Stress-induced fluid flow anisotropy in fractured rock

Anisotropic stress states are common in the upper crust and result in fracture apertures being dependent on fracture orientation. Fractured rocks should therefore display an anisotropic permeability determined by the aperture, length, and orientation of those fractures remaining open. In this paper, a numerical study of this effect is made for a rock containing two orthogonal fracture sets subject to a uniaxial compressive stress applied perpendicular to one of the sets. With increasing compressive stress, the decreasing aperture of fractures orientated perpendicular to the stress axis leads to a decrease in permeability both parallel and perpendicular to the stress. For flow parallel to the stress direction, this is a consequence of the finite length of the fractures, flow in fractures perpendicular to the stress being required to connect fractures orientated parallel to the stress direction. As the number of fractures is decreased towards the percolation threshold, the average permeability tensor is found to become increasingly isotropic. This behaviour results from the highly tortuous nature of the flow paths just at the percolation threshold.