MILLING SYSTEM PROVIDING CUTTINGS RE-CIRCULATION

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of, and priority to, United States Patent Application Serial No. 62/034,031 filed August 6, 2014, United States Patent Application Serial No. 62/034,052 filed August 6, 2014, United States Patent Application Serial No. 62/153,841 filed April 28, 2015, and United States Patent Application Serial No. 14/814,995 filed July 31, 2015, which applications are expressly incorporated herein by this reference in their entireties.

BACKGROUND

[0002] To increase the production of hydrocarbons, an oil and gas well may be stimulated by using perforating and fracturing processes. Perforation involves forming holes in the casing or liner. In particular, when a zone of interest is identified, holes may be formed by mechanical cutters, explosive charges, or other means to allow fluid communication between the reservoir and the wellbore. After the casing or liner has been perforated, a plug (e.g., a bridge plug or frac plug) may be set in the wellbore for hydraulically isolating the perforated zone from lower zones in the wellbore. By isolating the perforated zone, fracturing fluid pumped into the well may be limited to the particular zone of interest. The fracturing fluid is pumped at a high pressure to fracture the formation at the perforations through the casing or liner. The high pressure of the fracturing fluid propagates a fracture in the formation, which may increase the production of hydrocarbons from that zone of the wellbore.

[0003] The process of perforating the casing and isolating the zone of interest may be repeated at multiple locations within a single wellbore. A bridge plug may then be set at the lower end of each zone of interest where perforation and stimulation is to occur. After perforation and fracturing is completed for a zone, the set bridge plug may be removed. Removal of the bridge plugs may occur by using a retrievable bridge plug, or by milling out the bridge plug. The bridge plug may be formed of various different materials (e.g., rubber, composite materials, and metals). Milling the bridge plug may therefore involve using a mill that cuts into different materials with different material properties.

SUMMARY

[0004] Embodiments of the present disclosure relate to a downhole milling system. In at least some embodiments, the downhole milling system may include a milling bit having a face and cutting elements coupled to the face. The cutting elements may be used to mill a downhole tool. A barrier of the downhole milling system may re -circulate cuttings and debris generated by the cutting elements to the face of the mill.

[0005] According to another embodiment, a method of milling may include generating debris using a mill that includes cutting elements. A re-circulation zone may be created to promote re-circulation of the debris to the cutting elements. The debris may then be conditioned by re-milling the debris after it is recirculated to the cutting elements.

[0006] In accordance with another embodiment, a mill may include a face, blades, and junk channels. Cutting elements may be coupled to the blades, and a first nozzle may be used to expel fluid from the face to cool the cutting elements. Second nozzles may be located off the face and may expel fluid to create a hydraulic barrier restricting flow of debris away from the face of the mill.

[0007] In some embodiments of the present disclosure, a bit may be designed by selecting a bit design with a bit body having first nozzles configured to expel fluid to a face of the bit and second nozzles configured to expel fluid to create a re-circulation zone away from a face of the bit. Cuttings residence time and/or cuttings residence length can be calculated for materials to be cut by the bit for a plurality of angles of the second nozzles. The bit design may be modified to include an angle of the second nozzles based on the angle having an elevated cuttings residence time and/or cuttings residence length.

[0008] This summary is provided to introduce some features and concepts that are further developed in the detailed description. Other features and aspects of the present disclosure will become apparent to those persons having ordinary skill in the art through consideration of the ensuing description, the accompanying drawings, and the appended claims. This summary is therefore not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0009] In order to describe various features and concepts of the present disclosure, a more particular description of certain subject matter will be rendered by reference to specific embodiments which are illustrated in the appended drawings. Understanding that these drawings depict just some example embodiments and are not to be considered to be limiting in scope, nor drawn to scale for each embodiment contemplated hereby, various embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

[0010] FIG. 1 is a schematic illustration of an example downhole system for milling a plug, in accordance with one or more embodiments of the present disclosure;

[0011] FIG. 2-1 is a partial cross-sectional view of a milling system for milling a plug, in accordance with one or more embodiments of the present disclosure;

[0012] FIG. 2-2 is a partial cross-sectional view of another milling system for milling a plug, in accordance with one or more embodiments of the present disclosure;

[0013] FIG. 3 is a partial cross-sectional view of a milling system with an in-line filtering system, the milling system being used to reduce the size of cuttings of a milled plug, in accordance with one or more embodiments of the present disclosure; [0014] FIG. 4 is a cross-sectional view of a milling system with multiple milling components for reducing the size of cuttings of a milled plug, in accordance with one or more embodiments of the present disclosure;

[0015] FIG. 5 is a cross-sectional view of another milling system with multiple milling components for reducing the size of cuttings of a milled plug, in accordance with one or more embodiments of the present disclosure;

[0016] FIG. 6 is a perspective view of a portion of a milling bit for use in a downhole system for milling a plug, in accordance with one or more embodiments of the present disclosure;

[0017] FIG. 7 is a wireframe perspective view of a milling bit for use in milling a plug, in accordance with one or more embodiments of the present disclosure;

[0018] FIG. 8 is a cross-sectional view of a milling system for milling a plug, in accordance with one or more embodiments of the present disclosure;

[0019] FIG. 9 is a user interface illustrating plots of cuttings residence time for various materials at multiple nozzle angles, in accordance with one or more embodiments of the present disclosure; and

[0020] FIG. 10 is a user interface illustrating plots of cuttings residence length for various materials at multiple nozzle angles, in accordance with one or more embodiments of the present disclosure

DETAILED DESCRIPTION

[0021] In accordance with some aspects of the present disclosure, embodiments herein relate to downhole tools. More particularly, embodiments disclosed herein may relate to downhole tools and bottomhole assemblies ("BHA") that include a mill. An example BHA may include a mill for drilling and removing a bridge plug, frac plug, or other similar anchor or sealing device within a wellbore. In still other aspects, embodiments of the present disclosure may relate to mills that include hydraulic barriers to restrict cuttings of a bridge plug from being carried uphole and to promote re-circulation of the cuttings against the mill.

[0022] Referring now to FIG. 1, a schematic diagram is provided of an example downhole system 100 that may utilize milling systems, assemblies, devices, and methods in accordance with embodiments of the present disclosure. FIG. 1 shows an example wellbore 102 formed in a formation 104. In this particular embodiment the wellbore 102 includes a casing 106 installed therein. The casing 106 may extend along a full length of the wellbore 102; however, in other embodiments, the wellbore 102 may be an openhole wellbore that is uncased, or the wellbore 102 may include both cased portions and openhole portions. Casing 106 within the wellbore 102 may also include various types of casing, including surface casing, intermediate casing, conductor casing, production casing, production liner, and the like. In some embodiments, as the depth of the wellbore 102 increases, the diameter of the casing 106 may decrease.

[0023] In at least some embodiments, the casing 106 may provide structural integrity to the wellbore 102, isolate the wellbore 102 against fluids within the formation 104, or provide other aspects or features. In some applications, after the casing 106 is cemented or otherwise installed within the wellbore 102, a portion of the casing 106 may be perforated or removed to facilitate or stimulate production in the corresponding portion or zone of the formation 104. In FIG. 1, for instance, perforations 108 may be made in the casing 106 and may extend radially outward into the formation 104. Following formation of the perforations 108, fluid may be pumped into the wellbore 102 and through the perforations 108. The fluid may be pumped in at a sufficiently high pressure to cause the formation 104 to crack or fracture, thereby opening up fluid passageways to stimulate production of hydrocarbons, water, or other fluids in that particular zone within the formation 104. In some embodiments, proppant or other materials may be included in the fluid to assist in fracturing the formation 104 or to hold open the formed fractures.

[0024] A plug 110 may be set within the wellbore 102, and in some embodiments may facilitate use of the fluid in fracturing the formation 104. In this particular embodiment, the plug 110 may hydraulically seal a portion of the wellbore 102 below the plug 110 from a portion of the wellbore 102 above the plug 110. As fluid is then pumped into the wellbore 102, the plug 110 may restrict and potentially prevent the fluid from flowing downhole beyond the plug 110 and deeper into the wellbore 102. The fluid may thereby be forced into the formation 104 through the perforations 108. The plug 110 may include a so- called frac plug. A bridge plug may also be used to seal or isolate different portions of the wellbore 102. A frac plug may be a particular type of bridge plug for use in fracturing the formation 104, but bridge plugs may be used in myriad other applications. For instance, bridge plugs may also be used in wellbore abandonment, acidizing, cementing, selective single-zone operations, treatment, testing, repair/remedial, or other applications, or any combination of the foregoing. In other embodiments, the plug 110 may be a non- sealing plug (e.g., an anchor).

[0025] In the particular embodiment illustrated in FIG. 1, a BHA 112 may be provided to facilitate milling of the plug 110 so as to open the wellbore 102 and fluidly connect the upper and lower zones within the wellbore 102. The BHA 112 may be connected to a drill string 114. In FIG. 1, the drill string 114 is illustrated as extending from the surface and having the BHA 112 suspended therefrom. The drill string 114 may include one or more tubular members. The tubular members of the drill string 114 may themselves have any number of configurations. As an example, the drill string 114 may include segmented/jointed drill pipe or wired drill pipe. Such drill pipe may include rotary shouldered or other threaded connections on opposing ends to allow segments of drill pipe to be connected together to increase the length of the drill string 114 as the BHA 112 is tripped further into the wellbore 102, or disconnected to shorten the length of the drill string 114 and the BHA 112 is tripped out of the wellbore 102. The drill string 114 may also include continuous components such as coiled tubing. Couplings, drill collars, other drill string components known in the art, or combinations of the foregoing, may also be used.

[0026] The BHA 112 may include any number of components that may be used to perform one or more downhole operations. As an example, the BHA 112 may include a bit 1 16. In at least some embodiments, the bit 116 may be configured or otherwise designed to break-up the plug 110. For instance, the plug 110 may be a composite plug formed of multiple materials (e.g., ferrous materials, non-ferrous materials, composite materials, rubber, elastomers, etc.). The plug 110 may be configured to drill, mill, degrade, or otherwise break-up the different materials of the plug 110. The BHA 112 may also include any number of other components. By way of example, the BHA 112 may include stabilizers, mud motors, mills (e.g., section mills, follow mills, watermelon mills, etc.), logging-while-drilling or measurement-while-drilling components, memory or data storage devices, rotary steerable and directional drilling equipment, activation equipment, data processors and receivers, signal boosters, telemetry components, perforation or fracking equipment, drilling assistance devices (e.g., vibration tools, laser cutting tools, abrasive cutting tools, etc.), other devices or tools, or any combination of the foregoing.

[0027] The bit 116 may be a milling bit for milling the plug 110 to remove the plug 110 and open the wellbore 102 to fluid flow between upper and lower portions. The bit 116 may be a lead mill, taper mill, junk mill, or another type of mill that may be used to mill and grind away the plug 110 as the bit 116 is rotated and has weight-on-bit applied thereto. Uphole or downhole rotational power may be provided to rotate the bit 116. A drilling rig 118, for instance, may be used to convey the drill string 114 and BHA 112 into the wellbore 102. In an example embodiment, the drilling rig 110 may include a derrick and hoisting system 120, a rotating system, a mud circulation system, or other components. The derrick and hoisting system 120 may suspend the drill string 114, and the drill string 114 may pass through a wellhead 122 and into the wellbore 102. In some embodiments, the drilling rig 118 or derrick and hoisting system 120 may include a draw works, a fast line, a crown block, drilling line, a traveling block and hook, a swivel, a deadline, or other components. An example rotating system may be used, for instance, to rotate the drill string 114 and thereby also rotate the bit 116 or other components of the BHA 112. The rotating system may include a top drive, kelly, rotary table, or other components that can rotate the drill string 114 at or above the surface.

[0028] In other embodiments, the bit 116 may be rotated by using a downhole component. For instance, the BHA 112 may include a motor. The motor may include any motor that may be placed downhole, and expressly may include a mud motor, turbine motor, other motors or pumps, any component thereof, or any combination of the foregoing. A mud motor may include a positive displacement motor (PDM), progressive cavity pump, Moineau pump, other type of motor, or some combination of the foregoing. Such motors or pumps may include a helical or lobed rotor that is rotated by flowing drilling fluid. The drill string 114 may include coiled tubing, slip drill pipe, segmented drill pipe, or other structures that include an interior channel within a tubular structure so as to allow drilling fluid to pass from the surface to the BHA 112. In the mud motor, the flowing drilling fluid may rotate the lobed rotor relative to a stator. The rotor may be coupled to a drive shaft which can directly or indirectly be used to rotate the bit 110. In the same or other embodiments, the motor may include turbines. A turbodrill or other turbine motor may include one or more turbines or turbine stages that include a set of stator vanes that direct drilling fluid against a set of rotor blades. When the drilling fluid contacts the rotor blades, the rotor may rotate relative to the stator and a housing of the motor. The rotor blades may be coupled to a drive shaft (e.g., through compression, mechanical fasteners, etc.), which may also rotate and cause the bit 116 to rotate. [0029] Although the downhole system 100 is shown in FIG. 1 as being on land, those of skill in the art will recognize that embodiments of the present disclosure are also equally applicable to offshore and marine environments. Additionally, while embodiments herein discuss milling of a plug within a cased wellbore, in other embodiments a plug may be used in an openhole wellbore, or an openhole section within a wellbore. Further still, components other than plugs may be milled, or milling may occur above the surface rather than in a downhole environment.

[0030] Turning now to FIG. 2-1, a downhole milling system 200 is shown in accordance with some embodiments of the present disclosure. The downhole milling system 200 may include a milling bit such as mill 216 configured for use in milling or otherwise grinding a component or tool (e.g., a plug 210 set within a wellbore). In at least some embodiments, the plug 210 may include a bridge plug. As discussed herein, the plug 210 may be formed of one or more materials and, in some embodiments, may provide a hydraulic seal between an upper portion of the wellbore (i.e. , a portion of the wellbore uphole of the plug 210) and a lower portion of the wellbore (i.e., a portion of the wellbore downhole of the plug 210). The plug 210 may be formed of various materials, including metals (e.g., ferrous and non-ferrous metals), alloys, rubber or other elastomers, composite materials, other materials, or combinations of the foregoing.

[0031] The mill 216 may be inserted into a wellbore and moved downhole toward, and into engagement with, the plug 210. In at least some embodiments, the wellbore may have a casing 206 lining the inner surface of the wellbore, and the mill 216 may be inserted through the casing 206. For milling of the plug 210, the mill 216 may include a bit body 224 having one or more blades 226, knives, or other cutting structure thereon. These blades 226 or other cutting structures may further include or be coupled to cutting elements 228 configured to grind, mill, degrade, or break-up the plug 210. The blades 226 and the cutting elements 228 may have any suitable configuration. For instance, there may be multiple blades 226 circumferentially spaced around the bit body 224 of the mill 216. Any number of blades 226 may be provided. For instance, there may be between 1 and 20 blades 226 in some embodiments. More particularly, there may be 1, 2, 4, 6, 8, 10, 12, 15, 18, 20 blades, or any value therebetween. In other embodiments, there may be more than 20 blades 226, or there may be no blades and other cutting structures (e.g., roller cones, etc.) may be used. The blades 226 may each be the same, or different, and there may be equal or unequal spacing between the blades 226.

[0032] The cutting elements 228 may also have any suitable configuration and make-up. The cutting elements 228 may be formed of a material having sufficient hardness or abrasiveness to grind the plug 210 into cuttings and remove the plug 210 from the wellbore. In some embodiments, the cutting elements 228 may be formed of materials with material properties sufficient to cut steel or other ferrous metals. Examples of suitable materials useful for cutting steel or other ferrous metals may include, by way of illustration, tungsten, titanium, ceramics, metal carbides (e.g., tungsten carbide, cobalt-cemented tungsten carbide, cemented titanium carbide, cemented tantalum carbide), diamond (e.g., polycrystalline diamond), cubic boron nitride (e.g., polycrystalline cubic boron nitride), other so-called "superhard" or "super- abrasive" materials, or any combination of the foregoing. Such materials may also be suitable for cutting non-ferrous metals, alloys, composites, elastomers, and the like. In some embodiments, the cutting elements 228 may be formed as fixed cutters that can be brazed, welded, or otherwise secured within corresponding pockets in the bit body 224. In other embodiments, the cutting elements 228 may be components of hardfacing applied to the blades 226, may be distributed through the bit body 224 (e.g., impregnated), otherwise coupled to the bit body 224, or a combination of the foregoing may be used. For instance, one layer of the bit body 224 may be impregnated with cutting elements while another layer may have fixed cutters coupled to the bit body 224.

[0033] In accordance with some embodiments of the present disclosure, the blades 226 and cutting elements 228 may be part of a debris conditioning system 230 of the mill 216. The debris conditioning system 230 may be used to initially mill or grind the plug 210 into cuttings, and to re-grind or further mill the cuttings to have a size, shape, or other configuration that can be efficiently transported to the surface within the annulus 232 between the casing 206 and the mill 216, drill string, and BHA. For instance, drilling fluid flowing uphole within the annulus 232 may provide a solids transport mechanism for carrying the cuttings to the surface.

[0034] In operation, drilling fluid may flow through the downhole milling system 200 and may generally follow the block arrows shown in FIG. 2-1. The drilling fluid may, for instance, flow through a drill string (e.g. , drill string 114 of FIG. 1) and into an interior channel within the bit body 224 of the mill 216. The bit body 224 may define one or more ports, nozzles, or jets through which drilling fluid may exit the mill 216. For instance, the bit body 224 may include a first nozzle 234 which in the illustrated embodiment may convey drilling fluid from the interior of the bit body 224 to a location near the face of the mill 216. Drilling fluid flowing through the first nozzle 234 may be used to cool the blades 226 or cutting elements 228, and may exit and be jetted from the bit body 224 with sufficient velocity to evacuate cuttings from the face of the mill 216. A single first nozzle 234 is shown in FIG. 2-1 ; however, one skilled in the art should appreciate in view of the disclosure herein that 1, 2, 3, 4, 5, or more first nozzles 234 may be defined by the bit body 224 and included in the mill 216.

[0035] One or more second nozzles 236 may also be defined in the bit body 224 of the mill 216. In the embodiment shown in FIG. 2-1, the second nozzles 236 may cause drilling fluid to exit or be jetted from the mill body 224 in a direction that may be about perpendicular to a longitudinal axis of the mill 216. As indicated by the block arrows, the drilling fluid exiting the mill 216 through the first and second nozzles 234, 236 may enter the annulus 232 and return to the surface. Cuttings from the plug 210 may be suspended in the drilling fluid and also returned to the surface.

[0036] In at least some embodiments, the second nozzles 236 may be included as part of the debris conditioning system 230. For instance, as discussed in greater detail with respect to FIG. 2-2, the second nozzles 236 may be used to form a fluid shroud, curtain, or other barrier to restrict, and potentially prevent, cuttings or debris above a predetermined size from moving uphole past the fluid or hydraulic barrier and toward the surface. [0037] In some embodiments, the debris conditioning system 230 may include additional or other components. For instance, FIG. 2-1 illustrates a barrier 238 that may be used to restrict, and potentially prevent, cuttings or debris above a predetermined size from moving uphole past the barrier 238 toward the surface. The barrier 238 may include mechanical or other components. In at least some embodiments, the barrier 238 may include one or more expandable pads. The expandable pads may fill a portion of the annulus 232 between the mill 216 and the casing 206, thereby restricting the area through which debris or cuttings may pass toward the surface. Restricting flow of the debris and cuttings in this manner may be a result of gaps between the barriers 238 and the casing 206, and circumferential gaps between the barriers 238 themselves, having a size sufficient to allow passage of smaller cuttings and debris, while restricting passage of larger pieces.

[0038] Where the barriers 238 include expandable pads, the expandable pads may be selectively retractable or extendable. When the mill 216 is inserted into the wellbore, the expandable pads may be in an at least partially retracted state. As the mill 216 reaches the plug 210, the expandable pads may be expanded radially outward toward the casing 206. The expandable pad may pivot, slide along an inclined path, or otherwise move at least partially in a radial direction. Actuation of the expandable pad may occur in any suitable manner. For instance, the mill 216 or other bit or component of a downhole system may include one or more sensors (not shown) that sense weight-on -bit, proximity to the plug 210, or contact with the plug 210. In response to such detection, a mechanical, electrical, hydraulic, or other activation system may be deployed to expand the expandable pads or open a port to allow the drilling fluid in the mill 216 to expand the expandable pads. In other embodiments, actuation may be provided from an uphole actuation signal. The actuation signal may be conveyed using wireless, physical, or other mechanisms, or combinations of the foregoing. For instance, an actuation signal may be conveyed to the mill 216 by dropping a ball or dart which creates fluid pressure to expand the expandable pads. In other embodiments, an active or passive RFID tag may be conveyed from the surface through the drill string and to the mill 216. A wireless receiver may detect the RFID tag and expand the expandable pads. In other embodiments, the plug 210 may include an RFID tag so that proximity to the plug 210 can be detected. In still other embodiments, wireless signals or telemetry (e.g., mud pulse telemetry, pressure pulse patterns, drill string rotation patterns, etc.) may be used to convey an activation signal to the mill 216. The expandable pads of the barrier 238 may also be selectively retractable in a similar manner. For instance, when weight-on-bit, proximity to the plug 210, or contact with the plug 210 falls below a threshold value, the activation system may deactivate and retract the barriers 238. A second ball or dart may also be dropped, wireless or telemetry may be used, or the like.

[0039] With the expandable pads or other barriers 238 limiting annular or circumferential space between the barriers 238 and between the barriers 238 and the casing 206, debris larger than the size allowed by the spacing may be restricting the uphole directed flow of the debris or other cuttings from the plug 210 into the annulus 232. Optionally, flow through the second nozzles 236 or even the first nozzles 234 may be used to move the cuttings. As indicated by the curved arrows at the downhole end of the mill 216 in FIG. 2-1, drilling fluid passing through the first and/or second nozzles 234, 236 may cause the blocked cuttings to re-circulate. Re -circulation may push the cuttings back in front of the face of the mill 216 to allow the blades 226 and cutting elements 228 to re-grind and re-mill the cuttings to smaller sizes. The smaller cuttings may then attempt to pass by or through the barriers 238. Some of the cuttings may then be conveyed to the surface while other cuttings may still be too large and may be re-circulated one or more additional times.

[0040] While the barriers 238 are illustrated in FIG. 2-1 as being located on or radially adjacent the bit body 224, in other embodiments the barriers 238 may be positioned in other locations. For instance, the barriers 238 may be positioned above the bit body 224 or even above the mill 216. Additionally, while the barriers 238 may include expandable pads, the barriers 238 may include other components that expand, retract, or are fixed in place. Fixed pads, expandable filters or screens, or other components may also be used.

[0041] In other embodiments, the barriers 238 may be eliminated or may remain retracted while milling the plug 210. In particular, FIG. 2-2 illustrates a downhole milling system 200 with the barriers 238 (see FIG. 2-1) removed or retracted. In this embodiment, the debris conditioning system 230 may cause drilling fluid flowing through the second nozzles 236 to jet radially outward toward the casing 206 to form a shield, shroud, curtain, or other hydraulic barrier 240 between the outer surfaces of the mill 216 and the inner surface of the casing 206. More particularly, drilling fluid jetting from the second nozzles 236 may create an area of turbulence in the annulus, and may result in formation of a hydraulic barrier 240 which reduces the annular space between the mill 216 and the casing 206. The hydraulic barrier 240 may thereby restrict and potentially preventing cuttings or debris over a particular size (e.g., larger than circumferential gaps between multiple hydraulic barriers 240) from moving uphole past the hydraulic barrier 240 and into the annulus 232. In some embodiments, larger cuttings may not be efficiently conveyed to the surface and/or may clog the wellbore.

[0042] As also shown by the curved arrows at the face of the mill 216 in FIG. 2-2, the drilling fluid jetting from the second nozzles 236 may push the larger cuttings toward the face of the mill 216, thereby promoting re-circulation of the cuttings toward the face of the mill 216 re-milling and re-grinding. Re- milling or re -grinding of the cuttings may produce smaller or finer cuttings, or cuttings of a more desirable shape, thereby promoting efficient solids transport within the wellbore. In some embodiments, flexible materials (e.g., elastomers, rubber, etc.) may be more likely than rigid metals, alloys, and the like to be produced in larger sizes. In such embodiments, the debris conditioning system 230 may be configured to primarily recirculate flexible materials of the plug 210 for re-milling and re-grinding. In other embodiments, however, more metals or other rigid materials, or about equal quantities of different materials may be re-circulated. In some embodiments, the hydraulic barrier 240 of FIG. 2-2 may be used in combination with other barriers (e.g., barrier 238 of FIG. 2-1).

[0043] The hydraulic barrier 240 may be selectively activated in some embodiments. For instance, one or more check valves may restrict drilling fluid flow so that drilling fluid below a particular flow rate or pressure may not produce the hydraulic barrier 240. In other embodiments, the second nozzles 236 may be open but drilling fluid not meeting specified flow, weight, pressure, or other criteria may not produce a desired hydraulic barrier 240.

[0044] The number, location, angle, and other configurations of the second nozzles 236 may be varied to act as control jets that produce desired qualities in the hydraulic barrier 240. A single second nozzle 236 is shown in FIG. 2-2; however, one skilled in the art should appreciate in view of the disclosure herein that 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 15, or 20 or more second nozzles 236, or any number therebetween, may be defined or included in the bit body 224 and the mill 216. Including more second nozzles 236 may, in some embodiments, reduce the distance between the hydraulic barriers 240. Forming the second nozzles 236 at an angle that is non-perpendicular to the longitudinal axis of the mill 216 or the wellbore may allow recirculation patterns to change (e.g. , downhole directed second nozzles 236 may, in some embodiments, push cuttings downhole more efficiently). The second nozzles 236, and consequently the hydraulic barriers 240, may also be moved to be on-bit or off -bit. When on-bit, as shown in FIG. 2-2, the second nozzles 236 may extend through the bit body 224. In other embodiments, a collar, circulation sub, or other component located uphole of the mill 216 may include the second nozzles 236.

[0045] Debris conditioning systems of the present disclosure may also be configured to operate in other manners. FIG. 3, for instance, illustrates a downhole milling system 300 that includes a mill 316 and an additional debris conditioning system 330. In at least some embodiments, the debris conditioning system 330 may include one or more components or stages that may be used to reduce the size, change the shape, or otherwise condition debris or cuttings within a wellbore.

[0046] In the particular embodiment shown, the downhole milling system 300 and the debris conditioning system 330 may be used within a casing 306 lining a wellbore. The mill 316 may be coupled to the debris conditioning system 330 and a drive system 342 used to rotate the mill 316. As a result, as the mill 316 rotates and engages a downhole component (e.g., a plug), the downhole component may be milled or ground to form debris and cuttings. The drive system 342 may include any number of components. For instance, the drive system 342 may include drill string components that are rotated at the surface of the wellbore. In other embodiments, the drive system 342 may include a mud motor (e.g., a PDM, progressive cavity pump, Moineau pump, etc.), turbines, or the like. In an embodiment in which the drive system 342 includes a mud motor or a turbine motor, drilling fluid flowing through the downhole tool 300 may cause internal rotors to rotate to drive a drive shaft 344 coupled to the mill 316. The drive shaft 344 may extend through at least a portion of the drive system 342, and optionally through a housing 346 which may remain stationary, or which may have a rotation that is different than that of the drive shaft 344. The debris conditioning system 330 may be coupled to the housing 346 in some embodiments.

[0047] The mill 316 may be coupled to a downhole end portion of the drive shaft 344 and rotated to mill into, and grind away, a plug or other downhole component or tool (e.g., plug 210 of FIG. 2-2). The cuttings and debris produced by the mill 316 may be of a size that can be conveyed to the surface through drilling fluid within the annulus 332 between the outer surface of the downhole milling system 300 and an internal surface of a casing 306 of the wellbore. In other embodiments, the cuttings or debris, or a portion thereof, may have a size or shape that is not easily conveyed to the surface. As a result, multiple short- trips could be used to avoid plugging or clogging the annulus 332 of the wellbore.

[0048] In some embodiments, the debris conditioning system 330 may be used to reduce the number of short-trips by, for instance, re-milling, re -grinding, re-shaping, or otherwise conditioning the debris within the wellbore. As discussed herein, one mechanism for conditioning the debris or other cuttings may include the use of a barrier that promotes re-circulation of cuttings to the face of the mill 316. Mechanical pads, hydraulic jets, or other components discussed herein may therefore be included to define a barrier, curtain, or other device to limit the size of cuttings that may pass uphole, while further re -directing larger cuttings and debris back to the face of the mill 316 for re-grinding and re-milling. As discussed herein, such barriers may be located on or above the mill 316. In FIG. 3, for instance, a barrier 338 may be located above the mill 316.

[0049] More particularly, FIG. 3 illustrates an example embodiment in which a sleeve 348 may be coupled to the housing 346. Where the housing 346 is stationary, the sleeve 348 may also be stationary. In at least one embodiment, the housing 346 may be a housing of a mud motor, turbine motor, or other component of a drive system 342, and one or more connectors may be used to couple the housing 346 to the sleeve 348. For instance, external, pin threads may be formed on the outer surface of the housing 346 while corresponding internal, box threads may be formed on the inner surface of the sleeve 348. The sleeve 348 may then be threadingly coupled to the housing 346. In other embodiments, mechanical fasteners (e.g., screws, bolts, etc.), welding, other fastening techniques, or a combination of the foregoing, may be used to couple the sleeve 348 to the housing 346.

[0050] The barriers 338 may be permanently or temporarily used to block a portion of the annulus of the wellbore or to otherwise limit the passage of debris and cuttings uphole past the barriers 328. The barriers 338 may, for instance, be formed in or coupled to the sleeve 348 to occupy at least some of the space between the outer surface of the sleeve 348 and the inner surface of the casing 306. The barriers 338 may not be retractable and may therefore permanently be positioned in an expanded or active state. In other embodiments, the barriers 338 may operate as discussed herein, or otherwise be selectively expanded and/or retracted. For instance, the barriers 338 may include expandable pads that can expand or retract in response to hydraulic, mechanical, electrical, or other forces or signals. In still other embodiments, the barriers 338 may be formed using control jets, nozzles, or the like. For instance, as drilling fluid passes through the drill string, the drilling fluid may be routed inside the sleeve 348. Jets or nozzles corresponding to the position of the barriers 338 may then be used to expel the drilling fluid into the annulus and create a region of turbulence to form a fluid curtain, shroud, or other barrier 338 within at least a portion of the interior of the wellbore. This barrier 338 may be a hydraulic barrier that pushes down cuttings and debris toward the face of the mill 316 and thereby promotes re -circulation of at least some of the cuttings produced by the mill 316. [0051] In at least some embodiments, the barrier 338 may be formed between the inner surface of the casing 306 and the outer surface of the shroud, barrel, or other device forming the sleeve 348. Optionally, the sleeve 348 may extend downhole from the housing 346 but may fully to the mill 316 so that an axial gap may be formed between the mill 316 and the distal or downhole end of the sleeve 348. When debris and cuttings are milled or re-milled to have a shape and/or size suitable for solids transport within the drilling fluid, the debris and cuttings may pass uphole from the mill 316 and into the sleeve 348 to be carried to the surface. The sleeve 348 is optional, and may be omitted in other embodiments. For instance, the internal diameter of the casing 306 may be used as part of the debris conditioning system 330. As an example, the mill 316 may include crushed carbide or other cutting elements on the back of a blade, on the front of one or more gauge pads, and the like. Debris, cuttings, and the like that are between the blade and the casing 306 may then be milled and re-milled by the mill 316 even in the absence of the sleeve 348. Recirculation may therefore be used to re-circulate cuttings, debris, and the like to the face of the mill 316, to the back of the blades, to gauge portions that include cutting elements, or any combination of the foregoing.

[0052] To further condition the debris, promote re-circulation of debris and cuttings, or restrict the size of cuttings and debris passing to the surface, the debris conditioning system 330 may optionally include a filtering system 350. In FIG. 3, for instance, the filtering system 350 may be coupled to the interior surface of the sleeve 348 and/or to the outer surface of the drive shaft 344. The filtering system 350 may include a screen, slots, or other components configured to limit, and potentially prevent, debris and cuttings over a predetermined size from passing into the sleeve 348. For instance, cuttings having a diameter greater than a distance between slots of the filtering system 350, or greater than openings of a screen of the filtering system 350, may be restricted from passing through the filtering system 350. Optionally, such cuttings may be re -circulated to the face of the mill 316 (e.g., through drilling fluid, nozzles, jets, hydraulic barriers, etc.) for re-milling. In some embodiments, the filtering system 350 may be an in-line filtering system within the sleeve 348.

[0053] The debris and cuttings that are sufficiently small to pass through the filtering system 350 may be carried by drilling fluid to the surface. In at least some embodiments, however, the debris conditioning system 330 may include a secondary attrition system 352 which may be uphole relative to the filtering system 350 and/or the mill 316. The secondary attrition system 352 may operate as a second stage (the mill 316 may operate as the first stage) for further refining the shape or size of the debris and cuttings. The secondary attrition system 352 may thus operate as a secondary reduction system for reducing the size of debris and cuttings away from the mill 316. In FIG. 3, for instance, the interior surface of the sleeve 348 and/or the outer surface of the drive shaft 344 may include secondary cutting elements 354. The secondary cutting elements 354 may be positioned within the sleeve 348 and configured to re-mill and re-grind cuttings that pass through the filtering system 350. The secondary cutting elements 354 may therefore re- mill and re-grind the cuttings and debris away from the bit (e.g., mill 316). [0054] The secondary cutting elements 354 may refine the size of cuttings and debris through grinding and attrition, and may operate using abrasive, cutting, or other action. For instance, the cutting elements 354 may be included in hardfacing applied to the sleeve 348 and/or the drive shaft 344. In other embodiments, the cutting elements 354 may be part of an abrasive slurry. In still other embodiments, crushed carbide may be welded, brazed, or otherwise coupled to the interior surface of the sleeve 348 and/or the outer surface of the drive shaft 344 to facilitate debris grinding action. In at least some other embodiments, discrete cutting inserts, grooves, splines, teeth, or the like may be used as the cutting elements 354. In such embodiments, the cutting elements 354 may be spaced radially, angularly, and linearly. Thus, as the drive shaft 344 rotates relative to the sleeve 348, debris and cuttings may collect within the voids between the cutting elements 354, and may be crushed as the voids change location and shape by virtue of the rotating cutting elements 354.

[0055] In some embodiments, debris and cuttings may be milled by staged cutting structures within the sleeve 348. For instance, multiple sets of cutting elements 354 may be provided, which each set being configured to reduce the size of debris and cuttings to a particular target size. In some embodiments, the filtering system 350 may be removed and replaced by an additional secondary attrition system.

[0056] When debris and cuttings have passed through the secondary attrition system 352, and optionally been milled or ground to a desired size, the debris and cuttings may be conveyed to the surface. For instance, drilling fluid may carry the debris and cuttings to the surface. As shown in FIG. 3, the sleeve 348 may include one or more openings 356. The openings 356 may operate as exit ports to allow debris and cuttings to escape from the interior of the sleeve 348 and into the annulus 332. In some embodiments, the debris and cuttings that pass through the openings 356 may have a predetermined maximum size. For instance, the secondary attrition system 352 may be configured to reduce the size of the cuttings and debris to a maximum size that may be about equal to the distance between cutting elements 354 (e.g., axial distance between cutting elements 354 or radial distance between cutting elements 354), or the distance between the outer surface of the sleeve 348 and the inner surface of the casing 306.

[0057] FIG. 4 illustrates still another example embodiment of a downhole milling system 400, and may include a mill 416 and a debris conditioning system 430 for milling a plug 410. In at least some embodiments, the plug 410 may include a bridge plug. The mill 416 and the debris conditioning system 430 may collectively define multiple stages that may be used to reduce the size, change the shape, or otherwise condition debris or cuttings within a wellbore.

[0058] In the particular embodiment shown, the downhole milling system 400 and the debris conditioning system 430 may be used within a casing 406 lining a wellbore. The mill 416, may be coupled to the debris conditioning system 430 and a drive system 442. The drive system 442 may be used to rotate the mill 416. As a result, as the mill 416 rotates and engages the plug 410, the plug 410 may be milled and broken up. The drive system 442 may include any number of components. For instance, the drive system 442 may include drill string components that are rotated at the surface of the wellbore. In other embodiments, the drive system 442 may include a mud motor (e.g., a PDM, progressive cavity pump, Moineau pump, etc.) or turbine motor. In an embodiment in which the drive system 442 includes a mud motor, turbines, or other downhole motor, drilling fluid may cause internal rotors to rotate to drive a drive shaft 444 coupled to the mill 416. The drive shaft 444 may extend through the drive system 442 or be coupled to a downhole end of a rotating shaft of the drive system 442. For instance, the drive shaft 444 may be a component of a downhole motor, or an intermediate shaft that couples the downhole motor to the mill 416. In some embodiments, such as that shown in FIG. 4, the drive shaft 444 may be coupled directly to the mill 416 (e.g., to a stem of the mill 416). In other embodiments, one or more intermediate shafts, mandrels, or other components may couple the drive shaft 444 to the mill 416.

[0059] Whether the mill 416 is coupled directly or indirectly to the drive shaft 444, the mill 416 may be rotated to mill into, and grind away, plug 410. The cuttings (e.g., from metal or alloy portions of the plug 410) and debris (e.g., produced from elastomers, rubber, or composites of the plug 410) may be of a size that can be conveyed to the surface through drilling fluid within the annulus 432 between the downhole milling system 400 and the casing 406 of the wellbore. In other embodiments, the cuttings or debris, or a portion thereof, may have a size or shape that is not easily conveyed to the surface.

[0060] The debris conditioning system 430 may be used to reduce the number of short-trips used to avoid clogging the wellbore by, for instance, re-milling, re-grinding, re-shaping, or otherwise conditioning the cuttings and debris within the wellbore. As discussed herein, one mechanism for conditioning the debris or other cuttings may include the use of a barrier that promotes re -circulation of cuttings to the face of the mill 416. For instance, the mill 416 may include one or more nozzles 436 that may be defined in the bit body 424 of the mill 416, in the stem of the mill 416, in the drive shaft 444, or in some other component of the downhole milling system 400, and which may act as control jets for promoting re -circulation of the cuttings and debris. In the embodiment shown in FIG. 4, the nozzles 436 may cause drilling fluid conveyed through the mill 416 to exit or be jetted from the mill body 424 in a direction that is angularly offset from the longitudinal axis of the mill 416. As indicated by the block arrows, the drilling fluid exiting the mill 416 through the nozzles 436 may enter the annulus 432 and return to the surface. Debris and cuttings from the plug 410 may be suspended in the drilling fluid and also returned to the surface.

[0061] In at least some embodiments, the nozzles 436 may be included as part of debris conditioning system 430. For instance, as discussed herein, the nozzles 436 may be used to form a fluid shroud, curtain, or other barrier to restrict, and potentially prevent, cuttings or debris above a predetermined size from moving uphole past the fluid or hydraulic barrier and toward the surface. In particular, the fluid curtain or other barrier may limit the size of cuttings that may pass uphole and to re -direct larger cuttings back to the face of the mill 416 for re-grinding and re-milling by the cutting elements thereon. As discussed herein, such barriers may be located on the mill 416 or in other locations.

[0062] FIG. 4 further illustrates an example embodiment in which the debris conditioning system 430 may include a sleeve 448 defining an outer barrier for use in directing or limiting flow of drilling fluid, debris, or cuttings within the wellbore. In at least some embodiments, the sleeve 448 may not rotate, or may rotate at a different speed (or in a different direction) than the drive shaft 444 and/or the mill 416. In at least one embodiment, the sleeve 448 may be coupled to a housing of a mud motor, turbine motor, or other drive system. For instance, external, pin threads may be formed on the outer surface of the downhole motor or other component of the drive system 442, while corresponding internal, box threads may be formed on the inner surface of the sleeve 448. The sleeve 448 may then be threadingly coupled to the drive system 442. In other embodiments, mechanical fasteners (e.g., screws, bolts, etc.), welding, or other fastening techniques may be used to couple the sleeve 448 to the drive system 442 or other component of the downhole milling system 400.

[0063] The sleeve 448 may include, or cooperate with, one or more structures that can be used to further mill, grind, or condition the debris and cuttings produced from the plug 410. For instance, as the plug 410 is milled and cuttings are potentially re-milled through re-circulation promoted by the nozzles 436, the cuttings and debris may pass through the fluid or hydraulic barrier and into an annular region between the interior surface of the sleeve 448 and an outer surface of the drive shaft 444. Optionally, one or more secondary attrition systems 452 may be provided within the sleeve 448 to even further mill or grind the cuttings and debris. FIG. 4, for instance, illustrates two additional secondary attrition systems 452. The secondary attrition systems 452 may be staged and, in this embodiment, may be axially offset. The lowermost secondary attrition system 452 may first receive the cuttings and debris produced from the mill 416 and mill or grind the cuttings and debris into a finer size. When the debris and cuttings milled by the lowermost secondary attrition system 452 are of a size small enough to pass through the lowermost secondary attrition system 452, the debris and cuttings may then be carried by the fluid in an uphole direction to the upper secondary attrition system 452. The secondary attrition system 452 may similarly mill and grind the cuttings and debris, and optionally may be configured to reduce the size of the cuttings and debris. Each secondary attrition system 452 may therefore be configured to reduce the cuttings and debris to a different size, with upper secondary attrition system 452 optionally refining the cuttings and debris to an even smaller size.

[0064] The secondary attrition systems 452 may include cutting elements or other structures suitable to refine the size or shape of cuttings and debris through grinding and attrition, and may operate using abrasive, cutting, or other action. For instance, the secondary attrition systems 452 may include cutting elements included in hardfacing applied to the sleeve 448 and/or the drive shaft 444. In other embodiments, the cutting elements may be part of an abrasive slurry. In still other embodiments, crushed carbide may be welded, brazed, or otherwise secured to the interior surface of the sleeve 448 and to a longitudinally aligned portion of the outer surface of the drive shaft 444 to facilitate debris grinding action as debris passes through the crushed carbide. In at least some other embodiments, discrete cutting inserts, grooves, splines, teeth, or the like may be used as the cutting elements of the secondary attrition systems 452. In some embodiments, the cutting elements may be spaced radially, angularly, and linearly. Thus, as the drive shaft 444 rotates relative to the sleeve 448, debris and cuttings may collect within the voids between the offset cutting elements, and may be crushed as the voids change location and shape by virtue of the rotating cutting elements. [0065] When debris and cuttings have passed through the secondary attrition systems 452, and optionally been milled or ground to a desired size, the debris and cuttings may be conveyed to the surface. For instance, drilling fluid may carry the debris and cuttings to the surface. As shown in FIG. 4, the sleeve 448 may include one or more openings 456. The openings 456 may operate as exit ports to allow debris and cuttings to escape from the interior of the sleeve 448 and into the annulus 432. In some embodiments, the debris and cuttings that pass through the openings 456 may have a predetermined maximum size. For instance, the secondary attrition systems 452 may be configured to reduce the size of the cuttings and debris to a maximum size that may be about equal to the radial distance between cutting elements on the sleeve 452 and the drive shaft 444, or the radial distance between the outer surface of the sleeve 448 and the inner surface of the casing 406.

[0066] The downhole milling system 400 may also include additional or other components or features. For instance, to further condition the debris, promote re-circulation of debris and cuttings, or restrict the size of cuttings and debris passing to the surface, the debris conditioning system 430 may also include a filtering system 450. The filtering system 450 may be coupled to the outer surface of the mill 416 or the drive shaft 444, and may include a screen, slots, or other components configured to limit, and potentially prevent, debris and cuttings over a predetermined size from passing uphole and potentially into the sleeve 448. For instance, cuttings having a diameter greater than a distance between slots of the filtering system 450, or greater than openings of a screen of the filtering system 450, may be restricted from passing through the filtering system 450. Optionally, such cuttings may be re-circulated to the face of the mill 416 (e.g., through drilling fluid, nozzles, jets, hydraulic barriers, etc.) for re-milling. In FIG. 4, the filtering system 450 is shown as being positioned downhole of the sleeve 448; however, in some embodiments, the filtering system 450 may be an in-line filtering system within the sleeve 448.

[0067] In additional embodiments, the downhole milling system 400 may include other components, such as a stabilizer 458. The stabilizer 458 may have a fixed radial size or may be expandable through mechanical or hydraulic forces. Blades of the stabilizer 458 may be expandable to contact the interior surface of the casing 406 to centralize the downhole milling system 400 and to reduce vibrations within the system. In other embodiments, the blades of the stabilizer 458 may have a fixed radial size, may be undergauge within the casing 406, or may be otherwise configured.

[0068] Still another example embodiment of a downhole milling system 500 is shown in FIG. 5. The downhole milling system 500 may include features or components similar to those of other downhole milling systems described herein. For instance, the downhole milling system 500 may include a mill 516 that is driven by a drive system 542. Optionally, the downhole milling system 500 may include a debris conditioning system 530 to refine the shape and/or size of debris produced by the mill 516 when milling a plug 510 or other downhole tool. In some embodiments, the debris conditioning system 530 may include a barrier (e.g., a mechanical barrier, an expandable barrier, a hydraulic or fluid barrier, etc.), a filtering system, a secondary attrition system 552, or other components. Such components may be used to mill or grind cuttings and debris produced from the plug 410 away from the face of the mill 416, or to promote re- circulation of cuttings and debris for re-milling or re-grinding of the cuttings and debris at the face of the mill 416 or at secondary cutting structures (e.g., secondary attrition system 552).

[0069] In at least some embodiments, the debris conditioning system 530 may include a sleeve 548 coupled to a downhole motor or other component of the drive system 542. The sleeve 548 may encompass or enclose some features (e.g., the secondary attrition system 552 at a distal or downhole end or other location within the sleeve 548). Drilling fluid escaping the mill 516 (e.g., through nozzles to cool the face of the mill 516, through nozzles to form a hydraulic barrier, etc.) may flow toward the surface and carry cuttings and debris from the plug 510. The fluid and cuttings may enter the sleeve 548 and ultimately pass through one or more openings 556. The openings 556 may operate as exit ports to allow debris and cuttings to escape from the interior of the sleeve 548 and into the annulus between the outer surface of the sleeve 556 and the interior surface of a casing 506 lining the wellbore.

[0070] In some embodiments, the debris and cuttings that pass through the openings 556 may have a predetermined maximum size. For instance, the secondary attrition systems 552 may be configured to reduce the size of the cuttings and debris to a maximum size that may be about equal to the radial distance between cutting elements 528 on the interior surface of the sleeve 552 and the external surface of a drive shaft 544 or mandrel (or cutting elements on the drive shaft 544 or mandrel). In other embodiments, the sleeve 548 may be configured to produce cuttings and debris of a maximum size about equal the radial distance between the outer surface of the sleeve 548 and the inner surface of the casing 506.

[0071] The drive system 542 may be configured to operate as desired in connection with the mill 516, the drive shaft 544, the sleeve 548, and the secondary attrition system 552. In some embodiments, the drive system 542 may be a fit-for-purpose drive system that may be used to rotate the mill 516 while the sleeve 548 does not rotate or rotates at a different speed than the mill 516. In at least one embodiment, the sleeve 548 may be coupled to a housing of the drive system 542 while the mill 516 may be coupled to an output shaft 560 of the drive system 542. The output shaft 560 may, for instance, include a standard connection for coupling a downhole component to a motor or other drive system 542. As shown in the illustrated embodiment, an upper or uphole end portion of the drive shaft 544 may be coupled to the output shaft 560 while a lower or downhole end portion of the drive shaft 544 may be coupled to the mill 516. In particular, in FIG. 5, the mill 516 may include a stem 562 configured to be coupled to the drive shaft 544. In some embodiments, the stem 562 may include threads, shoulders, clamps, collars, welds, or other features enabling coupling of the stem 562 to the drive shaft 544. In other embodiments, one or more intermediate components may be used to couple the drive shaft 544 to the mill 516. For instance, the stem 562 may be an intermediate component rather than a portion of the mill 516, and may be threaded, welded, or otherwise coupled to the mill 516.

[0072] As discussed herein, various embodiments of the present disclosure may include systems and components for conditioning debris and cuttings produced when milling a downhole tool (e.g., a plug, a tubular, etc.). In some embodiments, a barrier may be used to restrict, and potentially prevent, larger debris (e.g., elastomers, rubber, etc. from a composite plug) from getting into the annulus of the wellbore. Larger debris may obstruct the wellbore, thereby causing multiple short-trips to be used to clear the wellbore. Using a barrier, the debris may be re -circulated to the face of the mill to promote re-milling and re-grinding into smaller pieces that may be more efficiently transported within drilling fluid, thereby reducing the number of short-trips. Optionally, secondary milling, grinding, or other attrition systems may further reduce the size of cuttings and debris away from the mill surface using one or more secondary stages.

[0073] One mechanism for promoting re-circulation of the debris and cuttings to the face of the mill or other bit is by using a fluid or hydraulic barrier. As discussed herein, a mill or other bit may include one or more nozzles. In some embodiments, the nozzles may be used to provide drilling fluid to a face of the mill to cool cutting elements on the mill. In the same or other embodiments, nozzles may be provided to direct drilling fluid against a casing or wellbore wall to create a fluid curtain, shroud, or other barrier that limits the upward or uphole flow of the cuttings and debris.

[0074] FIG. 6 illustrates an example embodiment in which a mill 616 may include various nozzles. In the particular embodiment illustrated in FIG. 6, the mill 616 is shown as having six (6) blades 626. Six (6) junk slots or junk channels 664 may also be defined in the regions between adjacent ones of the blades 626. In other embodiments, however, the number of blades 626 and junk channels 664 may be varied. For instance, there may be between 2 and 20 blades 626 or junk channels 664. More particularly, according to some embodiments, the number of blades 626 and/or junk channels 664 may be within a range having lower and upper limits including any of 2, 3, 4, 5, 6, 7, 8, 10, 12, 15, 18, 20, and any values therebetween. For instance, there may be between 2 and 10 blades 626 or junk channels 664, between 4 and 8 blades 626 or junk channels 664, or between 5 and 20 blades 626 or junk channels 664. In other embodiments, there may be more than 20, or fewer than 2, blades 626 or junk channels 664.

[0075] The mill 616 may be used in a system that provides a barrier to promote re-circulation of debris and cuttings to the face of the mill 616. For instance, the mill 616 may include one or more first nozzles 634, and one or more second nozzles 636. In at least some embodiments, the one or more first nozzles 634 may be used to jet drilling fluid and cool cutting elements 628-1, including those of the cutting elements 628-1 located at the face of the mill 616, or move debris and cuttings off the face of the mill 616. In contrast, the one or more second nozzles 636 may be used to jet drilling fluid and form a shroud, curtain, or other barrier restricting uphole movement of cuttings and debris. The second nozzles 636 may be selectively activated to create the barrier.

[0076] The number, location, shape, and configuration of the first and second nozzles 634, 636 may be varied based on a variety of factors, including the type of cutting elements 628-1 used, the size of the mill 616, the type of downhole tool being milled, and the like. Thus, while the illustrated embodiment depicts three (3) first nozzles 634 having a generally circular cross-sectional shape, positioned within the junk channels 664, and oriented to be about parallel to a longitudinal axis 668 of the mill 616, each of these features are merely illustrative. In other embodiments, for instance, the first nozzles 634 may have a triangular, rectangular, elongated, square, hexagonal, or other cross-sectional shape. Moreover, the first nozzles 634 may be located on a blade 626, off the face of the mill 616, adjacent gauge protection elements 628-2, in other locations, or in any combination of the foregoing, in other embodiments.

[0077] The first nozzles 634 may further be oriented at an angle that is non-parallel to the longitudinal axis 668. For instance, the angle between the longitudinal axis 668 and the first nozzles 634 (or the drilling fluid expelled from the first nozzles 634) may range between 0° and 30° in some embodiments. More particularly, according to some embodiments, the angle between the longitudinal axis 668 and the first nozzles 634 may be within a range having lower and upper limits including any of 0°, 2.5°, 5°, 7.5°, 10°, 15°, 20°, 30°, and any angles therebetween. For instance, the first nozzles 634 may jet drilling fluid along a path oriented at angle that is between 0° and 10°, between 0° and 5°, or between 5° and 20° relative to the longitudinal axis 668. In other embodiments, the first nozzles 634 may be angled more than 30° offset from the longitudinal axis 668.

[0078] Similarly, there may be more or fewer than three (3) first nozzles 634. For instance, there may be between 0 and 15 first nozzles 634. More particularly, according to some embodiments, the number of first nozzles 634 may be within a range having lower and upper limits including any of 0, 1, 2, 3, 4, 5, 7, 10, 12, 15, and any values therebetween. For instance, there may be between 2 and 10 first nozzles 634, between 3 and 7 first nozzles 634, or between 5 and 12 first nozzles 634. In other embodiments, there may be more than 15 first nozzles 634.

[0079] One or more second nozzles 636 may also be provided, and in some embodiments the second nozzles 636 may be used to form a fluid barrier around at least a portion of the mill 616. In FIG. 6, for instance, six (6) second nozzles 636 (2 are visible) may be formed, with one in each of the junk channels 664. Each of the one or more second nozzles 636 may be configured to jet drilling fluid therefrom and create a fluid or hydraulic barrier obstructing the flow or passage of debris and cuttings within the corresponding junk channel 664. As a result, the barriers may create re-circulation zones to re-circulate debris and cuttings back to the cutting elements 628-1 on the face of the mill 616.

[0080] The number, location, shape, and configuration of the second nozzles 636 may be varied based on a variety of factors, including the number, size, and shape of the blades 626 or junk channels 664, the size of the mill 616, the type of downhole tool being milled, the materials within debris produced by the downhole tool being milled, and the like. Thus, while the illustrated embodiment depicts six (6) second nozzles 636 having a flat or elongated cross-sectional shape, positioned within the junk channels 664 and along the gauge region of the mill 616 rather than at the face of the mill 616, and oriented to be at about a 30° angle relative to the longitudinal axis 668, each of these features are merely illustrative. In other embodiments, for instance, the second nozzles 636 may circular, triangular, square, hexagonal, or other cross-sectional shapes. Moreover, the second nozzles 636 may be located on a blade 626, on the face of the mill 616, in a stem of the mill 616, off the mill 616 entirely, in other locations, or in any combination of the foregoing.

[0081] In embodiments in which the second nozzles 636 are flat or elongated, the extent of the elongation may also be varied. In some embodiments, for instance, the ratio of the width of the second nozzles 636 to the height of the second nozzles 636 may be between 2: 1 and 20: 1. More particularly, according to some embodiments, the ratio of the width to the height of the second nozzles 636 may be within a range having lower and upper limits including any of 2: 1, 2.5: 1, 3: 1, 5: 1, 6.25: 1, 8: 1, 10: 1, 12.5: 1, 15: 1, 20: 1, and any values therebetween. For instance, the second nozzles 636 may have a width-to-height ratio that is between 2: 1 and 3: 1. As an example, the second nozzles 636 may have a width of 0.28 inch (7.1 mm) and height of 0.1 inch (2.5 mm), such that the width-to-height ratio may be 2.8: 1. In other example embodiments, the width-to-height ratio of the second nozzles 636 may be between 2.5: 1 and 8: 1. As an example, the second nozzles 636 may have a width of 0.50 inch (12.5 mm) and height of 0.08 inch (2.0 mm), such that the width-to-height ratio may be 6.25: 1. In still another example, the second nozzles 636 may have a width-to-height ratio between 5: 1 and 15: 1 (e.g., a width of 0.50 inch (12.5 mm) and a height of 0.05 inch (1.3 mm) or 0.04 inch (1.0 mm) producing ratios of 10: 1 and 12.5: 1, respectively. In other embodiments, the second nozzles 636 may have a width-to-height ratio between 2.5: 1 and 3.5: 1, while in further embodiments the second nozzles 636 may have a width-to-height ratio greater than 20: 1 or less than 2: 1. The second nozzles 636 may also be elongated in an opposite direction, with the height greater than the width. In at least some embodiments, the ratio of the width-to-height is configured to minimize or mitigate the Coanda effect. In particular, when a second nozzle 636 is sufficiently thin, the flow jetting therefrom may attach to a nearby surface such as the body of mill 616, and remain attached even when the surface curves away from the initial direction of the second nozzle 636. The width of the second nozzles 636 may therefore be sufficiently large to allow mitigate the Coanda effect and allow fluid to get from the second nozzle 636 to create an area of turbulence as a re -circulation zone.

[0082] The second nozzles 636 may further be oriented at any of various angles relative to the longitudinal axis 668. For instance, the angle between the longitudinal axis 668 and the second nozzles 636 (or the drilling fluid jetted from the second nozzles 634) may range between 15° and 120° in some embodiments. More particularly, according to some embodiments, the angle between the longitudinal axis 668 and the second nozzles 634 may be within a range having lower and upper limits including any of 15°, 20°, 25°, 30°, 35°, 45°, 60°, 75°, 90°, 105°, 120°, and any angles therebetween. For instance, the second nozzles 634 may jet drilling fluid along a path oriented at angle that is between 20° and 40°, between 25° and 35°, between 15° and 60°, between 20° and 90°, or between 60° and 120° relative to the longitudinal axis 668. In other embodiments, the second nozzles 634 may be angled more than 120° or less than 15° from the longitudinal axis 668.

[0083] Additionally, there may be more or fewer than six (6) second nozzles 636. For instance, there may be between 0 and 20 second nozzles 636. More particularly, according to some embodiments, the number of second nozzles 636 may be within a range having lower and upper limits including any of 0, 2, 4, 5, 6, 7, 8, 10, 12, 15, 20, and any values therebetween. For instance, there may be between 2 and 12 second nozzles 636, between 4 and 8 second nozzles 636, between 0 and 8 second nozzles 636, or between 6 and 20 second nozzles 636. In other embodiments, there may be more than 20 second nozzles 636. [0084] FIG. 7 illustrates an example embodiment in which a mill 716 may include various nozzles and regions for re-milling debris. In the particular embodiment illustrated in FIG. 7, the mill 716 is shown as having six (6) blades 726. Junk slots or channels may also be defined in the regions between adjacent ones of the blades 726. The number of blades 726 may be varied in other embodiments. For instance, the number of blades 726 (and corresponding number of junk slots) may be varied in a manner similar to the blades 626 discussed herein with reference to FIG. 6. In other embodiments, the number of blades 726 may be varied.

[0085] The mill 716 may be used in a system that promotes re-circulation of debris and cuttings to the face of the mill 716. For instance, the mill 716 may include one or more first nozzles 734, and one or more second nozzles 736. In at least some embodiments, the one or more first nozzles 734 may be used to jet drilling fluid and cool cutting elements 728-1, including those of the cutting elements 728-1 located at the face of the mill 716. The fluid exiting the first nozzles 724 may also be used to move debris and cuttings off the face of the mill 716. In contrast, the one or more second nozzles 736 may be used to jet drilling fluid and form a shroud, curtain, or other barrier restricting uphole movement of cuttings and debris. The second nozzles 736 may be selectively activated to create the barrier. The number, location, shape, orientation, and configuration of the first and second nozzles 734, 736 may be varied based on a variety of factors, as discussed herein with respect to FIG. 6.

[0086] As discussed herein, in some embodiments, the mill 716 may be used in a system that promotes re-circulation of cuttings and debris to the face of the mill 716 (e.g., proximate the first nozzles 734). In the same or other embodiments, however, the mill 716 may be used to re-mill at least some of the cuttings and debris away from the face of the mill 716. For instance, the mill 716 may include secondary cutting features configured for re-milling of debris and cuttings, rather than milling of a plug or other component or tool.

[0087] More particularly, FIG. 7 illustrates an example embodiment in which the mill may include re- milling features 770, 772. In operation, the mill 716 may rotate and the cutting elements 728-1 may be used to initially mill a plug or other tool or component. The cutting elements 728-1 may therefore be located on a front face or side of a blade 726 so as to cut, shear, or otherwise mill the plug or tool. In such operation, the rear face or side of a blade 726 may not engage the plug or tool being milled, and may therefore not include cutting elements 728-1. In some embodiments, however, the rear face or side of the blade 726 may include a re-milling feature 770.

[0088] The re-milling feature 770 may be a feature that is not primarily used for initial milling of a tool, plug, or the like, but which nonetheless may include cutting elements or components. In FIG. 7, for instance, the re-milling feature 770 may include a recess in the rear side or surface of the blade 726 (e.g., opposite a front or cutting face, edge, or surface). The recess may be used to mount cutting elements (e.g., crushed carbide, diamond particles, polycrystalline diamond compacts, etc.). As re-circulation of cuttings and debris occurs (e.g., through use of a debris conditioning system, barrier, or the like), the cuttings and debris may be trapped between wellbore casing or walls and the cutting elements of the re-milling feature 770. The re-milling feature 770 may then re-mill such cuttings and debris to reduce the size or change the shape thereof to facilitate efficient solids transport of the cuttings and debris within the wellbore.

[0089] The re-milling feature 770 may be located on the blades 726 and on or near the face of the mill 716. In the same or other embodiments, however, the mill 716 may include a re-milling feature 772 at a gauge portion of the mill 716. Gauge protection elements 728-2 may be located at the gauge portion of the mill 716, and optionally on extensions of the blades 726. In some embodiments, the re-milling feature 772 may be adjacent or proximate the gauge protection elements 728-2. In FIG. 7, for instance, the re-milling feature 772 may extend along a portion of the front face of the blades 726 proximate the gauge protection elements 728-2. The re-milling feature 772 may also be formed as a recess in the blades 726 or may otherwise facilitate the use of cutting elements (e.g., crushed carbide, diamond particles, polycrystalline diamond compacts, etc.) within the re-milling feature 772. As re-circulation of cuttings and debris occurs in a system using the mill 716, the cuttings and debris may be trapped between the wellbore casing or walls and the cutting elements of the re -milling feature 772. The re-milling feature 772 may then re-mill such cuttings and debris to reduce the size or change the shape thereof to facilitate efficient solids transport of the cuttings and debris within the wellbore.

[0090] In some embodiments, the re-milling feature 772 may be fully or partially located on a rear face of a blade 726. Moreover, while the re-milling features 770, 772 are illustrated as recesses extending along a face or edge of a blade 726, they may have any number of locations, shapes, orientations, or other configurations. For instance, the re-milling features 770, 772 may be located in a junk channel or slot, on an outer surface between front and rear faces of a blade 726, as a radial protrusion from a blade, or the like. Additionally, rather than extending longitudinally along an edge of a blade 726, the re-milling features 770, 772 may be shaped as circular, oval, elliptical, square, triangular, or other recesses, patches, protrusions, or the like.

[0091] Referring now to FIG. 8, a downhole milling system 800 is shown in accordance with some additional embodiments of the present disclosure. The downhole milling system 800 may include a milling bit such as mill 816 configured for use in milling or otherwise grinding a component or tool (e.g., a plug 810 set within a wellbore). In at least some embodiments, the plug 810 may include a bridge plug. As discussed herein, the plug 810 may be formed of one or more materials and, in some embodiments, may provide a hydraulic seal between an upper portion of the wellbore and a lower portion of the wellbore The plug 810 may be formed of various materials, including metals (e.g., ferrous and non-ferrous metals), alloys, rubber or other elastomers, composite materials, other materials, or combinations of the foregoing.

[0092] The mill 816 may be inserted into a wellbore and moved downhole toward, and into engagement with, the plug 810. In at least some embodiments, the wellbore may have a casing 806 lining the inner surface of the wellbore, and the mill 816 may be inserted through the casing 806. The mill 816 may include a bit body 824 having one or more blades, knives, or other cutting structure thereon. These blades or other cutting structures may (alone or in combination with additional cutting elements) be configured to grind, mill, degrade, or otherwise break-up the plug 810. There may be multiple blades circumferentially spaced around the bit body 824 of the mill 816, and any number of blades 826 may be provided as discussed herein.

[0093] In accordance with some embodiments of the present disclosure, the blades and any corresponding cutting elements may be part of a debris conditioning system 830 of the mill 816. The debris conditioning system 830 may be used to initially mill or grind the plug 810 into cuttings, and to re- grind or further mill the cuttings to have a size, shape, or other configuration that can be efficiently transported to the surface within an annulus 832 between the casing 806 and the mill 816, drill string, and BHA. For instance, drilling fluid flowing uphole within the annulus 832 may provide a solids transport mechanism for carrying the cuttings to the surface.

[0094] In operation, drilling fluid may flow through the downhole milling system 800 and into an interior of the bit body 824 of the mill 816. The bit body 824 may define one or more ports, nozzles, or jets through which drilling fluid may exit the mill 816. For instance, the bit body 824 may include a first nozzle 834 which in the illustrated embodiment may convey drilling fluid from the interior of the bit body 824 to a location near the face of the mill 816. Drilling fluid flowing through the first nozzle 834 may be used to cool and/or clean blades or cutting elements of the mill 816, and may exit and be jetted from the bit body 824 with sufficient velocity to evacuate cuttings from the face of the mill 816. Two first nozzles 834 are shown in FIG. 8; however, one skilled in the art should appreciate in view of the disclosure herein that 1, 2, 3, 4, 5, or more first nozzles 834 may be defined by or otherwise included in the bit body 824 and/or the mill 816.

[0095] One or more second nozzles 836 may also be defined by or otherwise included in or coupled to the bit body 824 of the mill 816. In the embodiment shown in FIG. 8, the second nozzles 836 may cause drilling fluid to exit or be jetted from the mill body 824 at a location that is uphole of the first nozzles 834. In some embodiments, the fluid may exit or be jetted from the mill body 824 at an angle Θ that is between 10° and 135° from a longitudinal axis 868 of the mill 816. For instance, the angle Θ may be within a range having lower and/or upper limits that include any of 10°, 20°, 25°, 30°, 35°, 45°, 60°, 75°, 90°, 105°, 120°, 135°, and any angles therebetween. For instance, the second nozzles 834 may jet drilling fluid along a path oriented at angle Θ that is between 25° and 65°, between 30° and 50°, between 37° and 45°, or between 75° and 120° relative to the longitudinal axis 868. In other embodiments, the second nozzles 834 may be angled more than 135° or less than 10° from the longitudinal axis 868.

[0096] As further discussed in relation to other embodiments herein, the second nozzles 836 may have any suitable cross-sectional shape, and may be circular, square, rectangular, flat, elongated, or have other suitable shapes of forms. For instance, in some embodiments, the second nozzles 836 may be elongated, and a ratio of the width of the second nozzles 836 to the height of the second nozzles 836 may be between 2: 1 and 20: 1. More particularly, according to some embodiments, the ratio of the width to the height of the second nozzles 836 may be within a range having lower and/or upper limits including any of 2: 1, 2.5: 1, 3: 1, 4: 1, 5: 1, 6: 1, 8: 1, 10: 1, 12: 1, 15: 1, 20: 1, and any values therebetween. For instance, the second nozzles 836 may have a width-to-height ratio that is between 2: 1 and 4: 1. In other example embodiments, the width-to-height ratio of the second nozzles 836 may be between 3.1 : 1 and 3.5: 1. In still another example, the second nozzles 836 may have a width-to-height ratio between 4: 1 and 12: 1. In other embodiments, the second nozzles 836 may have a width-to-height ratio greater than 20: 1 or less than 2: 1. The second nozzles 836 may also be elongated in an opposite direction, with the height greater than the width, may have a width-to-height ratio of 1: 1, may be symmetrical, or may be asymmetrical.

[0097] In at least some embodiments, the second nozzles 836 may be included as part of the debris conditioning system 830. For instance, as discussed herein, the second nozzles 836 may be used to form a fluid shroud, curtain, or other barrier to restrict, and potentially prevent, cuttings or debris above a predetermined size from moving uphole past the fluid or hydraulic barrier 840 and toward the surface. In some embodiments, the fluid jetted by the nozzles 836 may act as a hydraulic stabilizer or centralizer in addition to, or instead of, acting as a hydraulic barrier 840. According to some embodiments, the fluid jetted by the nozzles 836 may act as a hydraulic stabilizer that centers the mill 816 within the wellbore, and reduces whirl of the mill 816 when compared to a mill lacking the second nozzles 836. By reducing whirl, potentially damaging forces acting on the mill 816 may be avoided to prolong the life of the mill 816.

[0098] In some embodiments, the debris conditioning system 830 may include additional or other components, including expandable pads, secondary attrition systems, or other features. With our without additional components, the hydraulic barrier 840 between the outer surfaces of the mill 816 and the inner surface of the casing 806 may include an area of turbulence in the annulus, and which may promote recirculation of cuttings or debris within an area below the gauge of the mill 816. In FIG. 8, the mill 816 has a gauge diameter at the position of the mill 816 between lines 817-1 and 817-2. For instance, line 817-1 may be a downhole end of a gauge pad of the mill 816, and line 817-2 may be an uphole end of the gauge pad of the mill 816 (or location of a last gauge cutter). Re-circulation of the cuttings or debris may cause such cuttings or debris to be cut and potentially re-cut by blades or other features of the mill 816 below gauge line 817-1 and/or gauge line 817-2. In some embodiments, the second nozzles 836 may be between gauge lines 817-1, 817-2; however, in other embodiments, the second nozzles 836 may be downhole of the first gauge line 817-1, or uphole of the second gauge line 817-2.

[0099] In some embodiments, re-circulating the cuttings or debris may increase the amount of time cuttings or debris remain below the gauge (line 817-1 and/or line 817-2) of the mill 816 and/or the distance the cuttings or debris travel while below the gauge of the mill 816. The longer the time below the gauge and/or the larger the distance of travel, the increased likelihood the cuttings or debris have of being cut, milled, or ground (and re -cut, re-milled, and re-ground) to smaller and smaller sizes. Re-milling or re- grinding of the cuttings may produce smaller or finer cuttings, or cuttings of a more desirable shape, thereby promoting efficient solids transport within the wellbore.

[0100] In some embodiments, the mill 816 may be designed to optimize the time cuttings spend below the gauge of the mill and/or some other reference location (e.g., line 819). For instance, the line 819 may be positioned uphole or downhole of the gauge lines 817-1, 817-2, and may be above or below the exit of the fluid from the second nozzles 836. According to some embodiments, the second line 819 may be on a plane between 0.5 inch (1.3 cm) and 10 inches (25.4 cm) above the second gauge line 817-2. For instance, the second line 819 may be on a plane between 1 inch (2.5 cm) and 3 inches (7.6 cm), or between 1.5 inches (3.8 cm) and 5 inches (12.7 cm) above the second gauge line 817-2. In other embodiments, the second line 819 may be more than 10 inches (25.4 cm) or less than 0.5 inch (1.3 cm) above the second gauge line 817-2.

[0101] The number, location, angle, and other configurations of the second nozzles 836 may be varied to act as control jets that produce desired qualities in the hydraulic barrier 840. For instance, 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 15, or 80 or more second nozzles 836, or any number therebetween, may be defined or included in the bit body 824 and the mill 816. Including more second nozzles 836 may, in some embodiments, reduce the distance between fluid jets forming the hydraulic barrier 840. Forming the second nozzles 836 at an angle that is non-perpendicular to the longitudinal axis of the mill 816 or the wellbore may allow re-circulation patterns to change (e.g., downhole directed second nozzles 836 may, in some embodiments, push cuttings downhole more efficiently). The second nozzles 836 may be moved axially in uphole or downhole directions on the mill body 824. Further, the second nozzles 836, and consequently the hydraulic barrier 840, may also be moved to be on-bit or off-bit. When on-bit, as shown in FIG. 8, the second nozzles 836 may extend through the bit body 824. In other embodiments, a collar, circulation sub, or other component located uphole of the mill 816 may include the second nozzles 836. Changing the number, orientation, shape, or position of second nozzles 836 and the number of first nozzles 834 may each affect re-circulation, including time of cuttings and debris spent below a reference line (referred to as "residence time") and length of travel while below a reference line (referred to as "residence length").

[0102] When designing a mill for degrading a plug or other mill, a design process may include optimizing the mill by considering various factors, including the rate-of-penetration of the mill, the longevity or durability of the mill, the stability of the mill, the size of cuttings produced, and the like. In some embodiments, the amount of re-circulation of cuttings (e.g. , residence time and/or residence length) may be considered when designing and potentially optimizing a mill.

[0103] FIG. 9, for instance, illustrates a user interface 970 showing multiple plots 972-1 to 972-4. The plots 971-1 to 972-4 may reflect the residence time of cuttings below a reference line/plane for a particular mill design (e.g., number of first and/or second nozzles, number of mill blades, shape of mill blades, number and position of cutting elements, axial position of first and/or second nozzles, shape of nozzles, etc.) and different materials (e.g., four different materials, one for each of the plots 972-1 to 972-4) for different angles of the second nozzles. Plot 972-1, for instance, reflects the cutting residence time for cuttings of a first material at a particular rotational speed of the mill (e.g., 1500 rpm) for different drilling fluid flow rates (i.e., 105 GPM (6.6 L/s), 132 GPM (8.3 L/s), and 168 GPM (10.6 L/s)). As shown in the plot 972-1, the jet or nozzle angle may vary between 30° and 75°. In this particular embodiment, jet or nozzle angles between about 35° and 45° are shown as providing elevated cuttings residence times, with a maximum residence time of about 1.2 seconds at a nozzle angle of about 40°. [0104] Plot 972-2 shows the cuttings residence time for a second material. In this particular embodiment, nozzle angles between about 35° and 45° again show the largest, or elevated, residence times, with a maximum residence time of just over 2.1 seconds at a nozzle angle of about 37.5°. Plot 972-3 shows similar information for a third material, and again shows the largest cuttings residence times at nozzle angles between 35° and 45°, with a maximum residence time of about 1.1 seconds at a nozzle angle of about 40°. Plot 972-4 shows cutting residence time for a fourth material, with largest residence times again at nozzle angles between 35° and 45°, with a maximum residence time of about 0.6 second between nozzle angles of about 37.5° and about 40°. In each of plots 972-1 to 972-4, the lowest flow rate tended to provide the greatest cuttings residence time.

[0105] The different cuttings materials represented in plots 972-1 to 972-4 may include different materials that may be found in a plug or other component that is milled in accordance with embodiments of the present disclosure. For instance, a bridge plug may be formed of multiple materials (e.g., cast iron, aluminum, composite, rubber, etc.). In some embodiments, plot 972-1 may represent cast iron, plot 972-2 may represent aluminum, plot 972-3 may represent composite materials, and plot 972-4 may represent rubber or another elastomer. The plots 972-1 to 972-4 may also be varied if other materials are used, or if other parameters are used (e.g., flow rate, fluid type, nozzle shape, nozzle position, nozzle numbers, rotational speed, etc.). For instance, in some embodiments, higher rotational speeds may have increased cuttings residence time than lower rotational speeds, while in other embodiments higher rotational speeds may show decreased cuttings residence time relative to lower rotational speeds. In at least some embodiments, relative lower flow rates, higher rotational speeds, and higher weight cuttings may result in longer cutting residence times (and potentially lengths as discussed in more detail relative to FIG. 10). Residence times and/or lengths of larger cuttings may also be greater than for smaller size cuttings.

[0106] FIG. 10 illustrates another user interface 1070 showing multiple plots 1072-1 to 1072-4 of the residence length of cuttings (e.g., below a gauge or other reference line) for the same configuration and mill design used for the user interface 970 of FIG. 9, and for different angles of the second nozzles. Plot 1072-1, for instance, reflects the cutting residence length for cuttings of a first material at a particular rotational speed for various different drilling fluid flow rates. As shown in the plot 1072-1, the jet or nozzle angle may be varied between 30° and 75°. In some embodiments, jet or nozzle angles between about 35° and 50° are shown as providing elevated cuttings residence length, with a maximum of about 185 inches at a nozzle angle of about 40°.

[0107] Plot 1072-2 shows the cuttings residence length for a second material. In this particular embodiment, nozzle angles between about 35° and 45° show the largest cuttings residence length, with a maximum cuttings residence length of about 225 inches at a nozzle angle of about 37.5°. Plot 1072-3 shows similar information for a third material, and again shows the largest cuttings residence lengths at nozzle angles between 35° and 45°, with a maximum cuttings residence length of about 95 inches at a nozzle angle of about 40°. Plot 1072-4 shows cutting residence lengths for a fourth material, with largest residence lengths again at nozzle angles between 35° and 45°, with a maximum residence length of about 48 inches at a nozzle angle of about 37.5°. In each of plots 1072-1 to 1072-4, the lowest flow rate tended to give the greatest cuttings residence length.

[0108] In the design of a mill, a computational fluid dynamics, finite element analysis, or other analysis or simulation may be performed. Such analyses may provide data or plots similar to those illustrated in FIGS. 9 and 10. Using an iterative design, various features of a mill or other bit, including nozzle shape, nozzle size, nozzle position, nozzle number, nozzle angle, blade number, blade shape and configuration, fluid type, fluid flow rate, mill rotational speed, and the like may be analyzed. Using an iterative process, the mill may be modified or otherwise re-configured to optimize the mill or other bit design. For instance, the mill or other bit design may be modified based on data showing elevated cuttings residence time and/or length at particular nozzle angles. With reference to FIGS. 9 and 10, for example, a mill design may be selected which includes second nozzles at an angle between 30° and 60°, between 30° and 50°, between 32.5° and 47.5°, between 35° and 45°, or between 35° and 42.5°. In other embodiments, the angle may be less than 30° or greater than 60°.

[0109] As discussed herein, one aspect of a downhole milling system including a barrier (e.g., fluid or hydraulic barrier, mechanical barrier, etc.) may the ability to create re-circulation zones by which debris and cuttings may be re -circulated to the face of a mill for re-grinding and re-milling. In some embodiments, a secondary or additional system may be provided to re-grind and re-mill debris and cuttings away from the face of the mill. The secondary or additional system may operate in addition to, or in place of, a re-circulation system or barrier.

[0110] Regardless of whether a barrier promotes re -circulation, a secondary attrition system re-grinds and re-mills cuttings, and debris, or whether a combination of the foregoing is used, the debris and cuttings may be milled and ground to a size sufficiently fine to be conveyed to the surface in a solid transport system facilitated by drilling fluid within a wellbore. In accordance with at least some embodiments of the present disclosure, the downhole milling system may be configured to mill and grind debris and cuttings to produce a maximum cutting size between about 1/8 inch (3 mm) and about 1 inch (25 mm). For instance, debris originally produced may have a width up to about 3 inches (76 mm) and a length up to 10 inches (354 mm). By re-circulating the debris and/or by using a secondary attrition system, the width and length of the debris may be reduced. For instance, in some embodiments, the debris described above may be reduced to have a maximum width and/or length of 1 inch (25 mm), 1/2 inch (13 mm), 1/4 inch (6 mm), or less. In other embodiments, the maximum size of debris produced by the downhole system may be less than 1/8 inch (3 mm) or greater than 1 inch (25 mm).

[0111] Various assemblies, systems, tools, or other components may operate together to condition downhole cuttings and debris, regardless of the particular size of cuttings and debris the downhole milling system is configured to produce. A person having ordinary skill in the art should appreciate, with the benefit of the present disclosure, that example components may include, without limitation, hydraulic or fluid barriers to re -circulate cuttings and debris to the face of the mill, in-line or other filtering systems to screen cuttings and debris, and secondary attrition systems for re-grinding and re-milling cuttings in one or more stages. Each of these systems to restrict and potentially prevent larger debris (e.g., elastomers, rubber, etc.) and cuttings from getting into the annulus of a wellbore

[0112] In some embodiments, nozzles, jets, ports, or the like may be located in or above a bit to create a fluid curtain or other barrier to restrict movement of cuttings and debris from a milled downhole tool from moving above the bit by re-circulating them to the face of the bit. Fluid may create a barrier that reduces the annular space between the bit and the casing to restrict or even prevent larger cuttings/debris from getting past this region into the annulus, leading to re-grinding of cuttings to smaller sizes of more desirable shape for efficient solids transport. In some embodiments, a nozzle, port, or other similar feature may exit closer to face of the mill to cool cutters and with sufficient velocity to evacuate cuttings from the face of the mill.

[0113] Optional in-line or other filtering systems may be located above the mill to restrict and potentially prevent larger debris exiting the mill from moving uphole. In some embodiments, the filtering system may be provided as an alternative to a fluid or mechanical barrier, or as a redundant system. Similarly, a secondary attrition system may be an alternative to a fluid or mechanical barrier or a filtering system, or redundant to the fluid or mechanical barrier and/or the filtering system.

[0114] While some embodiments relate to a circulation system where drilling fluid may be provided through a drill string, out of the bit, and uphole with the cuttings in the annulus of the wellbore, other embodiments may be used in connection with reverse circulation systems. For instance, a baffle may be provided to direct drilling fluid in the annulus to the mill face. Cuttings and debris may then flow into the bit or milling system from the annulus and in an uphole direction. The same or a different baffle may also re-direct the flow within the bit or milling system to the annulus and uphole to the surface.

[0115] In view of the disclosure herein, one skilled in the art will appreciate that methods of milling may include generating debris by using a mill having one or more cutting elements. For instance, the debris may be generated by rotating a mill and applying weight to the mill while engaging a component to be milled (e.g., a downhole tool). During milling, a re-circulation zone may be created to promote recirculation of debris to the one or more cutting elements. The debris may then be conditioned by re- milling the debris following, and due to, re-circulation of the debris back to the one or more cutting elements.

[0116] In some embodiments, creating a re-circulation zone may include creating a hydraulic barrier using one or more nozzles. Fluid jetting through the one or more nozzles may create a curtain extending at least partially around the mill. For instance, the curtain may be created within junk channels of the mill. Re-circulating the debris may also include re-circulating debris larger than a predetermined or threshold size to the cutting elements. In at least some embodiments, conditioning the debris may include using the mill to re-mill the debris using the one or more cutting elements. In the same or other embodiments, conditioning the debris may include moving the debris to a secondary attrition system axially offset from the mill to re-mill the debris. Debris may also be filtered after re-circulation of the debris to the cutting elements. To generate debris, filter debris, or use a secondary attrition system, a drive system may rotate the mill. Optionally, the drive system does not rotate a sleeve of the secondary attrition system while causing the mill to rotate.

[0117] In at least some aspects, embodiments of downhole milling systems described herein may be used to reduce the time to complete a plug or other milling operation. Such reduction may occur as a result of reducing size of the cuttings and debris generated, so that multiple short trips may not be made (or so the number of short trips may be reduced) while continuing to effectively clean the wellbore from debris. The system may also operate in environments (e.g., coiled tubing) in which flow rate limitations may limit efficient solid transport of larger cuttings to the surface.

[0118] In the description herein, various relational terms are provided to facilitate an understanding of various aspects of some embodiments of the present disclosure. Relational terms such as "bottom," "below," "top," "above," "back," "front," "left," "right," "rear," "forward," "up," "down," "horizontal," "vertical," "clockwise," "counterclockwise," "upper," "lower," "uphole," "downhole," and the like, may be used to describe various components, including their operation and/or illustrated position relative to one or more other components. Relational terms do not indicate a particular orientation for each embodiment within the scope of the description or claims. For example, a component of a BHA that is described as "below" another component may be further from the surface while within a vertical wellbore, but may have a different orientation during assembly, when removed from the wellbore, or in a deviated borehole. Accordingly, relational descriptions are intended solely for convenience in facilitating reference to various components, but such relational aspects may be reversed, flipped, rotated, moved in space, placed in a diagonal orientation or position, placed horizontally or vertically, or similarly modified. Certain descriptions or designations of components as "first," "second," "third," and the like may also be used to differentiate between identical components or between components which are similar in use, structure, or operation. Such language is not intended to limit a component to a singular designation. As such, a component referenced in the specification as the "first" component may be the same or different than a component that is referenced in the claims as a "first" component.

[0119] Furthermore, while the description or claims may refer to "an additional" or "other" element, feature, aspect, component, or the like, it does not preclude there being a single element, or more than one, of the additional element. Where the claims or description refer to "a" or "an" element, such reference is not be construed that there is just one of that element, but is instead to be inclusive of other components and understood as "at least one" of the element. It is to be understood that where the specification states that a component, feature, structure, function, or characteristic "may," "might," "can," or "could" be included, that particular component, feature, structure, or characteristic is provided in some embodiments, but is optional for other embodiments of the present disclosure. The terms "couple," "coupled," "connect," "connection," "connected," "in connection with," and "connecting" refer to "in direct connection with," or "in connection with via one or more intermediate elements or members." Components that are "integral" or "integrally" formed include components made from the same piece of material, or sets of materials, such as by being commonly molded or cast from the same material, or commonly machined from the same piece of material stock. Components that are "integral" should also be understood to be "coupled" together.

[0120] Although various example embodiments have been described in detail herein, those skilled in the art will readily appreciate in view of the present disclosure that many modifications are possible in the example embodiments without materially departing from the present disclosure. Accordingly, any such modifications are intended to be included in the scope of this disclosure. Likewise, while the disclosure herein contains many specifics, these specifics should not be construed as limiting the scope of the disclosure or of any of the appended claims, but merely as providing information pertinent to one or more specific embodiments that may fall within the scope of the disclosure and the appended claims. Any described features from the various embodiments disclosed may be employed in combination.

[0121] A person having ordinary skill in the art should realize in view of the present disclosure that equivalent constructions do not depart from the spirit and scope of the present disclosure, and that various changes, substitutions, and alterations may be made to embodiments disclosed herein without departing from the spirit and scope of the present disclosure. Equivalent constructions, including functional "means- plus-function" clauses are intended to cover the structures described herein as performing the recited function, including both structural equivalents that operate in the same manner, and equivalent structures that provide the same function. It is the express intention of the applicant not to invoke means-plus- function or other functional claiming for any claim except for those in which the words 'means for' appear together with an associated function. Each addition, deletion, and modification to the embodiments that falls within the meaning and scope of the claims is to be embraced by the claims.

[0122] While embodiments disclosed herein may be used in oil, gas, or other hydrocarbon exploration or production environments, such environments are merely illustrative. Systems, tools, assemblies, methods, milling systems, and other components of the present disclosure, or which would be appreciated in view of the disclosure herein, may be used in other applications and environments. In other embodiments, milling tools, hydraulic or fluid barriers, methods of milling, or other embodiments discussed herein, or which would be appreciated in view of the disclosure herein, may be used outside of a downhole environment, including in connection with other systems, including within automotive, aquatic, aerospace, hydroelectric, manufacturing, other industries, or even in other downhole environments. The terms "well," "wellbore," "borehole," and the like are therefore also not intended to limit embodiments of the present disclosure to a particular industry. A wellbore or borehole may, for instance, be used for oil and gas production and exploration, water production and exploration, mining, utility line placement, or myriad other applications.

[0123] Certain embodiments and features may have been described using a set of numerical values that may provide lower and upper limits. It should be appreciated that ranges including the combination of any two values are contemplated unless otherwise indicated, and that a particular value may be defined by a range having the same lower and upper limit. Numbers, percentages, ratios, measurements, or other values stated herein are intended to include the stated value as well as other values that are about or approximately the stated value, as would be appreciated by one of ordinary skill in the art encompassed by embodiments of the present disclosure. A stated value should therefore be interpreted broadly enough to encompass values that are at least close enough to the stated value to perform a desired function or achieve a desired result. The stated values include at least experimental error and variations that would be expected by a person having ordinary skill in the art, as well as the variation to be expected in a suitable manufacturing or production process. A value that is about or approximately the stated value and is therefore encompassed by the stated value may further include values that are within 5%, within 1%, within 0.1%, or within 0.01 % of a stated value.

[0124] The Abstract included with this disclosure is provided to allow the reader to quickly ascertain the general nature of some embodiments of the present disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.