A new mechanistic model for two-phase flow in vertical and inclined pipes was proposed based on Drift-Flux approach. The proposed model, unlike the other mechanistic models (Ansari et al.1 , Xiao et al.2 , Unified Mechanistic Model3 ), which incorporate a system of nonlinear equation to solve, uses an explicit equation for liquid holdup prediction thus reducing computation time significantly. Coupled with some simplified assumptions on PVT, such simple form of liquid holdup prediction formula enables analytical integration of pressure gradient in two-phase flow along the pipe. This procedure is usually used to speed-up calculation of bottom-hole pressure for large number wells for oil production optimization purposes4 .

Drift-flux approach can predict liquid holdup for bubbly flow quite accurately. But for slug flow, it usually underestimates the void fraction. Since slug flow is the most common in producing wells, this leads to the pressure drop being overestimated significantly, that can be proved comparing computational results to the experimental data and mechanistic models. Small gas bubbles in liquid slugs should be taken into account to predict liquid holdup for slug flow more accurately. Gas in slug body is considered by adding a proper term to the void fraction expression. This term is based on correlation for liquid holdup in slug body. The model was evaluated using Rosneft field data and TUFFP databank. Evaluation of the model was made in comparison with three other mechanistic models for multiphase flow

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