Underground storage of liquid CO2
Dispensing of CO2 by storing it underground usually implies placing it in aquifer rocks deep enough (-800 m) to achieve sufficient pressure to maintain the CO2 in a supercritical (fluid) phase. This approach has many shortcomings as in this form, the CO2 can migrate, and potentially pollute surrounding waters, and acidify the rocks making them more likely to become porous and eventually release the gases. The British Geological Survey/University of Leicester are currently studying the feasibility of another potential approach which consists in storing CO2 both in liquid form and as a hydrate. Liquid CO2 has a higher density than supercritical CO2 , requiring less volume and reducing buoyancy forces that drive vertical migration. It implies very low temperatures for liquefaction, and storage must take place in cold deep-water sediments or below the permafrost regions. These temperatures can also increase viscosity and slow migration, but the containment could be further improved if the temperatures were sufficiently low for the CO2 hydrate to stabilize. The liquid CO2 could be injected below the hydrate stability zone, thus forming a hydrate cap which could help seal the surrounding rocks and under which a deeper store of liquid CO2 could be contained. Parallel research concerns CO2 and methane hydrate stability and the relationships between CO2 hydrates and the surrounding sediments.