Permeability anisotropy and fabric development: A mechanistic explanation

We develop a model to describe development of permeability anisotropy and fabric in clay-rich sediments due to clay grain reorientation during consolidation and shearing. The model considers porosity, grain aspect ratio, and average angle of grains with respect to the horizontal plane. To validate the model, we determined permeability anisotropy ratios (ratio of horizontal permeability to vertical permeability) of porous media composed of flat cylindrical particles by lattice-Boltzmann simulations. Over representative ranges of grain aspect ratio (diameter/ thickness = 1-20) and porosity (44%-82%) the simulation results match the predicted values well. We show that permeability anisotropy ratios up to similar to 20 can be attained within highly sheared (shear strain >20), low-porosity (<40%) materials with aspect ratios >20, and that the maximum anisotropy ratio attainable by grain rotation is limited by grain aspect ratio. We further show that the anisotropy ratio of mixtures of low aspect ratio and high aspect ratio particles, like silty clays, are low (<2). This occurs because the low aspect ratio particles reduce the difference in tortuosity in transverse directions. Our results demonstrate why larger permeability anisotropy ratios are possible only through diagenesis, layering, or development of aligned microcracks.