One of the risks associated with CO2 geologic sequestration is CO2 migrating upwards from the storage reservoir along vertical leakage paths, which may be abandoned or improperly cemented wellbores, or reactivated faults. CO2 migrating upwards is a process generally controlled by formation pressure gradient and CO2 buoyancy as controlled by the changes of CO2 properties such as density, viscosity, enthalpy and phase state along the paths, and companied by heat transfer between escaping CO2 and the leakage paths and heat conduction in the formation. Considering absorption or release of latent heat associated with phase changes, this paper develops models for CO2 flow and heat transfer during the leakage process. Then a procedure is developed to couple these models. Both steady and un-steady state models are presented in this paper. The steady state and unsteady state models generate similar temperature profiles with different magnitude. In terms of the temperature profile, the CO2 cooling effect caused by forming the super heated gas is dominant. It is also discovered that, during the CO2 migrating upwards process, CO2 cooling effect along the paths is essentially described by the saturation line in the CO2 P-T phase diagram. Case study shows that a simple relationship between the CO2 leakage rate and the maximum temperature difference between the original and the altered temperature profiles can obtained, so that the CO2 leakage rate could be estimated with the temperature profile logged along the wellbore in the field practice. (C) 2011 Elsevier Ltd. All rights reserved.