GB2190120A - Improvements in or relating to rotary drill bits - Google Patents

Abstract

A rotary drill bit for use in drilling or coring holes in subsurface formations comprises a bit body 10 having a leading face 11 and a gauge region 12, a plurality of preform polycrystalline diamond cutting elements 19,20,21,22 mounted at the leading face of the bit body, and a passage in the bit body communicating with a plurality of openings 26 in the leading face of the bit body for supplying drilling fluid to said face for cooling and cleaning the cutting elements. At least certain of the cutting elements nearer the gauge region 12 of the bit body are of greater height, for example by being circular elements of greater diameter, then other cutting elements disposed nearer the central axis of rotation of the bit. Since the outer cutting elements are normally subjected to greater wear, the arrangement permits greater wear of the outer elements, during the life of the bit, thus reducing the risk of early failure of the bit due to failure of the outer cutting elements. <IMAGE>

Description

SPECIFICATION Improvements in or relating to rotary drill bits The invention relates to rotary drill bits for use in drilling or coring deep holes in subsurface formations and ofthe kind comprising a bit body having a leading face and a gauge region, a plurality of preform polycrystalline diamond cutting elements mounted at the leading face ofthe bit body, and a passage in the bit bodyforcommunicating with a plurality of openings in the leading face ofthe bit bodyforsupplying drilling fluid to said face for cooling and cleaning the cutting elements.

There are many different designs of drill bit ofthis general type. For example, the bit body may be machined from solid metal, usually steel, or may be moulded using a powder metallurgy process in which tungsten carbide powder is infiltrated with metal alloy binder in afurnaceso as to form a hard matrix. The cutting elements may be mounted directly on the bit body or may be mounted on carriers, such as studs or posts, which are received in sockets in the bit body.

One common form of preform polycrystalline diamond cutting element used in such drill bits comprises a hard facing layer of polycrystalline diamond bonded two a backing layer of less hard material, such as cemented tungsten carbide. Since the backing layer is of less hard material than the facing layer, the two layer arrangement of the cutting element provides a degree of self-sharpening since, in use, the less hard backing layerwears away more easilythanthehardercutting layer.

Such preform cutting elements are often in the form of a circulartabletofsubstantiallyconstant thickness or are derived from such tablets. For example, the elements may be sectors or segments of such circulartablets. Otherpolycrystalline diamond preforms comprise a unitary body or layer of polycrystalline diamond formed without a backing layer, and such elements may be thermally stable so thatthey may be incorporated in a matrix bodied bit during formation ofthe matrix.

In orderthatthe entire surface ofthe bottom ofthe hole being drilled is acted on by the cutting elements, the elements are located at different distances from the central axis of rotation of the drill bit. However, cutting elementsfurtherfrom the axis of rotation, and nearerthe gauge region, move more rapidly relative to the formation than elements nearerthe axis of rotation and the annular area of formation swept by each such cutting element is greater. As a result, cutting elements nearerthe gauge region tend to wear more rapidly than elements nearer the axis of rotation, and in orderto combat this it is often the practice to position more cutting elements nearerthe gauge region.

It is usual for all the preform cutting elements on a bit body to be of the same size, although sometimes cutting elements immediately adjacent the gauge region are slightly smallerthan elements overthe remainder of the bit body. For example, preform polycrystalline diamond cutting elements are often in the shape ofcirculartablets and in this case elements adjacent the gauge region sometimes have a segment removed so as to present a substantially straight edge to the formation.

It is also known, in bits using natural diamonds,to use different sizes of diamond at different locations on the bit body but, again, more diamonds are usually located nearerthe gauge region.

In these known arrangements where more cutting elements are positioned near the gauge region, each cutting element is often positioned to make a shallower cut in the formation than a cutting element disposed radially inwardly thereof, in a further endeavourto reduce wear. However, for a given type of formation there is generally an optimum depth of cut and this difference in depth of cut between inner and outer cutting elements means that the outer elements are not operating at the optimum depth of cutandarethusnotoperating at maximum efficiency.

The present invention setsouttoovercomethese problems by providing larger preform cutting elements nearer the gauge region of the bit, since larger elements can better cope with the higher rates ofweartowhichtheoutercutting elements are subject and, as a result, can be positioned to operate with an efficient depth of cut without the risk of early failure of the elements due to wear.

Accordingly, the invention provides a rotary drill bit for use in drilling or coring deep holes in subsurface formations, comprising a bit body having a leading face and a gauge region, a plurality of preform polycrystalline diamond cutting elements mounted at the leading face of the bit body, and a passage in the bit body communicating with a pluralityofopenings in the leading face ofthe bit body for supplying drilling fluid to said face for cooling and cleaning the cutting elements, at least certain of said cutting elements nearerthe gauge region ofthe bit body being ofgreaterheightthan other cutting elements disposed nearer the central axis of rotation of the bit.

In this context, the "height" of a cutting element is defined as the maximum dimension ofthe element in a direction generally perpendicularto the surface of the formation being cut or abraded by the cutting element, in use, and transverse to its direction of movement during normal forward rotation of the bit.

Thus, in the case of a circular preform the height of the preform will be its diameter.

Generally speaking, therefore, during the life of the bit, as the cutting elements become worn, greater wearwill be permitted on one of said outer cutting elements than on one of said elements disposed nearerthe central axis of rotation ofthe bit, thus preventing early failure ofthe bitdueto earlyfailure of the outer cutting elements.

While not essential to the broadest aspect ofthe invention,the height of each cutting element is preferably not less than the height of every cutting element disposed inwardly thereof with respect two the central axis of rotation of the bit. In otherwords, each cutting element is either ofthe same height, or of larger height, than a cutting element disposed inwardly thereof.

In a preferred embodiment, said cutting elements are substantially circular, at least certain ofthe cutting elements nearerthe gauge region ofthe bit being of greaterdiameterthan other cutting elements disposed nearer the central axis of rotation of the bit.

The leading surface of the bit body may be formed with a plurality of upstanding blades extending generally radially outwardly from the central axis of the bit, a number of cutting elements being mounted on each blade.

As previously mentioned, the bit body may be machined from metal, such as steel, or may be moulded using a powder metallurgy process.

Each preform polycrystalline diamond cutting element may be directly mounted on the bit body or may be mounted on a carrier, such as a stud or post, received in a socket in the bit body.

Each cutting element may comprise a hard facing layer of polycrystalline diamond bonded to a backing layerof less hard material, such as cemented tungsten carbide. Alternatively, each cutting element may comprise a unitary body of polycrystalline diamond.

The following is more a detailed description of an embodimentofthe invention, reference being made to the accompanying drawings in which: Figure 1 is an end view of a drill bit in accordance with the invention, and Figure2 is a longitudinal section of the drill bit along the line 2-2 of Figure 1.

Referring to the drawings, there is shown a rotary drill bit for use in drilling deep holes in subsurface formations comprising a bit body 10 having a leading face 11 and a gauge region 12.

The bit body is machined from steel and the leading face 11 is integrally formed with four blades 13,14,15 and 16. As best seen in Figure 1,thefour blades are spaced substantially equally around the central axis of rotation of the bit. Each blade has an outer surface 17 which, in use ofthe bit, faces the surface oftheformation being drilled, and a front surface 18, facing in the direction of normal forward rotation of the bit.

Three cutting elements 19,20 and 21 are mounted along each ofthe blades 14 and 16 and a single cutting element 22 is mounted on each of the blades 13 and 15. Each cutting element is in the form of a preform comprising afront hard facing layer of polycrystalline diamond material bonded to a less hard backing layer, such as tungsten carbide. Each cutting element may be bonded to a post 23,for example of tungsten carbide, received in a socket in the associated blade.

Each ofthe elements 20 and 21 is a completely circular preform, whereas the elements 19 are each semicircular, comprising one half of a circular preform of the same diameter as the preforms 20. As best seen in Figure 2, the preform cutting elements 22, instead of being ful ly circular, are each in the form of a circulartabletfrom which a segment has been removed.

The gauge region 12 ofthe bit comprises four axially extending kickers or wear pads 24 in which abrasion elements (not shown) are mounted, junk slots 25 being formed between adjacent kickers.

On the forward side of each ofthe blades 13,14,15 and 16 is mounted a nozzle 26 which is located in a socket in the bit body which communicates through a passage 27 (see Figure 2) with a central passage 28 in the bit body. Drilling fluid is supplied under pressure through the central passage 28 and emergesfrom the nozzles 27 forthe purpose of cleaning and cooling the cutting elements as well as cooling the formation.

As previously mentioned, in known drill bits ofthis general type, it has been the practice for all the cutting elements along each blade to be similar and, in particular, to be of substantially constant diameter, the cutting elements being closerto one another nearthe gauge region 12 of the bitto compensate for the greater wear of the cutting elements which occurs in this region.

In accordance with the present invention, it will be seen that the innermost cutting element 19 on each blade 14 or 16, being only semicircular, is of lesser height (as previously defined) than the outwardly adjacent element 20 which is fully circular. The next outwardly adjacent cutting 21 on each blade is of greater diameter than the cutter 20, and the single cutting element 22 on each ofthe blades 13 and 15, which is disposed immediately adjacent the gauge region ofthe drill bit, is of still greater diameter.

As previously defined, the height of each cutting element is its maximum dimension in a direction generally perpendicularto the surface ofthe formation and transverse to its direction of movement during normal forward rotation of the bit.

The surface of the formation is indicated diagrammatically at29 in Figure 2 and it will be seen that, according to the above definition, the height of each of the cutting elements 20 and 21 is equal to the diameter of the circular tablet whereas the height of each element 19 is half of the diameter of the element 20. In the case of the elements 22, the height of each element is slightly less than the diameter of the basic circular tablet, due to the removal of the segment, but it will be seen that the height of each eiement 22 is still greaterthan that of the elements 19,20,21 inwardly thereof, in accordance with the invention. It will be appreciated that the invention includes within its scope cutting elements of many different configurations, such as square, sector-shaped and segment-shaped elements.

Itwill be seen from Figure 1 thatthecutting elements are so located on the blades with respect to the central axis of rotation of the bit that the entire surface of the bottom of the hole is swept by at least one cutting element. Since the outer cutting elements are of greater height than elements inwardly thereof, they are better able to withstand the greaterwearwhich occurs with increasing distance from the axis of rotation while at the same time operating with sufficient depth of cut to act efficiently.

Although it is preferred, as shown,thatevery cutting element is of a height greater than that of elements disposed inwardlythereof,the invention does include within its scope arrangements in which some elements at different radial positions in which some elements at different radial positions are of equal height, or in which somesmallercutting elements are disposed outwardly in relation to larger cutting elements. That is to say, in the arrangement of Figures 1 and 2 the advantageous effect ofthe cutting elements positioned as illustrated would still be derived even though other, smaller cutting elements, might, for example, be disposed outwardly of the larger cutting elements 21. Also, for example, it might be possible for the elements 19 to befully circular and of equal diametertothe elements 20 without losing the advantages ofthe invention as embodied in the other, more outwardly located, cutting elements.

Claims (11)

1. A rotary drill bitfor use in drilling orcoring holes in subsurface formations, comprising a bit body having a leading face and a gauge region, a plurality of preform polycrystalline diamond cutting elements mounted at the leading face of the bit body, and a passage in the bit body communicating with a plurality of openings in the leading face ofthe bit body for supplying drilling fluid to said face for cooling and cleaning the cutting elements, at least certain of said cutting elements nearerthe gauge region ofthe bit body being of greater height than other cutting elements disposed nearer the central axis of rotation of the bit.

2. Arotarydrill bit according to Claim 1,wherein the height of each cutting element is not less than the height of every cutting element disposed inwardly thereofwith respect to the central axis of rotation of the bit.

3. A rotary drill bit according to Claim 1 orClaim 2, wherein said cutting elements are substantially circular, at least certain of the cutting elements nearer the gauge region of the bit being ofgreater diameter than other cutting elements disposed nearer the central axis of rotation of the bit.

4. A rotary drill bit according to any of Claims 1 to 3, wherein the leading surface of the bit body is formed with a plurality of upstanding blades extending generally radially outwardly from the central axis of the bit, a number of cutting elements being mounted on each blade.

5. A rotary drill bit according to any of Claims 1 to 4, wherein the bit body is machined from metal.

6. A rotary drill bit according to any of Claims 1 to 4, wherein the bit body is moulded using a powder metallurgy process.

7. A rotary drill bit according to any of Claims 1 to 6, wherein each preform poiycrystalline diamond cutting element is directly mounted on the bit body.

8. A rotary drill bit according to any of Claims 1 to 6, wherein each polycrystalline diamond cutting element is mounted on a carrier received in a socket in the bit body.

9. A rotary drill bit according to any of Claims 1 to 8, wherein each cutting element comprises a hard facing layer of polycrystalline diamond bonded to a backing layer of less hard material.

10. A rotary drill bit according to any of Claims 1 to 8, wherein each cutting element comprises a unitary body of polycrystalline diamond.

11. A rotary drill bit substantially as hereinbefore described with reference to the accompanying drawings.