FAQs
Frequently asked questions
CCS: Storage
How is CO2 stored?
The CO2 is permanently stored in deep underground in geological formations which may be onshore or offshore. Far from being “caverns” or “voids”, these geological formations take the form of dense but porous rock. The storage of CO2 in such formations is the third element of the three-part CCS chain: capture, transport and storage.
At the storage site the CO2 is injected into a carefully surveyed and selected geological formation. After injection, the CO2 moves up through the storage site until it reaches the impermeable layer of rock known to overlay the storage site; this layer is known as the "caprock" or "seal". This mechanism of retention is the same one that has kept oil and natural gas securely under the ground for millions of years; this provides confidence that CO2, too, can be safely stored.
As the injected CO2 moves up through the geological storage site towards the caprock some of it is retained in the microscopic pore spaces of the rock. This CO2 is tightly trapped in the pore spaces by a mechanism known as “residual storage”.
Over time the CO2 stored in a geological formation will begin to dissolve into the surrounding salty water. This makes the salty water denser than before, causing it to sink to the bottom of the injection zone formation. This is known as “dissolution storage”.
Finally “mineral storage” may occur when the CO2 held within the storage formation binds chemically and irreversibly with the surrounding rock.
As the storage mechanisms change over time from structural to residual, to dissolution and perhaps to mineral storage the CO2 becomes less and less mobile. The longer CO2 is stored, the more securely it is retained.
There is already considerable experience with injecting CO2 deep underground for storage at a number of industrial-scale CCS projects. These storage sites have been carefully selected and the evidence from monitoring confirms that the CO2 has been completely and safely locked into the geological formations.
Where is CO2 stored?
In the UK, CO2 will predominantly be stored in deep rock formations, between 1 and 4km below the seabed of the North Sea. Depleted oil and gas fields and underground saline formations offer immense potential storage capacity for CO2. The UK has sufficient to store many decades of emissions.
Depleted oil and gas fields are well understood and have naturally stored oil, gas and CO2 for millions of years, and have been well explored and mapped. Saline formations consist of rocks that are saturated with salt water, too salty for human consumption or for agricultural and industrial uses. Over time, the rock absorbs CO2 (in a process called calcification) effectively becoming an integral part of the rock and resulting in increased security of storage over time. Saline formations are currently less well mapped than depleted oil and gas fields, and will need to be explored more extensively.
How do we know what happens to CO2 when it is stored?
The advanced modelling used for CCS storage predicts how CO2 behaves and migrates when stored. The credibility of these models is supported by practical experience “in the field”.
The monitoring mechanism put in place further ensures that operators have a clear picture of how CO2 is behaving.
How long will CO2 be stored for?
The properties of the rock formations in which CO2 is to be stored, and the care taken in selecting, developing, operating and monitoring storage sites, means it should be trapped safely and indefinitely. The longer the CO2 remains underground, the more securely it is stored.
A storage project has to undergo the strictest site characterisation. A dedicated regulator is responsible to ensure that storage sites will only be licensed if all evidence indicates that the CO2 will be permanently and safely stored within the site.
Who will monitor if CO2 stays stored?
The storage operator is responsible for monitoring the CO2 once stored.
Once CO2 injection is complete, responsibility remains with the operator while the site is sealed and the facility decommissioned. After a period of time and when it has been established that the stored CO2 will be contained indefinitely, the responsibility for the site will transfer from the operator to the state.
Why is the UK a good place to store CO2?
The UK has a unique geological advantage for storage given the wide number of suitable storage sites in the North Sea - in the form of depleted oil & gas fields and deep saline formations.
The British Geological Society has estimated that the UK has sufficient offshore capacity to store more than 100 years of CO2 emissions.
How can/where could CO2 be stored onshore in the UK?
UK has a unique geological advantage for storage given the wide number of suitable offshore storage sites in the North Sea. This availability of offshore storage means that onshore storage is not an option currently being considered by the UK.
What Health and Safety standards exist for the storage of CO2?
Industry has many years experience of transporting and storing CO2. In the US, a large quantity of CO2 is transported safely across country in 1000s of miles of pipeline for use in enhanced oil recovery.
Millions of tonnes of CO2 are currently being captured and stored in Europe each year. The Sleipner CCS plant in Norway is currently storing over 1 million tonnes each year, and has been doing so safely since 1996.
