Boring or penetrating the earth – Bit or bit element – Rolling cutter bit or rolling cutter bit element
Reexamination Certificate
2000-08-30
2003-03-04
Bagnell, David (Department: 3672)
Boring or penetrating the earth
Bit or bit element
Rolling cutter bit or rolling cutter bit element
C175S426000, C175S429000, C175S430000
Reexamination Certificate
active
06527065
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates generally to superabrasive cutting elements used in rotary drill bits, also referred to as drag bits, for use in drilling subterranean formations. More specifically, the present invention pertains to superabrasive cutting elements securable to rotary drill bits in a manner which minimizes unwanted stresses in the superabrasive member, particularly when the superabrasive cutting element is positioned at a high positive rake angle.
2. Background of the Invention
Superabrasive material such as polycrystalline diamond compact (PDC) and cubic boron nitride are commonly used in the fabrication of cutting elements employed in drill bits, particularly drill bits which are relied upon by the oil and gas industry for drilling wells in formations of earth in the exploration and production of oil and gas. Such superabrasive material may be formed into the bit body as a self-supporting member or may be employed in cutting elements which comprise a table or layer of superabrasive material joined to a substrate, or backing, of the cutting element. Typically, such cutting elements, such as representative PDC cutting element
214
depicted in cross-section in
FIG. 2A
, comprise a substantially planar superabrasive, or polycrystalline diamond table, such as table
216
, which is disposed on an underlying supportive substrate, or backing,
218
of a suitably strong material such as tungsten carbide (WC) or carbides mixed with other metals in which the diamond table is sintered or bonded to the substrate by methods known within the art. Superabrasive diamond table
216
typically will have a planar, generally circular cutting surface
226
, as can be seen in
FIG. 2B
which is a top view of cutting element
214
. As can be seen in
FIGS. 2A and 2B
, cutting element
214
is provided with a cutting surface
226
which is generally planar or flat in that it extends in only two directions or dimensions, and wherein the cutting surface itself does not extend in a third direction or dimension so as to provide cutting surface
226
with a nonflat or curved cutting surface. A superabrasive cutting element of this type is commonly known as a polycrystalline diamond compact cutter or PDC cutter.
A conventional cutting element, such as a PDC cutter, is positioned in the body of the drill bit so that the superabrasive material contacts and engages subterranean formations for cutting the formation as the drill bit is rotated by the drill string, or alternately a downhole motor in which it is connected. Several factors can contribute to how efficient or inefficient the cutting element performs. Traditionally, cutting elements such as PDC cutters are positioned on the bit body of a drill bit to have either a positive rake angle, zero rake angle, or a negative rake angle with respect to the formation to be engaged by the cutter as the bit rotates and proceeds into the formation being drilled. This terminology of positive, zero, and negative rake angles as used within the art in describing the rake angle of a given cutter is illustrated in FIG.
1
. Representative PDC cutters
200
,
208
, and
214
are all generally cylindrical in configuration and are each provided with respective superabrasive or diamond tables
202
,
210
, and
216
mounted on respective substrates
204
,
212
, and
218
. Each of the cutters are designed and positioned to laterally engage the formation in the direction of arrow
206
. Cutter
200
is regarded as having a positive rake angle due to cutting surface
222
of superabrasive table
202
thereof being inclined at an angle exceeding 90° with respect to formation
220
as illustrated. Thus, as the angle becomes more obtuse, or approaches 180°, it is regarded as being more “positive”. Cutter
208
is regarded as having 0° rake angle due to cutting surface
224
of superabrasive table
210
being generally perpendicular to formation
220
. Lastly, cutter
214
is regarded as having a negative rake angle due to cutting surface
226
of superabrasive table
216
being inclined less than 90° with respect to formation
220
as illustrated. Thus, as the angle becomes more acute, or approaches 0°, it is regarded as being more “negative”.
The characteristics of the formation being cut further influence the choice of cutting element design and placement on the body of the drill bit. For example, a PDC cutter is subjected to significant tangential loading as the drill bit rotates. Additionally, it is known that positioning the cutting element with a negative rake angle places the formation in compression. Contrastingly, positioning the cutting element with a positive rake angle results in the formation being placed in tension as the formation is engaged and cuttings or chips are sheared therefrom.
Further, it is known that conventional PDC cutter performance can be compromised by residual stresses which are induced within the cutting element itself and particularly in the area of the interface, designated as
228
in
FIG. 2A
, where the planar diamond table is joined with the substrate. That is, while the superabrasive diamond table is generally in compression and the substrate in tension, conventional PDC's display an undesirable amount of residual stress around the interface between the diamond table and the substrate, which stress is principally caused by different coefficients of thermal expansion in the diamond and the substrate. The high loading imposed on conventional PDC cutters during drilling, in combination with the residual stress, is known to cause unwanted spalling and delamination of the diamond table from the substrate.
Attempts have been made to remedy or lessen the failure of cutting elements employing PDCs during drilling by modifying or redirecting the residual stresses in PDC cutters by way of varying the configuration of PDC cutters. Examples of such efforts to modify the stresses in PDC's by modifying the configuration of the diamond table, the substrate, or both, are disclosed in U.S. Pat. No. 5,435,403 to Tibbitts, U.S. Pat. No. 5,492,188 to Smith, et al., and U.S. Pat. No. 5,460,233 to Meany, et al. Another type of improvement in drill bit design is disclosed in U.S. Pat. No. 5,437,343 to Cooley, et al., which discloses the use of multiple chamfers at the periphery of a PDC cutting face to enhance the resistance of the cutting element to impact-induced fracture.
It is known that conventional superabrasive cutting elements can be positioned in the bit body in a manner which optimizes cutting ability under the loading conditions of a particular formation. That is, the type of rock in the formation, the rock stresses, the filtration and the bit profile may all contribute to the performance of the cutting element. It has also been recognized that the location of the cutting element on the bit body influences the capability of the cutting element to withstand certain loading stresses. For example, it has been noted that a conventional planar cutting element located on the bit flank or shoulder may typically experience greater tangential loading than a cutting element located on the bit nose or bit gage. Further, positioning the cutting element in the bit body with a back rake (usually negative back rake) enables the cutting element to better withstand loading forces imposed upon it during drilling operations and lessens failure of the cutting element.
However, while a higher effective negative back rake permits the use of conventional planar PDC cutters, such higher effective back rakes reduce the aggressiveness of the cutter. This factor can be critical in cutting elements which are located on the bit flank or shoulder where the greatest amount of cutting of the formation occurs. Thus, it would be advantageous to provide a cutting element which is configured to effectively and aggressively cut a given earthen formation while being positioned at a high positive rake angle to place the formation in tension, thereby maximizing cutting performance and cutter durability, and it would be advantageous to p
Cooley Craig H.
Tibbitts Gordon A.
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