GB2367573A

GB2367573A - Drill bit with cutting fluid distributed according to cutting volumes

Info

Publication number: GB2367573A
Application number: GB0121250A
Authority: GB
Inventors: William R Trujillo; Craig H Cooley
Original assignee: Baker Hughes Inc
Current assignee: Baker Hughes Holdings LLC
Priority date: 1997-09-19
Filing date: 1998-09-03
Publication date: 2002-04-10
Anticipated expiration: 2018-09-03
Also published as: GB0121247D0; GB2363415B; GB0121250D0; GB2363415A; GB2367573B

Abstract

A rotary drag bit 310 has a series of cutting structures or blades 318, 320, 322, 324, 326, 328 which generate differing volumes of cuttings per revolution. Coolant nozzles 342, 344, 346, 348 deliver cutting fluid to each blade at a volumetric rate in proportion to the rate at which each blade generates cuttings. Individual nozzles 342, 344 may deliver coolant to a single fluid channel associated with a cutting structure, or alternatively, individual nozzles 346, 348 may apportion coolant between two coolant channels associated with adjacent cutting structures. Coolant may be flushed radially outwards by orientation of the nozzles 342, 344, 346, 348. Peripheral junk slots 330, 332, 334, 336, 338, 340 may have cross sectional areas proportional to the volume of coolant and cuttings flowing through them.

Description

2367573 EARTH-BORING DRILL BITS WITH ENHANCED FORMATION CUTTINGS REMOVAL

FEATURES AND METHODS OF DRILLING

TECHNICAL FIELD

The present invention relates generally to bits for drilling subterranean formations. More specifically, the invention relates to multiple nozzle rotary drag bits employing variations in nozzle size and orientation to apportion hydraulic flow volume 10 on the bit face in relationship to formation cuttings volume generated by groups of cutters on the bit, as well as to bits employing junk slots with cross- sectional areas apportioned in relationship to cuttings generated by groups of cutters with which the junk slots are respectively associated, such features providing enhanced formation cuttina s clearance from the bit face, through the junk slots, and into the well bore 15 annulus above the bit.

BACKGROUND ART

Design of rotary drag bits employing superabrasive cutters, usually in the form of so-called "polycrystalline diamond compacts", or "PDCs," has reached a high degree of 20 sophistication over the last several decades. Marked increases in rate of penetration (ROP) have been achieved. However, the inability of state-of-the-art rotary drill bits to clear formation cuttings at a rate commensurate with the bits' ability to generate such cuttings has proven to be a troublesome limitation to further increases in ROP.

Various designs and approaches have been employed in the art to facilitate 25 cuttinas remo-val from the bit, and thus facilitate increases in ROP. However, such designs and approaches have generally involved features which are not readily employable in bits of a variety of sizes and configurations, and many are limited to very specific configurations. Moreover, the prior art approaches have failed to consider and appreciate the tendency of poor cuttings clearance from a single blade of a multi-bladed 30 bit to hinder ROP.

One prior art approach to cuttings removal from the bit involving a specialized bit design is disclosed in U.S. Patent 5,417,296, wherein nozzles for supplying drilling fluid are placed both near the center of the bit and near the gage. An outer nozzle associated with one blade and fluid course on the bit face is oriented so as to provide a significant fluid flow component directed inwardly toward the centerline of the bit to augment the outward flow from an inner nozzle associated with another blade and fluid course through communication between the adjacent inner ends of the two fluid courses. Such an arrangement, in theory, enhances formation cuttings clearance, but it has been 5 reported that this is not the case in practice. Specifically, cuttings from the blade with which the outer nozzle is associated are carried inwardly to a constriction between blades, causing clogging of the fluid course fronting that blade and consequent balling of the bit.

Accordingly, the art is, to date, devoid of enhancements to rotary drag bit design 10 in terms of formation cuttings clearance readily applicable to improve the performance in terms of ROP of otherwise conventional bits.

DISCLOSURE OF INVENTION

The present invention provides enhancements to formation cuttings clearance 15 from rotary drag bits through design enhancements readily implementable in a wide variety of blade-type rotary drag bits.

In one aspect, the present invention provides enhanced formation cuttings clearance through optimized distribution of hydraulic energy in the form of drilling fluid flow apportionment in relationship to the total volume of cuttings generated by different 20 groups of cutters, typically those cutters grouped on each blade of a multi-bladed bit.

Such apportionment may be achieved by employing nozzles of differing aperture sizes, and thus relative flow volumes, in association with blades generating differing formation cuttings volumes. For example, in a four-bladed bit with two primary blades and two secondary blades, the terms "primary" and "secondary" being indicative of their relative 25 roles in volume of cuttings removed from the formation, the primary blades may each remove twice the cuttings volume as each of the secondary blades. Accordingly, in a one nozzle per blade bit, the nozzles associated with the primary blades are sized to provide substantially twice the fluid flow as those associated with the secondary blades.

In another aspect, the present invention provides optimized distribution of 3 0 hydraulic energy through selective orientation, or "tilt", of nozzles on the bit face in terms of angles relative to a line taken perpendicular to a tangent to the bit profile at the point the fluid jet from a nozzle impinges upon the formation being drilled. If the fluid jet is coincident with the line, substantially equal volumes of drilling fluid will flow outwardly toward the gage and inwardly toward the centerline or longitudinal axis, in the area defined between the bit face and the formation. A positive tilt, wherein a nozzle is oriented to direct a fluid jet from a point of origin radially inboard of the line, results 5 in a greater fluid flow outwardly through a fluid course toward the gage rather than inwardly toward the centerline, enhancing clearance of formation cuttings from the blade fronted by that fluid course, Conversely, a negative tilt, wherein a nozzle is oriented to direct a fluid jet from a point of origin radially outboard from the line, results in a greater fluid flow inwardly along a fluid course toward the centerline than outwardly toward the 10 gage, resulting in difficulty in clearing formation cuttings from the bit face. As noted with respect to the aforementioned '296 patent, such inward flow will tend to clog the fluid courses rather than clear them. The present invention employs positive tilt of the various nozzles on a bit face to ensure predominant outward flow of drilling fluid toward junk slots of the bit located proximate the bit gage, and to minimize cross-flow 15 on the bit face between fluid courses with which different nozzles are associated.

In a further aspect of the invention, it may be desirable or required, due to the configuration or size of the bit, that fewer nozzles are employed than blades. In such an arrangement, a single nozzle may provide drilling fluid to two fluid courses, for example, one lying in front of a primary blade and the other in front of a secondary blade.

20 Therefore nozzle orientation, or the orientation of the nozzle aperture, may be employed to allocate or apportion fluid flow from a single nozzle between the primary and secondary fluid courses, especially when the nozzle is placed at or near a convergence point of the two fluid courses. It should be noted that nozzle orientation may be altered in any direction, and not merely in terms of "tilt" along a radial line from 25 the centerline of the bit to the gage, in order to bias nozzle flow toward a fluid course.

In other words, to allocate or split flow between two fluid courses with which the nozzie is associated, normally by placement adjacent the radially inner ends of both, the "side to side" orientation of the nozzle or its aperture may be altered.

In yet another aspect, the present invention provides enhanced formation 30 cuttings clearance through sizing the cross-sectional areas of junk slots associated with various blades of a bit in similar proportion to the total formation cuttings volume generated by each of the blades. Again taking a four-bladed bit having two primary and two secondary blades by way of example, if the primary blades each generate mrice the formation cuttings volume of each secondary blade, the junk slots are sized in a similar ratio in terms of cross-sectional area transverse to the bit centerline.

In still another aspect of the invention, at least two of the abovedescribed 5 features are employed in the same bit to facilitate formation cuttings removal from the bit face and through the junk slots, to the well bore annulus above the bit.

The present invention also contemplates substantially balancing the cuttings volume removed by each of the primary blades of a multi-bladed bit with the volume removed by the other or others, and the cuttings volume removed by each of the 10 secondary blades with the volume removed by the other or others, so as to reduce the tendency of any particular blade to remove an excessive volume of cuttings, and thus exhibit a tendency to clog before the others, and inhibit ROP.

BRIEF DESCRIPTION OF DRAWINGS

15 FIG. I is a view of a four-bladed drill bit according to the invention, looking upwardly at the bit face from the formation ahead of the bit; FIG. I A is a schematic view of the bit face fluid course and associated junk slot pattern of the bit of FIG. 1, looking downwardly toward the formation being drilled and shov4na relative cross-sectional areas of the entrances to the junks slots transverse to 20 the Ion-aitudinal axis of the bit; FIG. 2A is a schematic, quarter-sectional side view of the bit of FIG. 1, showing a nozzle oriented with a positive tilt; FIG. 2B is a schematic, quarter-sectional side view of another bit, showing a nozzle oriented with a negative tilt-, 25 FIG. 3)A is a bar graph showing relative formation cuttings'volume generated by each blade of a cuttings volume-balanced four-bladed bit during a single revolution of the bit-, FIG. 3B is a bar graph showing relative formation cuttings volume generated by each blade of a cuttings volume-imbalanced four-bladed bit during a single revolution of 30 the bit; FIG. 4 is a view of a curved-bladed, six-blade bit according to the invention, looking upwardly at the bit face from the formation ahead of the bit, FIG. 4A is a half- sectional schematic view of the bit of FIG. 4 showing nozzle locations and orientations, and FIG. 4B is a quarter-sectional schematic with the nozzle locations rotated onto a common plane to highffight the locational and orientational differences-, and FIG. 5 is a view of a straight-bladed, six-blade bit according to the invention, 5 looking upwardly at the bit face from the formation ahead of the bit, FIG. 5A is a halfsectional schematic view of the bit of FIG. 5 showing nozzle locations and orientations, and FIG. 5B is a quarter-sectional schematic with the nozzle locations rotated onto a common plane to highlight the locational and orientational differences.

10 BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIGS. 1 and I A of the drawings, a rotary drag bit 10 of the present invention is illustrated. Drag bit 10 includes a body 12 having a face 14 radially extending outward from the centerline or longitudinal axis 16 of the bit body 12. Four C blades 18, 20, 22 and 24 extend over and above face 14 and radially outwardly 15 therebeyond, defining four longitudinally extending junk slots 26, 28, 30 and 3) 2 therebetween. An upper section 34 of the bit body 12 may be seen in FIG. I extending radially outwardly above and beyond the junk slots. A plurality or group of superabrasive cutters 40, preferably PDCs, is mounted to each blade 18 through 24 With their cutting faces 42 facing generally in the direction of bit rotation. Each group of 20 cutters 40, respectively mounted to blades 18 through 24, generates cuttings of formation material into its associated fluid course 50, 52, 54 and 56 located rotationally in front of that cutter group as the bit 10 is rotated by a drill string and weight is applied to the bit 10 through the drill string. Fluid courses 50, 52, 54 and 56 respectively communicate with the entrances to junk slots 26, 28, 3 0 and 3 2 at laterally peripheral 25 areas of bit body face 14. A plurality of nozzles 60, 62, 64 and 66 are shown on bit body face 14 adjacent the radially inner portions of fluid courses 50, 52, 54 and 56, the arrows in FIG. I showing the radial directions of the jets of drilling fluid discharged by each of the nozzles 60 through 66. The drilling fluid flow from the various nozzles 60 through 66 carries formation cuttings generated by each group of cutters 40 into fluid 0 courses 5 0 through 5 6, to junk slots 26 through 3 2, and ultimately into the well bore annulus above bit 10 between the drill string and the well bore sidewall.

In accordance with one aspect of the present invention, the drilling fluid flow volume, and thus the hydraulic energy, through each of the fluid courses 50 through 56, is generally proportional to the relative volume of formation rock or "cuttings" cut by the 14roups of cutters 40 respectively mounted to each of blades 18 through 24 with 5 which the fluid courses 50 through 56 are respectively associated. For example, if the cumulative volume of rock to be cut by the cutter groups of each of primary blades 18 and 22 is twice that cut by the cutter groups of each of secondary blades 20 and 24 (i.e., about 2: 1), driihng fluid flow is adjusted by appropriately locating and orienting the nozzles 60 through 66 and varying the aperture sizes thereof accordingly to apportion 10 the drilling fluid flowing to bit 10 through the drill string. While the proportioning of fluid flow into the various fluid courses need not exactly correspond to the relative volumes of cuttings from each blade with which the fluid courses are respectively associated, a variance of relative flow proportions within no more than about plus or minus twenty percent with respect to the rock volume proportions is desirable for 15 optimum results. In its simplest implementation, nozzle aperture size may be varied to achieve the desired proportioning. For example, in the bit of FIG. 1, the nozzle aperture sizes associated with the primary blades may be 1.03 cm (13/32 inch), and those associated with the secondary blades 0.71 cm. (9/32 inch), to provide the desired 2:1 flow volume proportioning.

20 In accordance with another aspect of the present invention, the entrances of junk slots 26 through 3)2 adjacent the lateral periphery of bit body face 14 are relatively sized, in terms of cross-sectional area transverse to the longitudinal axis 16, in similar proportion to the formation cuttings or rock volume cut by the cutter groups with which the junk slots are respectively associated. Again, using the previous example, if the rock 25 volume cut by each of the primary blade 18, 22 cutter groups is twice the rock volume cut by each of the secondary blade 20, 24 cutter groups, the junk slot entrance areas of each of primary junk slots 26 and 30 will be generally twice the entrance areas of each of secondary junk slots 28 and 3 0. In bit 10 of FIG. 1, the actual primary to secondary blade rock volume proportions are about 1.8 to 1, while the relative primary to 30 secondary junk slot entrance area proportions are about 1. 7 to 1. Further, in bit 10 the fluid flow volume ratios between each of primary fluid courses 50, 54 and secondary fluid courses 52, 56 is about 2. 1: 1, although a flow volume proportion range of about 1. 75 to 2. 'A between primary a d secondary fluid courses is contemplated as be n ing suitable for the practice of the invention in bit 10.

In accordance with yet another aspect of the invention, it will be understood by viewin!a FIGS. I and 1 A that the rock volume cut by the cutter groups of each primary 5 blade 18 and 22 will be substantially mutually balanced, and that the rock volume cut by the cutter groups of each secondary blade 20 and 24 will be substantially mutually balanced. In bits such as bit 3 10, discussed below with reference to FIGS. 5, 5A and 5B of the drawings, such substantial balancing is also extended to tertiary blades. Such balancing may be effected by employing the same number, size and exposure of cutters 10 40 on the blades to be balanced, although such balancing may be achieved even when emplo-ving a differing number of cutters by varying cutter size and, to some extent, exposure. It has been ascertained by the inventors that balancing rock volumes cut as described and proportioning associated drilling fluid flow volumes according to relative rock volumes (and thus balancing fluid flow volumes as well) will provide a noticeable 15 increase in rate of penetration (ROP) for the bit before clogging or "balling", in comparison to a similar, but unbalanced bit. Referring to FIGS. 3A and 3B of the drawin2s, FIG. 3A depicts the relative rock volume cut by each blade of a bit according to the present invention and sirmilar to bit 10, wherein it can readily be seen that the rock volumes (expressed as a percent of the total for all the blades) cut by each of the 20 priman- blades 18 and 22 (the same reference numerals as in FIG. 1 are employed for clarity) are in substantial balance, and that the rock volumes cut by each of the secondary blades 20 and 24 are in substantial balance. In contrast, another bit of similar design and size, but wherein design balance of relative rock volume to be cut or generated by the various blades was not effected, shows balance of secondary blades 25 120 and 124 but significant imbalance between primary blades 118 and 122. In head to head drilling tests, the balanced bit drilled at a significantly greater ROP than the unbalanced bit before clogging. Further, the dominant primary blade 122 of the non balanced bit clogged first on a consistent basis. In additional tests, it was found that proportioning fluid flow volumes according to rock volumes cut by the various blades 30 resulted in still further increases in ROP before balling occurred.

While sizing and locating nozzles on a bit body may be employed to effect drilling fluid flow proportioning as noted above, FIGS. 2A and 2B of the drawings illustrate that orientation, or tilt, of a nozzle 80 with respect to a line 82 perpendicular to the tangent to the bit profile (followed by the configuration 84 of the well bore bottom 86) at the point of fluid jet impingement on the formation may be desirably used to positively direct fluid flow over the bit face outwardly from the nozzle toward the gage 5 by varying the percentage of flow from a given jet which travels radially outwardly to the gage of the bit and radially inwardly toward the longitudinal axis. FIG. 2A shows that a "positive" jet tilt 88 from a nozzle 80 radially inboard of a line 82 results in a greater outward versus inward flow, in this instance, for example, an 11' positive tilt results in about a 60% outward flow to about a 40% inward flow proportion. In 10 contrast, FIG. 2B shows that a "negative" jet tilt 88 from a nozzle 80 radially outboard of a line 82 undesirably results in a greater inward versus outward flow; in this instance, for example, a 22' negative tilt results in about a 25% outward flow versus about a 75% inward flow. By ensuring a positive tilt of the fluid jet emanating from each nozzle, the large majonty of fluid flow volume and energy from each nozzle will be directed 15 outwardly toward the gage, enhancing drilling fluid management and minlimizing cross flow between fluid courses on the bit face to make most efficient use of the fluid energy in cooling the cutters and clearing formation cuttings from the bit.

Sirrfflarly, and with specific reference to FIGS. 4, 4A, 4B and 5, 5A and 5B of the drawings, it will be understood and appreciated that bits having fewer nozzles than 20 blades may nonetheless apportion fluid flow between adjacent or communicating primary and secondary blades (or even tertiary blades, as shown in FIG. 5) by nozzle placement in combination with appropriate orientation. As noted previously, while nozzle "tilt" in the context of distribution of fluid flow inwardly or outwardly is one design consideration, nozzle orientation, apart from tilt, relatively toward or away from 25 the entrance or inner end of a particular fluid course may be employed to apportion flow between several fluid courses.

Turning now to FIGS. 4, 4A and 4B of the drawings, a six-bladed drill bit 2 10 including a bit body 212 having a face 214 extending radially outwardly from longitudinal axis 216 is illustrated. Bit 210 includes three circumferentially spaced, 30 curved primary blades 218, 220 and 222, and three interspersed, curved secondary blades 224, 226 and 228. Primaryjunk slots 230, 232 and 234 are respectively associated with the primary blades 218, 220 and 222, and secondary junk slots 2_3) 6, 23 8 and 240 are respectively associated with secondary blades 224, 226 and 228. Each of the blades bears a plurality or group of superabrasive (PDC) cutters 40 having cutting faces 42. Unlike bit 10, bit 210 carries only half as many nozzles as there are blades, nozzles 242, 244 and 246 each respectively lying between adjacent fluid courses 248 and 5 250, 252 and 254, and 256 and 258 so that fluid fTom a single nozzle may feed two fluid courses. There may also be some crossflow across bit body face 214 between other fluid courses, but such is incidental, minirruized by the use of positive tilt of nozzles 242, 244 and 246, and comprises only a small portion of the total flow volume. The arrows in FIG. 4 depict the radial orientation of the fluid jets emanating from the nozzles 242, 10 244 and 246. It should be noted that impingement of the respective fluid jets from nozzles 242, 244 and 246 on radially inner ends of blades 224, 226 and 228 may also be employed as part of the flow apportionment mechanism, although such a technique may eventually cause erosion of blade material over an extended drilling inter-Val. As with bit 10, junk slot transverse entrance areas and fluid flow volumes associated with each of 15 the blades are each proportioned relative to the formation rock volume cut by the cutter group of each blade. Further, the rock volumes to be cut or generated by each priman, blade 218, 220 and 222 are substantially mutually balanced, and the rock volumes to be cut or generated by each secondary blade 222, 224 and 226 are substantially mutually balanced. Referencing FIGS. 4A and 4B, the respective positive tilts of nozzles 242, 20 244 and 246 are shown for a better appreciation of the manner in which such technique is employed to direct drilling fluid flow outwardly toward the gage in each fluid course.

In bit 2 10, all nozzle aperture sizes are equal.

Turning to FIGS. 5, 5A and 5B of the drawings, a six-bladed drill bit 3 10 including a bit body 312 having a face 314 extending radially outwardly from 25 longitudinal axis 3 16 is illustrated. Bit 3 10 includes two circumferentially spaced, straight primary blades 318 and 320, two secondary blades 322 and 324, and two tertiary blades 326 and 328. The terms "primary," "secondary" and "tertiary" are employed with regard to the relative volumes of rock cut by the cutter groups of the various blades. Primaryjunk slots 330 and 332 are respectively associated with the 3 30 primary blades 318 and 320, secondaryiunk slots 334 and 336 with secondary blades 322 and 324, and tertiaryjunk slots 338 and 340 with tertiary blades 326 and 328. Each of the blades bears a plurality or group of superabrasive (PDC) cutters 40 having cutting faces42. Bit 310 carries four nozzles 342, 344,346 and 348. Nozzles 342 and 344 feed drilling fluid to fluid courses 350 and 352 associated with the primary blades 318 and 320, while nozzles 346 and 348 each contribute flow to both a secondary fluid course and a tertiary fluid course, nozzle 346 feeding fluid courses 354 and 356 and 5 nozzle 348 feeding fluid courses 358 and 360. Again, as with bit 210, drilling fluid from a single nozzle may feed two fluid courses. As noted before, there may also be some crossflow across bit body face 314 between other fluid courses, but such is incidental, minimized by the positive tilts of the nozzle flows, and comprises only a small portion of the total flow volume. The arrows in FIG. 5 depict the radial orientation of the fluid 10 jets emanating from the nozzles 342 through 348. As with bits 10 and 210, fluid flow volumes associated with each of the blades are each proportioned relative to the formation rock volume cut by the cutter group of each blade. However, unlike bits 10 and 210, the junk slot entrance areas of the primary, secondary and tertiary junk slots are not proportioned in strict accordance with the invention. Primary junk slots 330 and 15 332 exhibit such proportioning. Secondary junk slots')-')4, 3:)36 and tertiary junk slots 3 3 8, 3) 40 are not individually sized with respect to relative rock volumes cut by their associated blades although the total entrance area of each pair of adjacent secondary and tertiary junk slots is generally proportioned to the rock volume cut by the blade pair associated with those junk slots. Further, the rock volumes to be cut or generated by 20 each blade of a type or category, primary, secondary or tertiary, are substantially mutualiv balanced. Referencing FIGS. 5A and 5B, the respective positive tilts of nozzles 342 through 348 are shown for a better appreciation of the manner in which such technique is employed to direct drilling fluid flow outwardly to the gage.

In the practice of flow apportionment or drilling fluid management according to 25 the present invention, it may be stated as a general guideline that nozzles should be tilted so that the flow emanating therefrom is directed outwardly toward the junk slots. The amount or degree of tilt may b e limited in some instances by bit geometry and the proximin.- of other nozzles, but in general it has been found that a positive tilt of between about 10' and about 25 ' is usually possible, and should be effected so as to direct the 30 predorninant portion of drilling fluid flow outwardly. Further, it may be generally stated as a rule of thumb for bits having a profile defined by an indented center cone portion and a nose radially outboard thereof, that a positive fluid flow (i.e., toward the gage of the bit) may be effected by placing and orienting a nozzle to cause a fluid jet from the nozzle to impinge on the formation at a radial location no greater than that defined by the farthest leading longitudinal extent of the nose.

NXIhile the present invention has been described with reference to certain 5 illustrated embodiments, those of ordinary skill in the art will understand and appreciate that it is not so limited. Many additions, deletions and modifications to the illustrated embodiments, as well as combinations of features from different embodiments, may be effected without departing from the scope of the invention as set forth in the claims. Further, one or more of the inventive features of the present invention may be employed 10 in a given bit to achieve perceptible benefits, although all such features may not be employed.

Claims

1 A rotary drag bit for drilling a subterranean formation, comprising:

a bit body having a longitudinal axis and a face extending radially outwardly from the longitudinal axis at one end of the bit body; a plurality of blades extending above and generally radially over the face and outwardly of the bit body, the blades separating and defining a plurality of fluid courses leading to a plurality junk slots extending -longitudinally away from the bit body face; a plurality of cutters mounted to each of the plurality of blades, the plurality of cutters of at least one blade differing in at least one of a number, size and exposure from a plurality of cutters mounted to at least one other blade such that a different formation cuttings volume is to be generated from engagement of the subterranean formation by the at least one blade cutters and the at least another blade cutters; and a plurality of nozzles disposed on said bit body and located to discharge drilling fluid between the bit face and the subterranean formation, at least one of the plurality of nozzles being located and oriented to apportion a discharge of drilling fluid between a fluid course positioned to receive formation cuttings generated by the at least one blade cutters and a fluid course positioned to receive formation cuttings generated by the at least another blade cutters in general proportion to the relative volumes of formation cuttings generate by each of the at least one blade cutters and the at least another blade cutters.

2. The rotary drag bit of claim 1, wherein the plurality of blades comprises at least two categories, primary and secondary, and wherein each primary blade will generate a substantially greater volume of formation cuttings than each secondary blade, and the nozzles are located and oriented to cause drilling fluid to flow through a fluid course associated with each of the plurality of blades to a junk slot entrance at a periphery of the bit face in general proportion to the relative formation cuttings volumes generated by the plurality of blades.

3. The rotary drag bit of claim 2, wherein the cutters on the primary blades are disposed, through variations in at least one of cutter number, size and exposure, to generate substantially similar formation cuttings volume from each of the primary blades.

4. The rotary drag bit of claim 3, wherein the cutters on the secondary blades are disposed, through variations in at least one of cutter number, size and exposure, to generate substantially similar formation cuttings volume from each of the secondary blades.

5. The rotary drag bit of claim 4, wherein each of said plurality of junk slots has a corss-section entrance area at a periphery of the bit face, measured transverse to the longitudinal axis, and the transverse crosssectional entrance areas of junk slots associated with each of the primary blades are substantially the same, and the transverse cross- sectional entrance areas of junk slots associated with each of the secondary blades are substantially the same.

6. The rotary drag bit of any preceding claim, wherein there are fewer nozzles than blades, and wherein the at least one nozzle, through variations in at least one of size, shape, orientation and location, provides substantially all the drilling fluid flow through two fluid courses.

7. The rotary drag bit of any preceding claim, wherein at least one junk slot is positioned to receive formation cuttings from the at least one blade cutters and at least another junk slot is positioned to receive formation cuttings from the at least another blade cutters, and wherein cross-sectional areas transverse to the longitudinal axis of the bit body at an entrance adjacent the bit body face of each of the at least one junk slot and the at least another junk slot are generally sized in proportion to the formation cuttings volume to be respectively generated by each of the at least one blade cutters and the at least another blade cutters.

8. A rotary drag bit for drilling a subterranean formation, comprising:

a bit body having a longitudinal axis and a face thereon extending radially outward from the longitudinal axis; at least two cutting structures, each secured to the bit body over the bit face, each of the at least two cutting structures being located and configured to generate a volume of formation cuttings into a different fluid course on the face of the bit body, each of the formation cuttings volumes being different; and a plurality of nozzles for discharging drilling fluid from the bit body face into an area between the bit body face the formation, the nozzles being located to provide drilling fluid flow to each of the different fluid courses in general proportion to the formation cuttings volume to be generated by each of the at least two cutting structures.

9. The rotary drag bit of claim 8, wherein each of the at least two cutting structures comprises at least one superabrasive cutter.

10. The rotary drag bit of claim 9, wherein each of the at least two cutting structures is carried on a blade extending -over the bit body face and radially outwardly past a periphery thereof.

11. The rotary drag bit of any one of claims 8 to 10, wherein at least two of the plurality of nozzles further differ in at least one of size and orientation to effect the provision of drilling fluid flow.

12. The rotary drag bit of any one of claims 8 to 11, further including a junk slot associated with each of the different fluid courses and having an entrance adjacent a radially peripheral portion of the bit face, each junk slot entrance having a cross- I i I I sectional area transverse to the longitudinal axis generally sized in proportion to the relative, different volumes of formation cuttings to be generated by each of the at least two cutting structures.

13. The rotary drag bit of anyone of claims 8 to 12, wherein the at least two cutting structures comprise at least four cutting structures, at least a first two of which are located and configured to generate substantially similar volumes of formation cuttings, and at least a second two of which are located and configured to generate substantially similar volumes of formation cuttings which are substantially less than the volumes generated by the at least first two cutting structures, and wherein the nozzles are located to provide substantially balanced volumes of drilling fluid flow to fluid courses associated with each of the at least first two cutting structures and lesser, substantially balanced volumes of drilling fluid flow to fluid courses associated with each of the at least second two cutting structures.

14. The rotary drag bit of any preceding claim, wherein said plurality of nozzles are located and oriented to apportion a discharge of drilling fluid between a fluid course positioned to receive formation cuttings generated by the at least one blade cutters and a fluid course positioned to receive formation cuttings generated by the at least another blade cutters in general proportion to the relative volumes of formation cuttings generated by each of the at least one blade cutters and the at least another blade cutters.

15. A method of drilling a subterranean formation, comprising:

applying weight and torque to a rotary drill bit to generate a volume of cuttings from the subterranean formation in a plurality of segments, each cuttings segment being associated with a fluid course on a face of the bit; and removing the respective cuttings segments from the face of the bit by directing flow of drilling fluid radially outwardly through the respective fluid courses, the relative volumes of drilling fluid flows through the respective fluid courses being in I f.

- substantial proportion to relative magnitudes of cuttings volume segments associated with the respective fluid courses.

i i Amendments to the claims have been riled as follows 1-71 CLAIMS 1. A rotary drag bit for drilling a subterranean formation, comprising:

a bit body having a longitudinal axis ano a face extending radially outwardly from the longitudinal axis at one end of the bit body; a plurality of blades extending above and generally radially over the face and outwardly of the bit body, the blades separating and defining a plurality of fluid courses leading to a plurality junk slots extending longitudinally away from the bit body face; a plurality of cutters mounted to each of the plurality of blades, the plurality of cutters of at least one blade differing in at least one of a number, size and exposure from a plurality of cutters mounted to at least one other blade such that a different formation cuttings volume is to be generated from engagement of the subterranean formation by the at least one blade cutters and the at least another blade cutters; and a plurality of nozzles disposed on said bit body and located to discharge drilling fluid between the bit face and the subterranean formation, at least one of the plurality of nozzles being located and oriented to apportion a discharge of drilling fluid between a fluid course positioned to receive formation cuttings generated by the at least one blade cutters and a fluid course positioned to receive formation cuttings generated by the at least another blade cutters in general proportion to the relative volumes of formation cuttings generate by each of the at least one blade cutters and the at least another blade cutters.

8. A rotary drag bit for drilling a subterranean formation, comprising:

10. The rotary drag bit of claim 9, wherein each of the at least two cutting structures is carried on a blade extending over the bit body face and radially outwardly past a periphery thereof 11. The rotary drag bit of any one of claims 8 to 10, wherein at least two of the 7 7 t plurality of nozzles further differ in at least one of size and orientation to effect the provision of drilling fluid flow.

12. The rotary drag bit of any one of claims 8 to 11, further including a junk slot associated with each of the different fluid courses and having an entrance adjacent a radially peripheral portion of the bit face, each junk slot entrance having a cross- sectional area transverse to the longitudinal axis generally sized in proportion to the relative, different volumes of formation cuttings to be generated by each of the at least two cutting structures.

13. The rotary drag bit of anyone of claims 8 to 12, wherein the at least two cutting structures comprise at least four cutting structures, at least a first two of which are located and configured to generate substantially similar volumes of formation cuttings, and at least a second two of which are located and configured to generate substantially similar volumes of formation cuttings which are substantially less than the volumes generated by the at least first two cutting structures, and wherein the nozzles are located to provide substantially balanced volumes of drilling fluid flow to fluid courses associated with each of the at least first two cutting structures and lesser, substantially balanced volumes of drilling fluid fiow to fluid courses associated with each of the at least second two cutting structures.

15. The rotary drag bit of any preceding claim, wherein each of the plurality of nozzles is orientated at a positive tilt angle.

16. The rotary drag bit of claim 15, wherein each of the plurality of nozzles is orientated at a positive tilt angle between about 100 and about 250.

17. The rotary drag bit of any preceding claim, wherein the rotary drag bit includes a profile comprising a cone proximate the longitudinal axis and a nose radially outward of the cone, wherein each of the nozzles of the plurality is orientated so as to cause a jet of drilling fluid emanating therefrom to impact the subterranean formation substantially radially inwardly of a farthest leading longitudinal extent of the nose.

18. A method of drilling a subterranean formation, comprising:

applying weight and torque to a rotary drill bit to generate a volume of cuttings from the subterranean formation in a plurality of segments, each cuttings segment being associated with a fluid course on a face of the bit; and removing the respective cuttings segments from the face of the bit by directing flow of drilling fluid radially outwardly through the respective fluid courses, the relative volumes of drilling fluid flows through the respective fluid courses being in substantial proportion to relative magnitudes of cuttings volume segments associated with the respective fluid courses.