At the International Energy Agency Greenhouse Gas (IEA GHG) Weyburn-Midale Project in Saskatchewan, Canada, CO2 storage research takes place alongside CO2 enhanced oil recovery (EOR) in the Weyburn oil field. Over four years of production well monitoring at Weyburn, measured changes in chemical and isotopic data for produced aqueous fluids and gases (i.e. an increase in Ca2+, Mg2+, K+, SO42-, HCO3-, and CO2 concentration and a decrease in ?13CHCO3- and ?13CCO2 values), confirm the integrity of CO2 storage, trace CO2 migration and dissolution in the reservoir fluids, and record a range of water-rock- CO2 reactions including carbonate mineral dissolution and alteration of K-feldspar. K-feldspar alteration buffers the pH decrease resulting from CO2 injection, enhances aqueous CO2 storage as HCO3- (ionic trapping) and can lead to mineral storage of CO2 as CaCO3. Geochemical reaction path simulations of the water-mineral- CO2 system reproduce the changes in measured data observed over the first few years, confirming proposed reaction pathways and rates. Extension of these history matched reaction path simulations over 100s of years shows that alteration of K-feldspar and other silicate minerals present in the Weyburn reservoir will lead to further storage of injected CO2 in the aqueous phase and as carbonate minerals.