Assessment of nonlinear flow behaviors through real rough rock fractures during shearing

In order to investigate the transition characteristics of the fluid flow regime of real rough rock fractures during shearing, a series of fractured samples with various joint roughness coefficients (3.21–12.16) and shear displacements (1–15 mm) were accurately engraved. Fluid flow tests were conducted under various confining pressures (0.5–20 MPa) and inlet volume flow rates. From the experimental results, the nonlinear relationships between the flow rate and the hydraulic gradient were fitted with the Forchheimer’s law. Both the linear and nonlinear coefficients decreased during the shearing and increased with the increase in the confining pressure. For all the test cases, the critical hydraulic gradient decreased by 81.37–94.99% with the shear displacement, but increased with the increase in the confining pressure. The equivalent hydraulic aperture increased by 2.757–8.364 times for the shear displacement range of 1–15 mm due to dilation and decreased by 1.306–3.023 times for the confining pressure range of 0.5–20 MPa due to closure of fractures. The transmissivity decreased with the increase in confining pressure, while it increased with the increase in shear displacement. In addition, during the shearing process, the change of transmissivity with Reynolds number can be characterized by a fifth-order polynomial function. The Forchheimer coefficient ( β ) increased by 71.05–153.15% with the applied confining pressure, while it decreased by 42.72–69.52% with the increase in shear displacement. Variations in the equivalent permeability coefficient with the hydraulic gradient can be divided into three stages of being constant, gentle decrease and significant decrease, and corresponded to linear Darcy flow, weak inertia and strong flow nonlinearity, respectively.