3D geomechanical modeling of potential areas for nuclear waste storage in southern Germany: application of sub-modeling techniques.

A robust prediction of the recent crustal stress state is essential for selecting and designing sites for high-level radioactive waste repositories to ensure long-term safety and environmental protection. While regional compilations provide data on the stress orientation in Germany (Reiter et al., 2016), reliable stress magnitude data are limited (Morawietz et al., 2020). To overcome this drawback, geomechanical-numerical modeling is used to provide a physics- and continuum mechanics-based prediction of the full stress tensor, which is consistent with the limited observational data.

The crustal stress field is controlled by factors of varying scales, such as plate boundary forces, density distribution and rock properties. This poses a challenge for modelling the stress, as there is a conflict between model size, resolution, and computing resources when numerical methods are used for the solution of the resulting partial differential equations of the equilibrium of forces. Small-scale models offer a high spatial resolution but lack incorporation of large-scale influencing factors, while large-scale models have limited resolution for specific locations. To resolve this conflict, we use models of different scales and a sub-modeling approach, combining them to achieve a comprehensive understanding of the crustal stress distribution.

The workflow that was used builds on the Germany-wide stress model of Ahlers et al. (2021). Subsequently, an embedded regional as well as a site model are used for geomechanical modeling with an increasing level of detail. The sub-modeling approach involves the repeated application of results from a larger-scale model as boundary conditions to a smaller one. Thus, more details can be included while maintaining a consistent large-scale stress pattern (Ziegler & Heidbach, 2024).

To test the practical applicability of the workflow, it is applied to Teilgebiet 001 as defined by the Bundesgesellschaft für Endlagerung (BGE). The study area is located in Baden-Württemberg and Bavaria, covering part of the Swabian Alb and the Molasse basin. The focus is on the middle Jurassic Opalinus clay as a potential stratigraphic unit for nuclear waste storage, which reaches up to 300 meters in thickness. The model covers the entire Mesozoic to Cenozoic succession from the top of the crystalline basement to the earth’s surface. The regional model, embedded in the German stress model of Ahlers et al. (2021), has dimensions of 150 km by 60 km and comprises 12 stratigraphic units. It encompasses the site model with dimensions of 30 km by 10 km and 20 units. The transfer of modeling results (stress and displacements) as boundary conditions to smaller-scale models allows for the incorporation of structural and lithological details without excessive calculation times. Consistent coupling of the various model scales is achieved using virtual calibration data from at least one stratigraphic unit which occurs with the same mechanical properties in all models. The case study shows the value of the sub-modeling approach for refined stress predictions on a local scale.