Figure 20-6. Photoelectric detection tool is used to follow an interface between fresh 

 water, and water made opaque by addition of a dye. 



surface, and the interface is followed down the hole by means of the photo- 

 electric tool. Changes in the rate of fall of the interface indicate fluid loss into 

 the zone being traversed. 



Another variation of the moving interface technique is to run the survey 

 in a normal manner while fluid is injected from the surface. The survey then 

 is rerun, and the fluid is injected at the bottom of the well bore through a tubing 

 string and follows the interface upward. This procedure provides not only a 

 comparative permeability profile, but also a log of the volume of the bore hole 

 at any depth interval, thus making a separate caliper survey unnecessary. This 

 method is not widely used because it is more expensive and time-consuming 

 than other methods. 



The static-interface method (fig. 20-7) of surveying involves a somewhat 

 different principle in determining the relative proportion of injected fluid that 

 different sections of the formation will accept. This method is largely experi- 

 mental and has not gained widespread use because of the necessity of running 

 tubing in the hole. It is based on the following principle: when two different 

 fluids (usually fresh water and brine) are injected into a well, one at the 

 bottom through a string of tubing and the other at the top of the formation 



401 



