Geological storage of COኼ is a crucial and upcoming technology to reduce anthropogenic greenhouse gas emissions. Due to the buoyant characteristic of injected gas, the security of underground storage is a major concern. To asses the security of COኼ storage, an accurate prediction of the gas plume behaviour is essential. In this study, a fully physical 2D model is developed to describe gas behaviour in a saline aquifer. In particular, we investigate the effect of gas impurities on injectivity, macroscopic dissolution and resulting plume migration. The model includes an investigation of 4-component buoyancy driven multiphase convective flow with miscible convective mixing. For an accurate description of the phase behaviour, a recently developed thermodynamic model based on a combination of cubic Equation of state with activity model has been implemented. The implemented thermodynamic model includes a specific description for the behaviour of water and an activity model describing component behaviour of the aqueous phase making this model more accurate than conventional cubic EoS. The phase behaviour based on this thermodynamic model and a consistent set of physical properties have been implemented in Delft Advance Research Terra Simulator, a new simulation framework developed at TU Delft. The results show that the presence of the gas impurities (tested here) have a negative effect on the solubility of CO2 which in turn reduces the security of gas storage in saline aquifers and increase the risk of leaking. This is the first time when this accurate physical model is applied for large-scale simulation of CO2 sequestration. Additionally a framework regarding the energy cost of the total CO2 sequestration process, from separation to injection, is implemented.