Effect of excavation damaged zone on gas migration in a KBS-3H type repository at Olkiluoto

The quantities of steel in the KBS-3H disposal concept with long horizontal deposition holes are moderate but the gas production rates are significant. Based on current understanding of the geological situation at the Olkiluoto site in Finland, a simplified model of gas pressure evolution and migration of corrosion gases in the disposal system was developed and the effect of EDZ on the gas transport was evaluated. The results of the analysis suggest that gas is predominantly transported by two-phase flow through the EDZ and the fracture network in the host rock. For the major part of the deposition hole sections, there is sufficient transport capacity for gas to escape through the host rock and EDZ and the calculated gas pressure increases are found to be moderate. In tight deposition hole sections, transport through the EDZ dominates and the gas pressure development is sensitive to EDZ permeability. If the EDZ permeability in tight sections is of the order of 10% of the measured intrinsic permeability, the pressure there may rise to near the minimum principal rock stress. However, if the permeability is of the order of the measured intrinsic EDZ permeability, the pressure will rise only slightly above hydrostatic pressure. The results of the analysis show that the EDZ is capable of transporting the gas gradually out from tight hole sections without significantly disturbing the isolation characteristics of the repository system.