PNG 520
Phase Relations in Reservoir Engineering

Specific Gravity

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Specific gravity is defined as the ratio of fluid density to the density of a reference substance, both defined at the same pressure and temperature. These densities are usually defined at standard conditions (14.7 psia and 60°F). For a condensate, oil or a liquid, the reference substance is water:

γ o = ( ρ 0 ) sc ( ρ w ) sc This equation is not rendering properly due to an incompatible browser. See Technical Requirements in the Orientation for a list of compatible browsers.
(18.3)

The value of water density at standard conditions is 62.4 lbm/ft3 approximately. For a natural gas, or any other gas for this matter, the reference substance is air:

γ g = ( ρ g ) sc ( ρ air ) sc This equation is not rendering properly due to an incompatible browser. See Technical Requirements in the Orientation for a list of compatible browsers.
(18.3a)

Or, equivalently, substituting Equation (18.2) evaluated at standard conditions ( Z sc ≈1 This equation is not rendering properly due to an incompatible browser. See Technical Requirements in the Orientation for a list of compatible browsers. for most gases),

γ g = M W g M W air This equation is not rendering properly due to an incompatible browser. See Technical Requirements in the Orientation for a list of compatible browsers.
(18.3b)

where the value of the molecular weight for air is MWair = 28.96 lbm/lbmol. Specific gravity is nondimensional because both numerator and denominator have the same units.