A micromechanical thermo-hydro-mechanical coupling model for fractured rocks based on multi-scale structures variations

Under the thermodynamics framework, a thermo-hydro-mechanical coupling model was established by a multi-scale homogenization technique for fractured rock mass containing both small-scale arbitrarily-distributed microcracks and large-scale grouped-directionally-distributed fractures. The proposed model simultaneously considers the deformation evolution and permeability and thermal conductivity variations induced by the damage growth, compression closure and frictional sliding of microcracks and the friction sliding and shear dilatancy of fractures under coupled thermo-hydro-mechanical loading. Model numerical implementation was carried out by the linkage between the self-developed FEM-based micromechanical code and TOUGH2, and verified with analytical solutions on a solid column under thermal consolidation. Laboratory compression test data with thermal conductivity measurements on Beishan granite was also used to validate the proposed model, with good agreement between the predicted and measured results. Finally, the proposed model was utilized to numerically study the heat extraction responses of a large scale three-dimensional enhanced geothermal reservoir, demonstrating the importance of considering the thermo-hydro-mechanical coupling behavior and the variations of both small-scale microcracks and large-scale fractures for better understanding the heat transfer and water pressure evolution responses and deformation-induced reservoir permeability change during heat mining.