Abstract:
A technique enables reduction in electrical power losses during operation of a submersible motor. The technique utilizes a submersible motor comprising a housing that encloses a stator and a rotor. A ferrofluid is located in the housing in a sufficient quantity to fill the gap between rotor and stator. The ferrofluid has substantially improved properties that facilitate a reduction in electrical power supplied and thus a greater efficiency in operation of the motor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is based on and claims priority to U.S. Provisional Application Ser. No. 61/141,875, filed Dec. 31, 2008. 
     
    
     BACKGROUND 
       [0002]    In a variety of well related applications, power for pumping or other work is provided by submersible electric motors. In electric submersible pumping systems, for example, oil-filled motors are used to power pumps that move fluid in the downhole environment. By filling the motors with oil, the submersible motors can be designed with relatively thin walled housings that can fit downhole and operate under wellbore pressure. However, an undesired side effect is large viscous power losses in the motor that are costly to supply via electrical power delivered downhole over long electric lines. The additional electric power required to overcome the viscous drag does no useful work. Instead, the added electrical current increases the heat dissipated within the motor windings and within the long power cable. Consequently, higher voltage is required at the surface to overcome losses in the power cable. All of these effects introduce added risks, stresses, and operating costs with respect to the pumping system. 
         [0003]    Conventional electric submersible pumping system motors typically run in dielectric oil filled housings to achieve a pressure balance between an interior of the motor and the wellbore fluid pressure along an exterior of the motor. The pressure balancing avoids the need for a thick walled pressure vessel able to withstand large pressure differentials. The concept of oil-filled, pressure balanced motors was incorporated by Armais Artunoff into his early electric submersible pumping systems around the year 1916. Although the dielectric oil helps to pressure balance and protect the submersible motor from the borehole fluid, the dielectric fluid does little to improve the electromagnetic performance of the motor because dielectric oil has approximately the same electromagnetic properties as air. 
         [0004]    Unfortunately, this characteristic results in significantly greater electrical current being applied to the motor&#39;s windings to overcome the added viscous friction from rotating the oil within the submersible motor. This additional current produces more heat and eddy current losses in the motor. Also, the additional current is carried downhole over long power cables which results in substantial resistive losses. The net result is higher operating costs, lower reliability, and reduced longevity because of the higher heat dissipated and higher voltages required in delivering sufficient power to the motor. 
       SUMMARY 
       [0005]    In general, the present application provides a technique for reducing electrical power losses in a submersible motor. A technique utilizes a submersible motor comprising a housing that encloses a stator and a rotor. A ferrofluid is located in the housing in a sufficient quantity to fill the gap between rotor and stator. The ferrofluid has substantially improved properties that facilitate a reduction in electrical power supplied and thus a greater efficiency in operation of the motor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Certain embodiments will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
           [0007]      FIG. 1  is a schematic illustration of one example of a motor system, according to an embodiment; 
           [0008]      FIG. 2  is a cross-sectional view of the motor system illustrated in  FIG. 1  taken generally across an axis of the motor, according to an embodiment; 
           [0009]      FIG. 3  is a schematic illustration of one example of wellbore equipment utilizing a submersible motor, according to an embodiment; and 
           [0010]      FIG. 4  is a flowchart illustrating one methodology for preparing a motor system for use in a submerged environment, according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In the following description, numerous details are set forth to provide an understanding of preferred embodiments. However, it will be understood by those of ordinary skill in the art that various embodiments may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
         [0012]    A preferred embodiment generally involves a system and methodology related to the construction and use of submersible motors. The system and methodology substantially improve the efficiency of motor operation and thus reduce the amount of electric power that must be directed to the motor at subterranean locations via a lengthy power cable. In some applications, the submersible motor is employed in artificial lift systems, such as electric submersible pumping systems. However, the motor also can be incorporated into other well related equipment for powering a variety of systems and/or components, such as formation tester pumps, electro-hydraulic actuators, drives for flow control valves, and other devices. The approach provides the electric motor with significantly lower operating costs, longer life, and higher reliability. 
         [0013]    According to one embodiment, the motor is designed to use an internal liquid magnetic material called ferrofluid. Depending on the application, the ferrofluid can be used to substantially fill the interior of the motor. For example, the ferrofluid is disposed within the motor housing to fill gaps between rotating parts and other gaps within the magnetic circuit of the motor. Use of the ferrofluid significantly reduces the magnetic reluctance of the motor. The reduced magnetic reluctance, in turn, increases motor performance and reliability by significantly reducing the required current and electrical power supplied to produce a required level of magnetic flux and hence power output. 
         [0014]    When the motor is used in electric submersible pumping (ESP) systems, for example, the unique motor construction significantly reduces the electrical current required to drive the ESP motor at its rated speed and power output compared to conventional ESP motors. In one embodiment of the motor suitable for use in electric submersible pumping systems, the ferrofluid is mixed with dielectric oil to improve the magnetic circuit performance of the critical gap between the rotor and stator of the motor. This approach again significantly reduces the magnetic reluctance of the motor to produce a required magnetic flux for a given amount of current. Consequently, there is a significant reduction in the amount of current that must be supplied downhole to produce a specified rotating magnetic field. 
         [0015]    In electric submersible pumping system applications, the unique motor leads to further increases in efficiency because less power must be generated and sent over long power cables routed downhole to the ESP motor. A lower current can be supplied to the motor without sacrificing operational functionality of the pumping system. Reducing the current for a given power output reduces the resistive losses in the power cable which also reduces the voltage required from a surface voltage source. 
         [0016]    In motors constructed with a rotor and stator, use of ferrofluid, including ferrofluid mixtures, in the gap between the rotor and the stator significantly adds to the reduction of reluctance of the magnetic circuit of the motor. In conventional submersible motors, the dielectric oil gap is very similar to an air gap between the rotor and stator. The dielectric oil/air gap dominates the reluctance of the magnetic circuit of the motor in which a magnetic flux, B, can be established by the electrical current, I, supplied from the surface via the long ESP cables. The relation between the current and the flux may be approximated by: 
         [0000]        NI=B{Lm/μ   m   +Lg/μ   o } 
         [0017]    In the above equation N is the number of turns through which the current flows through the motor; 
         [0018]    L m  is the equivalent magnetic path length through the motor&#39;s laminations and rotor; 
         [0019]    L g  is the air gap length, in the case of an ESP motor, the oil gap length; 
         [0020]    μ o  is the magnetic permeability of free space; 
         [0021]    μ m  is the magnetic permeability of the motor&#39;s iron alloy laminations. 
         [0022]    A comparison can be made between the current required in a ferrofluid filled ESP motor and a standard ESP motor, for the same power output from the same type ESP motor using the same number of turns and metal parts. For these two motors N, B, Lm and Lg, and μ m  are therefore the same. The ratio of the currents flowing in the ferrofluid motor and the standard motor can be calculated as: 
         [0000]        I   ff   /I={Lm/ (μ m )+ Lg/ (μ o )}/{ Lm/μ   m   +Lg/μ   o } 
         [0000]    Here μ ff  is the relative magnetic permeability of the ferrofluid compared to that of free space. 
         [0023]    The current reduction can be estimated to a first order approximation by assuming that the reluctance of the gap dominates the motor&#39;s reluctance in both cases; hence: 
         [0000]      I ff /I˜1/μ ff    
         [0000]    Because a ferrofluid&#39;s permeability, μ ff  is typically greater than 1; the current in the ferrofluid equipped motor is a fraction of the current required in a conventional oil-filled motor of the same dimensions and materials. The exact amount of the improvement depends on the motor&#39;s design and the specific ferrofluid formulated. 
         [0024]    Ferrofluids are stable colloidal suspensions of nano-size ferromagnetic particles in either aqueous or oil-based media. Typically, the magnetic particles are magnetite (an iron oxide) having diameters of about 10 nanometers (nm). These particles can be obtained as precipitates of simple chemical reactions. A surfactant layer covers the surface of the nano-particles and helps overcome the Van der Waals forces by preventing the particles from coming too close together and clumping or settling down due to gravity. Ferrofluids improve heat transfer, serve as good lubricants, and can also be formulated to operate over a wide temperature range up to, for example, 200° C. Ferrofluids improve motor cooling because ferrofluid magnetic properties vary inversely with temperature; the strong magnet fields of the motor&#39;s windings (which produce heat) attract cold ferrofluid more than hot ferrofluid thus forcing the heated ferrofluid away from the windings and toward cooler surfaces. This efficient cooling method requires up to no additional energy input. Ferrofluids have been discussed in various publications, such as R. E. Rosensweig, “Ferrohydrodynamics,” Cambridge University Press, Cambridge, (1985); and Elmars Blums (1995), “New Applications of Heat and Mass Transfer Processes in Temperature Sensitive Magnetic Fluids”, Brazilian Journal of Physics. Additionally, certain types of ferrofluids are available from Ferrotec Company of Tokyo, Japan. 
         [0025]    Referring generally to  FIG. 1 , an embodiment of a motor  20 , e.g. a submersible motor, is illustrated. By way of example, motor  20  is a submersible motor having a rotor  22  rotatably mounted within a stator  24  such that a gap  26  is formed around the rotor  22  between the rotor and the stator. The rotor  22  and stator  24  are enclosed by a motor housing  28 , and a ferrofluid  30  is disposed within motor housing  28 . For example, the ferrofluid  30  may be disposed in gap  26  to substantially increase the efficiency of motor  20 . In some embodiments, the interior of motor housing  28  is substantially filled with ferrofluid to substantially fill gaps between the rotating components of motor  20  and other gaps and spaces within its magnetic circuit. The ferrofluid  30  reduces the magnetic reluctance of motor  20  and increases its performance and reliability by significantly reducing the required current and electrical power that must be supplied to motor  20  to produce a required level of magnetic flux and resulting power output. In some applications, ferrofluid  30  may comprise ferrofluid mixtures including ferrofluid mixed with dielectric oil. 
         [0026]    Motor  20  also may comprise a variety of other components. For example, rotor  22  may be mounted on a rotatable shaft  32  for rotation within stator  24 , as further illustrated in  FIG. 2 . The rotatable shaft  32  may extend through one or both longitudinal ends  34  of motor housing  28 . In the embodiment illustrated, ferrofluid  30  is sealed within the interior of motor housing  28  by ferrofluid seals  36  that may be disposed around shaft  32  proximate longitudinal ends  34  of housing  28 . Additionally, motor  20  may comprise windings  38  and a variety of other or alternate components depending on the equipment with which motor  20  is engaged, the power requirements of the motor, the environment in which the motor is operated, and various design considerations. 
         [0027]    In one specific example, motor  20  is a submersible motor incorporated into an electric submersible pumping system  40 , as illustrated in  FIG. 3 . The electric submersible pumping system  40  may be constructed in a variety of forms with different components depending on the environment and application requirements. In the particular application illustrated, electric submersible pumping system  40  is deployed in a wellbore  42  drilled into a geological formation  44 . The wellbore  42  may be lined with a casing  46  that is perforated with a plurality of perforations  48  to allow well fluid to flow into the interior of casing  46 . 
         [0028]    The electric submersible pumping system  40  is deployed to a desired location in wellbore  42  via a conveyance  50  which may be in the form of a tubing  52 , e.g. coiled tubing, or other suitable conveyance that extends down from, for example, a wellhead  53 . The pumping system  40  is connected to conveyance  50  by a connector  54  and may comprise a variety of pumping related components. For example, electric submersible pumping system  40  may comprise a submersible pump  56  connected to a pump intake  58 . The pump intake  58  allows well fluid to be drawn into submersible pump  56  when pump  56  is powered by submersible motor  20 . In many applications, a motor protector  60  is located between submersible motor  20  and pump  56  to enable pressure equalization while isolating motor fluid from well fluid. 
         [0029]    In the embodiment illustrated in  FIG. 3 , power is supplied to submersible motor  20  via a power cable  62 . The use of ferrofluid  30  in submersible motor  20  enables substantially less current flow through power cable  62 , when compared to a conventional system, while achieving the same performance with respect to submersible motor  20  and pumping system  38 . The reduction in current required for a given output of motor  20  also reduces the resistive losses in power cable  62  and reduces the voltage required from a power source, such as power source  64  positioned at a surface location  66 . 
         [0030]    Referring generally to  FIG. 4 , a flow chart is provided to illustrate one approach for reducing electrical power consumption during operation of subterranean equipment, such as during operation of an electric submersible pumping system. In this example, motor  20  is initially assembled with a stator and a rotor deployed along an interior of the stator, as illustrated by block  68 . Depending on the specific application, a suitable ferrofluid  30  is selected, as illustrated by block  70 . The motor is filled with the ferrofluid  30  to enable substantially improved efficiency during operation of the motor, as illustrated by block  72 . 
         [0031]    The ferrofluid filled motor  20  is incorporated into the desired well equipment, as illustrated by block  74 . The motor  20  may be incorporated into an electric submersible pumping system, however the motor also may be incorporated into formation tester pump systems, electro-hydraulic actuator systems, electric motor driven flow control valve systems, and other well systems that can be powered via motor  20 . In at least some of these embodiments, the ferrofluid filled motor  20  and its associated equipment are deployed downhole into a wellbore, as illustrated by block  76 . Once deployed downhole, a relatively reduced amount of electrical power can be supplied via a suitable power cable or power cables to the motor  20 . 
         [0032]    Ferrofluid filled motor  20  provides substantially increased efficiency that is beneficial in a variety of environments. In downhole applications, the unique design of motor  20  simplifies the transfer and reduces the cost of delivering electrical power over substantial distances downhole. However, motor  20  can be used in a variety of systems, applications and environments. Additionally, individual motors or combinations of motors  20  can be used. In some electric submersible pumping systems, for example, a plurality of motors  20  is used to provide power to one or more submersible pumps. Depending on the specific motor application, the size, configuration, and materials used to construct motor  20  may vary. The ferrofluid may be contained within motor  20  via the ferrofluid seals, or other seals or motor protectors can be used to contain the fluid while enabling equalization of pressure. 
         [0033]    Although only a few embodiments have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this application. Accordingly, such modifications are intended to be included within the scope of this invention as defined in the claims. These embodiments are not meant to unduly limit the present claims herein or any subsequent related claims.