Monitoring the evolution and migration of a methane gas plume in an unconfined sandy aquifer using time-lapse GPR and ERT

Abstract

Fugitive methane (CH) leakage associated with conventional and unconventional petroleum development (e.g., shale gas) may pose significant risks to shallow groundwater. While the potential threat of stray (CH) gas in aquifers has been acknowledged, few studies have examined the nature of its migration and fate in a shallow groundwater flow system. This study examines the geophysical responses observed from surface during a 72day field-scale simulated CH leak in an unconfined sandy aquifer at Canadian Forces Base Borden, Canada, to better understand the transient behaviour of fugitive CH gas in the subsurface. Time-lapse ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) were used to monitor the distribution and migration of the gas-phase and assess any impacts to groundwater hydrochemistry. Geophysical measurements captured the transient formation of a CH gas plume emanating from the injector, which was accompanied by an increase in total dissolved gas pressure (P). Subsequent reductions in P were accompanied by reduced bulk resistivity around the injector along with an increase in the GPR reflectivity along horizontal bedding reflectors farther downgradient. Repeat temporal GPR reflection profiling identified three events with major peaks in reflectivity, interpreted to represent episodic lateral CH gas release events into the aquifer. Here, a gradual increase in P near the injector caused a sudden lateral breakthrough of gas in the direction of groundwater flow, causing free-phase CH to migrate much farther than anticipated based on groundwater advection. CH accumulated along subtle permeability boundaries demarcated by grain-scale bedding within the aquifer characteristic of numerous Borden-aquifer multi-phase flow experiments. Diminishing reflectivity over a period of days to weeks suggests buoyancy-driven migration to the vadose zone and/or CH dissolution into groundwater. Lateral and vertical CH migration was primarily governed by subtle, yet measurable heterogeneity and anisotropy in the aquifer.

Origin of Methane and Sources of High Concentrations in Los Angeles Groundwater

“…In this scenario, previously identified during the investigation of the Ross Department store explosion in 1985 (Jenden, ; Schoell et al, ), methane could migrate from deep oil pools along leaking oil well casings or up faults to the shallower groundwater. Recent studies have investigated the effects of the supersaturation of methane in groundwater resulting in a free gas phase migration (Cahill et al, ; Steelman et al, ). However, the theoretical solubility limits for LAB deep wells (at 30°C and 50 bar) would be ~903 mg/L methane, which is much greater than the highest concentration observed (LA‐19; 150 mg/L methane, 488 mbls; Tables and ), indicating that methane is not supersaturated in the aquifers, and dissolved gas phase transport is predominant.…”

Advancing knowledge of gas migration and fugitive gas from energy wells in northeast British Columbia, Canada

“…54 It was shown that, after entering the aquifer, free-phase methane preferentially migrated upward due to buoyancy while a significant portion migrated laterally five times faster than advective groundwater flow, due to subtle grain-scale bedding. 55 Results also showed that during the experimental leakage event, dewatered gas-migration channels formed and acted to facilitate emissions to the atmosphere 26 and limit retention in groundwater. After gas injection was stopped, these channels collapsed resulting in increased gas-water mixing, increased levels of dissolved hydrocarbon gases in groundwater, 56 and cessation of emissions to the atmosphere.…”

“…To circumvent such research challenges, a multidisciplinary controlled natural‐gas release investigation was recently completed, offering an opportunity to advance understanding of subsurface gas migration from a point source in a shallow groundwater system . It was shown that, after entering the aquifer, free‐phase methane preferentially migrated upward due to buoyancy while a significant portion migrated laterally five times faster than advective groundwater flow, due to subtle grain‐scale bedding . Results also showed that during the experimental leakage event, dewatered gas‐migration channels formed and acted to facilitate emissions to the atmosphere and limit retention in groundwater.…”

Groundwater Quality and Hydraulic Fracturing: Current Understanding and Science Needs

“…Methane injected into the shallow aquifer at Borden resulted in unexpected, strong lateral movement of free gas, along with a dissolved methane plume and dispersion in the direction of gas flow. Hydrochemical impacts persisted for over a year (Steelman et al ). These types of measurements must be done at many locations and under many different hydrogeologic conditions before a common pattern might emerge.…”