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.