Shear-induced flow anisotropy and scale-dependent permeability in rough fractures

Fluid flow through rough-walled rock fractures is governed by surface roughness, shear deformation, and scale, yet a unified description of permeability evolution remains elusive. Self-affine fracture surfaces with varying roughness levels (JRC = 2–18) were generated using a modified successive random addition algorithm, and progressive shear displacement (3–18 mm) was numerically simulated to resolve aperture field evolution. Fluid flow was subsequently modeled using the Reynolds equation to evaluate equivalent permeability and directional transport behavior across scales (20–200 mm). Three principal physical mechanisms are revealed: (i) a competitive interplay between shear-driven enhancement and roughness-induced resistance governs permeability evolution; (ii) shear deformation fundamentally reorganizes flow topology through channelization aligned with the shear direction and contact–structure anisotropy that restricts transverse transport; and (iii) scale dependence arises from statistical averaging of local geometric heterogeneity, with flow stabilizing as observation windows expand. These mechanisms are synthetically captured in a coupled empirical correlation unifying roughness attenuation, shear-induced enhancement, and scale effects. Validation against numerical simulations demonstrates reliable first-order permeability prediction across the investigated parameter space.