Numerical simulation of CO2 circulating in a retrofitted geothermal well

Abandoned oil and gas wells are proved to be effective in geothermal energy extraction, especially when the wellbore is located at a perfect geothermal reservoir. In this paper, CO2 is selected as the working fluid and a model is proposed to simulate the CO2 flow in the retrofitted wellbores. The CO2 is injected into the inner tube (injector) and then flows out through the annuli (producer) to surface. The proper length of the insulation layer installed outside the casing is predicted. Results show that (a) when the CO2 flows from well-bottom to wellhead in the producer, its temperature has an increase at first and then turns to decrease. An insulation layer outside the casing is an efficiency way to prevent CO2 from transferring heat to the stratum. (b) From the temperature point of view, a smaller mass flow rate is recommended to obtain a higher CO2 temperature at outlet of the producer. (c) A higher mass flow rate leads to a higher geothermal energy extraction rate but a lower temperature. (d) A small increase in temperature or the geothermal energy extraction rate need a large pressure increase, which can be high-cost for field practice.