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

Abstract The stable isotope composition of CO 2 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 CO 2 affects stable isotope fractionation of CO 2 with dissolved inorganic carbon (DIC) and H 2 O at elevated temperatures and pressures. We determined the influence of the supercritical state of CO 2 on stable carbon and oxygen equilibrium isotope fractionation between CO 2 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 CO 2 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 CO 2 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 CO 2 and H 2 O ( ϵ 18 O C O 2 − H 2 O ) 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, ϵ 18 O C O 2 − H 2 O was found to be identical at a given temperature irrespective of whether supercritical or subcritical CO 2 was present. Supercritical CO 2 did not result in carbon isotope effects that are different from those previously reported between sub-critical CO 2 and DIC ( ϵ 13 C D I C − C O 2 ). We conclude that supercritical conditions with respect to CO 2 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 CO 2 storage projects.