Contra rotating electric submersible pumping systems and methods

Abstract

A technique facilitates utilization of an electric submersible pumping system able to provide a desired production output with a reduced length. The electric submersible pumping system is sized for deployment in a borehole, e.g. within a borehole casing, and utilizes contra rotating impellers mounted on contra rotating shafts. To enable the contra rotation, the shafts may comprise a first shaft rotatably disposed within a hollow interior of a second shaft. This type of construction allows operation of the contra rotating impellers so as to enhance production without the length of sequential, traditional impellers.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of U.S. Provisional Application No. 63 / 497,220, filed Apr. 20, 2023, the entirety of which is incorporated by reference herein and should be considered part of this specification.BACKGROUND

[0002] In many oil and gas well applications, a wellbore is drilled into a desired subterranean formation. The wellbore is then completed for production of hydrocarbon fluids, such as oil and / or gas. Depending on the well, artificial lift may be required to move the fluid from the wellbore to the surface. By way of example, electric submersible pumping systems have been used for onshore oil wells to provide artificial lift, namely pumping of the fluid to the surface or to another desired collection location. The artificial lift can be used to get dead wells to flow or to otherwise increase production.

[0003] When operating electric submersible pumping systems, throughput is generally increased with increased diameter. However, wellbores typically are lined with casing which generally has a diameter in a range from 4.5 inches to 13.625 inches. Because the electric submersible pumping system is disposed within such casing, it must be sized to fit within this very limited diameter space. As a result, current electric submersible pumping systems tend to be constructed with pumps having a large number of pump stages which are operated via relatively long pump shafts. However, the long pump shafts limit the power that can be applied due to torsional strength and vibration, thus potentially limiting production output.SUMMARY

[0004] In general, a system and methodology provide an electric submersible pumping system able to pump fluid at a desired production output with a reduced pump length. The electric submersible pumping system is sized for deployment in a borehole, e.g. within a borehole casing, and utilizes contra rotating impellers mounted on contra rotating shafts. To enable the contra rotation, the shafts may comprise a first shaft rotatably disposed within a hollow interior of a second shaft. This type of construction enables operation of the contra rotating impellers so as to enhance production without the length of sequential, traditional impellers.

[0005] However, many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims.BRIEF DESCRIPTION OF THE FIGURES

[0006] Certain embodiments of the disclosure will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements. It should be understood, however, that the accompanying figures illustrate the various implementations described herein and are not meant to limit the scope of various technologies described herein, and:

[0007] FIG. 1 is an illustration of an example of an electric submersible pumping system deployed in a borehole, according to an embodiment of the disclosure;

[0008] FIG. 2 is a schematic cross-sectional illustration of at least a portion of the electric submersible pumping system illustrated in FIG. 1 in which a submersible pump utilizes contra rotating impellers, according to an embodiment of the disclosure;

[0009] FIG. 3 is a schematic cross-sectional illustration of another example of a submersible pump having contra rotating impellers, according to an embodiment of the disclosure;

[0010] FIG. 4 is a schematic cross-sectional illustration of another example of a submersible pump having contra rotating impellers, according to an embodiment of the disclosure;

[0011] FIG. 5 is a cross-sectional illustration of a portion of a submersible pump having contra rotating impellers, according to an embodiment of the disclosure; and

[0012] FIG. 6 is a cross-sectional illustration of another portion of a submersible pump having contra rotating impellers, according to an embodiment of the disclosure.DETAILED DESCRIPTION

[0013] In the following description, numerous details are set forth to provide an understanding of some embodiments of the present disclosure. It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. Specific examples of components and arrangements are described below to simplify the disclosure. These are, of course, merely examples and are not intended to be limiting. However, it will be understood by those of ordinary skill in the art that the system and / or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments are possible. This description is not to be taken in a limiting sense, but rather made merely for the purpose of describing general principles of the implementations. The scope of the described implementations should be ascertained with reference to the issued claims.

[0014] In the following description, numerous details are set forth to provide an understanding of some embodiments of the present disclosure. However, it will be understood by those of ordinary skill in the art that the system and / or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.

[0015] The disclosure herein generally involves a system and methodology for providing an improved electric submersible pumping system which is able to generate a desired production output with a reduced length. The electric submersible pumping system is sized for deployment in a borehole, e.g. within a borehole casing. For example, the electric submersible pumping system may be used in a wellbore for pumping desired well fluids, such as oil and oil-based liquids or wet gas (i.e., a fluid that is at least approximately 90% gas and approximately 10% or less liquid). In some applications the electric submersible pumping system may be constructed for use in compression applications, e.g. artificial lift of gas, although the overall construction facilitates use in a constrained space, e.g. a borehole, from which liquids or primarily liquid based fluids are pumped.

[0016] According to an embodiment, the electric submersible pumping system utilizes contra rotating impellers mounted on contra rotating shafts. To enable the contra rotation, the shafts may comprise a first shaft rotatably disposed within a hollow interior of a second shaft. This type of construction enables operation of the contra rotating impellers so as to enhance production without the length of sequential, traditional impellers. At least one motor is used to drive the contra rotating shafts. For example, a first motor may be coupled to the first shaft and a second motor may be coupled to the second shaft to independently drive the shafts in opposite / contra rotating directions.

[0017] Depending on the parameters of an anticipated use, the electric submersible pumping system may be constructed with both motors on the same side of the impeller section to allow simplified, more compact assembly and a simplified routing of power cables down to the motors. In some embodiments, the motor driving the outer shaft may have a hollow motor shaft while the other motor shaft is rotatable in an opposite direction within the hollow motor shaft. In some embodiments, the motors may comprise induction motors or permanent magnet motors, while other embodiments may utilize other types of motors, e.g. stator-less, double rotor motors to even further reduce the size and improve efficiency.

[0018] The contra rotating capability may be facilitated through the use of various bearings. For example, a contra rotating bearing may be located on a non-drive end of the submersible pump. Sometimes, further lateral support may be provided by a second contra rotating bearing located on the drive end of the submersible pump. Additionally, axially firm connections may be formed between the motor shafts and the corresponding pump shafts to reduce the number of thrust bearings that might otherwise be needed. One example of an axially firm connection is a connection formed via a through diaphragm coupling.

[0019] The submersible pump portion may be contained in various external pump housings of suitable diameters for a given borehole. The submersible motors also may be contained within the same external pump housing or within their own dedicated motor housing which is then coupled with the external pump housing. The sizing of the external housings, e.g. external pump housing, is selected so the diameter of the housing fits within a corresponding wellbore casing, e.g. within a 7 inch wellbore casing. However, the overall size of the electric submersible pumping system may be scaled to larger casings to improve head and flow capacity.

[0020] Additionally, the electric submersible pumping system may utilize thrust balancing to allow high differential pressures and a simplified barrier fluid pressure regulation. It should be noted the electric submersible pumping system may be constructed to serve as a generator when boosting, i.e. pumping, fluids is not necessary and the differential pressure from the well should be controlled. Flow through the impellers may be used to spin the impellers and thus drive the motors so as to generate electricity. As such, the system may work as a substitute for, or in addition to, a well choke during steady-state operation.

[0021] The contra rotating electric submersible pumping system also may be utilized in various other industrial, onshore applications because of its ability to provide high pump capacity in a small footprint. For some of these onshore applications, it may be less important to have both motors on the same side and this allows placement of at least one motor on each side of the impeller section. Additionally, the electric submersible pumping system works well in vertical borehole applications, but it can potentially be used in horizontal arrangements by constructing appropriately oriented well fluid inlets. In some embodiments, directing the flow of well fluid through the external pump housing in an upward direction can be beneficial in that it allows for an unchanged thrust direction between standstill weight support and operation.

[0022] Referring generally to FIG. 1, an example of an electric submersible pumping system 30 as illustrated is deployed in a borehole 32, e.g. a wellbore. The borehole 32 may be drilled down into a hydrocarbon bearing formation 34 from a surface 36, e.g. a land surface. The wellbore 32 (or other type of borehole) may be lined with a suitable casing 38. A wellhead 40 and / or other surface well equipment may be positioned at surface 36 above borehole 32.

[0023] The electric submersible pumping system 30 may comprise a submersible pump section 42 having one or more pump stages 44 enclosed in an external pump housing 46. In the example illustrated, the submersible pump section 42 comprises a plurality of pump stages 44, e.g. at least 10 pump stages and sometimes more than 20 pump stages, of contra rotating impellers as described in greater detail below.

[0024] The illustrated electric submersible pumping system 30 further comprises a submersible motor section 48 having at least one motor 50, e.g. a pair of motors 50, contained within an external motor housing 52. It should be noted the external motor housing 52 may be part of external pump housing 46 or it may be a separable housing secured to the external pump housing 46. Depending on the parameters of the pumping operation and / or environment, electric submersible pumping system 30 may include various other pumping system equipment 54 located above, below, or above and below the submersible pump section 42. Examples of pumping system equipment 54 include fluid intakes, gauges, sensor systems, gas separators, packers, and / or other equipment useful for given operation.

[0025] The electric submersible pumping system 30 is deployed downhole in borehole 32 via a suitable conveyance 56. By way of example, conveyance 56 may comprise production tubing, coiled tubing, cable, or other suitable conveyances 56 selected according to the particular pumping operation.

[0026] Furthermore, the electric submersible pumping system 30 may be constructed so as to pump well fluid to the interior of conveyance 56, e.g. to the interior of coiled tubing, for production to a surface collection area. In other applications, however, the well fluid may be pumped up along an annulus 58 disposed between conveyance 56 and the surrounding casing 38. In this latter configuration, the system equipment 54 may comprise a suitable packer or other device which prevents the pumped well fluid from moving downwardly below the electric submersible pumping system 30.

[0027] Referring generally to FIG. 2, an embodiment of one type of electric submersible pumping system 30 is illustrated. In this example, the electric submersible pumping system 30 comprises an impeller section 60 included within submersible pump section 42. The impeller section 60 may be separated into a plurality of the pump stages 44 having, for example, pairs of a first impeller 62 and a second impeller 64.

[0028] Each of the first impellers 62 is coupled with a first shaft 66 and each of the second impellers 64 is coupled with a second shaft 68. During operation, the second shaft 68 and thus the second impellers 64 may be rotated in a contra rotating direction (opposite direction) relative to the first shaft 66 and thus the first impellers 62. In this example, the first shaft 66 is rotatable within a hollow interior 70 of the second shaft 68 to facilitate the contra rotation of impellers 62, 64. The impeller section 60 is located within external pump housing 46.

[0029] In the embodiment illustrated, the shafts 66, 68 are rotated by corresponding motors 50. For example, one of the motors 50 may comprise a first motor shaft 72 engaged with the first shaft 66 so as to rotate the first impellers 62 in one direction. Similarly, the other motor 50 may comprise a second motor shaft 74 engaged with the second shaft 68 so as to rotate the second impellers 64 in a contra rotating direction relative to the first impellers 62. By way of example, the first motor shaft 72 may extend through a hollow interior 76 of the second motor shaft 74. This allows both motors 50 to be located at the same end of impeller section 60 and external pump housing 46. In this particular example, both motors 50 are located at an upper end of the impeller section 60. Placement of motors 50 at the upper end of impeller section 60 can be beneficial by achieving tensile loading of motor shafts 72, 74 when differential pressure is created during operation of impeller section 60.

[0030] In FIG. 3, another embodiment of electric submersible pumping system 30 as illustrated in which both motors 50 are located at a lower end of the impeller section 60. This latter example may be advantageous in certain types of wellbores 32 and pumping applications by, for example, providing for more efficient motor cooling. If space is less of an issue, some embodiments may utilize one or more motors 50 located at each end of the impeller section 60, as illustrated in FIG. 4. It should be noted that upper and lower generally refers to uphole and downhole directions, respectively, because the electric submersible pumping systems 30 may be adapted for use in deviated, e.g. horizontal, wellbores.

[0031] The use of the contra rotating first impellers 62 and second impellers 64 enables a desired fluid pumping capability with a much shorter submersible pump section 42 as compared to conventional systems with sequential impellers rotated in a common direction. As illustrated in the Figures, e.g. FIG. 2, the submersible pump section 42 may comprise a fluid inlet 78 by which well fluid is drawn in from annulus 58 to enable pumping of that well fluid linearly along impeller section 60 within external pump housing 46.

[0032] The well fluid is pumped to a fluid outlet or discharge 80 which, in this example, directs the well fluid back into the annulus 58. As described above, a packer or other suitable device would be used along annulus 58 between the inlet 78 and the outlet 80 to ensure production of the well fluid upwardly along the annulus 58. It should be noted, however, the path of produced well fluid may vary and may be routed internally through the electric submersible pumping system 30. Above electric submersible pumping system 30, the well fluid may be produced along annulus 58 or it may be produced in whole or in part along the interior of conveyance 56.

[0033] Electric power may be delivered downhole to electric submersible pumping system 30 via a variety of power cables 82. By way of example, the power cable 82 may be routed down along an exterior of conveyance 56 and coupled with the motors 50 via suitable electric leads 84. Placement of motors 50 at an upper end of impeller section 60 can simplify the routing of power cable 82. In other examples, the power cable 82 may be routed internally within conveyance 56 or within a wall of conveyance 56.

[0034] Additionally, various support systems 86 and seal systems 88 may be employed along the pump shafts 66, 68 and motor shafts 72, 74. By way of example, support system 86 may be constructed to provide thrust support and lateral support. As illustrated, the support system 86 may comprise a pair of lower bearings 90, e.g. thrust bearings, mounted to each pump shaft 66, 68 so as to provide a suitable bearing for both the straight rotating and contra rotating shafts 66, 68. The support system 86 also may utilize sleeves 92 positioned along specific shafts. For example, sleeves 92 may be positioned along the external, contra rotating second pump shaft 68 and second motor shaft 74. The internal shafts 66, 72 may be sized to receive lateral support via their surrounding contra rotating shafts 68, 74.

[0035] The seal system 88 may comprise multiple seals 94 positioned at selected locations along the various shafts. For example, seals 94 may be positioned above and below the impeller section 60 so as to provide suitable protection against leakage of well fluid from the impeller section 60. The seals 94 may be used on, for example, both pump shafts 66 and 68.

[0036] The motors 50, impeller section 60, shafts 66, 68, 72, 74, and other components of electric submersible pumping system 30 may be constructed according to certain parameters, e.g. operation within a 7 inch casing / envelope. Other examples of operational parameters may include torque ratings up to 2000 NM for each shaft and speed ratings having a maximum continuous rotational speed of 4500-6000 RPMs. Max fluid temperature ratings may vary depending on, for example, the operational environment and may be set at 120° C., 250° C., or other required heat ratings. Nominal combined motor power may be 2.0-2.5 MW, but various applications may be able to utilize lower powered motors. An example of a differential pressure target is 60-100 bar and a head target is 1000 m, and these targets may be achieved via selection of a suitable number of stages 44, e.g 20 or more stages. Flow may be in the 100-500 Am3 / h range with an overall life goal of the electric submersible pumping system 30 set at five years. However, these are simply examples of operational parameters / goals and these parameters / goals may vary substantially for different electric submersible pumping systems 30 constructed for use in different types of jobs and environments.

[0037] According to other aspects of the electric submersible pumping system 30, the motors 50 and adjoining mechanical sections may be submerged in barrier fluid to improve equipment lifespan. Various types of mechanical seals 94 may be used to separate process fluid, e.g. well fluid, and barrier fluid. The mechanical seals 94 may be located at the drive end (the motor end) as illustrated in FIG. 5. In some embodiments, mechanical seals 94 also may be located at the non-drive end as illustrated in FIG. 6. Cooling of the motors 50 may be achieved at least in part via heat transfer to the process / well fluid.

[0038] The wall thickness and the diameter of the various shafts 66, 68, 72, 74 may be selected to transmit desired torque with some margin. As discussed above, an example of a suitable torque rating for various applications is 2000 NM. The outside diameter of the inner shafts 66, 72 may be increased so as to utilize tighter clearances to provide radial support via the surrounding hollow shafts 68, 74.

[0039] Bearings 90 and sleeves 92 also may be selected and positioned so as to provide substantial radial support. For example, contra rotating bearings 90 may be positioned at the drive end and the non-drive end (see FIGS. 5 and 6). Additionally, thrust support on each shaft may be combined for the motor section 48 and impeller section 60 to reduce the number of thrust bearings that would otherwise be needed.

[0040] Thrust support may be enhanced by establishing an axially fixed connection between the motor shafts 72, 74 and the impeller shafts 66, 68. The axially fixed connection may be established by, for example, a diaphragm coupling, a hydro-mechanical coupling, or other suitable fixed couplings. Thrust balancing may be utilized to reduce thrust loading and to ensure barrier fluid pressure is related to either suction pressure or discharge pressure of the pumping system.

[0041] Various flow routes may be established for the barrier fluid. For example, a barrier fluid path may be created between the drive end and the non-drive end via a channel in the external pump housing 46. Another potential barrier fluid path may be established via a passage along the center of the inner shafts 66, 72.

[0042] In embodiments where the static height of the barrier fluid creates a very high pressure for the mechanical seals, a downhole pressure regulation system may be employed either downhole or at the wellhead 40. Sometimes barrier fluid leakage may occur across the mechanical seals 94, but new barrier fluid may be supplied continuously to the electric submersible pumping system 30 via a suitable supply line. The supply line may be routed down along the conveyance 56 via a hose, coiled tubing, or a long an interior of production tubing. In some embodiments, the power cable 82 may be routed downhole within the barrier fluid supply line.

[0043] Because the contra rotating construction enables a substantial reduction in the length of the pump section 42, e.g. a 50% reduction in length, torsional issues, such as torsional resonance, are substantially reduced. The size, thickness, and construction of the various shafts also may be adjusted to counter such torsional issues while enabling application of a maximum desired torque.

[0044] Furthermore, various types of motors 50 may be utilized in powering the impeller section 60. For example, the motors 50 may be constructed as induction motors; permanent magnet motors utilizing permanent magnet rotors; or stator-less, double rotor motors. Various features of the motors also may be adjusted to enhance operation in the downhole environment and in the relatively small envelope. For example, induction motors may be constructed with an increased number of poles to facilitate reduced stator diameter.

[0045] Depending on the specific well operation and well equipment employed, the electric submersible pumping system 30 may be used with many other types of well equipment and systems. Additionally, the size, components, materials, bearing systems, sealing systems, and other features of the submersible pump section 42 and submersible motor section 48 may vary. Similarly, the shaft size as well as the shaft configurations may change while still achieving the desired contra rotation. The configuration of the impellers 62, 64 also may be selected according to the type of fluid being produced, flow requirements, depth at which the electric submersible pumping system 30 is operated, and various other parameters affecting fluid production.

[0046] Although a few embodiments of the disclosure 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 disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims.

[0047] As used herein, a range that includes the term between is intended to include the upper and lower limits of the range; e.g., between 50 and 150 includes both 50 and 150. Additionally, the term “approximately” includes all values within 5% of the target value; e.g., approximately 100 includes all values from 95 to 105, including 95 and 105. Further, approximately between includes all values within 5% of the target value for both the upper and lower limits; e.g., approximately between 50 and 150 includes all values from 47.5 to 157.5, including 47.5 and 157.5.

[0048] As used herein, the terms “connect”, “connection”, “connected”, “in connection with”, and “connecting” are used to mean “in direct connection with” or “in connection with via one or more elements”; and the term “set” is used to mean “one element” or “more than one element”. Further, the terms “couple”, “coupling”, “coupled”, “coupled together”, and “coupled with” are used to mean “directly coupled together” or “coupled together via one or more elements”. As used herein, the terms “up” and “down”; “upper” and “lower”; “top” and “bottom”; and other like terms indicating relative positions to a given point or element are utilized to more clearly describe some elements. Commonly, these terms relate to a reference point at the surface from which drilling operations are initiated as being the top point and the total depth being the lowest point, wherein the well (e.g., wellbore, borehole) is vertical, horizontal or slanted relative to the surface.

[0049] Language of degree used herein, such as the terms “approximately,”“about,”“generally,” and “substantially” as used herein represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms “approximately,”“about,”“generally,” and “substantially” may refer to an amount that is within less than 10% of, within less than 5% of, within less than 1% of, within less than 0.1% of, and / or within less than 0.01% of the stated amount. As another example, in certain embodiments, the terms “generally parallel” and “substantially parallel” or “generally perpendicular” and “substantially perpendicular” refer to a value, amount, or characteristic that departs from exactly parallel or perpendicular, respectively, by less than or equal to 15 degrees, 10 degrees, 5 degrees, 3 degrees, 1 degree, or 0.1 degree.

[0050] Although a few embodiments of the disclosure 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 disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims. It is also contemplated that various combinations or sub-combinations of the specific features and aspects of the embodiments described may be made and still fall within the scope of the disclosure. It should be understood that various features and aspects of the disclosed embodiments can be combined with, or substituted for, one another in order to form varying modes of the embodiments of the disclosure. Thus, it is intended that the scope of the disclosure herein should not be limited by the particular embodiments described above.

Claims

1. A system for pumping well fluids, comprising:an electric submersible pumping system having:an external pump housing;a pair of contra rotating shafts located within the external pump housing and comprising a first shaft rotatable within a hollow interior of a second shaft;an impeller section comprising:a first impeller mounted to the first shaft; anda second impeller mounted to the second shaft, the first impeller and the second impeller being rotatable in contra directions to move a well fluid through the external pump housing for production to a desired location;a first motor driving the first shaft; anda second motor driving the second shaft.

2. The system as recited in claim 1, wherein the first motor and the second motor are both on the same end of the impeller section.

3. The system as recited in claim 2, wherein the same end is the uphole end.

4. The system as recited in claim 2, wherein the same end is the downhole end.

5. The system as recited in claim 1, wherein the first motor and the second motor are both on opposite ends of the impeller section.

6. The system as recited in claim 1, wherein the impeller section comprises a plurality of pump stages.

7. The system as recited in claim 1, wherein the first motor comprises a first motor shaft being in an axially firm connection with the first shaft and the second motor comprises a second motor shaft being in an axially firm connection with the second shaft.

8. The system as recited in claim 1, wherein the well fluid comprises wet gas.

9. The system as recited in claim 1, wherein the first motor and the second motor are induction motors.

10. The system as recited in claim 1, wherein the first motor and the second motor our stator-less double rotor motors.

11. A system, comprising:an electric submersible pumping system sized for deployment in a borehole, the electric submersible pumping system having an external pump housing containing contra rotating impellers mounted on contra rotating shafts, the contra rotating shafts comprising a first shaft and a second shaft, the first shaft being rotatably disposed within a hollow interior of the second shaft.

12. The system as recited in claim 11, wherein the first shaft is coupled with and driven by a first motor.

13. The system as recited in claim 12, wherein the second shaft is coupled with and driven by a second motor.

14. The system as recited in claim 13, wherein the first motor and the second motor are positioned at the same end of the first and second shafts.

15. The system as recited in claim 11, wherein the contra rotating impellers comprise multiple impellers arranged in at least 10 pump stages.

16. The system as recited in claim 11, wherein the well fluid comprises wet gas.

17. A method for pumping fluid from a cased wellbore, comprising:providing an electric submersible pumping system sized for deployment in a borehole, the electric submersible pumping system having an external pump housing containing contra rotating impellers mounted on contra rotating shafts, the contra rotating shafts comprising a first shaft and a second shaft, the first shaft being rotatably disposed within a hollow interior of the second shaft;deploying the electric submersible pumping system downhole in the cased wellbore; andusing at least one motor to rotate the first shaft and the second shaft in a contra rotating direction so as to pump well fluid comprising wet gas along the interior of the external pump housing via the contra rotating impellers.

18. The method as recited in claim 17, wherein using comprises using two motors with a first motor of the two motors connected to the first shaft and a second motor of the two motors connected to the second shaft.

19. The method as recited in claim 18, further comprising positioning the two motors at an uphole end of the external pump housing.

20. The method as recited in claim 18, further comprising positioning the two motors at a downhole end of the external pump housing

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