466 Subsurface Geologic Methods 



to determine the stratigraphic and structural position of a well which 

 might otherwise remain unknown. It may also reveal the presence of 

 probably porous beds that may be saturated because of their structural 

 position, and therefore the economic risk of drilling a new hole or aband- 

 oning a location may be substantially reduced. 



Since the publication of the first edition of this symposium, the 

 writer has received communication from J. A. Simons, geologist with 

 Creole Petroleum Corporation, regarding the use of drilling-time and 

 drilling-rate curves in Venezuela. He states, "This technique (drilling-time 

 logging) is extremely useful in this area (Pedernales District) and is used 

 as a method of bottoming a field well at the base of a known productive 

 sand lense and to prevent the penetration of the next lense, known to be 

 salt-water bearing ... It developed that the plotting 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 conversely exaggerates the effect of fast drilling 

 that cannot be caused by anything but the bit entering a zone of easier dig- 

 ging. The scale is chosen to fit the fastest drilling observed, and fluctua- 

 tions 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 218 and is introduced 

 in this paper as an illustration of individual adoption of variations in 

 selection of scale and method of plotting. It should be pointed out, how- 

 ever, that drilling-rate values cannot be determined without first measur- 

 ing 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. 



Commenting on the above, Mr. T. A. Banning, Jr., Chicago, Illinois, 

 writes*: "The answer to the foregoing question (drilling time or drilling 

 rate — which?) must depend on the use to which the information given is 

 to be put. For some purposes time will be the most important factor of 

 the ratio — for other purposes rate will be the important factor. In any case 

 drilling time and drilling rate are mathematically interchangeable, and 

 under the conditions presented in the case of well drilling these data are in 

 all cases sufficient to permit presentation of the ratio either way." 



* Personal communication. 



