Knowledge of the tectonic stress state is a key factor in site selection. In the first work package, the project aims to further develop and provide the geomechanical-numerical stress model for Germany (Ahlers et al., 2022). This stress model for Germany can be used, among other things, for sub-area and large-scale regional comparisons, but also provides necessary boundary conditions for more detailed regional and local models. Corresponding submodeling techniques for the consistent linking of the different model scales will be further developed in a second work package. In the third work package, the various measurement methods for determining stress magnitudes will be evaluated and generally applicable recommendations for optimizing exploration programs with regard to the parameterization and calibration of geomechanical-numerical models will be developed. Overall, the four-year R&D project will thus provide all the basic principles and modeling tools required for a robust prediction of the in situ stress state in Germany.

In the course of the SpannEnD project (www.spannend-projekt.de/en), a 3D stress model was created for Germany for the first time, taking into account published structural models and geomechanical data, and calibrated to measured data records on stress magnitudes and orientations. A new model is currently being created that will have a ~10-time higher vertical resolution, especially of the sedimentary units. For this purpose, 3 large-scale models (A-C), 19 regional models and ~100 geological sections are taken into account.

The local refinement of the geomechanical-numerical models for stress prediction is examined using a sub-area model as an example. The study area is located between the federal states of Bavaria and Baden-Württemberg and covers an area of 120 km2 (8 km x 15 km). The basis for the model is the reconstruction of the subsidence and uplift history using the current subsurface structure. The 3-D model takes into account 19 events and comprises 17 stratigraphic units from the crystalline basement to the Quaternary. The boundary conditions represent the development of the surface temperature and the basal heat flow as well as the changes in the supra-regional stress field over time. The model is calibrated using temperature and vitrinite data as well as information on the orientation and magnitude of the recent stress field. Based on the previous results, a larger geomechanical model with an area of 200 km x 80 km will be created and these two models will be linked with the multi-scale approach of submodeling.

For the search of a desposit, the requirements for exploration programs – for geomechanical data and in situ stress measurements – are significantly different from those of the hydrocarbon industry. In particular, the target horizon is generally shallower – at 300-1500 m below ground surface – and the host rocks have significantly different geomechanical properties than those of hydrocarbon reservoirs. In addition, besides the target horizon, i.e. the host rock, the strata in the overburden and underburden must also be geomechanically assessed, since those play an important role for geotechnical issues in repository construction, but also in modeling long-term safety, e.g. with respect to erosion or loads due to glaciation cycles. Likewise, the demands on the accuracy of model results are higher, so that the choice and planning of measurement programs in boreholes or their optimal localization in terms of parameterization and calibration of geomechanical models represent special challenges. Another central and controversially discussed question in the literature is the influence of disturbances on the components of the stress tensor (Yale, 2003), which has been investigated by Reiter et al. (2023).

Publications

• Reiter, K., Heidbach, O., and Ziegler, M.: Impact of faults on the remote stress state, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-1829, 2023.

Conference Contributions

• Röckel, L., Ahlers, S., Morawietz, S., Müller, B., Hergert, T., Reiter, K., Henk, A., Ziegler, M., Heidbach, O., and Schilling, F.: The slip tendency of 3D faults in Germany, Saf. Nucl. Waste Disposal, 2, 73–73, https://doi.org/10.5194/sand-2-73-2023, 2023, Abstract
• Ahlers, S., Reiter, K., Hergert, T., Henk, A., Röckel, L., Morawietz, S., Heidbach, O., Ziegler, M., and Müller, B.: SpannEnD – a 3D geomechanical model of Germany for the prediction of the recent crustal stress state, Saf. Nucl. Waste Disposal, 2, 59–59, https://doi.org/10.5194/sand-2-59-2023, 2023, Abstract
• Hergert, T., Ahlers, S., Röckel, L., Morawietz, S., Reiter, K., Ziegler, M., Müller, B., Heidbach, O., Schilling, F., and Henk, A.: On the influence of initial stress on final stress in data-calibrated numerical geomechanical models, Saf. Nucl. Waste Disposal, 2, 65–65, https://doi.org/10.5194/sand-2-65-2023, 2023, Abstract
• Reiter, K., Heidbach, O., Ziegler, M., Giger, S., Garrard, R., and Desroches, J.: Stress state estimation – new data and variability assessment of model results, Saf. Nucl. Waste Disposal, 2, 71–72, https://doi.org/10.5194/sand-2-71-2023, 2023, Abstract