For the rotary drilling system to work, fluids must be circulated continuously down the drill pipe, out of the bit nozzles, and then back to the surface. The lquid drilling fluids are commonly called drilling muds. Drilling muds can have a wide range of chemical and physical properties. These properties are specifically designed for drilling conditions and the special problems that must be handled in drilling agiven well.

For the rotary‐drilling system to work, fluids must be circulated continuously down the drill pipe, out of the bit nozzles, and then back to the surface. The lquid drilling fluids are commonly called drilling muds. Drilling muds can have a wide range of chemical and physical properties. These properties are specifically designed for drilling conditions and the special problems that must be handled in drilling agiven well.

Main functions of drilling fluids

The drilling fluid serves several functions, these are to:

• clean, cool and lubricate the drill bit

• suspend cuttings when circulation is stopped

• remove cuttings from the hole

• control sub‐surface pressures

• keep the hole walls from collapsing

• provide information about the formation penetration

• protect potential pay zones from damage

Figure 1 shows the functional relationship between the circulating mud system and the main components of the rotary rig. The drilling mud around the circulating system is illustrated in Figure 2.

Cooling and lubrication

As the bit drills into the rock formation, the friction caused by the rotating bit being forced against the rock generates heat. If this heat is not dissipated, the bit cutting structure, bearings, and other components become damaged and bit life is shortened. The drilling fluid absorbs much of this heat as it is circulated past the area where the bit contacts the rock.

In addition to the heat generated by friction between the bit and the rock, the static downhole temperature ‐ the temperature of the rock excluding the heat generated by friction and cooling effect of the drilling fluid ‐ increases with depth. This increase in formation temperature with depth is termed the temperature gradient and is usually assumed to be 1° F (5/9 °C) increase for each 100 feet (30.49 m) of depth.

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