Thermal-hydraulic finite element simulation of temperature decrease process during hot dry rock exploitation: A case study in the Qiabuqia area, Gonghe Basin, Qinghai Province

Hydraulic fracturing is widely employed for hot dry rock (HDR) exploitation, providing a fracture network that facilitates high-permeability of artificial reservoirs in Enhanced Geothermal System (EGS). This study establishes a 2D thermal-hydraulic coupling model for fracture-matrix media to simulate the extraction of HDR geothermal energy in the Qiabuqia area, Gonghe Basin using COMSOL Multiphysics software. The model incorporates insights from previous hydraulic fracturing experiments, numerical simulation, and on-site hydraulic fracturing monitoring data in the Gonghe Basin. This study investigates the influence of fracture aperture and matrix permeability on the temperature decrease process of HDR exploitation. The simulation results demonstrate that the fracture network is the main channel for fluid migration. The temperature decrease and early pressure change range extend along the fractures and then extend to the surrounding matrix divided by the fractures. The fracture aperture and matrix permeability significantly influence the temperature decrease process of HDR exploitation. When the fracture aperture is larger, the wider the fluid migration range, the faster the temperature decrease of the reservoir output water, and the shorter the thermal breakthrough time and the operating lifetime. Higher matrix permeability enhances fluid entry into the matrix for efficient heat exchange, leading to faster output water temperature decrease and earlier operating lifetime.