Stray Gas Migration: Multicomponent Mass Transfer Effects on Dissolution and Persistence of Gas in the Subsurface

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Ashmore, Nicholas
Stray gas , Gas migration , Multicomponent , Mass Transfer , Gas Persistence , Gas fate , Hydrocarbon leak , Fracking , Shale gas , Trapped gas , Natural gas , Gas ratio , Dissolved gas ratio , Multicomponent effects , Hydrogeology , Gas dynamics
As the development of deep geological gas reserves increases in the coming years, concern has been raised regarding potential environmental impacts including stray gas migration. The goal of this research was to improve investigation techniques at suspected stray gas migration sites by studying multicomponent mass transfer effects on the dissolution and persistence of trapped gas in the subsurface, and potential changes to one of the principal source identification tools, hydrocarbon ratios, over time. This goal was achieved by conducting small-scale (5.3 cm length, 7.2 cm diameter) column experiments, in which single- and multicomponent natural gas mixtures were trapped in sand. Water was pumped through the column, and dissolved gas concentrations were monitored over time. The experiments were simulated using a reactive transport code, MIN3P. Further simulations were conducted to investigate the effects of background dissolved gases on the persistence of gas in the subsurface, and potential changes in hydrocarbon ratios due to multicomponent mass transfer. Additionally, a set of calculations were conducted to determine potential errors for hydrocarbon (and other dissolved gas) ratios due to the misinterpretation of analytical results, and how the use of hydrocarbon ratios may be impacted in practice. MIN3P was successfully used to simulate the dissolution of both single- and multicomponent source trapped gases in porous media. Laboratory and numerical simulations showed that the aqueous concentrations during gas dissolution are generally insensitive to dissolved background gas concentration, but that gas persistence is highly sensitive to background gases. Multicomponent mass transfer can lead to variation in hydrocarbon ratios over time due to differences in gas partitioning properties, and as such they should be coupled with other identification methods. Furthermore, hydrocarbon ratios should be calculated using an equivalent gas pressure when interpreting dissolved gas concentrations, as other interpretations can lead to significant error (a factor of 1.15 to 2.67 under the conditions considered in this study). The findings of this research demonstrate that it is important to consider multicomponent effects in stray gas migration, as the presence of multiple components in both the source and the background has substantial impact on the dissolution of gas in the subsurface.
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