Numerical simulation of the heat extraction in 3D-EGS with thermal-hydraulic-mechanical coupling method based on discrete fractures model

•EGS reservoir comprising discrete fracture networks and matrix rock is modeled.•A THM coupling model is proposed for simulating the heat extraction in EGS.•The numerical model is validated by comparing with several analytical solutions.•An evaluation system and a case study is presented for evaluating the performance of EGS.•Energy efficiency and heat recovery remain at low level through a doublet system. The geothermal heat production from Enhanced Geothermal System (EGS) is influenced by complex thermal-hydraulic-mechanical (THM) coupling process, it is necessary to consider THM coupling effects on utilization efficiency and production performance of EGS. The geothermal reservoir regarded as a fractured porous media consists of rock matrix blocks and discrete fractures. Based on local thermal non-equilibrium theory, a mathematical model and an ideal 3D-EGS numerical model incorporating THM coupling process are established to simulate the heat production process in EGS, and the distribution regularities of pressure, temperature, stress and deformation in geothermal reservoir are analyzed. The results show that the connecting fractures are the main flow paths and the transmission characteristic of reservoir is altered due to displacement of fractures caused by the change of pressure and temperature in reservoir. The main parameters controlling the outlet temperature are also studied by sensitivity analysis. An EGS case from Desert Peak geothermal reservoir is simulated with a 3D stochastically generated fracture model to evaluate EGS heat production performance. The results indicate that heat production time, thermal output and power generation can meet the commercial standard with appropriate reservoir and operation parameters, however, energy efficiency and overall heat recovery remain at low level.