4D seismic imaging of an injected CO2 plume at the Sleipner Field, central North Sea

CO2 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
CO2 in the reservoir, image the CO2 plume as a number of bright sub-horizontal
reflections. These are interpreted as tuned responses from thin (< 8 m thick) layers of
CO2 trapped beneath intra-reservoir shales. A prominent vertical ‘chimney’ of CO2
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 CO2 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 CO2
accumulations. Velocity estimation using the Gassmann relationships suggests that
the observed pushdown cannot readily be explained by CO2 present only at high
saturations in the thin layers; a minor proportion of low saturation CO2 is also
required. This is consistent with the layer volume summation, but significant
uncertainty remains.