Window Mill for Sidetracking

Description

This application claims the benefit of U.S. Provisional Application No. 63/661,829, filed Jun. 19, 2024, which is incorporated herein by reference for all purposes.

FIELD OF INVENTION

This disclosure relates to mills for cutting a window in a casing in a wellbore.

BACKGROUND

In the oil and gas industry, “sidetracking” refers to drilling a deviated borehole that branches off an existing subterranean wellbore at a pre-planned angle depth below the surface. The deviated borehole is commonly referred to as a lateral or a sidetrack, depending on its purpose. The deviated borehole may be used, for example, to bypass an obstruction (fish) in the existing borehole, to explore an adjacent section of an oil and gas field, or to develop more economically an oil and gas field using multiple wellbores drilled from a single, preexisting wellbore. In an open hole (uncased) wellbore, the bit may directly access the surrounding formation for a directional drilling to create lateral wellbore. However, a wellbore is often cased with steel pipe. The steel pipe in the wellbore is referred to a “wellbore casing” or, simply, “casing.” Casing is typically formed by connecting pipe joints into a long string, a “casing string,” that is lowered into the wellbore. The casing may be secured and supported within a wellbore by cement pumped between the outer diameter of the casing and the borehole's sidewall. Although there are several ways to remove casing to gain access to the surrounding formation, one of two options are typically employed when sidetracking: (i) removing section of the to expose the formation; or (ii) cutting an opening or “window” in the side of the casing for a drill string to exit the wellbore while trying to maintain the integrity of the original casing.

The first option requires lowering a section mill to remove an entire section or length of casing and expose the formation. The removable of the section is followed by lowering directional drilling assembly, such as a bent-sub and a mud-motor assembly, to drill a bore hole in the exposed formation.

The second option, in which a window is cut, involves lowering a whipstock into the existing wellbore on the end of a drill string to a depth where the new branch will be started. The whipstock is used to deflect laterally a drill string lowered into a wellbore. In its simplest form, the whipstock has a surface slanted with respect to the axis of the wellbore. Its surface has groove or semicircular depression to support rotation of the drill string. After being positioned and oriented in the right direction, the whipstock is anchored to the wellbore to avoid being pushed downwardly as it is deflecting the drill string. Different anchoring methods are used depending on the circumstances. For example, if the whipstock is lowered to the bottom of the vertical wellbore, the whipstock may be anchored in place using cement that is pumped through the drill string, past the base of the whipstock, to form a plug. Alternatively, a whipstock is anchored mechanically. Anchors can take different forms and use different methods for holding the whipstock in place. For example, they may function like a packer. They may use slips that are extended outwardly to grab the casing. They may also incorporate or support other downhole tools and systems. The anchor can be set in place before the whipstock is lowered. However, it is generally preferred to lower the anchor and whipstock—a “whipstock assembly”—together as part of a sidetracking bottom hole assembly (BHA) in a single trip to avoid having to make multiple trips. Additional downhole operations such as, for example, cleaning or scraping the casing before the anchor is to be set helps to ensure that the inner diameter is clean enough for the anchor to set properly. The cleaning or scraping operation can also be performed before setting the anchor by incorporating those components into the bottom hole assembly.

The milling assembly, which includes one or more mills or other types of milling tools, on the end of a drill string are used to create the window. Cutting a window can require multiple trips. However, milling assemblies will typically include multiple milling tools. The whipstock deflects the milling assembly toward the casing as the drill string is rotated. Many different types, combinations, and configurations of window mills and other components have been used and are possible. The configuration of a milling assembly will depend on the requirements of the job, such as the size of the window that is needed, which in turn depends on the tools that will need to be passed through it, the build angle of the sidetrack, and other considerations and requirements that may be unique to the well. For example, a milling assembly may include a lead mill or bit that is used to initiate the cutting of the window followed by one or more mills above the lead mill that enlarge the window and finish cutting the window.

“One-trip” or “single-trip” sidetracking systems make use of bottom hole assemblies with tools to position and anchor a whipstock, create full size window in casing, and drill a full-size rathole without tripping the drill string out. “Drill-head” sidetracking has a BHA with a directional drilling unit and a cutting head that functions as both a mill lead mill and a drill bit to cut the window and drill a borehole through the adjacent formation. A “drill ahead” mill may include, for example, arrangements of PDC cutters placed on the mill that are capable of both milling the casing and drilling through a formation.

Many different shapes of window mills are known and have been used to cut windows in casing for sidetracking. Although a lead window mill may superficially share some similarities with drag bits used for earth boring, their design and use must contend with a different set constraints and problems. To start with, the mechanical properties of steel and rock are different. While it is rotating, the window mill must be pushed down to be deflected laterally by the whipstock against the wall of the casing. The orientation of the axis of rotation of the mill relative to the casing will be neither normal nor parallel to its surface, and it will change as it is moved down the whipstock and material is removed. Furthermore, the radius of curvature of the inside diameter of the casing is larger than the radius of the window mill. This mismatch can cause torsional and lateral vibrations until enough of an opening is milled to stabilize the rotating mill.

Window mills have employed many types of cutting elements in various configurations to mill steel casing. For example, it is common to “dress” the bodies of mills, including blades, using crushed tungsten carbide (pieces of tungsten carbide in different shapes and sizes) embedded in a matrix with random orientations.

However, it is more difficult to mill higher grade (harder) steel in casing pipe. Longer sidetracking windows are also increasingly common. To counter these trends, window mills have begun to dress mills at least in part using cylindrical inserts referred to as “cutters” that have mostly flat faces and are made of an ultra-hard material, such as carbide (e.g. tungsten carbide) cutters or PDC cutters. Cylindrical cutters will typically have a circular cross-section along most, but not necessarily all, of their central axes and a cutting face that is mostly or generally flat in contrast to, for example one that is domed, spherical, or conical, or shaped like or button or chisel. Around the perimeter of the cutting face is a circular cutting edge. The cutting face can, however, not be perfectly flat. It can have relative variations or contours. Furthermore, part of or all the cutting edge can be beveled or chamfered or cut to form a flat edge.

In the oil and gas industry, “carbide” refers a cemented metallic carbide, and a cylindrical cutter made from carbide is referred to as a carbide cutter or carbide insert cutter. The cemented metallic carbide is made using a powdered metallurgical process. A metallic carbide powders such as tungsten carbide is sintered under high pressure and temperature by infiltrating a liquid binder such as a cobalt alloy into the powder to create a composite matrix comprised of carbide grains and metal binder. Carbide is very hard with excellent wear resistance. Tungsten carbide is the most common carbide that is used.

“PDC cutters” have a polycrystalline diamond compact (PDC), which is sintered polycrystalline diamond (PCD), bonded to a carbide substrate. The PCD layer functions as the cutting face, which is the portion of the cutter that will have most of the contact with material being milled and the resulting swarf. Although the layer of PCD is harder and more wear resistant than carbide, it has the drawbacks of being more brittle and less stable at high temperatures.

These ultra-hard cylindrical cutters are mounted in fixed positions on the body of the mill and oriented to cut the casing using a shearing or scraping action. The cylindrical cutters are usually mounted by brazing or otherwise securing them within pockets formed along the leading edge of a raised surface feature on the mill's body called a “blade.” The blades are arrayed around the central axis of the body and define between channels called “junk slots” that are used to evacuate cuttings. Many different shapes and arrangements of blades and junk slots are possible. Most blades extend longitudinally along the body, in the general direction of the central axis of the mill. The blades can be straight, but they are usually helical. In a lead mill, one of more of the blades will start on the nose of the body and radially outwardly across its nose, shoulder, and gauge. The cutters can be removed, rotated, and remounted between jobs to prolong their useful life.

The mounted cutters are mounted with their cutting faces “back raked.” Back raking orients the cutting face of the cutter at an angle to the path of rotation of the cutter's cutting face. This allows its cutting edge to be pushed down into material for its cutting face to scrape away material using a shearing action. The ideal result from this cutting action a well-formed track in the material. The track helps to stabilize the cutters. Compared to drill bits, the cutters on sidetracking or mills are usually mounted with a less aggressive back rake, which limits their rate of penetration. Less aggressive back raking also limits their exposure, which is distance to which they extend above the surface on the body (which may include blades) of the mill on which they are mounted. The surface of the body of the mill effectively limits the depth to which the cutter can penetrate. A low depth of penetration means less steel is being removed on each revolution. However, steel is less forgiving than rock and the rate of penetration is not as important as when drilling a borehole though rock. Furthermore, too much penetration can slow milling and increase wear on the cutters. It can also apply too much torque to the drill string, which can cause the mill to become unstable and cause damage to the cutters.

The circular cutting faces of the cutters along the body of a mill when rotated through a plane extending through its central axis collectively define a mill's “cutting profile.” Because the pocket geometry of the pocket requires a minimum web thickness between the cutter pockets, cutters that are adjacent along the axis of the cutting profile are typically mounted on different blades. Adjacent cutting faces on the cutting profile also overlap to some degree along the axis of the body when rotated through the plane. The total number of cutters that can be mounted on a window mill is limited by the size of the window—it must fit through the window)—and the geometry of the mill's body, the number of blades that can be accommodated on the mill's body, and the desired exposure of the cutters. With a lower exposure, the cutters must be more densely packed along at least parts of profile, which means that adjacent cutters must overlap more.

FIG. 3 represents a cutting profile 300 for a representative example prior art lead mill that shows cutting faces 302 of cutters placed on the body or head of the mill. The cutting faces collectively define a cutting profile that extends along the tips of the cutting faces. The radial position of the cutter in the cutting profile is where a line perpendicular to the centerline or center axis 304 extends the tip of the cutter where it touches the profile. In this example, all the cutters placed on the leading edge of the blades function as primary cutters for window milling. Thus, the figure represents the primary cutting profile of the lead mill for milling a window. Additional cutters can be mounted on blades that are not intended to cut when the primary cutters are cutting. These additional cutters are typically located directly behind the primary cutters on the cutting profile, but not always. Mounting cutters that are not part of the primary cutting profile might be done for several different reasons or purposes. For example, these cutters might be backup cutters, which engage when a primary cutter fails, or part of secondary cutting profile intended to be used later. Lead mills can have different shapes or geometries depending on several considerations, including the whipstock profile (the inclination along its length) and many others. In this example, there are cylindrical cutters the mill's face 306, nose 308, taper 310, shoulder 312, and gauge 314. Cutters 316 are examples of underreaming cutters. Not every window mill, such as side-cutting follow mills and dress mills, has all the same or similar sections. Dashed line 318 represents the nominal gauge of the mill, which is outer diameter of the mill at the farthest point from the centerline. Solid line 320 represents the inner diameter of the casing pipe. Lines 322 represent the area of initial contact of the mill against the inside surface or inner diameter of the casing at the start of the milling process, after the lead mill has been disconnected from the whipstock, rotation of the milling assembly has begun, and the milling assembled lowered under the weight of the drill string to cause it to be deflected by the top portion of the in inclined ramp of the whipstock toward the inside surface of the casing. In this example of a lead mill, the cutters on the gauge portion 314 are the first to engage the casing. As can be seen in the figure, the highest density of cutters is in the taper section, with lower densities in on the gauge 314, shoulder, and nose 308 portions. The lowest density is on the face 302. The density reflects, at least to some extent, the blade area available on the body of the mill in the region for mounting cutters.

SUMMARY

Although carbide and PDC cutters are very good at cutting, they are relatively brittle. As cylindrical cutters on a window mill are first brought into contact with casing in the wellbore by a whipstock pushing the window mill against it experience during milling high magnitude lateral and torsional vibrations. These vibrations cause substantial frontal impact damage to the cutters, sometimes even before they can begin to cut the casing. Although carbide and PDC are very hard, they are also relatively brittle and those more prone to damage from impacts as compared to, for example, metal. Impact damage from such vibrations tends to occur early in the process of milling the window, though it may also occur at other times during the milling process, where the mill initially contacts the casing. Because carbide is tougher than PDC and better able to withstand impact forces, carbide cutters are commonly used where the mill first initiates contact the casing. However, carbide cutters still suffer significant impact damage.

Once the window mill has removed a sufficient length of the window, the cylindrical cutters will stabilize and begin milling the casing using their scraping cutting action. However, impact damage to the cutters will have reduced the milling efficiency of the window mill, making it less likely for the milling assembly to complete a full cutout and high-quality window before having to be raised to the surface and replaced. A second trip to complete the window is costly. So-called “button carbide stabilizers,” which are wear resistant and less prone to impact damage but do not cut, have been used to help stabilize the cutting structures on mills to reduce impact damage. However, they take up space on blades of the mill and can prevent the milling cutters from being exposed preventing the cutting action, which reduces milling efficiency.

Disclosed below are representative, nonlimiting examples of embodiments of window mills for sidetracking with cutting structures that address the problem of impact damage caused by lateral and torsional vibration at least during initiation of the window milling process by doing one or more of the following: increasing cutting ability of a window mill, reducing the amount impact damage, or reducing the effect of impact damage on milling efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a representative, nonlimiting example of a sidetracking bottom hole assembly.

FIG. 2A is a schematic illustration of a representative, nonlimiting example of a sidetracking bottom hole assembly on drill string being lowered into a cased wellbore.

FIG. 2B shows the representative example of a sidetracking bottom hole assembly of FIG. 2A after its whipstock has been anchored, the milling assembly detached from the whipstock, and the commencement of window milling.

FIG. 2C shows the representative, nonlimiting example of a sidetracking bottom hole assembly of FIGS. 2A AND 2B after an opening has been milled in the wellbore's casing.

FIG. 3 is a representative example of prior art cutting profile for a lead mill for a sidetracking assembly.

FIG. 4A is a view of one side of a representative example of a lead mill for a sidetracking assembly.

FIG. 4B is a view of the opposite side of the representative example of a lead mill for a sidetracking assembly illustrated in FIG. 4A.

FIG. 5A is a first perspective view of the representative example of the lead mill shown in FIGS. 4A and 4B.

FIG. 5B is a second perspective view of the representative example of the lead mill shown in FIGS. 4A and 4B.

FIG. 6 is an end view of the representative example of a lead mill for a sidetracking assembly shown in FIGS. 4A, 4B, 5A, and 5B.

FIG. 7 is a detail from FIG. 5B of the representative, nonlimiting example of a lead mill for a sidetracking assembly of FIGS. 4A, 4B, 5A, and 5B.

FIG. 8 is a first representative example of cutting profile for a lead mill for a sidetracking assembly.

FIG. 9 is a cutting profile for a second representative, nonlimiting example of cutting profile for a lead mill for a sidetracking assembly with non-cylindrical carbide inserts on its nose.

FIG. 10A is a detail of protectors and rectangular carbide inserts on the lead mill shown in FIG. 6.

FIG. 10B illustrates the detail of FIG. 9 after the protectors are worn.

FIG. 11 is an alternative embodiment of a representative example of a lead mill with a wear pad across brazed into a pocket on a blade that extends front-to-back across the blade.

FIG. 12 shows the body of the lead mill of FIG. 11 without the wear pad or cutters inserted into pockets.

FIG. 13 illustrates a representative example of the wear pad shown in FIG. 11.

FIG. 14 is an alternative embodiment of a representative example of a lead mill with a replaceable wear pad across mechanically connected to a blade behind cutters brazed into pockets on the blade using a representative example of a mechanical connection.

FIG. 15 is a detailed view of the lead mill of FIG. 14.

FIG. 16 is a is a detailed view of the lead mill of FIG. 15 with the wear pad removed to show part of a representative nonlimiting mechanical connection.

FIG. 17 shows the wear pad of FIGS. 14 and 15.

FIG. 18 illustrates a representative example of a follow mill for a sidetracking assembly.

FIG. 19 illustrates another representative example of a lead mill for a sidetracking assembly with representative examples of a contoured carbide cutting insert.

FIG. 20 is an end view of the lead mill of FIG. 19.

FIG. 21 illustrates the representative example of a contoured carbide cutting insert shown in FIG. 19.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Like numbers refer to like parts in the following description. The disclosed mills and the arrangement of cutters and other elements on the body of the mills are representative, nonlimiting examples of mills adapted for cutting windows in casing unless stated otherwise. Any representative structure, act, or embodiment is nonlimiting, even if not specifically stated. The terms “casing” and “wellbore casing” refer to steel pipe that cases a wellbore (with or without cement supporting it) unless explicitly stated otherwise.

FIG. 1 illustrates schematically a typical single-trip sidetracking bottom hole assembly (BHA) to provide context for the disclosure. For this reason, the FIGS. 1 to 3 are labelled “prior art.” It includes representative, non-limiting examples of a whipstock assembly connected with a milling assembly. The BHA and assemblies illustrated in FIGS. 1 and 2A-2C omit details not germane to the following disclosure for clarity. A directional drilling unit or assembly may also be incorporated into the BHA above or, alternatively, form part of the milling assembly to provide “drill ahead” capability. Other downhole tools and assemblies are commonly found sidetracking BHAs but have been omitted to allow focus on those features more relevant to the disclosure. FIGS. 2A-2C schematically illustrate a method of using the sidetracking bottom hole to cut a window in a cased wellbore and form a rat hole. FIGS. 1 and 2A-2C are not-to-scale.

The bottom hole assembly (BHA) 100 on the end of drill string 101 comprises a whipstock assembly 102 that releasably connected to a milling assembly 104 at connection point 107. The whipstock assembly includes at least a whipstock 108. The whipstock assembly commonly includes an anchor 110 and other tools. The method of anchoring a whipstock within the wellbore and, thus, the type of anchor used, will depend on several considerations.

The milling assembly includes one or more mills, referred to generally as window mills, for cutting an opening and, optionally, enlarging the opening. One mill, which might be referred to as a “lead mill,” “mill head,” “cutting head,” or “bit” in the context of milling a window for sidetracking, is configured or adapted to initiate cutting and, optionally, cut most if not the entire window in the casing starting from an inside the casing. One or more additional mills can be used to enlarge the opening cut by the lead mill and/or to dress (smooth and finish) the opening. The dimensions of the window must be large enough to allow a drill string with a directional drilling BHA to exit the existing wellbore and drill a borehole for the planned lateral through the adjacent formation. In any given sidetracking operation, the configuration of the milling assembly is determined, in part, by the size of the window that needs to be cut, the type and size of casing, and other requirements.

The representative example of milling assembly 104 illustrated in FIGS. 1 and 2A-2B includes a lead mill 106, or cutting head, and multiple side-cutting follow mills. The shape of the lead mill and the arrangement of cutting elements on its cutting face and sides are configured to mill the casing to cut through and then extend the opening longitudinally. The lead mill may optionally be configured to drill also through rock. Such lead mill might be referred as, for example, a “mill/drill bit” or similar designation. One or more side-cutting follow mills can be included in the milling assembly grind material from edges around the opening in the casing cut by the lead mill for purposes of enlarging it and/or smoothing and finishing it. In the illustrated example, the milling assembly 104 includes a first side-cutting follow mill 118. The illustrated example of the first side-cutting follow mill is sometimes referred to as “watermelon” mill because of its shape. In this example, the first side-cutting follow mill enlarges the opening as the lead mill extends it. A second side-cutting follow mill 120 smooths and/or finishes the edges of the opening. The milling assembly may have more mills or just the lead mill. The BHA, including the milling assembly 104, may incorporate “flexible” or “bendable” pipe joints 122 between one or more of the mills and/or between other tools in the BHA, to allow for a greater amount of bending or flexing than a typical drill pipe allows for without being damaged. The lead mill 106 and one or more side-cutting follow mills may, alternatively, be formed as a single, unitary body or sub.

The BHA 100 may include additional tools, which are represented by block 114, which is between the milling assembly 104 and the drill string 101, and block 112, which is below anchor 110. Examples of possible additional tools are stems, stabilizers, drill collars, motors, directional-drilling units, scrapers, cleaners, and centralizers. These additional tools and components may also be placed between one or more of the tools or components shown in the figure or elsewhere in the BHA, including below the whipstock assembly and between mills. However, one or more of these tools may, optionally, be incorporated into the milling or whipstock assemblies.

FIGS. 2A-2C illustrate the BHA 100 at different stages of cutting a window in casing of a wellbore before the BHA is converted downhole to a directional drilling mode. Though the method is shown and described with reference to the BHA 100, the method described below can be practiced with a different BHA that is specially configured or adapted to perform, or otherwise capable of performing, the steps of the method.

In FIG. 2A BHA 100 is being lowered on an end of a drill string (not shown) into an existing wellbore cased with casing 200, which is made of steel. Although not shown, the casing can be cemented by filling with cement an annulus between the outer diameter of the casing 200 and the side wall of the borehole (not indicated.) Once the whipstock assembly 102 is lowered to a kick-off point for the planned lateral, it is rotated to face the direction of the planned lateral by rotating the drill string and then anchored by setting the anchor 110. Once anchored, the BHA can be separated at connection point 107 to begin the milling, FIG. 2B illustrates the BHA after separation. The drill string is lowered to cause the lead mill 106 to be deflected or pushed sideways toward the inner diameter or surface of the casing 200. Rotating the milling assembly, either from the surface by rotating the drill string 101 or operating a motor in the BHA, initiates cutting an opening for window 204 in the casing. As the lead mill is rotated, downward force of the drilling pushes the lead mill against the ramp on the whipstock, which then deflects the lead mill further into the sidewall of the casing, creating the opening.

To elongate the opening to create the window, the drill string continues to be lowered and rotated to cut the opening for window 204. The side-cutting follow mill 118 enlarges the opening cut by the lead mill. The lead and follow mills are also being pushed beyond the outer diameter of the casing to begin to form a “rat hole” 206. Eventually, the whipstock pushes the lead mill beyond the outer diameter of the casing to allow the milling assembly to extend the rat hole. The second follow mill or dress mill 120 can be used to further enlarge and/or “dress” the opening. The disclosure below is not, however, limited to this example. Other milling assembly arrangements and milling tools are possible. Many different configurations are possible for the lead mill and follow mills to cut windows in casing.

Once the window is milled, the milling assembly is raised by tripping out the drill string and a directional drilling BHA is attached to the drill string and lowered into the wellbore. The whipstock 108 guides the directional drilling BHA and drill string toward and through the window. However, as mentioned, the BHA may optionally include a directional drilling unit or assembly to allow the drill string to cut the window and to continue by drilling a borehole through the formation in a single trip down the wellbore.

In the following description, starting with FIG. 4A and continuing through FIG. 21, the following terms should, unless plainly contrary to the context, are intended have the meanings given to them below. A “cutter” is intended to be a reference to a cutter with base made of an ultrahard material such as cemented carbide and a face made of sintered polycrystalline diamond, usually referred to as polycrystalline diamond compact (PDC) cutter. A PDC cutter is often, but does not have to be, cylindrical in cross-section. It usually has, but does not have to have, a flat cutting face. A cutter may also refer to carbide insert that is a generally cylindrical cross-section. In this specification, a reference to a “carbide insert,” “carbide cutting insert,” “insert,” or simply “insert” is intended to refer to a non-cylindrical insert (with a non-circular cross-section) made of a cemented carbide, such as but not limited to tungsten carbide—or similar ultrahard composite. It does not have a sintered polycrystalline diamond or natural diamond cutting face. A reference to a “window mill” or “sidetracking mill is intended to include a lead mill, follow mill, dress mill, or other type of mill used for milling a window in casing for sidetracking. A reference a lead mill, follow mill, or dress mill as a “window mill” implies that it is an example of a window mill.

FIGS. 4A, 4B, 5A, 5B, 6, 7, 8, 9, 10A, 10B, 11, 13, 14, 15, 16, 17, 18, 19, 20, and 21 illustrate representative, nonlimiting embodiments of window mills 400, 1100, 1400, 1800, and 1900. Mounted to their respective bodies 402, 1102, 1402, 1802, and 1902, are a plurality of cylindrical cutters 404 mounted on a plurality of blades 406. The cylindrical cutters 404 are, in these examples, mounted along leading edges of each of the blades with a back raked orientation by brazing them into pockets formed in the blades. This type of mounting is common, but not required, for ultrahard cutters on window mills. The cutters can be mounted in other orientations and in other ways. Though not shown, a lead mill may include cutters and cutting structures mounted on the blades or possibly in other places, which do not function as primary cutters for milling. Although not indicated, parts of any window mill prone to excessive wear may have hard facing or have wear features such as carbide insert buttons, or a crush carbide matrix can be added.

Each of the blades 406 extend longitudinally generally along each mill's central axis, about which it rotates. At least some of the blades partially wrap around the body in a helical fashion. Between each of the blades, extending from the front edge of each blade where the cutters are mounted, toward the back of an adjacent blade, are junk slots 407.

Although the window mills in the illustrated example are in many respects similar, the use of the same reference numbers to reference each of the cylindrical cutters does not imply that the cutters on a mill are identical, or that cutters on different mills are the same or identical. Similarly, the use of the same reference number for blades, sections or portions, and other similar aspects of the representative examples window mill do not imply that they are identical. The different examples and embodiments cutting structures on the similar mills with similar body geometries, blade geometries, and cutter placements, does not imply that the cutting structure is limited to similar mills with the shared characteristics. The cutting structures disclosed and described below are not limited to a window mill having a particular cutter size or layout, number of cylindrical cutters, orientation (rake angle), method of mounting, blade count or blade geometry, or body geometry except as might be explicitly stated.

Window mills 400, 1100, 1400, and 1900 are examples of lead mills. Their respective bodies 402, 1102, 1402, and 1902 may also be referred to as mill “heads.” They each have a pin connection 408 at end for connecting the mill with the milling assembly and a “face” 410 at the opposite end. The longitudinal or central axis of the mills extend through the center of the connection (such as an API connection) and a center of the face. These examples of lead mills have a single whipstock connection point 412 for connecting the mill to a whipstock. In this example, connection is made using a bolt is held in place inside the window mill by a pin 413, the head of which can be seen in FIG. 5B. However, other types of window mills do not need to be capable of being connected with a whipstock, and many types of whipstock connection methods are possible. This example also has a hydraulic connection point 414 for connecting a hydraulic line to supply pressurized fluid from the drill string to a tool downhole of the mill, such as a hydraulically actuated anchor below the whipstock or other tool.

Window mill 1800 is a representative example of a follow mill that is integrated into a sub 1804 with an API connection 1806 at one end. Although not shown, it may have an API connection at the other end, or it is otherwise connected or joined with another milling assembly components. The follow mill's central axis and axis of rotation is coincident with that of the sub 1804.

Window mill 400, which is shown in FIGS. 4A, 4B, 5A, 5B, 6, 7, 8, 9, 10A and 10B, has mounted on at least one of the blades 406 a representative examples of cutting structures that include one or more non-cylindrical carbide cutting inserts 416. In this representative example, there four carbide inserts 416 mounted on the gauge portion of the mill near the end of each of four blades. Cylindrical cutters—carbide cutters, preferably—are mounted on the remaining blades along the gauge. The non-cylindrical carbide inserts are mounted where cutting elements on the window mill will be subjected to the brunt of the high impact forces that are generated by lateral or torsional vibrations during initial contact of the window mill with the casing. For window mill 400, which is a lead mill, the point area of initial contact is its gauge 418. Because of their geometry, mounting one or more noncylindrical carbide inserts 416 provides more cutting edge on a blade than cylindrical cutters 404 mounted in the same position. The noncylindrical carbide inserts 416 are, in cross-section, elongated so that they have cutting edge on the blade. In other words, its width from side-to-side (or length from end-to-end) is greater than its back-to-front dimension where it is inserted into a pocket on a blade.

When mounted with its elongated dimensions extending generally in the longitudinal direction of the blade, it can present a larger cutting face and longer cutting edge as compared to cylindrical cutters. It thus allows a greater volume of carbide and a more cutting edge along part of the mill's cutting profile than would be otherwise possible using cylindrical cutters in the same positions. Although noncylindrical carbide insert might have drawbacks as compared to a cylindrical cutter, a non-cylindrical insert, such as one with a rectangular cross-section, will give more cutting edge per blade, resulting in better durability of the mill and reduced wear while completing the window as compared to cylindrical cutters.

In an embodiment shown in FIGS. 4A to 10B, each of the carbide inserts 416 have rectangular cross section starting with its base (which cannot be seen) and extending to its top surface 419. Although it can be square, the longer sides of the insert are generally oriented in the direction in which the blade extends, along the longitudinal axis of the mill. Part of its front surface is slanted backwardly to create a back-raked cutting face 420 when mounted within a complementary-shaped pocket on the blade. Its cutting edge 422 extends longitudinally along the long side of the insert, where its top surface 419 meets the cutting face 420. When mounted, its cutting face is, like the cutting faces of the cylindrical cutters 404, oriented generally in a direction in which the window mill rotates to mill casing, with its cutting edge also generally perpendicular to the direction in which the window mill rotates to mill casing. With its elongated axis extending longitudinally along the blade, it presents a much wider cutting face and cutting edge than a cylindrical cylinder, as can be seen in the cutting profile 800 shown in FIG. 8. Because of its flat sides, two or more of them can other each when mounted on the blade. They can be brazed in the same pocket, thus avoiding webbing between them to support the individual inserts as would be required with cylindrical cutters having a circular cross-section. This can be seen in the various views, such as FIG. 7, in which they are mounted in adjacent pairs. Furthermore, multiple pairs are mounted adjacent to each other, without webbing between them, with one pair laterally offset from the other due to the sweep of the blade.

Referring briefly only to FIG. 8, which shows the cutting profile 800 for this window mill, the centerline 802 of the mill is indicated by a dashed line. The centerline corresponds to its axis of rotation and longitudinal axis. Solid line 808 represents the inner diameter or inside surface of casing. The area of initial contact 806 of window mill 400 includes its gauge cutters 812. In this example, the radial position of the non-cylindrical carbide cutting inserts 416 overlap with gauge cutters 812 and at least partly overlaps with underreaming cutters 814 and shoulder cutters 816. Alternatively, or in addition, non-cylindrical carbide cutters may, for example, be placed in other areas of the mill, such as the nose area. As can be seen by comparing cutting profile 800 to the cutting profile for a similar window mill without carbide inserts, the densities of cylindrical gauge cutters 812, cylindrical underreamer cutters 814, and cylindrical shoulder cutters in profile 800 are lower due the cutters being replaced by carbide inserts on some of the blades. However, the density of shoulder taper cutters 818, nose cutters 820, and face cutters 822 engage later, in that order, after enough of the window is milled to stabilize the cutters enough to avoid excessive vibrations.

The dashed lines 804 indicates exposure of cylindrical cutters 404 on the gauge portion, the cylindrical gauge cutters 812 of the mill. The carbide inserts 416 in this example are mounted with an exposure above the exposure of cylindrical gauge cutters 812. The carbide inserts will therefore engage the casing before the cylindrical cutters, allowing them to take more of initial impact forces. Alternatively, the carbide inserts can have the same as or below the exposure of the cylindrical cutters 404 with which they overlap on the cutting profile.

Turning back to the FIGS. 4A to 10B, window mill 400 also includes non-cutting structures referred to for purposes of this disclosure as protectors 424. The protectors 424 are positioned on the window mill 400 where the mill first engages an unmilled surface of the casing as it is being pushed toward and against the casing by the whipstock. Each of the protectors made from a relatively softer material compared to the cutting elements, such as a ferrous or non-ferrous metal, a composite material, or an abrasive material in a soft matrix that can wear away to expose leading inserts or cutters. The protectors are mounted with an exposure indicated by dashed line 600 in FIGS. 6, 10A, and 10B, which is just beyond the outer cutting gauge 1000 (FIG. 10B) of the mill. Exposure of the protector is determined by the surface of the protector that extends furthest along a line normal with the centerline 802 (FIG. 8) of the mill. The exposures of the noncylindrical carbide inserts 416 are along an outer cutting gauge in this example.

The protectors take on the initial impact load at the onset of the milling process, to which cutters would otherwise be subjected to, without significantly impairing the cutting ability of the mill. Once rotation beings, they can wear progressively and rapidly during the initial part of the window milling until the carbide inserts begin to be exposed. After the mill is stabilized, the carbide inserts will wear down more gradually because the protectors limit exposure of the inserts, which means a smaller depth of cut for the carbide insert. A smaller depth of cut means lower cutting forces and, in turn, less damage to the carbide inserts.

The protectors may be placed behind the carbide insert on the same blade or in the shadow of carbide insert, meaning in approximately the same position on the mill's cutting profile as the carbide insert but on a different blade or, if on the same blade, not immediately adjacent to it. Though doing so provides some potential advantages, the protectors do not have to be mounted in the immediate shadow or necessarily on the same blade.

Using protectors with a rectangular cross-section reduces the amount of area required for mounting them when paired with a rectangularly shaped carbide insert. The front of the protector can be flush with the back of the carbide insert. However, different shapes are possible. Furthermore, the protectors have a contoured top surface 426. In this case, it curves from front to back, as can be best seen in FIGS. 7 and 10A. Curving the top surface reduces the total area contacting the casing and may encourage smoother initial contact, resulting in less vibration, and allow it to wear more quickly. However, other contours and non-rectangular geometries are possible for the protectors.

On representative window mill 400, a single protector is brazed in same cavity or pocket as a pair carbide insert carbide. However, the protectors can be brazed into a pocket or cavity separate from the carbide insert. A protector may, instead, be mechanically attached to the mill body using some sort of fastener, for example, to make replacement between runs easier. Furthermore, there is no requirement as to the number of carbide inserts per protector; there could be just one or more than two.

Not all the carbide inserts 416 need to have or can have a protector in their immediate shadow. There is no requirement that each carbide insert 416 have one. Several of the carbide inserts on window mill 400 do not have a protector in the immediate shadow. This is at least in part due to the lack of space on the blade for one. Furthermore, the protectors may be, optionally, entirely omitted, but doing could result in greater impact damage to the carbide inserts.

Although not shown in FIGS. 4A to 8B, carbide inserts 416 may also be used in in place of some of the cylindrical cutters—or possibly all of them—in other portions of the cutting profile that are also subject to high impact forces and/or where more cutting capacity may be needed. For example, a portion of the mill with a relatively small amount of area for mounting cutters may suffer from a greater amount of wear. Excessive wear reduces cutting capability and can lead to excessive wear on the body of the mill. In the example of cutting profile 900 in FIG. 9, which is the same as cutting profile 800 of FIG. 8, they can be used in place of at least some of the cutters in the nose portion of the mill, which have some of the highest steel removal rates of all the cutters typically and thus suffer more wear. Although this example also includes carbide inserts on the gauge to mitigate the effects of impact damage, other window mills may use carbide inserts 416 only where needed to augment cutting capacity and/or to avoid and address concerns over excessive wear.

Turning now to FIGS. 11-13, window mill 1100 is a representative example of an alternative embodiment of a window mill without carbide inserts. It has a non-cutting protector 1104 on the gauge of the body 1102 of window mill 1100, where the mill first engages an unmilled surface of the casing as it is being pushed toward and against the casing by the whipstock. In window mills of other types or geometries, the point of initial contact could be other than the gauge. More generally, it can be positioned on the window mill wherever its cutters are subjected to high magnitude impact forces caused by vibration.

Like the protectors 424 on window mill 400, protector 1104 is made from lower strength and lower abrasion resistant material as compared to cutters 404. Its exposure is slightly above the outer cutting gauge of the cylindrical cutters 404 that it protects. This allows the protector to take on the initial impact load at the onset of the milling process, to which the protected cylindrical cutters 404 would otherwise be subjected to, without significantly impairing the cutting ability of the mill. The protector will wear rapidly once rotation is turned on, during the initial part of the window milling, until the cylindrical cutters 404 begin to be exposed. After the mill is stabilized, the cylindrical cutters wear down gradually because the protectors limit exposure of the cutters, which means a smaller depth of cut. A smaller depth of cut means lower cutting forces and in turn, less damage to the cutter.

Each of the protectors 1104 in this representative example extends along a front edge of a blade and extend to the near the back edge of the blade. The top surface 1106 of the protector is shaped front to back to generally follow the gauge of the mill. Similarly, the top surface of the protector follows, in this example, the cutting profile of the cutters. However, in the alternative, surface contours or shapes are possible. With a broader top surface and placing several around the periphery of the mill in the same area of the cutting profile of the mill, the protectors 1104 will protect the cylindrical cutters 404 on other blades on the same area of the mill's profile, especially the ones on adjacent blades. Therefore, cylindrical cutters are preferable, but not required in all cases, to be placed on every other blade. Cutters could be, for example, placed on ever fewer blades, but this may result in cutter density that is insufficient. However, some blade and mill geometries may allow for more than one blade with cutters to be placed between the protectors 1104.

In this example, each protector 1404 has a generally rectangular shape. A cut out 1108 accommodates the widening of the blade as extends from the shoulder portion of the mill onto the gauge portion of the mills. Furthermore, it has a tapered end 1110 to follow the cutter of the cutters 404 at the very end of each blade that are used for underreaming. Although in this embodiment it covers most of the blade from back to front is advantageous, protector 1104 could alternatively be made in different shapes and with different coverages.

Turning only to FIG. 12, which shows the body 1102 of the window mill 1100 without any of the cylindrical cutters 404 mounted in cutter pockets 1200 or protectors 1104 mounted in protector pockets 1202. The protectors 1104 shown in FIG. 11 would otherwise be brazed into the protector pockets 1202.

Another representative embodiment of a protector is shown on window mill 1400 in FIGS. 14 to 17, which is lead mill like the window mill 1100. In this embodiment, at least one protector 1404 is mechanically fastened or attached to one of the blades 406 on the body 1402 of the window mill 1400, It is positioned on the body in an area along the profile of the mill where cutters 404 are subjected to impact damage from vibrations. In this example, it is positioned on the gauge portion of the window mill, which is the initial area of contact of the mill with the casing when it begins milling. The protector 1404 is made from a material that is softer and less wear resistant than the cylindrical cutters 404. Its exposure is slightly beyond the outer cutting gauge of the mill. It functions like protector 1104. It wears quickly upon initial contact. The cylindrical cutters are then exposed and wear down gradually because the protectors limit exposure of the cutters. The limited exposure means a smaller depth of cut. A smaller depth of cut means lower cutting forces and in turn, less damage to the cutter. Also, like protector 1104, the shape of the top surface 1406 of the protector 1404 generally conforms or follow the shape of the outer cutting gauge of the cutters 404 along that area of the body 1402 and to the cutting profile of the mill long its longitudinal axis.

Protector 1404 differs from protector 1104 in a couple of ways. First, the protector is mechanically fastened or attached to the body 1402 using a mechanical fastener 1408. It is not brazed into a pocket. This can make it easier to replace a won protector between runs. Second, the protector 1404 is mounted on a rear portion of a blade 406, behind cylindrical cutters 404 on the front edge of the blade. It does not cover the surface of the blade, front to back. It will tend to provide more protection for the cylindrical cutters in front of it on the same blade. But it will also provide some protection for the cutters that follow it on the next blade.

The example of a mechanical fastener 1408 used in this representative embodiment includes two parts. A first part 1410 is attached to cut out portion of on the rear of the blade; a second part 1412 is integrally formed as part of the protector 1404. Alternatively, the second part can be fabricated separately and joined with it. The two parts form, in this example, a sliding dovetail. The second part is in the form of a tail that slides into a socket 1414 formed in the first part. The tail and socket have complementary cross sections. The two parts are prevented from sliding during use by using one or threaded bolts 1416. Alternatively, or in addition, other fasteners, such as pins, or other means to lock them or prevent them from sliding can be used. Furthermore, the position of the respective parts can be reversed. Cross sectional shapes other than a dovetail could be substituted. Other types of fasteners could be substituted.

Note that FIG. 16 illustrates the body 1402 without any of the cylindrical cutters 404 mounted in cutter pockets 1418 or the protector 1404 joined to the blade body.

Referring now to FIG. 18, the window mill 1800 is a representative, nonlimiting example of a follow mill. Two carbide inserts 416 and a protector 424 are mounted on two of the blades 406 that can be seen in this view. The carbide inserts 416 and protector 424 are placed along the area of first contact of the follow mill with the casing in which a window is being cut, where cutters are most likely to receive impact damage. The carbide inserts and protectors are mounted and function in a manner like what is as described in connection with FIGS. 4A to 10.

Turning now to FIGS. 19-21, window mill 1900 is like window mill 400. Both are lead mills. Both have carbide inserts 416 and protectors 424 on the gauge areas of several of the blades 406. However, window mill 1900 also has contoured carbide cutting inserts 1904, which are an example of a carbide insert that can be used instead of, or in addition to, the carbide inserts 416, the combination of carbide inserts and protectors such those used on window mills 400 and 1800, or in place of cylindrical cutters and protectors such as those on window mills 1100 and 1400.

The contoured carbide cutting inserts 1904 have more mass than the cylindrical cutters and do not take up as much space on the blade. Each contoured carbide inserts 1904 has a rectangular cross section, with longer sides along its longitudinal axis. Its longitudinal axis generally extends in the same direction as the blade and the longitudinal axis of the mill. It has a front face 1906, a back (not visible), and side faces 1908. Its cutting edge 1914, where its top surface 1910 meets its front face 1906, is contoured, meaning not straight. Rather, in this example, it curves from side-to-side to define a tip portion 1912 of the cutting edge that can be exposed to engage and penetrate the surface of the casing initially. A contouring of the shape of the cutting edge enables more control on how the carbide insert engages the casing. For example, the carbide insert can be mounted to expose the tip portion 1912 initially so that it penetrates and engages first, before remaining portions of cutting edge engage.

Optionally, the top surface 1910 is curved front to back, as shown in this example.

The side-to-side and front-to-back contouring of the top of the cutter inserts 1904 do not follow, in this example, the curvature of the mill's gauge (around its centerline), the cutting profile (along its center line), or the cross-sectional geometry of the blade on which it is mounted. In alternative examples, the cutting edge 1914, which is defined by where the front face 1906 meets the top surface 1910, can be rounded, beveled, or chamfered, and the cutting insert may have a more complex cross-sectional shape. The top and front surfaces and the cutting edge may have other contours.

The rectangular cross section allows to be mounted with them abutting each other front-to-back and side-to-side and side when and be mounted in the same cavity or pocket formed on a blade. There is no back raked cutting face. Although pairs of two shown mounted adjacent to each on the blade, each pair having two carbide inserts 1904 placed front-to-back, one insert being in the shadow of the other, a contoured carbide cutting insert could, for example, be mounted without a second insert in its shadow.

Each of the carbide inserts 1904 is mounted so that its tip portion 1912 extends beyond the cutting profile of the cylindrical cutters 404 on the mill in positions that overlap with the contoured carbide cutting inserts 1904. Alternatively, one or more of them are mounted with their tips exposed beyond the cutting profile of the cylindrical cutters, while other contoured carbide cutting inserts 1904 can be mounted with lower exposures. Lower exposures include above, on, or below the cutting profile. The contoured carbide cutting inserts take the initial impact load and wear down more slowly than the previously described protectors, which are made of softer and less wear resistant material. Once worn down enough, the cylindrical cutters 404 are exposed and begin to engage and cut.

As an alternative, some or all the cylindrical cutters 404 in the embodiments and examples in the preceding description may, optionally, be replaced with a contoured cutting insert like contoured cutting inset 1904.

The foregoing description of examples and embodiments, including preferred embodiments, are unless otherwise noted representative and non-limiting examples of possible implementations, embodiments, and uses of inventive features for purposes of disclosing and explaining the principles of the claimed subject matter and how it can be put into practice.

The preceding description of embodiments, including any that might be “preferred,” are representative and nonlimiting examples of possible implementations, embodiments, and uses unless stated otherwise. They are disclosed for the purposes of explaining the claimed subject matter and its principles of operation so that skilled individuals in the field may understand and practice the claimed subject matter, including by adapting or modifying it to meet the requirements or limitations specific to their practice or implementation of it. Each disclosed embodiment or example may include multiple features, such as elements or sub-combinations of elements. Unless explicitly noted, nothing in the disclosure restricts or limits a feature from being practiced independently or in combination with other disclosed features. Modifications and substitutions of disclosed embodiments are, therefore, possible and should be expected when practicing the subject matter of a claim. None of the disclosed features are essential to the practice of claimed subject matter unless otherwise explicitly stated as being essential or required for the subject matter of a claim.

The meaning of the terms used in this specification are, unless stated otherwise, intended to have their ordinary and customary meaning to those in the relevant art. The meaning of a term is not intended to be limited to the specific structures or acts it is being used to identify or describe. A structure or act that is referenced by a common term for a class of structures or acts is intended only as a representative example for that structure or act.

Although the claims are to be interpreted in view of the specification, terms in claims are intended to have their ordinary and customary meaning to those skilled in the art without being limited to details of any described embodiments, examples, or function unless the term is explicitly defined except to extent reasonably necessary to understanding the claim and its scope The meaning of a term in a claim is not intended to be limited to the details of illustrated or described structures, acts, or processes referenced by the term unless explicitly indicated otherwise in the specification or defined. When a common term for a class of structures or acts, or a type of structure or act, is used to identify a specific element or act in a disclosed embodiment or example, the structure or act should be understood to be a representative, nonlimiting example or embodiment of the type or class of structures or acts understood to be referenced by the term. A statement made in reference to a named element, such as component, structure, act, function, or combination in one embodiment or example, or an illustration of it, does not by itself imply that that the statement or details of the illustration are also applicable to element referenced in to by the same term in a different embodiment or example unless expressly indicated otherwise.

References to geometries—such as perpendicular, normal, plane, radial, circular, square, and the like—or to other properties, including “predetermined” values, in a description of apparatus, structures, processes, and acts are to be understood to allow for variations or differences that are to be reasonably expected or encountered when making and using claimed subject matter. These variations may, by way of example and not limitation, result from manufacturing tolerances, natural variations in materials or environment, the precision and accuracy to which “predetermined” values can be reasonably determined or estimated prior to use, and variations that might occur during use.

For the avoidance of doubt, the terms mentioned below should be understood and interpreted to have the given meanings, with the understanding that the meaning given below does not exclude the common and ordinary meanings of the term unless it is explicitly stated otherwise or the given meaning creates a substantial ambiguity, in which case the additional meaning given below take precedence, but only to the extent necessary to resolve the ambiguity.

The use of the term “may” should be interpreted in its normal sense as expressing a possibility and not a requirement, even when not accompanied by the word “option” or “optionally.” No feature, aspect, or element is essential unless explicitly identified as such.

Each of the terms “couple,” “coupled,” “coupling,” “connect,” “connection,” “connected,” “in connection with,” and “connecting” encompasses a direct connection, an integral connection, and an indirect connection or coupling through one or more intermediate elements or members unless the accompanying text explicitly indicates otherwise.

The terms “comprise,” “have,” “include,” “contain,” and “involve,” and variations of them are open-ended linking verbs that signal a nonexclusive listing and thus permit the addition of other elements. When used in claims, the term “comprising” should be interpreted in the manner typically done in patents, which is “including but not limited to.” On the other hand, the phrase “consisting of,” when used in a claim, implies a closed set of elements.

When used in a claim, the phrase “consisting essentially of” excludes additional material elements but allows the inclusion of non-material elements. A material element substantively modifies, adds to, or subtracts from the functionality or nature of the subject matter recited in the claim.

If the specification states a component or feature “may,” “can,” “could,” “should,” “would,” “preferably,” “possibly,” “typically,” “optionally,” “for example,” “often,” or “might” (or other such language) be included or have a characteristic, that component or feature is not required to be included or to have the mentioned characteristic. Such a component, feature, or characteristic can be included or excluded.

A singular form of an element or components of an apparatus described herein is understood to include its plural form. Use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims or the specification means one or more than one unless context would otherwise make it indefinite. The term “or” in the claims is used to mean “and/or” unless the accompanying text explicitly indicates that it is referring only to alternatives or if the alternatives are by their nature mutually exclusive.

Terms like “up,” “uphole,” “upward,” “above,” and like terms when used in reference to a well, downhole tool, tool string, or the like should normally be interpreted as meaning towards the surface and away from the end of the well. Similarly, terms like “down,” “downward,” “downhole,” and “below” should be interpreted as meaning away from the surface and towards the bottom or end of a well.

A “work string,” for purposes of the disclosure, refers to tubing that can be used to lower, raise, and rotate downhole tools for downhole operations, including drilling, within a wellbore and to convey pressurized fluid from a pump at the surface to downhole tools. It is typically made of connected tubing joints but may also be coiled tubing.

Aspects of the embodiments and representative examples disclosed in preceding detailed description include, but are not limited to, the following: