Preliminary simulation of degassing of natural gases dissolved in groundwater during shaft excavation in Horonobe underground research project

Yamamoto, Hajime*; Shimo, Michito*; Kunimaru, Takanori; Kurikami, Hiroshi  

In Neogene-Quaternary sedimentary basins, natural gases such as methane are often dissolved in groundwater significantly. In such cases, the excavation of deep underground yields the degassing of natural gases due to decline of groundwater pressure. In this paper, two-phase flow simulations of multi-component gases (methane, and carbon dioxide) and groundwater, incorporating the degassing, were performed for the shaft excavation in Horonobe underground research project. Prior to the simulations, unknown parameters of a relative permeability curve were tested using gas flow data during a borehole pumping test. The results drawn from the simulations are summarized as follows. (1) As depth increases, degassing and gas inflow occurs significantly. (2) Degassing increases the compressibility of pore fluids, resulting in slow changes in groundwater pressures around the shaft. (3) Although the occurrence of gas phase decreases water mobility, the influence of the dissolved gas on the groundwater inflow rate to the shaft was small.