Stress state from data and modelling in the site selection process for a deep geological repository for radioactive waste in Germany - The SpannEnD Project

For a deep geological repository (DGR) for radioactive waste, the stress field is a key parameter for the assessment of potential siting regions, the DGR design, and the evaluation of its long-term safety. Therefore, a 3-D description of the stress state (orientation and magnitudes) is required not only for the repository host rock, but also for the under- and overlying formations that act as additional geological barriers. The main objectives of the project SpannEnD are to predict the spatial variability of the present-day stress state in Germany and to develop the tools that are required to set up and calibrate 3-D geomechanical models. Furthermore, we also developed a statistical framework that uses such models to investigate the number of stress magnitude data that are needed for a robust model calibration, and to test at which depth and in which lithologies microhydraulic fracturing and sleeve re-opening tests should be conducted that is used to derive the magnitudes of the minimum and maximum horizontal stress magnitudes S_hmin and S_Hmax, respectively.

Another objective is to investigate all relevant aspects of the stress state for Germany on a supra-regional scale. To describe the 3-D stress state with geomechanical-numerical models, stress data for the model calibration are required in combination with the knowledge of the geological structures of the subsurface and the rock properties. The orientation of S_Hmax has been compiled for decades in the World Stress Map database using a wide range of stress indicator. As part of the SpannEnD project, we updated the dataset for Germany and its surroundings to 1573 data records. For the compilation of stress magnitude data we developed for the first time a quality-ranking for the stress magnitude data applied it to the new open access German database with 1330 stress magnitude data records. These datasets are used to calibrate a large-scale 3-D geomechanical model covering the entirety of Germany. The numerical model is based on a comprehensive geological model that integrates all the structural information from the federal geological surveys and other publicly available sources. This model is populated with the rock properties of the individual lithological units and calibrated using the compiled stress data. The resulting geomechanical model for Germany enables an initial assessment of the crustal stress state for the entire country and can be used to investigate the impact of supra-regional structures. Furthermore, the first-order stress predictions are used to define initial conditions for regional-scale models.

In addition to stress data compilation and geomechanical modelling, the project also compiled a fault geometry database for Germany which, in combination with the model results, allows the prediction the slip tendency for these faults and a distribution function for all potential fault orientations. Furthermore, the usage of sub-modelling techniques, the impact of faults on the stress state and uncertainties in the predicted stress state due to the material properties and the calibration data has been investigated. The presentation will provide an overview of the main achievements of the SpannEnD project over the past eight years.