Storing captured carbon dioxide safely and at scale

Technology and space exists to store CO2 underground for at least 10,000 years

Mar 31 · 4 min read

Engineers have been injecting carbon dioxide (CO2) into geological formations in the earth’s crust for decades. In the main, they’ve been doing it to drive crude oil towards production wells. Now they’re doing it to store the CO2 and prevent it from reaching the atmosphere where it could contribute to climate change.

From extraction to insertion

Underground geological features are attractive potential sites to store CO2. In fact, some of the most promising are in oil and gas fields. This is because the type of rock they contain, and the formations they are made up of, are already known, mapped and understood.

What engineers are now discovering are special geological formations that can be used to store greenhouse gases. These need to be porous and permeable so that fluids such as CO2 can pass through them — in a one-way direction to be locked away.

To give one example, sandstone is a porous rock. It is made up of grains of sand that have been compressed over huge periods of time. These grains touch one another, but between them are gaps that fluids can flow through.

When transported in bulk, CO2 tends to behave more like a fluid than a gas. And when describing a formation as permeable, what we are talking about is how easy it is for a fluid — like CO2 in this case — to move between the pores.

These pores are where oil and natural gas are found. Once the oil or gas have been extracted, the empty pores can provide a space where CO2 can be stored.

Journey under the earth

Saline aquifers are also a promising option for CO2 storage. Like oil and natural gas reserves, these are formed of porous layers in the rock strata ­– only they contain brine or saline water.

Engineers inject CO2. into both these types of formation. This floods into the pores, and forces out the brine or any remaining oil and gas respectively, filling all the available space until it is trapped.

For a potential CO2. storage site to be a successful candidate, one more geological feature is required. This is a ‘cap rock’ or ‘seal rock’ sitting on top of the permeable formations into which the CO2 injected. This ‘seal rock’ acts as a block for the CO2 as it travels upwards, keeping it sealed into the porous rock.

In the long term, some of the stored CO2 will remain in its gas form trapped in the pores, some will dissolve in the water, and some may combine with other elements contained within the rock to become carbonate minerals — part of the rock formations themselves.

How long will this injected CO2. remain stored?

According to a study published in Nature, more than 98% of injected CO2 will stay in these underground stores for at least 10,000 years.

How much space do we have for carbon storage?

Experts at Imperial College London and energy and sustainability consultancy E4tech estimate that 100 billion tonnes of CO2 will need to be stored by 2060 if we are to keep global warming to an increase of two degrees Celsius.

100 billion tonnes may sound like a huge amount, but Europe alone may have more than 300 billion tonnes of storage capacity, while the USA could store an estimated 10 trillion tonnes more.

In fact, the North Sea offers the UK, Norway, Denmark and surrounding countries an estimated 70 billion tonnes of potential CO2 storage space.

Carbon storage technical terms explained

· Mineral trapping: Turning injected CO2 into a solid mineral.

· Dissolution trapping: Dissolving CO2 in the aquifer brine.

· Capillary trapping: Storing CO2 as bubbles in between the grains of sand that make up the reservoir rock. This is how the Endurance storage site being developed by the Northern Endurance Partnership will work. This saline aquifer is located in the Southern North Sea and will be used by Drax, its Zero Carbon Humber partners and energy and industrial capture projects in Net Zero Teesside.

How much storage space will be needed?

It is believed that by 2050, as much as 258 million tonnes of CO2 — but hopefully a lot less — will remain in the UK economy. Storing the removable emissions would require only a fraction of the available storage space beneath the North Sea.

It is clear that we already have the technology and the engineering expertise to store more than enough CO2 to achieve net zero emissions by 2050, along with plenty of space to keep it safely stored in the long term.

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