The annular flow pattern of two immiscible liquids having very different viscosities in a horizontal pipe-also known as 'core-annular flow'-provides an attractive means for the pipeline transportation of heavy oils since the oil tends to occupy the center of the tube, surrounded by a thin annulus of a lubricant fluid (usually water). The correspondent pressure drop is comparable to the flow of water only in the same pipe at the total volumetric flow rate. Recently, successful experiments led by the author and his group indicated that the core flow technology might be even more attractive for heavy oil production in vertical wells, due to the symmetry of the flow and the favorable effect of buoyancy. In this paper, several aspects of core-annular flow modeling are analyzed and discussed in the light of experimental data. First, criteria for existence of stable core flow are proposed, which show the essential role played by interfacial tension. Phenomenological models, based on mass and momentum balances, are developed for volume fraction and pressure drop and compared with data for both horizontal and vertical oil water core flows. The very good agreement observed is encouraging. (C) 2001 Elsevier Science B.V. All rights reserved.3241731SI127143