Increasing confidence and efficiency of process unit are major challenges for subsea field development. In such conditions, subsea liquid-liquid separation is still an important issue. Moreover, related to water depth, only compact separators with short residence times can be installed. The key point developed in this paper is to evaluate the water-oil separation regarding the oil properties including the effect of gas. A joint Industrial Program was performed in IFPEN on water separation efficiency in collaboration with Petrobras and ExxonMobil.
Experiment and numerical analysis of tests were performed on a facility platform. The GOwSP platform used is an industrial test facility designed to research and to test multiphase equipments. The closed loop is under pressure and temperature control to simulate real operating conditions. The material used is a crude oil supplied by Petrobras, synthetic brine and natural gas. The tests were performed in a horizontal separator unit implemented with specific instrumentation such as interface profiler gauge. The separation efficiency is calculated from flow mass balance. Various operating conditions were studied such as; gas and liquid flow rates, water fraction, temperature, pressure. Special attention was taken on pressure effect and gas/oil ratio.
Thus, a 1D model previously developed was used for tests simulation in order to predict the separation efficiency as a function of the operating production conditions. The 1D-model is based on an "experiment-modeling" methodology. The model is developed based on physical laws describing emulsion destabilization under only gravity effect and then it is extrapolated in case of fluid flows. Simulations shown a good accordance with experiment results. Whatever the conditions, most of simulation of the separation efficiency are within a 20% relative error margin. Classical behaviour of separation efficiency increase with oil residence time and temperature is confirmed as a decrease with initial water content increase. Simulations permitted to underline the important influence of oil characteristics viscosity, density on separation efficiency. At tested operating conditions, the gas had a slight influence on oil/water separation.