Gas gravity, as used in natural gas production and reservoir engineering, is the ratio of the molecular weight of a natural gas mixture to that of air, itself a mixture of gases. Gas gravity is perhaps the most important defining property of a natural gas because almost all properties and, in fact, the actual description of the natural gas itself, are intimately related to it. The molecular weight of air is usually taken as equal to 28.97 (approximately 79% nitrogen and 21% oxygen). Therefore the gas gravity, symbolized by γg, is
where yi and MWi are the mole fraction and molecular weight, respectively, of an individual component in the natural gas mixture.
Table 4-1 gives the molecular weights and critical properties for most hydrocarbon and nonhydrocarbon gases likely to be found in a natural gas reservoir. A light gas reservoir is one that contains primarily methane with some ethane and traces of higher molecular-weight gases. Pure methane has a gravity equal to (16.04/28.97) = 0.55. A rich or heavy gas reservoir may have a gravity equal to 0.75 or, in some rare cases, higher than 0.9. Such gravity would mean a significant concentration of ethane plus propane and butane. In commercial practice, the latter two components are removed and sold as natural gas liquids (NGLs).
Table 4-1. Molecular Weights and Critical Properties of Pure Components of Natural Gas
Example 4-1. Calculation of the Gravity of a Natural Gas
A natural gas consists of the following (molar) composition: C1 = 0.880, C2 = 0.082, C3 = 0.021, and CO2 = 0.017. Calculate the gas gravity to air.
Solution
With the data in Table 4-1 and the given composition, the contributions to this natural gas molecular weight are
Therefore, the gas gravity is 18.225/28.97 = 0.63.
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