Drilling fluid

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Drilling fluid

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.

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Drilling mud

In geotechnical engineering, drilling mud, also known as spud mud , is a drilling fluid used to drill boreholes into the earth. Often used while drilling oil well and natural gas wells and on exploration drilling rigs but can also be used for much simpler boreholes, such as water wells....
) 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:

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.
Keep the drilling bit cool & clean during drilling.
Bring out the cuttings generated by the bit.
Should be able to keep the cuttings in suspended form during the trips.
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.
Water-Based Mud (WBM) - A most basic water-based mud system begins with water, then clays and other chemicals are incorporated into the water to create a homogenous blend resembling something between chocolate milk and a malt (depending on viscosity). The clay (called "shale" in its rock form) is usually a combination of native clays that are disolved into the fluid while drilling, or specific types of clay that are processed and sold as additives for the WBM system. The most common of these is bentonite, frequently referred to in the oilfield as "gel". Gel likely makes reference to the fact that while the fluid is being pumped, it can be very thin and free-flowing (like chocolate milk), though when pumping is stopped, the static fluid builds a "gel" structure that resists flow. When an adequate pumping force is applied to "break the gel", flow resumes and the fluid returns to its previously free-flowing state. Many other chemicals (e.g. Potassium Formate) are added to a WBM system to achieve various effects, including: viscosity control, shale stability, enhance drilling rate of penetration, cooling and lubricating of equipment,......