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 back- ground 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 , ?13 C-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 condi- tions, and soil properties. Other light hydrocarbon gases, like C2 H4 and C2 H6 , 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