The number of wells drilled using underbalanced technique is increasing rapidly due to the advantages of this technology, i.e., increased penetration rate, minimized lost circulation and differential sticking, reduced formation damage and many environmental benefits. During Coiled Tubing (CT) underbalanced operations, gas-liquid mixtures are commonly used. In order to accomplish a successful drilling program, hydraulic calculations concerning two-phase fluid system should be carried out carefully. Since the flow geometry is mostly fully eccentric annuli in horizontal section of the wellbore, a realistic mechanistic model is essential. Although there are numerous studies available for two-phase flow through circular pipes, less is known about the flow of two-phase fluids though annular geometries. Commonly used methodology is using hydraulic diameter for representing the annular geometry, and using the circular pipe correlations by replacing the circular diameter with hydraulic diameter. It is observed that a better methodology is needed for basic hydraulic calculations. Therefore, in this study, horizontal flow of gas-liquid mixtures through fully-eccentric annuli is investigated. A mechanistic model is proposed for estimating frictional pressure losses, which can be used for both circular pipes and annular geometries by introducing a representative diameter term. Flow pattern identification is developed inheriting basic concepts proposed by Taitel-Dukler1 . The model results are compared with experimental data obtained from two different annular geometries (4.5in × 2.25in and 3.672in × 1.92in) at Middle East Technical University – Petroleum & Natural Gas Engineering Department – Cuttings Transport and Multiphase Flow Loop. It is observed that, for representing the annular geometries, introduced representative diameter works significantly more accurately than hydraulic diameter. Additionally, flow patterns can be estimated better if representative diameter term is used. It is concluded that, the proposed mechanistic model can identify the flow patterns correctly and estimate frictional pressure losses with an accuracy of less than 20% when compared with the experimental data.

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