Abstract:
In one embodiment, a pump assembly for pumping a wellbore fluid in a wellbore includes a pump, a fluid separator, a motor for driving the pump, and a shroud disposed around the fluid separator for guiding a gas stream leaving the fluid separator, wherein the gas stream is prevented from mixing with fluids in the wellbore.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention generally relate to an electrical submersible pump assembly adapted to efficiently reduce a gas content of a pumped fluid. Particularly, embodiments of the present invention relate to an electrical submersible pump assembly having a device to direct gas flow leaving the assembly. 
     2. Description of the Related Art 
     Many hydrocarbon wells are unable to produce at commercially viable levels without assistance in lifting formation fluids to the earth&#39;s surface. In some instances, high fluid viscosity inhibits fluid flow to the surface. More commonly, formation pressure is inadequate to drive fluids upward in the wellbore. In the case of deeper wells, extraordinary hydrostatic head acts downwardly against the formation, thereby inhibiting the unassisted flow of production fluid to the surface. 
     In most cases, an underground pump is used to urge fluids to the surface. Typically, the pump is installed in the lower portion of the wellbore. Electrical submersible pumps are often installed in the wellbore to drive wellbore fluids to the surface. 
     In a well that has a high volume of gas, a gas separator may be included in the ESP system to separate the gas from the liquid. The gas is separated in a mechanical or static separator and is vented to the well bore where it is vented from the well annulus. The separated liquid enters the centrifugal pump where it is pumped to the surface via the production tubing. 
     In a well that produces methane gas, the electrical submersible pump is generally used to pump the water out of the wellbore to maintain the flow of methane gas. Typically, the water is pumped up a delivery pipe, while the methane gas flows up the annulus between the delivery pipe and the wellbore. However, it is inevitable that some of the methane gas entrained in the water will be pumped by the pump. Wells that are particularly “gassy” may experience a significant amount of the methane gas being pumped up the delivery pipe. 
     For coal bed methane wells, it is generally desirable that no methane remain in the water. Methane that remains in the water must be separated at the surface which is a costly process. Therefore, a gas separator may be used to separate the gas from liquid to reduce the amount of methane gas in the pumped water. 
       FIG. 1  shows a prior art downhole electric submersible pump (ESP) assembly  10  positioned in a wellbore  5 . The ESP assembly  10  includes a motor  20 , a motor seal  25 , a gas separator  30 , and a pump  40 . The gas separator  30  is positioned between the pump  40  and the motor seal  25 . The motor  20  is adapted to drive the gas separator  30  and the pump  40 . A central shaft extends from the motor  20  and through the motor seal  25  for engaging a central shaft of the separator  30  and a central shaft of the pump  40 . Fluid enters the ESP assembly  10  through the intake port  32  in the lower end of the gas separator  30 . The fluid is separated by an internal rotating member with blades attached to the shaft of the gas separator  30 . The gas separator  30  may also have an inducer pump or auger at its lower end to aid in lifting the fluid to the blades. Centrifugal force created by the rotating separator member causes denser fluid (i.e. fluid having more liquid content) to move toward the outer wall of the gas separator  30 . The fluid mixture then travels to the upper end of gas separator  30  toward a flow divider in the gas separator. The flow divider is adapted to allow the denser fluid to flow toward the pump, while diverting the less dense fluid to the exit ports  38  of the gas separator  30 . Gas leaving the gas separator  30  travels up the annulus  7 . 
     One problem that arises is that the gas leaving the gas separator may commingle with the fluid flowing toward the intake port. In this respect, the gas content of the pumped fluid may be inadvertently increased by the gas leaving the separator. The increase in gas entering the gas separator when this occurs reduces the efficiency of the gas separator which may result in incomplete separation of the gas from the liquid. This has negative effects on pump performance and in a coal bed methane well will result in methane in the water being pumped from the well. 
     There is a need, therefore, for an apparatus and method for efficiently reducing a gas content of a pumped fluid. There is also a need for apparatus and method for maintaining a separated gas from a fluid to be pumped. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide methods and apparatus for preventing a separated gas leaving a pump assembly from mixing with a fluid in the wellbore. 
     In one embodiment, a pump assembly for pumping a wellbore fluid in a wellbore comprises a pump; a gas separator; a motor for driving the pump; and a shroud disposed around the gas separator for guiding a gas stream leaving the gas separator, wherein the gas stream is prevented from mixing with fluids in the wellbore. In one embodiment, the shroud guides the gas stream to a location above a liquid level in the well bore. 
     In another embodiment, a method of pumping wellbore fluid in a wellbore includes receiving the wellbore fluid in a separator; separating a gas stream from the wellbore fluid; exhausting the gas stream from the separator; and guiding a flow of the exhausted gas stream up the wellbore while substantially preventing the gas stream from mixing with fluids in the wellbore. The method further includes venting the gas stream above a fluid level in the wellbore and pumping the wellbore fluid remaining in the separator. In one embodiment, the method also includes disposing a shroud around the separator to guide the flow of the exhausted gas stream. 
     In another embodiment gas is vented above a zone where all the fluid is entering the well annulus. This can be a perforated zone or entry of multilateral legs in the well. 
     In yet another embodiment, a pump assembly for pumping a wellbore fluid in a wellbore includes a pump, a gas separator having a vent port, a motor for driving the pump, and a tubular sleeve in fluid communication with the vent port, wherein a gas stream in the tubular sleeve is prevented from mixing with fluids in the wellbore. 
     In yet another embodiment, a pump assembly for pumping a wellbore fluid in a wellbore includes a pump, a gas separator having a vent port, a motor for driving the pump, and a flow control device coupled to the vent port, wherein the vent port controls the outflow of a separated gas stream and the inflow of fluids through the vent port. In one embodiment, the flow control device includes an elastomeric tubular sleeve disposed around the vent port. In another embodiment, one end of the tubular sleeve is attached to the gas separator and another end of the tubular sleeve has a clearance between the tubular sleeve and the gas separator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  is a schematic view of prior art electric submersible pump. 
         FIG. 2  is a schematic view of an embodiment of an electric submersible pump assembly.  FIG. 2A  illustrates an alternative embodiment. 
         FIG. 3  is a cross-sectional view of a gas separator highlighting the separation of liquid and gas shown in  FIG. 2 . 
         FIG. 4  is a cross-sectional view of the top of a gas separator that has the gas vented in a conduit. 
         FIG. 5  is a cross-sectional view of the top of a gas separator that has a flapper valve on the gas vents. 
         FIG. 6A  is a partial view of a gas separator having a tubular sleeve type fluid control device.  FIG. 6B  is a partial view of another embodiment of a gas separator having a tubular sleeve type fluid control device. 
         FIGS. 7A-B  are partial views of a flap type fluid control device for a gas separator. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the present invention provide methods and apparatus for preventing a separated gas from commingling with fluids in the well bore. 
       FIG. 2  shows an embodiment of an electric submersible pump assembly  100  adapted to prevent the separated gas from commingling with the wellbore fluid. The ESP assembly  100  includes a motor  120 , a motor seal  125 , a gas separator  130 , and a pump  140 . The motor  120  is adapted to drive the gas separator  130  and the pump  140 . A central shaft extends from the motor  120  and through the motor seal  125  for engaging a central shaft  133  of the separator  130  and a central shaft of the pump  140 . The motor seal  125  may be used to couple the motor  120  to the separator  130  and the pump  140 . In one embodiment, the motor seal  125  is a barrier type seal having an elastomeric diaphragm or bag. Other suitable motors and motor seals known to a person of ordinary skill are also contemplated. 
       FIG. 3  illustrates an exemplary gas separator suitable for use with the electric submersible pump assembly  100 . In one embodiment, the gas separator  130  includes one or more intake ports  132  at its lower end and one or more exhaust ports  138  at its upper end. The separator  130  includes a rotating member  145  with blades (e.g., an impeller) that is attached to the shaft  133  of the separator  130  and is rotatable therewith. The separator  130  may optionally include an inducer pump or auger  147  at its lower end to aid in lifting the fluid to the blades. The separator  130  may further include a bearing support  151  to provide support to the shaft  133  during rotation. Rotation of the shaft  133  by the motor  120  causes the inducer  147  to rotate, thereby lifting the fluids entering the intake ports  132 . Rotation of the shaft  133  also causes the rotating member  145  to generate a centrifugal force in the gas separator  130 . The centrifugal force causes the denser fluid (i.e. fluid having more liquid content) to move toward the outer wall of the separator  130  and the less dense fluid (i.e., fluid having more gas content) to collect in the central area of the separator  130 . The fluid mixture then travels up the separator  130  and passes through a flow divider  135  positioned at an upper portion of the separator  130 . 
     In one embodiment, the flow divider  135  includes a lower ring  134  and a conical upper end, as illustrated in  FIG. 3 . Orientation of the flow divider  135  is parallel to and coaxial with the central shaft  133 . The lower ring  134  has a diameter that is smaller than the inner diameter of the separator  130 . An inner fluid passage  136  connects the interior of the lower ring  134  to exhaust ports  138  in the sidewall of the separator  130 . As the fluid flows up and toward the flow divider  135 , the more dense fluid located near the outer wall of the separator  130  are outside of the perimeter of the lower ring  134 . Thus, the denser fluid is allowed to flow around the flow divider  135  and up the outer passage  142  toward the conical upper end, which leads to the pump  140 . The less dense fluid (also referred to herein as “separated gas”) located in the inner part of the separator  130  are within the boundary of the lower ring  134 . Thus, the separated gas enters the lower ring  134  and is diverted into the fluid passages  136  and out through the exhaust ports  138 . In this respect, the flow divider  135  may be used to separate the gas from the liquid. It must be noted that other suitable fluid dividers known to a person of ordinary skill in the art may also be used, for example, a static gas separator. 
     Referring back to  FIG. 2 , the ESP assembly  100  is provided with a shroud  150  to guide the flow of the separated gas up the annulus  7 . In one embodiment, the shroud  150  is tubular shaped and is positioned around the separator  130  and the pump  140 , thereby creating an annular area between the separator  130  and the shroud  150 . The length of the shroud  150  is such that the lower end extends below the exhaust ports  138  and the upper end extends above the exhaust ports  138  to a height that is above the liquid level  9  in the wellbore  5 . As shown, the lower end of the shroud  150  remains open to the well bore  5 . The opening may allow venting of the gas below exhaust ports  138 , if the need arises. Alternatively, the lower end of the shroud  150  may be closed to the well bore (see  FIG. 2A ). The shroud  150  may be coupled to the ESP assembly  100  using a connection member such as a centralizer  137 . The centralizer  137  allows fluid flow in the annular area  139  while serving as a connector for the shroud  150  to the ESP assembly  100 . In another embodiment, the connection member may be one or more spokes or other suitable connection device capable of allowing fluid flow up the annular area. It must be noted that although the shroud is described as extending above the liquid level in the well, the shroud may be extended to any suitable length. For example, the upper end of the shroud may extend above the exhaust ports to a height that is above a zone where all of the fluids enter the well annulus. This zone may be the perforated zone or entry of multilateral legs in the well. 
     The ESP assembly  100  may optionally include a motor shroud  160  to guide the flow of wellbore fluid into the ESP assembly  110 . In one embodiment, the motor shroud  160  is tubular shaped and is positioned around the motor  120  and the intake port  132 . The inner diameter of the motor shroud  160  is larger than the outer diameter of the motor  120  such that fluid flow may occur therebetween. The upper end of the motor shroud  160  is connected to the separator  130  at a location above the intake port  132  and is closed to fluid communication. The lower end of the motor shroud  160  extends at least partially to the motor  120 , preferably, below the motor  120 . To enter the intake port  132 , wellbore fluid must flow down the exterior of the motor shroud  160 , around the lower end of the motor shroud  160 , and up the interior of the motor shroud  160  toward the intake port  132 . The wellbore fluid circulating the motor shroud  160  advantageously cools the motor  120 , thereby reducing overheating of the motor  120 . 
     In operation, the ESP assembly  100  may be used to pump water out of a coal bed methane well. The ESP assembly  100  is positioned in the well bore  5  such that the intake port  132  is below the perforations  8  in the wellbore  5 . Wellbore fluid  11 , which may be mixture of water and gas, may enter the annulus  7  through the perforations  8  and flow downward toward the intake port  132 . The fluid  11  may flow past the exterior of the motor shroud  160 , then up the interior of the motor shroud  160 . The wellbore fluid  11  enters the ESP assembly  100  through the intake port  132  of the separator  130 . The motor  120  rotates the rotating members  145  of the separator  130  to apply centrifugal force to the well bore fluid  11 . The centrifugal force causes the denser fluid to move toward the sidewall of the separator  130  as the wellbore fluid  11  travels up the separator  130 . As the wellbore fluid  11  nears the flow divider  135 , the denser, higher water content fluid located near the sidewall is allowed to flow past the inner ring  134  and up the outer passage  142  toward the pump  140 , where it is pumped to a tubing for delivery to the surface. The less dense, higher gas content fluid located in the inner area of the separator  130  enters the lower ring  134 , flows through the fluid passages  136 , and leaves the separator  130  through the exhaust ports  138 . After leaving the separator  130 , the separated gas is guided up the annular area  139  between the shroud  150  and the separator  130  by the inner wall of the shroud  150 . The separated gas is vented out of the shroud  150  at a location that is above the wellbore fluid level  9 . In this respect, the separated gas is substantially prevented from commingling with the wellbore fluid  11  flowing toward the lower end of the ESP assembly  100 . In this manner, water may be efficiently removed from the coal bed methane well. 
       FIG. 4  shows another embodiment of a ESP assembly. In this embodiment, the ESP assembly is equipped with a flow tube  239  connected to the exhaust port  238  of the separator  130 . The flow tube is adapted to guide the flow of separated gas from the separator and up the annulus  7 . The length of the flow tube  239  is such that the upper end extends to a height above liquid level in the wellbore  5 . 
       FIG. 5  shows another embodiment of a gas separator equipped with a valve to control the flow of separated gas out of the exhaust port  138 . In one embodiment, the valve is a flapper valve  236 . The flapper valve  236  may be adapted to open at a predetermined force. For example, the flapper valve  236  may be spring biased to close. In this respect, flapper valve will only open if the separated gas in the separator can generated enough force to open the flapper valve  236 . In the closed position, the flapper valve  238  keeps fluids from entering through the exhaust port  138 . Other suitable types of valves include one-way valves, backflow valve, check valve, and ball valve. 
       FIG. 6A  shows another embodiment of a flow control device for the gas separator  330 . The flow control device may be a tubular sleeve  310  and positioned around the exhaust port  338  of the gas separator  330 . One end  311  of the tubular sleeve  310  is attached to the outer surface of the gas separator  330  while the other end  312  is unattached. The free end  312  has an inner diameter that is slightly larger than the outer diameter of the gas separator  330 . The difference in diameters creates an opening  315  for the separated gas to vent. In one embodiment, the tubular sleeve  310  is made of an elastomeric material such as rubber. When a large amount of liquid tries to enter through the opening  315 , the liquid would force the elastomeric tubular sleeve  310  against the gas separator  330 , thereby closing the opening  315 . In another embodiment, the tubular sleeve  310  may be positioned in a recess  325  in the outer surface of the gas separator  330 , as illustrated in  FIG. 6B . The tubular sleeve  310  placed in the recess  325  would reduce the potential of liquid flowing into the gas separator  330 . 
     In another embodiment, the flow control device may be one or more flaps  350  disposed adjacent the exhaust port  338 , as illustrated in  FIGS. 7A-B . The flap  350  may be manufactured from an elastomeric material, but should have sufficient rigidity to remain substantially straight. In one embodiment, a metal support  360  may be attached to the flap  350  to provide additional rigidity to the flap  350 . Fasteners such as rivets  365  or adhesive may be used to attach the metal support  360  to the flap  350 . One end  351  of the flap  350  is anchored (or attached) to the gas separator while the other end  352  is unanchored. The anchor may be an elastomeric anchor or any suitable anchor capable of keeping the flap  350  substantially vertical. In operation, the flap  350  is hingedly attached to the gas separator. The flap  350  may be pushed open by the venting gas. Thereafter, the flap  350  swings back to the closed position. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follows.