Stress Field Sensitivity Analysis at a Reservoir Scale (Northern Switzerland) Using Numerical Geomechanical Modelling

A numerical geomechanical model is presented to characterize the stress field at a candidate site for a nuclear waste repository in Switzerland (Zürich Nordost). Lithological formations of approximately 20 to 200 m in thickness are considered in the model through specific rock properties as individual geomechanical units. Special attention is given to the Opalinus Clay (Lower Dogger), the designated host rock of high level waste at the candidate site.The modeled stress field is calibrated against stress data from borehole breakouts and hydraulic fracturing measurements conducted within the site. In general the state of stress strongly correlates with geomechanical properties. The stiff formations show much higher stress anisotropy with higher SH magnitudes and lower Sh magnitudes than the softer formations. In particular, it is concluded that the stress field in the Opalinus Clay is not very sensitive to changes in the boundary conditions as the stiffer formations (notably the limestones of the Upper Malm and the Upper Muschelkalk) take up the far-field tectonic stresses.