these formations the MicroLog does a spectacular job of showing the presence 

 of mud cake. Nevertheless, such information is only qualitative. In all 

 probability mud cake is formed equally well on formations with a few milli- 

 darcies as on sands with several thousand millidarcies. 



A method of evaluating the permeability quantitatively from the study of 

 resistivity gradients was offered in 1949. Observations often show that, when 

 a borehole has traversed a reservoir rock that is water saturated at the bottom 

 and oil bearing at the top, the resistivity shows a corresponding increase from 

 bottom to top. In a formation of low permeability, such as consolidated sand- 

 stone, it can be further observed that the average trend of the resistivity curve 

 in the transition zone from the water table up to the zone of maximum oil 

 saturation, is practically a straight line. This study shows that the value of the 

 basic gradient (i.e., the resistivity gradient divided by the resistivity at the 

 water level) provides a means for determining the value of the average perme- 

 ability when the in-place densities of both the hydrocarbons and the connate 

 water are known. 



In 1950 some theoretical studies were pursued on the matter of perme- 

 ability with the conclusion that a permeability range should be obtained without 

 much difficulty if values of porosity and irreducible water saturation are 

 correctly obtained from logs. It was shown that the permeability could be given 

 by a general formula 



K 1/2 = c — , w ith K the permeability in millidarcies, c a factor, <f> the 



porosity, and S w the irreducible water saturation. The coefficient c seems to 

 vary with the square of the porosity, and the general formula becomes 



K 1/2 = 250 — . Figure 14-25 shows this relation graphically. 



This simple solution is in general all that is necessary to give a working 

 range of permeability, which assists in evaluating the production ability of a 

 given formation. Great accuracy in determining permeability is not indispens- 

 able if only because, at the time of production, the permeability can be greatly 

 altered by the type of mud used or by techniques such as acidizing, shooting, 

 and hydraulic fracturing. 



Consequently, although there is no way of measuring the permeability 

 directly, in many instances a good idea of the average value of permeability 

 can be inferred with sufficient accuracy to permit a correct reservoir evaluation 

 from the logging techniques. 



325 



