Under the North Sea?

As environmentally friendly as possible. As risk-free as possible. In short: as safe as possible. This is how carbon storage should be implemented – in Germany and, of course, in other countries too. But what risks are actually involved in carbon storage, and how can they be minimised? Oeko-Institut is tackling these issues in various projects. The North Sea is one area of focus, as it is home to a number of European carbon storage projects.

There are numerous suitable geological formations in Europe for storing carbon dioxide. According to forecasts, the EU plans to store a total of 50 million tonnes of CO₂ by 2030. This figure is certain to rise considerably on the path to climate neutrality by 2040. “There are already many carbon storage projects in Europe. It is currently being driven forward primarily by Norway, Denmark, the UK and the Netherlands,” says Dr Florian Krob, Senior Researcher at Oeko-Institut. “Norway, for example, has been storing carbon dioxide since mid-2025 as part of the Northern Lights project near Bergen.”

In Germany, there are several areas that are suitable for CCS in principle. These are primarily saline aquifers, which are deep rock layers containing salt water, and – in theory – depleted natural gas fields. Saline aquifers are mainly found in northern Germany, but also in the Upper Rhine Graben and the Alpine foothills. “However, not all sites are actually suitable, for example where there are other uses for the land. Currently, Germany's plans are focused on the North Sea. The assumption here is a total storage capacity of between one and one-and-a-half gigatonnes of CO₂. However, I estimate that the actual figure is more likely to be below one gigatonne.”

Risks. What risks?

In the study titled ‘Securing the Underground – Managing the Risks of Carbon Storage through Effective Policy Design,’ which was initiated by the Nature and Biodiversity Conservation Union (NABU) and funded by the European Climate Foundation (ECF), Oeko-Institut examines the potential risks associated with carbon storage – focusing on storage beneath the seabed – and how these risks can be managed. “We are looking not only at the technologies themselves, but also at the regulation needed to minimise risks and improve the safety and sustainability of carbon storage.” But what risks are we actually talking about? “Experience gathered with previous projects shows that a wide variety of enormous operational difficulties can arise during injection. At the same time, there are many other challenges that interact with this. These include public perception, financial incentives, environmental impacts on marine ecosystems and a delay in the actual effectiveness of emission reductions.”

With a view to the legal and technical framework, the project team emphasises that the EU Directive on the geological storage of carbon dioxide provides a basis for managing the storage of CO₂. But it is also in need of improvement – for example, in terms of transparency and public trust. “We need independent third-party oversight, confidential reporting of irregularities and harmonised CO₂ purity standards,” states Krob. “Furthermore, if there is insufficient regulatory standardisation within the EU, this leads to inefficiency and higher operating costs for project operators.” Speaking of operators: the Sleipner and Snøhvit Norwegian storage projects saw costs significantly exceed expectations due to operational difficulties during CO₂ injection. “There is, for example, a risk that the rock layers will not show the injection and storage behaviour that was predicted. The correct selection of the site and the injection protocols – that is, the precise procedure for storage – is therefore crucial. Additional risks arise when existing infrastructure, such as pipelines from the fossil fuel industry, is to be reused. Technically, they are not compatible with the requirements of storing CO₂.”

Low environmental risks

According to the study, offshore storage is associated with relatively low environmental risks. However, these cannot, of course, be ignored. “These include direct effects such as the impact on marine ecosystems caused by CO₂ leaks, but also indirect consequences such as noise pollution resulting from increased shipping traffic.” The project team therefore recommends extending environmental impact assessments for carbon storage projects to include such indirect effects. Furthermore, the team calls for comprehensive and multidimensional spatial planning to avoid conflicts with other surface and subsurface uses, particularly in ecologically sensitive areas. “Efforts should also be made to find ways to reconcile these projects with other land uses.”

Low exposure risk

The risk of exposure to humans is low; nevertheless, comprehensive and continuous monitoring is required so that any irregularities in storage can be detected and mitigated. This monitoring is necessary both during the injection phase and after the storage site has been closed. “Furthermore, independent verifiers should be involved, and a clear framework for long-term liability is needed,” said Krob. “This must ensure that financial provisions are in place to cover remediation and maintenance costs following the closure of the storage site.” To date, EU guidelines stipulate a 20-year post-closure phase, while in Germany a 40-year period is envisaged. After this, the individual state authorities assume responsibility. “It is also important that policymakers regularly review and update the framework – for example, with a view to technological developments or cross-border carbon transport.”

The North Sea – our storage site?

In the study titled ‘Requirements in site selection to identify environmentally compatible storage sites’ commissioned by the German Environment Agency, researchers at Oeko-Institut examine in greater detail the suitability of the North Sea for German carbon storage projects. “From a purely geological perspective, it appears promising, but there are serious conflicts of use here – with nature conservation and marine protection, wind energy, fisheries and even military purposes. Unless compromises or opportunities for multiple use can be found, CCS will be very difficult to implement in the German sector of the North Sea.” CCS, for example, requires accompanying monitoring to oversee the storage process, says the researcher. “To this end, seismic measurements are carried out at regular intervals from ships – these must systematically survey quite large areas. This is not possible if there is a wind farm there, because ships cannot simply pass through it, and the vibrations from the wind farms can interfere with seismic measurements. Multiple use could also be difficult on areas managed by the German Armed Forces.” Furthermore, there are technical uncertainties that could significantly affect the potential for storage.

In this project, too, the researchers have considered environmental risks. There are direct risks – such as problems with storage or leaks. “Should large quantities of CO₂ be released uncontrollably, this would affect soil and water chemistry, resulting in acidification and risks to crustaceans and plankton, for example.” However, CCS could also have an indirect impact on the environment. “The technology can affect sensitive species such as porpoises – for example, during exploration, pipeline construction or seismic monitoring activities.” In principle, Krob sees only minor and acceptable risks from leaks, partly because these are highly unlikely. Impacts on marine ecosystems, such as those caused by noise, are, in his view, very likely and should be avoided at all costs. “Our study also aims to answer the question of how we can ensure long-term safety and environmental sustainability. Key aspects also include monitoring and an examination of processes and methods to prevent environmental impacts.”

In the North Sea, therefore, the storage of CO₂ should be carried out according to the principle ‘the less, the better’. “For it to be environmentally sustainable, a moderate and responsibly planned ramp-up is required, one that prevents undesirable developments and provides sufficient safety margins.” For example, clear exclusion criteria and binding minimum requirements are necessary when selecting sites. In addition to sufficient storage capacity and a favourable geological depth of at least 800 metres, these include a minimum thickness of the primary, sealing cap rock, as well as the long-term stability and integrity of this barrier. “Furthermore, safe and environmentally sound storage of CO₂ also requires what is known as favourable injectivity. This prevents technical difficulties during injection.” The exclusion criteria include, among other things, active faults in the subsurface, large-scale vertical movements, seismic or volcanic activity, and any adverse impact on usable groundwater caused by the storage projects.

From Dr Florian Krob’s perspective, it is crucial to establish the relevant processes at an early stage and in a standardised manner. “At present, for example, the legal framework only provides for an in-depth assessment once site investigations have been carried out by the operators – this is too late; we need a procedure that makes it clear at an early stage which areas are unsuitable due to environmental and safety concerns or conflicts of use.” Experience gathered in Denmark has also shown that an early, systematic and transparent process can significantly increase public acceptance of CCS. “It is important to involve the public at an early stage and on an ongoing basis to build trust,” emphasises the researcher from Oeko-Institut.

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Der Geowissenschaftler Dr. Florian Krob ist seit 2022 für das Öko-Institut tätig. Im Bereich Nukleartechnik & Anlagensicherheit befasst er sich neben der CO2-Speicherung in geologischen Schichten auch mit geotechnischen Fragestellungen im Kontext der Entsorgung und Endlagerung radioaktiver Abfälle.