The 3D stress state from geomechanical-numerical modelling and its uncertainties: a case study in the Bavarian Molasse Basin

Knowledge of the undisturbed stress state is a key parameter for borehole stability, productivity and induced seismicity hazard assessment. Due to the sparse and incomplete availability of data records, 3D geomechanical-numerical modelling is applied to estimate the stress state in a volume. To assess the quality of the model results, the model uncertainties have to be quantified. We present an approach that provides the uncertainties of the six independent stress tensor components at each location in the model volume by an average value and its standard deviation. We explore the uncertainties introduced to the model results with respect to the stress magnitude data used for model calibration. We test our approach on a model of the Bavarian Molasse Basin which includes the area around Munich with many geothermal projects. In the test area, we find large uncertainties in the modelled magnitude of the maximum horizontal stress (S-Hmax) in the order of 15-30%. The uncertainties in the magnitude of the minimum horizontal stress (S-hmin) are smaller between 5 and 20%. In connection with an adequate failure criterion, we compare our model results to the seismological observations at two neighbouring geothermal projects. Whilst Aschheim/Feldkirchen/ Kirchheim remained seismically quiet, induced seismicity was recorded in Poing. Our modelled undisturbed stress state was confirmed by these observations by showing in average a stable stress state in Aschheim/Feldkirchen/Kirchheim and a critical stress state in Poing. If the uncertainties in terms of two standard deviations are considered, this does not change. This demonstrates the increase in model significance when uncertainties are available.