Figure 29-3. A bit showing excessive wearing and rounding of the gage surface. 



because the softer the bit type, the faster the drilling rate provided the formation 

 is not too hard. The depth to which any bit type can be used economically varies 

 with the stratigraphy of the area and the drilling procedure. 



In general, excessive bearing wear should not be used as a criterion for 

 changing to a harder formation bit without first considering operating conditions 

 and the condition of the dulled cutting structure. Heavy or light weights in 

 combination with high rotary speeds can cause excessive bearing wear, particu- 

 larly if abrasive muds are used. A change to a harder bit type with a larger 

 bearing would reduce the bearing wear, but would probably result in a slower 

 penetration rate. An adjustment in operating practice might enable the operator 

 to continue to use the softer bit type more economically. 



The examination of the worn cutting structure offers an excellent means 

 of determining the proper bit for the formation being drilled. Excessive or un- 



643 



