U.S. patent application number 14/458847 was filed with the patent office on 2015-02-19 for downhole cutting tools having rolling cutters with non-planar cutting surfaces.
The applicant listed for this patent is Smith International, Inc.. Invention is credited to Yuri Burhan, Chen Chen, Bala Durairajan, Youhe Zhang.
Application Number | 20150047910 14/458847 |
Document ID | / |
Family ID | 52466021 |
Filed Date | 2015-02-19 |
United States Patent
Application |
20150047910 |
Kind Code |
A1 |
Chen; Chen ; et al. |
February 19, 2015 |
DOWNHOLE CUTTING TOOLS HAVING ROLLING CUTTERS WITH NON-PLANAR
CUTTING SURFACES
Abstract
A cutting tool may include a tool body having a plurality of
blades extending radially therefrom; each of the plurality of
blades having a plurality of cutter pockets formed therein; and at
least one rotatable cutting element having a cutting table with a
convex cutting surface mounted in at least one of the plurality of
cutter pockets, wherein the at least one rotatable cutting element
is mounted in a nose or shoulder region of the at least one of the
plurality of blades.
Inventors: |
Chen; Chen; (Houston,
TX) ; Burhan; Yuri; (Spring, TX) ; Zhang;
Youhe; (Spring, TX) ; Durairajan; Bala;
(Houston, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Smith International, Inc. |
Houston |
TX |
US |
|
|
Family ID: |
52466021 |
Appl. No.: |
14/458847 |
Filed: |
August 13, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61865859 |
Aug 14, 2013 |
|
|
|
Current U.S.
Class: |
175/336 ;
175/374 |
Current CPC
Class: |
E21B 10/62 20130101;
E21B 10/573 20130101; E21B 10/55 20130101; E21B 10/5673
20130101 |
Class at
Publication: |
175/336 ;
175/374 |
International
Class: |
E21B 10/14 20060101
E21B010/14; E21B 10/08 20060101 E21B010/08 |
Claims
1. A cutting tool comprising: a tool body having a plurality of
blades extending radially therefrom; each of the plurality of
blades having a plurality of cutter pockets formed therein; and at
least one rotatable cutting element having a cutting table with a
convex cutting surface mounted in at least one of the plurality of
cutter pockets, wherein the at least one rotatable cutting element
is mounted in a nose or shoulder region of the at least one of the
plurality of blades.
2. The cutting tool of claim 1, wherein the convex cutting surface
is a convex region extending radially from the center of the
cutting element a distance, after which the cutting surface
surrounding the convex region transitions to form a concave
rim.
3. The cutting tool of claim 1, wherein the convex cutting table
has a ratio of extension at apex to cutting table base diameter
ranging from 0.015 to 0.21.
4. The cutting tool of claim 3, wherein the convex cutting table
has a ratio of extension at apex to cutting table base diameter
ranging from 0.025 to 0.065.
5. The cutting tool of claim 1, further comprising a plurality of
rotatable cutting elements mounted in the shoulder region of at
least one of the plurality of blades.
6. The cutting tool of claim 5, wherein the plurality of rotatable
cutting elements mounted in the shoulder region have a cutting
table with a planar and/or non-planar cutting surface disposed on a
substrate, wherein a portion of the substrate has a reduced
diameter that is disposed within a sleeve.
7. The cutting tool of claim 5, wherein the plurality of rotatable
cutting elements located in the nose region have a portion of the
substrate extending above the sleeve that is greater than the
portion of the substrate extending above the sleeve for rotatable
cutting elements located in the shoulder region.
8. The cutting tool of claim 1, further comprising a plurality of
non-rotatable cutting elements mounted on at least one of the
plurality of blades.
9. A cutting tool comprising: a tool body having a plurality of
blades extending radially therefrom; each of the plurality of
blades having a plurality of cutter pockets formed therein; and at
least one rotatable cutting element having a cutting table with a
concave cutting surface mounted in at least one of the plurality of
cutter pockets, wherein the at least one rotatable cutting element
is mounted in the shoulder of the at least one of the plurality of
blades.
10. The cutting tool of claim 9, further comprising a plurality of
rotatable cutting elements mounted in the nose region of at least
one of the plurality of blades.
11. The cutting tool of claim 10, wherein the plurality of
rotatable cutting elements mounted in the nose region have a planar
and/or non-planar cutting surface.
12. The cutting tool of claim 10, wherein the plurality of
rotatable cutting elements mounted in the nose region have a
cutting table with a convex cutting surface disposed on a
substrate, wherein a portion of the substrate has a reduced
diameter that is disposed within a sleeve.
13. The cutting tool of claim 12, wherein the cutting table with a
convex cutting surface has a ratio of extension at apex to cutting
table base diameter ranging from 0.015 to 0.21.
14. The cutting tool of claim 13, wherein the cutting table with a
convex cutting surface has a ratio of extension at apex to cutting
table base diameter ranging from 0.025 to 0.065.
15. The cutting tool of claim 10, wherein the plurality of
rotatable cutting elements located in the nose region have a
portion of the substrate extending above the sleeve that is greater
than the portion of the substrate extending above the sleeve for
the rotatable cutting elements located in the shoulder region.
16. A cutting tool comprising: a tool body having a plurality of
blades extending radially therefrom; and a plurality of rotatable
cutting elements, wherein the plurality of rotatable cutting
elements have differing cutting surface geometries based on their
radial position along the plurality of blades.
17. The cutting tool of claim 16, wherein the plurality of
rotatable cutting elements are located in the nose and shoulder
region of the blades.
18. The cutting tool of claim 17, wherein the rotatable cutting
elements in the nose have a convex cutting surface geometry while
the rotatable cutting elements in the shoulder have a concave
cutting surface geometry.
19. The cutting tool of claim 17, wherein the rotatable cutting
elements in the nose have a concave cutting surface geometry while
the rotatable cutting elements in the shoulder have a convex
cutting surface geometry.
20. The cutting tool of claim 17, wherein the plurality of
rotatable cutting elements located in the nose region have a
thicker substrate layer supporting the cutting surface than the
rotatable cutting elements located in the shoulder region.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Patent
Provisional Application No. 61/865,859, filed on Aug. 14, 2013,
which is herein incorporated by reference in its entirety.
BACKGROUND
[0002] Drill bits used to drill wellbores through earth formations
generally are made within one of two broad categories of bit
structures. Drill bits in the first category are generally known as
"roller cone" bits, which include a bit body having one or more
roller cones rotatably mounted to the bit body. The bit body is
generally formed from steel or another high strength material. The
roller cones are also generally formed from steel or other high
strength material and include a plurality of cutting elements
disposed at selected positions about the cones. The cutting
elements may be formed from the same base material as is the cone.
These bits are generally referred to as "milled tooth" bits. Other
roller cone bits include "insert" cutting elements that are press
(interference) fit into holes formed and/or machined into the
roller cones. The inserts may be formed from, for example, tungsten
carbide, natural or synthetic diamond, boron nitride, or any one or
combination of hard or superhard materials.
[0003] Drill bits of the second category are generally referred to
as "fixed cutter" or "drag" bits. This category of bits has no
moving elements but rather have a bit body formed from steel or
another high strength material and cutters (sometimes referred to
as cutter elements, cutting elements or inserts) attached at
selected positions to the bit body. For example, the cutters may be
formed having a substrate or support stud made of carbide, for
example tungsten carbide, and an ultra hard cutting surface layer
or "table" made of a polycrystalline diamond material or a
polycrystalline boron nitride material deposited onto or otherwise
bonded to the substrate at an interface surface.
[0004] An example of a conventional drag bit having a plurality of
cutters with ultra hard working surfaces is shown in FIG. 1a. A
drill bit 10 includes a bit body 12 and a plurality of blades 14
that are formed on the bit body 12. The blades 14 are separated by
channels or gaps 16 that enable drilling fluid to flow between and
both clean and cool the blades 14 and cutters 18. Cutters 18 are
held in the blades 14 at predetermined angular orientations and
radial locations to present working surfaces 20 with a desired
backrake angle against a formation to be drilled. generally, the
working surfaces 20 are generally perpendicular to the axis 19 and
side surface 21 of a cylindrical cutter 18. Thus, the working
surface 20 and the side surface 21 meet or intersect to form a
circumferential cutting edge 22.
[0005] Nozzles 23 are often formed in the drill bit body 12 and
positioned in the gaps 16 so that fluid can be pumped to discharge
drilling fluid in selected directions and at selected rates of flow
between the cutting blades 14 for lubricating and cooling the drill
bit 10, the blades 14, and the cutters 18. The drilling fluid also
cleans and removes the cuttings as the drill bit rotates and
penetrates the geological formation. The gaps 16, which may be
referred to as "fluid courses," are positioned to provide
additional flow channels for drilling fluid and to provide a
passage for formation cuttings to travel past the drill bit 10
toward the surface of a wellbore (not shown).
[0006] The drill bit 10 includes a shank 24 and a crown 26. Shank
24 is generally formed of steel or a matrix material and includes a
threaded pin 28 for attachment to a drill string. Crown 26 has a
cutting face 30 and outer side surface 32. The particular materials
used to form drill bit bodies are selected to provide adequate
toughness, while providing good resistance to abrasive and erosive
wear. For example, in the case where an ultra hard cutter is to be
used, the bit body 12 may be made from powdered tungsten carbide
(WC) infiltrated with a binder alloy within a suitable mold form.
In one manufacturing process the crown 26 includes a plurality of
holes or pockets 34 that are sized and shaped to receive a
corresponding plurality of cutters 18.
[0007] The combined plurality of surfaces 20 of the cutters 18
effectively forms the cutting face of the drill bit 10. Once the
crown 26 is formed, the cutters 18 are positioned in the pockets 34
and affixed by any suitable method, such as brazing, provides the
pockets 34 inclined with respect to the surface of the crown 26.
The pockets 34 are inclined such that cutters 18 are oriented with
the working face 20 at a desired rake angle in the direction of
rotation of the bit 10, so as to enhance cutting. It should be
understood that in another construction (not shown), the cutters
may each be substantially perpendicular to the surface of the
crown, while an ultra hard surface is affixed to a substrate at an
angle on a cutter body or a stud so that a desired rake angle is
achieved at the working surface.
[0008] An example cutter 18 is shown in FIG. 1b. The conventional
cutter 18 has a cylindrical cemented carbide substrate body 38
having an end face or upper surface 54 referred to herein as the
"interface surface" 54. An ultra hard material layer (cutting
layer) 44, such as polycrystalline diamond or polycrystalline cubic
boron nitride layer, forms the working surface 20 and the cutting
edge 22. A bottom surface 52 of the ultra hard material layer 44 is
bonded on to the upper surface 54 of the substrate 38. The bottom
surface 52 and the upper surface 54 are herein collectively
referred to as the interface 46. The top exposed surface or working
surface 20 of the cutting layer 44 is opposite the bottom surface
52. The cutting layer 44 conventionally has a flat or planar
working surface 20, but may also have a curved exposed surface,
that meets the side surface 21 at a cutting edge 22.
[0009] Generally speaking, the process for making a cutter 18
employs a body of tungsten carbide as the substrate 38. The carbide
body is placed adjacent to a layer of ultra hard material particles
such as diamond or cubic boron nitride particles and the
combination is subjected to high temperature at a pressure where
the ultra hard material particles are thermodynamically stable.
This results in recrystallization and formation of a
polycrystalline ultra hard material layer, such as a
polycrystalline diamond or polycrystalline cubic boron nitride
layer, directly onto the upper surface 54 of the cemented tungsten
carbide substrate 38.
[0010] One type of ultra hard working surface 20 for fixed cutter
drill bits is formed as described above with polycrystalline
diamond on the substrate of tungsten carbide, often known as a
polycrystalline diamond compact (PDC), PDC cutters, PDC cutting
elements, or PDC inserts. Drill bits made using such PDC cutters 18
are known generally as PDC bits. While the cutter or cutter insert
18 is often formed using a cylindrical tungsten carbide "blank" or
substrate 38 which is sufficiently long to act as a mounting stud
40, the substrate 38 may also be an intermediate layer bonded at
another interface to another metallic mounting stud 40.
[0011] The ultra hard working surface 20 is formed of the
polycrystalline diamond material, in the form of a cutting layer 44
(sometimes referred to as a "table") bonded to the substrate 38 at
an interface 46. The top of the ultra hard layer 44 provides a
working surface 20 and the bottom of the ultra hard layer cutting
layer 44 is affixed to the tungsten carbide substrate 38 at the
interface 46. The substrate 38 or stud 40 is brazed or otherwise
bonded in a selected position on the crown of the drill bit body 12
(FIG. 1a). As discussed above with reference to FIG. 1a, the PDC
cutters 18 are conventionally held and brazed into pockets 34
formed in the drill bit body at predetermined positions for the
purpose of receiving the cutters 18 and presenting them to the
geological formation at a rake angle.
[0012] Bits 10 using conventional PDC cutters 18 are sometimes
unable to sustain a sufficiently low wear rate at the cutter
temperatures generally encountered while drilling in abrasive and
hard rock. These temperatures may affect the life of the bit 10,
especially when the temperatures reach 700-750.degree. C.,
resulting in structural failure of the ultra hard layer 44 or PDC
cutting layer. A PDC cutting layer includes individual diamond
"crystals" that are interconnected. The individual diamond crystals
thus form a lattice structure. A metal catalyst, such as cobalt may
be used to promote recrystallization of the diamond particles and
formation of the lattice structure. Thus, cobalt particles are
often found within the interstitial spaces in the diamond lattice
structure. Cobalt has a different coefficient of thermal expansion
as compared to diamond. Therefore, upon heating of a diamond table,
the cobalt and the diamond lattice will expand at different rates,
causing cracks to form in the lattice structure and resulting in
deterioration of the diamond table.
[0013] It has been found by applicants that many cutters 18 develop
cracking, spalling, chipping and partial fracturing of the ultra
hard material cutting layer 44 at a region of cutting layer
subjected to the highest loading during drilling. This region is
referred to herein as the "cutting region" 56. The cutting region
56 encompasses the portion of the ultra hard material layer 44 that
makes contact with the earth formations during drilling. The
cutting region 56 is subjected to high magnitude stresses from
dynamic normal loading, and shear loadings imposed on the ultra
hard material layer 44 during drilling. Because the cutters are
generally inserted into a drag bit at a rake angle, the cutting
region 56 includes a portion of the ultra hard material layer near
and including a portion of the layer's circumferential edge 22 that
makes contact with the earth formations during drilling.
[0014] The high magnitude stresses at the cutting region 56 alone
or in combination with other factors, such as residual thermal
stresses, can result in the initiation and growth of cracks 58
across the ultra hard layer 44 of the cutter 18. Cracks of
sufficient length may cause the separation of a sufficiently large
piece of ultra hard material, rendering the cutter 18 ineffective
or resulting in the failure of the cutter 18. When this happens,
drilling operations may have to be ceased to allow for recovery of
the drag bit and replacement of the ineffective or failed cutter.
The high stresses, particularly shear stresses, may also result in
delamination of the ultra hard layer 44 at the interface 46.
[0015] In some drag bits, PDC cutters 18 are fixed onto the surface
of the bit 10 such that a common cutting surface contacts the
formation during drilling. Over time and/or when drilling certain
hard but not necessarily highly abrasive rock formations, the edge
22 of the working surface 20 that constantly contacts the formation
begins to wear down, forming a local wear flat, or an area worn
disproportionately to the remainder of the cutting element. Local
wear flats may result in longer drilling times due to a reduced
ability of the drill bit to effectively penetrate the work material
and a loss of rate of penetration caused by dulling of edge of the
cutting element. That is, the worn PDC cutter acts as a friction
bearing surface that generates heat, which accelerates the wear of
the PDC cutter and slows the penetration rate of the drill. Such
flat surfaces effectively stop or severely reduce the rate of
formation cutting because the conventional PDC cutters are not able
to adequately engage and efficiently remove the formation material
from the area of contact. Additionally, the cutters are generally
under constant thermal and mechanical load. As a result, heat
builds up along the cutting surface, and results in cutting element
fracture. When a cutting element breaks, the drilling operation may
sustain a loss of rate of penetration, and additional damage to
other cutting elements, should the broken cutting element contact a
second cutting element.
[0016] Additionally, another factor in determining the longevity of
PDC cutters is the generation of heat at the cutter contact point,
specifically at the exposed part of the PDC layer caused by
friction between the PCD and the work material. This heat causes
thermal damage to the PCD in the form of cracks which lead to
spalling of the polycrystalline diamond layer, delamination between
the polycrystalline diamond and substrate, and back conversion of
the diamond to graphite causing rapid abrasive wear. The thermal
operating range of conventional PDC cutters is generally
750.degree. C. or less.
SUMMARY
[0017] This summary is provided to introduce a selection of
concepts that are further described below in the detailed
description. This summary is not intended to identify key or
essential features of the claimed subject matter, nor is it
intended to be used as an aid in limiting the scope of the claimed
subject matter.
[0018] In one aspect, embodiments disclosed herein relate to a
cutting tool that includes a tool body having a plurality of blades
extending radially therefrom; each of the plurality of blades
having a plurality of cutter pockets formed therein; and at least
one rotatable cutting element having a cutting table with a convex
cutting surface mounted in at least one of the plurality of cutter
pockets, wherein the at least one rotatable cutting element is
mounted in a nose or shoulder region of the at least one of the
plurality of blades.
[0019] In another aspect, embodiments disclosed herein relate to a
cutting tool that includes a tool body having a plurality of blades
extending radially therefrom; each of the plurality of blades
having a plurality of cutter pockets formed therein; and at least
one rotatable cutting element having a cutting table with a concave
cutting surface mounted in at least one of the plurality of cutter
pockets, wherein the at least one rotatable cutting element is
mounted in the shoulder of the at least one of the plurality of
blades.
[0020] In yet another aspect, embodiments disclosed herein relate
to a cutting tool that includes a tool body having a plurality of
blades extending radially therefrom; and a plurality of rotatable
cutting elements, wherein the plurality of rotatable cutting
elements have differing cutting surface geometries based on their
radial position along the plurality of blades.
[0021] Other aspects and advantages of the claimed subject matter
will be apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0022] FIG. 1A shows a perspective view of a conventional fixed
cutter bit.
[0023] FIG. 1B shows a perspective view of a conventional PDC
cutter.
[0024] FIGS. 2 and 3 show an embodiment of a rolling cutter with a
convex cutting surface.
[0025] FIG. 4 illustrates the characteristic dimensions of a
cutting element with a convex cutting surface.
[0026] FIGS. 5 and 6 show an embodiment of a rolling cutter with a
concave cutting surface.
[0027] FIG. 7 shows an embodiment of a rolling cutter with a
cutting surface having a concave region and a convex region.
[0028] FIG. 8 shows a profile of a bit as it would appear with each
blade and each cutting elements rotated into a single rotated
profile.
[0029] FIG. 9 shows a cutting structure profile of a bit according
to embodiments disclosed herein.
[0030] FIG. 10 shows a comparison between the effective back rake
angle of a cutting element having a convex cutting surface versus a
planar cutting surface.
[0031] FIG. 11 shows the change in effective back rake angle as the
cutting depth of a cutting element having a convex cutting surface
is adjusted.
[0032] FIGS. 12 and 13 show profile views of a drill bit according
to embodiments disclosed herein.
[0033] FIG. 14 shows an exploded view of a cutting element assembly
according to embodiments of the present disclosure.
[0034] FIG. 15 shows a finite element analysis of a rotatable
cutting element having a planar cutting face.
[0035] FIG. 16 shows a finite element analysis of a rotatable
cutting element having a convex cutting surface.
DETAILED DESCRIPTION
[0036] In one or more aspects, embodiments disclosed herein relate
to downhole tools (including fixed cutter drill bits) using
rotatable cutting structures. In one or more aspects, embodiments
disclosed herein relate to downhole tools (including fixed cutter
drill bits) using rotatable cutting structures having non-planar
cutting faces. Specifically, embodiments disclosed herein relate to
improving the life of a drill bit (or other downhole tool) by
positioning such rotatable cutting elements in particular
arrangements on the drill bit.
[0037] Generally, rotatable cutting elements (also referred to as
rolling cutters) described herein allow at least one surface or
portion of the cutting element to rotate as the cutting elements
contact a formation. As the cutting element contacts the formation,
the cutting action may allow portion of the cutting element to
rotate around a cutting element axis extending through the cutting
element. Rotation of a portion of the cutting structure may allow
for a cutting surface to cut the formation using the entire outer
edge of the cutting surface, rather than the same section of the
outer edge, as observed in a conventional cutting element. The
following discussion describes various embodiments for a rotatable
cutting element; however, the present disclosure is not so limited.
One skilled in the art would appreciate that any cutting element
capable of rotating may be used with the drill bit or other cutting
tool of the present disclosure.
[0038] Non-Planar Cutting Surfaces
[0039] In addition to using rolling cutters, other strategies may
be employed to enhance the utilization of rolling cutters.
Specifically, in accordance with embodiments of the present
disclosure, one or more of the rotatable cutting elements may have
non-planar cutting surfaces. In more particular embodiments, the
cutting surface of the rotatable cutting elements may be
substantially convex or concave. FIGS. 2-3 and FIGS. 5-6 show
representative embodiments of cutting elements with a substantially
convex or concave cutting surface, respectively. In addition to use
of non-planar cutting surfaces (and the extent of non-planarity),
the present disclosure also relates to the placement of such
non-planar cutting surfaces along a blade.
[0040] FIGS. 2 and 3 illustrate an embodiment of a rotatable
cutting element. As shown in FIGS. 2-3, a rotatable cutting element
300 possesses a diamond (or other ultrahard material) body 305
disposed on a substrate 301. The diamond body 305 has convex upper
or cutting surface 306 with an axial apex 307 extending a height
from the plane (extending perpendicular to an axis of the cutting
element 300) through cutting edge 304. Edge 304 is defined as the
edge formed between a circumferential side surface 302 and cutting
surface 306. Extending away from diamond table 305, substrate 301
includes a shaft portion 308, smaller in diameter than the portion
of the substrate 301 interfacing the diamond table 305. Further,
the incorporation of a shaft 308 may be particularly desirable for
a rotatable cutting element that is assembled with a sleeve;
however, the present disclosure is not so limited. Rather,
rotatable cutting elements used without sleeves may also be used,
and in such cases, the substrate may, but not necessarily, be
designed without a shaft having a reduced diameter. Further, it is
also within the scope of the present disclosure that a sleeve may
be used without a shaft portion. Thus, the present disclosure does
not limit the type of rolling cutter used, but instead is directed
to the shape of the cutting surface, and placement of the cutting
elements having such non-planar cutting surfaces on a downhole
cutting tool.
[0041] Referring to FIG. 4, a convex cutting surface 306 is shown.
Diamond table 305 has a base 303 forming the circumferential side
surface with a diameter D. Diamond table 305 extends axially away
from base 303 to convex cutting surface 306 terminating at an apex
307. Apex 307 is spaced a distance X from base (measured from the
plane extending through cutting edge 304). Incorporation of a
convex cutting surface may change the effective angle between the
cutting element and formation (i.e. effective backrake) as the
cutting element cuts into the formation. Specifically, such angle
may change depending on the depth of cut of the cutting element
into the formation. Thus, the inventors of the present application
have determined that a particular range of convexity results in an
advantageous effect on the impact strength of the cutting element.
The convexity may be considered as the ratio of X/D, i.e., the
axial rise of the cutting surface 306 to its apex 307 relative to
the size (diameter) of the diamond table 305. In one or more
embodiments, the ratio of X/D may range from 0.015 to 0.21. In more
specific embodiments, the ratios of x/d for the cutting elements
with convex cutting surfaces may range from 0.02 to 0.1, or from
0.025 to 0.065. In one or more other embodiments, the ratio of X/D
may have a lower limit of any of 0.015, 0.017, 0.020, 0.022, 0.025,
0.026, 0.03, or 0.04 and an upper limit of any of 0.21, 0.20, 0.15,
0.10, 0.075, 0.065, 0.06, 0.055 or 0.05 where any lower limit can
be used with any upper limit. The extension may reduce the stress
within the cutting element when the cutting element is subjected to
a frontal load. However, too much convexity results in higher
thermal residual stresses. The inventors of the present application
have found that a simultaneous reduction in stress under frontal
loading as well as thermal residual stress may be achieved when the
convexity is within the above described ranges for the X/D ratio.
Such reduction in stress may result in a cutting element that is
better suited for high impact applications as compared to a
non-planar cutting element or other shapes falling outside of this
ratio range.
[0042] The values for the diameter D of the base 303 of the diamond
table 305 may be in the range from 0.5 inches to 0.65 inches. The
values for the distance X, or extension height of the cutting
table, may be in the range from 0.005 inches to 0.25 inches. One
skilled in the art would appreciate that depending on the desired
convexity (and the size of cutting element desired for the tool
design), an appropriate extension height of the cutting table may
be selected. Convexity can also be considered as radius of
curvature; however, like the extension values, radius of curvature
also does not consider the size of the cutting element. The radius
of curvature of the cutting element may range, for example, from
0.375 to 4 inches, or from 1.0 to 3.8 inches. Further, in
particular embodiments, the convex cutting surface 306 may have a
substantially constant radius of curvature along the entire cutting
surface 306 from cutting edge 304 to apex 307.
[0043] The present disclosure also relates to the use of rotatable
cutting elements having concave cutting surfaces. Referring now to
FIGS. 5 and 6, another embodiment of a rotatable cutting element
having a non-planar cutting surface is shown. As shown in FIGS. 5
and 6, a rotatable cutting element 500 includes a diamond (or other
ultrahard material) table 505 disposed on a substrate 501. The
diamond body 505 has concave upper or cutting surface 506 with an
axial extremity (lowest point) 507 extending below the plane
through cutting edge 504. Thus, concave cutting surface 506 results
in a decreasing diamond table 505 thickness extending inwardly from
the outer radial or cutting edge 504 to the extremity 507, which
may (or may not be) near the radial center of the cutting element.
Extending away from diamond table 505, substrate 501 may include a
shaft portion 508, smaller in diameter than the portion of the
substrate 501 interfacing the diamond table 505. Further, the
incorporation of a shaft 508 may be particularly desirable for a
rotatable cutting element that is assembled with a sleeve; however,
the present disclosure is not so limited. Rather, rotatable cutting
elements used without sleeves may also be used, and in such cases,
the substrate may, but not necessarily, be designed without a shaft
having a reduced diameter. Further, it is also within the scope of
the present disclosure that a sleeve may be used without a shaft
portion. In a more particular embodiment, the rotatable cutting
element with a substantially concave cutting surface possesses a
substantially uniform radius of curvature along the concave cutting
surface 506 from cutting edge 504 to extremity 507. Advantageously,
the concave cutting surface may result in a sharper cutting edge,
which may provide for better cutting efficiency compared to a
planar cutting surface. The ratio of the depth of the concavity to
the diameter of the cutter may range from 0.01 to 0.25, with a
lower of any of 0.01, 0.025, 0.05, and 0.1 and an upper limit of
any of 0.1, 0.125, 0.15, 0.2, 0.225, and 0.25, where any lower
limit can be used in combination with any upper limit.
[0044] Further, the present disclosure also may incorporate a
rotatable cutting element having a variable non-planar cutting
surface, having both convex and concave portions. Referring now to
FIG. 7, an embodiment of such a cutting element is sown. As shown,
cutting surface 706 of rotatable cutting element 700 may have a
concave region 762 extending radially inward from a cutting edge
704 a selected distance which transitions to a convex region 764
extending radially inward from concave region 762 to the axis of
the cutting element 700. In one or more embodiments, the concave
region 762 may extend a distance of about 0.005 inches to 0.3
inches radially inward from cutting edge 704 before transitioning
to a convex region 764. Selection of the inward extension distance
of the concave region 762 before transitioning to a convex region
764 may be based upon the desired cutting efficiency as well depth
of cut control (the transition to the convex region 764 may dictate
the depth of cut into the formation). In one or more embodiments,
the apex 707 of the convex region 764 may extend above the axial
extent of the cutting edge 704. However, in one or more embodiments
the apex 707 of the convex region 764 may be at substantially the
same axial height as the cutting edge 704 or may be at a lower
axial height relative to the cutting edge 704.
[0045] Placement of Rolling Cutters
[0046] According to embodiments of the present disclosure, a bit
design consideration may include placement of rolling cutters on a
drill bit. Placement design of rolling cutters on a drill bit may
involve, first, predicting where conventional cutter (fixed cutter)
impact damage occurs most frequently or quickly on a drill bit. For
example, fixed cutter wear may be predicted using engineering and
design software, such as I-DEAS, "Integrated Design and Engineering
Analysis Software", or CAD software. Such engineering and design
software may also be used to optimize bit stabilization dynamics
using various placements of rolling cutters. Fixed cutter impact
damage may also be predicted by observing and/or measuring impact
damage on dull drill bits. In particular, as a drill bit having
conventional, fixed cutters contacts and cuts an earthen formation,
the cutting surface and cutting edge of a fixed cutter may wear and
form a wear flat.
[0047] Once fixed cutter impact damage is predicted, criteria for
the placement of rolling cutters having a non-planar cutting
surface may be set according to where the impact damage occurs. For
example, according to embodiments of the present disclosure,
rolling cutter placement design may include placing rolling cutters
having a convex cutting surface in a place where impact damage and
wear occurs. Further, in other embodiments, rolling cutter
placement design may include replacing cutters (rolling or not)
having planar cutting surfaces with rolling cutters having
non-planar cutting surfaces on certain blades of a drill bit.
[0048] In some embodiments of the present disclosure, rolling
cutter placement design criteria may be set so that rolling cutters
are positioned in the areas of the bit experiencing the greatest
wear. For example, rolling cutters may be placed in the shoulder
region of a drill bit. Referring to FIG. 8, a profile 39 of a bit
10 is shown as it would appear with each blade and each cutting
element (including primary cutting elements and back up cutting
elements) rotated into a single rotated profile. A blade profile 39
(most clearly shown in the right half of bit 10 in FIG. 8) may
generally be divided into three regions conventionally labeled cone
region 24, shoulder region 25, and gage region 26. Cone region 24
comprises the radially innermost region of bit 10 (e.g., cone
region 24 is the central most region of bit 10) and composite blade
profile 39 extending generally from bit axis 11 to shoulder region
25. As shown in FIG. 8, in most fixed cutter bits, cone region 24
is generally a concave portion of the blade. Adjacent cone region
24 is shoulder (or the upturned curve) region 25. Thus, composite
blade profile 39 of bit 10 includes one concave region of the
blade--cone region 24, and one convex region of the blade--shoulder
region 25. In most fixed cutter bits, shoulder region 25 is
generally the convex region of the blade. Moving radially outward,
adjacent shoulder region 25 is the gage region 26 which extends
parallel to bit axis 11 at the outer radial periphery 23 of
composite blade profile 39. Outer radius 23 extends to and
therefore defines the full gage diameter of bit 10. Cone region 24
is defined by a radial distance along the x-axis measured from
central axis 11. It is understood that the x-axis is perpendicular
to central axis 11 and extends radially outward from central axis
11. Cone region 24 may be defined by a percentage of outer radius
23 of bit 10. The actual radius of cone region 24, measured from
central axis 11, may vary from bit to bit depending on a variety of
factors including without limitation, bit geometry, bit type,
location of one or more secondary blades, location of back up
cutting elements 50, or combinations thereof. The axially lowermost
point of the convex shoulder region 25 and composite blade profile
39 defines a blade profile nose 27. At blade profile nose 27, the
slope of a tangent line 27a to convex shoulder region 25 and
composite blade profile 39 is zero. Thus, as used herein, the term
"blade profile nose" refers to the point along a convex region of a
composite blade profile of a bit in rotated profile view at which
the slope of a tangent to the composite blade profile is zero. For
most fixed cutter bits (e.g., bit 10), the composite blade profile
includes a single convex shoulder region on a blade (e.g., convex
shoulder region 25), and a single blade profile nose (e.g., nose
27). Advantageously, by placing rolling cutters with non-planar
cutting surfaces in areas of the bit experiencing the greatest wear
and load, for example at the shoulder region 26 and nose region 27
of a bit, the wear rate of the bit may be improved.
[0049] As discussed above, in one or more embodiments, depending on
the positioning of the rotatable cutting element along the blade,
the cutting surface geometry may be selected. While the non-planar
cutting surface roller cutters may be used in any location, in one
or more particular embodiments, at least one rotatable cutting
element with a convex cutting surface may be located in the cone
and/or nose of at least one of the blades on the bit.
Advantageously, the convex cutting surface geometry presents a
rotatable cutter with a higher impact resistance thereby making it
suitable for placement in the nose of the bit where the load is
higher on the cutters therein and greater impact resistance is
desirable. Further, in one or more embodiments, at least one
rotatable cutting element with concave cutting surface may be
located in the shoulder of at least one of the blades on the bit.
Such cutting elements located in the shoulder may benefit from a
greater cutting efficiency based on the greater scraping distance
(per bit revolution) experienced by those cutters, as compared to
more radially inward cutting elements. It is also within the scope
of the present disclosure that rotatable cutters with a any type of
non-planar cutting surface may be used in combination with
rotatable cutters having a planar surface and/or conventional
non-rotatable cutters. For example, in one or more embodiments,
rotatable cutting elements with a convex surface may be used in the
nose region, while non-rotatable cutting elements are used in the
cone and gage regions. In such embodiments, the cutters in the
shoulder region may be rotatable and have any type of cutting
surface.
[0050] Additionally, in one or more embodiments, rotatable cutting
elements with a concave surface may be used in shoulder nose
region, while non-rotatable cutting elements are used in the cone
and gage regions. In such embodiments, the cutters in the nose
region may be rotatable and have any type of cutting surface.
[0051] When placed on a blade, rotatable cutting elements (as well
as fixed cutters) may be oriented with a back rake and/or side
rake. Referring to FIG. 9, a cutting structure profile of a bit is
shown to aid in the understanding of the side rake and back rake of
cutting elements. As shown, cutters 2600 positioned on a blade 2602
may have side rake or back rake. Back rake is generally defined as
the angle subtended between the cutting face of the cutter 2600 and
a line parallel to the longitudinal axis 2607 of the bit. However,
for a non-planar cutting surface, the angle would upon where along
the cutting surface from which the angle is being taken. Thus, for
non-planar cutting elements, the plane extending through the
cutting edge (discussed above with respect to the measure of
convexity or concavity) may be used to determine the orientation of
the cutting element on the blade. Side rake is generally defined as
the angle between the cutting face and the radial plane of the bit
(x-z plane). For a non-planar cutting surface, the side rake may be
defined based on the plane 2605 extending through the cutting edge
(as discussed above). When viewed along the z-axis, a negative side
rake results from counterclockwise rotation of the cutter 2600, and
a positive side rake, from clockwise rotation.
[0052] In some embodiments, a planar cutter may have a back rake
ranging from about 5 to 35 degrees. In a particular embodiment, the
back rake angle of a rolling and/or fixed cutter with a planar
cutting surface may be >5 degrees, >10 degrees, >15
degrees, >20 degrees, >25 degrees, >30 degrees, and/or
<10 degrees, <15 degrees, <20 degrees, <25, <30
degrees, <35 degrees, with any upper limit being used with any
lower limit. Such back rake angles may be used for rolling and/or
fixed cutters in any of the cone, nose, shoulder or gage region of
the bit, but in particular embodiments, a back rake of between 10
and 35 degrees (or 15 to 35 degrees or 20 to 30 degrees in more
particular embodiments) may be particularly suitable for rolling
cutters in the nose and/or shoulder region of the bit. A cutter may
be positioned on a blade with a selected back rake to assist in
removing drill cuttings and increasing rate of penetration.
[0053] A cutter disposed on a drill bit with side rake may be
forced forward in a radial and tangential direction when the bit
rotates. In some embodiments, because the radial direction may
assist the movement of a rotatable cutting element, such rotation
may allow greater drill cuttings removal and provide an improved
rate of penetration. In one embodiment, a cutter may have a side
rake ranging from 0 to .+-.45 degrees, for example .+-.5 to .+-.35
degrees, .+-.10 to .+-.35 degrees or .+-.15 to .+-.30 degrees. In a
particular embodiment, the direction (positive or negative) of the
side rake may be selected based on the cutter distribution, i.e.,
whether the cutters are arranged in a forward or reverse spiral
configuration. For example, in embodiments, if cutters are arranged
in a reverse spiral, positive side rake angles may be particularly
desirable. Conversely, if cutters are arranged in a forward spiral,
negative side rake angles may be particularly desirable.
[0054] One of ordinary skill in the art may realize that any back
rake and side rake combination may be used with the cutting
elements of the present disclosure to enhance rotatability and/or
improve drilling efficiency. In one or more other embodiments,
cutting elements may be disposed in cutting tools that do not
incorporate back rake and/or side rake. When the cutting element is
disposed on a drill bit with substantially zero degrees of side
rake and/or back rake, the cutting force may be random instead of
pointing in one general direction. The random forces may cause the
cutting element to have a discontinuous rotating motion. Generally,
such a discontinuous motion may not provide the most efficient
drilling condition, however, in certain embodiments, it may be
beneficial to allow substantially the entire cutting surface of the
insert to contact the formation in a relatively even manner. In
such an embodiment, other inner rotatable cutting element and/or
cutting surface designs may be used to further exploit the benefits
of rotatable cutting elements. Further, in one or more other
embodiments, a bevel or chamfer size, angle, or design may be
selected to accommodate for a zero back or side rake.
[0055] As mentioned above, use of a non-planar cutting surface
changes the angle that the cutting surface makes with respect to
the formation (and changing the effective back rake angle of the
cutter from the perspective of the formation). Moreover, because of
the non-planarity, the angle also may also change depending on the
depth of cut of the cutting element into the formation. Referring
to FIG. 10, the depiction of the change in effective back rake
angle for a convex cutting surface is shown. As shown in FIG. 10, a
back rake of angle .beta. is formed between a line normal to the
formation 172 and the plane 174 extending through the cutting edge
304 (which would be the back rake of a cutting element with a
planar cutting surface). However, angle .alpha. is defined between
the line normal to the formation 172 and the tangent 176 of convex
cutting surface 306 at the cutting edge 304, and angle .alpha. is
clearly greater than angle .beta..
[0056] As also mentioned above, the effective back rake may also
change depending on the depth of cut into the formation. Referring
now to FIG. 11, such change is illustrated. Specifically, as
illustrated in FIG. 11, the impact on the effective back rake angle
with respect to the formation is dependent on the depth of cut
experienced by the cutting element. Specifically, as the cutting
element digs deeper into the formation, the leading point of the
cutting element that interfaces the formation will dictate the
effective back rake angle. Thus, as shown in FIG. 11, for a cutting
element with a convex cutting surface 306, as the depth of cut
(DOC) 602 increases from 0.05 to 0.10 to 0.15 to 0.2645 inches, the
observed effective back rake angle (defined as being the angle
between the line parallel to the longitudinal axis of the bit and a
line between the cutting edge 304 and the leading point of the
cutting element that interfaces the formation) correspondingly
decreases. Such decreasing angle is shown as the angles formed
between lines 601, 603, 605, and 607 with the line 606 parallel to
the longitudinal axis of the bit. However, because the effective
back rake angle with respect to the formation is dependent upon
both cutter orientation, non-planarity, and depth of cut, it may be
more simplistic to refer to back rake based on cutter orientation
and also contemplate a non-planar angle for the cutting element.
Such non-planar angle may be defined as the angle formed between
the plane extending through the cutting edge 304 and the tangent to
the convex cutting surface 306 at the cutting edge 304. In one or
more embodiments, such non-planar angle may be at least 2, 3, 4 or
5 degrees and less than 10, 9, 8, 7, 6, or 5 degrees, where any
lower limit can be used in combination with any upper limit.
[0057] According to embodiments of the present disclosure, rolling
cutter placement design criteria may be set so that rolling cutters
and fixed cutters on a drill bit have a plural set configuration.
Drill bits having a plural set configuration have more than one
cutting element at at least one radial position with respect to the
bit axis. Expressed in another way, at least one cutting element
includes a "back up" cutting element disposed at about the same
radial position with respect to the bit axis. For example,
referring to FIGS. 12 and 13, a face side profile view of a drill
bit 2400 having a plurality of cutting blades 2410 are shown,
wherein the bits rotate in direction R. Primary blades 2410a extend
radially from substantially proximal the longitudinal axis A of the
bit toward the periphery of the bit. Secondary blades 2410b do not
extend from substantially proximal the bit axis A, but instead
extend radially from a location that is a distance away from the
bit axis A. Cutting elements 2420, 2430 are positioned at the
leading side of blades 2410, wherein the leading sides of blades
2410 face in the direction of bit rotation R and trailing sides of
blades face the opposite direction. Further, as shown, cutting
element 2420 trails cutting element 2430 in plural set
configuration, i.e., cutting element 2420 "backs up" cutting
element 2430 at about the same radial position with respect to the
bit axis A. Either cutting element 2420 or cutting element 2430, or
both cutting elements 2420 and 2430, may be rolling cutters. In a
particular embodiment, a bit having a plural set cutter
configuration may have at least one trailing or backup cutting
element that is rotatable (a rolling cutter) and at least one
leading or primary cutting element that is a fixed cutter. In
another embodiment, a bit having a plural set configuration may
have at least one fixed cutter trailing cutting element and at
least one rolling cutter leading cutting element. Advantageously,
by using a plural set configuration having at least one rolling
cutter, the cutting structure may be more robust.
[0058] Further, a bit may have a single set configuration of
cutting elements, wherein each cutting element in a single set
configuration is at a unique radial position of the bit. In
embodiments having a single set configuration, a plurality of
rolling cutters may be placed at various unique radial positions
with respect to the bit axis. For example, a plurality of rolling
cutters may have a forward spiral or a reverse spiral single set
configuration, wherein the rolling cutters are placed in areas
experiencing wear. As used herein, a forward spiral layout refers
to a cutter placement where cutters having incrementally increasing
radial distances from the bit centerline are placed in a clockwise
distribution whereas a reverse spiral layout refers to a cutter
placement where cutters having incrementally increasing radial
distances from a bit centerline are placed in a counterclockwise
distribution. In some embodiments, the cutters may be placed in a
forward spiral, where rotatable cutters are at least placed in the
nose and/or shoulder region, are placed in the nose, shoulder, and
gage regions in particular embodiments, and are placed in the cone,
nose, shoulder, and gage regions in more particular embodiments. In
some embodiments, the cutters may be placed in a reverse spiral,
where rotatable cutters are at least placed in the nose and/or
shoulder region, are placed in the nose, shoulder, and gage regions
in particular embodiments, and are placed in the cone, nose,
shoulder, and gage regions in more particular embodiments.
[0059] Additionally, leading and trailing cutting elements may be
placed on a single blade. However, as used herein, the term "backup
cutting element" is used to describe a cutting element that trails
any other cutting element on the same blade when the bit is rotated
in the cutting direction. Further, as used herein, the term
"primary cutting element" is used to describe a cutting element
provided on the leading edge of a blade. In other words, when a bit
is rotated about its central longitudinal axis in the cutting
direction, a "primary cutting element" does not trail any other
cutting elements on the same blade. Suitably, each primary cutting
elements and optional backup cutting element may have any suitable
size and geometry. Primary cutting elements and backup cutting
elements may have any suitable location and orientation and may be
rolling cutters or fixed cutters. In an example embodiment, backup
cutting elements may be located at the same radial position as the
primary cutting element it trails, or backup cutting elements may
be offset from the primary cutting element it trails, or
combinations thereof may be used.
[0060] In particular, each blade on a bit face (e.g., primary
blades and secondary blades) provides a cutter-supporting surface
to which cutting elements are mounted. Primary cutting elements may
be disposed on the cutter-supporting surface of the blades and one
or more of the primary blades may also have backup cutting elements
disposed on the cutter-supporting surface of the bit. In one or
more embodiments, backup cutting elements may be provided on the
cutter-supporting surface of one or more of the bit primary blades
in the cone region. In a different example embodiment, backup
cutting elements may be provided on the cutter-supporting surface
of any one or more secondary blades in the shoulder and/or gage
region. In another example embodiment, backup cutting elements may
be provided on the cutter-supporting surface of any one or more
primary blades in the gage region. In yet another example
embodiment, the primary and/or secondary blades may have at least
two rows of backup cutting elements disposed on the
cutter-supporting surfaces.
[0061] Primary cutting elements may be placed adjacent one another
generally in a first row extending radially along each primary
blade of a bit and along each secondary blade of a bit. Further,
backup cutting elements may be placed adjacent one another
generally in a second row extending radially along each primary
blade in the shoulder region. Suitably, the backup cutting elements
form a second row that may extend along each primary blade in the
shoulder region, cone region and/or gage region. Backup cutting
elements may be placed behind the primary cutting elements on the
same primary blade, wherein backup cutting elements trail the
primary cutting elements on the same primary blades.
[0062] In general, primary cutting elements as well as backup
cutting elements do not have to be positioned in rows, but may be
mounted in other suitable arrangements provided each cutting
element is either in a leading position (e.g., primary cutting
element) or a trailing position (e.g., backup cutting element).
Examples of suitable arrangements may include without limitation,
rows, arrays or organized patterns, randomly, sinusoidal pattern,
or combinations thereof. Further, in other embodiments, additional
rows of cutting elements may be provided on a primary blade,
secondary blade, or combinations thereof.
[0063] Further, in a particular embodiment, a bit may have cutting
elements placed in a single set configuration with rolling cutters
with non-planar cutting surfaces placed in areas of the bit
experiencing the greatest wear. In another embodiment, a bit may
have cutting elements placed in a plural set configuration, wherein
at least one rolling cutter with a non-planar cutting surface is
placed in areas of the bit experiencing the greatest wear.
[0064] Embodiments of Rolling Cutters
[0065] Rolling cutters of the present disclosure may include
various types and sizes of rolling cutters. For example, rolling
cutters may be formed in sizes including, but not limited to, 9 mm,
13 mm, 16 mm, and 19 mm. Further, the type of rolling cutter is of
no limitation to the present disclosure. Rather, it may be of any
type and/or include any feature such as those described in U.S.
Pat. No. 7,703,559, U.S. Patent Publication Nos. 2011/0297454,
2012/0273280, 2012/0273281, 2012/0273281, and 2014/0054094, all of
which are assigned to the present assignee and herein incorporated
by reference in their entirety. Embodiments of rolling cutters are
also described below; however, the types of rotatable cutting
elements that may be used with the present disclosure are not
necessarily limited to those described below.
[0066] Referring now to FIG. 14, a rotatable cutting element
assembly according to embodiments of the present disclosure is
shown. Particularly, an exploded view of the cutting element is
shown in FIG. 14, including a rolling cutter 300, a retaining ring
320, and a sleeve 330. The rolling cutter 300 has an axis of
rotation extending longitudinally therethrough, a cutting face 306,
and a substrate 301 extending axially downward from the cutting
face 306. The substrate 301 includes a shaft portion 308 with a
reduced diameter as compared to the rest of the substrate 301. A
circumferential groove 310 is formed on a shaft 308 portion of the
substrate 301. Further, a cutting edge 304 is formed at the
intersection of the cutting face 306 and the outer surface 302 of
the rolling cutter 300. As shown, the cutting face 306 and cutting
edge 304 may be formed from a diamond or other ultra-hard material
table 305.
[0067] As assembled, the cutting element has a retaining ring 320
disposed in the circumferential groove 310, wherein the retaining
ring 320 extends at least around the entire circumference of the
shaft 308. For example, in the embodiment shown in FIG. 14, the
retaining ring 320 may extend greater than 1.5 times around the
circumference of the shaft 308. As shown in FIG. 14, the retaining
ring 320 protrudes from the circumferential groove 310 to extend
into an inner groove (not shown) of the sleeve 330, thereby
retaining the rolling cutter 300 within the sleeve 330.
[0068] Further, in one or more embodiments, the rotatable cutting
elements placed in the areas of the bit experiencing the greatest
wear may have a thicker substrate table extending above the sleeve
and supporting the cutting surface than cutting elements placed in
areas which experience less wear. In more particular embodiments,
the rotatable cutting elements placed in the nose region have a
thicker substrate table extending above the sleeve and supporting
the cutting surface than rotatable cutting elements located in the
shoulder region.
[0069] However, the present disclosure is not limited to the
rolling cutter type illustrated in FIG. 14, but instead, as
mentioned above, any type of rolling cutter may be used on the bits
and tools of the present disclosure.
[0070] In various embodiments, the cutting face of the inner
rotatable cutting element may include an ultra hard layer that may
be comprised of a polycrystalline diamond table, a thermally stable
diamond layer (i.e., having a thermal stability greater than that
of conventional polycrystalline diamond, 750.degree. C.), or other
ultra hard layer such as a cubic boron nitride layer.
[0071] As known in the art, thermally stable diamond may be formed
in various manners. A conventional polycrystalline diamond layer
includes individual diamond "crystals" that are interconnected. The
individual diamond crystals thus form a lattice structure. A metal
catalyst, such as cobalt, may be used to promote recrystallization
of the diamond particles and formation of the lattice structure.
Thus, cobalt particles are generally found within the interstitial
spaces in the diamond lattice structure. Cobalt has a different
coefficient of thermal expansion as compared to diamond. Therefore,
upon heating of a diamond table, the cobalt and the diamond lattice
will expand at different rates, causing cracks to form in the
lattice structure and resulting in deterioration of the diamond
table.
[0072] To obviate this problem, strong acids may be used to "leach"
the cobalt from a polycrystalline diamond lattice structure (either
a thin volume or entire tablet) to at least reduce the damage
experienced from heating diamond-cobalt composite at different
rates upon heating. Examples of "leaching" processes can be found,
for example, in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a
strong acid, such as hydrofluoric acid or combinations of several
strong acids may be used to treat the diamond table, removing at
least a portion of the co-catalyst from the PDC composite. Suitable
acids include nitric acid, hydrofluoric acid, hydrochloric acid,
sulfuric acid, phosphoric acid, or perchloric acid, or combinations
of these acids. In addition, caustics, such as sodium hydroxide and
potassium hydroxide, have been used to the carbide industry to
digest metallic elements from carbide composites. In addition,
other acidic and basic leaching agents may be used as desired.
Those having ordinary skill in the art will appreciate that the
molarity of the leaching agent may be adjusted depending on the
time desired to leach, concerns about hazards, etc.
[0073] By leaching out the cobalt, thermally stable polycrystalline
(TSP) diamond may be formed. In certain embodiments, a select
portion of a diamond composite is leached, in order to gain thermal
stability without losing impact resistance. As used herein, the
term TSP includes both of the above (i.e., partially and completely
leached) compounds. Interstitial volumes remaining after leaching
may be reduced by either furthering consolidation or by filling the
volume with a secondary material, such by processes known in the
art and described in U.S. Pat. No. 5,127,923, which is herein
incorporated by reference in its entirety.
[0074] In another embodiment, TSP may be formed by forming the
diamond layer in a press using a binder other than cobalt, one such
as silicon, which has a coefficient of thermal expansion more
similar to that of diamond than cobalt has. During the
manufacturing process, a large portion, 80 to 100 volume percent,
of the silicon reacts with the diamond lattice to form silicon
carbide which also has a thermal expansion similar to diamond. Upon
heating, any remaining silicon, silicon carbide, and the diamond
lattice will expand at more similar rates as compared to rates of
expansion for cobalt and diamond, resulting in a more thermally
stable layer. PDC cutters having a TSP cutting layer have
relatively low wear rates, even as cutter temperatures reach
1200.degree. C. However, one of ordinary skill in the art would
recognize that a thermally stable diamond layer may be formed by
other methods known in the art, including, for example, by altering
processing conditions in the formation of the diamond layer.
[0075] The substrate on which the cutting face is disposed may be
formed of a variety of hard or ultra hard particles. In one
embodiment, the substrate may be formed from a suitable material
such as tungsten carbide, tantalum carbide, or titanium carbide.
Additionally, various binding metals may be included in the
substrate, such as cobalt, nickel, iron, metal alloys, or mixtures
thereof. In the substrate, the metal carbide grains are supported
within the metallic binder, such as cobalt. Additionally, the
substrate may be formed of a sintered tungsten carbide composite
structure. It is well known that various metal carbide compositions
and binders may be used, in addition to tungsten carbide and
cobalt. Thus, references to the use of tungsten carbide and cobalt
are for illustrative purposes, and no limitation on the type
substrate or binder used is intended. In another embodiment, the
substrate may also be formed from a diamond ultra hard material
such as polycrystalline diamond and thermally stable diamond. While
the illustrated embodiments show the cutting face and substrate as
two distinct pieces, one of skill in the art should appreciate that
it is within the scope of the present disclosure the cutting face
and substrate are integral, identical compositions. In such an
embodiment, it may be desirable to have a single diamond composite
forming the cutting face and substrate or distinct layers.
[0076] The cutting elements of the present disclosure may be
incorporated in various types of downhole cutting tools, including
for example, as cutters in fixed cutter bits or as inserts in
roller cone bits, reamers, hole benders, or any other tool that may
be used to drill earthen formations. Cutting tools having the
cutting elements of the present disclosure may include a single
rotatable cutting element with the remaining cutting elements being
conventional cutting elements, each cutting element on the tool
being rotatable, or any combination therebetween of rotatable and
conventional cutting elements.
[0077] The cutting elements of the present disclosure may be
attached to or mounted on a drill bit by a variety of mechanisms,
including but not limited to conventional attachment or brazing
techniques in a cutter pocket, as well as by mechanical means. It
is also within the scope of the present disclosure that in some
embodiments, an inner rotatable cutting element may be mounted on
the bit directly such that the bit body acts as the outer support
element, i.e., by inserting the inner rotatable cutting element
into a hole that may be subsequently blocked to retain the inner
rotatable cutting element within.
[0078] While the above describes a situation where the rolling
cutters have a non-planar cutting surface, it is also contemplated
that planar cutting surfaces may also be used on rolling cutters.
In other words, it is expressly within the scope of the present
disclosure that the rolling cutters may comprise mixtures of planar
and non-planar cutting surfaces. Additionally, if each of the
cutters are not rolling cutters, the fixed cutters may possess
planar cutting surfaces, non-planar cutting surfaces, or mixtures
thereof.
EXAMPLE
[0079] A finite element analysis (FEA) was performed on a rotatable
cutting element having a planar cutting face (shown in FIG. 16) and
a convex cutting surface (shown in FIG. 17). The FEA results showed
that the cutting element with a planar cutting face has a maximum
principal stress of 118.8 ksi whereas the cutting element with the
convex cutting face has a maximum principal stress of 100.4 ksi,
with the higher stress being present particularly at the transition
region between the radial bearing surface and the shaft of the
rotatable cutting element. This FEA analysis shows that the cutting
element with the convex cutting surface shows an 18% better
strength than the planar cutting face under frontal loads.
[0080] Although only a few example embodiments have been described
in detail above, those skilled in the art will readily appreciate
that many modifications are possible in the example embodiments
without materially departing from this invention. Accordingly, all
such modifications are intended to be included within the scope of
this disclosure as defined in the following claims. In the claims,
means-plus-function clauses are intended to cover the structures
described herein as performing the recited function and not only
structural equivalents, but also equivalent structures. Thus,
although a nail and a screw may not be structural equivalents in
that a nail employs a cylindrical surface to secure wooden parts
together, whereas a screw employs a helical surface, in the
environment of fastening wooden parts, a nail and a screw may be
equivalent structures. It is the express intention of the applicant
not to invoke 35 U.S.C. .sctn.112, paragraph 6 for any limitations
of any of the claims herein, except for those in which the claim
expressly uses the words `means for` together with an associated
function.
* * * * *