Hybrid drill bit and design method

Abstract

A hybrid earth-boring bit comprising a bit body having a central axis, at least one, preferably three fixed blades, depending downwardly from the bit body, each fixed blade having a leading edge, and at least one rolling cutter, preferably three rolling cutters, mounted for rotation on the bit body. A rolling cutter is located between two fixed blades.

Claims

1. A hybrid bit, comprising: a cutting profile extending from a cone region to a gage region of the hybrid bit; a first rolling cutter assembly, a second rolling cutter assembly, and a third rolling cutter assembly, wherein each rolling cutter assembly of the first, second, and third rolling cutter assemblies is truncated in length and has a plurality of cutting elements configured to remove formation in nose and shoulder regions of the cutting profile, the first, second, and third rolling cutter assemblies establishing a rolling cutter aggressiveness for the hybrid bit; and a first fixed blade, a second fixed blade, and a third fixed blade, wherein each fixed blade of the first, second, and third fixed blades has a plurality of cutting elements configured to remove formation from at least the cone region adjacent a centerline of the hybrid bit, the first, second, and third fixed blades establishing a fixed blade aggressiveness for the hybrid bit, wherein at least one cutting element of the plurality on one of the first, second, and third fixed blades and at least one cutting element of the plurality on one of the first, second, and third rolling cutter assemblies are aligned to cut a same swath, wherein the first rolling cutter assembly and the first fixed blade are spaced apart by a first angular distance and the second rolling cutter assembly and the second fixed blade are spaced apart by a second angular distance, the first angular distance being different from the second angular distance, wherein the third rolling cutter assembly is angularly spaced apart from the third fixed blade by a third angular distance, the third angular distance being different from the first angular distance and the second angular distance, and wherein a bit aggressiveness of the hybrid bit is predetermined as a function of the rolling cutter aggressiveness and the fixed blade aggressiveness.

2. The hybrid bit of claim 1, wherein the bit aggressiveness is predetermined based at least partially on a predetermined angular distance between the first rolling cutter assembly and the first fixed blade.

3. The hybrid bit of claim 1, wherein a projection of the at least one cutting element on the first rolling cutter assembly is the same as a projection of the at least one cutting element on the first fixed blade.

4. The hybrid bit of claim 1, wherein the first fixed blade leads the first rolling cutter assembly to increase the bit aggressiveness.

5. The hybrid bit of claim 1, wherein the first fixed blade trails the first rolling cutter assembly to decrease the bit aggressiveness.

6. The hybrid bit of claim 1, wherein a means for establishing a predetermined bit aggressiveness comprises a predetermined angular distance between at least one of first and second rolling cutter assemblies and at least one of the first and second fixed blades.

7. The hybrid bit of claim 1, wherein the bit aggressiveness is predetermined based at least partially on an effective projection of one or more of the plurality of cutting elements on at least one of the first, second, or third rolling cutter assemblies.

8. The hybrid bit of claim 1, wherein the at least one of the first, second, or third fixed blades leads at least one of the first, second, or third rolling cutter assemblies to increase the bit aggressiveness.

9. The hybrid bit of claim 1, wherein at least one of the first, second, or third fixed blades trails at least one of the first, second, or third rolling cutter assemblies to decrease the bit aggressiveness.

10. The hybrid bit of claim 1, wherein the first rolling cutter assembly is angularly spaced about 120 degrees apart from the second rolling cutter assembly.

11. The hybrid bit of claim 1, wherein the first fixed blade is angularly spaced about 180 degrees apart from the first rolling cutter assembly.

12. A method of drilling subterranean formations, comprising: drilling with a first bit comprising: a first fixed blade, a second fixed blade, and a third fixed blade, wherein at least one of the fixed blades of the first, second, or third fixed blades has a first row of cutting elements arranged on a leading edge and configured to remove formation in cone, nose, and shoulder regions; a first truncated rolling cutter assembly, a second truncated rolling cutter assembly, and a third truncated rolling cutter assembly, wherein at least one of the truncated rolling cutter assemblies of the first, second, or third truncated rolling cutter assemblies has a plurality of rows of cutting elements configured to remove formation in at least the shoulder region, but not in the cone region; and wherein the first truncated rolling cutter assembly and the first fixed blade are spaced apart by a first angular distance and the second truncated rolling cutter assembly and the second fixed blade are spaced apart by a second angular distance, the first angular distance being different from the second angular distance, wherein the third truncated rolling cutter assembly is angularly spaced apart from the third fixed blade by a third angular distance, the third angular distance being different from the first angular distance and the second angular distance, and; determining aggressiveness of drilling with the first bit as a function of rate of formation penetration and weight-on-bit; and thereafter; and varying the aggressiveness of drilling by: drilling with another bit having an angular displacement between a truncated rolling cutter and a fixed blade cutter that is different than an angular displacement of first bit; or drilling with another bit having an effective projection between at least two adjacent cutting elements on a truncated rolling cutter that is different than an effective projection between adjacent cutting elements of at least one truncated rolling cutter assembly of the first bit; or drilling with another bit in which cutting elements on a rolling cutter lead cutting elements on a fixed blade more than on the first bit; or drilling with another bit in which cutting elements on a fixed blade lead cutting elements on a rolling cutter more than on the first bit; or drilling with another bit having cutting elements on opposing at least one fixed blade and cutting elements on at least one truncated rolling cutter such that the cutting elements track in the same kerf.

13. The method of claim 12, wherein the first bit further comprises a first cutting element and a second cutting element attached to the at least one truncated rolling cutter assembly of the first truncated rolling cutter assembly, a second truncated rolling cutter assembly, or a third truncated rolling cutter assembly are configured such that only one of the first cutting element and the second cutting element engages independently during drilling.

14. The method of claim 13, wherein at least one of the first fixed blade, the second fixed blade, or the third fixed blade of the first bit further comprises at least one row of back cutters aligned to cut formation in a same swath as cut by the first row of cutting elements.

15. The method of claim 12, wherein the first truncated rolling cutter assembly and the second truncated rolling cutter assembly of the first bit are angularly spaced about 120 degrees apart.

16. The method of claim 12, wherein the first bit comprises an equal number of fixed blades and truncated rolling cutter assemblies.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph illustrating the relative aggressiveness of a rolling-cutter bit, a fixed-blade cutter bit having polycrystalline diamond cutters or PDC bit, and embodiments of hybrid bits of the present disclosure.

(2) FIG. 2 is an elevation view of a hybrid earth-boring bit illustrative of the present invention.

(3) FIG. 3 is a bottom plan form view of the hybrid earth-boring bit of FIG. 2.

(4) FIG. 3A is a profile view of cutting elements of three fixed-blade cutters and cutting elements of three rolling cutters of an embodiment of a hybrid bit of the present disclosure of FIGS. 1 through 3.

(5) FIG. 3B is a profile view of cutting elements of a first fixed-blade cutter and cutting elements of a first rolling cutter of an embodiment of a hybrid bit of the present invention;

(6) FIG. 3C is a profile view of cutting elements of a second fixed-blade cutter and cutting elements of a second rolling cutter of an embodiment of a hybrid bit of the present invention;

(7) FIG. 3D is a view of cutting elements of a third fixed-blade cutter and cutting elements of a third rolling cutter of an embodiment of a hybrid bit of the present invention;

(8) FIG. 3E is a view of FIG. 3 showing a pair of a rolling cutter and a fixed-blade cutter of a hybrid bit of FIG. 3 of the present invention.

(9) FIG. 3F is a view of FIG. 3 showing another fixed-blade cutter and another rolling cutter of a hybrid bit of FIG. 3 of the present invention.

(10) FIG. 4 is a bottom plan form view of another embodiment of a hybrid earth-boring bit of the present invention.

(11) FIGS. 5 and 6 are partial schematic views of rolling cutters and cutting elements of rolling cutters interfacing with the formation being drilled.

DETAILED DESCRIPTION

(12) Turning now to the drawing figures, and particularly to FIG. 1, the characteristics of various embodiments of the present invention are described. FIG. 1 is a graph of rate-of-penetration (ROP on y-axis) versus weight-on-bit (WOB on x-axis) for earth-boring bits such as a fixed-blade cutter bit, a hybrid bit of the present invention, and a three rolling-cutter bit (three roller-cone bit). The data for the bits illustrated in the graph was generated using 12-inch bits on the simulator of Baker Hughes, a GE Company, formerly known as Hughes Christensen in The Woodlands, Tex. The conditions were 4000 pounds per square inch of bottom-hole pressure, 120 bit revolutions per minute, and 9.5 pounds per gallon drilling fluid or mud while drilling Carthage marble. The data used and reflected in FIG. 1 is intended to be general and to reflect general characteristics for the three types of bits, such as fixed-blade cutter bits having PDC cutting elements, hybrid bits including variations thereof of the present disclosure, and rolling-cutter bits (roller-cone bits) whose cutting aggressiveness characteristics are illustrated.

(13) The graph shows the performance characteristics of three different types of earth-boring bits: a three rolling-cutter bit (three roller cones), a six blade fixed cutter bit having PDC cutting elements, and a hybrid bit having both (three) rolling cutters and (three) fixed-blade cutters. As shown, each type of bit has a characteristic line. The six fixed-blade cutter bit having PDC cutting elements has the highest ROP for a given WOB resulting in a line having the steepest slope of the line showing cutting performance of the bit. However, the PDC bit could not be run at high weight-on-bit because of high vibrations of the bit. The three rolling-cutter bit (three roller-cone bit) has the lowest ROP for a given WOB resulting in a line having the shallowest slope of the line showing cutting performance of the bit. The hybrid bit in the three embodiments of the present invention exhibits intermediate ROP for a given WOB resulting in lines having an intermediate slopes of the lines showing cutting performance of the bit between the lines for the fixed-blade cutter bit and the three rolling-cutter bit.

(14) The slope of the line (curve) plotted for ROP versus WOB for a given bit can be termed or defined as the bit's cutting aggressiveness or simply Aggressiveness as used herein. Aggressiveness, for purposes of this application and the disclosure described herein, is defined as follows:
(1) Aggressiveness=Rate-of-Penetration (ROP)/Weight-on-Bit (WOB)(1)
Thus aggressiveness, as the mathematical slope of a line, has a value greater than zero. Measured purely in terms of aggressiveness, it would seem that fixed-blade cutter bits would be selected in all instances for drilling. However, other factors come into play. For example, there are limits on the amount of WOB and torque to turn the bit that can be applied, generally based on either the drilling application or the capacity of the drill string and drilling rig. For example, as WOB on a fixed-blade cutter bit increases the drill string torque requirement increases rapidly, especially with fixed-blade cutter bits, and erratic torque can cause harmful vibrations. Rolling-cutter bits, on the other hand, require high WOB which, in the extreme, may buckle a bottom hole assembly or exceed the load bearing capacity of the cutter bearings of the rolling cutters of the rolling-cutter bit. Accordingly, different types of bits, whether a fixed-blade cutter bit, a rolling-cutter bit, or a hybrid bit, have different advantages in different situations. One aspect of the present invention is to provide a method for the design of a hybrid earth-boring bit so that its aggressiveness characteristics can be tailored or varied to the drilling application.

(15) FIGS. 2, 3, and 4 illustrate embodiments of hybrid earth-boring bits 11 according to the present invention. Hybrid bit 11 comprises a bit body 13 that is threaded or otherwise configured at its upper extent for connection into a drill string. Bit body 13 may be constructed of steel, or of a hard-metal (e.g., tungsten carbide) matrix material with steel inserts. Bit body 13 has an axial center or centerline 15 that coincides with the axis of rotation of hybrid bit 11 in most instances. The illustrated hybrid bit 11 is a 12-inch bit. The hybrid bit 11 shown in FIG. 3 is used to exemplify the techniques of adjusting the aggressiveness of a hybrid bit according to the present invention, i.e., cutter-leading, blade-leading, and cutter-blade opposite, as described herein. One of the embodiments of the hybrid bits of the present disclosure illustrated in FIG. 3, is likely not a desirable production hybrid bit design when the hybrid bit is an all blade-leading design because aggressiveness of the hybrid bit is too great for certain types of formations, but not all types of formations. That is, if the hybrid bit is a hybrid bit having an all blade-leading design, it acts more as a fixed-blade cutter bit. As illustrated in FIG. 1, aggressiveness of such hybrid bit is high which might adversely affect its durability and dynamic stability.

(16) Illustrated in FIG. 2 and FIG. 3, at least one bit leg (two of three are shown in FIG. 2) 17, 19, 21 depends axially downwardly from the bit body 13. In the illustrated embodiment, a lubricant compensator is associated with each bit leg to compensate for pressure variations in the lubricant provided for the bearing. In between each bit leg 17, 19, 21, at least one fixed-blade cutter 23, 25, 27 depends axially downwardly from bit body 13.

(17) A rolling cutter 29, 31, 33 is mounted for rotation (typically on a journal bearing, but rolling element or other bearings may be used as well) on each bit leg 17, 19, 21. Each rolling cutter 29, 31, 33 has a plurality of cutting elements 35, 37, 39 arranged in generally circumferential rows thereon. In the illustrated embodiment, cutting elements 35, 37, 39 are tungsten carbide inserts, each insert having an interference fit into bores or apertures formed in each rolling cutter 29, 31, 33. Alternatively, cutting elements 35, 37, 39 can be integrally formed with the cutter and hardfaced, as in the case of steel- or milled-tooth cutters. Materials other than tungsten carbide, such as polycrystalline diamond or other superhard or superabrasive materials, can also be used for rolling-cutter cutting elements 35, 37, 39 on rolling cutters 29, 31, 33.

(18) A plurality of cutting elements 41, 43, 45 is arranged in a row on the leading edge of each fixed-blade cutter 23, 25, 27. Each cutting element 41, 43, 45 is a circular disc of polycrystalline diamond mounted to a stud of tungsten carbide or other hard metal, which is, in turn, soldered, brazed or otherwise secured to the leading edge of each fixed-blade cutter. Thermally stable polycrystalline diamond (TSP) or other conventional fixed-blade cutting element materials may also be used. Each row of cutting elements 41, 43, 45 on each of the fixed-blade cutters 23, 25, 27 extends from the central portion of bit body 13 to the radially outermost or gage portion or surface of bit body 13. On at least one of the rows on one of the fixed-blade cutters 23, 25, 27, a cutting element 41 on a first fixed-blade cutter 23 is located at or near the central axis or centerline 15 of bit body 13 (at or near meaning some part of the fixed cutter is at or within about 0.040 inch of the axial centerline 15). In the illustrated embodiment, the radially innermost cutting element 41 in the row on fixed-blade cutter 23 has its circumference tangent to the axial center or centerline 15 of the bit body 13 and hybrid bit 11.

(19) A plurality of flat-topped, wear-resistant inserts 51 formed of tungsten carbide or similar hard metal with a polycrystalline diamond cutter attached thereto are provided on the radially outer most or gage surface of each fixed-blade cutter 23, 25, 27. These serve to protect this portion of the bit from abrasive wear encountered at the sidewall of the borehole. Also, a row or any desired number of rows of backup cutters 53 is provided on each fixed-blade cutter 23, 25, 27 between the leading and trailing edges thereof. Backup cutters 53 may be aligned with the main or primary cutting elements 41, 43, 45 on their respective fixed-blade cutters 23, 25, 27 so that they cut in the same swath or kerf or groove as the main or primary cutting elements on a fixed-blade cutter. Alternatively, they may be radially spaced apart from the main fixed-blade cutting elements so that they cut in the same swath or kerf or groove or between the same swaths or kerfs or grooves formed by the main or primary cutting elements on their respective fixed-blade cutters. Additionally, backup cutters 53 provide additional points of contact or engagement between the hybrid bit 11 and the formation being drilled, thus enhancing the stability of hybrid bit 11.

(20) In the embodiments of the disclosure illustrated in FIG. 3, rolling cutters 29, 31, 33 are angularly spaced approximately 120 degrees apart from each other (measured between their axes of rotation). The axis of rotation of each rolling cutter 29, 31, 33 intersects the axial center 15 of bit body 13 (FIG. 2) or hybrid bit 11, although each or all of the rolling cutters 29, 31, 33 may be angularly skewed by any desired amount and (or) laterally offset so that their individual axes do not intersect the axial center of bit body 13 (FIG. 2) or hybrid bit 11. As illustrated, a first rolling cutter 29 is spaced apart 58 degrees from a first fixed-blade cutter 23 (measured between the axis of rotation of rolling cutter 29 and the centerline of first fixed-blade cutter 23 in a clockwise manner in FIG. 3) forming a pair of cutters. A second rolling cutter 31 is spaced 63 degrees from a second fixed-blade cutter 25 (measured similarly) forming a pair of cutters; and a third rolling cutter 33 is spaced 53 degrees apart from a third fixed-blade cutter 27 (again measured the same way) forming a pair of cutters.

(21) In FIG. 3A, a cutting profile for the fixed cutting elements 41, 45, 43 on fixed-blade cutters 23, 25, 27 (not shown) and cutting elements 35, 37, 39 on rolling cutters 29, 33, 31 are generally illustrated. As illustrated, an innermost cutting element 41 on first fixed-blade cutter 23 is tangent to the axial centerline 15 of the bit body 13 or hybrid bit 11. The innermost cutting element 43 on third fixed-blade cutter 27 is illustrated. Also, innermost cutting element 45 on second fixed-blade cutter 25 is also illustrated. A cutting element 35 on rolling cutter 29 is illustrated having the same cutting depth or exposure and cutting element 41 on first fixed-blade cutter 23 each being located at the same centerline and cutting the same swath or kerf or groove. Some cutting elements 41 on first fixed-blade cutter 23 are located in the cone of the hybrid bit 11, while other cutting elements 41 are located in the nose and shoulder portion of the hybrid bit 11 having cutting elements 35 of rolling cutter 29 cutting the same swath or kerf or groove generally in the nose and shoulder of the hybrid bit 11 out to the gage thereof. Cutting elements 35, 37, 39 on rolling cutters 29, 33, 31 do not extend into the cone of the hybrid bit 11 but are generally located in the nose and shoulder of the hybrid bit 11 out to the gage of the hybrid bit. Further illustrated in FIG. 3A are the cutting elements 37, 39 on rolling cutters 31 and 33 and their relation to the cutting elements 43 and 45 on fixed-blade cutters 27, 25 cutting the same swath or kerf or groove either being centered thereon or offset in the same swath or kerf or groove during a revolution of the hybrid drill bit 11. While each cutting element 41, 45, 43 and cutting element 35, 37, 39 has been illustrated having the same exposure of depth of cut so that each cutting element cuts the same amount of formation, the depth of cut may be varied in the same swath or kerf or groove, if desired.

(22) Illustrated in FIG. 3B is a cutting profile for the fixed cutting elements 41 on first fixed-blade cutter 23 and cutting elements 35 on rolling cutter 29 in relation to the each other, the first fixed-blade cutter 23 and the rolling cutter 29 forming a pair of cutters on hybrid bit 11. As illustrated, some of the cutting elements 41 on first fixed-blade cutter 23 and cutting elements 35 on rolling cutter 29 both have the same center and cut in the same swath or kerf or groove while other cutting elements 41 on fixed-blade cutter 23 and cutting elements 35 on rolling cutter 29 do not have the same center but still cut in the same swath or kerf or groove. As illustrated, all the cutting elements 41 and 41 on fixed-blade cutter 23 and cutting elements 35 and 35 on rolling cutter 29 have the same exposure to cut the same depth of formation for an equal cut of the formation during a revolution of the hybrid drill bit 11, although this may be varied as desired. Further illustrated in FIG. 3B in broken lines, backup cutters 53 on first fixed-blade cutter 23 located behind cutting elements 41 may have the same exposure of cut as cutting elements 41 or less exposure of cut as cutting elements 41 and have the same diameter or a smaller diameter than a cutting element 41. Additionally, backup cutters 53 while cutting in the same swath or kerf or groove as a cutting element 41 may be located off the center of a cutting element 41 located in front of a backup cutter 53 associated therewith. In this manner, cutting elements 41 and backup cutters 53 on first fixed-blade cutter 23 and cutting elements 35 on rolling cutter 29 will all cut in the same swath or kerf or groove while being either centered on each other or slightly off-centered from each other having the same exposure of cut or, in the alternative, a lesser exposure of cut.

(23) Illustrated in FIG. 3C is a cutting profile for the fixed cutting elements 43 on third fixed-blade cutter 27 in relation to the cutting elements 37 on third rolling cutter 33, the third fixed-blade cutter 27 and the third rolling cutter 33 forming a pair of cutters on hybrid bit 11. As illustrated, some of the cutting elements 43 on fixed-blade cutter 27 and cutting elements 37 on third rolling cutter 33 both have the same center and cutting in the same swath or kerf or groove while other cutting elements 43 on fixed-blade cutter 23 and cutting elements 37 on rolling cutter 33 do not have the same center but cut in the same swath or kerf or groove. As illustrated, all the cutting elements 43 and 43 on fixed-blade cutter 27 and cutting elements 37 and 37 on rolling cutter 33 have the same exposure to cut the same depth of formation for an equal cut of the formation during a revolution of the hybrid drill bit 11, although this may be varied as desired. Further illustrated in FIG. 3C in broken lines, backup cutters 53 on third fixed-blade cutter 27 located behind cutting elements 43 may have the same exposure of cut as cutting elements 43 or less exposure of cut as cutting elements 43 and have the same diameter or a smaller diameter than a cutting element 43. Additionally, backup cutters 53 while cutting in the same swath or kerf or groove as a cutting element 43 may be located off the center of a cutting element 43 associated therewith. In this manner, cutting elements 43 and backup cutters 53 on third fixed-blade cutter 27 and cutting elements 37 on third rolling cutter 33 will all cut in the same swath or kerf or groove while being either centered on each other or slightly off-centered from each other having the same exposure of cut or, in the alternative, a lesser exposure of cut.

(24) Illustrated in FIG. 3D is a cutting profile for the fixed cutting elements 45 on second fixed-blade cutter 25 in relation to cutting elements 39 on second rolling cutter 31 forming a pair of cutters on hybrid bit 11. As illustrated, some of the cutting elements 45 on second fixed-blade cutter 25 and cutting elements 39 on second rolling cutter 31 both have the same center and cutting in the same swath or kerf or groove while other cutting elements 45 on fixed-blade cutter 25 and cutting elements 39 on rolling cutter 31 do not have the same center but cut in the same swath or kerf or groove. As illustrated, all the cutting elements 45 and 45 on fixed-blade cutter 25 and cutting elements 39 and 39 on rolling cutter 33 have the same exposure to cut the same depth of formation for an equal cut of the formation, although this may be varied as desired. As illustrated, all the cutting elements 45 and 45 on fixed-blade cutter 25 and cutting elements 39 and 39 on rolling cutter 31 have the same exposure to cut the same depth of formation for an equal cut of the formation during a revolution of the hybrid drill bit 11. Further illustrated in FIG. 3D in broken lines, backup cutters 53 on second fixed-blade cutter 25 located behind cutting elements 45 may have the same exposure of cut as cutting elements 45 or less exposure of cut as cutting elements 45 and have the same diameter or a smaller diameter than a cutting element 45. Additionally, backup cutters 53 while cutting in the same swath or kerf or groove as a cutting element 45 may be located off the center of a cutting element 45 associated therewith. In this manner, cutting elements 45 and backup cutters 53 on second fixed-blade cutter 25 and cutting elements 39 on second rolling cutter 31 will all cut in the same swath or kerf or groove while being either centered on each other or slightly off-centered from each other having the same exposure of cut or, in the alternative, a lesser exposure of cut.

(25) When considering a pair of cutters of the hybrid bit 11 including a rolling cutter and a fixed-blade cutter, each having cutting elements thereon, having the same exposure of cut, and located at the same radial location from the axial center of the hybrid bit 11 cutting the same swath or kerf or groove, adjusting the angular spacing between rolling cutters 29, 31, 33, and fixed-blade cutters 23, 25, 27 is one way in which to adjust the cutting aggressiveness or aggressiveness of a hybrid bit 11 according to the present invention. When considering a pair of cutters having cutting elements thereon having the same exposure of cut and located at the same radial location from the axial centerline 15 of the hybrid bit 11 cutting the same swath or kerf or groove on the hybrid bit 11, the closer a rolling cutter 29 is to a first fixed-blade cutter 23 of the pair of cutters of the hybrid bit 11, the rolling cutter 29 is the primary cutter of the pair with the first fixed-blade cutter 23 cutting less of the pair. Spacing a rolling cutter 29 closer to a first fixed-blade cutter 23 of a pair of cutters on the hybrid bit 11 causes the rolling cutter 29 to have a more dominate cutting action of the pair of cutters thereby causing the hybrid bit 11 to have less cutting aggressiveness or aggressiveness. Spacing a rolling cutter 29 farther away from a first fixed-blade cutter 23 of a pair of cutters on the hybrid bit 11 allows or causes the cutting elements of the first fixed-blade cutter 23 to dominate the cutting action of the pair of cutters thereby increasing the cutting aggressiveness or aggressiveness of the hybrid bit 11.

(26) Another way of altering the cutting aggressiveness of a hybrid bit 11 is by having a rolling cutter to lead a trailing fixed-blade cutter of a pair of cutters (including one of each type of cutter) or to have a fixed-blade cutter lead a trailing rolling cutter of a pair of cutters (including one of each type of cutter). As illustrated in drawing FIG. 1, when a fixed-blade cutter leads a rolling cutter of a pair of cutters of a hybrid bit 11 (see line HBLC), the hybrid bit 11 has more cutting aggressiveness cutting more like a fixed-blade cutter polycrystalline diamond (PDC) bit. As illustrated in FIG. 1, when a rolling cutter leads a fixed-blade cutter of a pair of cutters of a hybrid bit 11 (see line HCLB), the aggressiveness decreases with the hybrid bit having aggressiveness more like a rolling-cutter (roller-cone) bit.

(27) In the illustrated hybrid bit 11 of FIG. 3E, for the purposes of illustrating different embodiments of the present invention, one rolling cutter 29 leads its trailing fixed-blade cutter 23 as a pair of cutters. As illustrated in FIG. 3F as another embodiment of the present invention, one second fixed-blade cutter 25 leads its trailing rolling cutter 33 as a pair of cutters. By leads it is meant that the cutting elements on the adjacent, trailing structure (whether fixed-blade cutter or rolling cutter) are arranged to fall in the same swath or kerf or groove as that made by the cutting elements on the leading structure (whether a fixed-blade cutter or rolling cutter), as indicated by phantom lines in FIG. 3E or FIG. 3F. Thus, the cutting elements 41 on first fixed-blade cutter 23 fall in the same swath or kerf or groove (see FIG. 3A, FIG. 3B) as the cutting elements 35 on rolling cutter 29. Similarly, the cutting elements 37 on third rolling cutter 33 fall in the same swath or kerf or groove (see FIG. 3A, FIG. 3C) as cutting elements 45 on second fixed-blade cutter 25. When a rolling cutter leads a trailing fixed-blade cutter, cutting aggressiveness or aggressiveness of the hybrid bit 11 is decreased. Conversely, when a fixed-blade cutter leads a trailing rolling cutter, cutting aggressiveness or aggressiveness of the hybrid bit 11 is increased. Such is illustrated in FIG. 1 in the broken lines labeled HCLB and HBLC therein.

(28) Also, in the embodiment of FIG. 3, rolling cutter 31 has its cutting elements 39 arranged to lead the cutting elements 43 on the opposing (if not directly opposite, i.e., 180 degrees) third fixed-blade cutter 27. Thus, being angularly spaced-apart approximately 180 degrees on the hybrid bit 11, third fixed-blade cutter 27 and second rolling cutter 31 bear load approximately equally on the hybrid bit 11. In most cases, where there are an equal number of fixed-blade cutters and rolling cutters, each fixed-blade cutter should be paired with a rolling cutter such that the cutting elements on the paired fixed-blade cutter and rolling cutter fall in the same swath or kerf or groove when drilling a formation. All rolling cutters can lead all fixed-blade cutters, making a less aggressive bit (see solid line HCLB in FIG. 1); or all fixed-blade cutters can lead all rolling cutters, making a more aggressive bit (see broken line HBLC in FIG. 1), or all the cutting elements of a rolling cutter can fall in the same swath or kerf or groove as the cutting elements on an opposing fixed blade (see broken line HCOB in FIG. 1), or any combination thereof on a hybrid bit of the present invention.

(29) FIG. 4 illustrates an embodiment of the earth-boring hybrid bit 111 according to the present invention that is similar to the embodiments of FIG. 3 in all respects, except that cutting elements 135, 137, 139 on each of the rolling cutters 129, 133, 131, respectively, are arranged to cut in the same swath or kerf or groove as the cutting elements 145, 141, 143 on the opposite or opposing fixed-blade cutters 125, 123, 127, respectively. Thus, the cutting elements 135 on rolling cutter 129 fall in the same swath or kerf or groove as the cutting elements 145 on the opposing fixed-blade cutter 125. The same is true for the cutting elements 139 on rolling cutter 131 and the cutting elements 143 on the opposing fixed-blade cutter 127; and the cutting elements 137 on rolling cutter 133 and the cutting elements 141 on opposing fixed-blade cutter 123. This can be called a cutter-opposite arrangement of cutting elements. In such an arrangement, rather than the cutting elements on a fixed-blade cutter or rolling cutter leading the cutting elements on a trailing rolling cutter or fixed-blade cutter, the cutting elements on a fixed-blade cutter or rolling cutter oppose those on the opposing or opposite rolling cutter or fixed-blade cutter.

(30) The hybrid bit 111 of FIG. 4, having the cutter-opposite configuration of pairs of cutters, appears to be extremely stable in comparison to all configurations of cutter-leading pairs of cutters or all blade-leading pairs of cutters. Additionally, based on preliminary testing, the hybrid bit 111 of FIG. 4 out drills a conventional rolling-cutter bit and a conventional fixed-blade cutter bit having polycrystalline diamond cutting elements (PDC bit), as well as other hybrid bit configurations (cutter-leading) in hard sandstone. For example, a conventional 12-inch rolling-cutter bit drills the hard sandstone at 11 feet/hour, a conventional fixed-blade cutter bit having polycrystalline diamond cutting elements (PDC bit) at 13 feet/hour, the hybrid bit with a cutter-leading pair of cutters configuration at 14 feet/hour and the hybrid bit with a cutter-opposite pair of cutters configuration at 21 feet/hour. Different types of hard sandstone is the material that are most difficult formations to drill using fixed-blade cutter bits mainly due to high levels of scatter vibrations. In that particular application, the balanced loading resulting from the cutter-opposite pair of cutters configuration of a hybrid bit is believed to produce a significant difference over other types and configurations of bits. In softer formations (soft and medium-hard), it is believed that the more aggressive blade-leading pair of cutter hybrid bit configurations will result in the best penetration rate. In any event, according to the preferred embodiment of the present invention, the aggressiveness of a hybrid bit can be tailored or varied to the particular drilling and formation conditions encountered.

(31) Still another way to adjust or vary the aggressiveness of the hybrid bit 11 is to arrange the cutting elements 35, 37, 39 on the rolling cutters 29, 31, 33 so that they project deeper into the formation being drilled than the cutting elements 41, 43, 45 on the fixed-blade cutters 23, 25, 27. The simplest way to do this is to adjust the projection of some or all of the cutting elements 35, 37, 39 on the rolling cutters 29, 31, 33 from the surface of each rolling cutter 29, 31, 33 so that they project in the axial direction (parallel to the bit central axis or centerline 15) further than some or all of the cutting elements 41, 43, 45 on fixed-blades cutters 23, 25, 27. In theory, the extra axial projection of a cutting element of the cutting elements on the rolling cutters causes the cutting element to bear more load and protects an associated cutting element of the fixed-blade cutter.

(32) In practice, it is a combination of the projection of each cutting element of a rolling cutter from the surface of its rolling cutter, combined with its angular spacing (pitch) from adjacent cutting elements that governs whether the cutting elements of a rolling cutter actually bear more of the cutting load than an associated cutting element on a fixed-blade cutter. This combination is referred to herein as effective projection, and is illustrated in FIGS. 5 and 6. As shown in FIG. 5, the effective projection A of a given cutting element of a rolling cutter, or that projection of the cutting element available to penetrate into earthen formation, is limited by the projection of each adjacent cutting element and the angular distance or pitch C between the adjacent cutting elements and the given cutting element. FIG. 6 illustrates full effective projection B in that the pitch is selected so that the adjacent cutting elements on either side of a given cutting element permit penetration of the cutting element to a depth equal to its full projection from the surface of a rolling cutter.

(33) From the exemplary embodiment described above, a method for designing a hybrid earth-boring bit of the present invention permits or allows the cutting aggressiveness of a hybrid bit to be adjusted or selected based on the relationship of at least a pair of cutters comprising a fixed-blade cutter and a rolling cutter, of a plurality of fixed-blade cutters and rolling cutters, wherein the relationship includes a fixed-blade cutter leading a rolling cutter in a pair of cutters, a rolling cutter leading a fixed-blade cutter in a pair of cutters, a rolling cutter being located opposite a fixed-blade cutter in a pair of cutters on the bit, and the angular relationship of a fixed-blade cutter and a rolling cutter of a pair of cutters regarding the amount of leading or trailing of the cutter from an associated cutter of the pair of cutters. The cutting aggressiveness of a hybrid bit of the present invention being achieved by defining a cutting aggressiveness of a hybrid drill bit and the various combinations of pair of a fixed-blade cutter and a rolling cutter, when compared to each other and to different types of drill bits, such as a rolling-cutter drill bit and a fixed-blade cutter drill bit, either as the ratio of torque to weight-on-bit or as the ratio of penetration rate to weight-on-bit. The cutting aggressiveness for a hybrid bit of the present invention being adjusted by performing at least one of the following steps: adjusting the angular distance between each rolling cutter and each fixed-blade cutter of a pair of cutters of the bit; adjusting the effective projection of the cutting elements on a rolling cutter; arranging the cutting elements of a fixed blade and the cutting elements of a rolling cutter so that at least one cutting element of a rolling cutter and at least one cutting element of a fixed blade cut the same swath or kerf or groove during a drilling operation; and arranging a pair of at least one fixed-blade cutter and a rolling cutter so that the rolling cutter either leads the fixed-blade cutter) [(<180) angular distance], the rolling cutter opposes the fixed-blade cutter [(=180) angular distance], or trails the fixed-blade cutter [(>180) angular distance].

(34) As described above, decreasing the angular distance between a leading rolling cutter and fixed-blade cutter decreases aggressiveness of the pair of cutters, while increasing the distance therebetween increases aggressiveness of the pair of cutters. Increasing the effective projection on cutting elements of a rolling cutter by taking into account the pitch between them increases the aggressiveness and the converse is true. Finally, designing the cutting elements on a fixed blade to lead the cutting elements on the trailing rolling cutter increases aggressiveness, while having a rolling cutter leading its trailing fixed-blade cutter has the opposite effect. According to this method, aggressiveness is increased, generally, by causing the scraping action of the cutting elements and fixed blades and to dominate over the crushing action of the cutting elements and the rolling cutters.

(35) Increased aggressiveness is not always desirable because of the erratic torque responses that generally come along with it. The ability to tailor a hybrid bit to the particular application can be an invaluable tool to the bit designer.

(36) The invention has been described with reference to preferred or illustrative embodiments thereof. It is thus not limited, but is susceptible to variation and modification without departing from the scope of the invention.