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. A previous in situ electrical resistivity survey indicated that 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 electrical resistivity.
Geological repository construction inevitably involves drainage of large volumes of groundwater from cracks and pores in surrounding rock, causing rock desaturation of the rock around galleries. Such desaturation is known to occur in the Horonobe Underground Research Laboratory (HURL), Japan, which is the subject of this study. Desaturation affects the hydraulic properties of rocks, increasing gas mobility relative to groundwater and leading to air infusion into rocks. Such a zone with moderate hydromechanical modification is termed an ‘excavation disturbed zone’ (EdZ), while a zone with significant modification of flow and transport properties is an ‘excavation damaged zone’ (EDZ) (Tsang et al., 2005). In a claystone formation of very low permeability, desaturation is confined more or less to the EDZ (Matray et al., 2007), indicating that permeability is a sensitive controlling factor for desaturation. A low permeability also allows atmospheric O2 to infuse into the EDZ because fluid flow from intact rock is limited.