the swivel. The kelly should be at least 6-inch, with a larger bore; and the 

 circulation holes through the bit should be made large to accommodate the in- 

 creased mud flow. 



5. Drilling fluid should be desanded to minimize wear on both the turbine 

 and mud pumps. The sand content should be kept to 1 or 2 percent if possible ; 

 it should never exceed 5 percent. 



ADVANTAGES OF The following advantages of turbodrilling tend 



TURBODRILLING to make it an important adjunct to the rotary 



system. In many instances, turbodrilling may 

 make obsolete other presently known drilling methods. 



1. Faster penetration rate: The availability of more power at the bottom 

 of the hole produces drilling rates that are not possible with the conventional 

 rotary. Then, too, as the hole deepens, the penetration rate of the turbodrill 

 can be maintained more uniformly than that of the conventional rotary. 



2. Straighter drilling: Because the bit speed in turbine drilling is higher 

 and the bit weight generally lower than with rotary drilling, less deviation of 

 the hole may be expected. 



3. Full-gage drilling: Since in turbine drilling the drill stem does not 

 whip, the hole has a more uniform caliper. This is an important advantage in 

 running or cementing casing. 



4. Better utilization of crew : Because of the potentially greater penetration 

 rate with less down-time for equipment repairs, use of the turbodrill will permit 

 a more efficient use of the drilling crew. 



5. Little wear on drill pipe: The hard wear on the drill pipe, which is 

 incident to rotary, is not a liability in turbodrilling. Because in turbodrilling 

 drill pipe is not used to transmit power to the bit, and rotates at most only 1 to 

 15 revolutions per minute — usually not at all — drill pipe is not subject to hard 

 wear. For this reason, drill pipe used in turbodrilling can be lighter and cheaper. 

 The lighter duty on the non-rotating drill pipe is said to cause 2 to 3 times 

 longer pipe life. Torque is less than with rotary, and it never approaches the 

 peak torque values created by the sudden reactions of the bit in rotary drilling. 



6. Fewer fishing jobs: Most of the causes of parted drill pipe are eliminated 

 by turbine drilling. The pipe is never subjected to excessive torque because of 

 the design characteristics of the turbine. Also, the pipe does not drag on the 

 wall of the hole except at extremely low speeds. Twisting off of drill pipe does 

 not occur. Turbine drilling does not create any new difficulties of this nature, 

 but instead eliminates or reduces most of the existing dangers and difficulties 

 encountered with rotary. 



7. Less wear on rig: The wear on the swivel and the rotary table is con- 

 siderably reduced in turbine drilling. 



672 



