A method of sealing the wall of a wellbore (1) as it is being drilled through a subterranean formation using a drill string (3) having a drill bit (4) on the lower end thereof comprises fitting to the lower end of a drill string (3) a device comprising (i) a cylindrically gathered pack of flexible tubing (6), (ii) a receptacle (5) for the gathered pack and (iii) a radially expandable locking means (8) having a first end of the tubing of the gathered pack connected either directly or indirectly thereto, and drilling a first section of wellbore, expanding the locking means (8) against the wellbore wall such that the first end of the tubing that is withdrawn from the gathered pack is locked in place in the wellbore, drilling a second section of wellbore with the movement of the drill string (3) through the wellbore causing the tubing to be withdrawn from the gathered pack (6) and to be turned inside out thereby forming a liner for the second section of wellbore.