IMPROVEMENTS IN OR RELATING TO ROTARY DRILL BITS

Abstract: A rotary drill bit for use in drilling or coring deep holes in subsurface formations comprises a bit body 10 having a shank for connection to a drill string and a plurality of cutting elements 15 mounted at the surface of the bit body. Each cutting element 15 is bonded to a stud 19 which is received in a socket in the bit body. Spaced rearwardly of each cutting element is a separate abrasion element comprising a stud 20 which is received in a socket in the bit body 10 and is impregnated with particles 21 of natural or synthetic diamond. The abrasion element 16 provides a back-up in the event of failure or excessive wear of the cutting element and its spacing from the cutting element prevents the damaging transfer of heat from the abrasion element 16 to the cutting element 15. The cooling may be enhanced by providing a channel 23 for drilling fluid between the cutting element and abrasion element.

Description: The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations, and of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutting elements mounted at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit body for cooling and/or cleaning the cutting elements, the bit body being formed from steel, and particles of superhard material being mounted rearwardly of each cutting element with respect to the normal direction of rotation of the bit. In DE-A-1054039 there is described a drill bit of this general kind in which the superhard particles are embedded in the bit body, over a large area, rearwardly of the cutting elements. The present invention is particularly, but not exclusively, applicable to drill bits of the above kind in which the cutting elements comprise preforms having a thin facing layer of polycrystalline diamond bonded to a backing layer of tungsten carbide. Various methods may be used for mounting such cutting elements on the bit body but such methods, and the general construction of bits of the kind to which the invention relates, are well known and will not therefore be described in detail. When drilling deep holes in subsurface formations, it often occurs that the drill passes through a comparatively soft formation and strikes a significantly harder formation. Also there may be hard occlusions within a generally soft formation. When a bit using preform cutters meets such a hard formation the cutting elements may be subjected to very rapid wear. In order to overcome this problem it has been proposed to provide particles of superhard material rearwardly of the cutting elements, as disclosed in DE-A-1054039. With such an arrangement, during normal operation of the drill bit the major portion of the cutting or abrading action of the bit is performed by the cutting elements in the normal manner. However, should a cutting element wear rapidly or fracture, so as to be rendered ineffective, for example by striking hard formation, the superhard particles take over the abrading action of the cutting element thus permitting continued use of the drill bit. Provided the cutting element has not fractured or failed completely, it may resume some cutting or abrading action when the drill bit passes once more into softer formation1. According to the invention there is provided a rotary drill bit for use in drilling or coring deep holes in subsurface formations, comprising a bit body having a shank for connection to a drill string, a plurality of preform cutting elements mounted at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit for cooling and/or cleaning the cutting elements, the bit body being formed from steel, and particles of superhard material being mounted rearwardly of each cutting element with respect to the normal direction of rotation of the bit, characterised in that each cutting element is mounted on a stud received in a socket in the steel bit body, said particles of superhard material being embedded in the stud, and at least the portion of the stud which includes the particles of superhard material projecting clear of the bit body. Each preform cutting element may comprise a thin facing layer of superhard material bonded to a less hard backing layer, or a unitary layer of thermally stable polycrystalline diamond material. Preferably the particles of superhard material are so positioned with respect to the leading surface of the drill bit that they do not come into cutting or abrading contact with the formation until a certain level of wear of the cutting elements is reached. The particles are preferably embedded in the surface of the stud so as to project therefrom. The stud may be formed from cemented tungsten carbide, and may be substantially cylindrical. In one embodiment there is provided on the surface of the bit body a plurality of blades extending outwardly with respect to the axis of rotation of the drill bit, each stud and its associated cutting element being mounted on a blade. The following is a more detailed description of embodiments of the invention, reference being made to the accompanying drawings in which: Figures 1 and 2 are front end views of rotary drill bits, Figure 3 is a diagrammatic section through a cutting element and associated abrasion element, Figure 4 is a front view of an abrasion element, and Figures 5 to 7 are similar views to Figure 3 of alternative arrangements. The arrangements of Figures 1 to 5 are not in accordance with the present invention but will be described by way of explanation of the invention. The rotary bit body of Figure 1 has a leading end face 10 formed with a plurality of blades 11 upstanding from the surface of the bit body so as to define between the blades channels 12 for drilling fluid. The channels 12 lead outwardly from nozzles 13 to which drilling fluid passes through a passage (not shown) within the bit body. Drilling fluid flowing outwardly along the channels 12 passes to junk slots 14 in the gauge portion of the bit. Mounted on each blade 11 is a row of cutting elements 15. The cutting elements project into the adjacent channel 12 so as to be cooled and cleaned by drilling fluid flowing outwardly along the channel from the nozzles 13 to the junk slots 14. Spaced rearwardly of the three or four outermost cutting elements on each blade are abrasion elements 16. In the arrangement shown each abrasion element lies at substantially the same radial distance from the axis of rotation of the bit as its associated cutting element, although other configurations are possible. Figure 2 shows an alternative arrangement in which some of the nozzles are located adjacent the gauge region of the drill bit, as indicated at 13 a in Figure 2. The flow from such a peripheral nozzle passes tangentially across peripheral portions of the leading face of the bit to the junk slots 14, thus ensuring a rapid and turbulent flow of drilling fluid over the intervening abrasion and cutting elements so as to cool and clean them with efficiency. In either of the arrangements described, the cutting elements 15 and abrasion elements 16 may be of many different forms, but Figure 3 shows, by way of example, one particular configuration. Referring to Figure 3, it will be seen that each cutting element 15 is a circular preform comprising a front thin hard facing layer 17 of polycrystalline diamond bonded to a thicker backing layer 18 of less hard material, such as tungsten carbide. The cutting element 15 is bonded, in known manner, to an inclined surface on a generally cylindrical stud 19 which is received in a socket in the bit body 10. The stud 19 may be formed from cemented tungsten carbide and the bit body 10 may be formed from steel or from matrix material. Each abrasion element 16 also comprises a generally cylindrical stud 20 which is received in a socket in the bit body 10 spaced rearwardly of the stud 19. The stud 20 may be formed from cemented tungsten carbide impregnated with particles 21 of natural or synthetic diamond or other superhard material. The superhard material may be impregnated throughout the body of the stud 20 or may be embedded in only the surface portion thereof. Referring to Figure 4, it will be seen that each abrasion element 16 may have a leading face which is generally part-circular in shape. The abrasion element 16 may project from the surface of the bit body 10 to a similar extent to the cutting element, but preferably, as shown, the cutting element projects outwardly slightly further than its associated abrasion element, for example by a distance in the range of from 1 to 10mm. Thus, initially before any significant wear of the cutting element has occurred, only the cutting element 15 engages the formation 22, and the abrasion element 16 will only engage and abrade the formation 22 when the cutting element has worn beyond a certain level, or has failed through fracture. In the arrangement shown, the stud 20 of the abrasion element is substantially at right angles to the surface of the formation 22, but operation in softer formations may be enhanced by inclining the axis of the stud 20 forwardly or by inclining the outer surface of the abrasion element away from the formation in the direction of rotation. In order to improve the cooling of the cutting elements and abrasion elements, further channels for drilling fluid may be provided between the two rows of elements as indicated at 23 in Figure 3. Figure 5 shows an arrangement where the cutting element 24 is in the form of a unitary layer of thermally stable polycrystalline diamond bonded without a backing layer to the surface of a stud 25, for example of cemented tungsten carbide, which is received in a socket in a bit body 26 which in this case is formed from steel. An abrasion element 27 is spaced rearwardly of each cutting element 24, but it will also be appreciated that the form of cutting element shown in Figure 5 may also be used in any conventional manner in a steel body bit without the additional abrasion elements. The present invention provides a modification of the arrangements described above. In accordance with the invention the bit body is formed from steel and each abrasion element is incorporated in the support stud which carries a cutting element. Such arrangements according to the invention are shown in Figures 6 and 7. In the arrangement of Figure 6 particles of diamond or other superhard material are impregnated into the stud 19 itself rearwardly adjacent the cutting element 15. In the alternative arrangement shown in Figure 7, a separately formed abrasion element impregnated with superhard particles is included in the stud. Any known form of cutting element 15 may be employed and the invention includes in its scope arrangements where the cutting element is mounted on another form of stud in the bit body, rather than on a cylindrical stud such as 19.

Claims: A rotary drill bit for use in drilling or coring deep holes in subsurface formations, comprising a bit body having a shank for connection to a drill string, a plurality of preform cutting elements (15) mounted at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit for cooling and/or cleaning the cutting elements (15), the bit body being formed from steel, and particles of superhard material being mounted rearwardly of each cutting element (15) with respect to the normal direction of rotation of the bit, characterised in that each cutting element (15) is mounted on a stud (19) received in a socket in the steel bit body, said particles of superhard material being embedded in the stud (19), and at least the portion of the stud (19) which includes the particles of superhard material projecting clear of the bit body. A drill bit according to Claim 1, wherein each preform cutting element (15) comprises a thin facing layer of superhard material bonded to a less hard backing layer. A drill bit according to Claim 1, wherein each preform cutting element (15) comprises a unitary layer of thermally stable polycrystalline diamond material. A drill bit according to any of Claims 1 to 3, wherein the particles of superhard material are so positioned with respect to the leading surface of the drill bit that they do not come into cutting or abrading contact with the formation until a certain level of wear of the cutting elements is reached. A drill bit according to any of Claims 1 to 4, wherein the particles of superhard material are embedded in the surface of the stud (19) so as to project therefrom. A drill bit according to any of Claims 1 to 5, wherein the stud (19) is formed from cemented tungsten carbide. A drill bit according to any of Claims 1 to 6, wherein the stud (19) is substantially cylindrical. A drill bit according to any of Claims 1 to 7, wherein there is provided on the surface of the bit body a plurality of blades extending outwardly with respect to the axis of rotation of the drill bit, each stud (19) and its associated cutting element (15) being mounted on a blade.

Inventors: FULLER, JOHN
Applicants: CAMCO DRILLING GROUP LIMITED
Application year: 1985
Main classification: E21B10
IPC codes: E21B10