Potential for Very Deep Ocean Storage of CO2 Without Ocean Acidification: A Discussion Paper

Abstract

Carbon Capture and Storage (CCS) is an essential contributor to the mitigation of climate change. CCS will require vast CO2 storage capacity. At present only geological storage is being considered. This paper revisits an alternative CO2 storage possibility in enclosed basins on the deep and very deep ocean floor.For example, the Indonesian Sunda trench, the Japanese Ryukyu trench and the Puerto Rico trench are more than 6km deep. If liquid CO2 were to be placed in such a trench, it would be 7% more dense than seawater and could remain permanently as a lake of liquid CO2 on the ocean floor, possibly becoming a solid hydrate over time which could inhibit mixing between the stored CO2 and ocean currents.At depths greater than about 4 to 5km metres, seawater is under-saturated in calcium carbonate, so ocean ecosystems are significantly different. Any impact on deep marine fauna would need to be investigated.The London Dumping Convention has provisions for disposal of material into the ocean provided the absence of adverse effects can be proven.Deep ocean CO2 entrapment is more certain than geological CO2 storage in deep aquifers. A CO2 delivery concept by ship and vertical pipe is suggested for exploratory trials, with subsea pipelines for permanent installations, which might be much cheaper than geological CO2 storage.There is vast capacity for storage of CO2 in the world's very deep ocean trenches. The Sunda trench below 6km has the capacity to accommodate 19,000 gigatonnes of liquid CO2, which is greater than the CO2 yield from all currently known global fossil fuel reserves. The Puerto Rico trench has capacity for 24,000 Gt of liquid CO2 deeper than 7km. Enclosed basins of limited area could easily accommodate captured CO2.China has the largest potential demand for CO2 storage from power generation and industrial sources, which could be 3 Gt per year by 2050. The Ryukyu trench, which is 700km from the Chinese coast and is in Japanese water, has two sections deeper than 7km. Those sections of the Ryukyu trench would have the capacity to accommodate all the CO2 captured in China at 3 Gt per year for over 200 years.In the event that very deep ocean storage of CO2 is found to be practicable and acceptable, the minimum practical depth would need to be determined as a criterions for acceptable additional storage locations. For consideration, there is an enclosed basin on the floor of the Mediterranean Sea 60km off Southern Greece, with capacity for 84 Gt of CO2 deeper than 4.5km. Also, there is an enclosed basin in the Arabian Sea, 320km south west of Karachi, with capacity for 86 Gt of CO2 deeper than 3.5km. The potential storage of CO2 in such locations would be temperature dependent.The global CCS community has previously considered ocean storage of CO2 on the basis of ultimate dissolution and dispersion of CO2 in ocean water. Those studies have dismissed ocean storage as environmentally unacceptable due to ocean acidification.This paper postulates that very deep ocean trenches (>6km) and deep ocean floor depressions (>4km) are environments for CO2 storage, where permanent storage without dissolution, acidification or adverse effects on fauna may be possible.The purpose of this paper is to pose the question “Why not?” to the CCS community and to suggest that active research is timely.