Shear failure plays an important role in fracture propagation in poorly consolidated formations. In past work, the effect of shear failure on the permeability and porosity of sands has been inconclusive. In this paper, a three-dimensional discrete element model is used to estimate changes in porosity and permeability due to mechanical deformation of sand-packs. Using pore network fluid flow simulations, the effect of shear failure and stress anisotropy on the permeability anisotropy and dilation of the granular specimens is analyzed.

Mechanical deformation data from experiments conducted on cylindrical sand packs is used to calibrate the simulations. Using these calibrated models, anisotropic stress regimes are modeled to mechanically deform the simulated samples. Post-processing tools have been developed to observe the preferential orientation of failure planes. Pore network fluid simulations describe the orientation of the permeability tensor. The principal permeability values of an anisotropically stressed sample are described in 3-D.

Deformation of the samples induces shear failure planes, which are preferentially oriented along the maximum horizontal stress direction. Deformed samples with the same minimum horizontal stress (50 psi) but increasing maximum horizontal stresses (50 psi to 300 psi) show an increase in the permeability in the maximum horizontal stress direction by 28-38%. An increase in horizontal stress anisotropy is shown to result in an increase in permeability in all directions due to dilation and failure. The effect of confining pressure, grain size distribution and sorting on failure, permeability anisotropy and dilation is also studied.

The resultant permeability anisotropy induced by shear failure and dilation is of vital significance for simulating fracturing and flow processes in soft rock formations.

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