Two-phase flow of oil and water is commonly observed in wellbores, and its behavior under a wide range of flow conditions and inclination angles constitutes a relevant unresolved issue for the petroleum industry. Among the most significant applications of oil-water flow in wellbores are production optimization, production string selection, production logging interpretation, down-hole metering, and artiflcial lift design and modeling. In this study, oil-water flow in vertical and inclined pipes has been investigated theoretically and experimentally. The data are acquired in a transparent test section (0.0508 m id., 15.3 m long) using a mineral oil and water {po/p^ = 0.85, MO/MW = 20.0 & Oo-^ = 33.5 dyne/cm at 32.22° C). The tests covered inclination angles of 90, 75, 60, and 45 deg from horizontal. The holdup and pressure drop behaviors are strongly affected by oil-water flow patterns and inclination angle. Oilwater flows have been grouped into two major categories based on the status of the continuous phase, including water-dominated and oil-dominated flow patterns. Waterdominated flow patterns generally showed significant slippage, but relatively low frictional pressure gradients. In contrast, oil-dominated flow patterns showed negligible slippage, but significantly large frictional pressure gradients. A new mechanistic model is proposed to predict the water holdup in vertical wellbores based on a driftflux approach. The drift flux model was found to be adequate to calculate the holdup for high slippage flow patterns. New closure relationships for the two-phase friction factor for oil-dominated and water-dominated flow patterns are also proposed. Introductory RemarksIn the petroleum industry, multiphase flow is a common occurence in production of oil and gas. Two-phase flow of oil and water is commonly encountered in wellbores; however, its hydro-dynamic behavior under a wide range of flow conditions and inclination angles constitutes a relevant unresolved issue for the oil industry.Multiphase flows are characterized by the existence of diverse flow configurations or flow patterns, which can usually be identified by a typical geometrical arrangement of the phases in the pipe. Inherent to each flow pattern are a characteristic spatial distribution of the interface, flow mechanisms, and distinctive values for design parameters such as pressure gradient, holdup, and heat transfer coefficient. There is clear evidence that accurate knowledge of the oil-water flow patterns, their ranges of existence as a function of flow rates and pipe inclination angles, are crucial in predicting many hydrodynamic parameters. These include pressure gradient and holdup in a number of production engineering applications, such as production optimization, optimum string selection, production logging interpretation, downhole metering, and artificial lift design and modeling.A fundamental problem in production engineering is to determine the actual volumetric flow rates of each of the phases flowing at any location in the wellbore. This in...