Micromechanics of Shear Localization under Elevated Temperature and Pore Pressure in Experimentally Deformed Quartz Sandstone

Abstract:

Triaxial compression experiments were conducted on a permeable quartz sandstone under argon pore fluid and vented conditions at temperatures to 900°C and effective pressures to 175 MPa. Vented tests exhibit a transition from brittle faulting towards thermally-activated cataclastic flow. Microfracture density and acoustic emissions indicate that the transition corresponds to a change in mechanism from rate-insensitive fracture to subcritical cracking. We conclude that the transition occurs when shear microfracture propagation and macroscopic shear localization are suppressed under increased grain-scale fracture energy and reduced dilatancy. A comparison of the two tests shows that pore fluid has little influence on the failure behavior at low temperatures. Conversely, pore fluid tests exhibit substantially smaller strengths and enhanced ductility than those of vented tests at high temperatures and pressures, similar to weakening induced by decreasing strain rate in the water-saturated sandstone at room temperature. We infer that the observed weakening results from an enhancement in intergranular frictional slip involving subcritical cracking where retained intracrystalline water reduced fracture energy. We are characterizing detailed microstructures and finite strain fields in samples deformed at varied strains, as well as evolution in volume strain and other inelastic properties, to further constrain the pore fluid effects on shear localization processes.