Gas migration through water-saturated, fractured rock: results of a gas injection test

Abstract

The generation of gases and possible development and migration of a gas phase at depth in groundwater-saturated, fractured rock has become an important aspect of assessing the performance and safety of radioactive waste disposal sites. To study gas-phase migration in this environment, helium gas was injected at constant pressure through an access borehole into an inclined fracture zone at a depth of about 40 m, in the 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 flow rate increased from 5 to 20 l min −1 over the 11-day period of the test indicating removal of water from the flow paths. Breakthrough of injected gas at the surface was detected within 2 days. Two areas of high concentrations of gas discharge were observed within 40 m of the injection borehole, indicating gas transport through near-vertical fractures. A larger area of trace He concentrations was detected 200 m away indicating transport along the fracture zone. The field results were compared with predictions of a simple analytical model derived from Braester and Thunvik (1983). Good agreement was found when the influence of fracturing in the bedrock and a low-permeability overburden and `excess porosity' due to non-uniformity of fracture apertures were included in the model. The model was then used to estimate and map the relative hydraulic conductivities of individual gas flow paths in the fractured rock.