Properties of Natural Gas

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GAS FIELD ENGINEERING

PROPERTIES OF NATURAL GAS

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 .

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 of hexanes, heptanes, octane, and the heavier gases. • The impurities found in natural gas include carbon dioxide, hydrogen sulfide, nitrogen, water vapor, and heavier hydrocarbons. • Usually, the propane and heavier hydrocarbon fractions are removed for additional processing because of their high market value as gasoline-blending stock and chemical-plant raw feedstock. • What usually reaches the transmission line for sale as natural gas is mostly a mixture of methane and ethane with some small percentage of propane.

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2.1 Introduction

• Physical properties of natural gases are important in solving gas well performance, gas production, and gas transmission problems. • The properties of a natural gas may be determined either directly from laboratory tests or predictions from known chemical composition of the gas. • In latter case, the calculations are based on the physical properties of individual components of the gas and on physical laws, often referred to as mixing rules, relating the properties of the components to those of the mixture.

Composition of Natural Gas

• There is no one composition or mixture that can be referred to as 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 can change composition as the reservoir is depleted. • Samples of the well stream should be analyzed periodically, since it may be necessary to change the production equipment to 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, gas produced with crude oil.

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Composition of Natural Gas

• Well stream 2 and 3 are typical non-associated low-pressure and high-pressure gases, respectively. • Figure 2.1 shows the structures of some.

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

The Ideal Gas Boyle’s Law • If the temperature of a given gas is constant, volume of gas varies inversely with the absolute pressure. • This relation is

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

The Ideal Gas

Charles’ Law

1. If the pressure on a particular quantity of gas is held constant, the volume will vary directly as the absolute temperature.

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

Boyle’s and Charles’ Laws

•Separate relations of Boyle’s and Charles’ laws may be combined to give

( 2.2 )

•It is one of the most widely used relations in gas measurement work.

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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)

Using Eqn 2.2

(b)

Using Eq. 2.2 again

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Avogadro’s 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 1 pound-mole of any gas. • A pound-mole of an ideal gas occupies 378.6 cu ft at 60oF and 14.73 psia.

The Ideal Gas Law

(2.3)

• Eqn 2.3 is only applicable at pressures close to atmospheric.

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The Ideal Gas Law • Since the number of pound-moles of a gas is equal to the mass of the gas divided by the molecular weight of the gas, ideal gas law can be expressed as

n=m/M, (2.4)

• Eqn 2.4 may be rearranged to give the mass and density, ρ, of the gas:

Example (3) • Using the fact that 1 pound-mole of an ideal gas occupies 378.6 scf, 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 gas processing 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 of hydrocarbon compounds, other chemicals, and some common gases.

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 as either the mole fraction, volume fraction, or weight fraction of its components.

• Mole fraction, yi, is defined as:

• Volume fraction, is defined as

• Weight fraction, wi, is defined as

• It is easy to convert from mole fraction (or volume fraction) to weight fraction and vice versa. These are illustrated in Eg. 2.6 and 2.7.

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Example ( 2.6)

Example (2.7)

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Apparent Molecular Weight

• Apparent molecular weight of a gas mixture is a pseudo property of the mixture and is defined as

• The gas laws can be applied to gas mixtures by simply using apparent molecular weight instead of the single-component molecular weight in the formulas.

Example (2.8)

• Therefore, the apparent molecular weight of the mixture is 17.08 lbm/lb-mol.

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Behavior of Real Gas

The gas deviation factor is defined as:

Real Gas Equation of State

• Real gas equation is

(2.17)

• z is dimensionless gas deviation factor. • z-factor can be interpreted as a term by which the pressure must be corrected to account for the departure from the ideal gas equation

(2.18)

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Real Gas Equation of State

• For a certain quantity of gas,

(2.19)

Real Gas Equation of State

• Eqn. 2.17

may be written in terms of specific volume v or density Rho, ρ and gas gravity Gamma g g ,

(2.20)

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Real Gas Equation of State

(2.22)

Real Gas Equation of State

• At standard conditions

(2.23)

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Assignment(1) 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.

Theorem of Corresponding States • Reduced temperature, reduced pressure and reduced volume are 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 gas mixture 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 Kay’s mixing rules as

Fig. 2.4 Gas Deviation Factor for Natural Gases

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• In cases where the composition of a natural gas is not available, 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 absolute temperature at which z-factor is required.

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

QUIZZ (1 ) QUIZZ(1) • Calculate compressibility factor for the following gas composition at operating pressure 2300 psia and temperature 130ºF. • 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.

Direct Calculation of z-factors

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Direct Calculation of z-factors

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Direct Calculation of z-factors

Other Equations of State

Van der Waal’s Equation of State • Van der Waals’ equation has limited application in engineering. It is accurate only at low pressures. Benedict- Webb- Rubin Equation (B-W-R) Equation of State • Equation of state describing the behavior of pure, light hydrocarbons 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 flow exerted by a fluid.

• The only accurate way to obtain the viscosity of a gas is to determine it experimentally.

• However, experimental determination is difficult and slow.

• Petroleum Engineer must rely on viscosity correlations.

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.

• . 50

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Gas Formation Volume Factor and Expansion Factor • In gas reservoir engineering, the main use of the real gas equation of state is to relate surface volumes to reservoir volumes of hydrocarbons. • This is accomplished by the use of the gas formation volume

factor Bg or gas expansion factor E. • Gas formation volume factor is the ratio of the volume of gas in the reservoir to its volume at standard conditions.

• Bg is usually expressed in units of reservoir cubic feet per standard cubic feet, sometimes expressed it in barrels per standard cubic foot. • Gas expansion factor is simply the reciprocal of the gas formation volume factor.

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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Gas Formation Volume Factor

• Dividing reservoir cubic feet by 5.615 to convert to reservoir barrels obtains

(2.90)

(2.91)

Example 2.13

• At a pressure of 2500 psia and reservoir temperature of 180º 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,

(2.88)

(2.89)

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Solution

(b) Gas in place

QUIZZ(2)

At a pressure of 3400 psia and reservoir temperature of 160º 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 with hydrocarbon liquids.

• is the liquid’s specific gravity at 60oF referred to that of water at 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 60oF and is independent of temperature. • Liquid specific gravity may be obtained by

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

Gas Gravity of Total Well Stream

• When the molecular weight of the tank oil is not known, it may be estimated using the formula:

• is equal to the average molecular weight of all the hydrocarbons flowing in the well stream divided by the molecular weight of air.

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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)

Some Gas Conversion Equations At conditions other than 14.7 psia and 60oF:

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Thank You

Q & A