Chap. 11] 



GEOPHYSICAL WELL TESTING 



835 



N and M, a maximiim potential difference, proportional to dip, will occur 

 between the electrodes when they are in the position shown. When the 

 configuration is then rotated to a position at right angles to the plane of 

 the paper, the potential difference vanishes, whereby the direction of strike 

 is established. In practice, the two directions are determined with ref- 

 erence to magnetic north, which may be established with an earth inductor 

 well compass. 



E. Determination of Casing Depth 



Since the casing is a much better conductor than the mud or the adjacent 

 rocks, its depth can be established from the abrupt change in apparent 

 resistivity when the electrodes pass through the casing shoe. Inside the 

 casing, the resistivity is zero. For these measurements, intermediate and 

 small electrode separations are used. 



F. Discussion of Results 



How closely electrical logging data may coincide with results of core 

 analysis is illustrated in Fig. 11-10. The section represents the oil zone 



Ptrmiabilltif MUlr S^ur»/MO g^^ivit^ Timfitrt/urt 



3340 



Fig. 11-10. Comparison of core analyses with results of electrical logging (after 



Schilthuis). 



of the Woodbine sand in the east Texas field. Porosities were determined 

 by the Washbum-Buntmg method,' permeabilities in accordance with 

 A.P.I. Code No. 27,^ and connate water and oil by vaporization and com- 

 bustion.* Porosities indicated by the self-potential curve are closely paral- 



' Schlumberger and Leonardon, loc. cit. 



"> Amer. Ceramic Soc. J., 4, 983-989 (1921). 



« Barnes, A.P.I. Drilling and Production Practice, 191-203 (1936). 



» R. J. Schilthuis, A.I.M.E. Tech. Publ. No. 869, 1937. 



