WO2020086194A1

WO2020086194A1 - Hybrid roller cone-mill completions bit

Info

Publication number: WO2020086194A1
Application number: PCT/US2019/051497
Authority: WO
Inventors: Matthew Charles Stroever; Johnathan Walter Howard; Todd Mitchell; Charles Johnson
Original assignee: Varel International Ind., L.L.C.
Priority date: 2018-10-25
Filing date: 2019-09-17
Publication date: 2020-04-30

Abstract

A completions bit for use in a wellbore includes: a body having a coupling formed at an upper end thereof and a plurality of lower legs; a plurality of roller cones forming a cutting face of the completions bit, each roller cone mounted to a bearing shaft of a respective leg for rotation relative thereto; a row of crushers mounted around each roller cone at an outer portion of the cutting face; and a mill structure mounted to each roller cone at an inner portion of the cutting face, each mill structure comprising a plurality mill cutters and a matrix material bonding the mill cutters to the respective roller cone.

Description

HYBRID ROLLER CONE-MILL COMPLETIONS BIT

BACKGROUND OF THE DISCLOSURE

Field of the Disclosure

[0001] The present disclosure generally relates to a hybrid roller cone-mill completions bit.

Description of the Related Art

[0002] US 4,372,404 discloses cutting teeth for rolling cutter bits, including cutter inserts, cutter teeth formed in place, or formed separately and welded in place, etc., are formed by powder metallurgy as a densified powder metallurgical composite of at least two varying phases, said composite having a substantially continuous mechanical property gradient therethrough. One of said phases is preferably a refractory compound and another phase is a binder metal or alloy. The gradient is from one mixture of said phases in one region having hardness or wear resistant properties to another mixture of said phases in another region having toughness properties.

[0003] US 5,887,655 discloses, at Fig. 31 C and col. 17, lines 57-65, a drill bit roller cone with a rotatable cone on a body (which is mountable or formable as part of a drill bit or mill-drill tool), the cone having thereon stubs of drilling material and a projecting body of milling material, e.g. welded to the body.

[0004] US 7,954,571 discloses a drill bit including a bit body having a face on which two different types of cutters are disposed, the first type being cutting elements suitable for drilling at least one subterranean formation and the second type being at least one of an abrasive cutting structure and an abrasive cutting element suitable for drilling through a casing shoe, reamer shoe, casing bit, casing or liner string and cementing equipment or other components, as well as cement. Methods of forming earth-boring tools are also disclosed.

[0005] US 8,905,1 17 discloses methods of forming at least a portion of an earth boring tool including providing at least one insert in a mold cavity, providing particulate matter in the mold cavity, melting a metal and a hard material to form a molten composition, and casting the molten composition. Other methods include coating at least one surface of a mold cavity with a coating material having a composition differing from a composition of the mold, melting a metal and a hard material to form a molten composition, and casting the molten composition. Articles comprising at least a portion of an earth-boring tool include at least one insert and a solidified eutectic or near eutectic composition including a metal phase and a hard material phase. Other articles include a solidified eutectic or near-eutectic composition including a metal phase, a hard material phase and a coating material in contact with the solidified eutectic or near-eutectic composition.

[0006] US 2017/0234092 discloses a hybrid bit for use in a wellbore including: a body having a shank for connection to a drilling motor or drill pipe and a plurality of legs attached to the shank; and a plurality of cutting structures. The cutting structures include a roller cone mounted to a first one of the legs and a fixed mill mounted to a second one of the legs and including a pad dressed with a cermet material.

[0007] US 2018/0355670 discloses a roller-mill bit for use in a wellbore including: a body having a coupling formed at an upper end thereof and a plurality of lower legs; a plurality of roller disks, each roller disk secured to a bearing shaft of a respective leg for rotation relative thereto; a row of crushers mounted around each roller disk; and a fixed mill mounted to the bearing shafts and comprising a pad for each roller disk. Each pad is dressed with a cermet material.

SUMMARY OF THE DISCLOSURE

[0008] The present disclosure generally relates to a hybrid roller cone-mill completions bit. In one embodiment, a completions bit for use in a wellbore includes: a body having a coupling formed at an upper end thereof and a plurality of lower legs; a plurality of roller cones forming a cutting face of the completions bit, each roller cone mounted to a bearing shaft of a respective leg for rotation relative thereto; a row of crushers mounted around each roller cone at an outer portion of the cutting face; and a mill structure mounted to each roller cone at an inner portion of the cutting face, each mill structure comprising a plurality mill cutters and a matrix material bonding the mill cutters to the respective roller cone.

[0009] In another embodiment, a completions bit for use in a wellbore includes: a body having a coupling formed at an upper end thereof and a plurality of lower legs; a plurality of roller cones, each roller cone mounted to a bearing shaft of a respective leg for rotation relative thereto; and a mill structure mounted to each roller cone, each mill structure comprising a plurality mill cutters and a matrix material bonding the mill cutters to the respective roller cone. Each mill cuter has a minimum dimension greater than one-eighth of an inch.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.

[0011] Figure 1 illustrates a hybrid roller cone-mill completions bit positioned for drilling out a frac plug set in a wellbore, according to one embodiment of the present disclosure.

[0012] Figures 2-5 illustrate the hybrid roller cone-mill completions bit.

[0013] Figures 6A, 6B, and 7 illustrate roller cones of the hybrid roller cone-mill completions bit.

DETAILED DESCRIPTION

[0014] Figure 1 illustrates a hybrid roller cone-mill completions (HRCMC) bit 1 positioned for drilling out a frac plug 2 set in a wellbore 3, according to one embodiment of the present disclosure. For a hydraulic fracturing operation, the frac plug 2 is set against a casing or liner string 4 to isolate a zone (not shown) of a formation adjacent to the wellbore 3. To set the frac plug 2, a setting tool (not shown) and the frac plug 2 may be deployed down the casing or liner string 4 using a wireline (not shown). The frac plug 2 may be set by supplying electricity to the setting tool via the wireline to activate the setting tool. A piston of the setting tool may move an outer portion of the frac plug 2 along a mandrel 5 of the frac plug while the wireline restrains a mandrel of the setting tool and the plug mandrel, thereby compressing a packing element 8 and driving slips 6 along respective slip cones 7 of the frac plug. The packing element 8 may be radially expanded into engagement with the casing or liner string 4 and the slips 6 may be wedged into engagement therewith.

[0015] The casing or liner string 4 may then be perforated above the set frac plug

2 and the isolated zone may be hydraulically fractured by pumping a ball 9 followed by fracturing fluid (not shown) down the casing or liner string 4. The ball 9 may land in a seat of the plug mandrel 5, thereby forcing the fracturing fluid into the zone via the perforations. Another frac plug (not shown) may then be set above the fractured zone and the casing or liner string 4 may again be perforated above the plug for hydraulic fracturing of another zone. This process may be repeated many times, such as greater than or equal to ten, twenty, or fifty times, until all of the zones adjacent to the wellbore

3 have been fractured.

[0016] After all of the zones have been fractured, a production valve at the wellhead may be opened to produce fluid from the wellbore in an attempt to retrieve the balls 9. The HRCMC bit 1 (only partially shown) may be deployed down the casing or liner string 4 using coiled tubing (not shown). A drilling motor (not shown), such as a mud motor, may connect the HRCMC bit 1 to the coiled tubing. The HRCMC bit 1 , drilling motor, and coiled tubing may be collectively referred to as a mill string. Milling fluid may be pumped down the coiled tubing, thereby driving the drilling motor to rotate the HRCMC bit 1 and the HRCMC bit may be advanced into engagement with the frac plug 2, thereby drilling out the frac plug. Once drilled out, the mill string may be advanced to drill out the next frac plug 2 until all of the frac plugs have been drilled out.

[0017] Alternatively, the mill string may include a string of drill pipe instead of coiled tubing with or without the drilling motor. Alternatively, the HRCMC bit 1 may be employed to drill out other types of downhole tools, such as packers, bridge plugs, float collars, float shoes, stage collars, guide shoes, reamer shoes, and/or casing bits. Alternatively, the HRCMC bit 1 may be assembled as part of a drill string for drilling the wellbore 3. Alternatively, the HRCMC bit 1 may be assembled as part of a drill string for drilling a wellbore to an aquifer or for geothermal use. Alternatively, the HRCMC bit 1 may be assembled as part of a drill string for drilling a blasthole.

[0018] Figures 2-5 illustrate the HRCMC bit 1 . The HRCMC bit 1 may include a body 10, a plurality of roller cones 1 1 a-c, a plurality of crushers 12a,b, and a plurality of mill structures 13a-c (schematically shown, see mill cutters 23 and matrix material 24 of Figures 6 and 7 (which are photographs of prototypes) of US 62/750,567, filed on October 25, 2018, with attorney docket number VT-18-019, which is herein incorporated by reference in its entirety). The roller cones 1 1 a-c, crushers 12, and mill structures 13a-c may form a lower cutting face 14 of the HRCMC bit 1 . The crushers 12a,b may be located at an outer portion of the cutting face 14 and the mill structures 13a-c may be located at an inner portion of the cutting face.

[0019] The body 10 may have an upper coupling 15, a lower leg 16a-c for each roller cone 1 1 a-c, and a dome 17 formed between the legs. The body 10 and the roller cones 1 1 a-c may each be made from a metal or alloy, such as steel. The body 10 may be made by attaching three forgings together, such as by welding. The legs 16a- c may be equally spaced around the body, such as three at one hundred twenty degrees. The upper coupling 15 may be a threaded pin for connection to the drilling motor or drill pipe. A bore may be formed through the coupling 15 and extend to a plenum (not shown) formed adjacent to the dome 17.

[0020] Each leg 16a-c may have an upper shoulder 18s, a mid shirttail 18h, a lower bearing shaft 18b, and a ported boss 18p. The shoulder 18s, shirttail 18h, ported boss 18p, and bearing shaft 18b of each leg 16a-c may be interconnected, such as by being integrally formed and/or welded together. Each ported boss 18p may be in fluid communication with the plenum via a respective port formed in the coupling 15 and may have a nozzle (not shown) fastened therein for discharging the milling fluid onto the respective roller cone 1 1 a-c. Each bearing shaft 18b may extend from the respective shirttail 18h in a radially inclined direction toward a center of the HRCMC bit 1 .

[0021] Alternatively, the HRCMC bit 1 may include a flow passage formed through the dome 17 for each roller cone 1 1 a-c instead of the ported bosses 18p. Each flow passage may be in fluid communication with the plenum and may have a nozzle (not shown) fastened therein for discharging the milling fluid onto the respective roller cone 1 1 a-c. The flow passages may be arranged about a center of the HRCMC bit 1 .

[0022] Each bearing shaft 18b and/or the respective cone 1 1 a-c may have one or more grooves and each groove may form a race for receiving a respective set of roller bearings (not shown). A thrust washer (not shown) may be disposed between each bearing shaft 16b and the respective cone 1 1 a-c and/or a pair of thrust washers may be disposed in opposing aligned grooves formed in each bearing shaft 18b and the respective roller cone. The roller bearing sets and thrust washers may support rotation of each cone 1 1 a-c relative to the respective leg 16a-c.

[0023] Alternatively, journal bearings may be used instead of the sets of roller bearings to support each roller cone 1 1 a-c from the respective bearing shaft 16b.

[0024] Each leg 16a-c may have a lubricant reservoir (not shown) formed therein and a lubricant passage (not shown) extending from the reservoir to the respective roller bearing set. The lubricant, such as grease, may be retained within each leg 16a- c by a respective seal (not shown), such as an o-ring, each seal positioned in a respective gland (not shown) formed in an inner surface of the respective roller cone 1 1 a-c. A pressure compensator 28 may be disposed in each reservoir and mounted to the respective leg 16a-c for regulating lubricant pressure in the interface between each bearing shaft 18b and the respective roller cone 1 1 a-c. An equalization passage may extend from each reservoir and through the dome 17 for operation of the respective pressure compensator to regulate the lubricant pressure to be slightly greater than wellbore pressure. Lubrication of the interface between each bearing shaft 18b and the respective roller cone 1 1 a-c may extend the service life of the drill bit 1 .

[0025] Each roller cone 1 1 a-c may be mounted to the respective bearing shaft 18b by a set of balls (not shown) received in a race formed by aligned grooves in each roller cone and the respective bearing shaft. The balls may be fed to each race by a ball passage formed in each leg 16a-c and retained therein by a respective keeper (not shown) disposed in the ball passage and a respective ball plug (not shown) closing the ball passage. Each ball plug may be attached to the respective leg 16a-c, such as by welding. [0026] Upper and lower edges of each shirttail 18h may be protected from erosion and/or abrasion by respective hardfacing 26u,b with a ceramic or cermet material. An outer surface of each shirttail 18h may also be protected from erosion and/or abrasion by protective inserts 27 secured into sockets thereof, such as by interference fit or brazing. Each protective 27 insert may be made from a cermet. Each roller cone 1 1 a- c may be treated to resist erosion and/or abrasion by case hardening, such as carburization.

[0027] Each roller cone 1 1 a-c may have a plurality of lands formed therein, such as a heel land and a gage land, and the second and third roller cones 1 1 b,c may have an inner land formed therein adjacent to the gage land. A gage row of crushers 12a may be mounted around each roller cone 1 1 a-c at the respective gage land and an inner row 12b of crushers may be mounted around each of the second and third roller cones 1 1 b,c at the respective inner land. Each crusher 12a,b may be an insert mounted in a respective socket formed in the respective roller cone 1 1 a-c by an interference fit. Each socket may be demarcated by a spot, such as a flat, formed in and/or between the respective lands to provide a visual indicator for and to receive carburization inhibitor. Each crusher 12a,b may be made from a cermet, such as a cemented carbide, and may have a cylindrical or conical portion mounted in the respective roller cone 1 1 a-c and a conical or chisel portion protruding from the respective spot of the respective roller cone.

[0028] Alternatively, a row of heel protectors may be mounted around each roller cone 1 1 a-c at a respective heel land. Each heel protector may be an insert mounted in a respective socket formed in the respective roller cone 1 1 a-c by an interference fit. Each heel protector may be made from a cermet, such as a cemented carbide, and may be cylindrical. Alternatively, the crushers 12a,b and/or heel protectors may be capped with polycrystalline diamond (PCD). Alternatively, each crusher 12a,b may be a hardfaced milled tooth or each row of the crushers may include both inserts and milled teeth.

[0029] The HRCMC bit 1 may further have junk slots 19 formed between adjacent legs 16a-c. Each junk slot 19 may be formed into the body 10, such as by milling and/or forging. Each ported boss 18p may be located in a respective junk slot 19. Each junk slot 19 may be sized to allow passage of debris (not shown) created during milling of the frac plugs 2 into an annulus formed between the mill string and the casing or liner string 4.

[0030] Figures 6A, 6B, and 7 illustrate roller cones 1 1 a-c of the HRCMC bit 1 . Each roller cone 1 1 a-c may have a respective outer rib 20a-c located adjacent to the gage land of the first roller cone 1 1 a and the inner land of the respective second and third roller cones 1 1 b,c. Each outer rib 20a-c may extend around an outer surface of the respective roller cone 1 1 a-c. Each roller cone 1 1 a-c may have an inner rib 21 a-c located at or defining the nose of the respective roller cone 1 1 a-c. Each of the first and third inner ribs 1 1 b,c may extend around an outer surface of the respective roller cone 1 1 b,c. The second inner rib 21 b may define the nose of the second roller cone 1 1 b. Each roller cone may have a channel 22a-c extending between the respective inner 21 a-c and outer 20a-c ribs. The inner portion of each roller cone 1 1 a-c may be fully dressed by the respective mill structure 13a-c. Each mill structure 13a-c may be disposed in and may protrude from the respective channel 22a-c and may be bonded, such as by brazing or welding, to the respective roller cone 1 1 a-c. Each mill structure 13a-c may include a plurality of mill cutters 23 and a matrix material 24 bonding the mill cutters to the respective roller cone 1 1 a-c. The mill cutters 23 may be disposed along and around the respective channel 22a-c and may protrude therefrom to an extent greater than that of the respective first and third inner ribs 21 a, c and the outer ribs 20a-c.

[0031] Each cone 1 1 a-c may have a length 29 and each mill structure 13a-c may have a length 30. The length 30 of each mill structure may range between twenty-five percent and seventy percent or between thirty percent and sixty percent of the length 29 of the respective cone 1 1 a-c. Each mill structure 13a-c may cover the respective outer surface of the inner portion of the respective cone 1 1 a-c.

[0032] Each mill cutter 23 may be a crushed cermet particle, such as cobalt- tungsten carbide, and the matrix material 24 may be a high strength metal or alloy, such as a copper alloy. Each mill cutter 23 may have a minimum dimension greater than or equal to one-eighth of an inch (three millimeters), three-sixteenths of an inch (five millimeters), or one-quarter of an inch (six millimeters). As used herein, the minimum dimension is the minimum of one of the three major dimensions (length, width, and height). Each mill cutter 23 may have a multi-edged random shape. The mill cutters 23 and matrix material 24 may be procured as composite rods and deposited onto the roller cones 1 1 a-c by oxyacetylene welding. The channels 22a-c and adjacent ribs 20a-c, 21 a-c may serve as guides to a technician or robot welding the mill cutters 23 and matrix material 24 onto the roller cones 1 1 a-c such that adequate clearance 25 (Figure 3) is maintained between the milling structures of the roller cones when the roller cones are mounted onto the bearing shafts 18b. The clearance 25 may range between one and ten percent or between one-half and fifteen percent of a size diameter (aka gage diameter) of the drill bit 1 .

[0033] Although not shown, each roller cone 1 1 a-c includes a receptacle formed therein for mounting the roller cone onto the respective bearing shaft 18b and for receiving the radial bearings and thrust bearings.

[0034] Alternatively, instead of the channel 22a-c, each roller cone 1 1 a-c may have a plurality of channels or a helical channel for receiving the respective mill structure 13-a-c. The plurality of channels maybe orientated circumferentially, longitudinally, or diagonally.

[0035] Alternatively, the ribs 20a-c, 21 a-c may be omitted. Alternatively, each roller cone 1 1 a-c may have one or more rows of milled teeth formed in the inner portion thereof and the respective mill structure 13a-c may be located between the rows of milled teeth or between milled teeth in the same row. Alternatively, each roller cone may have a nose row of milled teeth or a spearpoint and the respective mill structure 13a-c may be located between the nose row or spearpoint and the innermost row of crushers. Alternatively, the gage and inner rows of crushers 12a,b may be omitted and the mill structure can be deposited on each cone 1 1 a-c at the outer portion of the cutting face 14. Alternatively, the crushers 12a,b may be milled teeth or include a mixture of inserts and milled teeth.

[0036] Alternatively, each mill cutter 23 may have any pre-defined multi-edged shape and a minimum dimension greater than or equal to one-eighth of an inch (three millimeters), three-sixteenths of an inch (five millimeters), or one-quarter of an inch (six millimeters). The alternative mill cutter may be a block, such as a cubic block. The cermet material may be formed into the block by sintering, such as hot pressing. The alternative cutter block may have a pair of opposite rectangular sides and four profiled sides connecting the rectangular sides. The profiled sides may each have rectangular end portions located adjacent to the respective rectangular sides and profiled mid portions connecting the respective end portions. Each rectangular end portion may have chamfered corners adjacent to the respective rectangular sides. Each profiled portion may have a pair of opposed trapezoidal portions converging from the respective end portions toward a center of the mill cutter. Each profiled portion may further have a filleted rectangular center portion connecting ends of the trapezoidal portions distal from the respective end portions. Each rectangular side may have a raised peripheral portion and a recessed interior portion. Tapered walls may connect each raised peripheral portion to the respective interior portion. Each corner of the tapered walls may be shaved. Each alternative mill cutter may have a different shape than cubic or even rectangular, such as any polygonal shape, for example, star-shaped or triangular-shaped.

[0037] T o manufacture a typical one of the roller cones 1 1 a-c, the typical roller cone may start from a forged steel body of a conical shape. A handling socket may be formed into a rear of the steel body for mounting thereof onto a spindle of a machine tool, such as a lathe. The lathe may be manually operated or CNC. The lands and the channel 22a-c may be formed in an outer surface of the body, such as by turning, using the lathe.

[0038] Once the lands and the channel 22a-c have been formed, the spindle may be locked and a spot facer may be spun and plunged into an outer surface of one of the lands until a spot is formed. The spot facer may be plunged manually or be plunged and articulated by the CNC machine. Once the spot has been formed, the spot facer may be raised and the spindle may be unlocked and rotated by a predetermined increment. Once rotated by the increment, the spindle may be relocked and the spot facer used to form a second spot. The spot facing process may be repeated until a set of spots has been formed around the selected land.

[0039] Once the spots have been formed, the typical roller cone may be removed from the lathe and transported to a welding station. A technician or robot may operate an oxyacetylene welding machine to deposit the mill cutters 23 and matrix material 24 into the channel 22a-c of the typical roller cone, thereby forming the mill structure 13a- c. Once the mill structure 13a-c has been formed, the typical roller cone may be returned to the lathe and the handling socket may be enlarged, such as by machining, to form the bearing shaft receptacle for mounting the typical roller cone to the respective bearing shaft 18b.

[0040] Once the bearing shaft receptacle has been formed, carburization inhibitor (not shown) may be applied to the spots for protection of a region of the typical roller cone adjacent to the spot from carburization. Once the inhibitor has been applied to all of the spots and has been allowed to cure, the typical roller cone may be loaded into a carburization furnace (not shown). Before loading of the typical roller cone, the furnace may be preheated to a carburizing temperature and a reactor thereof operated to establish the carburizing atmosphere. The typical roller cone may be loaded and allowed to sit in the furnace for a predetermined period of time sufficient for carburization of an uninhibited portion thereof. The inhibitor may protect the spots from carburization thereof.

[0041] Once carburization of the typical roller cone is complete, the typical roller cone may be unloaded from the carburization furnace and quenched. The typical roller cone may then be loaded into a heat treatment furnace (not shown) and tempered therein. Once the typical roller cone has been quenched and tempered, a drill bit may be spun and plunged into each spot to form a pilot hole therein. Once the pilot hole has been formed, a reamer may be spun and plunged into each pilot hole to form the socket in the typical roller cone. Once the sockets have been formed, the crushers 12a,b may be pressed into the sockets. Once the crushers 12a,b have been pressed into the sockets, the typical roller cone may then be mounted onto the respective bearing shaft 18b.

[0042] Alternatively, the mill structure 13a-c may be applied to the typical roller cone after quenching and tempering and before forming the sockets therein.

[0043] Advantageously, because the plug 2 includes relatively softer materials in its inner portions and relatively harder materials in its outer portions, during drill out, the HRCMC bit 1 contacts and breaks apart both relatively harder and relatively softer materials. As such, during the drill out using the HRCMC bit 1 , the crushers 12a,b engage the harder slips 6 and the softer mandrel 5 is drilled out by the mill structures 13a-c. The mill structures 13a-c are more aggressive, efficient, and better suited for penetrating, gripping, and cutting the softer material of the mandrel 5. In contrast, the crushers 12a, b would be less efficient in cutting and ripping the material of the mandrel 5 and would also subject the cones 1 1 a-c to wear, which could result in loss of the crushers 12a,b.

[0044] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope of the invention is determined by the claims that follow.

Claims

Claims:

1. A completions bit for use in a wellbore, comprising:

a body having a coupling formed at an upper end thereof and a plurality of lower legs;

a plurality of roller cones forming a cutting face of the completions bit, each roller cone mounted to a bearing shaft of a respective leg for rotation relative thereto; a row of crushers mounted around each roller cone at an outer portion of the cutting face; and

a mill structure mounted to each roller cone at an inner portion of the cutting face, each mill structure comprising a plurality mill cutters and a matrix material bonding the mill cutters to the respective roller cone.

2. The completions bit of claim 1 , wherein:

each roller cone has an outer rib, an inner rib, and a channel formed between the ribs, and

the mill structure is mounted in the channel.

3. The completions bit of claim 2, wherein the mill cutters protrude from the channel past the ribs.

4. The completions bit of claim 1 , wherein each mill cuter has a minimum dimension greater than one-eighth of an inch.

5. The completions bit of claim 1 , wherein:

each row of crushers is a gage row, and

the completions bit further comprises a second row of crushers mounted around at least one of the roller cones at a location adjacent to the gage row.

6. The completions bit of claim 1 , wherein each mill cutter is a particle of crushed cermet and the matrix material is a copper alloy.

7. The completions bit of claim 1 , wherein each crusher is an insert mounted into a respective socket formed in the respective roller cone.

8. The completions bit of claim 1 , wherein:

a clearance is formed between the mill structures, and

the clearance ranges between one-half and fifteen percent of a size diameter of the drill bit.

9. The completions bit of claim 1 , wherein a length of each mill structure may range between twenty-five percent and seventy percent of a length of the respective roller cone.

10. The completions bit of claim 1 , further comprising a pressure compensator mounted to each leg for regulating lubricant pressure in an interface between each bearing shaft and the respective roller cone.

1 1. A method of drilling out a plug using the completions bit of claim 1 ,

assembling the completions bit as part of a mill string;

deploying the mill string into a casing or liner string set in the wellbore to the plug set in the casing or liner string; and

injecting milling fluid through the mill string, rotating the completions bit, and engaging the completions bit with the plug, thereby drilling out the plug.

12. A completions bit for use in a wellbore, comprising:

a plurality of roller cones, each roller cone mounted to a bearing shaft of a respective leg for rotation relative thereto; and

a mill structure mounted to each roller cone, each mill structure comprising a plurality mill cutters and a matrix material bonding the mill cutters to the respective roller cone,

wherein each mill cuter has a minimum dimension greater than one-eighth of an inch.