Fault Reactivation Experiments and Modeling Highlight Aseismic Fault Leakage in Caprock Analogue and the Risk of Induced Seismicity

Abstract:

Understanding how seismic events relate to the creation of leakage pathways in an overlying sealing caprock is a key concern in evaluating the feasibility of CO₂ sequestration in geological reservoirs. The Fault Slip (FS) experiment provides an access to a regional scale fault zone intersecting the Opalinus Clay, an analogue cap-rock layer, at 340m depth in the Mont Terri underground research laboratory (Switzerland). We conducted several experiments of fault pressurization from a sealed section of a borehole intersecting the ~3m thick fault zone with water in order to de-clamp the fault, trigger slip and seismicity. At the injection point, we observed that large leakage flow rates are much more associated to large normal displacement than to fault slip. Meters away from the injection, we observed that fault rupture is preceding the fluid pressurization. Seismicity occurred after a significant amount of injected water and aseismic fault rupture. We find that numerical models using a frictional stress-dependent permeability to simulate fluid flow in the parts of the fault activated in shear or tensile failure, and preventing flow from occurring in the remaining elastic parts were the most successful in reproducing the field observations. These models show the development of a shear stress perturbation zone outside the fluid leakage zone. The number, magnitude and location of the seismic events appears to depend on the characteristics of this stress perturbation zone which are highly sensitive to the initial background stress, the structural fault zone complexity and the change in fault permeability in the leakage zone. A new experiment in Mont Terri (FS-B) currently aims at time-lapse imaging these intricate relationships between aseismic flow paths and seismic volumes within a thick fault zone.