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Storing carbon under the North Sea: are wrong sites being looked at?

One effective way of reducing greenhouse gas emissions driving climate change is to prevent carbon dioxide from reaching the atmosphere by capturing and storing it. There are now 21 large-scale carbon capture and storage (CCS) facilities either operating or being built around the world, including in the US, Australia, Canada and Saudi Arabia.

The UK looked like joining them until the government cancelled its £1 billion competition in 2015 which had intended to lead to deployment of the technology. In October, however, a new £100m commitment was announced, potentially benefiting CCS projects in Grangemouth, Teesside and Aberdeenshire.

This includes a cost reduction drive aimed at having a fleet of CCS facilities by the 2030s. The UK government also recently published a new industrial strategy placing clean energy systems including carbon capture at its heart; and the Scottish government has continually promoted and supported feasibility studies.

The three current UK proposals all seek to store the carbon dioxide (CO₂) offshore, probably still the simplest option for the country. But it comes with a major presumption – that the geology works and the gas won’t escape.

Performing seals

The offshore options on the table are depleted petroleum fields and saline aquifers – massive porous sedimentary rock formations saturated with salt water. The petroleum fields are closed-off “traps” that sit within these aquifers. Potentially this makes traps more secure for storage, but aquifers have vastly more storage capacity and may well be required to store CO₂ in substantial quantities.

The Grangemouth and Aberdeenshire projects are both looking at depleted fields in the Moray Firth in the north of Scotland, while Teesside is looking at the Triassic Bunter Sandstone saline aquifer off east England in the southern North Sea. The two leading proposals in the UK government’s previous competition had similar plans – a Shell/SSE Aberdeenshire project would have used the Moray Firth while White Rose in Yorkshire would have used Triassic Bunter.

Electric dreams. Sitiardi21

Oil and gas is trapped in a field or saline aquifer by a robust seal – a layer of fairly impermeable rock surrounding the reservoir. To store CO₂ securely, it must not be able to leak or react with the seal either now or in future – or escape up faults that break the seal or leak along borehole walls.

It is essential to completely understand the physical properties and general integrity of seals in relation to CO₂. After all, it is a more mobile and smaller molecule (0.28nm) than gases more commonly trapped in petroleum reservoirs such as methane (0.38nm) or longer chained hydrocarbons.

Some gas accumulations do contain CO₂, which points to where storage will be viable. The Fizzy and Oak discoveries in the southern North Sea are examples, as are the North Morecambe and Rhyl gas fields in the East Irish Sea Basin off north-west England.

On the other hand, there was no CO₂ in the Goldeneye field in the Moray Firth that Shell/SSE considered. There are no indications that the adjacent Atlantic field proposed for the Grangemouth and Aberdeenshire projects contains CO₂ either. If not, was CO₂ once housed there and leaked? Or if it was never there, can we be confident CO₂ injection is safe?

Another potential issue is chemical reaction. Seals are unlikely to react with hydrocarbons because they are inert. But carbon dioxide reacts with water to produce carbonic acid, which may severely corrode the top seal and allow the gas an unwelcome return to the atmosphere. It all depends on what the seal is made of. The seal for Goldeneye and Atlantic, the Rodby Formation, is carbonate-rich so has reactive potential.

Aquifer anxieties

Aquifers are not traps but large migration pathways for oil and gas. Their vast storage potential has led some to champion them as more attractive sites for carbon storage. The aquifer in which the Goldeneye and Atlantic fields sits is known as the Lower Cretaceous Captain Sandstone. Some suggest it could store 1,700m tonnes of CO₂ – around five years of UK emissions. Goldeneye, in contrast, only had an estimated capacity of about 20m tonnes.

Yet new mapping suggests we should be cautious about the Captain Sandstone. The Moray Firth is riddled with faults reactivated by the uplift and easterly tilt that took place in Britain some 55m years ago.

These are evident in the seismic image of the Moray Firth below. The top of the picture shows the boreholes in the various oil drilling concessions. The coloured bands underneath are different rock formations – the yellow band with red lines on either side is the Captain Sandstone. The black lines cutting through the Captain Sandstone are fault lines that potentially allow CO₂ a route out.

John Underhill/Heriot Watt University

The next image shows the same area but east-west instead of north-south and shows that the Captain Sandstone rises up to the seabed. This raises more concerns about leakage. And like the depleted fields within it, the carbonate-rich formation sealing the entire aquifer is susceptible to corrosion from carbonic acid.

John Underhill/Heriot Watt University

Enthusiasts for the Triassic Bunter Sandstone aquifer in the southern North Sea face the same issue as it is affected by the same tilt as the Captain Sandstone – causing it to rise to the sea floor, a few kilometres off east England.

Plans to store CO₂ in either aquifer are therefore premature. It is better to look to use large traps containing CO₂ like Rhyl and North Morecambe in the East Irish Sea, where an active CO₂ gas processing plant already exists at Barrow.

When the country reaches the stage of a demonstrator project, it really needs to succeed. An early leakage could destroy national confidence in CCS. This means obtaining the best possible geological understanding of the sites and prioritising those fields that contain CO₂ already.

In some cases, these are places where drilling has found CO₂ ruling out commercial extraction of oil or gas. As such, the potential exists to turn an exploration failure into a storage opportunity and extend the life of the North Sea.

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