2. 1. empirical correlations for calculating z-factors
2. Gas Properties:
A. isothermal gas compressibility (Cg)
B. gas formation volume factor (Bg) and
gas expansion factor (Eg)
C. Gas Viscosity correlations
3. 1. Crude Oil Properties:
A. Density (rho), Gravity (gamma, API)
B. Gas Solubility (Solution gas) (Rs)
C. Bubble-point pressure (Pb)
4.
5. Properties of Crude Oil Systems
Petroleum (an equivalent term is crude oil) is a complex
mixture consisting
predominantly of hydrocarbons and containing
sulfur, nitrogen, oxygen, and helium as minor constituents
The physical and chemical properties of crude oils
vary considerably and are dependent on
the concentration of the various types of
hydrocarbons and minor constituents present.
An accurate description of physical properties of crude
oils is of a considerable importance in the fields of both
applied and theoretical science and
especially
in the solution of petroleum reservoir engineering problems
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6. Physical Properties of Petroleum
Main physical properties: Data on most of these
fluid properties are usually
Fluid gravity
determined by
Specific gravity of the
solution gas
laboratory experiments
performed on samples of
Gas solubility
actual reservoir fluids.
Bubble-point pressure
Oil formation volume factor In the absence of
experimentally measured
Isothermal compressibility
coefficient of
properties of crude oils,
undersaturated crude oils
it is necessary for the
Oil density
petroleum engineer to
determine the properties
Total formation volume
from
factor
empirically derived
Crude oil viscosity
correlations.
Surface tension
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7. Crude Oil density and
Crude Oil specific Gravity
The crude oil density (ρo)
is defined as the mass of a unit volume of the crude
𝑚
at a specified pressure and temperature.
𝜌𝑜 =
is usually expressed in pounds per cubic foot.
𝑉
The specific gravity of a crude oil (γo)
is defined as the ratio of the density of the oil
to that of water.
Both densities are measured
at 60°F and atmospheric pressure:
the liquid specific gravity is dimensionless, but
traditionally is given the units 60°/60°
to emphasize the fact that both densities
are measured at standard conditions.
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8. Crude Oil Gravity
Although the density and specific gravity are used
extensively in the petroleum industry,
the API gravity is the preferred gravity scale.
API gravity
This gravity scale is precisely related to the specific
gravity by:
The API gravities of crude oils usually range
from 47° API for the lighter crude oils to
10° API for the heavier asphaltic crude oils.
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9. Specific Gravity of the Solution Gas
γg is described by
weighted average of (based on separator gas-oil ratio)
the specific gravities of the separated gas
from each separator.
Where
n = number of separators,
Rsep = separator gas-oil ratio, scf/STB,
γsep = separator gas gravity,
Rst = gas-oil ratio from the stock tank, scf/ STB,
γst = gas gravity from the stock tank
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10.
11. Gas Solubility definition
The gas solubility Rs is defined as
the number of standard cubic feet of gas
that will dissolve in one stock-tank barrel of crude oil
at certain pressure and temperature.
The solubility of a natural gas
in a crude oil is a strong function of
the pressure,
temperature,
API gravity, and
gas gravity.
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12. Gas Solubility variation with pressure
For
a particular gas and
crude oil to exist
at a constant temperature,
the solubility increases with pressure
until the saturation pressure is reached.
At the saturation pressure (bubble-point pressure)
all the available gases are dissolved in the oil and
the gas solubility reaches its maximum value.
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13. Gas Solubility measurement
with pressure
Rather than measuring the amount of gas
that will dissolve in a given stock-tank crude oil
as the pressure is increased,
it is customary to determine the amount of gas that will come
out of a sample of reservoir crude oil as pressure decreases.
As the pressure is reduced from the initial reservoir
pressure pi, to the bubble-point pressure Pb,
no gas evolves from the oil and consequently
gas solubility remains constant at its maximum value of Rsb.
Below the bubble-point pressure,
solution gas is liberated and Rs decreases with pressure
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14. Gas-Solubility Pressure Diagram
A typical gas
solubility
curve,
as a function
of
pressure
for an
undersaturate
d crude oil
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15.
16. Empirical Correlations for
Estimating the Rs
The following five empirical correlations for
estimating the gas solubility are given below:
Standing’s correlation
The Vasquez-Beggs correlation
Glaso’s correlation
Marhoun’s correlation
The Petrosky-Farshad correlation
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17. Standing (1947) Correlation
Standing (1947) proposed
a graphical correlation
for determining the gas solubility as a function of
pressure, gas specific gravity, API gravity, and system
temperature.
The correlation was developed
from a total of 105 experimentally
determined data points on 22 hydrocarbon mixtures
from California crude oils and natural gases.
The proposed correlation
has an average error of 4.8%.
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18. Rs: Standing’s Correlation
Standing (1981) expressed
his proposed graphical correlation
in more convenient mathematical form of:
where
T = temperature, °R,
p = system pressure, psia γg = solution gas specific gravity
Standing’s equation is valid for applications
at and below the bubble-point pressure of the crude oil.
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19. Rs:
The Vasquez- Beggs (1980) Correlation
They presented an improved empirical correlation
The correlation was obtained by regression analysis
using 5,008 measured gas solubility data points.
predicting Rs with an average absolute error of 12.7%
Based on oil gravity, the measured data were divided
into two groups. (at a value of oil gravity of 30°API)
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20. gas gravity at the reference separator
pressure (Vasquez-Beggs Correlation)
the value of the specific gravity of the gas depends on
the conditions under which it is separated from the oil,
So the value of the gas specific gravity as obtained
from a separator pressure of 100 psig must be used
This reference pressure was chosen because
it represents the average field separator conditions.
Adjustment relationship for the gas gravity γg to the
reference separator pressure:
γgs = gas gravity at the reference separator pressure
γg = gas gravity at the actual separator conditions of psep and Tsep
psep (Tsep)= actual separator pressure (Temperature), psia (°R)
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21. Rs: Glaso’s Correlation
Glaso (1980) proposed a correlation for estimating
the gas solubility
as a function of
API gravity, pressure, temperature, gas specific gravity.
from studying 45 North Sea crude oil samples.
an average error of 1.28%, a standard deviation of 6.98%
p*b is a correlating number
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22. Rs: Marhoun’s Correlation
Marhoun (1988)
developed an expression
for estimating
the saturation pressure of
the Middle Eastern crude
where
oil systems.
γg = gas specific gravity
The correlation originates
γo = stock-tank oil gravity
from 160 experimental
T = temperature, °R
saturation pressure data.
a = 185.843208
The proposed correlation
b = 1.877840
can be rearranged and
c = −3.1437
solved
d = −1.32657
for the gas solubility:
e = 1.398441
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23. Rs: The Petrosky-Farshad Correlation
Petrosky and Farshad (1993)
used a nonlinear multiple regression software
to develop a gas solubility correlation.
The authors constructed a PVT database
from 81 laboratory analyses
from the Gulf of Mexico crude oil system.
p = pressure, psia, T = temperature, °R
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24. Rs: gas solubility calculation from the
experimental measured PVT data
The gas solubility can also be calculated rigorously
from the experimental measured PVT data
at the specified pressure and temperature.
The expression relates the gas solubility Rs to ρo, Bo,
γo, γg
ρo = oil density, lb/ft3
Bo = oil formation volume factor, bbl/STB
γo = specific gravity of the stock-tank oil
γg = specific gravity of the solution gas
the weight average of
separator and stock-tank gas specific gravities should be used
The error in calculating Rs by using the equation will
depend only on the accuracy of the available PVT data.
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25.
26.
27. Bubble-Point Pressure
The bubble-point pressure Pb
of a hydrocarbon system
is defined as the highest pressure at which
a bubble of gas is first liberated from the oil.
can be measured experimentally for a crude oil system
by conducting a constant-composition expansion test.
In the absence of the experimentally measured
bubble-point pressure, it is necessary
to make an estimate of this crude oil property
from the readily available
measured producing parameters
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28. Pb Correlations
Several graphical and
Several ways of combining
mathematical correlations the above parameters in a
for determining Pb have
graphical form or a
been proposed during the mathematical expression
last four decades.
are proposed by numerous
authors, including:
They are essentially based
on the assumption that
the bubble-point pressure
is a strong function of
gas solubility Rs, gas gravity
γg, oil gravity API, and
temperature T, or:
Pb = f (RS, γg, API, T)
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Standing
Vasquez and Beggs
Glaso
Marhoun
Petrosky and Farshad
Reservoir Fluid Properties Course:
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29. Pb: Standing’s Correlation
Standing (1947) graphical correlation
Based on 105 experimentally measured Pb
on 22 hydrocarbon systems from California oil fields,
The correlating parameters are Rs, γg, API, and system T
The reported average error is 4.8%
Standing (1981) mathematical correlation
pb = bubble-point pressure, psia, T = system temperature, °R
Standing’s correlation should be used with caution
if nonhydrocarbon components are known
to be present in the system.
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30. Pb: The Vasquez-Beggs Correlation
Vasquez and Beggs’ gas solubility
correlation can be solved for the pb
The coefficients C1, C2, and C3
have the following values:
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31. Pb: Glaso’s Correlation
Glaso (1980) used 45 oil samples,
mostly from the North Sea hydrocarbon system,
to develop an accurate correlation for Pb
Glaso proposed the following expression:
p*b is a correlating number and defined by:
Rs = gas solubility, scf/STB, t = system temperature, °F,
γg = average specific gravity of the total surface gases,
a = 0.816, b = 0.172, c = −0.989
• For volatile oils, the temperature exponent b, be 0.130.
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32. Pb: Marhoun’s Correlation
Marhoun (1988) correlation for estimating pb
used 160 experimentally determined bubble-point pressures
from PVT analysis of 69 Middle Eastern hydrocarbon mixtures
The correlating parameters are Rs, γg, γo, and T
average absolute relative error of 3.66%
when compared with the experimental data
used to develop the correlation.
T = temperature, °R
γo = stock-tank oil specific gravity
γg = gas specific gravity
a=5.3809×10−3, b=0.71508, c=−1.8778, d=3.144, e=1.3266
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33. Pb: The Petrosky-Farshad Correlation
The Petrosky and Farshad
gas solubility equation,
can be solved for the Pb to give:
where the correlating parameter x
is previously defined by.
the correlation predicts measured bubble point
pressures with an average absolute error of 3.28%.
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34. 1. Ahmed, T. (2010). Reservoir engineering
handbook (Gulf Professional Publishing).
Chapter 2