Shear enhanced nonlinear flow in rough-walled rock fractures

Nonlinear flow in 3D rough-walled rock fracture models are simulated by solving the Navier-Stokes equations in this paper. The emphasis is on the impacts of shear caused aperture changes (variable apertures and asperity contacts) and flow conditions (inertial term) upon nonlinear flow behaviors in 3D rough-walled rock fractures. In order to compare shear effects, two 3D fracture models, with and without shear process, were established with the identical initial rough-walled surfaces tomography of a realistic rock sample. Five groups of simulations with different inflow boundary conditions of flowrates/Reynolds numbers (Re) were conducted to demonstrate shear enhanced nonlinearity of flow fields and limitations of local cubic law (LCL) approach. The flow results clearly show channeling flow along the preferential fluid paths, transverse flow around the contact spots and eddy flows behind contact spots with increasing Re numbers, which cannot be observed in 2D models. The effective transmissivity of the 3D fracture model was calculated from the modeling results of velocity and pressure fields. The results showed that the effective transmissivity is a function of local apertures with important uncertainties even when Re is small (i.e. Re = 0.4 in this study), thus the validity of the transmissivity evaluation using LCL approach for nonlinear flow in 3D rough-walled rock fractures is questionable. The mechanical effects, i.e. stress and shear caused aperture space changes and asperity contacts should be considered for modeling flow and mass/energy transport processes in rough-walled fractures in 3D.QC 20161010