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Since its inception in 1996, the CO 2 injection operation at Sleipner has been monitored by 3D time-lapse seismic surveys. Striking images of the CO 2 plume have been obtained showing a multi-tier feature of high reflectivity. In the medium to longer term, the topmost layer of CO 2 , accumulating and migrating directly beneath the topseal, is the main determinant of storage site performance. Fortunately it is this topmost layer that can be most accurately characterized, its rate of growth quantified, and CO 2 flux arriving at the reservoir top estimated. The latter is mostly controlled by pathway flow through thin intra-reservoir mudstones. This has increased steadily with time, suggesting either that pathway transmissivities are increasing with time and/or the pathways are becoming more numerous. Detailed 3D history-matching of the topmost layer cannot easily match the observed rate of spreading. Isotropic permeabilities result in a stronger radial component than observed and a degree of anisotropic permeability, higher in a north-south direction, is possible. The main contributor to the mismatch, however, is likely to be small but significant uncertainty in the depth conversion. Irrespective of uncertainty, the observed rate of lateral migration seems to require very high permeabilities, and is, moreover, suggestive of a topseal which behaves like a 'hard' impermeable flow barrier. Detailed studies such as this will provide important constraints on longer term predictive models of plume evolution and storage performance which are key regulatory requirements.

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

No abstract

The significance of the potential impacts of microbial activity on the transport properties of host rocks for geological repositories is an area of active research. Most recent work has focussed on granitic environments. This paper describes pilot studies investigating changes in transport properties that are because of microbial activity in sedimentary rock environments in northern Japan. For the first time, these short experiments (39 days maximum) have shown that the denitrifying bacteria, Pseudomonas denitrificans, can survive and thrive when injected into flowthrough column experiments containing fractured diatomaceous mudstone material and synthetic groundwater under pressurised conditions. Although there were few significant changes in the fluid chemistry, changes in the Microbiological impacts -mudstone 2 permeability of the biotic column were quantitatively monitored. These same methodologies could also be adapted to obtain information from cores originating from a variety of geological environments including oil reservoirs, aquifers and toxic waste disposal sites to provide an understanding of the impact of microbial activity on the transport of a range of solutes, such as groundwater contaminants and gases (e.g. injected carbon dioxide).