Seismic Velocity Characterization and Modelling for Synergetic Utilisation of CO2 Storage Coupled with Geothermal Energy Extraction

While deep geothermal energy is seen as a zero-emission renewable energy source, bulk of the geothermal energy plants do emit carbon dioxide (CO2) as part of the produced steam. In the current ACT Consortium funded project SUCCEED, researchers are investigating the potential for injecting produced and captured CO2 into the reservoir with the aim of enhancing geothermal production as well as permanently storing CO2 at the Kizildere (Turkey) and Hellisheidi (Iceland) geothermal fields. The re-injection of CO2 will be monitored at both sites using a novel seismic monitoring system. Prior to conducting active seismic surveys, we are performing a combined experimental and modelling study in order to: i) select the proper acquisition configuration for both sites, and ii) help the interpretation of field data to be recorded. This research presents a well-controlled laboratory study on the relationship between axial and radial stress and seismic velocities. Our first experimental results show that the rate of velocity increase, as function of increasing stress, is largest at low absolute stresses. This most probably reflects the closing of microcracks at low stress values, resulting in increased velocities. The results obtained in the laboratory, that reveal seismic velocities as function of stress (i.e. depth) for each of the different lithologies, are used as an input for modelling seismic reflections at Kizildere (Turkey) and Hellisheidi (Iceland). For both sites, we perform simulations where the source-receiver configurations and the type of pore-fluid (brine or CO2) are varied. Our first simulations show that changing the pore-fluid from brine to CO2 yields an overall lowering of the bulk density and seismic velocity of the reservoir, the latter resulting in an increased acoustic impedance contrast. The modelling results will be used for designing an optimal active seismic survey at both project sites for monitoring the CO2 injection.