Investigating Methane Leaks Using Eddy Covariance and Footprint Analysis in an Injection Experiment

Monitoring fugitive gas leaks in case of oil and gas wells is a necessity, as gas migration can lead to undesired leaks. Various methods are being used to effectively monitor and minimize unwanted leaks. The micrometeorological technique of eddy covariance (EC) has been considered to be a promising method to monitor and detect these leaks, and potentially map them. We monitored methane fluxes continuously, using an open-path methane analyzer, LI-7700 (LI-COR Inc.) and a 3D sonic anemometer, CSAT-3B (Campbell Scientific Inc.), during and after a methane injection in the ground 15 km north of Hudson's Hope, BC. Measurements were made at 20 Hz and half-hourly flux averages were calculated. A total of 97.662 m³ of methane was injected at a depth of 26 m, over a period of 71 days. The EC tower was set up 26.4 m northeast of the injection point. Methane fluxes as high as 0.22 µmol m^-2 s^-1 were observed during the injection period when wind was from the injection area. In addition, continuous CO₂ and H₂O fluxes were also measured using an enclosed-path infrared gas analyzer, LI-7200 (LI-COR Inc.). A set of climate instrumentation including soil and radiation sensors were also installed in order to monitor weather conditions and energy balance closure.

Flux footprint techniques were used to analyze the EC fluxes and to compare these fluxes with those measured using non-steady-state chambers at the same site. Comparison showed that fluxes measured using the two techniques agreed well. An inversion analysis is being used to map methane leaks at the site using single-point EC measurements. Calculations dealing with inversion to map the leaks are currently underway. Also, controlled release tests have been carried out in order to interpret the footprint conditions and dependence of fluxes on along-wind distance, source height, release rate, turbulence conditions and atmospheric stability.