radially inward from radial bearing  96 . Typically, annular bushing  100  is rotationally affixed to shaft  66  such that a radially outer surface  102  of annular bushing  100  slides against a radially inward surface  104  of radial bearing  96 .  
         [0026]    As illustrated, one or more, e.g. two, O-rings  106  may be deployed between radial bearing  96  and bearing receiving area  98 . The O-rings  106  are resilient and allow for a slight amount of movement of radial bearing  96  to accommodate slight variations in shaft  66 . Additionally, a retainer ring  108  may be used to position radial bearing  96  within bearing receiving area  98 . Radial bearings  96  and corresponding annular bushings  100  can be deployed at each stage or selected stages, such as every other stage.  
         [0027]    An alternate embodiment of helico-axial pump  20 , labeled  20 ′, is illustrated in FIG. 6. In this embodiment, a separate bearing unit  110  is disposed between several of the helical impellers  76  and diffusers  82 . For example, the various components may be sequentially arranged from bottom to top in the order: helical impeller  76 , diffuser  82 , bearing unit  110 , helical impeller  76 , diffuser  82 , bearing unit  110 , etc. Each bearing unit  110  has a flow path  112  to permit the flow of fluid therethrough. Bearing units  110  typically are utilized in place of the bearing assemblies  89  discussed above with reference to FIGS. 4 and 5. Bearing units  110  can be designed, for example, to incorporate radial bearings and annular bushings similar to those described above with respect to bearing assemblies  89 .  
         [0028]    Because the gaseous phase has a tendency to accumulate in the radial center of the pump, lack of lubrication between bearing and shaft can become a problem in certain environments or applications. Accordingly, bearing structures  68 , radial bearings  96 , annular bushings  100 , and bearing units  110  can be designed with wear-resistant materials for such applications. Exemplary materials comprise ceramic materials, such as zirconia and silicon carbide. In the embodiment illustrated in FIGS. 4 and 5, for example, both the radial bearing  96  and annular bushing  100  can be made from ceramic materials. Use of such materials prolongs the useful life of helico-axial pumps  20  and  20 ′.  
         [0029]    It will be understood that the foregoing description is of exemplary embodiments of this invention, and that the invention is not limited to the specific forms shown. For example, the technique may be useful in other applications and environments in which multi-phase fluids are pumped from one location to another; a variety of electric submersible pumping system components may be added, changed or substituted for the components illustrated and described; the number of stages used in either the compressor pump or production pump can be adjusted; and the materials utilized may vary. These and other modifications may be made in the design and arrangement of the elements without departing from the scope of the invention as expressed in the appended claims.