IMPROVEMENTS RELATING TO DRILL BITS

Abstract: A polycrystalline diamond cutter (PDC) (12) is described which is elliptical or oval, rather than circular, in shape, having a major axis which extends outwardly from the drill bit body (10) when in use. The elliptical or oval shape of the PDC provides advantages in terms of the rate of development of the wear flat, the weight on bit requirement, and the useful cutting area and cooling characteristics of the PDC in comparison with circular configurations. The PDC's may be post mounted. Drill bits incorporating the PDC and methods of forming suitable bit bodies and mounting posts are also described.

Description: f. 1 "Improvements Relating to Drill Bits" 2 3 The present invention relates to drill bits for use in 4 oil drilling or the like and is particularly concerned 5 with improvements in polycrystalline diamond cutting 6 products for use in such drill bits. 7 8 Polycrystalline Diamond Compacts (PDC's), or cutters, 9 generally comprise a polycrystalline diamond layer 10 affixed to a tungsten carbide substrate, a plurality of 11 PDC's being mounted in a bit body to produce a 12 functional drill bit. Conventional PDC's are circular, 13 the substrate being a circular disc or cylinder, this 14 shape being determined by a number of factors, as 15 follows: 16 17 (a) Manufacture of the PDC's requires ultra high 18 pressures and temperatures, and a cylindrical vessel 19 provides the best solution for the containment of these 20 high pressures. Early PDC's had a diameter of 13 mm 21 (1/2 inch). However, much larger PDC's are now 22 produced. The substrate, which is a tungsten carbide 23 support material behind the polycrystalline diamond 24 layer, typically varies in thickness from 3 mm to 13 25 mm. (b) The cylindrical shape of the PDC's facilitates their attachment to the bit body, which is done in one of two basic ways: either the PDC is brazed directly into preformed circular holes in the bit body, or it is high temperature brazed onto a cylindrical post or stud (also of tungsten carbide) which in turn is press fitted or brazed into preformed circular holes in the bit body. In either case the circular configuration of the PDC's simplifies the formation of the corresponding holes in the bit body using conventional machine tools, and in the latter case it similarly simplifies manufacture of the posts. There are, however, a number of disadvantages in using cylindrical PDC's. (i) The generation of the wear flat is not ideal as it starts from a point contact which rapidly grows in the form of a chord to the circular diameter diamond layer on the face of the PDC cutter. (ϋ) The weight on bit requirement to maintain constant loading on the diamond layer rapidly increases with the area of the wear flat. (ϋi) The temperature of the cutter increases with the development of the wear flat and the mud's ability to cool the cutter drops. (iv) The spacing between cutters is restricted by their diameter and additionally, in the case of post-mounted cutters, by the diameter of the post, its depth of setting, and the curvature of the bit body upon which it is set. (v) In mounting cylindrical PDC's on a bit body there is a trade off between the depth of setting and the remaining cutter exposure; cylindrical cutters are usually set at or below the waist, leaving only half or less than half of the diameter exposed. The objects of the present invention include obviating or mitigating the abovementioned disadvantages of existing PDC's and of drill bits incorporating such PDC's. According to a first aspect of the invention, there is provided a polycrystalline diamond compact (PDC) comprising a polycrystalline diamond layer affixed to a substrate, said substrate being generally elliptical or oval and having a major axis and a minor axis. In the present disclosure the term "major axis" means the axis along the largest edge-to-edge dimension of an elliptical or oval body and "minor axis" means the axis along the largest edge-to-edge dimension in a direction at right angles to the major axis. According to a second aspect, the invention provides a post-mounted PDC, comprising an elliptical or oval PDC as defined above, mounted on a post having a corresponding cross-sectional shape. According to a third aspect, the invention provides a drill bit comprising a drill bit body having preformed elliptical or oval holes formed therein and having a plurality of elliptical or oval PDC's mounted in said holes with the major axes of the PDC's extending outwards from the surface of the bit body. The PDC's may either be mounted directly in the holes or may be post-mounted, the posts being mounted in the holes. Preferably, the elliptical or oval PDC's are cut from a circular PDC disc. Preferably also, the bit body with said elliptical or oval holes formed therein is produced by investment casting. The elliptical or oval cross-section posts for post-mounted PDC's may be produced in a similar manner. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic cross-sectional view of part of a drill bit body showing a comparison between the use of conventional, circular PDC's and elliptical or oval PDC's in accordance with the present invention; Fig. 2 is an end elevation of a PDC in accordance with the present invention; Fig. 3 is a side view of a first PDC having an end elevation as seen in Fig. 2; Fig. 4 is a side view of a second, post-mounted PDC also having an end elevation as seen in Fig. 2; Fig. 5 is a plan view of a PDC disc showing the manner in which nested PDC's as seen in Figs. 2, 3 and 4 may be cut from disc; and Figs. 6, 7, 8 and 9 are views corresponding to Figs. 2, 3, 4, and 5 for PDC's in accordance with the invention having different dimensions from the first and second PDC's. Referring n ' to the drawings, Fig. «. shows a sectional view of part- of a drill bit body 10, having a plurality of PDC's 12 mounted around the periphery thereof in correspondingly shaped holes or recesses. For purposes of comparison, the drawing shows both conventional, circular PDC's, 12a, and elliptical PDC's, 12b. As can be seen, the minor axes of the elliptical PDC's 12(b) are equal in length to the diameters of the circular PDC's 12(a) and the major axes of the elliptical PDC's 12b extend outwardly from the surface of the bit body 10. The use of elliptical or oval PDC's in accordance with the invention provides the following advantages in comparison with equivalent circular PDC's: (a) The rate at which the area of the wear flat develops as the PDC is worn away can be considerably reduced in comparison with circular PDC's. (b) The weight on bit required to maintain constant loading on the diamond layer does not have to be increased so significantly due to the slower development of the wear flat. (c) The area of the PDC's exposed to the cooling effects of drilling mud is increased in comparison with circular PDC's. (d) A greater overall cutting surface is available in comparison with circular PDC's owing to closer spacing of PDC's and/or the greater exposed areas of individual PDC's. Fig. 2 shows an end elevation of an elliptical I-DC 14 in accordance with the invention, having a major axis dimension of 13.44 mm and a minor axis dimension of 11.318 mm, i.e. an aspect'ratio of approximately 1.2. Fig. 3 shows a side view of a first PDC 16 comprising a polycrystalline diamond layer 18 (0.5 mm in thickness) on a tungsten carbide substrate 20, (giving an overall thickness of 8 mm) , and having a cross-sectional shape as seen in Fig. 2. Fig. 4 shows a post-mounted PDC 22 comprising a diamond layer 24 on a substrate 26 (giving an overall thickness of 3 mm) , located on a tungsten carbide post 2 (resulting in an overall length of 20 mm) and again having the cross-sectional shape seen in Fig. 2. Fig. 5 shows the manner in which a plurality of PDC's of this shape may be cut from a 2 inch PDC disc 30. 11 such cutters can be obtained from a 2 inch diameter disc, utilising 64.8% of the total disc area, as compared with 9 circular 13.4 mm diameter PDC's which could be obtained, utilising 62.99% of the disc area. Figs. 6 to 9 are equivalent illustrations for a 19.05 mm by 13.44 mm PDC 32 (having an aspect ratio of approximately 1:4), Fig. 7 showing a PDC 34 having a diamond layer 36 and substrate 38, Fig. 8 a post-mounted PDC 40 having a diamond layer 42, substrate 44 and post 46, and Fig. 9 showing the nesting of such PDC's on a 2 inch PDC disc 48. In this case, 5 PDC's are obtained from the disc, utilising 59.53% of its area in comparison with 4 circular, 19 mm diameter PDC's which could be obtained, utilising 56.25% of the area. Referring back to Figs. 2 and 3, the edge of the polycrystalline diamond layer 18 may be chamfered or radiused along at least a part of its periphery adjacent that end of the major axis which projects outwardly from the bit body in use, preferably along a chord determined by the chamfer angle and diamond layer thickness as indicated by lines 50 and 52. This reduces the possibility of the PDC fracturing upon first use. The production of bit bodies with elliptical or oval cutter recesses is advantageously accomplished using investment casting techniques such as are disclosed in WO90/01384. However, conventional steel and matrix techniques might also be employed. The elliptical or oval PDC's themselves can be cut from large circular PDC discs using EDM wire cutting techniques, and with efficient nesting might be produced at slightly lower cost than equivalent circular PDC's. Modifications and improvements may be incorporated without departing from the scope of the invention. MURGITROYD AND COMPANY CHARTERED PATENT AGENTS MITCHELL HOUSE 333 BATH STREET GLASGOW G2 4ER

Claims: 1 1 CLAIMS 2 3 1. A polycrystalline diamond compact (PDC) comprising 4 a polycrystalline diamond layer affixed to a substrate, 5 said substrate being generally elliptical or oval and 6 having a major axis and a minor axis, wherein the term 7 "major axis" means the axis along the largest 8 edge-to-edge dimension of an elliptical or oval body 9 and "minor axis" means the axis along the largest 10 edge-to-edge dimension in a direction perpendicular to 11 the major axis. 12 13 2. A PDC as claimed in Claim 1, wherein the ratio of 14 the length of said major axis to said minor axis is in 15 the range 1.2 to 1.4. 16 17 3. A PDC as claimed in Claim 1, wherein at least a 18 portion of the edge of said polycrystalline diamond 19 layer adjacent one end of said major axis is chamfered 20 or radiused. 21 22 4. A method of forming PDC's as claimed in any of 23 Claims 1 to 3, wherein a plurality of said oval or 24 elliptical PDC's are cut from a circular PDC disc. 25 26 5. A post-mounted PDC, comprising an elliptical or 27 oval PDC as claimed in any of Claims 1 to 3, mounted on 28 a post having a corresponding cross-sectional shape. 29 30 6. A drill bit comprising a drill bit body having 31 preformed elliptical or oval holes formed therein and 32 having a plurality of PDC's as claimed in any of Claims 33 1 to 3 or 5 mounted in said holes with the major axes 34 of the PDC's extending outwards from the surface of the 35 bit body. 7. A drill bit as claimed in Claim 6, wherein said PDC's are mounted directly in said holes. 8. A drill bit as claimed in Claim 6, wherein said PDC's are post-mounted and said posts are mounted in said holes. 9. A method of forming a post-mounted PDC as claimed in Claim 5, wherein said post is formed by investment casting. 10. A method of forming a drill bit as claimed in Claim 6 , wherein said bit body is formed by investment casting.

Inventors: BAIN JAMES, SIMPSON NEIL ANDREW ABERCROMBI, BAIN, JAMES, SIMPSON, NEIL, ANDREW, ABERCROMBIE
Applicants: BRIT BIT LIMITED
Application year: 1992
Main classification: E21B10
IPC codes: E21B10