Abstract:
A system for separating gas from a wellbore fluid as it is produced to the surface. The system includes a progressing cavity pump, a submergible electric motor and a fluid intake. The submergible electric motor is connected to the progressing cavity pump to drive the pump and draw wellbore fluid through the fluid intake. The fluid intake includes a hollow interior defined by a thick-walled section. Additionally, the fluid intake includes a plurality of fluid passageways extending through the thick-walled section. The passageways are oriented to create a reversal in fluid flow, and thus a release of gas, as the fluid is draw into the fluid intake.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a system and method for displacing fluids from a wellbore. More specifically, the present invention relates to a submergible pumping system utilizing a submergible electric motor, a progressing cavity pump, and a reverse flow gas separator to reduce the amount of gas pumped by the system. 
     BACKGROUND OF THE INVENTION 
     A variety of tools and other equipment are used in downhole, wellbore environments. For example, a progressing cavity pump may be utilized in producing petroleum and other useful fluids from production wells. When a progressing cavity pump system is used, production tubing is disposed within a wellbore to extend through the wellbore to the progressing cavity pump system disposed at a specific location within the well. The progressing cavity pump can be deployed or retrieved through the center of the production tubing, via a wireline or coiled tubing. 
     In operation, fluids contained in an underground formation enter the wellbore via perforations formed through a wellbore casing adjacent to a production formation. Fluids, such as petroleum, flow from the formation and collect in the wellbore. A progressing cavity pump moves the production fluids upwardly through the production tubing to a desired collection point. 
     A progressing cavity pump, consists of a single helical rotor which rotates inside a double internal helical stator. The rotor is typically made from a high strength steel while the stator is molded of an elastomeric material. When the rotor is placed within the stator, two chains of spiral cavities are formed. As the rotor turns, the cavities spiral up the length of the pump. Fluid within the cavities is carried along as the cavities progress up the length of the pump. Hence the name, progressing cavity pump. 
     A progressing cavity pumping system, typically includes a motor drivingly coupled to a progressing cavity pump. For oil field applications, the motor may be located on the surface and drivingly coupled from the surface down to a submergible progressing cavity pump in the wellbore. This is an example of a top-driven pumping system. Alternatively, the motor may be placed in the wellbore as part of an electrical submergible progressing cavity pumping system. Electric power is provided to a submergible electric motor drivingly coupled to a progressing cavity pump. The fluid displaced by the pump is communicated to the surface through production tubing. Spatial considerations among the pump, production tubing and motor encourage placement of the submergible electric motor below the progressing cavity pump. Such a system is an example of a bottom-driven pumping system. 
     A significant advantage of the progressing cavity pump is that the presence of gas in the fluid will not cause the progressing cavity pump to cavitate, as in other types of pumps. However, free gas in the fluid stream can occupy space in the cavities that could otherwise have been filled by desired liquids, such as oil. This reduces the pumps useful capacity and causes apparent pump inefficiency. 
     Rotary gas separators have been used to reduce the concentrations of gas in the fluid stream of submergible pumping systems utilizing other types of pumps, such as centrifugal pumps. Rotary gas separators use centrifugal force and differences in the specific gravities of fluids to separate a fluid into its constituent gases and liquids. Typically, the drive train of a submergible electric pumping system is coupled to the rotary gas separator. However, the drive train of a progressing cavity pump tends to produce oscillations and gyrations that propagate through the drive train during operation. Those oscillations and gyrations increase the stress on bearings supporting the drive train within the rotary gas separator and lead to a higher likelihood of bearing failure. 
     Additionally, the orientation of the motor, pump, and fluid intake in a bottom-driven system increases the complexity of using a rotary gas separator. Typically, in a bottom-driven system the system is oriented with the motor at the bottom of a tool string. The motor is coupled to the progressing cavity pump through a drive train. Fluid enters the system through a separate fluid intake that is located between the motor and the progressing cavity pump. Thus, the drive train coupling the motor to the progressing cavity pump must pass through the fluid intake to the progressing cavity pump. Consequently, the fluid intake and any other element between the motor and pump must provide structural support to the motor in order for the motor to provide torque to the pump. The structural member and torque requirements in a bottom-driven system, along with the oscillations and gyrations produced in a progressing cavity pumping system, must be factored into the design of any system incorporating a rotary gas separator into the tool string between the motor and pump. 
     Therefore, it would be advantageous to have a system that could reduce the quantity of gas pumped by a submergible electric progressing cavity pumping system without the use of a rotary gas separator. 
     SUMMARY OF THE INVENTION 
     The present invention features a submergible pumping system for displacing wellbore fluids. The system is comprised of a fluid intake and a submergible electric motor drivingly coupled to a progressing cavity pump. The fluid intake has a hollow interior defined by a thick-walled section. A plurality of fluid passageways extend through the thick-walled section and are oriented to create a reversal in fluid flow as fluid is drawn into the fluid intake. 
     According to another aspect of the invention, a pumping system for displacing wellbore fluids comprises a submergible electric motor, a progressing cavity pump operatively coupled to the submergible electric motor and disposed above the submergible electric motor when the system is oriented vertically, and a fluid intake disposed between the pump and motor. The fluid intake includes a body, a hollow interior within the body, and a sloped fluid passageway. The sloped fluid passageway extends through the body into communication with the hollow interior. When the system is oriented vertically the lowest point on an exterior end of a sloped fluid passageway is higher than the highest point on an interior end of the sloped fluid passageway. 
     According to another aspect of the present invention, a method of displacing wellbore fluids from a well is featured. The steps of the method are comprised of: drawing a wellbore fluid in a first direction along a fluid intake of a submergible pumping system, abruptly changing the flow of wellbore fluid to a second direction as the wellbore fluid enters the intake, maintaining a sufficient fluid flow rate, and maintaining a sufficient change from the first direction to the second direction to induce separation of a gas from the wellbore fluid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
     FIG. 1 is a front elevational view of a submergible pumping system, according to an embodiment of the present invention. 
     FIG. 2 is a detailed front elevational view of fluid flow in a wellbore and through a fluid passageway of a submergible pumping system, according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring generally to FIG. 1, a submergible pumping system  10  is shown in an exemplary downhole application, according to a preferred embodiment of the present invention. 
     In the particular example illustrated, a submergible pumping system  10  typically includes a submergible electric motor  12  drivingly coupled to a progressing cavity pump  14 . The progressing cavity pump  14  includes a helical rotor  16  that rotates inside a double internal helical stator  18 . The rotor  16  is made of high strength steel, and the stator  18  is made of an elastomeric material. Typically, two chains of lenticular, spiral cavities are formed when the rotor  16  is inserted into the stator. As the rotor  16  is rotated within the stator  18 , the cavities spiral up the stator  18  carrying fluid along within the cavities. 
     A fluid  20  enters the pumping system  10  through a fluid intake  22  that directs the fluid  20  to the progressing cavity pump  14 . The fluid intake  22  has a body  24  with a thick-wall section  25  defining a hollow interior  26 . The thick-wall section  25  includes at least one and preferably a plurality of fluid passageways  28  that allow fluid  20  to be drawn from the wellbore  30 , through the body  24 , and into the hollow interior  26 . The progressing cavity pump  14  intakes fluid from the hollow interior  26  and discharges the fluid to an external fluid receiving system through tubing  32 . 
     The fluid passageways  28  are oriented in the body  24  so that as fluid  20  is drawn from the wellbore  30  it will undergo an abrupt change in direction in passing from the wellbore  30  into the hollow interior  26 . The abrupt change in direction of the fluid  20  causes free gas  34  to break out of the fluid  20  and continue up the wellbore  30 . The release of free gas  34  from the fluid  20  reduces the concentration of free gas  34  in the fluid  20  that is drawn into the progressing cavity pump  14 , thus increasing the overall pumping efficiency of the pumping system  10 . 
     The output speed of an electric motor is, typically, too great to use directly to drive a progressing cavity pump. Therefore, in the illustrated embodiment, a gearbox  40  is used to reduce the speed of the submergible electric motor  12 . Additionally, a flexible drive  42  and shaft  44  are used to couple the gearbox  40  to the progressing cavity pump  14 . The flexible drive  42  helps to compensate for the oscillating motion of the pump rotor  16 . The flexible drive  42  and shaft  44  are housed within the hollow interior  26  of the fluid intake  22 . 
     A motor protector  46  also is included between the submergible electric motor  12  and the gearbox  40 . The submergible electric motor  12  and the gearbox  40  contain different fluids because of the specialized requirements of the submergible electric motor  12  and the gearbox  40 . These fluids are separated by the motor protector  46  and allowed to equalize with the well pressure. Keeping the fluids separate prevents contamination in one component from spreading into the other component and causing further damage. 
     A flow of fluid from the wellbore  30  into the hollow interior  26  is produced by the operation of the progressing cavity pump  14 . The progressing cavity pump  14  produces a low pressure region in the hollow interior  26  of the fluid intake. This creates a pressure differential between the fluid in the wellbore  30  and the fluid in the low pressure region of the hollow interior  26 . The fluid in the wellbore  30  is drawn toward the low pressure region producing a flow of fluid  20  through the fluid passageways. Fluid from the surrounding geologic formation is drawn into the wellbore  30  through perforations  48  in the wellbore casing  50 . 
     An important aspect of the present invention is the abrupt change in direction of fluid passing from the wellbore  30  into the hollow interior  26 . The illustrated embodiment utilizes fluid passageways  28  with a downward angle through the body  24 . A preferred method of operation is to position system  10  so the fluid passageways  28  are disposed above the perforations  48 . In this manner, fluid  20  is forced to flow upward through the wellbore  30  from perforations  48  to the fluid passageways  28 . 
     Because of the downward angle of the fluid passageways  28 , fluid  20  is forced to change direction from a generally upward flow in wellbore  30  to a generally downward flow through fluid passageways  28 . This effectively causes the fluid  20  to reverse its direction of flow. In other words, the direction of flow changes more than 90 degrees. 
     As illustrated in FIG. 2, reference number  60  represents the angle of deflection for a fluid flowing vertically through the wellbore  30 . If the fluid passageways were instead oriented with the perforations in the wellbore roughly horizontal to the entrance of the fluid passageways  28  fluid would flow horizontally towards the fluid passageways  28 . The change in direction of the fluid flow would not be as abrupt as if the flow were generally vertical. Reference number  62  represents the angle of deflection for fluid flowing horizontally through the wellbore  30 . If the perforations  48  were positioned at just the right height above the fluid passageways there would be no change in the fluid direction at all when entering the fluid passageways. 
     There are many factors that can affect the degree to which the fluid passageways  28  change the direction of fluid flow. The angle, size, shape and length of the fluid passageways  28  all affect the direction of fluid flow through the fluid passageways  28 . One method of changing the direction of fluid flow is to offset the entrance points and exit points of the fluid passageways  28 . For example, in the illustrated embodiment, fluid passageways  28  are formed at an angle through body  24  such that the highest point on the hollow interior side (labeled Side A) of a fluid passageway  28  is lower than the lowest point on the exterior side (labeled Side B) of a fluid passageway  28 . Reference number  64  represents the amount of offset between the highest point on the hollow interior side of a fluid passageway  28  and the lowest point on the wellbore side of a fluid passageway  28 . Generally, increasing the amount of offset will increase the angle of deflection of the fluid. It should be noted that the length of each fluid passageway is not necessarily as long as the entire flow path through body  24 . For example, some designs of body  24  may utilize flared regions or other formations at the interior side of certain fluid passageways  28 . Such regions are not considered part of the fluid passageway designed to separate a gas from the fluid. 
     The length of the fluid passageways  28 , often dictated by the thickness of the body  24 , also can affect the degree to which the direction of the fluid flow is changed. Generally, with the downwardly angled fluid passageways of the illustrated embodiment, increasing the thickness of the body  24  produces a greater amount of offset  64 . As described above, a greater amount of offset  64  generally means that a more abrupt change in direction of the fluid is achieved leading to greater separation of gas. 
     An additional aspect of the illustrated embodiment is that the diameter of the hollow interior  26  is preferably as small as practicable to allow the flexible drive  42  and shaft  44  to rotate and oscillate unobstructed. Although, the outer diameter of the fluid intake  22  is variable, it can be constrained somewhat by the typical use of the fluid intake as a coupling device for coupling the gearbox  40  to the submergible pump  14 . Maintaining the diameter of the hollow interior  26  as small as possible allows a thicker body  24  to be used for a given outer diameter of the body  24 . 
     Another operational consideration for submergible electric pumping system  10  is the provision of cooling for submergible electric motor  12 . The pumping system  10  preferably is positioned in wellbore  30  so both fluid passageways  28  and submergible electric motor  12  are located above the perforations  48  in wellbore casing  50 . Fluid is drawn upward by progressing cavity pump  14  past submergible electric motor  12 . The upward flow of fluid effectively carries away heat, thereby, cooling the submergible electric motor  12 . 
     It will be understood that the foregoing description is of preferred embodiments of this invention, and that the invention is not limited to the specific forms shown. For example, a variety of additional submergible pumping system components can be incorporated into the design and a variety of shapes, sizes, and number of fluid passageways can be utilized in the fluid intake. Additionally, the unique intake system may be used with other pumping systems and in a variety of other environments requiring separation of gas from liquid. 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.