Application of the Reflection Seismic Method in Monitoring CO2 Injection in a Deep Saline Aquifer in the Baltic Sea

Abstract: Time-lapse reflection seismic methods have proven effective fordetecting and monitoring the injection and spreading of geologicallystored CO2. These methods are based on interpreting changes in themedia’s elastic properties that result from replacing the native salinewater by the injected CO2, which in turn affects the seismic velocitiesof the media. Since applications of these methods in the field areexpensive, and the interpretation process is time consuming, pre-studyinvestigations should be done in order to determine whether or notreflection seismic surveys can successfully be applied to monitor theCO2 plume in the case of interest.In the present study, CO2 injection and migration in a deep salineaquifer based on a structure situated below the south-western part ofthe Baltic Sea was modeled. To determine the CO2 saturationdistributions at different times, the injection was numericallysimulated using TOUGH2/ECO2N. A radial-symmetric model withhomogeneous and isotropic properties was assumed and two differentinjection rates were studied, with the results analyzed at differenttimes after the start of the injection.The saturation and density values resulting from the TOUGH2simulation were converted to seismic velocities using the Biot-Gassmann model. A synthetic velocity model was built based on bothTOUGH2 and Biot-Gassmann models and synthetic seismic responsefields before and after injection were generated. The results show thatthe amplitude changes in the seismic response are detectable even forsmall amounts of injected CO2, while noticeable signs of velocitypushdown, as a signature of the CO2 substitution, could only beobserved if the injection rate is high enough.