AbstractsEarth & Environmental Science

The importance of fluid-injection experiments has widely increased in the past decades mainly due to their link with energy production activities (e.g. enhanced oil recovery, hydraulic fracturing/wastewater disposal, geothermal energy production and CO2 storage). These activities are frequently accompanied with the occurrence of induced micro-seismicity. While induced seismicity is mainly perceived as a problem for the public, it can also be used as a useful tool for efficient reservoir development and exploitation. For this reason, both seismological and geomechanical techniques can be combined to achieve a better understanding of the physical processes occurring in geological reservoirs. In this dissertation, the main aim is to improve the characterization of geo-reservoirs undergoing fluid injection under different scenarios. To do this, the seismic responses to fluid injection from a CO2 injection project (Penn West Pilot Project, Alberta, Canada) as well as from a geothermal field (The Geysers, California, USA) were analyzed and compared. Particular attention was given to the analysis of the stress field orientation by inverting focal mechanisms of induced seismicity, as well as investigating its relation with fluid injection/extraction processes. Also, a more general seismological and geomechanical (seismo-mechanical) reservoir analysis was performed to efficiently illuminate the processes that led to the occurrence of induced seismicity at The Geysers geothermal field. First, seismic and leakage signals from a CO2 injection – Enhanced Oil Recovery project in relation to a reported outflow of CO2/brine along the monitoring well were investigated. Looking for CO2 leakage signatures is important to ensure the safe storage of the CO2 within the target formation. Here, continuous seismic recordings from a borehole array near the reservoir as well as pressure/temperature data were examined. The seismological techniques applied revealed no associated micro-seismicity down to MW > -1, but remarkably elevated noise levels were observed in the seismic recordings framing the outflow time. Additionally, the pressure sensors located in-and above the reservoir reported leakage-related signals indicating the CO2 movement towards the surface. In the next step, the use of the stress tensor inversion as a tool for tracing the reservoir processes was explored. The understanding of the stress tensor inversion allowed developing an updated version of the linearized inversion scheme originally created by Hardebeck and Michael, (2006) and Michael, (1987). The software package was successfully designed together with corresponding extensive documentation system and it is now freely available to all users. To test the correct functioning of the software, it was applied to a natural seismicity catalog (North Anatolian Fault Zone), induced seismicity data (The Geysers geothermal field) as well as a synthetic catalog. Once the software package was ready, the stress field orientation was analyzed in detail at The Geysers geothermal…