Oil-gas-water three-phase upward flow through a vertical pipe: Influence of gas injection on the pressure gradient

► An oil-gas-water three-phase flow was investigated in vertical pipes. ► Both average in situ phase fraction and pressure gradient were measured. ► Pressure gradient is higher than oil-water flow around the phase inversion region. ► In situ gas fraction reached to its lowest value at the phase inversion point. ► Methods were evaluated to predict the pressure gradient of vertical bubbly flow.