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Introduction to Petroleum Engineering
PE 210
FLUID PROPERTIES
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The understanding of fluid properties allows petroleumengineers to design and predict the behavior of ahydrocarbon reservoir more accurately.
Natural Gas
Gases often occur in association with crude oil, either asfree gas cap or in solution. This is called "associated gas".
Natural gas that occurs alone is called "non-associated gas".
Methane (C1) is always the major component and occurswith or without other lightl hydrocarbons, and with orwithout inorganic gases.
If natural gas contains less than 0.1 gallons of liquid/1000ft3it is called "dry gas" and this is generally corresponds to
>90% methane. The opposite is "wet gas".
FLUID PROPERTIES
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Natural Gas
Ideal Gas Law:
Relationships that describe the pressure-volume-temperature
(PVT) behavior of gasses are called equation of state.
What is the ideal gas? Is the one in which the volume occupied
by the molecules is small compared to the total volume, i.e.
Vmolecules= 0.0
All molecular collisions are elastic (no lose in energy)
No attractive or repulsive forces among molecules
Why do we study ideal gasses behavior?
To make the understanding of the behavior of real gasses easier.
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Ideal Gas Law
The simplest gas equation of state is a combination of two fundamental
laws:
Boyles Law: (At constant temperature)
OR
Charles Law: (At constant pressure)
OR
Combining the Boyles law and Charles law results:
Where R is the gas constant equals to 10.73 psia.ft3/(ibm-mole R). Note
that avogadro number = 2.73x1026molecules/mole.
pV 1 tconsPV tan=
TV tconsT
Vtan=
nRTPV =
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Gas Specific Gravity
Specific Gravity, g
Defined as the ratio of the density of a gas at a given temperature and pressure
to the density of air at the same temperature and pressure.
Then the specific gravity, g, of a gas is
RT
pair
97.28=
97.2897.28W
W
air
g
gM
RT
PRT
MP
===
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Real Gas Law
Since the volume of a gas will be less than what the ideal gas volume would be,
the gas is said to be supercompressible. The number, which is a measure of the
amount of the gas deviates from the perfect or ideal behavior, is sometimes
called supercompressibility factor, usually shortened to the compressibility
factor (z-factor).
Note that: The z-factor is obtained from figures
Z-factor at standard condition = 1.0
PandTatgasofmolesnofvolumeIdeal
PandTatgasofmolesnofvolumeActual
V
Vz
i
a ==
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Z Correlation
Most correlations are based on the law of corresponding state which applies to the
hydrocarbon gases as they are closely related chemically. The law states at the same Tr
and Prall hydrocarbons have the same values of Z.
The pseudoreduced temperature and pseudoreduced pressure are defined for pure gasses
as:
Z = f (Pr, Tr); therefore,c
rT
TT = , and
c
rP
PP =
(Pc, Tc): The condition above which liquid and vapor phases of the compound cannot be
distinguished.
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Z Correlation
Natural Gases: are mixture of different hydrocarbon gases
They dont have a True Pc, and Tc.
For mixtures, the pseudoreduced temperature and pseudoreduced pressure need a
modification in which the compositions of the gas are taken into the consideration.
Gas analysis is done at atmospheremole fraction = vol. Fraction (Yi= vi) at atmosphere.
Note that Vol. of 1 mole = 379 SCF for all gases.
MW, Pc, and Tcare mole fraction weighted; therefore,
ci
n
i
i
n
i
ciic PVPYP
==
==11
= =
==n
i
n
i
ciiciic TVTYT
1 1
==
==n
i
ii
n
i
iig MWVMWYMW
11
If gas-condensate is produced with the gas its mole fraction must be included in the
composition (since the condensate is a gas in the reservoir).
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Examples
xample (1): What is the volume of 15 pounds of methane at 100oF and 900 psi?
Solution: Given that: at 100oF, 900 psi, 15 # C, (Mwc= 16.04)
PV = ZnRT
n = 935.0#04.16
#15==
wM
W
Gas is C1Fig. (6): Z (900 psia, 100oF) = 0.92
PV = ZnRT
R=10.73, T=100 + 460
V= 5.73 ft3
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Examples
xample (2): If the gas in Example 1 is pure.
Solution: Fig. 9 can be used:
34.1673
900
)(
900===
crc
rP
P
63.1460116
460100=
++=rT
From Fig. 9 Z = 0.92
V = 5.73 ft3
If it is ideal gas (Z=1):
PV = nRT
V = 6.24 ft3
Diff. in value = 9%
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Examples
Example (3):A gas mixture consists of 16 lbs of methane and 7.5 ibs of ethane.
Calculate its volume at 0oF and 1000 psi?
Solution: PV = ZnRT
nT = nc1+ nc2
moleM
mn
wc
cC 1
16
16
1
11 ===
moleM
mn
wC
c
C25.0
07.30
5.7
2
2
2 ===
nT= 1+ 0.25 = 1.25 moleGas Mixture (Fig. 10) Z (Pr, Tr)Tc= Yc1, Tc1+ Yc2Tc2
Yc1= %8080.025.1
11 ===T
c
n
n
Yc2= %2020.025.1
25.02 ===T
c
n
n
Tc = 0.8 (344) + 0.2 (549) = 385oR
Pc = 0.8 (673) + 0.2 (712) = 680.8 psi
Tr = 2.1385
4600=
+
Pr = 469.18.680
1000=
From (Fig. 10) Z = 0.645
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Examples
xample (4:)Estimate Z for 0.8 SG gas at 1390 psia and 980F?
Solution: Pr= 1.2662
1390==
c
P
P
Tr= 35.1413
46098=
+=
cT
T
Pc, Tcfrom (Fig. 11, Fig. 12)
And from (Fig. 10) Z = 0.72
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Gas Formation Volume Factor, Bg
Relates the actual volume of gas at P and T of the reservoir to the actual volume of gas
at standard conditions, pscand Tscon the surface.
sca
TPa
gV
VB
)
)
Scatgasofvol.actual
TP,atgasofvol.actual ,==
Where pscis 14.7 psia and Tscis 60 F,
pTZTpBsc
scg =
=
SCF
ft
p
ZTBg
3
,02829.0
=
SCF
bbl
p
ZTBg ,00504.0
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Gas Formation Volume Factor, Bg
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Isothermal Compressibility
A measure of change in volume with pressure under isothermal conditions.
Since, Therefore:
Note that the above equation can be solved if we know the slope of z-factor plot.
Viscosity:
The viscosity of natural gas depends on the temperature, pressure and composition
of the gas. It has units of (cp).
Carr, Kobayashi, and Burrows have developed correlation charts for estimating the
viscosity of natural gasses.
P
ZnRTV=
2P
ZnRT
P
Z
P
nRT
P
V
=
PP
ZnRT
P
Z
ZP
ZnRT
P
V 11
=
PP
Z
ZP
V
V
111
=
P
V
Vc
=
1
P
Z
Zpcg
=
11
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Crude Oil
A very complex mixture with hundreds of
thousands of compounds. Predominantly
hydrocarbons but also compounds of nitrogen,sulfur and oxygen. The mixture existed in a
liquid phase in an underground reservoir and
remains a liquid at surface after passingthrough surface separation equipment.
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Appearance: Ranges from black, dark brown,greenish through pale colors to water white.
Density: Most oil is less dense than water. Althoughany appropriate units can be used for density, it ismost commonly given as API gravity (AmericanPetroleum Institute)
Light oil are > 40oAPI (0.83 S.G)
Heavy oil are < 25oAPI (1.00 S.G)
( )5.131.
5.141
60/60=
FoGSAPI
Crude Oil Properties
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Crude Oil Properties
Solution Gas-Oil Ratio, Rso
It is a measure of the solubility of natural gas in crude oil.
Depends on the pressure, the temperature, and the composition of the gas and the
crude oil.
For a particular gas and crude oil at constant temperature, the quantity of solution
gas increases with pressure; and at constant pressure the quantity decreases with
increasing temperature. For any temperature and pressure, the quantity of solution
gas increases as the composition of the gas and crude oil approach each other.
A crude oil is said to be saturated with gas at any pressure and temperature if on a
slight reduction in pressure some gas is released from solution. Conversely, if no gas
is released from solution, the crude oil is said to be undersaturated at that pressure.
Rs: gas solubility: No. of SCF of gas which are in solution in 1 STB at reservoir T, P. Or
it is the volume (in SCF) of gas dissolved in 1 STB of oil , SCF/STB.
producedoilofSTB
SCFRs
producedgasof=
Bubble Point Pressure or Saturation Pressure: It is the pressure of a liquid system at
which gas starts bubbling.
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Solution Gas-Oil Ratio
Bubble point: is the condition at which the first bubble of gas
appears.
Dew Point: is the condition at which the first drop of liquid
condenses. It may have a unit of standard cubic feet of solution gas per
stock tank barrel of oil (SCF/STB).
Values of Rs ranges from 50 to 1400 SCF/STb. For most oil
reservoirs, the value of Rs falls between 200 to 1000 SCF/STB. Rs can be obtained from laboratory measurements or can be
estimated from correlation.
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Solution Gas-Oil Ratio
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Oil Formation Volume Factor, Bo
Relates the actual volume of oil at P and T of the reservoir to the actual
volume of oil at standard conditions, pscand Tscon the surface.
The reservoir volume includes dissolved gas, whereas the surface volume
is essentially dead oil and does not include the released gas.
It has units of reservoir barrels of oil per stock tank barrel of oil.
sco
TPo
oVVB
))
Scatoilofvol.actualTP,atoilofvol.actual ,==
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Oil Formation Volume Factor, Bo
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Viscosity
A measure of internal resistance of fluid to flow.
It has a unit of cp.
Below bubble point, the viscosity decreases with increasing pressure owing to
the thinning effect of gas entering solution, but above the bubble point, the
viscosity increases with increasing pressure.
Dead oil is defined as oil with no dissolved gas.
Live oil is defined as oil with dissolved gas.
Values vary widely from the very low value typical of gas to very high
values. In general, low API gravity oils tend to have higher viscosities.
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Water
Water is the dominant fluid in the subsurface. Thetotal quantity of dissolved salts varies from lowvalues in shallow fresh water to more than 400,000mg/liter (ppm) in some deep waters. Values are
usually reported as "Total Dissolved Solids" (TDS).Sea water has 35,000 mg/liter.
Chemical Composition of Reservoir Waters: Waters
are usually analyzed for Chloride (CI) Sulfate (SO4),bicarbonate (HCO3), Carbonate (CO3), Potassium(K+), Calcium (Ca++), magnesium (Mg++), andsodium (Na+).
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Water Properties
Solution Gas-Water Ratio, Rsw
It is a measure of the solubility of natural gas in water.
McCain developed a correlation for the solution gas-water ratio, Rsw(SCF/STB).
Water Formation volume factor, Bw
Relates the actual volume of water at P and T of the reservoir to the actual volume o
water at standard conditions, pscand Tscon the surface. It has units of bbl/STB.
In most cases, the water formation volume factors will be approximately 1.0.
Viscosity
A measure of internal resistance of fluid to flow. It has a unit of cp.
Pure water has a viscosity of 1.0 cp at standard conditions.
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Two phase Formation volume factor, Bt
Relates the actual volume of Liquid and gas at P and T of the reservoir to the actual
volume of Liquid at standard conditions, pscand Tscon the surface. It has units o
bbl/STB.
psi
FL
P
TgL
tSCV
sVVB
7.14
60
@
@
Re)( +=
Note:
The variables Z, Bg, Rs, Bo, and Bt are obtained either by Lab or from correlationcharts. They are all function of Pressure, Temperature, type of oil (S.Go, API) and type o
gas (S.Gg). Lab is more accurate than the correlation charts, but correlation charts are
cheap and take a short time.
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Phase Behavior of Single Component System: Vaporization of a
Pure Substance at constant Temperature
i f Si C S i i f
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Phase Behavior of Single Component System: Vaporization of a
Pure Substance at constant Pressure
When hydrocarbons are produced from the reservoir to the
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When hydrocarbons are produced from the reservoir to the
surface, both pressure and temperature are reduced. Phase
changes may therefore occur in the Produced fluids.
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Example
Given T=200oF, solve for the unknown variables in the following Table:
P L g G Bo Bg SGg Z Rs Bt
psia cc cc SCF bbl/STB bbl/SCfatSC SCF/STB bbl/STB
i 2950 Li 0 0 1.434 - - - 787.911 1.434
1 2630 621 0 0 1.465 - - - 787.911 1.465
2 2220 570 g2 0.977 1.345 0.0013 0.7 0.868 Rs2 1.821
3 900.144 445 455 0.867 1.05 0.0033 0.8 0.893 96.6 2.13SC,60oF 14.7 423.8 7248.9 0.256 1 0.178 0.85 1.0 0 BtSC
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Examples
( ) bblxLBL scoii 3108226.33.1589878.423434.1* =
==
Li= (3.8326 x 10-3) (158987.3) = 607.74 cc
2. bblxscfscf
bblGBg g
3
222 1027.1)977.0(0013.0* =
==
g2= (1.27x10-3bbl) (158987.3 cc/bbl) = 201. 9 cc
3.STBx
SCF
L
GG
L
GR
sc
sc
sc
s)10666.2(
)256.0867.0(3
3
2
+=
+==
Rs2 = 421.23 SCF/STB
4.cc
cc
L
gLB
sc
scsc
tsc)8.423(
)9.248.78.423( +=
+=
Btsc= 18.1 bbl/STB
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The Five Reservoir Fluids
The behavior of reservoir fluids during production isdetermined by the shape of its phase diagram (phase
behavior).
There are five different reservoir fluids:
black oil, volatile oil, retrograde gas, wet gas, and dry gas.
Each type is defined by reference to the shape of its phasediagram
Why do we need the fluid type:
- Type & size of surface equipment
- Estimating oil & gas in place
- Plan of depletion & secondary recovery techniques.
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Black Oil
At point 2, the oil is at its bubble-
point and is said to be saturated.
Rules of thumb:
Initial producing gas-oilratio of or < 2000 scf/stb.
The gravity of stock tank
liquid < 45 API.
The color of the stock-
tank liquid: very dark,often black, sometimes
with greenish cast, or
brown.
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Volatile Oil
Rules of thumb:
Initial producing gas-oil ratio: 2000 3300scf/stb.
The gravity of stocktank liquid > 45 API.
The color of the
stock-tank liquid:usually brown, orange,sometimes green.
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Retrograde Gas
Rules of thumb:
Initial producing gas-oilratio > 3300 scf/stb.
The gravity of stocktank liquid: 40 60 API.
The color of the stock-
tank liquid: brown,orange, greenish, orwater-white.
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Wet Gas
Rules of thumb:
Initial producing gas-
oil ratio > 50,000
scf/stb.
The gravity of stock
tank liquid: 40 60
API.
The color of the
stock-tank liquid:
water-white.
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Dry Gas
Rules of thumb:
Initial producing
gas-oil ratio: no
liquid production.
The gravity of
stock tank liquid:
Not applicable.
The color of the
stock-tank liquid:
colorless.