Effects of fracture intersection on the hydraulic properties of 3D crossed rough-walled fractures

Abstract

Characterization of hydraulic properties of crossed fractures is a key issue in understanding flow behavior of rock masses with complex fracture networks, which is of primary importance for many geoengineering practices. The effects of fracture intersection on fluid flow through 3D crossed rough-walled fractures in a linear flow regime were investigated by numerical simulations. A self-developed finite volume code was employed to solve the Navier–Stokes equation in 3D crossed fracture models. Different synthetic fractures were generated based on the data collected from three natural fracture surfaces. A series of numerical models were established by intersecting the different generations of the synthetic fractures to study the effect of surface roughness, intersection angle, intersection length, and scale on fluid flow through crossed rough-walled fractures. To evaluate the intersection effects, the effective transmissivity of the fracture segments, Teff, which is defined as the transmissivity affected by fracture intersection, was compared with the actual transmissivity of the fracture segments, Ta. The results showed that the ratio of Teff/Ta fluctuates more severely for the models with rougher fracture surfaces due to the formation of more complex geometry at fracture intersection. The ratio tends to reach a converged magnitude at large intersection lengths but with a wider ranges of variation for the models with rougher fracture surfaces. The results also showed that beside the complex geometry of intersection, the difference in the ratio of the mean aperture of the intersection facet to the actual hydraulic aperture of the corresponding fracture segment between different fracture segments may have an even more significant impact on the fluid flow in crossed fractures. The intersection angle affects the fluid flow through crossed fractures by altering the geometrical characteristics of the intersection in a heterogeneous manner, which stems from random nature of fracture surface roughness. The effects of intersection angle become negligible at large intersection lengths. The simulation results suggest one of the conditions to ignore the intersection effects on flow behavior of large-scale rough-walled fracture networks when flow is in a linear regime. Accordingly, all the fracture lengths in the fracture network should be larger than REV size of the fractures.