Improvement in Sachdeva's Multiphase Choke Flow Model Using Field Data

Abstract

Abstract Sachdeva's choke flow model has been found capable of predicting critical-subcritical boundary and liquid and gas flow rates for multiphase crude systems. Although this model was shown to be accurate by Sachdeva et al.(15) in their original paper using laboratory and field data, inaccuracy of the model has been found in other field applications. In this study, the accuracy of the choke model was evaluated using data from 239 oil wells and 273 gas condensate wells in Southwest Louisiana. Comparisons of results from measurements and model calculations indicate that the model is more accurate for oil wells than for gas condensate wells. It was found that the error of the model could be minimized using different values of choke discharge coefficient (CD). For oil wells, CD = 1.08 should be used for liquid rate prediction and CD = 0.78 should be used for gas rate predictions. For gas condensate wells, CD = 1.07 should be used for gas rate predictions and CD = 1.53 should be used for liquid rate predictions. Introduction Wellhead chokes are equipment used in the oil and gas industry to control fluid production rates from wells, to maintain stable pressure downstream from the choke and to provide the necessary backpressure to a reservoir to avoid formation damage from excessive drawdown. Because oil and gas production rates are extremely sensitive to choke size, accurate modeling of choke performance is vitally important for petroleum engineers in oil production simulation. Tangren et al.(1) performed the first investigation on gas-liquid two-phase flow through restrictions. They presented an analysis of the behaviour of an expanding gas-liquid system. They showed that when gas bubbles are added to an incompressible fluid above a critical flow velocity, the medium becomes incapable of transmitting a pressure change upstream against the flow. Several empirical choke flow models have been developed in the past half century. They generally take the following form for sonic flow: Equation (1) (Available in full paper) where: Pwh = upstream (wellhead) pressure, kPa Q = gross liquid rate, m3/day R = gas-liquid ratio, scm/m3 S = choke size, cm and C, m and n are empirical constants related to fluid properties. On the basis of the production data from Ten Section Field in Cal-ifornia, Gilbert(2) found the values for C, m and n to be 2.4644, 0.546 and 1.89, respectively. Other values for the constants were proposed by different researchers including Baxendell(3), Ros(4) and Achong(5). Poettmann and Beck(6) extended the work of Ros to develop charts for different API crude oils. Omana et al.(7) derived dimensionless choke correlations for water-gas systems. Fortunati(8) was the first investigator who presented a model that can be used to calculate critical and subcritical two-phase flow through chokes. Ashford(9) also developed a relation for two-phase critical flow based on the work of Ros. Gould(10) plotted the critical-subcritical boundary defined by Ashford, showing that different values of the polytropic exponents yield different boundaries. Ashford and Pierce(11) derived an equation to predict the critical pressure ratio.