SC RE Chap4- Phase Behaviour

PHASE BEHAVIOUR OF PHASE BEHAVIOUR OF HYDROCARBON SYSTEMSHYDROCARBON SYSTEMS

Adrian C Todd

Heriot-Watt UniversityHeriot-Watt University

DEPARTMENT OF PETROLEUM ENGINEERING

Heriot-Watt UniversityHeriot-Watt University

DEPARTMENT OF PETROLEUM ENGINEERING

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 behaviour is a key aspect in understanding nature and behaviour 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

Definitions

Phases Separate, physically homogeneous parts separated by

definite boundaries.

E.g. water - ice, liquid water and water vapour.

Equilibrium The system is in equilibrium when no change takes

place with respect to time in the measurable physical properties of the separate phases.

Definitions 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.

Impact of temperature and pressure Pressure Reflects the frequency of collision of the molecules on the walls of the

container.

More molecules increases the pressure.

Intramolecular 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 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 Behaviour 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.

Pressure - temperature diagram for single component system


Vapour 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 vapour 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.

General definition:

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 vapour co-exist under equilibrium conditions.

Not common for Petroleum engineers to deal with solid state. More recently 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 vapour exists.


Melting Point Line

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

where liquid exists.

Of relevance to wax and hydrates

Pressure - temperature diagrams

Consider behaviour of a PVT (pressure, volume, temperature ) cell charged with a pure substance and the volume varied by

frictionless piston.

P1 Single phase liquid at P1


Single phase liquid at P1

P1 P2

Significant pressure reductionSmall liquid volume change

Bubble point pressure

P2

Small gas bubble in

equilibrium with liquid


P1


P2

Bubble point pressure P2

P3

Further volumeexpansion

More gas phase.Liquid volume decreases

Pressure remains constant


P1


P2


P3


P4Dew point

pressure P4

Small liquid drop in equilibrium with

gas

Pressure remains constant


P1


P2


P3


P4

Dew pointpressure P4

P5Further gas expansion

Pressure reduces

Pressure - temperature diagramP1

P2

P3P4

P5

Behaviour across the vapour pressure line

From A to B via VP line

At A liquid phase

P1

Between A&B on VP line gas & liquid phases

P3

At B gas phase

P5

Behaviour around the critical point

At A liquid phase

At E compressed liquid phase

At G gas phase

At B gas phase

From E to F from liquid to gas with no

phase boundary

Beyond critical point

At 3 liquid phase ?

Dense phase fluid

At 4 gas phase

From 3 to 4 a change from liquid properties to gas properties with

no distinct phase change

Pressure - temperature diagram for ethane

Pressure Volume Diagram

1. Single phase liquid

Bubble pointfirst gas bubble

Dew pointLast drop of liquid

All gas

P1

P2

P4

P5

T = Tc critical temperature

T<Tc

Two phase region

Single phase region

T>Tc Smooth change from liquid to gas. No definite phase distinction.Reflects

compressibilty of liquid

Pressure Volume Diagram A series of

expansions at various constant temperatures yield pressure volume diagram.

Gives the locus of the bubble point and dew point values.

Liquid

GasTwo distinct phases

Single phase

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-Pressure Volume diagram

Isotherm similar to pure component

Pressure decreases as system goes from bubble

point to dew point

Composition of liquid and vapour phases change from

bubble point to dew point

Physical properties function of composition

e.g.density

Two Component Systems-Pressure Volume diagram

Compositions

Component light

Component heavy

Liquid mixture

At bubble point

Liquid Gas

At dew point

Liquid Gas

Gas mixture

Pressure volume diagram for specific two component mixture.

The diagram is for a 52.4 mole% mixture of

N-heptane and N-pentane

A ‘finger print’ of the mixture

Two Component Systems-Pressure Temperature diagram

Compared to a single line representing two phase behaviour for pure substances,

a broad region in which two phases co-exist characterises 2 component systems


Bubble point line and dew point line meet at critical point.

A mixture has a critical temperature and pressure.

These values are NOT the compositional averages of the critical values of the pure components.


Bubble Point

Dew point

Within 2 phase region proportions of liquid and gas. These are termed quality lines

By carrying out a series of constant temperature expansion phase envelope is defined


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

Pressure - Temperature Diagram for Mixtures of Ethane and Heptane

Critical points for various mixtures

Pure component critical values

Locus of the mixture critical points

Critical point loci for a series of binary

hydrocarbon

mixtures

Pure component critical values

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

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

Cricondentherm

Cricondenbar

Examine behaviour at constant temperature

between critical temperature and

cricondentherm of pressure decrease

1

2

At 1 single phase fluid

At 2 dew point3

At 3 maximum liquid

4

At 4 dew point5

At 5 single gas phase

Retrograde Condensation

Multi-component hydrocarbon Reservoir fluids contain hundreds of

components.

They are multicomponent systems.

Phase behaviour in liquid-vapour region similar to binary systems,

Mathematical and experimental analysis of phase behaviour is more complex.

Multi-component hydrocarbon phase behaviour

Liquid

First gas bubble

Bubble point Bubble point

Gas/40% liq’d

Last drop of liq’d

Dew point

Dew point

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 Behaviour of Reservoir Fluids

Black Oil Volatile Oil

Gas Condensate

Gas

Single Phase regionLiquid

Single Phase regionGas

Oil Systems-Black OilTc is higher than the reservoir temperature

Above the bubble the fluid is termed undersaturated

1. Undersaturated

2. Saturated Bubble pointpressure

Single phase between 1&2.

At the bubble point pressure fluid is saturated

Separator-two phasesliq’d/gas 85/15%.Hence term low shrinkage Separator

3. Two phases in reservoir

2>3 Reservoir fluid composition changes

Oil Systems-Black Oil

1. Undersaturated

2. Saturated Bubble pointpressure

Separator

3. Two phases in reservoir

Broad Phase Envelope

High percentage of liquid

High proportion of heavy HC’s

GOR<500scf/stb

Oil gravity 30oAPI or heavier

Liquid-black or deep colour

Oil Systems-Volatile Oil

Separator-two phasesliq’d/gas 65/35%.Hence term high shrinkage

Higher proportion of lighter & intermediate

HC’s

Tc is higher than the reservoir temperature

Separator

Oil Systems-Volatile Oil

Not so broad phase envelope as black oil

Fewer heavier hydrocarbons

Deep coloured

API<50o

GOR<8000scf/stbSeparator

Retrograde Condensate Gas

A single dense phase

Dew point

Maximum liquid drop-out

Region of retrograde

condensation

Dew point

Single gas phase

If reservoir temperature between critical point and cricondentherm - a retrograde gas condensate exists


Previously considered liquid drop out is immobile

Therefore lost to production

If liquid drop out is high

Gas cycling a possibility.

Very expensive

Gas Cycling Preventing retrograde condensation in the reservoir-keeping in single

phase condition

Imported gas

Production Well

Surface SeparationGas

Condensate

Condensate Sales

Gas Water Contact

Dry Gas Reinjection

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

Imported gas

Production Well

Surface SeparationGas

Condensate

Condensate Sales

Gas Water Contact

Dry Gas Reinjection

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

Exported gas

Production Well

Gas Sales through pipeline

Gas Water Contact

Can we not just blow the reservoir down ? If we just deplete the reservoir will not the

liquids vapourise 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 vapourise first are the light HC’s which we do

not need.

New Insights into Gas Condensates Research at Heriot-Watt Univ. has shown that

condensate liquids in the reservoir are more mobile than previously considered.

Depends on the relative position to critical point where interfacial tension forces are low.

At low IFT liquids move by gravity film flow causing flow at low liquid saturations.

A possible explanation for oil rims in gas condensate reservoirs


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 coloured.

Surface seepages- “white oil”.

Wet GasThe phase diagram for a mixture containing smaller molecules lies below the reservoir temperature.

The reservoir condition always remains outside the two phase

envelope

Condensates produced in separator

‘Wet’ because produces condensates.

Wet Gas

GOR<100,000 scf/stb

Condensate liquid >50oAPI

Dry Gas

The reservoir condition always remains outside the two phase envelope

Separator lies outside two phase envelopes

‘Dry’ because does not produce condensates

GOR>100,000 scf/stb

Relative positions of phase envelopes

Reservoirs with a gas capPhase 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

Critical Point Drying

Used to prepare delicate specimens

After conventional core cleaning some rock had higher than expected permeabilities.

Water injectivity test on well gave considerably lower permeability than conventional core cleaned rock.

Critical point drying reduces damage to rock

Critical Point Drying-Procedure Rock immersed in ‘formation water’ at well

sight.

Core recovered, prepared and loaded into core holder under formation water.

Critical Point Drying-Procedure

Water in core displaced with miscible fluid - alcohol.

Alcohol in core displaced with miscible fluid - high pressure carbon dioxide.

Pressure and temperature in core holder changed to go round critical point of carbon dioxide.

Gaseous carbon dioxide exists in core holder at end of test.

Critical Point Drying-Procedure

SC RE Chap4- Phase Behaviour

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