Effect of shearing on hydraulic properties of rough-walled fractures under different boundary conditions

Abstract

This study experimentally investigated nonlinear flow characteristics of rough-walled fractures during shear process under different boundary conditions, using the servo-controlled shear-flow apparatus. A series of shear-flow tests were conducted on two types of rough fractures with different shear displacements under various boundary conditions. The effects of shear process, fracture surface roughness, boundary normal stiffness, and initial normal stress on nonlinear flow behaviors are analyzed. The results show that Forchheimer equation provides a good description of the nonlinear relationship between flow rate and pressure gradient in rough-walled fractures. The linear and nonlinear coefficients decrease by 1-2 and 2-3 orders of magnitude during shear, respectively. Both the mechanical aperture and the hydraulic aperture increase with increasing shear displacement. The mechanical aperture is always larger than the hydraulic aperture, and their difference gradually increases with the increment of shear displacement. The contact area ratio exhibits a fast decrease as shear displacement ranges between 1 and 3 mm and then decreases gradually with the decreasing rate generally diminishes. With the increase in shear displacement, the critical Reynolds number shows an increasing trend. The critical Reynolds number is in range 3.22-35.41 for fracture G1 and 1.23-12.35 for fracture G3. As the boundary normal stiffness and initial normal stress increase, the critical Reynolds number decreases.