Patent Publication Number: US-2006011388-A1

Title: Drill bit and cutter element having multiple extensions

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation-in-part of U.S. patent application Ser. No. 11/113,747 filed Apr. 25, 2005, entitled “Multi-Lobed Cutter Element for Drill Bit;” which is a continuation application of U.S. patent application Ser. No. 10/355,493, filed Jan. 31, 2003, entitled “Multi-Lobed Cutter Element For Drill Bit;” this application is also a continuation-in-part application of U.S. patent application Ser. No. 10/371,388, filed Feb. 21, 2003, entitled “Drill Bit Cutter Element Having Multiple Cusps.” 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
      Not applicable.  
     FIELD OF THE INVENTION  
      The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone rock bits and to an improved cutting structure for such bits. Still more particularly, the invention relates to enhancements in cutting element design.  
     BACKGROUND OF THE INVENTION  
      An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by revolving the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or “gage” of the drill bit.  
      A typical earth-boring bit includes one or more rotatable cone cutters that perform their cutting function due to the rolling movement of the cone cutters acting against the formation material. The cone cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cone cutters thereby engaging and disintegrating the formation material in the bit&#39;s path. The rotatable cone cutters may be described as generally conical in shape and are therefore referred to variously as rolling or rotary cones, cone cutters, or rolling cone cutters.  
      Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pin shafts that extend downwardly and inwardly from the journal segment legs. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.  
      The earth disintegrating action of the rolling cone cutters is enhanced by providing the cone cutters with a plurality of cutting elements. Cutting elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are secured in apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “TCI” bits, while those having teeth formed from the cone material are commonly known as “steel tooth bits.” In each instance, the cutting elements on the rotating cone cutters break up the formation to form new borehole by a combination of gouging and scraping or chipping and crushing.  
      In oil and gas drilling, the cost of drilling a borehole 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 pipes, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string as been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which must be reconstructed, section by section. As is thus obvious, 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 which will drill faster and longer and which are usable over a wider range of formation hardness.  
      The length of time that a drill bit may be employed before it must be changed depends upon its ability to “hold gage” (meaning its ability to maintain a full gage borehole diameter), its rate of penetration (“ROP”), as well as its durability or ability to maintain an acceptable ROP. The form and positioning of the cutting elements (both steel teeth and tungsten carbide inserts) upon the cone cutters greatly impact bit durability and ROP and thus, are important to the success of a particular bit design.  
      The inserts in TCI bits are typically arranged in circumferential rows on the rolling cone cutters. Most such bits include a row of inserts in the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to align generally with and ream the sidewall of the borehole as the bit rotates. The heel inserts function primarily to maintain a constant gage and secondarily to prevent the erosion and abrasion of the heel surface of the rolling cone.  
      In addition to the heel row inserts, conventional bits typically include a circumferential gage row of cutting elements mounted adjacent to the heel surface but oriented and sized in such a manner so as to cut the corner of the borehole. Conventional bits also include a number of additional rows of cutting elements that are located on the cones in circumferential rows disposed radially inward or in board from the gage row. These cutting elements are sized and configured for cutting the bottom of the borehole, and are typically described as inner row cutting elements.  
      Typically positioned on or near the apex of one or more of the rolling cone cutters, are cutting elements commonly referred to as a nose cutter or nose row cutters. Such cutters are generally responsible for cutting the central portion (or core) of the hole bottom. They may be positioned as a single cutter at or very near the apex of the cone cutter, or may be in a circumferential row of several cutting elements disposed near to the cone apex.  
      Earthen formations generally undergo two types of fractures when penetrated by a cutting element. A first type of fracture is generally referred to as a plastic fracture, and is a fracture where the cutting element penetrates into the rock and volumetrically displaces the rock by compressing it. In this circumstance, shearing or tearing fracture, rather than tensile fracture, is the major mode of crack propagation. A plastic fracture generally creates a crater in the rock that is the size and shape of that portion of the cutting element that has penetrated into the rock.  
      A second principal type of fracture is what is referred to as a brittle fracture. A brittle fracture typically occurs after a plastic fracture has first taken place. That is, when the rock first undergoes plastic fracture, a region around the crater made by the cutting element will experience increased tensile stress and will weaken and may crack in that region, even though the rock in that region surrounding the crater has not been displaced. This region of increased stress is generally recognized as the “Hertzian” contact zone. However, in certain formations, when the cutting element displaces enough of the rock and creates enough stress in the Hertzian contact zone adjacent to the plastic fracture, that rock in the region of increased stress may itself break and chip away from the crater. Where this occurs, the cutting element effectively removes a volume of rock that is larger than the volume of rock displaced in the plastic fracture.  
      The characteristics of these fractures depend largely on the geometry of the cutting element and the properties of the rock that is being penetrated. In general, for a given formation, a sharper insert will generally create more of a plastic fracture whereas a more blunt cutting element will produce more of a brittle fracture. The more blunt insert will typically require a higher force, however, to penetrate to the same depth into the rock as compared to a sharper cutting element. Because a brittle fracture generally removes more rock material than a plastic fracture, it is advantageous to provide a cutting element suitable for inducing brittle fractures and that performs that function without requiring increased force or weight on bit. Thus, to increase a bit&#39;s rate of penetration (ROP), it is desirable to increase the bit&#39;s ability to initiate brittle fractures at the locations where the cutting element engages the formation material so that the volume of rock removed by each hit or impact of the cutting element is greater than the volume of rock actually penetrated by the cutting element.  
      A variety of different shapes of cutting elements have been devised. In most instances, each cutting element is designed to optimize the amount of formation material that is removed with each “hit” of the formation by the cutting element. At the same time, however, the shape and design of a particular cutting element is also dependent upon the location in the drill bit in which it is to be placed, and thus the cutting duty to be performed by that cutting element. For example, in general, heel row cutting elements are generally made of a harder and more wear resistant material, and have a less aggressive cutting shape for reaming the borehole side wall, as compared to the inner row cutting elements where the cutting duty is more of a gouging, digging and crushing action. Thus, in general, bottom hole cutting elements generally tend to have more aggressive cutting shapes than heel row cutters.  
      In many conventional TCI bits, conventional nose row cutters are typically of the chisel-shaped or conical designs. A chisel-shaped insert possesses a crest forming an elongated cutting edge that impacts the core portion of the hole bottom. It is particularly suited for softer formations. By contrast, as compared to a standard chisel-shaped cutter, a conical insert is considered less aggressive as it has a relatively blunt cutting surface, and does not include the relatively sharper cutting edge formed by the chisel&#39;s crest. As such, the conical design tends to be more durable than the chisel-shaped cutting element, particularly in harder formations. Regardless of its shape, conventional nose row cutters will only contact the core approximately 1.25 times per bit revolution. At the same time, due to their greater numbers, a row of cutting elements in other locations on each cone contact the hole bottom with much greater frequency, thereby removing formation material faster than at the borehole center. In certain formations, this may result in a core of material that remains uncut and builds up in the center of the borehole, causing the drilling of the borehole to be slower and more costly.  
      Accordingly, there remains a need in the art for a cutting element with a cutting structure that will allow it to remove more material from the hole bottom, and in particular—the hole core, with fewer revolutions of the bit. Such an enhanced design would hopefully provide a higher ROP and an increase in the footage drilled. It would be desirable to provide cutting elements designed and oriented so as to enhance brittle fracture of the rock formation being drilled, and to present to the formation multiple cutting edges as the cutting surface of the cutting element rotates through its cutting trajectory so as to take advantage of multiple cutting modes. At the same time, however, the cutting element should be able to withstand drilling in multiple formations as typically encountered when drilling with TCI bits. Thus, the desire for a more aggressive cutting element must be tempered by the need for providing a durable and relatively long-lasting cutter, one that will resist breakage.  
     BRIEF SUMMARY OF THE PREFERRED EMBODIMENTS OF THE INVENTION  
      Preferred embodiments of the invention are disclosed which provide an earth boring bit and cutting element design intended to provide the potential for increased ROP, as compared with bits employing cutting elements of conventional shape. The embodiments disclosed include cutting elements having aggressive cutting surfaces that have particular, but not exclusive, application in the nose region of a rolling cone cutter.  
      In one preferred embodiment, a cutting element for a drill bit includes a base portion and a cutting portion having two or more cutting extensions extending away from the base portion, and having valleys between the cutting extensions. The cutting surfaces of the various cutting extensions of the cutting element may be different in shape, or uniform in shape, and may have different extension heights. The cutting extensions may be crested extensions, such as extensions having a chiseled-shaped cutting surface. Further, with respect to crested cutting extensions, such cutting extensions include crests that may differ in crest length, and may form angles relative to the cutting element axis. The angles, referred to as twist angles or crest angles, may be the same or different for different cutting extensions on the cutting element. Further, the cutting extensions define extension angles relative to the longitudinal axis of the cutting element. The extension angles among the plurality of cutting extensions, of a cutting element, may differ, or may be the same. One embodiment includes a cutting element wherein the cutting portion includes a plurality of cutting extensions configured and arranged such that the cutting element includes an asymmetrical cutting surface. In certain embodiments, the cutting extensions are canted and may differ in cant angles.  
      In certain embodiments, the cutting portion of the cutting element includes a foundation surface adjacent to the base portion, and the cutting extensions extend from the foundation surface. In certain such cutting elements, the foundation surface is generally frustoconical, generally or partially dome-shaped, or other non-planar shape. Furthermore, as previously mentioned, the cutting extensions may include at least two cutting extensions that have cutting surfaces that differ in shape, height, extension angle, crest angle, or cant angle.  
      The cutting surfaces of the various cutting extensions (as well as the entire cutting extension itself) may be made of differing materials, in particular those having differing degrees of wear resistance, hardness and durability. The materials employed as the cutting surface, like the extension angles, extension height, cutting shapes, twist angle and cant angle may be varied to optimize the cutting element for the particular duty that is expected. For example, relatively long and relatively sharp crested cutting extensions may be included in a cutting element for particular use in soft formations. The same cutting element may include shorter and more rounded cutting extensions as being advantageous when the bit encounters harder formations. As stated, various combinations of the cutting extensions&#39; geometric characteristics or material properties allow the bit designer abundant latitude in optimizing a particular cutting element, where the term “optimizing” includes appropriate compromises in design.  
      Other embodiments of the invention include a drill bit for drilling through earthen formations including a bit body, at least one rolling cone cutter rotatably mounted on the bit body, and a plurality of cutting elements mounted in the cone cutter, wherein at least one of the cutting elements includes a base portion secured in the cone cutter and a cutting portion having a plurality of cutting extensions extending from the base and being separated by valleys. The drill bit may include such a cutting element located in the nose portion of the bit where the longitudinal axis of the cutting element may be aligned with the cone axis. Alternatively, the bit may have a plurality of cutting elements with multiple cutting extensions where the elements are mounted in the nose region of the cone cutter and disposed in a circumferential row about the cone axis. The cutting extensions may vary in size, shape, extension height, extension angle, crest length, as examples.  
      The cutting elements and drill bit described herein provide an aggressive cutting structure and cutting element with multiple cutting extensions. At least when employed in the nose region of a bit, these embodiments, offer potential in ROP enhancement given, in particular, that the cutter&#39;s multiple cutting extensions will engage and cut the borehole bottom more times per bit revolution than, for example, a conventional chisel-shaped element having only a single cutting surface or the conventional conical cutter having only a relatively blunt cutting surface.  
      Thus, the embodiments described herein comprise a combination of features intended to enhance the state of the art relating to bit and cutting element design. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For an introduction to the detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings, wherein:  
       FIG. 1  is an elevation view of a rolling-cone, earth-boring bit;  
       FIG. 2  is a partial cross sectional view of the bit of  FIG. 1  inside of a borehole;  
       FIG. 3  is another partial cross-sectional view of the bit of  FIG. 1  inside of a borehole;  
       FIG. 4  is an elevation view of a cutting element in the form of an insert for use in the drill bit of  FIG. 1 .  
       FIG. 5  is a top view of the cutting element of  FIG. 4 .  
       FIG. 6  is a top view, in schematic form, showing the orientation of the crests of the cutting extensions of the cutting element shown in  FIGS. 4 and 5 .  
       FIG. 7  is a top view, in schematic form, showing the orientation of the crests of the cutting extensions in another cutting element that may be employed in the rolling cone bit of  FIG. 1 .  
       FIG. 7A  is a side elevation view of the cutting element shown in  FIG. 7 .  
       FIG. 8  is a top view, in schematic form, showing the orientation of the crests of the cutting extensions in another cutting element that may be employed in the rolling cone bit of  FIG. 1 .  
       FIG. 9  is an elevation view, partly in cross-section, of the cutting element shown in  FIG. 4 .  
       FIG. 10  is an elevation view of another cutting element in the form of an insert for use in the drill bit of  FIG. 1 .  
       FIG. 11  is a top view of the cutting element of  FIG. 10 .  
       FIG. 12  is a top-view, in schematic form, showing the orientation of the crests of the cutting extensions of another cutting element for use in the bit of  FIG. 1 .  
       FIG. 13  is an elevation view, partly in cross-section, of the cutting element shown in  FIG. 12 .  
       FIG. 14  is an elevation view of another cutting element in the form of an insert for use in the drill bit of  FIG. 1 .  
       FIG. 15  is a top view of the cutting element of  FIG. 14 .  
       FIG. 16  is an elevation view of still another cutting element in the form of an insert for use in the drill bit of  FIG. 1 .  
       FIG. 17  is a top view of the cutting element of  FIG. 16 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Presently-preferred embodiments of the invention are shown and described below. These embodiments are exemplary only, and are not limiting. That is, the scope of the invention is not limited by the description of the specific embodiments described below, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims  
      As used herein to compare or claim particular features or characteristics (such as, for example, heights, lengths, angles) of different cutting extensions or cutting elements, the term “differs” or “different” means that the value or magnitude of the characteristic being compared varies by an amount that is greater than that resulting from accepted variances or tolerances normally associated with the manufacturing processes that are used to formulate the raw materials and to process and form those materials into a cutting element. Thus, particular characteristics selected so as to have the nominal value will not “differ,” as that term has thus been defined, even though the characteristics, if measured, would vary about the nominal value by a small amount.  
      In the description that follows, like parts or features are referred to throughout the specification and drawings with the same reference numerals. The drawing figures are not necessarily to scale. In the figures, certain features of the invention may be shown exaggerated in scale or in somewhat schematic form, and some details of certain elements may be omitted in the interest of clarity and conciseness.  
      Referring first to  FIG. 1 , an earth-boring bit  30  includes a central axis  31  and a bit body  32  having a threaded section  33  on its upper end for securing the bit to the drill string (not shown). Bit  30  has a predetermined gage diameter as defined by three rolling cone cutters  34 ,  35 ,  36  which are rotatably mounted on bearing shafts (not shown) that depend from the bit body  32 . The embodiments disclosed herein will be understood with a detailed description of one such cone cutter  34 , with cones  35 ,  36  being similarly, although not necessarily identically, configured. Bit body  32  is composed of three sections, or legs  37  (two shown in  FIG. 1 ), that are jointed together to form bit body  32 .  
      Referring now to  FIG. 2 , bit  30  is shown inside a borehole  29 , which includes sidewall  42 , corner portion  43  and bottom  44 . Cone cutter  34  is rotatably mounted on a pin or journal  38 , with an axis of rotation  39  oriented generally downward and inward towards the center of bit  30 . Cone cutter  34  is secured on pin or shaft  38  by ball bearings  40 . Cutters  34 - 36  include a plurality of tooth-like cutting elements  41 , for gouging and chipping away formation material to form the borehole  29 .  
      Referring still to  FIGS. 1 and 2 , each cone cutter  34 - 36  includes a backface  45  and nose portion  46  generally opposite backface  45 . Cutters  34 - 36  further include a frustoconical heel surface  47  that is adapted to retain cutting elements  51  that scrape or ream sidewall  42  of the borehole as cutters  34 - 36  rotate about borehole bottom  44 . Frustoconical surface  47  is referred to herein as the “heel” surface of cutters  34 - 36 , it being understood, however, that the same surface may be sometimes referred to by others in the art as the “gage” surface of a rolling cone cutter. Extending between heel surface  47  and nose  46  is a generally conical surface  48  adapted for supporting cutting elements  41  which gouge or crush the borehole bottom  44  as the cone cutters  34 - 36  rotate about the borehole.  
      Referring back to  FIG. 1 , conical surface  48  typically includes a plurality of generally frustoconical segments  49 , generally referred to as “lands,” which are employed to support and secure cutting elements  41 . Frustoconical heel surface  47  and conical surface  48  converge in a circumferential edge or shoulder  50 . Cutting elements  41  retained in cone cutter  34  include a plurality of heel row inserts  51  that are secured in a circumferential row  52  in the frustoconical heel surface  47 . Cone cutter  34  further includes a circumferential row  53  of gage inserts  54  secured to cone cutter  34  in locations along or near the circumferential shoulder  50 . Cone cutter  34  further includes a plurality of inner row inserts, such as inserts  55  and  56  secured to cone surface  48  and arranged in spaced-apart inner rows  57  and  58 , respectively.  
      Referring again to  FIG. 2 , heel inserts  51  generally function to scrape or ream the borehole sidewall  42  to maintain the borehole  29  at full gage and prevent erosion and abrasion of heel surface  47 . Cutting elements  55  and  56  of inner rows  57  and  58  are employed primarily to gouge and crush and thereby remove formation material from the borehole bottom  44 . Inner rows  57  and  58 , are arranged and spaced on cone cutter  34  so as not to interfere with the inner rows on each of the other cone cutters  35 ,  36 .  
      In the embodiment shown in  FIGS. 1 and 2 , each cone cutter  34 - 36  includes at least one cutting element on nose portion  46  spaced radially inward from inner rows  57  and  58 , herein referred to as a nose insert  60 . As cone cutters  34 - 36  rotate about their respective axis  39 , nose inserts  60  gouge and remove the central or core portion of the borehole. As shown in  FIG. 2 , nose insert  60  in cone  34  is positioned such that the insert is generally aligned with cone axis  39 .  
      Referring now to  FIG. 3 , bit  30  is disclosed in a borehole  29  with cone  35  shown in cross-section. All elements in  FIG. 3  are identical to those disclosed in  FIG. 2 , with the exception that nose inserts  60  on cone  35  are now arranged in a circumferential row  62  on nose portion  46 . Row  62  is disposed about cone axis  39 .  
      A cutting element in the form of an insert  70  is shown in  FIGS. 4 and 5 . Insert  70  is particularly suited for use as a nose row insert  60  shown in  FIGS. 2 and 3 ; however, it may also be employed at other locations in cone cutters  34 - 36 . Particularly, insert  70  may also be employed as a heel row insert  51 , a gage row insert  54 , or inner row inserts  55 ,  56 .  
      Insert  70  generally includes a base portion  71  and a cutting portion  75  connected to and extending form base  71 . Base  71  includes bottom surface  72  and a generally cylindrical side surface  73  that is formed about a central, longitudinal insert axis  74 . Cutting portion  75  intersects or joins base portion  71  at a generally circular junction  76 . Axis  74  extends generally perpendicular to bottom  72  and a plane containing junction  76 . In this embodiment, cutting portion  75  and base portion  71  are integrally-formed of tungsten carbide, although other materials and other manufacturing processes may be employed to form insert  70 . Base  71 , which may also be referred to as the insert&#39;s “grip,” is embedded and retained in cone  34 . Cutting portion  75  is that portion of insert  70  that extends beyond the steel of the cone cutter.  
      Cutting portion  75  generally includes foundation surface  78 . Foundation surface  78  intersects cylindrical side surface  73  of base  71  at junction  76 , and extends inwardly from junction  76  toward insert axis  74  and upwardly in the direction away from bottom surface  72 . In this manner, foundation surface  78  may be said to taper upwardly and away from base  71 .  
      Cutting portion  75  further includes cutting extensions that extend from the foundation surface in a direction upward and away from base  71 . In this embodiment, cutting portion  75  includes three cutting extensions  81 ,  82 ,  83  separated from one another by valleys  84 ,  85 ,  86 , best shown in  FIG. 5 . The intersection of valleys  84 - 86  form a central recess or valley  87  in this embodiment.  
      As best shown in  FIG. 4 , the embodiment of insert  70  includes three distinctly-shaped cutting extensions. Cutting portion  75  includes no plane of symmetry that passes through axis  74 , such that cutting portion  75  has an asymmetrical cutting surface.  
      Cutting extension  81  includes a generally dome-shaped cutting surface  90  which may be hemispherical or a greater or lower portion of a dome. Cutting surface  90  intersects with foundation surface  78  in a curved junction or fillet  91 . Cutting extension  83  includes a generally chisel-shaped cutting surface  92 , including sloping sides  93  and crest  94 , which forms a crest axis  95 . The chisel-shaped cutting surface  92  intersects foundation surface  98  in a rounded intersection or fillet  96 . Cutting extension  82  also includes a chisel-shaped cutting surface  98  having a crest  101  that generally extends along crest axis  104 . Cutting surface  98  includes sloping sides  100  that slope from crest  101  to intersect with foundation surface  78  in a rounded or radiused fillet  99 . Preferably, the radius of junctions  91 ,  96  and  99  is selected to be not less than 0.080. As best shown in  FIG. 5 , crest  101  of cutting extension  82  is narrower or sharper at outer end  102  as compared to inner end  103 . Further, crest  101  of cutting extension  82  is broader at both its inner and outer ends  101 ,  102  as compared with crest  94  on cutting extension  83 .  
      Cutting extensions  81 - 83  are spaced apart from one another and separated by valleys  84 - 87 , meaning that a planar cross-section of cutting portion  75  taken perpendicular to insert axis  74  will intersect the extensions in a plurality of spaced-apart closed figures when the cross-section is taken at at least one axial position along insert axis  74 . Thus, it is understood with reference to  FIG. 4  that a cross-section of insert  71  taken at plane  88  that is perpendicular to insert axis  74  will yield a cross-section having three spaced-apart closed figures, each represented by the intersection of plane  88  with the cutting surfaces  90 ,  98  and  92  of extensions  81 ,  82 ,  83 , respectively.  
      In the embodiment shown in  FIGS. 4 and 5 , foundation surface  78  is shown to be generally conical. This and other non-planar surfaces, such as generally or partially dome-shaped, conical, hemispherical, or other curved surfaces, are preferred for foundations surface  78 ; however, foundation surface  78  may be generally planar in some embodiments.  
      In the embodiment shown in  FIGS. 4 and 5 , cutting extensions  81 - 83  extend to outermost points that fall outside of an extending projection of cylindrical side surface  73 . As such, cutting extensions  81 - 83  have a negative draft in relation to base portion  71 . A cutting portion that does not extend beyond or outside of the upward projection of the outer cylindrical side surface  73  may also be employed, and would have what may be referred to as a positive draft with respect to base portion  71 . As compared to a cutting element having a cutting surface with a positive draft relative to its base, a design employing a negative draft would potentially allow a greater volume of hole bottom material to be cut with a given impact of the cutting element.  
      In the embodiment shown in  FIGS. 4 and 5 , the cutting surfaces  90 ,  98 , and  92  of cutting extensions  81 - 83  are continuously contoured to avoid sharp edges or discontinuities or abrupt changes in slope. As used herein, the term “continuously contoured” refers to surfaces that can be described as having continuously curved surfaces that are free of relatively small radii that are sometimes used to break sharp edges or round off transitions between adjacent distinct surfaces. Likewise, the intersections  91 ,  96  and  99  likewise may be formed to be free of such small radii, such that the entire cutting portion  75  of insert  70  may be said to have a cutting surface that is continuously contoured.  
      The cutting elements described herein as having a generally chisel-shape and crest may take many forms. For example, cutting extensions having the chisel-shape depicted and described with reference to  FIGS. 5-8  in U.S. Pat. No. 5,172,777 may be employed. The entire disclosure of U.S. Pat. No. 5,172,777 is hereby incorporated by reference.  
      Although the cutting surfaces of extensions  81 - 83  are shown and have been described as being continuously contoured, in order to illustrate certain aspects of insert  70 , it is useful to refer to certain portions of the cutting extensions as having distinct boundaries that do not actually exist. Thus, for ease of explanation only,  FIGS. 6-8  show top views of cutting element inserts having certain artificial boundaries superimposed in a schematic fashion. In particular, and referring to  FIG. 6 , a top view of insert  70  showing cutting extension  83  with crest region  97  and crest axis  95  radially disposed relative to insert axis  74 . That is, crest axis  95  is substantially aligned with radius R of insert  70 . Likewise, cutting extension  82  includes crest region  105  and is formed such that region  105  and crest axis  104  are radially aligned relative to insert axis  74 . As shown in this embodiment, cutting extensions  82  and  83  are arranged such that crest axes  104  and  95  intersect and form an angle of intersection of approximately 120°.  
      Referring to  FIG. 8 , another embodiment is shown. Like the embodiment of  FIGS. 4-6 , insert  114  of  FIG. 8  includes cutting extensions  81 - 83  as previously described. In this embodiment, however, cutting extension  83  is formed having cutting surface  116  with crest axis  95  twisted or rotated relative to its orientation shown in  FIGS. 6 and 7  where axis  95  was substantially aligned with or parallel to radius R. With cutting surface  116  of  FIG. 8 , crest axis  95  does not pass through insert axis  74 . The angle formed between crest axis  95  and radius R that passes through the midpoint MP of crest  94  forms what is described as the twist angle α. It should be understood that, in addition to either canting or rotating the cutting extension as shown in  FIGS. 7 and 8 , respectively, a cutting extension may be disposed such that it is both canted and twisted relative to a position in which a crest axis passes through the insert axis.  
       FIG. 9  illustrates other features relating to insert  71  previously described with reference to  FIGS. 4-6 . In particular,  FIG. 9  shows insert  71  in partial cross-section with the section taken along crest axis  95  and through insert axis  74 . As shown, cutting extension  83  includes an extension axis  106  that intersects insert axis  74  in an extension angle θ, which preferably is less than 60°. As used herein, the extension axis of a cutting extension may be described as the axis that passes through the midpoint of the extension&#39;s crest and that bisects the longitudinal cross-sectional area that is formed by the plane that contains the crest and that bisects the extension. Each of inserts  81 - 83  may include extension angles θ that are equal to one another, or they may differ. Likewise, in certain applications, it is believed advantageous to have cutting extensions  81 - 83  extend to different heights. As used herein, the extension height of a cutting extension is the distance measured axially from the junction  76  to the furthest point on the extension&#39;s cutting surface. As shown in  FIG. 9 , cutting extension  83  includes an extension height  108  that is greater than the extension height  109  of cutting extension  81 .  
      Referring now to  FIGS. 7 and 7 A, another embodiment of a cutting element is shown. In this embodiment, cutting element  110  includes foundation surface  78  and cutting extensions  81 ,  82  as previously described. Cutting element  110  further includes cutting extension  83 ′ which is substantially the same as cutting extension  83  previously described. However, in this embodiment, cutting extension  83 ′ is canted in relation to the position of cutting extension  83  in the embodiment shown in  FIGS. 4-6 . More particularly, as best shown in  FIG. 7A , cutting extension  83 ′ extends from the foundation surface  78  at a cant angle β as measured between extension axis  106 ′ and the insert axis  74 . In the embodiment of  FIGS. 4-6 , cutting element  83  included a cutting extension axis  106  having a projection substantially aligned with insert axis  74 . As shown in  FIG. 7A , canting the cutting extension to the position represented by  83 ′ moves the cutting extension axis  106 ′ such that, when viewed from the top of the cutting element ( FIG. 7 ), crest axis  95  is not aligned with a radius R. The axis  95  further does not pass through insert axis  74 .  
      The embodiments thus described include features believed to enable a nose row cutting element to cut the core portion of the borehole effectively, and to do so in a variety of formation hardnesses. For example, and referring to  FIG. 9 , the relatively long extension of cutting extension  83  and the relatively sharp cutting surface formed by its chisel shape is believed advantageous in cutting through relatively soft formations. In somewhat harder formations, it is believed that crested cutting extension  82  will provide an advantageous blend of aggressiveness and durability, given its intermediate extension length, relative to cutting extensions  81  and  83 , and given that it includes a chisel shape, albeit more rounded than the sharper chisel shape of cutting extension  83 . In still harder formations, although the cutting extensions  82  and  83  may wear and thereby not provide the rapid removal of formation material as they would in softer formations, the generally rounded, dome-shaped cutting surface  90  of cutting extension  81  is believed advantageous. In particular, the dome shape provides a durable, wear-resistant cutting surface.  
      Additional wear-resistance may be provided to cutting extensions  81 - 83 . In particular, some or all of the cutting surfaces of these cutting extensions may be coated with diamond or other super-abrasive material in order to optimize (which may include compromising) cutting effectiveness and/or wear-resistance. In the embodiment shown in  FIGS. 4 and 5 , for example, it is contemplated that only the dome-shaped cutting surface  90  of cutting extension  81  be coated with super abrasive material, with the cutting extensions  82 ,  83  being made entirely of tungsten carbide. In another embodiment, it may be that the cutting surfaces of all three of cutting extensions  81 ,  82 ,  83  are coated with a super abrasive.  
      Super abrasives are significantly harder than cemented tungsten carbide. As used herein, the term “super abrasive” means polycrystalline diamond (PCD), carbon boron nitron (CBN)thermal stable diamond (TSP), cubic boron nitride (PCBN), and any other material having a hardness of at least 2,700 Knoop (kg/mm 2 ). As examples, PCD grades have a hardness range of about 5,000-8,000 Knoop (kg/mm 2 ) while PCBN grades have hardnesses which fall within the range of about 2,700-3,500 Knoop (kg/mm 2 ). By way of comparison, conventional cemented tungsten carbide grades typically have a hardness of less than 1,500 Knoop (kg/mm 2 ).  
      Certain methods of manufacturing cutting elements with PDC or PCBN coatings are well known. Examples of these methods are described, for example, in U.S. Pat. Nos. 5,766,394, 4,604,106, 4,629,373, 4,694,918 and 4,811,801, the disclosures of which are all incorporated herein by this reference.  
      There are today a number of commercially available cemented tungsten carbide grades that have differing, but in some cases overlapping, degrees of hardness, wear resistance, compressive strength and fracture toughness. Some of such grades are identified in U.S. Pat. No. 5,967,245, the entire disclosure of which is hereby incorporated by reference. With this understanding, and referring to  FIG. 4  as an example, the cutting extensions  82  and  83  may be made of tungsten carbide having differing mechanical or physical properties. In particular, in this embodiment, it is desirable that the longer and generally sharper cutting extension  83  be made of a tungsten carbide having a higher degree of toughness than the cutting extension  82 . Similarly, the tungsten carbide employed in cutting extension  82  is preferably harder and more wear-resistant than the material of cutting extension  83 . As a still further example, cutting extension  81  would include a substrate material beneath the diamond coating and, in this example, may be made of a carbide material that is harder and more wear-resistant than the tungsten carbide used to form cutting extension  83 . Compared to the material used to form cutting extension  82 , the substrate of cutting extension  81 , depending upon the particular application, may be harder and more wear-resistant, but have a lower degree of facture toughness.  
      In another embodiment shown in  FIGS. 10 and 11 , cutting element  120  includes a base portion  121 , axis  122 , and a cutting portion  123 . Cutting portion  123  intersects the base at junction  124 . In this embodiment, cutting portion  123  includes foundation surface  78  and three identically configured cutting extensions  83 , having the shape and structure previously described. Best shown in  FIG. 11 , cutting extension  83  includes a chisel-shaped cutting surface  92  including sloping sides  93 , crest  94  and crest axis  95 . In this embodiment, crest axis  95  of each of the cutting extensions  83  intersect with insert axis  122 . It is preferred that the cutting surfaces of all cutting extensions  83  be continuously contoured.  
      The cutting extensions  83  may be oriented differently than shown in  FIG. 11 . For example, the cutting extensions  83  may be canted, twisted or both, such that in alternative orientations, the crest axis  95  of the cutting extension involved will not pass through the insert axis  122 . Such adjustments in the orientation of the cutting extensions and their crests may be desirable to optimize cutting effectiveness, or durability or other characteristics. As an example, one such alternative is shown in  FIG. 12 , in which cutting element  127  includes cutting portion  123  which includes chisel-shaped cutting extensions  83   a,    83   b,    83   c  defining asymmetrical cutting surface  126 . As shown in  FIG. 12 , cutting extension  83   a  is oriented such that crest axis  95   a  extends radially and intersects insert axis  129 . By contrast, cutting extension  83   b  is oriented such that its crest  95   b  is twisted or rotated away from being in alignment with the insert&#39;s radius, forming an angle α 1  equal to approximately 20°. Cutting extension  83   c  is oriented such that crest axis  95   c  is further rotated from being aligned with an insert radius, crest axis  95   c  intersecting with the insert radius at an angle α 2  of approximately 40°. Varying the twist angle, as defined above, among the various cutting extensions on the insert is believed to provide the opportunity to enhance cutting effectiveness in certain formations while maintaining acceptable durability in others. In addition to varying the twist angle, the cutting extensions may likewise be canted to differing degrees.  
      Further still, and referring to  FIG. 13 , the extension height and extension angle of cutting extensions may be varied to provide enhanced durability, greater rate of penetration, or a compromise in these characteristics. For example, as shown in  FIG. 13 , an insert  132  is shown that is substantially similar to the insert  120  described with reference to  FIGS. 10 and 11 . In the embodiment of  FIG. 13 , however, chisel-shaped cutting extension  83   d  has a greater extension height  133  than the extension height  134  of chisel-shaped cutting extension  83   e.  Likewise, relative to longitudinal element axis  135 , cutting extension  83   d  has a first extension angle θ 1  that is less than the extension angle θ 2  of cutting extension  83   e.  With these differences, extensions  83  provide an asymmetric cutting surface  136 . It is believed that advantages may be obtained by employing extension angles of between approximately 0 and 60°; however, other extension angles may be employed.  
      Referring to  FIGS. 14 and 15 , another cutting element  140  is shown having a cutting portion  142  extending from a base portion  144 . In this embodiment, cutting portion  142  includes three cutting extensions  82  that are substantially identical to cutting extensions  82  previously described with reference to  FIGS. 4-5 . In particular, each cutting extensions  82  includes a crest  101  and sloping sides  100  that intersect foundation surface  78  at fillet  99 . The crest  101  is oriented such that crest axis  104  extends radially and passes through insert axis  146 . The outer end  102  of each crest  101  is narrower or sharper than the inner end  103 . It is believed that this cutting element will be more durable than, for example, the cutting element  120  described with reference to  FIGS. 10 and 11  where relatively sharper chisel shapes are employed. As such, the insert  140  of  FIGS. 14 and 15  may be advantageous in formations that are expected to be harder than those in which the embodiment of  FIGS. 10 and 11  would be used.  
      Another embodiment is shown in  FIGS. 16 and 17 . Here, the cutting element  150  includes base portion  151  and cutting portion  152  having three cutting extensions  81  substantially the same as extension  81  previously described with reference to  FIGS. 4 and 5 . Each cutting extensions  81  includes a dome or partial dome-shaped cutting surface  90 . In the embodiment shown, each cutting extension  81  is oriented so as to have the same extension angle relative to element axis  154 , but the extension angle and extension height of the cutting extensions  81  may differ. Likewise, the spherical radius of curvature of each cutting surface  90  in this embodiment is substantially the same, it being understood that the spherical radius of curvature of the respective cutting surfaces may differ in other embodiments. It is believed that the insert of  FIGS. 16 and 17  may best be employed in hard formations where great durability is desirable. Accordingly, in such hard formations, the cutting surface  90  of one or more cutting extensions  81  may be diamond-coated or coated with any other super abrasive material.  
      While the embodiments described above are shown having three cutting extensions, it should be understood that the number of cutting extensions may vary depending upon the application. Thus, for example, the cutting elements shown herein may instead be formed having two or even four or more cutting extensions.  
      The cutting elements described herein may be advantageously employed in the nose region of a cone cutter, or in other locations. When employed in the nose region or portion, as shown in  FIGS. 1-3 , the multiple cutting extensions enhance the ability of the bit to cut the central core of the borehole as compared to a bit having conventional conical or chisel-shaped cutting element with only a single cutting extension. Compared to such conventional inserts, the cutting elements described herein will contact the core portion many more times per bit revolution than a conventional, single cutting extension, cutting element. The multiple cutting extensions provide more impacts or scrapes on the hole bottom per revolution of the bit, helping to prevent core buildup. Varying the cutting surface shape of the cutting extensions, varying their height, orientation, extension angle and materials provide the bit designer with numerous design features to provide the optimum cutting structure in light of the particular formation expected to be encountered and other factors.  
      The following co-pending patent applications are hereby incorporated by reference in their entireties: U.S. patent application Ser. No. 10/355,493, filed Jan. 31, 2003, entitled “Multi-Lobed Cutting element For Drill Bit” and of U.S. patent application Ser. No. 10/371,388, filed Feb. 21, 2003, entitled “Drill Bit Cutting element Having Multiple Cusps.”