Constraining O isotope equilibrium exchange between CO2 and water during fluid cycling in a closed system

Residual trapping is a critical component of secure geological CO2 storage. Quantification of the amount of CO2 residually trapped plays a key role in predicting CO2 plume migration, immobilisation and storage security. However, it is difficult to determine pore-space saturation of CO2 within a subsurface reservoir. δ18O of CO2 and reservoir fluids from a variety of field-scale CO2 injection settings have provided estimates of in-situ CO2 reservoir saturations. This is due to O isotope exchange between CO2 and H2O and subsequent changes in δ18OH2O values due to the presence of free phase CO2. Here, we present results from laboratory experiments to measure the O isotope equilibration time at ambient conditions and O isotope behaviour in both CO2 and water during fluid cycling in a closed system. This provides an analogue of O isotopic exchange within a single-well push-pull injection and production scenario and is primarily motivated to help understand varying estimates of the amount of residual trapping obtained from O isotope measurements during the Otway 2Bext single well push-pull test. We find that full O isotope equilibration between CO2 and water is established after 72 hours and water δ18O does not change after 48 hours. The maximum change in δ18O of water of 1.16 % during the back-production phase of our fluid cycling experiment using waters enriched in δ18O would be negligible in field-scale projects, when unenriched water and CO2 sources are used, considering standard analytical uncertainties. However, our results show that changes in δ18O values of CO2 during a back-production scenario may be larger than 3.52 %, hence it may be inaccurate to solely use O isotope composition of CO2 to quantify CO2 pore-space saturation in a single-well push-pull configuration.