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

• Main Authors: O.N. Forde, K.U. Mayer, A.G. Cahill, B. Mayer, J.A. Cherry, B.L. Parker
• Format: Article
• Language: English
• Published: Wiley 2018-09-01
• Series: Vadose Zone Journal
• Online Access: https://dl.sciencesocieties.org/publications/vzj/articles/17/1/180033

Shale gas development has led to concerns regarding fugitive CH migration in the subsurface and emissions to the atmosphere. However, few studies have characterized CH 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 m of natural gas (>93% CH) was injected into a shallow, unconfined sand aquifer at depths of 4.5 and 9 m. Methane and CO 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 CH concentrations exceeding the lower explosive limit (5% v/v). From these hot spots, episodic surface CH effluxes (temporally exceeding 2500 μmol m s [3465 g m d]) occurred during active injection. Higher injection rates led to increased average CH effluxes and greater lateral migration, as evidenced by a growing emission area approaching 25 m for the highest injection rate. Reactive transport modeling showed that high CH fluxes resulted in advection-dominated migration and limited CH oxidation, whereas lower CH effluxes were diffusion dominated with substantial CH oxidation. These results and our interpretations allowed us to develop a conceptual model of fugitive CH migration from the vadose zone to the ground surface.