Intermediate-Scale Laboratory Investigation of Stray Gas Migration Impacts: Transient Gas Flow and Surface Expression

Petroleum resource development is a significant contributor of greenhouse gas emissions to the atmosphere. A potential source of emissions may result from stray gas migration. However, its contribution to overall emissions and potential groundwater contamination is unknown, and quantification of flow and dissolution of stray gas is required. The environmental expression of stray gas was investigated using an intermediate-scale (150 × 150 × 2 cm³), two-dimensional flow cell packed in both homogeneous and heterogeneous sand configurations allowing for visualization and measurement of gas movement, collection of aqueous samples, and real-time measurement of gas fluxes escaping the surface of the sand. Results show that gas is transported to the surface of the system via varying dominant discontinuous conduits for flow dictated by geology, leading to surface expression that can be greater or less than the leakage rate of gas. This suggests that surface expression is not directly indicative of the expanse and magnitude of stray gas migration leaks. It was found that 35-39% of the methane was released to the aqueous phase and 61-65% to the atmosphere. The results underscore that subsurface characteristics and gas flow are the key drivers for the overall expression of stray gas in unconsolidated sand aquifers.