apparatus in contact with the rock face. When the borehole is smooth and true 

 to gauge, precise measurement of formation density can be made. When the 

 borehole diameter varies, a caliper log must be run to facilitate corrections in 

 the measurement. Figure 15-6 shows a family of curves indicating the extent 

 to which the measurement is affected by borehole diameter changes. More 

 serious are the effects on the log that result from abrupt variations in borehole 

 wall condition such as caving. In such instances the bow-spring may be unable 

 to maintain the device in contact with the rock over the sensitive length of the 

 instrument. Under such circumstances the device tends to measure the density 

 of the borehole fluid, and the log is of no value for quantitative interpretation. 

 Although the Densilog in favorable instances will indicate lithologic 

 changes behind casing, it is primarily useful only in uncased holes. In some 

 areas it has been found to be the best means for stratigraphic correlation and 

 identification of certain formations that contrast poorly with surrounding 

 formations on other logs. 



APPLICATIONS INVOLVING The gamma-ray curve is useful in observing 

 RADIOACTIVE TRACERS the position or movement of substances con- 



taining small amounts of radioactive tracer 

 material that have been introduced into the well,. Generally a base log is run 

 prior to the introduction of the tracer material in order to distinguish between 

 it and the naturally occurring radioactivity. A comparison of the subsequent 

 gamma-ray curves with the first reveals the distribution of the tracer along 

 the well. Some of the common uses of radioactive tracers in connection with 

 well logging are the following: 



1. Determining the distribution of cement in well completions or squeeze- 

 cementing operations. This is done in order to locate the top of the 

 cement or to show which zones have received relatively more or less 

 cement. 



2. Determining the relative permeability of producing zones or zones 

 selected for water or gas injection in secondary recovery operations. 



3. Studying the flow of fluid from one well to another as in waterflooding 

 gas repressuring. 



4. Finding casing leaks or zones of lost circulation. 



5. Locating zones opened up by fracturing or acidizing. 



The sensitivity of the gamma-ray logging apparatus is such that very small 

 quantities of radioactive tracer materials can be easily detected. Natural and 

 artificially produced gamma-ray emitting isotopes, having long or short half 

 life, and possessing a wide variety of chemical properties, are available. The 

 selection of a particular tracer element and of the physical and chemical form 



342 



