Injection Rate effects on failure of a saturated gouge-filled fault failure: Dilation vs. Diffusion

Understanding the underlying mechanisms controlling earthquake triggering by fluids has become increasingly important in recent decades, driven largely by observations that link subsurface fluid injections to subsequent nearby earthquakes. It is clear that understanding and predicting fluid-induced triggering is required in many energy-related activities (e.g. geothermal energy, CO2 injection), as well as for naturally triggered events. One of the main open questions is the effect of fluid-injection rate. Usually the ‘effective stress law’ is invoked to predict the failure of fluid-saturated granular or porous media. This law assumes that in fluid-pressurized faults the instantaneous value of pore pressure controls fault strength and failure. But recent laboratory results (Passelègue et. al., 2018) suggest that the level of pressure by itself cannot describe the full mechanics. These experiments show that the rate of fluid injection is also important: slower injections lead to failure at lower pressures than fast injection rates. 

 

We shall present results from a coupled hydromechanical-discrete element model that simulates the response of a pre-stressed, fully saturated fault, filled with a granular fault gouge, subject to fluid injection at different rates. Our simulation results find similar rate-dependence as seen in the laboratory experiment, i.e. that slow injection causes failure at lower fluid pressure than faster injection. Several mechanisms can be theorized to explain these observations, and we explore the two main end-member cases, dilation-driven and diffusion-driven rate-dependence, by comparing theoretical predictions for the poro-elastic response of the layer vs. pore-pressure diffusion. Our theoretical analysis provides upper and lower bounds to the numerically observed  rate dependence, suggesting the reason why the effective stress law is an insufficient approximation for failure of material exposed to pore fluid injection, when the pore-pressure injection rate is varied.