Title:
DRILL BIT WITH STAGED DURABILITY, STABILITY AND ROP CHARACTERISTICS
Kind Code:
A1


Abstract:
Drill bit with multiple stages of durability and ROP characteristics is disclosed. The drill bit has multiple layers of cutters established by deploying the cutters on blades of different heights or maintaining the blades at the same height and deploying the cutters to have different heights on one or more blades. Each layer provides independent bottom hole coverage and has independent stabilization, ROP, and durability characteristics so as to effectively drill through different subsurface formations. Cutters deployed on the different layers have their respective centers at substantially different radial positions. Due to the different radial positions, cutters in different layers cut different swaths in the subsurface formation. Cutters in different layers may also have different initial peripheral portions or shear lengths, resulting in different impact resistance characteristics for the different layers. This changes the wear and/or cutter deterioration processes for the different layers, resulting in different and/or improved toughness characteristics.



Inventors:
Mensa-wilmot, Graham (Spring, TX, US)
Application Number:
11/555547
Publication Date:
11/22/2007
Filing Date:
11/01/2006
Assignee:
VAREL INTERNATIONAL IND., L.P. (Carrollton, TX, US)
Primary Class:
International Classes:
E21B10/36
View Patent Images:
Related US Applications:



Primary Examiner:
ANDREWS, DAVID L
Attorney, Agent or Firm:
FOLEY GARDERE (WASHINGTON, DC, US)
Claims:
What is claimed is:

1. A drill bit, comprising: a drill bit body; blades formed on said drill bit body, said blades having a plurality of cutting element positions radially located thereon; and cutting elements deployed on said blades, said cutting elements forming a primary layer of cutting elements and a secondary layer of cutting elements, said primary layer of cutting elements having a different height relative to said drill bit body from said secondary layer of cutting elements; wherein at least one primary layer cutting element and a corresponding secondary layer cutting element occupy substantially different cutting element positions on said blades such that their cutting element profiles overlap, said at least one primary layer cutting element and said corresponding secondary layer cutting element together defining a cutting zone equal to a diameter of one of said at least one primary layer cutting element and said corresponding secondary layer cutting element plus a predetermined fraction of said diameter.

2. The drill bit according to claim 1, wherein said predetermined fraction is from approximately 1/10 to approximately ⅓.

3. The drill bit according to claim 1, wherein said predetermined fraction is approximately ⅕.

4. The drill bit according to claim 1, wherein said diameter is a diameter of said at least one primary layer cutting element.

5. The drill bit according to claim 1, wherein said diameter is a diameter of said corresponding secondary layer cutting element.

6. The drill bit according to claim 1, wherein said cutting elements in said primary layer and said cutting elements in said secondary layer have substantially different shear lengths.

7. The drill bit according to claim 6, wherein said shear lengths are average shear lengths derived from multiple shear lengths for said cutting elements in said primary layer and said cutting elements in said secondary layer.

8. The drill bit according to claim 1, wherein said cutting elements in said primary layer and said cutting elements in said secondary layer have substantially different sizes.

9. The drill bit according to claim 1, wherein said cutting elements in said primary layer and said cutting elements in said secondary layer have substantially different shape.

10. The drill bit according to claim 9, wherein said substantially different shapes result in said cutting elements in said primary layer and said cutting elements in said secondary layer having substantially different axial volumes.

11. The drill bit according to claim 1, wherein said cutting elements in said primary layer and said cutting elements in said secondary layer have substantially different abrasion resistances.

12. The drill bit according to claim 1, wherein said cutting elements in said primary layer and said cutting elements in said secondary layer have substantially different impact resistances.

13. The drill bit according to claim 1, wherein said cutting elements in said primary layer and said cutting elements in said secondary layer have substantially different thermal stabilities.

14. The drill bit according to claim 1, wherein said cutting elements in said primary layer and said cutting elements in said secondary layer have substantially different back rake angles.

15. The drill bit according to claim 14, wherein one or more cutting elements in said primary layer have a larger back rake angle than one or more cutting elements in said secondary layer.

16. The drill bit according to claim 14, wherein one or more cutting elements in said primary layer have a smaller back rake angle than one or more cutting elements in said secondary layer.

17. The drill bit according to claim 1, wherein said cutting elements form a tertiary layer of cutting elements and said primary layer of cutting elements have a different height relative to said drill bit body from said tertiary layer of cutting elements, and wherein at least one primary layer cutting element and a corresponding tertiary layer cutting element occupy substantially different cutting element positions on said blades such that their cutting element profiles overlap, said at least one primary layer cutting element and said corresponding tertiary layer cutting element together defining a cutting zone equal to a diameter of one of said at least one primary layer cutting element and said corresponding tertiary layer cutting element plus a predetermined fraction of said diameter.

18. A method of assembling a drill bit, comprising: providing a drill bit body having blades formed thereon said blades having a plurality of cutting element positions radially located thereon; and deploying cutting elements on said blades, said cutting elements forming a primary layer of cutting elements and a secondary layer of cutting elements, said primary layer of cutting elements having a different height relative to said drill bit body from said secondary layer of cutting elements; wherein at least one primary layer cutting element and a corresponding secondary layer cutting element occupy substantially different cutting element positions on said blades such that their cutting element profiles overlap, said at least one primary layer cutting element and said corresponding secondary layer cutting element together defining a cutting zone equal to a diameter of one of said at least one primary layer cutting element and said corresponding secondary layer cutting element plus a predetermined fraction of said diameter.

19. The method according to claim 18, wherein said cutting elements of said primary layer and said cutting elements of said secondary layer are mounted on blades having substantially different heights relative to said drill bit body.

20. The method according to claim 18, wherein said cutting elements of said primary layer and said cutting elements of said secondary layer are mounted on blades having substantially identical heights relative to said drill bit body.

21. The method according to claim 20, wherein said cutting elements of said primary layer and said cutting elements of said secondary layer are mounted on separate blades.

22. The method according to claim 20, wherein one or more cutting elements of said primary layer and one or more cutting elements of said secondary layer are mounted on a single blade.

23. The method according to claim 20, wherein said one or more cutting elements of said primary layer and said one or more cutting elements of said secondary layer are mounted on said single blade in multiple rows.

24. The method according to claim 23, wherein at least one of said rows contains a combination of cutting elements from said primary layer and cutting elements from said secondary layer.

25. The method according to claim 23, wherein at least one of said rows contains only cutting elements from said primary layer or only cutting elements from said secondary layer.

26. The method according to claim 18, wherein said primary layer of cutting elements establishes approximately 100% bottom hole coverage.

27. A drill bit body, comprising: blades formed on said drill bit body, and cutting element positions formed on said blades, said cutting element positions radially located thereon such that when cutting elements are deployed on said blades, said cutting elements form a primary layer of cutting elements and a secondary layer of cutting elements, said primary layer of cutting elements having a different height relative to said drill bit body from said secondary layer of cutting elements; wherein at least one primary layer cutting element and a corresponding secondary layer cutting element occupy substantially different cutting element positions on said blades such that their cutting element profiles overlap when said cutting elements are deployed on said blades, said at least one primary layer cutting element and said corresponding secondary layer cutting element together defining a cutting zone equal to a diameter of one of said at least one primary layer cutting element and said corresponding secondary layer cutting element plus a predetermined fraction of said diameter.

Description:

CROSS REFERENCE TO RELATED APPLICATION

This application for patent claims priority to, and hereby incorporates by reference, U.S. Provisional Application Ser. No. 60/746767, entitled “Drill Bit with Staged Durability and ROP Characteristics,” filed May 8, 2006 with the United States Patent and Trademark Office.

FIELD OF THE INVENTION

The present invention relates to rotary drill bits for rotary drilling of subterranean formations and, more specifically, to a rotary drill bit having multiple stages of durability and ROP (rate of penetration) characteristics.

BACKGROUND OF THE INVENTION

Subsurface formation drilling to recover hydrocarbons is well known in the art. The equipment for such subsurface formation drilling typically comprises a drill string having a rotary drill bit attached thereto that is lowered into a borehole. A rotary table or similar device rotates the drill string, resulting in a corresponding rotation of the drill bit. The rotation advances the drill bit downwardly, causing it to cut through the subsurface formation (e.g., by abrasion, fracturing, and/or shearing action). Drilling fluid is pumped down a channel in the drill string and out the drill bit to cool the bit and flush away any debris that may have accumulated. The drilling fluid travels back up the borehole through an annulus formed between the drill string and the borehole.

Many types of drill bits have been developed, including roller cone bits, fixed cutter bits (“drag bits”), and the like. For each type of drill bit, several patterns or layouts of cutting elements (“cutters”) are possible, including spiral patterns, straight radial patterns, and the like. Different types of cutting elements have also been developed, including milled cutting elements, tungsten carbide inserts (“TCI”), polycrystalline-diamond compacts (“PDC”), and natural diamond cutting elements. The selection of which particular drill bit, cutting element type, and cutting element pattern (i.e., cutting structure) to use for a given subsurface formation can depend on a number of factors. For example, certain combinations of drill bit, cutting element type, and cutting element pattern drill more efficiently and effectively in hard formations than others. Another factor is the range of hardness encountered when drilling through the different formation layers.

One common pattern for drill bit cutting elements in a fixed cutter drill bit is a spiral configuration an example of which is shown in FIGS. 1A-1B. As can be seen, a spiral pattern drill bit 100 is composed of several sections, including a bit body 102, a shank 104, and a threaded connector 106 for connecting the drill bit 100 to a drill string. Flats 108 on the shank 104 allow a tool, such as wrench to grip the drill bit 100, making it possible (or at least easier) to screw the drill bit 100 onto the drill string. Blades 110a, 110b, 110c, 110d, 110e, and 110f are formed on the drill bit 100 for holding a plurality of cutting elements 112. The cutting elements 112 include superabrasive faces that usually have identical geometries (i.e., size, shape, and orientation), but different positions and/or cutting angles (back rake angles) on the blades 110a-f Also visible are drill fluid outlets 114 that conduct the drilling fluid away from the drill bit 100, thereby removing any debris and cuttings that may have accumulated.

With existing drill bit configurations, it is known to have blades 110a-f and/or cutting elements 112 that are offset (i.e., have different heights) relative to other blades and/or cutting elements on the drill bit 100. The height of the blades 110a-f and/or cutting elements 112 is measured herein relative to the drill bit body 102. For blades 110a-f that are offset, the cutting element tips are set at the same height relative to each other, but one or more of the blades 110a-f have a height that is greater than one or more other blades 110a-f. Where the cutting elements 112 are offset, the blades 110a-f have the same height, but the tips of certain cutting elements are set at different heights relative to other cutting elements. In either case, the end result is a primary layer of cutting elements that performs the initial drilling, followed by a secondary layer of cutting elements, and in some applications, a tertiary layer and so forth as needed.

Because of the difference in height, the primary layer of cutting elements wears away or deteriorates faster than the secondary layer of cutting elements. As the primary cutting elements progressively wear away, however, the secondary cutting elements compensate increasingly more for the decreased effectiveness of the primary cutting elements in terms of bit durability and ROP. This allows the drill bit 100 to be able to drill at an acceptable ROP for longer durations before having to be replaced, in essence performing the work of multiple (e.g., two in this instance) drill bits 100.

The above arrangements are illustrated in FIGS. 2A-2B, where a segment of a drill bit profile 200 for a drill bit having multiple layers of cutting elements is shown. The term “profile,” as understood by those having ordinary skill in the art, refers to the area outlined by the cutting elements when rotated onto the same radial plane. As can be seen, the drill bit profile segment 200 includes two layers of cutting elements, a primary layer 202 and a secondary layer 204, that are offset from each other. The offset in the layers of cutting elements 202 and 204 is indicated here by the letter H. Each layer 202 and 204 is composed of a plurality of individual cutting elements 206a-n and 208a-n, respectively. For clarity purposes, the layers of cutting elements 202 and 204 shown here and in the remaining figures are depicted as relatively flat. Those having ordinary skill in the art will recognize that, in practice, the layers of cutting elements 202 and 204 may have a degree of curvature.

In existing drill bits, the cutting elements are essentially uniform in size and shape (typically a round shape). In addition, the cutting elements 206a-n of the primary layer 202 and the cutting elements 208a-n of the secondary layer 204 share substantially the same radial positions on their respective blades, or share a common reference axis. FIG. 2B illustrates an example of such radial position sharing where, for clarity purposes, only one cutting element from each layer 202 and 204 is shown. As can be seen, the centers C of the primary layer cutting elements (e.g., cutting element 206a) and the centers C′ of the secondary layer cutting elements (e.g., cutting element 208a) substantially line up on a common radial position (see radial displacement X) when rotated onto the same radial plane.

Furthermore, the cutting elements 206a-n of the primary layer 202 and the cutting elements 208a-n of the secondary layer 204 have substantially the same shear length (SL), shown in FIG. 2A by the heavy arcuate line. “Shear length” refers to the portion of a cutting element's periphery that is in direct contact with the formation being drilled when all cutting elements on the drill bit (and particularly in the vicinity of the cutting element being measured) are rotated onto a single radial plane. A cutting element's shear length is typically measured while the drill bit is new (i.e., unused) and depends in large part on the size of the cutting element and adjacent cutting elements, although it is possible to vary the shear length for different drill bits even when the cutting elements are all of the same size.

The shear length affects the ability of the drill bit to penetrate various types of formation material. For example, hard and abrasive formations requiring a high level of bit stabilization are more effectively drilled with drill bits having longer shear lengths. Soft formation materials, on the other hand, cause minimal impact damage and may therefore be effectively drilled with either longer or shorter shear lengths. Shear length also affects the level of stabilization needed to minimize impact damage, thus reducing the amount of cutting element deterioration For existing drill bits with the type of dual-layer profile shown in FIG. 2A, the cutting elements in the primary and secondary layers have identical shear lengths.

Because of the identical shear lengths, and also because of the shared radial positions, the cutting elements 206a-n and 208a-n in the primary and secondary layers 202 and 204 cut identical swaths through the subsurface formation. This is illustrated in FIGS. 3A-3B, where different implementations of drill bits having a dual-layer profile are shown.

Referring first to FIG. 3A, a dual-layer drill bit profile may be achieved by providing a drill bit 300 where the blades 302a and 302b have different heights. In the portion shown here, the blade in the lower half of the figure, blade 302a, has a greater height than the other blade 302b. Therefore, the cutting elements 306a-n on the first blade 302a constitute the primary layer of cutting elements, while the cutting elements 306a-n on the second blade 302b constitute the secondary layer of cutting elements.

Turning now to FIG. 3B, a dual-layer drill bit profile may also be achieved by providing a drill bit 310 where the blades 312a and 312b have the same heights, but the cutting elements 314a-n and 316a-n are set at different heights relative to each other. In the portion shown here, the non-shaded cutting elements 314a-n are set at a greater height than the shaded cutting elements 316a-n. These non-shaded cutting elements 314a-n are therefore part of the primary layer of cutting elements, whereas the shaded cutting elements 316a-n constitute part of the secondary layer of cutting elements. Both the shaded and non-shaded cutting elements 314a-n and 316a-n may be intermixed across the blades 312a and 312b, as depicted here, or cutting elements of different heights may be mounted on different blades, respectively (similar to the implementation of FIG. 3A).

Because of the same shear lengths and common radial positions, primary layer and secondary layer cutting elements at a given radial position necessarily cut the same swath (see dashed lines) in the subsurface formation This is the case regardless of the specific deployment of cutting elements used to achieve the primary layer and secondary layers. The width of the swath or “cutting zone” created by cutting elements on different blades sharing a common radial position is indicated here by the letter Z and is equal to the diameter D of the cutting elements. Because they cut the same swath the primary layer and secondary layer do not establish independent coverage of the bottom hole. In addition, and from a geometry standpoint and also based on their shear lengths, the wear and/or deterioration process on the cutting element typically starts from the same peripheral locations on the cutting elements for cutting elements in the different layers. This arrangement has a negative effect on overall bit performance, especially durability or longevity.

Thus, despite certain advances made in the industry, there remains a need for a drill bit having an improved cutting element arrangement that enhances stabilization as well as durability and ROP characteristics, and permits the drill bit to drill at economical ROPs for longer durations and through a wider range of formation materials without having to replace the drill bit, thereby reducing costly and time-consuming bit trips.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed to a drill bit, and method of assembling same, that can drill at economical ROPs for longer durations and in a wider range of formation materials. The drill bit has multiple layers of cutting elements established by deploying the cutting elements on blades of different heights or maintaining the blades at the same height and deploying the cutting elements to have different heights on one or more blades. Each layer provides independent bottom hole coverage and has independent stabilization, ROP, and durability characteristics so as to effectively drill through different subsurface formations. Cutting elements deployed in different layers have their respective centers at substantially different radial positions. Due to the different radial positions, cutting elements in different layers cut different respective swaths in the subsurface formation, and are thus loaded and deteriorate differently and independently of the other layers. Cutting elements in different layers may also have different initial peripheral portions or shear lengths, resulting in different impact resistance characteristics for the different layers. This drastically changes the wear and/or cutting element deterioration processes for the different layers, which results in different and improved toughness characteristics. In some embodiments, cutting elements deployed on different layers have different sizes, shapes, and/or back rake angles, respectively. In other embodiments, cutting elements deployed on different layers have different thermal stability, impact resistance, and/or abrasion resistance, respectively.

In general, in one aspect, the invention is directed to a drill bit. The drill bit comprises a drill bit body, blades formed on said drill bit body, said blades having a plurality of cutting element positions radially located thereon. The drill bit further comprises cutting elements deployed on said blades, said cutting elements forming a primary layer of cutting elements and a secondary layer of cutting elements, said primary layer of cutting elements having a different height relative to said drill bit body from said secondary layer of cutting elements. At least one primary layer cutting element and a corresponding secondary layer cutting element occupy substantially different radial cutting element positions on said blades such that their cutting element profiles overlap, said at least one primary layer cutting element and said corresponding secondary layer cutting element together defining a cutting zone equal to a diameter of one of said at least one primary layer cutting element and said corresponding secondary layer cutting element plus a predetermined percentage or fraction of said diameter.

In general, in another aspect, the invention is directed to a method of assembling a drill bit. The method comprises providing a drill bit body having blades formed thereon, said blades having a plurality of cutting element positions radially located thereon. The method further comprises deploying cutting elements on said blades, said cutting elements forming a primary layer of cutting elements and a secondary layer of cutting elements, said primary layer of cutting elements having a different height relative to said drill bit body from said secondary layer of cutting elements. At least one primary layer cutting element and a corresponding secondary layer cutting element occupy substantially different cutting element positions on said blades such that their cutting element profiles overlap, said at least one primary layer cutting element and said corresponding secondary layer cutting element together defining a cutting zone equal to a diameter of one of said at least one primary layer cutting element and said corresponding secondary layer cutting element plus a predetermined percentage or fraction of said diameter.

In general, in yet another aspect, the invention is directed to a drill bit body. The drill bit body comprises blades formed on said drill bit body and cutting element positions formed on said blades. The cutting element positions are radially located such that when cutting elements are deployed on said blades, said cutting elements form a primary layer of cutting elements and a secondary layer of cutting elements, said primary layer of cutting elements having a different height relative to said drill bit body from said secondary layer of cutting elements. At least one primary layer cutting element and a corresponding secondary layer cutting element occupy substantially different cutting element positions on said blades such that their cutting element profiles overlap when said cutting elements are deployed on said blades, said at least one primary layer cutting element and said corresponding secondary layer cutting element together defining a cutting zone equal to a diameter of one of said at least one primary layer cutting element and said corresponding secondary layer cutting element plus a predetermined percentage or fraction of said diameter.

In general, in another aspect, the invention is directed to a drill bit capable of drilling effectively in long intervals of formation material or sections having grossly different mechanical and/or geologic properties (i.e. sandstone, carbonates and chert or pyrite).

In general, in still another aspect, the invention is directed to a drill bit capable of effectively drilling in formations infested with chert, pyrite or nodules, where these specific materials are located at the top, middle or bottom sections of the formation interval, and where conventional drilling practices typically require the use of multiple drill bits, which may have drastic effects on drilling and operational costs.

Additional aspects of the invention will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments, which is made with reference to the drawings, a brief description of which is provided below.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparent from the following detailed description and upon reference to the drawings, wherein:

FIGS. 1A-1B, described previously, illustrate a side view and a bottom view of a prior art fixed cutting element drill bit;

FIGS. 2A-2B, described previously, illustrate a segment of a drill bit profile for a drill bit having multiple cutting element tip heights where cutting elements with different tip heights occupy substantially the same radial positions and have substantially the same size, shape, and shear length;

FIGS. 3A-3B, described previously, illustrate blades for drill bits having the profile shown in FIGS. 2A-2B;

FIGS. 4A-4B illustrate a segment of a drill bit profile for a drill bit having multiple layers of cutting elements where cutting elements with different tip heights occupy substantially different radial positions;

FIGS. 5A-5B illustrate exemplary implementations of a drill bit having the drill bit profile shown in FIGS. 4A-4B;

FIGS. 6A-6B illustrate alternative implementations for a drill bit having the drill bit profile shown in FIGS. 4A-4B;

FIGS. 7A-7B illustrate a segment of a drill bit profile for a drill bit having multiple layers of cutting elements where cutting elements with different tip heights occupy substantially different radial positions and have substantially different shear lengths;

FIGS. 8A-8B illustrate a segment of a drill bit profile for a drill bit having multiple layers of cutting elements where cutting elements with different tip heights occupy substantially different radial positions and have substantially different diameters;

FIGS. 9A-9B illustrate a segment of a drill bit profile for a drill bit having multiple layers of cutting elements where cutting elements with different tip heights occupy substantially different radial positions and have substantially different axial volumes;

FIGS. 10A-10C illustrate a segment of a drill bit profile for a drill bit having multiple layers of cutting elements where cutting elements with different tip heights occupy substantially different radial positions and have substantially different back rake angles; and

FIGS. 11A-11B illustrate a segment of a drill bit profile for a drill bit having multiple layers of cutting elements where cutting elements with different tip heights occupy substantially the same radial positions and have substantially different shear lengths.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE INVENTION

Following is a detailed description of the invention with reference to the drawings. It should be noted that the drawings are provided for illustrative purposes only and are not intended to be a blueprint or manufacturing drawings, nor are they drawn to any particular scale.

As mentioned above, existing drill bits have primary layer and secondary layer cutting elements that are uniform in size and shape and share the same radial positions. This results in the cutting elements of the secondary layers following an identical swath through the subsurface formation as the cutting elements of the primary layers. Consequently, the primary layer and secondary layer cutting elements do not establish independent coverage of the bottom hole. In this regard, the tips of the cutting elements in the different layers, which are the first points of contact with the formation being drilled, begin to fail or deteriorate along a common line. Such failure initiation at the tips of the cutting elements in the primary layer immediately exposes the cutting elements in the secondary or other layers to failure also because the mentioned tips are in a straight line. As such, the secondary or other layer's failure is dependent on the initiation and rate of failure of the secondary or other layer. This arrangement or deployment in existing drill bits has a negative effect on bit performance. In addition, the cutting elements of the primary layers and the cutting elements of the secondary layers have substantially the same shear lengths. This decreases the effectiveness of existing drill bits, in terms of their durability, stabilization and ROP, thus narrowing the range of formation materials with which they can be used.

Embodiments of the invention provide a drill bit where at least one cutting element of the primary layer occupies a substantially different radial position from a corresponding cutting element of the secondary layer. Specifically, the at least one primary layer cutting element and the corresponding secondary layer cutting element together define a cutting zone that spans a predetermined distance. In some embodiments, the predetermined distance may be the diameter of one of the cutting elements plus a certain percentage or fraction of that diameter. The diameter may be that of the primary layer cutting element or it may be that of the secondary layer cutting element, and the fraction may be about 1/10 to about ⅓ of that diameter. As a result, the different layers establish bottom hole coverages that are independent of each other. In one embodiment, the primary layer establishes nearly 100% coverage of the bottom hole and the secondary layer establishes at least 80% coverage. This determination is typically based on the intended application and may be influenced by bit size, blade count, bit profile, as well as the lengths of the secondary and tertiary blades used in the bit design.

The cutting elements of the different layers having different cutting element tip heights are deployed so as to define different layouts or cutting structures. In one embodiment, the different layers are adapted for specific applications based on the ROP, durability, and stabilization requirements of different drilling environments. The cutting elements may be any suitable type of cutting element known to those having ordinary skill in the art, including TCI cutting elements, PDC cutting elements, natural diamond cutting elements, and combinations thereof. Furthermore, cutting elements deployed on the primary layer and cutting elements deployed on the secondary layer may have substantially different shear lengths, sizes, shapes, back rake angles, thermal stability, abrasion resistance and/or impact resistance. Based on the blade count and cutting element type and deployment, the drill bit of the invention may be customized for specific subsurface formations, including formations infested with chert, nodules, and/or pyrite. The drill bit of the invention is capable of drilling through formations with such infestations at acceptable ROPs regardless of the location of the chert, nodules, and/or pyrite in a given formation interval or hole section Such an arrangement results in a drill bit that can drill at economical ROPs for longer durations and with a wider formation bandwidth, thus reducing costly and time-consuming bit trips. Note that while embodiments of the invention are described herein mainly with respect to drill bits having a primary layer and a secondary layer of cutting elements, the teachings and principles discussed are fully applicable to drill bits having one or more additional layers of cutting elements (e.g., a tertiary layer, etc.).

Referring now to FIGS. 4A-4B, a segment of a drill bit profile 400 for a drill bit having multiple layers of cutting elements, achieved by deploying the cutting elements on blades of different heights or maintaining the blades at the same height, then deploying the cutting elements so as to have different heights on one or more blades, is shown. As can be seen in FIG. 4A, the drill bit profile segment 400 includes a primary layer 402 that is offset in height relative to a secondary layer 404. Note that the two layers 402 and 404 are shown here in isolation (i.e., the amount of offset is exaggerated) so that the cutting elements 406a-n and 408a-n may be more easily seen. Those having ordinary skill in the art will understand that, in practice, the offset is much smaller than illustrated and more closely resembles the offset shown in FIGS. 2A-2B. It should also be noted that the number of layers of cutting elements is not necessarily limited to two. In some instances, depending on the drilling application and the challenges presented, there may be a primary layer, a secondary layer, a tertiary layer, as well as other layers, with each layer being offset in height relative to the immediately preceding layer. The amount of offset may be any value commonly used by those having ordinary skill in the art, but may depend on the expected ROP and durability of the primary layer as well as the specific location and presence of the formation material having grossly different mechanical and/or geologic properties (e.g., chert, pyrite, nodules, etc.) in a given interval. The amount of offset may also depend on the anticipated overall ROP and durability of the drill bit over the total interval to be drilled.

In accordance with embodiments of the invention the primary layer cutting elements and the secondary (and possibly tertiary) cutting elements occupy substantially different radial positions on their respective blades. This is illustrated in FIG. 4B, where one of the primary layer cutting elements 406a and a corresponding secondary layer cutting element 408a are shown. As can be seen, the primary layer cutting element profile 406a has a center C that lines up on one radial displacement X, whereas the corresponding secondary layer cutting element profile 408a has a center C′ that lines up on a substantially different radial displacement X′. As a result, the corresponding cutting element in the secondary layer (and tertiary layer where applicable) cuts a substantially different swath (i.e., follows a substantially different path) through the subsurface formation from the primary layer cutting element.

The width of the combined swath or cutting zone for the two corresponding cutting elements, in accordance with embodiments of the invention, spans a predetermined distance that, in some embodiments, is at least equal to the diameter of one of the cutting elements (i.e., where one cutting element has a larger diameter than the other) and at most equal to the diameter of one of the cutting elements plus a predetermined fraction of that diameter. This is illustrated FIGS. 5A-5B, with FIG. 5A showing blades of different heights and FIG. 5B showing blades of the same height, but corresponding cutting elements deployed so as to have different heights. For either case, however, the corresponding cutting elements in the different blades occupy different radial positions.

In FIG. 5A, a drill bit 500 has blades 502a and 502b that are offset at different heights, but with cutting elements 504a-n and 506a-n that are set at the same height on their respective blades. Specifically, the blade in the lower half of the figure, blade 502a, has a greater height than the blade 502b in the upper half of the figure. Therefore, the cutting elements 504a-n on the first blade 502a form part of the primary layer of cutting elements, while the cutting elements 506a-n on the second blade 502b constitute part of the secondary layer of cutting elements.

The cutting zone Z′, spanned by a given primary layer cutting element and corresponding secondary layer cutting element, is at least equal to the diameter D of one of these cutting elements and at most equal to the diameter D of one of the cutting elements plus a predetermined fraction of that diameter. That is, Z′=D+0.Y*D, where 0.Y is the predetermined fraction In some embodiments, the diameter D used to define the cutting zone Z′ is the diameter of the primary layer cutting element, while in other embodiments, the diameter D is that of the secondary layer cutting element. In still other embodiments, the diameter D used to define the cutting zone Z′ may be the diameter of whichever cutting element has the larger diameter. And as for the predetermined fraction of the diameter, in some embodiments, this value may be about 1/10 to about ⅓, and preferably about ⅕, of the diameter of whichever cutting element is used.

While the above arrangement has advantages, in some drilling applications, it is desirable to have all the blades be at the same height for stability and durability purposes. FIG. 5B shows a drill bit 508 with blades 510a and 510b that are the same height, but each blade has cutting elements 512a-n and 514a-n that are deployed so as to have different heights relative to each other. The technique used in deploying the cutting elements so as to have different heights may be any technique known to those having ordinary skill in the art. For example, the cutting elements may be recessed into the blades so as to have different degrees of exposure, or each cutting element may be exposed by the same amount, but the surface of the blades may be contoured so that certain cutting elements are seated higher than others.

In the drill bit portion shown here, the non-shaded cutting elements 512a-n are deployed so as to have a greater height than the shaded cutting elements 514a-n. These non-shaded cutting elements 512a-n accordingly constitute the primary layer of cutting elements, whereas the shaded cutting elements 514a-n constitute the secondary layer of cutting elements. Both shaded and non-shaded cutting elements 512a-n and 514a-n may be intermixed across different blades 510a and 510b, as depicted here, or cutting elements of different heights may be mounted on their own blades (similar to the implementation of FIG. 5A). In either case, the resulting cutting zone Z′ created by a given primary layer cutting element and corresponding secondary layer cutting element is at least equal to the diameter D of one of these cutting elements and at most equal to the diameter D of one of the cutting elements plus a predetermined fraction of that diameter (i.e, Z′=D+0.Y*D).

In the drill bits described thus far, multiple layers of cutting elements have been achieved using a single row of cutting elements on each blade. However, embodiments of the invention may also be implemented using multiple rows of cutting elements on a single blade. Examples of such an arrangement are illustrated in FIGS. 6A-6B. In the first implementation FIG. 6A, a drill bit 600 has blades that are of the same height, but at least one blade 602 has multiple (e.g., two) rows of cutting elements 604a and 604b deployed thereon. The first row 604a (lower half of the figure) has cutting elements 606a-n that are deployed so as to have a greater height than cutting elements 608a-n in the second row 604b. These cutting elements 606a-n, which may have different shear lengths, therefore constitute part of the primary layer of cutting elements, whereas the cutting elements 608a-n in the second row 604b, which may also have different shear lengths, constitute part of the secondary layer.

The implementation of FIG. 6B is similar to the implementation of FIG. 6A insofar as the drill bit 610 has blades that are of the same height, with at least one blade 612 having multiple (e.g., two) rows of cutting elements 614a and 614b. However, instead of the same cutting element height, each row 614a and 614b contains a mix of cutting element heights, with non-shaded cutting elements 616a-n deployed so as to have a greater height than shaded cutting elements 618a-n.

In both of the above implementations, the primary layer cutting elements 606a-n &616a-n and the corresponding secondary layer cutting elements 608a-n &618a-n occupy substantially different radial positions. As a result, they cut substantially different swaths in the subsurface formation. The width of the swath or cutting zone, indicated again as Z′, is at least equal to the diameter D of one of these cutting elements and at most equal to the diameter D of one of the cutting elements (e.g., the primary cutting element) plus a predetermined fraction of that diameter, or Z′=D+0.Y*D.

Referring back to the drill bit profile in FIGS. 4A-4B, in addition to occupying substantially different radial positions, the primary layer cutting elements 406a-n have a shear length SL that is substantially equal to the shear length SL′ of the secondary layer cutting elements 408a-n. This may be desirable in some drilling applications, but as explained above, the shear length affects a drill bit's stability and durability, as well as its ability to effectively drill much longer sections of different types of formation material. Therefore, in some embodiments, by endowing the primary and secondary layers of cutting elements with substantially different shear lengths SL and SL′, the range of formation material that may be effectively drilled is significantly increased.

FIGS. 7A-7B illustrate a segment of a drill bit profile 700 for another drill bit having primary and secondary cutting element layers 702 and 704 where the primary layer cutting elements 706a-n and the secondary layer cutting elements 708a-n have substantially different shear lengths SL and SL′. Referring to FIG. 7A, the drill bit profile segment 700 is similar to the drill bit profile segment 400 (see FIGS. 4A-4B) insofar as it represents a drill bit that may be achieved by deploying the cutting elements 706a-n and 708a-n on blades of different heights or maintaining the blades at the same height, then deploying the cutting elements 706a-n and 708a-n so as to have different heights on one or more blades. In addition, the primary layer cutting elements 706a-n and the secondary cutting elements 708a-n (and possibly tertiary layer cutting elements) also occupy substantially different radial positions on their respective blades. This is illustrated in FIG. 7B, where a center C of one of the primary cutting elements 706a lines up on a radial displacement X, and a center C′ of the corresponding secondary layer cutting element 708a lines up on a substantially different radial displacement X′.

Unlike the drill bit profile segment 400, however, the drill bit profile segment 700 represents a drill bit where the primary layer cutting elements 706a-n have a shear length SL that is different from the shear length SL′ of the secondary layer cutting elements 708a-n. In the specific embodiment of FIG. 7A, the primary layer shear length SL is smaller than the secondary layer shear length SL′. In other embodiments, however, the primary layer shear length SL may be larger than the secondary layer shear length SL′, depending on the particular subsurface formation to be drilled or application challenges presented. In embodiments where the primary layer cutting elements and/or secondary layer cutting elements (and/or tertiary layer cutting elements) have more than one shear lengths, the average shear length of the secondary (or tertiary) layer cutting elements is substantially different (e.g., larger or smaller) from the average shear length of the primary layer cutting elements. As a result of the different shear lengths and radial positions, the cutting elements in the different layers cut independent swaths in the formation being drilled, and will therefore have different characteristics in terms of durability, stability and ROP.

The width of the swath, or cutting zone, cut by a given primary layer cutting element and a corresponding secondary (or tertiary) layer cutting element in FIGS. 7A-7B spans a predetermined distance Z′ that, in some embodiments, is at least equal to the diameter D of one of these cutting elements and at most equal to the diameter D of one of the cutting elements plus a predetermined fraction of that diameter (i.e., Z′=D+0.Y*D). Note that with different shear lengths SL, the swath or cutting zone Z′ will vary with radial displacement (i.e., it is not a fixed value). As mentioned above, in some embodiments, the diameter may be the primary layer cutting element's diameter or it may be the secondary layer cutting element's diameter, and the predetermined fraction may be about 1/10 to about ⅓ percent of that diameter. In other embodiments, the diameter used to define the cutting zone Z′ may be the diameter of whichever cutting element has the larger diameter.

In some embodiments, the sizes and shapes, and hence the diameters, of the primary and secondary layer cutting elements are substantially the same. FIGS. 8A-8B illustrate an embodiment of the invention where the sizes of the primary and secondary layer cutting elements are substantially different. Referring to FIG. 8A, the embodiment shown here is similar to the embodiment shown in FIGS. 4A-4B in that the drill bit profile segment 800 represents a drill bit having primary and secondary cutting element layers 802 and 804. As before, the different layers 802 and 804 may be achieved by deploying the cutting elements 806a-n and 808a-n on blades of different heights or maintaining the blades at the same height, then deploying the cutting elements 806a-n and 808a-n so as to have different heights on one or more blades. Like the embodiment shown in FIGS. 4A-4B, the primary and secondary layers have cutting elements in substantially different radial positions (see FIG. 8B), and thus have independent and different bottom hole coverages and cut different swaths in the formation being drilled. In some instances, the different cutting elements belonging to the primary and secondary layers may also have substantially different shear lengths SL and SL′ (or average shear lengths as applicable), similar to the embodiment of FIGS. 7A-7B.

Unlike the embodiments shown in FIGS. 4A-4B and 7A-7B, the cutting elements 808a-n deployed on the secondary layer 804 in FIGS. 8A-8B have a substantially different size from the cutting elements 806a-n deployed on the primary layer 802. In the specific embodiment shown, the secondary layer cutting elements 808a-n have a diameter D′ that is larger than the diameter D of the primary layer cutting elements 806a-n. It is of course possible for the secondary layer cutting elements 808a-n to have a diameter D′ that is smaller than the diameter D of the primary layer cutting elements 806a-n, depending on the particular sequence of subsurface formation materials to be drilled in an interval. In embodiments where there are several sizes of cutting elements deployed on the primary and secondary layers, the average size of the secondary layer cutting elements 808a-n may be substantially different (e.g., larger or smaller) from the average size of the primary layer cutting elements 806a-n. Note again that with different shear lengths SL, the swath or cutting zone Z′ will vary with radial displacement (i.e., it is not a fixed value).

While the drill bits discussed thus far have primary layer and secondary layer cutting elements that are of substantially the same shape, namely, a round shape, other shapes may also be used. Examples of other shapes that may be used include elliptical shapes, egg shapes, pear shapes, and teardrop shapes (hereinafter, collectively referred to as oval shapes), as well as other common and customized shapes known to those having ordinary skill in the art. In some cases, even non-circular shapes may be used where at least a portion of the shape is flat (e.g., semicircular, diamond, rectangular, etc). Moreover, embodiments of the invention also provide a drill bit where the primary layer or primary cutting element tip profile and the cutting elements, and the secondary layer or secondary cutting element tip profile and the cutting elements, have substantially different shapes and geometries.

FIGS. 9A-9B illustrate an embodiment of the invention where the primary layer cutting elements and the secondary layer cutting elements have substantially different shapes. Referring to FIG. 9A, the embodiment shown here is similar to the previous embodiments insofar as the drill bit profile segment 900 represents a drill bit having primary and secondary cutting element layers 902 and 904. As before, the different layers 902 and 904 may be achieved by deploying the cutting elements 906a-n and 908a-n on blades of different heights or maintaining the blades at the same height, then deploying the cutting elements 906a-n and 908a-n so as to have different heights on one or more blades. Like the previous embodiments, the primary and secondary layers have cutting elements in substantially different radial positions (see FIG. 9B), and thus have independent and different bottom hole coverages and cut different swaths in the formation being drilled. In some embodiments, the different cutting elements belonging to the primary and secondary layers may also have substantially different shear lengths SL and SL′ (or average shear lengths as applicable), similar to the embodiment of FIGS. 7A-7B.

Unlike the embodiments shown in the previous figures, the cutting elements 908a-n deployed on the secondary layer 904 in FIGS. 9A-9B have a substantially different shape from the cutting elements 906a-n deployed on the primary layer 902. The difference in shape results in substantially different axial volumes Av for the primary and secondary layers 902 and 904. The term “axial volume,” as understood by those having ordinary skill in the art, refers to the distance from the center of the cutting element face to the cutting tip. In the specific embodiment shown, the secondary layer cutting elements 908a-n have an oval shape, whereas the primary layer cutting elements 906a-n are round. The oval cutting elements provide the secondary layer 904 with an axial volume Av′ that is greater than the axial volume Av of the round cutting elements of the primary layer 902.

The substantially different axial volumes affect the durability of the cutting elements in hard and abrasive formations. A larger axial volume increases the ability of the cutting element to withstand higher rotational speeds during the drilling process than a smaller axial volume due to the substantially higher diamond content. For this reason, oval cutting elements are known to be highly effective in abrasive formations or lithologies, such as sandstone and siltstone, from an axial volume perspective. In addition, oval shaped cutting elements are more effective at pre-fracturing of brittle formations, a characteristic that improves ROP in carbonate bearing formations. Round cutting elements, on the other hand, are more effective for shearing non-brittle formations or lithologies, such as shale, sandstones and siltstone.

By deploying oval cutting elements on the secondary layer 904 and round cutting elements on the primary layer 902, embodiments of the invention combine the advantages of both round and oval cutting elements. A similar benefit may be obtained by deploying the round cutting elements on the secondary layer 904 and the oval cutting elements on the primary layer 902. Alternatively, oval shaped cutting elements may be deployed on both the primary and secondary layers 902 and 904, but of substantially different types. For example, elliptical shaped cutting elements may be deployed on the primary layer 902 while teardrop cutting elements may be deployed on the secondary layer 904, and so on.

In addition to substantially different shapes, in some embodiment, the cutting elements of the primary layer and the cutting elements of the secondary layer may have substantially different back rake angles. The term “back rake angle,” as understood by those having ordinary skill in the art, refers to the angle formed between a line parallel to the cutting element face and a vertical line drawn through the center of the cutting element. Such back rake ankles control how aggressively the cutting element engages the subsurface formation. In general, a smaller back rake angle increases cutting element aggressiveness (i.e., high ROP), but leaves the cutting element vulnerable to impact breakage. On the other hand, a larger back rake angle decreases cutting element aggressiveness (i.e., low ROP), but gives the cutting element longer life.

FIGS. 10A-10C illustrate an embodiment of the invention where the primary layer cutting elements and the secondary layer cutting elements have substantially different back rake ankles. Referring to FIG. 10A, the embodiment shown here is similar to the embodiments shown in the previous figures in that the drill bit profile segment 1000 represents a drill bit having primary and secondary cutting element layers 1002 and 1004. As before, the different layers 1002 and 1004 may be achieved by deploying the cutting elements 1006a-n and 1008a-n on blades of different heights or maintaining the blades at the same height, then deploying the cutting elements 1006a-n and 1008a-n so as to have different heights on one or more blades. Like the previous embodiments, the primary and secondary layers have cutting elements in substantially different radial positions (see FIG. 10B), and thus have independent and different bottom hole coverages and cut different swaths in the formation being drilled. In some embodiments, the different cutting elements belonging to the primary and secondary layers may also have substantially different shear lengths SL and SL′ (or average shear lengths as applicable), similar to the embodiment of FIGS. 7A-7B.

Unlike the previous embodiments, the cutting elements 1008a-n deployed on the secondary layer 1004 here have a substantially different back rake angle from the cutting elements 1006a-n deployed on the primary layer 1002. FIG. 10C shows a side view of one of the cutting elements 1010 deployed on the primary layer 1002 and one of the cutting elements 1012 deployed on the secondary layer 1004. As can be seen, the secondary layer cutting element 1012 has a back rake angle A′ that is substantially different from the back rake angle A of the primary layer cutting element 1010 relative to a subsurface formation 1014. In the specific embodiment shown, the back rake angle A′ of the secondary layer cutting elements 1012 is larger (e.g., 30°) than the back rake angle A (e.g., 20°) of the primary layer cutting elements 1010. This substantial difference in back rake angle results in the secondary layer cutting element 1012 having decreased cutting element aggressiveness, but higher impact resistance. In other embodiments, it is possible to have the back rake angle A′ of the secondary layer cutting element 1012 be smaller than the back rake angle A of the primary layer cutting element 1010, depending on the levels of hardness and/or abrasiveness of the formation sequences to be drilled.

Although the embodiments described thus far have focused on the different radial positions and shear lengths (or average shear lengths as applicable), in some embodiments, it may be desirable to provide a drill bit where the primary layer cutting elements and the secondary layer cutting elements having different shear lengths. An example of such an embodiment is illustrated in FIGS. 11A-11B, where the drill bit profile segment of 1100 is shown representing a drill bit having primary and secondary cutting element layers 1102 and 1104. As before, the different layers 1102 and 1104 may be achieved by deploying the cutting elements 1106a-n and 1108a-n on blades of different heights or maintaining the blades at the same height, then deploying the cutting elements 1106a-n and 1108a-n so as to have different heights on one or more blades.

Unlike the previous embodiments, the primary and secondary layers 1102 and 1104 have cutting elements 1106a-n and 1108a-n mounted in substantially identical radial positions (see FIG. 11B). Nevertheless, in accordance with embodiments of the invention, the cutting elements belonging to the primary and secondary layers may still have substantially different shear lengths S L and SL′ (or average shear lengths as applicable). In some embodiments, the different shear lengths SL and SL′ (or average shear lengths as applicable) may be achieved by deploying cutting elements having different shapes (e.g., round, oval, etc.) in the primary layer 1102 versus the secondary layer 1104, or vice versa. In other embodiments, although not expressly shown, the different shear lengths SL and SL′ (or average shear lengths as applicable) may be achieved by deploying cutting elements having different sizes (e.g., 16 mm, 19 mm, etc.) in the primary layer 1102 versus the secondary layer 1104, or vice versa. Thus, although they share common radial positions, the cutting elements 1106a-n and 1108a-n of the primary and secondary layers 1102 and 1104 still provide independent and different bottom hole coverages and cut different swaths in the formation being drilled.

In some embodiments, based on the specifics of an application as well as the formation types to be drilled, the primary layer cutting elements and the secondary layer cutting elements may have substantially different properties in terms of abrasion and impact resistance. For example, either the primary layer cutting elements or the secondary layer cutting elements may be made more abrasion resistant (i.e., have a finer diamond grain), or both the primary layer cutting elements and secondary layer cutting elements may have improved abrasion resistance. In a similar manner, the primary layer cutting elements may be made more impact-resistant than the secondary layer cutting elements, or vice versa, or both the primary layer and secondary layer cutting elements may have improved impact resistance.

In other embodiments, either the primary layer cutting elements or the secondary layer cutting elements may be treated to remove catalyzing material (e.g., cobalt), a process commonly referred to as “leaching.” As is well known in the art, leaching or removal of catalyzing material from cutting elements can improve their thermally stability, thus allowing them to withstand much higher drilling temperatures before failing. Improved thermal stability drastically reduces the wear initiation process of the cutting elements. This process may be used to further enhance the performance properties of the primary layer cutting elements or the secondary layer cutting elements, as described herein. Techniques for removal of catalyzing material from cutting elements are generally known and may be found, for example, in U.S. Pat. No. 8,544,408 entitled “High Volume Density Polycrystalline Diamond with Working Surfaces Depleted of Catalyzing Material,” which is incorporated herein by reference.

It should be noted that regardless of the diamond material types (e.g., fine grain or coarse grain diamond materials) that may be used for the primary layer and/or secondary layer cutting elements, or the leaching or catalyzing material depletion processes employed, all advantages, principles and teachings herein discussed for the present invention remain valid and fully applicable to these various embodiments.

In operation, the cutting elements in the primary layer of the drill bit initially bear most of the load during drilling of a specific dominant formation type (e.g., sandstone, shale, siltstone, etc.). As the cutting elements in the primary layer wears and/or deteriorates due to formation hardness and/or abrasiveness, the cutting elements in the secondary and subsequent layers define a new bit, having independent bottom hole coverage that cut different swaths in the formation, and also have different and unique ROP, durability and stability characteristics. Based on the specific layout of a drill bit according to embodiments of the invention, but mainly due to the substantially different radial positions of the cutting elements in the different layers and/or substantially different shear lengths (SL) of the different layers, such drill bits are adapted to effectively drill in chert, pyrite and or nodules due to the controlled and specifically staged durability and ROP characteristics of the drill bit of the invention. In such instances, the cutting elements in the primary layer fail, but in do doing so, expose the cutting elements in the secondary layer (and possibly tertiary layer, and so forth), which are then able to re-establish the drill bit's ROP and durability characteristics, thus enabling the drill bit to continue drilling for longer periods of time at an effective ROP. In other words, because the secondary layer cutting elements have independent bottom hole coverage and may be customized with a substantially different shear length, size, shape, thermal stability, abrasion resistance, and/or impact resistance according to embodiments of the invention, the drill bit is able to continue drilling at an economical ROP through the subsequent formation type, eventually reentering the dominant formation type or a different formation that is devoid of chert, pyrite or nodules.

While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the scope of the invention Accordingly, each of the foregoing embodiments and obvious variations thereof is contemplated as falling within the scope of the claimed invention, as is set forth in the following claims.