314 Subsurface Geologic Methods 



an increase in total water content and a small drop in residual-oil satu- 

 ration. The oil-water ratio will be smaller than that for the oil-productive 

 zone. This type of transitional zone is of doubtful commercial value; 

 usually, water is produced with the oil upon initial completion, and, as 

 the oil saturation is decreased, the permeability to water rapidly increases 

 to the point where the entire zone is water-productive. 



The foregoing discussion emphasizes the importance of making ver- 

 tical permeability tests. Vertical permeability barriers should be used in 

 selecting the completion interval. Many core-analysis reports contain no 

 vertical permeability data because some core analysts have not recognized 

 the importance of these tests. The writer believes that every possible test 

 should be made on core samples while the cores are available because, 

 once the cores have been discarded, these additional data can only be 

 secured by recoring the formations. 



Secondary Migration of Water 



There is no way of recognizing oil- or gas-sand zones which have 

 been subjected to the secondary encroachment of water, and many wrong 

 predictions as to probable production may be ascribed to this phenomenon. 

 Cores from these zones can show exactly the same residual hydrocarbon 

 saturations as the cores from zones that have not been flushed by salt 

 water. 



The reason these zones cannot be recognized is that there is sufficient 

 gas left in the sand to drive out the excess water and thus make the water 

 saturations similar to those of gas-productive sands. Checks by the use 

 of the gas analyzer would show gas to be present. The only conclusive 

 test is to make a production test through the drill stem. Flooded oil sands 

 can show normal residual-oil saturations and still produce nearly 100 per- 

 cent salt water. 



The inability of the core analyst to recognize these zones solely on 

 the basis of core analysis should be recognized by the oil operators, and 

 all of the data as to the structural position of the well, the probability of 

 faulting, and electric-log correlation should be made available to the in- 

 terpreter. It is sometimes difficult to understand why the oil operators 

 withhold this information from the core analyst when they are paying for 

 the work. Any and all pertinent data that will help in arriving at the 

 right answer should be used. 



Oil in Place 



All of the oil in place cannot be recovered by any known means of 

 production. This volume is usually calculated at reservoir conditions and 

 is expressed in barrels per acre-foot of formation. The practice of some 

 engineers in using the residual-core oil as the oil in place may apply for 

 unflushed cores from shallow low-pressure reservoirs but does not apply 

 to cores from high-pressure reservoirs. As indicated in the following equa- 



