Gas migration in the shallow subsurface: Insights from a natural gas release into a deep vadose zone

Abstract:

The unintentional subsurface release of natural gas, comprised primarily of methane (CH₄), from a compromised oil or gas well can lead to gas migration (GM) causing aquifer contamination, explosive hazards, and greenhouse gas (GHG) emissions to the atmosphere. Northeastern British Columbia (NEBC), Canada, hosts extensive petroleum resource development where GM poses a continuing problem for industry and regulators. Understanding reactive transport pathways, physical migration mechanisms and attenuation potential for natural gas in the shallow subsurface remains a challenge. To address this shortcoming, we simulated gas leakage from a gas well through a controlled natural gas injection in NEBC. For five days a total of 30 m³ of natural gas (93.8% CH₄) was injected at 12 m depth into surficial unsaturated Quaternary deposits typical of the region. Gas migration was monitored using flux chambers and multi-level soil gas sampling systems combined with analyses of gas composition and stable carbon isotope ratios of CH₄ and CO₂.

Methane effluxes to the atmosphere were not uniformly distributed; a discrete surface hot-spot developed with emissions 10 times greater than at other locations. Oscillations in barometric pressure caused soil gas to be displaced upward and downward, accelerating and inhibiting CH₄ effluxes. Stable carbon isotope analyses indicate that CH₄ oxidation began within two days of injecting gas and continued throughout the monitoring period, attenuating CH₄ emissions to the atmosphere. However, periods of low barometric pressure drew significant fractions of the injected gas rapidly out of the subsurface, limiting CH₄ oxidation. Our results suggest that in a deep vadose zone, CH₄ can be attenuated through oxidation, but barometric pressure changes can decrease retention of gas in the subsurface and directly control the magnitude of CH₄ effluxes and GHG emissions to the atmosphere.