PDC cutter for high compressive strength and highly abrasive formations

Abstract

A PDC cutter utilizes the combination of an elliptical shape with higher thermal resistance obtained through leaching to provide a cutter which is more effective than a cutter using either concept alone. The PDC cutter includes a tungsten carbide portion with protrusions extending from a surface thereof in a pattern. The diamond volume is mounted to the surface wherein the protrusions allow for the diamond volume to be larger about a perimeter edge of the cutter and smaller/shallower in a center region of the cutter. The protrusions providing surfaces for the diamond layer (table) to bond to the underlying tungsten carbide portion. With this configuration, diamond volume is maximized around the edge of the cutter.

Description

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a PDC cutter used for drilling boreholes into the earth.

2. Description of Related Art

Drilling high compressive strength rocks that are highly abrasive has always been a challenge for PDC cutters. When a rock has a high compressive strength, high force is required for the cutter to penetrate the rock. High cutting forces cause problems. When the high cutting forces are transmitted to the drill string, drilling problems occur. For example, the drill string can buckle or bow causing unwanted deviation of the well. High forces cause problems within the cutter. The higher forces cause more frictional heating. The heating causes rapid wear of the PDC cutter. Heating of the cutter causes thermal stresses within the cutter and subsequent cracking. These problems make drilling the formation with PDC cutters uneconomical.

Currently high compressive strength/highly abrasive formations can be drilled with impregnated drill bits. Impregnated drill bits can resist the higher heat of high bit weights. However, they take small depths of cut. To drill sufficiently fast, they must be turned at a high rpm. Turning at a high rpm requires special equipment, increasing the cost of drilling operations.

Currently high compressive strength/highly abrasive formations can be drilled with insert rock bits. These bits require high levels of weight to drill efficiently. The high WOB increases the risk of the drilling problems mentioned above. It also causes rapid wear of the seals and bearings of the rock bit. This shortens bit life requiring the operator to frequently change the bit.

The following references are related to leached cutters:

	6,410,085	25 Jun. 2002	Griffin et al
	6,435,058	20 Aug. 2002	Matthias et al
	6,481,511	19 Nov. 2002	Matthias et al
	6,544,308	8 Apr. 2003	Griffin et al
	6,562,462	13 May 2003	Griffin et al
	6,585,064	1 Jul. 2003	Griffin et al
	6,589,640	8 Jul. 2003	Griffin et al
	6,592,985	15 Jul. 2003	Griffin et al
	6,601,662	5 Aug. 2003	Matthias et al
	6,739,214	25 May 2004	Griffin et al
	6,749,033	15 Jun. 2004	Griffin et al
	6,797,326	28 Sep. 2004	Griffin et al
	6,878,447	12 Apr. 2005	Griffin et al
	6,861,098	1 Mar. 2005	Griffin et al
	6,861,137	1 Mar. 2005	Griffin et al

The following reference is related to an elliptical cutter:

6,808,031

26 Oct. 2004

Wilmot et al

The foregoing references are incorporated herein by reference.

PDC cutters are formed from a mix of materials processed under high-temperature and high pressure into a polycrystalline matrix of inter-bonded super hard carbon based crystals. A common trait of PDC cutters is the use of catalyzing materials during their formation. The residue from the catalyzing materials imposes a limit upon the maximum useful operating temperature of the element while in service.

The common form of PDC cutter is a two or more layer PDC cutter where a layer of polycrystalline diamond is integrally bonded to a substrate of tungsten carbide. The PDC cutter may be brazed to a carrier, often also of cemented tungsten carbide. This is a common configuration for PDC's used as cutting elements in drill bits.

PDC cutters are most often formed by sintering diamond powder with a catalyst in a high-pressure, high-temperature press. One particular method is disclosed in U.S. Pat. No. 3,141,746, the disclosure of which is hereby incorporated by reference. In the typical process for manufacturing PDC cutters, diamond powder is loaded in an assembly with a preformed tungsten carbide substrate incorporating cobalt. The assembly is then subjected to very high temperature and pressure in a press. During this process, cobalt migrates from the substrate into the diamond layer and acts as a catalyst, causing the diamond particles to bond to one another with diamond-to-diamond bonding, and also causing the diamond layer to bond to the substrate.

The completed PDC cutter has a matrix of diamond crystals bonded to each other with many interstices containing the catalyst metal. The catalyst metal is most commonly cobalt. The diamond crystals form a continuous matrix of diamond, and the interstices form a continuous matrix of catalyst metal. These two matrices are usually known as the diamond layer or diamond table. The diamond table is in turn bonded to the tungsten carbide substrate.

Typical PDC cutters have a diamond table that is 85% to 95% diamond by volume. The other 5% to 15% volume is metal catalyst. The PDC cutters are subject to thermal degradation due to differential thermal expansion between the metal catalyst and the diamond matrix. This type of thermal degradation begins at temperatures of about 400 degrees C. Upon sufficient expansion the diamond-to-diamond bonding may be ruptured and cracks and chips may occur.

Also in polycrystalline diamond, the presence of the metal catalyst in the interstices leads to another form of thermal degradation. Due to the presence of the metal catalyst, the diamond is caused to graphitize as temperature increases, typically limiting the operation temperature to about 750 degrees C.

To reduce thermal degradation, “thermally stable” polycrystalline diamond components have been produced. A typical configuration is described in U.S. Pat. No. 4,224,380, the disclosure of which is hereby incorporated by reference. In this type of thermally stable PDC cutter the cobalt is leached from the interstices. While this increases the temperature resistance of the diamond to about 1200 degrees C, the leaching process also removes the cemented carbide substrate. Because there is no integral substrate or other bondable surface, there are severe difficulties in mounting such material for use in operation.

The fabrication methods for this “thermally stable” PDC cutter typically produce relatively low diamond densities, of the order of 80% or less. This low diamond density enables a thorough leaching process, but the resulting finished part is typically relatively weak in impact strength.

In an alternative form of thermally stable polycrystalline diamond, silicon is used as the catalyzing material. The process for making polycrystalline diamond with a silicon catalyzing material is quite similar to that described above, except that at synthesis temperatures and pressures, most of the silicon is reacted to form silicon carbide, which is not an effective catalyzing material. The thermal resistance is somewhat improved, but thermal degradation still occurs due to some residual silicon remaining. Again, there are mounting problems with this type of PDC cutter because there is no bondable surface.

Efforts to combine thermally stable PDC's with mounting systems to put their improved temperature stability to use have not been as successful as hoped due to their low impact strength. For example, various ways of mounting multiple PDC cutters are shown in U.S. Pat. Nos. 4,726,718; 5,199,832; 5,025,684; 5,238,074; 6,009,963 herein incorporated by reference for all they disclose. Although many of these designs have had commercial success, the designs have not been particularly successful in combining high wear and/or abrasion resistance while maintaining the level of toughness attainable in non-thermally stable PDC.

Other types of diamond or diamond like coatings for surfaces are disclosed in U.S. Pat. Nos. 4,976,324; 5,213,248; 5,337,844; 5,379,853; 5,496,638; 5,523,121; 5,624,068 all herein incorporated by reference for all they disclose. Similar coatings are also disclosed in GB Patent Publication No. 2,268,768, PCT Publication No. 96/34,131, and EPC Publications 500,253; 787,820; 860,515 (all herein incorporated by reference for all they disclose) for highly loaded tool surfaces. In these publications, diamond and/or diamond like coatings are shown applied on surfaces for wear and/or erosion resistance.

Some attempts have been made to improve the toughness and wear resistance of these diamond or diamond like coatings by application to a tungsten carbide substrate and subsequently processing in a high-pressure, high-temperature environment as described in U.S. Pat. Nos. 5,264,283; 5,496,638; 5,624,068 herein incorporated by reference for all they contain. Although this type of processing may improve the wear resistance of the diamond layer, the abrupt transition between the high-density diamond layer and the substrate make the diamond layer susceptible to wholesale fracture at the interface at very low strains. This translates to very poor toughness and impact resistance in service.

The most successful solution to the above problems is a conventional PDC cutter with only a portion of the catalyst metal removed. This type of cutter has greatly improved resistance to thermal degradation without loss of impact strength. In more detail, a conventional PDC cutter has a table with a continuous diamond matrix with metal catalyst remaining in the interstices. The working surfaces have had the cobalt leached to a depth of a few thousandths of an inch.

With this configuration, the PDC cutter retains the strength of a conventional PDC cutter. The PDC cutter can also easily be attached to the drill bit on the carbide side of the cutter. However, the working surfaces have a layer of the diamond that is more resistant to thermal degradation. This gives a good combination of properties that exceed the performance properties of conventional PDC cutters.

PDC cutters with a thermally stable layer have exceeded the performance of conventional PDC cutters. They have expanded the application of PDC bits into harder and more abrasive formations. However, they have not been successful in high compressive strength highly abrasive applications. In these applications, the PDC cutters continue to wear at a high rate, rendering the drill bit uneconomical for use.

A PDC cutter with improved durability uses elliptical shaped cutters. These cutters have been marketed as “Oval” cutters. They are PDC cutters that have an elliptical form with a long (major) axis and a short (minor) axis. The elliptical cutters are shown diagrammatically in FIG. 1 beside their conventional round cutter equivalents as a comparison.

Conventional cylindrical cutters are placed with the diamond table facing the direction of bit rotation. The round edge of the cutter is pushed into the formation by the weight on bit. When elliptical cutters are used, the small end of the cutter is presented to the formation. This has the effect of a “sharper” edge. The sharper tip profile of the elliptical cutter form generates high point loading at lower weights on bit, increasing penetrating ability thereby enhancing ROP. Simply put, the elliptical cutter converts applied weight on bit into ROP more efficiently than a round cutter. That is, less weight on bit is required to push the cutter into the formation than a bit made with conventional cylindrical cutters.

The large surface area to point of contact ratio of the elliptical cutter prolongs cutter life through improved thermal management. The cutting tip is better preserved, and performance is maintained over longer bit runs. As the elliptical cutter wears, it stays sharper than a round cutter due to the development of a smaller carbide bearing area than an equivalent round cutter. The smaller contact area maintains point loading and reduces heat build up in the substrate and diamond table.

The success of this approach has been documented in field results. By replacing 13 mm round cutters on a bit with 19 mm×13 mm elliptical cutters, the diamond volume (density) and cutter exposure (height), is increased significantly. The use of the elliptical cutter form delivers increased diamond volume without affecting the number of cutters on the bit, thus maintaining the cutter setting and ROP performance.

The use of increasingly thick diamond tables on elliptical cutters was resisted until a design was perfected that capitalized on the advantages, but overcame the downsides associated with thick diamond.

Thick diamond layers, offered by all cutter manufacturers, promised much in terms of improved durability and increased run length. The reality was that it proved difficult to retain the diamond layer on the substrate due to residual stresses inherent in the manufacturing process. The stresses often caused significant spalling, and in some cases, the total de-lamination of the diamond layer, resulting in early bit failure.

In cases where the bond did not fail and the diamond table was retained, many PDC bits were being pulled for low penetration rates without heavy wear. This was because normal progressive cutter wear resulted in a large diamond bearing area that due to its size was unable to shear the formation. Effectively, as the cutter wore, the surface area of the diamond in contact with the formation became so enlarged that it could no longer penetrate the formation. Successful shearing of formation is only possible when the cutter can penetrate the rock. A sharp diamond “lip” is essential to enable this to happen.

SUMMARY OF THE INVENTION

While the concepts of leaching cutters and using elliptical shaped cutters have improved the performance of PDC bits, neither has been successful in drilling high compressive strength formations that are highly abrasive. The present invention utilizes the combination of elliptical shape and higher thermal resistance through leaching to provide a cutter which is more effective than a cutter using either concept alone. There appears to be a synergy between the two concepts.

In an embodiment, a PDC cutter utilizes the combination of an elliptical shape with higher thermal resistance obtained through leaching. A tungsten carbide portion includes protrusions which extend from a surface thereof in a pattern. The diamond volume is mounted to the surface wherein the protrusions allow for better attachment as well as for the diamond volume to be larger about a perimeter edge of the cutter and smaller/shallower in a center region of the cutter.

In an embodiment, a PDC cutter comprises: a tungsten carbide substrate having a top surface including a protrusion pattern formed in a center portion of the top surface; and a diamond table mounted to the top surface of the tungsten carbide substrate. The diamond table is thicker at a perimeter of the top surface and thinner at the center portion of the top surface, and the diamond table is leached.

In another embodiment, a PDC cutter comprises: a tungsten carbide substrate; and a diamond table mounted to the top surface of the tungsten carbide substrate, the diamond table presenting a chisel forward shape. The cutter has an elliptical shape, and the diamond table is leached.

In an embodiment, a PDC cutter comprises: a tungsten carbide substrate having a top surface including a protrusion pattern; and a diamond table mounted to the top surface of the tungsten carbide substrate. The diamond table is thicker at a perimeter of the top surface and thinner at the center portion of the top surface, and the diamond table is leached.

The invented configuration of PDC cutter seeks to increase drilling efficiency by the following: The shape of the PDC cutter requires less weight on bit than conventional PDC cutters, lessening wear and cracking due to frictional heating. It is composed of preferred PDC materials to increase the cutters resistance to heat. The shape of the cutter is optimized for high strength to reduce damage from high down hole forces. The shape of the cutter is optimized for easier cooling from the flow of the surrounding fluid. These combined factors enable the economical drilling of formations that were not previously drillable by PDC cutters. The PDC cutters increase the rate of penetration beyond impregnated or rock bits.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and apparatus of the present invention may be acquired by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein:

FIG. 1 diagrammatically shows elliptical cutters beside their conventional round cutter equivalents as a comparison;

FIG. 2 is a perspective cut-away view of an elliptical PDC cutter in accordance with an embodiment of the invention;

FIGS. 3A-3C show various views, side, top and perspective, respectively, of a plow shaped cutter in accordance with an embodiment of the invention;

FIGS. 4A-4D which show various views, two side, top and perspective, respectively, of an elliptical PDC cutter with concave surface in accordance with an embodiment of the invention;

FIGS. 5A-4C which show various views, side, top and perspective, respectively, of an elliptical PDC cutter with concave surface in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally speaking, the present invention relates to the use of a leached PDC cutter which has an elliptical cutter geometry. This cutter has a unique diamond layer geometry and configuration that minimizes the risk of de-lamination, and delays the onset of wear, but as wear progresses, allows the cutter to “lip” thus preserving the cutter's rock shearing capability.

As shown in FIG. 2, diamond volume is maximized around the edge of the cutter. Protrusions from the tungsten carbide portion 10 of the cutter form a pattern 12 (in this exemplary illustration, cross-hatched) which allows for the diamond 14 volume to be larger about the perimeter edge, and smaller/shallower in the center region, while providing surfaces for the diamond layer (table) to bond to the underlying tungsten carbide substrate. Again, the cutter has an elliptical shape and the diamond table has been subjected to leaching. When the cutter is unworn, this thicker diamond resists wear at the cutting tip as it is better able to cope with abrasion. In addition, as diamond is a good thermal conductor, it quickly draws heat away from the cutting tip, preventing diamond loss through thermal degradation (graphitization).

Once the wear progresses beyond this thick diamond edge, it passes into the interior of the cutter. The diamond layer here is thinner and as the wear flat initiates, it will generate a sharp diamond lip, maintaining the ability to shear the formation.

Another version of the cutter, again leached as described above, would be a cutter with a plow shape (see, FIGS. 3A-3C which show various views, side, top and perspective, respectively). The plow shape would have a sharper tip in the direction of bit rotation. This would aid in the failure of the rock.

Another version of the cutter, again leached as described above, would be an elliptical cutter with concave surface (see, FIGS. 4A-4D which show various views, two side, top and perspective, respectively). The concave surface is oriented through the large side of the cutter. When cutter is engaged in the formation at a given DOC a sharper tip is presented to the formation. With this type of shape the chip flow will be different allowing better fluid flow and better cooling of the cutter.

Another version of the cutter, again leached as described above, would be an elliptical cutter with concave surface (see, FIGS. 5A-4C which show various views, side, top and perspective, respectively) wherein the concave surface is oriented through the small side of the cutter (compare to opposite orientation shown in FIGS. 4A-4D). The concavity can be adjusted resulting in a different angle at the cutter tip. With conventional cutters the front face is at 90° to the side of the cutter. With this type of shape we can break the relationship between back rake and relief angle. For example in the above cutter, the angle at the tip is 15°. With a pocket back rake of 20°, we have a relief angle of 20°. But we have an actual back rake at the tip of 5°. Low back rakes are advantageous in soft formation. Another example is with a relief and pocket angle of 10° resulting in a forward rake of 5°. Cutters of this type are known to have a self penetrating effect. Difficulties in that past were obtaining a self penetrating cutting structure without weakening the supporting structure.

The following configurations are of importance to the present invention: 1) A cutter with a shape sharper than a conventional cylindrical cutter with a leached layer on the working surfaces of the cutter; 2) A cutter with an elliptical shape with a leached layer on the working surfaces of the cutter; 3) A cutter with a plow shape with a leached layer on the working surfaces of the cutter; 4) An LCA cutter with a leached layer on the working surfaces of the cutter; 5) A cutter with an elliptical shape and a concave front surface and with a leached layer on the working surfaces of the cutter; 6) A cutter with an elliptical shape and a convex front surface and with a leached layer on the working surfaces of the cutter; 7) A cutter like any of the above formed on a cylindrical cutter with the working surface not parallel to the back surface; and 8) Any of the above configurations with layers of coarse and fine diamond.

Although preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.

Claims

1. A PDC cutter, comprising: a tungsten carbide substrate having a top surface including a protrusion pattern formed in a center portion of the top surface; and a diamond table mounted to the top surface of the tungsten carbide substrate; wherein the diamond table is thicker at a perimeter of the top surface and thinner at the center portion of the top surface; and wherein the diamond table is leached.

2. The cutter of claim 1 wherein the cutter has an elliptical shape having a major axis and a minor axis.

3. The cutter of claim 2 wherein a top surface of the diamond table has a concave shape oriented through the major axis.

4. The cutter of claim 2 wherein a top surface of the diamond table has a concave shape oriented through the minor axis.

5. A PDC cutter, comprising: a tungsten carbide substrate; and a diamond table mounted to the top surface of the tungsten carbide substrate, the diamond table presenting a chisel forward shape; wherein the cutter has an elliptical shape ; and wherein the diamond table is leached.

6. A PDC cutter, comprising: a tungsten carbide substrate having a top surface including a protrusion pattern; and a diamond table mounted to the top surface of the tungsten carbide substrate; wherein the diamond table is thicker at a perimeter of the top surface and thinner at the center portion of the top surface; and wherein the diamond table is leached.

7. The cutter of claim 6 wherein the cutter has an elliptical shape having a major axis and a minor axis.

8. The cutter of claim 7 wherein a top surface of the diamond table has a concave shape oriented through the major axis.

9. The cutter of claim 7 wherein a top surface of the diamond table has a concave shape oriented through the minor axis.