counter tool. The advantage of this method over the regular static-interface 

 method is that the tubing need not be raised to locate the interface. In this 

 instance, the Geiger counter is run inside the tubing and locates the interface in 

 the annulus, as though there were no tubing in the hole. 



In velocity surveys, a tracer method that has been developed uses a long 

 surveying tool containing a reservoir of radioactive material in the upper end 

 and a Geiger counter in the lower end. Fluid is injected into the well at a con- 

 stant rate from the surface as in normal velocity surveys. The combination tool 

 is suspended at a known depth in the well, and small increments of radioactive 

 material are ejected into the passing fluid stream. The fluid velocity is deter- 

 mined from the time interval required for the radioactive tracer to travel from 

 the top to the bottom of the tool. Similar determinations are made throughout 

 the well bore, and data for comparative-permeability profiles are obtained. 



Many other applications have been and are being developed whereby 

 valuable subsurface data may be obtained through the use of radioactive isotopes. 

 For example, casing and tubing leaks may be located without removing the 

 string from the well. Channeling behind cement can be detected. In several 

 instances the detection tool has been placed in the tubing just above a formation 

 packer for the purpose of detecting a packer leak immediately whenever one 

 should occur. Another successful application has been in determination of 

 zones of lost circulation in drilling wells. Radioactive tracers also can be used 

 to control the application of acid in selective acidizing treatments. 



Although there is still much to be done in perfecting application techniques 

 and properly interpreting the data, the use of radioactive isotopes appears to 

 have an unlimited application in the oil industry. Tracers are a valuable new 

 tool which will assist the engineer and operator in making proper application of 

 completion or remedial treatments. 



The growing interest in research on physical conditions and performance 

 of petroleum reservoirs has led to the development of many new and improved 

 instruments and techniques for permeability surveys. Continuing research 

 promises future discoveries and developments that will permit greater ease and 

 speed of surveying, and even greater accuracy of data. 



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