Effect of High-Pressure Deformation on Permeability of Ottawa Sand

Abstract

High-pressure laboratory experiments have been performed in which permeability and pore-volume changes were measured during hydrostatic and triaxial deformation of Ottawa sand. Under hydrostatic compression, the permeability and porosity of the Ottawa sand initially decreased fairly gradually. At a confining pressure of about 600 bars, however, an abrupt onset of grain crushing and pore collapse resulted in accelerated compaction and permeability loss. In the triaxial-deformation experiments, considerable grain crushing accompanied deformation and both large decreases in pore volume and extremely large decreases in permeability occurred. At higher confining pressures, even though relatively less pore-volume decrease accompanied deformation, there was greater permeability ecrease with axial strain. Dilatancy, or an increase in pore volume with deformation, occurred near the maximum in the stress-strain curves. This dilatancy preceded failure, apparently continued uniformly into the post-failure region, and resulted in a lessened rate of permeability decrease with strain. Comparison of the permeability changes in Ottawa sand in compression and extension showed that if granular material has been subjected to sufficient deformation, fluid flow can be quite anisotropic, the permeability in the direction of maximum compression being significantly lower than the permeability normal to that direction.