It is hoped that some procedure for quantitatively evaluating contributing 

 factors will enable drilling-time properties to be understood as fixed character- 

 istics after allowing for the amounts of time required in the drilling of a unit 

 of depth that are not attributed to the inherent lithology of the rock. Among 

 the contributing factors referred to are the size of the hole, the type of bit, the 

 drilling weight employed, the rotary speed, torque and friction, and the condition 

 of the mud. This subject is worthy of study as a research project in order to 

 determine the net-drilling-time value of a formation having uniform character- 

 istics over an area large enough that adequate drilling-time data could be ac- 

 cumulated and studied. 



The qualitative interpretative value of drilling-time data, however, is not 

 impaired by the absence of quantitative calculations. Observations by the 

 author in the drilling of hundreds of thousands of feet of hole have shown that 

 drilling conditions are insignificant in comparison to lithology in determining 

 the rate of penetration of a rock formation. Obvious exceptions have been noted, 

 but the foregoing observation holds true. It has been shown in many instances 

 that a change in formation will be reflected by a change in drilling time even 

 when very dull bits have been in use or where other conditions would be expected 

 to obliterate any evidences of change in drilling time. Perhaps the condition that 

 affects drilling time more than any other is holding up on drilling weight as 

 when straightening a hole tending to deviate. Under such disadvantageous con- 

 ditions, there may be no pronounced change in the actual time required to drill 

 a unit of depth when passing from one lithology into another, but the pattern 

 of the curve plotted from drilling-time data seldom fails to reflect the change in 

 lithology. Adverse drilling conditions do require more careful interpretation 

 than favorable drilling conditions, but the effect of changes in lithology is 

 seldom completely obscured. 



Because drilling time is a qualitative property of a rock, it is important 

 that correct identification of lithologies be based on the observation of relative 

 values. A foot of hole that is drilled in 5 minutes at one depth may be interpret- 

 ed as a sandstone, and another foot drilled under conditions differing from the 

 first-mentioned foot requiring also 5 minutes for drilling may be interpreted as 

 a shale. In each case the interpretation is based on the relative time in compari- 

 son to previously drilled feet. The value of the application of drilling-time data 

 to geologic and engineering problems lies in the recognition of this relative 

 interpretation. 



DRILLING TIME The two means of measuring rate of pene- 



AND DRILLING RATE tration have been defined; and, as shown, 



drilling time is a specific value for each foot, 

 and drilling rate is an average value involving the drilling of several feet. The 

 former is more exact and is useful where detailed lithologic information is re- 



370 



