Chap. 11] 



GEOPHYSICAL WELL TESTING 



833 



Frequently, the electrofiltration and 

 diffusion potentials reinforce each other. 

 Hence, self-potential records (taken with 

 the electrode arrangement illustrated 

 schematically in Fig. 11-lc) give a good 

 indication of the porosity of a forma- 

 tion and furnish valuable information 

 supplementary to resistivity data. Since 

 a low-resistivity indication may be inter- 

 preted as a salt-water sand or clay bed, 

 the porosity indication would decide which 

 interpretation is correct. The same is true 

 for high-resistivity indications which could 

 mean a limestone bed or an oil sand, un- 

 less a porosity record is available. 



Fig. 11-6. Origin of diffusion po- 

 tentials in drill holes (adapted 

 from Schiumberger). 



POROSITY POROSITY 



o .10-10 voirs o 



POROSITY 



.10 



fw " (^-6^ ohm-m |0 = 0.44 ohmm 



P (layer YMfer) ~ O.f Ohm-m 



f> = O.f 6 ohm-m 



Fig. 11-7. Variation of diffusion potential with resistivity of drilling mud (after 



Schiumberger). 



