Abstract:
A submersible pumping system for use downhole, wherein the system includes a pump, a pump motor, a seal section, a shaft coupling the pump motor to the pump, a bearing assembly for axially retaining the shaft in place, and an electrical insulator for electrically isolating the pump bearing assembly from electrical current leaking from the motor and through the shaft. The electrical insulator can be made from polyetheretherketone, polyimide, polyketone, and mixtures thereof.

Description:
BACKGROUND 
     1. Field of Invention 
     The present disclosure relates to downhole pumping systems submersible in well bore fluids. More specifically, the present disclosure involves insulated bearings for a submersible pump. 
     2. Description of Prior Art 
     Submersible pumping systems are often used in hydrocarbon producing wells for pumping fluids from within the well bore to the surface. These fluids are generally liquids and include produced liquid hydrocarbon as well as water. One type of system used in this application employs an electrical submersible pump (ESP). ESPs are typically disposed at the end of a length of production tubing and have an electrically powered motor. Often, electrical power may be supplied to the pump motor via wireline. The pumping unit is usually disposed within the well bore just above where perforations are made into a hydrocarbon producing zone. This placement thereby allows the produced fluids to flow past the outer surface of the pumping motor and provide a cooling effect. 
     With reference now to  FIG. 1 , an example of a submersible ESP disposed in a well bore is provided in a partial cross sectional view. In this embodiment, a downhole pumping system  10  is shown suspended on production tubing  18  within a cased wellbore  8 . The downhole pumping system  10  comprises a motor  12 , a seal section  14 , and a pump  16 . Energizing the motor drives a shaft coupled between the motor  12  and the pump section  16 . Rotors coaxially disposed on the shaft rotate with shaft rotation within the pump body. The centrifugal action of the rotors produces a localized reduction in pressure in the cavities thereby inducing fluid flow into the cavities. 
     The source of the fluid drawn into the pump comprises perforations  20  formed through the casing of the wellbore  10 ; the fluid is represented by arrows extending from the perforations  20  to the pump inlet. The perforations  20  extend into a surrounding hydrocarbon producing formation  22 . Thus the fluid flows from the formation  22 , past the motor  12  on its way to the inlets. 
     Because of the long length of the motor, the rotor is made up of a number of rotor sections. Each rotor section comprises a large number of flat disks called laminations secured by copper rods. The rotor sections are spaced apart from each other. A bearing assembly is located between each section for maintaining the shaft in axial alignment. The rotor sections are keyed to the shaft for rotation therewith, but are axially movable with respect to the shaft. 
     Traditionally, the bearing assemblies used in motors, seal sections and pumps of electrical submersible pumps (ESPs) are plain sleeve bearings, which provide radial support. These plain sleeve bearings are not heavily loaded since a large number of bearings are typically used and the ESP units are run in a near vertical orientation. The absence of a substantial load results in an unstable or marginally stable bearing operation that can result in metal-to-metal contact in the bearings, which accelerates bearing failure. 
     One example of a bearing assembly is provided in a cross sectional view in  FIG. 2 . Shown is a shaft  24 , circumscribed by a sleeve  26  and bearing assembly  32  within the pump housing  28 . The bearing assembly  32  radially encompasses a portion of the sleeve  26  and comprises an insert  34 , an outer race  36  and a T-ring  38 . The sleeve  26  is coupled to the shaft  24 , such as by a key, and rotates along with the shaft  24 . The sleeve  26 , and therefore the shaft  24 , is radially supported by the insert  34 . A lubricant film (not shown) allows for sleeve rotation within the insert. The T-ring  38  which is disposed in the space between the outer race  36  and the stator  30 , prevents bearing rotation. 
     Pump failure can be initiated by an electrical discharge from an electrical supply source into the bearing assembly. The discharge may produce sparks that in turn create pitting in the bearing components, such as between the sleeve and the insert. Although the pitting exceeds lubricant thickness enabling metal to metal contact, this condition often evades detection since the motor will continue to operate after the pitting episode and smear the evidence. Confirmation of this failure mode requires microscopic detection. 
     SUMMARY OF INVENTION 
     The present disclosure includes a downhole submersible pumping system disposable in a cased wellbore comprising, a housing, a pump disposed within the housing, a seal section, a pump motor disposed within the housing, a shaft coupling the pump to the pump motor, a bearing assembly circumscribing a portion of the shaft, and an electrical insulator disposed between the shaft and the housing. In one embodiment, the bearing assembly comprises an annular insert formed for coaxial displacement around the shaft and an outer race coaxially circumscribing said insert, and wherein the insulator is disposed between the insert and the outer race. The electrical insulator may be made from polyetheretherketone, polyimide, polyketone, and mixtures thereof. Optionally, a tubular sleeve may be included coaxially disposed around the shaft, wherein the sleeve comprises the electrical insulator. The electrical insulator may be disposed within the sleeve, on its outer diameter, or on its inner diameter. 
     A method is included for electrically insulating a bearing assembly on an electrical submersible pump, wherein the electrical submersible pump comprises a pump motor, a pump, and a drive shaft mechanically coupling the pump motor to the pump. The method comprises coaxially disposing a bearing assembly on the shaft and electrically insulating a portion of the bearing assembly from the shaft. The insulating step may include inserting an electrical insulating barrier between the shaft and a portion of the bearing. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a side view of a pumping system disposed in a wellbore. 
         FIG. 2  is a cross sectional view of a prior art bearing system for use in a submersible pumping system. 
         FIG. 3  is a cross sectional view of a bearing system for use in a submersible pumping system in accordance with the present disclosure. 
         FIG. 4  is a side view of a bearing system for use in a submersible pumping system in accordance with the present disclosure. 
         FIGS. 5   a - 5   c  are cross sectional views of a sleeve for use in an electrical submersible pumping system. 
     
    
    
     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF INVENTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be through and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     The present disclosure provides embodiments of a downhole submersible pumping system for producing fluids from within a wellbore up to the surface. The downhole submersible pumping system described herein includes a pump motor, a pump, and a shaft that mechanically couples of the pump motor with the pump. An electrical submersible pump is an example of a downhole submersible pumping system. Thus activation of the pump motor thereby produces corresponding rotation of the pump for pumping connate fluids from within a wellbore to the surface. 
     The embodiments herein described also include a bearing assembly disposed around a portion of the shaft, wherein the bearing assembly provides some radial support for the drive shaft of the pump motor. The bearing assembly described herein is insulated to prevent electrical transmission therethrough. Accordingly, any electrical leakage making its way to the drive shaft cannot be conducted through the bearing assembly to any surrounding hardware. 
       FIG. 3  illustrates in a cross sectional side view one embodiment of an insulated bearing assembly. In the embodiment of  FIG. 3 , the bearing assembly  40  comprises an insert having an outer race  44  radially circumscribing the bearing assembly  40  outer circumference. The insert  42  is a generally tubular member whose inner diameter is configured to slide over the pump drive shaft  43 . The line  41  represents the axis of the drive shaft  43 . In the event the shaft  43  includes an optional sleeve  47 , the bearing assembly  40  should be configured to slide over that as well. Thus, in the embodiment of  FIG. 3 , while the drive shaft  43  rotates, the insert  42  is designed to be relatively stationary. Rotating friction between the inner circumference of the insert  42  and the outer diameter of the drive shaft  43  (or the optional sleeve  47 ) will be addressed via a film of lubricant (not shown) disposed between these opposing surfaces. 
     The outer race  44 , which also has a generally annular configuration, is disposed within an opening formed on the outer circumference of the insert  42 . The outer race  44  has a generally rectangular cross-section with square like recesses  45  created on its outer radial surface. These optional recesses  45  are installed to receive a T-ring, wherein the T-ring may prevent radial rotation of the outer race  44 , thereby preventing bearing assembly  40  rotation. The T-ring outer circumference of mates with either a stator of an electrical submersible pump, or the inner diameter of the housing of the pumping system. The T-ring however, does not prevent axial movement of the bearing assembly  40  with respect to the drive shaft. 
     In the embodiment of  FIG. 3 , an annular space defined by the outer radial surface of the insert  42  and the outer race  44  inner radial circumference is shown. Disposed within this annular space is an insulating barrier  46 . The presence of the insulating barrier thereby prevents electrical communication from the insert  42  to the outer race  44 . Accordingly, in the event of leakage of electrical potential to the drive shaft  43  or the sleeve  47 , the bearing assembly  40  will not provide an electrical path from the shaft to the stator (not shown) or the pump housing. As such, the insulating barrier  46  prevents bearing assembly pitting or galling caused by an electrical discharge. 
     Examples of material making up the insulating barrier  46  include any non-conducting material. Polyetheretherketone (PEEK), polyketones, as well as polyimides each provide a suitable material for this insulator. Optionally, mixtures of these different compounds could be used to form an insulating barrier  46 . 
       FIG. 4  provides a side view of an embodiment of the bearing assembly. Here, the bearing assembly  40  is coaxially shown with reference to the drive shaft axis  41 . Here, oil circulation holes  49  can clearly be seen formed through the body of the outer race  44 . The oil circulation holes  49  provide a pathway for the flow of the dielectric oil packed within the pumping system. Also evident from this view is how the bearing assembly  40  comprises a pair of hemispherical sections coupled at their respective end by bolts  48 . However, the bearing assembly  40  can be comprised of a single section without the need for a fastening attachment. Additionally, the assembly  40  can be made up of more than two sections. Also shown in the side view of  FIG. 4  is the insulating barrier  46  represented by a dashed line there between the insert  42  and the outer race  44 . 
       FIGS. 5   a  through  5   c  illustrate cross sectional views of embodiments of an optional sleeve  52  for use with a drive shaft  43 . With respect to  FIG. 5   a , the sleeve  52  comprises an outer portion  54  and an inner portion  56  with an insulating barrier  46  sandwiched in between these respective portions. The sleeve  52  of  FIG. 5   a  would be configured such that the inner portion would be proximate to the outer circumference of a drive shaft and the upper portion would be in contact with a bearing assembly insert. Also, it should be pointed out that in this embodiment the insulating barrier  46   a  encompasses the entire region between the outer and inner portions ( 54 ,  56 ). Accordingly, in this view, the cross section can be from an axial view along the sleeve or radial view looking at a sleeve cutaway. 
       FIG. 5   b  illustrates a cross sectional view of a sleeve  52   a  embodiment comprising an outer portion  54   a  with an insulating barrier  46   b  disposed on the lower portion of the sleeve  52   a . Here, the sleeve would be disposed on the corresponding drive shaft such that the insulating barrier  46   b  is between the drive shaft and the outer portion  54   a . Thus a corresponding bearing assembly insert would be disposed proximate to the upper surface of the outer portion  54   a.    
     Similarly, as seen in  FIG. 5   c , another embodiment of a sleeve  52   b  is shown comprising an insulating barrier  46   c  on the outer radial surface of an inner portion  56   a . In this embodiment, the sleeve  52   b  would be coupled with a drive shaft wherein the inner portion  56   a  would ride on the drive shaft. Whereas, the corresponding insert of a bearing assembly would be proximate to the insulating barrier  46   c . Accordingly, using the insulating barrier in combination with the sleeve as shown in  FIGS. 5   a  through  5   c , a sleeve for use with a drive shaft of an electrical submersible pump could provide an electrically insulating barrier to prevent electrical arcing through a bearing assembly. 
     It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the invention is therefore to be limited only by the scope of the appended claims.