I'd like to add a small note: I like to think about this problem in terms of mass flow rate as opposed to superficial velocity. Superficial velocity of the individual phases is generally prescribed in terms of the total inlet area. However, Fluent requires separate areas to prescribe the velocity inlet condition. Fluent uses these areas and velocities to calculate the volume flow rates appropriately. Irrespective of what area fractions you choose at the inlet cross-section for oil and natural gas, you have to calculate the respective phase velocities (to prescribe the correct velocity BC) such that they match the individual flow rates given by the Flow Regime map.
I just thought this piece of information might be useful in this thread.