Correlations for Volumetric Properties
Molar Split Approximation
Given \(y_{i}\) at reservoir conditions and assume that \(y_{n+}\) goes to the liquid phase and \(y_{n}\) goes to the vapor phase at surface conditions.
Derivation
Given the definition of the gas FVF and the real gas law, the definition of the gas FVF can be rewritten as
From the molar material balance and the assumption that \(y_{n+}\) goes to the liquid phase at surface conditions and the fact that the Zfactor at surface conditions is equal to 1 yields
Inserting equation \eqref{eq:step2} in equation \eqref{eq:step1} and rearranging the fraction yields
Method
The approximation for the dry gas FVF using the real gas law becomes
where the Zfactor can be calculated by an EOS or by correlation.
Example
Given a mixture
\(y_i=[0.57116,0.04317,0.00416,0.01561,0.07881,0.13290,0.15417]\)
Assume that \(C_{5+}\) goes to the liquid phase at surface conditions then \(y_{5+}=0.36589\). The ratio of the dry and wet gas FVF then becomes
which indicates a widely different value of for the shrinkage of the gas!
Cragoe Correlation
Note
The units used in this correlation are field units, i.e. pressure: (psia), temperature: (R), OGR: (STB/scf) and FVF: (ft3/scf).
Method
where
and
In equation \eqref{cragoe_MW} \(\gamma_{API}\) is the API gravity of the surface oil.
Standing Oil FVF and GOR Correlations
In 1947 Standing published correlations for both oil FVF (for both saturated and undersaturated conditions), solution GOR and a bubblepoint correlation^{1}. The correlations were based on 105 different datapoints for 22 different hydrocarbon systems. In the following subsections the oil FVF and solution GOR correlations are given.
Note
The units used in this correlation are temperature: (F), pressure: (psia), solution GOR: (scf/STB).
Oil FVF (Saturated)
where \(\gamma_o\) and \(\gamma_g\) are the surface gas and oil specific gravities.
Oil FVF (Underaturated)
where \(c_o\) is the oil compressibility, \(p_b\) is the bubblepoint pressure and \(B_{ob}\) is the oil FVF at the bubblepoint pressure. The oil compressibility can be calculated by correlation or measured values.
Typically the bubblepoint is calculated by correlation. Standing provides a correlation for the bubblepoint given by rearranging equation \eqref{eq:standing_GOR} given by
where \(\gamma_{API}\) is the surface oil API gravity.
Solution GOR
where \(\gamma_{API}\) is the surface oil API gravity and \(\gamma_g\) is the gas gravity specific gravities.

M.B. Standing. A pressurevolumetemperature correlation for mixtures of california oils and gases. In Drilling and Production Practice, paper API–47–275. American Petroleum Institute, 1947. ↩