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A submersible pumping system for use downhole that includes a housing containing an expandable fluid, that when expanded pushes a piston that in turn pumps wellbore fluid to the surface. The expandable fluid can be a silicon based heat transfer fluid with a coefficient of thermal expansion of at lea

A submersible pumping system for use downhole that includes a housing containing an expandable fluid, that when expanded pushes a piston that in turn pumps wellbore fluid to the surface. The expandable fluid can be a silicon based heat transfer fluid with a coefficient of thermal expansion of at least about 0.0005 in3/in3/° F. The expandable fluid is expanded upon exposure to heat. A heat source is selectively activated for expanding the fluid.

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The invention claimed is: 1. A downhole submersible pumping system disposable in a cased wellbore comprising: an elongated housing having a first end and a second end; a piston formed for coaxial movement within the housing, said piston having a first side and a second side; an expandable fluid dis

The invention claimed is: 1. A downhole submersible pumping system disposable in a cased wellbore comprising: an elongated housing having a first end and a second end; a piston formed for coaxial movement within the housing, said piston having a first side and a second side; an expandable fluid disposed in the housing in pressure contact with the piston first side; an inlet in the housing formed to receive wellbore fluid within the housing and to be in pressure contact with the piston second side; an outlet in the housing formed to discharge wellbore fluid from the housing; and a heat source in thermal communication with the expandable fluid, wherein expansion of the expandable fluid in response to the applied heat source urges the piston towards the outlet. 2. The pumping system of claim 1 wherein the portion of the housing between the piston first side and the housing first end defines an expandable fluid section and the portion of the housing between the piston second side and the housing second end defines a wellbore fluid section. 3. The pumping system of claim 1, further comprising a discharge line connected to the outlet, wherein the discharge line is configured to deliver wellbore fluid from the well. 4. The pumping system of claim 3, further comprising a one way valve in the discharge line. 5. The pumping system of claim 2, further comprising a resilient member disposed in the wellbore fluid section contactable with the piston second side. 6. The pumping system of claim 5, wherein the resilient member comprises a spring. 7. The pumping system of claim 1, further comprising a one way valve disposed in the inlet. 8. The pumping system of claim 1 further comprising a seal disposed between the piston and the housing. 9. The pumping system of claim 1, wherein the expandable fluid comprises a silicon based fluid. 10. The pumping system of claim 1, wherein the coefficient of thermal expansion of the expandable fluid is at least about 0.0005 in3/in3/° F. 11. The pumping system of claim 1, wherein the wellbore fluid is selected from the group consisting of water, liquid hydrocarbons, and hydrate. 12. The pumping system of claim 1, wherein the heat source comprises an electrical resistance wire disposed in the housing. 13. A method of pumping fluid from a wellbore comprising: disposing a pumping system into the wellbore, wherein the pumping system comprises a housing having a motive fluid section and a working fluid section, a piston reciprocatingly disposed in the housing and separating the motive fluid section from the working fluid section, an expandable fluid disposed in the motive fluid section; admitting wellbore fluid into the working fluid section; and heating the expandable fluid thereby expanding the expandable fluid to urge the piston into the working fluid section, thereby forcing wellbore fluid from the working fluid section out of the housing into a discharge line. 14. The method of claim 13, wherein the step of heating the fluid comprises supplying electrical power to an electrical heater submerged in the motive fluid section. 15. The method of claim 13 wherein the expandable fluid comprises silicon based fluid. 16. The method of claim 13 wherein the coefficient of thermal expansion of the expandable fluid is at least about 0.0005 in3/in3/° F. 17. The method of claim 13 further comprising cooling the expandable fluid after the heating step for a selected time period. 18. A wellbore assembly having a wellbore lined with casing, and perforations providing fluid communication between a hydrocarbon producing zone and the wellbore, and a pumping assembly disposed in the wellbore, the assembly comprising: a pump housing having a piston coaxially slideable within the pump housing, wherein the piston separates the housing into an expanding fluid section and a working fluid section; an expansible fluid provided in the expanding fluid section; an inlet valve configured to allow selective ingress of wellbore fluid into the working fluid section when working fluid section pressure is less than wellbore pressure; an outlet valve in the housing to allow wellbore fluid to be discharged from the working fluid section when working fluid section pressure exceeds wellbore pressure; and a heat source in thermal communication with the expansive fluid. 19. The wellbore assembly of claim 18, wherein the expansive fluid comprises silicon based fluid. 20. The wellbore assembly of claim 18, wherein the heat source comprises an electrical heater element submersible in the expansible fluid. 21. The wellbore assembly of claim 18, further comprising a discharge line leading from the outlet valve to the upper end of the wellbore.