This example will demonstrate how to determine the compressibility factor (z) for gas with CO_{2} and H_{2}S. As it is described in the article,Determine Compressibility of Gases , it states that gas with CO_{2} and H_{2}S must be corrected. This is similar to a normal method for determining z-factor but it is required some correction. Please follow the steps below;

Gas component is shown in Table 1

Reservoir pressure = 6,000 psia

Reservoir temperature = 190 F

**1) Determine critical pressure and temperature of gas mixtures using Kay’s rule**

Pc‘ = Σyipci = 779.7 psia

Tc’ = ΣyiTci = -22.43 F = -22.43 +460 F = 437.57 R

**2) Determine ɛ**

Where

A =sum of mole fractions of carbon dioxide (CO_{2}) and hydrogen sulphide (H_{2}S) in the mixture

B = mole fraction of hydrogen sulphide (H_{2}S) in the mixture.

A =0.082 + 0.132 = 0.214

B= 0.132

**3) Determine corrected pseudo-critical temperature (Tc”) and corrected pseudo-critical pressure (Pc”)**

Corrected pseudo-critical temperature (Tc”)

Corrected pseudo-critical pressure (Pc”)

**4) Calculate pseudo reduced temperature and pressure**

Pr” = P ÷ Pc”

Pr” = 6,000 ÷ 729.94 = 8.22

Tr” = T ÷Tc”

Tr” = (190+460) ÷ (412.34)

Tr” = 1.58

Note: temperature must be in Rankin.

**5) Read the compressibility factor (z) from the chart.**

z = 1.022

**References**

Abhijit Y. Dandekar, 2013. Petroleum Reservoir Rock and Fluid Properties, Second Edition. 2 Edition. CRC Press.

L.P. Dake, 1983. Fundamentals of Reservoir Engineering, Volume 8 (Developments in Petroleum Science). New impression Edition. Elsevier Science.

Tarek Ahmed PhD PE, 2011. Advanced Reservoir Management and Engineering, Second Edition. 2 Edition. Gulf Professional Publishing.

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