The IEA Weyburn CO2 Monitoring and Storage Project has analysed the effects or a miscible CO2 flood into a carbonate reservoir rock at an onshore Canadian oilfield. Anthropogenic CO, is being injected as part of an enhanced oil recovery operation. The European research was aimed at analysing longterm migration pathways Of CO2 and the effects of CO, oil the hydrochemical and mineralogical properties of the reservoir rock. The long term safety and performance Of CO2 storage was assessed by the construction of a Features, Events and Processes (FEP) database which provides a comprehensive knowledge base for the geological storage of CO2. The pre-CO2 injection hydrogeological, hydrochemical and petrographical conditions in the reservoir,were investigated in order to rccognise changes caused by the CO2 flood and to assess the fate of the CO,. The Mississippian aquifer has a salinity gradient in the Weyburn area, where flows are oriented SW-NE. The baseline gas fluxes and CO2 concentrations in groundwater and soil were also studied. The dissolved gas in the reservoir waters has allowed potential transport pathways to be identified. Experimental studies of CO2-porewater-rock interactions in the Midale Marly unit have indicated slight dissolution of carbonate and silicate minerals, but relatively rapid saturation with respect to carbonate minerals. Equivalent studies oil the overlying and underlying units show similar reaction processes, but secondary gypsum precipitation was also observed. Carbon dioxide flooding experiments on samples of the Midale Marly unit demonstrated that porosity and gas permeability increased significantly and calcite and dolomite were shown to have undergone corrosion. Hydrogeological modelling indicates that if any dissolved CO, entered the main aquifers, it would be moved away from Weyburn in an E-NE direction at a rate of c. 0.2 metres per year due to regional groundwater now. Analysis of reservoir fluids proved that dissolved CO2 and CH4 increased significantly in the injection area between 2002 and 2003 and that solubility trapping accounts for the majority of the injected CO2 with little apparent mineral trapping. Twelve microseismic events were recorded and these are provisionally interpreted as possibly being related to small fractures formed by injection-driven fluid miggration within the reservoir. Pre- and post-injection soil gas data are consistent with a shallow biological origin for the measured CO2. Isotopic (delta C-13) data values are higher than in the injected CO2, and confirm this interpretation. No evidence for leakage of the injected CO2 to ground level has so far been detected.