Casing

Drilling a well modifies the mechanical and hydraulic equilibrium of the rocks around the borehole. Periodically this equilibrium has to be restored, by inserting a good casing. The casing is a steel tube that starts from the surface and goes down to the bottom of the hole and is rigidly connected to the rocky formation using cement slurry, which also guarantees hydraulic insulation. The casing transforms the well into a stable, permanent structure able to contain the tools for producing fluids from underground reservoirs. It supports the walls of the hole and prevents the migration of fluids from layers at high pressure to ones at low pressure. Furthermore, the casing enables circulation losses to be eliminated, protects the hole against damage caused by impacts and friction of the drill string, acts as an anchorage for the safety equipment and, in the case of a production well, also for the Christmas tree. At the end of drilling operations, a well consists of a series of concentric pipes of decreasing diameter, each of which reaches a greater depth than the preceding one. The casing is a seamless steel tube with male threading at both ends, joined by threaded sleeve joints. The dimensions of the tubes, types of thread and joints are standardized (API standards). There are also Special direct-coupling casings, without a sleeve joint.

The functions and names of the various casings vary according to the depth. Starting from the uppermost and largest casing first comes the conductor pipe, then the surface casing and the intermediate casing, and finally the production casing. The first casing is called the conductor pipe and is driven by percussion to a depth normally of 30 to 50 m. It permits the circulation of the mud during the first drilling phase, protecting the surface unconsolidated formations against erosion due to the mud circulation, which could compromise the stability of the rig foundations. The conductor pipe is not inserted in a drilled hole and is not usually cemented, and therefore it is not considered a casing in the true sense of the word. The first casing column is next and protects the hole drilled inside the conductor pipe. It is also called the surface casing and its functions are to protect the freshwater aquifers against potential pollution by the mud, to provide anchorage for the subsequent casing, and to support the wellhead. To increase its stiffness and make it capable of bearing the compressive loads resulting from the positioning of the subsequent casings, the surface casing is cemented up to the surface. Its length depends on the depth of the aquifers and on the calculated well-head pressure following the entry of fluids from the bottom hole into the casing.