The key to making the rotary drilling system work is the ability to circulate a fluid continuously down through the drill pipe, out through the bit nozzles and back to the surface.
The drilling fluid can be air, foam (a combination of air and liquid or a liquid.
Liquid drilling fluids are commonly called drilling mud.
All drilling fluids, especially drilling mud, 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 a well.
Purpose of Drilling Fluids
1. Cooling and lubrication. As the bit drills into the rock formation, the friction caused by the rotating bit against the rock generate heat. The heat is dissipated by the circulating drilling fluid. The fluid also lubricates the bit.
2. Cuttings removal. An important function of the drilling fluid is to carry rock cuttings removed by the bit to the surface. The drilling flows through treating equipment where the cuttings are removed and the clean fluid is again pumped down through the drill pipe string.
3. Suspend cuttings. There are times when circulation has to be stopped. The drilling fluid must have that gelling characteristics that will prevent drill cuttings from settling down at the bit. This may caused the drill pipe to be stuck.
4. Pressure control. The drilling mud can be the first line of defense against a blowout or loss of well control caused by formation pressures.
The hydrostatic head produced by the mud in psi is = 0.052 x G x H whereG = density of mud in ppg
H = depth of the hole in feet.
This hydrostatic head will counter the formation pressure in order to avoid a blowout while drilling.
For example, Lets say a well is being drilled in a salt-water basin (pressure gradient of 0.465 psi/ft), the pressure in the formation at 10,000 feet would be expected to be:
10,000 x 0.465 = 4,650 psi
The weight of mud required to counter this pressure is calculated as follows. P = 0.052GH
4,650 = 0.052 x G x 10,000
G = 8.94 ppg
5. Data source. The cuttings that the drilling mud brings to the surface can tell the geologist the type of formation being drilled.
6. To wall the hole with impermeable filter cake. This will give a temporary support to the wall of the borehole from collapsing during drilling.
Drilling fluid can solve problems
Many drilling problems are due to conditions or situations that occur after drilling begins and for which the drilling fluid was not designed.
Some of these problems can be solved by adding materials to the drilling fluid to adjust its properties.
Other cases, it may be necessary to replace the drilling fluid being used with another fluid system.
The most common changes is the mud weight or density. Weighting material is added when high-pressure formations are expected.
Some of the problems are:
1. Lost circulation
Lost circulation can occur in several types of formations, including high permeable formations, fractured formations and cavernous zones.
Lost circulation materials can be added to the mud to bridge or deposit a mat where the drilling fluid being lost to the formation. These materials include cane and wood fibres, cellophane flakes and even padi husks were used in oil drilling in Sumatra.
2. Stuck pipe
Stuck pipe can occur after drilling has been halted for a rig breakdown, while running a directional survey or when conducting other nondrilling operation.
The drill pipe may stick to the wall of the hole due to the formation of filter cake or a layer of wet mud solids on the wall of the hole in the formation.
3. Heaving or sloughing hole