In order to address the global warming threat posed by anthropogenic greenhouse gases, the European Union member states have committed themselves, through the Kyoto Protocol, to an 8% reduction in their greenhouse gas emissions from 1990 levels during the period 2008-2012.
In the medium to long term, however, it is becoming increasingly recognised that reductions of up to 60% will be needed in order to stabilise greenhouse gas levels in the atmosphere. Such a strategy requires several parallel approaches, including more efficient energy use, increased use of renewable energy sources, reduction of reliance on fossil fuels, removal of carbon dioxide (CO2) from the atmosphere and geological storage of CO2.
The capture and underground storage of industrial quantities of carbon dioxide is currently being demonstrated at the Sleipner West gas field in the Norwegian sector of the North Sea (see Chadwick et al., this volume). It has been suggested that such geological storage could offer potential long-term storage of significant quantities of CO2 that would otherwise be emitted to the atmosphere.
Although the storage duration needed may be a few hundred years, so as to reduce atmospheric emissions during the fossil fuel era, it is likely that CO2 will remain trapped for considerably longer, geological timescales. In order not to burden future societies with unacceptable environmental consequences, it is therefore necessary to have some understanding of site performance over the long term (104 to 105 years may be an appropriate timeframe).
This requires careful integration of several different approaches. Initial site characterisation will be necessary to provide a model of the geological system, as well as identifying past system behaviour. This site characterisation will include detailed baseline surveys that can be compared with future monitoring of site performance.
The prediction of site performance over long timescales also requires an understanding of, inter alia, CO2 behaviour within the reservoir, the possibilities and mechanisms of migration out of the reservoir, and the potential impacts of a leak on the near surface environment. The assessments of such risks will rely on a combination of predictive models of CO2 behaviour, that should in themselves include both a consideration of fluid migration and long-term CO2-porewater-mineralogical interactions (see for example Stenhouse et al., this volume). Such predictive models must therefore be verified through comparison with laboratory-based experiments that provide carefully constrained thermodynamic equilibrium data on specific geochemical processes (see for example Czernichowski et al., this volume). However, such experiments are necessarily limited to relatively short timescales and so other lines of evidence are required from natural systems where processes have operated for geological timescales.
This paper discusses some of the processes that have been studied at various analogue sites in western Europe during the EC-funded Nascent project (Pearce et al., 2004a) and serves as an introduction to other chapters in this book that provide much greater detail of specific studies (Lombardi et al., Chadwick et al., and Czernichowski et al., this volume; see also Schroot et al., 2005).
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