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Well design can be described as pipeline from the ground surface to the oil 
bearing formation. Oil comes to the surface through this pipeline. Such pipeline 
includes a number of special pipes connected between each other and called 
a casing string.
Casing has several important functions during the drilling and completing 
of a well. It is used to prevent the borehole from caving in during the drilling of 
the well, to provide a means of controlling fluids encountered while drilling, to 
prevent contamination of fluids to be produced, and to protect or isolate certain 
formations during the course of a well. Casing is also one of the most expensive 
parts of a well, around 20 % of the cost of a completed well. Casing is usually divided into five basic types: conductor casing, surface casing, intermediate 
casing, production casing, liner.
Conductor pipe or drive pipe if it is hammer-driven to depth, is the first 
string of casing to be used. The setting depth can vary from 10 ft to around 300 
ft. The normal size range for conductor pipe is from 16 to 36 inches (outside 
diameter). The conductor pipe must be large enough to allow the other casing 
strings to be run through it. Purposes of conductor pipe are:
1) to raise the level of circulating fluid so that fluid returns are possible; 
2) to prevent washouts in the near surface, normally unconsolidated for-
mations
The amount of surface casing used will depend on the depth of the uncon-
solidated formations. Surface casing is usually set in the first competent for-
mation. Normal size for surface casing is between 20 inch and 13 3/8 inch (out-
side diameter). Since temperature, pressure and corrosive fluids tend to increase 
with depth, different grades of casing will be required to handle the different 
well conditions. Purposes of surface casing are to:
1) protect fresh water formations; 2) seal off unconsolidated formations and 
lost circulation zones; 4) provide a place to install the B.O.P.'s; 5) protect 
«build» sections on deviated well; 6) provide for a sufficient «bottomhole» test 
to be conducted.
Intermediate casing is set after surface casing, normally to seal off a prob-
lem formation. The size of intermediate casing, will depend on the size of the 
surface casing and the grade required to withstand the subsurface conditions. 
Normal sizes are between 9 5/8 and 13 3/8 inch (outside diameter).
Production casing is usually the last full string of pipe set in a well. These 
strings are run to isolate producing formations and provide for selective produc-
tion in multi-zone production areas. The size of production casing will depend 
on the expected production rate, the higher the barrel per day production rate, 
the larger the inside diameter of the pipe. Common sizes are between 3 and 
7 inch (outside diameter).
A liner is a string of casing that does not reach the surface. They are usually 
«hung» (attached to the intermediate casing using an arrangement of packers 
and slips) from the base of the intermediate casing and reach to the bottom of 
the hole. The major advantage of a liner is the cost of the string is reduced, as 
are running and cementing times. During the course of the well, if the liner has 
to be extended to the surface (making it another string of casing), the string at-
taching the liner to the surface is known as a «tie-back» string.
Well equipment required for field operations includes: wellhead, valves and 
chokes, emergency shut down systems, blowout preventer, fluid tanks, separa-
tors, line heaters, dehydration equipment, gas sweetening equipment, compres-
sors.
Christmas tree is the main component of wellhead equipment. Christmas 
tree is attached to casing. It is designed to hold casing and tubing strings, isolate or seal annulus between these strings, provide fluid access to all strings, control 
processes of fluid and gas injection and production by means of valves.
Standard Christmas tree assembly includes adaptor flange, master valve, 
flanged cross, top adaptor, secondary master valve, wing valve, and a choke.
Pressure gauges are installed on Christmas tree to measure pressure in an-
nulus and tubing. When pressure values under different working conditions are 
known the well can be controlled much easier