PC-SAFT with New Pure-Component Parameters Fit to Density Data Collected At Temperatures to 260°C and Pressures to 276 Mpa

The perturbed chain – statistically associated fluid theory (PC-SAFT) gives reliable (±2%) PρT predictions for n-alkanes and other hydrocarbons at pressures below ~ 8000 psi.  However, PC-SAFT tends to overpredict densities at higher pressures (by as much as 5%), particularly for normal and branched alkanes; this phenomenon also increases with respect to temperature.  Since such conditions are frequently encountered in the field of petroleum recovery, PC-SAFT was evaluated to improve the density predictions at elevated temperatures (up to 260° C) and pressures (up to 276 MPa).  For most of the n-alkanes and other compounds studied by Gross and Sadowski, the three pure-component PC-SAFT parameters m, σ, and ε/k were obtained by fitting the equation predictions to subcritical PρT data only (occasionally both sub- and super-critical density data were used).  Therefore, we obtained a new, improved set of pure-component PC-SAFT parameters by fitting the PC-SAFT density predictions to high-temperature, high-pressure (HTHP) density data present in the literature that cover almost the entire pressure and temperature ranges of interest for deep reservoirs (instead of subcritical density data). For each data set, the minimum pressure value was typically in excess of 6.9 MPa and the maximum value was below 276 MPa.  Using a downhill simplex method, the three pure-component PC-SAFT parameters were allowed to vary until the value of the mean average percent deviation (MAPD) between the experimental density and the PC-SAFT prediction was minimized.  Density predictions made using the new, HTHP PC-SAFT pure-component parameters were typically within 0.5% of the experimental values for data points obtained at pressures between 6.9 and 276 MPa.  For compounds with a normal boiling point of higher than ~ 110° C, the HTHP PC-SAFT equation gives density predictions for data points obtained at pressures below 6.9 MPa that, on average, are only slightly inferior to those given by the original PC-SAFT equation.  Correlations were developed to accurately predict the HTHP PC-SAFT parameters for normal and branched alkanes for which there are either incomplete or nonexistent experimental density data sets.