Mechanisms for Slow Faulting in Porous Sandstones Deformed under Drained Conditions

Slow slip events on natural faults are often detected in regions with elevated pore fluid pressures. When the timescale for deformation is faster than the timescale for fluid diffusion, dilatant microcracking in rock leads to drops in pore fluid pressure that strengthen the rock and stabilize failure. Such strengthening is termed dilatant hardening and occurs when deformation is undrained. In laboratory experiments using the same differential pressure (the difference between confining and pore fluid pressures), slow faulting in compact rocks at conditions of high pore fluid pressure can be attributed to dilatant hardening, but whether high pore fluid pressure stabilizes failure in porous rock where deformation is drained remains unclear. We deformed highly permeable Adamswiller and Darley Dale sandstones with pore fluid pressures ranging from 2 to 180 MPa while maintaining a constant differential pressure of 10 MPa throughout each experiment. Three constant strain rates (10-4 s-1, 10-5 s-1 and 10-6 s-1) were used. Experiments completed on the Adamswiller samples did not show rate-dependent shear strength and similar faulting behaviors were observed at all pressure conditions, indicating negligible dilatant hardening during drained deformation. However, while failure behaviors of Darley Dale sandstone deformed at 10-4 s-1 and 10-5 s-1 are qualitatively similar to that of Adamswiller, Darley Dale samples deformed at a slower rate of 10-6 s-1 exhibited slow faulting at high pore fluid pressures. The observed slow faulting under drained conditions could not be explained by the conventional dilatant hardening model. We observed pervasive grain crushing and pore collapse in the sample failed by slow faulting. We propose that brittle creep plays an important role during faulting in Darley Dale deformed at 10-6 s-1. The interplay between dilatant hardening and stress corrosion facilitated fault growth is responsible to slow faulting in porous rocks under nominally drained conditions.