Abstract:
A motor protector for use in protecting an electric motor in a subterranean environment. The motor protector utilizes a dual chamber system for absorbing the expansion and contraction of the internal fluids of a submergible electric motor. One of the chambers provides for the expansion and contraction of the internal fluids of a submergible electric motor. The other chamber is in fluid communication with the subterranean environment. A gas pocket is disposed within the motor protector between and in fluid communication with both of the dual chambers. The volume of gas couples the fluid pressure of the subterranean environment to the internal fluids of the electric motor without direct contact between the internal fluids of the electric motor and fluids from the subterranean environment.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to downhole pumping systems, and more particularly to a motor protector for use with a downhole pumping system. 
     BACKGROUND OF THE INVENTION 
     Electric submergible pumping systems are widely used throughout the world for moving subterranean fluids to a desired location, e.g. the earth&#39;s surface. These electric submergible pumping systems include an electric motor that is drivingly coupled to a pump. Generally, the electric motors used for such applications are rotary motors containing a rotor and a stator. Typically, the rotor lies within a fluid filled cavity within the stator. The fluid not only lubricates the motor, but also cools the motor to prevent overheating. 
     However, deleterious materials, such as carbon dioxide, hydrogen sulfide and brine, can be found in the subterranean fluids. Those substances can corrode, or otherwise harm components within the electric motors, causing the motor to fail prematurely. Therefore, the cavities within the electric motors are usually filled with uncontaminated motor oil to ensure their long-term successful operation. 
     The temperature of the motor oil varies as a result of the intermittent operation of the electric motor and the temperature of the fluid surrounding the electric motor. As the temperature of the motor oil rises, for instance, the oil tends to expand and the pressure within the motor tends to increase. 
     In most submergible pumping systems in use today, this motor oil is partially contained within a device commonly referred to as a motor protector. Motor protectors may serve several different functions. The motor protectors may serve to prevent well fluids and gases from contaminating the motor oil, transmit the torsional power produced by the electric motor to the pump, provide a storage reservoir of motor oil, provide for the expansion and contraction of the motor oil due to changes in temperature, and to equalize the internal pressure of the motor with the pressure of the surrounding subterranean fluids. 
     Several different approaches have been used to construct motor protectors. One type of motor protector is generally referred to as a “labyrinth” system. Labyrinth systems are exposed at one end to the subterranean fluids and to the electric motor at the other end. These systems retard the migration of subterranean fluid into the electric motor. However, the subterranean fluids can eventually migrate through the labyrinth path to enter the electric motor. 
     Another approach, referred to as “bladder” or “bag” system, utilizes an elastomeric barrier between the motor oil and the subterranean fluids. However, elastomeric bags suffer from several limitations. First, the repeated expansion and contraction of the elastomeric bag can cause the bag to split or crack under certain conditions. Of course, once an elastomeric bag splits or cracks it no longer protects the motor oil from contaminants which are then free to enter and ultimately damage the motor. Second, elastomeric bags tend to lose their elasticity due to various conditions which may be present in a wellbore. Once an elastomeric bag loses its elasticity, it can no longer expand and contract as needed to satisfy the requirements of the motor oil which it contains. Eventually the bag ruptures, leaving the contaminants free to attack the motor. Third, most elastomers cannot survive in environments where the temperature rises above about 400° F. Above that temperature, most elastomers become brittle causing the bag to break during expansion or contraction. Finally, elastomeric compounds currently used for motor protector bags tend to be relatively permeable as compared to the contaminants within the wellbore fluid. Many wells contain contaminants, such as hydrogen sulfide for instance, which can permeate the motor protector bag and attack the motor. In fact, certain contaminants, such as hydrogen sulfide, also tend to alter the chemistry of some elastomers, causing the elastomers to harden. Once the elastomer has hardened, the bag eventually breaks. 
     It would be advantageous to have a submergible pumping system that provides the desired functions of a motor protector but without the limitations of the labyrinth or bag systems. 
     SUMMARY OF THE INVENTION 
     The present invention features a system for absorbing the expansion and contraction of internal fluids of an electric motor. The system is comprised of a motor protector having: a first chamber in fluid communication with electric motor internal fluid, a second chamber in fluid communication with an environmental fluid, a communication passage in fluid communication with the first chamber and the second chamber, and a volume of gas between the electric motor internal fluid in the first chamber and the environmental fluid in the second chamber. 
     According to another aspect of the invention, a pumping system is featured that is designed for submersion in a production fluid disposed in a wellbore. The pumping system includes a submergible pump and a submergible motor coupled to the submergible pump and having an internal motor fluid. A motor protector is coupled to the submergible motor. The motor protector interior is exposed to the production fluid and to the internal motor fluid. Additionally, the motor protector has a gas pocket disposed intermediate to and in contact with the production fluid and the internal motor fluid. 
     According to another aspect of the invention, a method is provided for allowing the expansion and contraction of the internal fluids of an electric motor of a submergible system. The method includes the step of fluidicly coupling an electric motor internal fluid from an electric motor to a first chamber. The first chamber has sufficient volume to contain any expected increase in electric motor internal fluid volume resulting from an increase in electric motor internal fluid temperature due to operation of the electric motor and heat from the surrounding environment. Other steps of the method include: providing a pressure barrier that allows fluid from the surrounding environment to flow into a second chamber at a predetermined pressure, fluidicly coupling the first chamber and second chamber in such a manner as to allow a gas pocket to form between the first chamber and second chamber, and inserting a sufficient volume of gas to maintain a gas pocket between the first chamber and the second chamber at the operating depth of the submergible system and with the electric motor operating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
     FIG. 1 is a front elevational view of a pumping system disposed in a wellbore, according to an embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of a submergible system taken generally along an axis of the system, according to an embodiment of the present invention; 
     FIG. 3 is a cross-sectional view of a submergible system taken generally along an axis of the system, according to an alternative embodiment of the present invention; and 
     FIG. 4 is a cross-sectional view of a submergible system taken generally along an axis of the system, according to an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring generally to FIG. 1, a submergible system  10  is illustrated according to an exemplary embodiment of the present invention. The submergible system  10  of the illustrated embodiment is an electric submergible pumping system. The submergible system  10  is comprised of a submergible electric motor  12 , a motor protector  14 , a fluid intake  16  and a submergible pump  18 . The submergible electric motor  12  is drivingly coupled to the submergible pump  18  through the motor protector  14  and the fluid intake  16 . Fluid is drawn into the submergible pump  14  through the fluid intake  16  when the submergible electric motor  12  is operated. The submergible electric motor  12  is comprised of a rotor and stator. The rotor sits within the stator in a fluid-filled cavity. The fluid serves to lubricate and cool the rotor and stator. 
     The submergible system  10  is deployed by a support system  20  into a wellbore  22 . The support system  20  also includes an electrical power cable  24  that couples electrical power to the submergible electric motor  12  from the surface. The support system  20  may be a wireline deployed system or, as illustrated, a coiled tubing deployed system. The coiled tubing also can serve to convey the fluid discharge of the submergible pump  18  to the surface. 
     As illustrated, the submergible system  10  is deployed into a wellbore  22  to displace fluids from the wellbore  22  to the surface. Fluids enter the wellbore  22  from a surrounding geologic formation through perforations  26  in a wellbore casing  28 . The pressure of the fluid surrounding the submergible system  10  increases as the submergible system  10  is lowered into the wellbore  22 . The pressure of the fluid in the wellbore  22  is coupled by the motor protector  14  to the fluid within the submergible electric motor  12 . Thus, any pressure difference between the wellbore  22  and the interior of the submergible electric motor  12  is minimized. Most importantly, the motor protector  14  prevents the fluids from the wellbore  22  from coming into direct contact with the fluid within the submergible electric motor  12 . 
     The submergible system  10  is lowered into the wellbore  22  until a desired depth is achieved. Once the desired depth in the wellbore  22  is achieved, the submergible electric motor  12  is energized to drive the submergible pump  18  to displace fluid from the wellbore  22 . The operation of the submergible electric motor  12  causes the fluids within the electric motor to heat up. The increase in temperature of the fluids within the submergible electric motor  12  causes these fluids to expand into the motor protector  14 . However, the motor protector  14  prevents the expanding fluids from coming into contact with the fluids from the wellbore  22 . 
     Referring generally to FIG. 2, an exemplary embodiment of a motor protector  14  is shown. In the illustrated embodiment, motor oil is used to fill the interior of the submergible electric motor  12  housing the rotor. The motor protector  14  absorbs the expansion and contraction of motor oil arising from heating and cooling of the motor oil. The motor protector  14  also couples the pressure in the surrounding wellbore  22  to the motor oil within the submergible electric motor  12  without direct contact between the fluids in the wellbore and the motor oil. Additionally, the motor protector  14  is part of the drive train that drivingly couples the submergible electric motor  12  to the submergible pump  18 . 
     The motor protector  14  is coupled to the submergible electric motor  12  by a first end  40 . The first end  40  serves several coupling functions. First, first end  40  mechanically couples the motor protector  14  to the submergible electric motor  12 . Additionally, the first end  40  drivingly couples the submergible electric motor  12  to a drive shaft  42 . Protector  14  further includes an axial thrust bearing assembly  43  mounted above end  40  as illustrated in FIG.  2 . The secure coupling of motor  12  with end  40  permits internal motor fluid, e.g. motor oil, to flow into motor protector  14 , and specifically into a first chamber  44  within the motor protector  14 . A first fluid passageway  46  conveys motor fluid, e.g. a motor oil  48 , to and from the first chamber  44  via a first chamber inlet tube  50  within the first chamber  44 . 
     Motor protector  14  also includes a second end  52 . The second end  52  couples the motor protector  14  to a subsequent tool in the tool string, e.g. fluid intake  16 . The second end  52  also fluidicly couples the fluid in the wellbore  22 , surrounding the motor protector  14 , to a second chamber  54  within the motor protector. A communication port  56  in the second end  52  and a wellbore fluid conduit  58  communicate wellbore fluids  60  to the second chamber  54 . 
     Communication port  56  preferably includes a pressure activated device  61 , such as a rupture disc. The rupture disc allows wellbore fluid  60  to enter the second chamber  54  when the pressure in the wellbore  22  surrounding the motor protector  14  reaches a predetermined value. Prior to rupture, the rupture disc maintains a seal to prevent fluid from entering or leaving through the communication port  56  until that wellbore pressure is achieved. Once the rupture disc has ruptured, the wellbore fluid conduit  58  will communicate wellbore fluid  60  from the communication port  56  to second chamber  54 . The pressure activated device  61  ensures that a gas pocket is contained within motor protector  14  intermediate motor oil  48  and the wellbore fluid, as will be more fully explained below. 
     The motor oil  48  in the first chamber  44  is exposed to the pressure of the wellbore fluid in the second chamber  54  by a communication passage  62 , such as a tube, within the motor protector  14 . In the illustrated embodiment, the second chamber  54  is disposed directly above the first chamber  44 . The communication passage  62  extends downward from cavity  64  in the top of the second chamber  54  to a bearing support structure  66 . A fluid passageway  67  is disposed through the support structure  66  to fluidicly couple the communication passage  62  to the top of the first chamber  44 . A pocket or volume of gas  68  occupies the space between motor oil  48  and wellbore fluid  60 . Preferably, the gas  68  occupies the upper region of the second chamber  54 , communication passage  62 , fluid passage  67  and the upper region of chamber  44 . The volume of gas  68  allows the pressure of the wellbore fluids to be conveyed to the motor oil  48  without direct contact between the wellbore fluids  60  and the motor oil  48 . In the exemplary embodiment, nitrogen is used as the gas in gas pocket  68 , although other gases or gas mixtures can be used. 
     During deployment of system  10 , the pressure of the wellbore fluids  60  surrounding the submergible system  10  rises as the submergible system  10  is lowered into the wellbore. At a predetermined depth, the wellbore fluid pressure overcomes the rupture disc and wellbore fluid flows through the communication port  56  and the wellbore fluid conduit  58  into the second chamber  54 . The pressure of the wellbore fluid  60  in the second chamber  54  on one side of the volume of gas  68  and the force of the incompressible motor oil in the first chamber on the other side of the volume of gas compresses the volume of gas  68  between the two fluids. Consequently, the volume of the gas decreases as the submergible system is lowered into the wellbore. 
     During operation of system  10 , the motor protector  14  of the illustrated embodiment provides an expansion volume for an increase in temperature of the motor oil. The temperature of the motor oil rises when the submergible electric motor is operated. The motor oil may also rise from the heat of the wellbore fluid surrounding the submergible system. Any increase in temperature corresponds to an increase in volume of the motor oil. The expansion in fluid volume causes the fluid to expand through the first end  40  and into chamber  44  of motor protector  14 . The pressure of the expanding motor oil in the first chamber on one side of the pocket of gas  68  is equalized with the pressure of the wellbore fluid  60  in the second chamber  54  via gas  68 . The pocket of gas  68  allows this equalization without mixing of motor fluid and wellbore fluid. Consequently, the internal motor oil is protected from contamination while allowing equalization of internal and external pressure to avoid damage to motor components, such as seals. 
     Mechanical seals  70  are placed along the drive shaft to prevent fluids from flowing along the drive shaft and contaminating either the motor oil or volume of gas  68 . Also, the drive shaft is supported by several bearings  72  along the length of the drive shaft  42 . 
     In operation, the motor protector  14  preferably maintains a gas pocket at the top of second chamber  54  and within communication tubing  62  when the system is operating at depth in the wellbore. As long as the volume of gas is at least in this condition it will prevent the wellbore fluids  60  from coming into contact with the motor oil  48 . A secondary cavity  64  is disposed at the top of second chamber  54  to receive the upper end of communication tube  62 . Cavity  64  is configured to minimize the volume of gas  68  needed to prevent wellbore fluids from entering communication passageway  62 . A more optimal condition allows the volume of gas  68  to occupy a volume extending from the top of the second chamber  54 , through the communication tube  62 , and into a portion of the first chamber  44  during even the worst-case scenario. 
     Preferably, factors tending to compress the volume of gas and the resulting change in the volume of the gas are considered in designing the system. The amount the volume of gas is compressed from the wellbore pressure depends on the initial pressure in the volume of gas and the pressure at the operating depth. The wellbore pressures that the motor protector may experience may rise dramatically as the system travels from the surface to the operating depth. Therefore, the volume of gas may be compressed dramatically between the surface and the operating depth. 
     Also, the amount the volume of gas is displaced due to increases in the motor oil temperature is directly related to the change in volume of the motor oil resulting from the increase in temperature. The change in volume of the motor oil is a function of the change in temperature, the volume of the motor oil, and the thermal coefficient of expansion of the motor oil. The change in temperature is, in turn, a function of the initial motor oil temperature, the surrounding wellbore temperature and the heat produced within the electric motor from the operation of the electric motor. 
     To provide more gas, the volume of gas  68  can be pressurized before lowering the system into the wellbore. With higher initial gas pressure, the volume of gas is compressed less as the system is moved downhole. However, the initial pressure to which the volume of gas can be pressurized is limited by the maximum differential pressure that can be withstood by internal components, such as mechanical seals, within the motor protector. In fact, limiting the differential pressure across these system components is the reason for exposing the motor oil to wellbore pressure. 
     Referring generally to FIG. 3, an additional supply of pressurized gas  100  can be used to supplement the initial volume of gas. The additional supply of pressurized gas  100  can be used to supplement the volume of gas  68 . Generally, the addition of pressurized gas  100  to gas  68  is designed to increase equalization with the wellbore pressure, to provide a larger pocket of gas between the motor fluid and the wellbore fluid. 
     The supply of pressurized gas  100  can be disposed in a tank or chamber within either of the first chamber  44  or the second chamber  54  or coupled to the volume of gas from a remote location. As illustrated, the supply of pressurized gas  100  is disposed below the submergible electric motor  12  in a lower chamber  102 . Motor oil  48  is coupled from the motor  12  through a third fluid passageway  104  to the lower chamber  102 . The motor oil pressure is felt by a flow initiator  106 , such as a pressure actuated valve. When the motor oil pressure has risen to a predetermined level, the flow initiator  106  is activated to permit pressurized gas  100  to flow into a conduit  107 , such as a high pressure hose, that is coupled to the top of the second chamber  54  to permit gas  100  to flow into volume of gas  68 . The pressurized gas also can be controlled in response to other parameters, such as the level of wellbore fluid in the second chamber, for example. 
     FIG. 3 also illustrates the internal construction of an exemplary submergible electric motor  12 . The submergible electric motor  12  is comprised primarily of a stator  120 , a rotor  122 , a top end coil  124 , a bottom end coil  126 , and a motor shaft  128 . Electrical power is supplied to the submergible electric motor  12  from the surface through electrical power cable  24  from the surface. The electrical circuit within the submergible electric motor is formed between the stator  120 , rotor  122 , the top end coil  124 , and the bottom end coil  126 . The motor shaft  128  transmits the rotational torque produced by the submergible electric motor  12 . The rotor  122  is coupled to the motor shaft  128 . Thus, when rotation is induced in the rotor  122 , the motor shaft  128  is able to drive a tool, e.g. pump, coupled to the motor shaft  128 . The interior of the submergible electric motor  12  contains motor oil  48 . 
     As illustrated in FIG. 4, the volume of gas  68  interfaces with the motor oil  48  in the first chamber  44 . It may be desirable to prevent contact between the motor oil  48  and the volume of gas  68  to help avoid gas becoming entrained within the motor oil. Therefore, a barrier can be placed in the first chamber to prevent contact between the volume of gas and the motor oil. As illustrated, one example of such a barrier is a piston  150  floating on the surface of the motor oil. A piston can be placed in the second chamber  54  to prevent or minimize contact between the wellbore fluids  60  and the volume of gas  68 . 
     It will be understood that the foregoing description is of preferred exemplary embodiments of this invention, and that the invention is not limited to the specific forms shown. For example, in the illustrated embodiments the two chambers are oriented with the second chamber above the first chamber. However, the two chambers can be oriented in a number of different orientations and still serve the same function. Reference has been made to motor oil as the fluid within the submergible electric motor. The present invention is not limited by the type of fluid used within the submergible electric motor. These and other modifications may be made in the design and arrangement of the elements without departing from the scope of the invention as expressed in the appended claims.