4D Seismic Imaging of an Injected C02 Plume at the Sleipner Field, Central North Sea

C0 2 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 C0 2 in the reservoir, image the C0 2, 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 C0 2 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 C0 2 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 C0 2 accumulations. Velocity estimation using the Gassmann relationships suggests that the observed pushdown cannot readily be explained by C0 2 present only at high saturations in the thin layers; a minor proportion of low saturation C0 2 is also required. This is consistent with the layer volume summation, but significant uncertainty remains.