Multiphase flow and multicomponent reactive transport model of the ventilation experiment in Opalinus clay

During the construction and operational phases of a high-level radioactive waste (HLW) repository constructed in a clay rock, ventilation of underground drifts will cause desaturation and oxidation of the rock. The ventilation experiment (VE) was performed in a 1.3 m diameter unlined horizontal microtunnel on Opalinus clay at Mont Terri underground research laboratory in Switzerland to evaluate these phenomena and their impact on rock properties. A multiphase flow and reactive transport model of VE is presented here. The model accounts for liquid and gas flow, evaporation/condensation and multicomponent reactive solute transport. Model results reproduce measured vapor flow, liquid pressure and chemical data and capture the trends of measured relative humidities, although such data are slightly overestimated near the rock interface due to uncertainties in the turbulence factor. Chloride exhibits anion exclusion and accesses only 54% of total porosity. Rock desaturation allows oxygen to diffuse into the rock and triggers pyrite oxidation, dissolution of calcite and siderite, precipitation of ferrihydrite, dolomite and gypsum and cation exchange. pH in the unsaturated rock varies from 7.8 to 8 and is buffered by calcite. Computed changes in the porosity of Opalinus clay in the unsaturated zone caused by oxidation and mineral dissolution/precipitation are smaller than 5%. Therefore, rock properties are not expected to be affected significantly by ventilation of underground drifts during construction and operational phases of a HLW repository in clay.