The geological storage of CO₂: Quantification of natural gas/groundwater interaction

The storage of anthropogenic CO₂ in geological reservoirs remains one of society’s preferred options for reducing theglobal emission of greenhouse gases. While there are several types of environment under investigation, including depleted oil and gas reservoirs, adsorption onto uneconomical coalbed surfaces and deep saline aquifers, it is the latter that has the potential to cope with the mass of CO₂ to be injected into the ground. Whatever the geological target, the security of the injected CO₂ must be assured both for efficacy and public confidence. Because CO₂ is both a reactive gas and highly soluble in groundwater understanding the fate of the injected CO₂ is not simple as this may be secured in the form of precipitated carbonate or remain mobile as either a free gas phase or dissolved in the groundwater. Nature has provided natural analogues. Volcanic CO₂ has been injected into a variety of reservoirs, differing both in their structure and reservoir lithology. We review the most recent advances we have made from case studies across the USA, Europe and China. We show how using a combination of noble gases and stable isotopes that we can constrain the hydrological environment into which these magmatic gases have been injected. We show that CO₂ removal from the gas phase is directly related to the degree of water in contact in all case studies. We show that in many of these natural analogues we can resolve and quantify the competing carbon sinks of dissolution in the groundwater from precipitation as a carbonate phase and conclude that carbonate precipitation is only a minor carbon sink in all analogue studies to date and probably not occurring at all in carbonate lithologies.