Oxygen isotope exchange between H₂O and CO₂ at elevated CO₂ pressures: Implications for monitoring of geological CO₂ storage

Traditionally, the application of stable isotopes in Carbon Capture and Storage (CCS) projects has focused on delta C-13 values of CO₂ to trace the migration of injected CO₂ in the subsurface. More recently the use of delta O-18 values of both CO₂ and reservoir fluids has been proposed as a method for quantifying in situ CO₂ reservoir saturations due to O isotope exchange between CO₂ and H₂O and subsequent changes in delta O-18(H₂O) values in the presence of high concentrations of CO₂. To verify that O isotope exchange between CO₂ and H₂O reaches equilibrium within days, and that delta O-18(H₂O) values indeed change predictably due to the presence of CO₂, a laboratory study was conducted during which the isotope composition of H₂O, CO₂, and dissolved inorganic C (DIC) was determined at representative reservoir conditions (50 degrees C and up to 19 MPa) and varying CO₂ pressures. Conditions typical for the Pembina Cardium CO₂ Monitoring Pilot in Alberta (Canada) were chosen for the experiments. Results obtained showed that delta O-18 values of CO₂ were on average 36.4 +/- 2.2 parts per thousand (1 sigma, n = 15) higher than those of water at all pressures up to and including reservoir pressure (19 MPa), in excellent agreement with the theoretically predicted isotope enrichment factor of 35.5 parts per thousand for the experimental temperatures of 50 degrees C. By using O-18 enriched water for the experiments it was demonstrated that changes in the delta O-18 values of water were predictably related to the fraction of O in the system sourced from CO₂ in excellent agreement with theoretical predictions. Since the fraction of O sourced from CO₂ is related to the total volumetric saturation of CO₂ and water as a fraction of the total volume of the system, it is concluded that changes in delta O-18 values of reservoir fluids can be used to calculate reservoir saturations of CO₂ in CCS settings given that the delta O-18 values of CO₂ and water are sufficiently distinct. (C) 2011 Elsevier Ltd. All rights reserved.