A method for pressure- and flow-prevetive fixing of well pipes, preferably liners and casings (14, 54, 60, 66) in a well when drilling the well, wherein the method also may be used in a completed well in order to place a pressure- and flow-preventive barrier in an annulus (16) surrounded by at least one leaking well pipe. The method comprises the use of granular particles of unconsolidated matter which, by means of their particle sorting, are arranged with a suitably small permeability, and wherein the particles of unconsolidated matter thereafter are mixed with water and potential other additives to become a fluidised mixture of unconsolidated matter (22) subsequently being placed, preferably by pumping, as a pressure- and flow-preventive barrier of unconsolidated matter (38, 56, 62, 68) in the pertinent annulus (16). The barrier of unconsolidated matter (38, 56, 62, 68) is placed in the annulus (16) in such a way that inflowing fluids are brought into contact with, and are prevented from flowing by, the barrier of unconsolidated matter (38, 56, 62, 68) which, owing to the method, thus also is arranged with a suitably small permeability.