456 Subsurface Geologic Methods 



drilling rate, each of which has its own specific uses and limitations. The 

 relationship between the two and their differences are discussed. Drilling- 

 time data may be applied to many engineering and drilling problems and 

 have proved of considerable value to contractors, drilling crews, and 

 operators. The multiple uses to the geologist involve general correlation 

 problems and detailed studies of lithology. The geologic application of 

 drilling-time data is an additional technique of particular value in the 

 determination of potentially productive sections of a well bore and in 

 the calculation of recoverable reserves. 



Many methods have been used to measure and record drilling time. 

 A technique used by the author is described and illustrated in which 

 mechanical recording of depth in reference to elapsed time is translated 

 into graphs or logs, which may be used in the solution of a great many 

 geologic and engineering problems. The amount of section so logged and 

 the scale employed are determined by the requirements of the individual 

 problem. Detailed instructions are included whereby a geologist not ex- 

 perienced in the use of this technique may learn how to prepare and in- 

 terpret drilling-time logs in any area in which he may be interested. 



Definitions 



Early methods of determining rate of penetration were crude and 

 approximate and generally consisted in recording the time required to 

 drill a certain number of feet of hole or the number of feet drilled per 

 hour. Data thus acquired may be adequate for some purposes, but as the 

 technique of using drilling time became more widely employed, certain 

 advantages were observed in determining the specific net amount of time 

 required to drill each foot. Before discussing the application of drilling- 

 time data, the distinction should be understood between drilling rate and 

 drilling time. 



By definition, rate of penetration is a fixed lithologic property, even 

 though it may be diagnostic only when used as a relative term. Drilling 

 time is the duration of time required in the actual drilling of a unit of 

 depth. Drilling rate is the number of units of depth drilled in a unit of 

 time. 



The foregoing may be illustrated by comparison to an automobile 

 speedometer. When a car travels a mile in a certain number of minutes 

 and seconds, it is a measure of speed comparable to the time occupied 

 in the drilling of one foot of formation, which we have defined as drilling 

 time. When a speedometer indicator points to 45, it indicates that the car 

 is traveling at the rate of 45 miles per hour. This is comparable to rate 

 of penetration measured in the number of feet drilled per hour, which is 

 the definition given for drilling rate. The distinction is more than academic 

 and should be clearly understood, because the interpretation of rate of 

 penetration is strongly influenced by the method of recording. 



