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CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. Provisional Patent Application No. 60/227,049 filed Aug. 21, 2000. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Field of the Invention.  
           [0003]    The present invention relates generally to drill bits, and, more particularly, to multi-directional cutters for a fixed cutter, drillout bi-center bit.  
           [0004]    2. Description of the Related Art.  
           [0005]    In the pursuit of drilling boreholes into the earth for the recovery of minerals, there are instances when it is desirable to drill a borehole with a diameter larger than the bit itself. Drill bits used to form these boreholes are generally known as bi-center type drill bits.  
           [0006]    Bi-center drill bits are well known in the drilling industry. Various types of bi-center drill bits are described in U.S. Pat. Nos. 1,587,266, 1,758,773, 2,074,951, 2,953,354, 3,367,430, 4,408,669, 4,440,244, 4,635,738, 5,040,621, 5,052,503, 5,165,494, 5,678,644 and European Patent Application 0,058,061 all herein incorporated by reference.  
           [0007]    Modern bi-center drill bits are typically used in difficult drilling applications where the earth formations are badly fractured, where there is hole swelling, where the borehole has a tendency to become spiraled, or in other situations where an oversize hole is desirable.  
           [0008]    In these difficult drilling applications, the top portion of the well bore is often stabilized by setting and cementing casing. The cement, shoe, float, and related cementing hardware are then typically drilled out of the casing by a drill bit that is run into the casing for this purpose. Once the cement and related hardware are drilled out, the drillout bit is tripped out of the hole and a bi-center drill bit is run back into the borehole. Drilling then proceeds with the bi-center drill bit, which drills a hole into the formation below the casing with a diameter that is greater than the inside diameter of the casing.  
           [0009]    To reduce drilling expenses, attempts have been made to drill the cement and related hardware out of the casing, and then drill the formation below the casing with a single bi-center drill bit. These attempts often resulted in heavy damage to both the casing and the bi-center drill bit.  
           [0010]    The casing tends to be damaged by the gauge cutting elements mounted on the bi-center drill bit because inside the casing the pilot section of the bit is forced to orbit about its center, causing the gauge cutters to engage the casing. The forced orbiting action of the pilot section can also cause damage to the cutters on the leading face of the bi-center drill bit.  
           [0011]    As is well known in fixed cutter drill bits, the cutting elements have cutting faces which are precisely oriented relative to the direction of travel of the cutter through the formation being drilled. However, cutters located in an area generally between the passthrough center and the drilling center of the bit face of drillout bi-center bits experience two different directions of travel as they drill. One direction of travel occurs when the bit is drilling out, and the other direction of travel occurs when the bit is drilling the full diameter borehole. The cutters which lie in line between the two centers, in fact, experience exactly opposite directions of travel.  
           [0012]    As previously stated, this has caused severe damage to the cutters in this area in the past. The typical solution to this problem has been to leave this area of the face of the bit devoid of cutters. Unfortunately, in some bi-center bit designs, particularly bi-center bits with large differences between the passthrough diameter and the drilling diameter, leaving this region devoid of cutters may cause the drilling performance of the bit to suffer.  
           [0013]    The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.  
         SUMMARY OF INVENTION  
         [0014]    In one aspect of the present invention, there is provided a drillout bi-center drill bit comprising a bit body with a first end adapted to be detachably secured to a drill string, a pilot section on a second, opposite end of the bit body and a reamer section intermediate the first and second ends. There is a first drilling center of rotation of the pilot section and a second passthrough center of rotation of the pilot section spaced apart from the first center of rotation by a non-zero distance D. There is also a first region of the pilot section centered about the first center of rotation having a radius D, a second region of the pilot section centered about the second center of rotation having a radius D, and a third region of the pilot section formed by the intersection of the first region and the second region. A cutting element is fixed on the bit body within the third region. The cutting element has a first cutting face generally oriented perpendicular to the direction of travel of the cutting element about the first center of rotation of the pilot section and a second cutting face generally oriented perpendicular to the direction of travel of the cutting element about the second center of rotation.  
           [0015]    In another aspect of the present invention, there is provided a drillout bi-center drill bit comprising a bit body with a longitudinal axis and a first end adapted to be detachably secured to a drill string, a pilot section on a second, opposite end of the bit body and a reamer section intermediate the first and second ends. There is a first drilling center of rotation of the pilot section and a second passthrough center of rotation of the pilot section spaced apart from the first center of rotation by a non-zero distance D. There is a first region of the pilot section centered about the first center of rotation having a radius D, a second region of the pilot section centered about the second center of rotation having a radius D, and a third region of the pilot section formed by the intersection of the first region and the second region. There are a plurality of first cutters in the third region, with superhard cutting faces generally oriented perpendicular to the direction of travel of the cutting element about the first center of rotation, projecting a distance from the bit body. At least one second cutter is fixed on the bit body within the third region and projecting a distance from the bit body greater than the projection of the first cutters, with a cutting face oriented generally perpendicular to the direction of travel of the second cutter about the second center of rotation. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0016]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:FIG. 1 provides a perspective view of a drillout bi-center drill bit in accordance with one embodiment of the present invention; FIGS. 2A, 2B and  2 C show side views perspectives of the drillout bi-center drill bit of FIG. 1; FIG. 3 shows an end view perspective of the drillout bi-center drill bit of FIG. 1; FIG. 4A shows an end view perspective of the drillout bi-center drill bit of FIG. 1 illustrating the iris shaped third region; FIG. 4B shows a simplified end view of the iris shaped third region of the drillout bi-center drill bit of FIG. 4A; FIGS.  5 - 7  show perspective views of various cutting elements that are mounted in the iris shaped third region on the drillout bi-center drill bit in accordance with the present invention; FIG. 8 is a partial end view of the face of the drillout bi-center drill bit showing an alternate cutter arrangement in accordance with another embodiment of the present invention; FIG. 9 shows a perspective view of a cutting element of the embodiment of the drillout bi-center drill bit of FIG. 8; FIG. 10 shows a perspective view of still another cutter arrangement in accordance with another embodiment of the present invention.  
         [0017]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     
    
     DETAILED DESCRIPTION  
       [0018]    Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers” specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0019]    Turning now to the drawings, and specifically referring to FIGS. 1, 2A,  2 B, and  2 C, a drillout bi-center drill bit  10  having multi-directional cutters is shown in accordance with one embodiment of the present invention. The drillout bi-center drill bit  10  has a longitudinal axis  11  upon which the drill bit  10  rotates, and a bit body  12  with a first end  14  adapted to be secured to a drill string (not shown) for driving the drill bit  10 . According to one embodiment, threads  16  may be used for coupling the drill bit  10  to the drill string. However, it will be appreciated to those of ordinary skill in the art that various other forms of attachment may be used in lieu of the threads  16  without departing from the spirit and scope of the present invention. At a second, opposite end of the bit body  12  is a pilot section  18  of the drillout bi-center drill bit  10  with an exposed drilling face  17 . A reamer section, shown generally by numeral  20 , is intermediate the first end  14  and the pilot section  18  of the bi-center drill bit  10 .  
         [0020]    While in operation, the bit body  12  is rotated via the drill string by some external means while the drillout bi-center drill bit  10  is forced into the material being drilled. The rotation under load causes cutting elements  24  exposed at the drilling face  17  to penetrate into the drilled material and remove the material in a scraping and/or gouging fashion.  
         [0021]    In accordance with one embodiment of the present invention, the bit body  12  includes internal passaging (not shown) that allows pressurized drilling fluid to be supplied from the drilling surface to a plurality of nozzle orifices  22 . These nozzle orifices  22  discharge the drilling fluid to clean and cool the cutting elements  24  as they engage the material being drilled. The drilling fluid also functions to transport the drilled material to the surface for disposal.  
         [0022]    According to one embodiment, the pilot section  18  may have a section with at least one fluid passage  26  provided for return flow of the drilling fluid. There also may be other fluid passages  26  provided in the reamer section  20  of the drillout bi-center drill bit  10  as well.  
         [0023]    Referring specifically to FIGS. 2B and 2C, side view perspectives of the drillout bi-center drill bit  10  of the present invention are shown. One important characteristic of the drillout bi-center drill bit  10  is its ability to drill a borehole  11  into the earth  13  with a gauge drilling diameter larger than the inside diameter of the casing  15 , or pipe or other type of conductor the bit  10  passes through, which is shown in FIG. 2C.  
         [0024]    Another important characteristic of the of the drillout bi-center drill bit  10  is its ability to drill out cement  19  (and related hardware, not shown) inside the casing  15  as shown in FIG. 2B without causing damage to the casing  15  or the cutting elements  24  of the drill bit  10 .  
         [0025]    Turning now to FIG. 3, an end view of a drillout bi-center drill bit  10  of the present invention is shown. The gauge drilling diameter, as indicated by the circle  28 , is generated by radius R 1  from a first center of rotation  30  of the pilot section  18 . In this drilling mode, the circular portion of the pilot section  18  will be concentric with the diameter  28 . The cutting elements  24  on the portion of the reamer section  20  radially furthest from the first center of rotation  30  actually drills the gauge drilling diameter of the borehole  11 , as indicated at numeral  31 . The reamer section  20  is formed eccentrically of the pilot section  18 , so only a portion of the wall of the borehole  11  is in contact with the cutting elements  24 , which cut the final gauge of the borehole  11  at any given time during operation.  
         [0026]    The drillout bi-center drill bit  10  also has a passthrough diameter, as indicated by the circle  32 , generated by radius R 2  from a second center of rotation  34  of the pilot section  18 . The shortest linear distance at the face of the bit between the centers of rotation  30 ,  34  is indicated as non-zero distance D. The second center of rotation  34  is on the centerline of the smallest cylinder that may be fitted about the drillout bi-center drill bit  10 . To be effective, the passthrough diameter that is indicated by circle  32  must be smaller than the inside diameter of the casing  15  that the drillout bi-center drill bit  10  must pass through.  
         [0027]    For optimal life, the cutting elements  24  must be oriented on the pilot section  18  in a known manner with respect to the direction of scraping through the material being drilled. This is no problem for bi-center drill bits that do not drill the cement  19  and related hardware out of the casing. However, when a drillout bi-center drill bit is drilling the cement  19  and related hardware in the casing, some of the cutting elements  24  may be subjected to reverse scraping while rotating about the second center of rotation  34 . Reverse scraping often causes rapid degradation of the cutting elements  24 .  
         [0028]    The cutting elements  24  are typically polycrystalline diamond compact cutters or PDC. A PDC is typically comprised of a facing table of diamond or other superhard substance bonded to a less hard substrate material, typically formed of but not limited to, tungsten carbide. The PDC is then often attached by a method known as long substrate bonding to a post or cylinder for insertion into the bit body  12 . This PDC type of cutting element  24  is particularly sensitive to reverse scraping because loading from reverse scraping can easily destroy both the diamond table bonding and the long substrate bonding.  
         [0029]    In prior art drill bits, the cutting elements are typically configured with a single cutting surface, where the cutting surface is properly oriented to cut through material being drilled when the drill bit rotates around a first center of rotation, such as center of rotation  30 , for example. However, when the drillout bi-center drill bit  10  rotates around a second center of rotation, such as center of rotation  34 , for example, the cutting surface of the cutting element is not properly oriented to optimally cut through the drilled material. That is, when the cutting element is configured with this single cutting surface, the drill bit is optimally utilized while drilling around the first center of rotation, but is not optimally positioned to cut material when the drill bit rotates around a second center of rotation. With this particular prior art configuration, the cutting element will undesirably wear at a faster rate when the drill bit is rotating around the center of rotation where the single cutting surface of the cutting element is not optimally positioned to cut material. As a result of the cutting elements wearing at a faster rate, the life of the drill bit is undesirably shortened.  
         [0030]    As previously stated, the distance D is the shortest linear distance between center of rotation  30  and center of rotation  34 . As shown in FIG. 4A, a first region  56  of the pilot section  18 , centered about the first center of rotation  30 , has a radius D. A second region  58  of the pilot section  18  is centered about the second center of rotation  34 , and also has a radius D. A third region  60  of the pilot section  18  is formed by the intersection of the first region  56  and the second region  58 . This iris shaped third region  60  is the critical area where reverse cutter scraping is possible.  
         [0031]    Turning now to FIG. 4B, a perspective view of cutting elements of the embodiment of the drillout bi-center drill bit of FIG. 8 is shown. Three cutting elements  72 ,  74 ,  76  (FIGS.  5 - 7 ) of the present invention are shown in the iris shaped third region  60  between the drilling center of rotation  30  and the passthrough center of rotation  34 .  
         [0032]    Cutter  72  has two cutting faces  78 ,  80 . When the drillout bi-center drill bit  10  is rotating about the drilling center of rotation  30 , cutting face  80  of cutter  72  is properly oriented for cutting along the path indicated by arrow  82 . Cutting face  80  is generally oriented perpendicular to the direction of travel of the cutter  72  in this operating mode, which is parallel to dashed line  86  passing through about the drilling center of rotation  30 .  
         [0033]    In a similar manner, when the drillout bi-center drill bit  10  is rotating about the passthrough center of rotation  34 , cutting face  78  of cutter  72  is properly oriented for cutting along the path indicated by arrow  84 . Cutting face  78  is generally oriented perpendicular to the direction of travel of the cutter  72  in this operating mode, which is generally parallel to dashed line  88  passing through about the passthrough center of rotation  34 .  
         [0034]    Cutter  72  may be formed of any material suitable for drilling earth formations. Since the wear rate of cutting elements near the center of the bit is generally low, cemented tungsten carbide may be a suitable material. It is understood that during drillout operation, only a small amount of wear is likely to occur on cutting face  78  of cutter  72 . It would be expected that much more wear would occur on face  80  when the bit is drilling into the earth. If the wear rates are unacceptably high, the cutter  72  may be formed of an infiltrated material comprising metallic powders such as tungsten carbide mixed with diamond particles and a binder.  
         [0035]    Cutter  74  operates in a manner similar to cutter  72 , although as described later, cutter  74  is intended for much more abrasive drilling than cutter  72 . Cutter  74  has two cutting faces  90 ,  92 . When the drillout bi-center drill bit  10  is rotating about the drilling center of rotation  30 , cutting face  90  of cutter  74  is properly oriented for cutting along the path indicated by arrow  94 . Cutting face  90  is generally oriented perpendicular to the direction of travel of the cutter  74  in this operating mode which is parallel to dashed line  98  passing through about the drilling center of rotation  30 .  
         [0036]    In a similar manner, when the drillout bi-center drill bit  10  is rotating about the passthrough center of rotation  34 , cutting face  92  of cutter  74  is properly oriented for cutting along the path indicated by arrow  96 . Cutting face  92  is generally oriented perpendicular to the direction of travel of the cutter  74  in this operating mode which is parallel to dashed line  100  passing through about the passthrough center of rotation  34 .  
         [0037]    In order to survive severe, abrasive drilling conditions the cutting face  90  of cutter  74  is quite different from that of cutter  72 . A PDC cutting element  102  is mounted on cutting face  90  a small distance  104  from the end  106  of cutter  74  exposed at the drilling face  17  of the drillout bi-center drill bit  10 . During the drillout phase, a small amount of wear will occur on end  106 . After drillout, the bit will then start drilling a full diameter hole in the earth. However, in abrasive drilling conditions, even cutters near the center will wear rapidly. Therefore, the end  106  of cutter  74  will wear rapidly, exposing the PDC element  102 . Once this happens, the cutter will wear at a rate comparable to other PDC cutters near the center.  
         [0038]    In this embodiment, the PDC is attached to the cutter  74  by a method known as long substrate bonding. The cutter  74  is then inserted into the bit body  12 , which gives the PDCs an alternative orientation with respect to the center of rotation about which the drill bit  10  rotates.  
         [0039]    It should be apparent that cutters  72  and  74  will generally have different orientations of cutting faces  78 ,  80 ,  90 ,  92  depending where they are located within the iris shaped third region  60  between the drilling center of rotation  30  and the passthrough center of rotation  34 . Although some mismatch of cutting faces  78 ,  80 ,  90 ,  92  would be tolerated, allowing some commonality of cutting face orientations, many different configurations of cutters  72  and  74  would still be necessary for most drillout bi-center drill bits  10 .  
         [0040]    Another embodiment of the invention which can be placed anywhere in the iris shaped third region  60 , a cone shaped cutter  76  suitable for very non-abrasive drilling condition, is shown in FIG. 7. In cutter  76 , the side  108  is generally conic and may terminate in a flat top  110  that is also exposed at the drilling face  17 . Since these cutters  76  are generally symmetrical, they may be placed anywhere within the iris shaped third region  60  between the drilling center of rotation  30  and the passthrough center of rotation  34 . In this cutter  76 , the cutting edge  112  is the intersection of the side  108  and the flat top  110 . Since cutter  76  is generally symmetrical, both drillout and passthrough drilling are readily accomplished. Although not particularly “sharp”, cutter  76  is suitable for non-abrasive drilling conditions.  
         [0041]    Shown in FIG. 8 is a particular bit configuration where the cutting functions for drillout and full diameter drilling are embodied in separate cutters. Two drilling face sections  17  are shown on bit body  12 . A plurality of conventional cutters  24  is shown with arrows  114  indicating their path of rotation about the drilling center of rotation  30 . A plurality of cutters  116  (shown in FIG. 9) have cutting faces  118  oriented for drilling out, with arrows  119  indicating their path of rotation about the passthrough center of rotation  34 .  
         [0042]    The tops  120  of cutters  116  are orientated relatively farther from the bit body  12  than the remainder of cutters  24  on the drilling face section  17  of the drillout bi-center bit  10 . Therefore the cutting faces  118  of cutters  116  will engage the drillout material and prevent damage to cutters  24  during drillout. Once drillout is complete, the cutters  116  will rapidly wear, allowing the cutters  24  to drill normally. The operation is therefore effectively the same as cutter  74 . Cutter  116  may be formed of any material suitable for drilling earth formations. However, similar to cutter  74 , a cemented tungsten carbide material or an infiltrated material comprising metallic powders such as tungsten carbide mixed with diamond particles and a binder is suitable. In this embodiment and similar to cutters  72 ,  74  and  76  the cutter  116  is oriented as necessary then fixed into the bit body  12 .  
         [0043]    An alternate embodiment for the arrangement of the drillout cutters shown in FIG. 8 is possible when the bit body  12  is an infiltrated powdered metal matrix material. When this is the case, the cutter  122  is formed as a bump in the matrix of the bit body  12 . The cutting face  118 , and top  120 , of cutter  122  function identically to cutter  116 . However, because matrix bits are made in a molding process, orienting and fixing the cutters  122  into the bit body  12  is not necessary. Cutter  122  is integral with the bit body. Methods of construction of matrix drill bits are well known in the art. Accordingly, the specific details of such will not be disclosed herein to avoid unnecessarily obscuring the present invention.  
         [0044]    The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below.  
         [0045]    Whereas the present invention has been described in particular relation to the drawings attached hereto, it should be understood that other and further modifications apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.

Summary:
A drillout bi-center bit includes first and second axes of rotation separated by a distance D, a first region of radius D centered on one of the axes of rotation, a second region of radius D centered on the other of the axes of rotation, and a third region defined by the overlap between the first and second regions, wherein the cutters provided within the third region are designed to withstand cutting in opposing cutting directions. This may be achieved by providing each cutter with two cutting faces, or by providing two groups of cutters, one group arranged to cut in one direction, the other group being designed to cut upon rotation in the other direction, one of the groups protruding from the bit body of the bit to a greater depth than the other group.