Stress state estimation – variability assessment of model results

The upper crust of the Earth is used more and more to transport and extract raw materials and energy. It is also used for the final disposal of radioactive waste in deep geological repositories. An accurate knowledge of the hosting rock and surrounding formation coupled to comprehensive modelling are fundamental to demonstrate the geological site has the required properties for safe and long-term underground storage. Besides other criteria, geomechanics plays an important role. Especially, the estimation of the contemporary stress state in the upper crust is a challenge. In-situ data of maximum and minimum horizontal stress magnitudes (S_Hmax and S_hmin) are required to calibrate 3-D geomechanical models.

During the recent exploration phase for a deep geological repository for radioactive waste in Switzerland, a unique dataset of stress magnitude data has been acquired from eight cored boreholes. Rock mechanical properties were constrained from geophysical logging and laboratory testing. The empirically correlated rock properties were not simply averaged, but a probability distribution was provided. The stress field was explored by conducting more than 120 tests in different stratigraphic units, to estimate the magnitudes of S_Hmax and S_hmin.

We present the results of a 3‑D geomechanical-numerical model that shows the best-fit with respect to the measured stress magnitudes. Considering the uncertainties of the tests and the ones resulting from rock property variability, the model can reproduce most of the measurements. However, we do show not only the best-fit result, but a bandwidth of individual stress components within a P₀₅-P₉₅ probability range.