Correlations Developed To Predict Two-Phase Flow Through Wellhead Chokes

Abstract

Abstract The predictive accuracy of ten critical two-phase flow correlations in combination with four PVT property correlations, is tested against field measured production data, from 210 well tests, covering a broad range of production rates, choke sizes, upstream pressures, gas-liquid ratios and oil API gravities. Test data are divided into four selected categories based on choke size (D) : D <6, 6 ≤D <10, 10 ≤D <30, and D ≥30/64 ". The average per cent error, absolute average per cent error and standard deviation are computed for each correlation combination. It is observed that the choice of empirical PVT correlation appears to have only a minor effect on final calculated statistical results. Also it is found that most of the compared correlations yielded unsatisfactory results, therefore an attempt is made to find correlations that best fit the measured data. As a result, four new correlations are developed (a correlation for each diameter category). Based on the statistical results, the new correlations clearly outperformed the original correlations. Introduction Several correlations have been published for describing critical two-phase flow through wellhead chokes. Most of these correlations were based on limited ranges of flow variables. Their validity is limited by the quality and scope of the data upon which they are based. The objectives of this study are:to determine the optimal combination of critical flow and PVT correlations for predicting flow rates; andto modify the existing correlations if necessary or develop new correlations that best fit the measured data. The correlations compared are those of Gilbert(1), Ros(2), Baxendall(3), Achong(4), Poettmann-Beck(5), Omana(6), Ashford(7), Pilehvari(8), Sachdeva et al.(9) and Hazim-Ghassan(10). Some of the correlations require values of fluid physical properties, combinations of the following PVT correlations are used to estimate them: Oil FVFStanding(11)Vazquez-Beggs(12)Ghassan-Naeema(13) Solution GORStanding(11)Lasater(14)Vazquez-Beggs(12) In addition, other PVT properties are calculated using the following correlations: Gas Compressibility : Hall-Yarborough(15) Gas-Oil Surface Tension : Baker-Swerdloff(16) The performance of each critical flow/PVT correlations was checked against the measured well data. Statistical tools were used to determine the optimum correlation combination for predicting flow rates. Well Test Data Fifty-six tests from Iraq wells, 108 from Poettmann's study(5), 27 from Ashford's study and 37 from Omana's study are combined in a data bank for this investigation. The approach developed by Sachdeva et al.(9) (Eq. 1) is used to verify that the test data collected are within the critical flow regime rather than subcritical flow regime. Equation (1) (Available In Full Paper) Where Yc = critical pressure ratio x1 = free gas quality VG = gas specific volume, ft3/lbm VL = liquid specific volume, ft3/lbm