Chapter3 Phase state of Reserviors Hydrocarbons and Gas-Liquid Equilibrium(59)

The characteristic of phase behavior

The equilibrium gas and liquid phases

The dissolution(溶解 ) and separation（分离） of system

Terminology:

Phase behavior: 相态 Bubble point ：泡点

Critical ponit ：临界点 Two phase region ：两相区Phase diagram: 相图 Dew point ：露点

Phase envelope ：相包络线 Quality lines: 等液量线Cricondentherm: 临界凝析温度 Cricondenbar: 临界凝析压力Locus ：轨迹 Shrinkage ：压缩 Condensate gas ：凝析气Curve ：曲线 Volatile ：挥发性的 Depletion ：衰竭，降低Retrograde gas condensate: 反凝析油 Separator ：分离器Equilibrium ratio ：平衡比

• Reservoirs may be radically different from each other in fluid composition.

diversified forms: gas reservoir

oil reservoir

oil reservoir with gas-cap

3.1.1Phase and the descriptive approaches of phase相态及其表示方法

Theoretically based on the thermodynamics( 热力学 ) and the physical chemistry,the theory of reservoir-fluid-phase usually takes a ”system”

as the object of study.

The term ”system” refers to the object which is artificially divided from the others for study and

conceived as a space enveloped( 包围 ) by boundaries.

3.1 Phase behavior of reservoir hydrovarbon fluids

For a constant(常量，不变的 ) composition system, the relationship between pressure, temperature and volume can be represented by EOS （ equation of state）

f(P ， v ， T) ＝ 0

Phase diagram is the graphic display of EOS.

( 将状态方程以图示法表示就是相图 )

3.1.1.1 3D phase diagram(P － V － T)

( 三维相图）

2 、 2D diagram 二维相图（ or Plane phase diagram 平面相图）

The P-T diagram is most convenient

P-V( 压力 - 比容图 ) and P-T( 压力 - 温度图 ) phase

diagram are generally used in petroleum industry.

2D phase diagram of propane( 丙烷的二维相图 )

C 1 ， %

C 2 ， %

C 3 ， %

C 4 ， %

C 5 ， %

C 6 ， %

C 7 ， %

4 8 .8 3

2 .7 5

1 .9 3

1 .6 0

1 .11 5

1 .5 9

4 11 5

6 4 .3 6

7 .5 2

4 .4 7

4 .1 2

2 .99 7

1 .3 8

1 4 .9 1

8 7 .0 1

4 .3 9

2 .2 9

1 .0 8

0 .88 3

0 .6 0

3 .8 0

9 5 .8 5

2 .6 7

0 .3 4

0 .5 2

0 .00 8

0 .1 2

0 .4 2

合计， % 1 0 0 .0 0 1 0 0 .0 0 1 0 0 .0 0 1 0 0 .0 0

C 7+ 分 子 量3 3气油比， m ／ m

3地 面 液 体 密 度 ， g ／ cm

液体颜色

225

111

0 .8 5 3 4

绿色

181

356

0 .7 7 9 2

桔黄色

112

3240

0 .7 3 5 8

浅稻草色

157

18690

0 .7 5 9 9

无色

3.1.1.3 Triangle phase diagram （ ternary diagram ） 三角相图 (P61)

组 分 黑 油 轻质油 凝析油 干 气

M1 M2 M3 ?

3.1.2Phase behavior of single/two-component system(P62)

3.1.2.1 The characteristic of single component system单组分体系的相态特征

1 、 Key parameters

(1)Bubble point pressure( 泡点压力 ): refers to pressure atwhich the first bubble occurs under constant temperaturewhen pressure decreases

(2)Dew point pressure( 露点压力 ) ： refers to pressure atwhich the first liquid dew occurs under constanttemperature when pressure increases

(3)Saturated vapor pressure curve( 饱和蒸汽压线 ) ：refers to the line showing that vapor and liquid phase

coexist for single component system

(4)Critical point( 临界点 ) ： refers to the end of saturated

vapor pressure curve (point C).

(5)Critical temperature (Tc) ， critical pressure (Pc) ：refers to the corresponding temperature and pressure at

the critical point

a 压 力 ， MPa

P-T diagram for ethane

C5 Pc

超临界区（气相区）F 。

液相区

4

3

2

1

0

-150 -100 -50 0 50 100 150

D 。。。BE

。A

蒸汽区Tc

温度， C

图 3—3 乙烷的 P-T 相图

(30C, 4.88MPa)

H2 SCO2

N2 CH4 C2 H6

C3 H8 C4 H1 0 C5 H1 2

a 压 力 ， MPa

3 、 P-T diagram for different pure system

109876543210

-200 -100 0 100 200 300

温度，℃

图 3—5 几种烷烃和非烃的饱和蒸汽压线

1.2 The characteristic of binary(二元的 ) component system双组分体系的相态特征

1 、 Key parameters(1)Bubble point curve( 泡点线 ): is defined as the line

separating the liquid phase region from the two phase region

(2)Dew point curve( 露点线 ):is defined as the line separating

the vapor phase region from the two phase region

(3)Cricondentherm( 临界凝析温度 ): is defined as the maximum

temperature above which liquid can not be formed regardless

of pressure.

(4)Cricondenbar( 临界凝析压力 ): is defined as the maximum

pressure above which no gas can be formed regardless of

temperature.

(5)Phase envelope ( 相包络线 ): the region enclosed by the

bubble-point curve and the dew-point curve, wherein gas and

liquid coexist in equilibrium, is defined as the phase envelope

of the hydrocarbon system.

(6)Quality lines( 等液量线 ): refers to the dash lines within the

phase diagram. They describe the pressure and temperature

conditions for equal volumes of liquids.

cricondenbar

2 、 P-T diagram for binary system

cricondentherm

3 、 Typical P-T diagram for a binary system

1) The two-phase region is between the saturated vapor line ofindividual pure substance.

2) The Pc is higher than one of pure substance, while Tc is betweenthat of both pure substance.

3) Point C will migrate to the side closer to the heavy substance withthe content increase of heavy one, accompanied with the P-T driftingto the saturated vapor pressure curve of heavy substance

4)The two phase region is larger if the proportion of both componentis close, contrary to the bigger proportion

1.3 The characteristic of multi-component hydrocarbon system(P66)

1 、 The key points:

1.Three phase regions: liquid

region, vapor region, two phase

region

2.Three points: cricondenbar,

critical point, cricondentherm

3. Three lines: bubble point

curve, dew point curve, quality

lines

4.Two iso-regions: isothermal

P － T diagramdecompression retrograde

region( 等温降压反凝析区 ),isobaric cooling retrograde

region( 等压降温反凝析区 )

Critical point( 临界点 ) ： intersection of bubble point curve

and dew point curve.

The Pc is the maximum pressure above which no gas can

be formed regardless of temperature (cricondenbar),

The Tc is defined as the maximum temperature above which

liquid cannot be formed regardless of pressure

(Cricondentherm).

isothermal decompression retrograde region( 等温降压反凝析区 )

isobaric cooling retrograde region( 等压降温反凝析区 )

2 、 The particular (P68)

isothermal decompression

retrograde region( 等温反凝析区 )

3 、 Judgment of reservoir types

GasVolatile oil Condensate gasBlack oil

Oil reservoirs—If the reservoir temperature T is less than the criticaltemperature Tc of the reservoir fluid, the reservoir is classified as an oilreservoir.Gas reservoirs—If the reservoir temperature is greater than the critical

temperature of the hydrocarbon fluid, the reservoir is considered a gasreservoir.

4 、 Phase behaviors of Typical reservoirs(P70)

1. Under-saturated oil

reservoir.

2. Saturated oil reservoir.

3. Gas-cap reservoir.

1 ） Phase diagram of conventional heavy oil reservoir( low shrinkage oil)

常规黑油油藏（低收缩油藏）相图1. The quality lines are closely

spaced near the dew-point curve,

2. The gas oil ratio is generally

lower than 90 m3/m3, and the

relative density is higher than 0.876.

The color of this type of oil is

generally black or dark brown.

3. The dash line simulates the path

of pressure and temperature

variation from wellbore to surface.

Oil accounts for about 85% at the

condition of separator.

2) Phase diagram of light oil reservoir (high shrinkage)

1. The phase diagram of

轻质油藏（高收缩油藏）相图

volatile oil reservoir is similarto that of black oil. But thequality lines are close togethernear the bubble-point and aremore widely spaced at lowerpressures.

2. The relative density is lowerthan 0.78, and gas oil ratio islower than 1800 m3/m3

3. Oil accounts for about 65%at ,.the condition of separator

5 、 Gas reservoirs

On the basis of their phase diagrams and the

prevailing reservoir conditions, natural gases can

be classified into four categories:

-Retrograde gas-condensate

-Near-critical gas-condensate

-Wet gas

-Dry gas

1 ） Phase diagram of retrograde condensate gas reservoir(P72)

反凝析气藏相图1. The initial reservoir pressureis higher than critical pressure ,but its temperature is betweencritical temperature andcricondentherm

2. The gas oil ratio ofcondensate gas reservoir couldreach 12600 m3/m3. The relativedensity of condensate oil couldreach 0.74. The color is light andtransparent.33. liquid accounts for about 25%under the condition of surfaceseparator. The produced liquid iscalled as condensate oil

2 ） Phase diagram of near critical gas condensate reservoir

近临界凝析气藏相图

1. The initial reservoir

temperature is near the

critical temperature

2. All the quality linesconverge ( 会聚 / 聚合 )at the critical point,

3.a rapid liquid buildup willimmediately occur below the

dew point as the pressure is

reduced to point 2.

3)Phase diagram of wet gas reservoir( 湿气气藏相图 )

1.The reservoir temperature isabove the cricondentherm ofthe hydrocarbon mixture, and itwill always keep gas statuswhen pressure decreases frompoint 1 to point 2.

2.It will be in two phase regionunder separator condition,therefore, there will be somelight oil in separator.

3.The relative density of lightoil is lower than 0.78, and thesurface gas oil ratio is lowerthan 18000 m3/m3.

4)Phase diagram of dry gas reservoir( 干气气藏相图 )

1.The natural gas withhigh methane content(70

％ ~98 ％ ） is referred asdry gas.

2.The reservoirtemperature andseparator temperature areoutside two phase region.There is no liquidreleased in undergroundand surface condition.The gas oil ratio is higherthan 18,000 m3/m3.

3.2 Gas-liquid phase equilibrium （ P73）汽 - 液相平衡

1 、 Ideal solution（溶液）

1) complete mixed between different constituents of

solutions;

2) no chemical reaction during mixing;

3) particle volume of each constituent is equal (that is ， the

repulsion force and attractive force between different

particle are equal)

2 、 Raoult’s Law ( 拉乌尔定律 ) ：The equilibrium partial pressure of any component in a multi-

component system is the product of its mole fraction xi in the

liquid phase and the vapor pressure of the pure component

pi:

pi xi pvi

where :pi = partial pressure of ith component.

pvi = vapor pressure of ith pure component.

xi= mole fraction of ith component in the liquid.

3 、 Dalton’s Law ( 道尔顿定律 )

The partial pressure pi of any component is the productof it’s mole fraction yi in the vapor phase and the total

pressure of the system p

pi yi p

where ： yi—mole fraction of component i ；p- total pressure of the system.

k

4 、 Vapor-liquid phase equilibrium of ideal solution理想溶液汽 - 液平衡计算

1) The equilibrium ratio

Raoult's and Dalton's law may be combined and written as

yi p xi Pvi

yi

xi

Pvi

Pk

f Li

f gi

yi

xi

Real solution at high pressure

yi ( f gi ) pure xi ( f Li ) pure

fugacity 逸度

The equilibrium ratio k:

The equilibrium ratio occurs underequilibrium conditions between vaporand liquid at any specified temperatureand pressure.

zi Ki∑yi ∑

2) Development of equations for calculating equilibrium relations

z j n x j nL y j ng

zi xi L yiV

xi zi

L KiV∑xi = 1 ∑yi = 1

zi

i

n n

1

Where : i 1 i 1 VKi L

L = moles of liquid at equilibrium V = moles of vapor at equilibriumzi xi yi = mole fraction of the ith component in the total/ liquid/ vapor phaseKi = yi/xi = equilibrium ratio of the ith component at the pressure andtemperature of the system

组分 组成，摩尔分数

丙烷丁烷戊烷

0.610

0.280

0.110

合计 Sum 1.000

Ex. 3-1: The composition of ideal liquor is listed in table

3-1, calculate the composition of vapor and liquid phase

under 65.5 (150 ) and 200 psia.℃ ℉

表 3-1

组分 组成Zj ， mole fraction

Pv j ,

Psia

yj ,

mole fraction

xj ,

mole fraction

C3 0.610 350 0.771 0.441

n-C4 0.280 105 0.194 0.370

n-C5 0.110 37 0.035 0.189

sum 1.0 y j 1 . 000 x j 1 .000

Solution:Step 1: Based on thechart of vapor pressure vs.T, presented thecomponent vapor

表 3-2

pressure, list at the 3thcolumn;;

组分 组成Zj ， mole fraction

Pv j ,

Psia

yj ,

mole fraction

xj ,

mole fraction

C3 0.610 350 0.771 0.441

n-C4 0.280 105 0.194 0.370

n-C5 0.110 37 0.035 0.189

sum 1.0 y j 1 . 000 x j 1 .000

∑j 1 y j ∑j 11 nL (

indicated that ∑y j 1

Step 4: The composition of each∑j 1

x j ∑1 ng (

m m

1−1)

z j

p

pvjj

Step 2: Assumed that nL=0. 487, use theequation yi to trial calculate, list at the4th;Step 3: If trial calculation is satisfied,

1

−1)

m

j 1

m

m

j 1

component in liquid xi is obtained.z j

pvj

p

表 3-2

zi∑y ∑ LV

zi Ki∑

∑z K

zi xi

Ki

pb ∑zi pvi

3) Calculate the bubble pressure

At the bubble-point pressure, p=pb ,it is consideredas a single phase liquid. L=1,V≈0

Based on the vapor equation ：

i①

1

② pvi

pb

Ki

n

i 1 VKi L

i i 1

n n

i 1 i 1

n

i 1

n

i 1

组分 组成， zj

Fractlon mole

饱和蒸汽压Pvj，Psia

zj Pv j

C3 0.610 350 213.5

n-C4 0.280 105 29.4

n-C5 0.110 37 4.1

Pb=247Psia

Ex. 3-2: Calculate the bubble point pressure of the above-

mentioned mixture (regarding as ideal liquor).

表 3-3

zi

∑i 1 xi ∑i 1 VK L 1

∑z / K

Ki

pd zi∑

4) Calculate the dew pressure

At the dew-point pressure, p=pd ,it is considered as asingle vapor phase. V=1,L≈0

Based on the vapor equation ：

n n

①i

zi yi

②

1

pvi

pd

ii

n

i 1n

1

i 1 pvi

组分 组成， zj fraction mole

饱和蒸汽压 ,

Pvj, Psia

zj ／ pvj

C3 0.610 350 0.00174

n-C4 0.280 105 0.00267

n-C5 0.110 37 0.00297

sum 1.000 0.00738

Ex. 3-3: Calculate the dew point pressure of the above-

mentioned mixture (regarding as ideal liquor).

最后 1

0 .00738136 Psia 。Pd

Calculating steps for saturated pressure

Yes

Given assumed Pb

Ki is obtained at Pb

No

calculatingΣniki=1

Yes

Output Pb

Given assumed Pd

Ki is obtained at Pd

No

Σni/ki=1

Output Pd

体 积

p5 p4 3

3 The solution and liberation of hydrocarbonsystem 油气体系中气体的溶解与分离 (P88)

3.1 liberation of system

p p2 p1

1 、 Flash liberation 闪蒸分离V5

V4

V3 Vb

V2

V1

pb

压力

The solution gas that is liberated from an oil sample during a stepdecline in pressure stays in the cell by contact with the oil, andthe change in the total hydrocarbon volume Vt is measured foreach pressure increment. (CCE)

1 、 Flash liberation 闪蒸分离

(CCE Constant Composition Expansion)

• Saturation pressure (bubble-point or dew-point pressure)

• Isothermal compressibility coefficients of the single-

phase fluid in excess of saturation pressure

• Compressibility factors of the gas phase

• Total hydrocarbon volume as a function of pressure

2 、 Differential liberation微分分离

The solution gas that is

liberated from an oil摩尔

液体

摩尔气体

液体

摩尔

液体

sample during a declinein pressure iscontinuously removedfrom contact with the oil,and before establishingequilibrium with the

（ b ） （ c ）（ a ）total hydrocarbonsystem. (CVD)

2 、 Differential liberation微分分离

(CVD Constant Volume Depletion)

• Amount of gas in solution as a function of pressure

• The shrinkage in the oil volume as a function of pressure

• Properties of the evolved gas including the composition of the

liberated gas, the gas compressibility factor, and the gas

specific gravity

• Density of the remaining oil as a function of pressure

3 、 multi-stage liberation (or separator)

多级分离

The solution gas that is liberated from an oil sample during a stepdecline in pressure is removed from contact with the oil, and thecell is constant.

A scheme using three stage separation at a fixed temperature

Optimum separation conditions

• A maximum stock-tank

API gravity

• A minimum oil

formation volume factor

(i.e., less oil shrinkage)

• A minimum producing

gas–oil ratio (gas

solubility)

Schematic illustration of n separation stages.

4 、 Comparison between flash liberation andmulti-stage separator 一次脱气与多级脱气的比较

reservoirs（ oil sample at the

gas oil ratio m3 ／ m3 Gas volumeincrement in

bottom hole ofproducing well ）

罗马什金次布仁斯克

新季米特里耶夫斯克

Flash

59.683.0175.4

Multi-stage

41.471.5144.5

different mothod

V ，％

30.613.917.6

In the flash (equilibrium) separation, the liberated

gas remains in contact with oil until its instantaneous

removal at the final separation pressure. A maximum

proportion of intermediate and heavy components is

attracted into the gas phase by this process, and this

results in a maximum oil shrinkage and, thus, a lower

oilil recovery.

In the differential separation, or multi-stage

separator, the liberated gas (mainly composed of lighter

components) is removed from contact with the oil as

the pressure on the oil is reduced. The maximum

amount of heavy and intermediate components will

remain in the liquid, minimum shrinkage of the oil will

occur, and, therefore, greater stock-tank oil recovery

will occur.

The gas volume released from oil sample by flash

liberation is more than by multi-stage separation, that

is the more gas oil ratio measured

The specific gravity of gas liberated from oil sample

by flash liberation is higher because of the more

molar fraction of light oil

3.2 The gas solution

1 、 Solubility

The gas solubility Rs is defined as the number of

standard cubic feet of gas which will dissolve in one

stock-tank barrel of crude oil at certain pressure and

temperature. The solubility of a natural gas in a crude

oil is a strong function of the pressure, temperature,

API gravity, and gas gravity.

2 、 influential factor about gas solubility

1) Temperature factor

Gas solution volume

reduces as the temperature

is increased, especially at

high pressure.

2) Pressure and API oil gravity factor

•Gas solution volume increases

as the pressure is increased

•With the increase of API oil

gravity, gas solution increases,

because of the composition of

hydrocarbon liquid is extremely

dynamic as P and T change.

3) Gas gravity (or component) factor

•Gas solution volume increases

as gas gravity is increased

4 、 The relationship between solution and liberation

1)The processes of solution and

liberation are opposite

and uniform. These two

processes occur at a specified

condition.

liberation curve and solution

curve by flash liberation are①all the same

Solution curve and②liberation curve by differential③liberation , two lines do not

coincide with each other

2) Two process lines of light component are

nearly the same whereas lines of heavier

component are just the opposite, especially

at low pressure.