FRAC PLUG MILL BIT

Abstract

A fixed cutter bit for milling a frac plug includes a body and a face. The face includes a base surface and a plurality of cutter support structures extending from the base surface. Each cutter support structure has a peripheral portion and an inner portion disposed radially internal of the peripheral portion. At least one first-type cutter is supported by each peripheral portion; at least one second-type cutter is supported by each inner portion. The first type cutter is adapted to mill a harder material than the second-type cutter, and the first-type is different from the second-type.

Description

TECHNICAL FIELD

The present invention relates generally to bits for drilling a wellbore, and more particularly to a frac plug mill bit for use in drilling out hydraulic fracturing equipment (e.g. frac plugs) or bridge plugs.

BACKGROUND

Different types of earth boring drill bits are used for drilling through different materials in in oil, gas, and mining fields operations to break through earth formations to shape a wellbore. In shaping the wellbore, the bit drills through different geological materials making up different rock formations. Although the drill bit encounters different formations at different depths in drilling through rock, generally speaking all parts of the drill bit are drilling the same type of rock formation at the same time.

In hydraulic fracturing operations, a frac plug is secured to a casing that lines the borehole. The frac plug is something of a disposable tool because after the frac plug has performed its function, it is drilled out, and the drilled out pieces of the plug are flushed up the wellbore by the drilling mud. A frac plug is a generally cylindrical component formed of different materials disposed at different radial positions moving from a generally hollow center. For example, a cast iron or other hard material generally annular slip is disposed at a radial perimeter of the frac plug. In contrast to drilling through rock formations, when drilling out a frac plug, the drill bit simultaneously drills through significantly different materials. The different materials create different penetration efficiencies and wear characteristics on different parts of the bit.

Conventional roller cone rock bits manufactured to International Association of Drilling Contractors (IADC) standards are used to drill out frac plugs. However, conventional roller cone rock bits were not designed for the bi-modal material encountered in drilling out frac plugs. Moreover, roller cone rock bits include relatively small journal bearings and a fragile lubrication system that may be have a higher incidence of malfunction or failure when subjected to the high forces of drilling through a cast iron slip portion of a frac plug.

Relatively long tapered milling tools are also sometimes used to drill out frac plugs. Such milling tool may be set with crushed tungsten carbide, tungsten carbide chisels, discs, or PDC cutters. In operation, long, tapered milling tools may over torque and over-strain a small diameter positive displacement motor and small diameter tubing associated with the long, tapered mill tool.

Reference is made to U.S. Pat. No. 4,538,691 to Dennis, the disclosure of which is incorporated by reference, which discloses a drill bit for drilling earth formations. The drill bit includes cutter elements that are disposed in a peripheral edge, a plurality of inclined side walls, and in a floor of a recess portion of the bit. The cutter elements are conventional polycrystalline diamond studs. Dennis discloses that the configuration of the drill bit, particularly a protrusion at its center, creates a generally balanced pattern of forces that tend not to divert the drill bit from its intended path of travel.

Reference is made to U.S. Pat. No. 7,958,940 to Jameson, which discloses an apparatus to drill through a composite frac plug. The apparatus includes a plurality of blade-type cutter support structures. One of the cutter support structures extends beyond the center of the mill to provide cutting action at the center. The support structures are disposed at between 18 and 26 degree angle with the horizontal in order to cut plug material from the outside to the inside of the frac plug. The support structures support conically shaped carbide inserts or cutters. Each blade-type cutter support structure may support only one cutter at a radial perimeter of the bit. Thus, according to the teaching of Jameson, five support structures collectively support five cutters radially disposed to attack the slip of a frac plug.

SUMMARY

In an embodiment, a fixed cutter bit for milling a frac plug includes a body and a face. The face includes a base surface and a plurality of cutter support structures extending from the base surface. Each cutter support structure has a peripheral portion and an inner portion disposed radially internal of the peripheral portion. At least one first-type cutter is supported by each peripheral portion; at least one second-type cutter is supported by each inner portion. The first type cutter is adapted to mill a harder material than the second-type cutter, and the first-type is different from the second-type.

The frac plug mill bit of the present disclosure is employed to drill out different materials of a frac plug simultaneously. The location of the first-type of cutters, referred to as hard material cutters, on a peripheral portion of the face of the bit, and the radial location of the second-type of cutters, referred to as soft material cutters, disposed on the inner portion of the face of the bit facilitate drilling out different materials of the plug. Specifically, the relatively harder material of a plug slip disposed on an outer diameter of the plug is effectively drilled out by the hard material cutters disposed on the peripheral portion of the bit, while the relatively softer material of the plug body is effectively drilled out by soft material cutters disposed radially inward of the hard material cutters on the inner portion of the face of the bit.

The term “hard material cutters” is used herein to refer to cutters that are particularly suited to fracture and break apart the hard material of the slip, as opposed to the comparatively softer material of the plug body. The term “soft material cutters” is used herein to refer to cutters that are particularly suited to tear and shear the comparatively softer material of the plug body. The terms “hard material cutters” and “soft material cutters” are not intended to describe the material composition of the particular cutters.

According to one embodiment, the concave profile created by wedge-shaped cutting structures allows the hard material of the slip of the frac plug to be drilled out ahead of softer material of the plug body. That is, the hard material cutters lead the softer material cutters as the mill bit is advanced within the wellbore. Also, the peripheral surface of each wedge cutting structure supports multiple hard material cutters. In this manner, the number of hard material cutters employed to attack the slip of a frac plug may be significantly increased.

Other aspects, features, and advantages will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, which are a part of this disclosure and which illustrate, by way of example, principles of the inventions disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts, in which:

FIG. 1 illustrates a frac plug mill bit disposed in a drill out position directly above a cross section of a frac plug set in a borehole;

FIG. 2 illustrates an isometric view of the frac plug mill bit of FIG. 1;

FIG. 3 illustrates a face of the frac plug mill bit of FIGS. 1 and 2;

FIG. 4 illustrates a cross section showing the concave profile of the frac plug mill bit of FIGS. 1-3;

FIG. 5 illustrates an isometric view of an alternate embodiment of a frac plug mill bit; and

FIG. 6 illustrates a cross section of the frac plug mill bit of FIG. 5.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is now made to FIG. 1, which shows a frac plug mill bit 10 in a borehole or wellbore 12 lined with a metal casing 16. The bit 10 is shown in a drill out position above a cross section of a casing plug, frac plug, or plug 14.

The frac plug mill bit 10 is a fixed cutter-type bit. The bit may be a random cutter bit where cutters are individually set on the bit, or the bit may be a blade-type bit where defined blades support the cutters. The bit 10 includes a bit body 18 formed from a matrix metal or any other material suitable for earth boring drill bits. The bit body 18 includes a plurality of wedge-shaped cutting structures defining a peripheral surface 20. The peripheral surface 20 supports a plurality of cutters 22 disposed closest to the casing 16 in the wellbore 12, which are adapted to cut or mill hard material. The cutting structures disposed more toward the center of the bit 12 are adapted to mill softer material.

The frac plug mill bit 10 is configured to drill out the entirety of a borehole and/or a frac plug 14 secured within a borehole. Thus, the frac plug mill bit 10 is configured to drill out either rock formation or portions of the frac plug from the centerline of the borehole and extending to the full radius of the borehole.

In certain borehole operations, such as hydraulic fracturing or fracking, a plug 14, such as a frac plug, is used to isolate a portion of a wellbore 12 to be fracked. The plug 14 acts as a one-way valve and allows a specific section of the borehole to be isolated and pressurized for the hydraulic fracking operation. After the plug 14 has performed its function, it is drilled out in a drill out operation using the frac plug mill bit 10 according to the teachings of the present disclosure. In a drill out operation, the frac plug mill bit 10 is attached to a drill string and is rotated such that its cutting elements crush, rip, and break apart the plug 14. Drilling fluid pumped through the bit 10 flushes the pieces of the plug 14 back to the surface. Plugs other than frac plugs may be secured in a borehole and may be drilled out with the frac plug mill bit 10 according to the teachings of the present disclosure. For example, the frac plug mill bit 10 may be used to drill out bridge plugs and other types of plugs that engage a casing 16.

In preparation for fracking, the plug 14 is positioned at the desired location in the borehole 12 such that an outer diameter portion of the plug 14 grips the casing 16 and secures or sets the plug 14 in position. Once set, the plug 14 will withstand pressurization of the zone in the borehole without moving or slipping. To set the plug 14, a slip 26 that is generally in the form of a ring surrounding a portion of a plug body 28 is caused to engage the casing 16 and create a type of seal. For purposes of this disclosure, the plug body 28 includes any portion of the plug not formed of relatively harder material that is engaged with the casing 16 to set the plug in position and create a seal. Although the plug body 28 is primarily disposed radially internal to the slip 26, some portions of the plug body 28 may be disposed above or below and aligned with the slips 26.

In the embodiment illustrated in FIG. 1, an upper and a lower slip 26 are shown. The slips 26 each include a plurality of ridges 29 that bite into the casing 16 to provide a robust grip. The slips 26 expand and may partially fracture such that some of the slips 26 embed into the metal casing 16. To maintain the grip of the plug 14 under high pressures, the slip 26 is generally formed from a hard material. In certain plugs 14, the slip 16 is formed from cast iron. Once set, the slip 26 occupies a space between the casing 16 and the plug body 28, which may be up to an inch inside the diameter of the casing. For example, a casing 16 of a borehole may have a diameter of approximately twelve inches and the slip 26 may have an outer diameter of approximately twelve inches and an inner diameter of approximately ten inches.

In certain embodiments, the slip 26 may include tungsten carbide or ceramic inserts that embed into the casing 16 for a better grip. A plug including such inserts is disclosed in U.S. Pat. No. 5,984,007 to Yuan (the disclosure of which is incorporated by reference). In contrast to the very hard material of the slip 26, the plug body 28 is generally formed of softer material than the slip 26 and/or any inserts that are included in the slip 26. For example, the plug body 26 is often formed of a composite material, a thermoplastic, phenolic, or a softer metal, such as brass. Because the plug 14 includes relatively softer materials in its inner portions and relatively harder materials in its outer portions, during drill out, the frac plug mill bit 10 simultaneously contacts and breaks apart both relatively harder and relatively softer materials.

Reference is made to FIGS. 2 and 3. FIG. 2 is an isometric view of the frac plug mill bit 10, and FIG. 3 illustrates a face 30 of the frac plug mill bit of FIG. 1. The frac plug mill bit 10 includes a bit body 18 and a face 30. A plurality of cutter support structures 32 are generally wedge-shaped and extend from a generally convex base surface 34. Together the cutting wedges 32 form a generally concave cutting structure. The bit 10 may include between two and six cutter support structures or cutting wedges 32. In the embodiment illustrated, the face 30 of the bit 10 includes three cutting wedges 32. Each cutting wedge 32 includes a peripheral surface 20 that merges into an inclined surface 36. The inclined surface 36 is delimited on one end by the peripheral surface 20 and on an opposite end by the base surface 34. The transition between the peripheral surface 20 and the inclined surface 36 may be rounded or the junction of the two surfaces may intersect at a more defined edge. The peripheral surface 20 is generally perpendicular to a rotational axis of the bit 10, and the inclined surface 36 forms an angle θ with respect to a horizontal (see FIG. 4). According to certain embodiments, θ is at least 30 degrees. For example, θ may be between 30 and 45 degrees.

The bit 10 also includes a plurality of ports or nozzles 38. Drilling fluid is pumped down the drill string and flows through the nozzles 38 where it can direct cuttings and other debris through junk slots 40 and back up the borehole. In the embodiment illustrated, each wedge 32 includes a nozzle 38, and an additional nozzle 38 is disposed near the center of the bit 10. However, more or less nozzles 38 may be employed according to the teaching of the present disclosure. A junk slot 40 is disposed between adjacent cutting wedges 36 and a floor of the junk slot is a portion of the base surface 34. The number of junk slots 40 generally corresponds to the number of cutting wedges 32. Thus, three junk slots 40 are illustrated. However, the bit 10 may include between one and six junk slots 40, depending on the number of cutting wedges 32. In the illustrated embodiment, the three junk slots 40 are each large enough to allow reasonably large portions of the broken-up slip 26 to pass through.

Another portion of the base surface 34 is a dome-shaped protrusion 42 disposed at the center of the face 30. This protrusion 42 functions to bring high point loading to frac plug balls 15 (see FIG. 1) encountered when drilling each frac plug. The center protrusion 42 also assists in driving a remainder of a lower portion of a drilled plug to engage a next plug deeper in the wellbore. To ensure full coverage of cutters supported by the bit 10, the protrusion 42 supports a plurality of cutters. In the embodiment illustrated, the protrusion 42 supports a plurality of cylindrical cutters that have a smaller diameter than the hard material cutters 22 supported by the peripheral surface 20.

The peripheral surface portions 20 of the cutting wedges 32 support a plurality of cutters 22 disposed in respective cutter pockets 23 to generally form a ring around the perimeter of the bit 10. The cutters 22 may be disposed in their respective cutter pockets 23 at any suitable back rake angle. The cutters 22 supported by the peripheral surface 20 are adapted to cut or mill hard material, such as the cast iron slips 26 imbedded or otherwise contacting the casing 16. These hard material cutters 22 are disposed on the wedge such that they will cut the frac plug material nearest the casing 16. Each cutting wedge 32 may support multiple hard material cutters 22 on its portion of the peripheral surface 20. In the embodiment illustrated, each cutting wedge 32 supports three hard material cutters 22 for a total of nine hard material cutters 22 disposed on the peripheral surface 20 and radially positioned to attack the slip 26. The increased number of cutters disposed to attack the slip 26 increases the effectiveness and the useful life of the frac plug mill bit 10 when used to drill out frac plugs 14. Larger diameter bits 10 may support more hard material cutters 22.

Reference is now made to FIG. 4, which is a cross section of the bit 10. The hard material cutters 22 are supported by a peripheral portion 35 of the cutter support structure 32, which in the illustrated embodiment generally corresponds to the peripheral surface 20. The peripheral portion 35 extends radially internal from the gage 33 and supports cutters 22 that are disposed to cut within approximately 1.5 to 2 inches of the casing 16. Alternatively, the radial distance of the peripheral portion 35 may be 1 to 0.5 inches, and therefore the hard material cutters 22 supported by the peripheral portion 35 are disposed to cut within 1 to 0.5 inches of the casing 16. As described in more detail below, an inner portion 37 is radially internal to the peripheral portion 35 and includes soft material cutters 24 adapted to tear and shear the softer material of the plug body 28.

The hard material cutters 22 may be cylindrical cutters. For example, the hard material cutters 22 may be any one of the following cutter-types: cubic boron nitride, tungsten carbide, tungsten carbide protected polycrystalline diamond compact (PDC), or coated tungsten carbide. According to one embodiment, a diamond layer of the PDC cutter includes a milling cap such as a tungsten carbide cap, a tungsten carbide or cubic boron nitride tipped cap, or a similar fitted cap made of a suitable material and fitted as an integral part of an existing PDC cutting structure, as described in U.S. Pat. No. 8,517,123 to Reese, which is hereby incorporated by reference.

In the illustrated embodiment, the inner portion 37 includes the inclined surface portions 36 of the cutting wedges 32 and the base surface 34. The inner portion 37 supports a plurality of cutters 24 also disposed in respective cuter pockets 23. More specifically, in one embodiment, the cutters 24 supported by inner portions 37 are adapted to cut through and mill the softer material that forms the inner portions of the body 28 of the frac plug 14. For example, these soft material cutters 24 may include standard PDC cutters, chisel or scribe shaped PDC cutters, chisel shaped tungsten carbide cutters, and disc shaped tungsten carbide cutters. According to an embodiment, the soft material cutters 24 are scribe cutters and may or may not be shark tooth-type cutters. The scribe cutters are pointed cutters that are particularly useful in deforming the softer material of the frac plug, such as plastic, such that the softer material shears and tears apart and can be flushed up the wellbore.

In an alternate embodiment, the soft material cutters 24 and the hard material cutters 22 may both be PDC cutters. However, the soft material cutters 24 are still a different type of cutter because they have a scribe shape that is particularly suited to shear and tear the softer material of the plug body 28. Similarly, the soft material cutters 24 and the hard material cutters 22 may both be PDC cutters, yet the hard material cutters may have a higher diamond composition than the soft material cutters 24 that makes the hard material cutters more suitable for fracturing and breaking apart the cast iron or other hard material of the slips 26 with reduced abrasion or other wear on the cutters.

The bit 10 may also include studs, such as Teflon studs, disposed on its outer periphery. The studs are not disposed to cut through the frac plug 14, but rather are disposed to maintain centering of the bit 10 in the casing 16. In addition, the bit 10 may include hemispherical tungsten carbide inserts disposed in the leading edge of the junk slots 40, or on the outer periphery of the bit 10 to assist in smoothly centering the bit 10 inside the casing 16 of the wellbore.

The generally concave shape of the cutting wedges 32 (see FIG. 4), and therefore the cutting structures of the bit 10 allow the hard material cutters 22 supported by the peripheral surface 20 to drill the slips 26 ahead of the drilling performed by the soft material cutters 24. As such, the more robust and stronger slips are fractured and removed ahead of the body 28 of the plug. When the last of the slip material is drilled in a specific plug the remaining body portion rides down the wellbore with the mill to engage the top of the following plug to be milled.

Each cutter 22, 24 is secured in its respective cutter pocket 23 using bonding techniques that are known in the art for the particular type of cutter being bonded. For example, the cutters 22, 24 may be secured into the cutter pockets 23 by brazing, welding, or adhering using an adhesive. The cutter pockets 23 are formed by casting or machining and are sized to receive a corresponding cutter 22, 24.

Reference is now made to FIGS. 5 and 6. FIG. 5 is an isometric view of an alternate embodiment of a frac plug mill bit 50, and FIG. 6 illustrates a cross section of the frac plug mill bit 50 of FIG. 5. The frac plug mill bit 50 is a fixed cutter bit. The frac plug mill bit 50 includes a bit body 52 and a face 54. Cutter support structures 56 extend from a base surface 55 and support a plurality of cutters, some of which are adapted to cut and fracture the hard material of the slip 26, and others are adapted to tear and shear the plug body 28, depending on the radial location on the bit of the particular cutter. According to this embodiment, the cutter support structures 56 are referred to as blades.

FIG. 6 is a cross section showing two blades. In the illustrated embodiment, the gage 58 portion of the blade cutter support structure 56 supports a pre-flatted gage cutter 60 and a full diameter gage cutter 62. Radially internal to the gage 58 is a shoulder 64. The shoulder 64 is generally an arcuate surface disposed between the gage 58 and the nose 66. In the embodiment illustrated, the shoulder 64 supports a near gage cutter 68. Radially internal to the shoulder is the nose 66 and the cone 70. The illustrated embodiment is a flat profile bit, but the teachings of the present disclosure apply to concave profile bits and fixed cutter bits having other profiles.

In order to fracture the hard material of the slip 26 when drilling out the frac plug 14, a peripheral portion 72 of each cutter support structure 56 supports a first-type of cutter 74 that is particularly suited to fracture and split apart the hard material of the slip 26. The peripheral portion 72 includes the gage 58 and the portion of the cutter support structure 56 extending radially inward approximately 1.5 to 2 inches from the gage 58. In this manner, the hard material cutters 74 are disposed to cut and mill a slip with a radial thickness of approximately 1.5 to 2 inches. The peripheral portion 72 also may extend radially inward from the gage 1 inch or 0.5 inches, according to alternate embodiments. In the embodiment illustrated, the cutters supported by the gage 58 and the shoulder 64 are hard material cutters 74 and the cutters supported radially internal to the shoulder at an inner portion 76 are soft material cutters 78 adapted to tear and shear the softer material of the plug body 28. In the illustrated embodiment, the inner portion 76 includes the nose 66 and the cone 70 portions.

Similarly as stated above, the hard material cutters 74 may be any one of the following cutter-types: cubic boron nitride, tungsten carbide, tungsten carbide protected polycrystalline diamond compact (PDC), or coated tungsten carbide. According to one embodiment, a diamond layer of the PDC cutter includes a milling cap such as a tungsten carbide cap, a tungsten carbide or cubic boron nitride tipped cap, or a similar fitted cap made of a suitable material and fitted as an integral part of an existing PDC cutting structure, as described in U.S. Pat. No. 8,517,123 to Reese.

The soft material cutters 78 include standard PDC cutters, chisel or scribe shaped PDC cutters, chisel shaped tungsten carbide cutters, and disc shaped tungsten carbide cutters. According to an embodiment, the soft material cutters 78 may or may not be shark tooth-type cutters.

The frac plug mill bits 10, 50 each differ from a reamer in that a reamer is not configured to drill out a central portion of a borehole proximate the centerline. Rather, a reamer is configured to ream a hole that has already been at least partially formed. However, with respect to the cutting dynamics and force balancing of the bit, the frac plug mill bits 10, 50 are similar to a reamer in that, when drilling out a frac plug, they are subjected to greater forces near their respective perimeters and reduced forces nearer their centers.

To that end, the mill bits 10, 50 of this disclosure may be force balanced by modifying the teachings of U.S. Pat. No. 8,162,081 to Ballard, in connection with the teachings of U.S. Pat. No. 5,010,789 to Brett, the disclosures of each of which are hereby incorporated by reference. The high compressive strength of the outer ring of the frac plug is modeled, along with the low compressive strength of the inner plug material to produce a force balancing of the overall cutting structure which overcomes the tendency to have “bit whirl” induced while drilling the bi-modal materials of the frac plug. For example, the force balance model may include parameters associated with a 1.5 inch thickness slip, such as 15,000 pounds/sq. inch as a value for the compressive strength of the slip. This thickness corresponds to a peripheral portion 35, 72 defined as the region 1.5 inches radially internal to the gage. The inner portion 37, 76 may be force balanced for a plug body having a compressive strength of 5,000 pounds/sq. inch. The same modeling procedure may be used to model a peripheral portion 35, 72 having a distance reduced by 0.5 inches to one inch, and the inner portion distance can be increased by 0.5 inches. This model corresponds to a slip having a radial thickness of one inch. This iterative modeling technique may be repeated for any suitable peripheral portion and using any suitable incremental distance. In this manner, the bit 10, 50 may be force balanced and adapted to attack frac plugs having slips of a variety of different radial thicknesses.

According to one embodiment, force balancing leads to a bit design that is densely set with cutting structure in the outer ring and significantly more lightly set with cutting structure across the remainder of the bit face.

According to embodiments of the present disclosure, during drill out of the frac plug 14 using the frac plug mill bit 10, 50 hard material cutters 22, 74 supported by peripheral portions 35, 72 of cutter support structures 32, 56 engage the slip 26 and/or the plug inserts that are adjacent, contacting, or embedded into the casing 16. Softer material cutters 24, 78 are supported by the inner portion 37, 76 and are adapted to drill out softer materials of frac plug body 28.

The foregoing describes only some embodiments of the invention(s), and alterations, modifications, additions and/or changes can be made thereto without departing from the scope and spirit of the disclosed embodiments, the embodiments being illustrative and not restrictive.

Claims

1. A fixed cutter bit, comprising: a body; anda face, comprising: a base surface;a plurality of cutter support structures extending from the base surface, each cutter support structure having a peripheral portion and an inner portion disposed radially internal of the peripheral portion;at least one first-type cutter supported by each peripheral portion; andat least one second-type cutter supported by each inner portion, the first-type cutter adapted to mill a harder material than the second-type cutter, the first-type being different from the second-type.

2. The bit of claim 1 wherein the plurality of cutter support structures form a generally concave cutting portion of the face.

3. The bit of claim 2 wherein each cutter support structure includes an inclined surface having an angle of at least 30 degrees from a horizontal.

4. The bit of claim 1 wherein the base surface is generally convex and the peripheral portions include peripheral surfaces that are generally perpendicular to a rotational axis of the bit.

5. The bit of claim 1 wherein the first-type is selected from a group consisting of: polycrystalline diamond cutters, cubic boron nitride cutters, cutting tool grade carbide cutters, and mill capped polycrystalline diamond cutters.

6. The bit of claim 5 wherein the second-type is either tungsten carbide cutters or polycrystalline diamond cutters.

7. The bit of claim 1 wherein the peripheral portions extend between 0.5 and 1.5 inches from a gage surface of the bit.

8. The bit of claim 1 wherein the cutter support structures are blades.

9. The bit of claim 1 wherein the first-type cutters supported by the peripheral portions and the second-type cutters supported by the inner portions are set such that the bit is force balanced when milling a slip having a thickness between 0.5 and 1.5 inches.

10. The bit of claim 9 wherein the cutters supported by the peripheral portions are more densely set than the cutters supported by the inner portions.

11. The bit of claim 1 wherein the peripheral portions collectively support at least six cutters.

12. A fixed cutter bit for milling a frac plug, comprising: a body; anda face having a peripheral portion and an inner portion disposed radially internal to the peripheral portion, the peripheral portion supporting a plurality of first-type cutters and the inner portion supporting a plurality of second-type cutters, the first-type being different from the second-type;the second-type cutters adapted to mill a plug body material; andthe first-type cutters adapted to mill a slip material, the slip material having a greater compressive strength than the plug body material.

13. The bit of claim 12 wherein the first-type cutters and the second-type cutters are individually set.

14. The bit of claim 12 wherein the first-type cutters and the second-type cutters are supported by blade-type cutter support structures.

15. The bit of claim 14 wherein the blade-type cutter support structures each include a gage portion and an arcuate shoulder portion, the gage portions and the shoulder portions supporting the first-type cutters.

16. The bit of claim 12 wherein the bit has a flat profile.

17. The bit of claim 12 wherein the bit has a concave profile.

18. The bit of claim 12 wherein the first-type is selected from a group consisting of: polycrystalline diamond cutters, cubic boron nitride cutters, cutting tool grade carbide cutters, and mill capped polycrystalline diamond cutters.

19. The bit of claim 12 wherein the second-type is either tungsten carbide cutters or polycrystalline diamond cutters.

20. The bit of claim 19 wherein the second-type are scribe-shaped cutters.

21. A fixed cutter bit for milling a frac plug, comprising: a body; anda face having a peripheral portion and an inner portion disposed radially internal to the peripheral portion, the peripheral portion supporting a plurality of first-type cutters and the inner portion supporting a plurality of second-type cutters, the first-type being different from the second-type;the second-type cutters adapted to mill a plug body material;the first-type cutters adapted to mill a slip material, the slip material having a greater compressive strength than the plug body material;wherein the first-type is selected from a group consisting of: polycrystalline diamond cutters, cubic boron nitride cutters, cutting tool grade carbide cutters, and mill capped polycrystalline diamond cutters;wherein the second-type is either tungsten carbide cutters or polycrystalline diamond cutters; andwherein the first-type cutters and the second-type cutters are set such that the bit is force balanced when milling a slip having a thickness between 0.5 and 1.5 inches.