A mechanistic model is formulated to predict the mixture behavior for upward two-phase flow in concentric annulus. The model is composed of a procedure for flow pattern prediction and a set of independent mechanistic models for calculating gas fraction and pressure drop in bubble, dispersed bubble, slug and annular flow.

Small-scale experimental data from the literature validate the predictions of the model. A full-scale experimental investigation is also executed to complete the evaluation. The experiments are performed in a 1278 m vertical well in the Petrobras research facility in Taquipe, Brazil, with an 88.9 mm x 159.4 mm (3.5 in. x 6.276 in.) annulus. Test matrix covered the whole range of possible combinations of liquid and gas injection rates for an underbalanced drilling operation in a similar geometry. The overall model performance is in good agreement with the experimental data.

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