Abstract:
An electric submersible pumping system includes a motor that is filled with motor lubricant fluid and a pump driven by the motor. The electric submersible pumping system further includes a fluid expansion module connected to the motor that is designed to accommodate the expansion and contraction of the motor lubricant fluid in the motor. The fluid expansion module preferably includes a piston seal housing in fluid communication with the motor and a bag seal housing in fluid communication with the piston seal housing. The fluid expansion module further includes at least one axially movable barrier in the piston seal housing and at least one expansible barrier in the bag seal housing. The axially movable barrier and the expansible barrier cooperate to permit the expansion of the motor lubricant fluid without contaminating the motor lubricant fluid with fluids or and solids from the wellbore.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to the field of submersible pumping systems, and more particularly, but not by way of limitation, to a system for accommodating the expansion of motor lubricants in high-temperature environments. 
       BACKGROUND 
       [0002]    Submersible pumping systems are often deployed into wells to recover petroleum fluids from subterranean reservoirs. Typically, the submersible pumping system includes a number of components, including one or more fluid filled electric motors coupled to one or more high performance pumps located above the motor. When energized, the motor provides torque to the pump, which pushes wellbore fluids to the surface through production tubing. Each of the components in a submersible pumping system must be engineered to withstand the inhospitable downhole environment. 
         [0003]    Components commonly referred to as “seal sections” protect the electric motors and are typically positioned between the motor and the pump. In this position, the seal section provide several functions, including transmitting torque between the motor and pump, restricting the flow of wellbore fluids into the motor, protecting the motor from axial thrust imparted by the pump, and accommodating the expansion and contraction of motor lubricant as the motor moves through thermal cycles during operation. Prior art seal sections typically include a “clean side” in fluid communication with the electric motor and a “contaminated side” in fluid communication with the wellbore. Bellows or bags have been used to separate the clean side of the seal section from the contaminated side. 
         [0004]    Recently, manufacturers have employed polymer expansion bags within the seal section to accommodate the expansion and contraction of motor lubricants while isolating the lubricants from contaminants in the wellbore fluid. Although generally effective at lower temperatures, the currently available polymers become somewhat permeable at extremely elevated temperatures and allow the passage of moisture across the membrane. The moisture reduces the insulating properties of polyimide and other films used to electrically isolate components within the downhole pumping system. Although piston-based systems may provide an alternative to the use of polymer expansion bags, prior art piston-based seal assemblies are susceptible to failure from sand, scale or other particulates. There is, therefore, a need for improved designs that can be used to accommodate expansion of motor fluids in elevated temperature applications. It is to this and other needs that the preferred embodiments are directed. 
       SUMMARY OF THE INVENTION 
       [0005]    In preferred embodiments, the present invention includes an electric submersible pumping system that is configured to pump fluids from a wellbore. The electric submersible pumping system includes a motor that is filled with motor lubricant fluid and a pump driven by the motor. The electric submersible pumping system further includes a fluid expansion module connected to the motor that is designed to accommodate the expansion and contraction of the motor lubricant fluid in the motor. 
         [0006]    The fluid expansion module preferably includes a piston seal housing in fluid communication with the motor and a bag seal housing in fluid communication with the piston seal housing. The fluid expansion module further includes at least one axially movable barrier in the piston seal housing and at least one expansible barrier in the bag seal housing. The axially movable barrier and the expansible barrier cooperate to permit the expansion of the motor lubricant fluid without contaminating the motor lubricant fluid with fluids or and solids from the wellbore. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  depicts a submersible pumping system constructed in accordance with a preferred embodiment of the present invention. 
           [0008]      FIG. 2  provides a cross-sectional view of the motor, lower fluid expansion module and seal section constructed in accordance with a presently preferred embodiment. 
           [0009]      FIG. 3  presents a cross-sectional representation of the motor of the pumping system from  FIG. 2 . 
           [0010]      FIG. 4  presents a cross-sectional representation of the lower fluid expansion module of  FIG. 2 . 
           [0011]      FIG. 5  presents a perspective view of a piston from the lower fluid expansion module of  FIG. 4 . 
           [0012]      FIG. 6  presents a cross-sectional view of the sealing ring from the piston of  FIG. 5 . 
           [0013]      FIG. 7  provides a cross-sectional view of the seal section of the pumping system from  FIG. 2 . 
           [0014]      FIG. 8  provides a cross-sectional view of a mechanical seal from the seal section of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    In accordance with a preferred embodiment of the present invention,  FIG. 1  shows an elevational view of a pumping system  100  attached to production tubing  102 . The pumping system  100  and production tubing  102  are disposed in a wellbore  104 , which is drilled for the production of a fluid such as water or petroleum. As used herein, the term “petroleum” refers broadly to all mineral hydrocarbons, such as crude oil, gas and combinations of oil and gas. 
         [0016]    The pumping system  100  preferably includes a pump  108 , a motor  110 , a seal section  112  and a fluid expansion module  114 . The production tubing  102  connects the pumping system  100  to a wellhead  106  located on the surface. Although the pumping system  100  is primarily designed to pump petroleum products, it will be understood that the present invention can also be used to move other fluids. It will also be understood that, although each of the components of the pumping system are primarily disclosed in a submersible application, some or all of these components can also be used in surface pumping operations. 
         [0017]    Generally, the motor  110  is configured to drive the pump  108 . In a particularly preferred embodiment, the pump  108  is a turbomachine that uses one or more impellers and diffusers to convert mechanical energy into pressure head. In alternate embodiments, the pump  108  is configured as a positive displacement pump. The pump  108  includes a pump intake  118  that allows fluids from the wellbore  104  to be drawn into the pump  108 . The pump  108  forces the wellbore fluids to the surface through the production tubing  102 . 
         [0018]    In the preferred embodiments, the seal section  112  is positioned above the motor  110  and below the pump  108 . The fluid expansion module  114  is positioned below the motor  110 . Although only one of each component is shown, it will be understood that more can be connected when appropriate, that other arrangements of the components are desirable and that these additional configurations are encompassed within the scope of preferred embodiments. For example, in many applications, it is desirable to use tandem-motor combinations, gas separators, multiple seal sections, multiple pumps, sensor modules and other downhole components. 
         [0019]    It will be noted that although the pumping system  100  is depicted in a vertical deployment in  FIG. 1 , the pumping system  100  can also be used in non-vertical applications, including in horizontal and deviated wellbores  104 . Accordingly, references to “upper” and “lower” within this disclosure are merely used to describe the relative positions of components within the pumping system  100  and should not be construed as an indication that the pumping system  100  must be deployed in a vertical orientation. 
         [0020]    Referring now also to  FIGS. 2 and 3 , shown therein is a cross-sectional view of the seal section  112 , motor  110  and fluid expansion module  114 . As depicted in the close-up view of the motor  110  in  FIG. 3 , the motor  110  preferably includes a motor housing  120 , stator assembly  122 , rotor assembly  124 , rotor bearings  126  and a motor shaft  128 . The stator assembly  122  includes a series of stator coils (not separately designated) that correspond to the various phases of electricity supplied to the motor  110 . The rotor assembly  124  is keyed to the motor shaft  128  and configured for rotation in close proximity to the stationary stator assembly  122 . The size and configuration of the stator assembly  122  and rotor assembly  124  can be adjusted to accommodate application-specific performance requirements of the motor  110 . 
         [0021]    Sequentially energizing the various series of coils within the stator assembly  122  causes the rotor assembly  124  and motor shaft  128  to rotate in accordance with well-known electromotive principles. The motor bearings  126  maintain the central position of the rotor assembly  124  within the stator assembly  122  and oppose radial and axial forces generated by the motor  110  on the motor shaft  128 . 
         [0022]    The motor  110  is filled with non-conductive lubricating oil during manufacture that reduces frictional wear on the rotating components within the motor  110 . As the motor  110  cycles during use and as the motor  110  is exposed to the elevated temperatures in the wellbore  104 , the lubricating oil expands and contracts. It is desirable to prevent the clean motor oil from becoming contaminated with fluids and solids in the wellbore. To permit the expansion and contraction of the lubricating oil under elevated wellbore temperatures, the seal section  112  and fluid expansion module  114  are connected to the motor  110  and placed in fluid communication with the motor oil. 
         [0023]    Continuing with  FIG. 2  and referring now also to  FIG. 4 , shown therein is a cross-sectional view of the fluid expansion module  114 . The fluid expansion module  114  includes a piston seal housing  130 , a bag seal housing  132 , one or more piston assemblies  134 , a bag seal assembly  136  and a fluid exchange assembly  138 . In a particularly preferred embodiment, the fluid expansion module  114  includes a pair of piston assemblies  134   a,    134   b.  The piston assemblies  134   a,    134   b  are placed in the piston seal housing  130  and are configured for axial movement within the fluid expansion module  114 . The inside surface of the piston seal housing  130  includes a polymer liner  140  that reduces friction and stiction. The polymer liner  140  is preferably manufactured from PTFE, PFA, PEEK and other high-temperature polymers. Alternatively, the inside surface of the piston seal housing  130  can be manufactured from polished chrome, stainless steel or other durable metal. 
         [0024]    Turning to  FIG. 5 , shown therein is a perspective view of one of the piston assemblies  134 . The piston assembly  134  preferably includes a piston  142  and a pair of spring-energized seals  144 . The piston  142  is preferably manufactured from a highly-polished metal, including chrome, stainless steel and related alloys and has an outside diameter that is only slightly smaller than the inside diameter of the piston seal housing  130 . Alternatively, the piston  142  can be manufactured from a high-temperature rated elastomer or polymer. Preferred polymers of the piston  142  include polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA) and polyethether ketone (PEEK). 
         [0025]      FIG. 6  presents a cross-sectional view of the seal  144 . The seal  144  includes an body  146  and an interior spring  148 . The body  146  is preferably manufactured from a durable, high-temperature and wear-resistant elastomer or polymer, such as polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), polyethether ketone (PEEK) and perfluoroelastomer. The interior spring  148  is configured to exert force against the body  146  in an outward radial direction. In this way, the spring  148  presses the body  146  against the inside surface of the piston seal housing  130 . The interior spring  148  is preferably configured as a coiled ring or series of connected Belleville washers. 
         [0026]    Turning back to  FIG. 4 , the bag seal housing  132  includes the bag seal assembly  136 . The bag seal assembly  136  preferably includes a bag support  150 , a bladder  152 , inlet ports  154  and discharge valves  156 . The bag support  152  is rigidly attached to the inside surface of the bag seal housing  132 . The bladder  152  is secured to the bag support  150  with compression flanges  158 . Alternatively, the bladder  152  can be secured to the bag support  152  with grips or hose clamps. The inlet ports  154  provide a path of fluid communication from the piston seal housing  130  into the inside of the bladder  152  and bag support  150 . Importantly, the bag support  150  permits the passage of fluids between the piston seal housing  130  and bag seal housing  132  only through the inlet ports  154 . Fluids external to the bladder  152  are not allowed to pass directly into the piston seal housing  130 . 
         [0027]    The discharge valves  156  are preferably one-way relief valves that are configured to open at a predetermined threshold pressure that exceeds the exterior wellbore pressure. In this way, if the fluid pressure inside the bladder  152  exceeds the set-point pressure, the discharge valves  156  open and relieve the pressure inside the bladder  152  by discharging a small volume of fluid into the wellbore  104 . In a particularly preferred embodiment, the bladder  152  is manufactured from a high-temperature polymer or elastomer. Suitable polymers and elastomers include polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA) and polyethether ketone (PEEK). 
         [0028]    The bag seal housing  132  also includes the fluid exchange assembly  138 . The fluid exchange assembly  138  includes a solids screen  160  and a plurality of exchange ports  162 . The exchange ports  162  allow fluids to pass from the wellbore  104  through the solids screen  160  into the bag seal housing  132  around the exterior of the bladder  152 . The solids screen  160  reduces the presence of particulates in the bag seal housing  132 . The solids screen  160  is preferably manufactured from a metal or polymer fabric mesh. 
         [0029]    During manufacture, the fluid expansion module  114  is filled with clean motor lubricant. The piston assemblies  134   a,    134   b  are then placed into the piston seal housing  130 . As the fluid in the motor  110  expands during operation, the increased volume exerts pressure on the upper side of the piston assembly  134   a.  In response, piston assembly  134   a  moves downward toward piston assembly  134   b.  When the volume between the piston assemblies  134   a,    134   b  decreases, the increased pressure on piston assembly  134   b  forces it downward toward the bag seal housing  132 . As piston assembly  134   b  moves downward it pushes clean motor lubricant through the inlet ports  154 , through the bag support  152  and into the bladder  152 . The bladder  152  expands to accommodate introduction of fluid from the piston seal housing  130 . As the bladder  152  expands, fluid external to the bladder  152  is expelled through the exchange ports  162  and solids screen  160 . If the pressure inside the bladder  152  exceeds the threshold pressure limit of the discharge valves  156 , the discharge valves  156  open and vent a portion of fluid into the wellbore  104 . 
         [0030]    Conversely, during a cooling cycle, the fluid in the motor  110  contracts and the movement of the components within the fluid expansion module  114  reverses. As the pistons  134   a,    134   b  are drawn upward, fluid is pulled out of the bladder  152 . As the volume and pressure inside the bladder  152  decreases, fluid from the wellbore is pulled into the bag seal housing  132  through the solids screen  160  and exchange ports  162 . The fluid expansion module  114  provides a robust mechanism for allowing expansion and contraction of lubricants from the motor  110  while maintaining an isolation barrier between the clean motor lubricants and the contaminated fluids from the wellbore  104 . Notably, the use of piston assemblies  134   a,    134   b  provide redundant barriers to the bladder  152  that are not susceptible to the increased permeability found in even high-temperature bladders. Accordingly, even if the bladder  152  is exposed to extremely high temperatures and permits the passage of some moisture from the wellbore  104  into the piston seal housing  130 , the moisture is isolated from the motor  110  by the redundant piston assemblies  134   a,    134   b.    
         [0031]    In certain applications, it may be desirable to place the pump  108  below the motor  110 . In those applications, the fluid expansion module  114  will be positioned above the motor  110  and the seal section  112  will be placed between the motor  110  and the pump  108 . In these alternative embodiments, the bag seal housing  132  will actually be positioned above the piston seal housing  130 . 
         [0032]    Turning to  FIG. 7 , shown therein is a cross-sectional view of the motor  110  and seal section  112 . The seal section  112  is attached to the upper end of the motor  110  and provides a second system for accommodating the sealing of the rotating shaft  128  to the equipment and support the thrust load of the pump  108 . The seal section  112  includes a seal section shaft  164 , a thrust bearing assembly  166 , one or more mechanical seals  168  and one or more relief valves  170 . During manufacture, the seal section  112  is filled with clean motor lubricant oil. 
         [0033]    The seal section shaft  164  is coupled to the motor shaft  128 , or formed as a unitary shaft with the motor shaft  128 , and transfers torque from the motor  110  to the pump  108 . The thrust bearing assembly  166  includes a pair of stationary bearings  172  and a thrust runner  174  attached to the seal section shaft  164 . The thrust runner  174  is captured between the stationary bearings  172 , which limit the axial displacement of the runner  174  and the motor shaft  128  and seal section shaft  164 . 
         [0034]    In a particularly preferred embodiment, the seal section  112  includes a plurality of mechanical seals. Two mechanical seals  168   a,    168   b  are depicted in  FIG. 7 . As best illustrated in the close-up view of the mechanical seal  168  in  FIG. 8 , the mechanical seals  168  each include bellows  176 , a coiled spring  178 , a runner  180  and a stationary ring  182 . These components cooperate to prevent the migration of fluid along the seal section shaft  164 . The stationary ring  182  has an internal diameter sized to permit the free rotation of the seal section shaft  164 . In contrast, the bellows  176 , springs  178  and runner  180  rotate with the seal section shaft  164 . The rotating runner  180  is held in place against the stationary ring  182  by the spring-loaded bellows  176 . The bellows  176  preferably includes a series of folds that allow its length to adjust to keep the runner  180  in contact with the stationary ring  182  if the seal section shaft  164  should experience axial displacement. The bellows  176  may be manufactured from thin corrugated metal or from elastomers and polymers, including AFLAS, perfluoroelastomer, polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA) and polyethether ketone (PEEK). 
         [0035]    Continuing with  FIG. 7 , the relief valves  170  are preferably one-way check valves that are spring-biased in a closed position. When a threshold pressure is exerted against the relief valves  170  by the internal pressure within the seal section  112 , the relief valves  170  temporarily open to release fluid from inside the seal section  112  into the pump  108  or wellbore  104  to relieve the excess internal pressure. 
         [0036]    Thus, during thermal cycling of the motor  110 , the motor lubricant may expand from the motor  110  into the seal section  112  and the fluid expansion module  114 . The fluid expansion module  114  provides the primary system for accommodating the expansion of fluid from the motor  110  and the seal section  112  provides a secondary system for accommodating the expansion of motor oil from the motor  110 . In the event that the fluid inside the seal section  112  exceeds a threshold pressure, the relief valves  170  temporarily open to prevent damage to the motor  110  or mechanical seals  168 . The primary function of the seal section  112  is to prevent migration of fluids along the shafts between the motor  110  and pump  108 . 
         [0037]    It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and functions of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. It will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other systems without departing from the scope and spirit of the present invention.