Carbon Dioxide (CO2) geological storage is the placement of CO2 into a geological formation in such a way that it will remain stored and not released to the atmosphere. Geological formations considered for CO2 storage are deep underground layers of porous rock that are capped by one or multiple layers of impermeable rock above them.
Amongst the types of formations that can be used to store CO2 the most promising settings are deep saline reservoirs, depleted oil and gas reservoirs and unmineable coal seams. In practice, highly pressurised CO2 is injected into the geological repository though a well drilled down into the porous rock. At depths below 800–1000 m, supercritical CO2 has a liquid-like density that provides the potential for efficient utilisation of underground storage space in the pores of sedimentary rocks. Once injected, the liquid CO2 tends to be buoyant and will flow upward until it encounters a confining layer of impermeable rock, which can trap the CO2 and prevent further upward migration. Other mechanisms for CO2 trapping include dissolution of CO2 molecules in the formation fluids, reaction with minerals in the storage formation and cap rock to form solid carbonates, retention as an immobile phase trapped in the pore spaces of the storage formation or adsorption onto organic matter in coal and shale.
By avoiding deteriorated wells, open fractures or faults, injected CO2 can be retained in these formations for very long periods of time. Moreover, CO2 becomes less mobile over time as a result of multiple trapping mechanisms, further lowering the prospect of leakage. The 2005 IPCC Special Report on CCS concluded that for large-scale operational CO2 storage projects, the fraction of CO2 retained in appropriately selected and managed geological reservoirs is very likely to exceed 99% over 100 years and is likely to exceed 99% over 1,000 years.
The degree to which a specific underground formation is amenable to CO2 storage very much depends on the formation characteristics. Models are being developed and validated using data from field demonstration sites in order to predict the behaviour of CO2 during the life time of a storage project. Laboratory and field experimental and theoretical research carried out at Imperial College includes:
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the thermodynamic properties and the impacts of impurities injected with the CO2 on fluid behaviour and geochemical reactions
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the long term geomechanical and geochemical behavior of reservoir rocks under CO2 storage induced fluid pressure and stress conditions
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different trapping mechanisms
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cap rock, seal and well integrity
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wellbore and reservoir injectivity
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CO2 monitoring
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CO2 storage performance and risk assessment
