Using oxygen isotope ratios to quantitatively assess trapping mechanisms during CO 2 injection into geological reservoirs: The Pembina case study

Abstract

Carbon Capture and Storage (CCS) is considered a viable option for reducing CO 2 emissions into the atmosphere from point sources such as coal-fired power plants. Monitoring of CO 2 storage sites is widely considered necessary for safety reasons and for verification of injected CO 2 in the reservoir. The latter is crucially dependent on the ability to determine CO 2 trapping mechanisms and to assess pore-space saturation of CO 2. Thus far, attempts to determine CO 2 pore-space saturations at CO 2 injection sites have had limited success. Here, we present data from the Pembina Cardium CO 2 Monitoring Project in Alberta, Canada, that demonstrate that changes in the oxygen isotope ratios (δ 18O) of reservoir water can be indicative of the extent of pore-space saturation with CO 2. The δ 18O value of injected CO 2 at the injection site was + 28.6‰ (V-SMOW) and δ 18O values of reservoir water at eight observation wells varied between − 13.5 and − 17.1‰ (V-SMOW) before CO 2 injection. After commencement of CO 2 injection the δ 18O values of reservoir water at three observation wells increased between 1.1 and 3.9‰ due to the presence of large quantities of injected CO 2 and equilibrium isotope exchange between water and CO 2. Our calculations revealed that reservoir water fully saturated with CO 2 would only result in increases of δ 18O H2O values of 0.4‰. Hence the observed larger increases in δ 18O values of reservoir water indicate free phase CO 2 with estimated pore-space saturations ranging from 12% (corresponding to a δ 18O increase of 1.1‰) to 64% (δ 18O increase 3.9‰) of the non-oil saturated pore-space. Contributions to oxygen in the system from mineral dissolution were calculated to be less than 0.01% of total oxygen and therefore did not alter the δ 18O value of the reservoir water significantly. Hence we conclude that changes in the δ 18O values of reservoir water caused by the presence of injected CO 2 can be used as a tracer for CO 2 plume migration in the subsurface provided that the injected CO 2 is isotopically distinct. Furthermore, we submit that the extent of the changes in the δ 18O values of the reservoir water provides a quantitative assessment of CO 2 stored in dissolved form (solubility trapping), assuming no density driven convective overturn, and as free-phase CO 2 (structural, stratigraphic and hydrodynamic trapping) thereby elucidating the trapping mechanisms within the reservoir.