New Correlations for Critical and Subcritical Two-Phase Flow Through Surface Chokes in High-Rate Oil Wells

Abstract

Abstract The critical and subcritical multiphase flow through wellhead restrictions of a prolific oil field in the Middle East is investigated and two sets of new correlations are presented. The first set of correlations is developed by using "40" field tests representing critical flow conditions. The second set of correlations is based on "139" field tests representing subcritical flow conditions of gas-liquid mixtures through wellhead chokes. For the critical multiphase flow condition, the predicted oil flow rates by the new set of correlations are in excellent agreement with the measured ones. The absolute average percent difference (AAPD) is between "1.88" and "4.37", and the corresponding standard of deviation (SD) is between "2.52" and "6.52". These results are found to be statistically superior to those predicted by other published correlations considered in this work. During the subcritical gas-liquid flow conditions through surface chokes, the accuracy of oil flow rates predicted by the new set of correlations seems to be sensitive to the type and size of the choke being used. For Cameron LD type and 144/64th -inch choke, the oil flow rates predicted by the proposed correlation are superior to those predicted by other methods available in the literature, with AAPD of "8.5". However, for smaller choke sizes of 96/64th and 64/64th -inch, the oil flow rates predicted by the new correlations and other methods are found to be close to each other. For Cameron F type and 144/64th -inch choke size, the oil flow rates predicted by the new correlation are closely matched by those predicted by other published methods, with AAPD of "13.7". For smaller choke sizes of 80/64th and 64/64th -inch few field tests are available and the predications of all methods, including the proposed ones, show similar statistical results. The above findings for Cameron F choke also seem to apply to Beaned wellhead assemblies for this particular oil field. Introduction The main problems associated with multiphase flow through restrictions are calculation of (i) pressure upstream from choke, (ii) liquid production rate, or (iii) choke diameter. Thus, various developments have been published that present theory and correlations for describing simultaneous liquid and gas flow through surface restrictions. Most of these correlations are for critical flow across the choke and very little work has been done for subcritical flow. Fonseca (1972) showed that the approach used by most investigators may be classified into one of the following:(i) Empirical correlations from field or laboratory data.(ii) Empirical correlations using dimensional analysis to select and group the most important variables.(iii) Theoretical approaches applying mathematical analysis to a simplified physical model with development of equations. Beggs and Brill (1975) stated that the two-phase flow through restrictions may fall into one of the following categories:Critical flow: this occurs when the fluid flows through the choke at velocities greater than that of sound in that fluid. To satisfy this condition in oil field work the upstream pressure must be approximately twice the downstream pressure.Subcritical flow: this occurs when the velocity of the fluid is less than the velocity of sound in that fluid.