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FIELD OF THE INVENTION 
   This invention relates in general to submersible rotary well pump installations, and in particular to a riser pipe assembly for separating gas in the well fluid prior to entry in the pump intake. 
   BACKGROUND OF THE INVENTION 
   One category of well pump is an electrically driven rotary pump that is driven by a downhole electrical motor. These types of pumps operate best when pumping fluid that is primarily liquid. If the well fluid contains large quantities of gas, a gas separator can be connected to the pump assembly upstream of the pump for separating gas in the well fluid and discharging it into the casing. A common type of gas separator has rotatable vanes that separate the gas by centrifugal force. 
   While a gas separator works well enough to separate gas prior to the entry in the pump, another problem exists, particularly in horizontal wells where slugging is a problem. The term “gas slugging” refers to large gas bubbles that are encountered and which may require several minutes to dissipate through the pump or gas separator and into the casing. Normally, the motor of the pump is located below the pump and in a position so that well fluid flows over it for cooling the motor as the well fluid flows into the intake of the pump. If large gas bubbles are encountered, the motor could heat drastically during the interim that no liquid is flowing over it. 
   One solution is to place the motor within a shroud and locate the inlet of the shroud below the perforations. This requires the well fluid to flow downward from the perforations into the inlet of the shroud, then back up to the intake of the pump within the shroud. As the well fluid flows downward, some of the gas will separate from the well fluid and flow upward, reducing the amount of gas that flows into the shroud. While this works well enough in areas where a shroud intake can be placed below the perforations, in some cases, it is not possible to locate a shroud intake below the perforations. 
   SUMMARY OF THE INVENTION 
   In this invention, a rotary pump is suspended in the well on a string of tubing. The pump has an intake for receiving well fluid and a discharge for discharging well fluid into the tubing. An electrical motor is coupled to the pump for rotating the pump. A barrier locates in the well below the intake of the pump and blocks well fluid from flowing below the barrier directly to the intake of the pump. A riser has an inlet in communication with the lower side of the barrier and an outlet above an effective level of the intake of the pump for flowing well fluid from below the barrier to above the effective level of the intake of the pump. This causes liquid components of the well fluid to flow back downward to enter the intake of the pump. This also results in gravity separation of gas components of the well fluid, which flow upward around the tubing in the casing. 
   In one embodiment, the motor is suspended below the barrier, which is run with the assembly of the motor and the pump. The pump has a discharge tube that extends to a Y-tube at the lower end of the tubing. An axial leg of the Y-tube aligns with the riser to enable a wireline to be lowered through the tubing and through the riser to below the barrier. 
   In another embodiment, the motor is located above the barrier. A feedback tube extends from one of the pump stages for delivering well fluid to below the motor for cooling the motor. 
   In another embodiment, a shroud encloses the motor and the intake of the pump. The shroud has an intake that is above the barrier. A riser has an inlet in communication with the lower side of the barrier and an outlet above the intake of the shroud. During installation, the barrier and the riser are installed in the well first, then the pump and shroud are lowered to the well. 
   In a fourth embodiment, a shroud is employed as mentioned above. In this embodiment, however, only the barrier is installed first, the barrier having a polished bore receptacle. The shroud has a stinger on its lower end that stabs into the barrier when running the pump and motor. An adapter connected to the stinger has one passage that leads to the riser. The adapter has another passage that leads from an intake of the shroud to the exterior. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view illustrating a well pump installation having a riser pipe gas separator constructed in accordance with this invention. 
       FIG. 2  is an enlarged view of a portion of the well pump installation of FIG.  1 . 
       FIG. 3  is a sectional enlarged view of an upper portion of the riser of FIG.  1 . 
       FIG. 4  is a schematic view of a lower portion of a second embodiment of a riser pipe gas separator. 
       FIG. 5  is a sectional view of the riser of  FIG. 4 , taken along the line  5 — 5  of FIG.  4 . 
       FIG. 6  is schematic view of a third embodiment of a riser gas separator for a well pump. 
       FIG. 7  is a sectional view of the riser of  FIG. 6 , taken along the line  7 — 7  of FIG.  6 . 
       FIG. 8  is a schematic view of another embodiment of a riser gas separator for a well pump installation. 
       FIGS. 9A and 9B  comprise a schematic view of another embodiment of a riser gas separator for a well pump installation. 
       FIGS. 10A and 10B  comprise a schematic view of another embodiment of a riser gas separator for a well pump installation. 
       FIG. 11  is an enlarged schematic sectional view of the adapter of the riser gas separator of  FIGS. 10A and 10B . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , the well has a casing  11  containing a set of perforations  13  to allow the flow of formation fluid into casing  11 . A string of production tubing  15  extends into the well. In this embodiment, a Y-tube  17  is secured to the lower end of tubing  15 . Y-tube  17  has a single upper end, an offset lower leg  19  and an axial lower leg  21 . Axial leg  21  is located coaxial with the axis of tubing  15 . Axial leg  21  extends only a short distance and contains a wireline profile for receiving a wireline plug  23 . 
   Offset leg  19  secures to a discharge tube  25  that extends upward from a rotary pump  27 . Pump  27  is shown in this example to be a centrifugal pump having a large number of stages, each stage having an impeller and diffuser. Alternately, rotary pump  27  could be a progressive cavity pump, which has an elastomeric stator with a double-helical cavity therein. A rotor having a helical configuration rotates within the stator. Pump  27  has an intake  29  on its lower end. 
   An electrical motor assembly connects to the lower end of pump  27  to rotate pump  27 . The motor assembly includes a seal section  31  and an electrical motor  33 . Seal section  31  contains a thrust bearing for absorbing downward thrust from pump  27 . Seal section  31  also equalizes pressure of lubricant contained in seal section  31  and motor  33  with the pressure of well bore fluid on the exterior. 
   A barrier  35  surrounds the upper portion of the motor assembly, particularly seal section  31  below intake  29 . Barrier  35  seals to casing  11  and may be a variety of types. Because the pressure differential between the lower and upper side of barrier  35  is very low, barrier  35  may comprise simply an elastomeric swab cup that slidingly engages casing  11  as pump  27  is lowered into the well. Barrier  35  could also be an inflatable or expandable type of packer. Motor  33  and the majority of seal section  31  extend below barrier  35 , terminating above perforations  13 . The thrust bearing in seal section  31  is preferably located in the portion of seal section  31  that is above barrier  35 . 
   A riser  37  extends sealingly through barrier  35  alongside seal section  31  and pump  27 . Riser  37  has an upper end above intake  29  of pump  27 . In the embodiment shown, the upper end of riser  37  is also above the upper end of pump  27 . Riser  37  may comprise simply a hollow cylindrical pipe or it could be a conduit of a variety of cross-sectional dimensions and shapes. A brace  39  secures the upper portion of riser  37  to discharge tube  25  above pump  27 . A funnel  41  optionally is located on the upper end of riser  37 . Riser  37  is preferably in axial alignment with axial leg  21  of Y-tube  17 . 
   All of the well fluid flowing from perforations  13  flows through riser  37 . Riser  37  optionally may have structure that causes swirling of the well fluid to enhance separation of gas from liquid. The embodiment shown in  FIGS. 2 and 3  has stationary, internal helical vanes  43  that extend continuously in a helical path in one section of riser  37 . A single helix may form helical vanes  43 , or they may comprise two separate vanes, as shown. Each helical vane  43  is parallel to the other, similar to a dual start thread. Each vane  43  is a short rib that is rigidly secured to the interior sidewall of riser  37  and protrudes a short distance inward, such as about ¼″. The central area within riser  37  that is surrounded by helical vanes  43  is completely open to enhance the upward passage of gas. The liquid components move to the interior sidewall of riser  37  due to centrifugal force. The spacing between helical vanes  43  may be varied. Helical vanes  43  need not extend the full length of riser  37 , rather preferably extend only the last two or three feet near the upper end of riser  37 . 
   Also, preferably a plurality of apertures  45  are formed in the sidewall of riser  37  adjacent vanes  43 . Apertures  45  allow some of the liquid to discharge out riser  37  as indicated by the arrows shown in FIG.  2 . The remaining portions of the liquid flow out the open upper end of riser  37  with the gas. Apertures  45  are preferably located only in the upper portion of vanes  43 . 
   In the operation of the embodiment of  FIGS. 1-3 , pump  27 , seal section  31 , motor  33 , barrier  35  and riser  37  are assembled together as shown, then lowered on tubing  15 . The assembly is positioned with motor  33  located above perforations  13 . Electrical power is supplied to motor  33 , which rotates pump  27 . Well fluid flows from perforations  13  around motor  33  and the lower portion of seal section  31  into the lower end of riser  37 . The well fluid flows upward when it encounters helical vanes  43 . The well fluid begins swirling, causing the liquid components to move to the interior sidewall of riser  37 . Some of the liquid components will discharge out apertures  45 . The gas remains in the open central area and flows out the upper end of riser  37  as indicated by the dotted arrow in FIG.  1 . After leaving riser  37 , the heavier liquid components flow downward by gravity into pump  27 , as indicated by the solid arrows in FIG.  1 . Pump  27  discharges the liquid components into tubing  15  for transport to the surface. If a large gas slug is encountered, it will flow over motor  33 , then up riser  37  and into casing  11 . 
   From time to time, it may be necessary to lower a wireline for various functions below barrier  35 . If so, the operator lowers a retrieval tool into tubing  15  and retrieves wireline plug  23 . The operator then lowers a wireline tool (not shown) down tubing  15 , out axial leg  21  and into guide funnel  41 . The wireline tool passes down riser  37  through the central open area surrounded by vanes  43 . The wireline tool is free to pass below for performing various operations. 
   Turning to  FIG. 4 , this downhole assembly is the same as in  FIG. 1 , with the exception of riser  47 . Riser  47  extends through barrier  49  alongside seal section  51 . In this embodiment, however, riser  47  has a closed lower end  52 , rather than open as in FIG.  1 . The inlet to riser  47  comprises a plurality of slots  53  located in the sidewall of riser  47  near lower end  52 . As shown in  FIG. 5 , each slot  53  is oblique or tangential. That is, slots  53  do not align radially with riser axis  55 . Rather they intersect the radial lines of axis  55  at acute angles. As indicated by the arrows, slots  53  cause the well fluid to flow tangentially inward in a swirling motion around the interior of riser  47 . Riser  47  may also have helical vanes  43  ( FIG. 3 ) as in the first embodiment. Tangential slots  53  may be utilized in all of the embodiments of this application. 
   In the embodiment of  FIG. 6 , pump  57  and seal section  59  are installed in a barrier  61  as in the embodiment of FIG.  1 . Riser  63  communicates from the lower side of barrier  61  to above pump  57  as in the first embodiment. However, in this embodiment, riser  63  has a different cross-sectional configuration than the cylindrical configuration of FIG.  2 . Riser  63  has a lower section  65  that may be cylindrical or have a different configuration, but is shown to be cylindrical in this embodiment. Riser  63  has an upper section  67  that is preferably cylindrical. Upper section  67  may have helical vanes within it, such as vanes  43  of FIG.  3 . Also, apertures  69  may be located along the helical vanes to discharge some of the liquid. 
   The intermediate section  71 , however, which is the portion that extends alongside pump  57 , is not cylindrical. Pump  57  has a larger diameter than its discharge tube  72 , thus restricts the amount of space available within the well casing for intermediate section  71 . Referring to  FIG. 7 , to provide the same flow area within intermediate section  71  as in lower section  65  and upper section  67 , a non-cylindrical configuration is utilized. The configuration is shown in the shape of a “D”, although it could be elliptical, oval, concave on one side and convex on the other, or other shape. Preferably, it has a minor axis or dimension and a major axis or dimension of different lengths. The minor axis  73  is located on a radial line of pump axis  75 . Major axis  77  is perpendicular to the minor axis  73  and is substantially greater. This configuration more effectively utilizes the space in the well casing on the side of pump  75 . The cross-sectional flow area through intermediate section  71  is preferably equal or greater than the cross-sectional flow areas in upper section  67  and lower section  65 . 
   The entire riser  63  could be constructed with a non-cylindrical configuration as described but if helical vanes are utilized in upper section  67 , a cylindrical configuration is preferred for upper section  67 . The embodiment of  FIG. 6  allows a cross-sectional flow area through riser  63  that would not be possible if the entire riser  63  were cylindrical because it would interfere with pump  57 . The non cylindrical cross-sectional shape of intermediate portion  71  of riser  63  could be utilized in all of the embodiments of this application. 
   In the embodiment of  FIG. 8 , pump  79  is a centrifugal pump having a plurality of stages, each stage having an impeller and a diffuser. Pump  79  has an upper section  81  and a lower section  83 . Lower section  83  has a few stages, while upper section  81  may have many stages more than lower section  83 . Pump  79  has a single intake  85  that is located at the lower end of lower section  83 . A feedback tube  87  taps into lower section  83  to cause some of the fluid being pumped up lower section  83  to be diverted back down feedback tube  87 . Feedback tube  87  extends alongside seal section  89  and terminates below the lower end of motor  91 . The pressure within pump  79  increases with each stage, beginning with the first stage in lower section  83 . 
   In this embodiment, motor  91  is located above barrier  93 , and feedback tube  87  is utilized to provide cooling liquid to flow over motor  91  during operation. Feedback tube  87  extends from one of the stages of lower section  83  to a point below motor  91 . Riser  95  extends alongside motor  91 , seal section  89  and pump  79  and has an open upper end above pump  79 . A brace  97  secures the upper end of riser  95  to discharge tube  99  of pump  79 . 
   In the operation of the embodiment of  FIG. 8 , barrier  93  and riser  95  are preferably run into the well along with pump  79  and motor  91 . The operation is the same as described in connection with FIG.  1 . All of the well fluid flows up riser  95 . Gravity separation occurs at the upper end of riser  95  with the gas flowing upward alongside discharge pipe  99  while the liquid flows downward to pump intake  85 . A portion of the well fluid will be discharged by feedback tube  87  below motor  91  to flow upward back into intake  85  for cooling of motor  91  and seal section  89 . The pressure of the fluid flowing down feedback tube  87  will be much less than the discharge pressure from pump  79  because feedback tube  87  taps into pump  79  at a point in the first few stages. 
   Referring to  FIG. 9B , in this embodiment, barrier  101  may be a conventional packer that is set in a conventional manner. A riser  103  is lowered and set with barrier  101  in casing  105 . Riser  103  has a lower end that is coaxial with barrier  101  and an offset section  107  that extends alongside pump  109 . 
   After barrier  101  and riser  103  are installed, pump  109  is lowered through the well. Pump  109  has a seal section  111  and electrical motor  113  attached to its lower end. In this embodiment, a shroud  115  extends around seal section  111  and motor  113 . The upper end of shroud  115  seals to the exterior of pump  109  above pump intake  117 . Shroud  115  is a tubular enclosure that has a tail pipe  119  extending from its lower end. The inlet  121  or open lower end of tail pipe  119  defines the effective level of intake  117 . The effective level is the elevation at which downward flowing well fluid turns to flow upward due to the suction of the pump. In this embodiment, the effective level is the elevation that fluid enters shroud  115 , this level being below the upper end of riser  103 . The effective level in the embodiments that do not employ a shroud, such as in  FIG. 1 , is the elevation of the actual intake  29  of the pump. In the embodiment of  FIG. 9B , riser  103  need not and does not have its upper end located above the actual level of pump intake  117 . However, the upper end of riser  103  is located above the effective intake  121  of pump  109 . 
   In the operation of the embodiment of  FIGS. 9A and 9B , barrier  101  and riser  103  are lowered into the well and set in a desired location above perforations (not shown). Pump  109 , seal section  111 , and motor  113 , all encased in shroud  115 , are lowered into the well. The operator lowers this assembly until the pump effective intake  121  is below the level of the outlet of riser  103 . 
   In a well with a static fluid level above the discharge of riser  103 , power is supplied to motor  113 , which causes fluid to flow up the riser. Power is supplied to motor  113 , which causes well fluid to flow up riser  103 . Gas will flow from the outlet around shroud  115  into casing  105 . Gravity will cause the liquid to flow downward from the outlet of riser  103  to pump effective intake  121 . The liquid flows up through shroud  115  around motor  113  and seal section  111  into intake  117 . As the well fluid flows past motor  113  and seal section  111 , it cools each component. 
   Referring to  FIG. 10B , in this embodiment, barrier  123  may also comprise a conventional packer that is set in a conventional manner in casing  125 . In this embodiment, barrier  123  has a polished bore receptacle  127  that has a flapper valve  129  on its lower end. 
   Pump  131  is secured to production tubing  133  and lowered into the well after barrier  123  is set. Pump  131  has a seal section  135  and a motor  137  suspended below it. A shroud  139  surrounds seal section  135  and motor  137  as well as pump intake  141 . Shroud  139  has a tail pipe  143  that extends downward. 
   Referring to  FIG. 11 , tail pipe  143  secures to an adapter  145 . Adapter  145  has a passage  147  that leads from the exterior to the interior of tail pipe  143  to deliver well fluid to the interior of shroud  139  (FIG.  10 A). A stinger  149  extends downward from adapter  145  for insertion into polished bore  127  ( FIG. 10B ) and past flapper valve  129 . Stinger  149  communicates with a passage  151  in adapter  145 . Passage  151  leads upward to a riser  153 . Riser  153  extends upward a selected distance, which in this case is below shroud  139 . Riser  153  is secured to tail pipe  143  by a brace  155 . 
   In this embodiment, the operator installs barrier  123  in a conventional manner. The operator then lowers the assembly shown in  FIG. 10A  into the well on tubing  133 . Stinger  149  stabs into polished bore  127  and opens flapper valve  129 . The operator supplies power to motor  137 , causing well fluid to flow up stinger  149 , passage  151 , and riser  153 . Gravity separation occurs with gas flowing upward in casing  125  and liquid flowing downward. The liquid flows downward to the effective intake of pump  131 , which is the entrance of passage  147 . This liquid flows up tail pipe  143  into shroud  139 . The well fluid flows past motor  137  and seal section  135  into the actual intake  141  of pump  131 . 
   The invention has significant advantages. The positioning of a riser above an effective intake of the pump allows a gravity separation to occur, causing gas to flow upward in the casing while liquid flows downward. The positioning of the assembly so that well fluid will flow past the motor enables cooling to occur. Consequently, if gas slugs encountered, the pump motor will not be exposed to a significant time period without liquid flow. 
   While the invention has been shown only in a few of its forms, it should be apparent to those skilled in the art that it is not so limited but susceptible to various changes without departing from the scope of the invention.

Summary:
A well pump has a riser gas separator for removing large slugs of gas prior to reentry into the pump. The riser extends upward from a barrier that is located in the well. The riser has an inlet that is located above an effective intake of the pump. Well fluid must turn to flow down to the pump, with gas separating by gravity flowing upward while the liquid flows downward. The downhole assembly has various configurations to assure that fluid flows past the motor for cooling.