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RESERVOIR ENGINEERINGRESERVOIR ENGINEERINGPAB4523PAB4523

CHAPTER CHAPTER 55--33Properties of Dry Gas Properties of Dry Gas

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LESSON LEARNING OUTCOME

At the end of the session, students should be able to:able to:

Describe the properties of Dry gas commonly used by the petroleum engineer.

Estimate the values of the properties using normally available information about the dry gas.

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Dry gases are the easiest to deal with because no liquid condenses from the gas

BEHAVIOUR OF DRY GASES

because no liquid condenses from the gas as it moves from the reservoir to the surface.

The gas compositions at the surface and reservoir conditions are the same and reservoir conditions are the same and therefore, the specific gravity would also be the same for both.

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Dry gases are the easiest to deal with because no liquid condenses from the gas

BEHAVIOUR OF DRY GASES

because no liquid condenses from the gas as it moves from the reservoir to the surface.

The gas compositions at the surface and reservoir conditions are the same and reservoir conditions are the same and therefore, the specific gravity would also be the same for both.

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33

The gas formation volume factor (Bg) is defined as the volume of the gas at the reservoir conditions required to produce one standard cubic foot of gas at the surface

GAS FORMATION VOLUME FACTOR

produce one standard cubic foot of gas at the surface conditions.

The reciprocal of the Bg sometimes is called gas expansion factor.

The usual units for the Bg are:g

Reservoir cubic feet per standard cubic feet, res. cu ft/ scf.Reservoir barrels per standard cubic feet, res. bbl/scf.

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GAS FORMATION VOLUME FACTOR

res bbl/scf or res. cu ft/scfWhat are the

Volume of the gas at the Volume of the gas at the reservoir P&Treservoir P&T

Volume of the same Volume of the same mass of gas at the mass of gas at the

f P&Tf P&T

standard conditions???

Pressure= 14.7 psiTemperature = 60 F Temperature = 520 R

SC

Rg V

VB =

surface P&Tsurface P&T

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GAS FORMATION VOLUME FACTOR

SC

Rg V

VB =

The volume of n moles of gas at reservoir conditiong

The volume of n moles of gas at standard condition

R

RRR P

nRTzV =sc

scscSC P

nRTzV =

Thus, the formation volume factor is

RT ftcuzT .02820

sc

scsc

R

RR

g

PnRTzPnRTz

B =

Psc = 14.7 psia, Tsc= 520 R, z = 1

=scf

ftcuPzTB g

.0282.0=

scfbblres

PzTB g

.00502.0=

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GAS FORMATION VOLUME FACTOR

Bg

Pressure

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55

EXAMPLE 1

Calculate the formation volume factor of a dry gas with a specific gravity of of a dry gas with a specific gravity of 0.818 at reservoir temperature of 220F and pressure of 2100 psig.

First, estimate the Ppc and Tpc

S d C l l t th P d T d t th f tSecond. Calculate the Ppr and Tpr and get the z factor.

Third, Calculate the Bg using the following equation:

scfbblres

PzTB g

.00502.0=9

First, from this figureand at 0.818 gravityPpc = 647psia Tpc = 406 R.

SOLUTION TO EXAMPLE 1 T =220FP = 2100 psig

Tpc 406 R.

Second calculate the Tprand Ppr

PRPC

TTT

= PRPC

PPP

=

Resource: Resource: McCain textbook pg. 119textbook pg. 11910

68.1406

)460220(=

+=

RRTPR

27.3647

)7.142100(=

+=

psiapsiaPPR

66

Third, Find z value for the dry gas using this chart

SOLUTION TO EXAMPLE 1

Ppr = 3.27 and Tpr = 1.68

z = 0.855

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Resource: McCain textbook pg. 112

EXAMPLE 1

Calculate the formation volume factor of a dry gas with a specific gravity of 0.818 at reservoir temperature of 220F

d f 2100 igand pressure of 2100 psig.

Once Z factor is estimated, then you can use the following equation for calculating the gas formation volume factor

scfbblres

PzTB g

.00502.0=f

scfbblresB g.00138.0

7.142100)460220)(855.0(00502.0 =

++

=

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77

COEFFICIENT OF ISOTHERMAL COMPRESSIBILITY OF GASES

The coefficient of isothermal compressibility (Cg) is defined as

the fractional change of volume as pressure is changed at a constant

temperature.

The coefficient normally is referred

to as gas compressibility (Cg)

Compressibility factor ,z, is not the

compressibility as

com

pre

ssib

ilit

y,

Cg

compressibility.

The unit for Cg is psi-1

mg

m

1 V 1 Vc or V P V P

∂ ∂⎛ ⎞ ⎛ ⎞= − = −⎜ ⎟ ⎜ ⎟∂ ∂⎝ ⎠ ⎝ ⎠

Reservoir pressure

Ga

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VISCOSITY DEFINITION & UNITS

Viscosity is a measure of the resistance to flow exerted by a fluid

This is called dynamic viscosity and has units of centipoise = g mass / 100 sec cm

Kinematic viscosity is viscosity / density, units are incentistokes = centipoise /g/cc

= cm2 /100 sec

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VISCOSITY OF GASESVISCOSITY OF GASES

Gas viscosity decreases as reservoir pressure decreases. The molecules are a apart at low pressure and move past each other more easily.

At l i i t t i i it h At low pressures an increase in temperature increases gas viscosity whereas at high pressure gas viscosity decreases as the temperature increases

osity

(cp)

osity

(cp)

100oF150oF

200oF

Visc

oVi

sco

PressurePressure

T increasingT increasing

100oF

150oF

200oF

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VISCOSITY OF GAS MIXTURES

The viscosity of the gas mixtures can be calculated using the following

Equation. This equation is only used when the gas composition and

i it f h k viscosity for each are known.

∑∑= 2/1

2/1

jj

jjgjg My

Myμμ

=gμ Viscosity of gas mixtures

=

=

=

j

j

gj

g

M

y

μ

μViscosity of component j in the gas mixtures

Mole fraction of component j in the gas mixtures

Molecular weight of component j in the gas mixtures

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99

Calculate the viscosity of the gas mixture given below at 200F and a pressure of one atmospheric absolute.

EXAMPLE 2

Mw

My jjj2/1∑μ

16.04

30.07

44.10

58.12

cpMy

My

jj

jjgjg 0125.02/1 ==

∑∑μ

μ

Read Molecular Weights Read Viscosities this figureApply formula…

Resource: McCain textbook pg. 180 17

VISCOSITY OF GAS MIXTURES

When the compositions of the mixture are not available, the viscosity of the mixture can be estimated as a function of the gas specific gravity with the aid of the following chart.

Resource: McCain textbook pg. 182 18

1010

VISCOSITY OF GASES AT HIGH PRESSURE

atmratiog μμμ ×=

make sure you checkthe specific gravity range

g

atmμ

where

Can be obtained as presented previously

ratioμ

previously

Can be obtained as a function of Pprand Tpr with the aid of this chart.

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