Characterisation of granite joint structures and their influence on permeability in the Beishan Underground Research Laboratory, China

To better understand fluid migration in fractured granite joints, an improved middle axis (IMA) method and 3D laser scanning are employed to accurately measure the aperture and roughness of real rock fractures, respectively. The Monte Carlo method and the principle of fractional Brownian motion (FBM) are used to generate fracture models based on the actual fracture structures observed in the field, overcoming the limitations of assuming random distributions in existing modeling methods. Furthermore, the lattice Boltzmann method (LBM) is used to investigate the effect of various fracture distributions on fluid seepage characteristics. The results showed that the distribution of aperture and roughness within fractures significantly influences fracture seepage flow, including fluid velocity and pressure distributions. Abrupt changes in roughness reduce the effective cross-sectional area of fluid flow, increase the hydraulic slope drop, and induce local vortices, leading to elevated fluid frictional resistance and energy losses. The permeability of fractured rock mass is influenced by the nonlinear interaction between aperture and roughness, with this effect being particularly strong in small apertures. As the aperture increases, the impact of roughness decreases, and the aperture becomes the dominant factor. When the ratio of average aperture geometry to maximum roughness undulation exceeds 10, the influence of roughness on fluid seepage is minimal, and the aperture dominates flow characteristics.