Page 54 - 4822

depend on the subsurface characteristics of the well, including the
diameter of the well (which is dependent on the size of the drill bit
used) and the pressures and temperatures experienced throughout
the well. In most wells, the diameter of the well hole decreases the
deeper it is drilled, leading to a type of conical shape that must be
taken into account when installing casing. The casing is normally
cemented in place.

Figure 5.2 - Well Casing

There are five different types of well casing. They include:
 Conductor casing, which is usually no more than 20
to 50 feet long, is installed before main drilling to prevent the top
of the well from caving in and to help in the process of circulating
the drilling fluid up from the bottom of the well.
 Surface casing is the next type of casing to be
installed. It can be anywhere from 100 to 400 meters long, and is
smaller in diameter than the conductor casing and fits inside the
conductor casing. The primary purpose of surface casing is to
protect fresh water deposits near the surface of the well from being
contaminated by leaking hydrocarbons or salt water from deeper
underground. It also serves as a conduit for drilling mud returning

54