EP0145422A2

EP0145422A2 - Improvements in rotary drill bits

Info

Publication number: EP0145422A2
Application number: EP84308323A
Authority: EP
Inventors: Michael Thomas Wardley; John Denzil Barr
Original assignee: NL Petroleum Products Ltd
Current assignee: NL Petroleum Products Ltd
Priority date: 1983-12-03
Filing date: 1984-11-30
Publication date: 1985-06-19
Also published as: GB2151283A; NO844770L; EP0145422A3; AU3592584A; GB2151283B; BR8406134A; GB8430291D0

Abstract

A rotary drill bit for use in drilling or coring deep holes in subsurface formations comprises a bit body (10) having a shank (11) for connection to a drill string, a plurality of cutting elements (14) mounted at the surface of the bit body, and a channel in the bit body for supplying drilling fluid to nozzles (17) in the surface of the bit body for cooling and/or cleaning the cutting elements. The cutting elements (14) each comprise a thin hard facing layer (19), defining a front cutting face, bonded to a less hard backing layer (20), and the cutting element is mounted on a carrier (22) which is received in a socket (23) in the bit body in such manner that, at least before any wear of the bit has occurred, the carrier (22) is wholly shrouded by the material of the bit body, the carrier 22 being formed from material which is less hard than the backing layer (20) of the cutting element (14).

Description

The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations and, in particular, to the mounting of cutting elements on such bits.
Rotary drill bits of the kind to which the invention relates comprise a bit body having a shank and an inner channel for supplying drilling fluid to the face of the bit. The bit body carries a plurality of so-called "preform" cutting elements. Each cutting element comprises a thin hard facing layer, which defines the front cutting face of the element, bonded to a less hard backing layer. For example, the hard facing layer may be formed of polycrystalline diamond or other superhard material, and the backing layer may be formed of cemented tungsten carbide. The two-layer arrangement of the cutting elements provides a degree of self-sharpening since, in use, the less hard backing layer wears away more easily than the harder cutting layer.
In many known forms of drill bit of this type the preform cutting elements are mounted on the bit body by being bonded, for example by brazing, to a carrier which may be in the form of a stud which is received and located in a socket in the bit body. Such studs have been formed of various materials.
In one known form of bit these studs have been formed from hardened steel. However, such arrangements suffer from the severe disadvantage that exposed portions of the steel stud are subject to erosion during use of the bit, such erosion being caused, generally, by the flow over the surface of the bit of drilling fluid carrying drilling debris. Such erosion can occur very rapidly and can progress to the stage where the mounting of the cutting elements is so weakened by erosion that the bit becomes unusable, even though the cutting elements themselves may not have reached the end of their working life due to the wear caused by drilling.
Hitherto, attempts have been made to overcome this deficency of steel carriers by coating the exposed portions of the carriers by a hard coating material, such as a layer of metal bonded tungsten carbide. However, not only is it a difficult and costly process to coat the carriers in this manner, it is also not particularly effective since it is found, in practice, that the hard coating, although more erosion-resistant than the underlying steel, still erodes fairly rapidly to an extent where the bit is rendered unusable before the cutting elements themselves are worn out. An alternative common solution to this problem, therefore, has been to form the carriers or studs themselves wholly from an erosion-resistant material such as cemented tungsten carbide.
While such carriers are resistant to erosion, their use brings other disadvantages. Not only are tungsten carbide carriers significantly more expensive and difficult to manufacture than steel carriers, but as the cutting elements and carriers wear during use, there is an increase in the area of hard carbide which is rubbing on the surface of the formation behind the cutting edge of the preform. This increases the drilling load and also reduces the self-sharpening effect, mentioned earlier, of the cutting elements in view of the large area which must be worn down behind the cutting edge.
The present invention sets out to overcome the disadvantages of the above-mentioned known arrangements.
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 cutting elements mounted at the surface of the bit body, and a channel in the bit body for supplying drilling fluid to the surface of the bit body for cooling and/or cleaning the cutting elements, at least some of the cutting elements each comprising a thin hard facing layer, defining a front cutting face, bonded to a less hard backing layer, the cutting element being mounted on a carrier which is received in a socket in the bit body in such manner that, at least before any wear of the bit has occurred, the carrier is substantially wholly shrouded by the material of the bit body, the carrier being formed from material which is less hard than said backing layer of the cutting element.
Since the carrier is initially shrouded by the material of the bit body it is not directly subjected to erosion during operation of the bit. As the cutting elements wear down, however, the backing layer and bit body to the rear of the cutting face wear down, eventually exposing part of the carrier, but since the carrier is exposed only due to wear, and not to erosion, it follows that any exposed surface of the carrier, during drilling, is in rubbing contact with the surface of the formation and is not therefore subject to erosion by drilling fluid. Since this exposed portion of the carrier is of less hard material than the backing layer of the cutting element, the self-sharpening effect of the whole cutting member is improved, since the softer carrier wears away at a faster rate than either the hard facing or the backing material.
In arrangements according to the invention each carrier may be formed from steel, and, as previously mentioned, steel carriers are less expensive to produce than carriers from harder material, such as cemented tungsten carbide. In order to achieve the enhanced self-sharpening effect, it is preferable for the material of the carrier to be less hard than the material from which the bit body is formed, although this is not essential.
In known manner, the hard facing layer of each cutting element may be formed of polycrystalline diamond and the backing layer may be formed of cemented tungsten carbide.
The backing layer of each cutting element may be of non-uniform thickness, being thicker adjacent the cutting edge of the facing layer than it is over the rest of the area of the facing layer. For example, the thickness of the backing layer may vary continuously and smoothly across the area of the cutting face. In a preferred embodiment, the rear surface of the backing layer is substantially flat so that the backing layer is generally wedge-shaped in cross-section.
Each cutting element may be substantially circular in cross-section, in known manner, although other shapes of cutting element may be employed. The carrier may be of similar cross-section to the associated cutting element and may form an axial extension thereof. Alternatively, the carrier may be in the form of a generally cylindrical stud having a surface inclined at less than 90° to the central axis of the stud and to which the rear surface of the backing layer is bonded.
The bit body may be formed from cemented tungsten carbide matrix, steel, or steel formed with a hard coating, or any conventional combination of such materials.
Each carrier may be secured within its associated socket by brazing or it may be an interference fit in the socket.
The following is a detailed description,by way of example, of embodiments of the invention, reference being made to the accompanying drawings in which Figure 1 is a side elevation of a typical drill bit of the kind to which the present invention is applicable,
Figure 2 is an end elevation of the drill bit shown in Figure 1, and
Figures 3 to 6 are diagrammatic sections through cutting elements mounted on a carrier in a drill bit body.
Referring to Figures 1 and 2, the body 10 of the drill bit is typically formed of tungsten carbide matrix infiltrated with a binder alloy, and has a threaded shank 11 at one end for connection to the drill string.
The operative end face 12 of the bit body is formed with anumber of blades 13 radiating from the central area of the bit and the blades carry cutting members 14 spaced apart along the length thereof.
The bit has a gauge section 15 including kickers 16 which contact the walls of the bore hole to stabilise the bit in the bore hole. A central channel (not shown) in the bit body and shank delivers drilling fluid through nozzles 17 in the end face 12, in known manner.
It will be appreciated that this is only one example of the many possible variations of the type of bit to which the invention is applicable, including bits where the body is formed from steel.
Each cutting member 14 comprises a preform cutting element mounted on a carrier in the form of a stud which is located in a socket in the bit body. Conventionally, each preform cutting element is usually circular and comprises a thin facing layer of polycrystalline diamond bonded to a thicker backing layer of tungsten carbide. The rear surface of the backing layer of each cutting element is bonded, for example by brazing, to a suitably orientated surface on the stud.
Figures 3 to 6 show, by way of example only, four typical arrangements of cutting member according to the invention. In each case, the stud is formed from a material which is less hard than the material, usually tungsten carbide, of the backing layer.
In the arrangement of Figure 3 the cutting element 18 itself is circular and comprises a thin hard facing layer 19 of polycrystalline diamond and a thicker backing layer 20 of cemented tungsten carbide. The facing layer 19 extends at right angles to the central axis of the cutting element.
The rear surface 21 of the backing layer 20 is bonded to the end face of a generally cylindrical stud 22, formed from steel, which is coaxial with the cutting element 18 and of similar cross-section, so as to form an axial extension of the cutting element. The cutting member 14 as a whole is received in a cylindrical socket 23 which is formed in the blade 13 of the bit body, the blade being formed from tungsten carbide matrix. The stud 22 may be secured within the socket by brazing or may be an interference fit. As shown in Figure 3, however, the depth of the socket 23 in relation to the axial length of the stud 22 is such that the stud portion of the cutting member is wholly received within the socket so that the material of the stud is wholly shrouded by the matrix material forming the bit body. As previously described, this arrangement protects the steel stud from the erosion to which it would otherwise be subject. However, as the cutting element 18 wears down in use, the thin layer of matrix at the lower part of the socket becomes worn away so that the material of the stud 22 itself rubs on the formation. This enhances the self-sharpening effect of the cutting member, since the softer material of the stud 22 will wear away more quickly than the harder material of the backing layer 20 which, in turn, wears away more quickly than the facing layer 19. At the same time, since the only exposed portion of the stud 22 is in rubbing contact with the formation, the remainder of the stud is still protected from erosion.
In the alternative arrangement shown in Figure 4 the backing layer 20 is not of uniform thickness, but the rear surface 21 of the backing layer is inclined at an angle of less than 90^o to the central axis of the cutting element. The backing layer 20 is thus generally wedge-shaped so as to be of increased thickness adjacent the cutting edge of the cutting element, which is indicated at 24.
The front surface of the steel stud 22 is similarly inclined to the central axis of the stud so that it registers with the rear surface of the backing layer 20, the two surfaces being bonded together, before the stud is inserted in the socket 23, by a high temperature bonding process, such as L S bonding. As in the Figure 4 arrangement, the dimensions of the socket 23 are such that the stud is wholly received in the socket so as to be protected from erosion during use of the bit.
Figures 5 and 6 show further alternative arrangements with different configurations of stud, the stud in each case again being wholly shrouded by the material of the bit body.
In the above-described arrangements the cutting members, each comprising the preform cutting element 18 and carrier 22, will normally be pre-assembled by bonding the cutting elements to the carriers before insertion in the sockets in the bit body. However the invention also includes within its scope arrangements where the carriers are mounted in the bit body before the cutting elements are bonded to the carriers. For example, the carriers may be cast in the bit body during formation of the bit body, or may be brazed into the sockets subsequently, the cutting elements in each case then being bonded to the carriers, for example by laser bonding, while the carriers are in situ.

Claims

1. A rotary drill bit for use in drilling or coring deep holes in subsurface formations comprising a bit body (10) having a shank (11) for connection to a drill string, a plurality of cutting elements (14) mounted at the surface of the bit body, and a channel in the bit body for supplying drilling fluid to the surface of the bit body for cooling and/or cleaning the cutting elements, at least some of the cutting elements (14) each comprising a thin hard facing layer (19), defining a front cutting face, bonded to a less hard backing layer (20), the cutting element being mounted on a carrier (22) which is received in a socket (23) in the bit body, characterised in that, at least before any wear of the bit has occurred, the carrier (22) is substantially wholly shrouded by the material (13) of the bit body, the carrier being formed from material which is less hard than said backing layer (20) of the cutting element.

2. A drill bit according to claim 1, characterised in that each carrier (22) is formed from steel.

3. A drill bit according to claim 1 or claim 2, characterised in that the hard facing layer (19) of each cutting element is formed of polycrystalline diamond.

4. A drill bit according to any of claims 1 to 3, characterised in that the backing layer (20) of each cutting element (14) is formed of cemented tungsten carbide.

5. A drill bit according to any of claims 1 to 4, characterised in that the backing layer (20) of each cutting element (14) is of non-uniform thickness, being thicker adjacent the cutting edge of the facirglayer (19) than it is over the rest of the area of the facing layer.

6. A drill bit according to claim 5, characterised in that the thickness of the backing layer (20) varies continuously and smoothly across the area of the cutting face.

7. A drill bit according to claim 6, characterised in that the rear surface of the backing layer (20) is substantially flat so that the backing layer is generally wedgeshaped in cross-section.

8. A drill bit according to any of claims 1 to 7, characterised in that each cutting element (14) is substantially circular in cross-section.
A drill bit according to any of claims 1 to 8. characterised in that the carrier (22) is of similar cross-section to each cutting element (14) and forms an axial extension thereof.

10. A drill bit according to any of claims 1 to 8, characterised in that the carrier (22) is in the form of a generally cylindrical stud having a surface inclined at less than 90° to the central axis of the stud and to which the rear surface of the backing layer is bonded.

11. A drill bit according to any of claims 1 to 10, characterised in that the bit body is formed from sintered tungsten carbide matrix, steel, or steel formed with a hard coating.

12. A drill bit according to any of claims 1 to 11, characterised in that each carrier (22) is secured within its associated socket (23) by brazing.

13. A drill bit according to any of claims 1 to 11, characterised in that each carrier is an interference ; fit in its associated socket.