It uses a 'model as input' approach whereby the conceptual and mathematical model itself forms part of the input to the software. QPAC is being used to model the performance of CO2 geological storage at the systems level, for example to support the development of monitoring strategies, and for investigating detailed issues such as the degradation of cement well seals. One application is to develop simplified models that can be used to investigate the potential significance of very low-probability hypothetical CO2 leakage scenarios. A systems model designed to explore the potential significance of hypothetical CO2 leakage via a fault plane was formed. The model was used to determine the combination of fault and aquifer permeabilities that would cause the fault to by-pass the intermediate and shallow aquifers. Here, after 5000 years, CO2 reaches the surface and very little enters these aquifers, which means that no credit should be taken for secondary storage capacity within them.