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1 British Geological Survey, Kings ley Dunham Centre, , Key worth, Nottingham, United Kingdom, NG12 5GG (e-mail: rach{at}bgs.ac.uk)
2 Netherlands Institute of Applied Geoscience TNO-National Geological Survey, , Kriekenpitplein 18, PO Box 80015, 3508 TA Utrecht, The Netherlands
3 Statoil Research Centre, , Rorvoll, N-7005 Trondheim, Norway
4 Sintef Petroleum Research, , N-7465 Trondheim, Norway
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.
