connected to recording instruments at the surface. The rate of fall of the 

 interface will remain constant until a permeable zone is passed. At this point, 

 the rate of fall of the interface is retarded since part of the injected fluid being 

 used to depress it now is entering the permeable zone just passed by the inter- 

 face. Each time a permeable zone is passed, the rate of fall is slowed still 

 further, until the bottom of the well is reached, or until a point is reached in 

 the well bore below which the formation will not take fluid. 



The flow rate into any given zone is obtained by taking the difference in 

 rate of fall above and below that zone. This value (feet per minute) is con- 

 verted readily into gallons of flow per minute. Thus, for any given rate of 

 fluid injection at the surface, the proportional quantity of fluid entering each 

 zone may be obtained and the comparative permeability profile determined (fig. 

 20-5 ) . These data are reliable as long as the diameter of the well bore is uniform. 

 Variations in hole size naturally will cause corresponding changes in fluid 

 velocity. These changes can be determined and corrected for by running a 

 caliper (hole diameter) survey. 



Several different surveying procedures have been developed, all of which 

 are based on the principle of following a moving fluid interface. The original 

 technique, still widely used, measures the electrical conductivity of the survey 



B^-*H 



>ACID 



ELECTRODES 

 IN OIL 



ELECTRODES 

 IN OIL AND ACID 



ELECTRODES 

 IN ACID 



Figure 20-4. Detection of an acid-oil interface is based on a measurement of conductivity 

 of external fluid between two insulated electrodes. 



399 



