Fluid induced fault slip behavior: frictional healing vs velocity dependence of friction

Fluid-induced fault reactivation and associated seismicity is a critical process in reservoir exploitation and emerging geo-energy activities such as Carbon Capture and Storage (CCS), Enhanced Geothermal Systems (EGS) and wastewater disposal. During fluid injection, the fault stress state progressively approaches the failure criterion τ = (σₙ - Pf) * µ + C , where τ is shear stress, σₙ normal stress, Pf fluid pressure, µ friction, and C cohesion. Once the stress state reaches the failure envelope, faults may reactivate either seismically or aseismically. However, the mechanisms governing aseismic versus seismic fault reactivation during fluid injection remain debated.

Previous laboratory studies suggest that this seismic vs. aseismic deformation may be influenced by fault frictional properties influenced by mineralogy, fault zone structure, stress state, and injection rate, yet the relative contribution of these factors remains unclear. To address this issue, we present an experimental study on binary and ternary fault gouges with variable fractions of quartz, calcite, and illite. These are minerals found along faults zones and within reservoir rocks commonly exploited for geo-energy applications.

For each mineralogical composition, two experimental datasets were acquired. In the first dataset, we performed slide–hold–slide and velocity-step tests to measure friction, frictional healing and the velocity dependence of friction. In the second dataset, we investigated fault slip behavior during fluid pressure-induced reactivation at three different stress states.

The frictional properties reveal a pronounced contrast between granular and platy phyllosilicate-rich gouges. Granular materials exhibit high friction (µ ≈ 0.6), positive frictional healing, and low a–b values, indicating velocity-weakening and potentially seismogenic behavior. In contrast, illite-rich gouges (illite > 40%) display low friction (0.28 < µ < 0.4), low to negative healing, and strongly positive a–b values, indicative of velocity-strengthening and aseismic behavior. Duringfluid injection induced-reactivation, granular-rich gouges reactivate through an exponential increase in slip velocity, mimicking seismic-like instability. Conversely, illite-rich gouges reactivate through aseismic but accelerated creep that does not evolve into dynamic failure.

Notably, reactivation in granular gouges is abrupt and occurs at stress states well above the predicted failure envelope, whereas in illite-rich gouges reactivation is gradual and occurs at or before the predicted failure envelope. In addition, at constant illite content, quartz-rich gouges reactivate faster than calcite-rich fault gouges.

The integration of these results suggests a conceptual framework in which fluid-induced fault reactivation is governed by the interplay between frictional healing and rate dependence, with mineralogy exerting a first-order control. In granular gouges, strong healing dominates the the fluid induced reactivation process, leading to delayed but abrupt fault reactivation that can overcome the stabilizing slight rate-strengthening effect, promoting an exponential acceleration under fluid pressurization. In contrast, in phyllosilicate-rich gouges, weak or negative healing combined with a marked rate strengthening behavior stabilizes slip, favoring continuous aseismic creep.

This framework demonstrates that the balance between healing and rate dependence, strongly linked to fault mineralogy, governs whether fluid-induced fault reactivation produces seismic slip or aseismic creep.