hole can be cleaned with the available air volume. The bit can sometimes drill 

 2 or 3 feet per minute, but generally at this rate the hole cannot be cleaned prop- 

 erly. The time gained in rapid drilling is lost in cleaning the hole before a con- 

 nection can be made or before a trip. If the weight and rotating speed are 

 reduced to the point where hole cleaning keeps up with pentration, then the bit 

 life is extended to maximum. 



In rigging up for air or gas drilling, it should be remembered that there is 

 no weight in the annulus to counteract possible high-pressure zones. For this 

 reason, it is advisable to maintain mud in the pits as a safety precaution. 



Another very important factor is the abrasive dust that returns to the 

 surface. If the moving parts of the rig are not protected properly, they will 

 be subject to severe wear. The crucial point of danger is the rotary table, where 

 the ascending air must be deflected into the flow line and carried away. Even 

 a small leak in this area will result in rapid accumulation of dust in the table 

 bearings. Normally some type of rotating stripper head is placed above the 

 flow line so that the packing of the head forms a seal around the kelly to pre- 

 vent the dust from entering the rotary table or from coming through to the rig 

 floor. The flow line should be of sufficient length and oriented in a direction 

 that will prevent the dust from being blown over the rig. If conditions permit, it 

 is advisable to set two flow lines in opposite directions so that a downwind line 

 is always available. 



It has been agreed generally that, for proper hole cleaning, the ascending air 

 velocity should be approximately 3000 feet per minute. To determine the volume 

 of air required, the hole size and the drill-pipe size must be considered. As 

 Table 36-1 shows, maintaining the proper annular velocity in the larger hole 

 sizes is impractical. In general, the velocity in large holes is allowed to drop 

 below that which is considered desirable because the cost of the additional 

 equipment to maintain 3000 feet per minute would be prohibitive. In this 

 instance, gas often has a decided advantage over air in that greater volumes 

 normally are available. Regardless of the conditions, it is advisable to have an 

 excess volume of air available to accommodate unusual situations that may 

 arise. 



Air-pressure requirements will vary with drilling conditions, but in general 

 they are quite low. For example, drilling a 7% -inch hole with 4-inch drill pipe 

 and 6-inch drill collars at 5000 to 6000 feet, a pressure of 100 pounds per 

 square inch is ample as long as the hole is dry. The pressure requirements 

 will increase with depth and will be further increased if damp or wet forma- 

 tions are encountered. The majority of the compressor equipment now used is 

 capable of sustained operations at pressures up to 350 pounds per square inch, 

 which are adequate for any drilling problems encountered thus far. When the 

 formation becomes wet, it is advisable to have an additional booster compressor 

 capable, not only of increasing the air pressure up to 1000 or 1500 pounds per 



737 



