Experimental and numerical studies on fluid flow through fractured rock masses based on an enhanced three-dimensional discrete fracture network model

Abstract

Fluid flow through rock fracture networks was experimentally and numerically studied based on an enhanced discrete fracture network (DFN) model that explicitly characterizes 3D void geometry within rough-walled fracture. Fluid flow tests under different hydraulic gradients J were conducted on a series of DFN samples created by a 3D printer. Meanwhile, numerical simulations were performed based on the enhanced DFN model solving the NS equations and conventional DFN model solving the Reynolds equation, respectively. The validity of the simulations was verified by comparison with flow tests. Then numerical investigations were extended to amend the permeability estimated by the Reynolds equation to seek for an acceptable approximation to the calculation of the Navier–Stokes (NS) equations. The results indicate that the enhanced DFN model can better capture the nonlinear flow caused by surface roughness and aperture heterogeneity, providing more realistic fracture representation and more accurate results. The conventional DFN model overestimates permeability by up to 82 % compared to the flow test result, while the enhanced DFN model give more accurate permeability with a fewer error of 5.3 %. As the fracture number or surface roughness increases, the critical hydraulic gradient J_c that defines the onset of the nonlinear flow decreases. For the linear flow regime under J < J_c, a model that can directly compare the equivalent permeability estimated by NS equations and Reynolds equation was proposed. This is important for assessment of permeability of fracture media where the conventional DFN model solving Reynolds equation is primarily utilized to reduce computational burden.