Seismic monitoring at the Sleipner underground CO ₂ storage site (North Sea)

The growing emissions of greenhouse gases, especially CO 2 , are seen worldwide as one of the major causes of climate change. International treaties like the Kyoto Protocol are supposed to contribute to reducing the emission of greenhouse gases. Underground sequestration has the potential to play an important role in keeping large volumes of CO 2 from escaping into the atmosphere in the short term. The first case of industrial scale CO 2 storage in the world (close to one million tonnes per year since 1996) is taking place at the Sleipner underground CO 2 storage site in the North Sea offshore Norway. Careful monitoring of the behaviour of the storage facility is required to establish its safety. To this end, two time-lapse seismic surveys have been acquired; the first repeat survey was completed in October 1999 and the second in October 2001. The presence of CO 2 beneath thin intra-shale layers within the reservoir has caused significant changes both in reflection amplitudes (up to a factor 10) and in travel time (more than 40ms) through the CO 2 plume (the velocity push-down effect). Some aspects of the interpretation of these time-lapse seismic surveys will be presented here.