The Bunter Sandstone in the UK sector of the Southern North Sea Basin is a reservoir rock that is typically 200 m or more thick and has variable but commonly fair to good porosity and permeability. East of the Dowsing Fault Zone it is folded into a number of large periclines as a result of post-depositional halokinesis in the underlying Zechstein salt. It is sealed by the overlying Haisborough Group and younger fine-grained strata and is underlain by the Bunter Shale and Zechstein Group. As such it appears to be an attractive target for industrialscale CO 2 storage. However, the very large masses of CO 2 that would have to be injected and stored if CCS is to be an effective greenhouse gas mitigation option are likely to cause (a) significant pore fluid pressure rise and (b) displacement of formation brines from the reservoir. A series of reservoir flow simulations of large-scale CO 2 injection was carried out to investigate these effects. A simple, 3D geocellular model of the Bunter Sandstone in the NE part of the UK sector of the Southern North Sea was constructed in the Tough2 reservoir simulator in which porosity and both horizontal and vertical permeability could be varied. The injection of CO 2 at various rates into the model through a variable number of wells for 50 years was simulated and the model was then run forward for up to 3000 years to see how pore fluid pressures, brine displacement and CO 2 distribution evolved. The simulations suggest that provided there is good connectivity within the reservoir, and 12 optimally distributed injection locations are used, 15 -20 million tonnes of CO 2 per year could be stored in the modelled area without the reservoir pore pressure exceeding 75% of the lithostatic pressure anywhere within the model. However, significant fluxes of the native pore fluid (saline brine) to the sea occurred at a point where the Bunter Sandstone crops out at the seabed. This suggests that the potential environmental impacts of brine displacement to the sea floor should be investigated. The injected CO 2 fills only up to about 1% of the total pore space within the model. This indicates that pore fluid pressure rise may be a greater constraint on CO 2 storage capacity than physical containment within the storage reservoir.3

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Selected European studies have illustrated the impacts of elevated CO 2 concentrations in shallow soils on pasture. For the first time, general unified conclusions can be made, providing CO 2 thresholds where effects on plants and soil microbiology are observed and making recommendations on how this information can be used when planning projects for CO 2 storage. The sites include those where CO 2 is being naturally released to the atmosphere from deep geological formations; and a non-adapted site, with no previous history of CO 2 seepage, where CO 2 has been injected into the unsaturated soil horizon. Whilst soil gas concentrations will be influenced by flux rates and other factors, the results suggest that a concentration of between 10-15% CO 2 soil gas at 20 cm depth, which is within the root zone, is an important threshold level for observing changes in plant coverage. Site-specific plant 'indicators' are also observed for CO 2 concentrations at ≥35%. Microbiological changes are seen where CO 2 soil gas concentrations are between 15-40%. As part of site characterisation, an evaluation of the risks of leakage and their potential environmental impacts should be undertaken.