Many underground formations have the potential to store CO2
In order for carbon capture and storage (CCS) to work, it is necessary not just to remove carbon before it is emitted from a coal-burning plant, but also to put the CO2 somewhere away from the atmosphere. The solution favored by many scientists and engineers is to bury it deep under ground, where they believe it can remain safely for a very long time, under the right conditions.
Carbon dioxide can be stored in depleted oil and gas wells and deep un-mineable coal seams. Other prospective locations are so-called saline aquifers, deep underground formations of porous rock and sand filled with salty water that is isolated from surface water and from underground fresh-water aquifers. Many areas of the US are underlain by saline aquifers.
For all these types of storage, it is first necessary to compress the CO2 gas enough that it behaves like a liquid. Then it must be pumped at least a half-mile underground so that the pressure exerted by the overlying rock and other material keeps it a liquid. In the case of porous, water-filled rock formations, some of the CO2 will dissolve over time into the salty water. Over long time periods it will react with surrounding minerals to create carbonate rock, which reduces the chance of escape even more.
Scientists and engineers continue to do research on the topic of underground CO2 storage, but enough is already known that large-scale projects are starting to be implemented.
The impetus for CCS comes from the fact that rising CO2 is causing the planet to warm and that the ongoing increase in atmospheric CO2 comes from burning fossil fuels, mostly coal.