the conventional rock bit, in areas where the depth and thickness of very hard 

 and abrasive formations are fairly well known, it has proved very economical. 

 Experience indicates that the best operating practice in drilling chert or 

 chert-bearing limestones is to carry a bit weight of approximately 4000 pounds 

 per inch of bit diameter, using a rotary speed of approximately 35 revolutions 

 per minute. Excess weight is not necessarily economical because it may result 

 in breakage of the compacts and also reduce the life of the bearing. 



Summary 



From the foregoing discussion, it can be seen that bits are available for 

 drilling any formation ranging from soft to hard. The big problem for the 

 operator is when and where to use these various rock bits to the best advantage. 

 Quite often the operator has no definite information as to the characteristics 

 of the formations to be drilled. In these instances, examination of dull bits can 

 be valuable in subsequent bit selection if the operator keeps in mind the funda- 

 mental differences in the various bit types. It should be remembered that each 

 bit type incorporates certain design features and cutting actions that will enable 

 it to drill a range of formations successfully. Bit selection by this method pre- 

 supposes that the subsequent bit will encounter approximately the same forma- 

 tion. Unfortunately this is not always true, but it is a better approach to better 

 bit selection than a blind guess. 



The most economical rock bit performance can be obtained if optimum 

 weights and rotary speeds are used. Sufficient weight must be applied to the 

 bit to exceed the compressive strength of the formation, thus enabling the teeth 

 to penetrate the rock; however, too much weight will cause the bit to ball up in 

 soft formations or cause excessive tooth breakage and/or bearing wear in the 

 more firm formations. Penetration rate tends to increase as rotary speed is 

 increased, but excessive rotary speed can cause excessive tooth breakage and/or 

 bearing wear, thus reducing the life of the bit. The operator must select his own 

 operating conditions to suit the particular formations in which he is drilling; 

 he must compromise between weights and rotary speeds in attempting to arrive 

 at a combination that will most economically drill the formation. Under certain 

 conditions, the operator may find it advisable to apply light weights; although 

 by using such weights, it may make the formation appear much harder than it 

 really is. In these cases it would be advisble to use a softer formation bit. 



The type of drilling fluid used should be considered in bit selection. A 

 change from water to mud will decrease bit performance without formation 

 variation being involved. This change might give the impression that the 

 formation is harder, and result in selecting the wrong bit type. 



In most areas it is possible to use group 1 types for spudding in and drilling 

 the upper portion of the hole. These types should be used as deep as possible, 



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