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Deep ocean storage has been proposed as a long-term storage site for captured atmospheric CO₂

Summary​

Deep ocean storage offers potential for sequestering captured CO₂ using the ocean's natural properties. Liquid CO₂ can be injected at depths where high pressure and low temperature enable stable liquid or hydrate formation. Three main mechanisms exist: dissolution into seawater (causing localized acidification), hydrate formation under specific conditions, and dense CO₂ puddles below 3000m depth. While offering advantages like vast storage capacity and lower infrastructure needs compared to geological methods, significant challenges remain. Environmental concerns focus on marine ecosystem impacts from acidification, and legal barriers exist under international agreements prohibiting ocean dumping of industrial waste. Current research suggests it could serve as a mid-term solution despite expected leakage rates around 10% over 50 years, leveraging slow ocean circulation patterns to delay CO₂ return to the atmosphere.

Deep ocean storage (not to be confused with hydrate storage which can occur below the sea floor) has been proposed as a long-term storage site for captured atmospheric CO₂. Already, the world's oceans have absorbed an estimated third of all anthropogenic CO₂ emissions since the industrial revolution 1. Deep ocean storage involves injecting liquid CO₂ into the deep ocean where it can be sequestered for long periods of time.

Characteristics of deep ocean storage​

  • Pressure and temperature: The world's oceans are vast, deep, and cold. The high pressures and low temperatures encountered in the deep ocean mean that it is possible for CO₂ to remain in its stable liquid phase, which is denser than salt water, or it can form a solid hydrate phase which can remain fixed on the ocean floor 1.
  • Ocean Circulation: The world's oceans are constantly circulating on timescales that can be as long as centuries, especially for deep ocean water. These slow circulation patterns mean that injected CO₂ can remain underwater for sufficient time scales to assist in reducing the amount of CO₂ in the atmosphere.

Trapping Mechanisms​

The mechanisms of deep ocean storage can occur in three main forms:

  • CO₂ dissolution: CO₂ dissolution is the main proposed mechanism for deep ocean storage. Dissolution simply involves injecting CO₂ into the ocean where it can dissolve. Once dissolved, the CO₂ can form carbonic acid (H₂CO₃), which has the negative effect of localized acidification of the ocean.
  • CO₂ hydrate formation: At certain depths, the ocean is under high pressure and low temperature conditions, which can lead to the formation of hydrates above the sea floor. Hydrates are solid structures that can resemble ice but are made up of CO₂ molecules trapped within the structural lattice of water ice structures. When pressures are high enough, hydrates can form at temperatures as high as 8°C (46.4°F) 2.
  • CO₂ puddling: Similar to hydrate formation, under the correct conditions, CO₂ can form a stable liquid phase which is denser than salt water at the same conditions. CO₂ puddling typically requires depths greater than 3000m (10,000ft) in order to be denser than the ambient seawater 2. Once the CO₂ puddling is formed, it can remain stable and fixed for long periods of time before ocean currents carry them to other locations where they can begin dissolving or returning to the surface.

Advantages​

Some advantages of deep ocean storage include cost, storage capacity, and timescale 2. Deep ocean storage does not require extensive underground infrastructure as with many other proposed geological storage methods such as saline aquifers or basalt formations. The world's oceans cover over 70% of the Earth's surface and have an average depth of around 3.8 km (2.4 miles), meaning that there is an abundance of potential storage sites. Lastly, although deep ocean storage is not considered a long-term storage solution, with high degrees of leakage into the atmosphere expected (at least 10% in a 50-year time scale), it is still seen as a viable mid-term solution due to the long residence time of CO₂ facilitated by the slow-moving ocean currents 1 2.

Disadvantages​

Deep ocean storage has a number of serious disadvantages that would need to be addressed in order to be a viable solution. The main obvious concern lies with its environmental impact. It is a foregone conclusion with deep ocean storage that it would lead to ocean acidification, which can have serious negative effects on marine life, at the very least on a localized level 1. Another concern with deep ocean storage is where it lies within international legislation. Current agreements such as the London Convention on the Prevention of Marine Pollution state that it is illegal for any to dump industrial waste into the ocean 1 2. Future decisions on whether to classify captured CO₂ as industrial waste will be required before deep ocean storage can be used as a viable solution.

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

  2. Sheps, K. M., Max, M. D., Osegovic, J. P., Tatro, S. R., & Brazel, L. A. (2009). A case for deep-ocean CO2 sequestration. Energy Procedia, 1(1), 4961-4968. https://doi.org/10.1016/j.egypro.2009.02.328 ↩ ↩2 ↩3 ↩4 ↩5