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 of gas, 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 and to 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
Petroleum reservoirs are broadly classified as oil or gas reservoirs.
These broad classifications are further subdivided depending on:
•
The composition of the reservoir hydrocarbon mixture•
Initial reservoir pressure and temperature•
Pressure and temperature of the surface productionThe conditions under which these phases exist are a matter of considerable
practical importance. The experimental 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 DiagramFigure 1-1 shows a typical pressure-temperature diagram of a multicomponent
system with a specific overall composition. Although a different
hydrocarbon system would have a different phase diagram, the
general configuration is similar.
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 Cricondenbar temperature 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)—The region 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
critical point (point C).
• Bubble-point curve—The bubble-point curve (line BC) is defined as
the line separating the liquid-phase region from the two-phase region.
• Dew-point curve—The dew-point curve (line AC) is defined as the
line separating the vapor-phase region from the two-phase region.
