Enhancements of geomechanical models for potential siting areas in Switzerland

After the upper earth's crust was mainly explored in the last century for the extraction of water, minerals and energy resources, the focus of exploration is increasingly shifting to the temporary storage of energy, the extraction of thermal energy and the disposal of CO₂ or radioactive waste. The geomechanical stability of the host rock is particularly relevant for the barrier integrity of a geological repository. The stresses acting on the lithologies and the waste-induced pore pressure evolution determine whether a potential failure can occur. Various methods are used to estimate individual components of the stress tensor on a meter scale. Since the local stress state may be strongly influenced by tectonic structures and spatially variable rock properties, it is necessary to apply 3-D geomechanical models, to predict the spatial distribution of the stress state beyond the in-situ data.

Geomechanical-numerical models require detailed information about the structural composition such as stratigraphic boundaries or faults, and the material properties of the subsurface lithologies. Technically the task is to estimate the equilibrium of forces between gravity and the elastic response due to lateral displacement boundary conditions that result in a best-fit with respect to pointwise stress magnitude data. With the increase in available data, the calibration process becomes more complex, but also provides a framework for testing the validity of the data and the models.

Motivated by the Sectoral Plan Deep Geological Repositories, a comprehensive exploration program was done in three sites in Northern Switzerland between 2016 and 2021, involving 3-D seismic surveys and drilling of nine boreholes. This enabled to refine 3-D geomechanical-numerical models for the potential repository sites. These models differ significantly not only due to the improved geometrical data, but the large number of stress magnitude data and the increased numerical resolution. The presentation is intended to provide an overview of the modelling process, the calibration procedure, the new features of the models and the key results.