While the concept of geosequestration of carbon dioxide as a means of reducing greenhouse gas emissions to the atmosphere has arisen only in the past decade or so, geosequestration utilises technologies that have been widely practiced in different industries for many years.
Carbon dioxide is injected into geological formations in many parts of the world for enhanced recovery of oil or natural gas. Oil and natural gas deposits demonstrate the capacity of specific geological structures to store material safely under pressure deep underground for millions of years. In many cases, depleted oil and gas wells could be used for geosequestration of carbon dioxide. Some deep saline formations are also suitable for long-term storage of carbon dioxide.
In every case, the characteristics of each storage site would be closely examined to ensure it could store carbon dioxide safely, and to calculate the storage capacity of the site. Sites would normally be chosen only after establishing the presence of one or more thick impermeable seals to prevent the carbon dioxide rising to the surface or migrating to sources of useable groundwater. The carbon dioxide can also be trapped through dissolving in saline water in deep formations or by combining with minerals in rocks to form stable carbonates. Sites will also be chosen to minimise any possibility of triggering seismic activity or reactivating any fault. The characteristics of individual storage sites also need to be well understood so that the physical and chemical processes which will take place during and after injection can be predicted with a high degree of confidence.
Recent research indicates that, when stored in saline aquifers, a proportion of the carbon dioxide dissolves in the formation water. Modelling indicates that the carbon-dioxide-rich water, which has a higher density, will tend over time to finger downwards, decreasing the risk of any trapped carbon dioxide leaking to the surface.
The Intergovernmental Panel on Climate Change considers that over 1000 years at a carefully chosen site, less than 1 per cent of the stored carbon dioxide may leak only as far as the next strata of rock, still deep underground, rather than to the land surface or atmosphere.