The accuracy of predicting compressibility factor for sour natural gases

This paper presents the initial stage of an effort aimed at developing a ne w correlation to estimate pseudo critical properties for sour gas when the exact composition is not known. Several mixing rules and gas gravity correl ations available in the literature are first evaluated and compared. The ev aluation is performed on a large database consisting of more than 2106 samp les of sour gas-compositions collected worldwide. Several evaluation criter ia are used including the average absolute deviation (AAD), the standard de viation (SD), the coefficient of correlation, R, and cross plots and error histograms. The mixing rules include: Kay's mixing rule combined with Wiche rt-Aziz correlation for the presence of non-hydrocarbons, SSBV mixing rule with Wichert and Aziz, Corredor et al. mixing rule, and Piper et al. mixing rule. These methods, in one form or another, use information on gas compos ition. Three different other methods that are based on gas gravity alone we re also analyzed. These are: Standing, Sutton, and Elsharkawy et al. gas gr avity correlations. While the methods based on knowledge of composition sho wed reasonable accuracy, those based on gas gravity alone showed weak accur acy with low correlation coefficients. A new gas gravity correlation that i s based on the fraction of non-hydrocarbons present in the sour gas was pro posed. Preliminary results indicate that a good improvement over past gravi ty correlations was achieved. The compositional correlations, still show, h owever, better accuracy. Research is still going on to come up with more ac curate correlations that are based on only readily available descriptors.