In CO2 geological sequestration, a combination of monitoring techniques needs to be in place to timely detect possible CO2 leakage from a primary storage along unanticipated pathways to shallower formations. This research aims to methodologically investigate the feasibility of a novel radiographic technique, i.e. cosmic-ray muon radiography, as a complementary continuous monitoring method. As an example, this method was tested on a geological model to monitor CO2 leakage into upper freshwater aquifers. The effectiveness of the method was preliminarily established by high-fidelity simulations, including the sensitivity for responding to CO2 leakage and the spatial resolution that can be achieved by the method. The simulation results indicate an increase of penetrating flux of the cosmic-ray muons with the increase of CO2 leakage in the monitored aquifers. The sensitivity tends to be higher in monitoring leakage taking place in shallower depths. At depths of about 200 m, the detectable CO2 can be as low as 3 % measured in volume fraction with a relatively high confidence level. The spatial resolution can be achieved within a range from 10 to 20 m for measurements at depths of no more than 520 m, demonstrating the effectiveness of the method.