In the context of carbon capture and storage (CCS), quantitative estimation of injected CO2 is of vital importance to verify if the process occurs without any leakage. From a geophysical perspective this is challenging as a CO2 plume has a severe imprint on seismic data. While this makes delineation of the plume rather straightforward, for quantitative interpretation a technique is required that takes complex wave propagation, including multiple scattering and mode conversions into account. In this contribution to the Special Topic, a wave-equation based AVO technique is discussed and successfully demonstrated on a seismic dataset from the Sleipner site. The technique solves the exact wave-equation which means that all complex wave propagation effects mentioned above are properly modelled. The scheme naturally inverts for compressibility and shear compliance and these parameters are more closely related to saturation, porosity and lithology than the conventional impedances. From compressibility the CO2 saturation can be found and the total amount of injected CO2 is calculated by integration over the plume. The result obtained was found to be in good agreement with the known value at the time when the data was acquired.