Abstract:
The present invention relates generally to drill bits used in drilling subterranean boreholes. More specifically, the invention relates to drill bits and cutting elements on the drill bits and the design of each. The invention utilizes a cutting element having a convex curved top portion with a shear face wherein said shear face is a plane formed by taking a planer slice from said convex curved top portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/794,943 filed Mar. 15, 2013, which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to drill bits used in drilling subterranean boreholes. More specifically, the invention relates to drill bits and cutting elements on the drill bits and the design of each. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a typical drilling operation, a drill bit is advanced into a soil or rock formation. The drill bit may chip and cut away rock to produce a borehole through rotary action, percussive action or both rotary and percussive action. Often to enhance drilling, especially through harder rock and prior equipment used downhole, the drill bit will utilize inserts or cutting elements. Conventional cutting inserts typically have a body consisting of a cylindrical grip or “post” portion from which a convex cutting end extends. In order to improve their operational life, these inserts are sometimes coated with a superhard material, sometimes also known as an ultrahard material. The coated cutting layer typically comprises a superhard material, such as a layer of polycrystalline diamond (PCD) or polycrystalline cubic boron nitride (PCBN). The substrate, which supports the cutting layer, is normally formed of a hard material such as tungsten carbide (WC). The grip is embedded in and affixed to the drill bit and the cutting end extends outwardly from the surface of the drill bit. The cutting end may be hemispherical, which is commonly referred to as a semi-round top (SRT). 
         [0004]    In oil and gas drilling, the cost of drilling a borehole is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. This process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits that will drill faster and longer and that are usable over a wide range of formation hardnesses. 
         [0005]    Accordingly, although cutting elements have significantly extended the life of drill bits and expanded the scope of formations for which drilling is economically viable, there is ever a need for improving the performance and durability of cutting elements to enhance the operation of drill bits. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with one embodiment of the invention, there is provided a cutting element comprising a substrate column comprising a first material. The substrate column has a convex curved top portion with a shear face, wherein the shear face is a plane formed by taking a planer slice from the convex curved top portion. An ultrahard material layer is disposed on the convex top portion. 
         [0007]    In accordance with another embodiment of the invention there is provided a drill bit comprised of a drill head having a plurality of cutting elements thereon. The cutting elements comprising a substrate column comprising a first material. The substrate column has a convex curved top portion with a shear face, wherein the shear face is a plane formed by taking a planer slice from the hemispherical top portion. An ultrahard material layer is disposed on the convex curved top portion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a front perspective view of a cutting element in accordance with an embodiment of the present invention. 
           [0009]      FIG. 2  is side cutaway view of a cutting element in accordance with the embodiment of  FIG. 1 . 
           [0010]      FIG. 3  is a side perspective view of a cutting element in accordance with the embodiment of  FIG. 1 .  FIG. 3  shows exemplary dimensions for the cutting element. 
           [0011]      FIG. 4  is a cutaway of an embodiment of a cutting element in accordance with another embodiment of the present invention. 
           [0012]      FIG. 5  is a partial perspective view of a rotary-percussive drill bit in accordance with the present invention. 
           [0013]      FIG. 6  is a side perspective view of a blade or drag bit, also referred to as a PDC drill bit, in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Referring now to the drawings, wherein like reference numbers are used herein to designate like elements throughout the various views, embodiments of the present invention are illustrated and described, and other possible embodiments of the present invention are described. The figures are not necessarily drawn to scale, and in some instances the drawings have been exaggerated and/or simplified in places for illustrative purposes only. One of ordinary skill in the art will appreciate the many possible applications and variations of the present invention based on the following examples of possible embodiments of the present invention. 
         [0015]    Referring now to  FIG. 1 , a cutting element  10  in accordance with an embodiment of the invention is illustrated. Cutting element  10  is a drill bit insert designed both for percussive or impact chipping and for cutting or shearing of subterranean material, which can be rock, soil, or structures and materials previously placed in the borehole such as a casing or shoe. Cutting element  10  includes a substrate column  12  and an ultrahard layer  20  formed on a top end  16  of substrate column  12 . Substrate column  12  comprises a base or grip  14  having outer surface  18 , and a top end  16  having an outer surface  22 . Substrate column  12  is formed from a material, typically a carbide material. Generally, the material is metal carbide such as tungsten carbide and can be formed by a cementing process. Accordingly, powdered metal carbide can be provided in a mold with a metal binder and heated under pressure to cause the binder to infiltrate and cement the metal carbide powder into a substrate body having the desired interface surface geometry. Alternatively, the desired surface geometry may be machined on the substrate column. 
         [0016]    Base or grip  14  is designed to mount into a drill bit and provided adequate elevation to top end  16  so that top end  16  extends out from the drill bit surface and contacts the subterranean material. Base  14  may be characterized as being a “post” shape and is, generally, a cylindrical—or disc-shaped body; however, other shapes can be utilized. 
         [0017]    Top end  16  is a convex curve in shape. Top end  16  can be a convex shape having a simple curve such as oval, hemisphere, or ballistic shape.  FIGS. 1-3  illustrate an embodiment where cutting element  10  has a top end  16  with a hemispherical shape. As used herein, the term “hemispherical” means true hemispheres (half of a sphere) and partial hemispheres or spherical caps. 
         [0018]    As can be seen from  FIGS. 1 and 2 , top end  16  has a planer slice removed so as to form shear face  24 , which is a planer section of top end  16 . Shear face  24  is generally circular or oval in shape with circular being currently preferred. As can be more clearly seen from  FIG. 2 , shear face  24  is at an angle to longitudinal axis  26 , as measured by inner angle α. Generally, shear face  24  is at an angle of less than 80°. For hemispherical top ends, inner angle α can typically be at an angle of from about 40° to about 80° with the longitudinal axis, and can be from 50° to 70°. As shown, in  FIG. 2  angle α is 60°. 
         [0019]    Generally, shear face  24  will be above interface  28  so as to leave a curved edge  30  between shear face  24  and interface  28 . Curved edge  30  will have a width  31 , as measured at its narrowest portion from interface  28 . Generally, width  31  can be at least 5% of the surface length from interface  28  to tip  27  and can be at least 10% or at least 15% of such surface length. Typically, width  31  can be no more than 30% of the surface length from interface  28  to tip  27  but can be no more than 25% or no more than 20% of such surface length. 
         [0020]    In general, shear face  24  has a radius RS less than 50% of the surface length of the top end from interface  28  to tip  27  to allow for curved edge  30 . Generally, radius RS is greater than 10% of the surface length from interface  28  to tip  27 , and can be from 25% to 45%. Where top end  16  is a hemisphere, shear face  24  has a radius RS that is about 50% or less than the radius Ri, which is the radius of the top end  16  at interface  28 . Also, width  31  generally can be at least 5% of radius Ri. 
         [0021]    Turning now to  FIG. 3 , exemplary dimensions for one embodiment of a cutting element in accordance with the invention is shown. The embodiment of  FIG. 3  has a hemispheric top end  16  in accordance with the embodiment described above for  FIGS. 1 and 2 . The dimensions illustrated are based on percentages of the height of cutting element  10 . The height of the insert from the bottom of the insert to where the dome curvature begins is represented by k. The height from the bottom of the insert to the center point of shear face  24  is represented by l. The distance between longitudinal axis  26  and the center of shear face  26  is represented by m. In the exemplary embodiment of  FIG. 3 , k is equal to 0.42, l is equal to 0.63 and m is equal to 0.12 of the height to cutting element  10 . Accordingly, if inner angle α is equal to 60° then RS would be equal to 0.20 of the height of cutting element  10 . 
         [0022]    An ultrahard material layer  20  is disposed on top end  16 . Generally, at least all of the convex portion of top end  16  will be covered with the ultrahard material so that, below interface  28 , the cutting element will have an outer surface  18  that is substrate material and, above interface  28 , the cutting element will have an outer surface  22  that is ultrahard material. Shear face  24  can have an outer surface that is substrate material or can have ultrahard material deposited thereon so that outer surface is ultrahard material. 
         [0023]    The ultrahard material forming layer  20  can be a polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or other suitable hard crystalline material. PCD is presently preferred as the ultrahard material. The ultrahard material can be deposited by methods known in the art. Typically, there will be a transition layer between ultrahard material  20  and the substrate material. In one exemplary method of making the cutting element, diamond powder is positioned adjacent to carbide substrate in a pre-shaped can, which is of the shape of the final cutting tool without shear face  24 . The diamond powder is positioned in that portion of the pre-shaped can that will form the top end of the cutting element. The pre-shaped can is sintered under high pressure and high temperature to form the cutting element. The pre-shaped can is removed and the top end  16  is machined to form shear face  24 . The resulting cutting element has no PCD on shear face  24 . 
         [0024]    Turning now to  FIG. 4 , another embodiment of the invention is illustrated in which a cutting element  48  has a the top end  16  having a ballistic shape. As used herein “ballistic” means a curve having a generally bullet shape; that is elongated in the direction of longitudinal axis of the cutting element. A ballistic top end  16  generally has a side  52  having an arcuate cross-section, which lies on a circle having a radius R1. Additionally ballistic top  16  has top spherical cap  50  in tangential relationship with side  52 . Spherical cap  50  has a radius less than R1 and as a width across opposing tangential points  54  and  56  of R2. In one embodiment of the invention the ratio of R1 to R2 is from 4 to 24 and can be 4 to 10. Additionally, top end  16  has a height H and a radius at interface  28  of Ri. 
         [0025]    Similar to the embodiment illustrated in  FIGS. 1-3  above, shear face  24  of the embodiment of  FIG. 4  will be above interface  28  so as to leave a curved edge  30  between shear face  24  and interface  28 . Curved edge  30  will have a width  31 , as measured at its narrowest portion from interface  28 . Generally, width  31  can be at least 5% of the surface length from interface  28  to tip  27  and can be at least 10% or at least 15% of such surface length. Typically, width  31  can be no more than 30% of the surface length from interface  28  to tip  27  but can be no more than 25% or no more than 20% of such surface length. 
         [0026]    In general, shear face  24  has a radius RS less than 50% of the surface length of the top end  16  from interface  28  to tip  27  to allow for curved edge  30 . Generally, radius RS is greater than 10% of the surface length from interface  28  to tip  27 , and can be from 25% to 45%. Where top end  16  is a ballistic shape, shear face  24  has a radius RS that can be about 50% or less of the radius R1. Generally, radius RS can be from 25% to 50% of radius R1. As indicated above, shear face  24  will be above interface  28  so as to leave a curved edge  30  between shear face  24  and interface  28 . Where top end  16  is a ballistic shape, curved edge  30  will have a width  31 , at its narrowest portion. Generally, width  31  can be at least 5% of radius R1. 
         [0027]    The inventive cutting element can be used in a variety of drill systems for drilling into subterranean formations. Thus, it is useful in rotary-percussive drill bits, PDC drill bits and fixed cutter bits. While applicable to a variety of drill bits, it is especially useful in rotary-percussive drill bits for the reasons explained below. 
         [0028]    Turning to  FIG. 5  a drill bit  32  a rotary-percussive drill bit utilizing cutting elements  10  is shown. As the name indicates, rotary-percussive drill bits use both rotary and percussive action in order to chip away rock and produce a hole. The combination of rotation and percussion helps the drill achieve a cutting and grinding action at the same time as a chipping action. A hole is formed when energy is transmitted pneumatically through the drill rod to the drill bit prompting it to thrust into the formation using a repeated hammering motion. The impact of the percussion component is enough to break and dislodge rock, which subsequently removes debris and cuttings with compressed air or water flow. The rotational component of the drill occurs as the drill is producing percussion strikes against the formation 
         [0029]    Drill bit  32  comprises drilling head  34  having a plurality of cutting elements  10  thereon. The cutting elements  10  are in accordance with the above description and are mounted in pockets  44 . Base  14  of cutting element  10  is inserted into pockets  44 , typically by pressing with a press. When a cutting elements  10  is mounted in drill bit  32 , base  14  is in pocket  44  and top end  16  extends out from pocket  44  so as to be able to interact with subterranean material during drilling. 
         [0030]    Drill bit  32  has a longitudinal axis  36  and includes radially innermost impact surfaces  38  and  39 , which are generally transverse to longitudinal axis  36  though, as shown, impact surface  38  can be concave, and impact surface  39  can annular and frustoconical so as to be slightly angled to transverse. Drill bit  32  includes a radially outermost gage surface  40 , which has a generally frustoconical annular shape. Moving radially inward from gage surface  40 , drill bit  32  includes an annular, generally frustoconical inner surface  41 . Gage surface  40  extends from inner surface  41  to skirt surface  42 , which is annular and generally frustoconical but angles oppositely from gage surface  40  so as to form edge or shoulder  43 . Gage surface  40  is generally at an inner angle β of from 35° to 42° degrees from longitudinal axis  36 . Cutting elements  10  can be mounted on impact surfaces  38  and  39 , inner surface  41 , gage surface  40  and skirt surface  42 . Alternatively, cutting elements  10  can be mounted on only gage surface  40 , and skirt surface  42  and impact surface  38  can use a traditional impact cutting element, such as one having a hemispherical top end without a shear surface. Cutting elements  10  incorporated on skirt surface  42  and can be truncated to provide shearing action to the bore wall and, thus provide a smoother hole. Additionally, cutting elements  10  can be mounted on only gage surface  40 , with the other surfaces using traditional impact cutting elements. 
         [0031]    The use of cutting elements  10  on gage surface  40  provide for one or more axial cutting elements for repeated axial impacts. Additionally, shear face  24  provides for shearing action. Thus, the above drill bit  32  allows the cutting elements  10  to absorb repeated impact loading of air hammers, and at the same time provide a shearing action of subterranean materials in response to rotary motion. The amount of deeper penetration that is desired depends on the hardness and type of subterranean material into which the borehole is being drilled. Harder rock formations require deeper penetration of the axial cutters. As the cutter elements  10  are rotated, the shear face  24  provides a shearing action to the rock thereby making the borehole smoother. 
         [0032]    It is not uncommon for diamond coated inserts to fail during cutting. Failure typically takes one of three common forms, namely spalling/chipping, delamination, and wear. External loads due to contact tend to cause failures such as fracture, spalling, and chipping of the diamond layer. The impact mechanism involves the sudden propagation of a surface crack or internal flaw initiated on the PCD layer, into the material below the PCD layer until the crack length is sufficient for spalling, chipping, or catastrophic failure of the enhanced insert. On the other hand, stresses resulting from manufacturing processes tend to cause delamination of the diamond layer, either by cracks initiating along the interface and propagating outward, or by cracks initiating in the diamond layer surface and propagating catastrophically along the interface. The inventive cutting elements incorporating a shear face have been demonstrated to be stronger than traditional cutting elements using a hemispherical top end and no shear face. For example, the shear face acts as a stress relief when the cutting element is pressed into the drill bit pockets; thus, reducing the deleterious effects of stress during production of the drill bit. Accordingly, the inventive cutting elements are more resistant to spalling/chipping, delamination, and wear than traditional cutting elements and the shear face not only provides for improved shearing action of the cutting elements, but for a stronger cutting element having a longer life. 
         [0033]    Turning now to  FIG. 6 , the use of the inventive cutting element on a blade or drag bit  60 , sometimes referred to as a PDC drill bit, is illustrated. A PDC drill bit uses polycrystalline diamond compact cutters to shear rock with a continuous scraping motion. As indicated above, the inventive cutting elements have improved shearing action and, thus, are exceptional for use in PDC drill bit applications. Drill bit  60  is a blade or drag bit utilizing the cutting elements  10 . Drill bit  60  comprises drilling head  62  having a plurality of blades  64  with a plurality of cutting elements  10  thereon. The cutting elements  10  are in accordance with the above description and are mounted in pockets  66 . Base  14  of cutting element  10  is inserted into pockets  66 , typically by pressing with a press. When cutting elements  10  are mounted in drill bit  60 , base  14  is in pocket  66  and top end  16  extends out from pocket  66  so as to be able to interact with subterranean material during drilling. 
         [0034]    Other embodiments of the current invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Thus, the foregoing specification is considered merely exemplary of the current invention with the true scope thereof being defined by the following claims.