Monitoring of near-surface gas geochemistry at the Weyburn, Canada, CO2-EOR site, 2001–2011

Soil gas and CO2 flux measurements were conducted above and in the vicinity of the Weyburn oil field during the period 2001–2005 and in 2011 to determine baseline values and distributions, and to monitor for surface leaks, above this well-established and intensely studied CO2-Enhanced Oil Recovery (CO2-EOR) project in southern Saskatchewan, Canada. Multiple sites were studied which had sample spacing that ranged from 25 to 200 m, including a 360 point regional grid above the CO2 injection field, a background site off the oil field, and a new site where a landowner claimed CO2 leakage on his property from the storage reservoir. Typically 400–500 points were sampled during each of the seven field campaigns and analysed for a wide range of components, thus yielding a large and varied database collected during different seasons and years. Results show no sign of leakage of the injected CO2. Spatial and seasonal trends and measured values from discrete sampling of soil gas CO2, O2 + Ar, N2, δ13C-CO2, He, Rn, and CH4, from continuous monitoring of soil gas CO2 and Rn, and from discrete sampling of CO2 flux can all be explained by the interplay between near-surface biochemical processes, seasonal environmental conditions, and soil properties. Other light hydrocarbon gases, like C2H4 and C2H6, were generally near or below the instrument detection limit. Lessons learned during the research are described to help improve future near-surface gas geochemistry surveys for site assessment, baseline definition, and leakage monitoring at active CCS sites.

► Gas geochemistry monitored above Weyburn site, 7 campaigns in 6 different years.
► Conducted regional and detailed surveys to examine spatial and seasonal variability.
► No evidence of leakage from the storage reservoir to the surface observed.
► Results explained by shallow biology, soil environment, meteorology, topography.
► Discuss baseline and site monitoring strategies in light of experience and results.