Why do we need permanent storage of CO₂?

Since the beginning of the industrial revolution, humanity has emitted approximately 2’000 billion tons of CO₂ equivalents. Roughly half of those additional greenhouse gases have been accumulating in the atmosphere. This imbalance of CO₂ in the atmosphere has been fuelling global warming. To restore a healthy balance of CO₂ in the air, a multitude of scientific studies indicate that by mid-century 10 billion tons of carbon dioxide will need to be permanently removed from the air every year. Direct air capture is one solution that can help achieve this goal.

What is the role of direct air capture?

To keep global warming to safe levels, there are a couple of things that we need to do. First, we have to drastically reduce our emissions to avoid an increase in CO₂ levels in the atmosphere.  Further, we have to balance the greenhouse gas emissions that cannot be avoided through CO₂ removal from the air. Lastly, we have to remove historic CO₂, as there is already too much CO₂ in the atmosphere. Climeworks’ direct air capture technology contributes to the last two points: it allows to remove unavoidable as well as historic CO₂ emissions from the air.

Climeworks' Orca plant in Iceland

Climeworks' Orca plant in Iceland

What happens to air-captured CO₂?

But what happens to the CO₂ once it is captured from the air? First option: it can be upcycled in the circular economy, for example, into sustainable aviation fuel, or in fizzy drinks. Thereby, carbon dioxide is captured from the air, used to create products, and released back into the atmosphere after the consumption of the products, captured from the air again and so on – a closed carbon cycle, which prevents an increase of CO₂ in the atmosphere.

But we have to face it: humanity has already emitted too much CO₂. As outlined above, we have to actively remove historic CO₂ from the air to keep global warming to safe levels on top of preventing new CO₂ from entering the atmosphere. Therefore, safe and permanent storage solutions that keep CO₂ out of the atmosphere are needed. And to enable Carbon Dioxide Removal technologies to fulfill their foreseen role in climate change mitigation, these storage solutions have to be scalable to billions of tons of CO₂ per year. Geological storage of CO₂ combines the scalability and technical maturity on the one hand with safety and permanence on the other.

What makes geological storage of CO₂ a permanent storage solution?

CO₂ storage happens naturally within the biosphere (land) in trees, soils, the oceans and in the geosphere (rocks on the surface and underground). When the CO₂ is stored in the geosphere, we speak of geological storage. Climeworks focuses on geological storage, as this is the most permanent form of storage. CO₂ is and has been naturally stored within the geosphere almost everywhere around the world. Most prominently, it can be found in fossil resources on almost any continent. However, it also naturally mineralizes as it crystallizes with certain minerals and changes phase to a solid material, such as carbonates. This happens above and below the ground and contributes to the formation of amazingly beautiful landscapes as seen in Oman or the Yellowstone National Park. Scientific estimates show that the global potential for geological storage of CO₂ outweighs all greenhouse gases ever emitted since the Industrial Revolution approximately by a factor of 3. Geological storage in suitable rock formations in the deep underground is seen as the safest, most scalable and permanent approach of CO₂ storage as it is a natural process that is able to store CO₂. That is why Climeworks focuses on it.  

CO₂ stored in a mineral phase in the Yellowstone National Parc

Geologic Storage of CO₂ in the Yellowstone National Parc

What criteria should geological storage solutions fulfill?

Natural CO₂ storage serves as a blueprint for managed CO₂ storage in geological formations. Suitable geological sites include a porous and permeable reservoir rock, that provides the required storage volume in the form of microscopic rock pores (e.g. a sandstone formation) and an impermeable (“tight”) caprock above it (e.g. a shale formation). The typical depth of CO₂ storage reservoirs is 800 to 2500 meters below ground. The following four natural mechanisms, so-called “trapping” mechanisms, help to ensure safe and permanent storage and prevent CO₂ from re-entering the atmosphere:

  1. Structural & stratigraphic trapping in the form of one or more impermeable rock formations ("caprocks"). These formations lay above the reservoir that contains the CO₂ and inhibits the CO₂’s tendency to move upwards. These impermeable rock formations can be hundreds of meters thick and there are often several caprock formations above a reservoir.
  2. Residual CO₂ trapping occurs as the CO₂ moves within the porous reservoir rock. Some of the CO₂ will be left behind as disconnected or remaining droplets in the pore spaces, which are immobile – just like water in a sponge.
  3. Solubility trapping means that the CO₂ dissolves in the saline formation water that is naturally present in the underground reservoir (making it a slightly fizzy brine). The CO₂ is thus no longer present as a separate phase; it unifies with existing geological brine. Thereby, the CO₂’s tendency to move upwards is eliminated due to the resulting solutions’ higher density.
  4. The final step, mineral trapping, relies on long timescales: the CO₂ contained in the formation water will undergo another geochemical reaction and mineralize towards stable carbonates, just as the ones known from above-ground mineralization in Oman and the Yellowstone National Park. 

While the proper characterization of the geological reservoir prior to CO₂ injection is mandatory to ensure containment by the caprock, it is conceptually wrong to assume a typical and steady leakage rate. Geological CO₂ storage is meant to be permanent by design, leakage only occurs in the case of unexpected failure. The (low) risk of leakage occurs mainly while CO₂ is injected and further decreases with time.[1,2] The techniques that enable the storage of CO₂ deep underground have been developed and used for decades in geothermal energy and, although for very different purposes, in the oil and gas industry. CO₂ storage has been proven to be technologically feasible and can also be economically viable.

Ophiolite in the Yellowstone National Parc

Ophiolite in the Yellowstone National Parc

Examples of managed geological storage

Based on the aforementioned trapping mechanisms, a wide range of managed CO₂ storage projects was able to imitate natural geological CO₂ storage successfully. Here are two prominent examples:

Sleipner

Probably the most prominent project is the Sleipner Project in the North Sea: this project is injecting roughly 1 million tons of CO₂ annually offshore into a saline aquifer at approximately 1 km below the seafloor (into the Utsira formation) since 1996. Like several dozens of others, this project demonstrates that geological CO₂ storage is technically mature, can be managed with high safety standards and is resulting in a de facto permanent removal of atmospheric CO₂ – which ultimately can help fight climate change.

Carbfix

In-situ mineral carbonation aims to accelerate the mineralisation process. With this approach, the CO₂ is stored via injection into reactive rocks, such as mafic or ultramafic rocks, which contain high concentrations of divalent cations. Carbon mineralisation can be further promoted by the dissolution of CO₂ into water before or during its injection, such as the Carbfix method, achieving solubility trapping immediately and mineral trapping within years.[4] Injection of dissolved CO₂ into reactive rocks for mineral carbonation results in a negligible risk of the CO₂ migrating back to the atmosphere both i) over the short term, due to the dissolution of CO₂ and the density-related inhibition of surface migration, and ii) the long term, due to the conversion of the CO₂ into carbonate minerals.

Change in contribution of the CO₂-trapping mechanism of CO₂ storage over time when injecting (a) pure, supercritical CO₂ in sedimentary basins and (b) water-dissolved CO₂ for rapid mineralization. Figure adapted from Snæbjörnsdottir et al.(2020) where part (a) is from the IPCC Special Report on Carbon Capture and Storage (2005). The timescale on the x-axis is indicative and is dependent on the geological characteristics and varying implementation of the CO₂ injection. Credit: Carbfix.

Basaltic rock with mineralized CO₂

Basaltic rock with mineralized CO₂

The Carbfix method was developed over the past decade by Reykjavik Energy and several academic partners.[4–6] When applied in combination with a CO₂ capture step, the Carbfix method involves the release of the captured CO₂ as fine bubbles into a stream of co-injected water at depth within injection wells (alternatively, for gas streams with high initial CO₂ concentration, the dissolution of CO₂ in water can take place in a scrubbing tower, essentially replacing the capture step). The gas completely dissolves into the water before entering the porous reservoir rocks. The water is typically sourced from the same underground reservoir. With this storage approach, which has been applied at depths between 400-2200 meters underground at temperatures between 30 and 260°C it is proven that after two years, already 95% of the CO₂ is converted into stable carbonate minerals.[4,7] Once mineralized, the CO₂ is permanently bound to the subsurface rocks and can hardly be released.

Key take-aways

  • We need to restore a healthy balance of CO₂ in the atmosphere by removing historic CO₂ emissions and storing them safely and permanently in natural sinks.
  • Geological storage is a safe and permanent storage solution and can be managed by humans. CO₂ storage in underground and above ground formations as well as in rocks results in CO₂ storage of typically several hundred thousand years.
  • The processes are relying on natural mechanisms and geological CO₂ sequestration is seen as the most scalable and permanent approach to CO₂ storage, with mineralization described as the “perhaps safest of all options” by the Royal Society of Sciences and the Royal Academy of engineering and medicine.
  • The global potential for geological storage of CO₂ is not limiting. It is said to outweigh all greenhouse gases ever emitted since the industrial revolution by a factor of approximately 3.
  • Geological CO₂ storage is technically mature. It can build a key mechanism in the much-needed scale-up of carbon dioxide removal.

[1]     M. E. Boot-Handford, J. C. Abanades, E. J. Anthony, M. J. Blunt, S. Brandani, N. Mac Dowell, J. R. Fernández, M.-C. Ferrari, R. Gross, J. P. Hallett, R. S. Haszeldine, P. Heptonstall, A. Lyngfelt, Z. Makuch, E. Mangano, R. T. J. Porter, M. Pourkashanian, G. T. Rochelle, N. Shah, J. G. Yao, P. S. Fennell, Energy Environ. Sci. 2014, 7, 130.

[2]    J. Blackford, S. Beaubien, E. Foekema, V. Gemeni, S. Gwosdz, D. Jones, K. Kirk, J. Lions, R. Metcalfe, C. Moni, K. Smith, M. Steven, J. West, F. Ziogou, A Guide to Potential Impacts of Leakage from CO₂ Storage, 2015.

[3]    IPCC, Carbon Dioxide Capture and Storage, Cambridge University Press, Cambridge, UK, 2005.

[4]    J. M. Matter, M. Stute, S. Ó. Snæbjörnsdottir, E. H. Oelkers, S. R. Gislason, E. S. Aradottir, B. Sigfusson, I. Gunnarsson, H. Sigurdardottir, E. Gunnlaugsson, G. Axelsson, H. A. Alfredsson, D. Wolff-Boenisch, K. Mesfin, D. F. de la R. Taya, J. Hall, K. Dideriksen, W. S. Broecker, Science (80-. ). 2016, 352, 1312 LP – 1314.

[5]    I. Gunnarsson, E. S. Aradóttir, E. H. Oelkers, D. E. Clark, M. Þ. Arnarson, B. Sigfússon, S. Ó. Snæbjörnsdóttir, J. M. Matter, M. Stute, B. M. Júlíusson, S. R. Gíslason, Int. J. Greenh. Gas Control 2018, 79, 117–126.

[6]    S. R. Gíslason, H. Sigurdardóttir, E. S. Aradóttir, E. H. Oelkers, Energy Procedia 2018, 146, 103–114.

[7]    P. A. E. Pogge von Strandmann, K. W. Burton, S. O. Snæbjörnsdóttir, B. Sigfússon, E. S. Aradóttir, I. Gunnarsson, H. A. Alfredsson, K. G. Mesfin, E. H. Oelkers, S. R. Gislason, Nat. Commun. 2019, 10, 1983.