Although the idea of intentionally storing CO₂ underground for extended periods is new, nature has in fact been doing it for millions of years. There are numerous natural CO₂ reservoirs throughout the Rocky Mountain states in geologic “domes” and “traps,” suggesting these types of formations will be excellent for storing CO₂ captured from industrial facilities. The overall capacity of these natural reservoirs, however, is far less than the storage capacity needed if industrial CO₂ capture is to be practiced on any meaningful scale.

Fortunately, these and other geologic formations capable of securely storing CO₂ are prevalent in WESTCARB's territory. In general, storage will be successful wherever a porous (high permeability) zone such as sandstone is sealed by a continuous layer of low-perm ability rock such as shale. Various "trapping" mechanisms also help keep any potentially mobile fluids such as CO₂ in place. Depleted natural gas reservoirs are an example; as they have trapped methane — which is more buoyant than CO₂ — for millions of years.

The practice of injecting gases and liquids underground for temporary or long-term storage is also well established. For example, natural gas companies routinely use depleted gas reservoirs to store gas received via pipeline, prior to its distribution.

Although health, safety, and environmental problems aren’t expected from geologic storage of CO₂, validation tests and large-scale demonstrations are needed to ascertain real-world site management needs.

Most experience to date with CO₂ injection comes from the oil industry, which has used it to “loosen” residual oil in mature producing fields. Known as enhanced oil recovery (EOR), this practice represents an excellent opportunity for initial geologic sequestration projects because of its economic value and established technical and regulatory procedures. However, EOR sites are ultimately too few and too geographically isolated to accommodate the quantity of CO₂ from large-scale industrial CO₂ capture operations.

The most promising geologic formations, nationally and in the West Coast region, are saline sedimentary basins. Essentially these are underground pockets of porous sandstone and salt water. They may lie beneath land or beneath the sea on the continental shelf. The potential capacity of these formations for CO₂ storage is quite large, equivalent to hundreds of years of the region’s man-made CO₂ emissions.

Unlike oil and gas reservoirs, these formations have been little studied and they are not used commercially. However, a major demonstration of CO₂ injection into a saline formation for sequestration is under way at an offshore gas platform in the North Sea near Norway. Results from this project (and others) suggest such formations will be reliable, long-term geologic sequestration sites.

Other geologic formations in the West Coast region also warrant investigation as candidates for CO₂ storage. Nevada's mafic and ultra-mafic rock formations are an example. WESTCARB continues to expand its studies in these areas.