Injecting carbon dioxide deep beneath the sea
WOR 8 The Ocean – A Climate Champion? How to Boost Marine Carbon Dioxide Uptake | 2024

Injecting carbon dioxide deep beneath the sea

Injecting carbon dioxide deep beneath the sea
> When carbon dioxide is captured during industrial activities or is removed directly from the atmosphere, the question of appropriate storage arises. Because underground land-based storage sites harbour risks and provoke protests from local communities, the search for storage options in rocks deep beneath the sea is intensifying. The technology for this already exists and has been employed for decades in various pilot projects.
Gas storage in sandstone layers and basalt rocks fig. 8.14: Heidi Wideroe/Bloomberg via Getty Images

Gas storage in sandstone layers and basalt rocks

> Rock layers at depths of 1000 to 4000 metres underneath the seabed are potential storage sites for carbon dioxide. These can be depleted oil or natural gas fields, or rocks whose pore spaces are presently filled with saltwater. Two approaches are currently being investigated. In one, carbon dioxide is injected into deep-lying sandstone formations, a method already used in the North Sea. In the second approach, the gas is injected into the reactive and porous upper basalt layer of the ocean crust in the hope that it will mineralize there relatively quickly.

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Carbon dioxide storage beneath the sea – a controversial practice on the horizon

Carbon dioxide can be captured either directly from the air or from exhaust streams. Both approaches are now playing an increasingly important role in the development of climate policy. Their application is hoped to offset residual emissions from industry and agriculture that are difficult to avoid, or to prevent their release in the first place. Moreover, carbon dioxide removal methods like widely-discussed combined electricity and heat production in biomass-fired cogeneration plants can similarly only contribute to offsetting emissions if the carbon dioxide produced during combustion is captured and then further processed into durable products, such as carbon fibres, or is safely stored. Carbon capture and storage (CCS) technology is therefore of vital importance in achieving the goal of greenhouse-gas neutrality by the year 2050.
The number of capture facilities operating worldwide is steadily increasing, but it is uncertain where the carbon dioxide that is removed can be permanently stored. Experts agree that most of the gas cannot be further processed over the longterm, but instead must be stored, preferably underground in rock layers that are sealed by an impermeable cap rock to prevent the carbon dioxide from escaping upwards. On land, there is strong resistance to such storage plans in many locations because the injection of carbon dioxide could increase the risk of earthquakes and of groundwater pollution.
Experts are therefore now directing their search for suitable storage rocks more toward the marine subsurface. The two most promising candidates here are sandstone formations and the porous upper basalt layer of the ocean crust. The technology for carbon dioxide storage in sandstone formations has been implemented successfully since 1996, mostly in Norwegian waters. To date, carbon dioxide has only been injected into the upper ocean crust in Iceland, because the basalt rocks there rise above sea level and are thus easily accessible. In contrast, there is still much that is not known about the storage potential of basalt rocks in the deep ocean subsurface. This is now being studied in various research projects.
One fundamental difference, however, is already known: Carbon dioxide injected into sandstone may linger for many thousands of years in the pore waters of the rock before it mineralizes and is safely bound in solid form. In the more reactive basalt rocks, on the other hand, the processes that facilitate mineralization operate much more rapidly.
Carbon dioxide injection beneath the sea is not without its risks. Reservoirs must be thoroughly investigated, carefully selected, and ultimately monitored for an extended time and in an environmentally responsible manner (noise). Furthermore, under some circumstances the injection of carbon dioxide may also conflict with other kinds of marine utilization in the area. Legally, carbon dioxide storage under the sea is regulated, for the most part, by new guidelines in the Protocol to the London Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (London Protocol). For example, it establishes what may be injected and how the monitoring of the storage site should be ensured. The final decision on approval of proposed projects, however, rests with the national authorities who are responsible for implementing the London Protocol at the national level.