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

Abstract

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 δ¹⁸O_H2O 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.