Beyond-Laboratory-Scale Prediction for Channeling Flows through Subsurface Rock Fractures

The present study evaluates aperture distributions and fluid flow characteristics for variously sized laboratory scale granite fractures under confining stress. As a significant result, the contact area in the fracture plane was found to be independent of scale. By combining this characteristic with the fractal nature of the fracture surfaces, a method for predicting fracture aperture distributions beyond laboratory scale is developed. The validity of the method is revealed through reproduction of the results of the laboratory investigation and the maximum aperture-fracture length relations, which have been reported in the literature. The present study finally predicts conceivable scale dependencies of fluid flows through joints and faults (fractures without and with shear displacement). Both joint and fault aperture distributions are characterized by a scale-independent contact area, a scale-dependent geometric mean, and a scale-independent geometric standard deviation of the aperture. The contact areas for joints and faults are approximately 60% and 40%, respectively. The changes in the geometric means of joint and fault apertures (μm), e_{m, joint} and e_{m, fault}, with fracture length (m), l, are respectively approximated by e_{m, joint} = 1×10²l^0.1and e_{m, fault} = 1×10³l^0.7, whereas the geometric standard deviations of both joint and fault apertures are approximately 3. Fluid flows through both joints and faults are characterized by the formations of preferential flow paths (channeling flows) with scale-independent flow areas of approximately 10%, whereas the joint and fault permeabilities (m²), k _joint and k _fault, are scale dependent and are approximated as k _joint = 1×10^–12l^0.2 and k _fault = 1×10^–8l^1.1, respectively.