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Abstract

Carbon capture and geological storage (CCS) has been discussed as one of several options contributing to the reduction of greenhouse gas emissions related to human activities for about two decades now and many countries have developed first projects investigating all aspects of the CCS process chain. The geological storage of carbon dioxide (CO2) is and has been investigated at a number of sites of various scales. Small scale pilot sites (storing less than 100,000 tons of CO2) provide ideal laboratory conditions for testing various monitoring approaches, including seismic methods. Demonstration and full scale storage sites (storing CO2 in the order of up to several million tons) show feasibility at large scale and demonstrate the performance of large scale 4D seismic measurements in tracking the CO2 plume in the reservoir. The ability of seismic measurements to image CO2 distribution in a storage formation depends on the contrast between the elastic properties of the storage formation before injection and with CO2 injected. Saline aquifers within sedimentary basins are almost ubiquitous over the globe and are therefore regarded as most prospective storage formations. They also, particularly at shallow to intermediate depth levels (down to around 1,500 m), provide favourable conditions for seismic monitoring due to the large impedance contrast between brine and free phase CO2. This has been demonstrated by pilot and full scale storage projects at Ketzin and Sleipner. Laterally heterogeneous geological conditions and greater depths pose larger challenges for surface based seismic monitoring and in these cases, borehole based observations become crucially important as shown at the Nagaoka site. Depleted oil and gas reservoirs are candidate structure for CO2 storage as well, but even with extremely high levels of repeatability, seismic monitoring of CO2 stored in such formations is hardly feasible from the surface, and still difficult using borehole based methods, due to a very small elastic contrast between initial reservoir conditions and those after starting CO2 injection. But also at sites where a direct seismic monitoring of CO2 in the primary reservoir is hardly possible, often saline aquifers at shallow to intermediate depth are potential areas of secondary CO2 accumulation after leakage, and in this case, seismic measurements are an indispensable tool for above reservoir monitoring assuring safe storage and the absence of leakage out of the storage complex.