One primary measure of performance for any proposed large-scale CO2 sequestration operation will be how the system responds to potential wellbore failure and subsequent CO2 migration during injection of large volumes of CO2. This will require detailed numerical simulations which will incorporate realistic wellbore details and capture near-wellbore conditions in a coarse 3-dimensional grid. In this paper, we will present results of detailed numerical simulations of a large-scale CO2 sequestration operation. The simulations are performed using FEHM, a multi-phase, heat and mass transfer simulator. We have developed a novel, computationally efficient approach to incorporate radial wellbores in a 3-dimensional Cartesian grid. With this approach the radial solution of fluid flow in the vicinity of wellbores is obtained at high grid resolution without a need for well functions such as the Peaceman approximation. The approach provides for flexible incorporation of wellbores in a computational grid without a need for grid refinement. Our study domain includes a hypothetical field site with a heterogeneous storage aquifer and multiple impermeable and permeable strata above it. CO2 output from a typical coal-fired power plant is injected, over a period of 50 years. Simulation of injection and subsequent migration is performed while capturing the physics of non-isothermal, multi-phase fluid flow. Leakage of CO2 through a wellbore within the field is simulated, both during and after the injection period, including any potential phase-change. We explore the sensitivity of predictions of CO2 migration through wellbores to changes in values of parameters such as location of wellbore, completion properties and local rock properties.