Abstract: A method and a device for in situ formation of a seal (17) in a region (2) of an annulus (18) located around a pipe structure (4) in a well (18), in which the method comprises the following steps: (A) to convey a perforation device into the pipe structure (4) to a location vis-a-vis said region (2) of the annulus (16); (B) by means of the perforation device, to make at least one hole (13) through the pipe wall of the pipe structure (4) at said annulus region (2); (C) to force a liquid sealing material, which is capable of entering into solid state, through said hole (13) and further into the annulus region (2) for the filling thereof, where-upon the sealing material enters into solid state and forms said seal (17).