A Comparison of Gas and Water Permeability in Clay-Bearing Fault and Reservoir Rocks: Experimental Results and Evolution Mechanisms

To better understand the discrepancy between gas and water permeability and the mechanism of permeability in response to the presence of water in clay-bearing rocks, we performed transport property experiments on synthetic quartz-clay mixtures and natural fault gouge, as well as clay-bearing sandstones for comparison purpose. The experiments were performed on a fluid flow apparatus with effective pressures (P-e) cycling between 5 and 105 MPa. Each sample was subjected to nine P-e cycles, along which permeability and porosity of either the gas-saturated (nitrogen) or water-saturated samples were measured. The results show that the permeability of all samples investigated decreases with increasing P-e. Gas permeability exhibits strong pore pressure dependence, which can be explained by the slippage (Klinkenberg) effect. Permeability results measured by water are commonly lower than the gas results. The decrease in permeability after the addition of water can be generally explained by the evolution of porosity, as can be determined from the bulk volume and solid mineral volume measurement results. The loss of permeability is associated with the hydration expansion of clay minerals that causes the reduction of porosity. A conceptual model for the evolution of porosity that incorporates clay expansion and grain rearrangement is proposed to describe the mechanism for water permeability in clay-bearing rocks. Based on these results, we discuss the effect of clay hydration on fluid transport and mechanical behaviors in clay-bearing fault zones and reservoir formations following fluid injection.