Bi-centered drill bit having improved drilling stability mud hydraulics and resistance to cutter damage

A bi-center drill bit includes pilot and reaming blades affixed to a body at azimuthally spaced locations. The blades have PDC cutters attached at selected positions. In one aspect, the pilot blades form a section having length along the bit axis less than about 80 percent of a diameter of the section. In another aspect, selected pilot blades and corresponding reaming blades are formed into single spiral structures. In another aspect, shapes and positions of the blades and inserts are selected so that lateral forces exerted by the reaming and the pilot sections are balanced as a single structure. Lateral forces are preferably balanced to within 10 percent of the total axial force on the bit. In another aspect, the center of mass of the bit is located less than about 2.5 percent of the diameter of the bit from the axis of rotation. In another aspect, jets are disposed in the reaming section oriented so that their axes are within about 30 degrees of normal to the axis of the bit. In another aspect, the reaming blades are shaped to conform to the radially least extensive, from the longitudinal axis, of a pass-through circle or a drill circle, so the cutters on the reaming blades drill at the drill diameter, without contact to the cutters on the reaming blades when the bit passes through an opening having about the pass-through diameter.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates generally to the field of polycrystalline diamond
 compact (PDC) drilling bits. More specifically, this invention relates to
 PDC bits which drill a hole through earth formations where the drilled
 hole has a larger diameter than the "pass-through " diameter of the drill
 bit.
 2. Description of the Related Art
 Drill bits which drill holes through earth formations where the hole has a
 larger diameter than the bit'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 108A in
 the pilot section and the blades 111B in the reaming section are typically
 straight, meaning that the cutters 110 are disposed at substantially the
 same relative azimuthal position on each blade 108A, 111B. In some cases
 the blades 108A in the pilot section 106 may be disposed along the same
 azimuthal direction as the blades 111B 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.
 SUMMARY OF THE INVENTION
 One aspect of the invention is a bi-center drill bit which includes a body
 having pilot blades and reaming blades affixed to the body at azimuthally
 spaced apart locations. The pilot blades and the reaming blades have a
 plurality of polycrystalline diamond compact (PDC) cutters attached to
 them at selected positions along each of the blades. In one example of the
 invention, the pilot blades form a pilot section having a length along an
 axis of the bit which is less than about 80 percent of a diameter of a
 pilot section of the bit. In one example of this aspect of the invention,
 the total make-up length of the bit, including the length of the pilot
 section and a reaming section formed from the reaming blades is less than
 about 133 percent of the drill diameter of the bit.
 In another aspect of the invention, selected ones of the pilot blades and
 reaming blades on a bi-center bit are formed into corresponding single
 (unitary) spiral structures to improve drilling stability of the bit.
 Selected ones of the reaming blade and pilot blades can be formed as
 spirals, where the azimuthal position of the cutters on each such spiral
 blade is different from that of the other cutters on that blade.
 In another aspect of the invention, the shapes and positions of the blades,
 and the positions of the PDC cutters thereon of a bi-center bit are
 selected so that the lateral forces exerted by the reaming section of the
 bit and by the pilot section of the bit are balanced as a single
 structure, whereby the forces exerted by each of the PDC inserts are
 summed without regard to whether they are located on the reaming section
 or on the pilot section. These forces are in one example preferably
 balanced to within 10 percent of the total axial force exerted on the bit.
 In another aspect of the invention, the center of mass of the a bi-center
 drill bit is located less than about 2.5 percent of the drilled diameter
 of the bit away from the axis of rotation (longitudinal axis) of the drill
 bit.
 In another aspect of the invention, a bi-center drill bit includes drilling
 fluid discharge orifices ("jets") in the reaming section of the bit which
 are oriented so that their axes are within about 30 degrees of normal to
 the axis of the bit.
 In another aspect of the invention, a bi-center bit includes reaming blades
 which are shaped to conform to whichever is radially least extensive, with
 respect to the longitudinal axis of the bit, at the azimuthal position of
 the particular blade, either a pass through circle or a drill circle. The
 drill circle and the longitudinal axis are substantially coaxial. The axis
 of the pass-through circle is offset from the longitudinal axis and
 defines an arcuate section wherein the pass-through circle extends
 laterally from the longitudinal axis past the drill circle. The leading
 edge cutters on the reaming blades are, as a result of this selected shape
 of the reaming blades, located radially inward of the trailing edge of the
 reaming blades with respect to the pass through circle where the reaming
 blades conform to the drill circle (in the arcuate section). This provides
 that the drill bit can pass through an opening having a diameter of about
 the pass-through diameter, for example casing in a wellbore, but can also
 drill out casing cementing equipment in a wellbore without sustaining
 damage to the leading edge cutters on the reaming blades.
 Another aspect of the invention is a bi-center drill bit comprising a body
 having pilot blades and reaming blades affixed to the body at azimuthally
 spaced apart locations. The pilot blades and reaming blades having
 polycrystalline diamond compact (PDC) cutters attached to them at selected
 positions along each of the blades. The pilot blades have additional
 cutters attached to them at locations which are proximate to a circle
 defined by precessing the pass-through axis of the bit about the
 longitudinal axis of the bit. In one example, the additional cutters are
 tungsten carbide cutters, PDC cutters or diamond cutters. In one example,
 the side rake or the back rake angle of the cutters proximate to the
 circle is changed. In another example, additional cutters can be provided
 proximate to the circle by adding a row of cutters on thickened blade
 portions proximate to the circle
 Another aspect of the invention is a method for drilling out a casing
 having float equipment therein. The method includes rotating in the casing
 a bi-center drill bit having pilot blade and reaming blades thereon at
 azimuthally spaced apart locations. The blades have PDC cutters thereon.
 The reaming blades are shaped to conform to whichever is radially least
 extensive, with respect to the longitudinal axis of the bit, at the
 azimuthal position of the particular blade, either a pass through circle
 or a drill circle. The drill circle and the longitudinal axis are
 substantially coaxial. The axis of the pass-through circle is offset from
 the longitudinal axis and defines an arcuate section wherein the
 pass-through circle extends laterally from the longitudinal axis past the
 drill circle. The leading edge cutters on the reaming blades are, as a
 result of this selected shape of the reaming blades, located radially
 inward of the trailing edge of the reaming blades with respect to the pass
 through circle where the reaming blades conform to the drill circle (in
 the arcuate section). This provides that the drill bit can pass through
 the casing, which has a diameter of about the pass-through diameter,
 without damaging the inserts on the reaming blades. When the bit fully
 penetrates the float equipment and exits the casing, the bit is then
 rotated about the longitudinal axis and then drills a hole, in the earth
 formations beyond the casing, which has the drill diameter.

DESCRIPTION OF PREFERRED EMBODIMENTS
 An example of a drill bit incorporating several aspects of the invention 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.sub.P (14A 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.times.R.sub.P. The pilot hole diameter can be maintained by gauge pads
 such as shown in FIG. 3 at 14G, disposed on the radially (laterally)
 outermost portion of the pilot blades 14.
 A reaming section 15A 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.sub.R (16A in FIG. 3),
 and which is larger than R.sub.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 16F
 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 16G. 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.times.R.sub.R as the blades 16 in the reaming section 15 which extend
 the full lateral distance R.sub.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.times.R.sub.R even while the pilot section 13 axial length (L.sub.p in
 FIG. 5) is less than about 80 percent of the diameter of the pilot section
 (2.times.R.sub.P). The pilot section length (L.sub.p in FIG. 5) is defined
 herein as the length from the end of the bit 10 to 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 10A) which
 is less than about 133 percent of the drilling diameter of the bit
 (2.times.R.sub.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 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 to 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.times.R.sub.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, Drag Bit Performance
 Modeling, 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 PDC cutters 12 can be calculated individually, and the locations of
 the If 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 B2, and B4 in FIG. 4. The
 reaming section 15 may include blades such as shown at B3, B5 and B6 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 same the
 azimuthal position as these particular reaming blades B3, B5, B6. It has
 been determined that having blades such as B2 and B4 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.
 Shown in FIG. 5 are the previously referred to jets, in both the pilot
 section, shown at 22P, and in the reaming section, shown at 22R. 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 22P, 22R is
 spent more efficiently, when the reaming jets 22R 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 bit 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 22R 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 12L, 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.times.R.sub.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 12L 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 B3
 and B4 in FIG. 4, so that the cutter locations will trace a circle having
 the full drill diameter (2.times.R.sub.R) when the bit rotates about the
 longitudinal axis 24. The radially most extensive reaming blades B3, B4,
 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 B3, B4 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 B3, B4 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 12L on blades B3 and B4
 can be positioned at "full gauge ", meaning that these cutters 12L are at
 the same radial distance from the axis 24 as the outermost parts of the
 blade B3, B4 onto which they are attached. However, the cutters 12L on
 blades B3, B4 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 B3, B4. Therefore, the outermost cutters
 12L 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.times.R.sub.R).
 The preferred shape of the radially outermost reaming blades B3, B4 and
 the position of radially outermost cutters 12L 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 12L, while at
 the same time drilling a hole which has the full drill diameter
 (2.times.R.sub.R).
 The reaming blades which do not extend to full drill diameter (referred to
 as "non-gauge reaming blades "), shown for example at B1, B2, B5, B6 and
 B7, 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 B1, B2, B5, B6 and B7 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 B1, B2, B5, B6 and
 B7 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 B1, B2, B5, B6 and B7. 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 blade which conform to the pass-through circle CP, the
 likelihood is reduced that the outermost cutters 12L on the gauge reaming
 blades B3, B4 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 12L 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.sub.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.sub.T, shown
 at 16X in FIG. 5, extending from the end of the bit to a make-up shoulder
 10A, 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 14G. The gauge pads for
 the reaming section blades 16 are shown generally at 16G.
 A bi-center bit according to another aspect of this invention can be
 modified to improve its performance particularly where the bit is used to
 drill through the previously mentioned float equipment (this drilling
 operation 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 go 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 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
 B1 and B4. As shown in FIG. 6, blades B1 and B4 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 12X. The
 additional cutters 12X 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 12X) which
 configuration is well suited for drilling earth formations. In the
 vicinity of the precession circle CX the additional cutters 12X and the
 other cutters 12 can be mounted on the blades B1, B4 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, 12X during drill
 out.
 Another aspect of the additional cutters 12X 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, 12X than
 do the pockets which hold the other cutters 12 distal from the precession
 circle CX, so that incidence of the cutters 12, 12X 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 ma cutters (12, 12X
 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.
 It will be appreciated by those skilled in the art that other embodiments
 of this invention are possible which will not depart from the spirit of
 the invention as disclosed herein. Accordingly, the invention shall be
 limited in scope only by the attached claims.