Storage

After capturing the CO₂ from industrial processes in order for it to be an effective climate change mitigation solution it must be stored safely and securely away from the atmosphere. In the case of CCS this would be geological storage - a process which has natural and industrial analogs.

There are a variety of rock formations that act as secure, natural traps, holding gases and liquids deep underground both on land and under the sea bed. Often located at depths of over 1km, these traps consist of layers of porous rock filled with oil, natural gas or very salty water - much like a solid sponge - overlain by a thick layer of impermeable rock, known as cap-rock. It is this cap-rock that initially prevents the fluids in the rock’s pores from making its way to the surface.

These traps are so effective that they have held such liquids and gases securely for tens of millions of years, before they were discovered by exploration and production companies in the quest for fossil fuels. CO₂ has also often been found naturally underground in traps like these, mixed in with the natural gas or oil, or dissolved in water.

As a result, these geological formations - whether depleted oil and gas reservoirs, which are proven containers for gases, or deep saline formations (rock containing very salty water unsuitable for other use), which can likewise be proven to have the characteristics to keep CO₂ underground. For CO₂ storage, the CO2 would be injected at pressure necessary to enter into the porous rock through wells drilled in the cap-rock.

How can CO₂ be stored in geological formations?

CO₂ can be stored securely underground because when it is injected into the porous rock, the higher temperatures and pressures that act on it deep underground, mean that it becomes a “supercritical fluid”, which has the properties of both a liquid and a gas.

These properties mean that it is easy to inject and diffuses readily through the pore spaces of the rock, but like a liquid, it takes up much less space than a gas would, increasing the amount of CO₂ that can be stored in a given volume of rock. To emphasize how much less space liquefied CO₂ occupies, the equivalent of 100 cubic meters of CO₂ in the atmosphere occupies only a quarter of one cubic foot of pore space when it is one kilometer below the earth’s surface. This property allows very large volumes of CO₂ emissions to be securely stored in comparatively small but deep geological formations.

Once the CO₂ is in the geological formation, there are a variety of interrelated geological mechanisms that work together to keep CO₂ securely underground once it is injected.

Find out more about CO₂ trapping mechanisms

CO₂ Storage Sites

There are two key types of potential storage sites - depleted oil and gas reservoirs or deep saline formations - that have the geological characteristics necessary to store large quantities of CO₂.

Depleted oil and gas reservoirs are attractive because their geology is well known and, by definition, are proven traps of liquids and gases.

Deep saline formations, rock formations that have their pore spaces filled with very salty water, exist in most regions of the world and appear to have a very large capacity for CO₂ storage. Unminable coal seams can also be used to store CO₂ because CO₂ adsorbs to the surface of the coal, in the process releasing previously absorbed methane gas, which can then be recovered. This process, however, has yet to be demonstrated.

Find out more about geological storage options

How would a CO₂ geological storage project work in practice?

By definition, every potential CO₂ geological storage site is unique in terms of its geology and structure, and therefore will have its own site-specific engineering and operational needs, drawing largely from the vast experience of the oil and gas industry. However, every geological storage project would be conducted in four phases, all of which are a critical part in ensuring the effectiveness of CO₂ underground storage.

Find out more about geological storage sites

Discover more about monitoring and verification