Patent Publication Number: US-10781642-B2

Title: Rotary drill bit including multi-layer cutting elements

Description:
RELATED APPLICATION 
     This application is a Divisional Application of U.S. patent application Ser. No. 15/034,143 filed May 3, 2016, which is a U.S. National Stage Application of International Application No. PCT/US2013/073583 filed Dec. 6, 2013, which designates the United States, and which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to downhole drilling tools and, more particularly, to rotary drill bits and methods for designing rotary drill bits with multi-layer cutting elements. 
     BACKGROUND 
     Various types of downhole drilling tools including, but not limited to, rotary drill bits, reamers, core bits, and other downhole tools have been used to form wellbores in associated downhole formations. Examples of such rotary drill bits include, but are not limited to, fixed cutter drill bits, drag bits, polycrystalline diamond compact (PDC) drill bits, and matrix drill bits associated with forming oil and gas wells extending through one or more downhole formations. Fixed cutter drill bits such as PDC bits may include multiple blades that each include multiple cutting elements. 
     In typical drilling applications, a PDC bit may be used to drill through various levels or types of geological formations with longer bit life than non-PDC bits. Typical formations may generally have a relatively low compressive strength in the upper portions (e.g., lesser drilling depths) of the formation and a relatively high compressive strength in the lower portions (e.g., greater drilling depths) of the formation. Thus, it typically becomes increasingly more difficult to drill at increasingly greater depths. Additionally, cutting elements on the drill bit may experience increased wear as drilling depth increases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present disclosure and its features and advantages thereof may be acquired by referring to the following description, taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
         FIG. 1  illustrates an elevation view of an example embodiment of a drilling system, in accordance with some embodiments of the present disclosure; 
         FIG. 2  illustrates an isometric view of a rotary drill bit oriented upwardly in a manner often used to model or design fixed cutter drill bits, in accordance with some embodiments of the present disclosure; 
         FIG. 3  illustrates a report of run information gathered from drilling a wellbore with a drill bit, in accordance with some embodiments of the present disclosure; 
         FIG. 4A  illustrates a graph of actual average rate of penetration (ROP) and revolutions per minute (RPM) as a function of drilling depth as estimated in accordance with some embodiments of the present disclosure; 
         FIG. 4B  illustrates a graph of actual average depth of cut as a function of drilling depth as estimated in accordance with some embodiments of the present disclosure; 
         FIG. 5  illustrates a graph of first layer cutting element wear depth, second layer cutting element critical depth of cut, and actual depth of cut as a function of drilling depth, in accordance with some embodiments of the present disclosure; 
         FIG. 6A  illustrates a schematic drawing for a bit face of a drill bit including first layer and second layer cutting elements for which a critical depth of cut control curve (CDCCC) may be determined, in accordance with some embodiments of the present disclosure; 
         FIG. 6B  illustrates a schematic drawing for a bit face profile of the drill bit of  FIG. 6A , in accordance with some embodiments of the present disclosure; 
         FIG. 7A  illustrates a flow chart of an example method for determining and generating a CDCCC, in accordance with some embodiments of the present disclosure; 
         FIG. 7B  illustrates a graph of a CDCCC where the critical depth of cut is plotted as a function of the bit radius of the drill bit of  FIG. 6A , in accordance with some embodiments of the present disclosure; 
         FIGS. 8A-8I  illustrate schematic drawings of bit faces of a drill bit with exemplary placements for second layer cutting elements, in accordance with some embodiments of the present disclosure; 
         FIG. 9  illustrates a graph of a CDCCC where the critical depth of cut is plotted as a function of the bit radius for a bit where the second layer cutting elements have different under-exposures, in accordance with some embodiments of the present disclosure; 
         FIG. 10  illustrates a flowchart of an example method for adjusting under-exposure of second layer cutting elements on a drill bit to approximate a target critical depth of cut, in accordance with some embodiments of the present disclosure; and 
         FIG. 11  illustrates a flowchart of an example method for performing a design update of a pre-existing drill bit with second layer cutting elements or configuring a new drill bit with second layer cutting elements, in accordance with some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure and its advantages are best understood by referring to  FIGS. 1-11 , where like numbers are used to indicate like and corresponding parts. 
       FIG. 1  illustrates an elevation of an example embodiment of a drilling system, in accordance with some embodiments of the present disclosure. Drilling system  100  is configured to provide drilling into one or more geological formations, in accordance with some embodiments of the present disclosure. Drilling system  100  may include a well surface, sometimes referred to as “well site”  106 . Various types of drilling equipment such as a rotary table, mud pumps and mud tanks (not expressly shown) may be located at a well surface or well site  106 . For example, well site  106  may include drilling rig  102  that may have various characteristics and features associated with a “land drilling rig.” However, downhole drilling tools incorporating teachings of the present disclosure may be satisfactorily used with drilling equipment located on offshore platforms, drill ships, semi-submersibles and drilling barges (not expressly shown). 
     Drilling system  100  may include drill string  103  associated with drill bit  101  that may be used to form a wide variety of wellbores or bore holes such as generally vertical wellbore  114   a  or generally horizontal wellbore  114   b  as shown in  FIG. 1 . Various directional drilling techniques and associated components of bottom hole assembly (BHA)  120  of drill string  103  may be used to form generally horizontal wellbore  114   b . For example, lateral forces may be applied to drill bit  101  proximate kickoff location  113  to form generally horizontal wellbore  114   b  extending from generally vertical wellbore  114   a . The term “directional drilling” may be used to describe drilling a wellbore or portions of a wellbore that extend at a desired angle or angles relative to vertical. Such angles may be greater than normal variations associated with vertical wellbores. Direction drilling may also be described as drilling a wellbore deviated from vertical. The term “horizontal drilling” may be used to include drilling in a direction approximately ninety degrees)(90°) from vertical. 
     BHA  120  may be formed from a wide variety of components configured to form a wellbore  114 . For example, components  122   a ,  122   b  and  122   c  of BHA  120  may include, but are not limited to, drill bits (e.g., drill bit  101 ) drill collars, rotary steering tools, directional drilling tools, downhole drilling motors, drilling parameter sensors for weight, torque, bend and bend direction measurements of the drill string and other vibration and rotational related sensors, hole enlargers such as reamers, under reamers or hole openers, stabilizers, measurement while drilling (MWD) components containing wellbore survey equipment, logging while drilling (LWD) sensors for measuring formation parameters, short-hop and long haul telemetry systems used for communication, and/or any other suitable downhole equipment. The number of components such as drill collars and different types of components  122  included in BHA  120  may depend upon anticipated downhole drilling conditions and the type of wellbore that will be formed by drill string  103  and rotary drill bit  101 . BHA  120  may also include various types of well logging tools (not expressly shown) and other downhole tools associated with directional drilling of a wellbore. Examples of such logging tools and/or directional drilling tools may include, but are not limited to, acoustic, neutron, gamma ray, density, photoelectric, nuclear magnetic resonance, rotary steering tools and/or any other commercially available well tool. 
     Wellbore  114  may be defined in part by casing string  110  that may extend from well surface  106  to a selected downhole location. Portions of wellbore  114  as shown in  FIG. 1  that do not include casing string  110  may be described as “open hole.” In addition, liner sections (not expressly shown) may be present and may connect with an adjacent casing or liner section. Liner sections (not expressly shown) may not extend to the well site  106 . Liner sections may be positioned proximate the bottom, or downhole, from the previous liner or casing. Liner section may extend to the end of wellbore  114 . Various types of drilling fluid may be pumped from well surface  106  through drill string  103  to attached drill bit  101 . Such drilling fluids may be directed to flow from drill string  103  to respective nozzles (item  156  illustrated in  FIG. 2 ) included in rotary drill bit  101 . The drilling fluid may be circulated back to well surface  106  through an annulus  108  defined in part by outside diameter  112  of drill string  103  and inside diameter  118  of wellbore  114 . Inside diameter  118  may be referred to as the “sidewall” or “bore wall” of wellbore  114 . Annulus  108  may also be defined by outside diameter  112  of drill string  103  and inside diameter  111  of casing string  110 . Open hole annulus  116  may be defined as sidewall  118  and outside diameter  112 . 
     Drilling system  100  may also include rotary drill bit (“drill bit”)  101 . Drill bit  101 , discussed in further detail in  FIG. 2 , may include one or more blades  126  that may be disposed outwardly from exterior portions of rotary bit body  124  of drill bit  101 . Rotary bit body  124  may have a generally cylindrical body and blades  126  may be any suitable type of projections extending outwardly from rotary bit body  124 . Drill bit  101  may rotate with respect to bit rotational axis  104  in a direction defined by directional arrow  105 . Blades  126  may include one or more cutting elements  128  disposed outwardly from exterior portions of each blade  126 . Blades  126  may include one or more depth of cut controllers (not expressly shown) configured to control the depth of cut of cutting elements  128 . Blades  126  may further include one or more gage pads (not expressly shown) disposed on blades  126 . Drill bit  101  may be designed and formed in accordance with teachings of the present disclosure and may have many different designs, configurations, and/or dimensions according to the particular application of drill bit  101 . 
     Drilling system  100  may include one or more second layer cutting elements on a drill bit that are configured to cut into the geological formation at particular drilling depths and/or when first layer cutting elements experience sufficient wear. Thus, multiple layers of cutting elements may exist that engage with the formation at multiple drilling depths. Placement and configuration of the first layer and second layer cutting elements on blades of a drill bit may be varied to enable the different layers to engage at specific drilling depths. For example, configuration considerations may include under-exposure and blade placement of second layer cutting elements with respect to first layer cutting elements, and/or characteristics of the formation to be drilled. Cutting elements may be arranged in multiple layers on blades such that second layer cutting elements may engage the formation when the depth of cut is greater than a specified value and/or when first layer cutting elements are sufficiently worn. In some embodiments, the drilling tools may have first layer cutting elements arranged on blades in a single-set or a track-set configuration. Second layer cutting elements may be arranged on different blades that are track-set and under-exposed with respect to the first layer cutting elements. In some embodiments, the amount of under-exposure may be approximately the same for each of the second layer cutting elements. In other embodiments, the amount of under-exposure may vary for each of the second layer cutting elements. 
       FIG. 2  illustrates an isometric view of rotary drill bit  101  oriented upwardly in a manner often used to model or design fixed cutter drill bits, in accordance with some embodiments of the present disclosure. Drill bit  101  may be any of various types of fixed cutter drill bits, including PDC bits, drag bits, matrix drill bits, and/or steel body drill bits operable to form wellbore  114  extending through one or more downhole formations. Drill bit  101  may be designed and formed in accordance with teachings of the present disclosure and may have many different designs, configurations, and/or dimensions according to the particular application of drill bit  101 . 
     Drill bit  101  may include one or more blades  126  (e.g., blades  126   a - 126   g ) that may be disposed outwardly from exterior portions of rotary bit body  124  of drill bit  101 . Rotary bit body  124  may be generally cylindrical and blades  126  may be any suitable type of projections extending outwardly from rotary bit body  124 . For example, a portion of blade  126  may be directly or indirectly coupled to an exterior portion of bit body  124 , while another portion of blade  126  may be projected away from the exterior portion of bit body  124 . Blades  126  formed in accordance with teachings of the present disclosure may have a wide variety of configurations including, but not limited to, substantially arched, helical, spiraling, tapered, converging, diverging, symmetrical, and/or asymmetrical. 
     In some embodiments, blades  126  may have substantially arched configurations, generally helical configurations, spiral shaped configurations, or any other configuration satisfactory for use with each downhole drilling tool. One or more blades  126  may have a substantially arched configuration extending from proximate rotational axis  104  of drill bit  101 . The arched configuration may be defined in part by a generally concave, recessed shaped portion extending from proximate bit rotational axis  104 . The arched configuration may also be defined in part by a generally convex, outwardly curved portion disposed between the concave, recessed portion and exterior portions of each blade which correspond generally with the outside diameter of the rotary drill bit. 
     Each of blades  126  may include a first end disposed proximate or toward bit rotational axis  104  and a second end disposed proximate or toward exterior portions of drill bit  101  (e.g., disposed generally away from bit rotational axis  104  and toward uphole portions of drill bit  101 ). The terms “uphole” and “downhole” may be used to describe the location of various components of drilling system  100  relative to the bottom or end of wellbore  114  shown in  FIG. 1 . For example, a first component described as uphole from a second component may be further away from the end of wellbore  114  than the second component. Similarly, a first component described as being downhole from a second component may be located closer to the end of wellbore  114  than the second component. 
     Blades  126   a - 126   g  may include primary blades disposed about the bit rotational axis. For example, in  FIG. 2 , blades  126   a ,  126   c , and  126   e  may be primary blades or major blades because respective first ends  141  of each of blades  126   a ,  126   c , and  126   e  may be disposed closely adjacent to bit rotational axis  104  of drill bit  101 . In some embodiments, blades  126   a - 126   g  may also include at least one secondary blade disposed between the primary blades. In the illustrated embodiment, blades  126   b ,  126   d ,  126   f , and  126   g  shown in  FIG. 2  on drill bit  101  may be secondary blades or minor blades because respective first ends  141  may be disposed on downhole end  151  of drill bit  101  a distance from associated bit rotational axis  104 . The number and location of primary blades and secondary blades may vary such that drill bit  101  includes more or less primary and secondary blades. Blades  126  may be disposed symmetrically or asymmetrically with regard to each other and bit rotational axis  104  where the location of blades  126  may be based on the downhole drilling conditions of the drilling environment. In some cases, blades  126  and drill bit  101  may rotate about rotational axis  104  in a direction defined by directional arrow  105 . 
     Each blade may have leading (or front) surface (or face)  130  disposed on one side of the blade in the direction of rotation of drill bit  101  and trailing (or back) surface (or face)  132  disposed on an opposite side of the blade away from the direction of rotation of drill bit  101 . Blades  126  may be positioned along bit body  124  such that they have a spiral configuration relative to rotational axis  104 . In other embodiments, blades  126  may be positioned along bit body  124  in a generally parallel configuration with respect to each other and bit rotational axis  104 . 
     Blades  126  may include one or more cutting elements  128  disposed outwardly from exterior portions of each blade  126 . For example, a portion of cutting element  128  may be directly or indirectly coupled to an exterior portion of blade  126  while another portion of cutting element  128  may be projected away from the exterior portion of blade  126 . By way of example and not limitation, cutting elements  128  may be various types of cutters, compacts, buttons, inserts, and gage cutters satisfactory for use with a wide variety of drill bits  101 . 
     Cutting elements  128  may be any suitable device configured to cut into a formation, including but not limited to, primary cutting elements, back-up cutting elements, secondary cutting elements or any combination thereof. Primary cutting elements may be described as first layer or second layer cutting elements. First layer cutting elements may be disposed on leading surfaces  130  of primary blades, e.g. blades  126   a ,  126   c , and  126   e . Second layer cutting elements may be disposed on leading surfaces  130  of secondary blades, e.g., blades  126   b ,  126   d ,  126   f , and  126   g.    
     Cutting elements  128  may include respective substrates with a layer of hard cutting material disposed on one end of each respective substrate. The hard layer of cutting elements  128  may provide a cutting surface that may engage adjacent portions of a downhole formation to form wellbore  114 . The contact of the cutting surface with the formation may form a cutting zone associated with each of cutting elements  128 . The edge of the cutting surface located within the cutting zone may be referred to as the cutting edge of a cutting element  128 . 
     Each substrate of cutting elements  128  may have various configurations and may be formed from tungsten carbide or other suitable materials associated with forming cutting elements for rotary drill bits. Tungsten carbides may include, but are not limited to, monotungsten carbide (WC), ditungsten carbide (W 2 C), macrocrystalline tungsten carbide and cemented or sintered tungsten carbide. Substrates may also be formed using other hard materials, which may include various metal alloys and cements such as metal borides, metal carbides, metal oxides and metal nitrides. For some applications, the hard cutting layer may be formed from substantially the same materials as the substrate. In other applications, the hard cutting layer may be formed from different materials than the substrate. Examples of materials used to form hard cutting layers may include polycrystalline diamond materials, including synthetic polycrystalline diamonds. 
     In some embodiments, blades  126  may also include one or more depth of cut controllers (DOCCs) (not expressly shown) configured to control the depth of cut of cutting elements  128 . A DOCC may include an impact arrestor, a back-up or second layer cutting element and/or a Modified Diamond Reinforcement (MDR). Exterior portions of blades  126 , cutting elements  128  and DOCCs (not expressly shown) may form portions of the bit face. 
     Blades  126  may further include one or more gage pads (not expressly shown) disposed on blades  126 . A gage pad may be a gage, gage segment, or gage portion disposed on exterior portion of blade  126 . Gage pads may contact adjacent portions of wellbore  114  formed by drill bit  101 . Exterior portions of blades  126  and/or associated gage pads may be disposed at various angles, positive, negative, and/or parallel, relative to adjacent portions of generally vertical wellbore  114   a . A gage pad may include one or more layers of hardfacing material. 
     Uphole end  150  of drill bit  101  may include shank  152  with drill pipe threads  155  formed thereon. Threads  155  may be used to releasably engage drill bit  101  with BHA  120  whereby drill bit  101  may be rotated relative to bit rotational axis  104 . Downhole end  151  of drill bit  101  may include a plurality of blades  126   a - 126   g  with respective junk slots or fluid flow paths  140  disposed therebetween. Additionally, drilling fluids may be communicated to one or more nozzles  156 . 
     Drill bit operation may be expressed in terms of depth of cut per revolution as a function of drilling depth. Depth of cut per revolution, or “depth of cut,” may be determined by rate of penetration (ROP) and revolution per minute (RPM). ROP may represent the amount of formation that is removed as drill bit  101  rotates and may be in units of ft/hr. Further, RPM may represent the rotational speed of drill bit  101 . For example, drill bit  101  utilized to drill a formation may rotate at approximately 120 RPM. Actual depth of cut (Δ) may represent a measure of the depth that cutting elements cut into the formation during a rotation of drill bit  101 . Thus, actual depth of cut may be expressed as a function of actual ROP and RPM using the following equation:
 
Δ=ROP/(5*RPM).
 
Actual depth of cut may have a unit of in/rev.
 
     Multiple formations of varied formation strength may be drilled using drill bits configured in accordance with some embodiments of the present disclosure. As drilling depth increases, formation strength may likewise increase. For example, a first formation may extend from the surface to a drilling depth of approximately 2,200 feet and may have a rock strength of approximately 5,000 pounds per square inch (psi). Additionally, a second formation may extend from a drilling depth of approximately 2,200 feet to a drilling depth of approximately 4,800 feet and may have rock strength of approximately 25,000 psi. As another example, a third formation may extend from a drilling depth of approximately 4,800 feet to a drilling depth of approximately 7,000 feet and may have a rock strength over approximately 20,000 psi. A fourth formation may extend from approximately 7,000 feet to approximately 8,000 feet and may have a rock strength of approximately 30,000 psi. Further, a fifth formation may extend beyond approximately 8,000 feet and have a rock strength of approximately 10,000 psi. 
     With increased drilling depth, formation strength or rock strength may increase or decrease and thus, the formation may become more difficult or may become easier to drill. For example, a drill bit including seven blades may drill through the first formation very efficiently, but a drill bit including nine blades may be desired to drill through the second and third formations. 
     Accordingly, as drill bit  101  drills into a formation, the first layer cutting elements may begin to wear as the drilling depth increases. For example, at a drilling depth of less than approximately 5,500 feet, the first layer cutting elements may have a wear depth of approximately 0.04 inches. At a drilling depth between approximately 5,500 feet and 8,500 feet, the first layer cutting elements may have an increased wear depth of approximately 0.15 inches. As first layer cutting elements wear, ROP of the drill bit may decrease, thus, resulting in less efficient drilling. Likewise, actual depth of cut for drill bit  101  may also decrease. Thus, second layer cutting elements that begin to cut into the formation when the first layer cutting elements experience a sufficient amount of wear may improve the efficiency of drill bit  101  and may result in drill bit  101  having a longer useful life. 
     Accordingly, to extend the bit life, it may be desired that (1) second layer cutting elements not cut into the formation until drill bit  101  reaches a particular drilling depth; (2) second layer cutting elements begin to cut into the formation at a particular drilling depth; (3) second layer cutting elements cut the formation effectively; and (4) approximately all second layer cutting elements cut into the formation substantially simultaneously. Hence, drill bit  101  optimized for maximizing drilling efficiency and bit life may include: 
     (a) first layer cutting elements that cut into the formation from the surface to a first drilling depth (D A ); 
     (b) second layer cutting elements that begin to cut into the formation at D A    
     (c) second layer cutting elements that cut efficiently based on formation properties; and 
     (d) second layer cutting elements that cut substantially simultaneously. 
     Improvement of the design of a drill bit may begin with actual performance of the bit when drilled into an offset well with a similar formation and similar operational parameters.  FIG. 3  illustrates a report of run information  300  gathered from drilling a wellbore (e.g., wellbore  114  as illustrated in  FIG. 1 ) with a drill bit, in accordance with some embodiments of the present disclosure. Drill bit run information may include, but is not limited to, rock strength, RPM, ROP, weight on bit (WOB), torque on bit (TOB), and mechanical specific energy (MSE). The run information may be measured at each foot drilled. 
     In the current example, rock strength, shown as plot  310 , remained substantially constant during drilling. RPM of the drill bit, which is the sum of RPM of the drill string and the RPM of the downhole motor, shown as plot  320 , and ROP, shown as plot  330 , decreased at a drilling depth of approximately 4,800 feet. Additionally, MSE may be calculated using the run information. MSE may be a measure of the drilling efficiency of drill bit  101 . In the illustrated embodiment, MSE increases after drilling approximately 4,800 feet, which may indicate that the drilling efficiency of the drill bit may decrease at depths over approximately 4,800 feet. Thus, drilling to approximately 4,800 feet may be described as high efficiency drilling  350 . MSE additionally increases again at approximately 5,800 feet. Drilling between approximately 4,800 feet and 5,800 feet may be described as efficiency drilling  360 , and drilling at depths over approximately 5,800 feet may be described as low efficiency drilling  370 . MSE may indicate a further drop in drilling efficiency. The data shown in  FIG. 3  may be obtained from various tools in the oil and gas drilling industry such as SPARTA™ analytical tools designed and manufactured by Halliburton Energy Services, Inc. (Houston, Tex.). 
     Using the gathered run information illustrated in  FIG. 3 , the average ROP and average RPM for a specified drilling section may be plotted as a function of drilling distance. Accordingly,  FIG. 4A  illustrates graph  400  of actual average ROP and actual average RPM as a function of drilling depth as estimated in accordance with some embodiments of the present disclosure. For example, from the drilling start point to a drilling depth of approximately 3,800 feet, actual average ROP, plot  410 , may be approximately 150 ft/hr. Corresponding average RPM, plot  420 , in this section of formation may be approximately 155. At a drilling depth of approximately 3,800 feet, actual average ROP, plot  410  may decrease to approximately 120 ft/hr while average RPM, plot  420 , remains approximately constant to a drilling depth of approximately 5,800 feet where it may begin to decrease. Thereafter, actual average ROP, plot  410 , may continue to decrease as the drilling depth continues to increase. 
     Similarly,  FIG. 4B  illustrates graph  430  of actual average depth of cut as a function of drilling depth as estimated in accordance with some embodiments of the present disclosure. Actual depth of cut as a function of drilling depth may be shown by plot  440 . For example, from the drilling start point to a drilling depth of approximately 3,800 feet, actual average depth of cut, plot  440 , may be approximately 0.19 in/rev. At a drilling depth of approximately 3,800 feet, actual average depth of cut, plot  440 , may decrease to approximately 0.15 in/rev. At a drilling depth of approximately 7,500 feet, actual average depth of cut, plot  440 , may begin to further decrease as the drilling depth increases. 
       FIG. 5  illustrates exemplary graph  500  of first layer cutting element wear depth, second layer cutting element critical depth of cut, and actual depth of cut for an example drill bit as a function of drilling depth, in accordance with some embodiments of the present disclosure. Critical depth of cut is a measure of the depth that second layer cutting elements cut into the formation during each rotation of drill bit  101 . Actual depth of cut is the measure of the actual depth that first layer cutting elements cut into the formation during each rotation of drill bit  101 . As first layer cutting elements become worn (and actual depth of cut decreases), the second layer cutting elements critical depth of cut may decrease such that second layer cutting elements engage the formation at a particular drilling distance. Based on run information  300  gathered as illustrated in  FIG. 3 , the actual wear of cutting elements may be plotted and then an average wear line may be estimated. Cutting element wear as a function of drilling depth may be shown as plot  510 . According to some embodiments of the present disclosure, a prediction of cutting element wear from drilling information may be made by utilizing a cutting element wear model, such as a model generated using SPARTA™ analytical tools designed and manufactured by Halliburton Energy Services, Inc. (Houston, Tex.). The cutting element wear models may be used to determine the cutting element wear of any drill bit, including drill bit  101 . One such model may be based on the accumulated work done by drill bit  101 :
 
Wear (%)=(Cumwork/BitMaxWork) a *100%
 
     where
 
Cumwork= f (drilling depth); and
 
     a=wear exponent and is between approximately 0.5 and 5.0. 
     Using the above model, cutting element wear as a function of drilling depth for a drill bit may be estimated and utilized during downhole drilling. Once the wear characteristics are obtained from the model, the drilling depth at which the first layer cutting elements may be worn to the point that the second layer cutting elements begin to cut into the formation (D A ) may be determined. For example, as illustrated in cutting element wear plot  510  in  FIG. 5 , after drilling to a depth of approximately 5,000 feet, the first layer cutting elements may have a cutting element wear depth of approximately 0.04 inches. Cutting element wear plot  510  in  FIG. 5  may depend on the material properties of the PDC layer and the bit operational parameters. As illustrated below with reference to  FIGS. 6A-7 , cutting element wear plot  510  may play a role in the optimization of the layout of the second layer cutting elements. 
     Second layer cutting element critical depth of cut as a function of drilling depth may be shown by plot  520  and actual depth of cut as a function of drilling depth may be shown by plot  530 . Second layer critical depth of cut if there was no first layer cutting element wear may be shown by plot  540 . A comparison of second layer depth of cut and actual depth of cut may identify when second layer cutting elements may engage the formation. For example, second layer cutting elements may have an initial critical depth of cut (plot  520 ) that may be greater than the actual depth of cut (plot  530 ). At a particular drilling distance, D A , second layer cutting element critical depth of cut, plot  520 , may intersect with the actual depth of cut, plot  530 . At a target drilling depth, second layer cutting element critical depth of cut, plot  520 , may be equal to approximately zero. Actual depth of cut, plot  530 , may be generated based on field measurements in accordance with  FIGS. 4A and 4B . 
     In some embodiments, the second layer cutting elements may be under-exposed by any suitable amount such that first layer cutting elements cut into the formation from the surface to a first drilling depth (D A ), and the second layer cutting elements begin to cut into the formation at D A  as the first layer cutting elements become worn. An analysis of  FIG. 5  indicates that the second layer cutting elements may begin to cut into the formation at drilling depth D A  of approximately 5,000 feet or when the actual depth of cut is approximately equivalent to the second layer critical depth of cut. 
     Thus, to ensure that second layer cutting elements do not cut into the formation until a particular drilling depth D A , the under-exposure of second layer cutting elements may be set to provide a critical depth of cut for second layer cutting elements greater than the actual depth of cut. Further, a critical depth of cut for the second layer cutting elements as a function of the drilling distance may be obtained based on the first layer cutting element wear depth. The under-exposure of the second layer cutting elements may approximate the first layer cutting element wear depth at a target drilling distance. 
     Accordingly, determining the amount of wear the first layer cutting element undergoes before second layer cutting elements engage the formation may be useful. In order to determine when the second layer cutting element may begin to cut into the formation, a critical depth of cut curve (CDCCC) for PDC bits having second layer cutting elements may be determined.  FIG. 6A  illustrates a schematic drawing for a bit face of drill bit  601  including first layer and second layer cutting elements  628  and  638  for which a CDCCC may be determined, in accordance with some embodiments of the present disclosure.  FIG. 6B  illustrates a schematic drawing for a bit face profile of drill bit  601  of  FIG. 6A , in accordance with some embodiments of the present disclosure. To provide a frame of reference,  FIG. 6B  includes a z-axis that may represent the rotational axis of drill bit  601 . Accordingly, a coordinate or position corresponding to the z-axis of  FIG. 6B  may be referred to as an axial coordinate or axial position of the bit face profile depicted in  FIG. 6B .  FIG. 6B  also includes a radial axis (R) that indicates the orthogonal distance from the rotational axis, of drill bit  601 . 
     Additionally, a location along the bit face of drill bit  601  shown in  FIG. 6A  may be described by x and y coordinates of an xy-plane of  FIG. 6A . The xy-plane of  FIG. 6A  may be substantially perpendicular to the z-axis of  FIG. 6B  such that the xy-plane of  FIG. 6A  may be substantially perpendicular to the rotational axis of drill bit  601 . Additionally, the x-axis and y-axis of  FIG. 6A  may intersect each other at the z-axis of  FIG. 6B  such that the x-axis and y-axis may intersect each other at the rotational axis of drill bit  601 . 
     The distance from the rotational axis of the drill bit  601  to a point in the xy-plane of the bit face of  FIG. 6A  may indicate the radial coordinate or radial position of the point on the bit face profile depicted in  FIG. 6B . For example, the radial coordinate, r, of a point in the xy-plane having an x-coordinate, x, and a y-coordinate, y, may be expressed by the following equation:
 
 r =√{square root over (x 2   +y   2 )}.
 
     Additionally, a point in the xy-plane (of  FIG. 6A ) may have an angular coordinate that may be an angle between a line extending orthogonally from the rotational axis of drill bit  601  to the point and the x-axis. For example, the angular coordinate (θ) of a point on the xy-plane (of  FIG. 6B ) having an x-coordinate, x, and a y-coordinate, y, may be expressed by the following equation:
 
θ=arctan( y/x ).
 
     As a further example, as illustrated in  FIG. 6A , cutlet point  630   a  (described in further detail below) associated with a cutting edge of first layer cutting element  628   a  may have an x-coordinate (X 630a ) and a y-coordinate (Y 630a ) in the xy-plane. X 630a  and Y 630a  may be used to calculate a radial coordinate (R F ) of cutlet point  630   a  (e.g., R F  may be equal to the square root of X 630a  squared plus Y 630a  squared). R F  may accordingly indicate an orthogonal distance of cutlet point  630   a  from the rotational axis of drill bit  601 . 
     Additionally, cutlet point  630   a  may have an angular coordinate (θ 630a ) that may be the angle between the x-axis and the line extending orthogonally from the rotational axis of drill bit  601  to cutlet point  630   a  (e.g., θ 630a  may be equal to arctan (X 630a /Y 630a )). Further, as depicted in  FIG. 6B , cutlet point  630   a  may have an axial coordinate (Z 630a ) that may represent a position of cutlet point  630   a  along the rotational axis of drill bit  601 . 
     The cited coordinates and coordinate systems are used for illustrative purposes only, and any other suitable coordinate system or configuration, may be used to provide a frame of reference of points along the bit face profile and bit face of a drill bit associated with  FIGS. 6A and 6B , without departing from the scope of the present disclosure. Additionally, any suitable units may be used. For example, the angular position may be expressed in degrees or in radians. 
     Returning to  FIG. 6A , drill bit  601  may include a plurality of blades  626  that may include cutting elements  628  and  638 . For example,  FIG. 6A  depicts an eight-bladed drill bit  601  in which blades  626  may be numbered 1-8. However, drill bit  601  may include more or fewer blades than shown in  FIG. 6A . Cutting elements  628  and  638  may be designated as either first layer cutting elements  628  or second layer cutting elements  638 . Each cutting element  628  or  638  may be referred to with an ending character, e.g., a-h, that corresponds to the blade, e.g., 1-8, on which the particular cutting element is located. For example, first layer cutting element  628   a  may be located on blade  1 . As another example, second layer cutting element  638   b  may be located on blade  2 . Second layer cutting elements  638  may be utilized to extend the life of drill bit  601  as first layer cutting elements  628  become worn. Second layer cutting elements  638  may be placed to overlap a radial swath of first layer cutting elements  628 . In other words, second layer cutting elements  638  may be located at the same radial position as associated first layer cutting elements  628  (e.g., second layer cutting elements  638  may be track set with respect to first layer cutting elements  628 ). Track set cutting elements have radial correspondence such that they are at the same radial position with respect to bit rotational axis  104 . Additionally, in some designs for drill bit  601 , second layer cutting elements  638  may not be configured to overlap the rotational path of first layer cutting elements  628 . Single set cutting elements may each have a unique radial position with respect to bit rotational axis  104 .  FIG. 6A  illustrates an example of a track set configuration in which first layer cutting elements  628   a  and second layer cutting elements  638   b  are located at the same radial distance from rotational axis  104 . 
     The critical depth of cut of drill bit  601  may be the point at which second layer cutting elements  638   b  begin to cut into the formation. Accordingly, the critical depth of cut of drill bit  601  may be determined for a radial location along drill bit  601 . For example, drill bit  601  may include a radial coordinate R F  that may intersect with the cutting edge of second layer cutting element  638   b  at control point P 640b . Likewise, radial coordinate R F  may intersect with the cutting edge of first layer cutting element  628   a  at cutlet point  630   a.    
     The angular coordinates of cutlet point  630   a θ   630a  and control point P 640b  θ P640b  may be determined. A critical depth of cut provided by control point P 640b  with respect to cutlet point  630   a  may be determined. The critical depth of cut provided by control point P 640b  may be based on the under-exposure (δ 640   b  depicted in  FIG. 6B ) of control point P 640b  with respect to cutlet point  630   a  and the angular coordinates of control point P 640b  with respect to cutlet point  630   a.    
     For example, the depth of cut at which second layer cutting element  638   b  at control point P 640b  may begin to cut formation may be determined using the angular coordinates of cutlet point  630   a  and control point P 640b  (θ 630a  and θ P640b , respectively), which are depicted in  FIG. 6A . Additionally, Δ 630a  may be based on the axial under-exposure (δ 640b ) of the axial coordinate of control point P 640b  (Z P640b ) with respect to the axial coordinate of cutlet point  630   a  (Z 630a ), as depicted in  FIG. 6B . In some embodiments, Δ 630a  may be determined using the following equations:
 
Δ 630a δ 640b *360/(360−(θ P640b −θ 630a )); and
 
δ 640b   =Z   630a   −Z   P640b .
 
     In the first of the above equations, θ P640b  and θ 630a  may be expressed in degrees and “360” may represent a full rotation about the face of drill bit  601 . Therefore, in instances where θ P640b  and θ 630a  are expressed in radians, the numbers “360” in the first of the above equations may be changed to “2π” Further, in the above equation, the resultant angle of “(θ P640b  and θ 630a )” (Δ θ ) may be defined as always being positive. Therefore, if resultant angle Δ 0  is negative, then Δ θ  may be made positive by adding 360 degrees (or  2 π radians) to Δ 0 . Similar equations may be used to determine the depth of cut at which second layer cutting element  638   a  at control point P 640b  (Δ 630a ) may begin to cut formation in place of first layer cutting element  628   a.    
     The critical depth of cut provided by control point P 640b  (Δ P640b ) may be based on additional cutlet points along R F  (not expressly shown). For example, the critical depth of cut provided by control point P 640b  (Δ P640b ) may be based the maximum of Δ 630a , Δ 630c , Δ 630e , and Δ 630g  and may be expressed by the following equation:
 
Δ P640b =max[Δ 630a , Δ 630c , Δ 630e , Δ 630g ].
 
     Similarly, the critical depth of cut provided by additional control points (not expressly shown) at radial coordinate R F  may be similarly determined. For example, the overall critical depth of cut of drill bit  601  at radial coordinate R F  (Δ RF ) may be based on the minimum of Δ P640b , Δ P640d , Δ P640f , Δ P640h  and may be expressed by the following equation:
 
Δ RF =min[Δ P640b , Δ P640d , Δ P640f , Δ P640h ].
 
     Accordingly, the critical depth of cut of drill bit  601  at radial coordinate R F  (ΔRF) may be determined based on the points where first layer cutting elements  628  and second layer cutting elements  638  intersect R F . Although not expressly shown here, it is understood that the overall critical depth of cut of drill bit  601  at radial coordinate R F  (Δ RF ) may also be affected by control points P 626i  (not expressly shown in  FIGS. 6A and 6B ) that may be associated with blades  626  configured to control the depth of cut of drill bit  601  at radial coordinate R F . In such instances, a critical depth of cut provided by each control point P 626i  (Δ P626i ) may be determined. Each critical depth of cut Δ P626i  for each control point P 626i  may be included with critical depth of cuts Δ P626i  in determining the minimum critical depth of cut at R F  to calculate the overall critical depth of cut ΔR F  at radial location R F . 
     To determine a CDCCC of drill bit  601 , the overall critical depth of cut at a series of radial locations R f (Δ Rf ) anywhere from the center of drill bit  601  to the edge of drill bit  601  may be determined to generate a curve that represents the critical depth of cut as a function of the radius of drill bit  601 . In the illustrated embodiment, second layer cutting element  638   b  may be located in radial swath  608  (shown on  FIG. 6A ) defined as being located between a first radial coordinate R A  and a second radial coordinate R B . Accordingly, the overall critical depth of cut may be determined for a series of radial coordinates R f  that are within radial swath  608  and located between R A  and R B , as disclosed above. Once the overall critical depths of cuts for a sufficient number of radial coordinates R f  are determined, the overall critical depth of cut may be graphed as a function of the radial coordinates R f  as a CDCCC. 
     The cutting edges of first layer cutting element  628   a  may wear gradually with drilling distance. As a result the shape of cutting edges may be changed. The cutting edges of second layer cutting element  638   b  may also wear gradually with drilling distance and the shape of second layer cutting element  638   b  may also be changed. Therefore, both under-exposure δ 640   b  and angle (θ P640b -θ P630a ) between cutlet point  630   a  and control point P 640b  may be changed. Thus, the critical depth of cut for a drill bit may be a function of the wear of both first layer and second layer cutting elements. At each drilling depth, a critical depth of cut for a drill bit may be estimated if wear of the cutting elements are known 
     Modifications, additions or omissions may be made to  FIGS. 6A and 6B  without departing from the scope of the present disclosure. For example, as discussed above, blades  626 , cutting elements  628  and  638 , DOCCs (not expressly shown) or any combination thereof may affect the critical depth of cut at one or more radial coordinates and the CDCCC may be determined accordingly. Further, the above description of the CDCCC calculation may be used to determine a CDCCC of any suitable drill bit. 
       FIG. 7A  illustrates a flow chart of an example method  700  for determining and generating a CDCCC in accordance with some embodiments of the present disclosure. The steps of method  700  may be performed at each specified drilling depth where cutter wear is measured or estimated. The steps of method  700  may be performed by various computer programs, models or any combination thereof, configured to simulate and design drilling systems, apparatuses and devices. The programs and models may include instructions stored on a computer readable medium and operable to perform, when executed, one or more of the steps described below. The computer readable media may include any system, apparatus or device configured to store and retrieve programs or instructions such as a hard disk drive, a compact disc, flash memory or any other suitable device. The programs and models may be configured to direct a processor or other suitable unit to retrieve and execute the instructions from the computer readable media. Collectively, the computer programs and models used to simulate and design drilling systems may be referred to as a “drilling engineering tool” or “engineering tool.” 
     In the illustrated embodiment, the cutting structures of the drill bit, including at least the locations and orientations of all cutting elements and DOCCs, may have been previously designed. However in other embodiments, method  700  may include steps for designing the cutting structure of the drill bit. For illustrative purposes, method  700  is described with respect to drill bit  601  of  FIGS. 6A and 6B ; however, method  700  may be used to determine the CDCCC of any suitable drill bit including bits with worn cutting elements at any drilling depth. 
     Method  700  may start, and at step  702 , the engineering tool may select a radial swath of drill bit  601  for analyzing the critical depth of cut within the selected radial swath. In some instances the selected radial swath may include the entire face of drill bit  601  and in other instances the selected radial swath may be a portion of the face of drill bit  601 . For example, the engineering tool may select radial swath  608  as defined between radial coordinates R A  and R B  and may include second layer cutting element  638   b , as shown in  FIGS. 6A and 6B . 
     At step  704 , the engineering tool may divide the selected radial swath (e.g., radial swath  608 ) into a number, Nb, of radial coordinates (R f ) such as radial coordinate R F  described in  FIGS. 6A and 6B . For example, radial swath  608  may be divided into nine radial coordinates such that Nb for radial swath  608  may be equal to nine. The variable “f” may represent a number from one to Nb for each radial coordinate within the radial swath. For example, “R 1 ” may represent the radial coordinate of the inside edge of a radial swath. Accordingly, for radial swath  608 , “R 1 ” may be approximately equal to R A . As a further example, “R Nb ” may represent the radial coordinate of the outside edge of a radial swath. Therefore, for radial swath  608 , “R Nb ” may be approximately equal to R B . 
     At step  706 , the engineering tool may select a radial coordinate R f  and may identify control points (P 1 ) at the selected radial coordinate R f  and associated with a DOCC, a cutting element, and/or a blade. For example, the engineering tool may select radial coordinate R F  and may identify control point P 640b  associated with second layer cutting element  638   b  and located at radial coordinate R F , as described above with respect to  FIGS. 6A and 6B . 
     At step  708 , for the radial coordinate R f  selected in step  706 , the engineering tool may identify cutlet points (C j ) each located at the selected radial coordinate R f  and associated with the cutting edges of cutting elements. For example, the engineering tool may identify cutlet point  630   a  located at radial coordinate R F  and associated with the cutting edges of first layer cutting element  628   a  as described and shown with respect to  FIGS. 6A and 6B . 
     At step  710  the engineering tool may select a control point P i  and may calculate a depth of cut for each cutlet point C j  as controlled by the selected control point P i  (Δ Cj ). For example, the engineering tool may determine the depth of cut of cutlet point  630   a  as controlled by control point P 640b  (Δ 630a ) by using the following equations:
 
Δ 630a =δ 640b *360/(360−(θ P640b −θ 630a )); and
 
δ 640b   =Z   630a   −Z   P640b .
 
     At step  712 , the engineering tool may calculate the critical depth of cut provided by the selected control point (Δ Pi ) by determining the maximum value of the depths of cut of the cutlet points C j  as controlled by the selected control point P i  (Δ Cj ) and calculated in step  710 . This determination may be expressed by the following equation:
 
Δ Pi =max{Δ C   j }.
 
     For example, control point P 340a  may be selected in step  710  and the depths of cut for cutlet point  630   a ,  630   c ,  630   e , and  630   g  (not expressly shown) as controlled by control point P 640b  (Δ 630a , Δ 630c , Δ 630e , and Δ 630g , respectively) may also be determined in step  710 , as shown above. Accordingly, the critical depth of cut provided by control point P 640b  (Δ P640b ) may be calculated at step  712  using the following equation:
 
Δ P640b =max [Δ 630a , Δ 630c , Δ 630e , Δ 630g ].
 
     The engineering tool may repeat steps  710  and  712  for all of the control points R identified in step  706  to determine the critical depth of cut provided by all control points P i  located at radial coordinate R f . For example, the engineering tool may perform steps  710  and  712  with respect to control points P 640c , P 640e , and P 640g  (not expressly shown) to determine the critical depth of cut provided by control points P 640c , P 640e , and P 640g  with respect to cutlet points  630   a ,  630   c ,  630   e , and  630   g  (not expressly shown) at radial coordinate R F  shown in  FIGS. 6A and 6B . 
     At step  714 , the engineering tool may calculate an overall critical depth of cut at the radial coordinate R f  (ΔR f ) selected in step  706 . The engineering tool may calculate the overall critical depth of cut at the selected radial coordinate R f  (ΔR f ) by determining a minimum value of the critical depths of cut of control points P i  (Δ Pi ) determined in steps  710  and  712 . This determination may be expressed by the following equation:
 
Δ Rf =min {Δ Pi} 
 
     For example, the engineering tool may determine the overall critical depth of cut at radial coordinate R F  of  FIGS. 6A and 6B  by using the following equation:
 
Δ RF =min [Δ P640b , Δ P640d , Δ P640f , Δ P640h].  
 
The engineering tool may repeat steps  706  through  714  to determine the overall critical depth of cut at all the radial coordinates R f  generated at step  704 .
 
     At step  716 , the engineering tool may plot the overall critical depth of cut (ΔR f ) for each radial coordinate R f , as a function of each radial coordinate R f . Accordingly, a CDCCC may be calculated and plotted for the radial swath associated with the radial coordinates R f . For example, the engineering tool may plot the overall critical depth of cut for each radial coordinate R f  located within radial swath  608 , such that the CDCCC for swath  608  may be determined and plotted, as depicted in  FIG. 5 . Following step  716 , method  700  may end. Accordingly, method  700  may be used to calculate and plot a CDCCC of a drill bit. The CDCCC may be used to determine whether the drill bit provides a substantially even control of the depth of cut of the drill bit. Therefore, the critical CDCCC may be used to modify the DOCCs, second layer cutting elements, and/or blades of the drill bit configured to control the depth of cut of the drill bit or configured to cut into the formation when first layer cutting elements are sufficiently worn in order to maximize drilling efficiency and bit life. 
     Method  700  may be repeated at any specified drilling depth where cutting element wear may be estimated or measured. The minimum of the CDCCC at each specified drilling depth may represent the critical depth of cut of the drill bit. Additionally, modifications, additions, or omissions may be made to method  700  without departing from the scope of the present disclosure. For example, the order of the steps may be performed in a different manner than that described and some steps may be performed at the same time. Additionally, each individual step may include additional steps without departing from the scope of the present disclosure. 
     Accordingly,  FIG. 7B  illustrates a graph of a CDCCC where the critical depth of cut is plotted as a function of the bit radius of drill bit  601  of  FIG. 6A , in accordance with some embodiments of the present disclosure. As mentioned above, a CDCCC may be used to determine the minimum critical depth of cut control as provided by the second layer cutting elements and/or blades of a drill bit. For example,  FIG. 7B  illustrates a CDCCC for drill bit  601  between radial coordinates R A  and R B . The z-axis in  FIG. 7B  may represent the critical depth of cut along the rotational axis of drill bit  601 , and the radial (R) axis may represent the radial distance from the rotational axis of drill bit  601 . For example, at a given under-exposure δ 640b  for second layer cutting element  638   b  and control points P 640b  of approximately 0.03 inches and a configuration shown in  FIG. 6A  (e.g., when second layer cutting element  638   b  is one blade  626  in front of first layer cutting element  628   a ), the critical depth of cut Δ 630a  is approximately 0.03246 in/rev. 
     The equation detailed above for critical depth of cut for first layer cutting elements  628   i  with cutlet points  630   i  may be rewritten more generally as:
 
Δ 630i =δ 640   i* 360/(360−(θ P640i −θ 630i ); and
 
δ 640i   =Z   630   −Z   P640i .
 
     If the angular locations of cutlet points  630   i  (θ 630i ) are fixed, then critical depth of cut, Δ 630i , becomes a function of two variables: under-exposure of second layer cutting elements at control points P 640i  (δ 640i ) and angular location of second layer cutting elements at control points P 640i  (θ 640i ). Thus, the equation for critical depth of cut, Δ 630i , may be rewritten as:
 
Δ 630i =δ 640i   *f (θ P640i )
 
The first variable, under-exposure of second layer cutting elements at control point P 640i  (δ 640i ), may be determined by the wear depth of first layer cutting elements  628 . Thus, an estimate of the wear depth of first layer cutting elements  628  may be determined as a function of drilling depth.
 
     Additionally, the second variable, f(θ P640i ), may be written as:
 
 f (θ P640i )=360/(360−(θ P640i −θ 630i )).
 
     Further, (θ P640i −θ 630i ) may vary from approximately 10 to 350 degrees for most drill bits. Thus, f(θ P640i ) may vary from approximately 1.0286 to approximately 36. The above analysis illustrates that f(θ p640i ) may act as an amplifier to critical depth of cut Δ 630i . Therefore, for a given under-exposure δ 640i , it may be possible to choose an angular location to meet a required critical depth of cut Δ 630i . 
       FIGS. 8A-8I  illustrate schematic drawings of bit faces of drill bit  801  with exemplary placements for second layer cutting elements  838 , in accordance with some embodiments of the present disclosure. For purposes of this disclosure, blades  826  may be numbered 1-n based on the blade configuration. For example,  FIGS. 8A-8I  depict eight-bladed drill bits  801   a - 801   i  and blades  826  may be numbered 1-8. However, drill bit  801   a - 801   i  may include more or fewer blades than shown in  FIGS. 8A-8I  without departing from the scope of the present disclosure. For an eight-bladed drill bit, blades  1 ,  3 ,  5  and  7  may be primary blades, and  2 ,  4 ,  6  and  8  may be secondary blades. Thus, there may be four possible blades  826  for placement of second layer cutting elements  838  in accordance with some embodiments of the present disclosure. Selection of the configuration of drill bit  801  may be based on the characteristics of the formation to be drilled and corresponding configuration of second layer cutting elements, e.g., under-exposure and/or blade location (as discussed below with reference to Table 1). In  FIGS. 8A-8D , first layer cutting element  828   a  with cutlet point  830   a  may be located on blade  1  and first layer cutting element  828   c  may be located on blade  3 . Cutting elements  828   a  and  828   c  may be single set. 
       FIG. 8A  illustrates second layer cutting element  838   b  and control point P 840b  located on blade  2  of drill bit  801   a  such that second layer cutting element  838   b  may be track set with first layer cutting element  828   a . Second layer cutting element  838   d  may be located on blade  4  and may be track set with first layer cutting element  828   c . Because second layer cutting elements are located on the blade rotationally in front of the corresponding first layer cutting element, drill bit  801   a  may be described as front track set. 
       FIG. 8B  illustrates second layer cutting element  838   h  and control point P 840h  located on blade  8  of drill bit  801   b  such that second layer cutting element  838   h  may be track set with first layer cutting element  828   a . Second layer cutting element  838   b  may be located on blade  2  and may be track set with first layer cutting element  828   c . Because second layer cutting elements are located on the blade rotationally behind the corresponding first layer cutting element, drill bit  801   b  may be described as behind track set. 
       FIG. 8C  illustrates second layer cutting element  838   f  and control point P 840f  located on blade  6  of drill bit  801   c  such that second layer cutting element  838   f  may be track set with first layer cutting element  828   a . Second layer cutting element  838   h  may be located on blade  8  and may be track set with first layer cutting element  828   c.    
       FIG. 8D  illustrates second layer cutting element  838   d  and control point P 840d  located on blade  4  of drill bit  801   d  such that second layer cutting element  838   d  may be track set with first layer cutting element  828   a . Second layer cutting element  838   f  may be located on blade  6  and may be track set with first layer cutting element  828   c.    
     In  FIG. 8E , first layer cutting element  828   a  with cutlet point  830   a  may be located on blade  1  of drill bit  801   e  and first layer cutting element  828   c  may be located on blade  3  such that cutting element  828   c  may be track set with first layer cutting element  828   a . First layer cutting elements  828   e  and  828   g  located on blades  5  and  7 , respectively, may also be track set. Second layer cutting elements  838   b  and  838   d , located on blades  2  and  4 , respectively, may be track set with first layer cutting elements  828   a  and  828   c . Second layer cutting elements  838   f  and  838   h , located on blades  6  and  8 , respectively, may be track set with first layer cutting elements  828   e  and  828   g . Second layer cutting element  838   b  may include control point P 840b . As such, cutting elements on blades  1 - 4  may be track set (more specifically, front track set), and cutting elements on blades  5 - 8  may be track set. 
     In  FIG. 8F , first layer cutting element  828   a  with cutlet point  830   a  may be located on blade  1  of drill bit  801   f . First layer cutting element  828   g  may be located on blade  7  and may be track set with first layer cutting element  828   a . First layer cutting elements  828   c  and  828   e  located on blades  3  and  5 , respectively, may also be track set. Second layer cutting elements  838   f  and  838   h , located on blades  6  and  8 , respectively, may be track set with first layer cutting elements  828   a  and  828   g . Second layer cutting elements  838   b  and  838   d , located on blades  2  and  4 , respectively, may be track set with first layer cutting elements  828   c  and  828   e . Second layer cutting element  838   h  may include control point P 840h . As such, cutting elements on blades  2 - 5  may be track set (more specifically, back track set), and cutting elements on blades  1  and  6 - 8  may be track set. 
       FIG. 8G  illustrates first layer cutting element  828   a  with cutlet point  830   a  located on blade  1  of drill bit  801   g . First layer cutting element  828   e  may be located on blade  5  and may be track set with first layer cutting element  828   a . First layer cutting elements  828   c  and  828   g  located on blades  3  and  7 , respectively, may also be track set. Second layer cutting elements  838   b  and  838   f , located on blades  2  and  6 , respectively, may be track set with first layer cutting elements  828   a  and  828   e . Second layer cutting elements  838   d  and  838   h , located on blades  4  and  8 , respectively, may be track set with first layer cutting elements  828   c  and  828   g . Second layer cutting element  838   b  may include control point P 840b . As such, cutting elements on blades  1 ,  2 ,  5  and  6  may be track set, and cutting elements on blades  3 ,  4 ,  7 , and  8  may be track set. 
       FIG. 8H  illustrates first layer cutting element  828   a  with cutlet point  830   a  located on blade  1  of drill bit  801   h . First layer cutting element  828   g  may be located on blade  7  and may be track set with first layer cutting element  828   a . First layer cutting elements  828   c  and  828   e  located on blades  3  and  5 , respectively, may also be track set. Second layer cutting elements  838   d  and  838   h , located on blades  4  and  8 , respectively, may be track set with first layer cutting elements  828   a  and  828   g . Second layer cutting elements  838   b  and  838   f , located on blades  2  and  6 , respectively, may be track set with first layer cutting elements  828   c  and  828   e . Second layer cutting element  838   d  may include control point P 840d . As such, cutting elements on blades  1 ,  4 ,  7  and  8  may be track set, and cutting elements on blades  2 ,  3 ,  5 ,  6  may be track set. 
       FIG. 8I  illustrates first layer cutting element  828   a  with cutlet point  830   a  located on blade  1  of drill bit  801   i . First layer cutting element  828   e  may be located on blade  5  and may be track set with first layer cutting element  828   a . First layer cutting elements  828   c  and  828   g  located on blades  3  and  7 , respectively, may also be track set. Second layer cutting elements  838   b  and  838   f , located on blades  2  and  6 , respectively, may be track set. Second layer cutting elements  838   d  and  838   h , located on blades  4  and  8 , respectively, may be track set. 
     For each of the angular locations of second layer cutting elements  838  shown in  FIGS. 8A-8I  and a given under-exposure  6840   i , critical depth of cut Δ 830i  may be calculated using method  700  shown in  FIG. 7A  or any other suitable method. Further, for a given critical depth of cut Δ 830i , under-exposure δ 840i  of second layer cutting elements  838  may be varied so that each of second layer cutting elements  838  engage the formation substantially simultaneously. 
       FIG. 9  illustrates graph  900  of CDCCC  910  where the critical depth of cut is plotted as a function of the bit radius for a bit where the second layer cutting elements have different under-exposures, in accordance with some embodiments of the present disclosure. In the illustrated embodiment, CDCCC  910  is generated for a drill bit configured with six second layer cutting elements track set with corresponding first layer cutting elements. The under-exposure of each of second layer cutting elements may be adjusted such that a target critical depth of cut may be achieved. For example, a target critical depth of cut may be specified as approximately 0.25 in/rev. In the illustrated embodiment, the under-exposure of each of second layer cutting elements  838 , which may be numbered 1-6 extending out from a bit rotational axis, may be adjusted such that each second layer cutting element 1-6 begins to cut into the formation at approximately 0.25 in/rev. 
       FIG. 10  illustrates a flow chart of example method  1000  for adjusting under-exposure of second layer cutting elements to approximate a target critical depth of cut, in accordance with some embodiments of the present disclosure. The steps of method  1000  may be performed by various computer programs, models or any combination thereof, configured to simulate and design drilling systems, apparatuses and devices. The programs and models may include instructions stored on a computer readable medium and operable to perform, when executed, one or more of the steps described below. The computer readable media may include any system, apparatus or device configured to store and retrieve programs or instructions such as a hard disk drive, a compact disc, flash memory or any other suitable device. The programs and models may be configured to direct a processor or other suitable unit to retrieve and execute the instructions from the computer readable media. Collectively, the computer programs and models used to simulate and design drilling systems may be referred to as a “drilling engineering tool” or “engineering tool.” 
     In the illustrated embodiment, the cutting structures of the drill bit, including at least the locations and orientations of all cutting elements and DOCCs, may have been previously designed. However in other embodiments, method  1000  may include steps for designing the cutting structure of the drill bit. For illustrative purposes, method  1000  is described with respect to drill bit  801   a  illustrated in  FIG. 8A ; however, method  1000  may be used to determine appropriate under-exposures of second layer cutting elements of any suitable drill bit. 
     Method  1000  may start, and at step  1004 , the engineering tool may determine a target critical depth of cut (Δ). The target may be based on formation characteristics, prior drill bit design and simulations, a CDCCC generated using method  700  shown in  FIG. 7 , or obtained from any other suitable method. For example, the engineering tool may determine a target critical depth of cut (Δ) of approximately 0.25 inches based on formation strength. 
     At step  1006 , the engineering tool may determine an initial under-exposure (δ) for second layer cutting elements. Initial under-exposure may be generated based on an existing drill bit design, formation characteristics, or any other suitable parameter. For example, initial under-exposure δ, for drill bit  801   a  may be defined as approximately 0.01 inches. 
     At step  1008 , the engineering tool may layout second layer cutting elements based on the initial under-exposure and a predetermined blade configuration. For example, drill bit  801   a  may have second layer cutting elements  838   b  configured on blade  2  and first layer cutting elements  828   a  configured on blade  1  as illustrated in  FIG. 8A . Second layer cutting elements may be track set with corresponding first layer cutting elements and under-exposed approximately 0.01 inches. 
     At step  1010 , the engineering tool may generate a CDCCC based on the initial second layer cutting element layout generated at step  1008 . The CDCCC may be generated based on method  700  shown in  FIG. 7  or any other suitable method. 
     At step  1012 , the engineering tool may analyze the CDCCC for each second layer cutting element and determine if the critical depth of cut for each second layer cutting element approximates the target critical depth of cut obtained in step  1004 . For example, at an initial given under-exposure of approximately 0.01 inches for the first second layer cutting elements, the critical depth of cut may be less than 0.25 in/rev. If a target critical depth of cut is approximately 0.25 in/rev, the under-exposure of the first second layer cutting element may be adjusted. Step  1012  may be repeated for all second layer cutting elements. 
     If all second layer cutting elements have a critical depth of cut that approximates the target critical depth of cut from step  1004 , the method ends. If any second layer cutting elements do not have a critical depth of cut that approximates the target critical depth of cut from step  1004 , then the method continues to step  1014 . 
     At step  1014 , the engineering tool may adjust the under-exposure of any second layer cutting elements that did not have a critical depth of cut that approximated the target critical depth of cut obtained in step  1004 . The process then returns to step  1008  until each of the second layer cutting elements achieves a critical depth of cut that approximates the target critical depth of cut obtained in step  1014 . For example, the under-exposure for each second layer cutting element 1-6 may be adjusted in order to approximate a target critical depth of cut of 0.25 inches. 
     Modifications, additions, or omissions may be made to method  1400  without departing from the scope of the present disclosure. For example, the order of the steps may be performed in a different manner than that described and some steps may be performed at the same time. Additionally, each individual step may include additional steps without departing from the scope of the present disclosure. 
     Table 1 illustrates example under-exposures for simulations performed for each of the drill bit  801  configurations illustrated in  FIGS. 8A-8I . The values in Table 1 are based on a given critical depth of cut equal to approximately 0.25 in/rev. The under-exposures of each of multiple second layer cutting elements were varied for each drill bit  801   a - 801   i  configuration shown in  FIGS. 8A-8I . The under-exposures in inches were ranked from minimum to maximum and the average under-exposure was calculated. 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                   
                 Minimum under- 
                 Maximum under- 
                 Average under- 
               
               
                 Drill bit 
                 exposure (inches) 
                 exposure (inches) 
                 exposure (inches) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 801a 
                 0.0775 
                 0.1787 
                 0.1426 
               
               
                 801b 
                 0.0313 
                 0.0537 
                 0.0410 
               
               
                 801c 
                 0.0627 
                 0.1106 
                 0.0868 
               
               
                 801d 
                 0.0775 
                 0.1699 
                 0.1350 
               
               
                 801e 
                 0.0313 
                 0.1669 
                 0.1012 
               
               
                 801f 
                 0.0313 
                 0.520 
                 0.0411 
               
               
                 801g 
                 0.0981 
                 0.1071 
                 0.1017 
               
               
                 801h 
                 0.0313 
                 0.1664 
                 0.0770 
               
               
                 801i 
                 0.0768 
                 0.1421 
                 0.1205 
               
               
                   
               
            
           
         
       
     
     For example, the average under-exposure for drill bit  801   a  shown in  FIG. 8A , in which the second layer cutting elements are positioned on blades rotationally in front of corresponding first layer cutting elements, may be approximately 0.1426 inches. As another example, the average under-exposure for drill bit  801   b  shown in  FIG. 8B , in which the second layer cutting elements are positioned on blades rotationally behind corresponding first layer cutting elements, may be approximately 0.0410 inches. Accordingly, the under-exposure for each second layer cutting element may be adjusted to achieve a critical depth of cut at which the second layer cutting elements may begin to cut into a formation. In other embodiments, the second layer cutting elements may be under-exposed by any suitable amount such that first layer cutting elements cut into the formation from a start point to a first drilling depth (D A ); the second layer cutting elements begin to partially cut into the formation at D A ; and the second layer cutting elements cut efficiently, as discussed with reference to  FIG. 5 . 
     In some applications, multiple bits may be utilized to drill a wellbore with multiple types of formations. For example, a drill bit with four blades may be utilized to drill into a first formation down to a particular depth. The four bladed drill bit may drill at approximately 120 RPM and a ROP of approximately 120 ft/hr. When the four bladed drill bit reaches a second formation, the cutting elements may be worn to a depth of approximately 0.025 inches. A different bit with eight blades may be utilized to drill into the second formation. In order to minimize the need to change from a four bladed to an eight bladed drill bit, a drill bit with eight blades may be designed to drill through both the first formation and the second formation. For example, first layer cutting elements, e.g., located on blades  1 ,  3 ,  5  and  7  shown with reference to  FIGS. 8A-8I , may be designed to cut into the first and second formations. Second layer cutting elements, e.g., located on blades  2 ,  4 ,  6  and  8 , may be designed to not contact the first formation and begin cutting when the drill bit reaches the second formation. For instance, second layer cutting elements may be designed to not cut under drilling conditions of approximately 120 RPM and ROP of approximately 120 ft/hr. Thus, second layer cutting elements may have a CDOC, Δ, of approximately 0.20 in/rev (120 ft/hr/(5*120 RPM)). Further, second layer cutting elements may have an under-exposure, 6, that is greater than approximately 0.025 inches, e.g., the wear depth of the first layer cutting elements when contacting the second formation. 
     In some embodiments, simulations may be conducted based on design parameters to determine a drill bit configuration, e.g., drill bits  801   a - 801   i  of  FIGS. 8A-8I , that meets the drilling requirements. For example, IBitS™ design software designed and manufactured by Halliburton Energy Services, Inc. (Houston, Tex.) may be utilized. For example, a behind track set configuration as shown in  FIG. 8B  may be selected for simulation. Selection of a drill bit configuration may be based on past simulation results, field results, calculated parameters, and/or any other suitable criteria. For example, selection of back track set drill bit configuration may be based on the average under-exposure shown in Table 1, above, with reference to drill bit  801   b . Parameters relating to the design may be input into the simulation software. A simulated layout may be generated and a determination may be made if the simulation meets the drilling requirements. For example, a simulation may be run with second layer cutting elements CDOC of approximately 0.20 in/rev, an RPM of approximately 120, and an ROP of approximately 120 ft/hr. The simulation may show that the second layer cutting elements under-exposure, δ, may be approximately 0.025 inches-0.040 inches. Thus, with a behind track set configuration, when first layer cutting elements are worn to between approximately 0.025 inches-0.040 inches, second layer cutting elements may begin to cut the formation. 
     As another example, a formation may exist that is relatively soft and abrasive. When drilling into a soft and abrasive formation, a drill bit with few blades, e.g., a four bladed drill bit, may be effective. An abrasive formation may wear cutting elements at a greater rate than a non-abrasive formation. Thus, when the cutting elements on a four bladed drill bit become worn, the drill bit may not drill as efficiently, e.g., experience a higher MSE. For example, cutting elements drilling into a formation at approximately 120 RPM and an ROP of approximately 90 ft/hr may have a wear depth of approximately 0.1 inches at a particular first drilling depth. Below the first drilling depth, a new four bladed drill bit may be utilized. In some embodiments, use of two layers of cutting elements on an eight bladed drill bit may improve the efficiency of a drill bit drilling into a soft and abrasive formation. For example, first layer cutting elements, e.g., located on blades  1 ,  3 ,  5  and  7  shown with reference to  FIGS. 8A-8I , may be designed to cut into the formation. Second layer cutting elements, e.g., located on blades  2 ,  4 ,  6  and  8 , may be designed to not contact the formation until a first drilling depth is reached. At that drilling depth, second layer cutting elements may begin cutting into the formation. For instance, second layer cutting elements may be designed to not cut under drilling conditions of approximately 120 RPM and ROP of approximately 90 ft/hr. Thus, second layer cutting elements may have a CDOC, Δ, of approximately 0.15 in/rev (90 ft/hr/(5*120 RPM)). Further, second layer cutting elements may have an under-exposure, δ, that is greater than approximately 0.1 inches, e.g., the wear depth of the first layer cutting elements when reaching the first drilling depth. 
     In some embodiments, a front track set configuration as shown in  FIG. 8A  may be selected for simulation. Selection of a drill bit configuration may be based on past simulation results, field results, calculated parameters, and/or any other suitable criteria. For example, selection of front track set drill bit configuration may be based on the average under-exposure shown in Table 1, above, with reference to drill bit  801   a . Parameters relating to the design may be input into the simulation software. A simulated layout may be generated and a determination may be made if the simulation meets the drilling requirements. For example, a simulation may be run with second layer cutting elements CDOC of approximately 0.15 in/rev. The simulation may show that the second layer cutting elements under-exposure, δ, may be approximately 0.085 inches-0.127 inches, with an average of approximately 0.109 inches. Thus, with a front track set configuration, when first layer cutting elements are worn to average approximately 0.109 inches, second layer cutting elements may begin to cut the formation. 
       FIG. 11  illustrates a flowchart of example method  1100  for performing a design update of a pre-existing drill bit with second layer cutting elements or configuring a new drill bit with second layer cutting elements, in accordance with some embodiments of the present disclosure. The steps of method  1100  may be performed by various computer programs, models or any combination thereof, configured to simulate and design drilling systems, apparatuses and devices. The programs and models may include instructions stored on a computer readable medium and operable to perform, when executed, one or more of the steps described below. The computer readable media may include any system, apparatus or device configured to store and retrieve programs or instructions such as a hard disk drive, a compact disc, flash memory or any other suitable device. The programs and models may be configured to direct a processor or other suitable unit to retrieve and execute the instructions from the computer readable media. Collectively, the computer programs and models used to simulate and design drilling systems may be referred to as a “drilling engineering tool” or “engineering tool.” 
     In the illustrated embodiments, the cutting structures of the drill bit, including at least the locations and orientations of all first layer cutting elements, may have been previously designed and bit run data may be available. However in other embodiments, method  1100  may include steps for designing the cutting structure of the drill bit. For illustrative purposes, method  1100  is described with respect to a pre-existing drill bit; however, method  1100  may be used to determine layout of second layer cutting elements of any suitable drill bit. Additionally, method  1100  may be described with respect to a designed drill bit similar in configuration to drill bit  801  as shown in  FIG. 8A-8I . 
     Method  1100  may start, and at step  1102 , the engineering tool may determine if a pre-existing drill bit exists that may be redesigned. If there is a pre-existing drill bit, method  1100  continues to step  1104 . If no pre-existing drill bit exists, method  1100  continues to step  1112 . 
     At step  1104 , the engineering tool may obtain run information for the pre-existing drill bit. For example,  FIG. 3  illustrates run information  300  for a pre-existing drill bit. As shown in  FIG. 3 , run information  300  may include RPM, ROP, MSE, and rock strength. 
     At step  1106 , the engineering tool may generate a plot of the actual depth of cut as a function of drilling depth for the pre-existing drill bit. For example,  FIG. 4B  illustrates an actual depth of cut plot as a function of drilling depth for a drill bit. 
     At step  1108 , the engineering tool may estimate the average first layer cutting element wear as a function of drilling depth of the pre-existing drill bit. For example,  FIG. 5  illustrates an estimate of first layer cutting element wear as a function of drilling depth for a drill bit. 
     At step  1110 , the engineering tool may generate a plot of the designed depth of cut as a function of drilling depth for second layer cutting elements of the pre-existing drill bit. The designed depth of cut may be based on the first layer cutting element wear estimated at step  1106 . For example,  FIG. 5  illustrates actual depth of cut, plot  530 , that begins at approximately 0.2 in/rev and as the first layer cutting elements wear, as shown in  FIG. 5 , the actual critical depth of cut may correspondingly decrease. 
     As noted above, if no pre-existing drill bit exists that may be redesigned at step  1102 , method  1100  may continue to step  1112 . At step  1112 , the engineering tool may obtain the expected drilling depth, D max , for the wellbore based upon exploration activities and/or a drilling plan. At step  1114 , the engineering tool may obtain the expected depth of cut as a function of drilling depth. For example,  FIG. 4A  may be generated based on expected RPM and expected ROP based on exploration activities and/or a drilling plan. 
     At step  1116 , the engineering tool may receive a cutting element wear model and may plot cutting element wear depth as a function of the drilling depth. For example,  FIG. 5  may represent the expected wear of first layer cutting elements based on a model generated by the equation:
 
Wear(%)=(Cumwork/BitMaxWork) a *100%
 
     where
 
Cumwork= f (drilling depth); and
 
 a =wear exponent and is between approximately 5.0 and 0.5.
 
     At this point in method  1100 , both step  1116  and step  1110  continue to step  1117 . At step  1117 , the engineering tool may determine an expected critical depth of cut for the second layer cutting elements. The critical depth of cut may be based on drilling parameters such as RPM and ROP. For example a critical depth of cut for second layer cutting elements for a drill bit operating at approximately 120 RPM with an ROP of 120 ft/hr may be approximately 0.20 in/rev. Additionally, second layer cutting elements may have an initial critical depth of cut that may be greater than the actual depth of cut or the expected depth of cut, as shown with reference to  FIG. 5 . Further, at a particular drilling distance, D A , second layer cutting element critical depth of cut, plot  520 , may intersect with the actual depth of cut, plot  530 . At a target drilling depth, second layer cutting element critical depth of cut, plot  520 , may be equal to approximately zero. 
     At step  1118 , the engineering tool may determine the drilling depth at which first layer cutting elements on the drill bit may be worn such that second layer cutting elements may begin to cut the formation based on bit wear and actual or expected ROP. This drilling depth may correspond to drilling depth D A . 
     At step  1120 , the engineering tool may determine the under-exposure of second layer cutting elements for the drill bit. The under-exposure may be approximately the amount of wear first layer cutting elements may have experienced while drilling to drilling depth D A . For example,  FIG. 5  illustrates an estimate of first layer cutting element wear as a function of drilling depth. Using D A  from step  1118  the engineering tool may determine the average under-exposure of second layer cutting elements as the amount of first layer cutting element wear at drilling depth D A . For example the under-exposure of second layer cutting elements may be determined to be greater than approximately 0.025 inches. The amount of underexposure may be further based on each second layer cutting element having an initial critical depth of cut greater than an actual depth of cut for a first drilling distance and a critical depth of cut equal to zero at a target drilling depth. At the target drilling depth or after a particular drilling distance, the first layer cutting elements may be worn such that at least one second layer cutting element may be cutting into the formation. 
     At step  1122 , the engineering tool may determine the optimal locations for second layer cutting elements and first layer cutting elements disposed on the drill bit. For example, based on the critical depth of cut for the second layer cutting elements and the under-exposure, a drill bit configuration may be selected from Table 1 shown above. As another example, the engineering tool may run multiple simulations to generate run information. Based on results of these simulations, the engineering tool may determine blade locations for both first layer cutting elements and second layer cutting elements. 
     At step  1124 , the engineering tool may determine if the second layer cutting elements begin to cut formation at drilling depth D A . For example, the engineering tool may generate a designed critical depth of cut as a function of drilling depth for second layer cutting elements of the drill bit. The engineering tool may run a simulation of the cutting element layout determined in step  1122  to generate designed critical depth of cut as a function of drilling depth curve. For example, the engineering tool may determine that second layer cutting elements  838  may begin to cut into the formation at drilling depth D A  of approximately 5,000 feet. If second layer cutting elements do not begin to cut formation at drilling depth D A , the process  1100  may return to step  1118  to reconfigure drill bit  801 . If the second layer cutting elements begin to cut formation at drilling depth D A , then the process may continue to step  1126 . 
     Based on these results, at step  1126 , the engineering tool may adjust under-exposure of each second layer cutting element in order for each second layer cutting element to have the same minimal depth of cut of the new drill bit. Following step  1126 , method  1100  may end. 
     Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alternations can be made herein without departing from the spirit and scope of the disclosure as defined by the following claims. For example, although the present disclosure describes the configurations of blades and cutting elements with respect to drill bits, the same principles may be used to control the depth of cut of any suitable drilling tool according to the present disclosure. It is intended that the present disclosure encompasses such changes and modifications as fall within the scope of the appended claims.