Q921 rfp lec6 v1

Reservoir Fluid Properties Course (2nd Ed.)

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

1. Crude Oil Properties:
A. Density (rho), Gravity (gamma, API)
B. Gas Solubility (Solution gas) (Rs)
C. Bubble-point pressure (Pb)

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
Fall 13 H. AlamiNia

Reservoir Fluid Properties Course:

5

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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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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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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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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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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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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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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Gas-Solubility Pressure Diagram
A typical gas
solubility
curve,
as a function
of
pressure

for an
undersaturate
d crude oil

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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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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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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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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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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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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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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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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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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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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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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


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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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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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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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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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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%.
Fall 13 H. AlamiNia


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1. Ahmed, T. (2010). Reservoir engineering
handbook (Gulf Professional Publishing).
Chapter 2

Q921 rfp lec6 v1

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