Stray Gas Migration in Shallow Groundwater: Gas Dynamics, Mass Transfer, and Environmental Expression

Abstract

The objective of this research was to systematically investigate the dynamics of flowing subsurface gases, leading to understanding of mass loading of stray gas in unconsolidated sands, then couple this knowledge to multicomponent mass transfer in homogeneous and heterogeneous systems. To do this, high spatial and temporal resolution visual techniques along with analytical methods were used to study multiphase flow, mass transfer, and surface expression of subsurface gases. Air injections in a small (25 × 25 × 1 cm3) and a large (150 × 150 × 2 cm3) two-dimensional flow cell were performed to quantify gas movement and understand the source architecture resulting from varying leak conditions. This knowledge of free-phase gas migration was then used to better understand mass transfer resulting from flowing gases. A novel visual technique to quantify gas-to-water mass transfer in the flow cells was developed. This technique was then used to quantify mass transfer from flowing carbon dioxide gas under varying aqueous flow velocities and gas injection rates. Finally, experiments were performed in which methane was injected into homogeneous and heterogeneous sand packs in the large flow cell and both aqueous and surface expression were measured. The results of the experiments performed have allowed for the refinement of the conceptual model for stray gas migration under homogeneous and heterogeneous conditions. Empirical insight on the transport and fate of stray gas has been developed, enhancing our understanding of free-phase gas migration, source architecture of free-phase gas, the complex coupling between gas flow and mass transfer, and the impact of gas dynamics on the surface expression of stray gas. The combined results of this research are intended to allow for the development of sound monitoring techniques, validate numerical modelling efforts, and educate future mitigation solutions for stray gas migration. This work has provided a unique suite of data that can be applied across a variety of applications including gas migration, carbon sequestration, groundwater remediation, and waste disposal.