Hydrocarbon Phase Behaviour

1. Hydrocarbon Phase Behaviour

2. What is a phase?
• A phase is any homogeneous and physically distinct
region that is separated from another such region by
a distinct boundary
• For example a glass of H2O with some ice in it
contains one component (the water) exhibiting three
phases; liquid, solid, and gaseous (the water vapour).
• The most relevant phases in the oil industry are
liquids (water & oil), gases (or vapours), and to a
lesser extent, solid.

3. General Hydrocarbon phase Behaviour
• As the conditions of pressure and temperature vary, the phases
in which hydrocarbons exist, and the composition of the phases
may change.
• It is important to understand the initial condition of fluids to be
able to calculate surface volumes represented by subsurface
hydrocarbons.
• It is also necessary to be able to predict phase changes as the
temperature and pressure vary both in the reservoir and as the
fluids pass through the surface facilities, so that the appropriate
subsurface and surface developments plans can be made.
• Phase behaviour describes the phase or phases in which a mass
of fluid exists at given conditions of pressure, volume and
temperature (PVT)

4. PHASE BEHAVIOUR OF HYDROCARBONS
When fluids are produced from a subsurface reservoir to the surface both
temperature and pressure are reduced.
The P-T changes result in two kinds of phase change in the produced
fluids:
1. Liquid may condense from the produced gas.
2. Gas may evolve from the produced liquid.
Similar changes take place in the subsurface reservoir as a result of the
isothermal (constant temperature) pressure change generated by fluid
production:
1. Condensate (liquid) may be produced in the reservoir from the gas
phase.
2. Solution gas may be evolved in the reservoir from the liquid phase.

5. Concept of Equilibrium
• Two phases reach the state of equilibrium when no changes-variation
of composition-occur with time if the system is left at the
prevailing constant pressure and temperature.
• Any multiphase system reaches equilibrium when it attains its
minimum energy level.
• The properties exhibited by any substance depend upon its phase,
namely, whether it is in solid, liquid, or gaseous phase.
• Substances can be classified into two types – pure or single
component, and multicomponent.
• Phase behaviour relationship can be determined from laboratory
pressure-volume-temperature (PVT) studies, or using theoretical /
empirical methods such as the use of equations of state ( PV = nRT)
• These relationships are frequently shown graphically as phase
diagrams.

6. Phase behaviour of a pure component
How isothermal pressure change affects fluid phase?
Fluid Pressure
Fluid Volume
Note: Isothermal pressure change is a common assumption for hydrocarbon production.

7. Phase behaviour of a pure component
How temperature affects fluid phase
(in a constant pressure)
A
B
C
D
Temperature
Fluid Volume

8. Phase behaviour of fluids
Liquid Phase
Solid Phase
Temperature
Pressure
Supercritical Phase
Gas Phase

9. Phase behaviour of a pure fluid (single component)
Liquid Phase
Temperature
Pressure
Supercritical
Phase
Gas Phase
Critical Point (CP)
Tcrit
Pcrit
The vapour pressure line separates the P-T stability field for liquid from
that for gas.
CP represents the critical point for the phase corresponding to the critical
pressure Pcrit and critical temperature Tcrit. Above this point the behaviour
of the two fluid phases are indistinguishable.

10. TWO-COMPONENT SYSTEM
P
T
PHASE 1&2
LIQUID
PHASE 1&2
GAS
PHASE 1
GAS
+
PHASE 2
LIQUID
For two separate individual phases, the vapour pressure lines are different.
Phase 1 represents a lower molecular weight alkane (e.g., ethane).
Phase 2 represents a higher molecular weight alkane (e.g., heptane).

11. oil condensate gas
Bubble-point Line
LIQUID
GAS
LIQUID
+
GAS
P
T
CB CP
CT
Phase Behaviour
of
a Multi-component
System
Dew-point Line
For a multi-component system, the bubble-point line divides the liquid
stability field from the liquid + gas field. The dew-point line divides the liquid +
gas field from the gas stability field.
The bubble-point (BPL) and dew-point (DPL) lines meet at the critical point
(CP).
CB = cricondenbar (max. P).
CT = cricondentherm (max. T)

12. PHASE BEHAVIOUR
CP
LIQUID
GAS
P
T
20%
X
40%
80% 60%
The various lines between the BPL and DPL are labeled with the
percentage of liquid in the liquid + gas stability field. On the BPL the fluid
is 100% liquid and on the DPL the fluid is 0% liquid (i.e., 100% gas.)
At the point X, the fluid consists of 70% liquid and 30% gas.

13. ISOTHERMAL OIL PRODUCTION
A
B
LIQUID
GAS
P
CP
T
40% 20%
80% 60%
In an oil reservoir, when an isothermal pressure drop occurs from A to
B, gas begins to be exsolved from solution in the liquid at the BPL.
Gas exsolution increases the compressibility of the reservoir and
makes liquid recovery less efficient. Pressure maintenance is
sometimes used to prevent liquid oil reservoirs from crossing the BPL.

14. D
E
CP
LIQUID
GAS
P
T
40% 20%
80% 60%
C A
B
ISOTHERMAL GAS PRODUCTION
Consider an isothermal reduction in pressure from point A to point B.
The fluid in the reservoir is dry gas throughout the P-T path.
Now consider an isothermal pressure drop from point C. At point D,
the P-T path crosses the DPL and liquid condenses from the gas. At
E, the P-T path recrosses the DPL and the condensate is
revapourized.

15. LIQUID
CP
GAS
P
T
“CONDENSATE” FLUIDS
Retrograde gas “condensate” fluids preferentially deposit the valuable,
heavier alkane fraction in the pore space when the P-T path crosses the
DPL. This loss of liquid moves the phase envelope down and to the
right in P-T space.
The result is that liquid is not revapourized and may not be recoverable.
Dry gas injection can prevent such losses by maintaining the reservoir
pressure above the DPL.

16. Phase enevelopes for natural gas
reservoir fluids
Initial conidtion within
the reservoir
From Ayala, 2006
Surface conidtion
conidtion within the
reservoir after
delpletion

17. Phase behavior of a Reservoir’s hydrocarbon during isothermal
production
Oil and Gas:
Two phase oil and/or gas below CP.
(Retrograde Gas) Condensate:
Single phase wet gas between the CP and CT.
Gas:
Single phase dry gas above CT.
Condensate

18. Schematic Pressure- Temperature Diagram
of a Binary Mixture
 The phase rule indicates that in a
binary vapour- liquid system, both
the temperature and the pressure
are independent variables
 The phase envelope, inside which
the two phase coexist, is bounded
by the bubble point and dew point
curve
 The two curves meet at the critical
(C), where all differences between
and two phases vanish and the
phases become indistinguishable
 Two phase can coexist at some
conditions above critical point
 The highest pressure (B) and the
highest temperature (D) on the
phase envelope are called the
cricondenbar and cricondentherm,
respectively
Pressur
e
Temperature
B
Critical
Point
C
Two Phase
Region
D

19. Vapor
Pressure
Curve for pure
Component A
Critical
Point
Cricondenbar
Two phase
envelope for
mixture A+B
Cricondentherm
Dew point
Temperature
Pressure
Pressure- Temperature Diagram for a Binary System
Bubble point
Vapor
Pressure
Curve for pure
Component B

20. Bubble Point
0% vapour,
100% liquid
A2
Critical
Point
Cricondenbar
Two phase
region
Cricondentherm
Dew point
100% vapour, 0% liquid
A1
Temperature
Pressure
Pressure- Temperature Diagram for a Binary System

21. As the pressure drops the compositions of both the liquid and the gas
phases change continuously: at the bubble point the first gas appears
and at the dew point vapour alone remains
One consequence of this behavior is that the pressure- temperature
plot is no longer a simple curve as for the single component; instead, it
is an envelope
The maximum pressure defined by this envelope is known as the
cricondenbar; above it, the liquid and gas phases cannot coexist
The maximum temperature defined by the envelope (the
cricondentherm) is one above which the two phase cannot coexist
The critical point is the point on the envelope at which the properties
of the gaseous and liquid phases become identical- it is not related in
any simple way to the cricondenbar or the cricondentherm
Also shown are lines of various volume percentage of the liquid phase

22. Pressure- Volume Diagram of Binary
Mixture
Pressur
e
C, Critical Point
Two Phase Region
Vapour
T <Tc
T3
Liquid
Volume
T >Tc
T =Tc

23. A series of expansions can be performed at various
constant temperatures and a pressure volume diagram
built up and the locus of the bubble point and dew point
values gives the bubble point and dew point lines which
meet at the critical point
Conditions under the bubble point and dew point lines
represent the conditions where two phase coexist whereas
those above these curves represent the conditions where
only one phase exist
At the critical temperature the P, T curve goes through the
critical point

24. Variation of Saturated Fluid Density
With Temperature
 The densities of vapour and
liquid phases approach each
other as the temperature
increases
 They become equal at
conditions known as the
critical point
 All the differences between
the phases are reduced as the
system approaches the
critical point
 The phases become the same
and indistinguishable at the
Critical
Point
Saturate
d Liquid
Saturate
d Vapour
Temperature critical point
Densit
y