APPLICATION OF Many of the uses for drilling-time logs have 



DRILLING-TIME LOGS been cited and illustrated in the literature. 



The value to drillers, contractors, and oper- 

 ators is well known. The chief concern is the drilling of the greatest amount of 

 hole in the shortest time possible consistent with good safety practices. At the 

 same time anyone connected with the drilling of an oil well knows that the 

 purpose of drilling a hole is to gain information, the use of which may lead to 

 production of oil or gas. The contractor may find drilling-time records most 

 useful in the analysis of operations and the study of down time as well as pay 

 time. The performance of different types of bits in various formations can be 

 observed directly on drilling-time charts which reflect the types of formations 

 being penetrated. The driller finds drilling-time charts of value as a record of 

 his tour showing exactly how much he drilled, what type of formations were en- 

 countered and their depths, and a record of time down for sundry purposes. 

 The guess work is eliminated. But the driller, like the operator, is concerned 

 with finding oil and he knows that reservoir beds are porous media overlain by 

 hard or impervious layers. If he knows he is drilling in a section where potential- 

 ly productive formations may occur and has not been given precise instructions 

 in respect to coring, logging, or testing, the driller uses his best judgment in the 

 interest of the operator. When he observes a drilling-time break, he may stop 

 drilling and circulate samples for examination and wait for orders before drilling 

 past any formation that might carry oil or gas. The drilling-time charts provide 

 an indisputable record of where the top of the break was encountered and how 

 many feet have been drilled in it. 



The geologic importance of drilling-time logs is evident primarily in the 

 fact that foot-by-foot information is available for correlation. As has been shown 

 in Figures 18-1 and 18-2, a drilling-time log, plotted on a time scale such that the 

 amplitude between fast and slow peaks generally corresponds to the range 

 between the shale base line and the maximum peak of an electric -potential curve, 

 provides the geologist with data that may be used, within reasonable limits, for 

 much the same purposes as an electric log itself. It is obvious, therefore, that if 

 a drilling-time log of a well corresponds in pattern to an electric-potential log of 

 that well after the hole has been drilled, the drilling-time log made during the 

 drilling could be used for the purposes served by the electric log. This has proved 

 true particularly in the Gulf Coast area where long-range or local correlation is 

 based on the succession of a series of beds predominantly shale and sandstone. 

 The stratigraphic position of any portion of a drilling well can be established in 

 advance of electric logging by observing the sequence of beds penetrated as re- 

 vealed by a drilling-time log. 



It would be easy, for example, for the sequence of beds illustrated in Figure 

 18-2 to correspond to a similar sequence of beds above or below the portion of the 

 well shown. It would be difficult, if not beyond the realm of possibility, for the 



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