Shear Induced Seepage and Heat Transfer Evolution in Hot Dry Rock Reservior

Abstract

In the Enhanced Geothermal System (EGS), the fluid injecting-induced shear sliding of rock fractures (e.g. hydroshearing) would increase the fracture aperture and improve the heat transfer efficiency of a geothermal reservoir. In this study, we combined the method of theoretical analysis, experimental research and numerical simulation to understand the permeability and heat transfer enhancement mechanism of the hot-dry-rock (HDR) mass under the effect of shearing, which would contribute to promote the application of the hydroshearing technique. By conducting fractured-rock direct shear test, the evolution of fracture aperture during shearing was obtained and linked with rock fracture permeability through cubic law. With the test result of shear-induced fracture dilation, the simulation study of shear-seepage-heat transfer in a fractured rock mass has been conducted to investigate the seepage characteristics and temperature distribution of a fractured HDR under the effect of shearing. The results show that: (1) the hydroshearing increases the dilation of granite fracture and enhances the permeability of the HDR rock mass, but it would in turn reduce the temperature around the HDR fracture. (2) Fracture roughness is vital to enhancing the permeability under shearing. Specifically, 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 would be controlled by the outlet water temperature since the fluid flux becomes stable after shear. Therefore, balancing the hydroshearing enhanced heat extraction efficiency and the shortened EGS reservoir lifespan would be significant to the sustainable development and utilization of geothermal energy.