The following four natural mechanisms, so-called “trapping” mechanisms, help to ensure safe and permanent storage and prevent CO₂ from re-entering the atmosphere:
- Structural & stratigraphic trapping in the form of one or more impermeable rock formations ("caprocks"). These formations lay above the reservoir that contains the CO₂ and inhibits the CO₂’s tendency to move upwards. These impermeable rock formations can be hundreds of meters thick and there are often several caprock formations above a reservoir.
- Residual CO₂ trapping occurs as the CO₂ moves within the porous reservoir rock. Some of the CO₂ will be left behind as disconnected or remaining droplets in the pore spaces, which are immobile – just like water in a sponge.
- Solubility trapping means that the CO₂ dissolves in the saline formation water that is naturally present in the underground reservoir (making it a slightly fizzy brine). The CO₂ is thus no longer present as a separate phase; it unifies with existing geological brine. Thereby, the CO₂’s tendency to move upwards is eliminated due to the resulting solutions’ higher density.
- The final step, mineral trapping, relies on long timescales: the CO₂ contained in the formation water will undergo another geochemical reaction and mineralize towards stable carbonates, just as the ones known from above-ground mineralization in Oman and the Yellowstone National Park.
Image: Geologic Storage of CO₂ in the Yellowstone National Parc