A main concern for any CO2 sequestration system is whether it may leak CO2 over a long-term time horizon. The outcome depends on the competition between sequestration and leakage processes. Leakages may occur from failure of manmade material or through faults in the formations above the reservoir. A simple and computationally efficient simulator was constructed based on the CQUESTRA model (LeNeveu, 2008). To assess the risk associated with uncertainty in the values of uncertain parameters in this model, thousands of runs were carried out with the simulator on a general-purpose graphics processing unit (CPU). The CPU implementation was up to 64 times faster compared to a CPU implementation. In the absence of active faults around a single injection well, the model suggests that leakages of more than 1% of the total CO2 are unlikely during the 1000 year period after dissipation of temperature and pressure transients associated with injection. Leakage amounts for ten leaky wells are considerably higher, suggesting the critical importance of monitoring equipment after sequestration. (C) 2011 Elsevier Ltd. All rights reserved.