Effect of Gas Ebullition on the Vertical Transport of Dissolved Solutes in the Subsurface

ABSTRACT: The production of oil and gas, including from unconventional resources, is crucial to satisfy the global fuel demand, but environmental and human health risks must be addressed. One potential hazard is leaking of natural gas wells, which may lead to fugitive hydrocarbon gases contaminating shallow aquifers and/or being released to the atmosphere. Previous field studies have found that the injection and upward flow of natural gas into a shallow aquifer coincided with an increase in dissolved ion concentrations in groundwater samples collected above the injection point. The goal of this study was to investigate the potential for deeper groundwater to be transported alongside upwards-migrating gas, leading to a degradation in groundwater quality in shallower systems. A series of laboratory experiments were performed to better understand the transport of dissolved solutes by gas flow through porous media. Nitrogen gas was injected into a water-saturated, sand-packed two-dimensional (30 x 30 x 1 cm3) flow cell that contained a plume of dyed water near the bottom of the sand pack. Movement of the plume was determined using image analysis, facilitated by a back light and a visual light transmission technique. The results indicated an optimal discontinuous (bubble) flow that produces maximum vertical transport of dissolved solutes. In the absence of horizontal water flow, gas flow at transitional rates (10 mL/min) and low discontinuous rates (0.1 mL/min) produced minimal rates (0.6 mm/h -1.1 mm/h) of vertical solute transport, while a high discontinuous rate (1 mL/min) produced higher rates of vertical contaminant transport (2.8 mm/h). Advective transport dominated in all experiments with high water flow (5.2 m/day), with minimal vertical plume movement over the length of the flow cell. These results demonstrate that both direct and indirect effects of leaking natural gas wells should be considered when assessing risks to shallow aquifers.