Two-phase flow in regionally saturated fractured rock near excavations

Hydrologic characterization for potential nuclear waste repositories relies upon data obtained from testing in excavations. The Simulated Drift Experiment in the Stripa Mine in Sweden, a fractured granitic formation below the water table, investigated excavation effects on hydrologic response. Measured water inflow to the drift at atmospheric pressure was nine times less than the value predicted from the inflow to boreholes with pressure held at 2.7 bars. This flow reduction may be due to dissolved gas that comes out of solution at pressures below 2.7 bars, creating a two-phase regime. To investigate this possibility, theoretical studies of flow through fractures when the water is super-saturated with respect to dissolved gas are carried out, using a simple analytical solution followed by a numerical model which relaxes some of the simplifying assumptions. Laboratory experiments that simulate degassing in transparent fracture replicas are conducted to test the assumptions used in the theoretical studies