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 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

Modelling carbon dioxide storage within closed structures in the UK Bunter Sandstone Formation

“…The pressure footprint caused by CO 2 injection reaches far beyond the extent of the CO 2 plume (Heinemann et al, 2012;Smith et al, 2010;Noy et al, 2012). Moreover, in this study, results show that tens of years may pass before the formation pressure recovers after injection has ceased.…”

“…This was chosen as a typical value reported for formations beneath the North Sea (Wiprut and Zoback, 2000;Wiprut, 2001), and is close to the fracture pressure gradient given by Noy et al (2012).…”

“…Modelling is therefore focussed on the region affected by the CO 2 plume. However, the pressure footprint due to injection extends much further (Noy et al, 2012;Smith et al, 2010), requiring pressure dispersal within the connected aquifer to be considered in order to avoid unrealistic pressure build-up within the model. In this study, we assumed an initial connected aquifer volume of 280 km 3 (from initial estimates made during the ETI UK SAP study).…”

“…It is known that the Bunter Sandstone outcrops at, or very near to, the sea bed less than 20 km from the model at well 43/28a-3. If this is considered, the pore volume could be regarded as being very large, as the seabed outcrop may provide a means of pressure relief for the open aquifer (Noy et al, 2012). To mimic the effect of a potentially very large aquifer, the pore volume of the surrounding aquifer was multiplied by 1000.…”

Pressure constraints on the CO 2 storage capacity of the saline water-bearing parts of the Bunter Sandstone Formation in the UK Southern North Sea

“…From a fracture-generation perspective, the maximum sustainable reservoir pressure could be assumed to be close to the leak-off pressure (LOP), the pressure at which fracture initiation occurs (Bell 1995), although a safety margin below this pressure is likely to be required for CO 2 storage. Noy et al (2012) presented a graph of LOP against depth to seabed for the UK sector of the SNS. This includes data from all stratigraphic intervals, to a depth of 3000 m. There is considerable variation in LOP v. depth.…”