Reservoir fluid phase behavior and PVT properties play a major role throughout the field life cycle from exploration, development, production and reservoir management up to EOR/IOR implementation. Complex reservoir fluids include the volatile oil, retrograde condensate, heavy oils, high impurities (such as CO2, H2S, etc.), high wax and asphaltenes content, fluids with high degree of compositional grading and variation, non-equilibrium fluids and also the change of phase behavior and composition through IOR/EOR processes. For instance, it is recognized that the phase and volumetric behavior of gas/oil system during gas injection had an important effect on ultimate recovery efficiency.