Importance of Multicomponent Mass Transfer on the Dissolution of Trapped Gas: A Focus on Geo-Energy Applications in the Shallow Subsurface

Migration of gases from geo-energy wells has the potential to impact groundwater and surface water and to emit greenhouse gases into the atmosphere. Gas migration is complex, which complicates detection and monitoring in the field. Multicomponent mass transfer is known to impact the dissolution of trapped gas; however, these effects have not yet been quantified in the context of gas migration. In this study, gases of varying compositions were trapped in a sand-packed column, and dissolved gas concentrations were measured during subsequent water flushing. These experiments were then simulated using MIN3P to provide further insights into the dissolution of multicomponent gases and the effects of dissolved background gases, which are abundant in shallow groundwater environments. Multicomponent mass transfer was shown to be a key process in the evolution of dissolved gas concentrations, molecular ratios, and persistence of gas in the subsurface, particularly due to the presence of background dissolved gases. Simulations showed that consideration of background gases improves the fit to the experimental dissolution data. The results highlight that consideration of the multicomponent nature of shallow subsurface systems when evaluating the impacts of geo-energy well leaks would improve future monitoring and modeling efforts.