Vadose Zone Gas Migration and Surface Effluxes after a Controlled Natural Gas Release into an Unconfined Shallow Aquifer

Abstract

Core Ideas Subsurface gas migration results in localized surficial CH4 releases. Surficial CH4 emissions show pronounced temporal variations. Methane concentrations in soil gas exceed lower explosive limits at low leakage rates. Increasing CO2 effluxes and stable C isotope signatures indicate vadose zone CH4 oxidation. Instantaneous surficial effluxes do not indicate the magnitude of subsurface gas leakage rates. Shale gas development has led to concerns regarding fugitive CH4 migration in the subsurface and emissions to the atmosphere. However, few studies have characterized CH4 migration mechanisms and fate related to fugitive gas releases from oil or gas wells. This paper presents results from vadose zone gas and surface efflux monitoring during a natural gas release experiment at Canadian Forces Base Borden, Alliston, Ontario, Canada. Over 72 d, 51 m3 of natural gas (>93% CH4) was injected into a shallow, unconfined sand aquifer at depths of 4.5 and 9 m. Methane and CO2 effluxes in combination with soil gas concentrations and stable C isotopic signatures were used to quantify the spatiotemporal migration and fate of injected gas. Preferential gas migration pathways led to vadose zone hot spots, with CH4 concentrations exceeding the lower explosive limit (5% v/v). From these hot spots, episodic surface CH4 effluxes (temporally exceeding 2500 μmol m−2 s−1 [3465 g m−2 d−1]) occurred during active injection. Higher injection rates led to increased average CH4 effluxes and greater lateral migration, as evidenced by a growing emission area approaching 25 m2 for the highest injection rate. Reactive transport modeling showed that high CH4 fluxes resulted in advection-dominated migration and limited CH4 oxidation, whereas lower CH4 effluxes were diffusion dominated with substantial CH4 oxidation. These results and our interpretations allowed us to develop a conceptual model of fugitive CH4 migration from the vadose zone to the ground surface.