Reservoir Fluid Properties Course (2nd Ed.)
1. Reservoir Fluid Behaviors
2. Petroleum ReservoirsA. Oil
3. Introduction to Physical Properties
1. Gas Behavior
2. Gas Properties: A. Z Factor:
a. Calculation for pure components
b. Calculation for mixture componentsI. Mixing rules for calculating pseudocritical properties
II. Correlations for calculating pseudocritical properties
c. Nonhydrocarbon adjustment
d. High molecular weight gases adjustment
Reservoir Fluid Properties
To understand and predict the volumetric behavior of oil and gas reservoirs as a function of pressure, knowledge of the physical properties of reservoir fluids must be gained.
These fluid properties are usually determined by laboratory experiments performed on samples of actual reservoir fluids.
In the absence of experimentally measured properties, it is necessary for the petroleum engineer to determine the properties from empirically derived correlations.
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Natural Gas Constituents
A gas is defined as a homogeneous fluid of low viscosity and density that has no definite volume but expands to completely fill the vessel in which it is placed.
Generally, the natural gas is a mixture of hydrocarbon and nonhydrocarbon gases. The hydrocarbon gases that are normally found in a
natural gas are methanes, ethanes, propanes, butanes, pentanes, and small amounts of hexanes and heavier.
The nonhydrocarbon gases (i.e., impurities) include carbon dioxide, hydrogen sulfide, and nitrogen.
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Properties of Natural Gases
Knowledge of PVT relationships and other physical and chemical properties of gases is essential for solving problems in natural gas reservoir engineering. These properties include:Apparent molecular weight, MaSpecific gravity, gCompressibility factor, zDensity, gSpecific volume, v Isothermal gas compressibility coefficient, cgGas formation volume factor, BgGas expansion factor, EgViscosity, g
The above gas properties may be obtained from direct laboratory measurements or by prediction from generalized mathematical expressions.
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For an ideal gas, the volume of molecules is insignificant compared with the total volume occupied by the gas.
It is also assumed that these molecules have no attractive or repulsive forces between them, and that all collisions of molecules are perfectly elastic.
Based on the above kinetic theory of gases, a mathematical equation called equation-of-state can be derived to express the relationship existing between pressure p, volume V, and temperature T for a given quantity of moles of gas n.
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The basic properties of gases
Petroleum engineers are usually interested in the behavior of mixtures and rarely deal with pure component gases. Because natural gas is a mixture of hydrocarbon
components, the overall physical and chemical properties can be determined from the physical properties of the individual components in the mixture by using appropriate mixing rules.
The basic properties of gases are commonly expressed in terms of the apparent molecular weight, standard volume,
density, specific volume, and specific gravity.
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Behavior of Ideal Gases
The gas density at any P and T:
Apparent Molecular Weight
Specific Volumethe volume occupied by
a unit mass of the gas
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ideal gas behavior
Three pounds of n-butane are placed in a vessel at 120F and 60 psia.
Calculate the volume of the gas assuming an ideal gas behavior.
calculate the density of n-butane.
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ideal gas behavior
Step 1. Determine the molecular weight of n-butane from the Table to give:M = 58.123
Step 2. Solve Equation for the volume of gas:
Step 3. Solve for the density by:
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Ideal Gases vs. Real Gases
In dealing with gases at a very low pressure, the ideal gas relationship is a convenient and
generally satisfactory tool.
At higher pressures, the use of the ideal gas equation-of-state may
lead to errors as great as 500%,
as compared to errors of 23% at atmospheric pressure.
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Behavior of Real Gases
Basically, the magnitude of deviations of real gases from the conditions of the ideal gas law increases with increasing pressure and temperature and varies widely with the composition of the gas. The reason for this is that the perfect gas law was
derived under the assumption that the volume of molecules is insignificant and that no molecular attraction or repulsion exists between them. Numerous equations-of-state have been developed in the
attempt to correlate the pressure-volume-temperature variables for real gases with experimental data.
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Gas Compressibility Factor Definition
In order to express a more exact relationship between the variables p, V, and T, a correction factor called the gas compressibility factor,
gas deviation factor, or simply the z-factor, must be introduced to account for the departure of gases from
The equation has the form of pV = znRT
Where the gas compressibility factor z is a dimensionless quantity and is defined as the ratio of the actual volume of n-moles of gas at T and p to the ideal volume of the same number of moles at the same T and p:
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Corresponding States Principle for Pure componentsthe critical point of a fluid is where the liquid and
vapor molar volumes become equal; i.e., there is no distinction between the liquid and vapor
phases. above Tc the two phases can no longer coexits.
Each compound is characterized by its own unique (Tc), (Pc) and (Vc)
Corresponding States Principle (CSP): All fluids behave similarly when described in terms of
their reduced temperature and pressureTr=T/Tc and Pr=P/Pc
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Deviation from law of ideal gases
Theory of Corresponding states
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one-fluid theory (mixtures)
Generally, we apply exactly the same equations for mixtures by treating the mixture as a hypothetical "pure" component whose properties are
some combination of the actual pure components that comprise it.
We call this the one-fluid theory.
To apply CSP, we use the same plot or table as pure components but we make the temperature and pressure dimensionless
with pseudo criticals for the hypothetical pure fluid instead of any one set of values as scaling variables from pure component values.
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Mixing rules form the pseudocritical of the hypothetical pure component (the mixture) bytaking some composition average of
each component's critical properties.
Many mixing rules are commonly used and provide more accuracy than kays mixing rule.You see other mixing rules in your thermodynamics class.
Kay's mixing rules is the simplest possible, It obtains the pseudocritical for
the hypothetical pure component.
It use a simple mole fraction average for both Tc and Pc: (,) = ,, Ppc( ,) = ,
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Pseudo-Reduced Properties Calculation (for mixtures)Studies of the gas compressibility factors for natural
gases of various compositions have shown that compressibility factors can be generalized with sufficient
accuracies for most engineering purposes when they are expressed in terms of the following two
dimensionless properties: Pseudo-reduced pressure and Pseudo-reduced temperature
These dimensionless terms are defined by the following expressions:
ppc and Tpc, do not represent the actual critical properties of the gas mixture and are used as correlating parameters in generating gas properties.
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Standing and Katz Compressibility Factors Chart
low pressure values (0
Standing and Katz Compressibility Factors Chart
for higher pressure values (15
Graphical method for Pseudo-Critical Properties approximationa graphical method for
a convenient approximation of the ppc and tpc of gases when only the specific
gravity of the gas is available.
Brown et al. (1948) Fall 13 H. AlamiNia Reservoir Fluid Properties Course: 27
Mathematical method for Pseudo-Critical Properties approximationIn cases where the composition of a natural gas is
not available, the pseudo-critical properties, i.e., Ppc and Tpc, can be predicted solely from the specific gravity of the gas.
Standing (1977) expressed brown et al graphical correlation in the following mathematical forms:Case 1: Natural Gas Systems
Case 2: Gas-Condensate Systems
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