- The main objective of this work is to present a model that predicts both flow regime and pressure gradient in a deviated wellbore. In modeling flow pattern transition and void fraction, an approach similar to that for vertical flow is taken; that is, four principal flow regimes are recognized - bubbly, slug, churn and annular. Gas void fraction data were obtained both from a 5-in circular pipe and from annular flow channels for deviation angles up to 32 degree from the vertical. The validity of the proposed model is demonstrated with these data as well as with laboratory data from other sources. Several field examples are presented to show the application of the model.