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Petroleum reservoir fluids are complex mixtures containing many hydrocarbon components that range in size from light gases such as methane (C1) and ethane (C2) to very large hydrocarbon molecules containing 40 or more carbon atoms. Nonhydrocarbon components such as nitrogen, H2S, or CO2 also may be present. Water, of course, is present in essentially all reservoirs. At a given temperature and pressure, the components distribute between the solid, liquid, and vapor phases present in a reservoir. A phase is the portion of a system that is homogeneous, is bounded by a surface, and is physically separable from other phases. Equilibrium phase diagrams offer convenient representations of the ranges of temperature, pressure, and composition within which various combinations of phases coexist. Phase behavior plays an important role in a variety of reservoir engineering applications, ranging from pressure maintenance to separator design to enhanced oil recovery (EOR) processes. This chapter reviews the fundamentals of phase diagrams used in such applications. Additional material on the role of phase equilibrium in petroleum/reservoir engineering can be found in Lake[1] and Whitson and Brulé.[2] The saturation curves shown inFigure 1.1 indicate the temperatures and pressures at which phase changes occur. At temperatures below the triple point, the component forms a vapor phase if the pressure is below that indicated by the sublimation curve and forms a solid phase at pressures above the curve. At pressures and temperatures on the melting curve, solid and liquid are in equilibrium. If the pressure is greater than the vapor pressure, a liquid forms; if the pressure is lower than the vapor pressure, a vapor forms. For a single component, the critical temperature is the maximum temperature at which two phases can exist.
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