Variation of density with composition for natural gas mixtures in the supercritical region

The densities of three different natural gas mixtures (Case A, Case B, and Case C) were evaluated at pressures from 14 to 38 MPa (2000 to 5500 psia) and temperatures from 230 K to 350 K by using SonicWare? and NIST-14 software packages. The chosen pressures and temperatures were based on the phase diagrams for each composition and the probability of encountering such conditions in reservoir or pipeline environment. For each isotherm, the heaviest hydrocarbon was varied from 0 to 1 mole percent in increments of 0.001 (Dx=0.001) and the density calculated for each composition. After the densities were obtained, the partial derivatives of the densities with respect to composition, were calculated numerically at fixed pressure and temperature. The results and calculations suggest that it is very difficult to obtain the desired accuracy (+ 0.1 %) in densities when using a combination of composition measurements and equation of state calculations