bed. This difference comes from the fact that the eddy currents induced in 

 the earth have a tendency to circulate through the most conductive media. If 

 the resistivity of the adjacent formations R s is greater than the resistivity R* 

 of the bed, the eddy currents concentrate within the boundaries of the bed. The 

 signal contributed by the adjacent formations is then comparatively small. 

 Computations and experience show that, in this instance, the measurements are 

 practically unaffected if the thickness of the bed is 6 feet or more. Approximate 

 charts have been computed for thinner beds. 



If a reservoir contains thin resistive streaks, such as lime or lignite beds, 

 the readings made opposite the porous and permeable sections of the reservoir 

 are not essentially affected by the presence of these streaks, which cause 

 important distortions on conventional logs. 



Effect of Invasion 



In contrast with the other resistivity methods, the induction log is at its 

 best for the investigation of permeable beds when Rj is greater than R t . In 

 this instance, most of the eddy currents flow in the uncontaminated zone, whereas 

 when Rj is smaller than R t they flow mostly in the invaded zone. For a given 

 value of Di, the proportion contributed by the uncontaminated zone to the 

 measured signal is greater in the first than in the second instance; in other 

 words, the measurement in the first instance is closer to the true resistivity. 



For obtaining the most accurate results with the induction log, the mud 

 resistivity R TO should preferably be at least 10 times as great as the connate- 

 water resistivity R w . This condition is fulfilled most of the time in those regions 

 where connate waters are very saline. The interpretation is still possible, even 

 if R m /R w is as low as 5, as is frequently the case when connate waters are 

 comparatively fresh. 



In short, it can be said that the effect of the invaded zone is negligible 

 when R m /R w is at least equal to 5 if the diameter of invasion does not exceed 

 about 20 inches. Approximate charts are also available to correct for the effect 

 of invasion, provided the diameter of the invaded zone is known or can be 

 reasonably surmised. 



Sand and Shales (Water-Base Mud) 



In these formations, the resistivity R f of the reservoirs is generally not too 

 great with respect to those of the adjacent formations. According to the above, 

 the induction log will usually give a direct record of the true resistivities when 

 the mud is not too saline, when the beds are not exceedingly thin (at least 5 

 feet), and when invasion is reasonably shallow (diameter of invasion not greater 

 than 20 inches). 



These conditions are very often encountered in sand-shale series. In 

 the instances where they are not fulfilled, the readings can be corrected for 



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