Patent Application
20080093076
In one aspect, embodiments disclosed herein relate to mill assemblies that include a lead mill, a second mill, and a shank therebetween on which each of lead mill and second mill are attached. Embodiments disclosed herein may also relate to mill assemblies having a single mill attached to the end of a shank, and mill assemblies having a lead mill, one or more second mills, and a shank therebetween. Further, embodiments disclosed herein may also relate to methods of designing a mill assembly, and methods of milling a window in a tubular. As used herein, “second mill” refers to any type of mill, e.g., dress mill, watermelon mill, string mill, follow mill, etc., that may elongate and/or dress the window to full gage.
Referring now to
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As shown in
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The cutters 116, 136 attached to lead mill 110 and second mill 130, according to some embodiments disclosed herein, may include tungsten carbide particles and a metal binder. Typical types of tungsten carbide include cemented tungsten carbide (crushed and spherical), cast tungsten carbide (crushed and spherical), macrocrystalline tungsten carbide, and carburized tungsten carbide. In a particular embodiment, the cutters 116, 136 may include crushed cemented tungsten carbide. For various embodiments using cemented tungsten carbide, the cemented tungsten carbide formed from carbide particles ranging in size from about less than 1 to 15 microns and cobalt in amount ranging from about 6 to 16 percent by weight. However, such sizes and amounts are not intended to be a limitation on the scope of the present invention. One skilled in the art would recognize that in using a cemented tungsten carbide, by varying the carbide particle size and/or cobalt content, the wear resistance/hardness and fracture toughness of the cutters may be optimized for a particular milling operation.
In a particular embodiment, the cutters 116, 136 may be formed from crushed cemented tungsten carbide particles ranging in size from about less than 1 to 10 microns and cobalt in amount ranging from 8 to 14 percent by weight. In various other embodiments, the cutters may also include other particles such as, for example, tantalum carbide, tantalum niobium carbide, titanium carbide, tungsten titanium carbide, and tungsten tantalum carbide.
In one embodiment, the cylindrically bodied cutters may be attached to each of the lead mill and the second mill in such a manner so as to provide for a desired workface angle between the cutting face of the cutters and the workface (material being cut) as the cutters engage the casing. The workface angle, shown as 13 in
The desired workface angle may be achieved, for example, by varying the cutter geometry or the placement of the cutters in cutter pockets in the blades of a mill to achieve a particular rake angle as known in the art. In one embodiment, the desired workface angle may be achieved by placing cylindrical cutters in angled cutter pockets. In another embodiment, the desired workface angle may be achieved by forming cylindrical bodied cutters having a cutting face angled with respect to the longitudinal axis of the cylindrical body of the cutter. One of ordinary skill in the art would recognize that one or more techniques may be used to achieve the desired workface angle.
In one embodiment, the workface angle may be achieved by varying the rake angle of the cutters. Rake angle, shown as β in
Referring now to
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In a particular embodiment, second mill cutters are complimentary to lead mill cutters, i.e., lead mill cutters and second mill cutters have substantially similar orientations with respect to the workface.
In some embodiments, a protective coating may be provided on a portion or all of various mill components of each of the lead mill and second mill, including for example, the cutters, blades, and bodies. In some particular embodiments, the coating may be applied on the cutters prior to insertion and brazing into the cutter pockets. In other embodiments, the coating may be applied to the cutters after the insertion and brazing of the cutters into the cutter pockets. In yet other embodiments, the coating may be applied to all or any portion of the cutters, blades and mill body before or after brazing the cutters. In a specific embodiment, both the lead and second mill may individually have select portions coated. Specifically, the cutters of the second mill may be coated while the entire head of the lead mill may be coated.
In a particular embodiment, a protective coating may provide an increase in hardness and surface lubricity (low coefficient of friction) to the tool surface. Increases in hardness and/or surface lubricity may provide for a reduction in abrasive wear, work hardening, smearing, galling, and/or welding. Examples of coatings suitable for use on the mill components as disclosed herein include an aluminum titanium nitride (AlTiN) coating and an aluminum chrome (AlCr) coating.
Other types of coatings that may provide increases in hardness and/or lubricity that may be used on a mill component as described herein may include titanium nitride, titanium aluminum nitride, titanium carbonitride, aluminum oxide, chromium nitride, aluminum chromium nitride, and chromium carbide. Other embodiments may include silicon based coatings, such as, for example, silicon nitride or silicon titanium nitride. In high heat applications, a coating with a high oxidation temperature or high heat hardness or a coating having aluminum therein may be desirable. Upon oxidation of aluminum, aluminum oxide may serve as a heat barrier. In a particular embodiment, the coating may include metal providing strength to the coating and a lubricious material, such as aluminum, fluoropolymers, including TEFLON® (E.I. DuPont de Nemours Corporation, Wilmington Del.). One of skill in the art would recognize that the selection of protective coating may be dependent upon several factors including, for example, wear resistance, surface lubricity, and oxidation temperature. In one particular embodiment, the protective coating has a static coefficient of friction of about 0.4 or less (against steel).
The coating may be applied by various techniques known in the art, including physical vapor deposition (PVD), chemical vapor deposition (CVD), plasma assisted chemical vapor deposition (PACVD), and plating. One of ordinary skill in the art would recognize that each coat of material may include multiple layers. In applying the coatings, depending on the desired thickness of the coating, the coating may include a single coat (or pass) of material or multiple coats with one or more coating compositions. In one embodiment, the coating may be applied to the lead mill and/or second mill in a total thickness ranging from about 2 to 15 microns. In a particular embodiment, the coating may include two coats of material, each coat ranging from about 3 to 4 microns, to provide a total coating thickness ranging from about 6 to 8 microns.
In one embodiment, the protective coating may have a hardness of at least about 1,000 HV. In another embodiment, the protective coating may have a hardness of at least about 2,000 HV.
As shown in
δmax=(F*L3)/(3E*I) (1)
where δmax is the maximum deflection; E is the modulus of elasticity; I is the moment of inertia; F is the total load; and L is the length of the shaft (i.e., the distance X in the above discussion).
For a given set of characteristics of a mill assembly (ID, OD of shaft, material type), the length of the shaft from the lead mill to a tangential engagement or contact point with the casing wall must be sufficient to allow for enough flexibility or deflection in the shaft to mill a window in the casing under the applied load without failure of the milling system. As used herein, the “engagement point” refers to the point on the mill assembly or BHA (or BHA component), i.e., second mill, motor, stabilizer, or drill string, which touches the casing wall, as the mill assembly begins to deflect and mill through the casing. For unit deflection (δmax=1) of a given shaft, the allowed load for a particular mill assembly may be determined by:
F=(3E*I)/L3 (2).
Thus, the design of a mill assembly, and in particular the distance between a lead mill and second mill or other type of engagement point, may be selected or optimized in accordance with the above relationship to allow for sufficient flexibility under applied loads. In other words, in order to select a location for the second mill, the above relationships may be used based on known values (of inertia and modulus of elasticity). Alternatively, the distance may be selected and a recommended load can be provided. Those having ordinary skill in the art will appreciate that other design considerations may also affect the ultimate placement of the second mill or length of the shaft from the lead mill to the engagement point.
In a particular example, in a 5 inch casing system (duplex 25% chrome casing) using a 2.25 inch OD, a 1.25 inch ID, and a 58 inch long shaft, unit deflection of the shaft may be achieved under an applied load of at least 525 pounds. One of ordinary skill in the art would recognize that for a given mill system, that as the length of the shaft is increased, the applied load that will result in unit deflection may decreased. In another embodiment, in a 5 inch casing system using a 2.25 inch OD, a 1.25 inch ID, and a 180 inch long shaft, unit deflection of the shaft may be achieved under an applied load of at least 17.56 pounds. Further, one of ordinary skill in the art would also recognize that for a given force required for unit deflection, the moment of inertia (f[OD,ID]) and length of the shaft may be varied in accordance with Equation 2 above to result in the same force for unit deflection. Additionally, one of ordinary skill in the art would recognize that there may be a critical load needed for unit deflection after which the mill may fail to cut open a window in the casing and instead cut into the whipstock.
In a particular embodiment, a mill assembly such as the one disclosed herein may be included a one-trip milling/whipstock system, such as those described in U.S. Pat. Nos. 5,771,972, 6,102,123, 6,648,068, which are herein incorporated by reference in their entirety. Briefly, a one trip mill system, as shown in
Blade 38 immediately adjacent the parallel surface 45 of whipstock 44 may be sufficiently wide to accommodate the shear bolt 39 threaded into the blade 38. The head of the shear bolt 39 is seated in the top of the whipstock 44 and the shank 54 of the shear bolt 39 is threaded into blade 38. The shank 54 may be hollow so that, once the bolt 39 is sheared, the shank 54 serves as a nozzle extension for nozzles 69 positioned at the base of shank 54 and at the entrance to flex conduit 37 that directs fluid to the whipstock anchor (not shown).
The whipstock 44 has a diameter DW that approximates the inside diameter DI of the interior wall of casing 11 which allows whipstock 44 to be lowered through cased borehole 9. Whipstock 44 also includes a profiled ramp surface 28 having a curved or arcuate cross section and multiple surfaces, each of the multiple surfaces forming its own angle with the axis 26 of whipstock 44. Profiled ramp surface 28 includes a starter surface 45 having a steep angle preferably 15°, a vertical surface 46 preferably parallel to the axis 26, an initial ramp surface 47 having a standard angle ranging from about 0.5 to 3°, a “kick out” surface 48 having a steep angle preferably 15°, and a subsequent ramp surface 49 having a standard angle ranging from about 0.5 to 3°. It should be appreciated that these angles may vary. For example, the starter ramp surface 45 may have an angle A in the range of 1 to 45° in one embodiment, 2 to 30° in another embodiment, 3 to 15° in yet another embodiment, and about 15° in still another embodiment. The vertical surface 46 may have a length approximately equal to or greater than the distance between mills 32 and 33. In a particular embodiment, ramp surfaces 46, 49 may range from greater than zero to 15°. One of ordinary skill in the art would recognize that the surfaces angles may be selected depending on the desired window dimensions.
The backside 62 of the whipstock 44, especially adjacent the upper end of the whipstock 44, is contoured to conform to the inside diameter DI of the interior wall of the pipe casing 11 for stability of the top of the whipstock 44. The opposite lower end of the whipstock 44 is secured to, for example, a hydraulically actuated anchor (not shown). A typical anchor is shown in U.S. Pat. No. 5,657,820, incorporated herein by reference in its entirety.
The mill 32 and whipstock 44 disclosed herein are configured such that the mill 32 tends to cut the wall of the casing 11 and not the whipstock 44. To achieve this objective, various factors are taken into consideration including the contact area and contact stress between the mill 32, casing 11, and whipstock 44 and the cutability of the metal of the casing and of the metal used for the whipstock 44.
Advantageously, embodiments disclosed herein may provide for at least one of the following. Mill assemblies incorporating cylindrical cutters on each of the lead and second mill may allow increased mill efficiency and mill life for ease in modification of existing mill assemblies. By forming a mill assembly that has a second mill distance from the lead mill that is selected to allow for flexibility, a mill assembly as disclosed herein may be effective in cutting a window in a casing that would otherwise be unobtainable, without otherwise altering the mill and drill assembly components.
When milling through a casing, one potential mode of failure of the mill is by galling and welding observed at the mill face, especially when milling through a chrome casing. The inclusion of a protective coating on the cutting structure may prevent or reduce such occurrences of galling. In the various embodiments that may include a lubricious material such as aluminum or silicon in the protective coating, the coating may also have an increased life span due to the preferential oxidation of aluminum, which further reduces galling and welding and contact temperatures.
Additionally, by coating the cutting structure, a dramatic increase in wear resistance of the mill may be observed. The mill assemblies disclosed herein may endure significant increases in downhole life as compared to a typical mill assembly, and even when tripped, mill assemblies made in accordance with embodiments disclosed herein, and specifically the gage of the mills disclosed herein, may have worn minimally while downhole.
Furthermore, as the cutting structure meets the casing wall, the cutting structure typically encounters severe vibrations that frequently lead to cracks in the cutters. By varying the cutter geometry and/or placement of cutters with respect to the workface surface, i.e., interior casing surface, the incidence of cracking may be decreased and the cutting efficiency, and thus mill life, may be increased. Additionally, by varying the cutter geometry by grinding or cutting the cutters to have the desired rake and workface angles, the number of cutters on a particular mill may be increased, and thus the amount of wear each cutter experiences may decrease.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
