Stable carbon and oxygen equilibrium isotope fractionation of supercritical and subcritical CO2 with DIC and H2O in saline reservoir fluids

The stable isotope composition of CO2 is often used as a tracer during carbon storage projects. To date it has not been investigated to what extent the transition from supercritical to subcritical CO2 affects stable isotope fractionation of CO2 with dissolved inorganic carbon (DIC) and H2O at elevated temperatures and pressures. We determined the influence of the supercritical state of CO2 on stable carbon and oxygen equilibrium isotope fractionation between CO2 and two types of saline waters: (a) a 80 g/L total dissolved solids (TDS) saline formation water from the Midale formation of the Weyburn CO2 Monitoring and Storage Project (2260 mg/L DIC); (b) a synthetic, DIC free NaCl saline brine with 250 g/L TDS, similar to fluids in the Basal Cambrian Sandstone targeted by the Shell Quest project in Alberta, Canada. The laboratory equilibration experiments between CO2 and saline water were conducted at pressures from 1.0 to 9.0 MPa and temperatures from 22 to 86 °C. We found that oxygen isotope fractionation between CO2 and H2O (ϵ18OCO2−H2O) for both investigated solutions ranged from 29.0 to 41.1‰ VSMOW and was generally about 1‰ lower than previously reported values for pure water. This discrepancy is likely due to salt effects. Also, ϵ18OCO2−H2O was found to be identical at a given temperature irrespective of whether supercritical or subcritical CO2 was present. Supercritical CO2 did not result in carbon isotope effects that are different from those previously reported between sub-critical CO2 and DIC (ϵ13CDIC−CO2). We conclude that supercritical conditions with respect to CO2 in or above CCS storage reservoirs do not cause additional isotope effects and hence do not compromise the use of stable isotopes as a tracer in CO2 storage projects.