1086 



EXPLORATION GEOPHYSICS 



An 18-shot gun is 6' 8^4" in length, 4^4" in diameter, and weighs 149 pounds. A 6-shot 

 gun is 18^" in length, 4" in diameter, and weighs 31 pounds. 



Field Operations 



The side-wall sampling operation usually follows the electrical survey, at which 

 time the electrical log is inspected for interesting uncored zones, and zones on which 

 more information is desired. Then the electrical logging sonde on the cable is replaced 

 with a side-wall sample gun, and samples secured at the points of interest indicated by 

 the electrical logs. 



By means of an electrode on the cable near the sample gun, the operator observes, 

 on his recorder in the electrical logging truck, the S.P. of the formations through 

 which the gun is moving. It is thus usually possible to verify the accuracy of depths 

 and to place the gun at the exact depth corresponding to a point of the electrical log 

 at which a core is desired. By this procedure the operator may take samples with great 

 accuracy with respect to the formations as they are indicated on the electrical log, 

 irrespective of absolute depth. 



When the gun is positioned at the proper depth, it is fired by the operator. The 

 firing of the bullet is observed by the operator as a sharp crack heard in a set of 

 headphones or a sharp kick of a galvanometer spot in the recorder.f The operator 

 then turns off the voltage and applies a tension to the cable to remove the sample from 

 the formation. If the sample bullet has penetrated the formation an unusually great 

 distance, so that it cannot be pulled loose, the wire fasteners joining the fired bullet 

 to the gun are designed to break well before the strain on the cable becomes dangerous. 

 In this case, the bullet is left in the wall of the hole. Nothing is left in the hole. 



When the bullet is pulled loose, the operator repeats the procedure at other coring 

 points until all the samples are taken. The gun is then pulled out of the hole and the 



cores are removed from the bullets, bottled, and deliv- 

 ered for the necessary core studies. If it is desired to 

 use the cores for permeability analysis, the cores may 

 be delivered intact in the bullets. 



The Cores. — The cores taken with present guns 

 are 54" in diameter and may be up to 2" in length, 

 depending on the recovery. The cores obtained, al- 

 though relatively small, are large enough to give cuts, 

 odors, and visual examination which will permit the 

 confirmation of interesting zones. They can also be 

 used, in favorable cases, for the usual porosity, perme- 

 ability, and other analyses. There is, in general, no 

 great compaction or crushing of the core, because of 

 the construction of the bullet, or core barrel. 



The fluid content of cores from porous formations 

 is fairly representative of the original conditions prior 

 to the penetration of the formations by the drill hole. 

 No core, mechanical or otherwise, can escape com- 

 pletely the washing effect of drilling mud. The side- 

 wall sample operations have shown, nevertheless, that, 

 while the invasion of the permeable formation takes 

 place, a true oil sand generally retains some of its oil, 

 and a salt-water sand retains much of its salt, at least 

 at a very short distance from the wall of the hole. 



Fig. 678.— Correlations between Efficiency and Safety.— The efficiency of the side- 

 electrical logging and side wall wall sample taker, like that of the rotary method, is not 

 samples. (Courtesy of Schlum- ^ „„ ^ i_ ^ ^i r i- r ^ 

 berger Well Surveying Corp.) 100 per cent, but the recovery of satisfactory cores, as 



t See seismo-electric effect, page 554. 



