GB2549751A

GB2549751A - Method of pumping a well with dual alternate submersible pumps

Info

Publication number: GB2549751A
Application number: GB1607333.0A
Authority: GB
Inventors: H Chang Tina; F Lawson Peter
Original assignee: Baker Hughes Inc
Current assignee: Baker Hughes Holdings LLC
Priority date: 2016-04-27
Filing date: 2016-04-27
Publication date: 2017-11-01

Abstract

An electrical submersible pump (ESP) for producing an oil well comprising an upper motor 127, connected by a cable 129 running uphole through tubing 99 and a lower motor 107 connected to a wet mate connector 101, which is connected to cable 103. At least one pump 113 may be driven by either motor independently of each other. Two pumps may be provided and each connected to one of the motors. A packer 117 may seal the tubing to form an upper and lower annulus, and a valve may permit flow from each pump to bypass the other pump. The system may provide redundancy in case of a faulty pump, motor or power cable.

Description

METHOD OF PUMPING A WELL WITH DUAL ALTERNATE

SUBMERSIBLE PUMPS

Field of the Disclosure

This disclosure relates in general to submersible well pump assemblies and in particular to two pump assembly motors installed within production tubing in a well, one powered by a power cable extending along the exterior of the tubing to a wet mate connection, and the other powered by a power cable lowered through the tubing to the upper end of the motor.

Background

Many hydrocarbon wells are produced by electrical submersible well pump assemblies (ESP). A typical ESP includes a centrifugal pump having a large number of stages, each stage having an impeller and a diffuser. An electrical motor couples to the pump for rotating the impellers. A pressure equalizer or seal section connects to the motor to reduce a pressure differential between lubricant in the motor and the hydrostatic pressure of the well fluid. Usually, the ESP is suspended on a string of tubing within the well. While operating, the pump discharges well fluid up the string of tubing.

Failures occur over a period of time, requiring that the ESP be pulled from the well for repair or replacement. While the mechanical components of the pump can fail, often the failure is the result of an electrical problem, either in the motor or the power cable. Pulling an ESP from a well and replacing the ESP is time consuming and expensive.

Redundant systems are known. For example, two ESPs may be suspended on a Y-shaped connector at the lower end of the string of tubing. However, the well casing must have a large enough diameter to install two ESPs laterally offset from each other.

Summary A method of producing a well includes providing an electrical submersible pump assembly (ESP) having at least one pump, an upper motor and a lower motor, the lower motor having a lower motor electrical wet mate connection. A string of tubing is installed in the well. The tubing has a tubing electrical wet mate connection and a lower motor power cable extending from the tubing wet mate connection alongside the tubing to a wellhead. An upper motor power cable connected to the upper motor is deployed while running the ESP assembly into the tubing. The lower motor wet mate connection electrically engages the tubing wet mate connection. Power may be supplied through the lower motor power cable and the wet mate connections to the lower motor while not supplying power through the upper motor power cable. Also, power may be supplied through the upper motor power cable to the upper motor while not supplying power through the lower ESP power cable.

In one embodiment, the ESP assembly includes an upper pump operatively connected to the upper motor and a lower pump operatively connected to the lower motor. A valve connects the upper pump to the lower pump. An annular packoff seals the valve to the tubing, defining an upper annulus separate from a lower annulus. While the lower pump is operating, the lower pump discharges well fluid from the lower annulus through the valve into the upper annulus, which flows around the upper pump and the upper motor. While the upper pump is operating, the upper pump draws well fluid from the lower annulus through the valve into an intake of the upper pump and discharges well fluid from the upper pump into the upper annulus.

In another embodiment, the upper motor is operatively connected to an upper end of the pump. The lower motor is operatively connected to a lower end of the pump. While power is supplied to the lower motor, the lower motor drives the pump while the upper motor remains dormant. While power is supplied to the upper motor, the upper motor drives the pump while the lower motor remains dormant.

Preferably, an upper disengaging device disengages a shaft of the upper motor from the pump while driving the pump with the lower motor. A lower disengaging device disengages a shaft of the lower motor from the pump while driving the pump with the upper motor.

Brief Description of the Drawings

So that the manner in which the features, advantages and objects of the disclosure, as well as others which will become apparent, are attained and can be understood in more detail, more particular description of the disclosure briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only a preferred embodiment of the disclosure and is therefore not to be considered limiting of its scope as the disclosure may admit to other equally effective embodiments.

Figures 1A and 1B comprise a schematic side view of a dual ESP assembly in accordance with this disclosure and shown with the lower ESP in operation;

Figure 2 is a schematic side view of the dual ESP assembly of Figures 1A and 1B shown with the upper ESP in operation;

Figure 3 is a sectional view of the valve of the dual ESP assembly of Figures 1A and 1B, shown in a first position;

Figure 4 is a sectional view of the valve of the dual ESP assembly of Figures 1A and 1B, shown in a second position; and

Figure 5 is a schematic side view of an alternate embodiment of an ESP assembly having two motors.

Detailed Description of the Disclosure

The methods and systems of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments are shown. The methods and systems of the present disclosure may be in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey its scope to those skilled in the art. Like numbers refer to like elements throughout.

It is to be further understood that the scope of the present disclosure is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation.

Referring to Figures 1A and 1B, a well 11 has a casing 13 cemented within. Casing 13 has perforations or other openings 15 to admit well fluid into well 11. A wellhead assembly or production tree (not shown) locates at the upper end of casing 13. The wellhead assembly supports a string of production tubing 17 extending into well 11. Well fluid flows through production tubing 17 to the wellhead. Production tubing 17 may comprise sections of pipe with threaded ends secured together, or it could comprise continuous coiled tubing. A lower section or sub of tubing 17 has an electrical wet mate connection 21 with mounted to it. Tubing wet mate connection 21 may have a variety of configurations, and in this example, comprises a pedestal or other structure having electrical contacts 19 and mounted within the interior of tubing 17. Electrical contacts 19 join an external power cable 23 that extends along the exterior of tubing 17 to the wellhead. Alternately, wet mate connection 21 could have electrical contacts 19 that are moved radially inward when in an engaged position.

Tubing 17 supports a dual electrical submersible pump assembly (ESP) 25, which includes a lower ESP 27 and an upper ESP 29. ESPs 27, 29 are secured together and have a single longitudinal axis 31. The terms “upper” and “lower” are used only for convenience as ESP 25 could be installed within a horizontal section of tubing 17. Lower ESP 27 has an electrical lower motor 33, normally a three-phase AC motor. Lower motor 33 has an electrical wet mate connection 35 that enters into electrical engagement with electrical contacts 19 of tubing wet mate connection 21 when dual ESP assembly 25 lands on tubing wet mate connection 21. External or lower ESP power cable 23 supplies power to operate lower motor 33.

Lower ESP 27 includes a protector, pressure equalizer, or lower seal section 39. In this example, seal section 29 secures to the upper end of lower motor 33. Lower motor 33 rotates a shaft assembly (not shown) that extends through lower seal section 39. Lower motor 33 and lower seal section 39 contain a motor lubricant, and lower seal section 39 has a movable element to reduce a pressure differential between the motor lubricant and the hydrostatic pressure of well fluid in well 11. The movable element may be, for example, a flexible bag or a metal bellows.

Lower ESP 27 includes a lower pump 41, preferably a rotary pump such as a centrifugal pump having a large number of stages, each stage comprising an impeller and a diffuser. Lower pump 41 has an intake 43 on a lower end and a discharge 45 on an upper end. Lower seal section 39 connects to lower pump intake 43. If a gas separator (not shown) is employed, the gas separator would connect to the lower end of lower pump 41, and pump intake 43 would be at the lower end of the gas separator. Other types of pumps rather than centrifugal pumps could be used for lower pump 41.

Upper ESP 29 includes an upper motor 47, which also may be a three-phase electrical motor. An internal power cable 49 secures to the upper end of upper motor 47. Internal power cable 49 may be installed closely within a string of coiled tubing 51 for supporting the weight of internal power cable 49 and dual ESP assembly 25. Internal power cable 49 supplies electrical power to operate upper motor 47.

Upper ESP 29 includes a protector, pressure equalizer, or upper seal section 53. In this example, upper seal section 53 secures to the lower end of upper motor 47. Upper motor 47 rotates a shaft assembly (not shown) that extends through upper seal section 53. Upper motor 47 and upper seal section 53 contain a motor lubricant, and upper seal section 53 has a movable element to reduce a pressure differential between the motor lubricant and the hydrostatic pressure of well fluid in well 11. The movable element may be, for example, a flexible bag or a metal bellows.

Upper seal section 53 connects to a discharge 55 on an upper end of an upper pump 57. Upper pump 57 may be the same type of pump as lower pump 41 or it may differ. Upper pump 57 has an intake 58 on a lower end. If a gas separator (not shown) is employed for upper ESP 29, the gas separator would connect to the lower end of upper pump 57, and pump intake 58 would be at the lower end of the gas separator.

In this embodiment, a valve 59 connects lower ESP 27 to upper ESP 29.

The shaft assembly of lower ESP 27 terminates at the upper end of lower pump 41 and does not extend into valve 59. The shaft assembly of upper ESP 29 terminates at the lower end of upper pump 57 and does not extend into valve 59. Thus lower motor 33 drives only lower pump 41, and upper motor 57 drives only upper pump 57.

An annular packer, packoff or seal element 61 seals between valve 59 and the inner wall surface of tubing 17. Packoff 61 could be a cup-shaped elastomeric member that slides down the inner wall surface of tubing 17 while dual ESP 25 is being installed. Alternatively, packoff 61 could be a type that is energized in various ways to provide an annulus seal after dual ESP 25 has landed on tubing wet mate connection 21. Packoff 61 defines and separates a lower annulus 63 in tubing 17 from an upper annulus 65.

As will be explained in more detail below, valve 59 has a lower pump inlet port 67 that is isolated from lower annulus 63. Lower pump inlet port 67 is in fluid communication with lower pump discharge 45. Valve 59 has a lower annulus inlet port 69 that is in fluid communication with lower annulus 63 and isolated from lower pump inlet port 67. Lower pump inlet port 67 and lower annulus inlet port 69 are located below packoff 61. Valve 59 also has an upper pump outlet port 71 that is in fluid communication with upper pump intake 58 and isolated from upper annulus 65. Upper pump outlet port 71 is in fluid communication with lower annulus inlet port 69. Valve 59 has an upper annulus outlet port 73 that is in fluid communication with upper annulus 65 and isolated from upper pump outlet port 71. Upper annulus outlet port 73 is in fluid communication with lower pump inlet port 67. Upper pump outlet port 71 and upper annulus outlet port 73 are located above packoff 61.

Valve 59 has a first position that creates an open flow path from lower pump inlet port 67 through upper annulus outlet port 73 and blocks flow from lower annulus inlet port 69 through upper pump outlet port 71. The arrows in Figures 1A and 1B show valve 59 in the first position. Valve 59 has a second position that creates an open flow path from lower annulus inlet port 69 through upper pump outlet port 71 and blocks flow from lower pump inlet port 67 through upper annulus outlet port 73. The arrows in Figure 2 show valve 59 in the second position. Valve 59 may shift between the first and second positions in various manners. In this embodiment, valve 59 is biased to the second position and shifts to the first position in response to fluid pressure caused by the discharge of lower pump 41.

In the operation of the embodiment of Figures 1A, 1B and 2, tubing 17 along with tubing wet mate connection 21 and lower motor power cable 23 will be installed first. The operator assembles upper ESP 29 to lower ESP 27 with valve 59 to make up dual ESP assembly 25. The operator lowers dual ESP assembly 25 into tubing 17 by reeling out coiled tubing 51 and lands lower motor wet mate connection 35 on tubing wet mate connection 21. Packoff 61 will seal to tubing 17. The operator has a choice of either operating lower ESP 27 first or operating upper ESP 29. Both ESPs 27, 29 are not intended to be operated at the same time.

Assuming that the operator chooses to operate lower ESP 27 first, power is supplied via lower motor power cable 23 to lower motor 33. No power is supplied over upper motor power cable 49 at this time. Lower motor 33 rotates the impellers of lower pump 41, causing well fluid to flow in lower pump intake 43 and be discharged out lower pump discharge 45, as indicated by the arrows in Figures 1A and 1B. The pressure caused by the discharge of lower pump 41 shifts valve 59 to the first position, which directs flow from lower pump discharge 45 through valve lower pump inlet port 67 and out valve upper annulus outlet port 73. The discharged well fluid flows up upper annulus 65 around upper ESP 29 to the wellhead. Upper ESP 29 remains dormant.

Then, at some time later, which could be years, trouble may occur with lower ESP 27, lower motor power cable 23, or the wet mate connections 21, 35. The operator ceases to apply power to lower motor power cable 23 and begins to apply power to upper motor power cable 49, as illustrated in Figure 2. Ceasing to operate lower ESP 27 causes valve 59 to shift to the second position. Well fluid from perforations 15 bypasses lower ESP 27 and flows into lower annulus inlet port 69. The well fluid flows from lower annulus inlet port 69 through valve upper pump outlet port 71 into upper pump intake 58. Upper pump 57 discharges the well fluid out upper pump discharge 55 into upper annulus 65. The well fluid flows around upper motor 47 up tubing 17 to the wellhead.

Figures 3 and 4 illustrate one type of valve 59, but other arrangements are feasible. A lower connector 75 connects valve 59 to lower pump 41 (Fig. 1B).

Lower connector 75 houses a lower valve seat 77, which is engaged by an axially movable valve element 79. Valve element 79 has a lower nose 80 on its lower end that seals to valve seat 77 while valve element 79 is in the lower or second position shown in Figure 4. A housing 81 encloses valve element 79 and has an enlarged inner diameter in a lower bypass area 83. When valve element 79 is in the upper or first position of Figure 3, lower nose 80 will be above seat 77, allowing fluid to flow upward around lower nose 80 and into lower pump inlet port 67, which is located within valve element 79. Valve element 79 has a hollow interior 85 that receives well fluid flowing in lower pump inlet port 67. Valve element 79 has openings 86 in its side wall that are positioned to block lower annulus inlet port 69 while valve element 79 is in the upper first position of Figure 3. Valve element openings 86 will register with upper annulus port 73 when valve element 79 is in the upper or first position of Figure 3. Well fluid flowing into valve element interior 85 flows out upper annulus outlet port 73. When valve element 79 is in the lower or second position of Figure 4, openings 86 register with lower annulus inlet port 69 to admit well fluid into valve element interior 85.

Valve 59 has an upper valve seat 87 in an upper connector 88, which connects to upper pump intake 58 (Fig. 1A). Valve element 79 has an upper nose 89 that seals to upper valve seat 87 while in the upper, first position of Figure 3. Valve housing 81 has an upper bypass area 91 located below upper seat 87 that is of a larger diameter that upper nose 89 to allow upward fluid flow past upper nose 89 while in the lower, second position of Figure 4. Upper pump outlet port 71 is located just below upper nose 89 to allow fluid flow from valve element interior 85 out and around upper nose 89 while valve element 79 is in the lower, second position.

In this embodiment, a coil spring 93 encircles valve element 79 and urges it upward to the second, upper position of Figure 4. Coil spring 93 bears against a shoulder 95 on valve element 79.

In the alternate embodiment of Figure 5, the well has a casing 97 and a string of tubing 99. As in the first embodiment, a tubing wet mate connector 101 mounts to a section or sub of tubing 99. An external power cable 103 joins tubing wet mate connector 101 and extends up alongside tubing 99 to the wellhead (not shown). ESP assembly 105 has a lower motor 107 with a lower motor wet mate connector 109 that makes electrical engagement with tubing wet mate connector 101. A lower seal section 111 connects the upper end of lower motor 107 for equalizing motor lubricant pressure with the hydrostatic pressure of the well fluid in the well. A pump 113 has an intake 115 that connects to the upper end of lower seal section 111. Pump 113 may also be a centrifugal pump as in the first embodiment. An annulus seal or packoff 117 is located above pump intake 115 and seals to the inner wall surface of tubing 99. Packoff 117 defines a lower annulus 119 and an upper annulus 121. Pump 113 has a discharge 123 that discharges into upper annulus 121 above packoff 117.

An upper seal section 125 connects to pump discharge 123. An upper motor 127 connects to the upper end of upper seal section 125. An internal power cable 129, which may be enclosed and supported in coiled tubing 131, secures to the upper end of upper motor 127 and extends up tubing 99 to the wellhead. Coiled tubing 131 supports the weight of ESP assembly 105 as it is lowered into tubing 99. Internal power cable 129 supplies electrical power to upper motor 127, but not to lower motor 107, which receives its power from external power cable 103.

Preferably, power is supplied only to one of the motors 107, 127 at one time.

Each motor 107, 127 is capable of driving pump 113. Lower motor 107 rotates a lower motor shaft 133 that extends through lower seal section 111 into engagement with the shaft (not shown) within pump 113. Lower motor shaft 133 is typically in separate sections within lower motor 107 and lower seal section 111 that are coupled by splined ends. Similarly, upper motor 127 rotates an upper motor shaft 135 that extends downward through upper seal section 125 into engagement with the shaft within pump 113.

Preferably, a lower clutch or disengaging device 137 will selectively uncouple lower motor shaft 133 from rotating engagement with the shaft in pump 113. Preferably, an upper clutch or disengaging device 139 will selectively uncouple upper motor shaft 135 from rotating engagement with the shaft in pump 113. Disengaging devices 137, 139 could be a variety of types that serve to avoid lower motor 107 from spinning upper motor shaft 135 while upper motor 127 is not being powered, and also to avoid upper motor 127 from spinning lower motor shaft 107 while lower motor 107 is not being powered. Disengaging devices 137, 139 could be electromechanical devices that when energized, either engage or disengage. Although shown at the upper end of lower motor 107, disengaging device 137 could be located at the lower end of pump 113. Similarly, disengaging device 139 could be located at the upper end of pump 113.

In the operation of the embodiment of Figure 5, tubing 99 along with tubing wet mate connector 101 and external power cable 103 will first be installed. The operator then lowers ESP 105 into tubing 99 by reeling out coiled tubing 131 until lower motor wet mate connector 109 engages tubing wet mate connector 101. The operator may operate either lower motor 107 or upper motor 127 first. If lower motor 107 is operated first, preferably, upper disengaging device 139 will place upper motor shaft 135 out of rotating engagement with the shaft in pump 113. The operator supplies power over external power cable 103 to operate lower motor 107, which drives pump 113. Pump 113 draws well fluid into pump intake 115 and discharges the well fluid out pump discharge 123 into upper annulus 121 for flow to the wellhead. Upper motor 127 remains in a back up or dormant state.

If lower motor 107, lower motor power cable 103, or the wet mate connectors 101, 109 fail, which could be years later, the operator will cause lower disengaging device 137 to disengage lower motor shaft 133 from rotating engagement with the shaft in pump 113. Upper disengaging device 139 will couple upper motor shaft 135 to the pump shaft to impart torque. The operator supplies power over upper motor power cable 129, causing upper motor 127 to drive pump 113. Lower motor 107 will remain in a dormant, non rotating position.

While the disclosure has been described in only a few of its forms, it should be apparent to those skilled in the art that various changes may be made.

Claims

1. A method of producing a well, comprising the following steps: (a) providing an electrical submersible pump assembly (ESP) having at least one pump, an upper motor and a lower motor, the lower motor having a lower motor electrical wet mate connection; (b) installing a string of tubing in a well with a tubing electrical wet mate connection at a selected depth and a lower motor power cable extending from the tubing wet mate connection alongside the tubing to a wellhead; (c) connecting an upper motor power cable to the upper motor and running the ESP assembly into the tubing, then engaging the lower motor wet mate connection with the tubing wet mate connection; (d) selectively supplying power through the lower motor power cable and the wet mate connections to the lower motor while not supplying power through the upper motor power cable; and (e) selectively supplying power through the upper motor power cable to the upper motor while not supplying power through the lower motor power cable.

2. The method according to claim 1, wherein: said at least one pump comprises an upper pump operatively connected to the upper motor and a lower pump operatively connected to the lower motor.

3. The method according to claim 1 or 2, wherein: said at least one pump comprises an upper pump operatively connected to a lower end of the upper motor and a lower pump operatively connected to an upper end of the lower motor.

4. The method according to claim 1, 2 or 3, wherein: the ESP assembly comprises: an upper pump connected below the upper motor; a lower pump connected above the lower motor; a valve connected between the upper pump and the lower pump; and an annular packoff that seals the valve to the tubing, defining an upper annulus separate from a lower annulus; step (d) comprises, with the lower pump discharging well fluid through the valve into the upper annulus, which flows around the upper pump and the upper motor; and step (e) comprises, with the upper pump drawing well fluid from the lower annulus through the valve into an intake of the upper pump and discharging well fluid from the upper pump into the upper annulus.

5. The method according to any preceding claim, wherein: the upper motor is operatively connected to an upper end of the pump; the lower motor is operatively connected to a lower end of the pump; and wherein: step (d) comprises driving the pump with the lower motor; and step (e) comprises driving the pump with the upper motor.

6. The method according to any preceding claim, wherein: the at least one pump of the ESP assembly is located between the upper motor and the lower motor; and wherein: step (e) comprises disengaging a shaft of the upper motor from the pump and driving the pump with the lower motor; and step (f) comprises disengaging a shaft of the lower motor from the pump and driving the pump with the upper motor.

7. A method of producing a well, comprising the following steps: (a) providing a lower electrical submersible pump assembly (ESP) having a pump and a motor with a motor electrical wet mate connection; (b) providing an upper ESP having a pump and a motor with an upper ESP power cable connected to the motor of the upper ESP; (c) installing a string of tubing in a well with a tubing electrical wet mate connection at a selected depth and a lower ESP power cable extending from the tubing wet mate connection alongside the tubing to a wellhead; (d) coupling the lower ESP to the upper ESP, defining a dual ESP assembly having a longitudinal axis, and running the dual ESP assembly into the tubing while feeding the upper ESP power cable into the tubing from the wellhead, then engaging the motor wet mate connection with the tubing wet mate connection; (e) selectively supplying power through the lower ESP power cable and the wet mate connections to the motor of the lower ESP and pumping well fluid up the tubing with the pump of the lower ESP while not supplying power through the upper ESP power cable; and (f) selectively supplying power through the upper ESP power cable to the motor of the upper ESP and pumping well fluid up the tubing with the pump of the upper ESP while not supplying power through the lower ESP power cable.

8. The method according to claim 7, further comprising: in step (e), isolating a discharge of the pump of the lower ESP from an intake of the pump of the upper ESP; and in step (f), isolating a discharge of the pump of the upper ESP from an intake of the pump of the lower ESP.

9. The method according to claim 7 or 8, wherein: step (d) comprises, with a packoff sealing between the dual ESP assembly and the tubing, defining an upper annulus surrounding the upper ESP and a lower annulus surrounding the lower ESP; step (e) comprises drawing well fluid from the lower annulus into an intake of the pump of the lower ESP and flowing a discharge of the pump of the lower ESP through the packoff into the upper annulus; and step (f) comprises flowing well fluid in the lower annulus through the packoff into an intake of the pump of the upper ESP.

10. The method according to claim 7, 8 or 9,wherein: step (d) comprises, with a packoff sealing between the dual ESP assembly and the tubing, defining an upper annulus surrounding the upper ESP and a lower annulus surrounding the lower ESP; the pump of the lower ESP has an intake in fluid communication with the lower annulus and a discharge in fluid communication with the upper annulus, and the pump of the upper ESP has an intake in fluid communication with the lower annulus and a discharge in fluid communication with the upper annulus; step (e) comprises drawing well fluid from the lower annulus into the intake of the pump of the lower ESP and flowing a discharge of the pump of the lower ESP through the packoff into the upper annulus; step (f) comprises flowing well fluid in the lower annulus through the packoff into the intake of the pump of the upper ESP and flowing a discharge of the pump of the upper ESP into the upper annulus; during step (e), the upper pump is not operating; and during step (f), the lower pump is not operating.

11. The method according to any of claims 7-10, further comprising: mounting a valve between the upper end of the lower ESP and the lower end of the upper ESP, the valve having a lower pump inlet port, a lower annulus inlet port, an upper pump outlet port and an upper annulus outlet port; wherein step (d) comprises sealing between the valve and the tubing, defining an upper annulus above the valve and a lower annulus below the valve; step (e) comprises flowing a discharge of the pump of the lower ESP through the lower pump inlet port and out the upper annulus outlet port to the upper annulus; and step (f) comprises flowing well fluid in the lower annulus through the lower annulus inlet port and out the upper pump outlet port to an intake of the pump of the upper ESP.

12. The method according to any of claims 7-11, further comprising: mounting a valve between the upper end of the lower ESP and the lower end of the upper ESP, the valve having a lower pump inlet port, a lower annulus inlet port, an upper pump outlet port and an upper annulus outlet port, the valve having a first position communicating the lower pump inlet port with the upper annulus outlet port, the valve having a second position communicating the lower annulus inlet port with the upper pump outlet port; wherein step (d) comprises sealing between the valve and the tubing, defining an upper annulus above the valve and a lower annulus below the valve; step (e) comprises placing the valve in the first position; and step (f) comprises placing the valve in the second position.

13. The method according to any of claims 7-12, further comprising: in step (d), sealing to the tubing an intermediate area of the dual ESP assembly between the upper ESP and the lower ESP, defining an upper annulus separated from a lower annulus; mounting a valve in the intermediate area of the dual ESP assembly, the valve having a lower pump inlet port in fluid communication with a discharge of the pump of the lower ESP, a lower annulus inlet port in fluid communication with the lower annulus, an upper pump outlet port in fluid communication with an intake of the pump of the upper ESP, and an upper annulus outlet port in fluid communication with the upper annulus; providing the valve with a first position communicating the lower pump inlet port with the upper annulus outlet port, and a second position communicating the lower annulus inlet port with the upper pump outlet port, and biasing the valve to the second position with a spring; wherein pressure caused by the discharge of the pump of the lower ESP in step (e) overcomes the spring and places the valve in the first position; and cessation of operation of the pump of the lower ESP in step (f) causes the spring to move the valve in the second position.

14. An apparatus for pumping well fluid from a well, comprising: a tubing wet mate sub adapted to be secured to a string of tubing within the well, the wet mate sub having a tubing electrical wet mate connection adapted to be secured to a lower motor power cable extending from the tubing electrical wet mate connection alongside the tubing to a wellhead; an electrical submersible pump assembly (ESP) having at least one pump, an upper motor and a lower motor, the lower motor having a lower motor electrical wet mate connection in electrical engagement with the tubing electrical wet mate connection to supply power to the lower motor; and an upper motor power cable connected to the upper motor and adapted to extend through the tubing to the wellhead to supply power to the upper motor.

15. The apparatus according to claim 14, wherein: said at least one pump comprises an upper pump operatively connected to the upper motor and a lower pump operatively connected to the lower motor.

16. The apparatus according to claim 14 or 15, wherein: said at least one pump comprises an upper pump operatively connected to a lower end of the upper motor and a lower pump operatively connected to an upper end of the lower motor.

17. The apparatus according to claim 14, 15 or 16, wherein the ESP assembly comprises: an upper pump connected below the upper motor; a lower pump connected above the lower motor; a valve connected between the upper pump and the lower pump; an annular packoff for sealing the valve to the tubing to define an upper annulus separate from a lower annulus; wherein, the valve has a first position defining a flow path for the lower pump to discharge well fluid from the lower annulus through the valve into the upper annulus, which flows around the upper pump and the upper motor; and the valve has a second position defining a flow path for the upper pump to draw well fluid from the lower annulus through the valve into an intake of the upper pump and discharge well fluid from the upper pump into the upper annulus.

18. The apparatus according to any of claims 14-17, wherein the ESP assembly comprises: an upper pump connected below the upper motor; a lower pump connected above the lower motor; a valve connected between the upper pump and the lower pump; an annular packoff for sealing the valve to the tubing to define an upper annulus separate from a lower annulus; wherein, the valve has a first position defining a flow path for the lower pump to discharge well fluid from the lower annulus through the valve into the upper annulus, which flows around the upper pump and the upper motor; the valve has a second position defining a flow path for the upper pump to draw well fluid from the lower annulus through the valve into an intake of the upper pump and discharge well fluid from the upper pump into the upper annulus; the valve is biased by a spring into the second position; and pressure caused by the discharge of the lower pump overcomes the spring and moves the valve to the first position.

19. The apparatus according to any of claims 14-18, wherein: the upper motor is operatively connected to an upper end of the pump; and the lower motor is operatively connected to a lower end of the pump.

20. The apparatus according to any of claims 14-19, wherein: the at least one pump of the ESP assembly is located between the upper motor and the lower motor and is driven selectively by the lower motor and by the upper motor; wherein the apparatus further comprises: means for disengaging a shaft of the upper motor from the pump and driving the pump with the lower motor; and means for disengaging a shaft of the lower motor from the pump and driving the pump with the upper motor.