A method for separation of fluid, in particular oil, gas and water, in connection with the extraction of such a fluid from formations under the surface of the earth or the sea bed. The fluid is transported in a supply pipe or transport pipe (4) to a separator (1) in the form of a tubular separator body, a gravitation tank or similar. The separated components, water and oil, are passed out of the separator separately via outlet pipes. The fluid upstream of the separator (1) is subjected to shear forces so that the drops in the supply flow are torn up to form drops that are so small that the interface generally becomes new and "uncontaminated" by surfactants. The shear forces are supplied by a phase inversion device (6) in the form of a valve or similar. Water can expediently be added to the fluid upstream of the phase inversion device (6) to achieve the desired phase inversion.