Example of Real Gas Calculation

This example will demonstrate how to calculate the compressibility of real gas in order to determine gas density and specific gravity at a specific condition.

Calculate the following based on the given condition:

1) Density of this gas under the reservoir conditions of 7,500psia and 220ºF,

2) Specific gravity of the gas.

Gas component is shown in Table 1

Table 1 - Gas Component

Table 1 – Gas Component

Average density of air = 28.96 lb/cu-ft


  • Determine critical pressure and temperature of gas mixtures using Kay’s rule
Table 2 - Critical Pressure and Temperature

Table 2 – Critical Pressure and Temperature

Note: critical pressure and temperature can be found from this link – http://www.drillingformulas.com/determine-compressibility-of-gases/

Pc’ = Σyipci = 660.5 psia

Tc’ = ΣyiTci = -46.2 F = -46.2 +460 F = 413.8 R

Table 3 - Pc' and Tr' by Kay's Rule

Table 3 – Pc’ and Tr’ by Kay’s Rule

  • Calculate Tr and Pr

Tr = T ÷Tc

Tr = (220+460) ÷ (-46.2+460)

Tr = 1.64

Note: temperature must be in Rankin.

Rankin = Fahrenheit + 460

For the critical temperature calculation, it can be converted the critical temperature from F to R before calculating Tc’. This will still give the same result. 

Pr = P ÷ Pc

Pr = 7500 ÷ 660.5 = 11.4

  • Read the compressibility factor (z) from the chart.

z = 1.22

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

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

  • Calculate average molar mass

Average Molar Mass = Σyi×Mi = 22.1 lb

Table 4 - Average Molar Mass of Gas

Table 4 – Average Molar Mass of Gas

  • Calculate density of gas from the equation below;

gas density

Gas Density = 18.6 lb/cu-ft

  • Calculate gas specific gravity from the equation below;

SG = Gas Density ÷ Air Density

SG = 18.6 ÷ 28.96

SG = 0.64


The answers for this answer are listed below;

Gas Density = 18.6 lb/cu-ft

SG = 0.64

We wish that this example will help you understand to determine z-factor and use it to calculate any related information.


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.

Related Post

Advantages and Disadvantages of Wait and Weight Me... We’ve learned so many topics regarding wait and weight well control method and I would like to share what advantages and disadvantages of wait and wei...
Pressure Loss and Equivalent Circulating Density R... We still have the same question as the previous post, Pressure Loss and Equivalent Circulating Density Review, but this case we will do reverser circu...
Pipe Displacement Calculation Pipe displacement, normally in bbl/ft, is steel volume to displace fluid volume.  When we either pull out of hole or trip in hole for any kind of pipe...
Effect of Frictional Pressure on ECD while reverse... A reverse circulation is another way to circulate by circulating into annulus up to a bit and drill string. The fluid outlet is at stand pipe side. Fo...
Share the joy

About DrillingFormulas.Com

Working in the oil field and loving to share knowledge.
Tagged , , , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *