Carbon sequestration can be referred to as the provision of long-term storage of carbon in the underground, the ocean or the terrestrial biosphere. This causes the concentration of carbon dioxide in the atmosphere to decrease. In some instances, this is done by maintaining or enhancing natural processes, while in other cases, novel techniques are developed for carbon disposal.
How Will Carbon Sequestration Offset Changes in Global Climate?
The capture and sequestration of carbon dioxide is a process consisting of the separation of CO2 from sources of emission. This CO2 is then transported to a storage facility, and ensuring the long-term isolation of CO2 from the atmosphere.
Carbon capture and sequestration is considered as one of the options targeted at stabilizing atmospheric greenhouse gas concentrations, and is the only available technology towards achieving this goal.
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What is Geologic Sequestration?
Geologic sequestration is a technique that involves the injection of in CO2 into underground reservoirs that have the ability to securely contain it. The research and development of geologic sequestration of CO2 focuses on five types of geologic formations:
- Oil and gas reservoirs,
- Deep saline formations,
- Unmineable coal seams,
- Oil- and gas-rich organic shale
- Basalt.
Oil and Gas Reservoirs
Oil and gas reservoirs consist of layers of porous rock formations that have trapped crude oil or natural gas for millions of years. An impermeable, overlying rock formation forms a seal that traps the oil and gas. The same mechanism would apply to CO2 storage.
As a value-added benefit, CO2 injected into these reservoirs can result in the recovery of oil and gas resources left behind by earlier recovery efforts. This process is known as enhanced oil recovery. CO2 can increase oil recovery from a depleting reservoir by an additional 10-20 percent of the original oil in place.
Saline Formations
Saline formations are composed of porous rock which is saturated with brine and capped by one or more regionally extensive impermeable rock formations, enabling the trapping of injected CO2. When compared to other geological sequestration techniques such as coal seams or oil and gas reservoirs, saline formations are more common and offer the added benefits of greater proximity to emission sources, higher CO2 storage capacity, and fewer existing well penetrations. On the other hand, much less is currently known about the potential of saline formations to store and immobilize CO2.
Unmineable Coal Seams
At depths beyond conventional limits of coal recovery, represent another promising opportunity for the storage of CO2. Coal beds normally contain large amounts of methane-rich gas that is adsorbed onto the coal’s surface. Current practice for the recovery of coal bed methane is to depressurize the bed, which is usually accomplished by pumping water out of the reservoir. An alternative approach is to inject carbon dioxide gas into the bed. Injecting CO2 into the coal bed results in enhanced coal bed methane recovery.
Similar to the by-product value gained from enhanced oil recovery, the recovered methane provides a value-added revenue stream to the carbon capture and storage process, reducing overall net costs.
Shale and Basalt
Many shale contain hydrocarbon materials that can provide an adsorption substrate for storage of CO2, which is similar to CO2 storage in coal seams.
The formation of basalt has a distinct chemical makeup which potentially allows for the conversion of the injected CO2 to a solid mineral form, thus isolating it from the atmosphere permanently.
Although oil- and gas-rich organic shale and basalt research is in its infancy, these formations may, in the future, prove to be optimal storage sites for stranded emissions sources.
What is Terrestrial Sequestration?
Terrestrial carbon sequestration is defined as either the net removal of CO2 from the atmosphere by plants and microorganisms in the soil or the prevention of CO2 net emissions from terrestrial ecosystems into the atmosphere. There is significant opportunity to use terrestrial sequestration both to reduce CO2 emissions and to secure additional benefits, such as habitat and water quality improvements that often result from such projects.
Terrestrial sequestration is the enhancement of the CO2 uptake by plants that grow on land and in freshwater and, importantly, the enhancement of carbon storage in soils where it may remain more permanently stored. Terrestrial sequestration is thought to be one of the most cost-effective means of reducing atmospheric levels of CO2.
A study in 2018 however shows that natural forests exhibit higher carbon sequestration than planted forests in China; additional benefits also included lower water consmption.
What is Ocean Sequestration?
CO2 is soluble in ocean water, and through natural processes the oceans both absorb and emit huge amounts of CO2 into the atmosphere. As a matter of fact, the amount of carbon stored in the ocean exceeds that of the carbon stored in terrestrial ecosystems.
It is widely believed that the oceans will eventually absorb 80-90 percent of the CO2 in the atmosphere and transfer it to the deep ocean. Although the ocean has huge potential as a carbon storage sink, the scientific understanding to enable ocean sequestration to be considered as a real option is not yet available.
“The tax credit is likely to stimulate significant investment in these emerging technologies”, says Julio Friedmann of the Energy Futures Initiative, who was previously principal deputy assistant secretary at the US Department of Energy’s Office of Fossil Energy, referring to ‘exploring ways to use carbon dioxide to produce alternative fuels, building materials, and other products.’
Sources and Further Reading
This article was updated on the 29th May, 2019.
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