Nature has been storing carbon dioxide (CO2) and other fluids such as oil and natural gas in underground geologic formations for millions of years. These types of formations are now being put to use for long-term storage of CO2 from industrial sources. Storing CO2 in this way prevents it from entering the atmosphere and contributing to climate change.
To date, most experience with injecting CO2 underground comes from the oil industry, which uses it to “loosen” residual oil in mature producing fields. Known as enhanced oil recovery (EOR), this practice represents an excellent opportunity for initial geologic storage projects because of its economic value and established technical and regulatory procedures.
Depleted natural gas fields are also viable candidates for storing CO2. In general, CO2 storage will be successful wherever a rock that is porous and permeable, such as sandstone, at a depth greater than a half mile, is sealed by an overlying continuous layer of low-permeability rock such as shale. Sufficient depth ensures that the CO2, which is injected in a liquid-like supercritical state, remains under enough pressure to keep it from becoming more buoyant.
Far more extensive than oil- and gas-bearing formations are saline formations, layers of porous rock that are filled with saltwater. Both nationally and in the West Coast region, saline formations represent the largest CO2 storage opportunity. Indeed, WESTCARB’s capacity estimates for the saline formations across its seven states are the equivalent of hundreds of years of the region’s man-made CO2 emissions.
Saline formations have been little studied because they do not contain hydrocarbons. However, three major CO2 injections into saline formations for storage are ongoing, two in offshore areas near Norway (by Statoil) and the other at In Salah in Algeria (a joint venture by BP, Sonatrach, and Statoil). Results from these projects and similar smaller projects suggest that saline formations can serve as reliable, long-term CO2 storage sites. Nonetheless, more field demonstrations are being undertaken to ascertain real-world site management needs and costs across a variety of locations.