IMPREGNATED DRILL BITS AND METHODS FOR MAKING THE SAME

[0001] 1. Field of the Invention

[0002] Embodiments disclosed herein relate generally to drill bits, and more particularly to drill bits having impregnated cutting surfaces and the methods for the manufacture of such drill bits.

[0003] 2. Background Art

[0004] An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. When weight is applied to the drill string, the rotating drill bit engages the earth formation and proceeds to form a borehole along a predetermined path toward a target zone.

[0005] Different types of bits work more efficiently against different formation hardnesses. For example, bits containing inserts that are designed to shear the formation frequently drill formations that range from soft to medium hard. These inserts often have polycrystalline diamond compacts (PDC's) as their cutting faces.

[0006] Roller cone bits are efficient and effective for drilling through formation materials that are of medium to hard hardness. The mechanism for drilling with a roller cone bit is primarily a crushing and gouging action, in which the inserts of the rotating cones are impacted against the formation material. This action compresses the material beyond its compressive strength and allows the bit to cut through the formation.

[0007] For still harder materials, the mechanism for drilling changes from shearing to abrasion. For abrasive drilling, bits having fixed, abrasive elements are preferred. While bits having abrasive polycrystalline diamond cutting elements are known to be effective in some formations, they have been found to be less effective for hard, very abrasive formations such as sandstone. For these hard formations, cutting structures that comprise particulate diamond, or diamond grit, impregnated in a supporting matrix are effective. In the discussion that follows, components of this type are referred to as “diamond impregnated.”

[0008] Diamond impregnated drill bits are commonly used for boring holes in very hard or abrasive rock formations. The cutting face of such bits contains natural or synthetic diamonds distributed within a supporting material to form an abrasive layer. During operation of the drill bit, diamonds within the abrasive layer are gradually exposed as the supporting material is worn away. The continuous exposure of new diamonds by wear of the supporting material on the cutting face is the fundamental functional principle for impregnated drill bits.

[0009] The construction of the abrasive layer is of critical importance to the performance of diamond impregnated drill bits. The abrasive layer typically contains diamonds and/or other super-hard materials distributed within a suitable supporting material. The supporting material must have specifically controlled physical and mechanical properties in order to expose diamonds at the proper rate.

[0010] Metal-matrix composites are commonly used for the supporting material because the specific properties can be controlled by modifying the processing or components. The metal-matrix usually combines a hard particulate phase with a ductile metallic phase. The hard phase often consists of tungsten carbide and other refractory or ceramic compounds. Copper or other nonferrous alloys are typically used for the metallic binder phase. Common powder metallurgical methods, such as hot-pressing, sintering, and infiltration are used to form the components of the supporting material into a metal-matrix composite. Specific changes in the quantities of the components and the subsequent processing allow control of the hardness, toughness, erosion and abrasion resistance, and other properties of the matrix.

[0011] Proper movement of fluid used to remove the rock cuttings and cool the exposed diamonds is important for the proper function and performance of diamond impregnated bits. The cutting face of a diamond impregnated bit typically includes an arrangement of recessed fluid paths intended to promote uniform flow from a central plenum to the periphery of the bit. The fluid paths usually divide the abrasive layer into distinct raised ribs with diamonds exposed on the tops of the ribs. The fluid provides cooling for the exposed diamonds and forms a slurry with the rock cuttings. The slurry must travel across the top of the rib before reentering the fluid paths, which contributes to wear of the supporting material.

[0012] An example of a prior art diamond impregnated drill bit is shown in FIG. 1. The impregnated bit 10 includes a bit body 12 and a plurality of ribs 14 that are formed in the bit body 12. The ribs 14 are separated by channels 16 that enable drilling fluid to flow between and both clean and cool the ribs 14. The ribs 14 are typically arranged in groups 20 where a gap 18 between groups 20 is typically formed by removing or omitting at least a portion of a rib 14. The gaps 18, which may be referred to as “fluid courses,” are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit 10 toward the surface of a wellbore (not shown).

[0013] Impregnated bits are typically made from a solid body of matrix material formed by any one of a number of powder metallurgy processes known in the art. During the powder metallurgy process, abrasive particles and a matrix powder are infiltrated with a molten binder material. Upon cooling, the bit body includes the binder material, matrix material, and the abrasive particles suspended both near and on the surface of the drill bit. The abrasive particles typically include small particles of natural or synthetic diamond. Synthetic diamond used in diamond impregnated drill bits is typically in the form of single crystals. However, thermally stable polycrystalline diamond (TSP) particles may also be used.

[0014] In one impregnated bit forming process, the shank of the bit is supported in its proper position in the mold cavity along with any other necessary formers, e.g. those used to form holes to receive fluid nozzles. The remainder of the cavity is filled with a charge of tungsten carbide powder. Finally, a binder, and more specifically an infiltrant, typically a nickel brass copper based alloy, is placed on top of the charge of powder. The mold is then heated sufficiently to melt the infiltrant and held at an elevated temperature for a sufficient period to allow it to flow into and bind the powder matrix or matrix and segments. For example, the bit body may be held at an elevated temperature (>1800° F.) for a period on the order of 0.75 to 2.5 hours, depending on the size of the bit body, during the infiltration process.

[0015] By this process, a monolithic bit body that incorporates the desired components is formed. One method for forming such a bit structure is disclosed in U.S. Pat. No. 6,394,202 (the '202 patent), which is assigned to the assignee of the present invention and is hereby incorporated by reference.

[0016] Referring now to FIG. 2, a drill bit 22 in accordance with the '202 patent comprises a shank 24 and a crown 26. Shank 24 is typically formed of steel and includes a threaded pin 28 for attachment to a drill string. Crown 26 has a cutting face 29 and outer side surface 30. According to one embodiment, crown 26 is formed by infiltrating a mass of tungsten-carbide powder impregnated with synthetic or natural diamond, as described above.

[0017] Crown 26 may include various surface features, such as raised ridges 32. Preferably, formers are included during the manufacturing process so that the infiltrated, diamond-impregnated crown includes a plurality of holes or sockets 34 that are sized and shaped to receive a corresponding plurality of diamond-impregnated inserts 36. Once crown 26 is formed, inserts 36 are mounted in the sockets 34 and affixed by any suitable method, such as brazing, adhesive, mechanical means such as interference fit, or the like. As shown in FIG. 2, the sockets can each be substantially perpendicular to the surface of the crown. Alternatively, and as shown in FIG. 2, holes 34 can be inclined with respect to the surface of the crown 26. In this embodiment, the sockets are inclined such that inserts 36 are oriented substantially in the direction of rotation of the bit, so as to enhance cutting.

[0018] As a result of the manufacturing technique of the '202 patent, each diamond-impregnated insert is subjected to a total thermal exposure that is significantly reduced as compared to previously known techniques for manufacturing infiltrated diamond-impregnated bits. For example, diamonds imbedded according to methods disclosed in the '202 patent have a total thermal exposure of less than 40 minutes, and more typically less than 20 minutes (and more generally about 5 minutes), above 1500° F. This limited thermal exposure is due to the shortened hot pressing period and the use of the brazing process.

[0019] The total thermal exposure of methods disclosed in the '202 patent compares very favorably with the total thermal exposure of at least about 45 minutes, and more typically about 60-120 minutes, at temperatures above 1500° F., that occurs in conventional manufacturing of furnace-infiltrated, diamond-impregnated bits. If diamond-impregnated inserts are affixed to the bit body by adhesive or by mechanical means such as interference fit, the total thermal exposure of the diamonds is even less.

[0020] With respect to the diamond material to be incorporated (either as an insert, or on the bit, or both), diamond granules are formed by mixing diamonds with matrix power and binder into a paste. The paste is then extruded into short “sausages” that are rolled and dried into irregular granules. The process for making diamond-impregnated matrix for bit bodies involves hand mixing of matrix powder with diamonds and a binder to make a paste. The paste is then packed into the desired areas of a mold. The resultant irregular diamond distribution has clusters with too many diamonds, while other areas are void of diamonds. The diamond clusters lack sufficient matrix material around them for good diamond retention. The areas void or low in diamond concentration have poor wear properties. Accordingly, the bit or insert may fail prematurely, due to uneven wear. As the motors or turbines powering the bit improve (higher sustained RPM), and as the drilling conditions become more demanding, the durability of diamond-impregnated bits needs to improve. However, generally, as durability of a bit increases (with a harder matrix), diamond exposure (and thus ROP) generally decreases, and vice versa. Accordingly, there exists a continuing need for improvements in diamond impregnated cutting structures to improve wear properties, rate of penetration, and diamond distribution.

[0021] In one aspect, embodiments disclosed herein relate to an impregnated cutting structure that includes a plurality of first encapsulated particles, each first encapsulated particle comprising a first abrasive particle encapsulated by a first matrix material shell; and a plurality of second encapsulated particles, the second encapsulated particles comprising a second abrasive particle encapsulated by a second matrix material shell, wherein the first encapsulated particles and the second encapsulated particles have at least one property difference.

[0022] In another aspect, embodiments disclosed herein relate to a drill bit that includes a bit body; and a plurality of ribs formed in the bit body, wherein at least one rib comprises: a plurality of first encapsulated particles, each first encapsulated particle comprising a first abrasive particle encapsulated by a first matrix material shell; a plurality of second encapsulated particles, each second encapsulated particle comprising a second abrasive particle encapsulated by a second matrix material shell, wherein the first encapsulated particles and the second encapsulated particles comprise at least one property difference therebetween.

[0023] In another aspect, embodiments disclosed herein relate to drill bit that includes a bit body; and a plurality of ribs formed in the bit body, wherein a portion of at least one rib has a height to width ratio of greater than about 1.75 with a minimum diamond concentration of 100 and comprises: a plurality of first encapsulated particles, each first encapsulated particle comprising a first abrasive particle encapsulated by a first matrix material shell.

[0024] In yet another aspect, embodiments disclosed herein relate to a method of forming an impregnated cutting structure that includes loading a plurality of first encapsulated particles and a plurality of second encapsulated particles into a mold cavity, each first encapsulated particle comprising a first abrasive particle encapsulated by a first matrix material shell and each second encapsulated particle comprising a second abrasive particle encapsulated by a second matrix material shell, wherein the first encapsulated particles and the second encapsulated particles comprise at least one property difference therebetween; and heating the mold contents to form an impregnated cutting structure.

[0025] Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

[0032] In one aspect, embodiments disclosed herein relate to encapsulated particles. In other aspects, embodiments disclosed herein relate to impregnated cutting structures or impregnated drill bits containing encapsulated particles. The use of encapsulated particles in cutting structures is described for example in U.S. Patent Publication No. 2006/0081402 and U.S. application Ser. Nos. 11/779,083 and 11/779,104, all of which are assigned to the present assignee, and herein incorporated by reference in their entireties.

[0033] Referring to FIG. 2, a cross-section of an embodiment of a cutting structure is illustrated. As shown in FIG. 3, cutting structure 200 includes encapsulated particles 210 and encapsulated particles 220. Each encapsulated particle 210, 220 is formed of an abrasive particle 212, 222 coated or surrounded by encapsulating shell 214, 224 of matrix powder material. Referring to FIG. 3, a cross-section of an embodiment of an alternative cutting structure is illustrated. As shown in FIG. 4, similar to FIG. 2, cutting structure 300 includes encapsulated particles 310 and encapsulated particles 320, where each encapsulated particle 310, 320 is formed of an abrasive particle 312, 322 coated or surrounded by encapsulating shell 314, 324 of matrix powder material. Additionally, a third matrix powder material 304 is infiltrated through the cutting structure 300.

[0034] As shown in FIGS. 2 and 3, encapsulated particles 210, 310 differ from encapsulated particles 220, 320 in several ways; however, in a particular embodiment, only one difference between two (or otherwise multiple) encapsulated particles in a cutting structure need exist. Of the types of differences that may exist, variation in total encapsulated particle size; matrix material composition, shell thickness, or wear properties; or abrasive particle type, size, strength, or retention coating may exist among encapsulated particles. As illustrated, encapsulated particles 210, 310 differ from encapsulated particles 220, 320, for example, in their total size, in the size of abrasive particles 212, 312 as compared to abrasive particles 222, 322, and the type of encapsulating shell 214, 314 as compared to encapsulating shell 224, 324. However, one of ordinary skill would appreciate that other combinations of the component parts may be used, depending on the particular application. Each of these component parts will be further discussed, including a description of embodiments of various impregnated cutting structures.

[0035] Abrasive Particles

[0036] In some embodiments, abrasive particles may be selected from synthetic diamond, natural diamond, reclaimed natural or synthetic diamond grit, silicon carbide, aluminum oxide, tool steel, boron carbide, cubic boron nitride (CBN), thermally stable polycrystalline diamond (TSP), or combinations thereof.

[0037] The shape of the abrasive particles may also be varied as abrasive particles may be in the shape of spheres, cubes, irregular shapes, or other shapes. In some embodiments, abrasive particles may range in size from 0.2 to 2.0 mm in length or diameter; from 0.3 to 1.5 mm in other embodiments; from 0.4 to 1.2 mm in other embodiments; and from 0.5 to 1.0 mm in yet other embodiments.

[0038] However, particle sizes are often measured in a range of mesh sizes, for example −40+80 mesh. The term “mesh” actually refers to the size of the wire mesh used to screen the particles. For example, “40 mesh” indicates a wire mesh screen with forty holes per linear inch, where the holes are defined by the crisscrossing strands of wire in the mesh. The hole size is determined by the number of meshes per inch and the wire size. The mesh sizes referred to herein are standard U.S. mesh sizes. For example, a standard 40 mesh screen has holes such that only particles having a dimension less than 420 μm can pass. Particles having a size larger than 420 μm are retained on a 40 mesh screen and particles smaller than 420 μm pass through the screen. Therefore, the range of sizes of the particles is defined by the largest and smallest grade of mesh used to screen the particles. Particles in the range of −16+40 mesh (i.e., particles are smaller than the 16 mesh screen but larger than the 40 mesh screen) will only contain particles larger than 420 μm and smaller than 1190 μm, whereas particles in the range of −40+80 mesh will only contain particles larger than 180 μm and smaller than 420 μm. Thus, in some embodiments, abrasive particles may include particles not larger than would be filtered by a screen of 10 mesh. In other embodiments, abrasive particles may range in size from −15+35 mesh. In a particular embodiment, a first encapsulated particle, i.e., encapsulated particles 210, 310 as shown in FIGS. 2 and 3, may include abrasive particles, i.e., particles 212, 312, ranging in size from −20+25 mesh, while a second capsulated particle, i.e., encapsulated particles 220, 320 as shown in FIGS. 2 and 3, may include abrasive particles, i.e., particles 222, 322, ranging in size from −25+35 mesh. However, one of ordinary skill would recognize that the particle sizes and distribution of the particle sizes of the abrasive particles may be selected to allow for a broad, uniform, or bimodal distribution, for example, depending on a particular application, and that size ranges outside the distribution discussed above may also be selected. Further, although particle sizes or particle diameters are referred to, it is understood by those skilled in the art that the particles may not necessarily be spherical in shape.

[0039] Further, as discussed above, various abrasive particles that may be selected for use in the encapsulated may vary in type (i.e., chemical composition) such that the multiple types of encapsulated particles may use different types of abrasive particles; however, one of ordinary skill in the art would appreciate that among these particles, there may also be a difference in compressive strength of the particles. For example, some synthetic diamond grit may have a greater compressive strength than natural diamond grit and/or reclaimed grit. Furthermore, even within the general synthetic grit type, there may exist different grades of grit having differing compressive strengths, such as those grades of grit commercially available from Element Six Ltd. (Berkshire, England).

[0040] In addition to varying the strength of the abrasive particles, the presence and identity of an interior, retention coating on the surface of the abrasive particle may also optionally be varied. Thus, in some embodiments, one type of encapsulated particle formed from abrasive particles having an interior, retention coating thereon may be used in combination with another type of encapsulated particle formed from abrasive particles which do not have an interior, retention coating. In other embodiments, different coatings may be used between the encapsulated particle type, such as for example, a weaker PVD coating on abrasive particles in a first type of encapsulated particles, and a stronger CVD coating on abrasive particles in a second type of encapsulated particles. Such interior coatings may be applied by conventional techniques such as CVD or PVD, and are in contrast to the encapsulating outer shell of matrix powder material used in embodiments of the present disclosure. One of ordinary skill in the art would appreciate that the interior, thin coatings (having a thickness of only a few micrometers as compared to the thicker encapsulating shell) may be more helpful for high temperature protection (e.g., SiC coatings) while others are helpful for grit retention (e.g., TiC). In certain embodiments, the “interior” coating (TiC in the above example) may help bond the diamond to the “outer” matrix coating. Additionally, in certain applications the interior coating may reduce thermal damage to the particles.

[0041] Encapsulating Shell

[0042] The encapsulating shell of matrix powder material may include a mixture of a carbide compounds and/or a metal alloy using any technique known to those skilled in the art. For example, encapsulating matrix material may include at least one of macrocrystalline tungsten carbide particles, carburized tungsten carbide particles, cast tungsten carbide particles, and sintered tungsten carbide particles. In other embodiments non-tungsten carbides of vanadium, chromium, titanium, tantalum, niobium, and other carbides of the transition metal group may be used. In yet other embodiments, carbides, oxides, and nitrides of Group IVA, VA, or VIA metals may be used. A binder powder may also optionally include a binder powder that may, for example, include cobalt, nickel, iron, chromium, copper, molybdenum and other transition elements and their alloys, and combinations thereof. Further, a non-metallic binder phase, such as polyethylene glycol (PEG) or organic wax.

[0043] Tungsten carbide is a chemical compound containing both the transition metal tungsten and carbon. This material is known in the art to have extremely high hardness, high compressive strength and high wear resistance which makes it ideal for use in high stress applications. Its extreme hardness makes it useful in the manufacture of cutting tools, abrasives and bearings, as a cheaper and more heat-resistant alternative to diamond.

[0044] Sintered tungsten carbide, also known as cemented tungsten carbide, refers to a material formed by mixing particles of tungsten carbide, typically monotungsten carbide, and particles of cobalt or other iron group metal, and sintering the mixture. In a typical process for making sintered tungsten carbide, small tungsten carbide particles, e.g., 1-15 micrometers, and cobalt particles are vigorously mixed with a small amount of organic wax which serves as a temporary binder. An organic solvent may be used to promote uniform mixing. The mixture may be prepared for sintering by either of two techniques: it may be pressed into solid bodies often referred to as green compacts; alternatively, it may be formed into granules or pellets such as by pressing through a screen, or tumbling and then screened to obtain more or less uniform pellet size.

[0045] Such green compacts or pellets are then heated in a vacuum furnace to first evaporate the wax and then to a temperature near the melting point of cobalt (or the like) to cause the tungsten carbide particles to be bonded together by the metallic phase. After sintering, the compacts are crushed and screened for the desired particle size. Similarly, the sintered pellets, which tend to bond together during sintering, are crushed to break them apart. These are also screened to obtain a desired particle size. The crushed sintered carbide is generally more angular than the pellets, which tend to be rounded.

[0046] Cast tungsten carbide is another form of tungsten carbide and has approximately the eutectic composition between bitungsten carbide, W₂C, and monotungsten carbide, WC. Cast carbide is typically made by resistance heating tungsten in contact with carbon, and is available in two forms: crushed cast tungsten carbide and spherical cast tungsten carbide. Processes for producing spherical cast carbide particles are described in U.S. Pat. Nos. 4,723,996 and 5,089,182, which are herein incorporated by reference. Briefly, tungsten may be heated in a graphite crucible having a hole through which a resultant eutectic mixture of W₂C and WC may drip. This liquid may be quenched in a bath of oil and may be subsequently comminuted or crushed to a desired particle size to form what is referred to as crushed cast tungsten carbide. Alternatively, a mixture of tungsten and carbon is heated above its melting point into a constantly flowing stream which is poured onto a rotating cooling surface, typically a water-cooled casting cone, pipe, or concave turntable. The molten stream is rapidly cooled on the rotating surface and forms spherical particles of eutectic tungsten carbide, which are referred to as spherical cast tungsten carbide.

[0047] The standard eutectic mixture of WC and W₂C is typically about 4.5 weight percent carbon. Cast tungsten carbide commercially used as a matrix powder typically has a hypoeutectic carbon content of about 4 weight percent. In one embodiment of the present invention, the cast tungsten carbide used in the mixture of tungsten carbides is comprised of from about 3.7 to about 4.2 weight percent carbon.

[0048] Another type of tungsten carbide is macro-crystalline tungsten carbide. This material is essentially stoichiometric WC. Most of the macro-crystalline tungsten carbide is in the form of single crystals, but some bicrystals of WC may also form in larger particles. Single crystal monotungsten carbide is commercially available from Kennametal, Inc., Fallon, Nev.

[0049] Carburized carbide is yet another type of tungsten carbide. Carburized tungsten carbide is a product of the solid-state diffusion of carbon into tungsten metal at high temperatures in a protective atmosphere. Sometimes it is referred to as fully carburized tungsten carbide. Such carburized tungsten carbide grains usually are multi-crystalline, i.e., they are composed of WC agglomerates. The agglomerates form grains that are larger than the individual WC crystals. These large grains make it possible for a metal infiltrant or an infiltration binder to infiltrate a powder of such large grains. On the other hand, fine grain powders, e.g., grains less than 5 μm, do not infiltrate satisfactorily. Typical carburized tungsten carbide contains a minimum of 99.8% by weight of WC, with total carbon content in the range of about 6.08% to about 6.18% by weight.

[0050] According to one embodiment of the present disclosure, the encapsulating shell of a first encapsulated particle is chosen to be different from the encapsulating shell of a second encapsulated particle. This difference(s) between the matrix powder materials of the encapsulated particles may include variations in chemical make-up or particle size ranges/distribution, which may translate, for example, into a difference in wear or erosion resistance properties of the encapsulating shell. Thus, for example, different types of carbide (or other hard) particles may be used among the different types of encapsulated particles. One of ordinary skill in the art would appreciate that a particular variety of tungsten carbide, for example, may be selected based on hardness/wear resistance. Further, chemical make-up may also be varied by altering the percentage s/ratios of the amount of hard particles as compared to binder powder. Thus, by decreasing the amount of tungsten carbide particle and increasing the amount of binder powder in an encapsulating shell, a softer encapsulating shell may be obtained, and vice versa.

[0051] Further, with respect to particle sizes, each type of matrix material (for respective types of encapsulated particles) may be individually be selected from particle sizes that may range in various embodiments, for example, from about 1 to 200 micrometers, from about 1 to 150 micrometers, from about 10 to 100 micrometers, and from about 5 to 75 micrometers in various other embodiments or may be less than 50, 10, or 3 microns in yet other embodiments. In a particular embodiment, each type of matrix material (for respective types of encapsulated particles) may have a particle size distribution individually selected from a mono, bi- or otherwise multi-modal distribution.

[0052] Thus, referring to FIG. 2, one of ordinary skill in the art would recognize that the wear properties of an encapsulating shell 214 relative to an encapsulating shell 224 may be tailored by changing their respective chemical makeup. Depending on the anticipated final use of the cutting structure, encapsulating shell 214 may be softer and less wear resistant than encapsulating shell 224. In another embodiment, encapsulating shell 214 may be substantially softer and less wear resistant than encapsulating shell 224. In such an embodiment, the relative ease of erosion of encapsulating shell 214 would allow the abrasive particles 212 to be exposed to the formation quickly, while providing the bit matrix with increased durability and life through encapsulating shell 224, which does not wear or erode near as quickly. Thus, variable wear among the encapsulated particle may allow for dual optimization of rate of penetration (ROP) and durability, which are otherwise inapposite performance characteristics. That is, for increased ROP, increased rates of diamond exposure are necessary (and thus less wear resistance of the matrix material in which the diamonds are impregnated); however, for durability, greater wear resistance of the matrix material is desirable so that the bit does not wear away as quickly. Further, the variable wear of the encapsulating shell may also allow for fluid pathways to be created in the cutting structure that may allow for efficient cuttings removal. However, as the variable wear is on a micro-level (the material around neighboring abrasive particle in the formed cutting structure may possess differential wear properties, as compared to a larger or macro-region), the fluids channels that form are similarly on such a micro-level.

[0053] A desirable shell thickness may vary depending on the final intended use of the cutting structure. Also, the thickness may vary depending on the sizes of abrasive grit used in forming encapsulated particle. In some embodiments, an encapsulating shell may have an average thickness ranging from 0.1 to 1.5 mm. In other embodiments, a shell may have an average thickness ranging from 0.1 to 1.3 mm; from 0.15 to 1.1 mm in other embodiments; and from 0.2 to 1.0 mm in yet other embodiments. In most embodiments, a shell may have an average thickness ranging from 750 micrometers to 1000 micrometers.

[0054] Further, while the encapsulated particles are shown in FIGS. 2 and 3 as having shells of approximately the same thickness, the present invention is not so limited. The thickness and chemical composition of the encapsulating shells may be tailored to achieve a desirable wear rate. Thus, abrasive grits may be of different sizes or of different kinds, and the encapsulating shells may be of various thicknesses and comprise matrices, which may wear at different rates thereby exposing the grits at different rates. The composition and thickness of this second matrix may also affect the rate at which the encapsulated grit is exposed. For example, it may take a longer time to expose the abrasive grit in an encapsulated particle with a larger shell thickness than a grit with a smaller shell thickness of same chemical composition.

[0055] Encapsulated Particles

[0056] Encapsulated particles may be formed by encapsulating or coating abrasive particles with matrix powder material using encapsulation techniques known to one skilled in the art. In a particular embodiment, at least two types of encapsulated particles are used to form a variable impregnated cutting structure. In embodiments where two types of encapsulated particles are used, the ratio of those particles may range from 20:80 to 80:20 in one embodiment, and 30:70-70:30 in another embodiment. However, one of ordinary skill in the art would appreciate that other number of types of encapsulated particles may find use in the cutting structures of the present disclosure. For example, where three encapsulated particle types are desired, a first particle type may represent 5 to 30 percent, a second particle type representing 10-40 percent, and a third particle type representing 30-85 percent of the total amount of encapsulating particles. However, one of ordinary skill in the art would appreciate the particular combination of encapsulated particle types and amounts may be varied depending on the particular application.

[0057] In some embodiments, each type of encapsulated particle may have an individually selected average diameter (or equivalent diameter) ranging from 0.3 to 3.5 mm. In other embodiments, encapsulated particles may have an average diameter ranging from 0.4 to 3.0 mm; from 0.5 to 2.5 mm in other embodiments; and from 0.7 to 2.0 mm in yet other embodiments. In other embodiments, encapsulated particles 38 may include particles not larger than would be filtered by a screen of 5 mesh. In other embodiments, encapsulated particles may range in size from −10+25 mesh. While the encapsulated particles are primarily shown as spheres, one of ordinary skill in the art would appreciate that the present disclosure is not so limited.

[0058] In various embodiments, encapsulated particles may be obtained from commercial sources, or synthesized using encapsulation techniques known to those of ordinary skill in the art.

[0059] Infiltrating Matrix Material

[0060] For embodiments where an infiltrating matrix material is used, the infiltrating matrix material may include hard particles and a binder phase. Such exemplary hard particles include tungsten (W) or a derivative such as tungsten carbide (WC), sintered tungsten carbide/cobalt (WC—Co) (spherical or crushed), cast tungsten carbide (particulate or crushed), macro-crystalline tungsten carbide, carburized tungsten carbide, other carbides, or combinations of these materials with an optional binder. In other embodiments, the infiltrating matrix material may be formed from hard particle materials such as carbides or nitrides of tungsten, vanadium, boron, titanium, or combinations thereof. Typically, a binder phase may be formed from a powder component and/or an infiltrating component. In some embodiments of the present invention, hard particles may be used in combination with a powder binder such as cobalt, nickel, iron, chromium, copper, molybdenum and their alloys, and combinations thereof. In various other embodiments, the first matrix material 44 may include a Cu—Mn—Ni alloy, Ni—Cr—Si—B—Al—C alloy, Ni—Al alloy, and/or Cu—P alloy. In other embodiments, the infiltrating matrix material may include carbides in amounts ranging from 50 to 70% by weight in addition to at least one binder in amount ranging from 30 to 50% by weight thereof to facilitate bonding of matrix material and impregnated materials. In one embodiment, the resulting infiltrating matrix material may be chosen to be very tough, yet maintain good cutting properties. Additionally, tungsten carbide, in particular a fine-grained tungsten carbide, may present an optimum matrix for controlled wear and cuttings removal.

[0061] In various embodiments, the infiltrating matrix may include hard particles ranging in size from about 1 to 200 micrometers, or about 5 to 150 micrometers, or about 10 to 100 micrometers. One of ordinary skill in the art would recognize that the particular combination of hard particle material and particle size used in the matrix material may depend, for example, on whether the particles disclosed herein are being used in a insert or a rib of a bit body so that desired properties such as wear resistance and ability to be infiltrated may be optimized.

[0062] One of ordinary skill in the art would recognize that the particular combination of carbides and binders used in the infiltrating matrix material may be tailored depending on the anticipated final use of the cutting structure. For example, the combination used may be customized for desired properties such as wear resistance and ability to be infiltrated. The infiltrating matrix material may be chosen to have sufficient hardness so that the impregnated materials, namely the encapsulated particles, exposed at the cutting face are not pushed into the matrix material under the very high pressures commonly encountered in drilling. In addition, the infiltrating matrix material may be selected to withstand continuous mechanical action such as rubbing, scraping, or erosion that typically occurs during drilling so that the impregnated materials are not prematurely released.

[0063] Manufacture of Cutting Structures Using Encapsulated Particles

[0064] In one embodiment, uniformly coated encapsulated particles are manufactured prior to the formation of the impregnated bit. An exemplary method for achieving “uniform coatings” is to mix the abrasive particles, and a matrix material in a commercial mixing machine such as a Turbula Mixer or similar machine used for blending diamonds with matrix. The resultant mix may then be processed through a “granulator” in which the mix is extruded into short “sausage” shapes which are then rolled into balls and dried. The granules that are so formed must be separated using a series of mesh screens in order to obtain the desired yield of uniformly coated particles. At the end of this process, a number of particles of approximately the same size and shape can be collected, and optionally pre-sintered. Another exemplary method for achieving a uniform matrix coating on the abrasive grits is to use a machine called a Fuji Paudal pelletizing machine. The uniformly coated particles may then be transferred into a mold cavity and formed into an insert or other cutting structure, i.e., rib of a drill bit. One such process is described in U.S. Patent Application Publication No. 2006/0081402, which is herein incorporated by reference in its entirety.

[0065] One of ordinary skill in the art would appreciate that the encapsulated particles disclosed herein may be used to form inserts, cutting structures or bit bodies using any suitable method known in the art. Heating of the material can be by furnace or by electric induction heating, such that the heating and cooling rates are rapid and controlled in order to prevent damage to the diamonds. The inserts may be heated by resistance heating in a graphite mold, while bit bodies may be formed by infiltration of a mold. The dimensions and shapes of the inserts and of their positioning on the bit can be varied, depending on the nature of the formation to be drilled.

[0066] Infiltration processes that may be used to form an infiltrated bit body of the present disclosure may begin with the fabrication of a mold, having the desired body shape and component configuration. Pellets of uniformly coated encapsulated particles may be loaded into the mold in the desired location, i.e., ribs, and, a matrix material, and optionally a metal binder powder, may be loaded on top of the encapsulated particles. The mass of particles may be infiltrated with a molten infiltration binder and cooled to form a bit body. In a particular embodiment, during infiltration at least a portion of the loaded matrix material may be carried down with the molten infiltrant to fill the gaps between the encapsulated particles. Depending on the size of the encapsulated particles, as well as additional properties, a size distribution of the additional matrix material may be likewise selected such that the additional matrix material possess a sufficient amount of “fine” particles that may be carried down between the encapsulated particles to fill the gaps therebetween.

[0067] It will further be understood that the concentration of diamond or abrasive particles in a consolidated insert, for example, can differ from the concentration of diamond or abrasive particles in the bit body. Diamond concentration may be obtained, for example by varying shell thickness and the matrix loading of the first matrix material. According to one embodiment, the concentrations of diamond in the inserts and in the bit body are in the range of 50 to 120 (100=4.4 carat/cm³). Other embodiments may have a diamond concentration greater than 110, while yet other embodiments may have a diamond concentration less than 85. A diamond concentration of 120 is equivalent to 30 percent by volume of diamond. Those having ordinary skill in the art will recognize that other concentrations of diamonds may also be used depending on particular applications. Further, in some embodiments, the various types of encapsulated particles may have a varied concentration, such as a concentration of at least 110 for one type of particle, and a concentration of at most 100 for another type of particles. However, one of ordinary skill in the art would appreciate that other combinations may be used.

[0068] Further, while reference has been made to a hot-pressing process above, embodiments disclosed herein may use a high-temperature, high-pressure press (HTHP) process. Alternatively, a two-stage manufacturing technique, using both the hot-pressing and the HTHP, may be used to promote the development of high concentration (>120 conc.) while achieving maximum bond or matrix density. The HTHP press can improve the performance of the final structure by enabling the use of higher diamond volume percent (including bi-modal or multi-modal diamond mixtures) because ultrahigh pressures can consolidate the bond material to near full density (with or without the need for low-melting alloys to aid sintering).

[0069] The HTHP process has been described in U.S. Pat. No. 5,676,496 and U.S. Pat. No. 5,598,621. Another suitable method for hot-compacting pre-pressed diamond/metal powder mixtures is hot isostatic pressing, which is known in the art. See Peter E. Price and Steven P. Kohler, “Hot Isostatic Pressing of Metal Powders”, Metals Handbook, Vol. 7, pp. 419-443 (9th ed. 1984).

[0070] Further, the processing times during sintering or hot-pressing, such as heating and cooling times, may be selected to be sufficiently short, as well as the maximum temperature of the thermal cycle may be selected to be sufficiently low, so that the impregnated materials are not thermally damaged during these processes.

[0071] In some embodiments, the multiple types of encapsulated particles on rib may include particles of varying size, varying composition, or combinations thereof. In other embodiments, the multiple encapsulated particles may include shells of varying thickness, varying composition, or combinations thereof. In yet other embodiments, the multiple encapsulated particles may include abrasive particles of varying size, varying composition, varying size distribution, and combinations thereof. In yet other embodiments, the drill bit or a rib on a drill bit may additionally include (be impregnated with) standard grit.

[0072] In various embodiments, the encapsulated particles disclosed herein may have localized placement in a drill bit. For example, encapsulated particles may be placed at the top of the bit being the first section of the bit to drill or solely imbedded deeper within the bit for drilling of the latter sections encountered during a bit run. Additionally, one of skill in the art would recognize that it may be advantageous to place the encapsulated particles at other strategic positions, such as, for example, in the gage area, and leading, or trailing sides of a rib/blade.

[0073] Further, as discussed above, the encapsulated particles may be used in a consolidated or hot pressed insert, such as the type described in U.S. Pat. No. 6,394,202, which is assigned to the present assignee and herein incorporated by reference in its entirety. As shown in FIG. 4, such inserts may be inserted into a drill bit. Bit 422 includes a shank 424 and a crown 426. Crown 426 has a cutting face 429 and outer side surface 430. According to one embodiment, crown 426 is formed by infiltrating a mass of tungsten-carbide powder impregnated with synthetic or natural diamond, or alternatively by infiltrating encapsulated particles as described herein.

[0074] Crown 426 may include various surface features, such as ribs 427, which may optionally be formed with spacers in the mold during the manufacturing process so that the infiltrated, diamond-impregnated crown includes a plurality of holes or sockets 434 that are sized and shaped to receive a corresponding plurality of diamond-impregnated inserts 436. Once crown 426 is formed, inserts 436 formed from the encapsulated particles of the present disclosure may be mounted in the sockets 434 and affixed by any suitable method, such as brazing, adhesive, mechanical means such as interference fit, or the like. As shown in FIG. 4, the sockets 434 may each be substantially perpendicular to the surface of the crown 426 so that once inserted into sockets 434, inserts are substantially perpendicular to the surface of crown 426 (and may be flush with or extend beyond surface of crown 426). Alternatively, holes 434 can be inclined with respect to the surface of the crown 426. In this embodiment, the sockets are inclined such that inserts 436 are oriented substantially in the direction of rotation of the bit, so as to enhance cutting.

[0075] Alternatively, inserts may be stacked within a rib, as shown in FIG. 5. Specifically, a rib 527 may include a plurality of inserts 536 (formed from encapsulated particles as disclosed herein) stacked within the rib, along its length, in a side by side fashion. Thus, the particular orientation of the diamond impregnated inserts of the present disclosure within a bit does not have any limitation on the scope of the present disclosure.

[0076] Further, it is also within the scope of the present disclosure that a bit is formed without impregnated inserts, but with the encapsulated particle loaded into bit mold cavity and infiltrated, as described above. FIGS. 6A-B illustrate partial views of an alternative embodiment of an impregnated bit, generally indicated by arrow 610. In FIGS. 6A-B, approximately one half of the circular face 611 is shown, with the other half being approximately the mirror image of the part that is shown. The bit body 613 is cylindrical in form, with the upper end thereof (not shown) forming a threaded pin which is adapted to be connected to the lower end of a drill string. The lower end of the bit body 613 forms the end face 611. The end face 611 has a plurality of elevated ribs 165 formed thereon, with channels 617 formed between the ribs 615.

[0077] The bit body 613 is preferably made of a steel core 612 having an outer shell 614 comprised of matrix material. The ribs 615 include diamonds (not illustrated) embedded within a matrix material, where the rib was formed from the encapsulated particles of the present disclosure. The diamond particles then function to wear away the bore hole formation as the bit rotates. The channels 617 function to allow drilling fluid to pass through a central plenum (not shown) from the interior of the bit body 613 and run along the channels 617 to cool the ribs 615 and to carry the formation cuttings up the annulus formed between the bit and the bore hole. Thus, in forming the bit, encapsulated particles and a matrix material are loaded into a mold cavity and heated, such as by infiltration or sintering, to form the resulting impregnated bit. Further, as the use of encapsulated particles may allow for a unique tailoring of the bit composition, taller ribs (for a given rib width) may be obtained, without high risk of failure by rib breakage. The inclusion of tall ribs may be determined by the ratio of the rib height (indicated as 640 on FIGS. 6A-B) to rib width (indicated as 650 on FIGS. 6A-B). Conventional bit designs generally require a ratio of rib height to width of no more than 1.5. However, by using at least one type of encapsulated particles (or at least two in other embodiments) of the present disclosure, bit having a ratio of rib height to rib width (along any portion of the rib) of greater than 1.75 may be obtained using a minimum diamond concentration of 100. In other embodiments, a ratio of rib height to width of greater than 2.0 may be obtained using a minimum diamond concentration of 75. In yet other embodiments, a ratio of rib height to width of greater than 2.25 may be obtained using a minimum diamond concentration of 50. Such ratios translate, for example, to a rib height of greater than 30 mm on an 8⅜ inch bit. In various embodiments, the specified ratio of bit height to width refers to a portion of the rib along the bit center (nose) or bit top (nose). Most of the catastrophic rib breakage typically occurs near these areas, the bit center (cone) and bit top (nose), because ribs typically have less width at these locations as compared to the gage area due to space restrictions and load concentrations when used in drilling.

[0078] Use of the encapsulated particles disclosed herein may provide increased TRS bending strength of at least 15 percent in some embodiment, at least 20 or 25 percent in other embodiments, and at least 30 percent in yet other embodiments. Further, one skilled in the art would appreciate that the recited ratios may be obtained by either reducing the rib width (to fit more ribs and thus more surface area for wear resistance) and/or by increasing blade height (to increase bit life before tripping), both of which may use the encapsulated particles disclosed herein.

[0079] Referring again to FIGS. 5-6B, impregnated bits may include a plurality of gage protection elements disposed on the ribs and/or the bit body. In some embodiments, the gage protection elements may be modified to include evenly distributed diamonds. By positioning evenly distributed diamond particles at and/or beneath the surface of the ribs, the impregnated bits are believed to exhibit increased durability and are less likely to exhibit premature wear than typical prior art impregnated bits.

[0080] Embodiments disclosed herein, therefore, may find use in any application in which impregnated cutting structures may be used. Specifically, embodiments may be used to create diamond impregnated inserts, diamond impregnated bit bodies, diamond impregnated wear pads, or any other diamond impregnated material known to those of ordinary skill in the art. Embodiments may also find use as inserts or wear pads for 3-cone, 2-cone, and 1-cone (1-cone with a bearing & seal) drill bits. Further, while reference has been made to spherical particles, it will be understood by those having ordinary skill in the art that other particles and/or techniques may be used in order to achieve the desired result, namely more even distribution of diamond particles. For example, it is expressly within the scope of the present invention that elliptically coated particles may be used.

[0081] An impregnated cutting structure formed in accordance with the present disclosure may include two types of encapsulated particles. The first type of particle may constitute 35% of the total volume of particles. It may include synthetic grit having a mesh size of −25+35, such as SDB1100, which is a strong grit commercially available from Element Six Ltd, coated with a TiC coating applied by chemical vapor deposition (CVD). A hard matrix comprising 70% WC (<10 micrometer standard WC type) and a binder mixture of Co and Cu may be used to encapsulate the particles sufficient to form a 110 diamond concentration. The second type of particle may constitute 65% of the total volume of particles. It may include synthetic grit having a mesh size of −20+25, such as MBS950, which is a medium strength grit commercially available from Diamond Innovations, Inc., coated with a TiC or SiC coating applied by CVD. A soft matrix comprising 30% WC (<10 micrometer standard WC type) and a binder mixture of Co and Cu may be used to encapsulate the particles sufficient to form a diamond concentration of 80. Such embodiment may be used to form hot pressed inserts, as described above, or may be used in conjunction with an infiltrating matrix material to form an impregnated bit body.

[0082] A second impregnated cutting structure formed in accordance with the present disclosure may also include two types of encapsulated particles. The first type of particle may constitute 35% of the total volume of particles. It may include synthetic grit having a mesh size of −25+35, such as MBS960, which is a high strength grit commercially available from Diamond Innovations, Inc., coated with a strong SiC coating applied by CVD. A soft matrix comprising 30% WC (<10 micrometer standard WC type) and a binder mixture of Co and Cu may be used to encapsulate the particles sufficient to form a 110 diamond concentration. The second type of particle may constitute 65% of the total volume of particles. It may include synthetic grit having a mesh size of −20+25, such as MBS960, which is commercially available from Diamond Innovations, Inc., coated with a SiC coating applied by CVD. A soft matrix comprising 30% WC (<10 micrometer standard WC type) and a binder mixture of Co and Cu may be used to encapsulate the particles sufficient to form a diamond concentration of 100. Such embodiment may be used in particular when high ROP is desired (due to the soft/high toughness nature of the encapsulating material).

[0083] A third impregnated cutting structure formed in accordance with the present disclosure may include three types of encapsulated particles. The first type of particle may constitute 55% of the total volume of particles. It may include synthetic grit having a mesh size of −18+20, such as NDG120, which is a strong grit commercially available from Element Six Ltd, coated with a TiC coating applied by CVD. A soft matrix comprising 30% WC (<10 micrometer standard WC type) and a binder mixture of Co and Cu may be used to encapsulate the particles sufficient to form a 120 diamond concentration. The second type of particle may constitute 30% of the total volume of particles. It may include synthetic grit having a mesh size of −25+35, such as SBD1100, which is a strong grit commercially available from Element Six Ltd, coated with a TiC coating applied by CVD. A medium hardness matrix comprising 50% WC (<10 micrometer standard WC type) and a binder mixture of Co and Cu may be used to encapsulate the particles sufficient to form a diamond concentration of 120. A third type of particle may constitute 15% of the total volume of particles. It may include synthetic grit having a mesh size of −35+40, such as MBS950, which is a medium strength grit commercially available from Diamond Innovations, Inc., coated with a SiC coating applied by CVD. A hard matrix comprising 70% WC (<100 micrometer mixture of standard WC and cast WC/W₂C types) and a binder mixture of Co and Cu may be used to encapsulate the particles sufficient to form a diamond concentration of 120. Such embodiment may be used to form hot pressed inserts, as described above, or may be used in conjunction with an infiltrating matrix material to form an impregnated bit body.

[0084] Advantageously, embodiments of the present disclosure may provide for at least one of the following. As discussed above, embodiments disclosed herein may provide more controllable wear properties, improved diamond retention, and increased diamond concentration (without diamond cluttering) for a given volume. Embodiments disclosed herein may also provide for the controlled exposure of fresh grit for increased ROP, as removal of the grit to expose fresh grit may be controlled by the hardness of the shell and the relative wear properties of the various matrix materials used, and may be tailored for the hardness of the earth formation.

[0085] Additionally, in the embodiments disclosed herein, the various combinations of encapsulated particle components that may be used may provide improved cutting structures to drill through formations of specific hardnesses and/or may make a bit particularly suitable for drilling through a variety of formations, including mixed formations, due to the adaptive nature of the bit. Further, a first encapsulating matrix material may be selected for its toughness, which may reduce blade breakage and allow the blade height to increase, which would increase the drilling life of the blade. Improvements in properties, such as bending strength, may be obtained by using encapsulated particles, which may allow for an increase in rib height to width ratios. Such ratio may be realized by either reducing rib width (to increase the number of ribs and thus surface area for wear resistance) and/or increasing the rib height (to increase bit life before tripping). Further, by blending at least two distinct pellets to form ribs having increased height to width ratios, it may be possible to effectively drill through mixed formation types. For example, a first pellet type may be selected to provide optimum drilling through abrasive sandstones, while a second pellet type may be selected to provide optimum drilling through shale, limestone, or chert (hard nodules). Typically, during formation changes, an operator is typically forced to pull an impregnated bit (despite having remaining bit life) due to reduced ROP to drill with another bit type, such as a roller cone bit, which have less life to bearing and seal failure. Thus, the combination of increased bit height and the unique encapsulated particles, may provide for increase bit life despite formation changes.

[0086] However, certain quantities of abrasive particles may be more readily exposed by the softer encapsulating material, which also increases ROP. Further, a second matrix material may be selected to be more wear resistant than the first matrix material in order to expose the concentrated grit at a slower rate. This may result in a robust cutting instrument wherein the grit is exposed in a controlled fashion.

[0087] Further, the disparity in wear properties between multiple matrices may allow for tailoring of the some of the properties of the cutting structure such as grit concentration, wear rate, controlled exposure of encapsulated grit to the formation, cuttings removal and robustness. If a high grit concentration is required for drilling a particularly hard formation, the shell thickness may be small. This may advantageously allow more encapsulated grit to be packed into the same cutting structure.

[0088] If more efficient cuttings removal is required, the cutting instrument may have a first matrix that is selected to be more wear resistant than the second matrix material. The second matrix may preferentially partially wear away creating fluid pathways within the cutting instrument, while exposing abrasive particles. This may result in a cutting instrument with superior cuttings removal properties.

[0089] Further, conventional bits rely on grit hot pressed inserts for a large portion of the wear; however, such segments are typically restricted to approximately thirty to forty percent of the rib volume due to design limitations. Because the cutting structures of the present disclosure may provide for improved rate of penetration by virtue of improved wear patterns, a bit that typically relies on grit hot pressed inserts for wear may instead be provided with ribs infiltrated with the encapsulated particles as disclosed herein. Such bits may possess improved wear across a larger volume of rib, as compared to conventional bits having grit hot pressed inserts.

[0090] Thus, embodiments disclosed herein may allow for an effective diameter of the encapsulated materials without such drastic increases in cost. Furthermore, some embodiments may include a hard particle, such as tungsten or silicon carbide, which has even lower costs as compared to diamond or other super abrasives. Therefore, cost savings may be achieved while maintaining or even improving rate of penetration (ROP), thus lowering the drilling cost per foot.

[0091] While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.