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The monitoring of CO₂ storage sites for leakages is a difficult task which often relies on observations of seismic activity and surface deformations.

Summary​

Geological storage of CO₂ carries risks of leakage that could reduce carbon removal effectiveness and pose environmental hazards. Storage sites typically rely on natural seals like caprocks or self-sealing mechanisms in formations like saline aquifers and basalt. Potential leakage pathways include well defects, gas diffusion, capillary pressure breaches, and fractures from seismic activity or injection pressure. Monitoring combines established techniques like seismic surveys with emerging methods like tracer detection. While well-sealing practices are mature, long-term integrity requires continuous monitoring due to CO₂'s corrosive potential. Different storage methods offer varying security levels, with depleted hydrocarbon reservoirs having proven caprocks, while other geologic storage methods depend on slower sealing processes or untested caprocks.

Carbon dioxide (CO₂) leakage is a major concern when considering potential storage methods and sites. CO₂ leakages are detrimental in two ways: they can undermine the effectiveness of atmospheric carbon dioxide removal, and they can pose a health risk to plants and animals due to the toxic nature of pure CO₂.

The majority of hypothetical storage sites rely on the stability of natural formations to form a seal between the CO₂ and the atmosphere. This makes it difficult to monitor for new fractures in the chosen rock formations as they are often very extensive and are located at large depths. Almost every geologic storage site relies on a 'caprock', essentially an impermeable rock layer, to form a seal between the CO₂ and the atmosphere. Depleted oil and gas wells (Enhanced oil recovery sites) have the advantage of having a relatively high degree of certainty that the caprock is present and stable; otherwise, much of the gas and oil would have dissipated long ago 1. Storage methods such as saline aquifers, basalt formations (mineralization), and hydrate storage have the ability to self-seal when a new fracture is created; however, some of these processes occur over yearly time scales and are not guaranteed to completely seal the fracture if ideal conditions are not met 1.

Leakage pathways​

Leakages in CO₂ storage sites can occur through a number of pathways, including:

  • Well Leakage: CO₂ can leak through the well or injection point. Well sealing is a relatively mature practice that has been utilized by the oil and gas industry for many decades. As CO₂ is slightly acidic and forms a weak acid when dissolved in water, it has the potential to corrode the industry-standard cement well caps 1. Constant monitoring of well cap integrity would be required for effective CO₂ storage.

  • Diffusion: As with many gases and liquids, CO₂ can diffuse through a caprock even if it is considered impermeable. However, the molecular diffusion of CO₂ is very slow, meaning that the rate of leakage would be relatively low 1.

  • Capillary leakage: Capillary leakage occurs when the pressure of the CO₂ (often in a brine mixture) is greater than the capillary entry pressure of the caprock 1. This essentially means that the pressure is so great the caprock loses its impermeability and CO₂ can flow through its pores.

  • Faults and fractures: Faults and fractures can occur in areas with seismic activity and can provide pathways for CO₂ to leak through. Fractures can also occur if the CO₂ injection rate is too high, thus fracturing the caprock 1.

Leakage monitoring​

Monitoring for leakages is a complex task that requires a combination of technologies and techniques. The integrity of a borehole can be monitored using tilt meters to observe whether a fracture has occurred. Regular seismic monitoring can also be utilized to detect vibrations in the ground which may indicate a fracture. Lastly, satellite geodesy can be utilized to map changes in surface elevation which may also indicate that seismic activity is occurring 1. The majority of CO₂ storage monitoring methods rely on the analysis of seismic and surface deformations as these are the most reliable indicators of a fracture. Other more complicated methods such as the use of tracers mixed into the CO₂ stream can be used to detect leakages on the surface as they will have a distinct signature compared to ambient atmospheric CO₂; however, this is still a relatively new technique and has not been widely adopted 2.

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 ↩4 ↩5 ↩6 ↩7

  2. Mayer, B., Humez, P., Becker, V., Dalkhaa, C., Rock, L., Myrttinen, A., & Barth, J. A. C. (2015). Assessing the usefulness of the isotopic composition of CO₂ for leakage monitoring at CO₂ storage sites: A review. International Journal of Greenhouse Gas Control, 37, 46-60. https://doi.org/10.1016/j.ijggc.2015.02.021 ↩