CCS stands for Carbon (Dioxide) Capture and Storage. CCS technology involves capturing, transporting and permanently storing CO₂. The basic idea is to capture CO₂ from the process or exhaust gases of e.g. large industrial plants, and store it in the deep subsurface instead of releasing it into the atmosphere. To have a significant impact on the climate, the CO₂ must remain underground for thousands of years. Only then CCS can help to limit global warming. Information on the basics of climate change can be found, for example, on the website of the German Federal Environment Agency.

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© J. Rütters

CO₂ capture from process or exhaust gases of industrial plants is technically challenging. Thus, this technology will be primarily applied at large stationary plants, such as various industrial plants, waste incineration plants as well as potentially also natural gas- or biomass-fired power plants.

For CO₂ capture, different technologies are currently applied. All capture technologies have in common that their application leads to an increase in overall energy demand of the production or industrial process and, in consequence, also to increasing process costs. Thus, a lot of effort is currently put into the optimisation of existing capture technologies and the development of alternative capture technologies

The next step in the process chain is the transport of the captured CO₂ to a storage site. If possible, storage sites close to the emitters are preferred to keep transport cost low. For the transport of large amounts of CO_2, pipelines are a cost-efficient option. Routing of CO₂ pipelines could follow existing pipeline routes to limit efforts, space use and costs. The captured CO₂ stream has to be compressed for pipeline transportation. In pipelines, CO₂ is typically transported at high pressures (typically between 80 to 150 bar) as a fluid of high density to maximise the amount of CO₂ that fits into a pipeline of a certain size. These high pressures are also typically encountered in high-pressure pipelines for the transportation of natural gas.

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© BGR

If the storage site is located in the offshore area, CO₂ can also be transported by ship. For this, the captured CO₂ stream is liquified by compression and cooling to below 0°C temperatures. At the storage site, the CO₂ is injected into the subsurface, e.g. from a platform or from the ship – after connecting to a subsea injection facility. Ship transportation tends to be more cost efficient than pipeline transportation for long transport distances. In addition, CO₂ transport by ship allows to flexibly connect different emitters and storage sites.

Small amounts of CO₂ can also be transported in tanks by rail or on roads which is already current practice as part of industrial supply chains.

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© BGR

Geological storage of CO₂ is most efficient in pore space of rocks at a depth of more than approximately 800 m. At the temperatures and pressure that typically are prevailing at these depths CO₂ has a much smaller volume (and a higher density) in comparison to atmospheric conditions. With increasing depths, temperature and pressure further increase in the subsurface. This further increase in temperature and pressure has only a small additional effect on the CO₂ density (as shown in figure). For long-term containment of the injected CO₂ in the storage reservoir, storage reservoir rocks must be overlain by barrier rock layers of sufficient thickness that are impermeable for CO₂.

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© BGR

In the subsurface, a given volume of CO₂ (in the CO₂ plume) is lighter than an equal volume of water (or formation water). This means that the CO₂ tends to move in the underground towards the earth's surface (buoyancy). Therefore, a sufficiently thick barrier, i.e. a sufficiently thick rock-layer impermeable to migrating CO₂, is required above the storage reservoir to retain the CO₂. Furthermore, an effective structural trap within the storage reservoir rock unit can additionally help to ensure CO₂ containment. Suitable barrier rocks are, in particular, salt and shales.

In addition to this ‘structural trapping’, the capillary forces within the rock pores ensure that once CO₂ has entered the pores, it cannot easily be removed again. Some of the CO₂ remains in the pores (‘capillary trapping’). In addition, CO₂ partly dissolves in the formation water that is naturally present in the pores. This CO₂-rich solution is heavier than the surrounding formation water and therefore tends to sink down (‘solution retention’). CO₂ can also react with formation water or the surrounding rock to form solid minerals (i.e. carbonates), in which the CO₂ is stored over the long term (‘mineral trapping’). Reactions with different rock components occur at different rates. For example, reactions with silicate minerals in the reservoir rocks are sometimes very slow, meaning that carbonates can continue to form hundreds or thousands of years after the CO₂ injection.

Overall, the various trapping mechanisms described above ensure that the CO₂ is contained in the storage reservoir with increasing safety over time. Prevailing type and extent of the individual trapping mechanisms depend on the specific conditions at each storage site.

The geological requirements for underground CO₂ storage largely correspond to the conditions of natural CO₂ and hydrocarbon deposits. Their existence proves that subsurface structures and barriers at a specific location are capable of retaining liquids or gases for many millions of years.

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© BGR

The German carbon dioxide storage act (Kohlendioxid-Speicherungsgesetz) stipulates that proposed storage projects may only be approved if the storage reservoir is demonstrably covered by effective barrier rocks. Existing geological faults must be either impermeable or constitute no obvious migration/leakage risk due to restricted extend and/or connectivity. In addition, the materials used to complete the well(s), must be resistant to corrosion by CO₂, other components of the CO₂ stream, if any, and saline formation water, over the long term.

A comprehensive risk assessment is required for each potential storage complex and site as part of the exploration process and the licensing procedure. Extensive risk statistics that could enable an assessment of the probability of occurrence and impact of adverse events are not yet available because there are too few CO₂ storage facilities in operation worldwide for this and because there have been no accidents, incidents or events with environmental relevance or personal injury at these facilities to date.

Each storage must be thoroughly monitored during and after CO₂ injection by various sampling, measuring and observation methods of the earth's surface or the seabed, water column and underground properties. This will enable to detect any deviations from original conditions and from the expected propagation behaviour of the CO₂. Moreover, type and extent of a potential leakage could be detected at an early stage of operations. Appropriate countermeasures could then be taken to avoid harming people and the environment.

For the safe injection of CO₂ into the subsurface (operational phase), we can draw on practical experience from ongoing CO₂ storage projects worldwide and corresponding international standards. Moreover, decades of experience in oil and gas production, enhanced oil recovery methods (also employing CO₂), the operation of natural gas storage facilities and the extraction of CO₂ from natural reservoirs exist. Unlike natural gas, CO₂ is neither flammable nor does it form explosive mixtures with air.

The German carbon dioxide storage act (Kohlendioxid-Speicherungsgesetz - KSpG) defines a storage complex as the carbon dioxide storage reservoir and the surrounding rock strata or parts thereof, which influence the overall integrity and safety of the carbon dioxide storage reservoir as a natural second barrier.

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© BGR

The definition of the storage complex is part of the permitting procedure for each CO₂ storage project. According to the KSpG and the EU CCS Directive, only the escape of CO₂ (or by-products of the carbon dioxide stream) from the storage complex is considered as "leakage" which must be avoided or stopped. This means that CO₂ may move out of the primary CO₂ storage reservoir and accumulate e.g. in the secondary storage rock layer, without countermeasures needing to be taken. However, it shall not leave the defined storage complex. The movement and dispersion of CO₂ (or its minor constituents) within the storage complex is referred to as migration in the KSpG.

Note: In their joint project ‘Storage Register of Germany’ (Speicherkataster Deutschland) for mapping rock layers that contain potentially prospective porous storage rock formations worthy of further investigation, the state geological services of Germany used the term ‘storage complex’ in a different sense.

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© BGR

The only site in Germany where CO₂ has been pumped into the ground for storage purposes so far is Ketzin, Havel. At the pilot injection site in Ketzin, a total of 67,271 tonnes of CO₂ were successfully injected and stored there between 2008 and 2013. The Ketzin site was planned, constructed and operated under the lead of the German Research Centre for Geosciences (GFZ) in Potsdam. The CO₂ injection was accompanied by extensive research with a focus on the development, testing and validation of monitoring technologies and concepts.

Further information on activities at the Ketzin site can be found here.

Between 2010 and 2014, there were several initiatives in Germany to capture CO₂ at selected power plants and store it underground at suitable locations. These projects were abandoned before the site characterisation took place, i.e. before storage license applications were submitted.

According to the national CO₂ storage and transportation act (KSpTG) that entered into force in November 2025, geological storage of CO₂ is now permissible in the subsurface of Germany's exclusive economic zone and its continental shelf in the North Sea and the Baltic Sea.

Porous and permeable rock strata that could be suitable for storage are found, in particular, in the deep underground on- and offshore northern Germany. Thus, in principle, ongoing or planned CO₂ storage projects in Norway or the other neighbouring countries in the North Sea realm are also conceivable in a similar manner in Germany.

The CO₂ captured from various emitters contains different concentrations of impurities, such as sulfur oxides, nitrogen oxides, hydrogen sulphide, carbon monoxide, oxygen and solvents. Impurities present and their concentrations depend on the emitter and the capture process. Impurities can have a corrosive effect on steel pipes or the well cement. They also affect the physical properties of the captured CO₂ stream and thus its transport and injection behaviour. In addition, impurities can affect the behaviour of the CO₂ stream in the storage reservoir.

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© modified from Rütters et al. 2017

Therefore, acceptable compositions of the CO₂ stream must be defined for each CCS project or for CO₂ transport and storage individually. The German carbon dioxide storage act, for example, requests that the carbon dioxide stream must consist overwhelmingly of carbon dioxide. The proportion of carbon dioxide must be as high as can be achieved with reasonable efforts using state of the art technologies for the respective type of emitter. The CO₂ stream composition must not result in risks to long-term safety, the environment or people. However, the higher the required purity of the CO₂ stream and the greater the effort necessary to purify the CO₂ stream, the more expensive capture and purification will be.

Many CCS projects that are currently being planned or implemented in various European countries strictly limit the permissible water content of the CO₂ stream for pipeline transport as well as the levels of reactive components such as sulfur oxides. This significantly reduces corrosion in pipelines and plant components. The state of the art for the CO₂ stream composition for transport is defined in a technical regulation of the German Association for Gas and Water (Deutscher Verein des Gas- und Wasserfachs, DVGW) which is intended to form the basis for a future European standard.

Impurities in CO₂ streams can also react with reservoir rocks and alter them - predominantly close to the injection wells. This can modify the storage rock properties and potentially lead to a decrease of the storage rock quality. Such rock alterations can only be remediated in close proximity of the injection well. The degree of rock alterations can be reduced by selecting sites with geochemically stable, quartz-rich sandstones as injection and storage formations.

In principle yes, like any other subsurface activity that is associated with pressure changes and the displacement of fluids in the underground. However, these "seismic events" are most of the time barely noticeable micro-earthquakes.

The changes in mechanical rock stress that arise due to pressure changes in the subsurface associated with the storage of CO₂ can and must be calculated in advance of a CO₂ injection using numerical models. Based on injection tests and the results of such models, injection rates and tolerable maximum pressures during operations are deduced. The values are set in order to minimise the risk of triggering earthquakes and to prevent compromising the barrier rock integrity. Whether earthquakes occur which can be detected by people on the earth's surface and whether these events cause any damage depends on the individual conditions of each storage site.

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© BGR

As mentioned above, CO₂ injections into the subsurface can trigger (micro-)earthquakes. This was the case e.g. during a CO₂ injection project in In Salah, Algeria. Here, the injection created overpressure in the reservoir rock and its surroundings, which ultimately induced new or reactivated existing fractures in the lower 100-200 m of the barrier rock above the storage reservoir. However, since the barrier rock layer at location is up to 950 metres thick, there was no impairment of storage safety. In storage projects like Sleipner in Norway and Weyburn in Canada, comparable injection rates to those in In Salah did not trigger any earthquakes or micro-earthquakes.

The storage of CO₂ in fractured basaltic rocks is currently being tested and developed on Iceland (e.g. in the Carbfix2 project) and at other locations. For example, since 2012 CO₂ dissolved in water (and later also CO₂ containing H₂S) has been injected into the subsurface by two wells near the Hellisheidi geothermal power plant during several test campaigns. The injection of one tonne of CO₂ requires about 27 tonnes of water and 5.3 tonnes of basalt. By mid-2020, more than 65,000 tonnes of CO₂ had been captured at Hellisheidi geothermal power plant and stored underground. A major advantage of this process is that young, un-weathered basic volcanic rocks predominantly consist of volcanic glass and minerals that are quickly dissolved by CO₂-rich water (carbonic acid). That way, they release high amounts of Ca²⁺, Mg²⁺ and Fe²⁺ ions. These ions can form new carbonate minerals with the dissolved CO₂, thus providing long-term storage of the CO₂ (‘mineral trapping’). A major part of the injected CO₂ was mineralised within a few months after injection at Hellisheidi. However, how quickly and to what extent this mineralisation occurs depends on the site and the site-specific conditions.

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© BGR

It is not yet clear whether this process can also be used for the injection of larger quantities of CO₂ through one well, as it is done in the Norwegian CCS projects Sleipner and Snøhvit, for example. Various projects on Iceland are currently testing the use of seawater instead of fresh water as the carrier for CO₂ and a CO₂ injection into older and less permeable basalts. Moreover, there are plans to set up a storage hub (‘Coda Terminal’) in the South-West of Iceland. The total storage capacity available there is estimated at 500 million tonnes of CO₂.

In principle, basic volcanic rocks are found in many parts of the world. They occur both on land and under the seabed, forming extensive rock volumes in some areas, e.g. ‘flood basalts’. These could potentially offer large storage capacities for CO₂. Various research and development projects are investigating to which extent these rocks are suitable for storing large quantities of CO₂ in a short injection period. The only other place in the world where CO₂ has been injected into basalt rocks to date is near the city of Pasco in eastern Washington State. Here, around 1,000 tonnes of CO₂ were injected for research purposes in July and August 2013.

Alongside USA and Iceland, also India, Australia, Canada and South Africa have widespread deposits of basic volcanic rocks. In contrast, in Germany these are only found in a few regions. Basic volcanic rocks usually forms small, isolated deposits in Germany which are also protected as raw material deposits, drinking water aquifers or for other reasons; expanding existing mining sites in Germany is hardly possible. Therefore, the potential for CO₂ storage in basic volcanic rocks in Germany is limited or negligible. The same applies to a different approach: the mining of basalt, which is crushed and spread on fields. Here, basalt powder can react with CO₂ from the air and can be partially converted into carbonates. The current annual production of basalt and lava in Germany (25 Mt in 2016) would be sufficient to bind just under 5 Mt of CO₂ per year.