Miscellaneous Subsurface Methods 645 



Soft Formation Bits 



The OSC-3 tricone bit (fig. 340) is designed for drilling unconsoli- 

 dated formations, such as soft shales, clays, redbeds, salt, and broken 

 shale, which have a low compressive strength and high drillability. 



The teeth on each cone of the OSC-3-type tricone are more widely 

 spaced and deeply cut than those on the OSC-1 or the OSC types. The 

 very widely spaced and deeply cut teeth permit effective penetration into 

 the low-strength formations and result in the excavation of large cuttings 

 at a rapid rate. The wide spacing of the deeply cut teeth provides more 

 space to handle cuttings extruded from the formation and facilitates clean- 

 ing of the cutting surface. Cleaning is of the utmost importance in the 

 softer range of drilling, because of the characteristic sticky properties of 

 strata frequently penetrated. 



In addition to the "ventilation" obtained through the use of widely 

 spaced and deep teeth, the full interfitting of the inner rows of teeth into 

 grooves of mating cutters provides mechanical cleaning of the grooves 

 between rows of teeth on each cone. In addition to mechanical cleaning, 

 interfitting provides necessary space for the long penetrating chisel teeth, 

 which are essential for the not uncommon penetration rates of 50 to more 

 than 100 feet an hour. 



In general, the soft, unconsolidated formations do not wear the teeth 

 rapidly, and a minimum number may be used. The use of relatively few 

 teeth in each row on the OSC-3 tricone means less tooth contact on bottom 

 at any instant, deeper penetration into the formation, and a faster drilling 

 rate. The teeth are strong but are made as slim as modern metallurgy will 

 permit, thereby prolonging the fast drilling rate. Further to assure a slow 

 rate of dulling and maximum footage per bit, Hughesite hard facing is 

 applied to surfaces of all teeth as well as the gage surface. 



Although the teeth are widely spaced, the teeth on successive revolu- 

 tions do not track in previous impressions made on bottom. Instead, the 

 outermost rows generate a rock-tooth pattern on bottom having crests 

 spaced approximately one-half the spacing of the cutter teeth. The inner 

 rows of teeth also generate a finer-spaced, rock-tooth pattern on bottom. 

 Thus, though teeth are widely separated, particles cut from bottom are 

 relatively small and cannot readily wedge between cutter teeth but are 

 easily washed away by the drilling fluid being ejected from the water- 

 course nozzles. 



To increase the cutting speed of the OSC-3 tricone further, especially 

 when light weight must be used to keep the hole straight, the cone axes 

 are offset forwardly off-center to provide a twisting-tearing action on bot- 

 tom as the teeth penetrate. This feature is effective especially in shale for- 

 mations having a tough structure not readily chipped. 



The OSC-1 Tricone 



The OSC-1 design (fig. 342) differs from the OSC-3 in that the teeth 

 are not so widely spaced or so deeply cut. The arrangement of teeth re- 



