Asymmetrical PDC cutter for a drilling bit

This invention relates to diamond drag bits. More particularly, this invention relates to diamond cutting elements for diamond drag bits.

Polycrystalline diamond compacts (PDC) are used extensively for cutters on drag bits for drilling soft to medium earthen formations in petroleum and mining exploration. One of the most common type PDC cutters used in diamond drag bits for drilling predominately ductile, medium strength formations is a cylinder type. A cylinder type PDC comprises a right cylinder tungsten carbide body with a thin layer (approximately 0.7 to 1 mm) of polycrystalline diamond chemically and metallurgically bonded to an end face of the cylinder using a high pressure/high temperature (HP/HT) sintering process.

Although cylindrical PDC type cutters serve a very useful purpose in drilling, there may be disadvantages in their use for certain applications. Typically, a cylinder type PDC cutter is fixedly mounted, by brazing, in a socket formed on the outer surface of a blade fabricated on the drilling face of a drag bit. The diamond face of the cutter is oriented substantially parallel to a radius of the borehole being drilled. The PDC cutter is positioned with back rake and heel clearance for the diamond cutting face by tilting the trailing end of the cutter body upward in relation to the borehole bottom.

For drilling many ductile rock formations, presently used PDC cylinder cutters do not have the necessary clearance from the diamond cutting edge to the supporting blade outer surface paralleling the formation bottom. Therefore, the displaced rock formation interferes with the aforementioned blade outer surface and greatly retards the drilling rate.

Also, because of the limited stand-off of the current diamond cutters from the blade surface, sufficient cooling and cleaning of the cutter often is not accomplished. This is because the entire exposed portion of the cutter is indented into the ductile rock leaving no room for the drilling fluid to flush across the cutter face.

Because of the relatively small exposure of the diamond cutting face of the prior art cutters, the drilling life of the bit is limited to the amount of wear the cutter can experience before the rock formation continuously bears on the insert supporting blade outer surface, effectively stopping the drilling process. This wear amount is normally somewhat less than one-half the cutter diameter.

Normally, prior art cylindrical PDC cutters only have approximately one half of the cutter body surface area brazed into the socket on the blade surface. In cases where the rock formations are tough in shear and high impact loads are experienced, the braze strength is often insufficient to keep the cutters in place, thereby contributing to the termination of the bit run.

As a normal PDC cylinder type cutter wears during drilling, an ever enlarging wear flat forms on the bottom side of the carbide cylinder body on the trailing side of the diamond layer, thus slowing the drilling rate. The possible magnitude of the wear flat is determined by the original amount of heel clearance between the diamond cutting point and the blade outer surface.

US-A-4,200,159 discloses (at Figure 3) a cutter insert having a cylindrical base end and a cylindrical cutter end having a diamond cutting surface thereon. The axis of the cutter end is inclined to the axis of the base end by an angle of about 12°.

A new PDC cutter for a drag type drilling bit is disclosed which overcomes the inadequacies of the prior art. The new asymmetric cutter provides more extension of the diamond cutting edge below the face of the drill bit. This permits better cleaning and cooling of the cutters and prevents the rock being drilled from bearing on the bit body surface, thereby significantly increasing the drilling rate and useful bit life.

It would be desirable to drill soft to medium ductility earthen formations at a faster rate and have a longer drilling life than is currently achieved with present day drag bits.

There is, therefore, provided in practice of this invention an improved cutter insert for a drag type of rock bit for drilling earthen formations comprising an asymmetric cutter insert comprising: an insert body having a first cylindrical base end and a second cutter end comprising an ultra-hard cutting surface thereon, the face of which is substantially 90 degrees to an axis of the cylindrical base end, characterised in that the second cutter end is non-cylindrical and a portion of the non-cylindrical cutter end of the insert projects beyond the circumferential wall formed by the cylindrical base end of the insert.

The invention also provides a drag rock bit for drilling earthen formations comprising: a rock bit body having a first threaded pin end and a second cutting end, the cutting end having at least a pair of generally radially disposed raised cutter blades and fluid channels formed therebetween, each fluid channel communicating with a fluid plenum formed within the bit body via at least one fluid exit port in the second cutting end of the bit body; and a plurality of asymmetric cutter inserts in each of the cutter blades, at least a portion of such cutter inserts being as recited above, the cylindrical base end of each such cutter insert being substantially completely encapsulated within a complementary cylindrical socket formed in the cutter blade, the non-symmetrical insert cutter end projecting beyond an end surface of the cutter blades for engaging an earthen formation.

The above noted features and advantages of the present invention will be more fully understood upon a study of the following description in conjunction with the detailed drawings wherein: <ul id="ul0001" list-style="none" compact="compact"><li>FIGURE 1 is a partial cross-sectional view of a blade formed on a drag bit having a prior art cylindrical type PDC cutter secured in place in the leading edge of the blade;</li><li>FIGURE 2 is a partial cross-sectional view of a blade formed on a drag bit illustrating a preferred embodiment of an asymmetric PDC cutter mounted in the leading edge of the blade;</li><li>FIGURE 3 is a perspective view of a preferred embodiment of the present invention illustrating a drag bit fitted with a new type asymmetric PDC cutter as illustrated by Figure 2;</li><li>FIGURE 4 is a perspective view of a preferred embodiment of a PDC cutter showing the diamond cutting end and the cylindrical mounting end of the cutter; and</li><li>FIGURE 5 is a face view of the diamond cutting end of the preferred embodiment of the PDC cutter.</li></ul>

With reference to the partial cross-sectional view of Figure 1, a blade, generally indicated at 10 is illustrated. The blade from a prior art drag bit is extended downward from a bit body toward a borehole bottom (not shown). The blade may be formed from steel or tungsten carbide matrix depending upon the specific field application. A cylindrical polycrystalline diamond compact (PDC) cutter 12 is shown brazed in a socket 14 which is formed into the leading edge 16 of the blade. A thin (0.7 - 1 mm) layer of polycrystalline diamond 20 is shown sintered to an end face 22 of the cylindrical carbide body 18.

The cutter socket 14 is tilted upwardly at the trailing end 15° to 20° in relation to the borehole bottom. This angled attitude of the cutter 12 provides negative back rake to the diamond cutter face 20 to give heel clearance "A" between the rock being drilled and the blade bottom surface 19. When using normal drilling weights, the PDC cutter is often buried in the rock formation to a depth "A" where the blade bottom 19 rides on the rock formation as the bit is rotated, thereby creating damaging heat. This also prevents drilling fluid from cleaning and cooling the cutter 10, thereby slowing the drilling rate and heat damaging cutter 10.

The cutter socket 14 envelops the cutter body 18 downward a small increment past the cutter centerline 21 as indicated by dashed line 23. This forms an interlock of the blade material that serves to hold the cutter 12 in place while it is being brazed into the blade socket 14. The braze is limited to an area that is only slightly more than one-half of the cylindrical surface 17 of the cutter body 18. This limited braze often fails from impact and tensile stresses encountered in the drilling process.

Figure 2 is a partial cross-sectional view of a drag bit blade 30 with the preferred embodiment of the asymmetric PDC cutter 32 mounted thereon. The blade 30 extends downwardly from the bit drilling head (48 on Fig. 3) to the borehole bottom (not shown). An asymmetric PDC cutter 32 is attached by brazing into a cylindrical socket 37 formed in the blade. The socket 34 is formed into the lower leading surface 35 of blade 30 at an angle of 15° to 35° in reference to the blade bottom surface 39, with the preferred angle being 20°.

The asymmetric PDC cutter has a cylindrical body or base 38 that is brazed into the complementary cutter socket 34. The drilling end 41 of cutter body 32 has an asymmetrical geometry with the body 38 being cylindrical forming a circumferential wall, 45 then blending into an off-set half cylindrical surface 41, which is positioned downward in the blade and forms the principal drilling end of the cutter. A thin (0.5 to 1 mm) polycrystalline diamond layer 40 is formed on the end surface 42 of the carbide body 38. The end face of the cutter is at an angle of 90° from the axis of the carbide body. The off-set or asymmetrical cutting end 41 provides the stand-off "B" between the bottom surface 39 of the blade and the rock formation being drilled.

This stand-off "B" is significantly greater than stand-off "A" as described and illustrated in Figure 1. This provides more clearance for drilling fluid to clean and cool the cutters and minimize the riding on the formation of the lower blade surface, thereby increasing the drilling rate. The shorter length "C" of the exposed cutter surface 41 reduces the amount of cutter bearing on the rock as the cutter wears while drilling, thereby drilling faster when using comparable drilling weights and rotational speeds as used with prior art bits.

As the drilling cutter socket 34 is a fully round cylindrical surface, the braze of the cutter body in the socket is very much superior to the prior art cutter braze which is only approximately 55% to 60% of that of the present invention.

Figure 3, a drag bit, generally designated as 45, comprises a bit body 47 having an open threaded pin end 46 and opposite cutting end generally designated as 48. Cutting end is comprised of a multiplicity of essentially radial raised lands or blades 30 and fluid channels 51 formed between. A number of fluid nozzles 40 are strategically positioned on the cutting end 48 to supply high velocity drilling fluid to fluid channels 51 to cool and clean the cutting end. A plurality of polycrystalline diamond compact (PDC) cutters 32 of the present invention are disposed strategically in the outer surfaces of the raised blades 30. As the bit 45 rotates on the bottom of a borehole (not shown), the diamond cutters 32 engage the rock formations with a shearing action to destroy the rock. The drilled rock cuttings are then entrained in the high velocity drilling fluid to exit up the borehole.

The asymmetric cutter insert has substantially larger heel relief than prior art cutters, and it can wear significantly more than a prior art cutter but still have a smaller wear flat.

Figure 4 is an isometric view of the preferred PDC cutter 32, as shown in Figures 2 and 3. Depicted is the asymmetrical off-set, essentially elliptical shaped diamond drilling layer 40, which is sintered to the drilling end face of the cemented tungsten carbide substrate portion. This off-set portion blends into the cylindrical carbide base end 38, which is brazed into the cylindrical socket (34 of Fig. 2) completely encapsulating the cylindrical section therein.

The asymmetrical portion 41 of the cutter is off-set 30% to 70% greater than the cylindrical diameter of base end 38, with 50% greater being the preferred off-set.

Figure 5 is a face view of the PDC cutter 32. It shows the diamond face layer 40, which is, for example, essentially elliptical in shape. This diamond drilling face 40 is comprised geometrically of two approximately semi-circular end surfaces having a common vertical axis 31. These semi-circular surfaces are joined by a rectangular surface whose sides 33 are tangent to the semi-circular arcs 43 at centerlines 44. In this embodiment the semi-circular ends have the same radius of curvature. If desired, the two approximately semi-circular end portions may have different radiuses of curvature. The end face may also be elliptical or other asymmetrical shape which has no sharp corners, cusps or the like.

It is well to note that the diamond layer 40 as illustrated in Figure 5 having two arcs with the same radii, can beneficially have arcs with differing radii depending upon the need for a sharper or blunter cutting tip.

It also should be noted that the diamond layer may be a curved surface or any other geometry, but the preferred embodiment is a planar diamond layer.

An advantage then of the present invention over prior art cutters is the designed asymmetric stand-off of the cutter's drilling edge and bit blade surface results in better cleaning and cooling of the cutter for increased drilling rates and bit drilling life. This asymmetric stand-off also prevents the blade's outer surface from riding on the rock formation, thus allowing greater depth of penetration of the cutter into the rock for higher drilling rates.

Another advantage of the present invention over prior art cylindrical cutters is the smaller wear flat surface formed on the carbide cutter body as the diamond cutting surface wears. This allows the cutter to penetrate the rock using lower drilling loads and still achieve better drilling rates.

Still another advantage of the present invention over prior art cutters is that by using a full-round mounting socket rather than a half-round socket a superior braze and better retention of the cylindrical base portion of the asymmetric diamond cutter to the bit blade is achieved.

Still another advantage of the present invention over prior art cutters is the generous relief formed behind the asymmetric diamond cutter face. This relief provides for less cutter body contact with the rock formation on the borehole bottom than is possible when using prior art straight cylinder cutters with the same amount of cutter wear. Thus, high drilling rates are achieved when using the same drilling weights as used with prior art cutters.

Although the diamond layer has heretofore been referred to as just a polycrystalline diamond layer, those skilled in the art realize that this diamond layer may be comprised of two or more transition layers of diamond powders and sintered tungsten carbide powders as needed for particular applications. If desired one may utilize cubic boron nitride instead of diamond for the ultra-hard cutting surface.

Thus, it will be realized that various other modifications can be made in the design and operation of the present invention without departing from the spirit thereof. Thus, while the principal preferred construction and mode of operation of the invention have been explained in what is now considered to represent its best embodiments, it should be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.