Passive seismic monitoring of carbon dioxide storage at Weyburn-Midale Field is in the Williston Basin of southern Saskatchewan, Canada has been reported. Passive seismic monitoring provides a different kind of information to controlled-source techniques. Controlled-source 4D seismicmonitoring has been largely successful in imaging the plumes of CO2 migrating away from the injection wells, with negative time-lapse amplitude differences marking zones of CO2 saturation. Passive monitoring at Weyburn has focused on a single pattern within the field. The distribution of injection- related event locations also appears to correlate with the regions of CO2 saturation that have been identified using 4D seismic. Geomechanical modeling shows that the evolution of stress during injection is likely to preferentially open the set, making it dominate the splitting results. Simple geomechanical modeling reveals that areas around and above production wells will be at greater risk of failure than around the injection well.