A special application of the conventional-type core barrel is encountered in 

 drilling for sulphur. The problem of the driller, in attempting to reach the 

 deposit, is comparable in many ways to those of the oil driller; however, in 

 some respects sulphur drilling is more complicated. Sulphur is found in dome 

 structures, but there is no definite means of predicting whether a given dome 

 will contain sulphur that is recoverable. This can be determined only by continu- 

 ous coring. Core recovery of sulphur is very difficult because the element occurs 

 in a calcium sulphate matrix which is brittle and easily fractured. Therefore, 

 it is advantageous to cut as large a core as possible so that the core will be 

 strong enough to withstand drilling stresses. Figure 33-2 shows a conventional- 

 type core barrel especially designed for sulphur coring or for coring other 

 frangible deposits. The core size of this barrel is as large as possible for the 

 size hole being drilled. The thin cutter head requires eight cutters rather than the 

 ordinary six. 



The thin cutters permit the head to run more smoothly, thus minimizing 

 core breakage. To further facilitate the entry of the frangible cored section 

 into the inner barrel, the barrel is supported on a special two-way rubber bearing 

 at the top in order to secure the barrel in both directions and to allow it to 

 float freely with the core during drilling operations. Since the sulphur-anhydrite 

 mixture is relatively firm, a slip type of core catcher is used that consists of long 

 fingers, with a tooth at the lower end, which fit into a tapered section in the head. 

 The spring fingers, projecting in a spiral pattern on the inside of the core 

 catcher, ride on the core and bend the fingers so that they are in close proximity 

 with the core. When the coring operation is completed, the drill string is raised, 

 and the teeth in the core catcher immediately dig into the core and wedge into 

 the taper in the cutter head. This action breaks off and retains the core. The 

 force to do this, which is considerable in a large-diameter core, is taken directly 

 by the cutter head, thereby protecting the thin inner barrel and the bearing 

 assembly. 



Figure 33-3 illustrates several types of cutter heads. At the top is a standard- 

 type diamond-core bit. The cutting surfaces consist of a multitude of industrial 

 diamonds set in a tungsten-carbide matrix. Suitable grooves are provided to 

 allow the circulating fluid to cool the cutting edges. This type of bit, although 

 expensive, can be used economically in coring very hard formations. It may 

 last ten times as long as a regular alloy bit. Core recovery is excellent because 

 of its smooth-running action and the large diameter core that it cuts. The 

 principal disadvantages of this head are the substantial loss that is possible if 

 the bit fails prematurely, and the inconvenience and expense of the extra long 

 conventional core barrels that are needed to take advantage of the bit's potential 

 life without excessive round trips. The center illustration in Figure 33-3 is a 

 roller-type coring bit, normally used in medium-hard formations. This core bit 

 consists of six cutters, three of which cut the bottom and the wall of the hole, 



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