Large quantities of produced water are generated in oil and gas fields all over the world. Produced water increases during the life of the field. In the world, the production of one barrel of crude oil generates three barrel of produced water but the ratio barrel of water/barrel of crude oil increases drastically. Produced Water Re-Injection (PWRI) can be used for injection purposes. However due to the combined presence of suspended solids and oil in water emulsion, the well injectivity and the possibility of topside filtration is often problematic. Membrane filtration is considered as a very attractive treatment process that may allow produced water re-injection even in difficult reservoir without loss of injectivity. Large quantities of sea water are also injected in oil and gas fields all over the world for pressure maintenance support and sweeping efficiency of the reservoir in order to maximize the hydrocarbon production. This is particularly right at the beginning of the field life when the produced water production is low. Membrane treatment is used worldwide in oil and gas to filter sea water for injection. However today the two process of filtration are different, organic micro or ultrafiltration membranes are used for sea water and ceramic membranes are used for produced water. TOTAL proposes to use only one process of membrane filtration for both sea and produced water alternatively or in comingle; it has been called the FLEX membrane process. After looking at the different processes, this paper will present the advantages of the FLEX design, such as the gain in space, weight and cost. The paper invites operators to envisage alternative strategies for water injection when developing fields and to consider the FLEX membrane solution.