Underground hydrate storage of CO₂ has been proposed as a novel method of storing CO₂ for its high potential storage density
Summary
Underground CO₂ storage using hydrate formation offers a promising method for carbon sequestration through crystalline structures where water molecules trap CO₂ under specific pressure and temperature conditions. This approach leverages natural geological formations like ocean floors or permafrost regions to create stable hydrate caps that prevent CO₂ leakage. While hydrate formation allows rapid sealing and self-healing properties, concerns remain about long-term stability due to potential changes to ambient conditions. Current research is primarily theoretical with limited practical testing, highlighting both the method's potential for high-density storage and the need for further investigation into its reliability.
Hydrate storage is a method that involves capturing CO₂ in the form of hydrates (also known as hydrated salts), which are solid crystalline compounds composed of CO₂ molecules trapped within the structural lattice of water ice structures. Hydrates are formed under high pressure and low temperature conditions (temperatures can be above zero degrees Celsius). A somewhat common example of a hydrate is Epsom salt (MgSO₄·7H₂O), which is the solid form of magnesium sulfate trapped between 7 water molecules in the ice lattice, stable at ambient temperatures and pressures.
Characteristics of hydrate storage of CO₂
- Composition: Water molecules form bonds to effectively create a 'cage' around each CO₂ molecule, trapping it within the hydrate structure.
- Formation conditions: High pressure and low temperature are generally required for hydrate formation, known as the 'hydrate stability zone'. These conditions mean that locations need to be chosen more specifically than other storage methods. Locations can include below thick permafrost or below the ocean floor 1.
Trapping Mechanisms
Hydrate storage for CO₂ would involve the injection of CO₂ into geological formations that contain the necessary temperature and pressures to facilitate hydrate formation. Hydrates can be stably formed up to temperatures approaching 10°C (50°F). However, for this to occur, the pressures need to approach 4 MPa, which is close to 40 times greater than atmospheric pressure 2.
One proposed method of hydrate storage is to inject CO₂ below the ocean floor or permafrost where the conditions for hydrate stability are met. Afterward, it would be possible to inject more CO₂ below the hydrate stability zone where the CO₂ would begin to return to its gaseous or liquid state due to the increasing ambient temperatures encountered the deeper the CO₂ is injected. This would mean that the CO₂ hydrate would effectively form a 'cap' similar to a caprock in saline aquifer storage, which would stop the non-hydrate CO₂ from escaping 1.
Advantages
The main advantage of hydrate storage is its fast formation 1. When ambient conditions lie within the hydrate stability zone, the CO₂ can be injected into the formation and will begin to form hydrates almost immediately, meaning losses back into the atmosphere would be minimal. This allows the hydrate to exhibit 'self-sealing' properties, whereby unintended fractures to a hydrate cap will naturally close off.
Disadvantages
The main disadvantage of hydrate storage relates to uncertainties in its long-term stability. As ambient conditions change, the formation may leave the hydrate stability zone and quickly decompose. The majority of the research into hydrate storage has been theoretical modeling, with only a small handful of field and lab tests conducted 1.
Sources
Footnotes
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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
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Hu, Q., & Xiao, X. (2023). Formation methods and applications of carbon dioxide hydrate: An overview. Carbon Capture Science & Technology, 7, 100113. https://doi.org/10.1016/j.ccst.2023.100113 ↩