Patent Publication Number: US-6668925-B2

Title: ESP pump for gassy wells

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to a means of improving the production capacity of a gaseous oil well by separating the gas from the crude oil before pumping the oil up the well. 
     2. Description of the Related Art 
     When an oil well is initially completed, the downhole pressure may be sufficient to force the well fluid up the well tubing string to the surface. The downhole pressure in some wells decreases, and some form of artificial lift is required to get the well fluid to the surface. One form of artificial lift is suspending an electric submersible pump (ESP) downhole in the tubing string. The ESP will provide the extra lift necessary for the well fluid to reach the surface. In gassy wells, or wells which produce gas along with oil, there is a tendency for the gas to enter the pump along with the well fluid. Gas in the pump decreases the volume of oil transported to the surface, which decreases the overall efficiency of the pump and reduces oil production. 
     In order to prevent the gas from interfering with the pumping of the oil, various downhole separators have been developed to remove gas from the well fluid prior to the introduction of the fluid into the pump. A typical gas separator is attached to the lower end of the pump assembly, which in turn is suspended on production tubing. Normal gas separators separate most of the gas and discharge the separated gas into the annulus outside the tubing string where the gas flows up the well to the surface. The separator discharges the liquid into the tubing to be pumped to the surface. 
     A first disadvantage of these separation and pumping systems is that intervention is costly and difficult because the pump assembly is attached to lower end of the tubing string. With the pump assembly attached to the lower end of the tubing string, the well must be “killed,” or the flow of gas and fluid through the perforations is stopped, in order to provide a safe working environment while the tubing string is out of the well. If the well is killed without a means of preventing downward flow, well fluid will flow back into the well where it could be too difficult to retrieve. A second disadvantage of these separation and pumping systems is that all the gas is not always separated from the well fluid, thus a significant portion of the gas may still flow into the pump. 
     Proposals have been made to suspend the pump assembly and separator on coiled tubing lowered into a liner or casing. However, improvements, particularly for gas separation, are desirable. 
     SUMMARY OF THE INVENTION 
     The present invention contemplates a means of pumping gas from gassy wells in which the gas is separated before entering the pump by a gas separator located below the pump within a section of a liner or conduit. The pump assembly is lowered into the liner and suspended above the gas separator. There is a set of valves and flow control devices located in the conduit below the pump that allows the pump to be installed and removed while the well is live. 
     The conduit lands and sealingly engages a packer set in the casing. The conduit has an opening in its lower portion for receiving the gas and well fluid flowing from the perforations in the well. A gas separator is located inside of the conduit for separating gas from the well fluid flowing from the well. The gas that is separated from the well fluid is discharged to an area surrounding the conduit where the gas will flow to the surface. The remaining well fluid is discharged up the conduit. 
     The pump assembly is made up of a pump, which has an inlet and an outlet, and a motor to drive the pump. The pump assembly lands in the conduit so that the pump and motor are above and not engaged with the gas separator. A pump seal located between the pump and the conduit seals the pump to the conduit when the pump assembly lands in the conduit. The gas separator below the pump separates most of the gas from the well fluid, therefore the pump is more efficient, and can produce more crude oil. The pump outlet is above the pump seal, accordingly the well fluid is discharged into the conduit, above the pump seal, where the fluid flows to the surface. 
     In the preferred embodiment, there will also be a flow control valve located in the conduit so that the well fluid cannot flow back into well. A circulation valve located above the flow control valve allows circulation from the conduit into the annulus surrounding the conduit. Finally, in the preferred embodiment, a pressure actuated downhole safety valve is located in the conduit to prevent well fluid from flowing up the conduit when it is closed. 
     In one embodiment of the well, the pump is suspended above the motor, and the pump seal creates a chamber inside of the conduit below the pump seal and above the gas separator. In this embodiment, some of the gas remaining with the well fluid after passing through the gas separator will collect in the chamber. A shroud located below the pump inlet makes it difficult for the remaining gas to enter the pump, so the gas flows past the inlet and collects at the top of the chamber. The remaining gas that collects in the chamber is vented out of the chamber by a vent. In one embodiment, the vent is through the pump seal and opens into the area inside the conduit above the seal. The gas then flows up the conduit to the surface. In another embodiment, the pump assembly is suspended by tubing, and the vent fluidly connects the chamber with the inside of the tubing. In this embodiment the gas flows up the tubing to the surface. In another embodiment, the vent is located in the conduit and discharges the remaining gas to the annulus, where the gas will flow to the surface. 
     In the final embodiment, the pump is located below the motor, the motor being suspended by a string of coil tubing. In this embodiment, the pump seals engage the conduit when the pump lands in an area of the conduit having a reduced diameter. The gas is separated from the well fluid by the gas separator and discharged into the annulus. The well fluid and some remaining gas flow into the pump inlet, where the fluid and remaining gas are pumped and discharged above the seal to flow to the surface inside the conduit. In this embodiment, there may also be a sand skirt for collecting sand that settles from the fluid flowing to the surface. With any of these embodiments, the pump assembly may be removed for intervention without having to kill the well by closing the safety valve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 comprises a schematic cross-sectional view of a lower end of a well constructed in accordance with this invention. 
     FIG. 2 comprises a schematic cross-sectional view of a second embodiment of a well constructed in accordance with this invention. 
     FIG. 3 comprises a schematic cross-sectional view of a third embodiment of a well constructed in accordance with this invention. 
     FIG. 4 comprises a cross-sectional view of a fourth embodiment of a well constructed in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the first embodiment of the means for pumping liquids from a gassy well. Referring to FIG. 1, a string of casing  11  has been hung and landed into the well. Perforations  12  in casing  11  allow well fluid to enter casing  11 . A packer  13  extends towards the center of the well from the inner surface of casing  11  along the lower section of casing  11  above perforations  12 . A conduit  15 , or liner, is hung in the well and landed in packer  13  so that the outer surface of conduit  15  is sealingly connected to packer  13 . Alternatively, packer  13  could be run into the well along with conduit  11 , then set. Conduit  15  is made up of sections of casing secured together. In the preferred embodiment, conduit  15  has a reduced diameter section  17  towards the lower end of conduit  15 . Reduced diameter section  17  has a diameter that is comparable to conventional production tubing. Reduced diameter section  17  is the portion of conduit  15  that lands and seals with packer  13 . The central opening in packer  13  has a flapper valve (not shown) that opens when the lower end of conduit  15  stabs into packer  13 . A conduit opening  19  is located at the lower end of conduit  15  for receiving well fluid from perforations  12 . Conduit opening  19  is the only means for the well fluid to flow to the surface because packer  13  is sealingly connected between casing  11  and conduit  15 . An annulus  21  is located in the area surrounding the outer surface of conduit  15  and above packer  13 . 
     A gas separator  23  is located in the string of conduit  15  above conduit opening  19 . In the preferred embodiment, gas separator  23  is a static gas separator which uses stationary vanes (not shown) having a geometry for creating a swirling motion as the well fluid passes through separator  23 . A portion of the gas in the well fluid is separated due to centrifugal force and discharged into an annulus  21  located in the area surrounding conduit  15  as indicated by the arrow. The gas discharged into annulus  21  flows up the well to be collected at the surface. In the preferred embodiment, gas separator  23  is located in reduced conduit diameter section  17 . The well fluid flowing upward from gas separator  23  will still have some gas in most cases. 
     A chemical injection chamber  25  optionally may be located in conduit  15 . Chemical injection chamber  25  is a passageway in which the well fluid is treated with chemicals. The types of chemicals injected into the well fluid in chemical injection chamber  25  will vary based upon different production conditions. For example, an operator may inject chemicals to reduce scaling, paraffin deposits, or to help lower the viscosity of the crude oil in the well fluid. A chemical supply line  27  which is in fluid communication with a chemical supply at the surface (not shown) provides the chemicals being injected into the well fluid in chemical injection chamber  25 . In the preferred embodiment, chemical injection chamber  25  is located above gas separator  23  and receives the well fluid after gas separator  23  discharges the gas into annulus  21 . 
     A flow control valve  29  is located in conduit  15  to prevent well fluid from flowing back down through conduit opening  19 . Flow control valve  29  is a check valve that allows upward flow of well fluid through conduit  15 , but prevents downward flow of well fluid so that well fluid does not go back through conduit opening  19 , thereby preventing fluid loss into perforations  12 . In the preferred embodiment, flow control valve  29  is located in reduced diameter section  17 , above gas separator  23 . 
     A downhole safety valve  31 , which is actuated by hydraulic pressure supplied by hydraulic fluid line  33 , is located in conduit  15  above flow control valve  29 . Downhole safety valve  31  is closed when there is no pressure applied to valve  31  from the hydraulic fluid supplied by hydraulic fluid line  33 . With downhole safety valve  31  closed well fluid cannot continue to flow up the well. Downhole safety valve  31  is opened by supplying hydraulic fluid through fluid line  33  to create the necessary hydraulic pressure. Well fluid continues flow up conduit  15  when downhole safety valve  31  is open. 
     A circulation control valve  35  is an operable valve located in conduit  15  below downhole safety valve  31  to allow circulation through conduit  15  if desired. In the preferred embodiment, circulation control valve  35  has a normally closed port  36  leading to annulus  21  and is located in reduced conduit inner diameter section  17 . Circulation valve  35  is pressure activated. In the preferred embodiment, circulation valve  35  is a tubing pressure activated communication sleeve (TPACS). Circulation valve  35  allows the well fluid to normally flow up conduit  15  towards downhole safety valve  31  while circulation valve  35  is in its open position. Circulation valve  35  redirects fluid flowing down conduit  15  through its port  36  to flow into annulus  21  while circulation valve  35  is in its circulation position. To place circulation valve  35  in its circulation position, the operator applies fluid pressure of a selected amount to the interior of conduit  15 . The fluid being discharged through port  36  flows back to the surface through annulus  21 . Flow control valve  29  prevents the fluid being pumped down conduit  15  from flowing downward into perforations  12 . Circulation valve  35  returns to its open position when the pressure inside conduit  15  is decreased by the operator. 
     A pump assembly  37  is hung and landed into conduit  15 . Pump assembly  37  may have an outer diameter greater than an inner diameter of conduit reduced diameter section  17 . Pump assembly  37  consists of a pump  39 , a motor  41 , a pump seal  43 , a seal section  55 , and optionally a flow sensor  45 . In the embodiment shown in FIG. 1, pump  39  is lowered down conduit  15  on the lower end of a string of tubing  47 . Tubing  47  in this embodiment comprises sections of tubing screwed together, with a power supply  49  running along the outside of tubing  47 . Tubing  47  could also be coiled tubing. Tubing  47  supports pump assembly  37 , and is also a passageway for transporting gas that is not separated by gas separator  23  up the well. Pump  39  has at least one pump inlet  51  located on the lower portion of the pump  39 , and at least one pump outlet  53  located on the upper portion of pump  39 . In the preferred embodiment, pump  39  is an electrical submersible pump (ESP), which can be a centrifugal type of ESP. 
     In the first embodiment, motor  41  is located below pump  39 . Motor  41  drives pump  39  through a motor drive shaft (not shown) connecting to the lower end of pump  39 , which is enclosed by seal section  55 . Power cable  49  provides motor with electricity. Power cable  49  runs down the well alongside tubing  47 , passes through a pump seal passageway  57  having sealed ends  59  where power cable  49  enters and exits passageway  57 , and connects with motor  41  below pump  39 . Flow sensor  45  is attached to the lower end of motor  41  for measuring the volume of fluid that is flowing into pump  39 . 
     Pump seal  43  is sealingly connected to the outer surface of pump  39  between pump inlet  51  and outlet  53 . Pump seal  43  lands and sealingly connects with a seal bore  61  located and protruding from on the inner surface of conduit  15 . Pump  39  discharges into conduit  15  above seal  43 . A chamber  63  is formed when pump seal  43  lands and seals with seal bore  61 , which is defined by the bottom surface of pump seal  43  and the inner surface of conduit  15 . Pump seal  43  is an annular elastomer. 
     A shroud  65  may be connected to the lower end of pump  39  to help prevent gas that remains in the well fluid after passing through gas separator  23  from entering pump  39 . Shroud  65  extends radially away from inlet  51  of pump  39  towards conduit  15 , and up so that the ends of shroud  65  are above pump inlet  51 . Shroud  65  retards gas remaining in the well fluid after separation from entering pump  39  by forcing the well fluid and gas to go up in chamber  63  above pump inlet  51 . The remaining gas, being the lighter substance, gathers in chamber  63  above pump inlet  51  while the well fluids flow back down to pump inlet  51 . In the first embodiment, a vent  67  with a lower end on the bottom surface of pump seal  43  and an upper end on the top surface of pump seal  43  vents gas collected in chamber  63 . A check valve in vent  67  prevents downward flow of well fluid. In the first embodiment, a collection tube  73 , which gathers and leads gas exiting chamber  63  via vent  67 , extends to tubing  47  to vent the gas in tubing  47 . 
     In the operation of the first embodiment, gas separator  23 , chemical injection chamber  25 , flow control valve  29 , circulation valve  35  and downhole safety valve  31  are mounted to conduit  15  at the surface. Conduit  15  is then lowered into the well and landed in packer  13 , which seals conduit  15  to casing  11 . Downhole safety valve  31  is placed in a closed position. Then pump  39  and its associated components are lowered into conduit  15  on tubing  47 . Pump seal  43  lands and seals to seal bore  61 . 
     Downhole valve  31  is opened to allow well fluid to flow into chamber  63  and power is supplied to motor  41 . The gas and well fluid flow up conduit  15  to gas separator  23 , where the gas from the well is separated and discharged into annulus  21 . The remaining well fluid flows from gas separator  23 , up conduit  15 , into chemical injection chamber  25  where the well fluid may be chemically treated with chemicals that are injected into the well fluid through chemical supply line  27 . The chemically treated well fluid flows up conduit  15  through flow control valve  29 , which allows the upward flow of well fluid and prevents the well fluid from flowing downward. The well fluid flows up conduit  15  through circulation valve  35 , through open downhole safety valve  31 , and into chamber  63 . 
     The well fluid enters chamber  63  from down hole safety valve  31  and flows past motor  41 . Shroud  65  prevents the well fluid from entering pump inlet  51  until the well fluid flows past and back down to pump inlet  51 . Some of the gas remaining in the well fluid upon entering chamber  63  flows up chamber  63 , past motor  41  until it reaches pump seal  43 , where the gas gathers. The gathering gas separates from the settling well fluid and enters vent  67 . Vent  67  leads the gas out of chamber  63  through pump seal  43 , to collection tube  73  which carries the gas into tubing  47 , where the gas flows up the well to the surface. The liquid components of the well fluid in chamber  63  flow downward to pump inlet  51  and enter pump  39 . Pump  39  increases the pressure of the well fluid and discharges the well fluid above pump seal  43 , into the interior of conduit  15  above pump seal  43  to flow up the well. 
     To repair or maintain pump  39  and motor  41 , hydraulic pressure to safety valve  31  is removed to close valve  31 . Tubing  47  is pulled from conduit  15  without having to kill the well. If it is desired for other reasons to kill the well, the operator increases the well fluid pressure inside of conduit  15 , which causes circulation valve  35  to actuate to its circulation position, allowing well fluid to flow from conduit  15  into annulus  21  and to the surface. Heavier liquid is circulated into conduit  15  and annulus  21  to kill the well. 
     In the embodiment shown in FIG. 2, there is no collection tube  73  (in FIG.  1 ). In this embodiment, the gas remaining in the well fluid after separation gathers below pump seal  43  in the top of chamber  63 . The gas separates from the settling well fluids and exits chamber  63  through vent  67 . Vent  67  carries the gas through and above pump seal  43  to the interior conduit  15  where the gas continues flow up the well. 
     In another embodiment there is no vent  67  extending through pump seal  43 . Rather, as shown in FIG. 3, a vent  75  is located above pump inlet  51  in a side pocket mandrel of conduit  15 . Vent  75  has a first end located on the interior surface of conduit  15 , and a second end located on the exterior surface of conduit  15 , for venting gas into annulus  21 . In operation, the gas collecting in chamber  63  above pump inlet  51  separates from the well fluid as the well fluid flows downward towards pump inlet  51 . The separated gas gathers along the top of chamber  63  until there is enough gas collected to flow into vent  75 . Vent  75  communicates the gas from chamber  63  to annulus  21 , where the gas flows up the well under normal natural gas-lift properties. A check valve in vent  75  prevents downward flow of well fluid and gas from annulus  21 . 
     A final embodiment, as shown in FIG. 4, shows a pump assembly  83  in which a pump  85  and a motor  87  are lowered on a string of coil tubing  89 . Motor  87  is suspended above pump  85  from the lower end of coil tubing  89 . A motor supply line  91 , which supplies electrical current to motor  87 , runs to motor  87  through the interior of coil tubing  89 . Motor  87  drives pump  85  with a drive shaft (not shown) that is enclosed in a seal section  93  between motor  87  and pump  85 . Pump  85  is an (ESP), normally a centrifugal type of pump, having at least one pump outlet  95  located on the upper section of pump  85 , and at least one pump inlet  97  located on the lower section of pump  97 . In the preferred final embodiment, a sand skirt  99  extends from the outer surface of pump  85 , below pump outlet  95 , to collect sand that flows down the well from above pump  85 . Sand skirt  99  is a conical flexible member extending radially outward from pump  85  to conduit  15  below pump outlet  95 . Sand skirt  99  collects sand that drops out of the flow stream. A flow meter  101  for measuring and monitoring the volumetric flow of well fluid may be located between pump inlet  97  and pump outlet  95 . 
     A set of pump seals  103  are located around the outer surface of the lower section of pump  85 , above pump inlet  97 . Pump seals  103  seal pump  85  to reduced diameter portion  17  of conduit  15  above pump inlet  97 . Pump assembly  83  is lowered by coil tubing  89  so that pump seals  103  seal pump  85  with the interior surface of conduit  15  when pump assembly is hung and landed. 
     In operation of the embodiment shown in FIG. 4, the chemically treated well fluid flows up conduit  15  through flow control valve  29 , circulation valve  35 , and downhole safety valve  31 , towards pump  85 . Pump seals  103 , which seal pump  85  with reduced conduit inner diameter section  17 , prevent the well fluid from flowing around pump  85  and force the well fluid to flow into pump inlet  97 . Flow meter  101  measures the flow rate of the well fluid as while the well fluid travels through pump  85 , and communicates the well fluid flow rate up the well. The well fluid exits pump  85  through pump outlet  95  and flows up the well alongside of coil tubing  89 . Sand skirt  99  catches and collects any sand particles settling from the well fluid so that the sand does not damage the pump seals  103  below. Separated gas by separator  23  discharges into annulus  21  and flows to the surface. 
     With all of these embodiments, the gas separator removes most of the gas from the well, so the pump does not have to pump as much gas as without a separator. Having less gas flowing into the pump increases the efficiency of the pump, which means greater oil production. Furthermore, with all of these embodiments, the pump assembly may be removed for intervention without having to kill the well. Further, it will also be apparent to those skilled in the art that modifications, changes and substitutions may be made to the invention in the foregoing disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in he manner consisting with the spirit and scope of the invention herein.