A well bore is inherently unstable due to the pressure gradient between the formations being drilled through and the hole through which the drilling fluid flows, and to the circulation of this fluid in the hole. To prevent the well bore from collapsing, which could result in the drill string becoming stuck or even abandonment of the borehole, steel cylinders called casings are lowered downhole section by section and screwed together in the same way as drill pipes. Once inserted, the entire length of each casing is cemented to the rock formation to ensure it remains stable. Each casing has a smaller diameter than the previous one.