US20110100714A1

US20110100714A1 - Backup cutting elements on non-concentric earth-boring tools and related methods

Info

Publication number: US20110100714A1
Application number: US12/608,832
Authority: US
Inventors: William A. Moss; David L. Rickabaugh; Mark E. Anderson
Original assignee: Individual
Current assignee: Baker Hughes Holdings LLC
Priority date: 2009-10-29
Filing date: 2009-10-29
Publication date: 2011-05-05
Also published as: WO2011059685A3; WO2011059685A2; WO2011059685A4

Abstract

An apparatus for engaging a subterranean borehole includes a bi-center bit having backup cutting elements thereon. The bi-center bit includes a pilot bit section and a reamer bit section adjacent to the pilot bit section. The pilot bit section includes at least one primary cutting element and at least one backup cutting element rotationally trailing and laterally offset from the at least one primary cutting element. Methods of drilling a subterranean borehole include engaging a first portion of a borehole with a reamer bit section of a drill bit and simultaneously engaging a second, opposing portion of the borehole with a pilot bit section adjacent to the reamer bit section.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to U.S. patent application Ser. No. 12/498,516, filed Jul. 7, 2009, pending, titled “BACKUP CUTTING ELEMENTS ON NON-CONCENTRIC REAMING TOOLS.”

TECHNICAL FIELD

Embodiments of the invention relate to drill bits and tools for subterranean drilling and, more particularly, embodiments relate to drill bits for enlarging the diameter of a subterranean borehole employing primary and backup cutting elements.

BACKGROUND

Boreholes are formed in subterranean formations for various purposes including, for example, extraction of oil and gas from subterranean formations and extraction of geothermal heat from subterranean formations. Boreholes may be formed in subterranean formations using earth-boring tools such as, for example, drill bits and reamer devices.
To drill a borehole with a drill bit, the drill bit is rotated and advanced into the subterranean formation under an applied axial force, commonly known as “weight on bit,” or WOB. As the drill bit rotates, the cutting elements or abrasive structures thereof cut, crush, shear, and/or abrade away the formation material to form the borehole, depending on the type of bit and the formation to be drilled. A diameter of the borehole drilled by the drill bit may be defined by the cutting structures disposed at the largest outer diameter of the drill bit.
The drill bit is coupled, either directly or indirectly, to an end of what is referred to in the art as a “drill string,” which comprises a series of elongated tubular segments connected end-to-end that extends into the borehole from the surface of the formation. Often various subs and other components, such as a downhole motor, a steering sub or other assembly, a measuring while drilling (MWD) assembly, a ream while drilling (RWD) assembly, one or more stabilizers, or a combination of some or all of the foregoing, as well as the drill bit, may be coupled together at the distal end of the drill string at the bottom of the borehole being drilled. This assembly of components is referred to in the art as a “bottom hole assembly” (BHA).
The drill bit may be rotated within the borehole by rotating the drill string from the surface of the formation, or the drill bit may be rotated by coupling the drill bit to a down-hole motor, which is also coupled to the drill string and disposed proximate the bottom of the borehole. The downhole motor may comprise, for example, a hydraulic Moineau-type motor having a shaft, to which the drill bit is mounted, that may be caused to rotate by pumping fluid (e.g., drilling fluid or “mud”) from the surface of the formation down through the center of the drill string, through the hydraulic motor, out from nozzles in the drill bit, and back up to the surface of the formation through the annulus between the outer surface of the drill string and the exposed surface of the formation within the borehole. As noted above, when a borehole is being drilled in a formation, axial force or “weight” is applied to the drill bit (and reamer device, if used) to cause the drill bit to advance into the formation as the drill bit drills the borehole therein.
Boreholes may be enlarged by using a non-concentric drilling tool such as an eccentric bit or a bi-center bit. Eccentric bits or bi-center bits may be particularly useful in enlarging a borehole below a “tight” or undersized portion thereof. Eccentric bits or bi-center bits may also be particularly useful when performing a RWD process. Examples of eccentric bits and bi-center bits are disclosed in U.S. Pat. Nos. 4,635,738 and 5,957,223.
A bi-center bit generally includes a pilot bit section, which may be similar in configuration to the drill bits discussed previously. The bi-center bit also includes an eccentrically laterally extended or enlarged reamer bit portion that, when the bit is rotated about its drilling axis, produces an enlarged borehole. The smaller diameter pilot section is employed to commence the drilling and establish the drilling axis. Rotation of the bit remains centered about the drilling axis as the second, upper and larger radius, reamer bit section extending beyond the pilot bit section diameter to one side of the bit engages the formation to enlarge the borehole.
Rather than employing a one-piece drilling structure, such as an eccentric bit or a bi-center bit, to enlarge a borehole, an extended bottom hole assembly (extended bi-center assembly) with a pilot bit at the distal end thereof and a reamer assembly some distance above may also be employed to enlarge a borehole. This arrangement permits the use of any standard bit type (e.g., a rock bit or a drag bit) as the pilot bit, and the extended nature of the assembly permits greater drill string flexibility when passing through tight spots in the borehole as well as the opportunity to effectively stabilize the pilot bit so that the pilot hole and the following reamer will take the path intended for the borehole. The assignee of the present invention has designed reaming structures (so-called “reamer wings”) which generally comprise a tubular body having a fishing neck with a threaded connection at the top thereof, and a tong die surface at the bottom thereof, also with a threaded connection. Such reamer wings are disclosed in, for example, U.S. Pat. No. RE 36,817 to Pastusek et al. and U.S. Pat. No. 5,765,653 to Doster et al. both of which are assigned to the assignee of the present invention and the disclosure of each of which is incorporated in its entirety by this reference. The upper mid-portion of the reamer wing includes one or more longitudinally extending blades projecting generally radially outwardly from the tubular body, the outer edges of the blades carrying superabrasive cutting elements (e.g., polycrystalline diamond compacts (PDC)). The lower mid-portion of the reamer wing may include a stabilizing pad having an arcuate exterior surface the same or slightly smaller than the radius of the pilot hole on the exterior of the tubular body and longitudinally below the blades. The stabilizing pad may also be sized so that the rotational diameter traversed by the stabilizing pad may be the same as, or even greater than, the physical diameter of the pilot bit to enhance the stabilization provided by the stabilizing pad when engaging a pilot borehole of greater diameter than a physical diameter of the pilot bit. The stabilizer pad is characteristically placed on the opposite side of the tubular body with respect to the reamer wing blades so that the reamer wing will ride on the pad due to the resultant force vector generated by the cutting of the blade or blades as the enlarged borehole is cut.

BRIEF SUMMARY

In some embodiments, the present invention includes a bi-center bit for drilling subterranean formations. The bi-center bit includes a pilot bit section having a first gage diameter and a reamer bit section adjacent to the pilot bit section. The pilot bit section includes at least one primary cutting element and at least one backup cutting element rotationally trailing the at least one primary cutting element disposed thereon for engaging a subterranean formation. The at least one backup cutting element is offset from the at least one primary cutting element in a direction substantially transverse to an intended rotational path of the at least one primary cutting element during rotational operation of the bi-center bit. The reamer bit section includes at least one blade extending radially beyond the first gage diameter for rotationally engaging a subterranean formation.
In additional embodiments, a bi-center bit for drilling subterranean formations includes an eccentric reamer comprising at least one radially extending blade. The at least one radially extending blade includes at least one reamer cutting element disposed thereon for rotationally engaging a first portion of a subterranean borehole. A pilot bit coupled to the eccentric reamer includes at least two laterally adjacent primary cutting elements disposed on the pilot bit for engaging a subterranean borehole and at least one backup cutting element rotationally trailing the at least two primary cutting elements and disposed at least partially laterally intermediate the at least two primary cutting elements.
In yet additional embodiments, the present invention includes a method of drilling a subterranean borehole. The method includes engaging a first portion of a borehole with a portion of a reamer bit section of a drill bit and simultaneously engaging a second, opposing portion of the borehole with a portion of a pilot bit section adjacent to the reamer bit section. The second, opposing portion of the borehole may be engaged with at least two laterally adjacent primary cutting elements and with at least one backup cutting element rotationally trailing and disposed laterally intermediate the at least two primary cutting elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a bi-center bit in accordance with an embodiment of the present invention;

FIG. 2 is a face view, or view looking up from the bottom of a borehole, of the bi-center bit depicted in FIG. 1;

FIG. 3 is a face view, or view looking up from the bottom of a borehole, of a bi-center bit having primary cutting elements and backup cutting elements in accordance with another embodiment of the present invention; and

FIG. 4 is a face view, or view looking up from the bottom of a borehole, of a bi-center bit having primary cutting elements and multiple rows of backup cutting elements in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION

Illustrations presented herein are not meant to be actual views of any particular drill bit or other earth-boring tool, but are merely idealized representations that are employed to describe the present invention. Additionally, elements common between figures may retain the same numerical designation.
The various drawings depict embodiments of the invention as will be understood by the use of ordinary skill in the art and are not necessarily drawn to scale.
FIG. 1 comprises a side view of a bi-center bit in accordance with an embodiment of the present invention. As shown in FIG. 1, the depicted bit is illustrated in its normal drilling orientation for clarity. In an embodiment of the invention, a non-concentric earth-boring tool such as, for example, a bi-center bit 100 may include a pilot bit section 112. The pilot bit section 112 may comprise a fixed-cutting element drill bit include blades 118 having superabrasive cutting structures such as, for example, primary polycrystalline diamond compact (PDC) cutting elements 120 and backup PDC cutting elements 121 mounted thereto. Fluid courses 122 extending between blades 118 carry drilling fluid laden with cuttings sheared by primary cutting elements 120 and backup cutting elements 121 of the blades 118 drilling the pilot borehole into junk slots 124, which extend longitudinally on gage 126 of the bi-center bit 100 between gage pads 128. The gage pads 128 may be provided with a wear-resistant gage surface in the form of tungsten carbide bricks, natural diamonds, diamond-grit impregnated carbide, thermally stable diamond (TSP), or a combination thereof, as known in the art. Drilling fluid is introduced into fluid courses 122 from ports 132 on the bit face 130, which may include a nozzle 134 disposed therein.
The bi-center bit 100 also includes reamer bit section 114 (e.g., an eccentric reamer). The reamer bit section 114 may include radially extending blades 140 that may have primary PDC cutting elements 120 mounted thereto. As shown in FIG. 1, the blades 140 comprise any suitable number of blades 140 based on the size of the bi-center bit 100. In some embodiments, the blades 140 may be circumferentially spaced about 90° from each other about the reamer bit section 114. Ports 142 (which, again, may include a nozzle 134 disposed therein), located intermediate blades 140, feed drilling fluid into fluid courses 144 located rotationally in front of (in the direction of bit rotation) blades 140, to carry away formation cuttings sheared by the primary cutting elements 120 of blades 140 when enlarging the pilot borehole to full gage diameter. Blades 140 include truncated gage pads 146, which may also include a wear resistant surface of the types previously mentioned. The blades 140 may include an elongated gage pad 147 thereon. A bit shank 152, having a threaded pin connection, may be used to connect bi-center bit 100 to a drill collar or to an output shaft of a downhole motor, as known in the art. It is noted that while the embodiment of FIG. 1 illustrates a bi-center bit 100 having a pilot bit section 112 and a reamer bit section 114, the bi-center bit 100 may comprise any suitable drill bit attached to a reaming apparatus. For example, the bi-center bit 100 may comprise an assembly of a drag bit coupled to a reaming apparatus such as, for example, the reamer tool described in U.S. Pat. No. 6,695,080 to Presley et al., which is assigned to the assignee of the present invention and the disclosure of which is incorporated in its entirety by this reference.
FIG. 2 comprises a face view, or view looking up from the bottom of a borehole, of the bi-center bit 100 depicted in FIG. 1. As shown in FIG. 2, the pilot bit section 112 includes blades 118 thereon. The primary cutting elements 120 may be disposed along the blades 118 proximate to the leading edge of the blades 118 (taken in the direction of rotational travel of blades 118). The primary cutting elements 120 and backup cutting elements 121 may be placed and oriented on the blades 118 with the backup cutting element 121 located behind (i.e., rotationally trailing) the primary cutting elements 120. The bi-center bit 100 may include backup cutting elements 121 secured to the blades 118 in the shoulder region 138 of the pilot bit section 112 of the bi-center bit 100. The bi-center bit 100 may also include backup cutting elements 121 secured to the blades 118 in the cone region 136 of the pilot bit section 112 of the bi-center bit 100. The backup cutting elements 121 may be offset from the primary cutting elements 120, taken in a direction transverse to an intended rotational path 160 of the primary cutting elements 120 during rotational operation of the bi-center bit 100. For example, each of the backup cutting elements 121 may be mounted in pockets 110 rotationally trailing the primary cutting elements 120 and at a location on the bit profile at least partially laterally intermediate two associated, rotationally leading primary cutting elements 120 laterally spaced from one another, taken transverse to a direction of intended bit rotation. In other words, the primary cutting elements 120 and the backup cutting elements 121 each have a longitudinal axis extending at a tangent to the intended rotational path 160 of the primary and backup cutting elements 120, 121 during rotational operation of the bi-center bit 100. The longitudinal axes of the primary cutting elements 120 and the longitudinal axes of backup cutting elements 121 may be laterally offset such that the longitudinal axes of the primary cutting elements 120 and the backup cutting elements 121 are not coplanar.
In some embodiments, the backup cutting elements 121 may be disposed in a position rotationally trailing the primary cutting elements 120 and the longitudinal axis of each of the backup cutting elements 121 may extend substantially between and, in some embodiments, parallel to, the longitudinal axes of the primary cutting elements 120. It is noted that while the embodiment of FIG. 2 illustrates the backup cutting elements 121 laterally intermediate two laterally adjacent primary cutting elements 120 on the same blade 118, in some embodiments, the backup cutting elements 121 may be disposed laterally intermediate two primary cutting elements 120 on a different blade 118. For example, the backup cutting elements 121 on one rotationally trailing blade 118 may be disposed such that the rotational path 160 of the backup cutting elements 121 is at least partially laterally intermediate two laterally adjacent primary cutting elements 120 located on another rotationally leading blade 118. In other words, the backup cutting elements 121 on one blade 118 may travel in a rotational path between the kerfs made by the laterally adjacent primary cutting elements 120 on another blade 118 (i.e., the width of the cut made by the primary cutting elements 120 as they are rotated against a subterranean formation).
Referring to FIGS. 1 and 2, it will be appreciated by those of ordinary skill in the art that, at some locations along the bit profile, which extends from the centerline C/L (drilling axis) of the bit along the outer face surface or profile of blades 118 to gage pads 128, the at least partially intermediate location of the backup cutting elements 121 will be somewhat more radially than longitudinally (in the direction of centerline C/L) intermediate the locations of associated primary cutting elements 120. On the other hand, when adjacent or near gage pads 128 as on the shoulder region 138 of the pilot bit section 112, the at least partially intermediate location of a backup cutting elements 121 may approximate the radial locations of its associated primary cutting elements 120 while being somewhat more longitudinally intermediate primary cutting elements 120.
Referring to FIG. 2, to form the backup cutting element pockets 110 in the primary portion of the bi-center bit 100 a flat bottom milling tool cuts the drill bit body by plunging directly into the blade 118, 140 and travels along the center line of the cutting element 120 located in front thereof. In some embodiments, the bi-center bit 100 or a portion thereof (e.g., the pilot bit section 112) may be fabricated to comprise a particle-matrix composite material. A so-called “infiltration” bit includes a bit body comprising a particle-matrix composite material and is fabricated in a mold using an infiltration process. Recently, pressing and sintering processes have been used to form bit bodies of drill bits and other tools comprising particle-matrix composite materials. Such pressed and sintered bit bodies may be fabricated by pressing (e.g., compacting) and sintering a powder mixture that includes hard particles (e.g., tungsten carbide) and particles of a metal matrix material (e.g., a cobalt-based alloy, an iron-based alloy, or a nickel-based alloy). It should be understood, however, that the invention is not limited to steel body or particle-matrix composite-type bits, and bits of other manufacture may also be configured according to embodiments of the invention.
If the bi-center bit 100 or portions thereof is a particle-matrix type bit formed of sintered tungsten carbide particles in a suitable matrix, the backup cutting element pockets 110 in the bi-center bit 100 are formed by casing the backup cutting element pockets 110 in the bi-center bit 100. Methods of manufacturing the bi-center bit 100 as a particle-matrix composite bit are described in, for example, pending U.S. patent application Ser. No. 11/271,153, filed Nov. 10, 2005 and entitled “Earth-Boring Rotary Drill Bits and Methods of Forming Earth-Boring Rotary Drill Bits,” and pending U.S. patent application Ser. No. 11/272,439, filed Nov. 10, 2005 and entitled “Earth-Boring Rotary Drill Bits and Methods of Manufacturing Earth-Boring Rotary Drill Bits Having Particle-Matrix Composite Bit Bodies,” each of which is assigned to the assignee of the present invention and the disclosure of each of which application is incorporated herein in its entirety by this reference.
Referring still to FIG. 2, the intermediate placement of the backup cutting elements 121 may afford wear protection to the bi-center bit 100. For example, a matrix portion 158 of the blades 118 (e.g., a portion of the blades 118 surrounding the pockets 110 for receiving the cutting elements 120, 121 that may comprise steel, a cemented material, etc., depending on the type of drill bit materials selected) have been observed to wear unduly in certain drilling situations involving imbalance forces of the bit, bit vibration, and precession, including bit whirl. Such situations may occur frequently when drilling with a bi-center bit 100 having an eccentric portion (e.g., the reamer bit section 114). When drilling with a bi-center bit 100 having an eccentric portion, the centerline of a bit may be canted or tilted, or offset, with respect to the axis of the borehole, and side loading of the bit is of substantial magnitude due to the presence of the reamer bit section 114. Positioning the backup cutting elements 121 intermediate the primary cutting elements 120 may provide protection of the bi-center bit 100 by preventing undue matrix wear at a matrix portion 158 of the bi-center bit 100 by reducing the amount of wear on a matrix portion 158 of the blades 118 as the backup cutting elements 121 may tend to contact the subterranean borehole rather than a matrix portion 158 of the bi-center bit 100. Further, the backup cutting elements 121 may prevent failure of the cutting elements 120, 121 due to wear of the surrounding blade material (e.g., the matrix portion 158).
In some embodiments, the backup cutting elements 121 may be placed to substantially oppose the blades 140 of the reamer bit section 114. For example, the backup cutting elements 121 may be placed on blades 118 of the pilot bit section 112, which are opposite to (i.e., on opposing lateral sides of the bi-center bit 100) the blades 140 of the reamer bit section 114. As discussed above, the backup cutting elements 121 may be located laterally intermediate the primary cutting elements 120 and may at least partially prevent matrix wear of a matrix portion 158 of the blades 118 between the laterally adjacent primary cutting elements 120. Such wear may be at least partially caused by imbalance forces (e.g., a resultant force vector generated by the eccentric blades 140 of the reamer bit section 114) due to the cutting forces created by the primary cutting elements 120 generated as the enlarged borehole is cut. Such imbalance forces may cause a matrix portion 158 of the blades 118 to contact (e.g., rub against) portions of the subterranean borehole during a drilling operation. Excessive contact between the matrix portion 158 of the blades 118 and the subterranean borehole may result in wear and, ultimately, failure of the bi-center bit 100. Locating the backup cutting elements 121 laterally intermediate the primary cutting elements 120 may reduce the amount of wear on a matrix portion 158 of the blades 118 as the backup cutting elements 121 may tend to contact the subterranean borehole rather than a matrix portion 158 of the bi-center bit 100.
The exposure of the backup cutting elements 121 (i.e., the distance the cutting elements 120, 121 extend away from the surface of the bi-center bit 100) may vary from the primary cutting elements 120 or may vary between the backup cutting elements 121. For example, the backup cutting elements 121 may be underexposed relative to the primary cutting elements 120 along the cutting element profile for a blade 118. In some embodiments, the exposure of the backup cutting elements 121 may vary depending on the location of the backup cutting elements 121. For example, the backup cutting elements 121 located in a cone region 136 of the pilot bit section 112 may exhibit relatively less exposure as compared to the backup cutting elements 121 located in the shoulder region 138 of the pilot bit section 112. For example, the backup cutting elements 121 located in the cone region 136 may be underexposed (e.g., by approximately 0.025 inch (0.635 millimeter)) from the primary cutting elements 120 while the backup cutting elements 121 located in the shoulder region 138 may have a exposure substantially equal to or only slightly less than the exposure of the primary cutting elements 120 (e.g., by approximately 0.01 inch (0.254 millimeter)).
In some embodiments, the primary cutting elements 120 and the backup cutting elements 121 of the bi-center bit 100 may each have similar or differing back rake and side rake angles such as the cutting elements described in, for example, U.S. patent application Ser. No. 12/498,516, which was filed Jul. 7, 2009 and entitled “Backup Cutting Elements on Non-Concentric Reaming Tools,” which is assigned to the assignee of the present invention and the disclosure of which is incorporated herein in its entirety by this reference.
As shown in FIG. 3, in some embodiments, backup cutting elements 221, 222 may have a differing orientation, geometry, or material composition than that of the primary cutting elements 120. A bi-center bit 200 may be substantially similar to the bi-center bit 100 shown and described with reference to FIGS. 1 and 2. As shown in FIG. 3, the backup cutting elements 221 may be positioned on a pilot bit section 212 in a similar manner and the backup cutting elements 121 shown and described with reference to FIGS. 1 and 2. However, the backup cutting elements 221 may have a relatively larger back rack angle than the primary cutting elements 120. For example, the backup cutting elements 221 may be positioned to have approximately a 90° back rake angle. Such a backup cutting element 221 is described in, for example, U.S. Pat. No. 6,408,958 to Isbell et al., which is assigned to the assignee of the present invention and the disclosure of which is incorporated in its entirety by this reference. The backup cutting elements 221 may be oriented substantially transverse to the bit face with the sides of superabrasive tables 223 of the backup cutting elements 221 facing in an intended rotational path 160 of the bi-center bit 200. The substantially transverse orientation of the backup cutting elements 221 may provide support to the pilot bit section 212 as the backup cutting elements 221 are a substantially radially unaggressive structure to a formation (e.g., a subterranean borehole). In some embodiments, the backup cutting element 221 may comprise a bearing block. In such an embodiment, the backup cutting element 221 may provide a designed bearing or rubbing area affording a surface area specifically tailored to provide support for the bi-center bit 200 under a force (e.g., an imbalance force, an axial WOB, etc.) on a selected formation being drilled without exceeding the compressive strength thereof.
As further shown in FIG. 3, in some embodiments, backup cutting elements 222 may have a differing geometry than that of the primary cutting elements 120. For example, the backup cutting elements 222 may have a substantially ovoid shape and may extend between the longitudinal axes of the primary cutting elements 120. It is noted that while the embodiment of FIG. 3 illustrates the backup cutting elements 221, 222 having a circular or ovoid shape, the backup cutting elements 221, 222 may comprise any suitable shape such as, for example, semi-circular, rectangular, tombstone, triangular, etc. In additional embodiments, the backup cutting elements 221, 222 may have a differing material composition than that of the primary cutting elements 120. For example, the backup cutting elements 221, 222 may comprise another type of synthetic diamond such as TSP. As also shown in FIG. 3, in some embodiments, the backup cutting elements 221, 222 may only be disposed on blades 218 of the pilot bit section 212 which are opposite to (i.e., on opposing lateral sides of the bi-center bit 200) the blades 140 of the reamer bit section 214.
FIG. 4 is a face view, or view looking up from the bottom of a borehole, of a bi-center bit 300 having a row of primary cutting elements 120 and multiple rows of backup cutting elements 321, 322 thereon, as may be desired or required. As shown in FIG. 4, the pilot bit section 312 have a first row of backup cutting elements 321 that may rotationally trail the primary cutting elements 120 and may be at a location on the bit profile at least partially laterally offset from rotationally leading primary cutting elements 120. As shown on blade 318, in some embodiments, a second row of backup cutting elements 322 may rotationally trail the first row of backup cutting elements 321 and may be at a location on the bit profile at least partially laterally offset from the rotationally leading first row of backup cutting elements 321. As shown on blade 319, in other embodiments, the second row of backup cutting elements 322 may rotationally trail the first row of backup cutting elements 321 and may be located rotationally behind the rotationally leading first row of backup cutting elements 321.
While the present invention has been disclosed herein with reference to illustrated embodiments, those of ordinary skill in the art will understand and appreciate that the invention is not so limited, and that additions, deletions and modifications to the disclosed embodiments may be made without departing from the scope of the invention. The present invention is limited only by the appended claims and their legal equivalents, which include within their scope all equivalent devices and methods according to principles of the invention as described.

Claims

1. A bi-center bit for drilling subterranean formations, comprising:

a pilot bit section having a first gage diameter and including at least one primary cutting element and at least one backup cutting element rotationally trailing the at least one primary cutting element disposed thereon for engaging a subterranean formation, wherein the at least one backup cutting element is offset from the at least one primary cutting element in a direction substantially transverse to an intended rotational path of the at least one primary cutting element during rotational operation of the bi-center bit; and

a reamer bit section adjacent to the pilot bit section, the reamer bit section comprising at least one blade extending radially beyond the first gage diameter for rotationally engaging a subterranean formation.

2. The bi-center bit of claim 1, wherein the pilot bit section comprises a plurality of blades, each blade of the plurality of blades having a plurality of primary cutting elements and a plurality of backup cutting elements rotationally trailing at least one primary cutting element of the plurality of primary cutting elements.

3. The bi-center bit of claim 2, wherein at least one backup cutting element of the plurality of backup cutting elements on at least one blade of the plurality of blades is offset from at least one primary cutting element of the plurality of primary cutting elements on at least another blade of the plurality of blades.

4. The bi-center bit of claim 1, wherein the reamer bit section comprises a reamer wing and wherein the pilot bit section comprises a fixed-cutting element drill bit, the reamer wing and the fixed-cutting element drill bit being coupled to form the bi-center bit.

5. The bi-center bit of claim 1, wherein the at least one blade of the reamer bit section comprises two blades extending radially beyond the first gage diameter.

6. The bi-center bit of claim 5, wherein the at least one backup cutting element comprises a plurality of backup cutting elements and wherein at least one backup cutting element of the plurality of backup cutting elements is positioned to substantially laterally oppose at least one blade of the two blades of the reamer bit section.

7. The bi-center bit of claim 6, wherein at least one backup cutting element of the plurality of backup cutting elements is positioned on a shoulder region of the pilot bit section, wherein at least another backup cutting element of the plurality of backup cutting elements is positioned on a cone region of the pilot bit section, and wherein the at least one backup cutting element of the plurality of backup cutting elements has an exposure greater than the exposure of the at least another backup cutting element of the plurality of backup cutting elements.

8. The bi-center bit of claim 1, further comprising at least one additional backup cutting element rotationally trailing the at least one backup cutting element.

9. The bi-center bit of claim 1, wherein the at least one backup cutting element is oriented at a back rake angle of about ninety (90) degrees.

10. The bi-center bit of claim 9, wherein the at least one backup cutting element comprises an ovoid shape.

11. A bi-center bit for drilling subterranean formations, comprising:

an eccentric reamer comprising at least one radially extending blade, the at least one radially extending blade including at least one reamer cutting element disposed thereon for rotationally engaging a first portion of a subterranean borehole; and

a pilot bit coupled to the eccentric reamer comprising:

at least two primary cutting elements disposed on the pilot bit for engaging a subterranean borehole; and

at least one backup cutting element rotationally trailing the at least two primary cutting elements and disposed at least partially laterally intermediate the at least two primary cutting elements.

12. The bi-center bit of claim 11, wherein the at least one reamer cutting element of the eccentric reamer is configured to rotationally engage a first portion of a subterranean borehole while the at least one backup cutting element of the pilot bit is configured to rotationally engage a second, opposing portion of a subterranean borehole.

13. The bi-center bit of claim 11, wherein the at least one backup cutting element is disposed laterally intermediate the at least two primary cutting elements at an equal lateral distance from each of the at least two primary cutting elements.

14. The bi-center bit of claim 11, wherein at least one radially extending blade of the eccentric reamer comprises two radially extending, circumferentially spaced blades.

15. The bi-center bit of claim 14, wherein the at least one backup cutting element comprises a plurality of backup cutting elements and wherein at least one backup cutting element of the plurality of backup cutting elements is positioned to substantially laterally oppose at least one radially extending blade of the two radially extending, circumferentially spaced blades.

16. A method of drilling a subterranean borehole, the method comprising:

engaging a first portion of a borehole with a portion of a reamer bit section of a drill bit; and

simultaneously engaging a second, opposing portion of the borehole with a portion of a pilot bit section adjacent to the reamer bit section, comprising:

engaging the second, opposing portion of the borehole with at least two primary cutting elements; and

engaging the second, opposing portion of the borehole with at least one backup cutting element rotationally trailing and disposed laterally intermediate the at least two primary cutting elements.

17. The method of claim 16, further comprising positioning at least two backup cutting elements on a shoulder region of the pilot bit section.

18. The method of claim 17, wherein positioning at least two backup cutting elements on a shoulder region of the drill bit comprises at least partially preventing wear of a matrix portion of the pilot bit by locating the at least two backup cutting elements on a shoulder region of the pilot bit section.

19. The method of claim 16, further comprising positioning a plurality of backup cutting elements on a portion of the pilot bit section.

20. The method of claim 19, wherein positioning a plurality of backup cutting elements on a portion of the pilot bit section comprises positioning at least one backup cutting elements of the plurality of backup cutting elements on a shoulder region of the pilot bit section to exhibit an exposure greater than an exposure of at least one backup cutting elements of the plurality of backup cutting elements on a cone region of the pilot bit section.