Miscellaneous Subsurface Methods 657 



compressive strength are encountered. These hard rocks such as hard 

 dolomite, chert, and limestone can be drilled at faster rates af penetra- 

 tion when all factors involved permit loading the rock bit in the heavy- 

 weight range. (See discussion under OWS and W7R tricones.) The 

 weight of the drill collars used should exceed the load applied on the 

 rock bit. In general, from 25 to 50 percent additional weight of drill 

 collars is considered good practice. In any case it is not good practice 

 for the neutral zone (change from compression to tension loading) to 

 occur near the upper end of the top drill collar, as under this condition 

 the joint of drill pipe attached to the top drill collar is not restrained 

 laterally and is subjected to severe bending stresses, and fatigue failure 

 is almost certain to occur in a relatively short time. The weight applied 

 on the bit and the number of drill collars used should be balanced so that 

 the neutral zone is well within the drill-collar string or in the drill 

 pipe several joints above the top drill collar. 



Rock-bit failures have been known to occur under lightweight loading 

 when long drill-collar strings were in use. It was found that the excessive 

 drill-collar weight, some 15,000 to 18,000 pounds, in conjunction with 

 rotary speed, rough-running characteristics of the formation, and light- 

 load application on the bit, sets up a harmonic vibration in the drill string 

 of a magnitude sufficient to cause the drill collars to act as a hammer. 

 The force transmitted to the rock bit exceeded the strength of the materials, 

 and fatigue failure occurred. A change in applied weight on the rock bit 

 or in rotary speed will eliminate this condition. 



The principal difficulty encountered with long tandem drill-collar 

 strings is failure of the threaded connections. Proper inspection of the 

 threads and shoulders on each trip is good practice. The connections be- 

 tween breaks should be checked for tightness; and, if makeup is noted, 

 the connections should be broken apart, cleaned, inspected, and recondi- 

 tioned, if necessary, to remove fins and slivers from the threads and 

 galls from the shoulders. The connection should be redoped and tonged 

 tight. 



When only a few drill collars (one to three) are used, most of the 

 applied weight is obtained by placing the lower portion of the drill pipe 

 in compression. As a result, the relatively limber drill pipe is deflected 

 off center until restrained by the wall of the hole; bending stresses invite 

 fatigue failures through the drill pipe; the pipe is subjected to O.D. 

 wear, which reduces the wall thickness and may cause failure; a portion 

 of the applied load is lost in friction established by the drill pipe and 

 tool joints forced against the wall of the drilled hole; tool-joint life is 

 materially reduced, particularly in abrasive formations; and the average 

 load on the rock bit is less owing to wall friction and the spring charac- 

 teristics in a long string of drill pipe. 



The compressive strength and drillability of formations vary widely 

 and prevent adoption of a universal drilling practice. With the proper 



