Drilling a very deep hole is a very costly business. The hole size is kept very small as the drilled depth increases because it is to cased and cemented after wards. The larger hole will require huge amount of steel casing, and other problems like caving and collapsing of the hole during the drilling. It is not possible to bring out the cuttings generated by drill bit manually from such depth as can be done while digging a well. To bring out the cutting from such depths, drilling fluid is used. Drilling fluid is generally a viscous, heavy fluid designed to perform a variety of functions. It is like the lifeline of a well.

Salient functions of Drilling Fluids a drilling fluid are given below:

1. To prevent the formation fluids from entering into the hole and result in disaster as a blow out by exerting sufficient pressure against the formations being drilled.
2. Keep the drilling bit cool & clean during drilling.
3. Bring out the cuttings generated by the bit.
4. Should be able to keep the cuttings in suspended form during the trips.
5. Should have minimum interaction with the exposed formations.

Drilling fluid also functions to minimize reservoir damage and limit corrosion.

Types of Drilling Fluid

Many types of drilling fluids are used on a day to day basis. Some wells require that different types be used at different parts in the hole, or that some types be used in combination with others. The various types of fluid generally fall into a few broad categories:

• Air - compressed air is pumped either down the bore holes annular space or down the drill string itself.
• Air/water - Same as above, with water added to increase viscosity, flush the hole, provide more cooling, and/or to control dust.
• Air/polymer - A specially formulated chemical, most often referred to as a type of polymer, is added to the water & air mixture to create specific conditions. A foaming agent is a good example of a polymer.
• Water - Water by itself is pumped to do very specific things in very specific formations.

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