Determine Compressibility Factor with Present of CO2 and H2S

This example will demonstrate how to determine the compressibility factor (z) for gas with CO₂ and H₂S. As it is described in the article,Determine Compressibility of Gases , it states that gas with CO₂ and H₂S 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

Table 1 – Gas Component

Reservoir pressure = 6,000 psia

Reservoir temperature = 190 F

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

Table 2 – Critical Pressure and Temperature

Pc‘ = Σyipci = 779.7 psia

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

Table 3 – Pc’ and Tr’ by Kay’s Rule

2) Determine ɛ

Where

A =sum of mole fractions of carbon dioxide (CO₂) and hydrogen sulphide (H₂S) in the mixture

B = mole fraction of hydrogen sulphide (H₂S) 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

Figure 1-z-factor from the Standing and Katz Chart

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.