1.2 Properties of Natural Gases in High Pressure

8/21/2019 1.2 Properties of Natural Gases in High Pressure

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1.2 Properties of Natural Gas under high pressure

Definition of natural gas

Composition and classification

Apparent molecular weight and specific

gravity

Equation of state (EOS)

Volume factor

Compressibility coefficient

Gas viscosity


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Terminology

1Apparent molecular weight

2Specific gravity

3Density

4

Compressibility factor

5Isothermalgas compressibility coefficient

6Gas formation volume factor

7Viscosity

8Pseudosiu:deu-reduced parameters


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1 Definition of natural gasP24Generally speaking,natural gas is flammable or

inflammable gaseous mixtures of hydrocarbons and

nonhydrocarbons,extracted from the underground,atnormal temperature and pressure.

2.1 Apparent Molecular weight and

desinity of natural gas


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Narrowly speaking,natural gas can beclassified into different types:

(1)according to the essential features of the deposits()

a. oil reservoir associated gasb. gas from gas reservoir

(2)according to the composition

a. dry gas

b. wet gas

c. poor gas

d. rich gas

(3) according to the content of acidic gasesa. sour natural gas

b. clean natural gas


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2 Composition and classification

Chemical composition:

Hydrocarbon: methane, ethane,.heavier

Non-hydrocarbon:CO2,H2S,N2

Expressed:()

molar composition

volume composition

mass composition


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1mole fraction(mole percentage)

2volume composition i

N

i

i

ii

n

n

1

y

N

i

i

i

V

V

1

i


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3mass fraction

4The relationship of three expressions

N

i

i

i

i

w

wG

1

N

i

ii

iii

MG

MGy

1

/

/


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Example 2-1

compentmass fraction

Gimoleculeweight Mi

Gi/Mi Mole Fraction yi

CH4 0.71

C2

H6

0.14

C3H8 0.09

C4H10 0.06

Total 1.00


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The molecular weight of natural gas

1Apparent molecular weight M

It represents the natural gas mass at

reference conditions that 01atm,

the gas volume occupied by 22.4L of gas.

076022 4 L22.4


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Example 2-2

calculate the molecular weight of the natural gas

N

i

iiMyM1

)(

component mole fraction yi Mi yiMi

methane 0.85

ethane 0.09

propane 0.04

butane 0.02

total 1.00


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Density (P27)

the density of natural gas is defined as the mass

of per unit volume of the natural gas

V

mg


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3Specific Gravity

The specific gravity is defined as theratio of the gas density to that of the air.

Assuming that the behavior of both the gasmixture and the air is described by the idealgas equation, the specific gravity can then

be expressed as:

agg

r /

29/Mrg


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2.2 Equation of state (EOS)for natural gasand principle of corresponding state

Equation-of-state describes the exact

relationship between the variables P, V,

and T of natural gas.


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1Behavior of ideal gases

PV nRTideal gas1The molecule particles are no volume and no mass2these molecules have no attractive or repulsive forces

between them, and that all collisions() of molecules areperfectly elastic.


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2Behavior of real gases (P29)(2)modifier formulaexperimental method

In order to express a more exact relationship

between the variables P, V, and T, a correction

factor called the gas compressibility factor, gas

deviation factor, or simply the z-factor

actual

ideal

pV ZnRT

VZ

V


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Definition Z

The gas compressibility factor z is defined as the

ratio of the actual volume of n-moles of gas to the

ideal volume of the same number of moles at the

same T and P.


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As Zattractive and repulsive forces are

equalAs Zattractive forces is less than

repulsive onesreal gas is

more difficult to be

compressed than ideal gas.

As Zattractive forces is less than

repulsive onesreal gas is easier to

be compressed than ideal gas.


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3compressibility factor calculations

1single component gas

chart by laboratory measurement

methane


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ethane


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Pseudo-reduced pressure

Ppr:

Pseudo-reduced tempeature

Tpr

Ppc, Tpc = pseudo-critical pressure and temperature, respectively

pr

pc i ci

p pP

p y p

pr

pc i ci

T TT

T y T

:

pc i ci

pc i ci

Where

p y pT y T


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Pseudo-critical parameters calculations:

:

1 equation method2 graphical method:


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4solving steps of z-factor

1. Calculate PcTc

2. Calculate PrTr,,

given by the PT

3. Based on the Tr

Pr , presented a

generalized gas

compressibilityfactor chart as

shown in Figure


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example 2-3.

Table shows the composition of a certain natural gas.Please determine its Z

value at 65and 12MPa ,and give the volume of 1mol of this gas.

component mole fraction yi

critical

temperature

Tci,K

yiTci

critical pressure

Pci,MPa yiPci

CH4 0.85 190.5 4.6408

C2H6 0.09 306.0 4.8835

C3H8 0.04 369.6 4.2568

C4H10 0.02 425.0 3.7928

00.1iy


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( )

5Adjust the z-factor(P35)

1 N2(>20%>3%)

ZZa y N 2 Z N 2 + 1 y N 2 Z gZ C Z a


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Pc 1.8Pc Tc

2 CO2(>4%>5%)

Z ZCO2 yCO2 + (1 yCO2) Zg

CO2N2

Za ZCO2 yCO2 + yN2ZN 2 + (1 yCO2 yN 2 )Zg

3 H2S

1.8Tc + yH 2S (1 yH 2S)


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solving steps of z-factor(P37 2-5)

1. Calculate PcTc

2. Calculate PrTr,

given by the PT

3. Based on the Tr

Pr , presented a

generalized gas

compressibility

factor chart as

shown in Figure

2 3 Ph i l i f l


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2.3 Physical properties of natural gas

under High pressure (P44)

1Conception: The volume factor is defined

as the actual volume occupied by a certainamount of gas at a specified pressure andtemperature PT, divided by the volumeoccupied by the same amount of gas atstandard conditions 200.1MPa.

Unit: m3/m3 , ft3/scf Bg


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In fact ,the natural gas volume factor Bg

describes the conversion coefficient of the

gas volumetric change from the reservoir

state to the land-surface state.

P

Zpt

pT

ZTp

pnRT

p

ZnRT

V

VB sc

sc

sc

sc

scsc

g

293

)273( +


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1mol

0,760mmHg(0.101325MPa) 22.4L

20(293K)0.101325MPa 24.04L

20(293K)0.101325MPa 24.04L

Regulation of standard state in different country

and situation

20.0(68), 0.101325MPa

20.0(68),1atm

24.04L

24.04L

60

(15.6

)

30in Hg

760mmHg

23.68L60(15.6)30in Hg760mmHg23.68L


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2.3.2 Isothermal Compressibility coefficient (P45)

1Conception: the isothermal gas

compressibility is the change in volume per

unit volume for a unit change in pressure, in

equation form:

()

.().

Unit : 1/MPa ,1/psia

1

( )g TV

C V p


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1single component gas

For ideal gases ,since Z=1 ,we can know

21 ( ) [ ( ) ]

1 1

g TV p nRT Z C p ZV p ZnRT p p

Z

p Z p

1gC

p


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Cg (

)T

2poly-component gas

P Pc Pr

PPc Pr

1 1 Z

P Z P1 Z

Z Pc Pr

1 V

V P1

Pc Pr

c r r c

1 1 Z Cgr

P P Z P P


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Cgr


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Example

A hydrocarbon gas mixture has a specific

gravity of 0.72. Calculate the isothermal gascompressibility coefficient at 20MPa and 60oC

by assuming:

a. An ideal gas behavior

b. A real gas behavior

Solution

a.

(1/MPa)1P

1/ 20 0.05Cg


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4.4

1.5

Solution

b.Displaying in the chart

PPcTPc

20

4.56

273+ 60

220

P

Ppc

T

Tpc

Pr

Tr

Cgr 0.18

1/MPa0.18

4.56 0.0392Cg Cgr /Pc


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2.3.3 Viscosity of Natural Gas

The natural gas is a viscous fluid.Its viscosity is much lowerthan that of water and oil,hence the unit used for .The viscosity of gas is a measure of the internalgas friction (resistance) to flow.

Viscosities are expressed in terms of poises,

centipoise, or micropoises.1 poise =100 centipoises

=1106micropoises()

P


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Pr

1.6

11.44

(P, T ,yi )

1.2

10.8

0.6

0.4

0.2

0

0 2 4 6 8 10Tr

29

The gas viscosity can be estimated precisely from

empirical correlations. The relationship simply states thatthe viscosity is a function of pressure, temperature, and

composition.


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1viscosity of n tur l g sat low pressure

(1) The equation calculation method

1

3 v

the equation indicates that the viscosity of gas has a connection with the parameters


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(2) The graphical method

(1)Unlike that for liquids,

viscosity increases as

the temperature is

raised;

(2)Reduces with the

increase of molar

weight;

(3)Non-hydrocarbongas viscosity is more;

By Donald L, Katz et al.


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2visocosity of natural gas at high pressure

The characteristic of natural gas :

1increases as the pressure is raised

2reduces as the temperature is raised

3increases with the increase of molar weight,

in a similar way of liquid viscosity


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Step 1:

Obtain 1 ,

the viscosity ofthe natural gas

at one

atmosphere

and the

temperature of

Carrs atmospheric gas viscosity correlation.

(Permission to publish by the Society of PetroleumEngineers of AIME. Copyright SPE-AIME.)

interest from

the followingfigure.


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Step 2:

Calculate Pc, Tc, Ma from the specific

gravity or the composition of the gas

Step 3:

Calculate Pr, Tr


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is calculated by 1

Step 4:

From the Pr, Tr, obtain

1the viscosity ratiofrom the figure.

1 1

Step 5:

The gas viscosity ,

at the pressure andtemperature of interest

multiplying the

viscosity at oneatmosphere andsystem temperature,

by viscosity ratio.


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exercises

1 Given that a certain gas well is in4554m depth, in which formation

pressure is 54.414MPa, ground average

temperature is 17.1, geothermalgradient 1/50mcompressibility

factor of natural gas 1.148specific

gravity 0.574solves that the

natural gas density in foramtion.


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component yi Mi PciMPa TciK

CH4 0.902 16.0 4.54 190.6

C2H6 0.045 30.1 4.82 305.4

C3H8 0.031 44.1 4.19 369.8

nC4H10 0.021 58.1 3.75 425.2

2 The following table displays the composition,

molar weight, critical parameters of a

certain

temperature and pressure is 32 ,8.3MPa in formation

respectively

1solve the compressibility factor of natural gas

2volume factor of natural gas3given that daily gas production rate in gas well

10000m3at standard statesolve that the subsurface

volume

1.2 Properties of Natural Gases in High Pressure

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