654 Subsurface Geologic Methods 



weight applied on an OSC-series tricone bit need not be of a high magni- 

 tude to establish a fast rate of penetration. These formations have a low 

 compressive strength, and good penetration by the rock-bit teeth is obtained 

 with a moderate load on the bit. An initial weight of 500 to 1,000 pounds 

 to the inch of rock-bit diameter and a rotary speed of 150 to 175 r.p.m. 

 are considered good practice. As the teeth on the rock bit wear and gradu- 

 ally dull, the applied weight should be increased to maintain a satisfactory 

 penetration rate. 



Rotary Speed 



Usually an increase in weight should be accompanied by a reduction 

 in rotary speed to compensate for the increased load on the bearings and 

 permit them to function properly during the normal life of the cone teeth. 

 Common practice, used in drilling low-strength formations, involves a 

 maximum applied load of 2,000 to 2,500 pounds to the inch of bit diameter 

 with the rotary-table speed decreasing from 175 r.p.m. to 75 r.p.m. as 

 additional weight is applied. 



In certain areas rotary speeds in the range of 250 r.p.m. to 350 r.p.m. 

 and applied loads in the range discussed above are accepted practice. The 

 formations in these areas are predominantly shale and sands and are 

 relatively free from any hard rocks, which cause severe shock forces that 

 would result in abnormal stress on the rock bit and the drill string. 



Volume of Drilling Fluid Circulated 



The volume of drilling mud circulated when drilling formations 

 having low strength and high drillability should be sufficient to provide 

 a return velocity up the annulus equal to at least three feet a second. 

 This volume will establish the required velocity through the rock-bit 

 water-course nozzles for good cleaning of the cutting structure and bottom 

 of the hole. Furthermore, the cuttings will reach the surface at a faster 

 rate, and the danger from a high density of cuttings in the annulus stick- 

 ing the drill string will be less. Since formation characteristics vary 

 widely, a definite pump capacity suitable for one area may be inadequate 

 for a different area. 



Bits for Formations on the Harder Side of Medium-Soft 



Weight Applied and Rotary Speed 



In drilling formations on the harder side of medium-soft, which in- 

 clude packed sands, anhydrite, medium-hard lime, and medium-hard 

 shales, moderate weight is applied on the OSQ-2-series tricone bit. Many 

 of these formations have a low strength, but harder zones usually present 

 warrant the application of more weight than is normally required in drill- 

 ing the relatively soft, low-strength formations. As a general rule the 

 OSQ-2 tricone, which is designed to drill these formations, will give satis- 

 factory performance with drilling weights ranging from 775 to 1,000 



