Utilization of Numerical Optimization to Model Gas Condensate Flow Through Chokes in a Rich Gas Condensate Reservoir in Saudi Arabia
Authors: Bokhamseen Senan H. , Tarihoran Anton R. , Al-Baqawi Ahmad M. , Al-Malki Bandar H. 
Published: November 2015

Production metering of a Saudi Arabian rich gas condensate reservoir with an average condensate to gas ratio (CGR) of 330 stock tank barrels/million standard cubic feet (STB/MMscf) is under evaluation. Inapplicability of Venturi meters, due to high liquid production, accompanied by the absence of multiphase flow meters and dedicated permanent test separator facility for individual testing, has resulted in a major metering challenge during the production life of this field. Infrequent deliverability tests with costly portable test separators are the only way to obtain direct and accurate gas and condensate rate measurements, which has severely affected the rate allocation accuracy. In this study, an alternative solution was employed to improve gas rate allocation and target rate compliance. Understanding the nature of multiphase flow through chokes and the principals of flow calculations to derive accurate correlations and models is the only alternative to mitigate the lack of surface metering devices in such a high CGR environment.

Choke equations rely heavily on choke parameters, surface measurements, and fluid properties with some assumed empirical constants. The majority of the published choke correlations have their unique empirical constants that fit the flow conditions and fluid properties of the studied fields. In this study, a numerical optimization technique was implemented to develop and brand two empirical choke correlations, which model the gas condensate flow through chokes under critical flow conditions in the subject field. An iterative methodology was performed by capitalizing on a nonlinear algorithm to analyze 64 well test data points, collected from 16 different separator tests, to identify the unique values of the empirical constants, which would yield the most accurate flow rate estimations.

Statistical evaluation results indicate the developed empirical choke models yielded encouraging results. The models successfully matched the measured gas flow rates, from 26 well tests, with less than 5% average absolute error, proving the high accuracy of these models in estimating the flow rates. In addition, back allocation task was greatly simplified by utilizing these developed choke equations. Comparison between the total field choke equation estimated rate and measured field gas rate at the gas plant for a period of 3 months showed an outstanding agreement with less than 5% average error and 1.02 average allocation factor. Moreover, the investigated empirical constants were examined in a different field with similar fluid properties and obtained excellent matching outcomes as well, reconfirming the precision of these correlations in estimating the gas rates in a high CGR environment.

The applications of these correlations will allow field operators to accurately estimate the flow rates without the necessity of conducting frequent costly separator tests, or installing expensive permanent multiphase flow meters.