areas where very hard and abrasive formations are encountered, because it may 

 be found that coring is more economical than drilling. 



Certain petroleum-engineering work is aided greatly by core-analysis data. 

 An understanding of the formation characteristics and composition may lead 

 to the location of (1) oil zones that subsequently might be overlooked, and (2) 

 impermeable zones that may aid greatly in work-over operations to shut off 

 undesirable water or free gas. Core data almost always help the engineer 

 evaluate subsequent work-over possibilities. One of the earliest engineering 

 uses of core-analysis data was for evaluating and planning secondary oil recovery 

 by water flooding. Today, in addition to that same use, it also aids in evaluating 

 gas cycling and pressure maintenance by gas-injection projects. Probably the 

 latest and perhaps one of the most valuable uses of modern core-analysis data 

 for an engineer is to provide basic data for reservoir-analysis studies. 



CORRELATION BETWEEN In considering the value of core data, one 

 CORING AND must remember that the examination of for- 



ELECTRIC LOGGING mation cores, either in the field or in the 



laboratory, provides the only direct informa- 

 tion concerning the physical properties of the formation that the drill penetrates 

 and that these core data are the basis of electric and other log interpretations. 

 An examination of formation cuttings and the use of other logging devices may 

 substantiate core-analysis data, but individually those methods are usually sub- 

 ject to variable factors and broad interpretations. When the geologist uses core 

 data for correlation, he is generally certain that they are accurate; and once 

 the reaction of an electric log in a particular formation in a certain area has 

 been established or substantiated by core-data interpretations, the log becomes 

 a very useful tool. Likewise, formation tests are necessary to determine electric- 

 log characteristics in regard to the probable type of fluid that a formation will 

 produce. It has been found that an interpretation of electric logs of certain holes 

 is often very misleading ; the logs are very valuable, however, in determining the 

 tops and thicknesses of certain sections when they are used in conjunction with 

 core data, especially when core recovery has been poor. 



Since electric logs are influenced sometimes by the type of drilling mud 

 and the mineral content of the section logged, core logs are often the only means 

 of formation interpretation. Sometimes a correlation between an electric log 

 and a core log on producing formations results in finding measurement errors, 

 which may cause subsequent difficulty. In using electric logs for purposes other 

 than correlations, limitations must be recognized, such as failure to register the 

 presence of sands or other zones in a producing horizon and the reverse reaction 

 on producing sands in certain areas. 



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