Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)The use of core-annular flow pattern. where a thin fluid surrounds a very viscous one, has been suggested as an attractive artificial lifting method for heavy oils in the current Brazilian ultra-deep water production scenario. This paper proposes a more refined pressure-loss prediction model for core-annular flow which includes a slip ratio term that implicitly accounts for the buoyancy of the oil core. Frictional pressure-gradient measurements in upward-vertical and horizontal core-annular flows in a 28.4-mm-i.d. 2.5-m-length glass tube (tap water and crude oil of 500 mPa s and 925 kg/m(3) at 20 degrees C) and 77-mm-i.d. 274-m-length steel pipeline conveying an ultra-viscous crude oil (36,950 mPa s and 972.1 kg/m(3) at 20 degrees C) respectively, are reported. The model shows very good agreement with the present data and data from the literature, improving some correlations previously developed. Regarding the onshore-field experiments, two pressure-gradient reduction factors are proposed. The observed improvement of oil production rates and pressure-loss reductions are remarkable. (c) 2009 Elsevier B.V. All rights reserved.65416716775Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [04/13374-7]