Introducing within a well or pipe a mandrel equipped with an inflatable membrane which can expand radially under the effect of an internal pressure. Positioning the mandrel opposite the portion to be treated, in introducing a pressurized fluid within the membrane such that its end regions form annular bulges pressing firmly against the wall, whereas its mid-region expands to a lesser degree. Forming a peripheral annular space into which is then injected a cement which is liquid but able to harden. Leaving the cement to set so as to form a solid sleeve. Deflating the tubular membrane. Withdrawing the mandrel together with the deflated membrane. The method can be used in the oil sector, particularly for repairing and/or plugging a portion of casing.