LIMITATION OF CORING Even though coring is the best tool available 

 TECHNIQUES AND for the correlation and interpretation of geo- 



APPLI CATIONS TO logic formations, it too has certain limitations 



GEOLOGIC PROBLEMS that must be recognized. Perhaps the greatest 



limitation of coring is that samples of the 

 complete section cored are rarely obtained for examination. The amount of 

 cored section recovered is often as low as 60 percent, and sometimes there is no 

 recovery. When full recovery is not obtained, it is usually difficult to place the 

 recovered section in the correct position in the formation log and to assign 

 physical values to the entire section from those obtained by analysis of the 

 recovered section. Then too, after a core has been recovered, it is not possible 

 to analyze the entire recovered section; thus more limitations are introduced, 

 because the core has to be sampled and tests obtained only on the sampled 

 portions. Work on gas-oil and water-oil contacts from core-analysis data has 

 certain limitations. Usually oil percentages, determined from core analysis, are 

 much lower above the gas-oil contact than below it, but this is not always true, 

 especially when the oil has a rather high gravity. Frequently a recovered core 

 near the water-oil contact appears to contain more oil than those higher in the 

 oil column. Past experience has indicated that cores usually are subjected to 

 considerable flushing by filtrates from drilling muds. When water-base muds 

 are used, oil is generally flushed from the core; therefore oil-saturation values 

 determined from core analyses may be too low. Likewise, when oil-base mud is 

 used, the water-saturation values determined on some cores, especially those 

 near or below a water-oil contact, may be erratic. It has also been found that 

 some oil sections contain argillaceous materials which may have various types of 

 nonproducible bound waters, and these in turn may introduce a considerable 

 error in core-saturation determinations. 



Another appreciable limitation of coring results from the fact that it is 

 usually very expensive and causes marked increases in over-all well costs when 

 it must be used. The development of more economical methods to evaluate sub- 

 surface formations accurately as well as to find ways to reduce coring costs 

 would be a major contribution to the oil industry. 



Reprinted from Subsurface Geologic Methods, 1951, p. 619-625. 



714 



