filtrate. This region, which generally extends over a distance of about 3 inches 

 from the wall, is called the flushed zone of resistivity R xo . 



If the bed is water bearing, the pores in the flushed zone are completely 

 filled with the mud filtrate. If the formation is substantially clean, the resis- 

 tivity (R^) is equal to F x R,„/, F being the formation factor and R m / the 

 resistivity of the mud filtrate. 



If the bed is oil bearing, the flushed zone contains a certain amount of 

 residual oil. In this instance, R^ is related to the resistivity of the filtrate (R,„/), 

 the formation factor (F), and the water saturation (S x0 ) in the invaded zone, 

 as shown by the following equation derived from equation 5: 



(6) 

 R^o 



S x0 - : 1 - - ROS. (ROS = residual oil saturation) 



Transition Zone 



Behind the flushed zone, the distribution of fluids in the invaded zone 

 varies continuously until a distance is reached where the formation has not 

 been disturbed. This variable region is called the transition zone. We do not 

 know clearly the distribution of the fluids in this zone. Accordingly, there is 

 no real definition of the depth of the invaded zone; but it is convenient to 

 introduce a factor, D;, called the electrically equivalent diameter of invasion. 

 The factor D; is equal to the diameter of a fictitious homogenous invaded zone 

 of resistivity, R xo , which would have the same effect on the measurements as the 

 actual invaded zone. This diameter, D;, roughly corresponds to the diameter of 

 a cylinder located midway in the transition zone. 



It is also convenient in interpretation practice to consider the average 

 resistivity of the invaded zone, R;, which includes the flushed zone and the 

 transition zone. The average water saturation in the invaded zone is called Si 

 and the average resistivity of its water R s , such that: 



S, - ( F X Rz \ (7) 



( F x R, \ 

 R; ) 



As a general rule, all other conditions being the same, the greater the 

 porosity and permeability, the smaller is the depth of invasion. If D; is ex- 

 pressed in terms of d, the hole diameter, it can be said that with usual muds D; 

 seldom exceeds 2d in high-porosity sands but will attain 5c? or more in low- 

 porosity formations. 



It has also been observed that in water-bearing sands with large vertical 

 permeability, the depth of invasion is extremely small, often less than 3 inches. 



275 



