Browsing by Subject "Otaniemi"

The need caused by climate change and the pursuit to use natural resources more efficiently has accelerated the search for new alternatives to fossil fuels. This has increased the popularity of geothermal energy, of which the most common and versatile energy system is the Enhanced Geothermal System (EGS). An unavoidable byproduct of EGS power plants is seismicity, which occurs particularly during hydraulic stimulation to enhance the flow of heat transfer fluid in the bedrock. Stimulation significantly increases the seismic risk in the area to a level well above the natural state, thus increasing the probability of significant earthquakes. To minimize the risks and damages caused by significant seismic events, it is important to determine the maximum credible earthquake magnitude (Mmax) for EGS projects. Mmax serves as a crucial parameter in seismic risk assessment, defining the largest possible magnitude event that can be reached during the injections. This work investigates the ST1 Deep Heat project that operated in Otaniemi, Espoo, during which two stimulations were conducted in 2018 and 2020. The combined seismic risk from these stimulations has not yet been evaluated in terms of the Mmax and the probability distribution of magnitudes. In this study, we aim to determine the Mmax value for Otaniemi based on a probabilistic approach, using the method presented by Shapiro et al. (2010), where the Gutenberg-Richter relation is modified by incorporating the seismogenic index and the volume of injected fluid. This work suggests that the value of Mmax produced by the Otaniemi stimulations is M2.33. Therefore, the probabilities for significant earthquakes are low, indicating a low seismic risk. When previous studies are complemented with the effects of the 2020 stimulation, this study shows that the 2018 stimulation is the main contributor to the seismic risk in Otaniemi and that the increase in seismic risk from the 2018 stimulation to the end of the 2020 stimulation is small. The results of this work show that the seismogenic index can be used to estimate Mmax and probabilities of significant magnitudes from statistical injection-based data and provide structural geological justifications for these results. The results support the idea that injection-based Mmax is the result of the interaction between injection strategy and natural seismotectonic conditions. The lower seismicity of the 2020 stimulation can be explained by these factors. In EGS operations, the injection strategy is crucial for managing the seismic risk. The seismogenic index determined in this study describes the seismotectonic state of the Otaniemi reservoir. Once determined, it can be used in future studies or projects focusing on the Otaniemi reservoir and boreholes. It is also possible that another geologically similar reservoir would have a similar seismogenic index, which would allow the findings of this study to be applied to the assessment and management of similar risks elsewhere. Continued and high-quality research is a requirement for the development of policies and methods to make geothermal energy production safer in the future. This research provides information on the relationship between EGS operations and induced seismicity and will increase the understanding of induced seismicity and its hazards in general.