Patent Publication Number: US-2023160286-A1

Title: Oil Circulation in an Electric Submersible Pump (ESP) Electric Motor

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not applicable. 
     BACKGROUND 
     Electric submersible pump (ESP) assemblies may comprise an electric motor, a seal section coupled to the electric motor, a fluid inlet coupled to the seal section, and a centrifugal pump coupled to the fluid inlet. A drive shaft of the electric motor is coupled to a drive shaft of the seal section, and the drive shaft of the seal section passes through the fluid inlet and couples to a drive shaft of the centrifugal pump assembly. When the electric motor is supplied electric power from the surface, the electric motor turns the drive shaft of the electric motor. The drive shaft of the electric motor then turns the drive shaft of the seal section, and the drive shaft of the seal section turns the drive shaft of the centrifugal pump assembly. The centrifugal pump assembly may comprise one or more pump stages, where each pump stage comprises an impeller coupled to the drive shaft of the centrifugal pump assembly and a diffuser that is coupled to an outer housing of the centrifugal pump assembly. The electric motor turns, the impellers turn, reservoir fluid is draw into the fluid inlet and lifted by the one or more pump stages to the surface. Electric motors of ESP assemblies are typically turned at rates between 3450 RPM and 3650 RPM and are operated continuously. It is desirable that the ESP assemblies operate for upwards of a year continuously without maintenance to achieve production goals and manage maintenance costs. Some ESP assemblies may incorporate a gas separator assembly located between the fluid inlet and the centrifugal pump whose purpose is to separate a gas phase fluid fraction (or higher gas liquid ratio fraction) of the reservoir from a liquid phase fluid fraction (or a lower gas liquid ratio fraction) of the reservoir fluid, exhaust the gas phase fluid into an annulus formed between the inside of wellbore and the outside of the ESP assembly, and flow the liquid phase fluid to the inlet of the centrifugal pump. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG.  1    is an illustration of a wellsite and an electric submersible pump (ESP) assembly in a wellbore at the wellsite according to an embodiment of the disclosure. 
         FIG.  2    is an illustration of an electric motor according to an embodiment of the disclosure. 
         FIG.  3 A  is an illustration of a stator of an electric motor according to an embodiment of the disclosure. 
         FIG.  3 B  is an illustration of another stator of an electric motor according to an embodiment of the disclosure. 
         FIG.  4 A  is an illustration of a rotor of an electric motor according to an embodiment of the disclosure. 
         FIG.  4 B  is an illustration of another rotor of an electric motor according to an embodiment of the disclosure. 
         FIG.  5 A  is an illustration of another rotor of an electric motor according to an embodiment of the disclosure. 
         FIG.  5 B  is an illustration of yet another rotor of an electric motor according to an embodiment of the disclosure. 
         FIG.  6    is an illustration of a drive shaft of an electric motor according to an embodiment of the disclosure. 
         FIG.  7    is a flowchart of a method of lifting reservoir fluid in a wellbore to a surface location according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood at the outset that although illustrative implementations of one or more embodiments are illustrated below, the disclosed systems and methods may be implemented using any number of techniques, whether currently known or not yet in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, but may be modified within the scope of the appended claims along with their full scope of equivalents. 
     As used herein, orientation terms “upstream,” “downstream,” “up,” “down,” “uphole,” and “downhole” are defined relative to the direction of flow of well fluid in the well casing. “Upstream” is directed counter to the direction of flow of well fluid, towards the source of well fluid (e.g., towards perforations in well casing through which hydrocarbons flow out of a subterranean formation and into the casing). “Downstream” is directed in the direction of flow of well fluid, away from the source of well fluid. “Down” is directed counter to the direction of flow of well fluid, towards the source of well fluid. “Up” is directed in the direction of flow of well fluid, away from the source of well fluid. “Downhole” is directed counter to the direction of flow of well fluid, towards the source of well fluid (towards a bottom of the wellbore). “Uphole” is directed in the direction of the flow of well fluid, towards a surface (towards a top of the wellbore). 
     ESP assemblies operate in a challenging environment. Wellbores in some environments are tight. For example, the trend is towards drilling narrower diameter wellbores, whereby to reduce drilling costs. Tighter wellbores impose technical obstacles, including transferring heat generated by the electric motor away from the motor. Heat generated by a variety of processes in the electric motor is transferred away from the heat source by a housing of the electric motor, for example to wellbore fluid surrounding the ESP assembly. Heat may be produced in the electric motor by current flow in electric motor windings and by core losses in the electric motor stator core and rotor core. Core loses can include eddy current losses and hysteresis losses. Heat may be produced in the electric motor by bearing/bushing friction, and other processes. The electric motor is located below the fluid inlet of the ESP assembly, hence wellbore fluid may flow upwards over the outside surface of the housing of the electric motor, receiving heat transferred from the housing. But heat may concentrate in an upper end of the electric motor, creating a “hot spot.” Often electrical failures occur in the upper ends of electric motors, probably due to excess heat in the upper ends of the electric motors. Heat also tends to concentrate in electric motors near the longitudinal axis of the electric motor and to flow radially outwards. Heat transfer occurs from a region of higher temperature to a region of relatively lower temperature. 
     The present disclosure teaches new structures for moving oil within the electric motor, whereby to improve the cooling of the electric motor and/or to promote even distribution of heat within the electric motor to avoid hot spots. In an embodiment, one or more grooves may be defined in an inside surface of an electric motor stator, in an outside surface of an electric motor rotor, in an inside surface of an electric motor rotor, and/or on an outside surface of a drive shaft of the electric motor. The one or more grooves can provide enhanced flow paths for oil within the electric motor, and the enhanced flow of oil can assist in transferring heat out of the electric motor. In an embodiment, the grooves may be parallel to the longitudinal axis of the electric motor. In an embodiment, the grooves may be defined in a helical form. 
     Turning now to  FIG.  1   , a wellsite  100  is described. The wellsite  100  comprises a wellbore  102  optionally lined with a casing  104 , an electric submersible pump (ESP) assembly  132  in the wellbore  102 , and a production tubing string  134 . The ESP assembly  132  comprises an optional sensor unit  120  at a downhole end, an electric motor  122  coupled to the sensor unit  120  uphole of the sensor unit  120 , a seal section  124  coupled to the electric motor  122  uphole of the electric motor  122 , a fluid intake  126  coupled to the seal section  124  uphole of the seal section  124 , a production pump assembly  128  coupled to the fluid intake  126  uphole of the fluid intake  126 , and a pump discharge  130  coupled to the production pump assembly  128  uphole of the production pump assembly  128 . The pump discharge  130  is coupled to the production tubing string  134 . In an embodiment, a motor head or pot head (not shown) is coupled between the electric motor  122  and the seal section  124 . 
     In an embodiment, the casing  104  has perforations  140  that allow reservoir fluid  142  to enter the wellbore  102  and flow downstream to the fluid intake  126 . The reservoir fluid  142  enters inlet ports  129  of the fluid intake  126 , flows from the fluid intake  126  into an inlet of the production pump assembly  128 , is pumped by the production pump assembly  128  to flow out of the production pump assembly  128  into the pump discharge  130  up the production tubing string  134  to a wellhead  156  located at the surface  134 . In an embodiment, an electric cable  136  is connected to the electric motor  122  and provides electric power from an electric power source located at the surface  158  to the electric motor  122  to cause the electric motor  122  to turn and deliver rotational power to the production pump assembly  128 . In an embodiment, the electric cable  136  attaches to the electric motor  122  via a motor head or pot head. In an embodiment, the production pump assembly  128  comprises one or more centrifugal pump stages, where each centrifugal pump stage comprises an impeller coupled to a drive shaft of the production pump assembly  128  and a diffuser retained by a housing of the production pump assembly  128 . The drive shaft of the production assembly is coupled to a drive shaft of the seal section  124 . The drive shaft of the seal section  124  is coupled to a drive shaft of the electric motor  122 . In some contexts, the production pump assembly  128  may be referred to as a centrifugal pump assembly. The production pump assembly  128  may be said to lift the reservoir fluid  154  to the surface  158 . 
     In an embodiment, the ESP assembly  132  may further comprise a gas separator assembly, for example located between the fluid intake  126  and the production pump assembly  128 . The gas separator assembly may induce rotational motion of the reservoir fluid  142  within a separation chamber such that high gas liquid ratio fluid concentrates near a drive shaft of the gas separator assembly and a low gas liquid ratio fluid concentrates near an inside housing of the gas separator assembly. The high gas liquid ratio fluid exits the gas separator by gas discharge ports to an exterior of the gas separator (e.g., into the wellbore  102  outside the ESP assembly  132 ), and the low gas liquid ratio fluid is flowed by liquid discharge ports to the inlet of the production pump assembly  128 . In this way, the gas separator assembly may provide a lower gas liquid ratio fluid to the production pump assembly  128  when the reservoir fluid  142  comprises a mix of gas phase and liquid phase fluid. In an embodiment, the gas separator assembly may comprise one or more fluid reservoirs that define empty annular spaces that may serve as fluid reservoirs that can continue to supply at least some liquid phase fluid during an extended gas slug impinging on the fluid intake  126 . The drive shaft of the gas separator assembly may be coupled to the drive shaft of the seal section  124  at a downhole end and coupled at an uphole end to the downhole end of the drive shaft of the production pump assembly  128 . 
     In an embodiment, the ESP assembly  132  may further comprise a charge pump assembly, for example located between the fluid intake  126  and the gas separator assembly. The charge pump assembly may comprise one or more fluid movers to urge the reservoir fluid  142  upwards to the gas separator assembly. The fluid movers of the charge pump assembly may be an auger coupled to a drive shaft of the charge pump assembly. The fluid movers of the charge pump assembly may be one or more centrifugal pump stages, where each centrifugal pump stage comprises an impeller coupled to a drive shaft of the charge pump assembly and a diffuser retained by a housing of the charge pump assembly. In an embodiment, the charge pump assembly may comprise one or more fluid reservoirs that define empty annular spaces that may serve as fluid reservoirs that can continue to supply at least some liquid phase fluid to the gas separator assembly during an extended gas slug impinging on the fluid intake  126 . The drive shaft of the charge pump assembly may be coupled at a downhole end to the drive shaft of the seal section  124  and coupled at an uphole end to the downhole end of the drive shaft of the gas separator assembly. 
     An orientation of the wellbore  102  and the ESP assembly  132  is illustrated in  FIG.  1    by an x-axis  160 , a y-axis  162 , and a z-axis  164 . While the wellbore  102  is illustrated in  FIG.  1    as having a deviated portion or a substantially horizontal portion  106 , the ESP assembly  132  may be used in a substantially vertical wellbore  102 . While the wellsite  100  is illustrated as being on-shore, the ESP assembly  132  may be used in an off-shore location as well. 
     Turning now to  FIG.  2   , further details of the electric motor  122  are described. It is understood that not all of the details of the electric motor  122  are depicted in  FIG.  2   . The electric motor  122  comprises a drive shaft  170  having male splines  171  at an upper end by which it may be coupled to a lower end of a drive shaft of the seal section  124 . For example, a coupler featuring female splines disposed in an inner opening may mate with the male splines  171  of the drive shaft  170  and with male splines in a lower end of the drive shaft of the seal section  124 . In an embodiment, the drive shaft  170  has a bore  172  that is concentric with a longitudinal axis  169  of the drive shaft  170  and that intersects at an upper end with a transverse through bore  173 . In an embodiment, the electric motor  122  comprises a first rotor  174   a , a second rotor  174   b , a third rotor  174   c , and a stator  176 . While  FIG.  2    depicts an electric motor  122  having three rotors  174 , in another embodiment, the electric motor  122  may have a single rotor, two rotors, or more than three rotors. The rotors  174   a ,  174   b ,  174   c  are coupled to the drive shaft  170 , for example by keyways in the rotors  174   a ,  174   b ,  174   c  and in the drive shaft  170  and a key (not shown) inserted into the keyways. The stator  176  is retained within a housing  182 . The electric motor  122  may be a 3-phase alternating current (AC) motor, for example a squirrel cage type induction motor. Alternatively, the electric motor  122  may be a 3-phase AC permanent magnet motor. 
     The rotors  174   a ,  174   b ,  174   c  and the stator  176  may be formed of a number of plates, referred to as laminations, in the form of disks with a hole in the center and a plurality of apertures between an inside diameter and an outside diameter of the disk to establish channels to accommodate electrical conductors in the assembled rotors  174   a ,  174   b ,  174   c  and the assembled stator  176 . These channels to accommodate electrical conductors are illustrated in later figures. Such laminations are employed to reduce eddy current losses in electric motor cores. These plates may be made of electrical steel. Electrical steel may be an iron alloy tailored to produce specific magnetic properties which result in low core losses and high permeability. In an embodiment, the surface of these plates may be chemically oxidized or treated with lacquer to reduce eddy current flows between plates. Alternatively, the plates may be made of other metal. The laminations may be formed by punching out the forms from sheets of metal, the traditional and conventional method of manufacturing laminations. The laminations may be formed by a process of 3-D printing, a relatively recently developed alternative method of manufacturing articles such as laminations. 
     In an embodiment, electrical conductors pass through channels formed in the stator  178  and are connected via the electric cable  136  to an electrical power source (not shown) at the surface  158 . The conductors in the stator  178  may be wires or copper bars. In an embodiment, electrical conductors pass through channels formed in the rotors  176   a ,  176   b ,  176   c  and are shorted at their upper ends and at the lower ends by end rings. In an embodiment (e.g., when the electric motor  122  is a squirrel cage type induction motor), the end rings may be formed of brass. In another embodiment (e.g., when the electric motor  122  is a permanent magnet motor), instead of conductors the channels formed in the rotors  176   a ,  176   b ,  176   c  retain permanent magnets. None of the conductors, copper bars, end caps, or permanent magnets are shown in  FIG.  2   . 
     The electric motor  122  comprises a plurality of bearings  178  coupled to the drive shaft  170  and associated bushings  180  coupled to the stator  176 . Each bearing  178  is located within a corresponding bushing  178 , and together the pairs of bearings  176  and bushings  178  support the drive shaft  170  and maintain it in proper axial alignment. In an embodiment, the bushings  178  define oil passageways providing flow communication from an upper side of the bushings  178  to a lower side of the bushings  178  and in the opposite sense as well. Oil within the electric motor  122  may flow upwards through the bore  172 , into the through bore  173 , and out the through bore  173 , through the oil passageways defined by the bushings  178 , into and through a gap between the stator  176  and the rotors  174   a ,  174   b ,  174   c , and complete an oil flow circuit by flowing back into the lower opening of the bore  172 . In an embodiment the oil within the electric motor  122  may flow in the opposite direction described above. In an embodiment, a fluid mover coupled to the drive shaft  170  or installed within the bore  172  may urge the flow of oil in a circuit within the electric motor  122 . In an embodiment, the oil in the electric motor  122  may be a dielectric oil. 
     Turning now to  FIG.  3 A , further details of the stator  176  are described. The stator  176  has a longitudinal axis  177  that is concentric with the longitudinal axis  169  of the drive shaft  170 . In an embodiment, an inside surface  185  of the stator  176  defines a plurality of helical grooves: a first groove  184   a , a second groove  184   b , a third groove  184   c , and a fourth groove  184   d . The stator  176  further defines a plurality of channels  186  for electrical conductors. To simplify the illustration in  FIG.  3 A  to better show the grooves  184 , only one lamination is shown at an upper end of the stator  176  and only one lamination is shown at the lower end of the stator  176 , but it is understood that the stator  176  is composed of many laminations as illustrated in  FIG.  2   . The grooves  184  provide channels to improve the flow of oil within the electric motor  122 , whereby to enhance the transfer of heat out of the electric motor  122 . While four grooves  184   a ,  184   b ,  184   c ,  184   d  are illustrated in  FIG.  3 A , it is understood that the inside surface  185  of the stator  176  may define two grooves, three grooves, or more than four grooves. The case of the inside surface  185  of the stator  176  defining a single groove  184  is illustrated and described with reference to  FIG.  3 B  below. 
     The grooves  184  may be cut in the stator  176  after laminations are assembled to form the stator  176 . Alternatively, the individual laminations may be cut with a groove slightly offset, and the helical groove established by aligning the individual laminations when assembling the stator  176 . Alternatively, the individual laminations may be 3-D printed with a groove slightly offset, and the helical groove established by aligning the individual laminations when assembling the stator  176 . In an embodiment, the grooves  184  may be cut in the stator  176  by machining the grooves  184  or by laser cutting the grooves  184  or by another method. 
     In an embodiment, the grooves  184  may be from 1/10000 (0.0001) inch deep to 1/100 (0.01) inch deep. In an embodiment, the grooves  184  may be from 1/10000 (0.0001) inch deep to 1/16 (0.625) inch deep. In an embodiment, the grooves  184  are about 2/10000 (0.0002) inch deep. In an embodiment, the grooves  184  are about 0.00025 inch deep to about 0.0005 inch deep. In an embodiment, the grooves  184  may be from about 1/10 (0.1) inch wide to about ½ (0.5) inch wide. In an embodiment, the groves  184  may be from about 1/16 (0.0625) inch wide to about 3/16 (0.1875) inch wide. In another embodiment, however, the grooves  184  may have a different depth and/or a different width. The depth of the grooves  184  may be limited by the separation between the inside diameter of the stator  176  and the channels  186 . In an embodiment, the grooves  184  may have a cross-sectional shape that is rectangular, square, half-round, semi-circular, oblong, V-shaped, or other shape. The grooves  184  may have any rate of twist or pitch. The grooves  184  may have a 1 turn in 4 inches rate of twist, a 1 turn in 8 inches rate of twist, a 1 turn in 12 inches rate of twist, a 1 turn in 16 inches rate of twist, a 1 turn in 20 inches rate of twist, a 1 turn in 24 inches rate of twist, a 1 turn in 28 inches rate of twist, a 1 turn in 32 inches rate of twist, a 1 turn in 36 inches rate of twist, a 1 turn in 40 inches rate of twist, a 1 turn in 44 inches rate of twist, a 1 turn in 48 inches rate of twist, or some other rate of twist. In an embodiment, the grooves  184  may have a rate of twist between a 1 turn in 4 inches rate of twist and a 1 turn in 48 inches rate of twist. In an embodiment, the grooves  184  may have a rate of twist between 1 turn in 4 inches rate of twist and a 1 turn in 24 inches rate of twist. In an embodiment, the grooves are not twisted (not helical in form) and extend axially along the inside surface  185  of the stator  176  and parallel to the longitudinal axis  177  of the stator  176 . While the grooves  184  are illustrated in  FIG.  3 A  as turning in a first sense, in another embodiment, the grooves  184  may turn in an opposite sense. 
     Turning now to  FIG.  3 B , another embodiment of the stator  176  is described. In  FIG.  3 B , only a single groove  184  is defined by the inside surface  185  of the stator  176 . The descriptions of the stator  176  above with reference to  FIG.  3 A  apply to  FIG.  3 B , with the restriction that there is the single groove  184 , and that the rate of twist of the groove  184  may have a higher rate of twist, for example a 1 turn in 1 inch rate of twist to a 1 turn in 12 inches rate of twist. 
     Turning now to  FIG.  4 A , further details of the rotor  174  are described. The rotor  174  has a longitudinal axis  179  that is concentric with the longitudinal axis  177  of the stator  176  and with the longitudinal axis  169  of the drive shaft  170 . In an embodiment, an outside surface  187  of the rotor  174  defines a plurality of helical grooves: a fifth groove  188   a , a sixth groove  188   b , a seventh groove  188   c , and an eighth groove  188   d . The rotor  174  further defines a plurality of channels  189  for electrical conductors or for permanent magnets, depending on the type of the electric motor  122 . To simplify the illustration in  FIG.  4 A  to better show the grooves  188 , only one lamination is shown at an upper end of the rotor  174  and only one lamination is shown at the lower end of the rotor  174 , but it is understood that the rotor  174  is composed of many laminations as illustrated in  FIG.  2   . The grooves  188  provide channels to improve the flow of oil within the electric motor  122 , whereby to enhance the transfer of heat out of the electric motor  122 . While four grooves  188   a ,  188   b ,  188   c ,  188   d  are illustrated in  FIG.  4 A , it is understood that the outside surface  187  of the rotor  174  may define two grooves, three grooves, or more than four grooves. The case of the outside surface  187  of the rotor  174  defining a single groove  188  is illustrated and described with reference to  FIG.  4 B  below. 
     The grooves  188  may be cut in the rotor  174  after laminations are assembled to form the rotor  174 . Alternatively, the individual laminations may be cut with a groove slightly offset, and the helical groove established by aligning the individual laminations when assembling the rotor  174 . Alternatively, the individual laminations may be 3-D printed with a grove slightly offset, and the helical groove established by aligning the individual laminations when assembling the rotor  174 . The grooves  188  may be cut in the rotor  174  by machining the grooves  188  or by laser cutting the grooves  188  or by another method. 
     In an embodiment, the grooves  188  may be from 1/10000 (one ten thousandths) inch deep to 1/100 (0.01) inch deep. In an embodiment, the grooves  188  are about 2/10000 (0.0002) inch deep. In an embodiment, the grooves  188  may be from about 1/10 (0.1) inch wide to about ½ % inch wide. In an embodiment, the grooves  188  may be from about 1/16 (0.0625) inch wide to about 3/16 (0.1875) inch wide. In another embodiment, however, the grooves  188  may have a different depth and/or a different width. The depth of the grooves  188  may be limited by the separation between the outside diameter of the rotor  174  and the channels  189 . In an embodiment, the grooves  188  may have a cross-sectional shape that is rectangular, square, half-round, semi-circular, oblong, V-shaped, or other shape. The grooves  188  may have any rate of twist or pitch. The grooves  188  may have a 1 turn in 4 inches rate of twist, a 1 turn in 8 inches rate of twist, a 1 turn in 12 inches rate of twist, a 1 turn in 16 inches rate of twist, a 1 turn in 20 inches rate of twist, a 1 turn in 24 inches rate of twist, a 1 turn in 28 inches rate of twist, a 1 turn in 32 inches rate of twist, a 1 turn in 36 inches rate of twist, a 1 turn in 40 inches rate of twist, a 1 turn in 44 inches rate of twist, a 1 turn in 48 inches rate of twist, or some other rate of twist. In an embodiment, the grooves  188  may have a rate of twist between a 1 turn in 4 inches rate of twist and a 1 turn in 24 inches rate of twist. In an embodiment, the grooves  188  may have a rate of twist between a 1 turn in 4 inches rate of twist and a 1 turn in 48 inches rate of twist. In an embodiment, the grooves are not twisted and extend axially along the outside surface  187  of the rotor  174  and parallel to the longitudinal axis  179  of the rotor  174 . 
     While the grooves  188  are illustrated in  FIG.  4 A  as turning in a first sense, in another embodiment, the grooves  188  may turn in an opposite sense. In an embodiment of the electric motor  122  where there are both grooves  184  on the inside surface  185  of the stator  176  and grooves  188  on the outside surface  187  of the rotor  174 , the grooves  188  may turn in the same sense as the grooves  184  turn, or the grooves  188  may turn in the opposite sense of the grooves  184  turn. In an embodiment, the grooves  188  in the outside surface  187  of the rotor  174  may act in part as fluid movers to urge the oil within the electric motor  122  to flow, for example like an auger might urge flow of fluids or particles. 
     Turning now to  FIG.  4 B , another embodiment of the rotor  174  is described. In  FIG.  48   , only a single groove  188  is defined by the outside surface  187  of the rotor  174 . The descriptions of the rotor  174  above with reference to  FIG.  4 A  apply to  FIG.  4 B , with the restriction that there is the single groove  188 , and that the rate of twist of the groove  188  may have a higher rate of twist, for example a 1 turn in inches rate of twist to a 1 turn in 12 inches rate of twist. 
     Turning now to  FIG.  5 A , further details of the rotor  174  are described. In an embodiment, the rotor  174  defines a plurality of helical grooves in an inside surface  191  of the rotor  174 , for example a ninth groove  190   a , a tenth groove  190   b , and an eleventh groove  190   c . In an embodiment, the rotor  174  may define one or more grooves  188  on the outside surface  187  of the rotor  174  (as described above with reference to  FIG.  4 A  and  FIG.  4 B  above) and also define the grooves  190  in the inside surface  191  of the rotor  174 . Alternatively, in an embodiment, the outside surface  187  of the rotor  174  does not define any grooves and grooves  190  are defined in the inside surface  191  of the rotor  174 . To simplify the illustration in  FIG.  5 A  to better show the grooves  190 , only one lamination is shown at an upper end of the rotor  174  and only one lamination is shown at the lower end of the rotor  174 , but it is understood that the rotor  174  is composed of many laminations as illustrated in  FIG.  2   . The grooves  190  provide channels to improve the flow of oil within the electric motor  122 , whereby to enhance the transfer of heat out of the electric motor  122 . While three grooves  190   a ,  190   b ,  190   c  are illustrated in  FIG.  5 A , it is understood that the inside surface  191  of the rotor  174  may define two grooves or more than three grooves. The case of the inside surface  191  of the rotor  174  defining a single groove  190  is illustrated and described with reference to  FIG.  5 B  below. 
     The grooves  190  may be cut in the rotor  174  after laminations are assembled to form the rotor  174 . Alternatively, the individual laminations may be cut with a groove slightly offset, and the helical groove established by aligning the individual laminations when assembling the rotor  174 . Alternatively, the individual laminations may be 3-0 printed with a grove slightly offset, and the helical groove established by aligning the individual laminations when assembling the rotor  174 . The grooves  190  may be cut in the rotor  174  by machining the grooves  190  or by laser cutting the grooves  190  or by another method. 
     In an embodiment, the grooves  190  may be from 1/10000 (0.0001) inch deep to 1/100 (0.01) inch deep. In an embodiment, the grooves  190  are about 2/10000 (0.0002) inch deep. In an embodiment, the grooves  190  are about 5/1000 (0.005) inch deep. In an embodiment, the grooves  190  may be from about 1/10000 (0.0001) inch deep to about 2/100 (0.02) inch deep. In an embodiment, the grooves  190  are between 0.01 inch deep and 0.03 inch deep. In an embodiment, the grooves  190  may be from about 1/10 (0.1) inch wide to about ½ (0.5) inch wide. In an embodiment, the grooves  190  may be from about 1/16 (0.0625) inch wide to about 3/16 (0.1875) inch wide. In an embodiment, the grooves  190  may be between 1/16 (0.0625) inch wide and % (0.25) inch wide. In another embodiment, however, the grooves  190  may have a different depth and/or a different width. In an embodiment, the grooves  190  may have a cross-sectional shape that is rectangular, square, half-round, semi-circular, oblong, V-shaped, or other shape. The grooves  190  may have any rate of twist or pitch. The grooves  190  may have a 1 turn in 4 inches rate of twist, a 1 turn in 8 inches rate of twist, a 1 turn in 12 inches rate of twist, a 1 turn in 16 inches rate of twist, a 1 turn in 20 inches rate of twist, a 1 turn in 24 inches rate of twist, or some other rate of twist. In an embodiment, the grooves  190  may have a rate of twist between a 1 turn in 4 inches rate of twist and a 1 turn in 24 inches rate of twist. In an embodiment, the grooves are not twisted and extend axially along the inside surface  191  of the rotor  174  and parallel to the longitudinal axis  179  of the rotor  174 . While the grooves  190  are illustrated in  FIG.  5 A  as turning in a first sense, in another embodiment, the grooves  190  may turn in an opposite sense. In an embodiment, the grooves  190  in the inside surface  191  of the rotor  174  may act in part as fluid movers to urge the oil within the electric motor  122  to flow, for example like an auger might urge flow of fluids or particles. Because the keyway and key that couple the rotor  174   a ,  174   b ,  174   c  to the drive shaft  170  may otherwise interrupt the oil flow pathway in the grooves  190   a ,  190   b ,  190   c  (e.g., when the grooves  190  have a helical configuration rather than a longitudinally parallel configuration), the key may be modified to have notches at positions where the grooves meet the key. Using grooves  190   a ,  190   b ,  190   c  on the inside surface  191  of the rotors  174   a ,  174   b ,  174   c  that are parallel to the longitudinal axis  179  of the rotors  174  may provide the advantage of omitting the notching of the key. 
     Turning now to  FIG.  5 B , another embodiment of the rotor  174  is described. In  FIG.  5 B , only a single groove  190  is defined by the inside surface  191  of the rotor  174 . The descriptions of the rotor  174  above with reference to  FIG.  5 A  apply to  FIG.  5 B , with the restriction that there is the single groove  190 , and that the rate of twist of the groove  190  may have a higher rate of twist, for example a 1 turn in inches rate of twist to a 1 turn in 12 inches rate of twist. 
     Turning now to  FIG.  6   , further details of the drive shaft  170  are described. In an embodiment, an outside surface  197  of the drive shaft  170  defines a plurality of helical grooves  196 : a twelfth groove  196   a , a thirteenth groove  196   b , and a fourteenth groove  196   c . The grooves  196  may be cut in the outside surface  197  of the drive shaft  170  during manufacturing and/or machining of the drive shaft  170 . The grooves  196  provide channels to improve the flow of oil within the electric motor  122 , whereby to enhance the transfer of heat out of the electric motor  122 . While  FIG.  6    illustrates a drive shaft  170  having an outside surface  197  defining three grooves  196   a ,  196   b ,  196   c , it is understood that the drive shaft  170  may define two grooves or more than three grooves. The grooves  196   a ,  196   b ,  196   c  may extend from a point less than 3 feet below, less than 2 feet below, less than 1 foot below, less than 9 inches below, or less than 6 inches below the male splines  171  to a lower end of the drive shaft  170 . In an embodiment, the grooves  196  may be cut in the drive shaft  170  by machining the grooves  196  or by laser cutting the grooves  196  or by another method. 
     In an embodiment, the grooves  196  may be from 1/10000 (0.0001) inch deep to 1/100 (0.01) inch deep. In an embodiment, the grooves  196  are about 2/10000 (0.0002) inch deep. In an embodiment, the grooves  196  are about 5/1000 (0.005) inch deep. In an embodiment, the grooves  196  are between 0.01 inch deep and 0.03 inch deep. In an embodiment, the grooves  196  may be from about 1/10 (0.1) inch wide to about ½ (0.5) inch wide. In an embodiment, the grooves  196  may be from about 1/16 (0.0625) inch wide to about 3/16 (0.1875) inch wide. In an embodiment, the grooves  196  may be between 1/16 (0.0625) inch wide and % (0.25) inch wide. In another embodiment, however, the grooves  196  may have a different depth and/or a different width. In an embodiment, the grooves  196  may have a cross-sectional shape that is rectangular, square, half-round, semi-circular, oblong, V-shaped, or other shape. The grooves  196  may have any rate of twist or pitch. The grooves  196  may have a 1 turn in 4 inches rate of twist, a 1 turn in 8 inches rate of twist, a 1 turn in 12 inches rate of twist, a 1 turn in 16 inches rate of twist, a 1 turn in 20 inches rate of twist, a 1 turn in 24 inches rate of twist, a 1 turn in 28 inches rate of twist, a 1 turn in 32 inches rate of twist, a 1 turn in 36 inches rate of twist, a 1 turn in 40 inches rate of twist, a 1 turn in 44 inches rate of twist, a 1 turn in 48 inches rate of twist, or some other rate of twist. In an embodiment, the grooves  196  may have a rate of twist between a 1 turn in 4 inches rate of twist and a 1 turn in 24 inches rate of twist. In an embodiment, the grooves  196  have a rate of twist between 1 turn in 4 inches rate of twist and 1 turn in 48 inches rate of twist. In an embodiment, the grooves are not twisted and extend axially along the outside surface  197  of the drive shaft  170  and parallel to the longitudinal axis  169  of the drive shaft  170 . While the grooves  196  are illustrated in  FIG.  6    as turning in a first sense, in another embodiment, the grooves  196  may turn in an opposite sense. In an embodiment, the grooves  196  in the outside surface  197  of the drive shaft  170  may act in part as fluid movers to urge the oil within the electric motor  122  to flow, for example like an auger might urge flow of fluids or particles. 
     Turning now to  FIG.  7   , a method  300  is described. In an embodiment, the method  300  is a method of lifting reservoir fluid in a wellbore to a surface location. At block  302 , the method  300  comprises assembling an electric submersible pump (ESP) assembly at a wellsite, wherein the ESP assembly comprises a production pump and an electric motor, where at least one of an inside surface of a stator of the electric motor, an outside surface of a rotor of the electric motor, an inside surface of a rotor of the electric motor, or an outside surface of a drive shaft of the electric motor defines at least one groove and where the drive shaft of the electric motor is coupled to a drive shaft of the production pump. In an embodiment, the ESP assembly further comprises a seal section between the electric motor and the production pump, wherein the seal section comprises a seal section drive shaft, the drive shaft of the electric motor is coupled to the drive shaft of the seal section, and the drive shaft of the seal section is coupled to the drive shaft of the production pump. In an embodiment, the ESP assembly further comprises a gas separator assembly between the electric motor and the production pump. In an embodiment, the ESP assembly further comprises a charge pump assembly between the electric motor and the gas separator assembly. 
     At block  304 , the method  300  comprises coupling the ESP assembly to a production tubing string. At block  306 , the method  300  comprises running the ESP assembly into the wellbore at the lower end of the production tubing string. At block  308 , the method  300  comprises providing electric power to the electric motor. 
     At block  310 , the method  300  comprises turning the drive shaft of the electric motor by the rotor of the electric motor. At block  312 , the method  300  comprises turning the drive shaft of the production pump by the drive shaft of the electric motor. At block  314 , the method  300  comprises lifting reservoir fluid in the wellbore by the production pump up an interior of the production tubing to the surface. 
     At block  316 , the method  300  comprises circulating oil within the electric motor via the at least one groove defined by the inside surface of the stator of the electric motor, the outside surface of the rotor of the electric motor, the inside surface of the rotor of the electric motor, or the outside surface of the drive shaft of the electric motor. In an embodiment, circulating the oil within the electric motor comprises circulating the oil via a bore in the drive shaft of the electric motor that is concentric with a longitudinal axis of the drive shaft of the electric motor. In an embodiment, the electric motor comprises a bushing retained by the stator of the electric motor and a bearing coupled to the drive shaft of the electric motor and located within the bushing, and circulating the oil within the electric motor comprises circulating the oil via a plurality of passageways defined by the bushing. 
     Additional Embodiments 
     The following are non-limiting, specific embodiments in accordance with the present disclosure. 
     A first embodiment which is an electric submersible pump (ESP) electric motor, comprising a motor housing; a stator retained within the motor housing; a drive shaft; and an at least one rotor mechanically coupled to the drive shaft and located concentric with and inside of the stator, wherein an inside surface of the stator defines a groove extending from an upper end to a lower end of the stator or an outside surface of the at least one rotor defines a groove extending from an upper end to a lower end of the at least one rotor. 
     A second embodiment which is the ESP electric motor of the first embodiment, wherein the groove extends helically from the upper end to the lower end of the inside surface of the stator or of the outside surface of the at least one rotor. 
     A third embodiment which is the ESP electric motor of the first or the second embodiment, wherein the inside surface of the stator defines a plurality of grooves extending from the upper end to the lower end of the stator. 
     A fourth embodiment which is the ESP electric motor of any of the first through third embodiment, wherein the outside surface of the at least one rotor defines a plurality of grooves extending from the upper end to the lower end of the at least one rotor. 
     A fifth embodiment, which is the ESP electric motor of any of the first through fourth embodiment, wherein an inside surface of the at least one rotor defines a groove extending from the upper end to the lower end of the at least one rotor. 
     A sixth embodiment, which is the ESP electric motor of any of the first through the fourth embodiment, wherein an inside surface of the at least one rotor defines a plurality of grooves extending from the upper end to the lower end of the at least one rotor. 
     A seventh embodiment, which is the ESP electric motor of any of the first through the sixth embodiment, wherein an upper end of the drive shaft defines male splines and an outside surface of the drive shaft defines a plurality of grooves extending from a point less than 1 foot below the male splines to a lower end of the drive shaft. 
     An eighth embodiment which is an electric submersible pump (ESP) electric motor, comprising: a motor housing; a stator retained within the motor housing; a drive shaft; and an at least one rotor mechanically coupled to the drive shaft and located concentric with and inside of the stator, wherein an inside surface of the at least one rotor defines a groove extending from an upper end to a lower end of the at least one rotor or an outside surface of the drive shaft defines a groove from an upper portion of the drive shaft adjacent an upper end of the at least one rotor to a lower portion of the drive shaft adjacent a lower end of the at least one rotor. 
     A ninth embodiment, which is the ESP electric motor of the eighth embodiment, wherein the groove is substantially parallel to a longitudinal axis of the at least one rotor or substantially parallel to a longitudinal axis of the drive shaft. 
     A tenth embodiment, which is the ESP electric motor of the eighth or the ninth embodiment, wherein the inside surface of the at least one rotor defines a plurality of grooves. 
     An eleventh embodiment, which is the ESP electric motor of any of the eighth to the tenth embodiment, wherein the outside surface of the drive shaft defines a plurality of grooves. 
     A twelfth embodiment, which is the ESP electric motor of any of the eighth to the eleventh embodiment, wherein the groove is between about 0.01 inch deep and about 0.03 inch deep. 
     A thirteenth embodiment, which is the ESP electric motor of any of the eighth to the twelfth embodiment, wherein the drive shaft defines a bore that is concentric with the longitudinal axis of the drive shaft. 
     A fourteenth embodiment, which is the ESP electric motor of any of the eighth to the thirteenth embodiment, wherein the groove is between about 1/16 inch wide and about % inch wide. 
     A fifteenth embodiment, which is a method of lifting reservoir fluid in a wellbore to a surface location, comprising assembling an electric submersible pump (ESP) assembly at a wellsite, wherein the ESP assembly comprises a production pump and an electric motor, where at least one of an inside surface of a stator of the electric motor, an outside surface of a rotor of the electric motor, an inside surface of a rotor of the electric motor, or an outside surface of a drive shaft of the electric motor defines at least one groove and where the drive shaft of the electric motor is coupled to a drive shaft of the production pump; coupling the ESP assembly to a production tubing string; running the ESP assembly into the wellbore at the lower end of the production tubing string; providing electric power to the electric motor; turning the drive shaft of the electric motor by the rotor of the electric motor; turning the drive shaft of the production pump by the drive shaft of the electric motor; lifting reservoir fluid in the wellbore by the production pump up an interior of the production tubing to the surface; and circulating oil within the electric motor via the at least one groove defined by the inside surface of the stator of the electric motor, the outside surface of the rotor of the electric motor, the inside surface of the rotor of the electric motor, or the outside surface of the drive shaft of the electric motor. 
     A sixteenth embodiment, which is the method of the fifteenth embodiment, wherein the ESP assembly further comprises a gas separator assembly between the electric motor and the production pump. 
     A seventeenth embodiment, which is the method of the the sixteenth embodiment, wherein the ESP assembly further comprises a charge pump assembly between the electric motor and the gas separator assembly. 
     An eighteenth embodiment, which is the method of any of the fifteenth through seventeenth embodiment, wherein circulating the oil within the electric motor comprises circulating the oil via a bore in the drive shaft of the electric motor that is concentric with a longitudinal axis of the drive shaft of the electric motor. 
     A nineteenth embodiment, which is the method of any of the fifteenth through eighteenth embodiment, wherein the electric motor comprises a bushing retained by the stator of the electric motor and a bearing coupled to the drive shaft of the electric motor and located within the bushing, and wherein circulating the oil within the electric motor comprises circulating the oil via a plurality of passageways defined by the bushing. 
     A twentieth embodiment, which is the method of any of the fifteenth through the nineteenth embodiment, wherein the ESP assembly further comprises a seal section between the electric motor and the production pump, wherein the seal section comprises a seal section drive shaft, the drive shaft of the electric motor is coupled to the drive shaft of the seal section, and the drive shaft of the seal section is coupled to the drive shaft of the production pump. 
     A twenty-first embodiment, which is an electric submersible pump (ESP) electric motor, comprising a motor housing; a stator retained within the motor housing; a drive shaft; and an at least one rotor mechanically coupled to the drive shaft and located concentric with and inside of the stator, wherein an inside surface of the stator defines a groove extending from an upper end to a lower end of the stator. 
     A twenty-second embodiment, which is an electric submersible pump (ESP) electric motor, comprising a motor housing; a stator retained within the motor housing; a drive shaft; and an at least one rotor mechanically coupled to the drive shaft and located concentric with and inside of the stator, wherein an outside surface of the at least one rotor defines a groove extending from an upper end to a lower end of the at least one rotor. 
     A twenty-third embodiment, which is an electric submersible pump (ESP) electric motor, comprising a motor housing; a stator retained within the motor housing; a drive shaft; and an at least one rotor mechanically coupled to the drive shaft and located concentric with and inside of the stator, wherein an inside surface of the at least one rotor defines a groove extending from an upper end to a lower end of the at least one rotor. 
     A twenty-fourth embodiment, which is an electric submersible pump (ESP) electric motor, comprising a motor housing; a stator retained within the motor housing; a drive shaft; and an at least one rotor mechanically coupled to the drive shaft and located concentric with and inside of the stator, wherein an outside surface of the drive shaft defines a groove extending from an upper end to a lower end of the at least one rotor. 
     While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods may be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted or not implemented. 
     Also, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component, whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.