level, and the casing seat or seats. This data then is utilized in determining 

 whether and where the well is likely to produce, where to make drill-stem 

 tests, where to apply acid or fracture treatment, where to set casing, and where 

 to perforate. 



Whether or not the well has heen newly drilled, a radioactivity log ac- 

 companies almost every perforating operation in order to determine the exact 

 position of the zone which it is intended to perforate. This requires a logging 

 process that responds to formation properties through casing. Further assistance 

 is provided by casing-collar locators on the logging instrument and on the per- 

 forating tool. These locators permit the formation depth to be determined with 

 respect to the nearest casing joint and then allow the perforating tool to be 

 similarly positioned. 



Gamma-ray and neutron logs are also widely applied in older wells, 

 particularly where well records or earlier logs are incomplete or suspected of 

 being inaccurate. By virtue of the ability of the radioactivity logging processes 

 to operate through casing and independently of the type of fluid in the well, 

 they can be applied under a wide variety of circumstances under which all other 

 logging methods are inoperative. Thus, in older wells, upper productive zones 

 can often be found when the original production has become depleted. Also, 

 reliable data concerning the location of casing seats and liners can be obtained 

 when other records are suspected of being inaccurate. 



DENSITY LOGGING The Densilog was developed to meet the need 



for a device capable of measuring the density 

 of subsurface formations and to aid in the interpretation of surface gravity 

 surveys. A more important application for the device was found to be the 

 measurement of formation porosity in many types of reservoir rocks. This 

 measurement is possible because most of the common types of rocks have 

 almost the same grain density. Bulk density varies from rock to rock and from 

 point to point in a particular formation, mainly on account of differences in 

 porosity. Thus a quantitative measurement of density amounts to a quantitative 

 determination of porosity, except for rock that has a peculiar chemical con- 

 stituency. This type of porosity determination is a valuable supplement to the 

 neutron log. Particularly in shaly or cemented sands and in porosities greater 

 than perhaps 15 percent, quantitative interpretation is least accurate for the 

 neutron log and may be satisfactory for the Densilog. 



To an even greater extent than the neutron log, the Densilog is sensitive 

 to borehole diameter and fluid consistency. It is also extremely sensitive to 

 the position and orientation of the instrument with respect to the borehole 

 wall. To maintain constant sensitivity to formation density, the instrument 

 is provided with a bow-spring backbone that is designed to hold the sensing 



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