Thermo-hydro-mechanical optimization of the enhanced geothermal system for commercial utilization

Abstract

Enhanced geothermal system (EGS) has been commercialized worldwide, yet the complex thermal-hydraulic-mechanical coupling processes were not well illuminated. With the reservoir regarded as the fractured porous media of rock matrix and discrete fractures, this work establishes a three-dimensional EGS model based on the thermal non-equilibrium theory to study the heat extraction in the EGS. After model validations, the evolution of three fields (i.e., temperature field, seepage field, and stress field) and the effects of several crucial operating parameters (e.g., injection flow rate, depth, and length) on heat extraction performance are discussed. The results show that the change of pressure and temperature in the reservoir alters the fracture displacement and the flow preferential paths and rock matrix permeability. For the given EGS reservoir, the total heat extraction rate reaches 60% at 50 years and approaches 86% at 100 years. Increasing the elastic modulus of the rock matrix promotes the overall recovery and then decreases the average temperature of the rock matrix. Decreasing injection flow rate, increasing length, increasing depth, and decreasing the elastic modulus of rock matrix give rise to (i) increasing the period for maintaining stable average outlet temperature; (ii) elevating the average outlet temperature after specific years. Running for 100 years, the final injection pressure of the EGS is 130 MPa, 48 MPa, and 45 MPa for the three fracture widths, 26%, 14%, and 10% lower than the initial injection pressure. Changing the fracture width from 0.005 m to 0.05 m is beneficial for the EGS operation on economic efficiency. Based on the commercial standards, the optimal operating conditions are proposed for the EGS.