oa Chimneys And Channels: History Matching The Growing CO2 Plume At The Sleipner Storage Site

A revised analysis of seismic data at Sleipner has revealed large-scale, roughly north-trending, channels at a range of levels in the Utsira Sand. The seismic data also reveal localised chimneys within the reservoir and overburden, some of which show evidence of having provided vertical conduits for earlier natural gas flow. Reservoir flow models were set up with flow properties constrained by the observed levels of CO2 accumulation in the reservoir and the arrival time of CO2 at the reservoir top just prior to the first repeat survey in 1999. The initial model with laterally homogeneous sand units separated by thin semi-permeable mudstones achieved a moderate match to the observed time-lapse seismics. Subsequent flow models, progressively incorporating higher permeability vertical chimneys through the mudstones and large-scale channelling within the reservoir sands, yielded a progressive and marked improvement in the history-match of key CO2 layers within the plume. The preferred plume simulation flow model was converted into a seismic property model using Gassmann fluid substitution with an empirical Brie mixing law. Synthetic seismograms generated from this show a striking resemblance to the observed time-lapse data, both in terms of plume layer reflectivity and also of time-shifts within and beneath the CO2 plume.