Patent Publication Number: US-6705402-B2

Title: Gas separating intake for progressing cavity pumps

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to progressing cavity well pumps and in particular to 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 a centrifugal electric submersible pump (ESP) downhole in the tubing string. The ESP provides the extra lift necessary for the well fluid to reach the surface. An ESP has a large number of stages, each stage having an impellor and a diffuser. 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. A gas separator may be mounted between the pump and motor to reduce gas entering into the pump. The gas separator rotates at the same speed as the pump and motor. 
     A progressive cavity pump is another type of well pump. A progressing cavity pump has a helical metal rotor that rotates inside a helical elastomeric stator. The liquid being pumped acts as a lubricator between the helical rotor and the stationary stator. If gas enters the pump, the gas may prevent the liquid from continuously lubricating the rotor and stator surfaces while flowing through the pump. The stator deteriorates quicker when there is not a thin layer of liquid on their surfaces acting as a lubricator. Quicker deterioration of the stator causes less time between maintenance and repairs of the pump. 
     Gas separators have not been used in conjunction with progressing cavity pumps, which operate at slower speeds than centrifugal pumps. Furthermore, the shaft in a rotary separator has a concentric or substantially circular path around the centerline of the shaft, while the rotor of a progressing cavity pump has an eccentric or elliptical path around the centerline of the rotor. 
     SUMMARY OF THE INVENTION 
     The downhole pump assembly in this invention has a progressing cavity downhole pump that is suspended by tubing in a well. The progressing cavity pump is a positive displacement pump. A cavity of liquid is forcibly pushed through the pump when a helical-shaped rotor rotates inside of the stator. A motor drives the rotor of the pump with a drive shaft. However the drive shaft from the motor typically rotates at a speed that is too fast for the rotor of the pump. A gear assembly between the motor and the pump transmits the rotations from the drive shaft to the pump rotor at a slower, operational speed of the pump. 
     A separator located below the pump separates the gas from liquids in the well fluid. The separator may have a helical inducer and a series of vanes rotated by a separator shaft inside of the separator housing, which in turn is driven by the motor. Alternatively, the separator may have a vortex chamber instead of vanes after the helical inducer. One end of the separator shaft is connected to the rotor of the pump. The separator shaft travels in a concentric or substantially circular path around the centerline of the shaft, while the rotor of the pump travels in an eccentric or elliptical path around the centerline of the rotor. A flexible shaft connects the shaft of the separator to the rotor of the pump. The flexible shaft compensates for different paths of the rotor and the separator shaft. 
     An annular passageway is located in the area between the flexible shaft and a shroud or housing that encloses the flexible shaft. The annular passageway is in fluid communication with the liquid outlet from the separator and the liquid inlets of the pump. In the first embodiment, the separator is also located above the gear reduction unit. Therefore, in this embodiment, the vanes and helical inducer of the separator rotate at the same speed as the rotor of the pump. 
     After suspending the pump assembly in the well, power is supplied to the motor to rotate the separator shaft and the pump rotor. The gear reduction unit located below the separator decreases the rotational speeds of the separator shaft and the pump rotor from that of the drive shaft from the motor. Well fluids enter the separator through separator inlets at the lower portion of the separator. The well fluid flows into an optional rotating helical inducer, and delivers the fluids into the separator vanes. The rotating vanes use centrifugal forces to push the heavier liquids in the well fluid to the outermost portion of the separator while the lighter gases remain in the innermost portions of the separator. 
     The liquids on the outer portion of separator exit the vanes to a passage on the outer surface of a crossover lip. The gases exit the vanes to the inner surface of the crossover lip. The crossover communicates the separated gases to gas outlets on the exterior surface on the upper portion of the separator. The gases exit the separator and rise to the surface under normal gas-lift properties. The passageway on the outside of the crossover lip communicates the separated liquids to the separator outlets on the upper portion of the separator, above the gas outlets. The separator liquid outlets communicate with the annulus surrounding the flexible shaft inside of the housing. The annulus communicates the liquids the to inlets of the pump. 
     The liquids enter the progressing cavity pump into a cavity between the rotor and the stator. The cavity travels up the pump as the rotor rotates inside the stator. Most of the fluid travels with the cavity and exits out of the pump outlets on the upper portion of the pump into the tubing with an increased liquid pressure to lift the liquids to the surface. A thin layer of liquid typically remains on the surfaces of the rotor and the stator when the cavity carrying liquid passes through the pump. The thin layer of liquid acts as a lubricant between the rotor and the stator. The liquid continues to lubricate the rotor and stator surfaces during operation. Therefore, the stator does not deteriorate due to lack of lubrication. 
     In another embodiment, the gear reduction unit is located between the separator and the pump. In this embodiment, the shaft of the separator rotates at the same speed as the drive shaft from the motor, while the rotor of the pump still rotates at the slower pump speed. The shroud surrounding the flexible shaft between the pump and the separator also extends down around the gear reduction unit to a point below the pump liquid outlets. Liquid communicates from the pump outlets into an annular passage between the shroud and the gear reduction unit to the annulus between the shroud and the flexible shaft to the pump inlets. This embodiment is good for situations in which the separator needs to operate at a faster speed in order to separate the gas from the liquids in the well fluid. 
     In the third embodiment, a motor on the surface at the upper end of the well drives the pump and separator. The drive shaft from the motor has a drive member extending down the well to the rotor of the pump. The separator is connected to the pump by a flexible shaft enclosed in a housing, as in the first embodiment. The separator is also driven by the motor located on the surface. The separator shaft is rotating at the same speed as the rotor of the pump. 
     In all three of these embodiments, gas in the well fluid is separated from the liquid before the liquids enter the pump. These embodiments increase the amount of time between repairs of the rotor and stator of the pump because the pump is continuously lubricated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B comprise a cross-sectional view of a downhole pump assembly constructed in accordance with this invention. 
     FIGS. 2A and 2B comprise a cross-sectional view of an alternative embodiment of a pump assembly constructed in accordance with the present invention, in which the gear reduction unit between the pump and separator. 
     FIGS. 3A-3C comprise a cross-sectional view of an alternative embodiment of a pump assembly constructed in accordance with the present invention, in which the motor is at the surface. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A downhole pump assembly  11  is shown in FIG.  1 . Pump assembly  11  is suspended from tubing  12  in a well in order to pump well fluid to the surface when ordinary gas-lift forces are not enough produce the oil and gas from the well. Pump assembly  11  has a progressing cavity pump  13 . Progressing cavity pump  13  has a rotor  15  having a helical shape that rotates within an elastomeric stator  17 . An inlet  19  is located at the lower portion of progressing pump  13  where liquids enter pump  13 . An outlet  21  is located at the upper portion of progressing cavity pump  13  for discharging the liquids up the string of tubing. 
     Liquids entering pump  13  flow into a double helical cavity  23  between rotor  15  and stator  17 . Rotor  15  rotates so that the helical shape of rotor  15  and stator  17  force liquid to travel up pump  13 . The liquid in cavity  23  is forcibly moved as portions of cavity  23  rise along rotor  15  to outlet  21 , where the liquid is discharged above pump  13  into the string of tubing  12  leading to the surface. The liquid leaves a thin layer of liquid on the surfaces of rotor  15  and stator  17  as the liquid in cavity  23  travels up rotor  15  through pump  13 . The thin layer of liquid left on the surfaces of rotor  15  and stator  17  acts as a lubricant, increasing the operational lifespan of rotor  15  and stator  17 . 
     A motor  25  rotates rotor  15  from below pump  13 . A multi-piece drive shaft  27  extends up from motor  25  in order to drive rotor  15  of pump  13 . A seal section  29  is located above motor  25  around the circumference of shaft  27  to equalize the pressure of the lubricant inside of motor  25  with the hydrostatic pressure in the well. A gear reduction unit  31  is located between motor  25  and pump  13 . Gear reduction unit  31  reduces the rotational speed of rotor  15  because pump  13  operates at a slower rotational speed than motor  25 . 
     A separator  33  for separating the gas from the liquids in the well fluid is located below pump  13 , between pump  13  and motor  25 . Separator  33  preferably has a housing  35  enclosing a helical inducer  37  and a plurality of vanes  39  axially mounted on a separator shaft  41 . Alternatively, separator  33  could have an empty chamber or vortex chamber (not shown) instead of vanes  39 , where the gases can separate from the liquids after being discharged from helical inducer  37 . The lower end of shaft  41  is connected to drive shaft  27  extending up from the motor  25 , and the upper end of shaft  41  extends towards pump  13 . A set of inlets  43  located at the lower portion of separator  33 , allow the well fluid from the well to enter separator  33 . Motor  25  rotates shaft  41 , which in turn rotates helical inducer  37  and vanes  39 . Well fluids entering separator  33  through inlets  43  flow to helical inducer  37 . Helical inducer  37  forces the well fluid upward to vanes  39 . The rotation of vanes  39  applies a centrifugal force to the well fluid, which causes the heavier liquids to flow to the outermost radial portions of separator  33  while the lighter gases remain in the innermost radial portions of separator  33 . 
     A crossover lip  45  located above vanes  39  acts as a physical barrier preventing the liquids and gases from recombining after exiting from vanes  39 . The heavier liquids exit vanes  39  and travel up separator  33  along the outside surface of crossover lip  45 , and the lighter gases travel up the inside surface of crossover lip  45 . Crossover  47  leads the lighter gases to gas outlet  49  located on the exterior surface on the upper portion of separator  33 . The lighter gases communicate through crossover  47  to gas outlet  49 , where the separated gases discharge into the annulus surrounding tubing  12  to rise to the surface under normal gas-lift properties. A passageway  51  defined by the exterior surface of crossover lip  45  and the interior surface of housing  35  receives the liquids separated from the well fluid by vanes  39 . The liquids flow through passageway  51  to outlet  53  located in the upper portion of separator  33 , which discharges the liquids towards pump  13 . 
     In this embodiment, separator  33  is above gear reduction unit  31 . Therefore, shaft  41  of separator  33  rotates at the same rotational speed as rotor  15  of progressing cavity pump  13 . A flexible shaft assembly  55  is located between pump  13  and separator  33  and connects rotor  15  to shaft  41 . Flexible shaft assembly  55  is needed because rotor  15  of pump  13  has an eccentric rotation while shaft  41  of separator  33  has a concentric rotation. Preferably, flexible shaft  57  is coupled to rotor  15  and shaft  41  by vertical spline or threaded couplings. Threaded and or vertically splined couplings allow each end of shaft  57  to orbit in unison with rotor  15  or shaft  41 . The eccentric rotation of rotor  15  means that rotor  15  travels in an elliptical path about the centerline of rotor  15  as it rotates. The concentric rotation of shaft  41  means that shaft  41  rotates in a substantially circular path about the centerline of shaft  41 . Flexible shaft assembly  55  has a flexible shaft  57  with the lower end connected to shaft  41  and the upper end connected to rotor  15 . Flexible shaft  57  is preferably made of a steel, however its length allows flexing to compensate for the different paths the centerlines of rotor  15  and shaft  41  travel when rotated. 
     A housing or shroud  59  encloses flexible shaft assembly  55 , defining an annulus  61  between the exterior surface of flexible shaft  57  and the interior surface of shroud  59 . Annulus  61  is in fluid communication with separator liquid outlet  53  and pump inlet  19 . Separator  33  discharges liquids separated from separator  33  through outlets  53  into annulus  61 , where the liquids travel up annulus  61  alongside flexible shaft  57  into pump  13  through inlets  19 . 
     In operation, downhole pump assembly  11  is lowered on tubing  12  into casing (not shown) in the well. Power is supplied to motor  25 . Motor  25  rotates drive shaft  27 , which in turn drives separator shaft  41  and rotor  15 . Gear reduction unit  31  decreases the rotational speed between drive shaft  27  and separator shaft  41 . Separator shaft  41  rotates helical inducer  37  and vanes  39 . Well fluid enters separator  33  through inlets  43 . Vanes  39  force the heavier liquids to the outermost portions of the inside of separator  33  and the lighter gases to inner portions of separator  33 . Crossover lip  45  provides a physical barrier preventing the separated liquids and gases from recombining after exiting vanes  39 . 
     Crossover  47  communicates the lighter gases from the inner portions of separator  33  to gas outlet  49 . The separated gases discharge into the annulus surrounding tubing  12 , where the gases will rise to the surface. The liquids flow along passageway  51  along the exterior of crossover lip  45  to separator outlet  53 , where the liquids discharge into annulus  61 . The liquids flow in annulus  61  between flexible shaft  57  and shroud  59  to pump inlet  19 . Separator shaft  41  communicates the reduced speed rotation from drive shaft  27  to rotor  15 . Flexible shaft  57  compensates for the different paths of the centerlines of pump rotor  15  and separator shaft  41 . 
     Liquids entering progressing cavity pump  13  through inlet  19  enter cavity  23  between rotor  15  and stator  17 . The rotation of rotor  15  causes cavity  23  to travel up pump  13  as helical rotor  15  rotates within stators  17 . The pressure on the liquids increases and the liquids discharge into tubing  12  to flow to the surface. 
     As the liquids travel along rotor  15  and past stator  17 , the liquids continually provide lubrication to the surfaces of rotor  15  and stators  17 . The reduction of gases in the fluid pumped by progressing cavity pump  13  reduces the chance for rotor  15  to rub against dry, non-lubricated stator  17 . Pump  13  can operate for longer periods of time because the lubricated surfaces will not deteriorate as quickly as surfaces constantly rubbing against each other without lubrication. Accordingly, pump assembly  11  as described above separates the gases from the well fluid in a manner that increases the time between repairs of pump  13 . Increasing the time period between repairs is an improvement which increases the production capabilities of the well. 
     Referring to FIG. 2, a second embodiment of downhole pump assembly  11  is shown. In this embodiment, motor  25  and seal section  29  are located below pump  13  and separator  33  as before. Gear reduction unit  31  is located in a different location, between pump  13  and separator  33 . In this embodiment, motor  25  rotates drive shaft  27 , which in turn rotates separator shaft  41 . Separator shaft  41  rotates at the same rotational speed as drive shaft  27  from motor  25 . The gas is separated from the well fluids in separator  33  in the same manner as in the first embodiment. 
     Gear reduction unit  31  connects separator shaft  41  with flexible shaft  57 , which is connected to rotor  15  on its other end. Gear reduction unit  31  decreases the speed of rotation of separator shaft  41  to the slower speed pump  13  needs rotor  15  to rotate. Accordingly, in this embodiment, separator  33  is operating at a higher rotational speed than pump  13 . 
     In this embodiment, shroud  59  extends downward and also encloses gear reduction unit  31 , defining a lower annular area  62  between the interior surface of shroud  59  and the exterior surface of gear reduction unit  31 . Lower annulus  62  is in fluid communication with annulus  61 . Separator outlet  53  discharges the separated liquids into lower annulus  62  and the liquids travel up lower annulus  62  past gear reduction unit  31  to annulus  61 . In an embodiment not shown in FIG. 2, the outlet of separator  33  is in fluid communication with annulus  61  via tubing. In this alternative embodiment not shown in FIG. 2, the liquids can communicate from separator  33  to annulus  61  in shroud  59  with tubing traveling around gear reduction unit  31 . 
     The liquids travel in annulus  61  between shroud  59  and flexible shaft  57  to pump inlets  19 , where the liquids are pumped to the surface using pump  13  as described in the first embodiment. This embodiment is preferable in conditions in which the separator  33  needs to operate at faster speeds in order for vanes  39  to create large enough centrifugal forces to separate the gases from the liquids in the well fluid. Like the first embodiment, the reduction of gas entering pump  13  allows the separated liquids to lubricate rotor  15  and stator  17  while traveling through pump  13 . 
     Referring to FIG. 3, a third embodiment of downhole pump assembly  11  is shown. In this embodiment, motor  25  is located above separator  33  and pump  13  at the surface or upper end of the well. Right angle gear reduction or belt drive unit  63  is located directly above the well. Gear reduction or belt drive unit  63  has a second shaft or rod  65  extending down into the well that drives pump  13 . Unit  63  also decreases the rotational speed of shaft  65  relative to motor drive shaft  27 . 
     Coupling  67  connects shaft  65  to the upper end of rotor  15  above pump  13 . Preferably, coupling  67  is a threaded coupling. In this embodiment, a coupling  69  connects the lower end of rotor  15  to flexible shaft  57 . Preferably, coupling  69  is a threaded coupling which prevents longitudinal movement of the rotor relative to the pump at coupling  69 . Welds  71  can further secure flexible shaft  57  and rotor  15  to coupling  69  after being threadedly coupled. However, coupling  67  could be a vertical spline coupling with a fastener extending through the coupling and the portion of flexible shaft  57  coupling  69  receives. Rotor  15  rotates flexible shaft  57  in flexible shaft assembly  55  and separator shaft  41  below pump  13 . Because gear reduction or belt drive unit  63  is located between motor  25  and pump  13 , separator shaft  41  rotates at the same rotational speed as pump rotor  15 . 
     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 the manner consisting with the spirit and scope of the invention herein.