The capture and storage of carbon dioxide is a technology that could make it possible to continue using fossil fuels during much of the 21st century. It particularly concerns coal, a central resource for many countries since there are still more than 2,500 thermal power plants in the world. This energy is used for the production of electricity and heat (cogeneration) for industrial and domestic purposes. Coal-fired and gas-fired thermal power plants are relatively abundant, affordable, available and located all over the world. As such, they strengthen the security and stability of energy systems.
Economy and demographics being what they are, the energy transition will take time – several decades at least. While we wait for the green hydrogen economy, we must nonetheless continue to live, all the while battling against the greenhouse effect caused by CO2 emissions. As such, carbon capture and storage offers a solution to help buy us some valuable time. CO2 emissions represent approximately 270 million tons every year but today only 0.1% of industrial emissions are captured. Needless to say, there is work to be done!
Storing CO2 underground
Normally, underground storage of CO2 is achieved through various methods of physical or chemical capture, and it requires strict geological conditions. As such, only very precise geological environments can be used. In particular, the geological formations must not only be capable of containing the CO2 but must also prevent lateral and/or vertical migration of the gas. Any leaks could contaminate potable groundwater at low depths, infiltrate the ground, or more importantly reach the atmosphere.
The geological formations used for CO2 storage are mainly oil and gas reservoirs, as well as deep saline aquifers found in sedimentary basins. The storage of gas (including CO2) in these environments has been proven to work on a large scale. It can even be performed during oil extraction operations (secondary recovery), natural gas storage, and acidic gas removal.
Some of the risks associated with CO2 capture and storage are similar and comparable to those of any other industrial activity for which safety and regulatory protocols are already established. At the moment, there are only few operations in the world where CO2 is injected and stored in the ground (USA, Australia, Canada, China and UK). Most of the time, if not exclusively, it is done in the context of an operation motivated by drivers other than climate change, such as oil production or regulatory requirements for the use of hydrogen sulfide (H2S).
A complicated start
Existing operations show that there is no major technological obstacle for the geological storage of CO2. Challenges and blocks thus lie elsewhere. They mainly stem from the high cost of the operation, particularly for diluted flows, like those from power plants and industrial combustion processes.
Specific risks associated with CO2 storage relate to the operational phase (the injection, to put it simply) and the post-operational phase. The greatest concern is linked with the possible risk of CO2 leakage in the short or long term. Negative effects include the global climate impact of the return of CO2 in the atmosphere, as well as the local health and environmental risks, which must therefore be correctly assessed and managed.
The other obstacle is thereby more media-driven. We are concerned that public opinion might reject this technology and that it could affect the large-scale implementation of CO2 geological storage. Indeed, who will accept such a storage site in their town? The risks associated with the transportation and injection of carbon dioxide are reasonably well understood. However, there exists a small possibility that the CO2 stored underground could leak from a reservoir, either by an unidentified migration pathway, or because of a well defect.
The threat that it could represent must be assessed in comparison with volcanic CO2 emissions, which are natural. Diffuse CO2 emissions from the soil or via carbonated sources in volcanic areas do not seem to represent a threat, provided that the CO2 can disperse in the atmosphere. However, CO2 is dangerous when it accumulates in closed spaces. Thus, large clouds of CO2 linked with sudden emissions coming from volcanic vents or craters are a deadly threat. The Lake Nyos disaster in 1986 in Cameroon, which resulted in 1,800 deaths from CO2 asphyxiation, serves as a reminder.
More acceptable solutions
Even if few analogies exist between such an event and a possible CO2 leak from a reservoir, the risk is not null. This disaster is therefore likely to come up in the media and will arouse hostility in populations living in proximity of a potential storage site. Murphy’s law will prevail over any other consideration.
In this case, only one option remains viable: storing CO2 in the open sea. In Europe, the Norwegian Sea is often cited. However, this does not mean that there would not be any impact in the event of a release of CO2. Leakage of gas under the sea would result in water acidification around the storage site, with possible damage for fauna and flora close by. This has been examined in ecotoxicology studies. But in any case, this CO2 release – even in the event of a significant leak – would not directly affect human health since it would be under the sea. This is therefore reassuring for the public. Social acceptance of this alternative is therefore the only variable capable of accelerating the implementation of technologies for reducing anthropogenic CO2 emissions in the atmosphere.