Pseudocritical Properties from Gas Gravity

In the absence of detailed composition of a natural gas, Figure 4-2 can be used to relate the gas gravity (to air) with the pseudocritical properties of gas mixtures. Using the results of Example 4-2, the calculated molecular weight is 18.92, leading to γ_g = 18.92/28.97 = 0.65. From Figure 4-2, p_pc = 670 psi and T_pc = 375°R, which compare with 671 psi and 378°R, calculated explicitly in Example 4-2.

Figure 4-2. Pseudocritical properties of natural gases. (From Brown, Katz, Oberfell, and Alden, 1948; inserts from Carr, Kobayashi, and Burrows, 1954.)

Figure 4-2 can be used as an approximation when only the gas gravity is known or when a quick calculation is indicated. Alternatively, Standing (1977) gave the following empirical equations for the curves in Figure 4-2:

for the miscellaneous gases, and

for condensate well fluids.

4.2.2. Presence of Nonhydrocarbon Gases

A gas with a high content of H₂S is often referred to as a “sour” gas. Natural gas with H₂S and/or CO₂ is called acid gas. Otherwise, natural gas without acidic components is called sweet gas. If the total gas gravity, γ_gM, is known, as well as the mole fractions of the nonhydrocarbon components, the hydrocarbon gas gravity can be computed as (Standing, 1977)

The inserts in Figure 4-2 can be used to adjust the pseudocritical properties of a gas mixture to account for the presence of nonhydrocarbon gases. Alternatively, the gas mixture pseudocritical values can be computed from hydrocarbon pseudocritical values as

using Equation (4-8) and Equation (4-9) with the hydrocarbon gravity from Equation (4-12). A more rigorous calculation using the complete gas composition is shown in Example 4-2.