The moving-interface survey is extremely accurate, even at very low fluid 

 velocities. Many operators, however, object to having both oil and water in- 

 jected into the formation. The conductivity electrode tool requires the use of 

 two dissimilar fluids. Although brine and fresh water can be used for this 

 purpose, the interface is less distinct than when oil and water are used. An ad- 

 vantage of the photoelectric tool is that only a single fluid is injected into the 

 formation. The soluble dye has no deleterious effect on the formations. On the 

 other hand, oil, or other foreign substances that sometimes coat the operating 

 lenses of the tool cause trouble. 



The static-interface method has the advantage of not requiring an auxiliary 

 caliper survey for interpretation. This is not true of the methods discussed 

 above. The cost and inconvenience of running tubing more than offsets this 

 advantage. The method is particularly adapted, however, to running surveys in 

 shot holes in which other methods fail. 



NEW DEVELOPMENTS Since the advent of atomic power, with the 



IN RADIOACTIVE accompanying availability of relatively in- 



TRACER TECHNIQUES expensive radioactive isotopes, new perme- 



ability-surveying techniques have been or are 

 being developed. Radioisotopes have been adapted for use with all four of the 

 above-described surveying methods. 



In one form of permeability logging using tracers, a log of the natural 

 radioactivity of the formation is obtained first. Fluid, containing a colloidal 

 dispersion of fine, insoluble radioactive particles, is then injected into the well. 

 A second radioactivity log of the well is then made and compared with the first. 

 Radioactive particles are deposited on the face of the well bore at the point the 

 fluid enters the formation. The amount of radioactive material deposited is 

 proportional to the amount of fluid entering. Accordingly, the increase in 

 radioactivity as determined from the two logs is used to locate and evaluate all 

 injection zones in the well. 



Another application of radioactive tracers is made as follows: A small 

 amount of concentrated solution of radioactive material is spotted by a bailer in 

 the fluid column. Injection is started at the surface, and the radioactive marker 

 is followed down the well bore by a Geiger counter. The resulting survey is 

 interpreted in the regular manner. This type of survey has the advantage of 

 using either oil or water in the hole. Also, the Geiger-counter readings are un- 

 affected by any contaminants in the well bore. 



In using radioactive tracers for the static-interface survey, normal pro- 

 cedures are used. The same fluid is used in both tubing and annulus, except that 

 one contains a lower concentration of dissolved radioactive material. The loca- 

 tion of the static interface at different pumping rates is detected by the Geiger- 



407 



