6- Properties of Dry Gases

1

PETPET--311 E311 EPetroleum Fluid PropertiesPetroleum Fluid Properties

Properties of Dry GasesProperties of Dry Gases

1

Dry gasesDry gases

Dry gases are the easiest to deal with because no liquid condenses from the gas as it moves from the reservoir to the surface. The composition of the surface gas is equal to the composition of the gas in thereservoir. Similarly, the specific gravity of the surface gas is equal to the specific gravity of the reservoir gas. Thus, a sample taken at the surface can be analyzed and the resulting composition can be used to todetermine the properties of the gas in the reservoir.

2

2

Gas formation volume factor (Gas formation volume factor (BBgg))

Definition :Definition :

Volume of gas at reservoir conditions required to produce one standard ft3 of gas at the surface.

Volume in reservoir Volume

at surface

cond.Std.

Reservoir

VVBg =

Production

3

Gas formation volume factor (Gas formation volume factor (BBgg) (continued)) (continued)

The volume of n moles of a gas at reservoir conditions may be obtained with the compressibility EOS :

SC

SCSC

PTRnzV =SC

The volume of the same number of moles of the gas at standard conditions :

R

R

PTRnzV =R

4

3

Gas formation volume factor (Gas formation volume factor (BBgg) (continued)) (continued)

Thus, the formation factor for the gas is

When the standard conditions are chosen such as TSC=520°R and PSC=14.65 psia and zSC=1.0, then

PTzPTz

PTRnz

PTRnz

VVB

SCSC

SC

SC

SCSCSC

Rg ===

)(00502.0

)(0282.03

scfBbl

PTzB

scfft

PTzB

g

g

=

=

5

Typical shape of gas formation volume factorTypical shape of gas formation volume factor

6

4

Isothermal gas compressibilityIsothermal gas compressibility

Isothermal gas compressibility Cg is defined as the fractional change of volume as pressure is changed at constant temperature. Cg may be written in different forms:

T

M

Mg

Tg

PV

VC

PV

VC

∂∂

−=

∂∂

−=

1

1

Units are psi-1.

7

Typical shape of isothermal compressibilityTypical shape of isothermal compressibility

8

5

Example #1Example #1

The following table gives volumetric data at 150°F for a natural gas. Determine the isothermal gas compressibility at 150°F and 1000 psia.

7.47.4800800

8.58.5700700

5.05.011001100

4.64.612001200

4.24.213001300

5.75.710001000

6.56.5900900

VVMM, ft, ft33/lb/lb--molemolePressure, Pressure, psiapsia

9

Solution #1Solution #1

psipsimolelbft

ftmolelbC

psimolelbftpsia

PV

g

F

M

10013.000723.07.5

1

00723.0800120007.718.41000@

3

3

3

150

=

−

−

−−=

−−=

−−

=

∂∂

•

•

10

6

Isothermal gas compressibility of an ideal gasIsothermal gas compressibility of an ideal gas

Ideal gas equation of state is

PPnRT

nRTPC

PnRT

VC

PnRT

PVPnRTV

nRTpV

g

g

T

1

1

2

2

2

=

−

−=

−

−=

−=

∂∂

=

=

11

Isothermal compressibility of real gasesIsothermal compressibility of real gases

The compressibility equation can be arranged to evaluate the partial derivative of volume :

Tg

Tg

T

T

Pz

zPC

zPzP

PnRT

znRTPC

P

zPzP

nRTPV

PznRTV

znRTpV

∂∂

−=

−

∂∂

−=

−

∂∂

=

∂∂

=

=

11

2

2

12

7

PseudoreducedPseudoreduced compressibilitycompressibility

The law of corresponding states can be used to express compressibility relation in terms of reduced properties. Pressure may be written as

prpc PPP =

prpc dPPdP =

If above equation is differentiated we get

In order to place the partial derivative of z into reduced form, the chain rule can be used

∂∂

∂∂

=

∂∂

pr

pr

Pz

PP

Pz

13

PseudoreducedPseudoreduced compressibility (continued)compressibility (continued)

Then the partial derivative of z can be written as

∂∂

=

∂∂

prpc Pz

Ppz 1

prTprpcprpc

g Pz

zPPPC

∂∂

−=11

Finally the gas compressibility takes the form of

or

prTprpr

pcg Pz

zPPC

∂∂

−=11

14

8


The product of Cg and Ppc is dimensionless. This product is called pseudoreduced compressibility Cpr.

prTprpr

pcgpr Pz

zPPCC

∂∂

−==11

The generalized z factor chart may be used to calculate the partial derivative of z. To simplify the slope calculation from the z chart the graph in the following page may be used. Methods presented in the previous chapters including adjustment for carbon dioxide and hydrogen sulfide can be used to estimate pseudocritical temperature and pressure.

15


16

9


Calculate the isothermal compressibility of 0.818 gravity gas at reservoir temperature of 220°F and reservoir pressure of 2100 psig.

First, determine the pseudocriticalpseudocritical propertiesproperties :

Tpc=406°R and Ppc=647 psia

Second, calculate the reduced propertiesreduced properties :

Tpr=1.68 and Ppr=3.27

Using the pseudoreducedpseudoreduced compressibility graphcompressibility graph

CprTpr=0.528

Cpr=0.528/1.68 =0.314

Cg=Cpr/Ppc =0.314/647 = 486 10-6 psi-1

17

Viscosity of gasesViscosity of gases

Viscosity is an important factor in petroleum and natural gas engineering calculations. Viscosity is the internal resistance exhibited as one portion or layer of a liquid is moved in relation to another layer. The molecules moving past each other cause internal friction. Insimpler terms, it is liquid’s resistance to flow. For the gases both temperature and pressure affect viscosity. Symbolically

dxdvAF

shearofratestressshear

//

==µ

whereµ : viscosity in poise (g/cm-sec)F : force in dynes (g-cm/sec2)A : area in cm2

v : velocity in cm/secx : distance in cm 18

10

Viscosity of gases (continued)Viscosity of gases (continued)

Usually viscosity is given in units of centipoise. The previous definition viscosity is known as dynamic viscosity. Kinematic viscosity is defined as dynamic viscosity divided by the density of the fluid :

3g g/cm

centipoisedensity

viscositydynamic==

ρµν

Gas viscosity decreases as reservoir pressure decreases. The molecules are simply further apart at lower pressure and move past each other more easily. The following page shows the change of viscosity with pressure and temperature.

19

Viscosity of gases (continued)Viscosity of gases (continued)

20

Increasing T

11

Viscosity of gas mixturesViscosity of gas mixtures

The following equation may be used to calculate the viscosity of a mixture of gases when the composition of the gas mixture is known and the viscosities of the components are known at the pressure and temperature of interest :

∑∑

=

iii

iiigi

g My

My

5.0

5.0µµ

The figure in the following slide may be used to obtain the viscosities of the usual components of natural gas at atmospheric pressure.

21

Viscosity of pure gases at atmospheric pressureViscosity of pure gases at atmospheric pressure

22

12


Calculate the viscosity of the gas mixture given below at reservoir temperature of 220°F and a pressure of one atmosphere absolute.

0.0400.040PropanePropane

0.0200.020nn--ButaneButane

1.0001.000TotalTotal

0.0900.090EthaneEthane

0.8500.850MethaneMethane

Composition, mole Composition, mole frfr..Component Component

Spreadsheet solution

23


If the composition of the gas is unavailable figure in the following page may be used to obtain viscosities of mixtures of hydrocarbon gases at one atmosphere. Small inserts on the graph show corrections to the value of hydrocarbon viscosity which may be applied to take intoaccount the effect of the presence of nitrogen and acid gases.

Results obtained from the figure agree with the experimental data within less than two percent. The presence of nonhydrocarboncomponents causes larger deviation.

24

13


25

Gas viscosity at high pressureGas viscosity at high pressure

In most instances, we are concerned with the viscosity of gases at pressures far removed from one atmosphere. The theory of corresponding states has been used to develop a gas viscosity correlation. This correlation is given in the next slides. Four figures are required to cover the specific gravity range. These figures give a viscosity ratio, µg/µ1, which is multiplied by the viscosity at one atmosphere to determine viscosity at high pressure.

As an example lets find the viscosity of the gas worked in Example #3 at 4000 psia.


26

14

Viscosity pressure correction (0.65Viscosity pressure correction (0.65--0.9)0.9)

27


28

15


29


30

16

Gas heating valueGas heating value

The heating valueheating value of a gas is the quantity of heat produced when the gas is burned completely to carbon dioxide and water. Heating value usually is expressed as British thermal units per standard cubic foot of gas, BTU/BTU/scfscf.

The petroleum and natural gas industry uses four adjectives to describe heating values :

• WetWet

•• DryDry

•• GrossGross

•• NetNet

31

Wet and dry explainedWet and dry explained

When used with heating values, the words wet and dry refer to the condition of the gas prior to combustionprior to combustion.

WetWet means that the gas is saturated with water vapor, about 1.75 volume percent.

DryDry means that the gas contains no water vapor. Sometimes, the termbone drybone dry is used.

32

17

Gross and net explainedGross and net explained

The words gross and net refer to the condition of the water of combustion after burning takes place.

Gross heating value Gross heating value is the heat produced in complete combustion under constant pressure with the combustion products cooled to standard conditions and the water in the combustion products condensed to the liquid state. This quantity is also called total heating total heating valuevalue.

Net heating valueNet heating value is defined similarly, except the water of combustion remains vapor at standard conditions. The difference between net and gross heating values is the heat of vaporization of water of combustion.

The value frequently used in the petroleum industry is gross heating value (dry).

33

Calculation of heating valuesCalculation of heating values

Ideal gas gross and net heating values (Lc)are given in Appendix A in your textbook for pure components. The values in Appendix A are on a dry basis. The ideal gas heating value of a gas mixture is calculated as

zL

L idealcc

,=

∑=i

ciiidealc LyL ,

Above equation can be used to calculate gross heating value or net heating value. In either case, the values must be converted from ideal gas to real gas heating values at standard conditions :

2)1(1 ii

i zyz −−= ∑Compressibility factor at standard conditions may be calculated as

34

18


Calculate the gross heating value of following separator gas

0.00320.0032NitrogenNitrogen

0.01670.0167Carbon dioxideCarbon dioxide

0.00030.0003CC7+7+

0.00270.0027HexanesHexanes

0.00270.0027nn--PentanePentane

0.00340.0034ii--PentanePentane

0.07510.0751PropanePropane

0.00890.0089ii--ButaneButane

0.01940.0194nn--ButaneButane

1.0001.000TotalTotal

0.15740.1574EthaneEthane

0.71020.7102MethaneMethane

Composition, mole Composition, mole frfr..Component Component


35

Converting heating valuesConverting heating values

Dry net heating value may be converted into wet net heating value by

)()0175.01()( dryLwetL cc −=

where 0.0175 is the mole fraction of water vapor in gas when saturated at standard conditions.

9.0)()0175.01()( +−= dryLwetL cc

Dry gross heating value may be converted into wet gross heating value by

where 0.9 accounts for the heat released during condensation of the water vapor which was in the gas prior to combustion.

36

19

JouleJoule--Thomson effectThomson effect

Temperature changes as pressure is reduced when a flowing stream of gas passes through a throttle like a valve, choke or perforations in casing. This iş called the Joule-Thomson effect. The change in temperature is directly related to the attraction of the molecules for each other. The equation

PCTz

zVT

Tp

P ∆

∂∂

=∆

gives the temperature change. Substitution of compressibility EOS into above equation results in

PC

VTVT

Tp

P ∆−

∂∂

=∆

37

JouleJoule--Thomson effect (continued)Thomson effect (continued)

The terms V, T, z, and Cp are always positive. Thus, the direction of the change in temperature depends on the sign of both the ∆P term and the temperature derivative of z at constant pressure.

Examination of generalized z factor chart gives us clues about the sign of the derivative. At moderate pressures (values of Ppr < 6.0) the sign is positive. Then ∆T is negative.

At high pressures (values of Ppr > 11) the sign is negative. Then temperature increases as pressure decreases. The effect is greater at higher pressures. This situation occurs when high pressure gas expands through perforations in the casing in a deep gas well. Temperature survey in the well shows hot spots where the gas expands and heats up.

At pressures below about 5000 psia, temperature decreases as pressure is decreased. The greatest temperature decrease occurs when the pressure decrease starts at values between 1500 and 2000 psia. This is observed as gas flows through the surface choke.

38

20

JouleJoule--Thomson effect (continued)Thomson effect (continued)

39

6- Properties of Dry Gases

Documents

Transcript of 6- Properties of Dry Gases