Monitoring of CO2 storage in a depleted natural gas reservoir: Gas geochemistry from the CO2CRC Otway Project, Australia

► CO2CRC Otway Project studied CO2 geosequestration in a depleted natural gas field. ► We injected 65K tonnes of a 3:1 mix CO2-rich supercritical fluid (3:1 CO2:natural gas) over a 17 month period. ► We collected pressurised downhole samples and measured their gas and isotope geochemistry together with added tracers (CD4, Kr, SF6). ► The analytical results identify breakthrough of both dissolved and free CO2 at monitoring well. ►Agreement between observed data and pre-injection model predictions were generally good.