PVT considerations are important in setting up the proper parameters when undergoing reservoir simulation. Phase behavior of a mixture with known composition consists of defining the number of phases, phase amounts, phase compositions, phase properties (molecular weight, density, and viscosity), and the interfacial tension (IFT) between phases. In addition to defining the phase behavior of mixtures at a specific reservoir pressure, knowing the derivatives of all phase properties with respect to pressure and composition is important in reservoir simulation. The calculation of phase behavior in a reservoir model can be made in one of the two following ways: * Using a "black-oil" approach based on simple interpolation of PVT properties as a function of pressure * Using a "compositional" approach based on a thermodynamically-consistent model such as a cubicequation of state (EOS). With either approach, the PVT quantities required by a reservoir simulator are essentially the same. Modern reservoir simulators are usually written with a general compositional formulation, whereas black-oil PVT properties are converted internally to a two-component "compositional" model; the two components are surface gas and surface oil. A reservoir simulator keeps track of overall composition in each computational grid cell as a function of time.