6. Weight on bit 



7. Mud velocity at bit nozzles 



8. Mud-annular velocity (between drill pipe and wall of the hole) 



9. Pressure losses in the circulating system 



10. Pressure losses at the bit nozzles 



11. Mud properties, i.e., viscosity, weight, solids content, etc. 



12. Horsepower expended at the bit 



13. Rock-bit type (jet or conventional) 



It will be noted that these factors are related to one another; that is, a change 

 in mud-pump liner size, or volume delivered, will result in a change in pressure 

 losses, annular velocity, etc. It is important, therefore, to understand that, due 

 to the number of variables involved in the basic requirement of drilling the hole 

 with a jet rock bit, an economic balance can be obtained to give the maximum 

 rock-bit performance. Of course, this balance is achieved more by experience 

 than by engineering calculation because the wide variation of the heterogeneous 

 formations respond differently to combinations of these variables. Many opera- 

 tors believe, however, that there is a definite relationship between the nozzle 

 velocity, which is a function of horsepower, and the rate of penetration, regard- 

 less of type of strata encountered. But despite this relationship, the remainder 

 of the controlled conditions so effect the net penetration rate that to minimize 

 the importance of any one of these conditions may result in a reduction of any 

 gains made by determining the proper jet velocity. It is necessary at this point 

 to clarify the relationship between the conditions and to determine how they 

 eventually affect horsepower at the jet bit. Because horsepower expended at 

 the rock bit is a function of several factors and can be expressed in a simple 

 formula, the study of this formula and related ones will help integrate all these 

 factors. 



(HHP) Bit-hydraulic horsepower 



(V) Bit-nozzle velocity 



(P) Pressure drop across bit nozzle 



GxP 



1714 



.32G 



12031 A 2 C 2 



WHERE: 



HHP - Hydraulic horsepower expended at bit nozzle 



V - Jet velocity (feet per second) 



P - Pressure drop across bit nozzle (pounds per square inch) 



A - Nozzle area in square inches 



G - Rate of mud flow (gallons per minute) 



p - Mud density (pounds per gallon) 



C - Orifice coefficient 



657 



