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Basalt formations have been proposed as a long-term storage site for captured atmospheric CO₂

Summary​

Basalt rock formations offer potential for permanent CO₂ storage through mineralization, where carbon dioxide reacts with reactive metal ions to form stable carbonate minerals. These abundant geological formations contain calcium, magnesium, and iron that enable chemical trapping. While mineralization provides more permanent storage than other methods like saline aquifers, the process requires months to years to complete and depends on formation characteristics like permeability. Recent demonstrations like Iceland's CarbFix project show promising mineralization rates, with 50% of injected CO₂ solidified within 4 – 9 months. Challenges include potential CO₂ leakage before mineralization completes and variable reaction rates depending on groundwater conditions. Both underground (in situ) and surface-level (ex situ) mineralization approaches are being explored globally.

Basalt formations have been proposed as long-term storage sites for captured atmospheric CO₂ via a process known as mineralization. Basalt is a type of igneous rock that forms from the cooling of lava or magma, and it forms approximately 8% of the Earth's continental crust 1.

Characteristics of basalt formations​

  • Composition: Basalt formations are abundant in highly reactive metal ions such as calcium (Ca), magnesium (Mg), and iron (Fe), which are vital in the mineralization trapping process.

  • Structure: Basalt formations have a highly heterogeneous structure, meaning that they have sections of differing permeability and porosity throughout the formation 1. It is within those areas of high permeability and porosity that are suitable for CO₂ storage.

  • Global availability: Basalt is a highly abundant rock type making up large areas of the Earth's continental crust above and below water.

Trapping Mechanisms​

Basalt formations trap CO₂ via a mechanism known as mineralization. This process involves the reaction of CO₂ with the reactive metal ions found abundantly in the basalt formation, forming stable carbonate minerals. For the mineralization process to occur, there needs to be some water contained within the formation; otherwise, some water may need to be added. The mineralization process can follow a number of different pathways depending on the type of basalt formation and the type of reactive metal ions present. Shown below is an example of the mineralization process with the reaction of calcium (Ca) ions 2.

  1. The CO₂ first dissolves in the water to form carbonic acid (H₂CO₃).
  2. The H₂CO₃ then reacts with metal ions present in the water that is trapped within the basalt formation. These metal ions are either already present in the groundwater, or they are released from a reaction with the H₂CO₃ and minerals such as calcium silicate (CaSiO₃) found in the basalt formation.
  3. The calcium ions (Ca²⁺) react with dissolved CO₂ to form calcium carbonate (CaCO₃), which is the solid species that traps the CO₂.

The simplified reaction can be shown as:

Ca2++CO2+H2O→CaCO3+2H+\begin{align*} \mathrm{Ca^{2+} + CO_2 + H_2O \rightarrow CaCO_3 + 2H^{+}} \end{align*}

Advantages​

The main advantage of carbon dioxide storage in basalt formations is their high abundance throughout the Earth's crust. Another advantage of basalt formation mineralization is its stable and long-term trapping of CO₂. Unlike other storage methods such as Saline aquifer storage, which relies on a stable caprock to maintain storage, mineralization is a more stable process as the CO₂ is trapped in the solid carbonate minerals that will not release the CO₂ back to the atmosphere.

Disadvantages​

A key disadvantage of basalt formation mineralization is the potential for CO₂ to be released back to the atmosphere before the mineralization process has occurred. Mineralization processes vary with conditions such as temperature, pressure, and acidity of the groundwater, but tend to occur within a matter of months and years 3. Results from the CarbFix project, the first large-scale demonstration of basalt formation mineralization, showed that 50% of the injected CO₂ was mineralized within 4 – 9 months 3. However, the mineralization rate increased after the initial 4-month period after the injection rate was doubled 3. This relatively long mineralization time means that many basalt formations might not be suitable for storage as they often contain fractured caprocks; however, the ability for the mineralization process to "self-heal" by sealing off these fractures is being explored 1.

Ex situ mineralization​

The process described above for basalt formations is known as in situ mineralization, where the CO₂ is injected into a target formation that contains the desired reactive metal ions. However, there is also the possibility of ex situ mineralization, where the mineralization process occurs above ground in a reactor with the addition of appropriate feedstock. Ex situ mineralization has been explored in countries such as Australia that do not contain large underground formations suitable for storage 4.

Sources​

Footnotes​

  1. Aminu, M. D., Nabavi, S. A., Rochelle, C. A., & Manovic, V. (2017). A review of developments in carbon dioxide storage. Applied Energy, 208, 1389-1419. https://doi.org/10.1016/j.apenergy.2017.09.01 ↩ ↩2 ↩3

  2. Matter, J. M., Broecker, W. S., Gislason, S. R., Gunnlaugsson, E., Oelkers, E. H., Stute, M., Sigurdardóttir, H., Stefansson, A., Alfreðsson, H. A., Aradóttir, E. S., Axelsson, G., Sigfússon, B., & Wolff-Boenisch, D. (2011). The CarbFix Pilot Project–Storing carbon dioxide in basalt. Energy Procedia, 4, 5579-5585. https://doi.org/10.1016/j.egypro.2011.02.546 ↩

  3. Clark, D. E., Oelkers, E. H., Gunnarsson, I., Sigfússon, B., Snæbjörnsdóttir, S. Ó., Aradóttir, E. S., & Gíslason, S. R. (2020). CarbFix2: CO₂ and H₂S mineralization during 3.5 years of continuous injection into basaltic rocks at more than 250 °C. Geochimica et Cosmochimica Acta, 279, 45-66. https://doi.org/10.1016/j.gca.2020.03.039 ↩ ↩2 ↩3

  4. Romanov, V., Soong, Y., Carney, C., Rush, G. E., Nielsen, B., & O'Connor, W. (2015). Mineralization of carbon dioxide: A literature review. ChemBioEng Reviews, 2(4), 231-256. https://doi.org/10.1002/cben.201500002 ↩