Laboratory hydraulic shearing of granitic fractures with surface roughness under stress states of EGS: Permeability changes and energy balance

Abstract

To offer key parameters and constitutive laws required for field-scale multiphysics simulations that can accurately predict created fracture network structures in enhanced geothermal systems (EGS) and resulting energy extraction, we explore the full spectral of hydraulic shear process of granite fracture and revisits the linkage between hydraulic and mechanical properties during shear slips. Key results from our novel lab-experiments include the followings: (1) fracture permeability of granite increases due to the hydraulic shear slip even at an effective normal stress of over 50 MPa, (2) shear slip and stress drop are in proportion and increment of the fracture permeability increases with increasing total shear slip displacement, and (3) although hydraulic shear slip tends to make fracture surfaces slightly smoother, the factual characteristics of surface are maintained after slip. By combining our experimental results with the seismological analysis, we first explore energy balance during hydraulic shearing of the preexisting rock fracture and point out the crucial role of the elastic potential energy stored in the surrounding bulk rock masses. Subsequently, we derive a constitutive model related to permeability change of granite fracture during hydraulic shearing under the typical crustal stress of EGS and presume that the maximum change in fracture permeability due to shear dilation is ∼20-fold, although scale effect is not considered. Finally, via the high resolution 2-D mapping of the surface damages, we illuminate the formation of preferential flow paths within rock fracture during hydraulic shearing and its possible role on the improvement of fracture permeability. Thus, we successfully demonstrate both new and sophisticated insights in hydro-mechanical coupled processes during hydraulic shearing to improve the accuracy of fracture network designs in EGS technology.