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During the heat extraction in enhanced geothermal systems (EGS), the porosity and permeability of the heat reservoir can be greatly affected by the multi-physical coupling of Thermal (T), Hydraulic (H), and Mechanical (M) processes. In the present work we develop a three-dimensional transient model fully coupling the subsurface THM behaviors during EGS heat extraction process. The local thermal non-equilibrium is assumed when describing the heat exchange between the rock matrix and heat transmission fluid, and the variable properties of the fluid are combined to realize the bi-directional coupling of thermal and hydraulic behaviors. The thermal-pore-elastic model is adopted to calculate the effective stress of the rock matrix and further to derive the local porosity and permeability. Case study with respect to an imaginary EGS demonstrates the validity and capability of the developed model. Results indicate that the model can be used for heat extraction simulation of more practical EGSs.