Effect of Invaded Zone 



In permeable beds, the current used for the measurement has to cross 

 the invaded zone radially before reaching the uncontaminated zone. The 

 apparent resistivity is proportional to a voltage, which includes the ohmic drop 

 of potential across the invaded zone and the ohmic drop of potential across 

 the uncontaminated zone. If the invasion is deep, the effect of the invaded 

 zone on the apparent resistivity may be comparatively important. 



Departure curves have been computed for thick beds, which give the 

 value of apparent resistivity read with the Laterolog 7 versus the true resistivity 

 for various depths of invasion and various resistivities of the invaded zone. 



It is possible to summarize the computed results into a few simple formulas 

 that enable a quick correction of the readings for invasion effect: namely, 



For D, = 20", R = .2 R, + .8 R, or R, = 1.25 R a — 0.25 R* 

 For D, = 4C, R a = .4 R, + .6 R, or R, = 1.66 R a — 0.66 R, 

 For D; = 80", R a = .6 R x0 + .4 R t or R t = 2.5 R a - 1.5 R^ 



In these form ilas, R x0 can be given by the MicroLog or the MicroLaterolog. 

 The value of Dj h-is to be assumed since there is no welblogging tool available 

 at the present tim^ to determine this directly. 



Because R x0 is a direct function of R m , the effect of the invaded zone 

 depends directly c n the mud resistivity. It is interesting to define what should 

 be the optimum value of R m so that the invaded zone effect will be small. 



To illustrate *he discussion, let us take the following numerical values: 



F = 30, R K = 0.05 



R — 1.5 for 100 percent water saturation 



R* = 24 for 25 percent water saturation 



Fresh Mud 



If one assumes that the mud resistivity is equal to 1, R^o will be approxi- 

 mately equal to 30. With such a mud, the invaded zone will be practically non- 

 disturbing when the formation is oil-bearing, since R xo = 30 is close to R t 

 = 24. 



For the water-bearing formations, the readings will be equal respectively 

 to 7.2, 12.9, and 18.6, according to the depths of invasion. The apparent 

 resistivities are, therefore, much above the true resistivity (R = 1.5), even 

 for a moderate diameter of invasion, as Dj = 2c?. The Laterolog, accordingly, 

 is not able to discriminate oil-bearing from water-bearing beds, even qualitative- 

 ly, except when invasion is very shallow. 



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