Abstract:
Embodiments of the present invention generally relate to an electric submersible pumping system for dewatering gas wells. In one embodiment, a method of unloading liquid from a reservoir includes deploying a pumping system into a wellbore to a location proximate the reservoir using a cable. The pumping system includes a motor, an isolation device, and a pump. The method further includes setting the isolation device, thereby rotationally fixing the pumping system to a tubular string disposed in the wellbore and isolating an inlet of the pump from an outlet of the pump; supplying a power signal from the surface to the motor via the cable, thereby operating the pump and lowering a liquid level in the tubular string to a level proximate the reservoir; unsetting the isolation device; and removing the pump assembly from the wellbore using the cable.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present invention generally relate to an electric submersible pumping system for dewatering gas wells. 
         [0003]    2. Description of the Related Art 
         [0004]    As natural gas wells mature, many experience a decrease in production due to water build up in the annulus creating back pressure on the reservoir. The gas industry have utilized varying technologies to alleviate this problem, however most do not meet the economic hurdle as they require intervention such as pulling the tubing string. 
       SUMMARY OF THE INVENTION 
       [0005]    Embodiments of the present invention generally relate to an electric submersible pumping system for dewatering gas wells. In one embodiment, a method of unloading liquid from a reservoir includes deploying a pumping system into a wellbore to a location proximate the reservoir using a cable. The pumping system includes a motor, an isolation device, and a pump. The method further includes setting the isolation device, thereby rotationally fixing the pumping system to a tubular string disposed in the wellbore and isolating an inlet of the pump from an outlet of the pump; supplying a power signal from the surface to the motor via the cable, thereby operating the pump and lowering a liquid level in the tubular string to a level proximate the reservoir; unsetting the isolation device; and removing the pump assembly from the wellbore using the cable. 
         [0006]    In another embodiment, a pumping system includes a submersible high speed electric motor operable to rotate a drive shaft; a high speed pump rotationally fixed to the drive shaft; an isolation device operable to expand into engagement with a tubular string, thereby fluidly isolating an inlet of the pump from an outlet of the pump and rotationally fixing the motor and the pump to the tubular string; and a cable having two or less conductors, a strength sufficient to support the motor, the pump, and the isolation device, and in electrical communication with the motor. A maximum outer diameter of the motor, pump, isolation device, and cable is less than or equal to two inches. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0008]      FIG. 1  illustrates an electric submersible pumping system deployed in a wellbore, according to one embodiment of the present invention. 
           [0009]      FIG. 2A  is a layered view of the power cable.  FIG. 2B  is an end view of the power cable. 
           [0010]      FIG. 3  illustrates an electric submersible pumping system deployed in a wellbore, according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  illustrates a pumping system  1  deployed in a wellbore  5 , according to one embodiment of the present invention. The wellbore  5  has been drilled from a surface of the earth  20  or floor of the sea (not shown) into a hydrocarbon-bearing (i.e., natural gas  100   g ) reservoir  25 . A string of casing  10   c  has been run into the wellbore  5  and set therein with cement (not shown). The casing  10   c  has been perforated  30  to provide to provide fluid communication between the reservoir  25  and a bore of the casing  10 . A wellhead  15  has been mounted on an end of the casing string  10   c.  An outlet line  35  extends from the wellhead  15  to production equipment (not shown), such as a separator. A production tubing string  10   t  has been run into the wellbore  5  and hung from the wellhead  15 . A production packer  85  has been set to isolate an annulus between the tubing  10   t  and the casing  10   c  from the reservoir  25 . The reservoir  25  may be self-producing until a pressure of the gas  100   g  is no longer sufficient to transport a liquid, such as water  100   w,  to the surface. A level of the water  100   w  begins to build in the production tubing  10   t,  thereby exerting hydrostatic pressure on the reservoir  25  and diminishing flow of gas  100   g  from the reservoir  25 . 
         [0012]    The pumping system  1  may include a surface controller  45 , an electric motor  50 , a power conversion module (PCM)  55 , a seal section  60 , a pump  65 , an isolation device  70 , a cablehead  75 , and a power cable  80 . Housings of each of the components  50 - 75  may be longitudinally and rotationally fixed, such as flanged or threaded connections. Since the downhole components  50 - 80  may be deployed within the tubing  10   t,  the components  50 - 80  may be compact, such as having a maximum outer diameter less than or equal to two or one and three-quarter inches (depending on the inner diameter of the tubing  10   t ). 
         [0013]    The surface controller  45  may be in electrical communication with an alternating current (AC) power source  40 , such as a generator on a workover rig (not shown). The surface controller  45  may include a transformer (not shown) for stepping the voltage of the AC power signal from the power source  40  to a medium voltage (V) signal, such as five to ten kV, and a rectifier for converting the medium voltage AC signal to a medium voltage direct current (DC) power signal for transmission downhole via the power cable  80 . The surface controller  45  may further include a data modem (not shown) and a multiplexer (not shown) for modulating and multiplexing a data signal to/from the PCM  55  with the DC power signal. The surface controller  45  may further include an operator interface (not shown), such as a video-display, touch screen, and/or USB port. 
         [0014]    The cable  80  may extend from the surface controller  45  through the wellhead  15  or connect to leads which extend through the wellhead  15  and to the surface controller  45 . The cable  80  may be received by slips or a clamp (not shown) disposed in or proximate to the wellhead  15  for longitudinally fixing the cable  80  to the wellhead  15  during operation of the pumping system  1 . The cable  80  may extend into the wellbore  5  to the cablehead  75 . Since the power signal may be DC, the cable  80  may only include two conductors arranged coaxially. 
         [0015]      FIG. 2A  is a layered view of the power cable  80 .  FIG. 2B  is an end view of the power cable  80 . The cable  80  may include an inner core  205 , an inner jacket  210 , a shield  215 , an outer jacket  230 , and armor  235 ,  240 . The inner core  205  may be the first conductor and made from an electrically conductive material, such as aluminum, copper, aluminum alloy, or copper alloy. The inner core  205  may be solid or stranded. The inner jacket  210  may electrically isolate the core  205  from the shield  215  and be made from a dielectric material, such as a polymer (i.e., an elastomer or thermoplastic). The shield  215  may serve as the second conductor and be made from the electrically conductive material. The shield  215  may be tubular, braided, or a foil covered by a braid. The outer jacket  230  may electrically isolate the shield  215  from the armor  235 ,  240  and be made from an oil-resistant dielectric material. The armor may be made from one or more layers  235 ,  240  of high strength material (i.e., tensile strength greater than or equal to two hundred kpsi) to support the deployment weight (weight of the cable and the weight of the components  50 - 75 ) so that the cable  80  may be used to deploy and remove the components  50 - 75  into/from the wellbore  5 . The high strength material may be a metal or alloy and corrosion resistant, such as galvanized steel or a nickel alloy depending on the corrosiveness of the gas  100   g.  The armor may include two contra-helically wound layers  235 ,  240  of wire or strip. 
         [0016]    Additionally, the cable  80  may include a sheath  225  disposed between the shield  215  and the outer jacket  230 . The sheath  225  may be made from lubricative material, such as polytetrafluoroethylene (PTFE) or lead and may be tape helically wound around the shield  215 . If lead is used for the sheath, a layer of bedding  220  may insulate the shield  215  from the sheath and be made from the dielectric material. Additionally, a buffer  245  may be disposed between the armor layers  235 ,  240 . The buffer  245  may be tape and may be made from the lubricative material. 
         [0017]    Due to the coaxial arrangement, the cable  80  may have an outer diameter  250  less than or equal to one and one-quarter inches, one inch, or three-quarters of an inch. 
         [0018]    Additionally, the cable  80  may further include a pressure containment layer (not shown) made from a material having sufficient strength to contain radial thermal expansion of the dielectric layers and wound to allow longitudinal expansion thereof. The material may be stainless steel and may be strip or wire. Alternatively, the cable  80  may include only one conductor and the tubing  10   t  may be used for the other conductor. 
         [0019]    The cable  80  may be longitudinally fixed to the cablehead  75 . The cablehead  75  may also include leads (not shown) extending therethrough. The leads may provide electrical communication between the conductors of the cable  80  and the PCM  55 . 
         [0020]    The motor  50  may be switched reluctance motor (SRM) or permanent magnet motor, such as a brushless DC motor (BLDC). The motor  50  may be filled with a dielectric, thermally conductive liquid lubricant, such as oil. The motor  50  may be cooled by thermal communication with the reservoir water  100   w.  The motor  50  may include a thrust bearing (not shown) for supporting a drive shaft (not shown). In operation, the motor may rotate the shaft, thereby driving the pump  65 . The motor shaft may be directly connected to the pump shaft (no gearbox). As discussed above, since the motor may be compact, the motor may operate at high speed so that the pump may generate the necessary head to pump the water  100   w  to the surface  20 . High speed may be greater than or equal to ten thousand, twenty-five thousand, or fifty-thousand revolutions per minute (RPM). Alternatively, the motor  50  may be any other type of synchronous motor, an induction motor, or a DC motor. 
         [0021]    The SRM motor may include a multi-lobed rotor made from a magnetic material and a multi-lobed stator. Each lobe of the stator may be wound and opposing lobes may be connected in series to define each phase. For example, the SRM motor may be three-phase (six stator lobes) and include a four-lobed rotor. The BLDC motor may be two pole and three phase. The BLDC motor may include the stator having the three phase winding, a permanent magnet rotor, and a rotor position sensor. The permanent magnet rotor may be made of a rare earth magnet or a ceramic magnet. The rotor position sensor may be a Hall-effect sensor, a rotary encoder, or sensorless (i.e., measurement of back EMF in undriven coils by the motor controller). 
         [0022]    The PCM  55  may include a motor controller (not shown), a modem (not shown), and demultiplexer (not shown). The modem and demultiplexer may demultiplex a data signal from the DC power signal, demodulate the signal, and transmit the data signal to the motor controller. The motor controller may receive the medium voltage DC signal from the cable and sequentially switch phases of the motor, thereby supplying an output signal to drive the phases of the motor. The output signal may be stepped, trapezoidal, or sinusoidal. The BLDC motor controller may be in communication with the rotor position sensor and include a bank of transistors or thyristors and a chopper drive for complex control (i.e., variable speed drive and/or soft start capability). The SRM motor controller may include a logic circuit for simple control (i.e. predetermined speed) or a microprocessor for complex control (i.e., variable speed drive and/or soft start capability). The SRM motor controller may use one or two-phase excitation, be unipolar or bi-polar, and control the speed of the motor by controlling the switching frequency. The SRM motor controller may include an asymmetric bridge or half-bridge. 
         [0023]    Additionally, the PCM may include a power supply (not shown). The power supply may include one or more DC/DC converters, each converter including an inverter, a transformer, and a rectifier for converting the DC power signal into an AC power signal and stepping the voltage from medium to low, such as less than or equal to one kV. The power supply may include multiple DC/DC converters in series to gradually step the DC voltage from medium to low. The low voltage DC signal may then be supplied to the motor controller. 
         [0024]    The motor controller may be in data communication with one or more sensors (not shown) distributed throughout the components  50 - 75 . A pressure and temperature (PT) sensor may be in fluid communication with the water  100   w  entering the intake  65   i.  A gas to liquid ratio (GLR) sensor may be in fluid communication with the water  100   w  entering the intake  65   i.  A second PT sensor may be in fluid communication with the reservoir fluid discharged from the outlet  65   o.  A temperature sensor (or PT sensor) may be in fluid communication with the lubricant to ensure that the motor and downhole controller are being sufficiently cooled. Multiple temperature sensors may be included in the PCM for monitoring and recording temperatures of the various electronic components. A voltage meter and current (VAMP) sensor may be in electrical communication with the cable  80  to monitor power loss from the cable. A second VAMP sensor may be in electrical communication with the motor controller output to monitor performance of the motor controller. Further, one or more vibration sensors may monitor operation of the motor  50 , the pump  65 , and/or the seal section  60 . A flow meter may be in fluid communication with the discharge  65   o  for monitoring a flow rate of the pump  65 . Utilizing data from the sensors, the motor controller may monitor for adverse conditions, such as pump-off, gas lock, or abnormal power performance and take remedial action before damage to the pump  65  and/or motor  50  occurs. 
         [0025]    The seal section  60  may isolate the water  100   w  being pumped through the pump  65  from the lubricant in the motor  50  by equalizing the lubricant pressure with the pressure of the reservoir fluid  100 . The seal section  60  may rotationally fix the motor shaft to a drive shaft of the pump. The shaft seal may house a thrust bearing capable of supporting thrust load from the pump. The seal section  60  may be positive type or labyrinth type. The positive type may include an elastic, fluid-barrier bag to allow for thermal expansion of the motor lubricant during operation. The labyrinth type may include tube paths extending between a lubricant chamber and a reservoir fluid chamber providing limited fluid communication between the chambers. 
         [0026]    The pump may include an inlet  65   i.  The inlet  65   i  may be standard type, static gas separator type, or rotary gas separator type depending on the GLR of the water  100   w.  The standard type intake may include a plurality of ports allowing water  100   w  to enter a lower or first stage of the pump  65 . The standard intake may include a screen to filter particulates from the reservoir fluid. The static gas separator type may include a reverse-flow path to separate a gas portion of the reservoir fluid from a liquid portion of the reservoir fluid. 
         [0027]    The pump  65  may be dynamic and/or positive displacement. The dynamic pump may be centrifugal, such a radial flow, mixed axial/radial flow, or axial flow, or a boundary layer (a.k.a. Tesla pump). The centrifugal pump may include a propeller (axial) or an open impeller (radial or axial/radial). The pump housing of the centrifugal pump may include a nozzle to create a jet effect. The positive displacement may be screw or twin screw. The pump  65  may include one or more stages (not shown). Each stage may be the same type or a different type. For example, a first stage may be a positive displacement screw stage and the second stage may be centrifugal axial flow (i.e., propeller). An outer surface of the propeller, impeller, and/or screw may be hardened to resist erosion (i.e., carbide coated). The pump may deliver the pressurized reservoir fluid to an outlet  65   o  of the isolation device  70 . 
         [0028]    The pumping system  1  may further include an actuator (not shown) for setting and/or unsetting the isolation device  70 . The actuator may include an inflation tool, a check valve, and a deflation tool. The check valve may be a separate member or integral with the inflation tool. The inflation tool may be an electric pump and may be in electrical communication with the motor controller or include a separate power supply in direct communication with the power cable. Upon activation, the inflation tool may intake reservoir fluid, pressurize the reservoir fluid, and inject the pressurized reservoir fluid through the check valve and into the isolation device. Alternatively, the inflation tool may include a tank filled with clean inflation fluid, such as oil for inflating the isolation device  70 . 
         [0029]    The isolation device  70  may include a bladder (not shown), a mandrel (not shown), anchor straps (not shown), and a sealing cover (not shown). The mandrel may include a first fluid path therethrough for passing the water  100   w  from the pump  65  to the outlet  65   o,  the outlet  65   o,  and a second fluid path for conducting reservoir fluid from the inflation tool to the bladder. The bladder may be made from an elastomer and be disposed along and around an outer surface of the mandrel. The anchor straps may be disposed along and around an outer surface of the bladder. The anchor straps may be made from a metal or alloy and may engage an inner surface of the casing  10  upon expansion of the bladder, thereby rotationally fixing the mandrel (and the components  50 - 75 ) to the tubing  10   t.  The anchor straps may also longitudinally fix the mandrel to the casing, thereby relieving the cable  80  from having to support the weight of the components  50 - 75  during operation of the pump  65 . The cable  80  may then be relegated to a back up support should the isolation device  70  fail. 
         [0030]    The sealing cover may be disposed along a portion and around the anchor straps and engage the casing upon expansion of the bladder, thereby fluidly isolating the outlet  65   o  from the intake  65   i.  The deflation tool may include a mechanically or electrically operated valve. The deflation tool may in fluid communication with the bladder fluid path such that opening the valve allows pressurized fluid from the bladder to flow into the wellbore, thereby deflating the bladder. The mechanical deflation tool may include a spring biasing a valve member toward a closed position. The valve member may be opened by tension in the cable  80  exceeding a biasing force of the spring. The electrical inflation tool may include an electric motor operating a valve member. The electric motor may be in electrical communication with the motor controller or in direct communication with the cable. Operation of the motor using a first polarity of the voltage may open the valve and operation of the motor using a second opposite polarity may close the valve. 
         [0031]    Alternatively, instead of anchor straps on the bladder, the isolation device may include one or more sets of slips, one or more respective cones, and a piston disposed on the mandrel. The piston may be in fluid communication with the inflation tool for engaging the slips. The slips may engage the casing  10 , thereby rotationally fixing the components  50 - 75  to the casing. The slips may also longitudinally support the components  50 - 75 . The slips may be disengaged using the deflation tool. 
         [0032]    Alternatively, instead of an actuator, hydraulic tubing (not shown) may be run in with the components  50 - 75  and extend to the isolation device  70 . Hydraulic fluid may be pumped into the bladder through the hydraulic tubing to set the isolation device  70  and relieved from the bladder via the tubing to unset the isolation device  70 . Alternatively, the isolation device  70  may include one or more slips (not shown), one or more respective cones (not shown), and a solid packing element (not shown). The actuator may include a power charge, a piston, and a shearable ratchet mechanism. The power charge may be in electrical communication with the motor controller or directly with the cable  80 . Detonation of the power charge may operate the piston along the ratchet mechanism to set the slips and the packing element. Tension in the cable  80  may be used to shear the ratchet and unset the isolation device  70 . Alternatively, hydraulic tubing may be used instead of the power charge. Alternatively, a second hydraulic tubing may be used instead of the ratchet mechanism to unset the packing element. Alternatively, the isolation device  70  may include an expandable element made from a shape memory alloy or polymer and include an electric heating element so that the expandable element may be expanded by operating the heating element and contracted by deactivating the heating element (or vice versa). 
         [0033]    Additionally, the isolation device  70  may include a bypass vent (not shown) for releasing gas separated by the inlet  65   i  that may collect below the isolation device and preventing gas lock of the pump  65 . A pressure relief valve (not shown) may be disposed in the bypass vent. 
         [0034]    In operation, to install the pumping system  1 , a workover rig (not shown) and the pumping system  1  may be deployed to the wellsite. Since the cable  80  may include only two conductors, the cable  80  may be delivered wound onto a drum (not shown). The wellhead  15  may be opened. The components  50 - 75  may be suspended over the wellbore  5  from the workover rig and an end of the cable  80  may be connected to the cablehead  75 . The cable  80  may be unwound from the drum, thereby lowering the components  50 - 75  into the wellbore inside of the production tubing  10   t.  Once the components  50 - 75  have reached the desired depth proximate to the reservoir  25 , the wellhead may be closed and the conductors of the cable  80  may be connected to the surface controller  45 . 
         [0035]    Additionally, a downhole tractor (not shown) may be integrated into the cable to facilitate the delivery of the pumping system, especially for highly deviated wells, such as those having an inclination of more than 45 degrees or dogleg severity in excess of 5 degrees per 100 ft. The drive and wheels of the tractor may be collapsed against the cable and deployed when required by a signal from the surface. 
         [0036]    The isolation device  70  may then be set. If the isolation device  70  is electrically operated, the surface controller  45  may be activated, thereby delivering the DC power signal to the PCM  55  and activating the downhole controller  55 . Instructions may be given to the surface controller  45  via the operator interface, instructing setting of the isolation device  70 . The instructions may be relayed to the PCM  55  via the cable. The PCM  55  may then operate the actuator. Alternatively, as discussed above, the actuator may be directly connected to the cable. In this alternative, the actuator may be operated by sending a voltage different than the operating voltage of the motor. For example, since the motor may be operated by the medium voltage, the inflation tool may be operated at a low voltage and the deflation tool (if electrical) may be operated by reversing the polarity of the low voltage. 
         [0037]    Once the isolation device  70  is set, the motor  50  may then be started. If the motor controller is variable, the motor controller may soft start the motor  50 . As the pump  65  is operating, the motor controller may send data from the sensors to the surface so that the operator may monitor performance of the pump. If the motor controller is variable, a speed of the motor  50  may be adjusted to optimize performance of the pump  65 . Alternatively, the surface operator may instruct the motor controller to vary operation of the motor. The pump  65  may pump the water  100   w  through the production tubing  10   t  and the wellhead  15  into the outlet  35 , thereby lowering a level of the water  100   w  and reducing hydrostatic pressure of the water  100   w  on the formation  25 . The pump  65  may be operated until the water level is lowered to the inlet  65   i  of the pump, thereby allowing natural production from the reservoir  25 . The operator may then send instructions to the motor controller to shut down the pump  65  or simply cut power to the cable  80 . The operator may send instructions to the PCM  55  to unset the isolation device  70  (if electrically operated) or the drum may be wound to exert sufficient tension in the cable  80  to unseat the isolation device  70 . The cable  80  may be wound, thereby raising the components  50 - 75  from the wellbore  5 . The workover rig and the pumping system  1  may then be redeployed to another wellsite. 
         [0038]    Advantageously, deployment of the components  50 - 75  using the cable  80  inside of the production tubing  10   t  instead of removing the production tubing string and redeploying the production tubing string with a permanently mounted artificial lift system reduces the required size of the workover rig and the capital commitment to the well. Deployment and removal of the pumping system  1  to/from the wellsite may be accomplished in a matter of hours, thereby allowing multiple wells to be dewatered in a single day. Transmitting a DC power signal through the cable  80  reduces the required diameter of the cable, thereby allowing a longer length of the cable  80  (i.e., five thousand to eight thousand feet) to be spooled onto a drum, and easing deployment of the cable  80 . 
         [0039]      FIG. 3  illustrates an electric submersible pumping system  1  deployed in a wellbore  5 , according to another embodiment of the present invention. In this embodiment, the casing  10   c  has been used to produce fluid from the reservoir  25  instead of installing production tubing. In this scenario, the isolation device  70  may be set against the casing  10   c  and the pump  65  may discharge the water  100   w  to the surface  20  via a bore of the casing  10   c.    
         [0040]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.