CN111032994A

CN111032994A - Bidirectional piston seal with pressure compensation

Info

Publication number: CN111032994A
Application number: CN201880042232.XA
Authority: CN
Inventors: 布莱恩·保罗·里夫斯; 王季锋; 王成豹; 翁普拉卡什·萨穆德腊拉
Original assignee: GE Oil and Gas ESP Inc
Current assignee: Baker Hughes ESP Inc
Priority date: 2017-06-24
Filing date: 2018-05-22
Publication date: 2020-04-17
Also published as: CA3068438A1; WO2018236515A1

Abstract

An electrical submersible pump system for pumping fluid from a wellbore comprising: a motor; a pump driven by the motor; and a fluid expansion module connected to the motor. The fluid expansion module comprises: a piston seal housing; and a piston assembly housed within the piston seal housing. The piston assembly includes: a piston body having an outer surface; a plurality of seals connected to the outer surface of the piston body; and a pressure equalization system. The pressure balancing system reduces a pressure differential between fluid in an annular space between the plurality of seals and fluid surrounding the first piston assembly.

Description

Bidirectional piston seal with pressure compensation

RELATED APPLICATIONS

This application is a continuation of part of U.S. patent application No. 14/075,656 entitled "Electric subassembly Motor oil expansion Compensator," filed on 8.11.2013, the disclosure of which is incorporated herein by reference.

Technical Field

The present invention relates generally to the field of submersible pump systems, and more particularly, but not by way of limitation, to a system for accommodating expansion of motor oil in high temperature environments.

Background

Submersible pump systems are typically deployed into oil wells to recover petroleum fluids from subterranean reservoirs. Typically, the submersible pump system includes a number of components, including one or more liquid-filled electric motors coupled to one or more high-performance pumps located above the motors. When energized, the motor provides torque to the pump that propels wellbore fluid through the production tubing to the surface. Each component in the submersible pump system must be energized to withstand the harsh downhole environment.

Components commonly referred to as "seal sections" protect the electric motor and are typically located between the motor and the pump. In this position, the seal section provides several functions, including transferring torque between the motor and the pump, restricting the flow of wellbore fluids into the motor, protecting the motor from axial thrust imparted by the pump, and accommodating expansion and contraction of the motor oil as it moves within thermal cycles during operation of the motor. Prior art seal segments typically include a "clean side" in fluid communication with the electric motor and a "dirty side" in fluid communication with the wellbore. A bellows or bag is used to separate the clean side from the contaminated side of the sealed section.

More recently, manufacturers have employed polymeric expansion pockets within the seal section to accommodate expansion and contraction of motor oil while isolating the oil from contaminants in the wellbore fluid. While generally effective at lower temperatures, currently available polymers become somewhat permeable at very high temperatures and allow moisture to pass through the membrane. The moisture may reduce the insulating properties of the polyimide and other films used to electrically isolate components within the downhole pump system. While piston-based systems may provide an alternative to the use of polymer inflation bags, prior art piston-based seal assemblies are prone to failure due to sand, dirt, or other particulates. Furthermore, the seal rings used in prior pistons may deform under differential pressure, apply unwanted pressure to the interior of the seal segment housing, and reduce movement of the piston. Accordingly, there is a need for improved designs that can be used to accommodate expansion of motor fluids in high temperature applications. The presently described embodiments are directed to this need and other needs.

Disclosure of Invention

In one aspect, an exemplary embodiment includes an electrical submersible pump system for pumping fluid from a wellbore. The electrical submersible pump system includes: a motor; a pump driven by the motor; and a fluid expansion module connected to the motor. The fluid expansion module comprises: a piston seal housing; and a first piston assembly housed within the piston seal housing. The first piston assembly includes: a piston body having an outer surface; a plurality of seals connected to the outer surface of the piston body; and a pressure equalization system. The pressure balancing system reduces a pressure differential between fluid in an annular space between the plurality of seals and fluid surrounding the first piston assembly.

In another aspect, embodiments of the invention include a system for accommodating expansion of motor oil in a motor within an electrical submersible pump for removing fluids from a wellbore. The system comprises: a seal section connected to a first end of the motor; and a fluid expansion module connected to a second end of the motor. The fluid expansion module has a longitudinal axis and at least one piston assembly. The at least one piston assembly moves along the longitudinal axis of the fluid expansion module in response to expansion of the motor oil. The at least one piston assembly includes: a piston body having an outer surface; a plurality of seals connected to the outer surface of the piston body; and a pressure equalization system. The pressure balancing system reduces a pressure differential between fluid in an annular space between the plurality of seals and fluid surrounding the at least one piston assembly.

In another aspect, embodiments of the invention include a fluid expansion module for use in an electrical submersible pump system that includes a pump driven by a liquid filled motor. The fluid expansion module comprises: a piston seal housing in fluid communication with the charge motor; and a first piston assembly housed within the piston seal housing. The first piston assembly includes: a piston body having an outer surface; a plurality of seals connected to the outer surface of the piston body; and a pressure equalization system. The space between the plurality of seals creates an annular space, and the pressure balancing system reduces a pressure differential between fluid in the annular space and fluid in the piston seal housing.

Drawings

FIG. 1 depicts a submersible pump system constructed in accordance with one embodiment of the present invention.

Fig. 2 provides a cross-sectional view of the motor, lower fluid expansion module, and seal section of the submersible pump system of fig. 1.

Fig. 3 presents a cross-sectional illustration of a motor of the pump system in fig. 2.

Fig. 4 presents a cross-sectional illustration of the lower fluid expansion module of fig. 2.

Figure 5 presents a cross-sectional view of a piston assembly constructed in accordance with a first embodiment.

Figure 6 presents a cross-sectional view of a piston assembly constructed in accordance with a second embodiment.

Fig. 7 presents a perspective view of the sealing ring in the piston assembly of fig. 5 and 6.

FIG. 8 provides a cross-sectional view of a motor and a seal section of the pump system in FIG. 2.

Fig. 9 provides a cross-sectional view of the mechanical seal in the seal section of fig. 7.

Detailed Description

Fig. 1 shows an elevational view of a pump system 100 attached to a production tubing 102. The pump system 100 and production tubing 102 are disposed in a wellbore 104 that is drilled for the production of fluids such as water or oil. As used herein, the term "petroleum" refers broadly to all mineral hydrocarbons, such as crude oil, natural gas, and combinations of oil and natural gas.

Pump system 100 includes pump 108, motor 110, seal section 112, and fluid expansion module 114. Production tubing 102 connects pump system 100 to wellhead 106 at the surface. While the pump system 100 is primarily designed to pump petroleum products, it will be understood that the pump system 100 may also be used to move other fluids. It will also be appreciated that although each of the components of the pump system are primarily disclosed in submersible applications, some or all of these components may also be used in surface pumping operations.

Generally, the motor 110 is configured to drive the pump 108. Power is provided to the motor 110 through a power cable 116. In some embodiments, pump 108 is a turbine that uses one or more impellers and diffusers to convert mechanical energy into head. In other embodiments, pump 108 is configured as a positive displacement pump. Pump 108 includes a pump intake 118 that allows fluid from wellbore 104 to be pumped into pump 108. Pump 108 forces wellbore fluid through production tubing 102 to the surface.

The seal section 112 is located above the motor 110 and below the pump 108. The fluid expansion module 114 is located below the motor 110. While only one is shown for each component, it will be understood that more components may be connected as appropriate, other arrangements of components are desired, and such additional configurations are contemplated within the scope of the exemplary embodiments. For example, in many applications, it is desirable to use an in-line motor combination, a gas separator, multiple seal sections, multiple pumps, sensor modules, and other downhole components.

Note that while the pump system 100 is depicted in a vertical deployment in fig. 1, the pump system 100 may also be used in non-vertical applications, including use in horizontal and deviated wellbores 104. Accordingly, references to "upper" and "lower" in this disclosure are used only to describe the relative positions of components within the pump system 100, and should not be understood as an indication that the pump system 100 must be deployed in a vertical orientation.

Referring now also to fig. 2 and 3, there are shown cross-sectional views of the seal section 112, the motor 110 and the fluid expansion module 114. As depicted in the close-up view of the motor 110 in fig. 3, the motor 110 includes a motor housing 120, a stator assembly 122, a rotor assembly 124, a rotor bearing 126, and a motor shaft 128. The stator assembly 122 includes a series of stator coils (not separately labeled) corresponding to various phases of power supplied to the motor 110. The rotor assembly 124 is keyed to a motor shaft 128 and is configured for rotation next to the fixed stator assembly 122. The dimensions and configuration of the stator and

rotor assemblies

122, 124 may be adjusted to suit application-specific performance requirements of the motor 110. Sequentially energizing the various strings of coils within the stator assembly 122 causes the rotor assembly 124 and the motor shaft 128 to rotate according to well-known motoring principles. Motor bearings 126 maintain the central position of rotor assembly 124 within stator assembly 122 and resist radial and axial forces generated by motor 110 against motor shaft 128.

The motor 110 is filled with non-conductive lubrication oil during manufacture, which reduces frictional wear to rotating components within the motor 110. The lubricating oil expands and contracts as the motor 110 rotates during use and when the motor 110 is exposed to the high temperatures in the wellbore 104. It is desirable to prevent contamination of the cleaned motor oil by fluids and solids in the wellbore. To permit the lubricating oil to expand and contract at high wellbore temperatures, the seal section 112 and the fluid expansion module 114 are connected to the motor 110 and placed in fluid communication with the motor oil.

With continued reference to fig. 2 and now also to fig. 4, a cross-sectional view of the fluid expansion module 114 is shown in fig. 4. In the embodiment depicted in fig. 4, the fluid expansion module 114 includes a piston seal housing 130, a bag seal housing 132, one or more piston assemblies 134, a bag seal assembly 136, and a fluid exchange assembly 138. However, it will be appreciated that in some embodiments, the fluid expansion module 114 may not include all of these components.

As shown in fig. 4, the fluid expansion module 114 includes a pair of

piston assemblies

134a, 134 b. The

piston assemblies

134a, 134b are located in the piston seal housing 130 and are configured to move axially within the fluid expansion module 114 in response to a pressure differential of the fluid surrounding the piston assemblies 134. Thus, if each piston assembly 134 is permitted to move freely within piston seal housing 130, then piston assembly 134 will maintain a relatively minimum pressure differential across piston assembly 134.

In some embodiments, the inner surface of the piston seal housing 130 includes a polymer liner 140 that reduces friction and adhesion. The polymer liner 140 may be fabricated from PTFE, PFA, PEEK and other high temperature polymers. Alternatively, the inner surface of the piston seal housing 130 may be made of polished chrome, stainless steel, or other durable metal. Note that piston assembly 134a is constructed in accordance with the first embodiment, while piston assembly 134b is constructed in accordance with the second embodiment. The similarities and differences between the first embodiment 134a and the second embodiment 134b of the piston assembly are described below.

Turning to fig. 5, a cross-sectional view of a first embodiment of a piston assembly 134a within a portion of the piston seal housing 130 is illustrated. The piston assembly 134a includes a solid piston body 142 and a pair of seals 144. In some embodiments, the piston body 142 is made of a highly polished metal. Suitable metals include chromium, stainless steel, and related alloys. Alternatively, the piston body 142 may be made of a high temperature rated elastomer or polymer. The polymer of the piston body 142 includes Polytetrafluoroethylene (PTFE), Perfluoroalkoxy (PFA), and Polyetheretherketone (PEEK). The piston body 142 has an outer diameter that is only slightly smaller than the inner diameter of the piston seal housing 130. A seal 144 is positioned around the exterior of the piston body 142 at the opposite end of the piston body 142.

Turning to fig. 6, a cross-sectional view of a second embodiment of a piston assembly 134b within a portion of the piston seal housing 130 is illustrated. Piston assembly 134b includes a piston body 142 and a pair of seals 144. A seal 144 is positioned around the exterior of the piston body 142 at the opposite end of the piston body 142. Piston assembly 134b also includes a pressure balancing system 146 configured to reduce a pressure differential across seal 144 to minimize pressure-induced deformation of seal 144. Reducing the pressure gradient across seal 144 allows piston assembly 134b to move more easily within piston housing 130 while maintaining an absolute barrier between the clean motor fluid above piston assembly 134b and the wellbore fluid below piston assembly 134 b.

The pressure equalization system 146 includes an inlet check valve 148, an equalization chamber 150, one or more pressure ports 152, and a pressure relief check valve 154. One or more pressure ports 152 extend from the balance chamber 150 through the piston body 142 to an annular space 158 between the piston body 142 and an inner wall of the piston seal housing 130 between the seals 144. In this manner, the balance chamber 150 is placed in fluid communication with the annular space 158.

The inlet check valve 148 is a one-way valve that opens when the pressure differential exceeds a predetermined threshold amount between the space above the piston assembly 134b and the balance chamber 150. When inlet check valve 148 is temporarily opened, the cleaned motor oil passes through inlet check valve 148 to increase the pressure within balance chamber 150 and annular space 158 to reduce the pressure gradient across seal 144. The amount of differential pressure required to open inlet check valve 148 may be set during manufacturing by adjusting the amount of closing force exerted by a spring within inlet check valve 148. In the exemplary embodiment, inlet check valve 148 is configured to open at a differential pressure that is greater than the amount of differential pressure expected to exist around piston assembly 134b under normal operating conditions. In this manner, the inlet check valve 148 will not permanently open and will reduce movement of the piston seal assembly 134b under normal operating conditions.

Pressure relief check valve 154 is calibrated to temporarily open if the pressure within balance chamber 150 exceeds the pressure below piston assembly 134b by a predetermined threshold amount. This relieves high pressure within piston assembly 134b to reduce any pressure gradient across seal 144. Once the elevated internal pressure is released, the pressure relief check valve 154 closes to prevent any fluid flow into the balance chamber 150.

Accordingly, pressure balancing system 146 ensures optimal performance of seal 144 by reducing deformation-based friction between piston assembly 134b and piston seal housing 130. This allows the piston assembly 134b to quickly respond to slight pressure imbalances within the piston seal housing 130. For example, if the pressure below piston assembly 134b increases during installation of pump system 100 in wellbore 104, piston assembly 134b may be forced upward to increase and equalize the pressure above piston assembly 134 b. If the increased pressure above piston assembly 134b is sufficiently greater than the pressure within balance chamber 150, inlet check valve 148 will temporarily open to balance the pressure around seal 144.

Fig. 7 presents a cross-sectional view of the seal 144. The seal 144 includes a body 160 and an internal spring 162. The body 160 is made of a durable, high temperature, and wear resistant elastomer or polymer, such as Polytetrafluoroethylene (PTFE), Perfluoroalkoxy (PFA), Polyetheretherketone (PEEK), and perfluorinated elastomers. The inner spring 162 is configured to apply a force to the body 160 in an outward radial direction. In this manner, the spring 162 presses the body 160 against the inner surface of the piston seal housing 130. The inner spring 162 may be configured as a helical ring or a finger spring.

Turning back to fig. 4, the bag sealing assembly 136 is housed within the bag seal housing 132. The bag seal assembly 136 includes a bag support 164, a bladder 166, an inlet port 168, and a discharge valve 170. The bag support 164 is rigidly attached to the inner surface of the bag seal housing 132. The bladder 166 is secured to the bag support 164 by compression flange 172. Alternatively, the bladder 166 may be secured to the bag support 164 by a grip or hose clamp. The inlet port 168 provides a path for fluid communication from the piston seal housing 130 into the interior of the bladder 166 and the bag support 164. Importantly, the bag support 164 permits fluid to pass between the piston seal housing 130 and the bag seal housing 132 only through the access port 168. Fluid outside of the bladder 166 is not allowed to flow directly into the piston seal housing 130.

The discharge valve 170 is a one-way relief valve configured to open at a predetermined threshold pressure that exceeds the external wellbore pressure. In this way, if the fluid pressure inside the bladder 166 exceeds the set point pressure, the discharge valve 170 opens and releases the pressure inside the bladder 166 by discharging a small amount of fluid into the wellbore 104. The bladder 166 may be made of a high temperature polymer or elastomer. Suitable polymers and elastomers include Polytetrafluoroethylene (PTFE), Perfluoroalkoxy (PFA), and Polyetheretherketone (PEEK).

The bag sealed housing 132 also includes a fluid exchange assembly 138. The fluid exchange assembly 138 optionally includes a solids screen 174 and a plurality of exchange ports 176. The crossover port 176 allows fluid to flow from the wellbore 104 through the solids screen 174 into the bag seal housing 132 around the exterior of the bladder 166. The solids screen 174 reduces the presence of particulates in the bag seal housing 132. The solids screen 174 is made from a metal or polymer fabric mesh.

During manufacture, the fluid expansion module 114 is filled with clean motor oil. The

piston assemblies

134a, 134b are then placed into the piston seal housing 130. As the fluid in the motor 110 expands during operation, the increased volume may exert pressure on the upper side of the piston assembly 134 b. In response, piston assembly 134b moves downward toward piston assembly 134 a. As the volume between

piston assemblies

134a, 134b decreases, the increasing pressure on piston assembly 134a urges it downward toward bag seal housing 132. As the piston assembly 134a moves downward, it pushes clean motor oil through the inlet port 168, through the bag support 152 and into the bladder 166. The bladder 166 expands to accommodate the introduction of fluid from the piston seal housing 130. As the bladder 166 expands, fluid outside of the bladder 166 is expelled through the exchange ports 176 and the solids screen 160. If the pressure inside the bladder 166 exceeds the threshold pressure limit of the discharge valve 170, the discharge valve 170 opens and discharges a portion of the fluid into the wellbore 104.

Conversely, during a cooling cycle, the fluid in the motor 110 contracts and the movement of the components within the fluid expansion module 114 reverses. When the

pistons

134a, 134b are drawn upward, fluid is pushed out of the bladder 166. As the volume and pressure inside the bladder 166 decreases, fluid from the wellbore is pushed into the bag seal housing 132 through the solids screen 174 and the exchange port 176. The fluid expansion module 114 provides a robust mechanism for allowing the lubrication oil in the motor 110 to expand and contract while maintaining an isolation barrier between the cleaned motor lubrication oil and the contaminated fluid from the wellbore 104. Notably, the use of the piston assembly 134 provides a redundant barrier to the bladder 166 that does not readily exhibit the increased permeability seen in high temperature bladders. Thus, although the bladder 166 is exposed to extremely high temperatures and permits some water from the wellbore 104 to be diverted into the piston seal housing 130, it is isolated from the motor 110 by the redundant piston assembly 134.

It will be appreciated that the fluid expansion module 114 may include one or more piston assemblies 134, which may include the first embodiment of the piston assembly 134a, the second embodiment of the piston assembly 134b, or a combination of the first and second embodiments of the

piston assembly

134a and 134b (as shown in fig. 4). Additionally, it will be appreciated that in some embodiments, the piston assembly 134 may be used without the bag sealing assembly 136. In some applications, it may be desirable to place the piston 108 below the motor 110. In those applications, the fluid expansion module 114 will be located above the motor 110, and the seal section 112 will be placed between the motor 110 and the pump 108. In these alternative embodiments, the bag seal housing 132 would be located above the piston seal housing 130.

Turning to fig. 8, a cross-sectional view of the motor 110 and the seal section 112 is shown. The seal section 112 is attached to the upper end of the motor 110 and provides a second system for accommodating sealing of the rotating shaft 128 and supports the thrust load of the pump 108 to the apparatus. Seal section 112 includes a seal section shaft 178, a thrust bearing assembly 180, one or more mechanical seals 182, and one or more relief valves 184. During manufacture, the seal section 112 is filled with clean motor oil.

The seal section shaft 178 is coupled to the motor shaft 128 or formed as an integral shaft with the motor shaft 128 and transfers torque from the motor 110 to the pump 108. Thrust bearing assembly 180 includes a pair of stationary bearings 186 and a thrust bearing slide bar 188 attached to seal section shaft 178. The thrust bearing slide bar 188 is captured between the fixed bearings 186, which limit axial displacement of the slide bar 188 and the motor shaft 128 and seal section shaft 178.

As shown in fig. 8, the seal section 112 includes a plurality of mechanical seals 182. As best shown in the close-up view of the mechanical seal 182 in fig. 9, the mechanical seals 182 each include a bellows 190, a coil spring 192, a sliding rod 194, and a retaining ring 196. These components cooperate to prevent migration of fluid along the seal segment axis 178. The retaining ring 196 has an inner diameter sized to permit the seal segment shaft 178 to rotate freely. In contrast, the bellows 190, the spring 192, and the sliding rod 194 rotate with the seal section shaft 178. The rotary slide 194 bears against a fixed ring 196 by a spring-loaded bellows 190. The bellows 190 includes a series of folds that allow its length to be adjusted to keep the sliding rod 194 in contact with the fixed ring 196 as the seal segment shaft 178 will undergo axial displacement. The bellows 190 may be made of thin corrugated metal or of elastomers and polymers including AFLAS, perfluorinated elastomers, Polytetrafluoroethylene (PTFE), Perfluoroalkoxy (PFA), and Polyetheretherketone (PEEK).

Exemplary embodiments include a method of accommodating expansion of motor oil by a fluid expansion module. The method includes the step of providing a fluid expansion module comprising a piston seal housing and one or more pistons having a pressure equalization system. The method further includes the step of connecting the fluid expansion module to a first end of the motor such that lubrication oil in the motor is in fluid communication with the fluid expansion module. The method may further comprise the step of connecting a seal section to a second end of the motor.

It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. Those skilled in the art will appreciate that the teachings of the present invention may be applied to other systems without departing from the scope and spirit of the present invention.

Claims

1. An electrical submersible pump system for pumping fluid from a wellbore, the electrical submersible pump system comprising:

a motor;

a pump driven by the motor; and

a fluid expansion module connected to the motor, wherein the fluid expansion module comprises:

a piston seal housing; and

a first piston assembly housed within the piston seal housing, wherein the first piston assembly comprises:

a piston body having an outer surface;

a plurality of seals connected to the outer surface of the piston body, wherein a space between the plurality of seals creates an annular space; and

a pressure balancing system, wherein the pressure balancing system reduces a pressure differential between fluid in the annular space and fluid surrounding the first piston assembly.

2. The electric submersible pump system of claim 1, wherein the pressure balancing system comprises:

a balancing chamber;

an inlet check valve connected to the balance chamber, wherein the inlet check valve is configured to permit introduction of fluid into the balance chamber; and

one or more pressure bleed holes connected between the balance chamber and the annular space.

3. The electric submersible pump system of claim 2, wherein the pressure balancing system further comprises a pressure relief check valve connected to the balancing chamber, wherein the pressure relief check valve is configured to permit fluid to drain from the balancing chamber.

4. The electric submersible pump system of claim 1, wherein each seal further comprises:

a main body; and

an inner spring, wherein the inner spring is configured to exert an outward radial force on the body.

5. The electric submersible pump system of claim 1, wherein the piston seal housing comprises a polymer liner.

6. The electric submersible pump system of claim 1, wherein the fluid expansion module further comprises a second piston assembly.

7. The electric submersible pump system of claim 6, wherein the second piston assembly comprises:

a piston body having an outer surface;

a pressure balancing system, wherein the pressure balancing system reduces a pressure differential between fluid in the annular space and fluid surrounding the second piston assembly.

8. The electric submersible pump system of claim 1, wherein the fluid expansion module further comprises:

a bag sealing housing; and

a bag sealing assembly within the bag sealing housing.

9. The electric submersible pump system of claim 8, wherein the bag sealing assembly further comprises:

a bag support; and

a pouch secured to the pouch support.

10. The electric submersible pump system of claim 9, wherein the bag sealing assembly further comprises one or more vents, and wherein each of the one or more vents is configured as a one-way check valve that, when opened, places an interior of the bladder in fluid communication with the wellbore.

11. The electric submersible pump system of claim 1, wherein the fluid expansion module further comprises a fluid exchange assembly and wherein the fluid exchange assembly comprises:

solid screening;

an exchange port; and is

Wherein the fluid exchange assembly is configured to place an exterior of the bladder in fluid communication with the wellbore.

12. The electrical submersible pump system of claim 1, further comprising a seal section between the motor and the pump, wherein the seal section comprises:

a shaft;

one or more mechanical seals; and

one or more pressure relief valves.

13. A system for accommodating expansion of motor oil in a motor within an electrical submersible pump for removing fluids from a wellbore, the system comprising:

a seal section connected to a first end of the motor; and

a fluid expansion module connected to a second end of the motor, wherein the fluid expansion module has a longitudinal axis, and wherein the fluid expansion module comprises at least one piston assembly, wherein the at least one piston assembly moves along the longitudinal axis of the fluid expansion module in response to expansion of the motor oil, wherein the at least one piston assembly comprises:

a piston body having an outer surface;

a pressure balancing system, wherein the pressure balancing system reduces a pressure differential between fluid in the annular space and fluid surrounding the at least one piston assembly.

14. The system of claim 13, wherein the fluid expansion module further comprises a bag seal assembly, wherein the bag seal assembly comprises a bladder that expands in response to movement of the at least one piston assembly.

15. The system of claim 14, wherein the fluid expansion module further comprises a fluid exchange assembly that places an exterior of the bladder in fluid communication with the wellbore.

16. A fluid expansion module for use in an electrical submersible pump system including a pump driven by a charge motor, the fluid expansion module comprising:

a piston seal housing in fluid communication with the charge motor; and

a piston body having an outer surface;

a pressure balancing system, wherein the pressure balancing system reduces a pressure differential between fluid in the annular space and fluid in the piston seal housing.

17. A fluid expansion module according to claim 16, wherein the pressure equalization system comprises:

a balancing chamber;

one or more pressure bleed holes connected between the equalizing chamber and the annular space; and

one or more check valves in fluid communication with the balance chamber.

18. A fluid expansion module as claimed in claim 16, wherein said fluid expansion module further comprises a second piston assembly.

19. A fluid expansion module according to claim 18, wherein said second piston assembly comprises:

a piston body having an outer surface;

20. A fluid expansion module according to claim 19, wherein the pressure equalization system comprises:

a balancing chamber;

an inlet check valve connected to the balance chamber, wherein the inlet check valve is configured to permit introduction of fluid from the piston seal housing into the balance chamber;

a pressure relief check valve connected to the balance chamber, wherein the pressure relief check valve is configured to permit fluid to vent from the balance chamber to the piston seal housing.