Brittle Faulting in Porous, Water-saturated Sandstone Deformed under High Pore Fluid Pressure

Abstract:

The effective stress law states that the shear strength of fluid-saturated rocks depends on the effective stress, which is equal to the difference between total stress and pore fluid pressure. Recent laboratory experiments on compact rocks (initial porosity < 1%) demonstrated that under the same effective stress, conditions of high pore fluid pressure resulted in stabilized failure (French and Zhu, 2017). Dilatant hardening (Rice, 1975) was thought to be responsible for the observed stabilizing effect. If the rate of fluid diffusion is slower than the rate of deformation, cracking will result in local increases in effective normal stress and impede further crack growth. In the current mechanical framework, dilatant hardening is controlled by a rock’s bulk permeability and should more readily occur in compact rocks.

In this study, we deformed water-saturated Darley Dale Sandstone (initial porosity ~14%) at various strain rates to investigate the effectiveness of high pore fluid pressures on stabilizing fault growth in high bulk permeability rocks. We deformed the samples at strain rates from 10^-4 to 10^-6 s^-1 while using a constant effective pressure of 10 MPa and pore fluid pressures ranging from 2 to 180 MPa. The mechanical data show that the shear strength is well-predicted by the effective stress law, but experiments completed at a pore fluid pressure of 180 MPa resulted in distinctly slower slip velocities, smaller stress drops, and longer weakening durations. The microstructural observations of two-end member experiments reveal that conditions of high pore fluid pressure resulted in pervasive grain comminution and higher average crack densities compared to the low pore fluid pressure case. Such grain crushing at high confinements is likely accompanied by local permeability reduction that results in dilatant hardening even in a porous, permeable sandstone. Permeability near rupture surfaces must be considered as a dynamic parameter that changes during fault slip. Ultimately, high pore fluid pressure may impede rupture on brittle faults and result in slow slip behavior regardless of the initial permeability of the fault zone.