A comprehensive thermo-hydro-mechanical framework for enhanced geothermal systems: thermal stimulation, energy recovery, and natural fracture activation

Geothermal energy plays a critical role in the transition to low-carbon energy systems, offering a stable and renewable source with minimal environmental impact. This study develops a comprehensive thermo-hydro-mechanical framework to investigate applications in geothermal systems. The applications include thermally induced fractures, geothermal energy extraction, and natural fracture activation under different reservoir conditions, to understand reservoir behavior and guide stimulation planning. The thermo-poroelastic theory is integrated to simulate coupled mechanical, hydraulic, and thermal processes in fractured geothermal reservoirs. The governing equations are implemented using the displacement discontinuity method, where fracture boundaries are discretized to evaluate stress, pressure, and temperature fields induced by displacement, fluid, and thermal sources. In addition, natural fracture deformation is modeled using the nonlinear Barton-Bandis relationship to account for stress-dependent closure and shear. Moreover, fracture propagation is governed by mixed-mode stress intensity criteria. Fluid flow within fractures follows Poiseuille's law, while thermal transport within fractures accounts for conduction, convection, and formation heat exchange. The coupled solution advances in time by iteratively solving for displacement, pressure, and temperature, ensuring full coupling across mechanical, hydraulic, and thermal domains. This framework integrates multiple modeling capacities that have previously been treated separately, enabling a comprehensive simulation of geothermal stimulation. The model was validated by comparing its numerical predictions with analytical solutions for thermo-poroelastic fracture responses under various loading conditions. The results show that injecting fluid 200 °C colder increased the maximum fracture width from approximately 0.7 mm to approximately 0.9 mm and reduced the time needed to reach a 32 m fracture length by about 1.75 × . Over five years, a single 200 m fracture sustained 3.9 × 10 4 J/s and yielded 6.2 × 10 12 J, whereas a 50 m fracture produced 1.6 × 10 4 J/s and 3.1 × 10 12 J. Increasing rock thermal conductivity from 2 to 15 W/m·°C raised cumulative recovery from 2.2 × 10 12 J to 5.3 × 10 12 J and maintained production temperature near 125 °C after five years. In natural-fracture activation tests, final widths reached 2.25 mm in tight formations, while high-permeability cases showed minimal change (0.1 mm). This study helps operators design more efficient and cost-effective EGS projects by optimizing injection strategies, fracture geometry, and site selection. In addition, it offers actionable guidance to improve heat recovery, reduce stimulation volumes, and manage risks like fluid loss and induced seismicity. • Developed a fully coupled thermo-hydro-mechanical DDM framework for simulating geothermal reservoirs. • Incorporated fracture propagation, natural fracture deformation, and fluid flow and thermal transport in fracture. • Validated model against benchmark thermo-poroelastic solutions under mechanical, hydraulic, and thermal loading conditions. • Quantified the influence of rock thermal conductivity on heat extraction efficiency, identifying optimal conductivity ranges. • Analyzed natural fracture activation under varying permeability, showing tight formations promote earlier activation.