Abstract:
Seal sections for electric submersible pumps. Seal sections are described which include a combination of a barrier fluid to protect a seal from unfavorable ingress and control of differential pressure across the face of the seal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates generally to electric submersible pump assemblies used for hydrocarbon production recovery. In particular aspects, the invention relates to systems for limiting oil leakage from downhole motors used in electric submersible pump assemblies. 
         [0003]    2. Description of the Related Art 
         [0004]    A typical electrical submersible pump (ESP) system includes a pump that is driven by a motor. Because the ESP system may be disposed at great depths and are inaccessible at this time, the motors are designed to operate for a long period of time without maintenance. Motor oil is used to help lubricate the motor and to dissipate the heat the motor generates during operation. A seal section is used between the motor and pump to isolate the clean motor oil from the wellbore fluid. This seal section also provides for volume change of the motor oil during operation due to changes in pressure and temperature. By allowing the volume of motor oil to change, the internal motor pressure is equalized with the wellbore annulus pressure. A shaft in the seal section transmits torque from the motor to the pump. 
         [0005]    Controlling leakage around rotating shafts is often achieved by utilizing mechanical seals. Some leakage will always occur across the faces of a mechanical seal in operation. By design, a small amount of leakage of clean motor oil through the seal is desired to lubricate the faces of the seal. This can be accomplished by having a greater internal oil pressure than external. If the external fluid pressure is higher, leakage will be driven in the other direction, causing external fluid to contaminate the motor oil through the mechanical seal. Over time, as the seal starts to wear, the leakage rate increases and can become problematic to the operation of the equipment. If the seal is not properly lubricated, the wear will be accelerated. Mechanical seal flush plans of various types are used to keep mechanical seals working properly by maintaining proper operating conditions of the seals in order to maximize run life and control leakage. Employing flush plans is relatively straightforward in applications where the mechanical seals are accessible, but it becomes more difficult with rotating equipment that is installed in remote locations, such as downhole pump applications. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides mechanical arrangements for downhole motors with seal assemblies that promote lubrication of the mechanical seals. The arrangements of the present invention are particularly useful for extending the run life of an ESP. 
         [0007]    Exemplary seal sections are described which include a combination of a barrier fluid to protect a seal from unfavorable ingress and control of differential pressure across the face of the seal. The barrier fluid is a heavy specific gravity blocking fluid that is placed on top of a mechanical seal to prevent fluid ingress into the ESP seal section. The barrier fluid can be a relatively heavy specific gravity fluid that does not mix well with water or other substances. In certain embodiments, a barrier fluid pocket or reservoir is recessed inside the head or guides of the seal assembly to facilitate the addition of the barrier fluid. The purpose of the barrier fluid is to displace and block water or well fluid from coming into contact with the rotating face of the mechanical seal to prevent water or well fluid ingress past the seal. The barrier fluid would also allow lighter fluids, such as lubricating fluid (motor oil) to pass upward through the barrier. As a result, motor oil within the seal section can leak out, as desired, while fluid ingress is prevented. 
         [0008]    In addition, the invention provides mechanisms that produced positive fluid pressure within the seal section and thereby dictate the preferred direction of leakage across the mechanical seals of the seal section and provide improved or even optimum pressure across the mechanical seal faces. In certain embodiments, the positive fluid pressure mechanisms include a metallic, bellows-based pressure compensator having a natural spring rate of the bellows. The positive fluid pressure mechanism also includes a biasing means that increases fluid pressure of motor oil retained within the bellows assemblies of the seal section. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    For a thorough understanding of the present invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, wherein like reference numerals designate like or similar elements throughout the several figures of the drawings and wherein: 
           [0010]      FIG. 1  is a side view of an exemplary electric submersible pump assembly located within a wellbore. 
           [0011]      FIG. 2  is a side, cross-sectional view of upper portions of an exemplary seal section which incorporates features in accordance with the present invention. 
           [0012]      FIG. 3  is a side, cross-sectional view of lower portions of an exemplary seal section which incorporates features in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]      FIG. 1  illustrates an exemplary wellbore  10  that has been drilled through the earth  12  from the surface  14  down to a hydrocarbon-bearing formation  16 . The wellbore  10  has been lined with metallic casing  18  of a type known in the art. Perforations  20  are disposed through the casing and into the formation  16 , thereby allowing hydrocarbons to enter the wellbore  10 . 
         [0014]    An electric submersible pump (ESP) assembly, generally indicated at  22 , is shown disposed within the wellbore  10  by production tubing  24 . An annulus  26  is defined between the casing  18  and the production tubing  24 /ESP  22 . The ESP assembly  22  includes a pump section  28 , a seal section  30  and a motor section  32 . As is known, the motor section  32  drives the pump section  28  to draw hydrocarbon fluid in from the wellbore  10  via fluid inlets  34  and flow it to the surface  14 . A power cable  36  provides power to the motor section  32  from the surface  14 . As is known, the motor section  32  includes an outer housing, a stator and a rotor that is rotatable with respect to the stator. The rotor rotates a shaft that will, in turn, power the pump section  28 . It is noted that, while the motor section  32  and seal section  30  are shown in the drawings to be located below the pump section  28 , this is not necessarily the case in practice. The pump section  28  might be located below the motor section  32 . 
         [0015]      FIGS. 2 and 3  are cross-sectional views of portions of the interior of the seal section  30 .  FIG. 2  illustrates the upper axial end  38  of the exemplary seal section  30  having threaded openings  40 , which permit the seal section  30  to be affixed to the pump section  28  with suitable connectors (not shown). The outer housing  42  of the seal section  30  is made up of a top cap  44 , one or more outer housing sleeves  46 , and intermediate guides  48 , as are known in the art and which are typically threaded together. 
         [0016]    A central shaft  50  passes through the axial center of the housing  42  and, as is known, is used to transmit rotational power from the motor section  32  to the pump section  28 . A bellows chamber  52  is defined radially within each housing sleeve  46 . A well fluid chamber  54  is defined within the top cap  44 . Mechanical seal assemblies, generally indicated at  56 , are disposed axially between each of the bellows chambers  52  as well as above the upper most bellows chamber  52  (see  FIG. 2 ). Those of skill in the art will understand that there may be a series of bellows chambers  52  and seal assemblies  56  within the seal section  30 , depending upon the needs and desires of the well operators. Mechanical fluid seals  58  of known construction form part of each mechanical seal assembly  56  and surround the shaft  50 . A seal guide  60  is formed axially adjacent to and above mechanical seal  58 . Portions of the seal assembly  56  reside within the seal guide  60 . A barrier fluid reservoir  62  is formed within each guide  60  and presents an open upper end  64 . Barrier fluid  66  resides within each reservoir  62 . The barrier fluid  66  is a heavy specific gravity blocking fluid that is placed on top of the mechanical seal  58  to prevent well fluids from moving downwardly across the seal  58  and into the bellows chamber  52  below. The barrier fluid  66  can be a relatively heavy specific gravity fluid that does not mix well with water or other substances. In particular embodiments, the barrier fluid  66  has a density that is significantly greater than water. According to further particular embodiments, the barrier fluid  66  has a specific gravity that is from about 1.8 to about 2.2. The purpose of the barrier fluid  66  is to displace and block water or well fluid from coming into contact with the rotating face of the mechanical seal  58  to prevent water or well fluid ingress past the seal assembly  56 . The barrier fluid  66  allows lighter fluids, such as lubricating fluid (motor oil) to pass upward through the barrier. 
         [0017]    A metallic bellows assembly, generally shown at  70  is located in each bellows chamber  52  and includes a radially outer bellows  72 , a radially inner bellows  74  and a sleeve  76  which interconnects the inner and outer bellows  74 ,  72 . Each of the bellows  72 ,  74  is expandable and contractible axially in the manner of an accordion bellows. Motor oil is retained within a bellows reservoir  77  that is formed by the bellows assembly  70 . The natural spring force of the bellows assembly  70  will tend to cause the motor oil to migrate up between the seals  58  and the shaft  50 , as illustrated by arrows  78  in  FIGS. 2 and 3 . The barrier fluid  66  within the reservoir  77  will prevent well fluid from reaching the seals  58 , instead redirecting well fluids around the seal guide  60  and to exterior space  82 , as indicated by arrows  80  in  FIGS. 2 and 3 . 
         [0018]    During typical operation, well fluid resides within the well fluid chamber  54  as well as the annular spaces  82  which radially surround the outer bellows  72 . Well fluid may also enter the seal section  30  via port  84 . Motor oil to be supplied to the adjacent motor section  32  is contained within each bellows assembly  70 . Motor oil resides within each outer bellows  72  below the sleeve  76  and can be transmitted, under pressure, upwardly between the inner bellows  74  and the shaft  50 . 
         [0019]    Also in accordance with particular embodiments of the present invention, biasing mechanisms are provided that produce positive fluid pressure within the seal section  30  and thereby dictate the preferred direction of leakage across the mechanical seals  58  of the seal section  30  and provide improved or even optimum pressure across the mechanical seal faces.  FIG. 2  depicts a biasing mechanism in the form of a weighted member  86  which is disposed within the outer housing  42  in order to exert pressure upon the motor oil within the bellows assembly  70 . Provided the assembly is in the vertical position, the pressure applied will be proportional to the weight of the weighted member  86 . The pressure applied will largely not depend on the position of the bellows assembly  70  within the outer housing  42 . The weighted member  86  is preferably cylindrical and resides atop the sleeve  76  radially outside of the inner bellows  74 . In the depicted embodiment, O-ring seals  87  are compressed against sleeve  76  and help to hold the weighted member  86  in place as well as protect the bellows assembly  70  from potential damage from the weighted member  86  resulting from vibration.  FIG. 3  illustrates an alternative biasing mechanism for the bellows assembly  70  in the form of a compression spring  88  which urges the sleeve  76  axially downwardly. The compression spring  88  is shown compressed in  FIG. 3 . In this embodiment, the pressure applied to the motor oil within the bellows assembly  70  will be dependent upon the spring rate of the spring and the position of the bellows assembly  70  within the outer housing  42 . This pressure will be substantially unaffected by an orientation other than vertical in the wellbore  10 . 
         [0020]    Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof.