Miscellaneous Subsurface Methods 



611 



needed in addition to most other routine surface drilling equipment. The 

 complete core-barrel assembly, which is made up on the bottom of the 

 drilling string, consists essentially of a cutter head, an outer barrel, a 

 floating inner barrel, and a finger-type "catcher," which retains the core 

 in the barrel when the assembly is raised from the bottom of the hole. 

 Mud circulates from the drill pipe between the two barrels to the cutter 

 head. Either drag-type or roller-type cutters may be used, depending on 

 the formation to be cored. Ordinarily, the conventional core barrel will 

 accommodate a twenty-foot core, but sometimes cores of much shorter 



HEAD PIECE 



CHECK VALVE 

 SWIVEL NUT 



SWIVEL BEARINGS 

 SWIVEL SHAFT 



OUTER BARREL 

 INNER BARREL 



CORE CATCHER 

 CORE BIT 



Figure 315. Core barrel used for diamond coring. 



lengths are cut because of local conditions or requirements. Weight on 

 the bit, rotary speed, and circulation rate are dependent upon local con- 

 ditions, but usually they are varied appreciably from routine drilling 

 techniques. To recover a core cut while using the conventional barrel, the 

 entire drilling string must be hoisted. Advantages of this type of coring 

 include the following: (1) the obtaining of a large-diameter core for a 

 given hole size, (2) the usual allowance of a maximum percentage of re- 

 covery of the formation cored, (3) its adaptability to all except the most 

 abrasive types of formations, and (4) the usual requirement of no addi- 

 tional surface drilling equipment Disadvantages include (1) the limita- 

 tion of cutting only a twenty-foot core during each run and (2) the nee- 



