Experimental Study of the Nonlinear Flow Characteristics of Fluid in 3D Rough-Walled Fractures During Shear Process

To understand the influence of shear on the hydraulic properties of rock fractures, shear-flow tests were carried out on rock fractures with different surface roughnesses. Each rough-walled fracture was replicated in four specimens, which were sheared at different displacements under normal stresses that varied from 0.5 to 2.0 MPa. At each shear displacement, a series of hydraulic tests with different hydraulic gradients were performed, and the nonlinear flow regimes of the fluid within the fractures were investigated. The results show that Forchheimer’s law can well describe the nonlinear relationship between the flow rate and the hydraulic gradient in rough-walled fractures. Both the linear coefficient and nonlinear coefficient decrease during shearing but increase as the normal stress increases. The critical hydraulic gradient increases with an increase in the shear displacement and normal stress. With an increase in the joint roughness coefficient, the critical hydraulic gradient decreases. The normalized transmissivity exhibits a strong correlation with the Reynolds number. As the shear displacement increases, the fitted curves of the normalized transmissivity versus the Reynolds number shift upward but the curves shift downward with an increase in normal stress. Additionally, the Forchheimer coefficient decreases with an increase in the shear displacement but increases with an increase in the applied normal stress. Visualization tests show that the number of flow paths is large when the shear displacement is small due to various distributions of the contact areas and that the flow of dyed water over the entire fracture decreases. As the shear displacement increases, the flow resistance decreases due to the shear dilation-induced increase in the aperture, and the advantage channel flow is distinct in the fracture. The contact ratio rapidly decreases as the shear displacement increases from 1 to 3 mm and then slightly varies with a continuously increasing maximum shear displacement of 9 mm