The use of geothermal energy, as a more sustainable alternative than currently used energy sources/technologies, bears a great potential to reduce greenhouse gas emissions and therefore, to reach the set climate goals. A more widespread use of geothermal energy could especially aid in the decarbonization of the heating and cooling sector. Therefore, it is essential to better understand induced seismicity associated with geothermal operations. A more detailed understanding of induced seismicity, the mechanisms behind it as wells as assessing the risk for induced seismic events related to geothermal projects play a crucial role in further establishing geothermal energy production in our society.
The first part of the thesis provides an overview of worldwide reported induced seismic events with a magnitude ≥ 2 that were related to geothermal energy production. The focus was laid on the geological setting, the present-day stress field, and on characterizing the geothermal operation as well as the induced seismic events. Furthermore, the basic processes leading to induced seismicity and commonly used methods for risk assessment are briefly discussed. The second part of the thesis focuses on the analysis of core samples from fault zones in the Vienna Basin. The sample set consists of 9 mudstones of Badenian age, all originating from a depth of approximately 1650 m. The fault rocks are investigated concerning their mineralogy, the pore system, the mechanical properties, and the degree of tectonic deformation (micro-tectonics).
The porosity measurements – broad ion beam – scanning electron microscopy (BIB-SEM), mercury intrusion capillary pressure (MICP), He-Pycnometry – show in general systematically lower porosity values for the faulted rocks compared to un-faulted rocks from a similar depth range. However, the capillary displacement pore radii are larger resulting in smaller hydrocarbon column heights (HCH) and therefore, in worse seal capacities. The overall pore characteristics are similar for both the tectonically impacted samples and the samples showing no signs of tectonic influence. The mechanical properties - reduced elastic modulus (Er) and hardness (H) - are in a similar range as well. Cation exchange capacity (CEC) measurements as well as the clay mineralogy measured with X-ray diffractometry (XRD) show that a higher degree of deformation seems to lead to a more advanced illitization. In general, the fault rocks show indications of only low deformation in combination with low temperature-pressure conditions far below the greenschist facies.