Scalable En Echelon Shear-Fracture Aperture-Roughness Mechanism: Theory, Validation, and Implications

Shear fractures can facilitate fluid conductivity through rock. Aperture and roughness are controlling characteristics for a fracture's fluid conductivity. Inspired by en echelon fractures, we develop a shear fracturelet model that predicts anisotropic aperture with respect to the direction of shearing, rougher (nonplanar) rather than smoother (planar) fractures, and the bounds of this roughness for a coalesced fracture. This tendency for rougher fracture creation is validated by in situ X-ray images and fluid conductivity measurements from triaxial direct shear experiments on anhydrite and shale. These experiments were conducted at confining stresses from 4 to 30MPa and shear displacement magnitudes from 0 to 2mm on initially intact rock specimens. Hydraulic, dilatational, and local fracture apertures were measured in the experiments. Apertures exhibited strong anisotropy with more conductive flow paths forming perpendicular to the direction of shearing. Local and dilatational aperture were found to be positively correlated with increasing shear displacement but hydraulic aperture was found to vary significantly, always having values smaller than the other aperture measures at factors ranging from 0.6 to 0.0. An implication of these results is that shear fractures have a mechanism for simultaneously exhibiting very low fluid conductivity and high fluid storage volume.