Construction of a thermo-hydro-mechanical multi-field coupling model and its application in analyzing the thermal performance of hot dry rock mining

Thermo-hydro-mechanical (THM) coupling plays a vital role in analyzing the heat extraction efficiency of geothermal. To study the extraction performance of hot dry rock (HDR) in the Matouying Uplift area, a THM coupling model that describes the entire extraction process is first established, and then, the main controlling factors affecting the thermal performance are determined and the influence mechanism is elucidated; finally, the feasible recommendations to enhance HDR extraction efficiency are proposed. Results show that the injection temperature, pressure difference, well spacing, matrix permeability, and fracture aperture exhibit certain correlation with production well temperature, heat extraction power, heat extraction quantity, and heat extraction rate, which can be described by linear, exponential, power, binomial, and logistic functions. The low injection temperature, high-pressure difference, high matrix permeability, and appropriate well spacing and fracture aperture are most beneficial for enhancing heat extraction efficiency. In the presence of natural and artificial fractures, heat extraction is over an order of magnitude higher than in the absence of fractures. Under the same reservoir and process conditions, the thermal breakthrough time and heat extraction performance with the SCCO2 as the working fluid are 16.2% and 1–3 times of water. To achieve optimal results in HDR extraction, the following recommendations are made: first, select areas with well-developed natural fractures and conduct reservoir modification to create a dense fracture network; second, utilize SCCO2 as the working fluid for heat exchange within the reservoir; and finally, choose a higher-pressure difference, appropriate well spacing, and lower injection temperature.