| Summary: | The safe sequestration of CO2 is pivotal to a successful carbon capture and storage scheme. Saline aquifers and depleted hydrocarbon fields in the North Sea are currently used as storage repositories for captured CO2 from anthropogenic sources. This is because they are deemed safe due to the presence of a sealing cap rock and large storage volume they provide. One such field is the Sleipner field, with over 13 million tonnes of CO2 injected into the Utsira Saline +Formation from 1996 to date. Careful monitoring of the injected CO2 into the formation has revealed growing reflections on nine identified sub-horizontal horizons, referred to as intra reservoir shales. The enhanced reflectivity of the shale layers is mainly caused by the high compressibility of the CO2, trapped beneath them, and by constructive tuning effects of the top and bottom reflections at the CO2 accumulations. Within the same data are chimneys – high permeability pathways that show up in the time lapse seismic images as zones of disturbed layering that cut nearly vertically through the interbedded thin shale layers in the reservoir sands. The presence of these intra-reservoir shales and chimneys affects the distribution pattern of CO2 in the reservoir and distorts the verification of known injected mass of CO2. The aim of the research is to interpret intra-reservoir shales and chimneys on the pre-injection seismic data, these features have previously only been identified in the post-injection time lapse seismic data. The characterisation of the 3D geometry of a reservoir from seismic data is crucial to understanding the parameters that control fluid distribution. The 1994 3D pre injection dataset was interpreted with the help of volumetric seismic attributes tied to a well log. This led to the characterisation of layers from the Utsira top layer to intra-Utsira Shales (IUTS) one to ten and Utsira base layer. A multi-attribute analysis was also used to identify chimneys within the data set. The results from the interpretation workflow were then tested, against the post-injection seismic image data. The CO2 plume visualised across the 4D seismic data set were recreated into geobodies to delineate their form and extent across the reservoir. These geobodies were analysed alongside the interpreted geometry (layers) to understand the effect the layers have in controlling the spatial distribution of the injected CO2. Further analysis was conducted on the geobodies (CO2 plume) to calculate the reservoir volume of CO2 and compare against the known injected amounts of CO2. The interpreted geometry of the plume was used to simulate the impact of the reservoir geometry on injected CO2. Models were created with input parameters derived from well logs and published data. Although limited (real time measurements), results from simulations reveal close resemblance with 4D seismic data set. This study has highlighted the possibility of identifying intra-reservoir shales and chimneys to inform site characterisation that can be performed before any CO2 injection project commences. Attribute and spectral analysis can be used to add resolution to seismic data to enable detailed interpretation of the geometry of a reservoir and the volume of CO2 within a reservoir can be verified using seismic geobodies. The current monitoring techniques can employ the characterisation and verification procedure described in this study to characterise a reservoir, verify and quantify the injected amounts of CO2 to avert and mitigate for CO2 leakage.
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