Frequently asked questions
The carbon dioxide 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 carbon dioxide in such formations is the third element of the three-part CCS chain: capture, transport and storage.
At the storage site the carbon dioxide is injected into a carefully surveyed and selected geological formation. After injection, the carbon dioxide 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 carbon dioxide, too, can be safely stored.
As the injected carbon dioxide moves up through the geological storage site towards the caprock some of it is retained in the microscopic pore spaces of the rock. This carbon dioxide is tightly trapped in the pore spaces by a mechanism known as “residual storage”.
Over time the carbon dioxide 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 carbon dioxide 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 carbon dioxide becomes less and less mobile. The longer carbon dioxide is stored, the more securely it is retained.
There is already considerable experience with injecting carbon dioxide 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 carbon dioxide has been completely and safely locked into the geological formations.
Where is carbon dioxide stored?
In the UK, carbon dioxide 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 carbon dioxide. 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 carbon dioxide 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 carbon dioxide (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 carbon dioxide when it is stored?
The advanced modelling used for CCS storage predicts how carbon dioxide 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 carbon dioxide is behaving.
How long will CO2 be stored for?
The properties of the rock formations in which carbon dioxide 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 carbon dioxide 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 carbon dioxide will be permanently and safely stored within the site.
Who will monitor if carbon dioxide stays stored?
The storage operator is responsible for monitoring the carbon dioxide once stored.
Once carbon dioxide 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 carbon dioxide 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 carbon dioxide?
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 carbon dioxide emissions.
How can/where could carbon dioxide 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.
Is carbon dioxide storage safe?
The geological storage of carbon dioxide is safe, and industry has many years experience of safely transporting, injecting and storing carbon dioxide. There is a strong legal framework, under the European CCS Directive, to ensure the safe geological storage of carbon dioxide.
In the US, a large quantity of carbon dioxide is transported safely across country in over 6000 km of pipeline, primarily for the injection of carbon dioxide into geological formations for enhanced oil recovery. Most of this carbon dioxide originates from naturally occurring carbon dioxide reservoirs, where carbon dioxide has been contained for millions of years.
The Sleipner CCS project in Norway is currently storing 1 million tonnes of carbon dioxide each year, and has been doing so safely since 1996. The project is designed to increase the amount of carbon dioxide injected and stored at Sleipner to 1.7 million tonnes per year from 2014*. Further projects that have been safely storing carbon dioxide include the In Salah project in Algeria, the Snøhvit Liquefied Natural Gas (LNG) plant in Norway and the Weyburn project in Canada.
What Health and Safety standards exist for the storage of carbon dioxide?
The European CCS Directive sets out a legal framework for the safe geological storage of carbon dioxide and has been transposed into UK law (through the Energy Act 2008*). The International Organisation for Standardisation (ISO) is also developing standards for the storage of carbon dioxide.
Industry has many years experience of transporting, injecting and storing carbon dioxide. In the US, a large quantity of carbon dioxide is transported safely across land in over 6000 km of pipeline, primarily for use in enhanced oil recovery. Most of this carbon dioxide originates from naturally occurring carbon dioxide reservoirs, where carbon dioxide has been contained for millions of years.