The Bunter Sandstone Formation in the UK Southern North Sea has the potential to become an important CO2 storage unit if carbon dioxide capture and storage becomes a widely deployed option for the mitigation of greenhouse gases. A detailed geological model of a region of the Bunter Sandstone consisting of four domed structural closures was created using existing seismic, well log and core data. Compositional simulation of CO2 injection was performed to estimate the storage capacity of domes within the system. The injection was constrained by both pressure and CO2 migration criteria, and the storage efficiencies of the domes (volume of stored CO2 divided by the pore volume of the dome) were calculated when injection ceased. A sensitivity study evaluated the effect of varying the total aquifer volume, reservoir heterogeneity and injection well location. A wide range of storage efficiency values were obtained across the different simulation cases, ranging from 4% (closed dome) to 33% (homogeneous model). Intrareservoir heterogeneity, specifically in the form of continuous low permeability layers has an important effect on storage capacity in dome-like structures, because it increases the tendency for CO2 to migrate laterally from the storage complex via structural spill points. (c) 2013 Natural Environment Research Council (NERC). Published by Elsevier B.V. All rights reserved.

In the UK, by far the largest CO2 storage opportunities lie offshore. The North Sea in particular has a long and complex geological history, with potential reservoirs geographically widespread and occurring at multiple stratigraphic levels. Diverse storage estimates have been made, using a range of working methods, and yielding different values, e.g. SCCS (2009); Bentham (2006). Consequently the UK Storage Appraisal Project (UKSAP), commissioned and funded by the Energy Technologies Institute (ETI), is undertaking the most comprehensive assessment to date, using abundant legacy seismic and borehole data. This study has a remit to use best current practice, consistent between locations, to calculate the CO2 storage capacity of the entire UK Continental Shelf (UKCS) within saline aquifers and hydrocarbon fields. The potential storage formations have been subdivided into units for assessment, and filtered to remove units with only a small estimated storage capacity to concentrate resources on more viable units. The size of potential storage units approximate to a power law distribution, similar to that of hydrocarbon fields, with a large number of small units and a small number of large units.

Large-scale underground storage of CO2 has the potential to play a key role in reducing global greenhouse gas emissions. Typical underground storage reservoirs would lie at depths of 1000m or more and contain tens or even hundreds of millions of tonnes of CO2. A likely regulatory requirement is that storage sites would have to be monitored both to prove their efficacy in emissions reduction and to ensure site safety. A diverse portfolio of potential monitoring tools is available, some tried and tested in the oil industry, others as yet unproven. Shallow-focused techniques are likely to be deployed to demonstrate short-term site performance and, in the longer term, to ensure early warning of potential surface leakage. Deeper focused methods, notably time-lapse seismic, will be used to track CO2 migration in the subsurface, to assess reservoir performance and to calibrate/validate site performance simulation models. The duration of a monitoring programme is likely to be highly site specific, but conformance between predicted and observed site performance may form an acceptable basis for site closure.

Saline aquifers represent a promising way for CO2 sequestration. Storage capacities of saline aquifers are very important around the world. The Sleipner site in the North Sea is currently the single case world-wide of CO2 storage in a saline aquifer. A general review is given on the specific risks for CO2 storage in saline aquifer. The regional distribution of CO2 storage potential is presented. Finally, the knowledge gaps and the future research in this field are defined.

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