656 Subsurface Geologic Methods 



are encountered below the soft, unconsolidated formations having high 

 drillability previously discussed, and proper pump capacity for drilling 

 these formations should insure that an adequate volume of drilling fluid 

 is being circulated when drilling with OWS tricone bits. 



Drilling Hard to Hard, Abrasive Formations 

 Weight Applied and Rotary Speed 



The application of heavy weight on the W7R tricone bit will ma- 

 terially improve performance in drilling hard to hard, abrasive forma- 

 tions, which generally are firmly cemented, have a high compressive 

 strength, and are the most expensive to drill. This class of formations 

 include chert, chat, pyrite, granite, quartzite, dolomitic sandstone, cherty 

 limestone, quartzitic sandstone, and the like. 



During the first few minutes of operation a moderate weight in the 

 range or 1,000 to 1,500 pounds to the inch of diameter of the bit is good 

 practice. The cutter teeth will "pattern bottom," and all rows of teeth 

 will function properly. Having "patterned bottom," the weight may be 

 increased into the range of 3,000 to 4,000 pounds to the inch of diameter 

 of the bit to establish an economical rate of penetration. Under heavy- 

 weight operating practice the rotary-table speed is usually within the 

 range of 45 to 60 r.p.m. The desired chipping-crushing action can be 

 realized only when the load supported by the rock-bit teeth is in excess 

 of the compressive strength of the formation. 



In extremely hard, abrasive formations the application of weight in 

 the range of 4,000 to 6,000 pounds to the inch of diameter of the bit has 

 been successfully used. With these extreme weights applied on the bit, 

 it is advisable to limit the rotary speed from 40 to 50 r.p.m. 



Volume of Drilling Fluid Circulated 



In general, balling up of the cutting structure is not liable to occur 

 when drilling in hard to hard, abrasive formations. However, shale breaks 

 are sometimes encountered, and a strong jetting action may be needed to 

 remove any formation from the cutter teeth readily. In addition a return 

 velocity of the drilling fluid in the range of two to three feet a second 

 will give a low density of cuttings in the mud stream and lessen the danger 

 of sticking the drill pipe. 



Weight From Tandem Drill Collars 



The use of drill collars for providing the weight carried on rock 

 bits offers several advantages. The weight is concentrated closer to 

 bottom and is more effective in forcing the cutter teeth into the forma- 

 tion. The drill pipe is subject principally to tensile loading only, and 

 fatigue failure resulting from reversal of stress is materially reduced. 



Long strings of 10 to 20 drill collars, each 30 feet long, are be- 

 coming increasingly popular in areas where hard rocks having a high 



