CROSS-REFERENCES TO RELATED APPLICATION

This application claims the benefit of U.S. Application No. 62/022504, filed on July 9, 2014, which is incorporated herein by reference in its entirety.

BACKGROUND

1. Field of the Disclosure

[0001] The present disclosure is related to a milling apparatus for milling casing exits and to perform other cutting operations in wellbores.

2. Background Of The Art

[0002] Conventional cylindrical mills are commonly utilized for milling windows (or sections) in metal casings (such as pipes) placed in wellbores to provide exits for forming lateral wellbores and to perform other downhole cutting operations. Often, three mills (a window mill, a lower mill and an upper mill) are used on a bottomhole assembly (BHA) to perform the milling operations. Such mills generally include an enlarged pipe section (larger diameter section) that transitions to a smaller diameter pipe on both sides at a taper angle, typically 15°, with a small blending radius at both ends of the taper. Blades are welded over the tapered sections and the enlarged section. Although such tapers or tapered sections along with the small blending radiuses appear to provide a smooth transition between the two diameters to avoid stress concentration, the analysis and operational experience show a relatively high concentration of stress at such transitions. Additionally, the lower mill is the most highly stressed member of the bottomhole assembly (BHA). During milling operations, as the window mill moves down the ramp and laterally through the casing wall, the lower mill body is bent. High stress concentration occurs at the end of the blades, causing cracks to first appear near the ends of such blades. The lower mill is also subject to the substantial torque required to drive the mill, and to torsional impacts from the blades engaging (hitting) the side of the window and the cut slot. The torsional stress is sufficiently high to promote crack growth.

[0003] The disclosure herein provides a milling apparatus that addresses at least some of the above- described deficiencies of the mills. SUMMARY OF THE DISCLOSURE

[0004] In one aspect, the present disclosure provides a mill for use in milling a section of a casing in a wellbore that in one embodiment includes a first concave curved section followed by a substantially flat section and a second concave curved section following the substantially flat section and a number of blades attached to the first concave curved section, substantially flat section and the second concave curved section, wherein each such blade includes a first convex curved section that corresponds to the first concave curved section and a second convex curved section that corresponds to the second concave curved section.

[0005] In another aspect, a method of milling a section of a casing in a wellbore is disclosed that in one embodiment includes: placing a ramp at a selected location in the casing; conveying a bottom hole assembly in the wellbore, the bottom hole assembly including a mill that contains a plurality of blades attached to a blade body, wherein at least one blade in the plurality of blades includes a tapered curved section attached to a

corresponding curved tapered section along downhole side of the blade body for reducing stress on an end of the such blade; and milling a section of the casing above the ramp by the mill. In another aspect, one or more blades may further be attached to a curved tapered section on an uphole side of the blade body and wherein each such blade includes a curved section that corresponds to the curved section on the uphole side of the blade body. In another aspect a one or more bosses may be provided downhole and/or uphole of the blades to reduce the stresses on and extend the fatigue life of the blades.

[0006] Examples of certain features of the apparatus and method disclosed herein are summarized broadly in order that the detailed description thereof that follows may be better understood. There are, of course, additional features of the apparatus and method

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The disclosure herein is best understood with reference to the accompanying figures in which like numerals have generally been assigned to like elements and in which:

FIG. 1 (Prior Art) shows a line diagram of a prior art mill having a larger diameter (raised) section that tapers on both sides to a smaller diameter pipe and a blade attached to the tapered and larger diameter sections;

FIG. 2 shows a line diagram of a blade body that includes a raised section on a pipe with curved tapered sections on both sides of the pipe having a curved outer surface, according to one embodiment of the disclosure; FIG. 3 shows a blade attached to the curved tapered sections and the raised section of the blade body shown in FIG. 2, wherein the underside of the blade profile matches or substantially matches the curved tapered sections;

FIG. 4 shows a bottom hole assembly carrying one or more mills made according to an embodiment of the disclosure;

FIG. 5 shows results of an analysis showing concentration of stress in and in front of a blade on a mill made according to the prior art embodiment shown in FIG. 1; and

FIG. 6 shows results of an analysis showing stress concentration on a blade of a mill made according to the embodiment shown in FIG. 3;

DESCRIPTION OF THE EMBODIMENTS

[0008] FIG. 1 shows a line drawing of a partial prior art mill 100 that includes a body (also referred to as the "blade body") 110 that includes a raised section 120 having a diameter Dl and a first tapered section or lower tapered section 130a that transitions or extends from an end 132a of the raised section 120 to a point 115a on a pipe 115 having a diameter D2 toward a downhole side of the mill 100 (in this example, the right side). A second tapered section (or upper tapered section or ramp) 132b transitions or extends from the second end 132b of the raised section 120 to a point 115b on the tapered section 130b toward an uphole side (in this example, the left side) of the mill 100. The diameter Dl is greater than the diameter D2. The tapered sections 130a and 130b have respectively straight or flat outer surfaces 134a and 134b. A number of blades, such as blades 150a and 150n are attached circumferentially on surface 134a, the raised section 120 and the surface 134b. It is known that the ends 154a and 154n of blades 150a and 150n at the pipe 115 are subject to damage during milling of an opening in a casing. In some cases the prior art mills, such as mill 100, provide a relief radius (not shown) covered with a wear resistant material to limit wear during milling. The application of the wear resistant material, typically tungsten carbide, is a high temperature process that can reduce the strength of the steel pipe, such as pipe 115. During milling operations, the lower tapered section, such as section 130a, moves the mill 100 laterally into the casing, bending the blade body 110, which develops a high stress concentration at the end of the blades, such as end 154n of blade 154. Therefore, cracks typically first appear near the ends of the blades. The mill, such as mill 100, is also subject to the substantial torque required to drive the mill and torsional impacts from the blades hitting the side of the window and the slot cut in the casing. The cracks formed are typically formed at an angle or even turn as they propagate or grow. [0009] FIG. 2 shows a line diagram of a blade body 200, according to a non-limiting embodiment of the disclosure. The blade body 200 includes a raised section 220 having a diameter or an outer dimension Dl and a first or lower tapered section 230a that transitions or extends from a first end 232a of the raised section 220 to a point 225a on a pipe 225 having a diameter or an outer dimension D2 toward the lower or downhole side of the blade body 200 (in this example, the right side). A second or upper tapered section 230b transitions or extends from the second or upper end 232b of the raised section 220 to a point 225b of the pipe 225 toward an upper or uphole side (in this example, the left side) of the blade body 200. The diameter Dl is greater than the diameter D2. The tapered sections 230a and 230b include curved surfaces (also referred to herein as "radiused" surfaces") 234a and 234b respectively. The tapered section 230a may include more than one curved surfaces, such as surfaces with the same or different curvatures. In one aspect, the radius or curvature of surface 234a may be made as large as possible before it approaches a straight line in nature such that the stress reducing benefits are compromised. Up to a point as the curvature on the downhole or front side of the raised section is made larger, the stress in the blades and body near the end of the blades placed on such surface is reduced. Large radius surfaces, such as surfaces 234a and 234b provide additional length on the outer surface (compared to the flat surfaces 134a and 134b, FIG. 1) that elongate when the blade body 200 is bent, without excessive stretching and stress. A curved or radiused surface provides more surface length compared to a non- radiused surface. The relatively greater surface length prevents bending stress accumulation as the radiused surface is bent into a straight surface, compared to a non-radiused surface exposed to bending, since the area near the blade will experience stress from the cumulative stretching along the blade surface. In one aspect, a computer model is used to determine the optimum contour for minimum or optimal bending stress on the curved surface 234a. The computer modes or equations for determining torsional stress concentration are similar to the bending stress models and thus a low bending stress concentration will have a low torsional stress concentration as well. Such models are known in the art and are not described herein. In one aspect, the blade body 200 may further include an enlarged section or boss 280, on downhole side (in this example, the right side) of the raised section 220, having a tapered section 282a on the downhole or lower side and section 282b on the upper or uphole side. The surfaces 284a and 284b of sections 282a and 282b may be curved. The curvature or radius 284a of section 282a and curvature or radius 284b of section 282b may be same or different. The diameter D3 of boss 280 is between the diameters Dl and D2. In another aspect, the boss 280 may be a cusp or may have a flat surface. The diameter or outer dimension of the boss 280 is between the diameters Dl and D2. In another aspect, the outer most surface of the boss 280 may have a length less than half the diameter D2. In yet another aspect, another boss (not shown) may be provided uphole of the upper tapered section 230b to reduce bending stresses. In yet another aspect, more than one boss may be provided on one or both sides of the raised section 220. A boss on the upper side of the raised section 220 is more useful in reducing bending stress in larger mills.

[0010] FIG. 3 show the blade body 200 of FIG. 2 with a blade 250 attached, such as by welding, on the upper curved surface 234b, raised section 220 and the lower curved surface 234a. In one aspect, the underside 252a of the lower blade section 256a includes a curved surface 254a that matches or conforms or substantially matches or conforms to the contours of curved surface 234a of section 230a and the underside surface 252b of the upper section 256b of the blade 250 matches or conforms or substantially matches or conforms to the contour of the surface 234b of the curved section 230b.

[0011] Referring to FIGS. 2 and 3, as discussed above in reference to FIG. 1, the prior art mills provide a relief radius covered with a wear resisting material to prevent grooves from being worn in the pipe. The application of the wear resisting material, typically tungsten carbide, is a high temperature process which compromises the strength of the steel pipe, such as pipe 225. In one aspect, the disclosure herein addresses such a problem by providing the enlarged section or boss 280 adjacent to the radiused section 230a. In one aspect, such an enlargement holds the mill 200 away from the casing being milled to avoid damage to the ends of the blades, such as end 288a of blade 250. In another aspect, the boss 280 may have a narrow width or outside diameter surface 284. Thus, in the non-limiting configuration shown in FIG. 3, the mill includes a narrow diameter enlargement or boss 280 formed by a continuation of the neck radius 284b on one side, and radius 284a on the other side. Forming one side with the neck radius, such as radius 284b, near the blade ends reduces the stress on the blades during milling compared to the mills, such as mill 100 shown in FIG. 1. In one aspect, the surface width 284 of the enlarged section 280 is relatively small. In another aspect, the width 284 may be a point (cusp), as shown in FIGS. 2 and 3. In another aspect, the "point" 284 may be about .060". In another aspect, the upset 295 protects the front ends of the blades, which alleviates the need for applying wear resisting material, such as carbide to the radiused section of the mill. Consequently, there is less reduction in the strength of the steel in the maximum stress area, as shown in reference to FIG. 6.

[0012] FIG. 4 shows a wellbore system 400 with a bottom hole assembly 420 utilizing a lower mill 430 and a mill 450 made according to one or more embodiments of this disclosure for milling a casing section and forming a lateral wellbore from the milled section. The system 400 shows a wellbore 401 that is lined with a casing 410, such as a casing made from steel sections. A ramp 480 containing a whipstock 482 is shown anchored in the casing 410 by an anchor 484 at a location 485 where a window is to be milled in the casing 410. A drill string 412 containing a conveying member 414 and a bottomhole assembly 420 is shown placed inside the casing 410. The bottom hole assembly 420 includes a cutting device 425, a lower mill 430 above the cutting device 425 and a second mill 450 above the mill 430. Mill 430 or both mills 430 and 450 are made according to an embodiment of this disclosure. In the particular bottom hole assembly of FIG. 4, mills 430 and 450 are respectively shown to include blades 432a-432n and 452a-452n attached to a respective blade body, such as body 200 shown in FIG. 2. To form a window or mill a section 460 in the casing, 410, the cutting device 425 travels along the whipstock 482 and makes an initial cut in the casing 410 at location 485. The mill 430 then mills the window 460 as shown in FIG. 4. The stresses introduced in the mill 430 are reduced, compared to prior art mills, such as shown in FIG. 1 due to the boss, such as boss 280 shown in FIG. 2 and corresponding radiused tapered surfaces (such as surface 234a) on the blade body 200 and radiused underside surfaces on the blades (such as blade 254a), as described above in reference to FIGS. 2 and 3. The bottom hole assembly may be further used to form a lateral wellbore, as known in the art.

[0013] FIG. 5 shows results of a FEA stress analysis for a prior art mill 500, similar to the mill 100 shown in FIG. 1. For this FEA analysis, the mill 500 was constrained at the back end 510 and on the gage, and the front end 520 bent to the side as dictated by the whipstock geometry. The results shows high stress 530 in front 535a of the blade 540 on the gage at the point of contact with the casing wall 535c, and at the back 535b of the blade, as shown in FIG. 5. In this analysis, the maximum stress was 34,000 psi just in front of the blade 540.

[0014] FIG. 6 shows results of a stress analysis for a mill 600 made according to an embodiment of the present disclosure, similar to the mill 300 shown in FIG. 3. The analysis shown in FIG. 6 was performed in a manner similar to the analysis performed for mill 500 of FIG. 5. The results show a relatively lower stress 630 in front 635a of the blade 640 on the gage at the point of contact with the casing wall 635b, and at the back 635c of the blade 640, as shown in FIG. 6. The results show that the stress reduced from 34,000psi to 16,000 psi in front of the blade 640. Also, the results show that the stress in mill 600 is more uniformly distributed compared to the mill 500. While the concepts of the mill are described herein in reference to a window cutting or casing exit mill design and application, a mill made as described herein or with obvious modifications, is equally applicable in various other applications, including, but not limited to, reaming tools and operations.

[0015] While the foregoing disclosure is directed to the certain exemplary embodiments of the disclosure, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.