Influence of injection rate on the dynamic of fluid-induced aseismic slip fronts

Fluid injections can induce aseismic slip that propagates either behind or beyond the pore pressure diffusion front, depending on the initial stress state along the fault. In the latter case, the aseismic slip front may trigger seismicity at considerable distances from the injection well. In this study, we investigate the influence of the initial stress state and injection rate on the transition between these two end-member scenarios.

We performed triaxial experiments on a saw-cut Westerly granite sample oriented at an angle of 30° relative to the maximum principal stress. The fault was preloaded to 60% and 90% of its frictional strength before fluid injection was initiated. Fluid was injected along the fault through a borehole positioned at one edge of the fault, with fluid pressure rates ranging from 0.015 to 15 MPa/s.

The propagation of the fluid pressure front was monitored using three pressure sensors placed at varying distances along the fault. A deterministic inversion approach was employed to reconstruct the spatial and temporal evolution of hydraulic diffusivity during injection, up to the onset of instability. This method provided an optimal solution for the diffusion of pore pressure throughout the injection process. The slip front propagation was monitored using strain gauges distributed around the experimental fault. The initiation times of strain release recorded by these gauges were used as proxies for the passage of the slip front. The slip front velocity was inferred by assuming a quasi-circular geometry, as defined by the elastic properties of the tested rock.

Our results reveal that, regardless of the initial stress state, increasing the injection rate reduces the stress injection parameter induces T, allowing the transition between the two end-member cases: (1) aseismic slip front propagating behind the pore pressure diffusion front (λ<1) and (2) aseismic slip front propagating beyond the pore pressure diffusion front (λ>1). Furthermore, higher injection rates result in increased slip front velocities. These experimental observations are interpreted within the framework of fracture mechanics. Specifically, we demonstrate that reducing the initial stress state along the fault enhances the energy release rate (G) promoting the initiation of the slip front propagation. Secondly, higher injection rates generate larger values of G at the crack tip, explaining the observed increase in slip front velocities, and the transition between the two end member cases with increasing injection rates.