Resistivity 



10 20 30 40 50 60 70 80 90 100 



e = 5d 



Normal^ 

 AM = l.75d: : 



:\ l^-L a t e r a I AO = 25d 



^p_ ^ 



/S •' ^Lateroloc 



-True 

 , O f Resistivity 



2 



0,0 2 = l.5d A2 



A| A 2 = 9d 



Figure 14-19. Response of laterolog and conventional devices. 



MicroLaterolog 



It has been said that the MicroLog is not the proper tool for an accurate 

 determination of R x0 when the porosity of the formations under study is lower 

 than about 15 percent. The limitation of the MicroLog is due essentially to the 

 by-passing influence of the mud cake on the measuring circuit. 



The MicroLaterolog is a microdevice involving a focusing system whereby 

 the effect of the mud cake on the measurement is reduced and even negligible 

 if the mud thickness is small. Under this condition, the MicroLaterolog is 

 adapted to the determination of formation factor and porosity in all types of 

 formations. 



Principle 



The MicroLaterolog device comprises one center electrode, A , of very 

 small size and three circular (ring) electrodes, M i5 M 2 , and A*, each concentric 

 with A and spaced with short gaps (about Vg" to 1") between successive rings 

 (fig. 14-20). These electrodes are imbedded in an insulating pad which is 

 applied against the wall of the borehole by means of an appropriate spring 

 system similar to the MicroLog. 



A current of known and constant intensity is sent through the current 

 electrode, and another current of the same polarity is fed through bucking- 

 current ring, which is automatically adjusted so that the potential difference 

 between measure and monitor electrodes is kept practically equal to zero. 



312 



