Patent Publication Number: US-6659207-B2

Title: Bi-centered drill bit having enhanced casing drill-out capability and improved directional stability

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of application Ser. No. 09/345,688 filed on Jun. 30, 1999 now U.S. Pat. No. 6,269,893 and assigned to the assignee of the present invention. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates generally to the field of fixed cutter drill bits used to drill wellbores through earth formations. More specifically, the invention relates to bi-center drill bits which drill a hole larger in diameter than the diameter of an opening through which such bits may freely pass. 
     2. Background Art 
     Drill bits which drill holes through earth formations where the hole has a larger diameter than the bit&#39;s pass-through diameter (the diameter of an opening through which the bit can freely pass) are known in the art. Early types of such bits included so-called “underreamers”, which were essentially a drill bit having an axially elongated body and extensible arms on the side of the body which reamed the wall of the hole after cutters on the end of the bit had drilled the earth formations. Mechanical difficulties with the extensible arms limited the usefulness of underreamers. 
     More recently, so-called “bi-centered” drill bits have been developed. A typical bi-centered drill bit includes a “pilot” section located at the end of the bit, and a “reaming” section which is typically located at some axial distance from the end of the bit (and consequently from the pilot section). One such bi-centered bit is described in U.S. Pat. No. 5,678,644 issued to Fielder, for example. Bi-centered bits drill a hole larger than their pass through diameters because the axis of rotation of the bit is displaced from the geometric center of the bit. This arrangement enables the reaming section to cut the wall of the hole at a greater radial distance from the rotational axis than is the radial distance of the reaming section from the geometric center of the bit. The pilot section of the typical bi-centered bit includes a number of PDC cutters attached to structures (“blades”) formed into or attached to the end of the bit. The reaming section is, as already explained, typically spaced axially away from the end of the bit, and is also located to one side of the bit. The reaming section also typically includes a number of PDC inserts on blades on the side of the bit body in the reaming section. 
     Limitations of the bi-centered bits known in the art include the pilot section being axially spaced apart from the reaming section by a substantial length. FIG. 1 shows a side view of one type of bi-center bit known in the art, which illustrates this aspect of prior art bi-center bits. The bi-center bit  101  includes a pilot section  106 , which includes pilot blades  103  having PDC inserts  110  disposed thereon, and includes gauge pads  112  at the ends of the pilot blades  103  axially distant from the end of the bit  101 . A reaming section  107  can include reaming blades  111  having PDC inserts  105  thereon and gauge pads  117  similar to those on the pilot section  106 . In the bi-center bit  101  known in the art, the pilot section  106  and reaming section are typically separated by a substantial axial distance, which can include a spacer or the like such as shown at  102 . Spacer  102  can be a separate element or an integral part of the bit structure but is referred to here as a “spacer” for convenience. As is conventional for drill bits, the bi-center bit  101  can include a threaded connector  104  machined into its body  114 . The body  114  can include wrench flats  115  or the like for make up to a rotary power source such as a drill pipe or hydraulic motor. 
     An end view of the bit  101  in FIG. 1 is shown in FIG.  2 . The blades  108 A in the pilot section and the blades  111 B in the reaming section are typically straight, meaning that the cutters  110  are disposed at substantially the same relative azimuthal position on each blade  108 A,  111 B. In some cases, the blades  108 A in the pilot section  106  may be disposed along the same azimuthal direction as the blades  111 B in the reaming section  110 . 
     Prior art bi-center bits are typically “force-balanced”; that is, the lateral force exerted by the reaming section  110  during drilling is balanced by a designed-in lateral counterforce exerted by the pilot section  106  while drilling is underway. However, the substantial axial separation between the pilot section  106  and the reaming section  110  results in a turning moment against the axis of rotation of the bit, because the force exerted by the reaming section  110  is only balanced by the counterforce (exerted by pilot section  106 ) at a different axial position. This turning moment can, among other things, make it difficult to control the drilling direction of the hole through the earth formations. 
     Still another limitation of prior art bi-centered bits is that the force balance is calculated by determining the net vector sum of forces on the reaming section  110 , and designing the counterforce at the pilot section  106  to offset the net vector force on the reaming section without regard to the components of the net vector force originating from the individual PDC inserts. Some bi-center bits designed according to methods known in the art can have unforeseen large lateral forces, reducing directional control and drilling stability. 
     A bi-center bit such has shown in U.S. patent application Ser. No. 09/345,688 filed on Jun. 30, 1999 and assigned to the assignee of the present invention avoids a number of limitations of prior art bi-center drill bits. It has been observed, however, that even these bi-center bits are subject to “dropping angle” during directional drilling operations, meaning that they have a tendency to turn the direction of a directionally drilled wellbore back toward vertical. Further, some of the cutting elements on these bits may move in a direction counter to the direction of rotation of the bit about its “pass-through” axis when the bit is used to drill out float equipment and is thus constrained to rotate in an opening having about the “pass-through” diameter of the bit. 
     SUMMARY OF INVENTION 
     One aspect of the invention is a bi-center drill bit including a bit body having pilot blades and reaming blades thereon distributed azimuthally around the bit body. Selected ones of the blades have cutting elements attached to them at selected locations. Selected ones of the blades include, longitudinally between the pilot blades and the reaming blades, a pilot hole conditioning section. The pilot hole conditioning section on each of the selected blades includes a gage face. The gage faces together define a diameter intermediate a pilot hole diameter and a pass-through diameter defined, respectively, by the pilot blades and the reaming blades. 
     Another aspect of the invention is a bi-center bit having at least one cutting element disposed in a portion of a pilot section thereof which has a cutting surface oriented to cut earth formation when moving in a direction substantially opposite a direction of rotation of the bit. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows a side view of a prior art bi-center drill bit. 
     FIG. 2 shows an end view of a prior art bi-center drill bit. 
     FIG. 3 shows an oblique view of one embodiment of a bi-center drill bit. 
     FIG. 4 shows an end view of one embodiment of the drill bit of FIG.  3 . 
     FIG. 5 shows a side view of one embodiment of a bi-center drill bit. 
     FIG. 6 shows an end view of one embodiment of the bit wherein additional cutters are attached to pilot blades near a precession circle. 
     FIG. 7 shows a side view of locations of cutters on one of the blades in the embodiment of the bit shown in FIG.  6 . 
     FIG. 8 illustrates an area on a bi-center bit susceptible to reverse rotation during drill out of a casing. 
     FIG. 9 shows a profile view of a blade structure including one embodiment of an aspect of the invention called a pilot hole conditioning section. 
     FIG. 10 shows an example of a “seesaw”, or bidirectional cutter which may be used in some embodiments of a bi-center drill bit. 
     FIG. 11 shows a cross section of an example blade on a bi-center bit having a cutter mounted in a reverse direction. 
     FIG. 12 shows a plan view of a pilot section on a bi-center bit having a blade as shown in cross-section in FIG.  11 . 
    
    
     DETAILED DESCRIPTION 
     An example of a bi-center drill bit is shown in oblique view in FIG. 3. A bi-center drill bit  10  includes a body  18  which can be made from steel or other material conventionally used for drill bit bodies. One end of the body  18  can include thereon a threaded connection  20  for attaching the bit  10  to a source of rotary power, such as a rotary drilling rig (not shown) or hydraulic motor (not shown) so that the bit  10  can be turned to drill earth formations (not shown). 
     At the end of the body  18  opposite the threaded connection  20  is a pilot section  13  of the bit  10 . The pilot section  13  can include a set of azimuthally spaced apart blades  14  affixed to or otherwise formed into the body  18 . On each of the blades  14  is mounted a plurality of polycrystalline diamond compact (PDC) inserts, called cutters, such as shown at  12 . The pilot blades  14  typically each extend laterally from the longitudinal axis  24  of the bit  10  by the same amount. The pilot section  13  thus has a drilling radius, which can be represented by R P  ( 14 A in FIG. 3) of about the lateral extent of the pilot blades  14 . The radially outermost surfaces of the pilot blades  14  generally conform to a circle which is substantially coaxial with the longitudinal axis  24  of the bit  10 . When the bit  10  is rotated about its longitudinal axis  24 , the pilot section  13  will thus drill a hole having a diameter about equal to 2×R P . The pilot hole diameter can be maintained by gauge pads such as shown in FIG. 3 at  14 G, disposed on the radially (laterally) outermost portion of the pilot blades  14 . 
     A reaming section  15  is positioned on the body  18  axially spaced apart from the pilot section  13 . The reaming section  15  can also include a plurality of blades  16  each having thereon a plurality of PDC cutters  12 . The reaming blades  16  can be affixed to or formed into the body  18  just as the pilot blades  14 . It should be understood that the axial spacing referred to between the pilot section  13  and the reaming section  15  denotes the space between the axial positions along the bit  10  at which actual cutting of earth formations by the bit  10  takes place. It should not be inferred that the pilot section  13  and reaming section  15  are physically separated structures, for as will be further explained, one advantageous aspect of the invention is a unitized spiral structure used for selected ones of the blades  14 ,  16 . Some of the blades  16  in the reaming section  15  extend a maximum lateral distance from the rotational axis  24  of the bit  10  which can be represented by R R  ( 16 A in FIG.  3 ), and which is larger than R P . 
     The bit  10  shown in FIG. 3 has a “pass-through” diameter (the diameter of an opening through which the bit  10  will fit), which, as will be further explained, results from forming the reaming blades  16  to conform to a circle having the pass-through diameter. The center of the pass through circle, however, is offset from the longitudinal axis  24  of the bit. As a result of forming the blades  16  to conform to the axially offset pass-through circle, some of the reaming blades  16 , such as shown at  16 F in FIG. 3, will not extend laterally from the axis  24  as much as the other reaming blades. The laterally most extensive ones of the reaming blades  16  thus formed can include gauge pads such as shown at  16 G. During drilling, as the bit  10  is rotated about the longitudinal axis  24 , the hole which is drilled by the reaming section  15  will have a diameter about equal to 2×R R  as the blades  16  in the reaming section  15  which extend the full lateral distance R R  from the longitudinal axis  24  rotate about the longitudinal axis  24 . 
     The bit  10  includes a plurality of jets, shown for example at  22 , the placement and orientation of which will be further explained. 
     In one aspect of the invention, it has been determined that a bi-center bit can effectively drill a hole having the expected drill diameter of about 2×R R  even while the pilot section  13  axial length (L p  in FIG. 5) is less than about 80 percent of the diameter of the pilot section (2×R P ). The pilot section length (L p  in FIG. 5) is defined herein as the length from the end of the bit  10  to the top of the reaming section  15 . In this example, the bit  10  also has an overall axial make-up length (measured from the end of the bit to a make up shoulder  10 A) which is less than about 133 percent of the drilling diameter of the bit (2×R R ). Prior art bi-center bits have pilot section axial lengths substantially more than the 80 percent length-to-diameter of the bit  10  of this invention. It has been determined that drilling stability of a bi-center bit is not compromised by shortening the pilot section axial length and overall axial make-up length of the bit in accordance with the invention. 
     Conversely, it should be noted that the reaming section  15  necessarily exerts some lateral force, since the blades  16  which actually come into contact with the formation (not shown) during drilling are located primarily on one side of the bit  10 . The lateral forces exerted by all the PDC cutters  12  are balanced in the bit of this invention in a novel manner which will be further explained. However, as a result of any form of lateral force balancing between the pilot section  13  and the reaming section  15 , the pilot section  13  necessarily exerts, in the aggregate, a substantially equal and azimuthally opposite lateral force to balance the lateral force exerted by the reaming section  15 . As will be appreciated by those skilled in the art, the axial separation between the lateral forces exerted by the reaming section  15  and the pilot section  13  results in a turning moment being developed normal to the axis  24 . The turning moment is proportional to the magnitude of the lateral forces exerted by the reaming section  15  and the pilot section  13 , and is also proportional to the axial separation of the reaming section  15  and the pilot section  13 . In this aspect of the invention, the axial separation of the pilot section  13  and the reaming section is kept to a minimum value by having a pilot section length  13  and overall length as described above. By keeping the axial separation to a minimum, the turning moment developed by the bit  10  is minimized, so that drilling stability can be improved. 
     In another aspect of the invention, it has been determined that the drilling stability of the bi-center bit  10  can be improved when compared to the stability of prior art bi-center bits by mass-balancing the bit  10 . It has been determined that the drilling stability will improve a substantial amount when the bit  10  is balanced so its center of gravity is located within about 2.5 percent of the drill diameter of the bit (2×R R ) from the axis of rotation  24 . Prior art bi-center bits were typically not mass balanced at all. Mass balancing can be performed, among other ways, by locating the blades  14 ,  16  and selecting suitable sizes for the blades  14 ,  16 , while taking account of the mass of the cutters  12 , so as to provide the preferred mass balance. Alternatively, gauge pads, or other extra masses, can be added as needed to achieve the preferred degree of mass balance. Even more preferable for improving the drilling performance of the bit  10  is mass balancing the bit  10  so that its center of gravity is within 1.5 percent of the drill diameter of the bit  10 . 
     In another aspect of the invention, it has been determined that the drilling stability of a bi-center bit can be further improved by force balancing the entire bit  10  as a single structure. Force balancing is described, for example, in, T. M. Warren et al.,  Laboratory Drilling Performance of PDC Bits,  paper no. 15617, Society of Petroleum Engineers, Richardson, Tex., 1986. Prior art bi-center bits were force balanced, but in a different way. In this embodiment of the invention, the forces exerted by each of the PDC cutters  12  can be calculated individually, and the locations of the blades and the PDC cutter  12  thereon can be selected so that the sum of all the forces exerted by each of the cutters  12  will have a net imbalance of less than about 10 percent of the total axial force exerted on the bit (known in the art as the “weight on bit”). The designs of both the pilot section  13  and the reaming section  15  are optimized simultaneously in this aspect of the invention to result in the preferred force balance. An improvement to drilling stability can result from force balancing according to this aspect of the invention because the directional components of the forces exerted by each individual cutter  12  are accounted for. In the prior art, some directional force components, which although summed to the net lateral force exerted individually by the reaming section and pilot section, can result in large unexpected side forces when the individual cutter forces are summed in the aggregate in one section of the bit to offset the aggregate force exerted by the other section of the bit. This aspect of the invention avoids this potential problem of large unexpected side forces by providing that the locations of and shapes of the blades  14 ,  1  and cutters  12  are such that the sum of the forces exerted by all of the PDC cutters  12 , irrespective of whether they are in the pilot section  13  or in the reaming section  15 , is less than about 10 percent of the weight on bit. It has been determined that still further improvement to the performance of the bit  10  can be obtained by balancing the forces to within 5 percent of the axial force on the bit  10 . 
     An end view of this embodiment of the invention is shown in FIG. 4 which illustrates several features intended to improve drilling stability of the bi-center bit  10 . The blades  14  in the pilot section ( 13  in FIG. 3) are shown azimuthally spaced apart. Each pilot section blade  14  is preferably shaped substantially in the form of a spiral. The spiral need not conform to any specific spiral shape, but only requires that the blade be shaped so that the individual cutters ( 12  in FIG. 3) on each such spirally shaped blade are at different azimuthal positions with respect to each other. Although the example shown in FIG. 4 has every blade being spirally shaped, it is within the contemplation of this invention that only selected ones of the blades can be spiral shaped while the other blades may be straight. Each cutter on such straight blades may be at the same azimuthal position. 
     In another aspect of the invention, selected ones of the pilot blades  14  can be formed into the same individual spiral structure as a corresponding one of the reaming blades  16 . This type of unitized spiral blade structure is used, for example, on the blades shown at B 2 , and B 4  in FIG.  4 . The reaming section  15  may include blades such as shown at B 3 , B 5  and B 6  in FIG. 4 which are not part of the same unitized spiral structure as a pilot blade  14 , because there is no corresponding pilot blade  14  at the same azimuthal position as these particular reaming blades B 3 , B 5 , B 6 . It has been determined that having blades such as B 2  and B 4  shaped substantially as a unitized spiral structure, encompassing both the pilot blade  14  and the azimuthally corresponding reaming blade  16 , improves the drilling stability of the bit  10  when compared to the stability of bi-center bits using straight-blades and/or non-unitized pilot/reaming blades as previously known in the art. 
     Also shown in FIG. 4 are the previously referred to jets, in both the pilot section, shown at  22 P, and in the reaming section, shown at  22 R. In another aspect of this invention, it has been determined that cuttings (not shown) generated by the bit  10  as it penetrates rock formations (not shown) are more efficiently removed from the drilled hole, and hydraulic power used to pump drilling fluid (not shown) through the jets  22 P,  22 R is spent more efficiently, when the reaming jets  22 R are oriented so that their axes are within about 30 degrees from a line normal to the axis ( 24  in FIG. 3) of the bit  10 . Prior art bi-center bits typically include reaming jets which are oriented so that their axes are in approximately the same directions as the pilot jets, this being generally in the direction along which the bit drills. Other prior art bits have reaming jets which discharge directly opposite the direction of the bottom of the drilled hole. Either type of reaming jet previously known in the art has reduced hydraulic performance as compared to the bi-center bit of this aspect of the invention. It has been determined that the performance of the reaming jets  22 R can be improved still further by orienting them so that their axes are within 20 degrees of a line normal to the longitudinal axis  24 . 
     Another advantageous aspect of the invention is the shape of the reaming blades  16  and the positions of radially outermost cutters, such as shown at  12 L, disposed on the reaming blades  16 . In making the bit according to this aspect of the invention, the outer surfaces of the reaming blades  16  can first be cut or otherwise formed so as to conform to a circle having the previously mentioned drill diameter (2×R R ). This so-called “drill circle” is shown in FIG. 4 at CD. The drill circle CD is substantially coaxial with the longitudinal axis ( 24  in FIG. 3) of the bit  10 . In FIG. 4, the previously referred to pass-through circle is shown at CP. The outer surfaces of the reaming blades  16 , after being formed to fit within the drill circle CD, can then be cut or otherwise formed to conform to the pass-through circle CP. The pass-through circle CP is axially offset from the drill circle CD (and the longitudinal axis  24 ) by an amount which results in some overlap between the circumferences of pass through circle CP and the drill circle CD. The intersections of the pass-through circle CP and drill circle CD circumferences are shown at A and B in FIG.  4 . 
     The radially outermost cutters  12 L can then be positioned on the leading edge (the edge of the blade which faces the direction of rotation of the bit) of the radially most extensive reaming blades, such as shown at B 3  and B 4  in FIG. 4, so that the cutter locations will trace a circle having the full drill diameter (2×R R ) when the bit rotates about the longitudinal axis  24 . The radially most extensive reaming blades B 3 , B 4 , however, are positioned azimuthally between the intersections A, B of the drill circle CD and the pass through circle CP. The drill circle CD defines, with respect to the longitudinal axis  24 , the radially outermost part of the bit at every azimuthal position. The reaming blades  16  are generally made to conform to the pass-through circle CP; however, the reaming blades B 3 , B 4  located between intersections A and B will be formed to conform to the drill circle CD, because the drill circle CD therein defines the radially outermost extension of any part of the bit  10 . Between intersections A and B, the drill circle CD is radially closer to the longitudinal axis  24  than is the pass-through circle CP, therefore the blades B 3 , B 4  within the arcuate section between intersections A and B will extend only as far laterally as the radius of the drill circle CD. As shown in FIG. 4, the radially outermost cutters  12 L on blades B 3  and B 4  can be positioned at “full gauge”, meaning that these cutters  12 L are at the same radial distance from the axis  24  as the outermost parts of the blade B 3 , B 4  onto which they are attached. However, the cutters  12 L on blades B 3 , B 4  are also disposed radially inward from the pass-through circle CP at the same azimuthal positions because of the limitation of the lateral extent of these blades B 3 , B 4 . Therefore, the outermost cutters  12 L will not contact the inner surface of an opening having a diameter about equal to the pass-through diameter as the bit  10  is moved through such an opening. When rotated about the longitudinal axis  24 , however, the bit  10  will drill a hole having the full drill diameter (2×R R ). The preferred shape of the radially outermost reaming blades B 3 , B 4  and the position of radially outermost cutters  12 L thereon enables the bit  10  to pass freely through a protective casing (not shown) inserted into a wellbore, without sustaining damage to the outermost cutters  12 L, while at the same time drilling a hole which has the full drill diameter (2×R R ). 
     The reaming blades which do not extend to full drill diameter (referred to as “non-gauge reaming blades”), shown for example at B 1 , B 2 , B 5 , B 6  and B 7 , have their outermost cutters positioned radially inward, with respect to pass-through circle CP, of the radially outermost portion of each such non-gauge reaming blade B 1 , B 2 , B 5 , B 6  and B 7  to avoid contact with any part of an opening at about the pass-through diameter. This configuration of blades and cutters has proven to be particularly useful in efficiently drilling through equipment (called “float equipment”) used to cement in place the previously referred to casing. By positioning the cutters  12  on the non-gauge reaming blades as described herein, damage to these cutters  12  can be avoided. Damage to the casing can be also be avoided by arranging the cutters  12  as described, particularly when drilling out the float equipment. Although the non-gauge reaming blades B 1 , B 2 , B 5 , B 6  and B 7  are described herein as being formed by causing these blades to conform to the pass-through circle CP, it should be understood that the pass-through circle only represents a radial extension limit for the non-gauge reaming blades B 1 , B 2 , B 5 , B 6  and B 7 . It is possible to build the bit  10  with radially shorter non-gauge reaming blades. However, it should also be noted that by having several azimuthally spaced apart non-gauge reaming blades which conform to the pass-through circle CP, the likelihood is reduced that the outermost cutters  12 L on the gauge reaming blades B 3 , B 4  will contact any portion of an opening, such as a well casing, less than the drill diameter. 
     It should also be noted that the numbers of gauge and non-gauge reaming blades shown in FIG. 4 is only one example of numbers of gauge and non-gauge reaming blades. It is only required in this aspect of the invention that the gauge reaming blades conform to the drill circle CD, where the drill circle is less radially extensive than the pass-through circle CP to be able to locate the outermost cutters  12 L at full gauge as in this aspect of the invention. It is also required that all the reaming blades conform to the radially least extensive of the drill circle CD and pass-through circle CP at any azimuthal blade position. 
     FIG. 5 shows a side view of this embodiment of the invention. As previously explained, the pilot section ( 13  in FIG. 3) can have an overall length, L p , which is less than about 80 percent of the drill diameter of the pilot section ( 13  in FIG.  3 ). The overall make-up length, L T , shown at  16 X in FIG. 5, extending from the end of the bit to a make-up shoulder  10 A, in this embodiment of the invention can be less than about 133 percent of the drill diameter of the bit  10 . The gauge pads for the pilot section blades  14  are shown in FIG. 5 generally at  14 G. The gauge pads for the reaming section blades  16  are shown generally at  16 G. 
     A bi-center bit can be modified to improve its performance, particularly where the bit is used to drill through the previously mentioned float equipment (this drilling operation is referred to in the art as “drill out”). During such operations as drill out, a bi-center bit will rotate with a precessional motion which generally can be described as rotating substantially about the axis of the pass through circle, while the longitudinal axis  24  generally precesses about the axis of the pass through circle (CP in FIG.  4 ). This occurs because the bit is constrained during drill out to rotate within an opening (the interior of the casing) which is at, or only slightly larger than, the pass-through diameter of the bit. Referring to FIG. 6, the precessional motion of the longitudinal axis ( 24  in FIG. 3) about the pass-through circle axis defines a circle CX (hereinafter called a “precession circle”) having a radius about equal to the offset between the longitudinal axis ( 24  in FIG. 3) and the axis of the pass through circle (CP in FIG.  4 ). The improvements to the drill bit in this aspect of the invention includes increasing the thickness of the blades, particularly in the vicinity of the precession circle CX. These thickened areas are shown at  116  on blades B 1  and B 4 . As shown in FIG. 6, blades B 1  and B 4  can be the previously described unitized spiral structures forming both a reaming and pilot blade, although this is not to be construed as a limitation on the invention. The thickened blade areas  116  can be formed on any blade in the part of the blade proximate to the precession circle CX. The thickened blade areas  116  can be used to mount additional cutters, shown at  12 X. The additional cutters  12 X can be PDC inserts as are the other cutters  12 , or can alternatively be tungsten carbide or other diamond cutters known in the art. Tungsten carbide cutters provide the advantage of relatively rapid wear down. The wear down, if it takes place during drill out, will leave the bi-center bit after drill out with a cutter configuration as shown in FIG. 4 (which excludes the additional cutters  12 X), which configuration is well suited for drilling earth formations. In the vicinity of the precession circle CX the additional cutters  12 X and the other cutters  12  can be mounted on the blades B 1 , B 4  at a different back rake and/or side rake angle than are the cutters  12  away from the precession circle CX to reduce damage to the cutters  12 ,  12 X during drill out. 
     Another aspect of the additional cutters  12 X and the other cutters  12  proximate to the precession circle CX is that they can be mounted in specially formed pockets in the blade surface, such as shown at  117 , which have greater surface area to contact the individual cutters  12 ,  12 X than do the pockets which hold the other cutters  12  distal from the precession circle CX, so that incidence of the cutters  12 ,  12 X proximate to the precession circle CX breaking off during drilling can be reduced, or even eliminated. 
     Referring to FIG. 7, another aspect of this invention is shown which can improve drilling performance of the bi-center bit, particularly during drill out. FIG. 7 shows a side profile view of the locations of cutters on the pilot blades ( 14  in FIG.  3 ). The positions of the cutters ( 12 ,  12 X in FIG. 6) along the blade are shown by circles  114 . In this aspect of the invention, the improvement is to include a greater volume of diamond per unit length of the blade in areas such as shown at A′ in FIG. 7 than at other locations, such as at B′, further away from the pass-through circle axis PTA. The increased diamond volume per unit blade length preferably is proximate to the pass-through circle axis PTA in FIG.  7 . 
     The increased diamond volume can be provided by several different techniques. One such technique includes mounting additional cutters in a row of such additional cutters located azimuthally spaced apart from the other cutters on the same blade. This would be facilitated by including pockets therefor, such as at  117  in FIG. 6 in thickened areas on the blade (such as  116  in FIG.  6 ). Other ways to increase the diamond volume per unit length include increasing the number of cutters ( 12  in FIG. 6) per unit length along each blade. Still another way to increase the diamond volume would be to increase the thickness of the diamond “table” on the cutters proximate to the pass-through axis. Irrespective of how the diamond volume is increased, or irrespective of the ultimate cutter density selected near the pass-through axis PTA, the cutter forces and the mass of the bit are preferably balanced by the methods described earlier herein. 
     The bi-center drill bit described herein is particularly well suited for drill out of the float equipment used to cement a casing in a wellbore. To drill out using the bi-center bit of this invention, the bit is rotated within the casing while applying force along the longitudinal axis ( 24  in FIG. 3) to drill through the cement and float equipment at the bottom of the casing. While constrained within the casing (not shown), the reaming blades ( 16  in FIG. 3) are constrained to rotate substantially about the pass-through axis PTA because the reaming blades conform to the pass-through circle (CP in FIG.  4 ). The radially most extensive reaming blades do not contact the casing during drill out because they are located in the arcuate section where the drill circle (CD in FIG. 4) is radially less extensive than the pass through circle (CP in FIG.  4 ). As the float equipment is fully penetrated, and the bit leaves the casing, the bit will then rotate about the longitudinal axis ( 24  in FIG. 3) so that the hole drilled will have the full drill diameter. 
     An improvement to the drill out capability for a bi-center drill bit as described above can be explained by referring to FIG.  8 . The example in FIG. 8, generally at  10 B, includes blades B 1 A-B 7 A similar in configuration to the blades of the previous embodiments (for example B 1 -B 7  in FIG.  4 ). In the embodiment of FIG. 8, blades B 4 A and B 5 A are disposed within the arcuate section (A-B in FIG. 4) and are shaped generally to conform to the drill circle (CD in FIG.  4 ). Blades B 1 A-B 3 A, B 6 A and B 7 A generally conform at their outermost lateral extent to the pass-through circle (CP in FIG.  4 ). In the embodiment of FIG. 8, blades B 6 A and B 3 A extend laterally inward (in the pilot section) to about the position of the longitudinal axis  24  of the bit so that the pilot hole will be properly drilled. In the embodiment shown in FIG. 4, for example, the corresponding blades extending inward to about the axis  24  include blades B 1  and B 4 . The blades shown in FIG. 8, however, are arranged so that substantially no cutting elements are disposed proximate a line  10 C which extends between the longitudinal axis  24  and the pass-through axis PTA. When a bi-center bit is rotated inside an opening having a diameter about equal to the pass-through diameter, it rotates about the pass through axis PTA, as previously explained. By arranging the blades B 1 A-B 7 A such as shown in FIG. 8 to avoid having cutting elements proximate the line  10 C, reverse-rotating cutting elements are avoided when the bit rotates about the pass through axis PTA. For purposes of the various embodiments of the invention, the expression “proximate the line  10 C” further includes within its scope an area roughly defined by a triangle including as two of its sides lines extending from the pass-though axis PTA at an angle of about 45 degrees from the line  10 C. The third side of the triangle intersects these lines and the longitudinal axis of the bit  24 . The area within this triangle is susceptible to reverse rotation when the bit is rotated about the pass-through axis PTA. 
     In some cases, and according to one aspect of the invention, it may be advantageous to arrange some of the blades on the pilot section to extend to a position proximate to the line ( 10 C in FIG.  8 ), as defined above. This arrangement of pilot blades and cutting elements thereon is shown in FIG. 4, as previously explained. As explained with respect to the example bit shown in FIG. 8, however, any cutting elements disposed proximate to the line  10 C may rotate in a direction opposite to the direction of rotation of the bit when the bit is constrained to rotate within an opening having a diameter about the same as the pass through diameter, and therefore about the pass-through axis PTA. In such cases where the pilot blades are arranged to include cutting elements proximate the line  10 C, such as the arrangement in FIG. 4, a particular type of cutting element may improve the ability to drill out casing. An example of such a special cutting element is shown in FIG.  10 . The special cutting element  12 Q is affixed to the blade B 1  in a position proximate the line ( 10 C in FIG.  8 ). The special cutting element  12 Q includes a substrate  12 Q 3  made in any manner as is conventional for making PDC cutting element substrates, and a primary diamond table  12 Q 1  affixed thereto in an orientation adapted to cut earth formation as the bit is rotated so that the special cutting element  12 Q moves along the direction of rotation. The special cutting element  12 Q also includes a secondary diamond table  12 Q 2  mounted so that it cuts earth formation when the bit is rotated so that the special cutting element  12 Q moves in a direction opposite the rotation of the bit. This occurs, as previously explained, when the bit is rotated inside an opening having about the pass-through diameter, or about the pass through axis. Special cutting elements such as shown at  12 Q in FIG. 10 may be affixed to any one or more cutting element positions on the blades proximate the line ( 10 C in FIG.  8 ). 
     An alternative to the special cutting element ( 12 Q in FIG. 10) is shown in FIG. 11, which shows a cross-section of one of the blades B 4  that extends to a position proximate the line ( 10 C in FIG.  8 ). The blade B 4  includes at least one cutting element or cutter  112  proximate the line ( 10 C in FIG. 8) oriented to cut earth formation when the portion of the blade B 4  proximate the line reverse rotates during casing drill out. For ordinary drilling where the bit rotates about its longitudinal axis ( 24  in FIG.  8 ), the blade B 4  preferably includes at least one normally-oriented cutter  12  proximate the line ( 10 C in FIG.  8 ). The normally oriented cutter  12  cuts earth formation during rotation in the same direction as the rotation of the bit. 
     The arrangement of the at least one reverse-oriented cutter  112  and normally oriented cutter  12  shown in cross section in FIG. 11 is shown in plan or end view in FIG. 12 to illustrate a preferred placement of the at least one reverse-oriented cutter  112  and the normally oriented cutter  12 . A bit made according to the present aspect of the invention may have better performance during casing drill out. 
     Another aspect of the invention can improve the ability of a bi-center drill bit to maintain drilling direction when used in directional drilling applications. FIG. 9 shows a cross section of an example blade structure, extending from the pilot section  13  to the reaming section  16  thereof. The blade in this example is one of the blades which is disposed in the arcuate section (A-B in FIG.  4 ), for example B 4 A in FIG.  8 . It should be understood that any one of the blades on a bi-center bit may include a structure such as shown in cross section in FIG.  9 . The blade B 4 B shown in cross section FIG. 9 includes a tapered face  17 A and a gage face  17 G. Preferably at least one, and more preferably a plurality of cutting elements  12 , which may be PDC cutters as are the other cutting elements on the bit, are disposed on the tapered face  17 A. The intermediate diameter gage face  17 G is disposed below the tapered face  17 A and may include thereon any form of gage protection (not shown) known in the art, or may include at least one cutting element (not shown in this example) disposed at an intermediate gage diameter defined by the gage face  17 G. Preferably, the tapered face  17 A, having cutting elements  12  thereon, and the intermediate diameter gage face  17 G are included on a plurality of the blades azimuthally distributed around the circumference of the drill bit. The longitudinal position of the tapered face  17 A and gage face  17 G is generally between the pilot section  13  and the reaming section  16 . Preferably the tapered face  17 A and gage face  17 G are formed into the same blade structure as selected ones of the pilot blades  14  or reaming blades  16 , but this is not intended to be a limitation on the invention. In combination, the intermediate diameter gage faces  17 G, and in the embodiment of FIG. 9 the tapered faces  17 A, distributed azimuthally around the bit, form a pilot hole conditioning section  17  disposed longitudinally between the pilot section  13  and the reaming section  16  on the bi-center bit. Having a pilot hole conditioning section  17  such as shown in FIG. 9 may improve the ability of a bi-center bit to “hold angle” or otherwise maintain intended wellbore trajectory when used in directional drilling applications. 
     It should also be understood that other embodiments of a pilot hole conditioning section may not require a tapered face on any one or all of the blades. It is only required in this aspect of the invention that blades, or a portion thereof, distributed around the circumference of the bit define an intermediate gage diameter. The tapered face  17 A in the embodiment of FIG. 9 is only to provide a convenient form of transition between the pilot hole diameter and the intermediate diameter. Accordingly, other embodiments of a pilot hole conditioning section may include blade profiles other than the one shown in FIG.  9 . Additionally, the gage faces  17 G need not be formed integrally with the blade structure of either the pilot blades or the reaming blades as shown in FIG.  9 . In other embodiments, the gage faces  17 G may be formed as separate structures. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.