Reservoir Fluid & It Properties

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Characteristics of Natural Gas

Natural gas is a mixture of hydrocarbon gases with some impurities, mainly

nitrogen, hydrogen sulfide, and carbon dioxide.

Gases containing significant amounts of H2S or CO2or both are called

sour or acid gases. These impurities must be removed before the gas is used

as a fuel.

The hydrocarbon gases are methane, ethane, propane, butanes, pentanes,

and small amounts of hexanes, heptanes, and some heavier fractions.

In gas used for fuel, methane is the largest component, usually 95 to 98%.

Natural gas is normally considered to be a mixture of straight chain or

paraffin hydrocarbon compounds.; general formula is CnH2n+2where n is

the number of carbon atoms.


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Occurrence of Natural Gas in Conventional

ReservoirsGas is found in sedimentary subsurface strata composed of sandstone, limestone, or

dolomite.

Natural gas is frequently characterized in terms of its nature of occurrence

underground, as follows:

1.Non-associated

. Found in reservoirs with no crude oil, and is typically richer inCH4, poorer in heavier compounds.

2. Dissolved or associated. Gas in solution with crude oil is termed dissolved gas,

whereas the gas found in contact with the crude oil as gas cap is termed associated

gas. Typically, associated gas is poorer in CH4, but richer in heavier components.

3. Gas condensates. These gases have high amounts of hydrocarbon liquids andmay occur as gas in the reservoir.

The most desirable gas is the non-associated type, because it can be produced at high

pressure. Associated or dissolved gas is separated from the crude oil at lower

separator pressures and, therefore, entails more compression expenses. Such gas isoften flared or vented.


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Properties of Natural Gases

Introduction

This chapter presents methods for estimating reservoir fluid

properties required for calculations of gas reservoir and production

engineering.

When laboratory measurements are not available, correlations for

estimating properties of natural gases will be presented andapplied.


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Moles and Mole Fraction

A pound-mole (lbm-mol) is a quantity of matter with a mass inpounds equal to the molecular weight;n = m /M

For a system with nccomponents, the mole fraction is:

where

yi= mole fraction of the i-th component,

ni= number of pound-moles of i-th component, and

nc= number of components in the system.


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Molar Volume

The concept of molar volume, Vm (Vm =V/n), is used to convert agiven mass of gas to its vapor volume at standard pressure and

temperature conditions.For a given set of standard conditions, the

molar volume is constant.


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Real-Gas Behavior

The real-gas law is simply the pressure/volume relation (calledequation of state, or EOS) predicted by the ideal-gas law modifiedby a correction factor that accounts for the non ideal behavior ofthe gas. The real-gas law is:

PV = Z n RTWhere

Z = the z factor (dimensionless quantity)

= compressibility factor, or gas deviation factor.

Thus, it is the ratio of the actual volume occupied by a mass of gas atsome pressure and temperature to the volume the gas wouldoccupy if it behaved ideally.

Z = Vactual/Videal or Vactual= Z Videal


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Principle of Corresponding States

All pure gases have the same Z-factor at the same values of

reduced pressure and reduced temperature. It works better for gasesof similar molecular characteristics such as paraffinic hydrocarbons.

Reduced pressure and reduced temperature for pure compounds aredefined as:

where

pc= critical pressure for a pure gas, pisa;

Tc= critical temperature for a pure gas, R;Table 1.1 shows the physical properties of pure gases at standardconditions.


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This Law has been extended to cover mixtures of gases that are chemicallysimilar. Since it is difficult to obtain critical points for multicomponentmixtures, the quantities ofPpcand Tpchave been conceived.

According to Kays mixing rule, Ppc and Tpc for gas mixtures can be

estimated as follows.Tpc = SyiTci & Ppc = SyiPci

The pseudoreduced pressure and pseudoreduced temperature for gasmixtures are defined as:

Ppc= pseudo critical pressure for a gas mixture,

Tpc = pseudo critical temperature for the mixture,


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Standing-Katz Z-Factor Chart; Z= f (Ppr,Tpr)

Fig.2 shows the Standing and Katz correlation for Z as a function ofPprandTprfor sweet (non-H2S or CO2containing) natural gases.

The applicability of the Law of corresponding states is assumed, and Kaysmixing rules are used for gas mixture properties.

This chart is generally reliable for sweet natural gases with minor amounts ofN2.

Wichert and Aziz proposed a correction factor ( corrects TpcandPpcdetermined by Kays mixing rule), which can be used to extend theapplicability of this chart to sour gases.

The modified values (after correcting for contaminants) of the pseudocriticalpressure (P'pc) and pseudocritical temperature (T'pc) are used to calculate

modified pseudoreduced pressure and pseudoreduced temperature,respectively.

These modified pseudoreduced properties are then used in the Standing-Katzchart to determine the Z-factor for sour gases.


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Apparent Molecular Weight of a Gas Mixture

It is defined for a gas mixture as follows:

Where

M = apparent molecular weight of gas mixture, Ibm/lbm-mol;

Mi = molecular weight of the ith gas component, Ibm/lbm-mol; and

yi = mole fraction the gas phase of the ithcomponent, fraction.


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Specific Gravity of a Gas

The ratio of the densities, in lbm/cu.ft, of the gas mixture and

dry air when both are measured at the same temperature andpressure:

gg= rgra

Knowing that r= m /Vand n= m /M, we can also express the

specific gravity of a gas mixture as:

gg= M/MairWhere Mair= molecular weight of air = 28.9625 lbm/lbm-mole;

M = apparent molecular weight of the gas mixture, lbm/lbm- mole.


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In some cases the composition of a gas will be given in weight or

mass percent rather than mole percent.

Composition must be first converted to mole fraction or percent

before the mixture properties can be calculated.

Example 2; a gas mixture consists of 50% C1, 30% C2, and 20%

C3by weight. Assuming 100 lbmof this gas as a basis, calculate

apparent molecular weight and specific gravity of this mixture.


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If the volume fraction is given at conditions other than standard,

the volume fraction must be converted to a mole fraction basis,

taking into account the deviation from ideal behavior.

Example 3; a gas has the following composition at 2,500 psia and

300 F.

Assuming 100 cu.ft. as a basis, calculate the composition in molefraction.

Component Volume%

Methane, C1 67Ethane, C2 30

Propane, C3 1

CO2 2


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If gas composition (yi) is not known, Fig.1 or Fig.3.1 may be

used to determinePcand Tcfrom gas gravity.

When using Fig.3.1, corrections may be made for the presence

of non-hydrocarbon components.


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Other More Complex Equations of States

One limitation in the use of the real-gas EOS (PV = ZnRT), is that the

Z-factor, being a function of pressure, temperature, and composition,is not constant.

Therefore, mathematical manipulations cannot be made directly but

must be accomplished through graphical or numerical techniques.

Other equations of states are devised so that correction factors, which

correct the ideal gas law for non-ideality, may be assumed constant,

thus permitting differentiation or integration.

However, they all involve a trial and error type of solution scheme.


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The most commonly used equations of states are:

1. Van der Waals Equation.2. Peng-Robinson Equation.

3. Benedict-Webb-Rubin Equation.

4. Redlich-Kwong Equation.


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Calculation of Natural Gas Pseudo-critical Properties

Stewart et al-Mixing Rules-Method

The Stewart methodis a set of mixing rules developed to

calculate pseudo-critical properties of gas mixtures.

I t requi res the gas composition to be knownand more

calculations than Kay's procedure.

The Stewart et al's mixing rules has been proved more accurate.


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Steps to apply the Stewart et al. method are:

1. A. First, estimate the boiling temperature of the C7+fraction,


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1. B. Estimate the pseudo-critical pressure of the C7+fraction,


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1.C. Estimate the pseudocritical temperature of the C7+fraction,


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2. Determine the correction factors Fj, Ej, and Ek for high

molecular- weight components using Sutton's method


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4. B. Correct the parametersJandK for the C7+fraction.

4. C. Calculate the pseudo critical temperature and pressure.

Practice Examples 1-1 and 1-2 in your text book for calculation of Pseudo critical

Properties for a Sweet and sour Natural Gases With the Stewart et aI . Mixing

Rules.


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Estimating Pseudo-critical Properties When

Gas Composition Is Unknown (Suttons Correlations)

This method is based on Sutton's correlation curves, shown inFig. 1.1, and also based on a large database. The method empirical

equations relating pseudocritical properties of the hydrocarbons to

specific gas gravity:

where

Ppch = pseudocritical pressure of the hydrocarbon components, psia;

Tpch = pseudocritical temperature of the hydrocarbon components, R;

gh = specific gas gravity of the hydrocarbon components (air = 1.0).


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Eqs. 1.25 and 1.26 and Fig. 1.1 are applicable for 0.57 12 mol% CO2, > 3 mol% N2, or any H2S,then the hydrocarbon gas gravity should be calculated by

where gw= ggif the separator gas gravity is being used.

O h ifi i f h h d b i


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Once the specific gas gravity of the hydrocarbon components is

estimated, the pseudocritical properties of the hydrocarbon mixture

are calculated with Sutton's correlations given by Eqs. 1.25 and 1.26

or Fig. 1.1.

The pseudocritical properties of the entire mixture, including

contaminants, are estimated with the following equations.

Note that the pseudocritical properties calculated with eqns. 1.28 and 1.29 are not

correct if the gas mixture is contaminated with nonhydrocarbon components (see

next Section).

Practice Examples 1.3 and 1.4, for sweet and sour gas, respecively, with the

Suttonsmethod.


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Practice Example 1.5


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* Caseys method.

Practice Example 1.6.


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Gas Formation Volume Factor (FVF)

The FVF of a gas,Bg, is defined as

where

VR = Volume occupied by gas at reservoir temperature and pressure and,

VSC= Volume occupied by the same mass of gas at standard conditions.

The reciprocal of the gas formation volume factor is called the gas

expansion factor and is designated by the symbolEg


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The volume of n moles of gas at reservoir conditions can be

obtained from the real-gas law,

The volume of n moles of gas at standard conditions can be

obtained

Gas FVF can be obtained as:


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Gas Density

Gas density is defined as the mass of gas per unit volume, or simply

the reciprocal of specific volume, as follows:

Gas density can be calculated as:


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Isothermal gas compressibility

By definition, the isothermal gas compressibility is the change

in volume per unit volume for a unit change in pressure

For a liquid phase, the compressibility is small and usually

assumed to be constant. For a gas phase, the compressibility

is neither small nor constant.


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Isothermal Gas Compressibility


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Isothermal Gas Compressibility

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Reservoir Fluid & It Properties