Various types and shapes of earth boring bits are used in various applications in the earth drilling industry. Earth boring bits have bit bodies which include various features such as a core, blades, and cutter pockets that extend into the bit body or roller cones mounted on a bit body, for example. Depending on the application/formation to be drilled, the appropriate type of drill bit may be selected based on the cutting action type for the bit and its appropriateness for use in the particular formation.
Drag bits, often referred to as “fixed cutter drill bits,” include bits that have cutting elements attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by a binder material. Drag bits may generally be defined as bits that have no moving parts. However, there are different types and methods of forming drag bits that are known in the art. For example, drag bits having abrasive material, such as diamond, impregnated into the surface of the material which forms the bit body are commonly referred to as “impreg” bits. Drag bits having cutting elements made of an ultra hard cutting surface layer or “table” (typically made of polycrystalline diamond material or polycrystalline boron nitride material) deposited onto or otherwise bonded to a substrate are known in the art as polycrystalline diamond compact (“PDC”) bits.
PDC bits drill soft formations easily, but they are frequently used to drill moderately hard or abrasive formations. They cut rock formations with a shearing action using small cutters that do not penetrate deeply into the formation. Because the penetration depth is shallow, high rates of penetration are achieved through relatively high bit rotational velocities.
In PDC bits, polycrystalline diamond compact (PDC) cutters are received within cutter pockets, which are formed within blades extending from a bit body, and are typically bonded to the blades by brazing to the inner surfaces of the cutter pockets. The PDC cutters are positioned along the leading edges of the bit body blades so that as the bit body is rotated, the PDC cutters engage and drill the earth formation. In use, high forces may be exerted on the PDC cutters, particularly in the forward-to-rear direction. Additionally, the bit and the PDC cutters may be subjected to substantial abrasive forces. In some instances, impact, vibration, and erosive forces have caused drill bit failure due to loss of one or more cutters, or due to breakage of the blades.
In a typical PDC cutter, a compact of polycrystalline diamond (“PCD”) (or other superhard material, such as polycrystalline cubic boron nitride) is bonded to a substrate material, which is typically a sintered metal-carbide to form a cutting structure. PCD comprises a polycrystalline mass of diamond grains or crystals that are bonded together to form an integral, tough, high-strength mass or lattice. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired.
PCD may be formed by subjecting a volume of diamond grains to certain high-pressure/high-temperature (“HPHT”) conditions in the presence of a sintering aid or binder. Conventionally, the sintering aid or binder is provided in the form of a solvent metal catalyst material, such as one or more elements from Group VIII of the Periodic table. The solvent metal catalyst may be added and mixed with the diamond grains prior to HPHT processing and/or may be provided during the HPHT process by infiltration from a substrate comprising the solvent metal catalyst as one of its constituent materials.
A conventional PDC cutter may be formed by placing a cemented carbide substrate into a HPHT container. A mixture of diamond grains or diamond grains and catalyst binder is placed atop the substrate in the container and the container is loaded into a HPHT device that is configured and operated to subject the container and its contents to a desired HPHT condition. In doing so, metal binder migrates from the substrate and passes through the diamond grains to promote intergrowth between the diamond grains. As a result, the diamond grains become bonded to each other to form the diamond layer, and the diamond layer is in turn bonded to the substrate. The substrate often comprises a metal-carbide composite material, such as tungsten carbide. The deposited diamond body is often referred to as a “diamond layer”, a “diamond table”, or an “abrasive layer.”
An example of a prior art PDC bit having a plurality of cutters with ultra hard working surfaces is shown in FIGS. 1A and 1B. The drill bit 100 includes a bit body 110 having a threaded upper pin end 111 and a cutting end 115. The cutting end 115 typically includes a plurality of ribs or blades 120 arranged about the rotational axis L (also referred to as the longitudinal or central axis) of the drill bit and extending radially outward from the bit body 110. Cutting elements, or cutters, 150 are embedded in the blades 120 at predetermined angular orientations and radial locations relative to a working surface and with a desired back rake angle and side rake angle against a formation to be drilled.
A plurality of orifices 116 are positioned on the bit body 110 in the areas between the blades 120, which may be referred to as “gaps” or “fluid courses.” The orifices 116 are commonly adapted to accept nozzles. The orifices 116 allow drilling fluid to be discharged through the bit in selected directions and at selected rates of flow between the blades 120 for lubricating and cooling the drill bit 100, the blades 120 and the cutters 150. The drilling fluid also cleans and removes the cuttings as the drill bit 100 rotates and penetrates the geological formation. Without proper flow characteristics, insufficient cooling of the cutters 150 may result in cutter failure during drilling operations. The 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 100 toward the surface of a wellbore (not shown).
Referring to FIG. 1B, a top view of a prior art PDC bit is shown. The cutting face 118 of the bit shown includes six blades 120-125. Each blade includes a plurality of cutting elements or cutters generally disposed radially from the center of cutting face 118 to generally form rows. Certain cutters, although at differing axial positions, may occupy radial positions that are in similar radial position to other cutters on other blades.
Referring to FIG. 2, a profile of prior art PDC bit 200 is shown as it would appear with all blades 220 and all cutting elements 250 (including primary cutters and backup cutting elements) rotated into a single rotated profile. As shown, the rotated profile of the plurality of blades 220 includes blade profiles 225. Blade profiles 225 and the bit face 218 may be divided into three different regions labeled cone region 222, shoulder region 224, and gage region 226. Cone region 222 is concave in this embodiment and comprises the inner most region of bit 200 (e.g., cone region 222 is the central most region of bit 200). Adjacent cone region 222 is shoulder (or the upturned curve) region 224. It should be noted that in some prior art PDC bits, the shoulder region 224, as it is depicted in FIG. 2, may be defined as including both a nose region and a shoulder region, wherein the nose region is the region that first contacts the bottom of the wellbore. Next to shoulder region 224 is the gage region 226 which is the portion of the bit face 218 which defines the outer radius 205 of bit 200. Outer radius 205 extends to and therefore defines the full gage diameter of bit 200.
Still referring to FIG. 2, cone region 222 is defined by a radial distance along the x-axis measured from central axis L. It is understood that the x-axis is perpendicular to central axis L and extends radially outward from central axis L. Cone region 222 may be defined by a percentage of the outer radius 205 of the bit 200. The actual radius of cone region 222, measured from central axis L, may vary from bit to bit depending on a variety of factors including without limitation, bit geometry, bit type, location of backup cutter elements, or combinations thereof.
Conventional PCD includes 85-95% by volume diamond and a balance of the binder material, which is present in PCD within the interstices existing between the bonded diamond grains. Binder materials that are typically used in forming PCD include Group VIII elements, with cobalt (Co) being the most common binder material used.
Conventional PCD is stable at temperatures of up to 700-750° C., after which observed increases in temperature may result in permanent damage to and structural failure of PCD. In particular, heat caused by friction between the PCD and the work material causes thermal damage to the PCD in the form of cracks, which lead to spalling of the diamond layer and delamination between the diamond layer and substrate. This deterioration in PCD is due to the significant difference in the coefficient of thermal expansion of the binder material, which is typically cobalt, as compared to diamond. Upon heating of PCD, the cobalt and the diamond lattice will expand at different rates, which may cause cracks to form in the diamond lattice structure and result in deterioration of the PCD. High operating temperatures may also lead to back conversion of the diamond to graphite causing loss of microstructural integrity, strength loss, and rapid abrasive wear.
In order to overcome this problem, strong acids may be used to “leach” the cobalt from the diamond lattice structure (either a thin volume or the entire body) to at least reduce the damage experienced from different expansion rates within a diamond-cobalt composite during heating and cooling. Examples of “leaching” processes can be found, for example, in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a strong acid, typically nitric acid or combinations of several strong acids (such as nitric and hydrofluoric acid) may be used to treat the diamond table, removing at least a portion of the co-catalyst from the PDC composite. By leaching out the cobalt, thermally stable polycrystalline (“TSP”) diamond may be formed. In certain embodiments, only a select portion of a diamond composite is leached, in order to gain thermal stability with less effect on impact resistance. As used herein, the term thermally stable polycrystalline (TSP) includes both of the above (i.e., partially and completely leached) compounds. Interstitial volumes remaining after leaching may be reduced by either furthering consolidation or by reinfiltrating the volume with a secondary material. An example of reinfiltration can be found in U.S. Pat. No. 5,127,923.
However, some of the problems described above that plague PCD cutting elements, i.e., chipping, spalling, partial fracturing, cracking or exfoliation of the cutting table, are also often encountered in TSP cutters or other types of cutters having an ultra hard diamond-like cutting table such as polycrystalline cubic boron nitride (PCBN) bonded on a cemented carbide substrate. In particular, it has been observed that TSP cutters are slightly more prone to spalling and delamination under severe loads. These problems result in the early failure of the cutting table and thus, in a shorter operating life for the cutter.
Furthermore, in drag bits having fixed PDC cutters, a common cutting surface contacts the formation during drilling. Over time, the edge of the cutting surface that constantly contacts the formation begins to wear down, forming a local wear flat, or an area worn disproportionately to the remainder of the cutting element. Local wear flats may result in longer drilling times due to a lose of rate of penetration caused by dulling of edge of the cutting element. Additionally, fixed cutters are under constant thermal and mechanical load. As a result, heat builds up along the cutting surface, and results in cutting insert fracture. When a cutting element breaks, the drilling operation may sustain a loss of rate of penetration, and additional damage to other cutting elements, should the broken cutting element contact a second cutting element.
Accordingly, there exists a continuing need for developments in improving the life of cutting elements.