Summary

Laboratory investigations using multi-scale experimental testing is under way to improve our fundamental understanding of how natural gas concentrations detected at the land surface are affected by the subsurface and atmospheric conditions. This paper presents preliminary results from a set of experiments conducted in a small test sand tank interfaced with a wind tunnel. Both homogeneous and heterogeneous sand-packing configurations were used. While atmospheric conditions were held constant and the water content in the sand was maintained at residual saturation, a diluted mixture of methane and nitrogen was injected at the bottom of the sand pack to simulate a natural gas leak. Three different gas leakage rates were simulated. For each leakage case, methane gas concentration was measured at different heights above the land-atmospheric interface. The data from these preliminary experiments suggest that subsurface heterogeneity significantly impacts gas transport from the subsurface. First, the detected natural gas signal in the atmosphere at elevations less than 10 cm above the sand surface was much higher for the homogeneous cases. However, as the elevation increased by only a few cm, the difference in measured concentration is much smaller between the homogeneous and heterogeneous cases studied which is important for designing detection methods. Also, the estimated leakage rate from the homogeneous configuration was almost twice that estimated from the heterogeneous conditions. Finally, as the injection rate increased the percentage that was estimated to be leaking into the atmosphere markedly decreased. These results have implications for accurately estimating total leakage flux if the estimate relies solely on measurement of the increases in concentration above atmospheric background concentration near a leakage source.

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