Abstract:
A submersible pumping system includes a pump, a motor that drives the pump and a separator assembly. The separator assembly is for separating gas from the fluid and includes an intake and a vent above the intake. Fluid enters the separator assembly at the intake and the vent returns a portion of the fluid into wellbore for recirculation into the intake.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to the field of downhole pumping systems, and more particularly to gas separators for separating gas from well fluid prior to pumping. 
   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 pump assemblies. Production tubing is connected to the pump assemblies to deliver the petroleum fluids from a production stream in the subterranean reservoir to a storage facility on the surface. 
   Production streams usually contain a combination of liquids and gases, and excessive amounts of gases in the production stream can cause the pump to malfunction or operate inefficiently. In progressive cavity pumps, gas pockets occupy space in the pump that could otherwise be occupied by desirable liquids, thereby lowering the efficiency of the pump. Most pumps work best with a gas concentration in the production stream of less than twenty five percent. 
   Rotary gas separators have been used to remove gas from production streams before entry into the pump. Rotary gas separators take advantage of the difference in specific gravities of gas and liquids by using centrifugal force to separate the gas and liquid components. Rotary mechanisms such as spinning chambers force the liquids to the outside radius of the rotary separator and the gases remain near the inside radius of the rotary separator because liquids are heavier than gases. 
   The radial positions of the liquids and gases after centrifugal separation are disadvantageously located for the desired venting of the gases to the wellbore and the axial pumping of the liquids. To solve this problem, rotary separators often employ crossover mechanisms that transfer the liquids to the center of the separator for entry into the pump and transfer the gases to the outer radius of the separator for venting away from the pump. These mechanisms include passages that route the gases and liquids to the desired location for venting into the wellbore or for pumping to the surface. The rotary and crossover mechanisms add complexity and cost to the separators, and can result in costly downtime for the submersible pumping system when repairs are needed. 
   It would therefore be desirable to separate liquids and gases in a production stream without the use of complex mechanisms that increase manufacturing and maintenance costs. 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 provides a submersible pumping system for producing a fluid from a wellbore. The submersible pumping system includes a pump, a motor that drives the pump and a separator assembly. The separator assembly is for separating gas from the fluid and includes an intake and a vent above the intake. Fluid enters the separator assembly at the intake and the vent returns a portion of the fluid into the wellbore for recirculation into the intake. 
   In alternate preferred embodiments, the separator assembly includes a shaft rotated by the motor, an inducer rotated by the shaft, and an orifice positioned between the vent and the pump. 
   The present invention provides a method for separating gas from a wellbore fluid. The method includes moving the wellbore fluid from an intake through a separator assembly, diverting a portion of the wellbore fluid from the separator assembly into the wellbore, and recirculating the diverted wellbore fluid into the intake. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an elevational view of an electric submersible pumping system disposed in a wellbore constructed in accordance with a preferred embodiment of the present invention. 
       FIG. 2  is an elevational view of a separator assembly for use with the electrical submersible pumping system  FIG. 1 . 
       FIG. 3  is a cross section view of the separator assembly of  FIG. 2 . 
       FIG. 4  is a top plan view of a support bearing for use with the separator assembly of  FIG. 2 . 
       FIG. 5  is a top plan view of a support bearing for use with the separator assembly of  FIG. 2   
       FIG. 6  is a top view of an orifice plate for use with the separator of  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In accordance with a preferred embodiment of the present invention,  FIG. 1  shows an elevational view of a pumping system  100  attached to production tubing  102 . The 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. As used herein, the term “petroleum” refers broadly to all mineral hydrocarbons, such as crude oil, gas and combinations of oil and gas. Petroleum enters the wellbore  104  through perforations  105 . 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 substances that consist of other fluids. 
   The pumping system  100  preferably includes some combination of a pump assembly  108 , a motor assembly  110  and a separator assembly  112 . Although not shown, the pumping system  100  can also include components such as seal sections, gear boxes and various sensors. The motor assembly  110  is provided with power from the surface by a power cable  114 . The motor assembly  110  preferably drives the pump assembly  108  to move a fluid from the wellbore  104  to the surface through the production tubing  102 . 
   Turning to  FIGS. 2 and 3 , shown therein are elevational and cross-sectional views of the separator assembly  112 , respectively. The separator assembly  112  preferably includes an intake  116 , a chamber  118 , a neck  119 , and vents  120 . The intake  116  permits well fluid to enter the separator  112  and is preferably fitted with a screen  117  that blocks large pieces of rock, dirt or other debris that may be present in the wellbore. 
   The chamber  118  acts as a conduit for the flow of fluid through the separator assembly  112 , and is generally defined to be cylindrically shaped by a housing  121 . The neck  119  is preferably situated towards the top of the separator assembly  112 , and in a presently preferred embodiment, is characterized by a narrowing of the housing  121  and includes vents  120  that link the chamber  118  to the wellbore  104 . The vents  120  may optionally include one-way valves  123  that restrict fluid flow by only allowing fluid to exit the chamber  118 . It will be understood that the size and angular disposition of the vents  120  can be varied to control the amount of fluid in the chamber  118  that exits the separator assembly  112 , as discussed in more detail below. 
   The separator assembly  112  can also include a shaft  122 , an inducer  124 , an orifice plate  126 , and support bearings  128 ,  130 . The inducer  124 , which is preferably affixed to the rotating shaft  122  by a keyed connection or other known methodology, imparts energy to the fluid as the inducer  124  spins with the rotating shaft  122 . The inducer  124  preferably increases the pressure in the chamber  118  to a level greater than the pressure in the wellbore  104 . The positive head pressure created by the inducer  124  prevents well fluid from flowing into the chamber  118  through vents  120 . For applications in which the separator assembly  112  is located between the pump  108  and the motor  110 , the shaft  122  also transfers rotational energy from the motor  110  to the pump  108 . 
   Turning to  FIG. 4 , shown therein is a top plan view of support bearing  128 . The support bearing  128  includes a sleeve  132  and a collar  134 . The sleeve  132  is fixed to the shaft  122  and the collar  134  is fixed to the housing  121 . The sleeve  132  rotates with the shaft  122  while the collar  134  remains stationary. The support bearing  128  is located in the chamber  118  where the fluid flows from the intake  116  to the top of the separator assembly  112 . To permit the flow of fluid through the chamber  118 , the support bearing  128  includes fluid passages  136 . In this way, the support bearing  128  provides axial alignment to the shaft  122  without hindering the flow of fluid through the chamber  118 . 
   Turning now to  FIG. 5 , shown therein is a top plan view of support bearing  130 . The support bearing  130  preferably includes a sleeve  132  fixed to the shaft  122  and a collar  134  fixed to the housing  121 . Because the support bearing  130  is positioned below the intake  116 , fluid passages are not necessary. Support bearings such as support bearing  130  do not require fluid passages if they are located in areas where the flow of fluid is not needed or desired. 
   Turning to  FIG. 6 , shown therein is a top plan view of the orifice plate  126 . The orifice plate  126  is fixed to the housing  121  of the separator assembly  112  and provides an orifice  138  through which well fluid flows out of the chamber  118  toward the pump  108 . The size of the orifice  138  affects the volumetric flow of fluid from the separator assembly  112  into the pump assembly  108  and recirculation. Various sizes of orifice  138  can be chosen to regulate fluid flow based on factors such as pump capacity and the desired flow of fluid in the separator assembly  112 . It will be understood that the movement of well fluid through the separator assembly  112  is caused by the cooperative operation of the motor  110  and the pump assembly  108 . 
   During operation, well fluid enters the separator assembly  112  at intake  116 . As the well fluid in the chamber  118  reaches the vents  120 , well fluid from an outer diameter of the chamber  118  is diverted into the wellbore  104  through the vents  120  and the remaining portion of the well fluid in the chamber  118  flows toward the pump  108 . 
   As well fluid exits the vents  120  into the wellbore  104 , gas in the vented fluid ascends toward the top of the wellbore and the remaining well fluid (with a higher concentration of liquid) descends toward the intake  116  for recirculation through the separator assembly  112 . As the recirculation continues, well fluid entering the separator assembly  112  becomes less encumbered with gas. The gas level of the well fluid in the separator assembly  112  thereby decreases with continuous recirculation of the vented well fluid. 
   Because untreated well fluid is constantly introduced into the intake  116  from the perforations  105  in the well, the maximum reduction of gas content is limited by the amount of gas in the untreated well fluid. It is thought that the percentage by which the gas content of the well fluid is reduced is approximately equal to the percentage of well fluid vented into the wellbore from the chamber  118  after the system has reached a steady state. For example, well fluid from a formation that produces a gas concentration of twenty percent is expected to be reduced to a gas concentration of about ten percent if half the well fluid is vented back into the wellbore. Likewise, seventy five percent venting should result in a fluid stream of five percent gas content that reaches the pump. 
   In the presently preferred embodiment, the vents  120  direct about fifty percent of the well fluid moving through the separator assembly  112  from the chamber  118  to the wellbore  104 . This amount can be varied by changing the size and angular disposition of the vents  120 , and by adjusting the size of the orifice  138 . If a greater reduction of gas is desired, more fluid should be vented and recirculated. Variations in recirculation rates can be chosen based on characteristics such as the performance of the pump and the gas content of the well. For example, some types of pumps that are sensitive to a high gas content will require more well fluid to be recirculated. Similarly, wells with a high gas content may also require more well fluid to be recirculated. 
   The reduction of gas content is also affected by the amount of time the separator assembly  112  is in operation. The commencement of recirculation of fluid in the separator assembly  112  begins the process of reducing gas content from the level found in untreated well fluid. Only after sufficient time has elapsed will the separator assembly  112  reduce the gas content to the theoretical limit. 
   The efficiency of the recirculation process depends at least in part on the amount of time in which gas is allowed to separate from liquid as it recycles outside the separator assembly  112 . The “separation time” can be controlled by adjusting the velocity of the recycle stream and/or the length of the recycle path. The velocity of the recycle stream can be controlled by varying the outer diameter of the separator assembly  112  with respect to the inner diameter of the wellbore  104 . The length of the recycle path can be controlled by modifying the length of the separator assembly  112 , and more particularly the distance between the vents  120  and the intake  116 . 
   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.