The primary purpose for giving these formulae is to show the effects of one 

 variable on the others. An examination of the established conditions will show 

 that variations of diameter and length through the hydraulic system, mud-pump 

 size, horsepower in the mud system, etc., will result in losses or gains at the 

 rock bit. A variation in the controllable conditions will similarly result in 

 hydraulic losses or gains at the rock bit. Although the established conditions 

 noted all pertain to jet-bit hydraulics, some of the controllable conditions do 

 not. Specifically, reference is made to the revolutions per minute of the rotary 

 table and the weight on the bit. Although these factors do not pertain to jet- 

 rock-bit hydraulics, they do pertain directly to the over-all rate of penetration 

 because they are factors affecting the life of the rock-bit bearings and cones as 

 well as the ability of these bits to break up the formation physically. These two 

 factors are of utmost importance because they determine the rate at which the 

 chips are broken away from the formation; they are not related to the jet-rock- 

 bit principle. They are as important in drilling with any rotary bit. 



In summary, rotary drilling with rock bits may be divided into two 

 parts: (1) the physical breaking up of the formation and, (2) the removal of 

 the cuttings from the bit teeth. The former pertains in part to the action of the 

 rotary speed and bit weight, whereas the latter pertains to the jet-rock-bit 

 principle. 



Earlier in this chapter mention was made of the fact that, although it is 

 possible to calculate the losses involved in the hydraulic system, experience is 

 the primary factor in determining the practicability of the jet bit in various 

 formations. It is from experience, therefore, that many operators have establish- 

 ed arbitrary figures on required mud volumes and velocities. It has been noted 

 that 180 feet per minute is the minimum annular velocity required to carry the 

 cuttings to the surface. It also has been established that the minimum effective 

 jet-nozzle velocity satisfactory for the use of jet bits is 200 feet per second. It 

 is these two figures that have created the jet-bit controversy now prevalent in 

 the drilling industry. On numerous occasions, it has been demonstrated that 

 annular velocities below the arbitrary minimum would be sufficient. The impli- 

 cations are that, with a lower annular velocity requirement in a given hole 

 with a given drill-pipe size, lower circulating volumes may be used. The effect 

 of the lower volume requirement results in the use of less horsepower, and all 

 related losses are decreased. The opportunity then arises to reduce rock-bit- 

 nozzle sizes and thereby to increase horsepower expended at the bit without 

 overloading slush pumps and engines. This factor would enable some rigs, now 

 equipped for conventional bit drilling, to utilize jet rock bits with existing 

 facilities, provided that it would be possible to attain the established jet-nozzle 

 velocity of 200 feet per second. On the other hand, some operators believe that 

 a jet-nozzle velocity of 200 feet per second is not adequate and that the higher 

 the jet-nozzle velocity, the higher the penetration rate. As has been stated 



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