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Pseudo-Critical Properties

Introduction

Pseudo-critical properties are the properties of a mixture of components which each pure component contributes for the overall pseudo-critical property estimation. When the composition of the fluid mixture (natural gas) is known, the calculation of pseudo-critical pressure and temperature can be calculated as:

where:

is the pseudo-critical temperature;

is the pseudo-critical pressure;

is the critical temperature of the component i ;

is the critical pressure of the component i ;

is the molar fraction of the component i.

However, in the case where the composition of the natural gas is not available, the pseudo-critical properties can be estimated from some correlations that has a input specific gravity of the gas, for example.

Standing (1977)

Standing developed a correlation to predict pseudo critical pressure and temperature based on the specific gravity of the gas and divided it in two different of gas type, dry gas and wet gas.

  • Dry Gas

  • Wet Gas

where

is the pseudo-critical temperature [oR];

is the pseudo-critical pressure [psia];

is the specific gravity of the gas mixture [-];

Effect of Non-hydrocarbon components in Natural Gas

Natural gas can have frequently a large amount of contaminants, such as , , and others. The presence of these contaminants deviate and change the behaviour of Z-factor. Therefore the Z-factor needs to be correct based on the concentration of these contaminants in the natural gas. There are different correlation and methods to to correct Z-factor.

Witchert and Aziz (1992)

Wichert and Aziz, 1992 developed a correlation to re-estimate the pseudo critical properties of natural gas in the presence of and . They introduced a correction factor as following:

where

are the mole fraction of and , respectively.

The corrected and are:

Whitson and Brule (2000)

Whitson and Brule, 2000 developed a correlation based on gas specific gravity and non-hydrocarbon contents to estimate natural gas pseudo critical properties to further adjust these properties using Witchert-Aziz method. The calculation procedure is as following:

Step 1: Calculate hydrocarbon specific gravity, excluding the non-hydrocarbon components:

Step 2: Estimate pseudo critical properties using the hydrocarbon specific gravity as following:

Step 3: Adjust these two values and to account for non-hydrocarbon components by applying the following relationships:

Step 4: Use the and calculated in step 3 as a input of Wichert and Aziz correlation.

References

  • El-Banbi, Ahmed / Alzahabi, Ahmed / El-Maraghi, Ahmed  PVT Property Correlations: Selection and Estimation, 2018, Gulf Professional Publishing.

  • Ahmed, Tarek, Equations of State and PVT Analysis: Applications for Improved Reservoir Modelling, 2007, Gulf Publishing