Whatever well functions are being carried out, there are approximately 7 functions which a mud must perform. This is true regardless of which hole section is being drilling, although the emphasis changes in different hole sections. The 7 primary functions are :

1. Control formation pressure
2. Prevent the hole from caving in
3. Seal off permeable formations
4. Cuttings Transport
5. Cool and lubricate the bit and drillstring
6. Physicochemical Functions
7. Allow easy formation evaluation

Of course the mud also does other things. For example it transfers hydraulic horsepower downhole, to the bit and to downhole motors. MWD uses the mud to transfer data to surface. The mud also helps to support the weight of the drill string. But these things do not really affect the way in which drilling fluid is run.

1. Controlling Formation Pressure
A drilling fluid is the first line of defense against well-control problems. The drilling fluid balances or overcomes formation pressures in the wellbore. Typically, this is accomplished with weighting agents such as barite, although there are other chemicals that can be used. In addition, surface pressure can be exerted to give the equivalent pressure needed to balance a formation pressure. An overbalanced condition occurs when the drilling fl uid exerts a higher pressure than the formation pressure. An underbalanced condition occurs when the drilling fluid exerts a lower pressure than the formation pressure. Therefore, in underbalanced drilling operations, the borehole is deliberately drilled with a fluid/pressure combination lower than the formation pressure. A balanced condition exists if the pressure exerted in the wellbore is equal to the formation pressure.

2. Prevention of Caving
It is desirable to drill the hole as close to gauge as possible. This makes the hole easier to clean, to log, and helps to ensure that the casing is properly cemented in place. If the hole collapses during drilling there is a danger of the pipe becoming stuck.
Too low a Mud Weight may allow the walls of the hole to fall in. The hydrostatic pressure from the mud should normally match or exceed the formation pressure.
Mud Chemistry is also important. Reactions between the mud and the formation may cause the formation to swell. The additional pressure due to formation swelling may cause caving. Where the formation being drilled can dissolve, as for example salt formations, we have to treat the mud to stop or slow down this from happening. Otherwise large caverns will be formed downhole where the formation has been dissolved away.

3. Sealing off Permeable Formations
Permeable formations are those where the pore spaces are interconnected, allowing fluid to flow to and from the well-bore.
To cake off these formations the drilling fluid requires particular filtration properties. As the mud passes into the formation, the solids in the mud are left behind, forming a filter cake. This keeps the hole in a stable condition. It also cuts down the quantities of mud and filtrate entering the formation.

4. Cuttings Transport
The drilling fluid should be able to remove rock fragments or cuttings from beneath the drilling bit, transport them up the wellbore drillstring annulus, and permit their separation at the surface using solids-control equipment. The density and viscosity of the drilling fluid are the properties that control the process of lifting particles that fall down through the flowing fluid by the effect of gravity. The fluid must also have the ability to form a gel-like structure to hold cuttings and weighting materials when circulation is interrupted. Horizontal and high-angle wells require specialized fluid formulations and “sweep” protocols to minimize the risk of barite sag (in which the barite slurry separates downhole) and of low-side cuttings settling, both of which can lead to stuck pipe and loss of the well. The lubricity of the drilling fluid is also a key factor in controlling torque and drag in these types of operations.

5. Cooling and Lubrication
The drilling mud cools and lubricates when the rotating bit drills into the bottom of the hole and when the drillstring rotates against the wellbore walls. The fluid should have the ability to absorb the heat generated by the friction between metallic surfaces and formation. In addition, the fluid should not adversely affect the bit life nor increase the torque and drag between the drillstring and the borehole.

6. Physicochemical Functions
The drilling fluid system should remain stable when exposed to contaminants and hostile downhole conditions. Among the common natural contaminants are reactive drill solids, corrosive acid gases (e.g., H2S), saltwater flows, and evaporites (e.g., gypsum). The cement used in setting casing and liner strings is also a contaminant to some water-based muds. Wells in certain areas have extremely high bottomhole temperatures, at times approaching 500°F, and, likewise, arctic locations may expose the drilling fluid to subzero temperatures at surface.

7. Formation Evaluation
Formations are evaluated in three different ways :
a) from the cuttings
b) from electric logs
c) by coring
The cuttings are collected and checked by the Mud Loggers and the Geologist. The condition of these cuttings depends mainly on the Mud Chemistry. The information gained from logs and cores depends on the Filtration Properties of the mud. Thick filter cake can make logging difficult. Filtrate invasion can give misleading results for both logs and cores.