458 Subsurface Geologic Methods 



and fifteen seconds; nor, conversely, that any rock which drills at that 

 rate is necessarily that particular type of sandstone. Under one set of 

 conditions it may drill in exactly that amount of time, and with other 

 drilling conditions it may require much less or much more time. Never- 

 theless, we are defining rate of penetration as a fixed lithologic property, 

 comparable to electric, radioactive, or mineralogic properties, and the 

 hypothetical fixed time required to drill one foot of sandstone such as 

 that described above could be defined as diagnostic of that rock. 



It is hoped that some procedure for quantitatively evaluating contrib- 

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

 characteristics 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 characteristics over an area large 

 enough that adequate drilling-time data could be accumulated 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 ex- 

 ceptions have been noted, but the foregoing observation holds true. It 

 has been shown in many instances that a change in formation will be re- 

 flected 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 evi- 

 dences of change in drilling time. Perhaps the condition that affects drill- 

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

 straightening a hole tending to deviate. Under such disadvantageous 

 conditions, 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 impor- 

 tant that correct identification of lithologies be based on the observation 

 of relative values. A foot of hole that is drilled in five minutes at one 

 depth may be interpreted as a sandstone, and another foot drilled under 

 conditions differing from the first-mentioned foot requiring also five min- 

 utes for drilling may be interpreted as a shale. In each case the interpreta- 

 tion is based on the relative time in comparison to previously drilled feet. 

 The value of the application of drilling-time data to geologic and engineer- 

 ing problems lies in the recognition of this relative interpretation. 



