日本地球惑星科学連合2021年大会

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セッション記号 H (地球人間圏科学) » H-SC 社会地球科学・社会都市システム

[H-SC05] 地球温暖化防⽌と地学(CO2地中貯留・有効利⽤、地球⼯学)

2021年6月4日(金) 13:45 〜 15:15 Ch.15 (Zoom会場15)

コンビーナ:徂徠 正夫(国立研究開発法人産業技術総合研究所地圏資源環境研究部門)、薛 自求(公益財団法人 地球環境産業技術研究機構)、愛知 正温(東京大学大学院新領域創成科学研究科)、今野 義浩(東京大学)、座長:今野 義浩(東京大学)

14:00 〜 14:15

[HSC05-14] Numerical estimation of leakage suppression using CO2 hydrate on the oceanic field scale

若生 拓也1、澤野 陽介1、*佐藤 徹1 (1.東京大学)

キーワード:海底下CO2貯留、地層、CO2ハイドレート、数値シミュレーション、貯留層スケール

CO2 capture and storage is a promising strategy against global warming. Although there is concern about the risk of CO2 leakage from deep saline aquifers, it is expected that leaking CO2 forms gas hydrate and this hydrate formation suppresses the CO2 leakage. When the water depth of a storage site is large: say, about 400 m or more, leaked CO2 changes its form to gas hydrate, which may block CO2 rise in the sandy seabed sediment. Therefore, hydrate formation in the sub-seabed sand sediments is one of key factors in lowering the risk of CO2 leakage and it is important to know conditions under which CO2 leakage is suppressed by hydrate formation.

On the other hand, using this leakage trap mechanism, it is also possible to store CO2 in the form of gas hydrate without a cap rock in the sub-seabed geological formation. To estimate the sealing effect of CO2 hydrate against CO2 leakage beneath the seafloor, it is necessary to evaluate the permeability of the stratum after CO2 hydrate forms.

In this study, numerical simulations of hydrate formation in sand sediment are conducted for CO2 injected in the sub-seabed geological formation. When modelling CO2 hydrate formation, it is assumed that the hydrate forms on the CO2-water interface and on the surface of the sand particles after CO2 plume front passes by during its rise. Permeability reduction during hydrate formation was modelled using the results of microscale hydrate simulations with the phase field model and lattice Boltzmann method in the literature.

First, the processes of CO2-water two-phase flow and CO2 hydrate formation in the sand sediment under the experimental conditions are analysed by the present numerical model. Simulation results are compared with the experimental data, so that unknown parameters in the models are determined by parameter-fitting.

Simulation results suggest that the total hydrate formation rate is higher near the boundary of the reaction vessel, and CO2 hydrate is mainly distributed near the boundary of the reaction vessel due to the cooling effect of the temperature-controlled boundary. Furthermore, it is also indicated that CO2 hydrate formation on the hydrate film behind the gas front makes the most important contribution to the high CO2 hydrate saturation near the boundary; on the contrary, CO2 hydrate formation on the gas front and on the surface of the sand particles behind the gas front only have has a very small effect on CO2 hydrate saturation in the sand sediment. Therefore, on the basis of the analysis above, we can draw the conclusion that the sharp permeability reduction of the sand sediment during CO2 injection is mainly caused by the part of CO2 hydrate formation on the hydrate film behind the CO2 plume front, which is likely to exist between the sand particles and occupy the pore space of the sand sediment, resulting in blockage of the gas flow.

Then, this numerical method was applied to 3D realistic geological formation to simulate whether hydrate seal is formed to block the leakage of CO2 and to store CO2 safely in the form of gas hydrate in the reservoir scale. The results from numerical simulations showed that the formation of a CO2 hydrate layer could block the upwards migrating CO2 under most conditions of injection depth, seafloor depth, seafloor temperature, and injection rate, while in some cases, CO2 leaks before hydrate forms to occupy the pore space enough to block the leakage. Therefore, it is important to choose storage sea sites: particularly, absolute permeability.

Acknowledgement
This work has been conducted under “Sustainable CCS project” by the Ministry of the Environment, Government of Japan.