Balancing heat extraction and reservoir stability in enhanced geothermal system: mechanisms and synergistic evaluation based on thermo-hydro-mechanical coupling

To address the fully coupled Thermo-Hydro-Mechanical (THM) effects during the heat extraction process of Enhanced Geothermal System (EGS), this paper establishes a Discrete Fracture Network (DFN) numerical model that considers the dynamic evolution of porosity and permeability. To overcome the limitations of previous studies that pursued high heat extraction efficiency while neglecting irreversible reservoir damage, this study innovatively proposes a synergistic evaluation system based on range analysis and a four-quadrant matrix. Utilizing critical heat flux ( Q = 0.3 W/m²) and critical permeability retention rate ( η = 90%) as core constraints, this system achieves a quantitative trade-off between heat flux gain and the cost of reservoir mechanical damage. The results indicate that the spatiotemporal evolution of thermal stress and pore pressure governs the nonlinear response of heterogeneous reservoir physical properties. A hydraulic conduction saturation threshold of 0.3 mm exists for the fracture aperture; arbitrarily increasing the aperture or the injection-production pressure differential can enhance thermal output in the short term, but it exacerbates cold front fingering, triggers premature thermal short-circuiting, and induces significant compaction damage. Initial permeability is the absolute dominant factor controlling the system's heat extraction capability (heat extraction sensitivity coefficient S _Q = 322.39%), whereas initial porosity, by virtue of its prominent pressure buffering effect, acts as a critical physical barrier for maintaining long-term reservoir mechanical stability ( η > 90%). The intrinsic relationship between the reservoir's dynamic response and heat extraction efficiency in a multiphysics coupled environment investigated herein, along with the proposed multi-parameter synergistic evaluation matrix, accurately delineates the feasible zone for highly efficient and stable EGS production. This reveals the nonlinear control mechanisms of various geological and engineering parameters on system performance, providing theoretical support for productivity prediction and parameter optimization in EGS projects.