Abstract: A process for insulating a tubing string placed in a pipe and through which a fluid circulates, the insulant being a non-rigid foam produced from a liquid solution and placed in the annular space defined by the outside of the tubing string and the walls of the pipe in which these tubings are placed. A production well (1) opens, in the lower part thereof, onto a producing rock containing a fluid to be extracted. Tubing (2) is placed in production well (1). A zone (3) represents the annulus defined between the outside (5) of the tubing string and the walls of well (1) in which the tubing string is placed. A tube for injecting a liquid and/or gaseous fluid (4) is placed in annulus (3) and a liquid and/or gaseous fluid injection nozzle (6) opens into zone (3).