surface and pumped down the drill pipe. After leaving the bit, it moves from 

 the restricted drill pipe and drill-collar bore into the annulus. The air travels 

 at high speeds through the openings in the bit and is directed against the bottom 

 of the hole. This action has two immediate benefits : it greatly assists in loosen- 

 ing and lifting the bit cuttings, and it cools the bearings and teeth of the bit. 

 Although the cuttings have a density considerably greater than that of the air, 

 the high velocity of the air carries them up the annulus. As a formation chip 

 moves upwards, the constant contact with the drill pipe, the walls of the bore- 

 hole, and other chips reduces the size of the chips and makes it easier for the 

 air to carry them to the surface. As the cutting-laden air reaches the surface, 

 it is diverted away from the rig, preferably downwind, and exhausted to the 

 atmosphere. 



Besides the chipping action of the bit teeth and the blasting effect of the air, 

 one other condition assists in removing cuttings. The formation being pene- 

 trated has been, through an extended period of time, subjected to the weight of 

 the formations lying above. In normal drilling with mud, this formation weight 

 is partially replaced by the head of drilling fluid in the hole. When air is sub- 

 stituted for mud, most of this weight is removed, and the penetrated formation 

 is relieved of nearly all of the downward compression. The removal of this 

 stress is probably responsible for partial breakage of the formation. 



DRILLING METHODS Air drilling involves no major change-over 



from conventional rotary drilling. Air com- 

 pressors simply replace mud pumps, and air is substituted for circulating mud. 

 Normally a rotating stripper head is added on the top of the blowout preventer. 

 Minor modifications are made to suit the particular situation or the desires of 

 the company. 



The basic principles of rotary drilling do not change just because air has 

 been substituted for mud. The drilling bit is cleaned by circulation. It has been 

 found that drilling with air requires considerably less weight on the bit and that 

 the bit may be rotated at a much slower speed. The combination of weight and 

 rotating speed most suitable for penetrating a particular formation is determined 

 on the basis that the rotating hours obtained from a bit vary inversely with the 

 weight and speed used. In general, it has been found that a good starting point 

 is about one half the weight used with mud and from one half to three quarters 

 of the rotary speed used with mud. No rule of thumb can be followed to deter- 

 mine what combination is best or what rate of penetration should be attempted. 

 It must be realized, however, that the greater the bit life, the greater the percent- 

 age of productivity time obtained from the equipment. 



Another factor to bear in mind, particularly in the drilling of softer forma- 

 tions, is that the drilling rate should not be faster than the rate at which the 



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