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[Ｂ−５６.１　Study on Evaluation of Deep Ocean Storage Method of Recovered Carbon Dioxide]

[Contact person]

[Total Budget for FY1998-2000]

　36,207,000 Yen
　(FY 2000; 12,731,000 Yen)

［Abstract］

　The deep ocean CO₂ storage, in which liquid CO₂ is stored in a dented ocean floor deeper than 3500 m where liquid CO₂ is heavier than ambient seawater, is a promising measure to mitigate the global warming, because its sequestration term is expected longer than 2000 years, the vertical ocean circulation period. In order to get the necessary data for the evaluation of this method, the properties of CO₂ hydrate, dissolution and diffusion process of stored CO₂ and the biological influence of dissolved CO₂ were investigated through the land-based experiments, the numerical simulation and the in situ experiments.
　On the properties of CO₂ hydrates, the hydrate membrane strength was measured by a modified Du-Nouy type surface tension meter, in fresh water, in saline water and in CO₂ rich water. As a result, it was found that the strength increases about 10 times just below the dissociation temperature and in CO₂-saturated water. And it was also found that the membrane deforms a lot under stress in sub-saturated water but it shows almost no deformation in saturated water. These unexpected phenomena are to be considered for the evaluation of CO₂ ocean sequestration. "Free water molecule model" proposed by the Aya et al. can well explain the above phenomena and made clear that the deformation is a kind of rebuilding process of hydrate membrane. Another experiment succeeded in simulating the overflow phenomenon discovered by Dr. Brewer et al. in their in situ experiment at Monterey Bay.
　On the CO₂ dissolution and diffusion process, the numerical simulations for two and three dimensional systems around CO₂ storage site were conducted as a trust research to the Sojo University. The results show that the stratified layer formed above stored liquid CO₂ well suppresses the dissolution rate of CO₂ from the storage site and the pH lowering is only 0.2 above stratified layer, which implies the sequestration term longer than 2000 years can be expected
　As another trust research, the Tokyo University of Fisheries conducted several in situ surveys in Suruga Trough and a deep seafloor depression south east of Haha Island, potential sites of CO₂ storage. The site in the latter location seems to be suitable from the scanty biomass. An investigation on the possibility of the endemic fauna, however, must be done from the point of conserving biodiversity. A preliminary study on the lowering effect of pH on a deep-sea Copepod (Neocalanus cristatus) was also conducted to show an equivalent tolerance range with oceanic zooplankton in shallow waters. Considering long-term effect of low pH and topographical character of the site, it is necessary to have intensive investigation around the site before considering storage experiments.
　From the joint in situ experiment with the Monterey Bay Aquarium Research Institute, it was suggested that sea animals such as hagfish might be insensitive to the dissolved CO₂.

［Key Words］

　Carbon Dioxide, Deep Ocean Storage, Hydrate, Membrane Strength, Dissolution, Diffusion, Ecosystem, Storage Site, pH Dependence