A method for forming a flange collar constituting a cement foundation in a casing annulus, includes the steps of installing a casing section comprising an internal sleeve axially displaceable by a conveyed tool for opening a casing cementing outlet; the internal sleeve connected through radial bolts through axial-parallel slots through the casing wall to a casing-external sleeve, the casing-external sleeve abutting against a first edge of a deformable metal sleeve having its opposite, second end abutting axially against a radial shoulder on the casing section; using the drill pipe conveyed tool for axially displacing the internal sleeve thus forcing the external sleeve to deforming the metal sleeve to a radially extending flange collar radially extending across the casing annulus, and ejecting cement via the casing cementing outlet to the annulus, allowing the cement to distribute above the formed radially extending flange collar, and allowing the cement to settle in the annulus.