Gas migration phenomena in potential components of near-field barrier system. 2

Gas production resulting from some mechanisms, including metal corrosion and microbial degradation in a geological repository, enables to lead overpressurization of pore pressure and enhanced release of pore water with dissolved radionuclides. In this study, gas injection experiments under three-dimensional confinement pressure, a gas-water capillary pressure test using high speed centrifuge method and high pressure mercury injection test were performed on tuff core samples to clarify flow parameters of gas and water. Results of the gas injection tests under the condition of constant injection pressure and axial flow were described. The water permeability of saturated samples were existing between 8.4 x 10-19 and 8.9 x 10-19 m2, although the gas permeability of dried sample were varied between 2.3 x 10-16 and 2.5 x 10-15 m2 depending on gas injection pressure. Gas breakthrough phenomenon after rapid rise in outflow rate of water was indicated rather slow increase of gas outflow than that observed on clay materials. Obtained excess pressure when the breakthrough phenomena occurred were 700 and 1,700 kPa. Grater the gas injection pressure, higher the excess pressure at breakthrough. Threshold pressure obtained from the capillary pressure test was order to 101 kPa which was two order of magnitude lower than the value of clay materials. Based on these experimental results, the gas migration process and the retention curve of the tuff sample indicated typical properties observed on porous media. Thus it is expected that the conventional continuum two-phase flow model will be applied reasonably for the tuff sample. (author)