Patent Publication Number: US-7896106-B2

Title: Rotary drag bits having a pilot cutter configuraton and method to pre-fracture subterranean formations therewith

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 60/873,349, filed Dec. 7, 2006, for “ROTARY DRAG BITS HAVING A PILOT CUTTER CONFIGURATION AND METHOD TO PRE-FRACTURE SUBTERRANEAN FORMATIONS THEREWITH,” the entire contents of which is hereby incorporated herein by this reference. 
     This application is also related to U.S. patent application Ser. No. 12/019,814, filed Jan. 25, 2008, for ROTARY DRAG BIT, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/897,457 filed Jan. 25, 2007, for ROTARY DRAG BIT. This application is also related to U.S. patent application Ser. No. 12/020,399, filed Jan. 25, 2008, for ROTARY DRAG BIT AND METHODS THEREFOR, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/897,457 filed Jan. 25, 2007, for ROTARY DRAG BIT. This application is also related to U.S. patent application Ser. No. 12/020,492, filed Jan. 25, 2008, for ROTARY DRAG BIT AND METHODS THEREFOR, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/897,457 filed Jan. 25, 2007, for ROTARY DRAG BIT. 
    
    
     FIELD OF THE INVENTION 
     The present invention, in several embodiments, relates generally to a rotary drag bit for drilling subterranean formations and, more particularly, to rotary drag bits having at least one cutter set including a pilot cutter and a rotationally trailing primary cutter, and a method for pre-fracturing subterranean formations therewith. 
     BACKGROUND 
     Rotary drag bits have been used for subterranean drilling for many decades, and various sizes, shapes, and patterns of natural and synthetic diamonds have been used on drag bit crowns as cutting elements. A drag bit can provide an improved rate of penetration (ROP) over a roller cone bit or impregnated diamond drill bit in many formations. 
     Over the past few decades, rotary drag bit performance has been improved with the use of a polycrystalline diamond compact (PDC) cutting element or cutter, comprised of a planar diamond cutting element or table formed onto a tungsten carbide substrate under high temperature and high pressure conditions. The PDC cutters are formed into a myriad of shapes including, circular, semicircular or tombstone, which are the most commonly used configurations. Typically, the PDC diamond tables are formed so the edges of the table are coplanar with the supporting tungsten carbide substrate or the table may overhang or be undercut slightly, forming a “lip” at the trailing edge of the table in order to improve the wear life of the cutter as it comes into formations being drilled. Bits carrying PDC cutters, which for example, may be brazed into pockets in the bit face, pockets in blades extending from the face, or mounted to studs inserted into the bit body, have proven very effective in achieving high ROP in drilling subterranean formations exhibiting low to medium compressive strengths. The PDC cutters have provided drill bit designers a wide variety of improved cutter deployments and orientations, crown configurations, facilitated optimal nozzle placements and other design alternatives previously not possible with small natural diamond or synthetic diamond cutters. While the PDC cutting element improves drill bit efficiency in drilling many subterranean formations, however, the PDC cutting element is nonetheless prone to wear when operationally exposed to drilling conditions and lessens the life of a rotary bit. 
     Thermally stable diamond (TSP) is another synthetic diamond, PDC material which can be used as a cutting element or cutter for a rotary drag bit. TSP cutters, which have had catalyst used to promote formation of diamond-to-diamond bonds in the structure removed therefrom, have improved thermal performance over PDC cutters. The high frictional heating associated with hard and abrasive rock drilling applications, creates cutting edge temperatures that exceed the thermal stability of PDC, whereas TSP cutters remains stable at higher operating temperatures. This characteristic also enables them to be furnaced into the face of a matrix-type rotary drag bit. 
     While the PDC or TSP cutting elements provide better ROP and manifest less wear during drilling as compared to some other cutting element types, it is still desirous to further the life of rotary drag bits and improve cutter life regardless of the cutter type used. Researchers in the industry have long recognized that as the cutting elements wear, i.e., wearflat surfaces develop and are formed on each cutting element coming in contact with the subterranean formation during drilling, the penetration rate (or ROP) decreases. The decrease in the penetration rate is a manifestation that the rotary drag bit is wearing out, particularly when other drilling parameters remain constant. Various drilling parameters include formation type, WOB, cutter position or rake angle, cutter count, cutter density, drilling temperature and RPM, for example, without limitation, and further include other parameters understood by a person of skill in the subterranean drilling art. 
     While researchers continue to develop and seek out improvements for longer lasting cutters or generalized improvements to cutter performance, they fail to accommodate or implement an engineered approach to achieving longer drag bit life by maintaining or increasing penetration rate or ROP by taking advantage of cutting element wear rates. In this regard, while ROP is many times a key attribute in identifying aspects of the drill bit performance, it would be desirable to utilize or take advantage of the cutting element wear in extending or improving the life of the drag bit. 
     Accordingly, there is an ongoing desire to improve or extend rotary drag bit life regardless of the subterranean formation type being drilled. There is a further desire to extend the life of a rotary drag bit by beneficially orienting and positioning cutters upon the bit body. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, a rotary drag bit having a pilot cutter configuration is provided. The rotary drag bit life is extended by the pilot cutter configuration, making the bit more durable and extending the life of the cutting elements. Further, the pilot cutter configuration on the rotary drag bit improves fracturing of subterranean formation material being drilled, providing improved bit life and reduced stress upon the cutters. 
     In accordance with an embodiment of the invention, a rotary drag bit configured for formation fracturing is provided. The rotary drag bit comprises a bit body having a face, and a plurality of cutters coupled to the face surface of the bit body. The plurality of cutters comprises at least one pilot cutter and a primary cutter rotationally following the at least one pilot cutter. The at least one pilot cutter is of smaller lateral extent than the primary cutter and may be exposed to a greater extent than the primary cutter to pre-fracture and clear a portion of the formation being drilled before contact therewith of the primary cutter during drilling. 
     In other embodiments of the invention, a rotary drag bit having improved life is provided. The rotary drag bit comprises a bit body and at least one cutter set comprising a pilot cutter and a rotationally trailing primary cutter coupled to the bit body. 
     In further embodiments of the invention, a bit body comprising at least one blade, at least one fluid course rotationally leading a pilot cutter coupled to the blade and adjacent the fluid course, and a primary cutter coupled to the blade rotationally following the pilot cutter and rotationally removed from the fluid course. 
     A method to drill subterranean formations using a rotary drag bit having a pilot cutter configuration is also provided. 
     Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the various embodiments of the invention when taken in conjunction with the attached drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a face view of a rotary drag bit in accordance with a first embodiment of the invention. 
         FIG. 2  shows a face view of a rotary drag bit in accordance with a second embodiment of the invention 
         FIG. 3  shows a cutter and blade profile for the first embodiment of the invention. 
         FIG. 4  shows a cutter profile for a first blade of the bit of  FIG. 1 . 
         FIG. 5  shows a cutter profile for a fourth blade of the bit of  FIG. 1 . 
         FIG. 6  shows a cutter profile for a seventh blade of the bit of  FIG. 1 . 
         FIG. 7A  shows a cutter profile for a bit having a cutter set in accordance with a third embodiment of the invention. 
         FIG. 7B  shows another bit having at least one pilot cutter having a first exposure lesser than a second exposure of at least one primary cutter in accordance with another embodiment of the invention. 
         FIG. 8  is a graph of cumulative diamond wearflat area during simulated drilling conditions. 
         FIG. 9  is a graph of drilling penetration rate during simulated drilling conditions. 
         FIG. 10  shows a representative formation cut segment for a bit having one cutter combination set in accordance with the first embodiment of the invention. 
         FIG. 11  shows an illustration of the cutter set in accordance with the third embodiment of the invention. 
         FIG. 12  shows a cutter profile for the second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a face view of a rotary drag bit  110  in accordance with a first embodiment of the invention. While the rotary drag bit  110  of this embodiment comprises nine pilot or cutter sets  160 , it is contemplated that the drag bit  110  may include one cutter set or a plurality of cuter combination sets greater or less than the nine illustrated. Before turning to a detailed description of the cutter sets  160 , the general description of the drag bit  110  is first discussed. 
     The rotary drag bit  110  as viewed by looking upwardly at its face or leading end  112  as if the viewer were positioned at the bottom of a bore hole. Bit  110  includes a plurality of cutting elements or cutters  114  bonded, as by brazing, into pockets  116  (as representatively shown) located in the blades  118  extending above the face  112  of the drag bit  110 , as is well known to those of ordinary skill in the art. The drag bit  110  depicted is a matrix body bit, but the invention is not so limited. The bit may also be formed as a so-called “steel body” or other bit type. “Matrix” bits include a mass of metal powder, such as tungsten carbide particles, infiltrated with a molten, subsequently hardenable binder, such as a copper-based alloy. Moreover, while this embodiment of the invention includes blades  118  extending above the face  112  of the bit  110 , the use of blades  118  is not critical to, or limiting of, the present invention. 
     Fluid courses  120  lie between blades  118  and are provided with drilling fluid by nozzles  122  secured in nozzle orifices  124 , orifices  124  being at the end of passages leading from a plenum extending into a bit body  111  from a tubular shank at the upper, or trailing, end of the bit  110 . Fluid courses  120  extend to junk slots  126  extending upwardly along the side of bit  110  between blades  118 . Gage pads (not shown) comprise longitudinally upward extensions of blades  118  and may have wear-resistant inserts or coatings on radially outer surfaces  121  thereof as known in the art. Formation cuttings are swept away from the cutters  114  by drilling fluid F emanating from nozzles  122  and which moves generally radially outwardly through fluid courses  120  and then upwardly through junk slots  126  to an annulus between the drill string from which the bit  110  is suspended and supported. The drilling fluid F provides cooling to the cutters  114  during drilling and clears formation cuttings from the bit face  112 . 
     Each of the cutters  114  in this embodiment are PDC cutters. However, it is recognized that any other type of cutting element may be utilized with the embodiments of the invention presented. For clarity in the various embodiments of the invention, the cutters are shown as unitary structures in order to better describe and present the invention. However, it is recognized that the cutters  114  may comprise layers of materials. In this regard, the PDC cutters  114  of the current embodiment each comprise a diamond table bonded to a supporting substrate, as previously described. The PDC cutters  114  remove material from the underlying subterranean formations by a shearing action as the drag bit  110  is rotated by contacting the formation with cutting edges  113 . As the formation is cut, the flow of drilling fluid F comminutes the formation cutting and suspends and carries the particulate mix away through the junk slots  126  mentioned above. 
     The blades  118  comprise primary blades in the form of first, fourth and seventh blades  131 ,  134 , and  137 , respectively, and further comprise secondary blades in the form of second, third, fifth, sixth, eight and ninth blades  132 ,  133 ,  135 ,  136 ,  138 , and  139 , respectively. Each blade  118  generally projects longitudinally from the face  112  and extends generally radially outwardly thereover to the gage of the bit body  111 . The plurality of cutters  114  are arranged upon the blades  131 ,  132 ,  133 ,  134 ,  135 ,  136 ,  137 ,  138 ,  139  as shown by a cutter and blade profile  130  in  FIG. 3 . Each of the cutters  114  shown in  FIG. 3  are representative of cutter placement upon the bit body  111  as understood by a person of skill in the art of cutter profiles, are numbered  1  through  61  extending from lead lines and will be referenced by the same numerals  1  through  61 , respectively, for purposes of describing this embodiment of the invention. Each of the cutters  1  through  61  include a subscript numbered between  1  and  12  indicating its placement within cutter rows  141  through  152 , respectively, arranged upon the blades  118 . Each cutter row  141  through  152  rotationally trails the cutter row immediately preceding it. For example, cutters  16  and  17  include subscripts  1  and  2 , respectively, indicating that the cutter  16  belongs to the first cutter row  141  and the cutter  17  belongs to the second cutter row  142  rotationally trailing the first cutter row  141 . Cutters  16  and  17  are both disposed upon the first blade  131 . While the cutters  114  are placed in twelve rows upon the drag bit  110  having nine blades, the drag bit  110  may have any suitable number of cutter rows or any number blades. Specifically, embodiments of the invention are particularly suited for a drag bit having two cutter rows disposed upon one blade. A cutter row may be determined by a radial path extending from the centerline C/L of the face  112  of the drag bit  110  and may be further defined by having one or more cutting elements disposed substantially along or proximate to the radial path. 
     The Cutter sets  160  include: cutters  12 / 13 ; cutters  16 / 17 ; cutters  20 / 21 ; cutters  24 / 25 ; cutters  28 / 29 ; cutters  32 / 33 ; cutters  36 / 37 ; cutters  40 / 41 ; and cutters  44 / 46 . The cutter sets  160  are located primarily in a nose region  172 , a flank region  174  and a shoulder region  175  of the bit body  111 . The cutter sets  160  may also be located in the cone region  170  and the gage region  176  of the bit body  111 , or in any given region, without limitation. 
     Each cutter set  160  includes a pilot cutter  162  of relatively smaller lateral extent rotationally leading a primary cutter  164  of relatively larger lateral extent in substantially the same rotational path, at substantially the same radius from the centerline C/L. The cutter sets  160  are illustrated in profile in  FIG. 4  which shows a cutter profile  127  for a first blade  131 , in  FIG. 5  which shows a cutter profile  128  for a fourth blade  134 , and in  FIG. 6  which shows a cutter profile  129  for a seventh blade  137  for the drag bit  110 , respectively. For example, primary cutter  17  rotationally trails pilot cutter  16  along substantially the same rotational path as shown in  FIG. 4 . Optionally, a cutter set  160  may be placed upon any blade, e.g., primary, secondary or tertiary blades, without limitation, but are included upon the primary blades  131 ,  134 ,  137  in this embodiment. 
     The pilot cutter  162  may have a particular exposure to the formation, the exposure being the extent to which a cutter protrudes above the surrounding bit face, such as the face of a blade  137  as illustrated in  FIG. 6 . The cutters distributed along one or more blades together exhibit a cutter profile as shown in  FIGS. 3 through 6  and identified at  166  in  FIG. 6 . In use, the cutters engage the formation to a depth of cut usually limited by the surrounding surface on the bit face to which each cutter is mounted, but in other instances limited by so-called penetration or depth of cut limiters, as is well known in the art. The larger, primary cutter  164 , rotationally trailing the pilot cutter  162 , is under exposed with respect to the pilot cutter  162 . While the larger, primary cutter  164 , is under exposed with respect to the pilot cutter  162  in this embodiment of the invention, the primary cutter  164  may have the same exposure. The underexposure may, of course, be varied based upon formation characteristics, relative cutter sizes, cutter shapes, the presence or absence of chamfers on the cutting faces of the cutters, cutter backrakes, rotational spacing between cutters, and other factors. In this regard the selected underexposure is an engineered exposure. Also, the engineered exposure of a pilot cutter may include the same exposure with respect to other primary cutters. In this configuration the smaller, more highly exposed pilot cutter  162  is enabled to apply focused energy applied to the bit from weight on bit (WOB) and bit rotation to pre-fracture the formation while the larger cutter  164  clears and widens the cut made in the formation by the pilot cutter  162 . The larger cutter  164  may have any under exposure such that it remains in subsequent contact with the formation while substantially trailing the pilot cutter  162  prior to other cutters  114  cutting the uncut formation material when cutting along the rotational path spaces  168  between cutters on the depicted blade. 
       FIG. 2  shows a frontal view of a rotary drag bit  210  in accordance with a second embodiment of the invention. Simultaneous reference may be made to  FIG. 12 , which shows a cutter profile  230  for the second embodiment of the invention. The rotary drag bit  210  comprises six blades  218  and a plurality of cutters  214  coupled thereto. For purposes of describing  FIGS. 2 and 12  of the second embodiment of the invention, the cutters are numerically numbered between  1 - 57 , and the drag bit  210  also include wear knots numerically numbered  58 - 62 . In this regard, the cutter numerals  1  through  61  for the first embodiment of the invention is not to be confused with the cutter numerals  1  through  57  and the wear knot numerals  58  through  62  as shown in the cutter profile  230  in  FIG. 12  for the second embodiment of the invention. The blades  218  include three primary blades  231 ,  234 ,  237  and three secondary blades  232 ,  235 ,  238 . Each of the cutters  1 - 57  and each of the wear knots  58 - 62  include a subscript numbered between  1  and  6  indicating its placement upon blades  231 ,  232 ,  234 ,  235 ,  237 ,  238 , respectively, and further arranged within cutter rows  241  through  252  for each blade  231 ,  232 ,  234 ,  235 ,  237 ,  238 . 
     The cutters  214  are arranged in first cutter rows  241 ,  243 ,  245 ,  247 ,  249 ,  251  and in second cutter rows  242 ,  244 ,  246 ,  248 ,  250 ,  252  on blades  231 ,  232 ,  234 ,  235 ,  237 ,  238 , respectively. The second cutter rows  242 ,  244 ,  246 ,  248 ,  250 ,  252  each rotationally trail the first cutter rows  241 ,  243 ,  245 ,  247 ,  249 ,  251 , respectively preceding them. The cutters  214  include smaller cutting elements  262  in first cutter rows  241 ,  243 ,  245 ,  247 ,  249 ,  251  leading larger cutting elements  264  in second cutter rows  242 ,  244 ,  246 ,  248 ,  250 ,  252  in order to pre-fracture or improve fracturing of a formation during drilling. In this regard, the smaller cutting elements  262  in first cutter rows  241 ,  243 ,  245 ,  247 ,  249 ,  251  may be considered “pilot” cutter set  260  when paired with respective larger, primary cutting elements  264  in second cutter rows  242 ,  244 ,  246 ,  248 ,  250 ,  252  disposed substantially along or proximate to the radial path created by the smaller cutting elements  262 . 
     In this embodiment of the invention, the cutter sets  260  are located substantially in a nose region  272 , of the drag bit  210 . The cutters  214  located within the nose region  272  experience significant cutter load, by providing cutter sets  260  the work load distributed across cutters  262  and  264  improving removal of formation material while decreasing individual cutter loading. The cutter sets  260  may also be located in a cone region  270 , a shoulder region  274  and the gage region  276  of the bit body  111 , or in any given region, without limitation. The cutter sets  260  include cutters  11 / 12 ,  13 / 14 ,  15 / 16 ,  17 / 18 ,  19 / 20 ,  21 / 22 ,  25 / 26 ,  29 / 30  and  33 / 34  as shown in  FIG. 12 . 
     In this embodiment of the invention, the smaller cutting element  262  is a pilot or core cutter providing a primary means of fracturing a formation allowing the larger cutting element  264  with its larger diameter coming in behind, i.e., rotationally following, the smaller cutting element  262  to further remove the formation. The larger cutting element  264  shears the formation material as in conventional drag bits, but because the formation has already been fractured, and thus weakened, by the rotationally leading smaller cutting element  262 , the cut may be completed with less energy. In this regard, it is easier for the larger cutting element  264  to remove the formation material weakened but unremoved by the smaller cutting element  262  without being exposed to as much stress. In another aspect, the same amount of formation removal is accomplished with the smaller “pilot” cutting element  262  in front of the larger cutting element  264 , allowing the smaller cutting element  262  to leave a smaller footprint on the working formation in terms of wearflat area (discussed below) allowing the cutter combination  260  (smaller cutting element  262  in front of the larger cutting element  264 ) to maintain an improved efficiency for a longer period of time as the cutters  214  wear, (again in terms of wearflat area as discussed below). 
       FIG. 7A  shows a cutter profile  330  for a bit  310  having a cutter set  360  in accordance with a third embodiment of the invention. The cutter set  360  includes a first cutter  362  and a second cutter  364 , both being coupled to a bit body  311  of the bit  310 . The second cutter  364  is larger than the first cutter  362 , and is underexposed with respect to and rotationally trails the first cutter  362 . While the second cutter  364  rotationally trails the first cutter  362 , it need only rotationally trail in a substantially adjacent or similar rotational or helical path created by the rotation of the bit  310 . Assuming that the applied force for fracturing the formation is held constant upon the bit  310 , the first cutter  362  may apply greater stress upon the formation because of its smaller face surface area  363  and engaged cutting edge in comparison to the second cutter  364  with its larger face surface area  365 . In this regard, the first cutter  362  may provide the primary force for pre-fracturing a formation due to its greater applied force per unit area, while the second cutter  364  is able to clear and open the cut made in the formation with its lower applied force per unit area. 
     Initially, at the time of formation drilling, i.e., before wearflat areas develop upon the cutters  114 , the energy supplied by the drill string primarily is transmitted into the cutters  362  and  364  and through their face surface areas  363  and  365 , respectively, providing stress upon the formation  366  to fracture it (the penetration force). Reference may also be made to  FIG. 11 , wherein it is shown that as the cutters  362  and  364  wear, wearflat areas develop upon the normal cutter surfaces  380  and  381 , respectively. As the wearflat areas increase or grow on the normal cutter surfaces  380  and  381  the indentation force increases, requiring a greater WOB to effect a given depth of cut. While the energy transfer effect is true for conventional cutters, the embodiments of the invention advantageously harness and control the growth of the wearflat areas by optimizing interaction of the cutter set  360  to maintain a lesser required WOB during drilling by reducing cutter wear, which enhances and prolongs the life of the drag bit  310 . 
       FIG. 7B  shows another embodiment of a rotary drag bit, the rotary drag bit having a bit body  312  with a face and a longitudinal axis, the bit body  312  configured to rotate about the axis. The rotary drag bit further including at least one pilot cutter  361  disposed at a radius from the longitudinal axis and including a cutting surface of a first lateral extent protruding at least partially from the face at a first exposure and at least one primary cutter  367  disposed at substantially the same radius from the longitudinal axis and including a cutting surface of a second, greater lateral extent protruding at least partially from the face at a second exposure. In this embodiment, the first exposure of the at least one pilot cutter  361  is lesser than the second exposure of the at least one primary cutter  367 . 
     In embodiments of the invention, the life of a drag bit is increased as compared to a substantially equivalent, conventional drag bit. Specifically, by using a smaller diameter or lateral extent, rotationally leading cutter with a wider or trailing space before a larger cutter of greater lateral extent or diameter follows in the same radial path, less cutter density is needed, i.e., cutter density is decreased when compared with a similar conventional bit, although the cutter count may be the same. The cutter density, in effect, leaves a smaller footprint upon the formation as compared to a conventional bit having the same number of cutters, enabling greater penetration as the cutters wear. In this regard, the smaller footprint by the cutters upon the formation improves the energy transfer, particularly in terms of the force being applied to the drill bit which is utilized more efficiently by the cutters for a longer period of time. 
       FIG. 10  shows a representative formation cut segment  167  for a bit  110  having one cutter combination set  160  in accordance with the first embodiment of the invention. The cut segment  167  is shown as if looking toward the bit  110  when looking up from the bottom surface of a bore hole in a formation. The set  160  comprises a smaller cutter  162  rotationally leading or in front of a larger cutter  164 . Both cutters  162 ,  164 , of the set  160 , are aligned on a blade  118  of a bit body of the bit  110  in combination in order to facilitate pre-fracture and removal of subterranean formation to achieve the cut segment  167  when drilling. The cutting face of the larger cutter  164  trails the cutting face of the smaller cutter  162  by a rotational segment or space  161  and cutters  162 ,  164  are placed on the blade  118  such that the center of both cutters  162 ,  164  lie in slightly different or substantially the same radial paths. The radial path  169  is representative of the helical path the cutters  162 ,  164  travel when cutting the formation during drilling. The larger cutter  164  is slightly underexposed with respect to the smaller cutter  162 . In this regard, the smaller cutter  162  pre-factures the formation after which the underexposed larger cutter  164  enlarges the cut segment  167  and removes additional formation material while cutting. The amount of underexposure will be determined by the desired ROP and the rotational segment or space  161 . In this embodiment, as the desired ROP is increased or the rotational space  161  is increased, the designed underexposure of the cutter  164  will necessarily increase in order to allow the smaller cutter  162  to primarily contact the formation with the larger cutter  164  trailing to open up the cut segment  167 . 
     As with other embodiments of the invention, the rotational space  161  between the cutters  162 ,  164  may be such that the smaller cutter  162  is aligned within a first cutter row  141  with other cutters  114  and the larger cutter  164  is aligned within a second cutter row  142  having other cutters  114 . Optionally, the rotational space  161  may be larger or smaller such that placement of either cutter  162 ,  164  is in its own cutter row. 
     As depicted, smaller cutter  162  and the larger cutter  164  are both PDC full round face cutters providing suitable cutting capability for multiple formations types. Optionally, the smaller cutter  162  and larger cutter  164  may each be made from different cutting element materials, e.g., TSP, without limitation, and may include various cutter shapes, e.g., scribed cutters, without limitation, suitable for cutting different formation types. 
     Representatively,  FIG. 10  shows the formation cut segment  167  before the cutters  162 ,  164  begin to develop wearflats. As the bit  110  wears, wearflats  190  develop upon the cutters  162 ,  164 . As the bit  110  continues to wear, the surface area  191  of the wearflats  190  continues to increase. The other cutters  114  also develop wearflats as the bit  110  wears. The wearflats  190  represent the cutter area of the cutters coming in contact generally in the axial or normal direction of the bit  110  with respect to the formation. As the surface area  191  of the wearflats  190  increase, the force required to penetrate the formation with the cutters increases and resultantly reduces the amount of force (or energy) available for penetration causing the ROP to decrease. Also, as the bit  110  wears, the increase in energy transfer to penetrate the formation accelerates the rate of wearflat growth and ultimately shortens the life of the bit  110 . Advantageously, the life of the bit  110  is extended by the cutter combination set  160  when compared to a conventional bit. The cutter combination set  160  distributes the work load upon the cutters  162 ,  164 . Specifically, the smaller cutter  162  pre-fractures the formation and the larger cutter  164  enlarges the cut in the pre-fracture formation, which lowers the stress upon the cutter set  160  allowing the wearflat area  191  of the bit  110  to increase at a lower rate for a given ROP. 
     Performance improvement obtained through use of an embodiment of the invention is shown in  FIGS. 8 and 9 .  FIG. 8  is a graph  400  of cumulative diamond wearflat area and  FIG. 9  is a graph  410  of drilling penetration rate, for two different drag bits simulated under the same drilling conditions. 
     The graph  400  of  FIG. 8  includes a vertical axis indicating total diamond wearflat area of all the cutting elements in square inches, and a horizontal axis indicating distance drilled in feet. The graph  410  of  FIG. 9  includes a vertical axis indicating penetration rate (or ROP) in feet per hour, and a horizontal axis indicating distance drilled in feet. The results shown in  FIGS. 8 and 9  were based upon a computer model of the drag bits drilling a vertical hole in a single, hard abrasive sandstone formation while maintaining 25,000 lbs WOB at a constant bit rotation of 120 RPM over the entire drill run. The bits were 7⅞ inches in size and included the same number of bit blades. Also, the simulation maintained the bit temperatures at 100° C. by providing cooling fluid to the bits. Further, there where no dynamic dysfunctions and offset forces in the model of the simulation. 
     The responses  402  and  412  shown in  FIGS. 8 and 9 , respectively, are of a conventional bit. The responses  404  and  414  shown in  FIGS. 8 and 9 , respectively, are for a pilot cutter bit according to an embodiment of the invention. Both bits have the same number of cutting elements; in this regard the conventional bit and the pilot cutter bit are functionally identical in design. However, the actual diamond or cutter density for the conventional bit was greater than that for the pilot cutter bit, i.e., the diamond density of the pilot cutter bit was less because of smaller or pilot cutting elements used. Diamond or cutter density is a measure of the cutter area, cutter size and the cutter volume of all the cutters on a bit, for example, without limitation. Looking at graph  400 , the response  402  of the wearflat area of the conventional bit increases at a faster rate than the response  404  of the wearflat area of the pilot cutter bit. In this regard, the life of the pilot cutter bit is extended beyond the life of the conventional bit. 
     Looking at graph  410 , the response  414  shows penetration rate of the pilot cutter bit is greater than the penetration rate shown in response  412  for the conventional bit for a given distance drilled, correspondingly correlating to wearflat area for the same distance drilled as shown in graph  400 . Accordingly, by providing a bit configured according to an embodiment of the invention, the rate of wearflat area increase of the cutting elements is reduced and reduction in ROP over the course of the run is also reduced for a given distance drilled as compared to a conventional bit. 
     Also, the penetration rate, i.e., response  414  of the pilot cutter bit is greater than the penetration rate, i.e., response  412 , of the conventional bit at a given distance drilled, in part because the “pilot cutter” bit has lower cutter density, despite the fact that both bits have the same cutter count. In this regard, as the cutters of the pilot cutter bit wear, a smaller “footprint” or wearflat area is comparatively maintained over the life of the bit, providing more force, i.e., energy, to removing and penetrating the formation and less force into the “footprint” or wearflat area. In the conventional bit, more force, i.e., energy, is transferred into its “footprint” or wearflat area comparatively because of its larger diamond density, which accelerates the growth of the wearflats and decreases its drilling life. 
     In embodiments of the invention, the primary or larger cutters may be spaced together as close as possible without interfering with other cutters. Because the pilot or smaller cutters lead the larger cutters, the pilot cutters will be spaced wider apart and the cutter density will be less than conventionally expected for a similar bit profile. Increasing the spacing of the pilot and larger cutters improves the life of the bit by leaving a smaller “imprint” or wearflat area as compared to conventional bit cutter and further improves penetration rate over the life of the drag bit as the cutters wear. Further, by increasing the spacing of the cutters by having pilot cutters upon the drag bit allows more bit or blade body material to surround the cutters, providing additional surface area to absorb any impact or dynamic dysfunctional energy that might damage the primary cutters or the pilot cutters. 
     In embodiments of the invention, the primary or larger cutters may have an engineered exposure. The engineered exposure may include the same exposure for a pilot cutter and the primary cutter rotationally trailing the pilot cutter in substantially the same rotational path where the pilot cutter includes a smaller cutter density than the primary cutter. 
     In other embodiments of the invention, all of the primary or larger cutters may have an engineered exposure and all of the pilot cutters may have an engineered exposure. The engineered exposure may include the same exposure for all of the pilot cutters and all of the primary cutters rotationally trailing each of the pilot cutters in each of the substantially same rotational path for each pilot cutter and each primary cutter groupings. Each of the pilot cutters includes a smaller cutter density than each of the primary cutters. 
     In still other embodiments of the invention, all of the secondary cutters may have an engineered exposure and all of the pilot cutters may have an engineered exposure. The engineered exposure may include the same exposure for all of the pilot cutters and all of the secondary cutters rotationally trailing each of the pilot cutters in each of the substantially same rotational path for each pilot cutter and each secondary cutter groupings. Each of the pilot cutters includes a smaller cutter density than each of the primary cutters. 
     In yet another embodiment of the invention, all of the primary cutters may have an engineered exposure. The engineered exposure may include the same exposure for all of the primary cutters. Some of the primary cutters are positioned upon a blade of the bit body approximately trailing a junk slot that immediately rotationally precedes the blade, and other primary cutters rotationally trail their respective pilot cutters on the blade in substantially same rotational path for each pilot cutter and each primary cutter grouping. At least one of the pilot cutters includes a smaller cutter density than the primary cutter that it rotationally trails on the blade. 
     While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited in terms of the appended claims.