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BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to cutting elements for rotary drill bits for subterranean drilling, and more specifically to cutting elements providing a controlled superabrasive contact area during a predominant portion of the useful life of the cutting element, as well as bits so equipped and methods of drilling therewith. 
     2. State of the Art 
     Rotary bits are the predominant type of drill bits employed for subterranean drilling to oil, gas, geothermal and other formations. Of the types of rotary bits employed, so-called fixed cutter or “drag” bits have garnered an ever-increasing market share over the past few decades. This market share increase is attributable to a number of factors, but significant ones must be acknowledged as the wide availability and performance of superabrasive cutting elements. 
     Superabrasive cutting elements in their present state typically take the form of a polycrystalline diamond compact (PDC) layer or “table” formed onto a supporting substrate, typically of a cemented or sintered tungsten carbide (WC), in a press under ultra-high pressure and temperature conditions. Other superabrasive materials are known, including thermally stable PDCs, diamond films, and cubic boron nitride compacts. The present invention has utility with cutting elements employing any superabrasive material. 
     Several physical configurations of superabrasive tables for cutting elements are known, including square, “tombstone” shape, and triangular. However, the most common shape is circular, backed by a circular substrate of like size. These circular superabrasive tables are usually formed substantially to size in a press, but may be cut from larger, disc-shaped blanks. The other referenced shapes are generally required to be cut from a larger, disc-shaped blank, thus generating a large volume of scrap, reducing yield during fabrication and increasing fabrication costs. 
     As can be seen in FIGS. 1 and 2 of the drawings, state-of-the-art, disk-shaped cutting element  10  includes a circular, PDC superabrasive table  12  of substantially constant depth mounted to a disk-shaped WC substrate  14 . Superabrasive table  12  includes a cutting face  16 , a cutting edge  18  at the periphery of cutting face  16 , and a side  20  to the rear of cutting edge  18  (taken in the direction of cutting element travel, cutting face-first). Cutting element  10  would typically be oriented on a drill bit with at least a nominal negative backrake so that cutting face  16  “leans” away from the formation being drilled. As the cutting edge  18  and side  20  of superabrasive table  12  of cutting element  10  first contact the formation under application of weight on bit (WOB) at location  22  of cutting edge  18 , it can be seen that the superabrasive contact area is extremely small in both longitudinal depth or thickness as well as width, in part due to the aforementioned backrake. Thus, for a given WOB, the responsive loading per unit surface area at the side  20  of superabrasive table  12  contacting the formation being drilled is extremely high. 
     Due to the circular shape of the superabrasive table  12 , however, as the cutting element  10  begins to wear and a so-called “wear flat” forms at one side of cutting face  16 , superabrasive table  12  and the WC substrate  14  therebehind, the contact area of the superabrasive material under WOB, or so-called Normal force applied along the axis of the drill string to which the bit is secured, increases markedly in width and therefore in total area. The increasing contact area consequently requires an increase in WOB to maintain cutting element loading in terms of load per superabrasive unit surface area in contact with the formation to continue an acceptable rate of penetration (ROP). However, as WOB increases, so does wear on the superabrasive table, as well as the likelihood of spalling and fracture damage thereto. In addition, the requirement to increase WOB may undesirably affect drilling performance in terms of reducing steerability of a bit, as well as precipitate stalling of a downhole motor when the torque required to rotate under excessive WOB is exceeded, with consequential loss of tool face orientation. As can readily be visualized by looking at the relative contact area widths at location  22 , location  24  (as the cutting element is about 20% in diameter worn) and location  26  (as cutting element  10  is about 40% in diameter worn and typically approaching, if not well past, the end of its useful life), the superabrasive contact area may increase by more than an order of magnitude from the time a cutting element first engages a formation until the end of its useful life, thus requiring an attendant increase in WOB to maintain ROP in a given formation. 
     This undesirable increase in superabrasive contact area is present in conventional PDC cutting elements bearing constant-thickness superabrasive tables of about 0.030 inch thickness. However, as cutting elements bearing tables of greater thicknesses are developed, for example 0.070 inch and 0.100 inch uniform-thickness tables, the contact area increase is exacerbated. The increase in wear flat area for such PDC cutting elements of 13 mm (0.529 inch) diameter is illustrated in FIG. 9, wherein superabrasive contact area versus percentage of cutting face diametric wear is shown respectively by lines A, B and C for cutting elements of 0.030, 0.070 and 0.100 inch superabrasive table thickness. For each of the 0.030 inch, 0.070 inch and 0.100 inch thickness tables, the contact area more than doubles between 5% and 30% diametric wear of the superabrasive table. More significantly, for the 0.070 inch and 0.100 inch thickness superabrasive tables, contact area quickly increases in absolute terms to in excess of 0.02 square inch (the maximum superabrasive contact area for a 13 mm, 0.030 inch thick table PDC cutting element), thus necessitating substantial and undesirable WOB increases extremely early in the life of the cutting element in order to maintain the load per unit surface area of superabrasive material contacting the formation. While use of a square or tombstone-shaped cutting face, would obviously provide a relatively constant superabrasive contact area, as noted above such configurations are undesirable for other reasons. Consequently, there is a need in the art for a cutting element exhibiting a circular cutting face and superabrasive table, the term “circular” as used herein including a segment of a circle a segment or which otherwise exhibits an arcuate or nonlinear cutting edge, which provides a relatively constant superabrasive contact area during a large portion of the useful life of the cutting element. 
     BRIEF SUMMARY OF THE INVENTION 
     In contrast to the circular or disk-shaped cutting elements comprising the state of the art, the cutting elements of the invention are configured with superabrasive tables having configurations such that the surface area of superabrasive material in contact with a formation being cut by the cutting element responsive to WOB quickly reaches a relatively stable value, which value remains relatively constant over a substantial portion of the useful life of the cutting element, for example, from about 5% to about 30% wear across the diameter of the cutting face. The present invention provides this relatively stable value of a relatively small magnitude, for example, from about 0.018 to about 0.021 square inch for a 13 mm (0.529 inch) diameter cutting element. 
     One embodiment of the cutting element of the present invention is configured with a planar cutting face and a non-planar interface between the superabrasive table and the supporting substrate, wherein at least one radially-oriented, substantially isosceles triangular projection of increased superabrasive table thickness lies adjacent the periphery of the superabrasive table with the triangle base oriented toward the formation. The superabrasive projection gradually decreases in thickness and width from a location adjacent the cutting edge at the periphery of the as-formed, unworn superabrasive table toward the center of the cutting element. During drilling, the decrease in thickness and width of the superabrasive projection as the cutting element wears is substantially offset by an increase in width of contact with the formation of the superabrasive table as a whole, attributable to the increasing lateral contact span of the thinner portions of the table laterally flanking the projection as the cutting element wears during use. In actual practice, it may be desirable to fabricate such a cutting element with, for example, four such triangular projections at 90° rotational intervals, so as to maintain symmetrical stress patterns at the superabrasive table-to-substrate interface. Such an embodiment may employ projections which immediately commence a decrease in depth from the cutting face periphery, or may maintain an initial constant depth or even increase in depth for a measurable distance from the table periphery, to provide a robust superabrasive mass to effect and sustain the initial contact with the formation until the wear flat is well-established. 
     Another embodiment of the invention features a cutting element employing a superabrasive table which features a thicker portion of constant width lying along a radius of the cutting element, the table decreasing non-linearly in thickness toward the center of the cutting element in proportion to the increase in contact area width of the superabrasive table, so as to maintain a substantially constant superabrasive contact area for a significant portion of the cutting element life. 
     It is contemplated that cutting elements according to the invention having superabrasive tables employing superabrasive projections or thickness increases leading or projecting from the cutting faces of the tables may be employed. For example, a triangular or other shape projection may lie on the cutting face, or the cutting face may be of a convex configuration, with the increased superabrasive depth exhibited as a domed, diametrically-extending ridge. 
     It is further contemplated that cutting elements according to the present invention may be configured with cutting tables of varying depth, wherein the depth variances are manifested both internally (at the substrate interface) and externally (as a projection from the cutting face, or non-planar cutting face), or both. 
     It is also contemplated that the invention may be embodied in the form of a half-circular, one-third circular, or other circular fraction cutting element having an internal or external superabrasive table projection, or both, of appropriately varying depth and/or width, as the case may be, extending from an arcuate cutting edge at a periphery of the table toward a center point from which the radius defining the cutting edge extends. The invention may also be employed with cutting elements exhibiting cutting edges of other than constant radius, such as ellipsoidal cutting edges, to compensate for increases in superabrasive contact area. 
     Finally, it may be recognized that extreme variations in backrake of a cutting element when mounted to a drill bit may necessitate some adjustment in the configuration in terms of variations in thickness and width of the deeper portions of the superabrasive table to ensure a substantially constant superabrasive contact area responsive to WOB, since a highly backraked cutting element will present a larger contact area to the formation than a slightly backraked one and the contact areas of cutting elements bearing particularly thick superabrasive tables will be particularly affected by large backrakes. 
     The invention also includes methods of drilling with bits equipped with cutting elements of the invention, wherein a relatively constant superabrasive contact area with the formation is maintained, and a substantially constant ROP may be maintained throughout a substantial portion of cutting element life under a relatively constant applied WOB. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIGS. 1 and 2 comprise, respectively, side and frontal views of a prior art, circular, superabrasive cutting element; 
     FIGS. 3A,  3 B and  3 C comprise, respectively, perspective, frontal and side sectional views of a substrate for a first embodiment of the invention; 
     FIG. 4 comprises a perspective view of a cutting element of the first embodiment of the invention; 
     FIGS. 5A,  5 B and  5 C comprise, respectively, side, frontal and perspective views of one variant of the first embodiment, FIG. 5D is an enlarged side view of the cutting edge area of the superabrasive table, and FIG. 5E is a perspective view of the leading face of a substrate for that variant; 
     FIGS. 6A,  6 B and  6 C comprise, respectively, perspective, frontal and side sectional views of a substrate for another variant of the first embodiment; 
     FIGS. 7A and 7B comprise, respectively, frontal and side sectional views of a second embodiment of the invention; 
     FIGS. 8A and 8B comprise, respectively, frontal and side views of a third embodiment of the invention; 
     FIG. 9 comprises a graph of superabrasive wear flat area as a function of percent of circular superabrasive table diametrical wear; 
     FIGS. 10A,  10 B and  10 C depict, respectively, additional cutting element embodiments of the invention exhibiting arcuate cutting edges and other than circular cutting faces; and 
     FIG. 11 depicts a rotary drag bit having cutting elements according to the invention mounted thereto. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 3A-3C and  4 , a first embodiment  100  of the cutting element of the present invention will be described. Cutting element  100  includes substrate  102  in the shape of a preformed, longitudinally truncated cylinder fabricated of sintered or cemented WC or other suitable material, as known in the art. The trailing face  104  of substrate  102  as shown is flat, while the leading face  106  carrying superabrasive table  130  (see FIG. 4) is non-planar, comprising a plurality of substantially triangular indentations  108  at 90° intervals, the indentations  108  being separated by ridges  110  which converge at the center  124  of the substrate  102 , the top surfaces  111  of the ridges  110  lying substantially on the same plane transverse to the longitudinal axis L of cutting element  100  so as to exhibit a “cross” shape to the viewer. The substantially triangular indentations  108  may be characterized as isosceles in general character, and are each bounded by two linear sides  112  defining about a 60° angle α therebetween, a short inner arcuate boundary  114  connecting converging linear sides  112 , and an outer arcuate edge or base  116  extending between sides  112  and coincident with the outer periphery or side  122  of the substrate  102  in a finished cutting element  100 . The transitions, as at  120 , from the floors  118  of the indentations  108  to sides  112  and boundary  114  and from sides  112  and boundary  114  to ridge top surfaces  111  are preferably radiused rather than sharply angled, for example, along about a 0.02 inch radius. As shown, indentation floors  118  are relatively flat, angled or tilted along a radius of substrate  102  at about a 10° angle of inclination β to ridge top surfaces  111  of the ridges  110 , and located so that a line extending from each floor  118  toward center  124  would intersect a line parallel to the ridge top surfaces  111  and about 0.010 inch therebelow (i.e., within substrate  102 ) at about a 0.060 inch radial distance from center  124 , so as to provide a decrease in thickness of the indentations  108  as they extend from the side  122  of the substrate  102  toward the center  124  thereof. 
     As can be seen in FIG. 4, superabrasive table  130 , preferably comprised of a PDC, is formed on leading face  106  of substrate  102  as known in the art. Table  130  exhibits a substantially planar imperforate cutting face  132 , and superabrasive projections  134  fill indentations  108  of substrate  102 . The depth of superabrasive table  130  at projections  134  may be, for example, about 0.080 inch at the cutting edge  136 . The remainder of table  130 , other than projections  134  and substantially comprising the table area lying over the “cross” of ridges  110 , and center  124  of substrate  102 , comprises portions of lesser and substantially constant superabrasive thickness, for example, about 0.040 inch. Further, the surface of cutting face  132  preferably exhibits a high degree of smoothness, as disclosed and claimed in U.S. Pat. Nos. 5,447,208 and 5,653,300 to Lund et al., assigned to the assignee of the present invention. It is preferred that at least a portion of the cutting face surfaces of all of the embodiments of the invention exhibit a high degree of smoothness as taught by the Lund et al. patents. 
     In use, cutting element  100  is preferably placed with one of the substrate indentations  108  and its associated superabrasive material projection  134  oriented away from the face of the bit on which cutting element  100  is mounted, and toward the formation to be cut by cutting element  100  in a shearing-type cutting action. Such an orientation ensures, after an initial rapid increase in superabrasive contact area as an initial contact point at cutting edge  136  of table  130  wears laterally into a flat during the first 5% or less of diametric cutting face wear, that further lateral increases in the wear flat will be substantially offset by decreases in depth and width of the projection  134  until the cutting face is diametrically worn in excess of about 30%. Thus, as shown by line D in FIG. 9, the superabrasive contact area for the cutting element embodiment  100  in question will, for a 13 mm diameter cutting element, only increase from about 0.018 square inch to about 0.021 square inch as cutting element  100  wears through the aforementioned range, and to only about 0.028 square inch by the time the cutting face is 40% diametrically worn, a point well past its typical useful life. 
     Referring now to FIGS. 5A-5E, a first variant cutting element  200  of the first embodiment is depicted. Cutting element  200  includes a substrate  202  having indentations  208  lying between radially-extending ridges  210  disposed at 90° circumferential intervals, as with cutting element  100 . However, unlike cutting element  100 , ridges  210  are defined by sloping side surfaces  212  (see FIGS.  5 A and  5 D), which extend downward on each side of a ridge  210  from ridge top  214  to meet floors  218  of laterally adjacent indentations  208 . In this variant  200 , the indentation floors  218  lie substantially parallel to the plane of the cutting face  232  and transverse to the longitudinal axis of cutting element  200 , rather than sloping as in cutting element  100 . Further, unlike in cutting element  100 , the sides of the ridges  210  are substantially parallel and the ridges  210  remain of substantially constant transverse cross section until meeting adjacent ridges  210  toward the center  224  of substrate  202 , rather than the ridges necking down as they approach the center. The thickness T 1  of superabrasive table  230  at projections  234  of superabrasive table  230  lying over the indentation floors  218  is about 0.080 inch, while the table thickness T 2  over the tops  214  of the ridges  210  is about 0.040 inch. In variant  200 , the superabrasive contact area is maintained relatively constant during wear of the cutting element by appropriate selection of the relative thicknesses of the table portions over the floors  218  and ridge tops  214 , the degree to which indentations  208  decrease in width as cutting element  200  wears, and the angles of the side slopes of the ridge side surfaces  212  extending between ridge tops  214  and indentation floors  218 . 
     Further, in cutting element  200 , the cutting edge  236  is chamfered to about a 0.015 inch radial width at a 45° angle to the cutting face  232 , and (as shown in FIG. 5A) at least part of the side of the table  230  may be angled at about a 10° angle γ to the side  222  of the substrate  202  as taught by U.S. Pat. No. 5,437,343 to Cooley et al, assigned to the assignee of the present invention. Alternatively, as shown in FIG. 5C, a chamfer and an angled table side may be eliminated, as desired. 
     FIGS. 6A through 6C depict a substrate  302  for another variant  300  of the first embodiment of the cutting element of the invention. Substrate  302  is similar to substrate  102 , except that leading face  306  includes substantially isosceles triangular indentations  308  having composite topography floors  318 , each comprising an outer, arcuate, flat shelf  317  oriented substantially parallel to the ridge top surfaces  311  of ridges  310 , shelf  317  extending radially inwardly a measurable distance D 3  (for example, about 0.030 inch) to an inner, substantially flat surface  319 . Surface  319  may actually be characterized as a very shallow, barely perceptible concavity comprising a section of a cone of revolution. Surface  319  is inclined along a radius of substrate  302  at an angle β, for example, about 10° for a 0.529 inch or 13 mm diameter cutting element, to the ridge top surfaces  311  of ridges  310  and located to intersect a line parallel to and  0 . 010  inch below ridge tops  311  about 0.060 inch radially outward of center  324 , so as to reduce the depth of the indentation  308  as the radial distance from the center  324  of the substrate  302  decreases. Composite topography floors  318  are bounded by a pair of linear, convergently-oriented sides  312  of adjacent ridges  310  (again defining about a 60° included angle) connected at their radially inner ends by arcuate boundary  314  and at their radially outer ends by outer arcuate base or edge  316  extending therebetween and substantially coincident with the outer periphery or side  322  of substrate  302  in a finished cutting element  300 . The boundary  321  between shelf  317  and inner, flat surface  319  is preferably arcuate or radiused, rather than sharp, for example, on about a 0.125 inch radius. The exterior of a cutting element formed with substrate  302  would look substantially identical to cutting element  100  (see FIG.  4 ), and so is not separately illustrated, although reference numerals applicable to cutting element  300  are shown in FIG. 4 for clarity. The transitions as at  320  between the outer periphery of shelf  317  and surface  319  and sides  312  and boundary  314  and between sides  312  and boundary  314  and ridge tops  311  are radiused, as with substrate  302 . The presence of shelf  317  at the outer periphery of each indentation  308  provides a larger depth of superabrasive material (see FIG. 4) in projections  334  of superabrasive table  330  at the cutting edge  336  to sustain initial impacts with the formation until a wear flat is formed, and thus may form a more robust cutting element. It is also contemplated (see FIG. 6C) that shelf  317  may even dip downward as it extends radially inward from the side  322  of substrate  302 , as shown in broken lines  317 ′, to provide an even greater effective thickness of superabrasive table  330  in a projection  334  oriented toward the formation and aligned with the resultant force acting on the cutting edge of the imperforate cutting face  332  and, further, that the angle of inclination β of surface  319  may be greater than 10° (again, as shown in broken lines  319 ′) to accommodate this configuration of shelf  317 . 
     FIGS. 7A and 7B depict a second embodiment  500  of the cutting element of the present invention. Cutting element  500  includes a substrate  502  onto which is formed a superabrasive table  530 . Table  530  includes at least one radial or diametric projection  534  of substantially constant widths and of increased thickness with respect to the remainder of table  530 . Projection  534  is thickest adjacent cutting edge  536 , and decreases in thickness non-linearly (such as along a radius of curvature R) as it approaches the center  524  of substrate  502 . Thus, as cutting face  532  and table  530  wears toward center  524  during use, the decreasing thickness of projection  534  is offset by the increase in superabrasive contact area with the formation afforded by the increasing width of the thinner table areas  533  flanking projection  534 . 
     FIGS. 8A and 8B depict a third embodiment  600  of the cutting element of the present invention. Cutting element  600  includes a substrate  602  onto which a superabrasive table  630  is formed, there being a substantially planar interface or boundary between the two elements. Table  630  includes a radial projection  634  protruding from the cutting face  632 , projection  634  decreasing in both depth and width toward the center  624  of substrate  602  so that the superabrasive contact area with the formation remains substantially constant as cutting edge  636  wears into a flat during drilling and the increase in the lateral width of the wear flat is offset by the decrease in the footprint size of the projection  634 . Optionally, as shown in broken lines  640 , projection  634  may extend from the rear of table  630  as well as, or in lieu of, from cutting face  632 . 
     FIGS. 10A,  10 B and  10 C respectively depict cutting elements exhibiting arcuate cutting edges and other than circular superabrasive tables and cutting faces. Cutting element  700  of FIG. 10A is of half-cylindrical configuration, with half-circular superabrasive table  730 , projection  734  extending to the rear thereof into the supporting substrate. Cutting element  800  of FIG. 10B is of one-third cylindrical configuration, with one-third circular superabrasive table  830 , projection  834  extending to the rear thereof into the supporting substrate. Cutting element  900  of FIG. 10C is of ellipsoidal configuration, with ellipsoidal superabrasive table  930 , projection  934  extending to the rear thereof into the supporting substrate. 
     FIG. 11 depicts a drill bit in the form of a rotary drag bit  1000  having cutting elements  100 ,  200  and  300  mounted thereon in accordance with the present invention. 
     As noted previously, the cutting elements of the present invention may employ any known superabrasives, including without limitation, PDCs, thermally stable PDCs, diamond films, and cubic boron nitride compacts. It is contemplated that superabrasive tables according to the invention may be formed as free-standing superabrasive masses and employed as cutting elements secured directly to the bit face as by brazing or during infiltration of a matrix-type bit, in addition to being formed onto supporting substrates as is conventional in PDC fabrication. Substrates may take the form of cylinders or studs, as desired, the manner of securement of the cutting elements to the bit face being of no consequence to the invention. 
     It will be appreciated by those of ordinary skill in the art that the cutting elements of the invention permit maintenance of WOB for a given ROP (or range of ROPs) within a controlled, non-disadvantageous magnitude through control of the superabrasive contact area of the cutting elements on the bit with a formation being drilled. Thus, the present invention includes novel and unobvious methods of drilling. 
     While the cutting elements and drill bits of the present invention have been described in terms of certain illustrated embodiments, those of ordinary skill in the art will understand and appreciate that it is not so limited. Rather, additions, deletions and modifications to the illustrated embodiments may be effected, as well as combinations of features of different embodiments, without departing from the scope of the invention as set forth hereinafter in the claims.

Summary:
Cutting elements providing a relatively constant superabrasive area in contact with the formation responsive to weight on bit during a substantial portion of the useful life of a circular cutting face cutting element or other cutting element exhibiting a non-linear cutting edge, for example, from about 5% diametrical wear to in excess of about 30% diametrical wear in the case of a circular cutting element, measured across the cutting face. The superabrasive table of the cutting element is configured, internally, externally, or both, to vary in depth radially and laterally, as required, so that an increase in width of the contact or wear flat area with the formation and the variation in table depth as the cutting element wears, are substantially offsetting. The rate of penetration of a drill bit so equipped may thus be maintained at a desirable magnitude without a substantial increase in weight on bit as the cutting element wears, since the superabrasive contact area is maintained relatively constant.