Will stored CO₂ leak?

The most important public concern about CCS is whether stored CO2 will leak back to the surface (land or sea) and into the atmosphere (Photo: Dex Image PTE Ltd)

The most important public concern about CCS is whether stored CO₂ will leak back to the surface (land or sea) and into the atmosphere.

The most important public concern about CCS is whether stored CO₂ will leak back to the surface (land or sea) and into the atmosphere. This would be a hazard because CO₂ at high concentrations is an asphyxiant, but it would also mean that the process would not be working as a climate change mitigation method.

However, for well-selected, designed and managed geological storage sites, the IPCC says that risks are low. CO₂ could be trapped for millions of years, and well selected stores are likely to retain over 99 per cent of the injected CO₂ over 1000 years.

Demonstration sites for CCS

It is important to study examples of CO₂ storage. Norway 's Sleipner gas field is a demonstration site for CCS and is the oldest store of CO₂ on an industrial scale. Environmental assessment of the gas field has concluded that storage is successful and that the Utsira Formation (the saline aquifer formation that stores the CO₂) is stable and a suitable site for CO₂ storage.

At another test site at Weyburn, Saskatchewan, Canada, the likelihood of stored CO₂ release has been determined to be less than one per cent in 5000 years.

Studies of volcanic areas in Italy and Germany where carbon dioxide is naturally coming to the surface helps us learn more about how CO₂ affects ecosystems. They show how ecosystems are coping with exposure to high CO₂ concentrations and how effects are very localised.

Will earthquakes cause CO₂ to leak?

In tectonically-active parts of the world CO<sub>2</sub> may have to be stored closer to potential earthquake zones but pipeline technology developed for natural gas and oil is able to cope with earthquakes. (Photo: BGS/P510383)

In tectonically-active parts of the world CO₂ may have to be stored closer to potential earthquake zones but pipeline technology developed for natural gas and oil is able to cope with earthquakes.

CO₂ storage sites will normally be positioned away from seismic (earthquake) activity and active faults. However in some tectonically-active parts of the world like Japan, New Zealand or Greece, CO₂ may have to be stored closer to potential earthquake zones.

In October 2004, the Nagaoka CO₂ storage and monitoring project in Japan experienced a large earthquake during a CO₂ injection trial. The injection was stopped immediately and the site was monitored for possible CO₂ leakage, however, no leakage was detected. In December 2004, injection was started again and a total of 10400 tonnes of CO₂ has been safely stored.

If a site is well understood and characterised even a large earthquake in close proximity to a CO₂ storage site should not result in leakage.

Seismic activity can also be caused by human activities. The most common form of ‘induced seismicity' is that caused by large surface water reservoirs, large buildings, and mining and extraction of minerals, groundwater, oil and natural gas.

In the case of extraction of groundwater, oil and natural gas, instability is caused by changes in the pressure in the affected geology, usually a de-pressurisation, which causes it to shrink or collapse slightly like a deflating ball. This process of de-pressurisation and pressurisation is well known and understood and can be managed in oil and gas fields, thus should not be a problem in CCS.

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