A method, system, apparatus and computer program product for determining values of flow rates of components of a liquid hold up of a gaseous and liquid (water and oil) multiphase flow in an oil well. Including measuring total quantities of flow (Qtot), gas (Qgas) and liquid phase (Qliq) with a multiphase flow meter. Disposing an inlet of a core sampler inside a main pipe of the oil well at a place where a concentration of the liquid phase is greater than an average concentration of the liquid phase in the main pipe, to obtain a derived sample having a greater concentration of oil and water than the average concentration. Measuring quantities of water (Qwater) and oil (Qoil) of the derived sample. And calculating from Qwater, Qoil and Qliq, quantities of water and oil in the flow.