Abstract: A method for producing an oil reservoir, comprising establishing an oil producing well, producing a production fluid containing oil and water, transporting the production fluid to a downhole liquid/liquid hydrocyclone, separating the fluid in the liquid/liquid hydrocyclone into an oil enriched stream at the hydrocyclone underflow, transporting the oil enriched stream to the surface, reinjecting the water enriched stream at a downhole site, cooling the transported oil enriched stream by sending a counterflow of a cooling medium relative the oil enriched stream, thereby keeping the oil enriched stream in a substantially liquid phase.

Abstract: The invention relates to a system for separating oil/water/gas from a marginal field by using a combination of a coarse separator with short treatment duration which separates into three flows where one obtains an oil concentration of over 70%, a water concentration of over 70% and a gas quality for burning off, where the oil and water flows are treated in cyclone systems having defined geometric proportions and a gas portion is reinjected into the water injection flows without the use of a compressor. Gas/liquid cyclones may also be used before the choke valve so that the gas pressure is kept, or optionally reinjected or used for injector pump operation of low pressure wells.