Geomechanical Assessment of Potential for Induced Seismicity

The Bavarian Molasse Basin is one of the largest European areas for low-enthalpy hydrothermal applications. In the last decades, more than 20 geothermal applications for district heating and even power generation have been established, and more are planned or under construction. At about one third of the projects, seismicity of up to Ml 2.4 has been observed after the onset of production and reinjection of fluids while no seismicity has been observed at the other project sites. In order to assess the potential for induced seismicity at a specific site, three parts of information are required: (1) The initial stress state, (2) the changes in the stress state due to production and reinjection, (3) a fault geometry (strike, dip), and (4) a fault properties in terms of a failure criterion. While a modelling of the production and/or injection induced stress changes is performed commonly and basic information on the failure behaviour of rocks is available, information on stress magnitudes is rare and unevenly distributed. Thus, 3D geomechanical-numerical modelling is used to estimate the stress state in a target area based on the few data records available.

We present a 3D geomechanical-numerical model of the initial stress state in the Bavarian Molasse Basin in order to assess the individual potential for induced seismicity at different geothermal sites. Our model area contains several lithological units with different rock properties. Additionally, in our approach, we quantify the uncertainties introduced by the variability of the 13 stress magnitude data records used for the calibration of the model. We further reduce the large uncertainties by introduction of additional observables that limit the range of acceptable stress states. From the model results, we extract the stress state and its uncertainties at the two geothermal sites Aschheim/Feldkirchen/Kirchheim and Poing that are in a distance of 4 km. While seismicity of up to Ml 2.1 has been observed in Poing, the other site remained seismically quiet. Our modelled stress state at the two sites in combination with according failure criteria on optimally oriented faults is in agreement with the seismological observations. Even considering uncertainties of 2σ, the modelled stress state in Aschheim/Feldkirchen/Kirchheim is stable, while in Poing even the average stress state is already critical. These results indicate the relevance of a geomechanical assessment of sites of subsurface applications in order to minimize the potential for induced seismicity.