BORE-HOLE INVESTIGATIONS 



1107 



As previously described, the shale value on the neutron curve remains a relatively 

 constant base line throughout the traverse. Each additional layer of steel, such as 

 casing or a liner surrounding the neutron instrument, reduces the curve intensity 

 because of the shielding efifect on the neutron source. Thus the entrance of the instru- 

 ment into an additional string of casing or liner overlap results in a shift to the left of 

 the entire base line. This new base line is assumed until some other physical change is 

 encountered. When short ionization chambers are used, these physical changes can be 

 located within one foot. 



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FLUID LEVEL 



Fig. 691. — Additional effects of physical bore-hole conditions on 

 neutron curves. (A) Neutron curve is affected by hole diameter. The 

 curve shifts as shown for each change in hole diameter. A change in 

 the amount of casing surrounding the instrument also causes the curve 

 to shift as indicated. No change in bore-hole diameter: note length of 

 shift. (B) Neutron curve is affected by well fluid-level. The curve 

 shifts to the right at the fluid level. 



The neutron curve is also useful in determining or checking fluid level, as shown 

 in Figure 691. Being very sensitive to the amount of fluid surrounding the instrument, 

 a drastic change such as that encountered at fluid level records as a large base line shift 

 to the right. After the shift has taken place, however, the curve continues in a normal 

 manner on the newly-assumed base line. If the fluid level is at a critical or interesting 

 zone, where relative intensity comparisons of adjacent formations are desired, it is 

 recommended that the hole be filled to a higher level and a re-traverse made to eliminate 

 the curve shift at this point. 



Comparison of Radioactivity Logs and Electrical Logs 



Figures 692 and 693 show comparison plots of radioactivity and electrical logs for 

 limestone and sandstone fields, respectively. 



The close resemblance between the gamma-ray curve and the S.P. curve on the one 

 hand and the neutron curve and the resistivity curves on the other is very striking. 

 Although this close resemblance is usually observed, it should be pointed out that it is 

 mainly coincidental because each of the two methods of logging measures different 

 parameters. 



The resemblance is quite useful when attempting to make correlations between 

 wells having electric logs and those having radioactivity logs. 



Applications of Radioactivity Well Logs 



In the early stages of the development of this type of logging it was thought that 

 some direct relationship might exist between natural radioactivity and petroleum. Expe- 



