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. In addition, some aspects of churn flow such as the slug/churn transition and the predictions of pressure drop are discussed. A comprehensive experimental program was also launched to collect data. An extensive full-scale investigation was executed in a 1278 m vertical well with an 88.9 mm x 159.4 mm (3.5 in. x 6.276 in.) annulus. The test matrix covered the whole range of possible combinations of liquid and gas injection rates for an UBD operation in a similar geometry. Small-scale tests were also performed in a U-tube, which simulates the simultaneous injection of liquid and gas through the drill string. The U-tube apparatus comprehended a descending pipe (ID=34 mm=1.34 in.) and a 41 mm x 74 mm (1.61 in. x 2.91 in.) ascending annulus. Small-scale data available in the literature were also collected and catalogued. The model is validated against the acquired database and shows a good performance for pressure drop prediction. Further, the performance of the model is also compared with other multiphase empirical or mechanistic models from the literature. The proposed model performs better than the other alternatives.