Two-phase flow occurs during the production of oil and gas in the wellbores. Modeling this phenomenon is important for monitoring well productivity and designing surface facilities. Since transient time period in the producing well is usually shorter than reservoir time steps, stabilized flow is assumed in the wellbore. As such, semi-steady state models are used for modeling wellbore dynamics. However, in the case that flow variations occur in a short period of time (i.e., gas kick during drilling) the use of a transient two-phase model is crucial.

A great deal of research has been conducted to study transient two-phase flow in wellbores. However, there is lack of a comprehensive two-fluid model in the petroleum literature. In this paper, we present an implementation of a pseudo-compositional, thermal, fully implicit, transient two-fluid model for two-phase flow in wellbores. In this model, we solve gas/liquid mass balance, gas/liquid momentum balance, and two-phase energy balance equations to obtain five primary variables: liquid velocity, gas velocity, pressure, holdup, and temperature. This simulator can be used as a stand-alone code or can be used in conjunction with a reservoir simulator to mimic wellbore/reservoir dynamic interactions. In our model, we consider stratified, bubbly, intermittent and annular flow regimes using appropriate closure relations for inter-phase and wall shear stress terms in the momentum equations. In our simulation, we found that the inter-phase and wall shear stress terms for different flow regimes can significantly affect the model's results. In addition, the inter-phase momentum transfer terms mainly influence the holdup value.

The outcome of this research leads to a more accurate simulation of multiphase flow in the wellbore and pipes, which can be applied to the surface facility design, well performance optimization, and wellbore damage estimation.

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