Evolutions of Anisotropic Hydraulic Properties of Rough-Walled Rock Fractures under Different Shear Displacements

Abstract

Four cylindrical sandstone samples were extracted from the original rectangular sample with a rough-walled fracture. Each drilling angle (θ) of cylindrical sandstone samples is different to consider the anisotropies of rough-walled rock fractures. For each sample, different flow velocities ranging from 0 m/s to 13 m/s were designed. For a given flow velocity, a series of different confining pressures ( ${\sigma }_n$ ), including 1.5 MPa, 2.5 MPa, and 3.5 MPa, were applied on the fractured samples. The hydraulic properties of each cylindrical sandstone sample were tested under different shear displacements ( $u_s$ ) and ${\sigma }_n$ . The results show that the hydraulic gradient ( $J$ ) shows an increasing trend with the increment of ${\sigma }_n$ . With the increment of the Reynolds number ( $\mathrm{Re}$ ), the transmissivity ( $T$ ) decreases in the form of the quadratic function. The normalized transmissivity ( $T/T_0$ ) decreases with the increment of $J$ . The variations in $T/T_0$ with $J$ can be divided into three stages. The first stage is that $T/T_0$ approximately holds a constant value of 1.0 when $J$ is small indicating that the fluid flow is in the linear regime. The last two stages are that $T/T_0$ decreases with the continuous increase of $J$ , and the reduction rate first increases and then decreases. The critical Reynolds’ number ( ${\mathrm{Re}}_{\text{c}}$ ) of the sample angle with a drilling angle of 90° is different from that of other samples. The corresponding ${\mathrm{Re}}_{\text{c}}$ is 6.52, 28.73, and 32.1 when the shear displacement $\left(u_{\text{s}}\right)=2\text{mm}$ , 3 mm, and 4 mm, respectively. The variations in ${\mathrm{Re}}_{\text{c}}$ and $J$ along different drilling angles are significantly obvious. When the confining pressure is large, the effect of anisotropy on Rec is much greater than that of confining pressure.