Separator Sample Analysis and Calculating Wellstream Composition

Abstract

Abstract This paper discusses the use of separator gas and separator oil samples as the primary method used in tight unconventional wells. Separator samples provide direct measurement of surface product compositions and properties, but they are primarily used in the calculation of wellstream composition. Our discussion encompasses a comprehensive and quantitative QC and correction of separator sample compositions, also providing improved accuracy in the estimation of wellstream composition, both for conventional and tight unconventional wells. The standard QC of separator sample compositions assumes that the samples collected should represent equilibrium gas and oil phases at conditions of pressure and temperature in the separator at the time of sampling. The Hoffman-Crump-Hocott method plots the log of component K-value and separator pressure log(Kpsp) versus a component factor F. The Hoffman "KpF" plot should result in a linear trend, with deviations from the linear trend indicating that the sampled phases are not in equilibrium. The problem with this QC method is that it does not provide a method to correct erroneous separator sample compositions. Our paper presents such a correction method for separator gas, separator oil, and wellstream compositions. The proposed method uses the lab-reported wellstream composition based on the sampled gas and oil separator compositions using a recombination ratio computed from the separator gas-oil ratio at the time of sampling, and separator oil molar volume. This wellstream composition is flashed with K-values applicable at separator temperature and pressure, e.g. from the Standing correlation using a Rachford-Rice phase split calculation, or doing a flash with an appropriate equation of state. The resulting flash calculation provides corrected separator gas and separator oil compositions that are more accurate than lab-reported separator gas and oil compositions, and can be used to calculate a corrected wellstream composition using the lab-reported recombination ratio. We have studied the utility of this method for a wide range of wellstream mixtures, separator conditions, and the four potential sources of errors in separator samples: (1) leakage during transportation to the laboratory, (2) samples being collected at different times when pressure-temperature conditions differ for the gas and oil samples, (3) liquid carryover in the separator gas and gas carryover in the separator oil, and (4) gas chromatography errors in laboratory compositional measurements. We show that the proposed method of correcting separator gas, separator oil, and wellstream compositions are accurate for all fluid types and errors that may be found in separator sampling. The approach is consistent with, but not dependent on, the Hoffman KpF method. The method does require K-value estimates at separator conditions, but this is almost always available from the Standing correlation (often used in connection with the Hoffman KpF QC plot) or an available EOS model.