Proper sizing of flow lines in the upstream energy industry depends on accurate modeling of gas–liquid flow, which has a common occurrence in production wells and has been studied thoroughly for many decades. However, data of flow in duct geometries different from circular pipes and when the liquid viscosity is much higher than that of water are scarce. Proper prediction of pressure gradient, heat and mass transfer, and corrosion depends on the accuracy of the model used to calculate the volumetric phase fraction. In pumped directional wells with inverted-shroud gravitational separators, there is flow through an annular duct formed between the wells’ casing and the separator itself that can have some tens of meters. The present work is an investigation on upward vertical/inclined high-viscous-oil/gas flow in a large and narrow annulus (30 mm hydraulic diameter with an outer diameter equal to 155 mm), using a radial geometry comparable with those found in real production systems. Air–water and air–oil mixtures, the latter with two oil viscosity ranges, were used as working fluids. The experimental test section used was 9.67 m long positioned at 90 deg (vertical) and 45 deg and made of two concentric pipes. Flow pattern transitions from the literature were analyzed and compared with the collected experimental data. Drift-flux parameters were obtained from multiple working conditions. These drift-flux parameters were used in the development of a novel flow pattern-independent correlation, compared against the present data and other data sets from the literature in which other geometries and fluids were used. The predictions of the proposed drift-flux correlation are significantly superior in comparison with correlations selected from the literature in all cases.