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.

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At the Sleipner fields in the North Sea, CO2 is being injected into sands of the Miocene-Pliocene Utsira Formation, which is overlain by thick Pliocene shales. The highly porous (35%–40%) and extremely permeable (approximately 2 D) Utsira sands are organized into approximately 30 m thick packages. These packages are separated by thin (predominantly 1 m thick), low-permeability shale layers, which are assumed to contain potential fluid pathways of erosive or deformational origin. A 6.5 m thick shale layer close to the top of the sands separates an eastward thickening sand wedge from the main sand package below. Migration simulations indicate that the migration pattern of CO2 below the shale layer would differ strongly from that within the sand wedge above. Time-lapse seismic data acquired prior to the start, and after three years, of injection confirmed a reservoir model based on these findings and showed that the thin shale layers act as temporary barriers and that the 6.5 m thick shale layer does not fully inhibit upward migration of CO2.

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C02 produced at the Sleipner field is being injected into the Utsira Sand, a major saline aquifer. Time-lapse seismic data acquired in 1999, with 2.35 million tonnes of C02 in the reservoir, image the C02, plume as a number of bright sub-horizontal reflections. These are interpreted as tuned responses from thin (< 8 m thick) layers of C02 trapped beneath intra-reservoir shales. A prominent vertical 'chimney' of C02 appears to be the principal feeder of these layers in the upper part of the reservoir. Amplitude-thickness scaling for each layer, followed by a layer summation, indicates that roughly 80% of the total injected C02 is concentrated in the layers. The remainder is interpreted to occupy the feeder 'chimneys' and dispersed clouds between the layers. A prominent velocity pushdown is evident beneath the C02 accumulations. Velocity estimation using the Gassmann relationships suggests that the observed pushdown cannot readily be explained by C02 present only at high saturations in the thin layers; a minor proportion of low saturation C02 is also required. This is consistent with the layer volume summation, but significant uncertainty remains.