Geothermal energy development by circulating CO2 in a U-shaped closed loop geothermal system

At present, geothermal energy is a promising research area but with a series of unknowns waited to be explored. Recently, the U-shaped closed loop geothermal extraction system was proposed to improve the geothermal recovery performance. However, there is a lack of mathematical model to simulate the circulating process of CO2 in the wellbore. In this paper, a model is developed for simulating CO2 flow in the descending, horizontal and ascending sections of the geothermal well. Besides, in order to properly evaluate the geothermal recovery performance, two new concepts of critical position and effective distance are proposed. Simulation results show that: (a) There exists a critical position in the ascending wellbore where the working fluid temperature is equal to the formation temperature. (b) The effect of mass flow rate on pressure drop becomes more obvious when the gravity is neglected, especially under the condition that the mass flow is at a higher value. (c) The rapid increase of working fluid temperature does not mean that the geothermal recovery performance is better. At this time, the effective distance should be adopted as an additional reference.