Patent Publication Number: US-8109346-B2

Title: Drill bit supporting multiple cutting elements with multiple cutter geometries and method of assembly

Description:
PRIORITY CLAIM 
     The present application is a divisional of United States application for patent Ser. No. 11/406,470 filed Apr. 18, 2006, now issued as U.S. Pat. No. 7,677,333, the disclosure of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to rotary drill bits for rotary drilling of subterranean formations and, more specifically, to a rotary drill bit having cutting elements with multiple geometries and arranged so that the drill bit becomes more stable and mechanically more efficient with increasing wear on the cutting elements. 
     BACKGROUND 
     Subsurface formation drilling to recover hydrocarbons is well known in the art. The equipment for such subsurface formation drilling typically comprises a drill string having a rotary drill bit attached thereto that is lowered into a borehole. A rotary table or similar device rotates the drill string, resulting in a corresponding rotation of the drill bit. The rotation advances the drill bit downwardly, causing it to cut through the subsurface formation (e.g., by abrasion, fracturing, and/or shearing action). Drilling fluid is pumped down a channel in the drill string and out the drill bit to cool the bit and flush away debris that may have accumulated. The drilling fluid travels back up the borehole through an annulus formed between the drill string and the borehole. 
     Many types of drill bits have been developed, including roller cone bits, fixed cutter bits (or “drag bits”), and the like. For each type of drill bit, several patterns of cutting elements (or “cutters”) are possible, including spiral patterns, straight radial patterns, and the like. Different types of cutting elements have also been developed, including milled cutting elements, tungsten carbide inserts (“TCI”), polycrystalline-diamond compacts (“PDC”), and natural diamond cutting elements. The selection of which drill bit, cutting element type, and cutting element pattern to use for a given subsurface formation can depend on a number of factors. For example, certain combinations of drill bit, cutting element type, and cutting element pattern drill more efficiently and effectively in hard formations than others. Another factor is the range of hardness encountered when drilling through the different formation layers. 
     One common pattern for drill bit cutting elements is to arrange them in a spiral configuration, an example of which is shown in  FIGS. 1A-1B . As can be seen, a spiral pattern drill bit  100  is composed of several sections, including a bit body  102 , a shank  104 , and a threaded connector  106  for connecting the drill bit  100  to a drill string. Flats  108  on the shank  104  allow a tool, such as wrench, to grip the drill bit  100 , making it possible (or at least easier) to screw the drill bit  100  onto the drill string. Blades  110   a ,  110   b ,  110   c ,  110   d ,  110   e , and  110   f  are formed on the drill bit  100  for holding a plurality of cutting elements  112 . The cutting elements  112  include superabrasive faces that usually have identical geometries (i.e., size, shape, and orientation), although different positions and/or cutting angles on the blades  110   a - f . Also visible are drill fluid outlets  114  that conduct the drilling fluid out of the drill bit  100 , carrying away any debris and cuttings that may have accumulated. 
     In the spiral configuration and other radial configuration drill bits, the cutting elements  112  are placed at selected radial positions with respect to a central longitudinal axis A. In addition, the positions of the cutting elements  112  on one blade  110   a - f  are staggered relative to the positions of the cutting elements  112  on another blade  110   a - f . The result is that a cutting surface of one cutting element  112  overlaps the cutting surface of at least one other cutting element  112  in their cutting profiles, which is the area outlined by the cutting surfaces when the cutting elements are rotated onto the same radial plane. Thus, each cutting element  112  removes a lesser volume of material than would be the case if it were positioned so that no overlapping occurred. 
       FIGS. 2A-2C  illustrate the overlap via a segment of a drill bit&#39;s cutting profile  200  for the drill bit  100 . Note that the portions of the blades  110   a - f  shown in  FIGS. 2A-2C  have been flattened out in order to more clearly illustrate the shortcomings of existing drill bits. Those having ordinary skill in the art will recognize that, in practice, the blades of a drill bit frequently have some degree of curvature. 
     As can be seen, the profile segment  200  is composed of several individual cutting profiles  202   a ,  202   b ,  202   c , and  202   d  representing the various cutting elements  112  (see  FIG. 1 ) on the blades  110   a - f . The cutting profiles  202   a - d  show the area outlined by the cutting surfaces of the cutting elements  112  when they are all rotated onto the same radial plane. Thus, the profile segment  200  may be shared by the cutting elements  112 , denoted herein by cutting profiles  202   a ,  202   b ,  202   c , or  202   d , even though the cutting elements  112  may physically reside on different blades  110   a - f , have different radial positions on the blades  110   a - f , and follow different paths in the subsurface formation as the bit is rotated. This arrangement ensures substantially complete coverage of the bottom hole as the bit is rotated during the drilling process. 
     The overlap can be seen in more clearly  FIG. 2B , where the cutting surfaces represented by the first and second cutting profiles  202   a  and  202   b  overlap when rotated onto the same radial plane. Similarly, the cutting surfaces represented by the second and third cutting profiles  202   b  and  202   c  overlap when rotated onto the same plane. Likewise, the cutting surfaces represented by the third and fourth cutting profiles  202   c  and  202   d  overlap when rotated onto the same plane. 
     The overlap helps provide greater coverage for the borehole bottom, but can result in a specific wear pattern that, depending on the location of the wear, may drastically blunt the cutting elements  112 , causing severe reductions in ROP. In the specific example shown, the overlap occurs mainly on the sides  204  of the cutting profiles  200   a - d . As a result of the overlaps, the cutting element density in those areas  204  is necessarily greater than the density in the tip regions  206  of the cutting profiles  200   a - d . Consequently, the cutting elements, as shown by the individual cutting profiles  202   a - d  along the segment of the bit&#39;s profile  200 , tend to wear down more quickly in the tip regions  206 , which happen to be the most mechanically efficient portion of the cutting element. This is indicated by the cutting profiles  202   a ′-d′ of  FIG. 2C . 
     Accelerated or pronounced wear in the most mechanically efficient portions of the cutting elements is not a great hindrance in comparatively soft formation materials, where rates of penetration (ROP) are usually higher and less energy is usually required to fail the rock being drilled. However, for hard formations, the tip regions of the cutting surfaces are the most effective portions for shearing (in the case of shale, sandstone, and siltstone) or fracturing (in the case of limestone and dolomite) the rock being drilled. For these subsurface formations, a drill bit where the cutting elements exhibit accelerated cutter tip wear (based on the cutting profile) can significantly reduce the ROP. This wear pattern can also minimize a drill bit&#39;s effectiveness at combating damaging vibrations, specifically lateral vibrations and bit whirl, due to the resulting bottomhole pattern that is created as a result of the wear. Stabilization forces that normally act to re-stabilize the bit at the initiation of an off-center movement and/or rotation are minimized, making bits with pronounced cutter tip wear patterns prone to intense vibrations. 
     Thus, despite certain advances made in the industry, there remains a need for a drill bit having an improved cutting element arrangement that will permit the bit to drill effectively at good or economical ROPs, and provide increased stability and enhanced mechanical efficiency as wear occurs, especially in hard formations, and in deep harsh drilling environments, where the time and expense needed to retrieve and replace ineffective and un-stable drill bits substantially increase overall drilling operational costs. 
     SUMMARY 
     Embodiments are directed to a drill bit, and method of assembling same, that becomes more effective mechanically, and also gains in stability with increasing wear. The drill bit has cutting elements with multiple cutting surface geometries that are positioned so that their cutting profiles overlap, but do not completely contain or engulf one another. The different cutting surface geometries and specific overlap of the cutting elements define zones of different cutting element densities in the cutting surface and along the bit&#39;s profile. In one implementation, the overlap occurs in the middle regions of the cutting profiles, resulting in a zone of higher density in the middle regions that extends to the tip, but lower density on the periphery. The higher density middle regions and lower density periphery has the effect of sharpening the tip regions of the cutting surfaces as wear progresses, making the cutting elements increasingly effective during the drilling process. Moreover, cutting elements having larger axial volumes may be combined with cutting elements having smaller axial volumes, resulting in an even more effective drill bit in terms of durability and ability to drill efficiently in hard and abrasive formations. 
     In general, in one aspect, a drill bit comprises a drill bit body and first and second blades formed on the drill bit body. The drill bit further comprises a first cutting element mounted on the first blade, the first cutting element having a first cutting surface geometry corresponding to a first cutting profile, and a second cutting element mounted on the second blade, the second cutting element having a second cutting surface geometry corresponding to a second cutting profile. The first and second cutting elements are positioned on the first and second blades, respectively, so that the first and second cutting profiles partially overlap each other without completely containing each other, the overlap creating a high-density zone in a middle region of the first and second cutting profiles and a low-density zone on a periphery of the first and second cutting profiles. 
     In general, in another aspect, a method of assembling a drill bit comprises providing a drill bit body having first and second blades formed thereon and mounting a first cutting element on the first blade, the first cutting element having a first cutting surface geometry corresponding to a first cutting profile. The method further comprises mounting a second cutting element on the second blade, the second cutting element having a second cutting surface geometry corresponding to a second cutting profile. The first and second cutting elements are positioned on the first and second blades, respectively, so that the first and second cutting profiles partially overlap each other without completely containing each other, the overlap creating a high-density zone in a middle region of the first and second cutting profiles and a low-density zone on a periphery of the first and second cutting profiles. 
     In general, in still another aspect, a method of assembling a drill bit comprises providing a drill bit body having first and second blades formed thereon, at least one of the first and second blades being capable of supporting two rows of cutting elements. The method further comprises mounting a first row of cutting elements and a second row of cutting elements on the at least one of the first and second blades, the cutting elements of the first and second rows having different cutting surface geometries, respectively. The first row of cutting elements is spaced angularly apart from the second row of cutting elements, and at least one cutting element on the first row and at least one cutting element on the second row have cutting profiles that overlap radially, but without completely containing each other. 
     In general, in still another aspect, a method of drilling through a subsurface formation using a drill bit comprises drilling through a first formation material using the drill bit, the drill bit having cutting elements with multiple cutting surface geometries that partially overlap one another, but without completely containing each other, when rotated onto a same radial plane. The method further comprises drilling through a second formation material using the drill bit, the second formation material being located below the first formation material and harder and more abrasive than the first formation material. Drilling through the second formation material causes a periphery of at least one of the cutting elements to wear away faster than a middle region of the at least one of the cutting elements. 
     Additional aspects of the invention will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments, which is made with reference to the drawings, a brief description of which is provided below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other advantages of the invention will become apparent from the following detailed description and upon reference to the drawings, wherein: 
         FIGS. 1A-1B , described previously, illustrate a side view and a bottom view of a prior art fixed cutter drill bit; 
         FIGS. 2A-2C , described previously, illustrate cutting profiles for a prior art fixed cutter drill bit; 
         FIG. 3  illustrates an exemplary arrangement for a fixed cutter drill bit having cutting elements with multiple cutting surface geometries according to embodiments; 
         FIG. 4  illustrates another exemplary arrangement for a fixed cutter drill bit having cutting elements with multiple cutting surface geometries according to embodiments; 
         FIGS. 5A-5C  illustrate cutting profiles for an exemplary fixed cutter drill bit having cutting elements with multiple cutting surface geometries according to embodiments; 
         FIG. 6  illustrates exemplary axial volumes (Av) for cutting elements with different cutting surface geometries according to embodiments; 
         FIG. 7  illustrates exemplary cutting profiles for another exemplary fixed cutter drill bit having cutting elements with multiple cutting surface geometries according to embodiments; 
         FIG. 8  illustrates exemplary cutting profiles for yet another exemplary fixed cutter drill bit having cutting elements with multiple cutting surface geometries according to embodiments; and 
         FIGS. 9A-9B  illustrate an exemplary cutting element layout for still another exemplary fixed cutter drill bit having cutting elements with multiple cutting surface geometries according to embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Following is a detailed description of embodiments with reference to the drawings. It should be noted that the drawings are provided for illustrative purposes only and are not intended to be a blueprint or manufacturing drawings, nor are they drawn to any particular scale. 
     As mentioned above, existing fixed cutter drill bits have cutting elements that have identical cutting surface geometries and are arranged on the blades so that they radially overlap on the periphery of their cutting profiles. The term “geometry” refers to the size, shape, and orientation of the cutting surfaces, but not their positioning or cutting angle on the blades. For bits where each radial position is unique, the peripheral overlap creates a smooth wear pattern that can drastically reduce ROP especially in hard formations. For bits where multiple cutting elements share a common radial position, the peripheral overlaps initially locate cutter wear at the tips of the cutting elements. The resulting wear patterns from existing cutter arrangements have the same negative effects on ROP and bit stabilization. The wear patterns in these instances reduce mechanical efficiency and thus ROP, especially in hard formations, forcing the use of high energy levels (e.g., via weight on bit (WOB) and/or RPM (revolutions per minutes)) in order to achieve acceptable ROPs, conditions that further compound the wear process and its negative effects. In addition, bit stabilization is also compromised as cutter wear progresses. 
     Embodiments provide a fixed cutter drill bit where the cutting elements have different cutting surface geometries that overlap each other in the middle regions of their cutting profiles. The multiple cutting surface geometries and specific overlap of the cutting profiles create a zone of higher cutter element density in the middle regions extending to the tips, but lower density on the periphery. The result is a drill bit where the cutting elements acquire more sharply defined tip regions as wear progresses. Such an arrangement can produce higher ROPs and greater stability, especially when the drill bit advances into hard and abrasive formation materials. Moreover, cutting elements having larger axial volumes are also used to enhance the durability and, hence, effectiveness of the drill bit even further, advantages that are critical in order to achieve needed performance improvements in hard and abrasive formation. 
     Referring now to  FIG. 3 , a partial view of a fixed cutter drill bit  300  having cutting elements with multiple cutting surface geometries according to embodiments is shown. The cutting elements may be any suitable type of cutting element known to those having ordinary skill in the art, including TCI cutting elements, PDC cutting elements, natural diamond cutting elements, or some combination thereof. Note in  FIG. 3  and all remaining applicable drawings that the portions of the blades shown have been flattened out in order to more clearly illustrate the principles and teachings of the present invention. Those having ordinary skill in the art will understand that actual blades may have some degree of curvature. In addition, to avoid unnecessarily cluttering the drawing, various details of the drill bit  300  have been omitted and only the cutting surfaces of a few cutting elements are illustrated. It should also be reiterated that the drawings are provided for illustrative purposes alone and are not drawn to any specific scale, particularly with respect to the location of and spacing between the various cutting elements. 
     As can be seen, at least one blade  302  (similar to blades  110   a - f  of  FIGS. 1A-1B ) of the drill bit  300  has a plurality of cutting elements  304   a - d  having one cutting surface geometry mounted thereon, and at least one other blade  306  (similar to blades  110   a - f  of  FIGS. 1A-1B ) has a plurality of cutting elements  308   a - d  having a different cutting surface geometry mounted thereon. These cutting surface geometries may have shapes such as, for example, round shapes, oval shapes, including elliptical, egg-shaped, pear-shaped, teardrop, and the like, as well as other common and customized shapes known to those having ordinary skill in the art. The term “oval” as used herein refers to any shape that is closed and smooth (i.e., has a finite derivative at all points). In some cases, even non-circular shapes may be used where at least a portion of the cutting surface shape is flat. 
     For the specific implementation of  FIG. 3 , the first set of cutting elements  304   a - d  has an oval cutting surface oriented so that a major (i.e., long) axis X extends generally perpendicular to a profile of the drill bit and  300 , but may also be generally parallel to a central axis (see  FIG. 1A ) of the drill bit, while a minor (i.e., short) axis Y extends generally parallel to the profile of the drill bit, but may also be generally perpendicular to the central axis of the drill bit. The second set of cutting elements  308   a - d , on the other hand, have a round cutting surface, with a cutting surface diameter that is equal to, less than, or greater than a cutting surface width of the oval cutting elements  304   a - d  (as measured along the minor axis Y) and less than a cutting surface length of the oval cutting elements  304   a - d  (as measured along the major axis X). The result is that the round cutting elements  308   a - d  do not fully contain or engulf the oval cutting elements  304   a - d , and vice versa, when rotated onto the same radial plane. 
     In accordance with embodiments, the oval cutting elements  304   a - d  and the round cutting elements  308   a - d  are positioned on their respective blades  302  and  306  so that at least one oval cutting element  304   a - d  and at least one round cutting element  308   a - d  partially overlap in the middle when rotated onto the same radial plane. That is, the major axis X of at least one oval cutting element  304   a - d  and a corresponding axis Z of at least one round cutting element  308   a - d  substantially line up when the cutting elements are rotated onto the same radial plane. Such an arrangement causes the overlap to occur mostly in the middle regions and not on the periphery, resulting in a zone of higher density in the middle regions extending to the tips, but lower density on the periphery of the cutting elements. 
       FIG. 4  illustrates an alternative arrangement to  FIG. 3  in that the same blade may contain cutting elements with different cutting surface geometries. As can be seen in  FIG. 4 , a drill bit  400  includes at least one blade  402  (similar to blades  110   a - f  of  FIGS. 1A-1B ) containing a mix of oval and round cutting elements  404   a - d , and at least one other blade  406  (similar to blades  110   a - f  of  FIGS. 1A-1B ) also containing a mix of oval and round cutting elements  408   a - d . The oval cutting elements  404   a - d  are again oriented so that a major axis X′ of their cutting surface extends generally perpendicular to a profile of the drill bit  400 , but may also be generally parallel to a central axis of the drill bit, while a minor axis Y′ extends generally parallel to the profile of the drill bit, but may also be generally perpendicular to the central axis of the drill bit. 
     The mixing of the different cutting surface shapes on individual blades  402  and  406  on the drill bit  400  shown in  FIG. 4  results in substantially the same partial overlap of cutting surfaces as the drill bit  300  shown in  FIG. 3 . However, based on the different rock failure mechanisms of round cutting elements (primarily shear) and oval cutting elements (primarily pre-fracture), the drilling efficiencies of the different blades will be dissimilar for the two layouts. The cumulative total wear flat areas on the individual blades will also be different and will have dissimilar effects on overall drilling performance. 
     In the embodiment of  FIG. 4 , the major axis X′ of at least one oval cutting element  404   a  and  404   b  on one blade  402  and a corresponding axis Z′ of at least one round cutting element  408   c  and  408   d  on one other blade  406  substantially line up when the cutting elements are rotated onto the same plane. However, for both  FIGS. 3 and 4 , benefits may be achieved even when the X and X′ axes of the oval cutting elements  304   a - d  and  404   a  and  404   b  and the Z and Z′ axes of the round cutting elements  308   a - d  and  408   c  and  408   d  do not line up, so long as neither cutting element type is totally contained or engulfed within the periphery of the other cutting element type (based on their cutting profiles). 
       FIGS. 5A-5C  show cutting profiles for an exemplary fixed cutter drill bit having cutting elements with multiple cutting surface geometries according to embodiments. As mentioned above, a cutting profile is the region outlined by the cutting surfaces of the cutting elements when they are rotated onto the same radial plane. The cutting profiles in  FIGS. 5A-5C  may represent a fixed cutter drill bit similar to that shown in  FIG. 3 , or they may represent a fixed cutter drill bit similar to that shown in  FIG. 4 , or they may represent some other fixed cutter drill bit. 
     Referring first to  FIG. 5A , a cutting profile  500  may include cutting profiles  502   a - d  representing cutting elements having one cutting surface geometry and cutting profiles  504   a - d  representing cutting elements having a different cutting surface geometry. These cutting surfaces may result in cutting profiles  502   a - d  and  504   a - d  having shapes such as, for example, round shapes, oval shapes (e.g., elliptical, egg-shaped, pear-shaped, teardrop, etc.) and even non-circular shapes. For the particular embodiment shown, the first set of cutting profiles  502   a - d  has an oval shape oriented so that a major axis (see  FIGS. 3-4 ) extends generally perpendicular to a profile of the drill bit (not expressly shown) and a minor axis (see  FIGS. 3-4 ) extends generally parallel to the profile of the drill bit. The second set of cutting profiles  504   a - d , on the other hand, are round, with a diameter that is at least equal to or greater than the width of the oval cutting profiles  504   a - d  (as measured along the minor axis) and equal to or less than the length of the oval cutting profiles  504   a - d  (as measured along the major axis). In a preferred embodiment, at least a portion of the oval cutting profiles  502   a - d  extends beyond the round cutting profiles  504   a - d , and vice versa, so that the cutting profiles  502   a - d  and  504   a - d  do not completely contain or engulf one another. Additionally, the tips  510  of the oval cutting profiles  502   a - d  and the round cutting profiles  504   a - d  reflect the fact that the cutting elements are set to substantially the same height when rotated onto the same plane. 
     In accordance with embodiments, the cutting elements are arranged so that the oval cutting profiles  502   a - d  overlap the round cutting profiles  504   a - d  in their middle regions  506 , as shown in  FIG. 5B . Consequently, the cutting element density in the middle regions  506  extending to the tips  510 , is higher than the density on the periphery  508 . This causes the round cutting elements to wear down faster in the periphery  508  than in the reinforced middle regions  506  so that the cutting profiles  502   a - d  and  504   a - d  gradually become sharper, resembling the cutting profiles  502   a ′-d′ and  504   a ′-d′ of  FIG. 5C . Such a self-sharpening process makes the cutting elements mechanically more efficient, thus improving ROP, especially in harder formations. In addition, the bottomhole pattern created as a result of the wear becomes more scalloped, leading to an increase in the magnitude of the restoration force needed to re-stabilize against vibrations, especially lateral and bit whirl. These advantages are critical for drilling performance improvements, especially in harder formations. 
     Note in the foregoing embodiments that the oval cutting elements have a cutting surface length (as measured along the major axis) that is greater than the cutting surface diameter of the round cutting elements. The longer cutting surface length provides the drill bit with an increased axial volume (“Av”). By way of background, the axial volume indicates how much of the superabrasive cutting surface of a cutting element is available for cutting/fracturing/breaking the formation material. The axial volume is typically defined in terms of the distance from the center of the superabrasive cutting surface to its tip. This is illustrated in  FIG. 6 , where a round cutting element  600  and an oval cutting element  602  are shown with the tips of their cutting surfaces contacting a borehole bottom  604 . As can be seen, the oval cutting element  602  has an axial volume Av oval  that is greater than an axial volume Av round  of the round cutting element  600 . 
     Those having ordinary skill in art understand that the axial volume of a cutting element affects the durability of that cutting element in hard and abrasive formations, such as limestone, dolomite, and other materials of high compressive strength values. Having a large axial volume also increases the ability of the cutting element to withstand high rotational speeds during the drilling process. Thus, a higher axial volume translates to a larger superabrasive area available for drilling and a longer lifespan for the drill bit in hard and abrasive formation material. For this reason, oval cutting elements, because of their comparatively higher Axial volume (Av), which maximizes their superabrasive material content, are known to be highly effective in abrasive formations or lithologies, such as sandstone and siltstone. In addition, any elongated cutting element (and even non-circular cutting elements) having a length (as measured along a major axis) that is greater than a diameter of the round cutting element is likely to be mechanically more effective at pre-fracturing of brittle formation material, such as limestone or dolomite, than the round cutting element. The advantage becomes more pronounced as the brittle materials become harder. 
     A round cutting element, however, is sometimes more effective than an oval cutting element in certain applications. Round cutting elements, for example, are more effective for shearing non-brittle formations or lithologies, such as shale, sandstones and siltstone. In addition, a round cutting surface, based on its peripheral curvature, generally has higher resistance to impact damage. In comparison to round cutting elements, an oval cutting element, based on its geometry, and specifically its major to minor axis ratio, has a relatively lower resistance to impact damage, particularly where the minor axis of the oval cutting element is less than the diameter of the round cutting element. Thus, in terms of application specificity, both oval cutting elements and round cutting elements, when used by themselves, are effective in only a limited number of applications. 
     In accordance with embodiments, oval cutting elements are employed in conjunction with round cutting elements. Such an arrangement combines the advantages of both round and oval cutting element types. The different cutter surface types establish nearly complete and independent bottomhole coverages. A drill bit in accordance with these embodiments lasts longer and is more effective for penetrating hard, brittle formation material (e.g., limestone, dolomite, carbonate, etc.) as well as non-brittle formation material (e.g., shale, sandstone, siltstone, etc.). In addition, wear is controlled so that it occurs more quickly in the periphery, thereby promoting sharpening of the cutting surfaces and improving bit stabilization. The improved stabilization minimizes cutting element impact damage, which further improves bit longevity. 
     The specific cutting surface dimensions of the round cutting elements and oval cutting elements, as well as the degree of elongation for the oval cutting elements, depend on the particular subsurface formation to be drilled. For example, a subsurface formation with high carbonate content may require cutting elements that are more oval or elongated for pre-fracturing purposes than a formation with high shale content. In one embodiment, the round cutting elements may have a cutting surface diameter of 16 mm and the oval cutting elements may have a cutting surface width of 16 mm and length of 19 mm (as measured along the minor and major axes, respectively). Of course, other diameters, widths, and lengths may also be used without departing from the scope of the invention. For example, in some embodiments, the oval cutting elements may have a cutting surface width that is larger than the cutting surface diameter of the round cutting elements. In a preferred embodiment, however, no oval cutting element completely contains or engulfs a round cutting element, and vice versa, as viewed according to their cutting profiles. 
     In operation, as the drill bit drills through non-brittle formation materials (e.g., sandstone, shale, siltstone, etc.), the lower density periphery of the cutting elements are worn down faster than the reinforced middle regions. This process promotes self-sharpening of round cutting elements and allows the drill bit to maintain or increase its effectiveness in hard and abrasive formation materials (e.g., limestone, carbonate, dolomite, etc.). In addition, the controlled wear pattern aligned to the periphery of the cutting surfaces due to the different density distributions also promotes stability, which is desirable for hard formation drilling. Furthermore, the larger axial volume (Av) of the oval cutting elements also enhances durability in hard and abrasive formations as well as in high rotational speed applications. Consequently, the drill bit is able to continue performing at an acceptable or economical ROP for longer periods of time or over longer intervals of drilling, especially upon encountering hard formation materials in comparison to conventional drill bits. 
     Thus far, only one type of oval cutting element, namely, a cutting element with an elliptical cutting surface, has been shown. As previously stated, however, other types of oval cutting elements may also be used so long as the oval cutting elements do not completely contain or engulf the round cutting elements (based on their cutting profiles), and vice versa. Examples of other oval cutting elements that may be used include cutting elements with egg-shaped, pear-shaped, teardrop, and similarly shaped cutting surfaces. In general, all oval cutting elements as well as non-circular and various common and customized cutting elements known to those having ordinary skill in the art may be used. 
       FIG. 7  illustrates an embodiment in which an egg-shaped cutting element is used. As can be seen, an exemplary cutting profile  700  includes round cutting profiles  702   a - d  representing round cutting elements and egg-shaped cutting profiles  704   a - d  representing egg-shaped cutting elements. The cutting profiles  702   a - d  and  704   a - d  overlap one another in the middle, but do not completely contain or engulf one another. The disparate cutting element shapes and specific overlap create a zone of lower density on the periphery, but higher density in the middle regions. The zone of higher density has the effect of sharpening the tip regions of the cutting elements as wear progresses, making them increasingly effective and stable during drilling in hard and abrasive formations. 
       FIG. 8  illustrates an exemplary cutting profile  800  for an embodiment where the cutting elements have substantially identical cutting surface sizes and shapes, but different orientations. As can be seen, the exemplary cutting profile  800  includes cutting profiles  802   a - d  and  804   a - d  having substantially identical sizes and shapes, namely, oval shapes. One set of oval cutting profiles, for example, the first set  802   a - d , has a major axis oriented generally perpendicular to a profile of the drill bit (not expressly shown), while the other set of oval cutting profiles  804   a - d  has a major axis oriented generally parallel to the profile of the drill bit, such that perpendicular and parallel cutting profiles overlap, but do not completely contain or engulf one another. 
     The above arrangement of cutting profiles  802   a - d  and  804   a - d  creates a zone of lower density on the periphery, but higher density in the middle regions. The benefits of such an arrangement are similar to those described previously in  FIGS. 3-7  in terms of the effectiveness, stability, and durability of the drill bit for drilling in hard and abrasive formations. An additional benefit of the arrangement in  FIG. 8  is that a single type of cutting element may be used to achieve multiple cutting profiles  802   a - d  and  804   a - d . This is possible due to the difference between the orientation of the major and minor axis of such a cutting element. In such a layout, all the benefits as discussed earlier are achieved. 
     In some embodiments, one of the overlapping cutting elements may be made more abrasion-resistant. For example, where round cutting elements and oval cutting elements are used, the round cutting elements may be made more abrasion-resistant than the oval cutting elements, or the oval cutting elements may be more abrasion-resistant than the round cutting elements. Or both the round and the oval cutting elements may have improved abrasion resistance. In a similar manner, the round cutting elements may be made more impact-resistant than the oval cutting elements, or the oval cutting elements may be more impact-resistant than the round cutting elements. Or both the round and the oval cutting elements may have improved impact resistance. 
     Based on the specifics of an application, as well as the formation types needed to be drilled, the different geometries will have different performance properties, in terms of abrasion and impact resistance, as well as thermal stability. In such instances, the material needs are used to augment and support the effects of the cutting element densities within the overlapping surfaces so as to promote or accelerate the peripheral wear. In instances where the round cutting elements are made with finer grain diamond material (giving them higher abrasion resistance in comparison to the oval cutting elements), the wear rate in the zone of reduced cutting element density is delayed. Likewise, when the oval cutting elements are made with finer grain diamond material (in comparison to the round cutting elements), the wear process in the zone of reduced cutting element density is accelerated. Through this process, the self-sharpening and improved stabilization benefits can be tailored to match the performance requirements of specific applications, based on formation types, levels of shearing and/or pre-fracturing, expected run length, and ROP. 
     In another embodiment, the overlapping cutting elements may be treated to remove catalyzing material (e.g., cobalt), a process commonly referred to as “leaching.” As is well known in the art, leaching or removal of catalyzing material from cutting elements can improve their thermally stability, thus allowing them to withstand much higher drilling temperatures before failing. Improved thermal stability drastically reduces the wear initiation process of the cutting elements. This process may be used to further enhance the performance properties of the circular and oval (and even non-circular) cutting elements, as described herein. Techniques for removal of catalyzing material from cutting elements are generally known and may be found, for example, in U.S. Pat. No. 6,544,308 entitled “High Volume Density Polycrystalline Diamond with Working Surfaces Depleted of Catalyzing Material,” which is incorporated herein by reference. In accordance with embodiments, the round cutting elements may be treated to remove catalyzing material, or the oval cutting elements may be treated to remove catalyzing material. Or both the round and the oval cutting elements may be treated to remove catalyzing material. 
     It should be noted that regardless of the diamond material types (e.g., fine grain or coarse grain diamond materials) that may be used for the round and/or oval and/or non-circular cutting elements, or the leaching or catalyzing material depletion processes employed, the advantages, principles and teachings herein discussed for the present invention will all remain valid and fully applicable to these various embodiments. 
     Moreover, cutting profiles similar to the exemplary cutting profiles shown in  FIGS. 5A-5C  may also be derived from a drill bit where a single blade supports multiple rows of cutting elements. An exemplary implementation of these latter embodiments may be seen in  FIGS. 9A-9B , which show a top view of an exemplary cutting element layout  900  for a blade  902  of a drill bit and the resulting cutting profile, respectively. As can be seen, the blade  902  has two rows of cutting elements, a front row  904  and a back row  906 . Each row  904  and  906  is separated from the other row by a predetermined angle α and supports a plurality of cutting elements  908   a - d  and  910   a - d , respectively. 
     In accordance with embodiments, the cutting elements  908   a - d  on the front row  904  and the cutting elements  910   a - d  on the back row  906  have different cutting surface geometry. In one implementation, the cutting surface geometry of the front row cutting elements  908   a - d  have an oval shape while the cutting surface geometry of the back row cutting elements  910   a - d  have a round shape. In addition, the radial positioning of the oval cutting elements  908   a - d  and the round cutting elements  910   a - d  along the front and back rows  904  and  906  is such that at least one oval cutting element  908   a - d  and at least one round cutting element  910   a - d  partially overlap in the middle regions, but without completely containing or engulfing each other when rotated onto the same radial plane. 
     The above cutting element layout  900  results in the drill bit profile segment  912  shown in  FIG. 9B . As can be seen, oval cutting profiles  914   a - d  representing the oval cutting elements  908   a - d  and round cutting profiles  916   a - d  representing the round cutting elements  910   a - d  overlap in their middle regions without completely containing or engulfing each other. Consequently, the cutting element density in the middle regions is higher than the cutting element density on the periphery. This causes the round cutting elements  910   a - d  to wear down faster in the periphery than in the reinforced middle regions. Such a self-sharpening process makes the cutting elements mechanically more efficient (particularly in harder formations), thus improving ROP. In addition, the bottomhole pattern created as a result of the controlled wear becomes more scalloped, thereby increasing the magnitude of the restoration force needed to re-stabilize the drill bit against vibrations (especially lateral vibrations and bit whirl). These advantages are critical for drilling performance improvements (particularly in harder formations). 
     Of course, the cutting surface geometries of the front row cutting elements  908   a - d  and the back row cutting elements  910   a - d  may be switched and the types of cutting elements present on each row may be intermixed together without departing from the scope of the invention. In addition, non-circular shapes known to those having ordinary skill in the art may also be used, including common and customized shapes. Finally, improved abrasion resistance, impact resistance, and/or thermal stability may be applied to either or both types of cutting elements in the manner described above without departing from the scope the invention. 
     While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the scope of the invention. Accordingly, each of the foregoing embodiments and obvious variations thereof is contemplated as falling within the scope of the claimed invention, as is set forth in the following claims.