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GAS FIELD ENGINEERING
PROPERTIES OF NATURAL GAS
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CONTENTS
- Introduction
- Composition of Natural Gas
- Ideal Gas Law
- Properties of Gaseous Mixtures
- Real Gas Equation of State
- Determination of Compressibility Factor
- Gas Conversion Equations
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Lesson Learning Outcome
At the end of the session, students should be able to:
Explain the governing laws of gas behavior.
Calculate basic parameters for determination of Gas flow performance, volume measurement and Gas reserves .
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2.1 Introduction Natural gas is a mixture of hydrocarbon gases and impurities.
Hydrocarbon gases normally found in natural gas are methane,ethane, propane, butanes, pentanes, and small amounts ofhexanes, heptanes, octane, and the heavier gases.
The impurities found in natural gas include carbon dioxide,hydrogen sulfide, nitrogen, water vapor, and heavierhydrocarbons.
Usually, the propane and heavier hydrocarbon fractions areremoved for additional processing because of their high marketvalue as gasoline-blending stock and chemical-plant rawfeedstock.
What usually reaches the transmission line for sale as naturalgas is mostly a mixture of methane and ethane with some smallpercentage of propane.
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2.1 Introduction
Physical properties of natural gases are important in solvinggas well performance, gas production, and gas transmissionproblems.
The properties of a natural gas may be determined eitherdirectly from laboratory tests or predictions from knownchemical composition of the gas.
In latter case, the calculations are based on the physicalproperties of individual components of the gas and on physicallaws, often referred to as mixing rules, relating the properties ofthe components to those of the mixture.
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Composition of Natural Gas
There is no one composition or mixture that can be referred toas the natural gas.
Each gas stream produced has its own composition.
Same reservoir may have different compositions.
Each gas stream produced from a natural gas reservoir canchange composition as the reservoir is depleted.
Samples of the well stream should be analyzed periodically,since it may be necessary to change the production equipmentto satisfy the new gas composition.
Table 2.1 shows some typical natural gas streams.
Well stream 1 is typical of an associated gas, that is, gasproduced with crude oil.
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Composition of Natural Gas
Well stream 2 and 3 are typical non-associated low-pressureand high-pressure gases, respectively.
Figure 2.1 shows the structures of some.
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Composition of Natural Gas
Table 2.1 Typical Natural Gas Analyses
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Composition of Natural Gas
Paraffin Compounds (saturated straight chain)
Fig (2.1) Hydrocarbon Gas Molecule Structures
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The Ideal Gas
If the temperature of a given gas is constant, volume of gasvaries inversely with the absolute pressure.
This relation is
Boyles Law
OR OR
( 2.1 )
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Example (1)
A quantity of gas at a pressure of 50 psig has a volume of 1000 cu
ft. If the gas is compressed to 100 psig, what volume would it
occupy? Assume the barometric pressure is 14.73 psia and the
temperature of the gas remains constant.
Solution
Substituting in Eqn 2.1 would give
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The Ideal Gas
1. If the pressure on a particular quantity of gas is held constant,
the volume will vary directly as the absolute temperature.
Charles Law
OR OR
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The Ideal Gas
2. If the volume of a particular quantity of gas is held constant, the
absolute pressure will vary directly as the absolute temperature:
OR OR
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Boyles and Charles Laws
Separate relations of Boyles and Charles laws may be combined to give
It is one of the most widely used relations in gas measurement work.
( 2.2 )
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Example(2)(a) How many cubic feet of an ideal gas, measured at standard
conditions of 60oF and 14.73 psia, are required to fill a 100-cu
ft tank to a pressure of 40 psia when the temperature of the
gas in the tank is 90oF? Atmospheric pressure is 14.4 psia.
(b) What would be the reading on the pressure gauge if the tank in
the above example is cooled to 60oF after being filled with the
ideal gas?
Solution
(a)
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Using Eqn 2.2
(b)
Using Eq. 2.2 again
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Avogadros Law
Under the same conditions of temperature and pressure,equal volumes of all ideal gases contain the same number of
molecules.
It has been shown that there are 2.733 x 1026 molecules in 1pound-mole of any gas.
A pound-mole of an ideal gas occupies 378.6 cu ft at 60oFand 14.73 psia.
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The Ideal Gas Law
Eqn 2.3 is only applicable at pressures close to atmospheric.
(2.3)
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The Ideal Gas Law
Since the number of pound-moles of a gas is equal to the massof the gas divided by the molecular weight of the gas, ideal gas
law can be expressed as
(2.4)
Eqn 2.4 may be rearranged to give the mass and density, , ofthe gas:
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n=m/M,
Example (3)
Using the fact that 1 pound-mole of an ideal gas occupies 378.6scf, calculate the value of the universal gas constant, R.
Solution
Using Eqn 2.4,
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Properties of Gaseous Mixtures
Physical properties that are most useful in natural gasprocessing are molecular weight, boiling point, freezing point,
density, critical temperature, critical pressure, heat of
vaporization and specific heat.
Table 2.2 is a tabulation of physical constants of a number ofhydrocarbon compounds, other chemicals, and some common
gases.
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Table 2.2 Physical Properties of Gases at Standard Pressure and Temperature
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Composition
Composition of a natural gas mixture may be expressed aseither the mole fraction, volume fraction, or weight fraction
of its components.
Mole fraction, yi, is defined as:
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Volume fraction, is defined as
Weight fraction, wi, is defined as
It is easy to convert from mole fraction (or volume fraction) toweight fraction and vice versa. These are illustrated in Eg. 2.6
and 2.7.
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Example ( 2.6)
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Example (2.7)
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Apparent Molecular Weight
Apparent molecular weight of a gas mixture is a pseudoproperty of the mixture and is defined as
The gas laws can be applied to gas mixtures by simply usingapparent molecular weight instead of the single-component
molecular weight in the formulas.
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Example (2.8)
Therefore, the apparent molecular weight of the mixture is17.08 lbm/lb-mol.
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Behavior of Real Gas
The gas deviation factor is defined as:
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Real Gas Equation of State
Real gas equation is
z is dimensionless gas deviation factor.
z-factor can be interpreted as a term by which the pressuremust be corrected to account for the departure from the ideal
gas equation
(2.17)
(2.18)
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For a certain quantity of gas,
(2.19)
Real Gas Equation of State
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Eqn. 2.17
may be written in terms of specific volume v or density Rho, and gas gravity Gamma g ,
Real Gas Equation of State
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(2.20)
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Real Gas Equation of State
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(2.22)
At standard conditions
Real Gas Equation of State
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(2.23)
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Assignment(1)
(1) Find and tabulate boiling point, freezing point, density,
critical temperature, critical pressure, heat of vaporization and
specific heat of different hydrocarbons and some of the
common gases.
(2) Define the followings in your ownwords:
Boiling point
Freezing Point
Density
Critical Temperature
Critical Pressure
Heat of Vaporization
Specific heat
To be submitted individually by 12 February 2013 not later than 5:00 pm into pigeon hole.
Assignment (1)
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Theorem of Corresponding States
Reduced temperature, reduced pressure and reduced volumeare the ratios of the actual temperature, pressure and specific
volume to the critical temperature, critical pressure, and critical
volume.
By the theorem of corresponding states, z-factor for any gasmixture is defined solely by reduced temperature and reduced
pressure:
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Determination of z-factor
z-factor Correlation of Standing and Katz
Pseudo-properties are given by Kays mixing rules as
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Fig. 2.4 Gas
Deviation Factor
for Natural
Gases
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In cases where the composition of a natural gas is notavailable, pseudo-critical pressure and pseudo-critical
temperature may be approximated from
Pseudo-reduced pressure and temperature:
where p and T are the absolute pressure and absolutetemperature at which z-factor is required.
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Example 2.9 (Sweet Natural Gas)
Note: No Hydrogen Sulphide
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At a pressure of 2000 psia and temperature of 150oF.
Using the z-factor chart, Fig. 2.4
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QUIZZ (1
) QUIZZ(1) Calculate compressibility factor for the following gas
composition at operating pressure 2300 psia and temperature130F.
Component mole fraction
CH4 0.6136
C2H6 0.1828
C3H8 0.1414
C4H10 0.0253
C5H12 0.0029
C6H14 0.0024
C7H16 0.0021
N2 0.0140
CO2 0.0155
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Direct Calculation of z-factors
1. The Hall Yarborough Method
2. Dranchuk, Purvis and Robinson Method
3. Gopal Method
All of these methods have their own Equations. Useful in
developing computer programs. Standing-Katz
correlation chart is handy to put in a program.
Be referred to Equations 2.40,2.41,2.42,2.43, and 2.44 for
above three methods.
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Direct Calculation of z-factors
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Direct Calculation of z-factors
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Direct Calculation of z-factors
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Direct Calculation of z-factors
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Other Equations of State
Benedict- Webb- Rubin Equation (B-W-R) Equation of State
Van der Waals Equation of State
Van der Waals equation has limited application in engineering.It is accurate only at low pressures.
Equation of state describing the behavior of pure, lighthydrocarbons over single and two-phase regions, both
below and above critical pressure.
Redlich-Kwong (R-K) Equation of State
Applicable to mixtures
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Viscosity of Natural Gases
Coefficient of viscosity is a measure of the resistance to flowexerted by a fluid.
The only accurate way to obtain the viscosity of a gas is todetermine it experimentally.
However, experimental determination is difficult and slow.
Petroleum Engineer must rely on viscosity correlations.
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Viscosity of Natural Gases
Viscosity of a gas can be calculated from
Composition
Gas Gravity
Stiel and Thodas(1961) equation can be used if
composition is known.
Carr et al(1954) method can be used if gas gravity is
known.
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Gas Formation Volume Factor and Expansion Factor
In gas reservoir engineering, the main use of the real gasequation of state is to relate surface volumes to reservoir
volumes of hydrocarbons.
This is accomplished by the use of the gas formation volumefactor Bg or gas expansion factor E.
Gas formation volume factor is the ratio of the volume of gasin the reservoir to its volume at standard conditions.
Bg is usually expressed in units of reservoir cubic feet perstandard cubic feet, sometimes expressed it in barrels per
standard cubic foot.
Gas expansion factor is simply the reciprocal of the gasformation volume factor.
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Gas Formation Volume Factor
(2.86)
(2.87)
At standard conditions of 14.73 psia and 60oF assuming Zsc=1
(2.88)
(2.89)
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Dividing reservoir cubic feet by 5.615 to convert to reservoirbarrels obtains
(2.90)
(2.91)
Gas Formation Volume Factor
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Example 2.13
At a pressure of 2500 psia and reservoir temperature of 180 F,the gas deviation factor, z for the sour natural gas is 0.850.
(a) Calculate the formation volume factor, Bg and gas
expansion factor, E.
(a) How many standard cubic feet of this gas are contained in
a reservoir with a gas pore volume of 1.0 x 109 cu ft?
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Solution
(a) Using Eqn. 2.88, and 2.89,
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(2.88)
(2.89)
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(b) Gas in place
Solution
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QUIZZ(2)
At a pressure of 3400 psia and reservoir temperature of 160 F,
the gas deviation factor for the sour natural gas is 0.784.
(1) Calculate the formation volume factor and gas expansion
factor.
(2) How many standard cubic feet of this gas are contained in
a reservoir with a gas pore volume of 1.3 x 1012 cu ft?
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API Gravity
API gravity is another gravity term that is used withhydrocarbon liquids.
is the liquids specific gravity at 60oF referred to that of waterat 60 deg F, that is, specific gravity of 1.0, will have an API
gravity of 10o API.
Gravity of a liquid in oAPI is determined by its density at 60oFand is independent of temperature.
Liquid specific gravity may be obtained by
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Gas Gravity of Total Well Stream
Total Well stream gas specific gravity differs from surface
gas specific gravity where the gas oil ratio is low.
Many correlations use the specific gravity as an index to
various fluid properties.
This should be the Well Stream Gas Gravity.
Following is the procedure for calculating well stream gas
gravity.
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Gas Gravity of Total Well Stream
Well stream gas specific gravity (air=1) is given by
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Gas Gravity of Total Well Stream
is equal to the average molecular weight of all the
hydrocarbons flowing in the well stream divided by the
molecular weight of air.
When the molecular weight of the tank oil is not known, it maybe estimated using the formula:
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Some Gas Conversion Equations
At standard conditions of 14.7 psia and 60 degree F:
Molecular weight of gas = 28.79 *(sp gr)
Density of gas, (lbm/cu ft)=0.0764 * (sp gr)
=mol wt/379
=28.97 (sp gr)/379
Specific volume of gas (cu ft/lbm)=13.08/sp gr = 379/mol wt
Gas flow (moles/day)=Gas flow rate(cfd)/379
Mass flow rate( lbm/hr)=3185 (MMscfd)(sp gr)
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At conditions other than 14.7 psia and 60oF:
Some Gas Conversion Equations
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Thank You
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Q & A
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