best suited for use depends upon a number of factors pertaining to the 

 individual application. In some applications, as for example in determining 

 permeability, it is important that the radioactive material adhere at the point 

 of entry of the carrier fluid, whereas in other applications the tracer element 

 must remain with the injected fluid. In determining permeability, the radio- 

 active material is attached to particles of resin, just sufficiently large to prevent 

 their entry into the porous rock. In other instances, the tracer may be applied 

 in the form of a highly soluble salt. 



CURRENT DEVELOPMENTS Radioactivity logging techniques appear to 



offer the only feasible means of making quan- 

 titative measurements of formation properties after a well is cased. Even in 

 uncased holes it is believed by many experts that nuclear logs offer the best 

 prospect of determining certain rock and fluid characteristics. This is par- 

 ticularly true in many special cases such as air-drilled wells or wells drilled 

 with oil-base mud where one or another of the other logging techniques may 

 not work successfully. 



For this reason research and development in this field has received much 

 emphasis in recent years. As a consequence, there has been rapid advancement 

 in the quality and diversification of instruments available commercially. One 

 example is the development of logging instruments for use through tubing in 

 permanent-type well completions. Simultaneous gamma-ray and neutron log- 

 ging instruments now in service have an outside diameter of only 1% inches. 

 Instruments having a larger diameter, typically 3% inches, have been improved 

 in sensitivity, stability, and reliability. As a consequence, the quantitative in- 

 terpretation of radioactivity logs has become more accurate and more wide- 

 spread in application. 



Logging instruments have been designed to use many of the techniques of 

 modern nuclear physics. Neutron sources for logging instruments have been 

 built that use a high-voltage ion accelerator to produce artificially neutrons of 

 energy higher than those emitted by conventional capsuled neutron sources. 

 Much research has been conducted on the spectral characteristics of the radia- 

 tion detected in neutron and gamma-ray logging. Commercial application of 

 these techniques may be expected to increase the benefits obtainable from 

 radioactivity logging. 



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