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. 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 Llouisiana. 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 predictions, 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 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 modelling of choke performance is vitally important for petroleum engineers in oil production simulation.

Tangren. 1 performed the first investigation on gasliquid two-phase flow through restrictions. He presented an analysis of the behavior of an expanding gas-liquid system. He showed that when gas bubbles are added to an incompressible fluid, above a critical flow velocity, the medium becomes incapable of transmitting pressure change upstream against the flow. Several empirical choke flow models have been developed in the past halfcentury. They generally take the following form for sonic flow: (equation (1) is available in full of paper).

and C, m and n are empirical constants related to fluid properties. On the basis of the production data from Ten Section Field in California, Gilbert2 found the values for C, m and n to be 435, 0.546 and 1.89, respectively. Other values for the constants were proposed different researchers including Baxendell, 3 Ros, 4 and Achong. 5 Poettmann and Beck6 extended the work of Ros to develop charts for different API crude oils. Omana7 derived dimensionless choke correlations for water-gas systems.

Fortunati8 was the first investigator who presented a model that can be used to calculate critical and subcritical two-phase flow through chokes. Ashford9 also developed a relation for two-phase critical flow based on the work of Ros. Gould10plotted the criticalsubcritical boundary defined by Ashford, showing that different values of the polytropic exponents yield different boundaries. Ashford and Pierce11 derived an equation to predict the critical pressure ratio. Their model assumes that the derivative of flow rate with respect to the downstream pressure is zero at critical conditions. One set of equations was recommended for both critical and subcritical flow conditions.

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