Phase Behavior of Hydrocarbon

Systems
Introduction
 Oil and gas reservoir fluids are mixtures of a large number of components which
when subjected to different environments of P & T may exist in different forms.
These forms are termed phases.
 Phase behavior is a key aspect in understanding nature and behavior of fluids
both in the reservoir and also during the production and transport process.
Definitions
System
 Amount of substance within given boundaries under specific conditions composed of a
number of components.
 If anything moves across the boundary system will have changed.
Components
Those pure substances which produce the system under all conditions. E.g. methane,
ethane, carbon dioxide, water
Phases
Separate, physically homogeneous parts separated by definite boundaries. E.g. water - ice,
liquid water and water vapor.
Equilibrium
The system is in equilibrium when no change takes place with respect to time in the
measurable physical properties of the separate phases.
Intensive Properties
Independent of the quantity of material present. e.g. density, specific volume and
compressibility factor
Extensive Properties
Dependant on the total quantity of material present. e.g. volume and mass
Impact of temperature and pressure
Temperature
 An indication of the kinetic energy of the molecules.
 A measure of the average kinetic energy of the molecules.
 Increases as heat is added.
 Causes an increase in the motion of the molecules.
 Molecules move further apart.
Pressure
 Reflects the frequency of collision of the molecules on the walls of the container.
 More molecules increases the pressure.
Intramolecular forces
 Forces are attractive and repulsive forces between molecules.
 Attractive forces increase as distance between molecules decreases until electronic field
of molecules overlap.
 Then further decrease in distance causes a repulsive force, which increases as
molecules are forced together.
Gases and Liquids
Gases
Molecules are widely spaced.
Attractive forces exist between the molecules.
Liquids
Molecules are closer together.
Repelling force which causes liquid to resist further compression.
Phase Behavior of Pure System.
Useful to examine pure systems to gain insight into more complex hydrocarbon
systems.
Phase diagrams
Plots of “pressure versus temperature” or “pressure versus volume”
Show the phases that exist under varying conditions.
P-T diagram for single component system
Vapor pressure line:
 Divides the regions where the substance is a liquid from regions where it is a gas
 Conditions on the line indicate where both liquid and gas coexist

Critical Point:
 The limit of the vapor pressure line
 Defines the critical temperature, Tc, critical pressure, Pc of the pure substance
 For pure component, the limiting state for liquid and gas to coexist.
 The point at which all intensive properties of the gas and liquid are equal

Triple Point:
Represents the pressure and temperature at which solid, liquid and vapor co- exist
under equilibrium conditions.
 It is an issue in the context of wax, ashphaltenes and hydrates.

Sublimation -Pressure Line:

Represents the pressure and temperature at which solid exists from the area where
vapor exists

Melting Point Line:

Represents the pressure and temperature at which solid exists from the area where
liquid exists. E.g. wax and hydrates.
 P-T Diagram: Consider behavior of a PVT (pressure, volume, temperature ) cell
charged with a pure substance and the volume varied by frictionless piston.
Bubble point
pressure P2 Further Small liquid drop in Dew point
volume equilibrium with gas pressure P4
Significant pressure expansion
reduction Small liquid
volume change More gas
Small gas bubble in phase. Further gas expansion
equilibrium with liquid Liquid
volume
Single phase decreases P4
Pressure reduces P5
liquid at P1 P2 P3
P1

Pressure Pressure
remains remains
constant constant
Pressure - Temperature Diagram
Pressure - Temperature diagram for ethane
Pressure Volume Diagram
1. Single phase liquid P1
T = Tc critical temperature
P2
Single phase region

Bubble point Reflects compressibility

first gas bubble of liquid

P4

P5 T<Tc
Dew point Last Two phase region
drop of liquid

2. All gas

4. T>Tc Smooth change from liquid to gas. No definite

phase distinction.
Pressure Volume Diagram Single phase

A series of
expansions at
various constant
temperatures yield
pressure volume
diagram.
Gives the locus of
the bubble point
and dew point
values.

Liquid

Two distinct phases Gas

Pressure Volume Diagram -Ethane
Three Dimensional Phase Diagram for a Pure Component
Two Component Systems
Such a system is called a binary
One component more volatile than the other.
Components are miscible
Two Component Systems: P-V diagram
Compositions
Component light
Component heavy

Liquid mixture

At bubble point
Liquid Gas

At dew point
Liquid Gas

Gas mixture
 P-V diagram for two component mixture.

The diagram is for a mixture of N-Heptane and N-pentane (52.4 mole% of Heptane)
 Two Component Systems: P-T diagram
Critical pressure of mixture

Critical pressure of A Critical point of mixture

Critical point of A

Critical temperature
of mixture

Critical point of B

Critical
temperature of B

Compared to a single line representing a broad region in which two phases co-exist
two phase behavior for pure substances, characterizes 2 component systems
Two Component Systems: P-T diagram

A specific mixture will give a

specific phase envelope.

Increase the proportion of A

and the envelope will change
shape and move to the left.

Increase the proportion of

B and the envelope will
change shape and move to
the right.

PCAB much greater than PCA and PCB

P-T Diagram for Mixtures of Ethane & Heptane

Critical points for various

mixtures

Pure component critical

values

Locus of the mixture

critical points
Critical point for a
series of binary HC
mixtures

Mixture of methane and

n-octane has a critical point much
greater than pure component values

Two phases exist within

this boundary

Methane is a significant
component of reservoir fluids

Pure component critical

values
Retrograde Condensation
Within the two phase region of our two component system there can be
temperatures higher than the critical temperature and pressures higher than
the critical pressure.
The cricondentherm.
Is the maximum temperature at which two phases can exist in
equilibrium.
The cricondenbar.
Is the maximum pressure at which two phases can exist in equilibrium.
Retrograde Condensation
Examine behavior at constant temperature between critical temperature and
cricondentherm of pressure decrease

Cricondenbar 1
At 1 single phase
flui
d
2 At 2 dew point
3

4 At 3 maximum
liquid

5 At 4 dew point

At 5 single gas
phase
Cricondentherm
Retrograde Condensation
Multi-component hydrocarbon
Reservoir fluids contain hundreds of components.
They are multicomponent systems.
Phase behavior in liquid-vapor region similar to binary systems,
Mathematical and experimental analysis of phase behavior is more
complex.
Multi-Component HC Phase Behavior

Liquid

Bubble point

Bubble point
Gas/40% liq’d Dew point

First gas bubble Dew point

Last drop of liq’d

All gas
Classification of Reservoir Fluids
 Black Oil
 Heavy Oil
 Low-shrinkage oil
 Volatile Oil
 High-shrinkage oil
 Retrograde condensate gas
 Wet gas
 Dry Gas
Phase Behavior of Reservoir Fluids
Single Phase region
Single Phase region Gas Gas
Liquid Condensate
Black Oil Volatile Oil
Gas
 Oil Systems-Black Oil
Above the bubble the Tc is higher than the reservoir temperature
fluid is termed
undersaturated

At the bubble point 1. Undersaturated

pressure fluid is
saturated

Single phase 2. Saturated Bubble point

between 1&2. pressure

Separator-two
phases 3. Two phases in reservoir
liq/gas: 85/15%.
Hence term low
shrinkage Separator

2>3 Reservoir fluid

composition changes
Oil Systems-Black Oil

Broad Phase Envelope

1. Undersaturated
High percentage of
liquid
High proportion of 2. Saturated Bubble point
heavy HC’s pressure
GOR<500scf/stb
3. Two phases in
Oil gravity 30 API or
o
reservoir
heavier
Separator
Liquid-black or deep
color
 Oil Systems-Volatile Oil
Tc is higher than the reservoir
temperature
Separator-two phases
liq/gas 65/35%.
Hence term high
shrinkage

Higher proportion of
lighter & intermediate
HC’s

Separator
 Oil Systems-Volatile Oil

Not so broad phase

envelope as black oil
Fewer heavier
hydrocarbons
Deep colored
API<50o
GOR<8000scf/
stb
Separator
Retrograde Condensate Gas
If reservoir temperature between critical point and
cricondentherm - a retrograde gas condensate exists A single dense
phase

Dew point
Maximum liquid
drop-out

Region of
retrograde
condensation

Dew point

Single gas phase

Retrograde Condensate Gas

Previously considered
liquid drop out is
immobile
Therefore lost to
production
If liquid drop out
is high
Gas cycling is a
possibility.
Very expensive
Gas Cycling explain and why cant we blow down reservoir

Preventing retrograde condensation in the

reservoir-keeping in single phase condition

Imported gas
Surface Separation

Gas
Condensate
Sales Dry Gas Reinjection
Condensate
Production Well

Gas Water Contact

Gas Cycling
The project should operate until dry gas break through.
Early gas breakthrough could occur due to reservoir heterogeneity
Dry gas less viscous than wet gas

Gas Imported gas

Surface Separation

Condensate Sales

Dry Gas Reinjection

Condensate

Production Well

Gas Water Contact

Gas Cycling
When gas break through occurs operated as a dry gas reservoir

Gas Sales through pipeline Exported gas

Production Well

Gas Water Contact

Can we not just blow the reservoir down ?
If we just deplete the reservoir will not the liquids vaporize and therefore be
produced?

When separation occurs in the

reservoir the reservoir fluid
composition changes causing the
mixture to get richer

The phase diagram moves to

the right

The components which vaporize

first are the light HC’s which we
do not need.
Retrograde Condensate Gas
Contains more lighter HC’s and fewer heavier HC’s than volatile oil API
up to 60o API
GOR up to 70,000 scf/stb
Stock tank oil is water-white or slightly colored.
Surface seepages- “white oil”.
Wet Gas
The phase diagram for a mixture containing smaller molecules lies below the
reservoir temperature.
 The reservoir condition always remains outside the two phase envelope
 ‘Wet’ because produces condensates.

 GOR<100,000 scf/stb
 Condensate liquid >50oAPI

Condensates produced in separator

Dry Gas
 The reservoir condition always remains outside the two phase envelope
 GOR>100,000 scf/stb
 ‘Dry’ because does not produce condensates

Separator lies outside two

phase envelopes
Relative positions of phase envelopes
Relative positions of phase envelopes
Reservoirs with a gas cap
Phase diagram of fluid
representing mixture of gas
cap in oil in their respective
proportions

Phase diagram of gas cap

fluid

Phase diagram of reservoir

liquid

Oil at bubble point

pressure

Gas at dew point

pressure