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Free keywords:
3D geomechanical–numerical model, Uncertainties, Variability, Stress state, Stress model, Bavarian Molasse Basin, Northern Alpine Foreland Basin
Abstract:
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 ( SHmax ) in the order of 15–30%. The uncertainties in the magnitude of
the minimum horizontal stress ( Shmin ) 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.
