BASIC DRILLING METHODS

ROTARY DRILLING WITH MUD

In the rotary method of drilling, action is accomplished by rotating a drill pipe by means of a power driven rotary table or hydraulic powered top head drive, with a bit attached to the bottom of the pipe. The bit cuts and breaks up the material as it penetrates the formation. Drilling fluid for mud is pumped through the rotating drill pipe and through holes in the bit. This fluid swirls in the bottom of the hole picking up material broken by the bit, then flows upward in the space outside the drill pipe, carrying the cuttings to the ground surface and clearing the hole. The greater the fluid flow the faster the drilling. The drill pipe and bit move downward deepening the hole as the operation proceeds. At the surface, drilling mud flows into a ditch to a settling pit where the cuttings settle to the bottom. From the settling pit the fluid overflows into another pit from which it is picked up through the suction hose of the mud pump and re - circulated through the drill pipe. In the rotary drilling method the casing pipe is not introduced until after the drilling operations are completed. The walls of the hole are held in place by the pressure of the mud pump against the sides of the hole.


Drilling mud has a number of functions in rotary drilling operations. These are as follows:-

1. Cooling the bit. This is the simplest and one of the most essential duties of drilling mud. Practically any type of mud that can be pumped will perform this function. Otherwise the task of keeping the bits from burning might be a difficult one.

2. Removing cuttings. This is another essential of drilling mud. The removal of cuttings of a given size and density requires an upward mud velocity of sufficient pressure to force the cuttings out of the hole.

3. Preventing caving. By providing a hydrostatic pressure the drilling mud tends to hold loosely consolidated, cave - in formations in place. A column of fluid or mud in a water well exerts pressure equally in every direction at any point. In consolidating loose or cavy formations such as gravel, sand, glacial drift, etc., the drilling mud should surround the particles of the formation and consolidate them by holding.

4. Lubricating drill pipe. Good drilling mud covers everything with a slippery film that is an excellent lubricant. Thus drilling speeds can be maintained and power costs reduced.

5. Checking corrosion and rust.

When prepared mud's are not available, native clays can be used for drilling purposes in emergencies but are inferior. Most of them contain too much sand which interferes with the sealing properties of the mud.