Vattenfall - Storage mechanisms

Storage mechanisms

After injecting the CO₂ into an underground reservoir a combination of physical and geochemical processes takes place, which prevents the CO₂ from leaving the storage site.

At the earth's surface CO₂ is a very stable gas that is heavier than air. Before storing it in geological formations, the CO₂ must first be compressed to a dense fluid state known as "supercritical". In this supercritical state, CO₂ has a density similar to that of a liquid and occupies less than one per cent of the space it would occupy in a gaseous state. The three principal storage mechanisms are: physical trapping, solubility trapping, and mineral trapping.

Physical trapping

CO₂ remains trapped within the underground formation due to the presence of an impermeable caprock above the reservoir. This provides a natural seal that prevents the CO₂ from moving upwards.

Solubility trapping

The CO₂ dissolves into the formation waters that fill the pore spaces in the reservoir rock.

Mineral trapping

CO₂ may over the long-term react with minerals present in the reservoir and become immobilised in the form of carbonate minerals, similar to those found in natural limestone.