Fluid-induced transition from precursor to lag-type critical slowing-down in shale failure

Injection-induced seismicity during hydraulic fracturing of shale reservoirs poses a growing operational risk, yet the role of pore fluids in shaping failure precursors remains poorly resolved. This study applies critical slowing-down (CSD) theory to paired laboratory experiments on Longmaxi shale to identify how fluids alter the emergence of failure precursors and to elucidate the underlying mechanism. Pseudo-triaxial compression tests on one dry and one water-injected specimen, with real-time acoustic emission (AE) monitoring, were analyzed using the variance and autocorrelation of strain, AE count, and AE energy as CSD indicators tracking the transition from stable deformation to macroscopic failure. The results reveal a clear divergence in precursor timing: in the dry specimen, CSD indicators rose 1.7–4.9 s before the macroscopic stress drop, consistent with a true pre-failure precursor response; in the water-injected specimen, CSD indicators emerged 9.6–14.0 s after the initial stress drop, representing a post-onset lag rather than a pre-failure warning. This contrast is interpreted as a consequence of fluid-driven processes — chiefly lubrication, clay softening, and effective-stress reduction — that may shift the dominant macroscopic failure mode from brittle shear toward progressive tensile fracture, delaying cooperative instability. Together, the observations support a mechanistic hypothesis that pore fluids regulate the temporal position of CSD signals relative to macroscopic failure, and motivate a dual-regime monitoring framework in which pre-failure warning is appropriate for dry, brittle formations while post-onset hazard tracking is appropriate for fluid-saturated conditions.