GAS MIGRATION THROUGH WATER-SATURATED, FRACTURED ROCK - RESULTS OF A GAS INJECTION TEST

The generation of gases and possible development and migration of a ga s phase at depth in groundwater-saturated, fractured rock has become a n important aspect of assessing the performance and safety of radioact ive waste disposal sires. To study gas-phase migration in this environ ment, helium gas was injected at constant pressure through an access b orehole into an inclined fracture zone at a depth of about 40 m, in th e granitic Lac du Bonnet Batholith, southeastern Manitoba, Canada. The gas flow rate, arrival time and pattern of distribution of gas at the surface were monitored by soil gas surveys. Gas Bow rate increased fr om 5 to 20 l min(-1) over the 11-day period of the test indicating rem oval of water from the Bow paths. Breakthrough of injected gas at the surface was detected within 2 days. Two areas of high concentrations o f gas discharge were observed within 40 m of the injection borehole, i ndicating gas transport through near-vertical fractures. A larger area of trace He concentrations was detected 200 m away indicating transpo rt along the fracture zone. The field results were compared with predi ctions of a simple analytical model derived from Braester and Thunvik (1983). Good agreement was found when the influence of fracturing in t he bedrock and a low-permeability overburden and 'excess porosity' due to non-uniformity of fracture apertures were included in the model. T he model was then used to estimate and map the relative hydraulic cond uctivities of individual gas flow paths in the fractured rock. (C) 199 7 Elsevier Science B.V.