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The Effect of dissolved gas on rock desaturation in artificial openings in geological formations

地下施設の建設に伴う岩盤中の不飽和領域の形成に対する溶存ガスの役割

宮川 和也  ; 青柳 和平 ; 笹本 広; 赤木 俊文*; 山本 肇*

Miyakawa, Kazuya; Aoyagi, Kazuhei; Sasamoto, Hiroshi; Akaki, Toshifumi*; Yamamoto, Hajime*

高レベル放射性廃棄物の地層処分場の掘削により、坑道周辺岩盤の損傷や、溶存ガスの発生等による不飽和領域の形成といった掘削影響領域が生じる。そこでは、岩盤の透水性の増大や、地下水の水質変化といった、地層が有する放射性核種の移行を遅延させる機能に影響を与えることが考えられる。そのため、大規模地下施設の建設・操業・閉鎖に伴う地質環境(水理地質構造や地下水の流動特性・物質移動特性・地球化学特性等)の変化過程や定常状態に達するまでの時間やプロセスを、確度の高い情報に基づきモデル化するための一連の技術開発が必要である。本研究では、溶存ガスが岩盤中の不飽和領域の形成に与える影響を調べるために、幌延深地層研究所の深度140m, 250m, 350m調査坑道の条件を模擬した数値・感度解析を実施した。その結果、溶存ガス濃度は、不飽和領域の形成に対して、飽和度と領域の広がりの両方に影響する一方で、岩盤の透水性は、主に不飽和領域の広がりのみに影響することが分かった。

The construction and operation of geological repositories require excavation and ventilation of galleries, with significant groundwater drainage. Desaturation of rock around galleries is unavoidable and may affect hydraulic properties and redox conditions. This study used numerical modeling to assess the influence of dissolved gas on the degree of saturation of rock surrounding excavated galleries, focusing on siliceous mudstone rock in the 140 m, 250 m, and 350-m-deep galleries of the Horonobe Underground Research Laboratory, Japan. Based on previous $$in situ$$ electrical survey, the degree of saturation in the 250 m gallery was higher than that in the 140 m and 350 m galleries. In the Horonobe area, deep groundwater contains high concentrations of dissolved methane, and exsolution of this methane from pore water can affect desaturation. Simple numerical modeling, including simulation of multiphase flows, was undertaken for each gallery to confirm the effect of dissolved gas and rock permeability on desaturation. A sensitivity analysis was performed by varying dissolved gas contents and permeability. Results indicate that the dissolved gas content affects both the degree of saturation and its spatial extent, whereas rock permeability affects only the latter. Higher dissolved gas concentrations result in lower degrees of saturation with a greater spatial extent of desaturation, and higher permeability leads to greater extents of desaturation. It is therefore likely that gas content, rather than rock permeability, caused the observed variations in the saturation degree.

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