ABSTRACT:

A fully coupled thermal-hydraulic-mechanical (THM) model was developed to investigate the heat extraction process of an artificial enhanced geothermal system (EGS). The random fracture network in the stimulated geothermal reservoir was generated by the fractal theory. The local thermal non-equilibrium (LTNE) theory was adopted to simulate the heat exchange between the rock matrix and the injection cold fluid. Temperature-dependent fluid thermodynamic properties and pressure-dependent fracture/pore permeability were also incorporated into the thermo-poroelastic coupling model by some empirical formulas. The proposed multiphysics model was validated by several analytical solutions. The evolution of temperature, effective stress and reservoir permeability during heat extraction was analyzed in detail. The sensitivity of heat production performance to heat convection coefficient, injection rate, injection temperature, fracture network morphology was discussed. Results indicate that at the early stage, the interconnected large-scale fractures around wellbore dominate the mass and heat transport. Some scattered small-scale fractures contribute to uniformly propelling the cooling of the heat rock mass. The change in effective stress associated with the thermo-poroelastic effect may induce fracture shear dilation and pore expansion, resulting in the permeability enhancement of the overall reservoir. In some regions, where the temperature drop is insignificant and the pore pressure decreases, thereby inducing compressive stress to make fractures closure. It is more practical to use the LNTE theory to analyze the heat extraction process in fractured geothermal reservoirs. The heat extraction efficiency, thermal breakthrough time and service-life of an EGS are seriously affected by the hydraulic conductivity and connectivity of fracture networks under constant injection rates and injection temperatures. Generating a complex and scattered fracture network but without preferential channels is conducive to extracting more heat from geothermal reservoirs.

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