Soil Gas as a Monitoring Tool of Deep Geological Sequestration of Carbon Dioxide: Preliminary Results from the Encana EOR Project in Weyburn, Saskatchewan (Canada)

Carbon dioxide injection into the EnCana (formerly Pan Canadian) oil reservoir, located south of Weyburn, Saskatchewan, Canada, is being undertaken for enhanced oil recovery (EOR) purposes, while at the same time, the site is being studied for the feasibility of environmental sequestration of this gas. The systematic measurement of soil gas concentration and gas flux at locations above the Phase 1A injection site provides data to construct a baseline of near surface soil gas geochemistry. Data for CO2, CO2 flux, O2, CH4, radon (222Rn), thoron (Tn or radioisotope 22°Rn), and helium, along with other permanent gases, sulphur species, and light hydrocarbons are being collected at yearly intervals for a period of three years, while radon is also being continuously monitored for seasonal and diurnal variations using in situ probes. The Istituto Nazionale di Geofisica e Vulcanologia (INGV) CO2 flux survey measured all 360 points, whereas the initial URS soil gas survey measured 217 points for Rn and Tn, 291 points for field gas chromatography, and 292 sites for laboratory measured gases. The BGS soil gas sampling program completed the southern third of the grid except for 16 sampling sites in the southeast comer that could not be sampled due to a late harvest. The British Geological Survey (BGS), Bureau de Recherches Grologiques et Minières (BRGM) and INGV also completed gas measurements along 4 profiles. The profiles were defined from anomalies detected during the initial grid sampling, in conjunction with structural data for the immediate area. Natural background levels and concentration distributions have been established for the measured soil gases, and by comparing these results with future data sets, it will be possible to estimate CO2 storage integrity for the reservoir rocks.