In the context of supercritical CO2 geological storage, the study of leakage event through free media leads to a turbulent two-phase flow model with Reynolds number Re > 500. In the present work, we compute analytically and characterize the 1-D CO2 rising velocity in a water column due to buoyancy effect. We also give numerous orders of magnitude concerning side effects during gas migration, such as CO2 solubility and Joule-Thomson cooling. Most interesting is the CO2 phase transition at ~800 m depth, where gas bubbles may expand a lot and accelerate, or condensate into liquid, slow down and accumulate.