Miscellaneous Subsurface Methods 



613 



technique, which was discussed previously. The inner tube is supported 

 on two open ball bearings so that it will remain stationary while the core 

 is being cut. Improvements in the design of the barrel are being made as 

 more experience is obtained. Diamond-core bits are all basically of the 

 same construction. The diamonds are placed in a powdered metal, usually 

 tungsten alloy, in a mold of desired size and shape. The matrix under heat 

 and pressure forms a solid mass with the diamonds exposed at the surface. 

 This section, known as the "crown of the bit," is then fastened to a steel 

 bit hub by brazing or mechanical pinning. Four general designs of dia- 

 mond-core bits are shown in figure 316. The techniques employed in 

 diamond coring involve the use of little weight on the bit, low pump pres- 



FiGURE 317. Wire-line coring reel. 



sure, and normal to high rotating speeds. Diamonds drill by abrasion, 

 and, while increased weight on the bit may increase drilling rates slightly, 

 high weights tend to fracture the diamonds. Pump pressures and fluid 

 velocity are kept low to prevent washing of the core and to reduce erosion 

 of the bit-matrix metal. Since the cutting rate of a diamond should in- 

 crease with its linear velocity, high rotating speeds should be best for 

 diamond coring. In actual practice, however, rotating speeds are limited 

 by the drilling equipment available. Experience has also indicated that 

 the removal of all junk iron from the well bore before beginning to core 

 is absolutely essential. When using the proper design of barrel, cores up 

 to ninety feet in length can be cut during each run, if no mechanical diffi- 

 culty develops. 



Advantages of diamond coring include (1) the fact that in areas 



