Using chemical and isotopic data to monitor geological carbon dioxide storage at the International Energy Agency Weyburn Carbon Dioxide Monitoring and Storage Project, Saskatchewan, Canada

Carbon capture and geological storage (CCS) is one of the most promising technologies for reducing atmospheric CO2 emissions. In western Canada the proximity of the Western Canadian Sedimentary Basin to many large stationary sources of CO2 provides an opportunity to implement CCS. The IEA GHG (International Energy Agency Greenhouse Gas) Weyburn CO 2 Monitoring and Storage Project involves the injection of 1.5 million tonnes of CO2 annually into a hydrocarbon reservoir for the purpose of enhanced oil recovery and geological CO2 storage. Monitoring of geological storage is essential to evaluate the different storage processes and confirm the integrity of the storage reservoir.;In 2000, prior to CO2 injection at Weyburn, produced fluids and gases were sampled from over forty production wells, with a range of chemical and isotopic parameters measured; a further 11 monitoring events followed at circa four month intervals. Geochemical data from the fluid and gas monitoring events were used to trace injected CO2, investigate the origin and evolution of sulfur species, evaluate CO2-aqueous fluid-mineral reactions, quantify the mass of injected CO2 stored as HCO 3-, and estimate molecular CO2 storage in the reservoir fluids.;Geochemical data from selected production wells over four years of CO 2 injection demonstrate that ionic trapping of injected CO2 is the most important source of HCO3- during CO 2 injection, with dissolution of carbonate minerals a secondary HCO 3- source. These geochemical data, supported by history matched reaction path simulations, suggest that alteration of K-feldspar has buffered the pH decrease, enhancing aqueous CO2 storage (as HCO 3-) and mineral storage of CO2 as CaCO 3. In a selected part of the field, reconstruction of measured data to reservoir conditions suggests that approximately 185,000 tonnes of injected CO2 are stored in the reservoir fluids.;An investigation of sulfur species concentrations and isotopic compositions, while highlighting the influence of bacterial sulfate reduction (BSR) and injected water, demonstrates that BSR is unlikely to have been an important source of HCO3- following CO2 injection.;As well as evaluating CO2 storage processes, the geochemical data provide a means of monitoring the ongoing response of the reservoir matrix and cap rock to CO2 injection.