in this area (Pedernales District) and is used as a method of bottoming a field 

 well at the base of a known productive sand lens and to prevent the penetration 

 of the next lens, known to be salt-water bearing ... It developed that the plot- 

 ting of drilling time in minutes per foot had been tried and poor results were 

 obtained. A different method of plotting — feet per hour per one foot — was tried 

 and has been adopted as a standard practice." 



Simons states further, "If it requires a certain number of minutes to drill 

 one foot, that is the drilling time for that foot. But that foot was also drilled at 

 a certain rate which can be expressed in units of depth per units of time . . . 

 When drilling time is plotted in feet per hour for one-foot intervals, a semi- 

 logarithmic form of curve is obtained which dampens the effect of very slow 

 feet caused by harder streaks or by the inattention of the driller, and which con- 

 versely exaggerates the effect of fast drilling that cannot be caused by anything 

 but the bit entering a zone of easier digging. The scale is chosen to fit the fastest 

 drilling observed, and fluctuations in the hardness of shale do not cause a widely 

 varying curve, and the S.P. log in the shale section is more closely approximated." 



The above discussion is illustrated in Figure 18-4 and is introduced in this 

 chapter as an illustration of individual adoption of variations in selection of scale 

 and method of plotting. It should be pointed out, however, that drilling-rate 

 values cannot be determined without first measuring drilling-time values and it 

 would appear that calculating the reciprocal values is an unnecessary step. The 

 use of the zero base line at the right and the plotting of reciprocal values, even on 

 a straight arithmetic basis, might have advantages in the analysis of special 

 problems. 



METHOD OF PREPARING The experience of the writer in the use of 

 DRILLING-TIME LOGS drilling-time data has been based on records 



obtained from geolograph charts for the most 

 part. Although there are other means by which usable data may be collected, 

 perhaps the most practical source of complete drilling-time records is the 

 geolograph. For this reason it is considered in place here to describe in detail 

 the technique recommended in translating the orginal record into the drilling- 

 time log for which multiple uses have been described. 



It is unnecessary to include the maintenance of the equipment, which is 

 the responsibility of the service company. A few points should be kept in mind, 

 however, by the geologist desiring to obtain as perfect records as possible. A 

 drilling-time log is the plotted curve of two components, time and depth, each 

 requiring accuracy within the limits of observational errors. The geolograph 

 machine provides two inking pens, generally supplied with inks of different 

 colors. One pen indicates by vertical and horizontal lines the amount of time 

 that drilling is in progress and when it has been stopped or when the bit is off 



379 



