Miscellaneous Subsurface Methods 655 



pounds to the inch of the diameter of the bit in its early life to a maximum 

 of 3,000 pounds to the inch of the diameter of the bit when it is well worn, 

 with rotary speeds decreasing from 150 to 75 r.p.m. as the drilling weight 

 is increased. 



Volume of Drilling Fluid Circulated 



The pump capacity should be equivalent to that suggested for drilling 

 soft formations. The average penetration rate is less, but the softer, sticky 

 zones are apt to be present in formations classed as "on the harder side 

 of medium-soft," and high drilling-fluid velocity through the rock-bit 

 nozzles is good insurance against the cutting structure balling up. To a 

 certain extent drilling is "blind," and, as formations are not uniformly 

 free of sticky breaks, the pumps should be designed and operated to main- 

 tain a clean, free-cutting bit at all times. 



Bits for Medium-Hard to Hard, Nonabrasive Formations 

 Weight Applied and Rotary Speed 



Medium-hard to hard, nonabrasive formations, like all others, can 

 be drilled with relatively light weight on the rock bit. The rate of pene- 

 tration with light weight will be slow, particularly when the cutter teeth 

 are partly dulled from their initial sharpness. When drilling conditions 

 permit, it is better practice to apply sufficient weight on the OWS tri- 

 cone bit to establish effective chipping-crushing action of the cutter teeth 

 working on the formation. This action, in combination with the twisting- 

 tearing action always present in the OWS tricone bit, will materially 

 increase the rate of penetration and footage obtained from the bit. 



During the early life of the bit, it is good practice to drill with 

 moderate weight, in the range of 1,000 to 1,500 pounds to the inch of dia- 

 meter of the bit, with rotary speed in the range of 85 to 125 r.p.m. The 

 cutter-teeth "pattern bottom" and breakage of sharp teeth are minimized. 

 As the teeth dull, the weight applied on the OWS tricone bit should be 

 increased and the rotary speed decreased to retain a fast penetration 

 rate. It is common practice to apply weight in the range of 2,500 to 

 3,500 pounds to the inch of diameter of the bit, with a rotary-table speed 

 in the range of 60 to 70 r.p.m. 



In some areas where other factors such as limited hours on bottom, 

 smooth drilling characteristics of formation, and flat-formation bedding 

 planes will permit, weights in the range of 3,500 to 5,000 pounds to the 

 inch of diameter of the bit are accepted practice. With the OWS tricone 

 bit carrying this heavy weight, the rotary-table speed should not exceed 

 60 r.p.m. 



Volume of Drilling Fluid Circulated 



The presence of sticky shales, gypsum, and the like is to be con- 

 sidered in the selection of pump capacity. In general, harder formations 



