Modelling carbon dioxide storage within closed structures in the UK Bunter Sandstone Formation
Introduction
Despite environmental concerns about the long-term impacts of releasing vast quantities of CO2 into the atmosphere, it is expected that the combustion of fossil fuels will continue to account for the majority of the world's energy needs in the foreseeable future (IPCC, 2005). Capturing CO2 from large stationary emission sources such as power plants, and storing it in subsurface reservoirs is one of the tools that may be used to reduce greenhouse gas emissions, mitigating the impacts of global climate change while still allowing societies to meet their energy requirements. The Triassic Bunter Sandstone Formation of the UK sector of the Southern North Sea (SNS) is considered likely to have significant CO2 storage potential (Holloway et al., 2006a, Holloway et al., 2006b). It has fair to good reservoir properties as required for large-scale CO2 storage in saline aquifers (Chadwick et al., 2008), and there is a regional seal immediately above it provided by the mudstones of the Triassic Haisborough Group. This seal is enhanced over much of the SNS by one or more of the three widespread but not ubiquitous halite members within the Haisborough Group. The stratigraphy and structure of the SNS are described in detail by Cameron et al. (1992) and Underhill (2003).
Structurally, the Bunter Sandstone contains several large periclines (henceforth referred to as Bunter domes) formed by post-depositional halokinesis in the underlying halite-dominated Zechstein Group. Many of the Bunter domes are faulted but, where satisfactorily sealed, they form structural traps that could immobilise and store significant quantities of injected CO2. Some of the Bunter domes contain gas fields (Bifani, 1986, Ketter, 1991, Ritchie and Pratsides, 1993), which demonstrates (a) the ability of the Bunter Sandstone to store buoyant fluids, and (b) that, at least where unfaulted, the overlying strata are capable of sealing significant gas accumulations. The non-gas-bearing domes are saturated with highly saline brine.
Previous studies of the CO2 storage potential of the Bunter Sandstone indicate that more detailed appraisals are needed to obtain more realistic estimates of its capacity to store CO2 (Bentham, 2006, Holloway et al., 2006a). Hence this study investigates the storage potential of one of the Bunter domes in detail using geocellular models and reservoir simulation.
Section snippets
Methodology and geocellular modelling
The aims of the study were to examine the likely storage efficiency and storage capacity by (1) simulating the injection of realistic industrial quantities of CO2 into a single Bunter dome, (2) investigating potential pore fluid pressure and hence injectivity interactions resulting from injection into multiple domes, and (3) investigating the sensitivity of injection to a range of boundary conditions and other uncertain parameters. The following methodology was used:
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Selection of an appropriate
Lithological modelling
The reservoir was divided into three lithology categories on the basis of petrophysical analysis; non-cemented sandstone, cemented sandstone (sandstone with occluded porosity) and shale.
A discrete lithofacies log was generated for each well in order to upscale lithology into the model grid (Fig. 5). These logs were upscaled to the 3D grid, and distributed throughout each reservoir zone in the model in accordance with the reservoir description for each zone (Fig. 6). The degree of upscaling is
Reservoir simulations
The primary aim of the reservoir simulation was to determine the storage capacity of Dome A, given imposed limitations to pressure build-up and migration out of the dome via its structural spill-points. The pressure controls and the spill-points are described in Sections 4.2 Control of injection wells, 4.3 Spill point criterion, respectively.
A base-case dynamic simulation of CO2 injection into Dome A was performed, followed by a sensitivity study which focussed on the effect of varying the
Parameter sensitivity analysis
Due to a lack of site-specific data many of the parameters in the geological model are uncertain. To deal with the uncertainty, two sets of sensitivity studies were carried out based on aquifer size and heterogeneity.
Effect of injection strategy
Section 5 described sensitivity analyses aimed at addressing the uncertainty of poorly defined parameters. Other factors which affect the CO2 storage efficiency such as well completion depth, and the number of domes utilised by the storage complex were also investigated. These cases utilise the base case model parameters.
Results of the sensitivity studies
Results of all the sensitivity cases are summarised in Table 5, Table 6, which list the results for the single-dome sensitivity study and the multi-dome study respectively. In addition to showing the storage efficiency, the tables also give the time taken for 0.01% of the injected CO2 by mass to reach the spill point (spill time) and they list the CO2 storage capacity, taken as the total mass of CO2 stored. There is a wide variation in the storage efficiency throughout the sensitivity studies. E
Discussion
The results presented here show that the derived storage efficiency depends on the aquifer size, the heterogeneity, well positioning and the injection strategy. The simulations indicate that it may be possible to increase storage capacity by controlling the ratio of viscous/gravity forces acting on the CO2. If the injection rate is high (high viscous force), the pressure will rise, necessitating a reduction of the injection rate. Alternatively, if the limiting pressure is not reached, the CO2
Summary and conclusions
In this study a detailed geological model of part of the Bunter Sandstone Formation has been generated in order to estimate the CO2 storage efficiency of domed structural closures using numerical simulations. The geological model allowed an evaluation of the effects of reservoir heterogeneity on the storage efficiency, which is important for understanding the factors affecting geological storage of CO2.
The injection of CO2 was carefully controlled in the simulations, using pressure control at
Acknowledgements
This study was carried out as part of the UK Storage Appraisal Project, commissioned and funded by the Energy Technologies Institute. We would like to thank the Energy Technologies Institute for permission to publish this work, and the project consortium for their input. The authors would also like to thank PGS for use of the 3D seismic data and interpretation used in this study, and for the use of their data in the images published in this paper. We thank Schlumberger for the use of the PETREL
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