Note: Descriptions are shown in the official language in which they were submitted.
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DRILL BIT
BACKGROUND OF THE INVENTION
O1 A challenge in underground drilling is to provide a drill bit with extended
life, that
cuts quickly through earth formations of various types and that avoids balling
up of cuttings in
the vicinity of the drill bit. The balling up of cuttings in the vicinity of
the drill bit may cause
the drill bit to cease cutting as the cutting elements no longer contact the
earth formation.
02 Modern drilling bits typically are formed of a body, blades extending from
the body,
mostly forwardly but also extending somewhat radially outward of the body, and
polycrystalline diamond cutters (PDCs) embedded in the cutting faces of the
blades. Two
main types of PDC drill bits on the market are the matrix body and steel body.
Matrix body
bits are one piece construction and are made in a mould as for example
disclosed in United
States patent #6,823,952. The material is a mixture of steel and tungsten
carbide. Steel body
bits are also one piece construction but are cut on a lathe and made from 4140
steel, 4145
steel or a similar material. The blades on PDC bits are typically set in a
vertical plane, or may
be canted forward slightly towards the cutting surface. Some bits have forward
sweeping
cutting elements, as for example disclosed in United States patent no.
5,443,565. The PDC
cutting elements provide hard wearing surfaces that cut the formation. Junk
slots between the
blades provide pathways for the removal of cuttings away from the bit face
into the annular
space of the wellbore. Most PDC bits make the junk slot area as wide and as
obstruction free
as possible for the pathway to remove cuttings. To further assist in removal
of cuttings, drill
bits are provided with openings or nozzles in the forward end of the drill bit
that direct fluid
jets between the blade surfaces. The drilling fluid, which is also typically
used in a mud
motor to power the drill bit, passes through the inside of the drill bit,
through the nozzles and
the junk slots, and draws cuttings away from the drill bit towards the
surface.
03 In a further problem with PDC type drill bits, cutter surfaces often fail
as a result of
high temperatures created from friction between the cutter and the rock it is
cutting. When a
PDC cutter reaches a critical temperature known as the thermal degradation
temperature, the
diamond surface will separate from the tungsten carbide substrate. The thermal
degradation
temperature ranges from 300°C to 700°C. Heat is removed from the
bit face and the cutters by
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the drilling fluid as it removes the cuttings from the surface of the drill
bit. Heat is also
transferred through the tungsten carbide cutter into the blade and bit body.
Tungsten carbide is
a much better conductor of heat than steel. Therefore the transfer of heat
away from the
cutters into the blades and bit body is not very efficient.
SUMMARY OF THE INVENTION
04 According to an aspect of this invention, there is provided a drill bit,
and a method of
manufacturing a drill bit, that uses the design of junk slots between the
blades of the drill bit
to enhance removal of cuttings from the drill bit. In a method of construction
of a drill bit,
according to an aspect of the invention, a drill bit is made of a shank, lid
welded to the shank
and blades welded in slots in the lid.
OS According to further aspects of the invention, junk slot impingement is
used to
increase cuttings removal through alternating junk slots. Provision of high
angle nozzles in
the forward end of the drill bit, which is facilitated by the method of
construction, also assists
in cuttings removal. According to a further aspect of the invention, a drill
bit with PDC cutters
is provided with a cooling feature to remove heat from the PDC cutters more
efficiently. A
high conductivity conduit leading from the cutters guides heat away from the
cutters into the
blade and hence into the bit body.
06 These and other aspects of the invention are set out in the claims, which
are
incorporated here by reference.
BRIEF DESCRIPTION OF THE FIGURES
07 Preferred embodiments of the invention will now be described with reference
to the
figures, in which like reference characters denote like elements, by way of
example, and in
which:
Fig. 1 is perspective view of a drill bit according to the invention;
Fig. 2 is a view of a shank for use with the drill bit of Fig. 1;
Fig. 3 is a perspective view of a cutting end of a lid for use with the drill
bit of Fig. 1;
Fig. 4 is a perspective view of the opposed end to the cutting end of the lid
of Fig. 3;
Figs. SA-SD illustrate method of making a blade for use with the drill bit of
Fig. 1;
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Fig. 6 shows a method of assembling blades into the drill bit of Fig. 1;
Fig. 7 is a cross-section through a blade showing a cooling feature according
to an
aspect of the invention;
Fig. 8 is a cross-section through the bit showing a nozzle aspect of the
invention;
Fig. 9 illustrates offset cutters on succeeding blades; and
Fig. 10 is a section through a blade and cutter showing a soft metal insert
behind the
cutter.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
08 In the claims, the word "comprising" is used in its inclusive sense and
does not
exclude other elements being present. The indefinite article "a" before a
claim feature does
not exclude more than one of the feature being present.
09 Referring to Fig. 1, there is shown a drill bit 10 formed of a threaded
shank 12, lid 14
and blades 16, 18 extending out from the cutting end of the drill bit 10. The
shank 12 is
threaded in conventional fashion for threading onto a downhole end of a drill
string. There
should be at least four and preferably eight blades 16, 18. The blades 16, 18
alternate
between primary blades 16 and secondary blades 18. The drill bit 10 is
intended to be rotated
counterclockwise in use when taking a view of the cutting, bladed, end or face
of the drill bit,
which would be clockwise looking in the downhole direction in use. The
operational
direction of rotation defines a forward and rearward direction. The blades 16,
18 are
preferably canted rearward at an angle of about 5° to 10° to the
vertical (the central axis A of
the drill bit 10 is vertical in a desired operating position). The shank 12,
lid 14 and blades 16,
18 are manufactured separately from steel using a machine lathe type of
construction and then
welded into one unit, for example using electric arc welding. Not seen in Fig.
1, but shown in
Fig. 2, is a cylindrical central passage 20 in the shank 12 that widens
towards the lid 14 and
supplies drilling fluid to the cutting end of the drill bit 10. The shank 12
is made from circular
steel bar stock on a machine lathe. The passage 20 is drilled through the
shank 12. The shank
12 is a cylindrical body having a central axis indicated by the arrow A about
which the drill
bit 10 rotates.
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Referring to Figs. 3 and 4, the lid 14 is shown separately from the shank 12
and blades
16, 18. The lid 14 has openings 22 passing from the shank end 14A to the
cutting end 14B of
the lid 14. Nozzles 24 are inserted in the openings 22. The nozzles 24 are
hardened tubes,
made for example of tungsten carbide, that protect the steel of the lid 14
from excessive wear
from drilling fluid. Typically, there are as many openings 22 as there are
blades 16, 18 and the
exit of each opening in the cutting end 14B is located at an inward end of a
corresponding
blade 16, 18, adjacent the forward face of the blade 16, 18. The openings 22
are oriented at
an angle to the central axis A preferably greater than 15°, and for
openings closer to the
central axis of the drill bit, greater than 30°, and directed so that
fluid exiting the nozzles 24
flows between adjacent blades 16, 18. The greater the angle of an opening 22,
the more the
fluid is directed between the corresponding blades. The nearer the opening 22
is to the central
axis A of the drill bit, the greater the angle of the opening to the central
axis. The openings 22
may be drilled through the lid 14 after it has been machined into the general
shape shown in
Figs. 3 and 4. Use of a lid 14 facilitates a high angle of the nozzles 24 as
opposed to a one
piece design, which makes high angle nozzles difficult to achieve. The nozzles
24 provide a
flow path for drilling fluid pumped through the central passage 20 of the
shank 12. The
nozzles 24 may be provided for example in different sizes, such as 10 mm for
the primary
blades 16 and 7.5 mm for the secondary blades 18.
11 As shown in Figs. l, 3 and 6, rectangular slots 26, 28 are provided in the
cutting end
14B of the lid 14 for the blades 16, 18. The blades 16, 18 are secured in the
slots for example
by welding. The blades 16, 18 and the corresponding slots 26, 28 may have a
front edge 27
that is parallel to but offset rearward from a radius extending outward from
the central axis A.
The amount of offset may be in the order of 2-4 mm. The blades 16, 18 may be
provided with
laminated steel backings welded to the lid 14 to strengthen the blades 16, 18
and dampen
vibrations of the blades 16, 18. The underside 30 of the lid 14 (Fig. 4) in
the area around the
nozzles 24 is coated with a tungsten carbide material to protect the area from
erosion as the
drilling fluid is pumped through the nozzles 24. The lid 14 is also made from
circular steel
bar stock on a machine lathe. The under side of the lid 14 is milled to a
diameter suitable for
the intended use and the openings 22 are drilled through the lid for the
nozzle inserts 24.
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12 Referring to Figs. SA-SD, a blade 16 (or 18) is made from steel flat bar
stock of a
suitable width, thickness and length for the intended application. Multiple
holes 32, depending
on the required number of cutters, are drilled in the blade 16, and the blade
16 is then cut
along the line B in Fig. SC to yield cutter holes. Polycrystalline diamond
cutters 34 are
inserted in the holes 32 and soldered in place. The blade 16 is coated with a
tungsten carbide
hard metal to protect the blade from erosion. The cutters 34 are shown in
Figs. 1, SA-SC and
6 as fully penetrating the blades, but in practice there will be a small
amount of steel left
behind the cutters 34, as shown in Fig. SD.
13 Referring again to Fig. 1, junk slots 36, 38 are formed by each pair of
adjacent blades.
Each blade 16, 18 separates adjacent junk slots 36, 38. The junk slots
alternate in pairs 36, 38
around the cutting end 14B. Each pair of junk slots 36, 38 includes an impeded
junk slot 38
with a higher resistance to fluid flow and an unimpeded junk slot 36 with a
lower resistance to
fluid flow, with an intervening blade 16 between the pair of junk slots 36,
38. In operation,
the junk slot 38 of each pair of junk slots with higher resistance to fluid
flow forces drilling
fluid from the junk slot 38 with higher resistance into the junk slot 36 with
lower resistance.
Drilling fluid exiting the nozzles 24 is forced by the resistance of the junk
slots 38 across the
intervening blade 16 and between the cutters 34 of the intervening blade 16.
The drilling fluid
passes across the intervening blade through openings created by previous
cutters, which may
be ensured by radially offsetting and overlapping the cutters 34 on succeeding
blades. The
amount of offset and overlap may be varied. Increasing overlap creates a more
aggressive
cutting action, at the expense of decreasing the size of the flow path between
the cutters.
Thus, for cutters of radius R, the cutters on one blade may be spaced by R/2
and the centers of
cutters on succeeding blades offset by 3R/4. When the blades 16, 18 alternate
between
primary blades 16 and secondary blades 18, the primary blades 16 are the
intervening blades.
14 The higher resistance of the junk slots 38 may be caused by a variety of
means. For
example, the resistance may be caused by a restriction in the junk slot 38,
such as an
enlargement or extension 40 of a secondary blade 18 rearward. The extension 40
may sweep
circumferentially under the intervening or primary blade 16 as shown in Fig.
1. The extension
40 may be machined from a steel flat bar stock and welded to the outer
periphery of the shank
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12 and lid 14. The extension 40 is shaped to be continuous with the blade 18.
The radially
outward surface of the extensions 40 may be fluted in conventional fashion for
a stabilizer.
15 As seen in Fig. 1, the blades 16, 18 extend axially forward of the cutting
end 14B to
engage an earth formation during drilling. The blades 16, I8 also extend
radially outward
from the cutting end 14B and extend axially rearward along the outer periphery
42 of the drill
bit body to form stabilizers. The blades 16, 18 include blades that extend
axially rearward for
different distances, and in the preferred embodiment alternate between longer
blades 18 and
shorter blades 16. As seen in Fig. 3, the blades 16, 18 and the corresponding
slots 26, 28 are
also oriented on the cutting end 14B of the drill bit body such that a linear
extension of the
blade passes behind the central axis in the direction of rotation in
operation. The linear
extension may be part of the blade itself or an extrapolation of a blade that
terminates
inwardly of the central axis. Such off center orientation of the blades, where
the blades do
not all point towards the same center, assists in stabilizing the drill bit.
16 As the cutters 34 rotate around the central axis A, and cut into an earth
formation, they
leave gouges in the formation. Cutters 34 on succeeding blades deepen the
gouge. It is
conventional for cutters 34 on succeeding blades to overlap, and typically the
gouges created
by cutters of succeeding blades lie midway between the gouges of the preceding
blades. In a
preferred embodiment shown in Fig. 9, the cutters 34 in succeeding blades
preferably
differentially overlap cutters in preceding blades in the direction of
rotation such that a cutter
on a succeeding blade overlaps more of one, outer, cutter, on a preceding
blade than it
overlaps an adjacent, inner, cutter on a preceding cutter. The overlap of the
outer cutter
should be more than 25% but less than 100%, for example 60-75% of the outer
cutter. In Fig.
9, cutters 34A are in a leading or preceding blade in the direction of
rotation. Cutters 34B are
in the following or succeeding blade. The cross-hatched areas 37 indicates the
areas being cut
by the following blades. The hatched area 35 in the path of the cutters 34A
shows where
cuttings from the drilling activity of the following blades may slide sideways
away from the
cutters and be cleared from the cutting area. With the overlap system
described here, the
cutters of a preceding blade cut a slot in the formation through which fluids
can pass during
cutting by the cutters of a succeeding blade.
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17 The cutters, which are cylindrical or conical objects having an axis of
rotation, are
oriented on the respective blades with their axes of rotation tangential to a
circle centered on
the central axis A of the drill bit. The cutters 34 are also preferably
oriented on the respective
blades with their cutting faces parallel to the forward faces of the blades,
or may be canted
outward from the center of rotation by a side rake of 4°-11°.
Inner cutters may have a side
rake of 6-11°, while cutters at the gauge may have a side rake of
6°. With the blades behind
center and canted rearward, and the cutters on circle, vibration of the blades
during use tends
to sweep particles away from the cutting face and help prevent balling. It is
preferred to keep
the number of cutters 34 on the periphery or gauge of the drill bit to a
minimum required to
make a good gauge in the hole, with the cutters 34 concentrated on the forward
cutting end
14A. For example, for given gauge there need only be a single cutter set at
the outside edge
of each of the primary blades to produce that gauge. There need not be
multiple cutters 34
running axially rearward along the outer periphery of the blades.
18 Once the components are manufactured they are assembled. The lid 14 is
welded to
the shank 12 and the weld is ground smooth. The blades 16, 18 are set in the
rectangular slots
26, 28 in the top of the lid and welded in place as shown in Fig. 6.
19 With the design of the drill bit shown in Figs. 1-SD, greater angle can be
achieved on
the nozzle orientation because the nozzle holes 22 are drilled from the
underside of the lid 14
before it is welded to the shank 12. The nozzle orientation is important to
the cleaning
characteristics of PDC bits. If the nozzles 24 can be oriented at the correct
angle, cleaning is
enhanced, thus the bit will drill faster and cutter wear life is extended. In
addition, with the
method of manufacture shown in Fig. 6, the blades 16, 18 can be easily
replaced, unlike with
a matrix body or one piece steel body bit. When the blade of a one piece steel
or matrix body
bit is damaged, the bit may be un-repairable. To remove one of the blades 16,
18, the weld is
cut using a grinder, the blade is heated up and pops out or is easily pulled
out. Use of a lid 14
allows more blades to be used.
20 As shown in Fig. l, the blades 16, 18 are canted back away from the cutting
structure.
This improves cleaning and cuttings removal. The faster cuttings can be moved
away from the
blades the higher the rate of penetration (ROP) will be. This prevents bit
balling. Moving the
cuttings away from the blades quickly also prevents regrinding of the
cuttings, which can
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increase the temperature of the cutters. Increased temperature can cause
premature cutter
failure.
21 The flow restrictor 40, which also acts as a stabilizer, creates pressure
between the
primary and secondary blades 16, 18 for more efficient cuttings removal. The
flow restrictor
actually forces cross flow across the blades 16 between the cutters 34. That
is, the cuttings are
forced between the spaces in the cutters 34. This actually works better than
trying to get all
the cuttings to leave the bit face via the junk slot area. The higher
resistance may be achieved
by other means such as putting the secondary blades closer to the primary
blades. This will
create a higher pressure in the narrow passage between the primary and
secondary blades.
More generally, the concept is to force the cuttings to crossflow between the
cutters 34 on
every second blade.
22 As shown in Fig. SD and Fig. 7, during blade construction small diameter
holes
(conduits) 46 are drilled from the base of the blades 16, 18 and terminate in
the tops of the
blades 16, 18 below the cutters 34 but in heat conducting proximity to the
cutters 34, for
example 1-4 millimeters away. Heat conducting proximity means sufficiently
close to
provide a cooling effect to the cutters 34. The hollow conduits 46 are then
filled with a
material with high heat conductivity, at least higher than the heat
conductivity of the blade
material, such as copper. This high conductivity conduit can remove heat
quickly form the
PDC cutters 34 and dissipate the heat into the surrounding blade. The ends of
the heat
conducting conduits 46 near the cutters 34 may have small holes, not filled
with the high heat
conductivity material, drilled through the blade from the cutter to the metal
in the conduit.
Fig. 10 also shows a backing part 48 of the blade 16 behind the cutter 34, and
the hardened
cutting surface 50 of the cutter. A softer metal such as brass 52 may be
placed between the
cutter 34 and the backing part 48 to help reduce cutter vibration, as shown in
Fig. 10.
23 Immaterial modifications may be made to the embodiments of the invention
described
here without departing from the invention.
