An accurate characterization of phase behavior is critical to the prediction of oil recovery. Often, sufficient pressure-volume-temperature (PVT) experimental data is not available, and mathematical models that are "tuned" to experimental data are needed.Equation of state (EOS) calculations are used for this purpose. EOS models are typically easy to implement in a numerical simulator. Calculation of vapor pressure * 2.4.5 Example calculation of two-phase envelope * 3 Nomenclature * 4 References * 5 Noteworthy papers in OnePetro * 6 External links * 7 See also The phase behavior of a typical pure fluid can be represented on a pressure-temperature diagram, as illustrated inFigure 1. From the Gibbs phase rule, the number of degrees of freedom are 3 -np, which means that one, two, or three phases can be present at equilibrium. For simplicity, these phases are shown as a solid, liquid, and a vapor, although numerous additional solid and liquid phases are possible, as long as no more than three of those phases coexist at equilibrium at the same temperature and pressure. Water, for example, has nine different solid phases, each of which has a different crystalline structure.[1] At the pressure and temperature of the Earth's surface, however, we experience only one solid, liquid, and vapor phase of water. The path indicated from point A, a vapor, to point B, a liquid, would never encounter an interface. According to the Gibbs phase rule, there are no degrees of freedom when three phases are in equilibrium. This necessarily implies that three phases must be in equilibrium only at one temperature and pressure; this is the triple point indicated inFigure 1.