Shear-enhanced compaction and permeability reduction: Triaxial extension tests on porous sandstone

Abstract

Triaxial extension experiments were conducted to investigate the influence of radial stress on porosity and permeability (for hydraulic flow along the axial direction) in three porous sandstones. The effective mean stresses were sufficiently high that the samples failed by cataclastic flow, with development of strain hardening and shear-enhanced compaction. Comparison of the new data with triaxial compression data from a previous study shows that the critical stress states for the onset of shear-enhanced compaction are comparable for the two different loading paths. The initial yield stress data for each sandstone map out an approximately elliptic envelope in the stress space. Stress-induced permeability anisotropy was inferred from synthesis of the triaxial compression and extension data. Before the onset of shear-enhanced compaction, permeability and porosity reduction are primarily controlled by the effective mean stress and stress-induced anisotropy is negligible. With the onset of shear-enhanced compaction and development of cataclastic flow, coupling of the deviatoric and hydrostatic stresses induces considerable permeability and porosity reduction. The permeability for flow along the direction of the maximum (compressive) principal stress is greater than that along the minimum principal stress. Microstructural observations on the shear-compacted samples show appreciable increase of grain crushing and pore collapse, which explain the overall decrease in permeability. The damage from grain crushing is highly anisotropic, with the stress-induced microcracks preferentially aligned with the maximum principal stress direction. Because more microcrack conduits are available to focus the flow in this direction, the permeability is relatively enhanced.