Numerical modeling of groundwater and air flow between compacted bentonite and fractured crystalline rock

Sammanfattning: The geological repository for final storage of spent nuclear fuel, envisioned by the Swedish Nuclear Fuel and Management Company (SKB), relies on several barriers: copper canisters deposited in holes in the floor of underground tunnels in deep bedrock, embedded in a buffer of compacted bentonite. The initially unsaturated buffer would take up water from the surrounding rock mass and swell to seal any potential gap. This initial two-phase (gas and liquid) regime with two components (air and water) may impact the final density, swelling pressure and biogeochemical conditions in the buffer. A main objective of this work is to identify factors and mechanisms that govern deposition hole inflow and bentonite wetting under the prevailing two-phase flow conditions in sparsely fractured bedrock. For this purpose, we use the numerical code TOUGH2 to perform two-phase flow simulations, conditioned by a companion field experiment (the Bentonite Rock Interaction Experiment or BRIE) performed in a 417 m deep tunnel of the Äspö Hard Rock Laboratory in southeastern Sweden. The models predict a significant de-saturation of the rock wall, which was confirmed by field data. To predict the early buffer wetting rates and patterns, the position of local flowing fractures and estimates of local rock matrix permeability appear more important than the total open hole groundwater inflow. A global sensitivity analysis showed that the buffer wetting time and the persistence of unsaturated conditions over extended periods of time in the rock depend primarily on the local fracture positions, rock matrix permeability, ventilation conditions in the tunnel and pressure far in the rock. Dismantling photographs from BRIE were used to reconstruct a fine-scale snapshot of saturation at the bentonite/rock interface, showing tremendous spatial variability. The high level of heterogeneity in the rock generates complex two-phase flow phenomena (air trapping, dissolution), which need to be accounted for in buffer design and rock suitability criteria. In particular, results suggest that uncertainties regarding two-phase flow behavior are relatively high close to residual air saturation, which may also have important implications for other applications involving two-phase flows, such as geological storage of carbon dioxide.