06*fite*3O 



JET BITS 



W. M. Booth 



and 



R. M. Borden 



A jet bit is any drilling bit in which nozzles directed at the bottom of the hole 

 utilize hydraulic power to assist in the drilling process. Certain basic differences 

 between jet bits and conventional bits must be understood in discussing theory 

 of drilling operation. In a conventional bit, the mud is discharged through rel- 

 atively large orifices, usually one to three in number, so that the streams impinge 

 on the cones or cutters. This discharge produces a cleaning action on the cones 

 or cutters, and the mud deflects sideways toward the wall of the hole. In a jet 

 bit, the mud is discharged through somewhat smaller orifices at a higher velocity 

 and is directed so that it impinges on the bottom of the hole. The theory of 

 operation of the jet bit, then, is that the mud-stream energy is used to remove 

 loosened chips of formation from the bottom of the hole so that repeated cutting 

 action by the bit teeth is not required to remove the chips. Conserving tooth 

 structure in this manner extends bit life and improves penetration. 



The cleaning of the cones or cutters of the jet bit is accomplished by the 

 turbulence created by the high-velocity streams as they strike the bottom of the 

 hole. It is believed that the amount of cleaning necessary is reduced with jet 

 bits because sticky particles of the formation are removed before they have an 

 opportunity to adhere to the cutting units. Thus, high volumes of mud can be 

 used to advantage and are essential to efficient operation of the jet bit. With a 

 conventional bit, high mud volumes may have an undesirable erosional effect 

 on the cones or cutters. 



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