THE PERMEABILITY OF OIL-BEARING STRATA 379 



the possibility of some practical use for it. For example, this can be done 

 in evaluating the total mean water saturation of a reservoir under the condi- 

 tion of established distribution of the permeability. 



Wyllie and Rose^^^) suggested the determination of permeabihty of 

 petroleum-bearing strata from the specific resistance and gave formula (■'^^) 

 which expresses the dependence of the residual water saturation on the 

 permeabihty. However, this method has not been used. Walstrom (^^) 

 justifies this in the following way: "It is possible to evaluate the lower limit 

 of the mean permeability from the interstitial water saturation and the 

 relative resistance, obtained from data of electrical logging. In exploratory 

 drilHng, however, the relative resistance is determined from the core taken 

 from the productive sandstones, in which the permeability can be meas- 

 ured directly. Consequently, it is not necessary to try to determine perme- 

 ability from the data of electrical logging, since cores must be used for 

 this. 



"Tiks'e shows that at the present time there is no method of logging which 

 would make it possible to determine the permeabihty of rocks cut in a bore- 

 hole" (16- P^°^ 349) _ 



The data given fully agreed with the conclusion which we obtained in 

 testing the method for determining permeability from the specific resist- 

 ance. 



Let us consider the possibility of determining permeability from the 

 specific resistance from another point of view. 



It is generally known that the permeability varies considerably along the 

 stratum and over wide limits. This is readily supported by the large differ- 

 ences in the values of permeability of separate cores from the stratum. 

 Fig. 12 shows a core diagram illustrating the sharp change in permeabihty 

 along the stratum. 



In contrast to this, the curve for the resistance against the petroleum- 

 bearing stratum differs comparatively gently; it does not have any sharp 

 changes of apparent and specific resistances, typical of changes in perme- 

 ability. The deviations of the curve of resistance against the petroleum- 

 bearing stratum are caused by the form of the resistance curve against the 

 high resistance strata. This serves as an indirect indication of the fact that 

 the change in permeability of the stratum is not reflected in the variations 

 of the specific resistance and that between the permeability, the interstitial 

 water saturation and the coefficient of increase in resistance there is no 

 direct unambiguous connection. Thus, the shape of the resistance curves 

 indicates unfavourable conditions for using the specific resistance method 

 in measuring the permeability. 



