Methane Migration in the Vadose Zone Affected By Subsurface Heterogeneities and Land/Atmospheric Interactions.

The world’s energy picture is changing drastically as the United States is undergoing a major transition to unconventional oil and gas extraction, particularly light oil and gas from shale reservoirs.  Methane (CH₄) leakage occurs during the production, processing, storage, transmission, and distribution of natural gas. Extraction of shale gas through hydraulic fracturing represents another leakage source for CH₄. As a greenhouse gas, CH4, is more than 20 times as potent as carbon dioxide (CO₂) over a 100 year period. CH4 emission from natural gas development was estimated to be around 221.2 million metric tons of CO₂ equivalent in 2009. To evaluate the benefits of transitioning from burning oil and coal to natural gas in the context of reduction of greenhouse gas loading, it is necessary to get better estimates of the total CH4 loading from unconventional energy development. This talk presents ongoing research that attempts to fill knowledge gaps on how migration of CH4 in the vadose zone is affected by the subsurface soil variability and soil moisture conditions and land/atmospheric interactions to obtain better estimates of atmospheric loading of CH₄ from subsurface sources. A set of experiments was conducted in a newly developed intermediate test facility that couples a soil tank to a climate controlled low-velocity wind tunnel.  A sample of these results is presented to demonstrate how the heterogeneity of the vadose zone and soil moisture in combination with the conditions in the boundary layer affect CH₄ flux observed at the land surface.  This fundamental knowledge will also be of applicable to other problems in gas migration in the vadose zone that include vapor intrusion and leakage of sequestrated CO₂ from deep geologic formations into the shallow subsurface.