The use of core-annular flow pattern, where a thin fluid surrounds a viscous one, may be attractive to heavy oil transportation and as an artificial lifting method in heavy oil wells,a situation that can become frequent in the Brazilian offshore scenario. However, in petroleum production, operations gas is frequently present. Therefore, the study of three-phase flow of heavy oil, water and gas is in order. This paper reports pressure drop measurements and three-phase flow patterns observed in horizontal and vertical 2.84-cm W. glass pipes. The focus was a mixture of heavy crude oil (3400 mPa s, 970 kg/m(3) at 20 degrees C), water an air at several combinations of the individual flow rates. Three-phase pressure drop data were compared with single-phase oil and two-phase oil-gas flows to assess the gains due to water injection. In addition. three-phase flow patterns formed inside vertical and slightly inclined 1.0 cm i.d. pipes,are also presented. Thus, scale-up and inclination effects could be qualitatively analyzed. Full-scale onshore-field experiments were conducted in order to investigate the applicability of using water to transport heavy oil in actual lines in the presence of gas. A big steel pipeline (7.7 cm W. and 274 m) conveying a very viscous crude oil (36950 mPa s, 972.1 kg/m(3) at 20 degrees C), natural gas (GOR 15 m(3)/m(3)) and water was used. Onshore-field three-phase flow tests were carried out and pressure gradient data are reported. The observed improvements in oil production rates and the pressure drop reductions obtained are remarkable. (c) 2009 Elsevier B.V. All rights reserved.6541671113ANPFINEPPETROBRAS