Almost every well constructed, and virtually all drilled wells, use casing to support the hole and allow access to the producing horizon, be it oil, gas, water or most anything else under the ground. The design of the casing is very important to the success of the well. Some offshore wells have as many as six or eight different strings of casing, set at different depths, to complete the hole. Most common house wells use only one string of casing with the possible addition of a telescoping screen, or liner. This is usually done to isolate an undesirable zone or assure that production comes from a very specific horizon.
Regardless of the casing design, there are some things that must be taken into account to run casing successfully. First, the hole should be as straight as possible. Casing is usually much stiffer than drill pipe, and doesn’t like to go around corners very well. In the case of directional wells, the bends should be as long and gradual as conditions permit. Abrupt dog-legs should be avoided.