[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）

[HSC05-P11] Effect of Hydrate Saturation on Methane Recovery Efficiency from Methane Hydrate by N₂−CO₂ Gas Mixture Injection Method

*安江 正宏、増田 昌敬¹、Yunfeng Liang¹ (1.東京大学工学系研究科システム創成学専攻)

キーワード：メタンハイドレート、CO2−CH4 置換、多孔質媒体、シミュレーション、N2-CO2 ガス圧入、生産

Methane hydrate (MH) is the best-known unconventional energy resource and methane (CH₄) gas recovered from hydrate is the low carbon energy. MH has begun to open its promising future especially for Japan. In 2017, the second offshore gas hydrate field test was conducted in the Eastern Nankai Trough, Japan, where a depressurization technique was adopted for producing methane gas [1]. The depressurization method is thought to be the most effective for recovering gas from methane hydrate-bearing sediments, but it does not work well for low-temperature sediments like the permafrost area because this method uses the sensible heat of sediments as energy for hydrate dissociation. Alongside the depressurization method, the replacement of CH₄ from gas hydrates by sole carbon dioxide (CO₂) gas injection method was suggested and studied to increase performance for both methane gas recovery and carbon dioxide sequestration. Since CO₂ hydrate is more stable than methane hydrate, CO₂ can replace CH₄ molecules in hydrate when it is exposed to methane hydrate in a porous medium. However, this method has a serious problem that CO₂ hydrate formation easily blocks the pore of sediment to prevent continuous injection. One possible process to overcome this weakness is the injection of nitrogen-carbon dioxide (N₂-CO₂) gas mixture presented herein. The amount of abundant methane hydrates is estimated to exist in the permafrost area; therefore, this method, which does not require any heat source, can be used as a standard method for recovering methane and sequestrating CO₂ in the ground in future.
In this study, experiments to continuously inject N₂-CO₂ gas mixture (about 59 mol% CO₂) into hydrate-bearing cores with different methane hydrate saturations were conducted [2]. The hydrate saturation varies from 41 to 67 percent on 16 cores. At these experiments, the CH₄ recovery factor (the ratio of the number of moles of CH₄ produced to the initial molar quantity of CH₄ in the hydrate and the free-gas in the core) was 23 to 57 percent. The exchange ratio (the ratio of the number of moles of CH₄ produced from the hydrate to the initial number of moles of CH₄ contained in the hydrate in the core) was 0 to 29 percent. The result showed the exchange ratio is inversely proportional to hydrate saturation. With the lesser saturation the more CH₄ molecules in the hydrate phase can be replaced and recovered.
Simultaneously, the numerical simulation model was constructed and in order to express the gas exchange phenomenon of CH₄-(N₂+CO₂), the phase equilibrium analyses were utilized between gas mixture and hydrates. So far agreements were obtained between experiments and simulations. The validation of the simulation with experimental results will strongly support our study to find the efficient injection gas method for methane recovery.
[1] Yamamoto, K.; Wang, X. -X.; Tamaki, M.; Suzuki, K. The second offshore production of methane hydrate in the Nankai Trough and gas production behavior from a heterogeneous methane hydrate reservoir. RCS Adv. 2019, 9, 25987-26013
[2] Yasue, M.; Masuda, Y; Liang, Y. Estimation of methane recovery efficiency from methane hydrate by the N₂-CO₂ gas mixture injection method. Energy Fuels 2020, 34, 5236-5250.