Industry shows growing interest in saline aquifers for carbon capture and storage (CCS). Do their high storage capacities make up for the lack of data? A geological focus on porosity and faults appears crucial to identifying new, safe storage sites.

CCS is on a tear, with the sector set to play a major role in the decarbonisation of industry and achieving net zero targets. One challenge is in finding suitable locations for CO2 injection.

Typically, the focus has been on depleted oil and gas fields, on which there is both infrastructure and institutional knowledge.

A 2021 paper in the International Journal of Greenhouse Gas Control reported the UK could store 18.2 billion tonnes of CO2 in oil and gas fields, with the Forties field alone being able to store 3bn tonnes.

But saline aquifers are gaining attention. Saline potential is even higher, though, with the same paper reporting 48 aquifers could hold 110bn tonnes of CO2.

The International Energy Agency (IEA) has estimated saline aquifers could hold more than 100 years’ worth of emissions. The Intergovernmental Panel on Climate Change (IPCC) has put total storage capacity at between 8 trillion and 55 trillion.

Aquifers under the sea incur higher costs to develop, but have higher storage potential.

Saline aquifers to watch

The Norwegian North Sea has driven activity. This area is home to one of the highest profile saline aquifers in Europe, Aurora, part of the Longship CCS project.

Northern Lights – the transport and storage part of Longship – is due to begin operations this year, but Norway has been in the CCS business for longer. The country’s Sleipner project began in 1996, following the introduction of Norway’s emissions tax in 1991.

Sleipner uses a seabed saline aquifer. It was the world’s first offshore CCS project. It has captured more than 23 million tonnes of CO2 via a well into the Utsira Sand.

The UK North Sea has been slower to start, but is working to similar ends. Endurance, in the Southern North Sea, will store CO2 from Net Zero Teeside.

The project is approaching a possible final investment decision (FID) by September this year. It is a joint venture, after the North Sea Transition Authority (NSTA) awarded a carbon storage licence to allow the Northern Endurance Partnership (NEP) to develop CO2 store sites for the East Coast Cluster.

The cluster aims to bring the hub into operation by 2027. It will store CO2 from various industries in a series of saline aquifers, with increasing injection volumes expected as the facility develops.

Europe’s North Sea developments

As of May 2024, the Netherlands granted an exploration licence to Shell Offshore Carbon Storage NL (SOCS NL). This covered CO2 storage in North Sea saline aquifers. The exploration programme aims to assess the use of aquifers for CO2 storage alongside depleted reservoirs.

In February, Denmark granted its first full-scale CO2 storage permits in joint industry projects for its part of the North Sea. The Danish Energy Agency has recommended CO2 exploration in both saline aquifers and depleted fields.

The 2021 paper estimated Denmark could store 3.8bn tonnes of CO2 in depleted fields, noting permeability was low. However, it said, “they are sufficient to offset the country’s emissions”. It assessed the Netherlands as having 147bn tonnes of storage capacity.

The NSTA is also working on its CCS ambitions. In September 2023, it issued 21 licences to 14 companies around the UK for depleted and saline fields. In April, it opened a competitive process inviting UK applications for appraisal and storage licences in part of the English Channel.

This offering covers exploration and appraisal rights. The submission deadline is approaching, with bids due in by June 5.

Depleted vs saline storage

There are pros and cons to CO2 storage in depleted reservoirs vs saline aquifers. Saline aquifers can come with a higher cost upfront because of their lack of infrastructure. However, it is not mean always better to have infrastructure to re-use with depleted reservoirs, IEAGHG explained in a webinar. Sometimes, infrastructure can be a liability.

Certain sites with depleted hydrocarbons also have mechanically compromised sites. These have less capacity to accept the supercritical state of the CO2 required during injection, unless artificially pressurised.

There is less data on saline aquifers than depleted reservoirs. Therefore, understanding their reservoir properties and mechanisms can only be gained by more exploration, appraisal and research.

Once CO2 is injected, the gas flows up, or gets trapped in a number of ways, which can be challenging to predict.

Prior to injection, CO2 is pressurised in a supercritical state – between a gas and a liquid. This then dissolves in the brine, reacts with the surrounding minerals, or get trapped in the porosity of the aquifers. Chemical reactions may transform the CO2 into stable minerals, trapping it for longer timescales.

Most of the gas at Sleipner, for example, is trapped via buoyancy. Residual trapping accounts for over one third of storage after 20 years, while trapping by mineralisation should account for 22%, 10,000 years post-injection, according to CC Reservoirs.

Being a dense, supercritical fluid brings more diversity in how it is stored across the field.

*Trapping mechanisms scale over time intervals. Source: MDPI*

Key considerations

The lack of diverse geological data around saline aquifers ramps up project costs, TGS has said, recommending that leakage risks be thoroughly defined before sequestration operations go ahead.

The British Geological Society (BGS) has also backed careful characterisation work on prospective aquifer storage sites. This is to ensure storage security, integrity, feasibility and cost-effectiveness.

Simulations may give more confidence on how the gas will respond, where it would be important to remember the reactions in the reservoir and how the caprock can differ to depleted fields.

Research shows CO2 injection brings a risk of overpressure-induced caprock failure or fault activation as the brine displaces. There also needs to be care around how CO2 reacts with water, cement and steel. After dropping the pH of water, it can dissolve the cement used to plug injection wells in some situations. Corroded cement can create escape pathways for the CO2.

This may make depleted fields seem a safer bet, along with their proven record of trapping petroleum for millions of years. This does not necessarily mean they are “simpler” storage sites.

Looking at a pore-level of detail is essential, according to Professor Martin Blunt. He told E-FWD there are “distinct pore-scale flow processes that control the effectiveness of CO2 storage.”

“I think understanding these are indeed increasingly important, as we need to assure government, the public and other stakeholders that we can safely and securely store the carbon dioxide over thousand-year time-scales.”

However, he said, from a physics perspective, “storage in a saline aquifer is simpler than in depleted hydrocarbon fields. This is because there is only interaction between the brine in the aquifer and carbon dioxide, rather than between brine, carbon dioxide and the oil or gas present.”

Taking petroleum out of the equation might simplify things. However, as they are less understood than petroleum plays, the saline storage sites come with their own mysteries. The interaction of CO2 as a supercritical fluid with the brine and rock minerals at the pore scale drives the migration and trapping mechanisms of the gas.

*The Northern Lights Carbon Capture and Storage facilities in Øygarden, outside Bergen*

Geology in focus

Fracture networks can also be important to consider in determining whether there is surface seepage or successful trapping.

The presence of interbeds in aquifers cans sometimes increase sealability. Research from Lisong Zhang geomodelled an irregular fault distribution in an interbedded saline aquifer. In a paper, the researchers said it “displays a great CO2 storage potential, as the fault and the interbed could combine to enhance the CO2 storage efficiency”.

Interbedded saline aquifers may provide important storage reservoirs. More research is needed to understand how different fault frameworks affect migration and storage.

Reducing risk

Depth influences storage capacity, but ideal depth can vary case-by-case. A 2005 paper highlighted the gas can occupy less pore space in liquid form at 600-800 metres below sea level. A 2023 presentation by SUT indicated that a depth of more than 1,000 metres was better for CO2 in its supercritical state.

Identifying the best sites for the safe storage of CO2 depends on ideal geological properties and structural integrity.

Detailed characterisation of the aquifer’s properties is beneficial, before project planning. This should include porosity, permeability, wettability and the presence of faults that could compromise storage integrity.

Long-term monitoring will help to ensure the cohesiveness of these storage sites, along with pressure management to mitigate risks of caprock failure and to ensure the sites are gas-tight.

Trapping tech improving

Some artificial measures could help along the way to better contain CO2. These range from foam co-injection, to alter the CO2 plume’s flow and improve sweep efficiency within the aquifer, to the use of nanoparticles to increase the chances of trapping.

New technologies and methods could improve efficiency, safety or monitoring of CO2 storage, specifically in saline aquifers. Some recent research insights have already showed promise.

One 2024 review of carbon storage in saline aquifers shares recent advancements to improve trapping. This includes foam co-injection, applying microbial methods or nano particles. Hydraulic fracturing is a recommended technique to improve injectivity and storage in the narrower low permeability reservoirs.

Government support

There is clearly rising interest in saline aquifers, which could complement storage sites along with depleted reservoirs for CO2 storage. There is less knowledge on aquifers so more geological research is crucial to identify suitable sites for safe containment. This must also pay attention to geological details, such as pore mechanisms and fault networks.

Saline aquifers may be more uncertain compared to depleted fields, but research and development has been advancing.

Government support could help to buffer risk, as CCS is within the nation’s interest as an important part in 2050 net zero plans. What is more, while companies will carry out the CCS work and operations, ultimately they will hand these sites back to the government for oversight.

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