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This lecture slides describes the properties of dry gas from the reservoir
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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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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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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
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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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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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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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