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Since October 1996, Statoil and its Sleipner partners have injected CO 2 into a saline aquifer, the Utsira Sand, at a depth of approximately 1000 m. The aquifer has a thickness of more than 200 m near the injection site and is sealed by thick shales. A multi-institutional research project SACS (Saline Aquifer CO 2 Storage) was formed to predict and monitor the migration of the injected CO 2 . To this end two time-lapse seismic surveys over the injection area have been acquired, one in October 1999 after 2.3 million tonnes of CO 2 had been injected and the second in October 2001 after approximately 4.4 million tonnes of CO 2 had been injected. Comparison with the base seismic survey of 1994 prior to injection provides insights into the development of the CO 2 plume. In this paper some selected results of the seismic interpretation of the CO 2 plume at the two different time-steps will be shown.

CO2produced at the Sleipner natural gas field is being injected into the Utsira Sand, a major saline aquifer. Time-lapse seismic data were acquired in 1999 and 2001, with 2.35 and 4.26 million tonnes of CO2in the reservoir respectively. The CO2plume is imaged as a number of bright sub-horizontal reflections within the reservoir unit, growing with time, and underlain by a prominent velocity pushdown. No leakage has been detected from the repository reservoir. The reflections are interpreted as tuned responses from thin (<8 m thick) layers of CO2trapped beneath thin intra-reservoir mudstones and the reservoir caprock. However, these alone are unable to account for the amount of observed pushdown. A two-component 3D saturation model is therefore developed for the 1999 dataset, with high-saturation CO2forming the layers and a lesser component of low-saturation CO2between the layers. Saturations are calculated from the observed reflectivity and velocity pushdown and the resulting model contains 85% of the known injected mass of CO2. A 2D synthetic seismic section through the saturation model matches the observed seismic response well and the model is considered to provide an acceptable description of the CO2distribution. Signal attenuation is more pronounced within the 2001 plume and its effects are likely to become more significant with time, perhaps reducing the efficacy of seismic verification techniques as the plume grows further. Other geophysical methods, such as microgravimetry, may become increasingly useful at this stage.

Exposure to simulated altitude of ~4,000 m does not seem to worsen the whole body mass and fat-free mass reductions or alter resting energy expenditure and appetite during a 21-day simulated microgravity.