Safe, long-term sequestration of super-critical CO2 in large-scale aquifers is expected to be achieved through the mechanisms of capillary, solubility, and mineral trapping. The total amount of CO2 that can be stored in an aquifer is primarily a function of the rate at which CO2 is immobilized by the different trapping mechanisms, as well as, the geologic quality of the aquifer based on its overall size, permeability characteristics, the relative permeability of cap-rock and the absence of major geologic faults and fractures. Vertical and up-dip migration of large-scale CO2 plumes over long periods of time also adds the risk of leakage. In order to maximize the overall amount of injected CO2 while minimizing the risk of leakage, the complex dynamics associated with the various trapping mechanisms and their interactions must be analyzed in detail and the governing equations must be formulated rigorously. Consequently a detailed characterization model for storage target and high-resolution numerical simulations should be used to make prediction of complex dynamics related to field-scale CO2 sequestration.