Permeability enhancement due to microcrack dilatancy in the damage regime

A three dimensional microscopically based permeability model incorporating inelastic deformation has been developed to account for the modification of transport properties due to fracturing. The basic hypothesis investigated is that permeability enhancement during brittle deformation is caused by th e formation of dilatant microcracks which are associated with friction slid ing on a preexisting random population of shear cracks. Additional dilatanc y, produced as a result of frictional sliding over asperities, is also acco unted for by the introduction of a crack roughness parameter. Linear elasti c fracture mechanics is used to calculate the evolution of crack length and crack area as a function of applied stress and fluid pressure. After makin g a geometrical simplification the microcrack parameters derived from the d eformation model can be used to calculate the permeability tensor assuming that fluid transport results from Poiseuille flow through a connected distr ibution of cracks. The model enables investigation of the macroscopic perme ability variation as a function of two loading parameters, three constant m aterial parameters, and the crack roughness parameter. Results demonstrate that permeability is a smooth but strongly increasing: (near power law) fun ction of the Terzaghi effective stress ratio and is strongly dependent on t he initial crack density, Young's modulus, the friction coefficient, and th e effective confining: pressure. Numerical results are in quantitative agre ement with published experimental measurements and display behavior similar to results obtained with a relatively simple analytical model. The model e nables calculation of the degree of stress-induced anisotropy, which is sho wn is be relatively small (< x 10) for resonable effective stress ratios. T he modeling presented provides a quantitative tool with which the effects o f microcrack-induced permeability enhancement can be investigated within th e broader context of coupled fluid flow and brittle deformation in the crus t.