Additional Quality Controls
In this section, several quality checks that are used in various PVT experiments like CCE, CVD, and DLE or sample validations are presented.
Zfactor Quality Check
To estimate the gas Zfactor, Hall and Yarborough correlations are used. The inputs of the correlations are pseudocritical properties of the gas mixture, \(T_{pc}\) and \(p_{pc}\). To calculate them, we use Sutton correlations, Standing correlations, or Kay's mixing rule. For more information, see the pages ZFactor and Real Gas Law.
When gas composition is available, Kay's mixing rule is usually applied^{1}.
where the pseudocritical properties of \(\text{C}_{7+}\) can be estimated from Matthews et al. correlations^{2},
When significant quantities of \(\text{CO}_2\) and \(\text{H}_2\text{S}\) nonhydrocarbon components are present, pseudocritical properties are corrected using Wichert and Aziz correlations^{3}.
where,
If only the gas gravity and nonhydrocarbon content are known, the hydrocarbon specific gravity is first calculated from
Hydrocarbon pseudocritical properties are then calculated using Sutton correlations. These values are adjusted for nonhydrocarbon content based on Kay's mixing rule.
Hall and Yarborough correlations^{4} are used to calculate the Zfactor. First, the following parameters are calculated.
Then, the reduceddensity parameter, \(y\), is obtained by solving
with
An initial value of \(y = 0.001\) can be used with a NewtonRaphson procedure to solve Equation \eqref{eq:hall_y} for \(y\)^{1}. Then, Zfactor is given by
Figure 1 shows an example of Zfactor QC for the data given in this Excel file.
Figure 1: An example of Zfactor QC.
Hoffman et al. Kvalue Quality Check
Hoffman et al. proposed correlations for prediction of \(K\)values. Standing used the Hoffman et al. method to generate a lowpressure \(K\)value equation for surfaceseparator calculations with \(p_{\text{sep}} < 1000 \text{ psia}\) and \(T_{\text{sep}} < 200\ ^{\circ}\text{F}\). The Standing equations are given below.
First, the parameters of the correlation are calculated.
For all components up to , Table 1 is used for parameters \(b_i\) and \(T_{bi}\).
Table 1 Values of \(b_i\) and \(T_{bi}\) for use in Standing lowpressure \(K\)value correlation^{1}.
Component  \(b_i\)(cycle\(^\circ\)R)  \(T_{bi}\) (\(^\circ\)R) 

N\(_2\)  470  109 
CO\(_2\)  652  194 
H\(_2\)S  1136  331 
C\(_1\)  300  94 
C\(_2\)  1145  303 
C\(_3\)  1799  416 
iC\(_4\)  2037  471 
nC\(_4\)  2153  491 
iC\(_5\)  2368  542 
nC\(_5\)  2480  557 
C\(_6\)  2738  610 
For \(\text{C}_{7+}\) fraction, the following equations are used.
\(K\)values are then estimated using
Figure 2 shows an example of Hoffman et al. quality check for a separator test given in this Excel file.
Figure 2: An example of Hoffman et al. QC for a separator test .
References

C. H. Whitson and M. R. Brulé. Phase behavior. Volume 20. Henry L. Doherty Memorial Fund of AIME, Society of Petroleum Engineers …, 2000. ↩↩↩

TA Matthews, CH Roland, and DL Katz. High pressure gas measurement. Petroleum Refiner, 1942. ↩

Edward Wichert and Khalid Aziz. Compressibility factor of sour natural gases. The Canadian Journal of Chemical Engineering, 49:267–273, 1971. ↩

K. R. Hall and L. Yarborough. A new equation of state for zfactor calculations. Oil Gas J, 71:82, 1973. ↩