Shear Induced Seepage and Heat Transfer Evolution in a Single-Fractured Hot-Dry-Rock

Abstract

In the enhanced geothermal system (EGS), the injected fluid will induce shear sliding of rock fractures (i.e., hydroshearing), which consequently, would increase the fracture aperture and improve the heat transfer efficiency of the geothermal reservoir. In this study, theoretical analysis, experimental research and numerical simulation were performed to uncover the permeability and heat transfer enhancement mechanism of the Hot-Dry-Rock (HDR) mass under the impact of shearing. By conducting the direct shear test with the fractured rock samples, the evolution process of fracture aperture during the shearing tests was observed, during which process, cubic law was adopted to depict the rock fracture permeability. To investigate the seepage characteristics and temperature distribution of the fractured HDR under the influence of shearing, a simulation study of shear-seepage-heat transfer in a fractured rock mass has been conducted to validate the observed shear-induced fracture dilation during the direct shear test. The results demonstrate that (1) the hydroshearing increases the dilation of granite fracture and enhances the permeability of the HDR rock mass, while the temperature around the HDR fracture will reduce. (2) Fracture roughness is of vital importance to enhance the permeability during the shearing tests. To be more specific, a rougher fracture always implies a higher permeability and a greater heat extraction efficiency. (3) The shear induced heat extracting efficiency is dominated by the increased fluid flux in the earlier period of the EGS reservoir, and this efficiency is controlled by the outlet water temperature since the fluid flux becomes stable after the shearing test. Therefore, balancing the hydroshearing enhanced heat extraction efficiency and EGS reservoir lifespan would be significant to the sustainable development and utilization of geothermal energy