Compressibility Factor for Sour Gas Reservoirs

This paper presents the initial stage of an effort aimed at developing a new correlation to estimate pseudo critical properties for sour gas when the exact composition is not known. Several mixing rules and gas gravity correlations available in the literature are first evaluated and compared. The evaluation is performed on a large database consisting of more than 2000 samples of sour gas compositions collected worldwide. Several evaluation criteria are used including the average absolute deviation (AAD), the standard deviation (SD), the coefficient of correlation, R, and cross plots and error histograms. The mixing rules include: Kay’s mixing rule combined with Wichert-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 composition. Three different other methods that are based on gas gravity alone were also analyzed. These are: Standing, Sutton, and Elsharkawy et al. gas gravity correlations. While the methods based on knowledge of composition showed reasonable accuracy, those based on gas gravity alone showed weak accuracy with low correlation coefficients. A new gas gravity correlation that is based on the fraction of non-hydrocarbons present in the sour gas was proposed. Preliminary results indicate that a good improvement over past gravity correlations was achieved. The compositional correlations, still show, however, better accuracy. Research is still going on to come up with more accurate correlations that are based on only readily available descriptors. Introduction Gas compressibility factor is involved in calculating gas properties such as formation volume factor, density, compressibility, and viscosity. All these properties are necessary in the oil and gas industry for evaluating newly discovered gas reservoirs, calculating initial and gas reserves, predicting future gas production, and designing production tubing and pipelines. The industry standard is to measure gas properties, PressureVolume-Temperature (PVT), in laboratory using reservoir samples. The draw back is that these isothermally measured PVT data is applicable at measured pressured and reservoir temperature. Calculation methods such as correlations and equations of state are used to predict properties at other pressures and temperatures. Also, laboratory analyses for PVT behavior are sometimes expensive and time consuming. Correlations, which are used to predict gas compressibility factor, are much easier and faster than equations of state. Sometimes these correlations have comparable accuracy to equations of state. Predicting compressibility factor for sour gases is much more difficult than that of sweet gases. Therefore, several attempts have been made to predict compressibility factor for sweet gases . Wichert and Aziz presented corrections for the presence of hydrogen sulfide and carbon dioxide for determining compressibility factor of sour gases. Because there is no exact method for predicting the PVT behavior of natural gases several approximations have been proposed. The most common method is to use one of the forms of the principle of corresponding states. In this form, gas compressibility factor is expressed as a function of pseudo reduced pressure and temperature (Ppr,Tpr). Standing and Katz (SK) presented a chart for determining gas compressibility factor based on the principle of corresponding states. The SK chart was prepared for binary mixtures of low molecular weight sweet gases. Several mathematical expressions fitting the SK chart, have been proposed to calculate the gas compressibility factor. Evaluation of these methods by Takacs and Elsharkawy et al. concluded that Dranchuk-Abou-Kassem (DK) correlation is the most accurate representation of SK chart. When dealing with gas mixtures, the mixture critical pressure (Ppc) and temperature (Tpc) are required. Critical properties of natural gas are calculated from either gas composition or gas gravity. Several mixing rules have been proposed to calculate mixture critical properties of natural SPE 64284 Compressibility Factor for Sour Gas Reservoirs Adel M. Elsharkawy and Ali Elkamel College of Engineering and Petroleum Kuwait University, P.O. Box 5969, Safat 13060, Kuwait