Effective measurement of the presence and rate of methane gas migration (GM) outside the casing of energy wells is important for managing social and environmental impacts and financial liabilities in the upstream petroleum industry. Practitioners typically assess GM by above-background methane gas concentrations in-soil or at-grade; however, factors influencing the potential variation in these measurements are not well represented in industry recommended best-practices. Inexpensive chemoresistive sensors were used to record a one-minute frequency methane gas concentration time series over 19 days. Time series were recorded at three soil depths (0, 5, and 30 cm) at two locations <30m cm radially from a petroleum well with known GM, in addition to two ‘control’ locations. Observed concentration variations ranged over several orders of magnitude at all depths, with generally lower concentrations and more variation observed at shallower depths. Varying concentrations were correlated to meteorological factors, primarily including wind speed and shallow groundwater table elevation. The gas concentration patterns were affected by a 3.5 mm rainfall event, suggesting soil moisture changes affected preferential gas migration pathways. Results indicate potential variability in repeated snapshot GM test results. Although currently recommended GM detection methods would have effectively identified the presence/absence of GM, they would not have quantified order of magnitude changes in concentration. GM detection success at this site was increased with measurement at more than one location spatially within 30 cm of the well casing, lower concentration detection limits, and greater measurement depth. These findings indicate that meteorological factors should be considered when conducting gas migration surveys (particularly for improving at-grade test reliability). The low-cost approach for long-term concentration measurement facilitates insight into variable gas concentrations and may be advantageous in comparison to snapshot measurements in some circumstances.

Well integrity failure resulting in migration of natural gas outside of the surface casing can cause atmospheric greenhouse gas emissions and groundwater quality impacts from existing and historic energy wells. Spatial and temporal variability in gas migration can result in errors in detection (i.e., presence/absence) and efflux estimations. This field-based case study used automated dynamic closed chambers to record repeated (~ every 18 minutes) CO2 and CH4 efflux measurements over a two-week period around a single petroleum production well in Alberta, Canada. Long-term efflux measurements supplemented soil gas compositional and isotopic characterization, along with surface concentration measurements. Effluxes were spatially concentrated around the wellhead and only occasionally detectable more than a few meters away. Estimated total emissions attributable to gas migration ranged from 48 - 466 g CH4 d-1 (or 0.07 - 0.7 m3 CH4 d-1). Methane effluxes and concentrations were temporally variable on second-to-hourly and diel scales. Multivariate stepwise regression analysis indicates that multiple meteorological factors, particularly wind speed and air temperature, were related to the temporal variability. Despite temporal variability, elevated concentrations and effluxes were consistently detectable around the well. Major soil gas composition suggests that gas migration near the wellhead causes advective displacement of soil gas, while more distal measurements are indicative of episodic and diffusion-dominated transport. Values of 13C-CO2 and 13C-CH4 samples were consistent with CH4¬ oxidation within the unsaturated zone. Although these results reflect a single well, the findings are salient to gas migration detection and emission estimation efforts.