Analysis of the 3D Displacement Data during a Fault Reactivation Experiment in the Mont Terri Rock Laboratory

The Fault Slip (FS) experiment gives insights into the stability of clay fault zones. The tests targeted the Main Fault located in the Opalinus Clays at the Mont Terri underground research laboratory. This tectonic zone is up to 6m wide and consists of various facies including scaly clays, fault gouge and fractured zones. To trigger fault slip, fluid was injected with stepwise increases in pressure into the packed-off boreholes within and in the vicinity of the Main Fault. The injection probe includes the capability of monitoring the 3D displacement in the injection chamber together with the fluid pressure and flow rate (SIMFIP probe). With the additional information about deformation obtained with this probe compared to standard packer probes, it is intend to develop a methodology for measuring the in-situ stress in deep boreholes. This study shows the first detailed results from the FS data analysis using the detailed structural model of the fault zone. The 3D displacement shows pressure dependent and pressure independent response interpreted as elastic and plastic deformation, respectively. The elastic response was mostly observed below the Fracture Opening Pressure (FOP) and is related to borehole expansion. The orientation of the elastic displacement component aligns with the intermediate principal stress, which was independently determined by slip-tendency approach. Plastic response is dominating above the FOP and controlled by the reactivation of fault planes. The rock mass response to hydraulic injection during the FS experiment was also numerically simulated using the 3DEC code. The model identifies limitation of stress inversion methodologies based on slip orientation and slip sense caused by non-uniform stress field related to a fluid pressure source and complex fault geometry. A sensitivity study has been performed in order to quantify the impact of these limitations for stress-inversion methodology.