Basic concepts of phase behavior
1) Single component system.
Ethane is taken as an example for single
component system.
Ethane exists as gas
phase at ordinary conditions. At other than
ordinary conditions, ethane may exists as a
liquid as is shown in figure.
The plotted curve is called vapor-pressure curve.
All the pressures and
temperatures located on the curve means that both liquid and vapor phases
are in equilibrium. Meanwhile, at any condition of pressure and temperature
which does not located on the curve means only one phase exists, i.e., either
liquid or gas.
As it can be seen from the diagram, ethane exists in the liquid state at those
pressures lying above the vapor-pressure curve for the appropriate
temperatures. It can be noted further that ethane is vapor or gas at those
pressures lying below the vapor-pressure curve for same values of
temperatures.
 Point C (where the curve terminates) is the critical point of the system which
defines as the highest value of pressure and temperature at which two
phases can coexist.
 At point A, ethane has the properties of vapor.
 By increasing temperature at constant pressure (A→B), ethane still remains
in vapor phase.
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 By increasing Pressure at constant temperature (A→F) or (B→D), ethane
states changes from vapor to liquid.
 By decreasing temperature at constant pressure (D→E), ethane remains as
liquid and does not change to vapor.
 By decreasing pressure at constant temperature (E→A) or (F→A), ethane
changes from liquid to vapor.
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2) Binary system.
When a second component is added to a hydrocarbon system, the phase
behavior becomes more complex because of the introduction of another
variable composition to the system. The effect of this variable can be noted in
the figure below.
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 For a single component system the vapor-pressure curve represents the
trace of both bubble point and dew point curves on the pressuretemperature diagram. In contrast, for a binary or other multi-component
system, the bubble point and dew point lines no longer coincide.
 The traces of bubble point and dew point curves for the multi-component
system form a phase diagram. Moreover, for each possible composition a
distinct phase diagram exists.
 On the left of the figure, terminating at the point C, is the vapor-pressure
curve of pure ethane.
 On the extreme right of the figure, terminating in point C7, is the vaporpressure curve for pure n-heptane.
 Between the vapor-pressure curves of pure constituents lie the phase
diagrams of the various binary mixtures of ethane-n-heptane system.
 A3 – C3 → Bubble point curve.
 C3 – B3 → Dew point curve.
 Above and to the left of the bubble point curve A 3 – C3, the mixture exists as
a liquid.
 Below and to the right of the dew point curve C 3 – B3, the mixture exists as a
gas.
 Within the region enclosed by bubble point and dew point curves, the
mixture exists as two phase.
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3) Multicomponent systems
Naturally occurring hydrocarbon systems are composed of a wide range of
constituents.
These constituents include not only the paraffin series of
hydrocarbons but components from various other series. The phase behavior
of a hydrocarbon mixture is dependent on the composition of the mixture as
well as the properties of individual constituents.
A number of important concepts associated with the diagram must be defined.
Critical point (C): The state of pressure and temperature at which the intensive
properties of each phase are identical.
Intensive properties: The properties that are independent of the amount of
material under consideration.
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Extensive properties: Those properties that are directly proportional to the
amount of material under consideration.
Bubble point curve: The locus of the points of pressure and temperature at
which the first bubble of gas is formed in passing from liquid to the two-phase
region.
Dew point curve: The locus of the points of pressure and temperature at which
the first droplet of liquid is formed in passing from the vapor to the two-phase
region.
Two-phase region: That region enclosed by the bubble point line and dew
point line wherein gas and liquid coexist in equilibrium.
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).
Cricondentherm (M): The highest temperature at which a liquid and vapor can
coexist in equilibrium.
Cricondenbar or Crivaporbar (N): The highest pressure at which a liquid and
vapor can coexist in equilibrium.
Retrograde region (shaded area): Any region where condensation or
vaporization occurs in reserve to the conventional behavior.
Retrograde condensation: In which liquid is condensed upon either lowering
the pressure at constant temperature (lines A, B, D) or increasing the
temperature at constant pressure (lines H, G, A).
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Retrograde Vaporization: In which vapor is formed upon decreasing the
temperature at constant pressure (lines A, G, H) or increasing the pressure at
constant temperature (lines D, B, A).
In general, reservoirs are conveniently classified on the basis of the location of
the point representing the initial reservoir pressure Pi and temperature T with
respect to the pressure-temperature diagram of the reservoir fluid.
Accordingly, reservoirs can be classified into basically two types. These are:
1. Oil reservoirs: If the reservoir temperature T is less than the critical
temperature of the reservoir fluid, the reservoir is classified as an oil
reservoir.
2. Gas reservoirs: If the reservoir temperature is greater than the critical
temperature of the hydrocarbon fluid, the reservoir is considered a gas
reservoir.
Oil Reservoirs
Depending upon initial reservoir pressure Pi, oil reservoirs can be sub-classified
into the following categories:
1. Undersaturated oil reservoir. If the initial reservoir pressure Pi is greater
than the bubble-point pressure Pb of the reservoir fluid, the reservoir is labeled
an undersaturated oil reservoir.
2. Saturated oil reservoir. When the initial reservoir pressure is equal to the
bubble-point pressure of the reservoir fluid, the reservoir is called a saturated
oil reservoir.
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3. Gas-cap reservoir. If the initial reservoir pressure is below the bubble point
pressure of the reservoir fluid, the reservoir is termed a gas-cap or two-phase
reservoir, in which the gas or vapor phase is underlain by an oil phase. The
appropriate quality line gives the ratio of the gas-cap volume to reservoir oil
volume.
Gas Reservoirs
In general, if the reservoir temperature is above the critical temperature of the
hydrocarbon system, the reservoir is classified as a natural gas reservoir. On
the basis of their phase diagrams and the prevailing reservoir conditions,
natural gases can be classified into four categories:
1. Retrograde gas-condensate reservoir. If the reservoir temperature
lies
between the critical temperature and cricondentherm of the reservoir fluid,
the reservoir is classified as a retrograde gas-condensate reservoir.
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2. Near-critical gas-condensate reservoir. If the reservoir temperature is near
the critical temperature, as shown in Figure, the hydrocarbon mixture is
classified as a near-critical gas-condensate.
3. Wet-gas reservoir. A typical phase diagram of a wet gas is shown in Figure
below, where reservoir temperature is above the cricondentherm of the
hydrocarbon mixture. Because the reservoir temperature exceeds the
cricondentherm of the hydrocarbon system, the reservoir fluid will always
remain in the vapor phase region as the reservoir is depleted isothermally,
along the vertical line A-B.
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4. Dry-gas reservoir. The hydrocarbon mixture exists as a gas both in the
reservoir and in the surface facilities. The only liquid associated with the gas
from a dry-gas reservoir is water. Usually a system having a gas-oil ratio
greater than 100,000 scf/STB is considered to be a dry gas.
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