THE TRANSITION FROM BRITTLE FAULTING TO CATACLASTIC FLOW - PERMEABILITY EVOLUTION

Triaxial compression experiments were conducted to investigate influen ces of stress and failure mode on axial permeability of five sandstone s with porosities ranging from 15% to 35%. In the cataclastic flow reg ime, permeability and porosity changes closely track one another. A dr astic decrease in permeability was triggered by the onset of shear-enh anced compaction caused by grain crashing and pore collapse. The compa ctive yield stress C maps out a boundary in stress space separating t wo different types of permeability evolution. Before C is attained, p ermeability and porosity both decrease with increasing effective mean stress, but they are independent of deviatoric stresses. However, with loading beyond C, both permeability and porosity changes are strongl y dependent on the deviatoric and effective mean stresses. In the brit tle faulting regime, permeability and porosity changes are more comple x. Before the onset of shear-induced dilation C', both permeability an d porosity decrease with increasing effective mean stress. Beyond C', permeability may actually decrease in a dilating rock prior to brittle failure. After the peak stress has been attained, the development of a relatively impermeable shear band causes an accelerated decrease of permeability. Permeability evolution in porous sandstones is compared with that in low-porosity crystalline rocks. A conceptual model for th e coupling of deformation and fluid transport is proposed in the form of a deformation-permeability map.