Page 31 - Petroleum and Gas Field Processing

Drilling Fluid (Mud)
The drilling fluid is a very important element of the drilling operation. Its
importance stems from the many essential functions it serves. Some of
these functions are as follows:
1. Transporting the cuttings from the bottom of the hole to the
surface
2. Cooling of the bit and lubrication of the drill string
3. Exerting hydrostatic pressure to overbalance the pressure of the
formation and thus prevent flow of formation fluids into the well
4. Supporting the walls of the hole to prevent it from caving in
5. Enhancing drilling by its jetting action through the bit nozzles
The drilling fluid can be prepared in different formulations to
provide the desired properties (density, viscosity, and filtration) under the
bottom hole conditions. The basic drilling fluid consists of water and clay
(water-base mud ). Other materials and chemicals are also added to control
the properties of the fluid. Other fluids such as foam and air have also
been used in drilling operations.

1.6.2 Casing the Well
The casing is a steel pipe that is placed in the drilled hole (well) to support
the wall of the hole and prevent it from collapsing. When cemented to the
wall, it seals the subsurface formation layers and prevents communications
between the various layers.
Normally, four strings of casing of different diameters are installed in
the well at various depths that are specified by the geologist. These are the
conductor, the surface casing, the intermediate casing and the production
casing. The conductor has the largest diameter and shortest length of the
four casing strings; the production casing has the smallest diameter and
longest casing.
Casings of various outside diameters are available in different grades
and weights. The grade refers to the type of casing steel alloy and its
minimum yield strength. Commonly available grades are H-40, J-55, N-80,
C-75, L-80, and P-105. The letter (H, J, etc.) identifies the type of alloy
and heat treatment; the number (40, 55, etc.) refers to the minimum yield
strength in thousands of pounds per square inch (psi). For a given outside
diameter and grade, casings are available in different weights (i.e., various
inside diameters) expressed in pounds per linear foot of casing. The weight
and grade of the casing specify its resistance to various loads such as
burst, collapse, and tension loads. In designing casing strings, weight and

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