already, some believe that there is a definite relationship between nozzle velocity 

 and penetration rate, but this relationship has not been consistent in practice; 

 hence, is not universally accepted. The result of the higher nozzle velocity 

 through increased mud circulation, with the resultant higher annular velocity, 

 appears to be gaining in popularity at this writing and has brought about a 

 minor modification in the specifications of the drilling rig. Five-inch drill pipe 

 has shown a very marked increase in popularity due to its improved hydraulic 

 characteristics over the more common 41/2-inch pipe. Although engines for 

 drilling rigs were formerly matched to the drawworks or hoist, they are now 

 being matched to the slush pumps to provide adequate pumping horsepower, 

 a setup that invariably results in an excess of power for the hoist. 



The purpose of this chapter is not to discuss methods for determining when 

 or when not to use a jet bit, but basically to review the jet bit as an existing 

 oil-well drilling tool. However, at this point we must discuss how to determine 

 whether a rig is capable of drilling a hole with jet bits. When it has been 

 established that a given annular velocity is required, or that a given minimum 

 jet-nozzle velocity is required, it must be determined whether the total losses in 

 horsepower will exceed the available horsepower on the surface. If it is establish- 

 ed that this horsepower requirement is in excess of the existing power, a com- 

 promise must be made and a revised approach to the problem worked out. To 

 calculate horsepower losses, manufacturers provide charts, formulae, and curves. 

 With this information, horsepower losses in the entire system can be established, 

 and resultant annular velocities and jet velocities can be determined. From these 

 figures the operator is able to set the maximum depth limit of the jet-bit opera- 

 tion. 



Although industry is reasonably sure that the jet-rock-bit principle is 

 sound and of considerable value if properly applied, the lack of sufficient empir- 

 ical data to supplement the arbitrary established data sometimes results in failure 

 to improve penetration rates sufficiently to make jet-bit drilling economically 

 feasible. There can be no doubt that, when applied properly, the jet bit has 

 indicated improved penetration rates. 



JET- BIT ECONOMICS The justification for the use of jet bits lies 



in their ability to reduce drilling cost. The 

 economic evaluation of jet bits is complex and involves use of estimates for 

 some variable-cost items such as service life of mud pumps, prime-mover main- 

 tenance, and maintenance of mud lines. Because the use of jet bits requires 

 higher mud-pump pressure and horsepower, it is apparent that operating costs 

 of the pump and associated equipment will be higher. To arrive at exact costs 

 would require exhaustive testing over the entire life span of the equipment, which 

 may be from 8 to 10 years. This compilation has yet to be done; therefore esti- 



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