Multicomponent Mass Transfer Effects on the Dissolution and Persistence of Stray Gas

ABSTRACT: As the development of deep geological gas reserves increases, concern has been raised regarding potential environmental impacts. Among those concerns are uncontrolled releases of natural gas into the subsurface, referred to as stray gas migration. These incidents can lead to degradation of surface water and groundwater resources, greenhouse gas emissions and can present explosive risk. When elevated methane concentrations are detected at a receptor, investigations are conducted to determine whether the gas is of natural (biogenic) or anthropogenic (thermogenic) origin. Biogenic gas from shallow microbial sources and thermogenic gas from deep sources are distinguished by their isotopic signature and their hydrocarbon composition, typically as a ratio of methane to ethane and propane. To improve stray gas investigation techniques, multicomponent mass transfer effects on the dissolution and persistence of trapped gas in the subsurface were studied. A series of small-scale (5.3 cm length, 7.2 cm diameter) column experiments were conducted, in which single- and multicomponent natural gas mixtures were trapped in sand. Water was pumped through the column, and dissolved gas concentrations were measured over time. The experiments were simulated using the reactive transport code MIN3P. MIN3P was successfully used to simulate the dissolution of both single- and multicomponent trapped gases. Further simulations were conducted to investigate the effects of background dissolved gases on gas persistence, and potential changes in hydrocarbon ratios due to multicomponent mass transfer. Results from laboratory and numerical simulations demonstrated that variation in hydrocarbon ratios over time can occur due to differences in source gas partitioning properties. Furthermore, gas persistence is highly correlated to background gas concentrations, as background gases can sustain a gas phase long after the source gas has dissolved. Therefore, the effects of multicomponent mass transfer, due to both source and background gases, should be considered in gas migration investigations.