Disturbed zone effects: Two phase flow in regionally water-saturated fractured rock

Field evidence suggests that two-phase flow may develop near underground excavations in regionally-saturated fractured crystalline rock, resulting in lower inflow rates compared to undisturbed rock. Mechanisms for the development of two-phase flow conditions include depressurization of formation water that is supersaturated with dissolved gas and buoyancy-driven air invasion into fractures from the drift. Models that assume gas-liquid phase equilibrium indicate that for constant head boundary conditions, the build-up of pressure behind the gas phase evolving from depressurization should redissolve the gas and maintain higher flowrates, requiring unreasonably high dissolved gas concentrations to produce observed flow reductions at the Stripa Mine in Sweden. This discrepancy initiated a laboratory-scale investigation. Gas evolution following depressurization is simulated in two different 8 cm x 8 cm transparent fracture replicas for linear flow with constant head boundary conditions. Gas forms and accumulates in the large apertures and the extent of flow reduction is greater when the flow through the fracture is controlled by a large aperture channel, compared to a fracture where large aperture regions are relatively isolated. An effective continuum numerical model (TOUGH2) is used to describe the development of two-phase flow under degassing conditions. Numerical simulations were made for a homogeneous porous mediummore » and for a heterogeneous medium using the aperture distribution of one of the fractures used in the laboratory experiments, which allows a direct comparison between laboratory and numerical results. The incorporation of kinetic expressions into the numerical model will allow the prediction of resaturation rates of a repository following closure.« less