Phase Behavior Notes

Reservoir Engineering 1

CGE 567

Department Oil & Gas, Faculty of Chemical Engineering

Phase Behavior

Department Oil & Gas, Faculty of Chemical Engineering

CGE 567

Reservoir Engineering

1

Fluid Chemical Composition

What is petroleum ?

Petroleum comes from the Latin word “petra” meaning rock or stone & oleum” meaning oil.

Petroleum includes both gaseous and liquid forms (sometimes they even exists as solids)

Petroleum

North Sea: An example of a crude oil.

Australia: An example of a light crude oil.

Utah: An example of Petroleum in solid state at room temperature.

• In the world around us, we can see that petroleum’s physical appearance varies from invisible gases to clear liquids to very dark and thick solids. What causes these variations in the physical properties of petroleum?

• The variation of the petroleum compound is a function of its chemical composition and the pressure and temperature that the petroleum substance is subjected to.

Petroleum

Petroleum Chemistry

• Consists of thousands of chemical compound mainly C & H molecules + other minor amounts of N2, CO2, S, O2.

• In chemistry compounds which contains Carbon are called Organic compounds eg Methane (CH4), Ethanol (C2H5OH) etc. The opposite term is called Inorganic eg Salt (NaCl), Laughing gas (N2O) etc.

• Petroleum can consists of chain of the smallest molecules ie CH4 or chains of the largest molecules up to C50++

Basically, hydrocarbons are divided into two groupings based on their structure ;

• Bonds between the carbon molecules

(single, or multiples )

• Arrangements of C molecules

(open chains or cyclic/rings)

Petroleum Chemistry

Hydrocarbons

Alkanes (Paraffins)

CnH22n+2

Alkenes CnH22n+2

Alkynes CnH22n-2

Cyclic Aliphatics

(Naphthenes)

Aliphatic Aromatics

Unstable

Hydrocarbon classification

Unstable Hydrocarbons - Unsaturated

• Unsaturated hydrocarbons (or olefins) have double or triple bonds between carbon atoms.

• Have the potential to add more hydrogen or other elements. Hence, structure is unstable

• Two types

alkenes

e.g ethylene

CH2=CH2

alkynes

e.g, acetylene

CH-CH

Stable Hydrocarbons - Saturated

• Paraffins

• Naphthenes

• Aromatics

Alkanes

• The simplest Hydrocarbon is methane, CH4. It is made up of 1 Carbon atom + 4 Hydrogen atoms.

• It is a member of a homologous series of hydrocarbons called alkanes which has a general formula of CnH2n+2

• Alkanes are also known as saturated hydrocarbons or paraffin hydrocarbons.

• Each successive member of the series has one more Carbon atom than the preceding member.

Alkanes

• The lighter ones are gases and used as fuels. The middle ones (7 Carbons to 12 Carbons) are liquids used in petrol (gasoline). The higher ones are waxy solids. Candle wax is a mixture of alkanes.

• Polythene is a very large alkane with millions of atoms in a single molecule. Apart from being flammable, alkanes are stable compounds found underground.

Alkanes

• In the alkanes, all four of the Carbon valency bonds are taken up with links to different atoms. These types of bonds are called single bonds and are generally stable and resistant to attack by other chemicals. Alkanes contain the maximum number of Hydrogen atoms possible. They are said to be saturated.

• The alkanes are mainly the primary hydrocarbon in the reservoir.

Alkanes or Paraffin Hydrocarbons No of carbon atoms Name State ( NTP* )

1 Methane Gas

2 Ethane Gas

3 Propane Gas

4 Butane Gas

5 Pentane Liquid

6 Hexane Liquid

7 Heptane Liquid

8 Octane Liquid

9 Nonane Liquid

10 Decane Liquid

C5 – C17 Liquid

C18+ Solid

Alkenes

• Another series of compounds is called the alkenes. These have a general formula:

CnH2n. • Alkenes have fewer hydrogen atoms than the

alkanes. The extra valencies left over occur as double bonds between a pair of Carbon atoms. The double bonds are more reactive than single bonds making the alkenes chemically more reactive.

• The simplest alkenes are: C2H4 - Ethene • Used as an industrial starter chemical. These

compounds are named in a similar manner to the alkanes except that the suffix is -ene.

Alkynes

• A third series are the alkynes. These have the

following formula: (CnH2n-2). • Alkynes have two carbon atoms joined by a tripple

bond. This is highly reactive making these compounds unstable.

• Examples of alkynes are: C2H2 - Ethyne Better known as acetylene which is used for welding underwater. These highly reactive substances have many industrial uses. • Again the naming of these compounds is similar to

the alkanes except that the suffix is -yne.

Carbon Rings

• Alkanes, alkenes and alkynes all contain Carbon atoms in linear chains. There are also hydrocarbons arranged in rings. Some examples follow: – C6H12 - Cyclohexane - A saturated hydrocarbon with the atoms

arranged in a hexagonal ring. In organic chemistry, the presence of Hydrogen atoms is often assumed and this compound can be represented by a hexagonal ring:

– C6H6Benzene - an industrial solvent. The Benzine Ring is one of the most important structures in organic chemistry. In reality, its alternate double and single bonds are "spread around" the ring so that the molecule is symmetrical. This structure is represented by a hexagon with a circle

– C7H8Toluene - an important solvent and starter chemical.

• Isomers are substances of the same molecule compositions but having different molecular structure and properties.

• Notice that both compounds of C4H10 contain 2 Carbon atoms and 10 Hydrogen atoms.

• C4H10 can exist as the straight chain butane molecule or the branched methylpropane. Both of these are shown below.

Isomers

Butane 2 Methyl Propane

Isomers

• Even though the atoms are the same, they are arranged differently. This yields two different compounds with the same number of atoms. These compounds are isomers and the phenomenon is called Isomerism.

• Isomerism increases the number of Organic compounds. The more Carbon atoms in a compound, the more ways of arranging the atoms and the larger number of isomers.

Isomers

One more example of a structural isomer is C6H10. This can exist as an alkene in the 1, 2 or 3 position and as a cyclic alkane.

Hex-1-ene Hex-2-ene Hex-3-ene Cyclohexane

The more Carbon atoms in a compound, the more ways of arranging the atoms and the larger number of isomers.

Structure of the Four Lightest Paraffin Series

Compounds

C

H

H H

H

Methane

C

C

H

H

H

H

H

H

Ethane

C

C

C

H

H H

H H

H

H H

Propane

C

C

C

C

H H

H

H H

H H

H

H

H

Butane

Structure Formula

C H C C C C C

H

H

H

H

H

H

H H

H H

H

H

H

Normal Hexane C6H14

(Paraffin Series)

C C C C C C H

H

H

H

H

H

H

H

H H H H

Normal Hexene C6H12

(Olefin Series)

Structure Formula

H

H H

H

H

H

C

C C

C

C C

Benzene C6H6

(Aromatic Series)

C

C

C

C

H H

H

H H

H H

H

H

H

Butane

Physical Properties of Hydrocarbons

• Formula CnH2n

• Sometimes termed cycloparaffins or alicyclic hydrocarbons.

• Single bonds but carbon chain is closed and saturated.

• Very stable

• Important constituents of crude oil.

• Properties similar to paraffins.

• Crude oil termed napthenic with high napthene content

CHEMISTRY OF HC - Naphthenes

• Aromatic series unsaturated closed-ring

• Formula CnH2n-6

• Based on the benzene compound.

• Characterised by strong aromatic odour.

• Various compound found in crude oil.

• Closed ring gives greater stability than open chain compounds.

CHEMISTRY OF HC - Aromatics

Structure Formula

H

H H

H

H

H

C

C C

C

C C

Benzene C6H6

(Aromatic Series)

C C

C

C

C

C

H H

H

H

H

H H

H H

H

H

H

Cyclohexene C6H12

(Napthene Series)

• Commonly found components are: – Nitrogen

– Carbon Dioxide

– Hydrogen Sulphide (H2S)

• Reservoir fluids that contain H2S are called sour gases/crudes.

• Reservoir fluids that are devoid of H2S are called sweet gases/crudes.

CHEMISTRY OF HC - Nonhydrocarbons

• They are unique phases that resulted from a particular pressure, temperature, compositional or chemical changes occurring in reservoir fluids.

• It can severely restrict flow of fluids • They includes:

– Gas hydrates (dirty ice) – Waxes (heavier paraffins) – Asphaltenes (aromatic, naphthenis compound with nitrogen,

sulfur and oxygen molecules

SOLID COMPONENTS

Hydrates

Asphaltene

Typical Compositional Analyses of a Crude Oil & A Natural Gas

Components Crude Oil Mole Fraction

Natural Gas Mole Fraction

C1

C2

C3

nC4

nC5

nC6

C7+

0.09

0.10

0.11

0.12

0.13

0.15

0.30

0.70

0.14

0.08

0.05

0.03

0.00

0.00

1.00 1.00

EXAMPLE OF FLUID COMP

COMPONENTS MOLE PERCENT

Methane 47.96

Ethane 5.66

Propane 5.87

i-butane 1.33

N-butane 2.18

i-pentane 1.09

N-pentane 1.04

Hexanes 2.05

Heptanes Plus 32.29

Nitrogen 0.2

Carbon Dioxide 0.33

Baronia RV2

COMPONENTS MOLE PERCENT

Methane 5.15

Ethane 1.81

Propane 2.81

i-butane 1.5

N-butane 1.44

i-pentane 1.30

N-pentane 0.89

Hexanes 2.08

Heptanes Plus 82.73

Nitrogen 0.06

Carbon Dioxide 0.68

Angsi I-35

Phase Behavior

INTRODUCTION Phase Behaviour

• Reservoir hydrocarbons exist as vapour, liquid or solid phases

• A phase is defined as a part of a system which is physically distinct from other parts by definite boundaries

• A reservoir oil (liquid phase) may change form into gas (vapour phase) during depletion

• The evolved gas initially remains dispersed in the oil phase until more and more gas is evolved. When this happens, large clusters will form and be mobile.

• Either mobile or not, both this condition is considered as a two-phase fluid.

Phase Behaviour….cont. • The subject of phase behaviour, however, focuses

only on the state of equilibrium, where no changes will occur with time if the system is left at the prevailing constant pressure and temperature

• A system reaches equilibrium when it attains it minimum energy level

• Fluids at equilibrium are also referred to as saturated fluids

• The state of a phase is fully defined when its chemistry, composition, temperature and pressure are specified

KEY POINTS

PHASE

PURE SUBSTANCE

STATE OF A PHASE

EQUILIBRIUM

PHASE BEHAVIOUR

Part of a system which is homogeneous and physically distinct from other parts by definite boundaries – gas, liquid, solid

Has a fixed chemical composition throughout

Behaviour of phases under different pressure and temperature

A state where there is no changes will occur with time if the system is left at the prevailing constant pressure and temperature

Defined by chemistry, composition, pressure and temperature

The Phase Diagram

• Phse diagrams are generally plots/graphs of pressure versus temperature (PT) OR pressure versus volume (PV).

• It is beneficial to study the behaviour of a pure hydrocarbon under varying pressure and temperature to gain an insight into the behaviour of more complex hydrocarbon system

• 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.

Phase Diagram Terminology

Solid

Pre

ssure

Temperature

Liquid

Liquid & Gas coexist

Gas/vapour

Liquid & Solid coexist

Gas & Solid coexist

Phase Diagram Terminology Vapour Pressure Line

Solid

Pre

ssure

Temperature

Liquid

Gas

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.

Phase Diagram Terminology Melting Point

Solid

Pre

ssure

Temperature

Liquid

C

Gas

Separates the pressure and temperature at which solid exists from the area where liquid exists. Conditions on the line indicates where solid and liquid coexist


Triple Point

Solid

Pre

ssure

Temperature

Liquid

C

Gas T

• 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 Line

Solid

Pre

ssure

Temperature

Liquid

C

Gas T

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

Phase Diagram Terminology Critical Point

Solid

Pre

ssure

Temperature

Liquid

Tc

C

The limit of the vapour pressure line

Defines the Critical temperature, Tc & Critical pressure, Pc of the pure substance

For pure component, it is the limiting state for liquid and gas to coexist

Gas

Pc

The point at which all intensive properties of the gas and liquid are equal

Pressure-Temperature diagram for ethane

Supercritical fluid

P-V Diagram for a Pure System

Pre

ssu

re

C - Critical Point

Vapour

Liquid

T1

T2

T3

Volume

Two Phase Region

3

4

1

2

Three Dimensional Phase Diagram for a Pure Component

C

A

D

B

Solid

Triple Point

Vapour

Liquid

Critical Point

Temperature

Pre

ssu

re

Mel

tin

g p

oin

t cu

rve

P-T Diagram for a Pure System

Definition Bubble point

The state of a system characteristic by the coexistence of a liquid phase with an infinitesimal quantity of gas phase in equilibrium

Bubble point pressure The fluid pressure system at its bubble point

Cricondentherm The maximum temperature at which liquid and vapour phases can

coexist in equilibrium for a constant composition, multicomponent system

Cricondenbar The maximum pressure at which liquid and vapour phases can coexist

in equilibrium for a constant composition, multicomponent system

Critical state The state of a system at which all properties of the coexisting vapour

and liquid phases become identical

Critical pressure and/ or temperature The pressure and/ or temperature in a hydrocarbon system at the

critical state

Definition

Dew point The state of a system characterized by the coexistence of a vapour

phase with an infinitesimal quantity of liquid phase in equilibrium

Dew point pressure The fluid pressure in a system at its dew point

Phase A homogeneous body of material which differs in its intensive

properties from its neighbouring phases

Producing gas: oil ratio, GOR The ratio of gas production rate to crude oil production rate

expressed as volume/ volume; for example, cubic feet per barrel measured under standard conditions

Properties, extensive and intensive Properties that are directly proportional to the quantity of

material making up the system are termed extensive properties. Those that independent of the quantity of material and therefore describe its condition at a particular state are termed intensive properties

Definition

Pseudo- critical pressure and temperature Fictitious critical pressure and temperature values

ascribe to a multicomponent system in order that the reduced pressure- volume- temperature states of the system conform to the reduced states of pure gases

Reduced pressure and temperature The ratio of pressure in a system to the critical pressure

(or pseudo- critical pressure) of the system. The reduced temperature is the ratio of the temperature of a system to the system’s critical pressure

Saturated liquid A liquid that is in equilibrium with vapour at a given

pressure and temperature state

Saturated vapour A vapour that is in equilibrium with a liquid at a given

pressure and temperature state

Definition Saturated pressure

The pressure at which vapour and liquid are in equilibrium (also bubble point pressure or dew point pressure)

Stock tank oil

Crude oil in equilibrium with a portion of its evolved gases at standard atmospheric conditions

Undersaturated fluid

A liquid or vapour capable of holding additional gaseous or liquid components in solution at the specified pressure and temperature

Vapor Pressure Curve for pure Component A

Critical

Point Cricondenbar

Two phase

envelope for

mixture A+B

Cricondentherm

Temperature

Pre

ssu

re

P-T Diagram for a Binary System

Vapor Pressure Curve for pure Component B

Bubble Point

0% vapour,

100% liquid

A2

Critical

Point Cricondenbar

Two phase

region

Cricondentherm

Dew point

100% vapour, 0% liquid

A1

Temperature

Pre

ssu

re

P-T Diagram for a Binary System

P-V Diagram for a Binary System

Pre

ssu

re

Two Phase Region

C, Critical Point

Vapour

Liquid

T <Tc

T3

Volume

T >Tc

T =Tc

P-T 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

Pre

ssu

re

Temperature

Critical

Point

C

Two Phase

Region

B

D

Multi- Component Hydrocarbon

Reservoir fluids contain hundreds of component and therefore are multicomponent system

The phase behaviour of multicomponent hydrocarbon systems in the liquid- vapour region however is very similar to that of binary system however the mathematical and experimental analysis of the phase behaviour is more complex.

Understanding the phase behavior of a binary system allows appreciation of the more complex multi-component systems.

Additionally, crude oils also contain appreciable amount of relatively non- volatile constituents which affect the systems phase behaviour such that dew point are practically unattainable

Phase Diagram For Multicomponent System

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 reduction Small liquid volume change

Bubble point pressure

P2

Small gas bubble in equilibrium

with liquid




P1 P2 P3 More gas phase.

Liquid volume decreases

Further volume expansion

Pressure remains constant


Bubble point pressure P2

P1 P2 P3 P4

Pressure remains constant

Dew point pressure P4

Small liquid drop in equilibrium with gas



Pressure-Temperature diagrams Single phase liquid at

P1 Bubble point pressure P2

Dew point pressure P4

P5 P1 P2 P3 P4

Further gas expansion

Pressure reduces


Phase Diagram of a Multicomponent Mixture P

ress

ure

Temperature

Critical Point

C Bubble Point Curve

Liquid Volume %

Dew

- P

oin

t C

urv

e

10 20 30 0

60

80

50

100

A

B

D

Phase Diagram of Segregated Oil & Gas in the

Vicinity of Gas/Oil Contact P

ress

ure

Temperature

Critical

Point

Critical

Point GC

OC

Gas Phase Envelope

Res.

Pres.

Oil Phase Envelope

Reservoir

Temp.

Pressure- Volume Diagram For A Two- Component Mixture

Bubble Point

Dew Point

Pre

ssu

re

Specific Volume

Pressure- Composition Diagram for Two- Component Mixtures

Combination of the composition and pressure which plot above the envelope indicate conditions at which the mixture is completely liquid

Combinations of composition and pressure which plot below the envelope indicate conditions at which the mixture is gas

Any combinations of pressure and composition which plot within the envelope indicate that the mixture exists in two phases, gas and liquid

The bubble- point line is also the locus of compositions of the liquid when two phases are present

The dew- point line is the locus of composition of the gas and liquid are in equilibrium

The line which ties the composition of the liquid with the composition of gas in equilibrium is known as an equilibrium tie- line

Tie-lines are always horizontal for two- component mixtures

Liquid

Pre

ssu

re, p

sia

Tie line

Gas

2 1 3

0 50 100

Composition, mole % component A

Typical pressure-composition diagram of a two-

component mixture with one tie line, 123

Consider that a mixture of composition represented by point 1 is brought to equilibrium at the indicated pressure and the temperature of the diagram

The composition of the liquid is indicated by point 2, and the composition of the equilibrium gas is given by point 3

The tie- line can also be used to determine the quantities of gas and liquid present at 1

The length of line 12 divided by the length of the tie- line 23, is the ratio of moles of gas to total moles of mixture

The length of line 13 divided by 23 is the ratio of moles of liquid to total moles of mixture

Pressure- Composition Diagram for Two- Component Mixtures

Liquid

Pre

ssu

re, p

sia

Tie line

Gas

2 1 3

0 50 100

Composition, mole % component A

Typical pressure-composition diagram of a two-

component mixture with one tie line, 123

Isothermal pressure- composition diagram of mixtures of methane and

ethane There are four saturation envelopes

corresponding to four different temperatures

The edge of the diagram labeled 100 mole percent methane represents vapor pressure of methane

The edge of the diagram labeled zero mole percent methane gives vapor pressures of ethane

When the temperature exceeds the critical temperature of one component, the saturation envelope does not go all the way across the diagram; rather, the dew point and bubble point lines join at a critical point

E.g., when the critical temperature of a mixture of methane and ethane is minus 100°F, the critical pressure is 750 psia, and the composition of the critical mixture is 95 mole percent methane and 5 mole percent ethane

Example

Determine the compositions and quantities of gas and liquid formed when 3 lb moles of mixture of 70 mole percent methane and 30 mole percent ethane are brought to equilibrium at –100 ºF and 400 psia

SOLUTION

Plot 70 mole percent methane and 400 psia on the –100 ºF saturation envelope

Draw the tie- line and read the composition of the equilibrium liquid on the bubble point line and the composition of equilibrium gas on the dew- point line

Calculate fractions of the gas and liquid from length of the tie- line

fraction gas = 70.0 – 52.2 91.8 – 52.2 = 0.45 lb mole of gas/lb mole total fraction liquid = 91.8 – 70.0 91.8 – 52.2 = 0.55 lb mole of liquid/lb mole total quantity of gas = ( 0.45) (3 lb mole)

= 1.35 lb mole gas quantity of liquid = ( 0.55) (3 lb mole)

= 1.65 lb mole liquid

SOLUTION

Component Composition of

liquid, mole percent

Composition of

gas, mole percent

Methane 52.2 91.8

Ethane 47.8 8.2

100.0 100.0

Ternary Diagram

0

0

0

100

100 100

% C7+ % C1

% C2 – C6

C7+

C1

C2 – C6

M

Three- Component Mixtures

Compositional phase diagram for three- component mixtures must be plotted in such way that the compositions of all three components can be displayed

Diagrams formed from equilateral triangles are convenient for this purpose These are called ternary diagrams

Component A

Ternary Diagrams

Each apex of the triangle corresponds to 100% of a single component The usual convention is to plot the lightest component at the top and the heaviest

component at the lower left Each side of the triangle represents two- component mixtures The left side of the triangle represents all possible mixtures of the light and the heavy

components Point within the triangle represents three- component mixtures Composition is usually plotted in terms of mole fraction or mole percent For a single diagram, both pressure and temperature are constant; only composition

change

Component A

Point 1 represents pure component B Point 2 represents a mixture of 30 mole percent component A and 70 mole percent

component C Point 3 represents a mixture which consists of 50 mole percent A, 30 mole percent B,

and 20 mole percent C The composition of the mixture represented by point 3 is best determined by imagining

three lines from point 3 perpendicular to the sides of the triangular diagram The length of line 43 represents the composition of component A in the mixture The length of line 53 represents the composition of component B, and the length of line

63 represents the composition of component C

Ternary Diagrams Component A

Line 21 represents a process of interest to the petroleum engineer Point 2 represents the composition of a mixture of component A and component C with

no component B present (A = 30 % & C =70 %) Line 12 represents the compositions of all mixtures formed by the addition of

component B to the original mixture of component A and C E.g., point 7 represent a mixture of equal parts of the original mixture if A and C with

component B The composition is 50% component B, 15% component A, and 35% component C The ratio of component A to C, 15:35, is the same as the ratio of A to C in the original

mixture, 30:70

Ternary Diagrams

Component A

Example

Determine the compositions and quantities of

equilibrium gas and liquid when 6 lb moles of a mixture

of 50 mole percent methane, 15 mole percent propane,

and 35 mole percent n-pentane are brought to

equilibrium at 160°F and 500 psia

Solution Plot composition of the mixture on the

ternary diagram for the given temperature and pressure (point 1)

Read composition of equilibrium gas at point where the tie- line through point 1 connects with dew- point line (point 2)

composition of gas: 14 mole percent propane 75 mole percent methane 12 mole percent n- pentane 100 mole percent Read composition of equilibrium liquid at

point where tie- line through point 1 connect with bubble-point line (point 3)

composition of liquid: 13 mole percent methane 17 mole percent propane 70 mole percent n-pentane 100 mole percent

Solution

Calculate fraction of mix which is gas fraction gas = 0.65 inches 1.07 inches = 0.607 lb mole of gas/lb mole total Quantity of gas = (0.607) (6 lb mole) = 3.6 lb moles Calculate fraction of mix which is liquid fraction liquid = 0.42 inches 1.07 inches = 0.393 lb mole liquid/ lb mole total quantity of liquid = (0.393) (6 lb mole) = 2.4 lb moles

Reservoir Fluids Classifications

Common Types of Petroleum

5 reservoir fluid The behavior of reservoir fluid during production is determined by

the shape of its phase diagram and the position of its critical point

5 types – black oil, volatile oil, retrograde gas, wet gas, and dry gas

Type of reservoir fluids have been define because each requires different approaches by reservoir engineer and production engineer Method of fluid sampling, type and size of surface equipment,

cal.procedure to determine oil and gas in place, plan of depletion, selection of EOR


Black oil ; Exist as liquid in the reservoir. Will exhibit bubble point behavior as pressure of

reservoir decreases through the field life.

As the liquid oil is being produced through surface, dry gas (mainly C1) will evolved due to pressure reduction.The gas remain gaseous through the reservoirs, tubular, separators till the surface.

The higher the number of API degree, the lighter is the oil. A rough classification of crude oil is sometimes used based on the API gravity. Conventional black oil has viscosity low enough to flow naturally into a well, usually in the range 20- 45°API, and is the most common form of reservoir liquid

Oil prices vary with specific gravity, heavy oil of less than 20 °API having relatively low value and lighter oil between 20 °API and 45 °API having progressively higher values

GORs are in the range 100-2000 scf/stb (20 – 360m³/ m³ ); specific gravity () from 0.6 to 1.0; and viscosities range from below 1 cp (liquids that are about as thin as water) to those that are > 100cp

They are black to green- black in colour

Black oil Volatile oil

Volatile oil

This has low specific gravities and viscosities, 45- 70 °API

GORs are in excess of 2000 scf/bbl (360m³/ m³)

They are pale red to brown in colour


Gas condensate

Hydrocarbon which are gaseous in the reservoir but which, when temperature and pressure are reduced, partially condense to yield condensate in liquid form

The hydrocarbons mixture gravity is usually above 45°API

The liquids that condense (6- 60 m³/ m³, 30- 300 bbl/ MMscf) are straw-coloured

If the condensation occurs in the reservoir fluid is termed a gas condensate fluid

This isothermal condensation behaviour is opposite to normal experience, and the phenomenon is known as retrograde condensation

Gas condensate reservoirs are an important class of hydrocarbon accumulation


Oil & Condensate from Australia

Natural gas ( gas )

A mixture of hydrocarbons that consist mainly of methane, but also includes ethane and minor quantities of natural gas liquids

Natural gas liquid ( NGL )

A light hydrocarbon that consist mainly of propane and and butane, which is liquid under pressure at normal temperature

Associated gas

The natural gas and NGL which, under reservoir conditions, are dissolved in the crude oil or are present as a gas cap above the reservoir


Sour A petroleum is considered sour when other substances such as sulphur compounds, carbon

dioxide and so on that are often mixed with the hydrocarbons in various proportions and caused problems in production and processing

Oil or gas is considered sweet if it contains few sulphur components

Sour natural gas contains an appreciable amount of hydrogen sulphide and carbon dioxide

if there is any measurable sulphur content (more than one part per million) then the sulphur components, particularly hydrogen sulphide (H2S), can cause considerable damage to the production facilities unless they are designed for, are poisonous to human, and lower the commercial values of the oil or gas

They therefore have to be extracted, but can be converted to sulphur and sold on as a useful product

The production equipment has to use special quality steels to prevent rapid corrosion

The water found in the reservoir at discovery is termed ‘connate water’ and can occupy 5 – 50% of the pore volume

It is also usually very salty ( sometimes more concentrated than seawater, 35000 ppm salts)


Liquid Density

• Specific gravity of a liquid

• API gravity

),(

),(

11

11

TP

TP

w

oo

5.1315.141

o

o API

Typical Compositions of Reservoir Fluids

Component Black Oil Volatile Oil Gas Condensate Wet Gas Dry Gas

C 1 48.83 64.36 87.07 95.85 86.67

C 2 2.75 7.52 4.39 2.67 7.77

C 3 1.93 4.74 2.29 0.34 2.95

C 4 1.60 4.12 1.74 0.52 1.73

C 5 1.15 3.97 0.83 0.08 0.88

C 6 1.59 3.38 0.60 0.12

C 7 +

42.15 11.91 3.80 0.42

M w C 7 +

225 181 112 157

GOR 625 2000 18,200 105,000 -

Tank o API 34.3 50.1 60.8 54.7 -

Liquid

Color Greenish

Black

Medium

Orange

Light

Straw

Water

White

-

Production Path

Wellhead Gas

Wellbore

Separator

Water

Oil

Reservoir

Oil Reservoir

Solution Gas

Stock Tank Oil

Rsi scf/stb

+

1 stb. oil

Bo res. Bbl. oil

Concept to understand •Undersaturated •Saturated

Production Path

Schematic Diagram of Stabilising Produced Oil As

Stock Tank Oil & Gas at Standard Condition

The reservoir fluid is produced and measured at the surface as the stock tank oil and gas at standard conditions, as shown schematically

Sep

ara

tor

Stock Tank

Oil

Gas

Reservoir

Oil

Gas

Reservoir Thermodynamic Engineering Data

Physical properties are needed accurately to describe the fluids for pressures up to 1500 bar ( 22000psia), the possibility of high temperatures (up to 250°C) and corrosive fluids (water more saline than sea water, which is approximately 35000ppm)

Empirical relationships are often used to extrapolate this physical understanding to applications to the real system

The comprehension of such complex natural fluids comes from an understanding of simple and ideal systems, which starts with visualization in the laboratory

The data required include density, compressibility, formation volume factors of oil and gas: oil ratios for determination of recovery factors, viscosity and gas: oil ratios for production rates, and interfacial tension for recovery efficiency, as it has a major influence on oil trapping

The Thermodynamic Path From Reservoir To Stock Tank

T & P

Formation Volume Factor

GOR

Density

Shrinkage

Bubble/ dew points

Flash/ differential

Viscosity

Flow rates

Stock tank

Ambient conditions

Reservoir

Up to 1500 bar, 250°C

water

separators

Up to 35 bar, 0 - 60°C gas

To sell

oil

Wel

l bore

Typical pressure, & temperature

Location Pressure (psia)

Temperature (oF)

Reservoir 500-10,000 100-300 (500+ thermal)

Separator 100-600 75-150

Stock tank 14.7 Ambient

Standard

Conditions

14.7 60

Faculty of Chemical Engineering

Overview of Reservoir Engineering

Phase diagram Reservoir fluid

Bubble point pressure – the pressure at which the first bubble of gas appears as the pressure of a liquid is reduced at constant temperature Dew point

pressure – the pressure at which the first drop of liquid appears as the pressure of a gas is increased at constant temperature

Critical point – the temperature and pressure at which the properties of liquid and vapor phase are identical


Hydrocarbon phase behavior

•Present as a single liquid in the reservoir {point (a)} •Remains a single phase liquid at the wellbore (significant reduction in pressure and small change in temperature during flow in reservoir) {point (b)} •Starts to evolve gas {point (c)} as temperature and pressure are reduced during flow up the tubing •Evolves increasing amounts of gas {points (d) and (e)} until the separator {point (f)} is reached



Phase diagram Oil reservoir



Phase diagram Oil Reservoir

If the initial Pres and Tres are at point 2 – oil is at its reservoir bubble point and is said to be saturated ( the oil contains as much dissolved gas as it can; and a further reduction in pressure will cause formation of gas)

If the initial Pres and Tres are at point 1 – oil is said to be undersaturated ( fluid behaviour in the reservoir is single phase – oil)

As the oil being brought up to the surface (separator) a further reduction on the pressure will increase a number of gas produced

BLACK OIL RESERVOIRS:

– GOR less than 1,000 SCF/STB

– Density less than 45 ºAPI

– Reservoir temperatures less than 250 ºF

– Oil FVF less than 2.00 (low shrinkage oils)

– Dark green to black in color

– C7+ composition > 30%

Classification of Reservoirs based on

Production and PVT data

(from Mc Cain’s textbook )

Phase Diagram of a Black Oil Reservoir

Temperature

Pre

ssu

re

Initial Reservoir

Conditions

CPPath of Production

Separator Conditions

25%

50%75%

VOLATILE OIL RESERVOIRS:

– GOR between1,000-8,000 SCF/STB

– Density between 45-60 ºAPI

– Oil FVF greater than 2.00 (high shrinkage oils)

– Light brown to green in color

– C7+ composition > 12.5%




Phase Diagram of a Volatile Oil Reservoir

Temperature

Pre

ss

ure

Initial Reservoir

Conditions

CP

Path of Production


75%

50%25%

GAS CONDENSATE RESERVOIRS:

– GOR between 70,000-100,000 SCF/STB

– Density greater than 60 ºAPI

– Light in color

– C7+ composition < 12.5%






Phase diagram Retrograde Gas Condensate Reservoir



Phase diagram Retrograde Gas Condensate Reservoir

Initially when Pres and Tres are at point 1– it is totally gas in the reservoir

As Pres decreases, the retrograde reach dew point at point 2

As Pres continue reduced, liquid condenses from the gas to form a free liquid in the reservoir

Liquid produced from retrograde gas reservoir often called as condensate

Phase Diagram of a Retrograde Gas

Temperature

Pre

ssu

re

Initial Reservoir

Conditions

CP

Path of Production


Temperature

Pre

ssu

re

Initial Reservoir

Conditions

CP

Path of Production


Temperature

Pre

ssu

re

Initial Reservoir

Conditions

CP

Path of Production


DRY GAS RESERVOIRS: – GOR much greater than 100,000 SCF/STB

– No liquid produced at surface

– Mostly compose of methane

WET GAS RESERVOIRS: – GOR > 100,000 SCF/STB

– No liquid is formed in the reservoir

– Separator conditions lie within phase envelope and liquid is produced at surface






Phase diagram Wet Gas Reservoir



Phase diagram Wet Gas Reservoir

A wet gas exist solely as a gas in the reservoir throughout the reduction in Pres

The pressure path (point 1 -2 ) does not enter the phase envelope

No liquid is formed in the reservoir

Separator conditions lie within the phase envelope causing some liquid to be formed at the surface

The 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.

Phase Diagram of Wet Gas

Phase Diagram of a Wet Gas

Temperature

Pre

ssu

re

Path of Production

Initial Reservoir

Conditions


CP



Phase diagram Dry Gas Reservoir



Phase diagram Dry Gas Reservoir

Dry gas is primarily methane with some intermediates

Hydrocarbon mixture is solely gas in the reservoir

Normal surface separator conditions fall outside the phase envelope thus no liquid is formed at the surface

Phase Diagram of a Dry Gas

Temperature

Pre

ssu

re

Path of Production

Initial Reservoir

Conditions


CP

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

Additional Guidelines

Reservoir Surface GOR range Gas specific API Typical composition, mole %fluid appearance gravity gravity C1 C2 C3 C4 C5 C6

Dry gas Colorless gas Essentially 0.60 - 0.65 96 2.7 0.3 0.5 0.1 0.4no liquids

Wet gas Colorless gas Greater than 0.65 - 0.85 60o-70o

with small amount 100 MSCF/bblof clear or strawcolored liquid

Condensate Colorless gas 3 to 100 0.65 - 0.85 50o-70o 87 4.4 2.3 1.7 0.8 3.8with significant MSCF/bblamounts of light- (900-18000 m3/m3)colored liquid

“Volatile” or Brown liquid About 0.65 - 0.85 40o-50o 64 7.5 4.7 4.1 3.0 16.7high shrinkage with various 3000 SCF/bbloil yellow, red, or (500m3/m3)

green hues

“Black” or low Dark brown 100-2500 SCF/bbl 30o-40o 49 2.8 1.9 1.6 1.2 43.5shrinkage oil to black (20-450 m3/m3)

viscous liquid

Heavy oil Black, very Essentially no gas 10o-25o 20 3.0 2.0 2.0 2.0 71viscous liquid in solution

Tar Black substance Viscosity >10,000cp <10o _ _ _ _ _ 90+

There are no definite boundaries between these classifications and usage may vary depending on location. Gravities and GOR are alsodependent on separation conditions.

Phase envelopes of mixtures with different proportions of same HC components

0

1000

2000

3000

4000

5000

6000

7000

Pre

ssu

re (

psia

)

-200 -100 0 100 200 300 400 500 600 700 800

Temperature o

F

Critical Points

Dry Gas

Wet Gas

Condensate

Volatile I

Black Oil

TR

Volatile I

Volatile II

Pres, Tres

Relative positions of

phase envelopes

Phase Behavior Notes

Documents

Transcript of Phase Behavior Notes