CO2 Storage Options

What are the CO2 storage options for?

Oil and gas reservoirs, many containing carbon dioxide (CO2), as well as natural deposits of almost pure CO2, can be found in many places in the United States and around the world. These are examples of long-term storage of CO2 by nature, where “long term” means millions of years. Their existence demonstrates that naturally occurring geologic formations and structures of various kinds are capable of securely storing CO2 deep in the subsurface for very long periods of time. Because of the economic importance of oil and gas, scientists and engineers have studied these natural deposits for many decades in order to understand the physical and chemical processes which led to their formation. There are also many decades of engineering experience in subsurface operations similar to those needed for CO2 storage. The most directly applicable experience comes from the oil industry, which, for 40 years, has injected CO2 in depleted oil reservoirs for the recovery of additional product through enhanced oil recovery (EOR). Additional experience comes from natural gas storage operations, which have utilized depleted gas reservoirs, as well as reservoirs containing only water. Scientists and engineers are now combining the knowledge obtained from study of natural deposits with experience from analogous operations as a basis for studying the potential for large-scale storage of CO2 in the deep subsurface.

Carbon Storage Options
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Carbon dioxide storage can take place in several geologic formations. One option is storage in mature oil and gas reservoirs. Once the oil and gas is extracted from an underground formation it leaves a permeable and porous volume that can be readily filled with CO2. Injecting CO2 can also enhance oil production by pushing fluids towards producing wells through a process called enhanced oil recovery (EOR). Most mature oil and gas fields have been extensively studied and documented. The information gathered through the process of oil and gas production is useful in evaluating and demonstrating the suitability of these fields as secure CO2 storage sites. Since the equipment and infrastructure to inject CO2 and to remove oil and gas are similar, few field modifications may be required.

A second option for CO2 storage is injection into saline formations. Saline formations consist of porous rock filled with brine, or salty water, and span large volumes deep underground. Studies have revealed that saline formations have the largest potential volume for storing CO2 around the world. Using current U.S. CO2 emission rates, studies have shown over 450 years of storage potential in the identified areas1. Current studies are underway to more fully understand saline formations and to determine their suitability for long-term CO2 storage.

Carbon dioxide can also be stored within unmineable coal seams. Some coal seams may be too deep or too thin to be mined but may still serve as locations to store CO2. Coal seams may also contain methane (CH4), which can be produced in conjunction with CO2 injection. In coal seams the injected CO2 can be chemically trapped by being adsorbed to the coal. This trapping mechanism allows for permanent storage of CO2. Although the estimated storage volume of coal seams is lower than the first two options, the availability of CH4 as a byproduct makes it an attractive alternative.

Two final options currently under investigation are basalt formations and organic shale basins formations for CO2 storage. Basalt is a type of volcanic rock and has the potential to chemically absorb the stored CO2 through mineralization, thereby permanently trapping the CO2. A major challenge in using basalt is that the formation typically has low permeability. Research is currently being done to evaluate the suitability of basalt for CO2 storage. Shale formations are found across the United States and are typically made up of low porosity and low permeability rocks best suited as confining zones. However, some shales, like coal, have the ability to trap CO2 through adsorption, making them potentially attractive for storage. The advents of new drilling and field technologies that can enable injection of CO2 into shale formations have opened the possibility of shale as a potential option for CO2 storage. As these new methods continue to develop, shale may become another viable option for long-term CO2 storage.

Estimates of CO2 stationary source emissions and estimates of CO2 storage resources
for geologic storage sites.

Terrestrial storage, which is defined as the uptake of CO2 by soils and plants, provides another opportunity for CO2 storage. Activities such as tree planting, no-till farming, wetlands restoration, land management on grasslands and grazing land and forest preservation all provide opportunities to increase the storage of CO2.

Myth: There are limited options to store CO2 underground, and little is known about these options.
Reality: There are a wide range of geologic formations that can store CO2 and geologic storage of oil, gas, and CO2 in the subsurface has been occurring naturally for millions of years.

12010 Carbon Sequestration Atlas of the United States and Canada, Third Edition.