Note: Descriptions are shown in the official language in which they were submitted.
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SHEAR CUTTER DRILL BIT
FIELD OF THE INVENTION
The present application relates generally to shear cutter drill bits for
drilling
subterranean formations. More specifically, a high rate of penetration (ROP)
shear
cutter drill bit is provided having cutters arranged at low backrake.
BACKGROUND OF THE INVENTION
Shear cutter drill bits have a plurality of fixed cutters, usually comprising
polycrystalline diamond compacts ("PDCs"), located on the bit face that are
set at a
fixed backrake angle. The drill bit is attached to the end of a drill string
and is rotated by
either rotating the drill string from surface or by using a downhole motor to
form a
borehole in a subterranean formation.
The rate of penetration (ROP), which generally defines how fast a given drill
bit
drills, and the durability of a drill bit depends on a number of factors, for
example, cutter
densities, number of cutters, size of cutters, number of blades, arrangement
of cutters
on the bit face, and the individual cutter's backrake angle. A drill bit that
drills with a
high ROP is generally referred to as an aggressive drill bit. The overall
aggressiveness
of a drill bit is generally defined by the aggressiveness of the cutters
disposed on the
face of the bit, i.e., how aggressively each cutter will bite into the
formation. One of the
primary factors that determine the aggressiveness of an individual cutter is
the backrake
angle at which it is set. Backrake is defined as the angle between the axis of
the cutter
and the formation that it is cutting, and, in general, the smaller the
backrake angle, the
more aggressive the cutter.
Traditionally, the industry has used relatively high backrake angles, as bits
with
higher backrake tend to suffer less damage than bits with lower backrake. The
industry
standard has been to have cutters with backrake angles above 20 . It is
generally
believed that drill bits having cutters with backrakes lower than 200 will be
too
aggressive, which will lead to premature cutter breakage. It is further
believed that drill
bits with low backrake have poor durability as they present less diamond
surface area at
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the cutter - rock interface. However, use of such high backrake cutters
results in
relatively unaggressive drill bits requires greater weight on bit ("WOB") in
order to
achieve a higher ROP.
U.S. Patent 7,000,715 describes a drill bit having cutters with higher
backrakes
primarily located in the center of the bit, with the backrakes getting
generally lower with
increasing radial position of the cutters on the bit face, with the lowest
backrake cutters
being positioned on the shoulder of the bit. However, having cutters with the
lowest
backrake present on the shoulder of the bit may lead to premature cutter
breakage. In
addition to the higher likelihood of breakage on the low backrake shoulder
cutters,
breakage also becomes more of an issue in the other regions of the bit, in
particular, the
nose region, due to excessive axial loading on these regions. U.S. Patent
7,000,715
also describes a`fast-drilling' drill bit having cutters with the lowest
backrake located in
the center of the bit, with the backrakes getting generally higher with
increasing radial
position of the cutters on the bit face, with the highest backrake cutters
being positioned
on the shoulder of the bit. However, having cutters with the lowest backrake
present in
the center of the bit may also lead to premature cutter breakage, and having
higher
backrake further out on the profile inhibits the bit's ability to drill at a
high ROP. Further,
in general, it has been discovered that it is the cutters in the nose region
that dictate the
efficiency of the overall cutting action of the bit rather than the cutters
located outside of
the nose region.
There is a need for a shear cutter drill bit which can drill with a very high
ROP
without compromising the durability of the bit (i.e., minimizing cutter
breakage). The
present applicants have discovered that providing a specific arrangement of
cutters with
low backrakes on the drill bit face will result in fast (aggressive) drill
bits that are still
durable.
SUMMARY OF THE INVENTION
In one broad aspect, the present application provides a shear cutter drill bit
for
drilling a subterranean formation, having:
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= a bit body;
= a bit face on the bit body having at least a center zone, an efficiency
zone, and
an outer zone, the efficiency zone being located between the center zone and
the outer zone; and
= a plurality of cutters having a backrake angle located on the bit face;
wherein a majority of the cutters located in the efficiency zone of the bit
face have a
more aggressive backrake angle than a majority of the cutters located in the
center
zone of the bit face and a majority of the cutter located in the outer zone of
the bit face.
In one embodiment, the majority of the cutters located in the efficiency zone
have
a backrake angle less than 200. In another embodiment, the majority of the
cutters
located in the efficiency zone have a backrake angle less than 150. In another
embodiment, the majority of the cutters located in the efficiency zone have a
backrake
angle less than 10 . In another embodiment, the majority of the cutters
located in the
efficiency zone have a backrake angle less than 5 .
In another broad aspect, the present application provides a shear cutter drill
bit
where a majority of the cutters located in the efficiency zone have the most
aggressive
backrake angles, the majority of the cutters located in the center zone have
intermediately aggressive backrake angles, and the majority of the cutters
located in the
outer zone have the least aggressive backrake angles. In one embodiment, the
majority of the cutters located in the efficiency zone have a backrake angle
in the range
of about 50 to about 15 , the majority of the cutters located in the center
zone have a
backrake angle in the range of about 15 to about 20 , and the majority of the
cutters
located in the outer zone have a backrake angle in the range of about 150 to
about 30 .
In another broad aspect, the application provides a method for drilling a
subterranean formation, comprising:
= providing a shear cutter drill bit for drilling a subterranean formation,
the shear
cutter drill bit having a bit body, a bit face on the bit body having at least
a center
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zone, an efficiency zone and an outer zone, the efficiency zone being located
between the center zone and the outer zone, and a plurality of cutters having
a
backrake angle located on the bit face, wherein a majority of the cutters
located
in the efficiency zone of the bit face have a more aggressive backrake angle
than
a majority of the cutters located in the center zone of the bit face and a
majority
of the cutters located in the outer zone of the bit face;
= positioning the face of the drill bit towards the subterranean formation so
that at
least one of the center zone, efficiency zone and outer zone contacts the
subterranean formation; and
= rotating the drill bit while applying a weight on the drill bit so as to
penetrate the
subterranean formation;
wherein the weight applied to the drill bit is less that the weight needed to
be applied to
a conventional shear cutter drill bit for obtaining the same rate of
penetration.
Other features will become apparent from the following detailed description.
It
should be understood, however, that the detailed description and the specific
embodiments, while indicating preferred embodiments of the invention, are
given by
way of illustration only, since various changes and modifications within the
spirit and
scope of the invention will become apparent to those skilled in the art from
this detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates an end view of an embodiment of a shear cutter drill bit
according to the invention.
Figure 2 illustrates the side perspective view of the drill bit embodiment
shown in
Figure 1.
Figure 3 illustrates a side view of a cutter as used in an embodiment of the
invention depicting the cutter backrake.
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Figure 4 is a side cross-sectional elevation of an embodiment of a five bladed
shear cutter drill bit according to the invention.
Figure 5 graphically illustrates a profile of the drill bit embodiment shown
in
Figure 4 highlighting cutter height versus bit radius.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
While the invention is described below with reference to what are presently
considered to be the preferred embodiments, it is to be understood that the
invention is
not limited to the disclosed embodiments. To the contrary, the invention
herein is
intended to cover various modifications and equivalent arrangements included
within
the spirit and scope of the appended claims.
The applicants have discovered that the cutters of a shear cutter drill bit
that are
located generally in a region adjacent to the cone region of the bit and which
extends
partially into the shoulder region of the drill bit appear to have the
greatest impact on the
overall efficiency of the shearing action of the drill bit. This region is
referred to
hereinafter as the "efficiency zone". It is understood by a person skilled in
the art that
the cone region of a drill bit generally refers to the center region of the
drill bit
(hereinafter referred to as the "center zone") and that the shoulder region
generally
refers to the region that starts where the profile angle is approximately 15 ,
the profile
angle being defined as the angle between a line drawn perpendicular to the bit
profile
and the centerline of the bit when viewing the bit from a profile view.
Of course, it is understood to those skilled in the art that the beginning of
the
efficiency zone will vary according to a number of variables: the intended RPM
of the
application, the type of rock being drilled, the overall size of the drill
bit, the shape of the
drill bit profile, etc. However, for example purposes only, and not meant to
be limiting,
the efficiency zone generally starts at a radius of approximately 1" from the
centerline of
the drill bit and, in general, the efficiency zone ends where the profile
angle is
approximately 45 in the shoulder region.
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It was further discovered that the beginning of the efficiency zone generally
correlates to a radial distance where the relative surface speed of the
cutters becomes
more significant. The cutters located in the center zone (i.e., closest to
center of the bit)
move at a very slow surface speed (surface speed being defined as 2-rrRw,
where "R" is
the radial distance of the tip of the cutter to the center of the bit, and w
is the angular
velocity - or RPM) with respect to the formation because they are positioned
at such a
small radial distance from the center of the bit. As a result of that slow
surface speed,
and as a result of the fact that these cutters are drilling rock that has been
stress
relieved, the cutters in the center generally have a reduced effect on the
efficiency of
the cutting structure. As the bit drills through formation, the rock left in
the center of the
bit no longer has the stress of the rock around it to support it, and when
rock is under
less stress, it is much easier to drill. In other words, the cutters do not
need to be very
aggressive in the center zone for the overall drill bit to be aggressive.
Cutters located on the opposite side of the efficiency zone, i.e., those
cutters in
the shoulder and gage region past the efficiency zone (hereinafter referred to
as the
"outer zone"), are oriented generally more laterally than axially, and
therefore have less
effect on the efficiency of the axial cutting action (i.e., the ability of the
bit to drill ahead).
Because the cutters located in the outer zone are the furthest away from the
center of
the cutter, these cutters have a higher surface speed with respect to the
formation.
Thus, cutters in the outer zone will still remove rock relatively fast even if
they are not
set at a particularly aggressive backrake angle, due to this increased surface
speed.
Conversely, cutters in the efficiency zone are generally oriented more axially
than
laterally, and therefore have a greater effect on the efficiency of the axial
cutting action.
The rock being removed by the cutters near the nose of the bit is still
supported by rock
on three sides and, as such, is much harder to drill. Thus, it is desirable
that the cutters
in this zone be as aggressive as possible, i.e., have the lowest backrake, and
that these
lower backrake cutters extend past the beginning of the shoulder of the bit.
Hence, the
cutters in the efficiency zone will have the greatest effect on the overall
ROP of the drill
bit.
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There may be additional advantages in having the cutters located in the center
zone and outer zone set at a less aggressive angle. For example, the cutters
in the
center zone tend to be more susceptible to impact damage and, in particular,
can easily
break if set at a low backrake. Thus, setting the cutters in the center zone
at higher
backrake angles will tend to minimize such potential breakage without
significantly
affecting the overall drilling speed and efficiency of the drill bit.
With respect to the cutters in the outer zone, it is believed that having a
high
backrake on these cutters may help balance the axial loads on all of the
cutters. Since
the cutters in the efficiency zone will generally see the greatest amount of
axial load (if
all of the cutters on the drill bit were set at the same backrake), and since
those cutters
are set at the most aggressive backrake, by increasing the backrake on the
cutters
outside the efficiency zone it effectively increases the axial load on those
cutters outside
the efficiency zone, therefore helping to protect the cutters in the
efficiency zone from
excessive axial loading leading to cutter breakage.
Another possible advantage with high backrake angles on cutters in the outer
zone may be that the cutters in that region tend to see the most damage from
any
lateral vibration or lateral movement of the bit. Since lateral vibration is a
common
problem with shear cutter drill bits, the overall performance of such drill
bits may be
improved by having the cutters in the outer zone at a higher backrake to
prevent any
breakage resulting from lateral movement/vibration.
Hence, since the efficiency of the overall cutting action of the bit is
dictated by
cutters in the efficiency zone, the cutters outside that zone (either inside
of it or outside
of it) have less effect on the efficiency of the cutting action, and therefore
less effect on
the resulting ROP of the bit for a given WOB (weight on bit). Thus, according
to one
embodiment of the invention, the majority of the cutters located in the
efficiency zone
will have a more aggressive backrake angle than the majority of the cutters
located in
the center zone and the majority of the cutters located in the outer zone.
An embodiment of the invention will now be described with reference to Figures
1-5. With reference first to Figures 1 and 2, shear cutter drill bit 20
comprises bit body
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22 having a shank portion 24 and a bit face 26. A plurality of generally
radially oriented
blades 28 generally define bit face 26 and each blade 28 further comprising a
plurality
of cutting elements or cutters 30, each cutter having a particular backrake
angle and
disposed about the blades surface in a conventional fashion, e.g., by brazing
or force
fitting. The most common type of cutters used in the industry are cutters
cylindrical in
shape such as PDCs
The backrake angle of a cutter is defined as the angle between the axis of the
cutter and the formation that it is cutting, measured perpendicular to the
profile of the
drill bit. This can be seen more clearly with reference to Figure 3. In Figure
3, the
formation is represented by flat horizontal line 10, the axis of the cutter by
hatched line
12, and the backrake angle of the cutter is the angle between the axis of the
cutter and
the formation (8).
Drill bit 20 further comprises a plurality of nozzles or jets 32, which
provide
drilling fluid to the bit face 22. In Figure 1, drill bit face 22 is shown
divided into three
distinct regions by concentric hatched circles A and B, forming center zone
34,
efficiency zone 36 and outer zone 38. Those cutters present in the efficiency
zone 36
have been marked with an asterisk (*) in Figure 2.
With reference now to Figure 4, the profile of bit face 26 of drill bit 20 as
shown
in Figures 1 and 2 is illustrated, including blades 28 upon which a plurality
of cutters 30
are oriented. The axis of the drill bit 20 is represented by hatched line 40
and the profile
terminates at gage 42. Although all of the cutters 30 are depicted as being
oriented on
a single blade 28, Figure 4 merely depicts the position of cutters 30 relative
to one
another with respect to both the longitudinal axis 40 of the bit 20 and a
vertical position
44 along longitudinal axis 40. In actuality, cutters 30 are carried on various
blades 28,
the cutter positions having been rotated in a single plane for clarity. Zone
34, which
extends from the axial line 40 to hatched line A, is the center zone and it
can be seen
from the profile that, in this embodiment, there are four cutters 30 in the
center zone.
Zone 36, which is the efficiency zone, extends from hatched line A to hatched
line B,
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and includes the nose point C. Zone 34, the outer zone, extends from hatched
line B to
gage 42.
The profile of the blades 30 in Figure 4 can be seen in more detail in Figure
5.
The x axis is taken along the longitudinal axis and generally represents the
distance of
each cutter from the center of the bit or the bit radius. The y axis generally
represents
the height of the cutters. In this embodiment, the individual cutters have
been labeled
30A to 30CC, for a total of 29 cutters. Cutters 30A, 30B and 30C are present
in the
center zone and in this embodiment have a backrake angle of about 15 . Cutters
30D
to 300 are present in the efficiency zone and in this embodiment have a
backrake angle
of about 10 . Cutters 30P to 30CC are present in the outer zone. In this
embodiment,
cutters 30P to 30S have a backrake angle of about 15 , cutters 30T to 30W have
a
backrake angle of 20 , and cutters 30X to 30AA have a backrake angle of about
30 .
It is understood, however, that the embodiment in Figure 5 is only one example
of a shear cutter drill bit of the invention. According to the invention,
generally, more
than half of the cutters present in efficiency zone will have a backrake angle
of about
or less, in some instances about 15 or less, in some instances about 10 or
less, or
in some instances about 5 or less. The majority of the other cutters, which
are located
in the center zone and the outer zone, will have higher backrake angles than
the
majority of those in the efficiency zone.
20 For example, in another embodiment, the majority of the cutters located in
the
efficiency zone have a backrake angle in the range of about 5'- to about 15 ;
the
majority of the cutters located in the center zone have a backrake angle in
the range of
about 15 to about 20 , and the majority of the cutters located in the outer
zone have a
backrake angle in the range of about 15 to about 25 . In another embodiment,
the
majority of the cutters in the outer zone have a backrake angle of about 20
to about
.
The previous description of the disclosed embodiments is provided to enable
any
person skilled in the art to make or use the present invention. Various
modifications to
those embodiments will be readily apparent to those skilled in the art, and
the generic
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principles defined herein may be applied to other embodiments without
departing from
the spirit or scope of the invention. Thus, the present invention is not
intended to be
limited to the embodiments shown herein, but is to be accorded the full scope
consistent
with the claims, wherein reference to an element in the singular, such as by
use of the
article "a" or "an" is not intended to mean "one and only one" unless
specifically so
stated, but rather "one or more". All structural and functional equivalents to
the
elements of the various embodiments described throughout the disclosure that
are
known or later come to be known to those of ordinary skill in the art are
intended to be
encompassed by the elements of the claims. Moreover, nothing disclosed herein
is
intended to be dedicated to the public regardless of whether such disclosure
is explicitly
recited in the claims.
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