Direct measurement of contact area and stress dependence of anisotropic flow through rock fracture with heterogeneous aperture distribution

Direct measurement of a contact area using pressure-sensitive film was performed for artificially generated granite tensile fractures, with different shear displacements of 2, 3, and 5 mm and at various normal stresses of up to approximately 100 MPa, to clarify the relationship between anisotropic flow and normal stress in a rock fracture with shear displacement. Aperture distributions with contact areas were then numerically generated using fracture surface geometry data, and anisotropic flow was evaluated through a bi-directional flow simulation. The directional distributions of contacting asperities in the fracture plane were clearly observed for every given condition and the contacting asperities tended to align in the direction perpendicular to shear displacement. Generally, the contact area increased and the anisotropic distribution was enhanced with increasing normal stress. However, the contact areas for ≥ 3 mm shear displacement were less stress dependent. Anisotropic flow with higher permeability in the perpendicular direction was always observed for aperture distributions with an anisotropic distribution of the contacting asperities. Flow paths created in the aperture distributions seemed less tortuous in the perpendicular direction due to the distribution of the contacting asperities. The anisotropic flow was enhanced with increasing normal stress to the fracture, and was more evident for the smaller shear displacements of 2 mm due to the high sensitivity of the contact area to the normal stress. The results suggested that the anisotropic flow was likely for a wide range of normal stresses (and correspondingly, at varying depths in the Earth's crust), at which anisotropic flow was enhanced with increasing normal stress based on the stress dependence of the contact area. In addition, the stress dependent anisotropic flow is expected for much higher normal stresses because complete fracture closure with 100% contact area may be unexpected under the crustal stress conditions at depths of several to ten thousand meters.