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Figure 33-8. Sample-taker bullet and cores (Courtesy Schlumberger Well Surveying Com- 

 pany) . 



fact that the tool is run in the hole on a standard multiconductor cable used in 

 electrical logging, permits a simple but positive method of locating the coring 

 point. A logging electrode is run adjacent to the sample taker, and the coring 

 point is determined by monitoring the electric log. Because the choice of coring 

 points was determined with reference to the original logs, it is a simple matter 

 to pick up any selected point independent of the actual depth of the tool. 

 The complete operation consists of running the tool to a selected point for 

 coring, firing a charge, then hoisting the tool to the next point. 



Another view of the bullet and examples of retrieved cores are illustrated 

 in Figure 33-8. This style of bullet, which produces cores 1 3/16 inches in 

 diameter and up to 2*4 inches long, will penetrate most sedimentary formations. 

 These cores can be subjected to regular core analysis for oil saturation, porosity, 

 and permeability. In fact, such analysis represents a majority of the work 

 done by Gulf Coast laboratories. In softer formations, the cores yield acceptable 

 porosity and permeability determinations, but the tendency for them to shatter 

 makes results unreliable in harder formations. 



Since the explosive sample-taker was introduced in 1936, it has become 

 increasingly popular because of its economy of operation. The hard-rock model 

 has greatly extended the application of the tool because it will core most forma- 

 tions and will secure a sample of sufficient size for evaluation. It is run by the 

 regular electric-logging unit and can obtain 30 samples from a 10,000-foot well 

 in about 2 hours. Other more expensive side-wall coring methods are used only 

 if this tool fails to produce desired results. 



710 



