Observing the Invisible – CO2 Feeder Chimneys on Seismic Time-Lapse Data

Summary CO2 has been injected and stored at the Sleipner injection site since 1996 and the CO2 distribution has been monitored through a total of nine repeat time-lapse surveys. The time-lapse response when CO2 replaces brine is very strong and has enabled us to quantify at least nine strong seismic reflections which are interpreted as CO2 being trapped beneath thin shale layers within the otherwise very homogenous storage formation. The CO2 accumulation beneath each shale layer can be described and modelled by capillary seal theory and Invasion Percolation modelling; however, some unexpected behaviour is observed which is best explained by CO2 migrating through a main feeder chimney of dim amplitudes observed near the location of the injection point. There are also examples of layers growing at leakage points offset from the main chimney, without any obvious feeder chimney attached to them. In this paper we discuss the detection of these 'invisible seismic response' features and the consequences for reservoir understanding and simulation. The Sleipner CO2 plume observations can also serve as a proxy for and give understanding of migration and leakage detection using seismic time-lapse data.