Decrease in the Permeability of Microcracked and Macrocracked Granite at Elevated Pressure and Temperature

Abstract

Pressure and temperature increase with depth, modifying the microstructure of crustal rocks. The opening or closing of micro- and macrocracks resulting from variations in the in-situ conditions influences the permeability of crustal rocks. While confining pressure is known to close pre-existing cracks, reducing permeability, the influence of temperature has received less attention. Here, we measured the permeability of micro- and macrocracked granite at confining pressures and temperatures up to 50 MPa and 150°C, respectively. We find that the permeability of micro- and macrocracked granite decreases with confining pressure. At constant pressure, increasing temperature reduces permeability by up to an order of magnitude, interpreted as the closure of microcracks due to mineral thermal expansion. Using a simplified microstructural model, we show that microcrack aperture is reduced by up to 50% at 150°C compared to room temperature. However, temperature does not appear to influence the permeability of a tortuous macrocrack.