Numerical study of the long-term performance of EGS based on discrete fracture network with consideration of fracture deformation

Abstract

During heat extraction in fractured reservoir, the perturbation of pressure, stress, temperature could alter the fracture aperture, which feeds back to the long-term performance of EGS. In this work, a 3D THM coupled model was developed for the long-term performance based on the discrete fracture network. The apparent improvement of this model is both shear dilatancy and normal closure/opening are considered in a complete constitutive relationship under the THM coupled framework TOUGH2M-FLAC3D. The feasibility is verified and validated through comparing with analytical solution and previous numerical work. In application of the new approach in a fractured reservoir, it is found that the phenomenon of channel flow will concentrate the heat mining in partial fractures, and the fracture enhancement further reinforce the channel flow. The pressure effect on fracture transfers faster than the thermal effect. In the sensitivity analysis, the zero-stress aperture has a positive impact on heat extraction. Higher thermal expansion coefficient promotes the heat extraction as well. While, the dilation angle in this work has a neglectable impact on heat extraction performance. The thermal effect accounts for the most fracture variation under high thermal expansion coefficient. Therefore, the thermal effect should be considered especially in high temperature geothermal reservoir.