Abstract
Fluid flow tests on three-dimensional (3D) transparent fracture networks were performed to clarify the effect of aperture heterogeneity on the hydraulic properties of rough fractures. The five-axis machining combined with the 3D construction technique was applied to create the transparent sample, which ensures the direct visualization and quantification of flow behavior through the fractures. The effects of the anisotropic aperture on flow channeling and permeability of the fracture networks were evaluated using a developed numerical code, the validity of which has been verified by the flow test results. The numerical investigation was extended to complex 3D discrete fracture networks with large fracture densities. The results show that five-axis machining can reproduce a 3D fracture network sample with spatially distributed apertures by first machining the fracture blocks and subsequently assembling them. An obvious channeling flow in the sample was observed during the flow tests. With decreasing mean and/or increasing deviation of the aperture fields following a normal distribution, the localization of fluid flow is increasingly obvious; the main flow channels account for approximately 26–67% of the fracture planes. The ratio of permeability of the model with apertures following truncated lognormal distributions to those following complete lognormal distributions first showed remarkably increase and subsequently approached 1.0 with the increasing truncation threshold of the aperture distribution. A mathematical expression is proposed for predicting the critical truncation threshold, which is defined when the normalized permeability is equal to 0.9. The suitability of upscaling the proposed expression to complex 3D discrete fracture networks is verified, which can help to determine the proper truncation threshold when simulating a fracture aperture with lognormal distribution functions in fractured rock masses.
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