ENHANCED CUTTER PROFILE FOR FIXED CUTTER DRILL BITS

Abstract

In one aspect, a cutting structure for a drill bit includes cutters arranged to form a cutter profile. Substantially all of the cutters in a shoulder section of the cutter profile are positioned at a profile angle of at most 65°. In another aspect, substantially all of the shoulder cutters are arranged so that the longitudinal force exerted by each shoulder cutter onto an earthen formation is at least 25 percent of the normal force exerted by the shoulder cutter onto the earthen formation.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority is claimed from U.S. Provisional Application No. 61/265,030 filed on Nov. 30, 2009.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to fixed cutter drill bits used for drilling wellbores in subsurface earthen formations. More particularly, the present invention relates to the arrangement of cutting elements on drill bits used to shear earthen formations in so-called “fixed cutter” drill bits.

2. Background Art

Fixed cutter wellbore drill bits known in the art generally include cutting elements arranged in such a way that if the bit were rotated about its longitudinal (rotational center) axis, the edges of all of the cutters would define a “cutter profile” or “bottom hole pattern.” When viewed as a section through the longitudinal axis, a two dimensional (2D) cutter profile drawn through the tips of all of the cutters typically includes a linear ‘cone’ shaped section toward or proximate the rotational center of the bit, arcuate ‘nose’ and ‘shoulder’ sections, and a linear ‘gage’ section substantially tangent to the arcuate shoulder section and substantially parallel to the bit's longitudinal axis. Such profile shape is the result of decades of development in the fixed cutter drill bit industry, and is so shaped for a variety of reasons, one of the most important being the ability to increase the density of cutter elements with increasing radial (lateral) displacement from the longitudinal axis along the profile.

Generally, fixed cutter bits that are used to drill “hard” earthen formations have a longer bit profile than bits used to drill softer formations. Longer bit profiles allow the placement of additional cutting elements compared to shorter bit profiles, and it is generally desired to have more cutting elements in the profile when drilling harder formations. The cutters on a fixed cutter drill bit are arranged to create a smooth transition between the different sections of the bit profile, and are also generally arranged in such a way that the ‘profile angle’ of the cutters increases with increasing radius from 0° at the nose position to a maximum value around 90° at the ‘gage point’. The ‘profile angle’ of a cutter is defined as the angle between a line drawn through the center of the cutter, perpendicular to the 2D bit profile, and the longitudinal axis of the drill bit. A table of cutter locations including profile angles can be found in U.S. Pat. No. 5,678,644 issued to Fielder. The ‘gage point’ is defined as the point of the furthest radial extent of the bit profile, at the closest longitudinal location to the nose of the drill bit. Examples of prior art bit profiles can be observed in many patents including U.S. Pat. No. 6,575,256 issued to Doster.

Fixed cutter, polycrystalline diamond compact (PDC) cutting element bits were first used in the field of drilling earth formations in the 1970's. For many years, fixed cutter drill bits were primarily used for drilling low-strength formations, as the cutters could not withstand the forces created when drilling harder formations. More recently, however, fixed cutter bit design and cutter technology have improved to such an extent that enables fixed cutter bits to drill many harder formations more effectively than other types of drill bits. However, there are still many formations that are too hard or too abrasive to be effectively drilled with fixed cutter bits, and these formations represent the most expensive drilling in the industry as penetration rates are very low, and drill bit life is very short.

Many attempts have been made to further increase the wear resistance of fixed cutter bits when drilling hard and abrasive formations, but those attempts have primarily focused on either adding additional diamond to the drill bit (refer to U.S. Pat. No. 7,594,554, the “554 patent”), or on developing new types of cutters that can better withstand the conditions experienced when drilling harder formations (refer to U.S. Pat. No. 6,544,308 issued to Griffin et al.). So called ‘double-row’ fixed cutter layouts, such as described in the '554 patent, are now commonplace in state of the art fixed cutter PDC drill bits due to their success in extending drill bit life in hard and abrasive formations. This improvement in drill bit life is simply a result of adding more diamond to the drill bit to increase overall wear resistance. However, in applications where the formation is even more hard and abrasive than those drilled using a bit as described in the '554 patent, even double-row fixed cutter bits still experience very short life due to the rapid deterioration of the PDC cutters.

As is well known in the art, the wear life of PDC cutters is greatly affected by the temperature the cutter experiences while drilling, and by the depth of cut achieved by the cutter while drilling. It is well documented in the literature that, in a given formation, when a fixed cutter bit drills with a larger depth of cut, the wear life of the cutters increases dramatically (ref. IADC/SPE paper no. 39306 authored by Sinor et al.). This effect is described in the literature as ‘shearing’ versus ‘scraping’. Fixed cutter diamond drill bits have been used to drill earthen formations for over 100 years, but until the introduction of the PDC cutter in the 1970's, drilling efficiency was generally poor due to the fact that the previous fixed cutter bits generally drilled by scraping or grinding rather than shearing the formation. With the introduction of the PDC cutter, also called a ‘shear cutter’, drilling efficiency was dramatically improved, allowing boreholes to be drilled faster and more economically than ever before. However, in so-called “ultra-hard” formations it can be very difficult to achieve a “shearing” depth of cut with a fixed cutter drill bit. Generally, when attempting to drill ultra-hard and abrasive formations with fixed cutter drill bits, in order to achieve an effective bit life, fixed cutter bits are designed with a large plurality of cutters, which is achieved through a combination of high blade count, long cutter profile, and double-row layouts. In order to achieve shear with such a heavy-set fixed cutter bit, it is necessary to apply a very large weight on bit (WOB). However, when high WOB is used, there is an increased tendency to cause mechanical failure of the cutters. Also, because frictional heating of the cutters is directly proportional to WOB, using a high WOB increases the temperature of the cutters, which further contributes to cutter deterioration. As a result, fixed cutter bits are generally not effective for drilling in ultra-hard and abrasive formations.

Accordingly, there is a need for fixed cutter drill bits that exhibit increased durability when drilling hard and abrasive formations. A fixed cutter drill bit with enhanced durability, using existing materials, will allow the bit to drill longer sections in hard and abrasive formations.

SUMMARY OF THE INVENTION

A cutting structure for a fixed cutter drill bit according to one aspect of the invention includes cutters arranged to form a cutter profile. Substantially all of the cutters in a shoulder section of the cutter profile are positioned at a profile angle of at most 65 degrees.

A cutting structure for a fixed cutter drill bit according to another aspect of the invention includes cutters arranged to form a cutter profile. Substantially all of the cutters in a shoulder section of the cutter profile are arranged such that the longitudinal force exerted on an earthen formation by each cutter in the shoulder section is at least 25 percent of a normal force exerted by that cutter on the earthen formation.

A fixed cutter drill bit according to another aspect of the invention includes a bit body having a plurality of blades extending laterally away from a longitudinal center of the bit body. A plurality of cutters affixed to the at least one of the plurality of blades define a cutter profile having a shoulder section. Substantially all of the cutters in the shoulder section of the profile are positioned to at least one of (a) a profile angle of at most 65 degrees and (b) such that a longitudinal force exerted on an earthen formation by each cutter is at least 25 percent of a normal force exerted by such cutter on the formation.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an oblique view of an example fixed cutter drill bit.

FIG. 1 shows a prior art cutter profile.

FIG. 2 shows an example profile according to the present invention.

FIG. 3 shows an exemplary cutter oriented at a profile angle α, acting on a formation.

DETAILED DESCRIPTION

FIG. 1 shows a prior art cutter profile 10 having a cone section 12, a nose section 14, a shoulder section 16, a gage point 18, and a gage section 20. On the cutter profile 10 is an example cutter of circular cross-section 22, positioned in the shoulder section 16 at a profile angle α with respect to the longitudinal axis 24 of the drill bit. It is common for cutters in the shoulder section 16 on prior art fixed cutter bits to be arranged with a maximum profile angle of between 70-90 degrees The normal force (F_N) that the cutter exerts on the formation is created by the axial loading or weight on bit (“WOB”), which is the force applied by the drill string along the longitudinal drill bit axis. As shown in FIG. 3, the normal force can be further resolved into its components which include a longitudinal force (F_L) and a radial force (F_R), both of which components are resultants of the normal force F_N and the profile angle. The longitudinal component of the normal force defines a cutter's ability to remove formation that is longitudinally adjacent to (ahead of) the cutter, thus defining a fixed cutter drill bit's ability to penetrate earthen formations. Therefore, the larger the profile angle of a cutter, the smaller the corresponding longitudinal force on the cutter, and the lesser the ability to penetrate the formations longitudinally forward of the cutter. The cutters in the shoulder section 16 of the cutter profile 10 that are at the largest radial extent from the bit axis, and therefore also oriented at the highest profile angle, therefore exert the least amount of longitudinal force on the formation.

Having explained a prior art cutter profile, various implementations of a cutter profile according to the invention and a drill bit made therewith will now be explained. A drill bit made using a cutter profile according to any of the examples explained herein below is shown in generalized form in FIG. 1A, wherein the drill bit 11 includes a bit body 13 of a type well known in the art for affixing thereto a plurality of PDC cutters 15. Any or all of the cutters shown in the example bit 11 in FIG. 1A may be arranged according to any of the example profiles explained herein, and it is to be clearly understood that the bit body and cutter configuration shown in FIG. 1A are only meant to serve as an example of a drill bit made using cutter profiles according to the invention. Accordingly, the bit and cutting structure and numbers of cutters on the bit as shown in FIG. 1A are in no way intended to limit the scope of the present invention. Generally, the cutter profile is defined by the edges of cutters arranged on one or more blades 17, which extend laterally away from a center or longitudinal axis of the bit body 13. The invention relates to the profile angle of certain of the cutters 15 disposed in a selected portion of the blade (and thus the profile).

Now referring to FIG. 2, in an example embodiment of the present invention, the cutter profile 10 includes a cone section 12, a nose section 14, a shoulder section 16, a gage point 18, a gage section 20, and an example cutter 22 positioned in the shoulder section 16 at a profile angle β with respect to the longitudinal axis 24 of the drill bit. The foregoing profile components are similar qualitatively to those in the prior art profile of FIG. 1. Unlike the prior art cutter profile, however, a cutter profile according to the present invention includes a very sharp transition at the gage point 18 and is arranged such that the cutters in the shoulder section 16 remain at a relatively low profile angle. In one embodiment of the present invention, the profile angle may be at most 65 degrees. In other embodiments, the profile angle may be at most 45 degrees. In still other embodiments, the profile angle may be at most 30 degrees. Because the cutters in the shoulder section 16 are oriented on the cutter profile 10 at a low profile angle as contrasted to those of prior art fixed cutter bits, the longitudinal force exerted by those cutters in the shoulder section 16 on the formation is much larger than that of the cutters in the shoulder section on prior art fixed cutter bits. As a result of the larger longitudinal force exhibited by the cutters in the shoulder section 16 of a bit according to the present invention, it is possible to achieve shear cutting of the formations even in ultra-hard formations with a relatively low axial force or weight on bit (WOB). Finally, because the WOB can be kept at a relatively low value, frictional heating on the cutters is lessened as contrasted with prior art profile fixed cutter bits, thus further protecting the cutters from thermally induced wear. FIG. 3 shows an exemplary cutter 22 oriented at a profile angle α, acting on a formation, represented by line 26, and showing the forces that result from this interaction. The normal force, F_N, is parallel to a line through the center of the cutter 22, and is perpendicular to the cutter profile. The longitudinal force, F_L, is the component of the normal force acting in a direction parallel to the longitudinal axis of the drill bit. The radial force, F_R, is the component of the normal force acting in a directional normal to the longitudinal axis of the drill bit. As can be observed in the example of FIG. 3, because the total driving force for fixed cutter drill bits is the WOB, and because that force is directed to the cutting structure along the bit axis, the larger the profile angle, α, of a cutter, the lower the longitudinal force acting on the formation. When drilling ultra-hard formations, a very large longitudinal force is required to allow a large enough depth of cut to shear the formation. Because prior art drill bits are designed with long profiles that have the shoulder cutters arranged at very high profile angles, those cutters do not exert sufficient longitudinal force to penetrate the formation, and therefore only cut the formation by scraping, which is a very inefficient mechanism to cut or fail abrasive formations and results in very high temperatures being generated. It is primarily for this reason that cutters on the outer shoulder of prior art fixed cutter drill bits experience rapid wear when drilling ultra-hard abrasive formations.

In some embodiments of a drill bit according to the invention, the profile angle of the cutters in the shoulder section 16 is such that the longitudinal force exerted on the formation by each cutter in the shoulder section is at least 25% of the normal force exerted by said cutter on the formation. In other embodiments, the cutter profile angle of the cutters in the shoulder section is selected such that the longitudinal force exerted by such cutter on the formation is at least 50% of the normal force exerted by such cutters on the formation. In still other embodiments, the cutter profile angle of the cutters in the shoulder section is selected such that the longitudinal force exerted by each cutter on the formation is at least 75% of the normal force exerted by such cutter on the formation.

In another embodiment the bit is designed and built with backup gage cutters positioned at the gage point, where the cutter profile meets the gage. The radially outermost cutter before the gage point is positioned so that its edge is tangent to the gage of the bit. All gage cutters may be positioned with sufficient side rake to provide clearance between the carbide substrate of the PDC cutter and the gage of the bit where it would contact formation. Additionally, if consistent with other drilling parameters and objectives, cutters with a 16 millimeter diameter may be used as the primary cutter size, but backup gage cutters may be 13 millimeter diameter. While the present invention is described in terms of polycrystalline diamond compact (“PDC”) cutter drill bits and circular cross-section cutters, the invention is equally applicable to all types and shapes of cutters used in fixed cutter drill bits. Accordingly, the material composition and the cross sectional shape of the cutter used in any embodiment is not a limitation on the scope of the present invention.

Further, while the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Claims

1. A cutting structure for a fixed cutter drill bit, comprising:

cutters arranged to form a cutter profile on part of a bit body; and

wherein substantially all of the cutters in a shoulder section of the cutter profile are positioned at a profile angle of at most 65 degrees.

2. The cutting structure of claim 1 wherein substantially all of the cutters in the shoulder section are positioned at a profile angle of at most 45 degrees.

3. The cutting structure of claim 1 wherein substantially all of the cutters in the shoulder section are positioned at a profile angle of at most 30 degrees.

4. A cutting structure for a fixed cutter drill bit, comprising:

cutters arranged on part of a bit body to form a cutter profile; and

wherein substantially all of the cutters in a shoulder section of the cutter profile are positioned such that a longitudinal force exerted on an earthen formation by each cutter is at least 25 percent of a normal force exerted by that cutter on the earthen formation.

5. The cutting structure of claim 4 wherein each cutter in the shoulder section is positioned such that the longitudinal force exerted on the earthen formation by that cutter is at least 50 percent of the normal force exerted by that cutter on the earthen formation.

6. The cutting structure of claim 4 wherein each cutter in the shoulder section is positioned such that the longitudinal force exerted on the earthen formation by that cutter is at least 75 percent of the normal force exerted by that cutter on the earthen formation.

7. A fixed cutter drill bit comprising:

a bit body having a plurality of blades extending laterally away from a longitudinal center of the bit body;

a plurality of cutters affixed to at least one of the plurality of blades and defining a cutter profile having a shoulder section; and

wherein substantially all of the cutters in the shoulder section of the cutter profile are positioned to at least one of (a) having a profile angle of at most 65 degrees and (b) such that a longitudinal force exerted on an earthen formation by each cutter is at least 25 percent of a normal force exerted by such cutter on the earthen formation.

8. The bit of claim 7 wherein substantially all of the cutters in the shoulder section are positioned to at least one of (a) having a profile angle of at most 45 degrees and (b) such that the longitudinal force exerted on an earthen formation by each cutter is at least 50 percent of the normal force exerted by such cutter on the earthen formation.

9. The bit of claim 7 wherein substantially all of the cutters in the shoulder section are positioned to at least one of (a) having a profile angle of at most 30 degrees and (b) such that the longitudinal force exerted on an earthen formation by each cutter is at least 75 percent of the normal force exerted by such cutter on the earthen formation.

10. The bit of claim 7 wherein the cutters comprise polycrystalline diamond compacts.

11. The bit of claim 8 wherein the cutters comprise polycrystalline diamond compacts.

12. The bit of claim 9 wherein the cutters comprise polycrystalline diamond compacts.