Subsurface natural gas migration in heterogeneous Quaternary deposits of the Montney Resource play, northeastern British Columbia (NE BC), Canada: Insights provided by a controlled release experiment

Ongoing petroleum resource development raises concerns about the unintentional release of fugitive gas into shallow aquifers. Given that limited field investigations have examined transport and fate of fugitive gas from real cases of wellbore leakage, we conducted a controlled-release experiment in a shallow confined aquifer typical of NE BC, where intensive petroleum resource development occurs. The subsurface at the site is highly complex, consisting of a 12 m thick silty diamict confining layer, underlain by 14 m of heterogeneous glacial outwash comprised of interbedded sands, silts and clays. Prior to the experiment the site was rigorously characterized at high resolution by sonic cores, cone penetrometer testing, downhole and surface-based geophysical surveys. We injected 100 m³ of synthetic Montney natural gas (85% CH₄, 8% C₂H₆, 5% C₃H₈) at the base of the confined aquifer over 66 days and monitored gas migration using 12 multilevel monitoring wells with 34 sample ports, 5 single screen piezometers and 4 in-situ pressure transducers. After 12 days of injection, a subtle increase in dissolved CH₄ was detected (~ 0.1 compared to a baseline of ~10^-3 mg/L) in 1 port at 16 m depth, 10 m up-gradient of the injection point. 5 ports showed clear increases in CH₄ (ranging from ~0.1 - 16 mg/L) during injection, whereas 17 ports showed elevated CH₄ 274 days post-injection. Only half of the ports (n = 8/17) where elevated CH₄ was detected also contained elevated ethane (~0.003 - 1.9 mg/L), potentially a result of a higher effective solubility causing a chromatographic effect during gas migration. The greatest levels of dissolved CH₄ occurred mostly at 12 and 18 m depths concurring with an increase in subsurface resistivity, likely a result of capillary trapping caused by low-permeability units as identified in sediment cores. In-situ pressure and aqueous chemistry data suggest injection induced-groundwater mixing in proximity to free-gas flow. Generally under saturated dissolved CH₄ conditions were observed, likely due to channeling of free-phase gas and limited mixing with groundwater in the complex and fine-grained soils. Gas migration in this Quaternary geological system appears to move in a highly discrete manner through localized, small preferential channels controlled by the presence and nature of capillary barriers.