The use of core-annular flow pattern may be attractive as an artificial lift method in heavy oil wells. This flow pattern can be induced by the lateral injection of relatively small quantities of water, in order to get a lubricated oil core along the pipe. Frictional pressure drop measurements for upward vertical core flow in a 1-in. pipe, using a 17.6-Pa.s, 963-kg/m(3) oil and water at room temperature reported a decrease by over 1000-fold with respect to single-phase oil flow, being comparable to the flow of water alone in the pipe at mixture flow rate. The total pressure drop was reduced by over 45-fold. The frictional pressure drop model proposed includes both irreversible and buoyancy terms. The model was adjusted to fit our data and shows excellent agreement with laboratory data available.