Surface Casing
The principal functions of the surface casing string are to: hold back unconsolidated shallow formations that can slough into the hole and cause problems, isolate the freshwater-bearing formations and prevent their contamination by fluids from deeper formations and to serve as a base on which to set the blowout preventers. It is generally set in competent rocks, such as hard limestone or dolomite, so that it can hold any pressure that may be encountered between the surface casing seat and the next casing seat.
Setting depths of the surface casing vary from a few hundred feet to as much as 5,000 ft. Sizes of the surface casing vary from 7 to 16 in. in diameter, with 10| in. and 13| in. being the most common sizes. On land, surface casing is usually cemented to the surface. For offshore wells, the cement column is frequently limited to the kickoff point.