CO2 Underground

What happens to the carbon dioxide (CO2) underground?

Permeability, porosity, wettability are among the factors that affect CO2 trapping.
Source: Courtesy of Cal Cooper and the CO2 Capture Project, 2009.

Geologic formations suitable for storage of CO2 occur in places where thick accumulations of sediments have been deposited over geologic time periods of millions of years. Rocks in sedimentary basins are composed of rock grains, organic material, and minerals that form after the rocks are deposited. The pore space between grains or minerals is occupied by fluid (mostly water, with occasional presence of oil and gas). The same kinds of geological settings where oil and gas deposits are found are suitable for geologic storage. These settings are distinguished by the presence of alternating layers of rocks with different textures. Some of the layers consist of fine-textured materials such as clay, silt, and salts. These form impermeable barriers, or seals, that trap oil and gas underground-and are also essential for trapping CO2 underground.

Alternating with these low-permeability layers are coarser textured layers, consisting typically of sand or carbonate rocks (mainly limestone and dolomite), that form the reservoir in which the oil and gas reside. These coarse-textured sand and carbonate layers can also be used for underground CO2 storage. In general, CO2 will be stored at depths one-half mile or more below the ground surface. At these depths, CO2 performs more like a liquid than a gas, resulting in more efficient use of the underground storage space. In addition, deep storage enhances the security of underground containment, which is influenced by a number of factors including increased probability of multiple barriers between the storage formation and the ground surface, a smaller number of old, abandoned wells that penetrate to such depths, and smaller density differences between the CO2 and in place fluids.

Porosity in Rocks and Rock Permeability

It is common to hear CCS experts talk about storage of CO2 in the “supercritical” condition. Supercritical CO2 means that the CO2 is at a high temperature and pressure. At higher 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 much less than if the CO2 were at “standard” (room) pressure conditions. This reduction in volume is illustrated in the figure. The blue numbers represent 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 approximately 800 meters (about 2,600 feet), in most places on Earth, the natural temperature and fluid pressures are in excess of the critical point of CO2. This means that CO2 injected at these temperatures and pressures will be in the supercritical condition. The pressure of CO2 must be greater than the naturally existing fluid pressure in order to get the CO2 into the reservoir. Large temperature differences between the injected CO2 and the surrounding rock are not recommended, but, the CO2 will take on the temperature of the surrounding rock as it moves into the reservoir. Hence, even if not injected under supercritical conditions, it will—in most cases—end up in the supercritical condition in the reservoir.

In nature, over long time periods, CO2 slowly reacts with mineral matter to form stable compounds. There was a large reduction of CO2 in the atmosphere from Cretaceous to post-Cretaceous (after the age of dinosaurs) times. Natural processes caused these reductions, such as weathering of silicates to carbonates, and calcium carbonate (limestone and chalk) formation from CO2 deposition. While CO2 mineralization is an ongoing natural process, it is a slow process when left to nature. The rate of mineralization also varies from site to site. There has been research aimed at accelerating these reaction rates (research at the National Energy Technology Laboratory-Albany site and elsewhere) to be able to capture and store CO2 in a solid mineral form. This can provide another safe, long-term option for CO2 storage.

Myth: When injected underground, CO2 behaves like a gas, and it is likely to migrate to the surface.
Reality: Because CO2 is injected deep underground, it behaves more like a liquid than a gas. In addition, layers of impermeable rock above the injected CO2 will act to keep it in place. Over the longer term, CO2 is expected to react with the rocks to form stable compounds.
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