CO₂ Capture and Storage

5. How can CO₂ be stored underground?

• 5.1 What are the possibilities of geological storage?
• 5.2 How expensive is geological storage?

5.1 What are the possibilities of geological storage?

5.1.1 Geological formations suitable for the storage of CO₂ are oil and gas reservoirs, deep saline formations, and un-minable coal seams. Storage sites must generally be located at a depth of 800m or deeper, where prevailing pressures keep CO₂ in either a liquid or a supercritical state. Under such conditions, CO₂ is less dense than water and it must be trapped from above to prevent it from moving back up to the surface. It can for instance be physically trapped under a well-sealed cap rock and in pore spaces within the rock, or chemically by dissolving in water and reacting with rock minerals to form carbonate minerals.

Compressed CO₂ can be injected into porous rock formations below the earth’s surface using many of the same methods already used by the oil and gas industry: well-drilling technology, injection technology, computer simulation of storage reservoir dynamics and monitoring methods.

Industrial scale storage projects are underway in the North Sea, Canada, Algeria and in Texas, each storing every year more than a million tonnes of CO₂ that would otherwise be released into the atmosphere. More...

5.1.2 Potential geological storage sites exist around the globe both onshore and offshore. Estimates of the total storage space available vary widely, but they generally indicate that space exists for tens to hundreds of years of CO₂ emissions at current levels. Furthermore, a large portion of existing power plants and other industrial sources lie within 300 km of areas with storage potential (see Figure TS.2a/2b)

While the available storage capacity in geological reservoirs is “likely” to be sufficient for contributing significantly to CO₂ emission reductions, the true amount is yet uncertain. More...

5.1.3 Leakage of CO₂ from storage entails global risks for the climate, but also potential local risks for humans, ecosystems and groundwater in the case of sudden and rapid CO₂ releases. These risks are expected to be quite small: most of the CO₂ should remain underground for centuries, and leakages should be controlled well before causing local damage. Monitoring of storage sites may however be required for very long periods and methods are expected to evolve as technology improves. A legal framework with a long-term perspective is also needed as storage times extend over many generations. The general public appears not to know much about this technological option, and is “reluctant” to accept it, sometimes because of the perception that CO₂ storage is required because of a failure to reduce CO₂ emissions in other ways. More...

5.2 How expensive is geological storage?

Storage in geological formations is the cheapest and most environmentally acceptable storage option for CO₂. The cost of storage in saline formations and depleted oil and gas fields would typically be between 0.5–8 US$/tCO₂ injected, with additional monitoring costs of 0.1–0.3 US$/tCO₂ The lowest storage costs are for onshore, shallow, high permeability reservoirs, and/or storage sites where wells and infrastructure from existing oil and gas fields may be re-used. The geological storage of CO₂ could even yield net benefits, for instance in the case of enhanced oil or gas recovery where CO₂ could be injected underground to displace and recover the fuel. More...