GB2327443A - Drill bit with canted gage insert - Google Patents

Abstract

A rolling cone drill bit has gage inserts 30 on the first row from the bit axis to cut to full gage diameter. The gage inserts have a cutting portion enhanced with a layer (fig 4, 42) of super abrasive material. The gage cutting surface has a center axis 41 which is canted to be more normal to the gage curve 22 such that its point of contact at gage is away from the thinner portion of the layer of super abrasive material. The axis 41 of each gage cutter insert 30 is no longer perpendicular to the cone axis 13. The surface of the cone 31 around the insert is reshaped so that it remains perpendicular to the center axis of the insert 41.

Description

DRILL BIT WITH CANTED GAGE INSERT The present invention relates to earth-boring drill bits.

An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied by the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or "gage" of the drill bit.

A typical earth-boring bit includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cutters thereby engaging and disintegrating the formation material in its path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones. Such bits typically include a bit body with a plurality of journal segment legs. Each rolling cone is mounted on a bearing pin shaft that extends downwardly and inwardly from a journal segment leg. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones remove chips of formation material that are carried upward and out of the borehole by drilling fluid that is pumped downwardly through the drill pipe and out of the bit. The drilling fluid carries the chips and cuttings in a slurry as it flows up and out of the borehole. The earth disintegrating action of the rolling cone cutters is enhanced by providing the cutters with a plurality of cutter elements.

The cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. This process, known as a "trip" of the drill string, requires considerable time, effort and expense.

Accordingly, it is desirable to employ drill bits that will drill faster and longer and are usable over a wider range of formation hardnesses.

The length of time that a drill bit may be employed before it must be changed depends upon its rate of penetration (',ROP"), as well as its durability or ability to maintain an acceptable ROP. The form and positioning of the cutter elements on the cutters greatly impact bit durability and ROP and thus are critical to the success of a particular bit design.

Bit durability is, in part, measured by a bit's ability to "hold gage," meaning its ability to maintain a full gage borehole diameter over the entire length of the borehole.

Gage holding ability is particularly vital in directional drilling applications. If gage is not maintained at a relatively constant dimension, it becomes more difficult, and thus more costly, to insert drilling assemblies into the borehole than if the borehole had a constant full gage diameter. For example, when a new, unworn bit is inserted into an undergage borehole, the new bit will be required to ream the undergage hole as it progresses toward the bottom of the borehole. Thus, by the time it reaches the bottom, the bit may have experienced a substantial amount of wear that it would not have experienced had the prior bit been able to maintain full gage. This unnecessary wear will shorten the bit life of the newly-inserted bit, thus prematurely requiring the time-consuming and expensive process of removing the drill string, replacing the worn bit, and reinstalling another new bit downhole.

Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone.

Bits having tungsten carbide inserts are typically referred to as "TCI" bits, while those having teeth formed from the cone material are known as "milled tooth bits." In each case, the cutter elements on the rotating cutters functionally breakup the formation to form new borehole by a combination of gouging and scraping or chipping and crushing.

While the present invention is described by reference to bits having inserts rather than milled teeth, it is to be understood that the concepts disclosed herein can also be utilised in milled tooth bits.

In Figure 1 the positions of all of the cutter inserts from all three cones are shown rotated into a single plane.

As shown in Figure 1, to assist in maintaining the gage of a borehole, conventional rolling cone bits typically employ a row of heel cutters 14 on the heel surface 16 of each rolling cone 12. The heel surface 16 is generally frustoconical and is configured and positioned so as to generally align with the sidewall of the borehole as the bit rotates. The heel cutters 14 contact the borehole wall with a sliding motion and thus generally may be described as scraping or reaming the borehole sidewall. The heel cutters 14 function primarily to maintain a constant gage and secondarily to prevent the erosion and abrasion of the heel surface of the rolling cone.

In addition to heel row cutter elements, conventional bits typically include a row of gage cutter elements 30 mounted in gage surface 31 and oriented and sized in such a manner so as to cut the corner of the borehole. For purposes of the following discussion, the gage row is defined as the first row of inserts from the bit axis of a multiple cone bit that cuts to full gage. This insert typically cuts both the sidewall of the borehole and a portion of the borehole floor.

Cutting the corner of the borehole entails cutting both a portion of the borehole side wall and a portion of the borehole floor. It is also known to accomplish the corner cutting duty that is usually performed by the gage cutters by dividing it between adjacent gage and nestled gage cutters (not shown) such that the nestled gage cutters perform most of the sidewall cutting and the adjacent gage cutters cut the bottom portion of the corner.

Conventional bits also include a number of additional rows of cutter elements 32 that are located on the main, generally conical surface of each cone in rows disposed radially inward from the gage row. These inner row cutter elements 32 are sized and configured for cutting the bottom of the borehole and are typically described as inner row cutter elements.

In Figures 1, 3, 5 and 7, the positions of all of the cutter inserts from all three cones are shown rotated into a single plane. As can be seen, the cutter elements in the heel and gage rows typically share a common position across all three cones, while the cutter elements in the inner rows are radially spaced so as to cut the borehole floor in the desired manner. Excessive or disproportionate wear on any of the cutter elements can lead to an undergage borehole, decreased ROP, or increased loading on the other cutter elements on the bit, and may accelerate wear of the cutter bearing and ultimately lead to bit failure.

Relative to polycrystalline diamond, tungsten carbide inserts are very tough and impact resistant, but are vulnerable to wear. Thus, it is known to apply a cap layer of polycrystalline diamond (PCD) to each insert. The PCD layer is extremely wear-resistant and thus improves the life of a tungsten carbide insert. Conventional processing techniques have, however, limited the use of PCD coatings to axisymmetrical applications. For example, a common configuration for PCD coated inserts can be seen in Figures 1 and 2, wherein insert 30 comprises a domed tungsten carbide base or substrate 40 supporting a hemispherical PCD coating 42. Inserts of this type are formed by forming a nonreactive container also known as a "can", corresponding to the external shape of the insert, positioning a desired amount of PCD powder in the can, placing the substrate in the can on top of the PCD powder, enclosing and sealing the can, and applying sufficient pressure and temperature to sinter the PCD and adhere it to the substrate. If required, the resulting diamond or substrate layers can be ground into a final shape following demolding.

The shape of PCD layers formed in this manner is based on consideration of several factors. First, the difference in the coefficients of thermal expansion of diamond and tungsten carbide gives rise to differing rates of contraction as the sintered insert cools. This in turn causes residual stresses to exist in the cooled insert at the interface between the substrate and the diamond layer. If the diamond layer is too thick, these residual stresses can be sufficient to cause the diamond layer to break away from the substrate even before any load is applied. On the other hand, if the diamond layer is too thin, it may not withstand repetitive loading during operation and may fail due to fatigue. The edge 61 of the diamond coating is a particular source of stress risers and is particularly prone to failure.

For all of these reasons, PCD coated inserts have typically been manufactured with a hemispherical top, commonly referred to as a "semi-round top" or SRT.

Referring again to Figure 2, the SRT 103 is aligned with the longitudinal axis 41 of the substrate such that its center point lies approximately on axis 41. The inner surface of the diamond coating corresponds to the domed shape of the substrate. Thus, the thickness of the diamond coating is greatest on the axis of the insert and decreases toward the edge of the coating layer. While inserts in which the diamond coating is of uniform thickness are known, e.g. U.S. Patent No. 5,030,250, it is more common to form a diamond layer that decreases in thickness as distance from the center point increases, resulting in the crescent-shaped cross-section shown in Figure 2.

Nevertheless, it is contemplated that diamond layer 42 can be other than crescent-shaped. For example, the thickest portion of diamond layer 42 could comprise a region rather than a point. The diamond layer typically tapers toward the outer diameter of the substrate (the diamond edge 61).

This tapering helps prevent cracks that have been known to develop at the diamond edge when a substantially uniform diamond layer is used.

Because of the interrelationship between the shape of each cone and the shape of the borehole wall, cutter elements in the heel row and inner rows are typically positioned such that the longitudinal axes of those cutter elements are more or less perpendicular to the segment of the borehole wall (or floor) that is engaged by that cutter element at the moment of engagement. In contrast, cutter elements in the gage row do not typically have such a perpendicular orientation. This is because in prior art bits, the gage row cutter elements are mounted so that their axes are substantially perpendicular to the cone axis 13. Mounted in this manner, each gage cutter element engages the gage curve 22 at a contact point 43 (Figure 2) that is close to the thin edge of the diamond coating on the hemispherical top of each cutter element.

Still referring to Figure 2, the angle between the insert axis 41 and a radius terminating at contact point 43 is hereinafter designated a. In prior art bits, the angle a has typically been in the range of 540 to 750, with a being greater for harder formation types. For example, in a typical 12 " rock bit, a may be about 57".

The prior art configuration described above is not satisfactory, however, because contact point 43 is at the edge of diamond layer 42, where the diamond layer is relatively thin, and is subjected to particularly high stresses and is therefore especially vulnerable to cracking and breaking, which in turn leads to premature failure of the inserts in the gage row.

The present invention seeks to provide a drill bit having a gage insert which is more durable than those conventionally known.

According to the present invention there is provided an earth-boring drill bit for drilling a borehole of a predetermined gage, comprising a bit body having a bit axis, a plurality of rolling cone cutters, each rotatably mounted on a bit body about a respective cone axis and having a plurality of rows of cutting inserts thereon; one of the rows being a gage row with gage inserts, the gage inserts having a generally cylindrical base portion secured in the cone and defining an insert axis, and a cutting portion extending from the base portion, the cutting portion comprising a generally convex gage cutting surface with a centre axis which is inclined with respect to said cone axis.

The invention also extends to an earth-boring drill bit for drilling a borehole of a predetermined gage which comprises a bit body having a bit axis and a plurality of rolling cone cutters, each rotatably mounted on the bit body about a respective cone axis and having a plurality of rows of cutting inserts thereon. One of the rows is a gage row with gage inserts located such that it is the first row of inserts from the bit axis that cuts the predetermined gage and the borehole corner substantially unassisted. The gage inserts have a generally cylindrical base portion secured in the cone and defining an insert axis, and a cutting portion extending from the base portion. The cutting portion comprises a generally convex gage cutting surface with a center axis that is at an acute angle to the cone axis and at least a portion of the gage cutting surface is enhanced with a super abrasive material.

The present invention also relates to a drill bit in which the axis of the gage cutting surface of a gage insert is repositioned so that it is more normal to the gage curve and less normal to the cone axis whereby the angle alpha is decreased so that the contact point on the gage insert is farther from the edge of a diamond layer, thereby providing a thicker diamond layer at the contact point.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side schematic view of one leg and one rolling cone cutter of a known rolling cone bit; Figure 2 is an enlarged view of the gage insert of Figure 1; Figure 3 is a side schematic view of one leg and one rolling cone cutter of a rolling cone bit of an embodiment of the present invention; Figure 4 is an enlarged view of the gage insert of Figure 3; Figure 5 is a side schematic view of one leg and one rolling cone cutter of a rolling cone bit of a second embodiment of the present invention; Figure 6 is an enlarged view of the gage insert of Figure 5; Figure 7 is a side schematic view of one leg and one rolling cone cutter of a rolling cone bit of a further embodiment of the device of Figure 5; Figure 8 is an enlarged view of the gage insert of Figure 7; Figure 9 is a side schematic view of one leg and one rolling cone cutter of a rolling cone bit of a still further embodiment of the present invention; Figure 10 is an enlarged view of the gage insert of Figure 9; Figures 11 and 12 are side views of a diamond enhanced insert, showing one technique for constructing an insert having a canted diamond layer; and Figures 13 and 14 are side views of alternative axisymmetric diamond coated inserts which may be canted.

In Figures 1, 3, 5, 7 and 9, the positions of all of the cutter inserts from all three cones are shown rotated into a single plane.

Referring now to Figures 3 and 4, which show a first embodiment of the present invention, each gage cutter insert 30 is repositioned such that its axis 41 is no longer perpendicular to the cone axis 13. Instead, the axis 41 of each gage cutter insert is rotated around the center of its hemispherical top such that its base is shifted toward the tip of the cone 12 and its axis 41 is more normal to gage curve 22. Rotation in this manner has the desired effect of moving contact point 43 away from the edge 61 of diamond layer 42. Because the insert is rotated about the center of its hemispherical top, the gage curve 22 remains tangential to the surface of the insert and the cutting load is not altered.

Surface 31, which defines a land 35 around each insert, is reshaped so that it remains perpendicular to axis 41. Modification of surface 31 in this manner is preferred because it provides better support for each cutter and because it is generally easier to carry out the drilling and press-fitting manufacturing steps when the hole into which the insert is set is perpendicular to the land surface. Moreover, it allows all of the grip on base 40 to be maintained while also allowing the extension portion of cutter element 30 to be unchanged.

For example, axis 41 is rotated until the angle a is between Oc and SOC, and more preferably is no more than 400.

It would be preferable to reduce a to 0, if possible, but rotation of axis 41 is limited by geometry of the cone.

That is, either the clearance between the bottom of an insert in the gage row and an insert in the next, inner row becomes inadequate to retain the insert, or the holes for adjacent inserts run into each other. Thus, it is generally preferable to keep a in the range of about 250 to 550.

Referring now to Figures 5 and 6, each gage cutter insert 30 is reconfigured such that the center point of its diamond insert layer 42 no longer coincides with axis 41.

Instead, diamond layer 42 and the axisymmetric SRT cutting surface defined thereby are canted with respect to axis 41 such that the thickest portion of diamond layer 42 is closer to the gage curve 22. Canting the SRT 103 in this manner has the desired effect of moving contact point 43 away from the edge 61 of diamond layer 42. It is preferred but not necessary that the thickest portion of diamond layer 42 be between axis 41 and contact point 43.

Cone surface 31 is reshaped so that each land 35 remains aligned with the lower edge of the SRT. Thus, in this embodiment, surface 31 is no longer perpendicular to axis 41. Modification of surface 31 in this manner allows the amount of extension of insert 30 to remain unchanged.

While the hole into which insert 30 is pressfit is no longer perpendicular to surface 31, this method has the advantage of maintaining a larger clearance between the base of each gage insert and the bases of adjacent inserts.

In one embodiment, the center point of the diamond layer 42 is shifted until the angle (Figure 6), defined as the angle between axis 41 of insert 30 and a radius through the thickest portion of diamond layer 42, is at least 5", and more preferably at least 10 . It is not typically possible to cant the SRT by more than about 450. Canting the SRT results in a being reduced by an amount approximately equal to , so that o preferably ranges from about 25C to about 550.

When SRT 103, which extends outward from land 35, is canted, a wedge-shaped portion 101 is defined between SRT 103 and the cylindrical portion of base 40. Because both SRT 103 and the base portion 40 have circular cross-sections with substantially the same diameter, the outer surface of wedgeshaped portion 101 forms a transition between the surface of base 40 and the surface of SRT 103.

Referring now to Figures 7 and 8, an alternative embodiment of the insert shown in Figures 5 and 6 again comprises an insert having a canted SRT. In this embodiment, however, the outer surface of base 40 is maintained as a right cylinder and the geometry of the SRT is re-shaped so as to conform to the outer surface of base 40. Thus, the footprint of the diamond enhanced portion becomes an ellipse, rather than a circle, with its minor diameter equal to the diameter of base 40 and its major diameter equal to the diameter of base 40 divided by the cosine of 9 and cutting portion of insert 30 is no longer axisymmetric.

In the embodiment shown in Figures 9 and 10, concepts described with respect to Figures 3 to 8 above are combined. Thus, in this embodiment, the axis 41 of each gage cutter insert 30 is rotated around the center of its hemispherical top and each gage cutter insert 30 is reconfigured such that the center point of its diamond insert layer 42 no longer coincides with axis 41. Together these modifications preferably result in a reduction of a to a range of about 150 to about 450. For a typical 12 " rock bit, a may be about 290 in this embodiment.

Referring now to Figures 11 and 12, one technique for creating an insert having a canted diamond layer is to form an axisymmetric diamond-coated insert 70 having a cylindrical base 72. By cutting insert 70 on a plane 71 that forms an angle e with respect to a plane perpendicular to the axis of the insert 70, a top portion 74 is generated, as shown in Figure 11. When top portion 74 is rotated 180 and re-attached to base 72, it will be canted with respect to base 72 at an angle that is equal to 20.

Figures 13 and 14 illustrate a conical insert extension and a bullet-shaped extension, respectively.

Both of these axisymmetric shapes can be used in inserts having a diamond layer which is canted as described. It will be recognized that the conical insert of Figure 13 is conical only at the lower portion of its extension, its tip being rounded to form a curved cutting surface.

It will be understood that the concepts described above are applicable to diamond enhanced inserts in the gage row. Nevertheless, the principles disclosed herein can be applied to inserts in other rows, such as a nestled gage row, if the configuration of the cone and borehole wall would otherwise cause each insert in that row to contact the wall at a point that is close to the edge of its diamond layer. For example, if desired, the canted SRT can be used on inserts occupying what is sometimes referred to as the nestled gage row. Likewise these concepts can be used to advantage in inserts having a non-tapered diamond layer of uniform thickness. Such inserts tend to be prone to cracking near the edge of the diamond layer, so that moving the contact point away from the diamond edge results in a longer-lived insert.

Whilst various embodiments of the invention have been shown and described, it will be appreciated that modifications thereof can be made without departing from the scope of the invention as defined by the appended claims.

Claims (35)

1. An earth-boring drill bit for drilling a borehole of a predetermined gage, comprising: a bit body having a bit axis; a plurality of rolling cone cutters, each rotatably mounted on a bit body about a respective cone axis and having a plurality of rows of cutting inserts thereon; one of the rows being a gage row with gage inserts, the gage inserts having a generally cylindrical base portion secured in the cone and defining an insert axis, and a cutting portion extending from the base portion, the cutting portion comprising a generally convex gage cutting surface with a center axis which is inclined with respect to said cone axis.

2. A drill bit as claimed in Claim 1, wherein the insert axis is at an acute angle with respect to the cone axis.

3. A drill bit as claimed in Claim 1 or Claim 2 wherein the insert axis is aligned with the centre axis of the gage cutting surface.

4. A drill bit as claimed in any preceding claim, wherein the cutting portion is axisymmetric about the insert axis.

5. A drill bit as claimed in any preceding claim, wherein the cutting portion is generally hemispherical.

6. A drill bit as claimed in Claim 5, wherein the angle between the insert axis and the radius of the generally hemispherical cutting portion through its point of contact at gage is between about 0 and about 50 degrees.

7. A drill bit as claimed in Claim 6, wherein the angle is between about 25 degrees and about 40 degrees.

8. A drill bit as claimed in Claim 6, wherein the angle is between about 15 degrees and about 45 degrees.

9. A drill bit as claimed in any of Claims 1 to 3, wherein the cutting portion is non-axisymmetric about the insert axis.

10. A drill bit as claimed in Claim 9, wherein the gage cutting surface is axisymmetric about its center axis.

11. A drill bit as claimed in Claim 9 or Claim 10, wherein the gage cutting surface is generally hemispherical.

12. A drill bit as claimed in Claim 9 or Claim 10, wherein the gage cutting surface is generally conical.

13. A drill bit as claimed in Claim 9 or Claim 10, wherein the gage cutting surface is generally bullet-shaped.

14. A drill bit as claimed in Claim 11, wherein the angle between the insert axis and the radius of the generally hemispherical gage cutting surface through its point of contact at gage is between about 0 and about 50 degrees.

15. A drill bit as claimed in Claim 14, wherein the angle is between about 25 degrees and about 40 degrees.

16. A drill bit as claimed in Claim 14, wherein the angle is between about 15 degrees and about 45 degrees.

17. A drill bit as claimed in any preceding claim, wherein the gage row is located such that it is the first row of inserts from the bit axis which cuts the predetermined gage and the borehole corner substantially unassisted.

18. A drill bit as claimed in any preceding claim, wherein at least a portion of the gage cutting surface is enhanced with a super abrasive material.

19. A drill bit as claimed in Claim 18, wherein the super abrasive material comprises polycrystalline diamond.

20. A drill bit as claimed in Claim 18 or Claim 19, wherein the gage cutting surface is enhanced with a layer of the super abrasive material.

21. A drill bit as claimed in Claim 20, wherein the layer of the super abrasive material is of a varying thickness with a maximum thickness and a minimum thickness and the layer contacts gage where its thickness is closer to the maximum thickness than the minimum thickness.

22. A drill bit as claimed in Claim 21, wherein the crosssection of the layer of super abrasive material is generally crescent shaped.

23. A drill bit as claimed in any of Claims 20 to 22, wherein the cutting portion is fully capped by the layer of super abrasive material.

24. A drill bit as claimed in any of Claims 20 to 22, wherein the layer of super abrasive material has an edge and a centre, and wherein the layer contacts gage at a point closer to the centre than the edge.

25.N A cutting insert for use in an earth boring drill bit, comprising: a generally cylindrical base portion defining an insert axis; a cutting portion extending from the base portion and having a generally convex gage cutting surface, a centre axis of the gage cutting surface being canted with respect to the insert axis of the base portion.

26. A cutting insert as claimed in Claim 25, wherein the cutting portion is generally hemispherical.

27. A cutting insert as claimed in Claim 25 or Claim 26, wherein the centre axis is canted with respect to the insert axis by at least about 5 degrees.

28. A cutting insert as claimed in Claim 25 or Claim 26, wherein the centre axis is canted with respect to the insert axis by at least about 10 degrees.

29. A cutting insert as claimed in any of Claims 25 to 28, further comprising a wedge-shaped portion transitioning between the base portion and the cutting surface such that the cutting surface has a generally circular footprint.

30. A cutting insert as claimed in any of Claims 25 to 28, wherein the cutting surface has a generally elliptical footprint.

31. A cutting insert as claimed in any of Claims 25 to 30, wherein the cutting surface is enhanced with a super abrasive material.

32. A cutting insert as claimed in Claim 31, wherein the super abrasive material comprises polycrystalline diamond.

33. A method of making a cutting insert as claimed in Claim 25, comprising the steps of: making an insert with a generally cylindrical base portion defining an insert axis and a generally hemispherical cutting portion with an apex coincident with the insert axis; cutting the base portion at an oblique angle with respect to the insert axis to create a top that includes the cutting portion and some of the base portion and a bottom that includes the remainder of the base portion; rotating the top about 180 degrees with respect to the bottom about the insert axis; and attaching the top to the bottom to generally match the elliptical footprints of the top and the bottom.

34. An earth-boring drill bit substantially as hereinbefore described with reference to Figures 3 to 14 of the accompanying drawings.

35. A cutting insert for use in an earth-boring drill bit substantially as hereinbefore described with reference to Figures 3 to 14 of the accompanying drawings.