658 Subsurface Geologic Methods 



design and type of tricone on bottom, the weight on the bit, the speed of 

 rotation, and the volume of drilling fluid are all important factors in 

 obtaining a maximum penetration rate for efficient rotary drilling. 



The graphs reproduced in figures 349 to 354 show the compressive 

 strengths of various formations, the drilling rates in various formations 

 with diff"erent loads applied on the rock bit, and common drilling practice 

 in balancing rotary-table speed with the weight applied on different sizes 

 of rocks bits. 



All rotary rock bits require weight to make them cut. Up to the 

 Eoundering point Hughes tricones will generally drill faster and make 

 more hole the greater the weight applied. With weights greater than the 

 foundering load, particularly in soft, low-strength formations, the teeth 

 become fully embedded and the cutter shell bears on bottom and slows 



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m AUSTIN CHALK (, SAMPL.) UNSHADE^D MREA mOlCATES 



V7V\ GLEN ROSE LIMESTONE (2 SAMPLES) STRENGTH 



'////A 1 EDWARDS LIMESTONE (2 SAMPLES) 



'/////////A 1 PRE- CAMBRIAN CALCITE MARBLES (8 SAMPLES) 



■//////////////\ ~l CAMBRO-ORDOVICIAN CALCITE MARBLES (7 SAMPLES) 



V//'////y>//////\ Ipimk r.RAKiiTF (laSAMPLEs) 



'///////////////\ I r.RAY r.RANlTF (|0 SAMPLES) 



'////////////////A ~l PRF-r.AMRRIAN DOLOMITE MARBLES (6 SAMPLES) 



'/////////////////A ~l r AMRRfi-nRnnviriAN DOLOMITE MARBLES(7SAMPLES) 



'//////////////////// ///A BURNET COUNTY DOLOMITE (l SAMPLE) 



^////////////////////y/y^\W^ QUARTgiT^ I'l tt.UPL t)'////////777A 



10,000 20,000 30,000 40,000 50^000 60,000 70,000 



Figure 349. Chart showing compressive strength (pounds per square inch) 

 of various rock types. 



up progress. The determination of optimum weight, as well as other factors 

 affecting drilling practice, must be the result of actual field testing and 

 the experience thus acquired. 



The efficiency of a rock bit requires an adequate volume of fluid 

 circulation, particularly when formations in the softer range are being 

 drilled. The harder range of formations may not require the large vol- 

 umes used in soft formations. The presence of sticky-shale breaks, which 

 are often encountered, however, warrants the selection of pumps de- 

 signed to deliver an adequate volume of drilling fluid. The volume of 

 drilling fluid circulated can be established when the pumps are purchased. 



