Carbon Storage

HOW CAN CO2 BE STORED UNDERGROUND? 

Carbon dioxide (CO2) can be stored underground as a supercritical fluid. Supercritical CO2 means that the CO2 is at a temperature in excess of 31.1°C and a pressure in excess of 72.9 atm (about 1,057 psi); this temperature and pressure defines the critical point for CO2. At such high temperatures and pressures, the CO2 has some properties like a gas and some properties like a liquid. In particular, it is dense like a liquid but has viscosity like a gas. The main advantage of storing CO2 in the supercritical condition is that the required storage volume is substantially less than if the CO2 were at “standard” (room) pressure conditions.

illustration-of-pressure-effects-on-co2
Illustration of Pressure Effects on CO2 (based upon image from CO2CRC). The blue
numbers show the volume of CO2 at each depth compared to a volume of 100 at the surface.

Temperature naturally increases with depth in the Earth’s crust, as does the pressure of the fluids (brine, oil, or gas) in the rocks. At depths below about 800 meters (about 2,600 feet), the natural temperature and fluid pressures are in excess of the critical point of CO2 for most places on Earth,. This means that CO2 injected at this depth or deeper will remain in the supercritical condition given the temperatures and pressures present.

Myth: The CO2 gas behaves the same in the atmosphere as it does when injected deep underground.
Reality: The elevated temperatures and pressures that exist at the depths where CO2 is injected changes its characteristics, allowing for storage of much greater volumes of CO2 than at the surface.


Once the CO2 is injected underground, how does it stay there?