Process for heating a fluid in a geothermal formation

A method of heating a fluid in a brine-containing geothermal reservoir formation penetrated by an injection well and a production well which comprises:

• a. introducing an organic fluid having a low solubility in water into the formation through the said injection well,
• b. forcing the said organic fluid through the said formation thereby heating the said fluid,
• c. recovering the said heated organic fluid substantially free of brine through the said production well in liquid phase,
• d. reducing the pressure in said liquid phase thereby converting the organic fluid to a vapor phase,
• e. expanding the organic fluid in vapor phase in a power extracting gas expansion device, and
• f. condensing the organic fluid in vapor phase after expansion thereby forming an organic fluid liquid phase and a separate water liquid phase derived from water dissolved in the organic fluid recovered from the production well and wherein the temperature of the said geothermal, reservoir formation is substantially above the temperature of the fluid introduced into the formation in step (a).