Shear fracture geometry including dilation, roughness, and anisotropy exerts a primary control on fracture permeability. Predictive models of the geometry of shear fracture formation are few, resulting in limited understanding of the permeability of subsurface shear fractures as a function of stress conditions, rock type and other factors. We conduct experiments with triaxial direct shear methods at subsurface stresses that provide new insights into intrinsic mechanisms controlling fracture roughness and the significant anisotropy of fracture aperture distributions. Observed fracture systems contain multiple segments at a variety of scales that initiate as shear fractures and which are out-of-plane with the imposed direct-shear plane and interconnect via tensile fractures. We use a combination of theory and finite-discrete element models to explore the origins of this behavior and the significance for fracture permeability. We find that the simulations are consistent with a theory that predicts that multiple segment fracture systems (en échelon structures) are favored (in terms of stress to cause failure) relative to simple planar fractures. These en échelon structures create permeability anisotropy as well as imparting roughness consistent with field and laboratory observations.

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