Drillstring Stability Based on Variable Material Specific Force and Using a Sharp Three-Insert Polycrystalline Diamond Compact (PDC) Coring Bit

Previous research in stability of drillstrings was based on the assumption of constant material specific force, i.e., a bit force that is proportional to the area of cut. Moreover, earlier work used a simplified bit model that consisted of planar radial blades. In this paper, correlation between the material specific force and area of cut is obtained for Sierra White granite and Berea sandstone from test data developed at Sandia National Laboratories in Albuquerque, NM. These correlations, together with an improved bit model in which cylindrical cutters are arrayed in an overlapping pattern over a flat surface, are used to obtain the stability equations. Laboratory testing shows good correlation between measured bit vibrations and relative instability as predicted by the stability equations. These results are useful in predicting the appropriate operating conditions for stable drilling and serve as a basis for future development of more accurate models of PDC bits.

A Review of Attachment of StratapaxR Using Gas Pressure Diffusion Bonding

The development of the gas pressure diffusion bonding technique for attachment of StratapaxR diamond cutters to studs or bit blanks is reviewed in this paper. This paper updates the results presented previously on a continuing project conducted for the Division of Oil, Gas, Shale and In Situ Technology, Department of Energy. The process has been developed to a point where bonded parts can be obtained from commercial suppliers. A complete description is given of the latest bonding process including surface conditioning, metallization, canning, and gas pressure bonding. Results from experiments designed to facilitate reliable fabrication are included. Bond strengths of samples obtained from commercial suppliers have shear strengths (500 MPa) comparable to those developed at Sandia Laboratories. Results from laboratory drilling experiments are included. These tests were run to demonstrate the reliability of the bond in a cyclic load environment. With a suitable attachment technique and adequate cooling, the Stratapax has demonstrated superior cutting capabilities in rock with rates of penetration to 7.6 mm/s in Sierra White Granite.

Lyonesse revisited: the early walls of Scilly

The Isles of Scilly form an archipelago some 27 miles (43 km) WSW of Land’s End, Cornwall, and lie within an oval area about 12 miles (19km) SW-NE and 5 miles (8km) NW-SE. Five islands (St Mary’s, Tresco, St Martin’s, Bryher and Agnes; Fig. I) are inhabited, most of the permanent population of 2,000-plus being on St Mary’s. Some 40 further smaller isles bear vegetation, several with signs of former occupation, and there are several hundred more descending to mere rocks and reefs. The total exposed land surface at HWNT is c. 3,900 acres (1,600 ha). Scilly is almost entirely granite, the lowlying area between the isles being mainly a fine, white, granite-derived sand. Scilly is also the most southerly detached landmass of Britain and is botanically just within the extreme northern range of various species (Lousley, 1971). The islands are constitutionally quite separate from Cornwall, with a divergent recent social history (best accounts: Matthews, 1960; Gill, 1975). and a separate, only partly Celtic, linguistic one (cf. Thomas, 1979b), relevant here where place-names can reflect physical development. The archaeology of Scilly, first brought to wider notice by Borlase (1756), has long centred around the inordinate number of post-neolithic entrance-grave cairns (Hencken, 1932; Daniel, 1950), and has only recently been accorded a full length study (Ashbee, 1974).

III.—Notes on the Tourmaline of the White Granite of Meldon, Dartmoor

This “remarkable variety of granite” was briefly described by Mr. J. J. Harris Teall, F.E.S., in his “British Petrography” (1888), p. 316, and an interesting account was given in a footnote of the process by which the author was able to identify the topaz found in the rock.