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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to electrically driven submersible well pumps, and in particular to a gas dissipation chamber for removing the gas processed by a through-tubing conveyed gas separator, thereby preventing such gas from entering the pump intake and gas locking the pump 
     2. Description of the Related Art 
     Most oil wells being pumped by a downhole electrical pump typically will also produce some gas. If the gas is of sufficient volume, it can reduce the performance of the pump. In these circumstances, gas separators are mounted in the assembly below the pump to separate gas from the well fluid entering the intake of the pump. 
     Typically, prior art gas separators utilize a rotatably driven rotor within a cylindrical housing. The rotor has at least one blade and often an inducer vane. The blade will impart a centrifugal force to the well fluid flowing through the housing. This centrifugal force tends to separate the liquid components from the gas components because of difference in densities, with the liquid components locating near the outer wall of the housing, and the gas remaining near the shaft. 
     A discharge member, mounted above the rotor, provides a passage from the central portion of the rotor to the exterior of the gas separator to discharge gas. The discharge member also provides a liquid passageway for the remaining portion of the well fluid to flow upward toward the intake of a pump. In most systems the pump is suspended on and discharges into the production tubing. The separated gas flows up the annular space in the casing surrounding the tubing. 
     In other types of installations, the pump assembly is lowered into and suspended within the production tubing. Preferably the motor is mounted to the lower end of the production tubing, and the pump assembly stabs into engagement with the drive shaft of the motor. The pump discharges into the production tubing. If a through tubing gas separator is desired, it would be lowered along with the pump assembly through the tubing. In such case, there would be very little clearance around the gas separator and the pump for the separated gas to dissipate up the tubing. Therefore a gas bubble could be created around the intake, causing a gas lock. 
     SUMMARY OF THE INVENTION 
     A gas dissipation chamber for through tubing conveyed ESP (electrical submersible pump) pumping system prevents gas discharged from the gas separator from entering the pump intake and subsequently gas locking the pump system. The gas dissipation chamber is installed in the string of tubing between the tubing crossover to the motor and the production tubing string. The gas dissipation chamber is a tubular device having a series of slots and ports and is located above the seal section and motor. The pump and a gas separator are lowered through the tubing and land in the gas dissipation chamber. 
     The gas dissipation chamber has a larger inner diameter than the production tubing to provide an annular flow area above the gas separator. Lower ports on the gas dissipation chamber allow the well fluid to enter the gas separator, while the gas discharged from the gas separator will flow up the annular flow area and be vented out through upper slots in the chamber, thereby permitting principally liquid to enter the pump. The gas dissipation chamber shunts the discharged gas from the gas separator and the pump intake, thereby preventing the gas locking of the pump system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B comprise a partially sectional view of an electrical submersible pump assembly and a gas dissipation chamber constructed in accordance with this invention. 
     FIGS. 2A and 2B comprise a side elevational view of the submersible pump assembly and gas dissipation chamber of FIGS. 1A and 1B. 
     FIG. 3 is a cross-sectional view of the gas separator of the submersible pump assembly of FIGS. 1A and 1B. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 and 2, a string of production tubing  1  extends from the surface into a cased well. Production tubing  1  is a conduit, typically made up of sections of pipe, for example four inches in diameter, screwed together. Production tubing  1  supports a submersible pump assembly. 
     Referring to FIGS. 1B and 2B, the submersible pump assembly includes a motor  5  that is in this embodiment a three-phase A.C. electric motor. A power cable (not shown) connects to motor  5  and extends alongside tubing  1  to the surface for delivering power. Motor  5  is filled with a lubricant and coupled to a seal section  7 , which seals well fluid from the interior of motor  5  and also equalizes pressure differential between lubricant in motor  5  and the exterior. Motors other than three-phase electrical motors are also feasible. 
     A multi-piece drive shaft  9  extends upward through seal section  7  and is driven by motor  5 . Drive shaft  9  has a splined upper end that is rotatably supported within a tubular cross-over housing  11  by bushings. Cross-over housing  11  includes an adapter  12  with a threaded upper end. Adapter  12  may be integrally formed with cross-over housing  11  or secured by threads as shown. 
     A gas dissipation chamber  13  has a lower end secured to adapter  12 . The upper end of gas dissipation chamber  13  is secured by an adapter  15  to production tubing  1 . The weight of motor  5  and seal section  7  is thus supported by chamber  13 . Referring to FIGS. 2A and 2B, chamber  13  has a set of lower ports or slots  17  located in its side wall near the lower end of chamber  13 , and a set of upper ports or slots  19  located in the side wall near the upper end of chamber  13 . Chamber  13  has a larger inner diameter than production tubing  1 . Normally, however, the maximum outer diameters of the motor assembly comprising seal section  7  and motor  5  are greater than the inner diameter of chamber  13 . Preferably, the maximum outer diameter of chamber  13  is approximately the same as the maximum outer diameters of seal section  7  and motor  5 . 
     Referring again to FIG. 1B, a gas separator  21  is located entirely within chamber  13 . Gas separator  21  has an intake on its lower end for receiving well fluid flowing inward through lower ports  17 . Gas separator  21  separates gas from the liquid of the well fluid and may be of different types. FIG. 3 illustrates one suitable type. Gas separator  21  has a tubular housing  23  through which a shaft  25  rotatably extends. An adapter (not shown) mounts to the lower end for making a stabbing engagement of shaft  25  with the splines of drive shaft  9 . A head  27  secures to the upper end of housing  23  by threads. Head  27  is coupled to a lower end of a submersible pump  29  (FIGS.  1 A and  2 A). Head  27  has an axial discharge passage  31  for discharging liquid. into the intake of pump  29 . The upper end of shaft  25  connects to a drive shaft contained in pump  29 . A plurality of intake ports  31  are located at the lower end of separator housing  23 . Intake ports  31  incline upward for drawing fluid into the lower end of housing  23 . Optional screens  32  may be employed over inlet ports  31 , if desired. 
     In this embodiment, an inducer  33  comprising a helical vane is mounted within separator housing  23  for rotation with shaft  25 . A set of blades  35  are mounted above inducer  33  and rotate with shaft  25  for forcing heavier components of the well fluid outward due to centrifugal force. A cross-over  37  formed in head  27  collects the centrally located lighter components, such as gas, and directs them outward through a gas outlet port  39  in the side wall of housing  23 . The heavier liquid components flow upward through axial passage  31  to the intake of pump  29  (FIGS.  1 A and  2 A). 
     In this embodiment, pump  29  is a centrifugal pump, having a plurality of stages of inducers and impellers, however, other types of pumps are also feasible. Pump  29  has a tubular adapter  40  (FIG. 1A) on its upper end that is adapted to be coupled by a running tool (not shown) to a line, such as coiled tubing or a cable, for lowering and retrieving pump  29  through tubing  1 . Adapter  40  also has a seal  41  that is actuated by the running tool to seal adapter  29  to the interior of production tubing  1 . Seal  41  thus seals the discharge end of pump  29  to the interior of tubing  1 . 
     Gas dissipation chamber  13  encompasses gas separator  21  and preferably substantially the entire length of pump  29  so as to place upper ports  19  as far as practical from lower ports  19 . This results in the gas being released into the casing a considerable distance from the intake of well fluid into chamber  13 . In some cases, the distance between lower ports  17  and upper ports  19  may be 30 feet or more. However, it is not necessary that the entire length of pump  29  locate within chamber  13 . The maximum outer diameter of gas separator  21  and pump  29  is smaller than the inner diameter of chamber  13  by a significant amount so as to create an annulus around gas separator  21  and pump  29  for gas discharged from gas outlet port  39  to flow upward. For example, the maximum outer diameter of gas separator  21  and pump  29  may be only about 2.7 inches, while the inner diameter of chamber  13  may be more than 4.5 inches. The lower ports  17  on the gas dissipation chamber  13  permit the well fluid and entrained gas to enter the gas separator  21 . The upper ports  19  of the gas dissipation chamber  13  permit the gas discharged from the gas separator  21  to be vented out, thereby permitting substantially only liquid to enter the intake of pump  29 . 
     In the operation, motor  5  and seal section  7  are secured to the lower end of chamber  13  by adapter  12 . Chamber  13  is secured to the lower end of tubing  1  by adapter  15 . Tubing  1  is then lowered into the well to a desired depth, while the power cable for motor  5  is strapped alongside tubing  1 . Then pump  29  and gas separator  21  are lowered through tubing  1 . The adapter on the lower end of gas separator  21  stabs separator drive shaft  25  into engagement with drive shaft  9 . The running tool (not shown) and coiled tubing are detached from adapter  40  and retrieved to the surface. 
     When power is supplied, motor  5  will rotate drive shaft  9 , which in turn will rotate shaft  25  of gas separator  21  and the drive shaft extending through pump  29 . Pump  29  will draw fluid through intake ports  31  of gas separator  21 . Gas separator  21  will proceed to separate the gas from the liquid and will vent the discharged gas from the gas separator  21  through outlets  39  into chamber  13 . Gas separator  21  delivers the liquid directly into the lower end of pump  29 . The discharged gas will travel up the annular space in chamber  13  around gas separator  21  and pump  29  and exit chamber  13  through upper ports  19 . The separated liquid is discharged by pump  29  into tubing  1 , where it flows to the surface. The gas discharged into the casing flows to the surface for gathering. There may be a packer between tubing  1  and the casing to isolate a hydrostatic head of well fluid in the casing from perforations in the casing. If so, passages with check valves may be provided in the packer to allow the upward flow of gas in the casing. 
     Periodically, the pump assembly comprising pump  29  and gas separator  21  may be retrieved through tubing  1  to the surface for repair or replacement. A running or retrieval tool is lowered through tubing  1  into engagement with adapter  40  for retrieving pump  29  and gas separator  21 . Motor  5  and chamber  13  will remain downhole with tubing  1 . 
     The invention has significant advantages. The discharge of the gas into the chamber and out the upper ports in the chamber prevents the discharged gas from forming into a gas bubble near the pump intake. 
     Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention maybe utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein or in the steps or in the sequence of steps of the methods described herein without departing from the spirit and the scope of the invention as described.

Summary:
A gas dissipation chamber, installed between the tubing crossover and the production tubing string, for a through tubing conveyed ESP pumping system prevents gas discharged from the gas separator from entering the pump intake and subsequently gas locking the pumping system. The gas dissipation chamber secures to a lower end of production tubing. An electrical motor assembly is suspended on the lower end of the chamber. The gas separator and the pump are lowered through the tubing and land in the chamber in operative engagement with the motor assembly. Well fluid flows into the chamber to the separator, and gas separated by the separator vents out of the chamber into the casing. Liquid separated from the well fluid by the separator is pumped by the pump into the production tubing.