Fault reactivation by fluid injection: Controls from stress state and injection rate

TL;DR

This study investigates how stress state and fluid injection rate influence fault reactivation, revealing that higher injection rates lead to increased fluid pressure and reactivation, driven by nonlocal rupture initiation rather than traditional Coulomb failure.

Contribution

The paper demonstrates that injection rate controls fault reactivation through fluid pressure heterogeneities, challenging conventional Coulomb failure models with a nonlocal rupture criterion.

Findings

Higher injection rates increase peak fluid pressure for reactivation.

Fluid pressure heterogeneities are induced by high injection rates.

Reactivation depends on nonlocal rupture initiation, not just Coulomb law.

Abstract

We studied the influence of stress state and fluid injection rate on the reactivation of faults. We conducted experiments on a saw-cut Westerly granite sample under triaxial stress conditions. Fault reactivation was triggered by injecting fluids through a borehole directly connected to the fault. Our results show that the peak fluid pressure at the borehole leading to reactivation depends on injection rate. The higher the injection rate, the higher the peak fluid pressure allowing fault reactivation. Elastic wave velocity measurements along fault strike highlight that high injection rates induce significant fluid pressure heterogeneities, which explains that the onset of fault reactivation is not determined by a conventional Coulomb law and effective stress principle, but rather by a nonlocal rupture initiation criterion. Our results demonstrate that increasing the injection rate enhances the transition from drained to undrained conditions, where local but intense fluid pressures perturbations can reactivate large faults.