Keeping CO2 stored

Geological storage options suitable for the injection of supercritical CO2 include depleted oil and gas reservoirs, enhanced oil recovery methods, deep unused saline water-saturated reservoir rocks, deep unmineable coal seams and enhance coal-bed methane recovery methods (IPCC 2005). For well selected, designed, operated and monitored sites, it is likely that 99% or more of the CO2 injected into these stores would be retained for 1000 years (IPCC 2005).

The IPCC (2005) have estimated that 460–3,030 Pg of carbon can be stored in geological reservoirs (oil and gas fields, unmineable coal seams and deep saline formations).

Oceanic injection of CO2 as been proposed as an alternative to geological storage, however there is assumed to be significant detrimental impact on ecosystems, so this method is generally considered to be unviable (Lenton, 2011).

The IPCC (2005) have ruled out other geologic storage options such as basalts, oil or gas rich shales, salt caverns or abandoned mines as having no significant contribution to make.

One more recent storage idea is to inject CO2 into deep-sea sediments at a depth where it is gravitationally stable (<3,000 m water depth and a few hundred metres sediment depth) (House et al. 2006). CO2  would stay in its liquid phase at such high pressures and low temperatures and would be denser than overlying pore fluid with CO2 hydrates forming a cap over the stored liquid CO2. 

Concerns highlighted include the unknown implications of the pore water displaced into the ocean and the importance of site selection as landslide events could release the CO2. In continental USA alone storage capacity for this method is measured at. >104 Gt CO2 (>2700 Pg C) (House et al. 2006). It is proposed that no verification or monitoring would be required due to the chemistry and physics of the over and underlying hydrates and fluids —an idea that may not be well received (Harvey and Huang 1995).

If this suggested form of carbon storage stands up to subsequent investigation, then when combined with storage options investigated within the IPCC’s SRCCS there may be sufficient capacity to store in excess of all the known fossil fuel resources of ∼3700 Pg C (IPCC 2007).

Next up, stratospheric aerosol injection.