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 an improved seal section for use with a submersible pumping system. 
       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. Each of the components and sub-components in a submersible pumping system must be engineered to withstand the inhospitable downhole environment, which includes wide ranges of temperature, pressure and corrosive well fluids. 
         [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 provides several functions, including transmitting torque between the motor and pump, restricting the flow of wellbore fluids into the motor, absorbing axial thrust imparted by the pump, and accommodating the expansion and contraction of the dielectric motor lubricant as the motor moves through thermal cycles during operation and pressure equalization. Many seal sections employ seal bags to accommodate the volumetric changes and movement of fluid in the seal section. Seal bags can also be configured to provide a positive barrier between clean lubricant and contaminated wellbore fluid. 
         [0004]    Modern seal sections may include two or more seal bags connected in parallel or series configurations. When seal bags are placed in series, the oil from one bag is kept separate from the oil in another bag by the use of a shaft seal between each section. In this way, seal bags connected in a series configuration function as redundant seals. If the first seal bag is compromised or avoided, the foreign fluid is prevented from going into the motor by the second seal bag. 
         [0005]    In contrast, multiple seal bags connected in a parallel configuration do not provide a redundant layer of protection. Instead, seal bags connected in a parallel configuration are intended to simply increase the overall effective volume change capacity within the seal section. In prior art parallel seal bag configurations, there is typically no shaft seal placed between adjacent seal bags and fluid is encouraged to communicate concurrently between bag sections along the shaft. Although effective at increasing fluid exchange capacity, the use of directly connected parallel seal bags presents a concern if a contaminated fluid is allowed to quickly migrate through the parallel seal bags. There is, therefore, a need for an improved seal section that overcomes the deficiencies of the prior art while retaining the benefits of parallel seal bag sections. It is to this and other needs that the present invention is directed. 
       SUMMARY OF THE INVENTION 
       [0006]    In a preferred embodiment, the present invention provides a downhole pumping system that includes a motor, a pump driven by the motor and a seal section positioned between the pump and the motor. The seal section preferably includes a first seal bag assembly, a second seal bag assembly and an interconnect module connected between the first seal bag assembly and the second seal bag assembly. The interconnect module includes a plenum, at least one fluid exchange passage connected to the plenum, and a shaft seal assembly. The shaft seal assembly is configured to divert fluid from the plenum into the at least one fluid exchange passage. 
         [0007]    In another aspect, the plenum, the at least one fluid exchange passage and the shaft seal assembly cooperate to form a fluid labyrinth through the interconnect module. In a first group of interconnect modules within the seal section, the shaft seal assemblies are oriented in a first direction and within a second group of the interconnect modules the shaft seal assemblies are oriented in a second direction to apply an axial force to position the shaft in the operative position or to balance the axial force generated by the shaft seal assemblies in the first group of interconnect modules. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a front perspective view of a downhole pumping system in a non-vertical installation. 
           [0009]      FIG. 2  is an elevational view of a seal section constructed in accordance with a presently preferred embodiment. 
           [0010]      FIG. 3  is a cross-sectional view of a portion of the seal section of  FIG. 2 . 
           [0011]      FIG. 4  is a cross-sectional perspective view of the bag section of  FIG. 3 . 
           [0012]      FIG. 5  is a close-up cross-sectional view of the interconnect module and bag sections from the seal section of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    In accordance with a preferred embodiment of the present invention,  FIG. 1  shows a front perspective view of a downhole pumping system  100  attached to production tubing  102 . The downhole 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. The downhole pumping system  100  is shown in a non-vertical well. This type of well is often referred to as a “horizontal” well. Although the downhole pumping system  100  is depicted in a horizontal well, it will be appreciated that the downhole pumping system  100  can also be used in vertical wells. 
         [0014]    As used herein, the term “petroleum” refers broadly to all mineral hydrocarbons, such as crude oil, gas and combinations of oil and gas. 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  100  are primarily disclosed in a submersible application, some or all of these components can also be used in surface pumping operations. 
         [0015]    The pumping system  100  preferably includes some combination of a pump assembly  108 , a motor assembly  110  and a seal section  112 . In a preferred embodiment, the motor assembly  110  is an electrical motor that receives its power from a surface-based supply. The motor assembly  110  converts the electrical energy into mechanical energy, which is transmitted to the pump assembly  108  by one or more shafts. The pump assembly  108  then transfers a portion of this mechanical energy to fluids within the wellbore, causing the wellbore fluids to move through the production tubing to the surface. In a particularly preferred embodiment, the pump assembly  108  is a turbomachine that uses one or more impellers and diffusers to convert mechanical energy into pressure head. In an alternative embodiment, the pump assembly  108  is a progressive cavity (PC) or positive displacement pump that moves wellbore fluids with one or more screws or pistons. 
         [0016]    The seal section  112  shields the motor assembly  110  from mechanical thrust produced by the pump assembly  108 . The seal section  112  is also preferably configured to prevent the introduction of contaminants from the wellbore  104  into the motor assembly  110 . Although only one pump assembly  108 , seal section  112  and motor assembly  110  are shown, it will be understood that the downhole pumping system  100  could include additional pumps assemblies  108 , seals sections  112  or motor assemblies  110 . 
         [0017]    Referring now to  FIG. 2 , shown therein is an elevational view of the seal section  112 . The seal section  112  includes a head  114 , a base  116  and four bag sections  118   a - 118   d . The head  114  is configured for connection to the pump assembly  108  and the base  116  is configured for connection to the motor assembly  110 . Unless otherwise noted, each of the bag sections  118  includes the same components. It will be understood, however, that the seal section  112  may include bag sections  118  that include different components or components arranged in different configurations. 
         [0018]    Continuing with  FIG. 2 , but referring now also to  FIG. 3 , shown therein is a cross-sectional view of two of the bag sections  118   a ,  118   b . As depicted in  FIG. 3 , the seal section  112  includes a common housing  120  and a common shaft  122 . The shaft  122  transfers mechanical energy from the motor assembly  110  to the pump assembly  108 . Each bag section  118  within the seal section  112  includes an interconnect module  124  and a seal bag assembly  126 . It will be understood that the housing  120  may be segmented, with separate sections joined by a threaded connection to the interconnect module  124 . In turn, each seal bag assembly  126  includes a seal bag  128 , bag support tube  130  and a seal bag retention mechanism  132 . In a first preferred embodiment, the seal bag retention mechanism  132  includes a conventional flange and locking clamp arrangement. 
         [0019]    Turning now to  FIG. 4 , shown therein is a cross-sectional view of a presently preferred embodiment of the seal bag assembly  126 . The seal bag assembly  126  is configured to prevent the contamination of clean motor lubricants with wellbore fluids. The bag support tube  130  provides support for the seal bag  128  and shields the shaft  122  as its passes through the seal bag  128 . In a preferred embodiment, the seal bag  128  is fabricated from a suitable plastic, polymer or elastomer, which are commercially available from a number of sources, including E.I. du Pont de Nemours and Company and Daikin Industries. Suitable plastics include PTFE, AFLAS® and other fluoropolymer plastics that exhibit favorable resistance to corrosive chemicals and elevated temperatures. 
         [0020]    The seal bag retention mechanism  132  secures the seal bag  128  within the seal bag assembly  126 . In a preferred embodiment, the seal bag retention mechanism  132  includes an inner flange  134  secured to the bag support tube  130  and an outer locking clamp  136 . The inner flange  134  is preferably threadingly engaged or pinned with the bag support tube  130 . Alternatively, the inner flange  134  can be configured to rest on a shoulder formed on the bag support tube  130 . 
         [0021]    The inner flange  134  has an outer diameter slightly larger than the inner diameter of the seal bag  128 . In this way, the open end of the seal bag  128  can be pushed onto the flange  134 . The elasticity of the bag material allows the seal bag  128  to stretch to conform to the shape of the flange  134 . The seal bag  128  is held in place over the flange  134  by the locking clamp  136 , which applies a compressive force on the end of the seal bag  128 . The compressive force of the locking clamp  136  further improves the sealed engagement between the seal bag  128  and the flange  134 . The locking clamp  136  is preferably provided with a worm gear mechanism configured to adjust the clamping force exerted by the locking clamp  136 . 
         [0022]    The seal bag assembly  126  is configured to permit the exchange of fluids in and out of the seal bag  128 . In the preferred embodiment, at least one of the flanges  134  includes ports  138  that allow fluid to pass through the inner flange  134  from, or to, the seal bag  128 . Similarly, the bag support tube  130  includes vents  140  that permit the exchange of fluid between the interior space of the bag support tube  130  and the seal bag  128 . 
         [0023]    Turning to  FIG. 5 , shown therein is a cross-sectional view of the interconnect module  124  and the adjacent seal bag assemblies  126 . The interconnect module  124  is used to connect adjacent seal bag assemblies  126 . The interconnect module  124  preferably includes an inlet plenum  142 , a shaft bearing  144 , a shaft seal assembly  146  and one or more fluid exchange passages  148 . The interconnect module  124  is configured to accept the inner flange  134  and end of the support tube  130  of the seal bag assembly  126 . In a particularly preferred embodiment, dowels or pins (not separately designated) are used to maintain positional registration between the seal bag assembly  126  and the interconnect module  124 . The shaft bearing  144  is preferably configured as a hydrodynamic bearing that includes an outer stationary member fixed within the interconnect module  124  and a rotary member fixed to the shaft  122 . The shaft bearing  144  aligns and stabilizes the shaft  122 . 
         [0024]    The shaft seal  146  is preferably configured as a spring-biased mechanical seal. The shaft seal  146  discourages the migration of fluid along the shaft  122 . In alternate preferred embodiments, the shaft seal  146  can include a wiper seal that includes a compliant wiping mechanism in contact with the shaft  122 . As depicted in  FIG. 3 , the shaft seals  146  are preferably oriented in alternating bellows-up and bellows-down position in adjacent interconnect modules  124 . By alternating the orientation of the shaft seals  146 , the resultant axial force imposed by the collection of shaft seals  146  is minimized. In a highly preferred embodiment, the number and disposition of shaft seals  146  within the seal section  112  is designed to offset or compliment the downthrust imposed on the shaft  122  by the pump assembly  108 . 
         [0025]    Continuing with  FIG. 5 , the bag support tube  130  includes an annular space  150  between the interior surface of the bag support tube  130  and the exterior surface of the shaft  122 . The annular space  150  permits the movement of fluid between the shaft  122  and the bag support tube  130 . Due to the rapid rotation of the shaft  122  within the bag support tube  130 , the fluid within the annular space  150  is subject to turbulence and shear forces. If oil-based fluids encounter water-based fluids in the annular space  150 , the turbulent mixing effect may cause the fluids to partially or completely emulsify. Accordingly, it is desirable to divert wellbore fluids (which may include water-based fluids) away from the turbulent region within the annular space  150 . 
         [0026]    The plenum  142  is connected to the fluid exchange passages  148  and the annular space  150  within the interior of the bag support tube  130 . In this way, the plenum provides a fluid path from the adjacent seal bag  128  to the fluid exchange passages  148  extending through the interconnect module  124 . Notably, the interconnect module  124  is configured to move fluid from the turbulent annular space  150  into the more stagnant region within the bag seal  128 . In a preferred embodiment, the interconnect module  146  includes two or more fluid exchange passages  148  extending from the plenum  142  through the interconnect module  146  to the adjacent bag section  118 . As best illustrated in  FIG. 5 , the fluid exchange passages  148  communicate with the ports  138  of the seal bag retention mechanism  132 . 
         [0027]    Thus, the combination of the annular space  150 , plenum  142 , shaft bearing  144 , shaft seal  146  and fluid exchange passages  148  create a labyrinth  152  that causes fluid to pass from an upstream seal bag assembly  126  through the interconnect module  124  to the seal bag assembly  126  of a downstream bag section  118 . Unlike prior art parallel bag configurations, the use of an intervening shaft seal  146  causes the fluid to be rerouted in an indirect, tortuous manner. During horizontal applications (as depicted in  FIG. 1 ), the benefit of the labyrinth  152  is increased. Due to the tortuous nature of the indirect passages through the interconnect module  124 , foreign fluids may settle out of solution in the relatively static area within the seal bag  128 . 
         [0028]    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.
A downhole pumping system includes a motor, a pump driven by the motor and a seal section positioned between the pump and the motor. The seal section preferably includes a first seal bag assembly, a second seal bag assembly and an interconnect module connected between the first seal bag assembly and the second seal bag assembly. The interconnect module includes a plenum, at least one fluid exchange passage connected to the plenum, and a shaft seal assembly. The shaft seal assembly is configured to divert fluid from the plenum into the at least one fluid exchange passage. In another aspect, a first group of interconnect modules within the seal section each includes a shaft seal assembly oriented in a first direction and a second group of the interconnect modules each includes a shaft seal assembly oriented in a second direction to selectively apply an axial force on the shaft.