Method of cementing deformable casing inside a borehole or a conduit

A method of cementing casing inside a borehole or a conduit that is approximately cylindrical, using a tubular preform that is deformable by being expanded radially from a folded first state in which its maximum transverse dimension is considerably smaller than the diameter of the well or conduit, to an unfolded second state in which it is cylindrical in shape and has a diameter that is slightly smaller than that of the well or the conduit, the preform being settable on site in order to constitute the casing, the method comprising steps of:

• a) inserting the preform (1) in the folded state into the well or into the conduit (P), and lowering said preform down to the desired level;
• b) closing the well or the conduit (P) at the bottom of the preform (1) by substantially inflating in said zone a hydraulically deformable closure member (4);
• c) injecting a cement (L1) that is fluid and settable above the closure member (4) so as to surround the bottom portion of the preform, with the volume of said cement (L1) corresponding substantially to the volume required for cementing the casing in the well or in the conduit;
• d) deforming the preform (1) to cause it to take up its unfolded state, said deformation taking place progressively from the bottom upwards, so that the cement is displaced little by little along the annular space between the preform (1) and the wall of the well or of the conduit, along the entire height of the preform;
• e) allowing the preform (1) to set to obtain the casing (1'), and allowing the cement (L1) to set;
• and
• f) deflating the closure member and then withdrawing the closure member (4) from the well or the conduit.