PVT (Hydrocarbon Phase Behaviour)

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Petroleum Fluid Properties

Copyright of Core Laboratories (1999 Rev 001) Aberdeen Advanced Technology Centre

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CHAPTER 2.

HYDROCARBON PHASE BEHAVIOUR

2-1 BASICS OF PHASE BEHAVIOUR

Petroleum systems are known to exhibit multi-phase behaviour over extremely wideranges of pressures and temperatures. In fact hydrocarbon mixtures have been observedwith two or more phases in equilibrium at temperatures as low as -250 oF and higher than1,000oF. The molecular compositions of the gas and liquid phases in a reservoir rock aredifferent, and hence the physical properties of the phases differ. This difference in phaseproperties, in conjunction with the reservoir (rock) relative permeability characteristics,results in the complex behaviour of hydrocarbon reservoirs.

The phase behaviour of hydrocarbon systems takes on a further degree of complexity whenthe variables of pressure and temperature are introduced. To circumvent some of thecomplexity, a stepwise discussion of phase behaviour of hydrocarbons will be developedthrough the following topics:

- Basic Thermodynamics- Phase Behaviour of a Pure Component System- Binary (two-component) systems- Generalised multi-component systems

2-1-1

THERMODYNAMIC TERMINOLOGY

Before considering the effect of temperature and pressure on hydrocarbon systems, we willdefine some terms:

SYSTEM: amount of substances within given boundaries underspecified conditions.

COMPONENTS: The pure substances that make up the system. For thispurpose they will be considered as molecular species of fixedelemental composition and molecular weight.

PHASES: Physically and chemically homogeneous sub-systems

separated by definite boundaries, but which allow masstransfer to other phases.

EQUILIBRIUM: An isolated system is in thermodynamic equilibrium if theproperties of the system do not alter as long as the externalenvironment remains unchanged.

INTENSIVE

PROPERTIES: Physical properties are termed either intensive orextensive. Intensive properties are independent of thequantity of material present. i.e. Density, specific volume,viscosity and compressibility factor, pressure and

temperature


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EXTENSIVEPROPERTIES: Properties such as volume and mass are termedextensive; their values are determined by the total quantity ofmatter present.

2-1-2 PHASE BEHAVIOUR OF PURE COMPONENT SYSTEMS

To begin, it is useful to review the behaviour of a pure component under varying pressuresand temperatures to gain an insight into the behaviour of more complex hydrocarbonsystems. Figure 1 shows the pressure-temperature phase boundaries for a single-component system (CO2). The boundaries and their intersections are annotated as follows:

C

100 200 300 400

80

60

40

20

0

P/bar

T/K

(s+l)

(l+g)

(s+g) T

s l g

Figure 1: Phase Diagram for Carbon Dioxide


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Vapour Pressure Line (l+g)

For a pure substance, the vapour pressure line divides regions where the substance is a

liquid (l)from regions where the substance is a gas (g). Pressure and temperatures abovethe line indicate conditions for which the substance is a liquid; points below the linerepresent conditions under which it is a gas. Two other terms in common use in relationto the vapour pressure of a system are the bubble pointand the dew point. The bubblepoint for a liquid is the first appearance (bubble) of a gas phase on reaching the two-phaseequilibrium line, while for a gas the dewpoint is the first appearance of liquid (dew) at thetwo-phase line. For a pure component the bubble point and dew point are co-incident onthe vapour pressure curve; for a mixture there is an important distinction which will bediscussed in detail below.

Critical point (C)

The upper (p,T) limit of the vapour pressure curve is the critical point (C). The temperatureand pressure represented by this point are called the critical temperature, (Tc) and the

critical pressure, (Pc). For a pure substance Tc is the temperature above which a vapourcannot be 'liquefied' regardless of the applied pressure and above P c a liquid cannot be'vaporised' regardless of the temperature. For a pure substance above its critical point (ineither T or p), no distinction can be made between liquid and vapour phases; it is simplyreferred to as a 'fluid'. Consider the path shown around the critical point in Figure 2for apure substance confined over mercury in a visual PVT cell. The schematic diagramsillustrate the behaviour that would be observed, although the phase volumes are not toscale. It is possible to move from A to D via either: i) the sample undergoing a phasechange (A-E-D)or ii) the sample remaining monophasic (A-B-C-D).

A B C D

A D

}

E

Mercury

A

B C

DE

liquid gas


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Figure 2: Phase Behaviour of Carbon Dioxide around the Critical Point

Triple point (T)

This point represents the (only) pressure and temperature at which solid, liquid andvapour coexist for a pure substance under equilibrium conditions.

Sublimation Curve (s+g)

The sublimation curve is simply the vapour pressure curve for the solid. It is nota smoothcontinuation of the gas-liquid vapour pressure curve; there is a discontinuity in curvatureat the triple point. The line divides the area between solid (s)and vapour (g)and continuesdown to (T=0, p=0) on absolute scales.

Melting point Line (s+l)

This line divides solid from liquid. For pure hydrocarbons (as for most substances), themelting point generally increases with pressure so the slope of the line is positive. Water(and a few other substances) is exceptional in that its melting point initially decreases withpressure; ie: the melting point line has negative slope from the triple point. At very highpressures, the melting curve for water reverts to a positive slope (dp/dT>0). Unlike thevapour pressure curve, the melting line has no known upper limit analogous to the criticalpoint for liquid and gas.

2-1-3 USE OF PHASE DIAGRAMS

Consider the behaviour of a PVT cell charged with a pure substance and the volume variedby a piston at constant temperature as shown in Figure 3.Following the path 1-2-3-4, thegas initially undergoes compression and the pressure rises. When the vapour pressure isreached (at fixed temperature T) condensation of liquid commences (dewpoint). As thepiston is moved in further, the pressure remains constant and the volume of the gas phasedecreases, while the liquid phase volume increases. When the last bubble of gasdisappears (3) (bubble point), the pressure commences to rise again rapidly, as the liquidis compressed (4).


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2-5

Gas Liquid

LIQUIDCOMPRESSION

GASCOMPRESSION

2-PHASE REGION

P

V

1 2 3 4

1

23

4

Figure 3: p,V Behaviour of a Pure Substance

2-1-4 BINARY SYSTEMS

As described above, pure substances are unique in one sense in that their bubble and dewpoints are coincident on the vapour pressure curve. For binary (and all multi-componentsystems) vapour liquid equilibrium occurs within a phase envelope. The distinction isshown schematically in Figure 4. Note that the critical point is no longer necessarily atthe maximum temperature and/or pressure at which gas and liquid coexist. In terms of ap,V diagram for a mixture (Figure 5), the distinction between bubble and dew point at aparticular temperature is clearly apparent. As a result, mixtures exhibit a number ofadditional phase behaviour phenomena not obtainable with pure substances. By way ofexample a binary system will be considered in more detail.


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T

P

T

P

LL

GG

(L + G)

Critical Point

PURE SUBSTANCE MIXTURE

Figure 4: Transition of Vapour Pressure Curve for Pure Component to Phase Envelopefor Multi-component Systems

Ethane/n-Heptane Mixture

Figure 6is the pressure-temperature diagram for a mixture of ethane and n-heptane. Atall pressures and temperatures outside of the phase diagram, the mixture is in singlephase. At all pressures and temperatures within the phase diagram, the mixture exists in

two phases. Line A-Cis the bubble-point locus and B-Cis the dewpoint locus. As before,Cis the critical point where the properties of the liquid and gas phases become identical.

Two additional terms in common use to describe the pressure and temperature extremesof the phase envelope are the cricondenbarand cricondentherm(occasionally referred toas the maxcondenbar and maxcondentherm respectively).


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Gas Liquid

LIQUIDCOMPRESSION

GASCOMPRESSION

2-PHASE REGION

P

V

1 2 3 4

1

23

4

Figure 5: Volumetric (p,V) Behaviour for a Multi-

component System

b

b

a a

Single PhaseRegion(liquid)

bubb

lepointl

ocus Single Phase

Region(gas)

dewp

ointlo

cus

Two PhaseRegion

C

-100 0 100 200 300

200

400

600

800

1000

1200

T/ Fo

Pressure/psia

96.8 mol% ETHANE + 3.17 mol% HEPTANE

A

B

Figure 6: Phase Envelope for the binary system ( 0.968 Ethane

+ 0.032 Heptane)


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Cricondenbar (a---a)

The cricondenbar is defined as the maximum pressure at which gas and liquid can exist in

equilibrium.

Cricondentherm (b---b)

The cricondentherm is defined as the maximum temperature at which gas and liquid canexist in equilibrium.

Note that for this system the dewpoint curve has two solutions (one high pressure, one lowpressure) for a range of temperatures between the critical point and the cricondentherm.

This is a direct consequence of the critical point no longer being at the extremes of thephase envelope and that ethane/heptane system exhibits the phenomenon of retrograde

condensation. Retrograde condensation is commonly observed in many petroleumsystems and is illustrated more clearly in Figure 7.

(A)

(B)

(C)

(D)

(E)

T

P

(B) (C) (D) (E)

Figure 7: Schematic Representation of Retrograde Condensation

The path A-B-C-D-E is at constant temperature and crosses the dew curve twice. Theobserved behaviour is shown schematically at the bottom of the Figure. As the pressure isdecreased (volume increased) liquid is observed to condense from the single phase fluid.

The volume of liquid builds up to a maximum and as the system is expanded further, theliquid re-evaporates until the lower dewpoint is reached. A similar phenomenon can beobserved by varying the temperature and moving across the top of the phase envelope at a

constant pressure between the critical pressure and the cricondenbar. Again, the dewcurve is crossed twice and liquid is observed to 'condense' and 'evaporate'.

2-1-5 GENERALISED MULTI-COMPONENT SYSTEMS

Figure 8 is a typical pressure-temperature diagram of multi-component hydrocarbonsystems. The lines within the diagram boundary are called iso-volume or quality lines.

They describe the pressure and temperature conditions for equal volume percentages ofthe liquid phase. Note that the quality lines converge at the critical point.

As reservoir fluid is produced, reservoir pressure drops while reservoir temperature

remains constant. The reservoir fluid often enters the two-phase region. The location at


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which the constant temperature line enters the two phase region depends somewhat onthe reservoir temperature and heavily on the composition of the reservoir fluid.

C

Temperature

Pressure

10%2

0%30%4

0%50%

60%

70%

80%

90%

BlackOil

Volatile

Oil

CriticalFluid

GasCondensate

'Gas'

LeanGas

Condensate

Dry Gas

Wet Gas

Phase Envelope (Bubble & Dew Point Curves)

'Quality' Lines (Constant Phase Volume Fraction)

P,T Path of Producing Fluid

Figure 8: Generalised Phase Diagram for Reservoir Fluids


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2-2 COMPARISON OF MULTI-COMPONENT HYDROCARBON SYSTEMS

Figure 8illustrates various hydrocarbon systems and the general position at which they

enter a phase envelope. The compositions of fluids, which enter their phase envelopetowards the left boundary, contain more heavy hydrocarbons than those that enter theirphase envelope towards the right boundary. The fluids that enter to the left of the criticaltemperature exist as liquidsin the reservoir. Those, which enter to the right of the criticaltemperature, exist as gasesin the reservoir.

It follows that any suitable hydrocarbon mixture of fixed composition can exhibit all typesof phase behaviour highlighted in the figure, depending upon the temperature. But inpetroleum reservoirs temperature, pressure and fluid composition are not uncorrelatedparameters. Figure 9 illustrates the interrelation of composition, pressure andtemperature for the various types of fluid. Black and heavy oils contain very involatilematerial and their phase envelopes only extend to completion in principle. Attemperatures above 300 oC, thermal cracking of components prevents any experimental

determination of the phase envelope.

>350 Co

T

P

Dry GasesGas Condensates

Black Oils

Heavy Oils

Thermal Decomposition

Figure 9: Evolution of Reservoir Fluid Phase Diagrams with Composition

2-2-1 BLACK OILS

Figure 10 is the pressure-temperature diagram of a hypothetical black oil mixture.Assume the reservoir temperature is as shown. If the liquid volumes were measured asthe pressure was lowered, the resultant liquid shrinkage curve is also illustrated in theFigure. Note that the quality lines of Figure 10are essentially equally spaced at reservoirtemperature. This results in a liquid volume curve that approximates a straight lineexcept at very low pressures. This behaviour characterises what is commonly called an

"ordinary black oil".


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PHASE ENVELOPE

C(T , P )res re s

Temperature

Pressure

10%20

%30%40

%50%

60%

70%

80%

90%

0

20

40

60

80

100

Pressure

Liquid

Volume

(%)

RES. OIL VOLUME

Figure 10: Phase Envelope and Reservoir Oil Volume for a typical 'Black' Oil

Note that the phase envelope is a pressure-temperature diagram of the reservoir fluidunder constant composition conditions. However, the fluid in the reservoir that originallyexists as a single-phase liquid and subsequently exists as two phases, does not produceunder constant composition conditions. Below the bubble point pressure, both the fluidproduced and that remaining in the reservoir have variable compositions but constantreservoir volumes. The liquid volume curve represents the shrinkage of the liquid phaseoccurring in the reservoir under normal pressure depletion conditions.

Typical Black Oil Characteristics

API Gravity


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0

20

40

60

80

100

Pressure

Liquid

Volume(%)C

(T , P )res res

Temperature

Pressure

10%2

0%30%4

0%50%6

0%70%

80%

90%

PHASE ENVELOPE

RES. OIL VOLUME

Figure 11: Phase Envelope and Reservoir Oil Volume for a typical Volatile Oil

2-2-3 GAS CONDENSATES

Figure 12 illustrates a reservoir fluid where the reservoir temperature is located at theright of the critical point, but still to the left of the cricondentherm. Pressure depletion atreservoir temperature indicates the retrograde dewpoint is encountered. At a lowerpressure level, the depletion path encounters the phase boundary at the normal dewpoint

where essentially 100 percent gas exists.

However, this phenomenon cannot happen in the reservoir. Figure 12 is a pressure-temperature diagram of the reservoir fluid under constant composition conditions.However, the fluid in the reservoir which originally exists as a single-phase gas and whichsubsequently exists as two phases, does not produce under constant compositionconditions. Below the dewpoint pressure level, both the fluid produced and thatremaining in the reservoir have variable compositions but constant reservoir volume.Since the gas saturation is high, the relative permeability to the gas phase is virtuallyinfinite when compared to that of the liquid phase, thus causing the liquid phase toremain inside the reservoir.

The associated liquid volume curve at the bottom of the figure is a retrograde liquid curvetypical of a gas condensate system. Again it does not reflect directly the quality lines ofthe phase envelope because of the depletion process described. Gas condensate fluidsexist which condense trace amounts of less than 0.1 percent (wet gases) up to 55 percentretrograde liquids during pressure depletion of the reservoir.


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0

4

8

12

16

20

Pressure

Liquid

Volume

(%)

CONDENSATE VOLUME

Lean

Rich

C

Temperature

Pressure

10%2

0%30%4

0%50%

60%

70%

80%

90%

PHASE ENVELOPE

(T , p )res res

Figure 12: Phase Envelope and Condensate Volume for a Gas Condensate

2-2-4 NEAR CRITICAL SYSTEMS

Figure 13illustrates the similarity of near critical systems, whether the system is that of avolatile oil or of a gas condensate. The compositions of these two systems are similar andthe saturation pressures are similar; however, the original reservoir phases are different(one is termed an 'oil' and one is treated as a 'gas') at pressures above their saturation

level. At pressure levels within the two-phase region and very close to the saturationlevels, the two curves converge rapidly. At small pressure increments below thesaturation pressure of each system, the percentages of liquid and gas phases areessentially equal and follow similar liquid shrinkage.

The surface gas/liquid ratios of these two different reservoir systems are very similar,making it virtually impossible to infer whether the reservoir is oil or gas. In both cases,the gas phase saturation within the reservoir will be high and the gas phase will be theprincipal fluid produced.


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0 2 4 6

Pressure (kpsi)

0

20

40

60

80

100

LiquidV

olume

(%)

Gas Condensate

Volatile Oil

NEAR CRITICAL SYSTEMS

Figure 13: Liquid Volume Curves for Near Critical Systems

Because the primary fluid produced from either system is the gas phase, the phase isexamined thoroughly when laboratory reservoir fluid studies are made. A laboratory"constant volume depletion" study is routinely performed on these types of systems to

examine the produced gas in detail. This depletion is discussed further in the "ReservoirFluid Study - Gas Condensate" section of this manual.

2-3 TYPICAL SOLUTION GAS/LIQUID RATIO FOR VARIOUS HYDROCARBON

SYSTEMS

On first appearance Figure 14 appears similar to the pressure-temperature diagramsexamined in previous sections. However, it should be noted that the X-axis has beenchanged from temperature to Gas-Liquid Ratio(GLR), ie:composition. The oil systems areat GLR's to the left of the critical GOR, while the gas systems are located to the right of thecritical point.


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C

Composition (GLR)

Pressure

10%

20%

30%

40%5

0%60%

70%

80%90%

Figure 14: Generalised Pressure-Composition (p,x) Phase Diagram for Oil-Gas Systems

2-3-1 RESERVOIRS WITH GAS CAP AND ASSOCIATED OIL ZONE

Figure 15illustrates the relationship of the pressure-temperature diagram of a typical oiland a gas cap associated with the oil. The oil and gas each has its unique phase diagram,

yet they are related since the bubble-point pressure of the oil will be equal to the dewpointpressure of the gas cap. Moreover, both of these values will be equal to the static

reservoir pressure at the gas-oil contact. These relationships are theoretically true onlyat the gas-oil contact; however, they can be considered true from a practical viewpoint ifthe reservoir is small in area and is relatively thin.


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Tres

Pres

T

P

gasc

ap

reservoir oil

Figure 15: Interrelation between the Phase Envelopes of a Reservoir Oil and itsAssociated Gas Cap

Other Considerations

When reservoirs are large in area, or there is a tall hydrocarbon column, the composition(and hence the physical properties) of the hydrocarbon fluids will usually change withinthe reservoir. Such compositional changes occur for several reasons, such as an activeaquifer below the oil zone, secondary influx into the reservoir or simply due togravitational segregation. Samples should be taken to investigate if the need exists toaccount for these variations in subsequent reservoir engineering calculations.

Both the static and bubble-point pressures vary with depth in reservoirs with large verticalclosure. The rate of change with depth below the gas-oil contact is greater for the staticpressure. This results in continually increasing undersaturation of the oil as depthincreases. Conversely, the bubble-point pressure increases as a function of the reservoirtemperature increase with depth. However the effect of pressure dominates and the

overall saturation pressure of the fluid decreases with depth, in the hydrocarbon column.


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2-4 PHASE BEHAVIOUR OF PRODUCING FLUIDS

Up to this point we have considered in detail the phase relationships of fluids in thereservoir. However, once in the wellbore the temperature and pressure of the flowing fluid

begins to fall rapidly and it is normal for gas and oil to be produced from the wellhead,even if the fluid entered the wellbore single phase at the perforations. Figure 16illustrates this process schematically. The separation of gas and oil is occurringcontinuously, although during flow in the wellbore, they are in intimate contact andremain near to compositional equilibrium. Once in the separator they are still assumed tobe equilibrium phases as shown in Figure 17 before undergoing separate processing.Providing the separator conditions are stable, this assumption of equilibrium is usedroutinely to validate separator samples in the laboratory.

P

T

(a)

P

T

(a)

(b)

P

T

(a)

(b)

(c)

(d)

(d)

GAS

OIL

(c)

(c)

GAS

OIL

P

T

(a)

(b)

(c)

(d)

OIL

GAS

ProducingFluid SeparatePhases

RESERVOIR

STOCKTANK

SEPARATOR

(a)

(b)

(c)

(d)

WELLBORE

CHOKE

Figure 16: Phase Relationship for Producing Fluids


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Tsep

Psep

T

Psep

.gas

sep. oil

feed

Figure 17: Phase Relationship Between Separator Gas and Oil

PVT (Hydrocarbon Phase Behaviour)

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