Modelling large-scale carbon dioxide injection into the Bunter Sandstone in the UK Southern North Sea

Abstract

The Bunter Sandstone in the UK sector of the Southern North Sea Basin is a reservoir rock that is typically 200m 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 industrial-scale CO2 storage. However, the very large masses of CO2 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 CO2 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 CO2 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 CO2 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 CO2 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 CO2 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 CO2 storage capacity than physical containment within the storage reservoir.