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FUNDAMENTALS OF RESERVOIR FLUID BEHAVIOR

Naturally occurring hydrocarbon systems found in petroleum reservoirs are mixtures of organic compounds which exhibit multiphase behavior over wide ranges of pressures and temperatures. These hydrocarbon accumulations may occur in the gaseous state, the liquid state, the solid state, or in various combinations ofgas, liquid, and solid. These differences in phase behavior, coupled with the physical properties of reservoir rock that determine the relative ease with which gas and liquid are transmitted or retained, result in many diverse types of hydrocarbon reservoirs with complex behaviors. Frequently, petroleum engineers have the task to study the behavior and characteristics of a petroleum reservoir andto determine the course of future development and production that would maximize the profit. The objective of this chapter is to review the basic principles of reservoir fluid phase behavior and illustrate the use of phase diagrams in classifying types of reservoirs and the native hydrocarbon systems.

CLASSIFICATION OF RESERVOIRS AND RESERVOIR FLUIDS
Petroleum reservoirs are broadly classifiedas oil or gas reservoirs. These broad classifications are further subdivided depending on:

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Reservoir Engineering Handbook

• The composition of the reservoir hydrocarbon mixture • Initial reservoir pressure and temperature • Pressure and temperature of the surface production The conditions under which these phases exist are a matter of considerable practical importance. Theexperimental or the mathematical determinations of these conditions are conveniently expressed in different types of diagrams commonly called phase diagrams. One such diagram is called the pressure-temperature diagram.
Pressure-Temperature Diagram

Figure 1-1 shows a typical pressure-temperature diagram of a multicomponent system with a specific overall composition. Although a different hydrocarbon systemwould have a different phase diagram, the general configuration is similar.

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Figure 1-1. Typical p-T diagram for a multicomponent system.

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Fundamentals of Reservoir FluidBehavior

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These multicomponent pressure-temperature diagrams are essentially used to: • Classify reservoirs • Classify the naturally occurring hydrocarbon systems • Describe the phase behavior of the reservoir fluid To fully understand the significance of the pressure-temperature diagrams, it is necessary to identify and define the following key points on these diagrams: • Cricondentherm(Tct)—The Cricondentherm is defined as the maximum temperature above which liquid cannot be formed regardless of pressure (point E). The corresponding pressure is termed the Cricondentherm pressure pct. • Cricondenbar (pcb)—The Cricondenbar is the maximum pressure above which no gas can be formed regardless of temperature (point D). The corresponding temperature is called the Cricondenbartemperature Tcb. • Critical point—The critical point for a multicomponent mixture is referred to as the state of pressure and temperature at which all intensive properties of the gas and liquid phases are equal (point C). At the critical point, the corresponding pressure and temperature are called the critical pressure pc and critical temperature Tc of the mixture. • Phase envelope (two-phase region)—Theregion enclosed by the bubble-point curve and the dew-point curve (line BCA), wherein gas and liquid coexist in equilibrium, is identified as the phase envelope of the hydrocarbon system. • Quality lines—The dashed lines within the phase diagram are called quality lines. They describe the pressure and temperature conditions for equal volumes of liquids. Note that the quality lines converge at the...