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As defined by NORSOK: One of several dependent barrier elements, which are designed to prevent unintentional flow of a formation fluid. A barrier is an envelope preventing hydrocarbons from flowing unintentionally from the formation, into another formation or, to surface. Barrier elements that make up the Primary barrier are those elements, which are or might be in direct contact with well pressure during normal operation. These elements provide the initial and inner envelope preventing unintentional flow of reservoir fluid to surface, or another zone. Barrier elements that make up the secondary barrier are those, which are or might be exposed to contact with well pressure should any of the elements described as a Primary barrier fail. These elements provide an envelope outside the Primary barrier envelope providing a second barrier preventing unintentional flow of reservoir fluid to surface, or another zone.