Full Gauge Milling Bottom Hole Assembly with Optimal Contact Force and Build Rate Capability
A milling bottom hole assembly (BHA) for use in cutting a full gauge window in a wellbore casing wall, the resultant length of the window being greater than or equal to the whipstock ramp length. A milling BHA is described which includes two shaft portions, a window mill and two bearing mills. The design, which involves strategically placed bearing mills, allows the milling BHA to stay on the whipstock ramp for the entire casing window milling operation and, thereafter, to optimally rapidly build angle and move laterally away from the whipstock and casing, creating a significantly long window which allows for easy passage of directional drilling BHAs through the milled window.
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This application is a continuation-in-part of U.S. patent application Ser. No. 12/325,184 filed Nov. 29, 2008 which claims priority to provisional patent application Ser. No. 60/991,432 filed Nov. 30, 2007.
BACKGROUND OF THE INVENTION1. Field of the Invention
The invention relates generally to the arrangement and design of mills on bottom hole assemblies that are used to cut windows in casing strings for the creation of lateral wellbores.
2. Description of the Related Art
In modern hydrocarbon production, it is common to create one or more lateral production wellbores which extend outwardly from a central, generally vertical wellbore. In order to form a lateral production wellbore, a window must be cut into the side of casing in the central wellbore. Thereafter, drilling tools are used to form an extended lateral wellbore. Traditionally, whipstocks and milling tools are used to create the window in the central wellbore casing wall.
SUMMARY OF THE INVENTIONThe invention provides an improved milling bottom hole assembly (BHA) for use in cutting a window in a wellbore casing wall. An exemplary milling BHA is described which includes a shaft that is made up of two shaft sections. The distal end of the shaft carries a window mill. A pair of bearing mills is carried by the shaft sections above the window mill. Preferably, each of the bearing mills is carried by a different shaft section. Placement of the bearing mills permits the milling BHA to cut a window having a greater length and quality as it allows the milling BHA to stay on the whipstock ramp for the entire milling operation and then exit the ramp and casing rapidly, such that the lateral build rate of the milling BHA away from the whipstock and its anchor is optimum and both risks of casing reentry of the milling BHA and excessive damage to the milling BHA are mitigated. The resultant milled casing exit window is superior for subsequent ingress and egress of long and stiff directional drilling BHAs. A full gauge arrowhead-shaped mill is preferably used for the lower bearing mill. A full gauge watermelon-shaped mill is preferably used for the upper bearing mill. All three mills, the window mill, the arrowhead-shaped mill and the watermelon-shaped mill, present the same full gauge diameter.
The advantages and further aspects of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout the several figures of the drawing and wherein:
A first bearing mill 26 is located on the lower shaft section 20 above the window mill 24. The first bearing mill 26 is preferably of full gauge and is preferably of an arrowhead-shaped configuration, as illustrated in
Also of note is that the angle of the taper on the lower portion 27b of the arrowhead-shaped first bearing mill 26 is derived from the predicted angular position between the centerlines of the first bearing mill 26 and the whipstock 10 when the first bearing mill 26 transitions from the primary wellbore 44 into the window 40. Because maximum forces are encountered at this transition point, the angle of the taper is such that the surface area on the cutting surface is optimized, damage to the mill 26's cutting structure is minimized, and cutting structure life expectancy is maximized.
A second bearing mill 28 is located on the upper shaft section 18. The second bearing mill 28 preferably presents a cross-section that is curved and oblong, thereby presenting a substantially flat center segment 30 and arcuately curved end sections 32. The second bearing mill 28 may be of the type generally known in the industry as a “watermelon mill.” In an alternate embodiment, the second bearing mill 28 presents a cross-section that is arcuately rounded, in the same manner as the first bearing mill 26. Both the first and second bearing mills 26, 28 extend radially outwardly to full gauge.
The overall length “L” of the milling BHA 12 (the milling BHA length) exceeds the longitudinal length “l” of the ramp 34 of the whipstock 10 (the whipstock ramp length). The second bearing mill 28 is preferably located at a distance “x” from the window mill 24 that is from about 1.0 to about 1.25 times the length “l” of the ramp 34. Most preferably, the distance “x” is about 1.15 to about 1.20 times the length “l” of the ramp 34. The first bearing mill 26 is preferably located at a distance “d” from the window mill 24 that is from about one-fifth to about one-half of the length “x”. Most preferably, the distance “d” is about one-third of the length “x”. It is further noted that the spacing (“d1”) between the first and second bearing mills 26, 28 preferably exceeds the distance “d”.
The distance “x” of the second bearing mill 28 from the window mill 24 is also preferably from about 75% to about 90% of the overall milling BHA length “L”. More preferably, the distance “x” is from about 80% to about 85% of “L”.
In operation, the drill string 16 and milling BHA 12 are rotated within the casing 42, and the milling BHA 12 is lowered within the wellbore 44 until the milling BHA 12 encounters the whipstock 10 proximate the kick-off point 43. As
During the milling operation, as illustrated by
The design of the milling BHA 12 provides high constraining forces at the window mill 24 while it traverses the midsection of the ramp 34 of the whipstock 10. The use of a milling BHA 12 constructed in accordance with the present invention produces a milled window 40 having an extended length, as measured from the upper end 50 to the lower end 52. The proximity of the first bearing mill 26 to the window mill 24 creates restraining forces on the window mill 24 to urge it properly along the departure path 46 from the primary wellbore 44. Additionally, the proximity of the first bearing mill 26 to the window mill 24 helps in harnessing the efficiency of the cutters of the first bearing mill 26 for additional cutting of the upper end 50 of the window 40. This results in a longer window 40 than with many conventional techniques.
As noted, the first bearing mill 26 preferably has an arcuate cross-section, thereby providing for point-type contact between the bearing mill 26 and the surrounding casing 42 or the whipstock 10. Point-type contact results from the fact that the surface of the curved bearing mill 26 cross-section will contact the surrounding casing 42 or whipstock 10 at a single point.
It can be seen that the milling BHA 12 and the whipstock 10 collectively provide a window cutting arrangement that is operable to form a window in surrounding wellbore casing. It should also be understood that the invention provides an improved method for forming a window within wellbore casing.
In order to achieve a high build rate, the lower mill 26, which follows the window mill 24, will experience a contact force/restoring force that is in a direction towards the whipstock 10 at the time after the window mill 24 has exited the casing 42. Also, generally the magnitude of the contact force on the lower mill 26 should be equal to or greater than the maximum contact force experienced by the window mill 24.
Contact forces at defined intervals are experienced by the mills 26, 28 (which are at drift OD) when they contact the casing 42 as they pass through the deviated well profile. The contact force plots are generated for the window mill 24, lower mill 26 and the upper mill 28. For comparison purposes, these respective contact forces are superimposed on the same plot in
The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope and the spirit of the invention.
Claims
1. A bottom hole assembly for use in milling a window in a wellbore casing in association with a whipstock with an angled ramp having a whipstock ramp length, the bottom hole assembly having a bottom hole assembly length and comprising:
- a shaft providing a bottom hole assembly length;
- a window mill located proximate a lower end of the shaft;
- a first bearing mill upon the shaft;
- a second bearing mill upon the shaft;
- the window mill and the second bearing mill being spaced from each other by a first distance;
- the first bearing mill being spaced from the window mill at a second distance that is from about one-fifth to one-half of the first distance; and
- the first distance is from about 75% to about 90% of the bottom hole assembly length.
2. The bottom hole assembly of claim 1 wherein the first distance is from about 80% to about 85% of the bottom hole assembly length.
3. The bottom hole assembly of claim 1 wherein the second distance is about one-third of the first distance.
4. The bottom hole assembly of claim 1 wherein the first and second bearing mills are separated from each other by a third distance that is at least as great as the second distance.
5. The bottom hole assembly of claim 1 wherein the first distance is greater than the whipstock ramp length.
6. The bottom hole assembly of claim 1 wherein the first bearing mill has an arrowhead-shaped configuration.
7. The bottom hole assembly of claim 6 wherein:
- the angled ramp presents a whipstock scoop angle;
- the first bearing mill includes a lower portion which presents a mill blade taper angle; and
- the mill blade taper angle is from 1.5 times to 3 times the whipstock scoop angle.
8. The bottom hole assembly of claim 1 wherein the second bearing mill presents a cross-section having a substantially flat bearing surface.
9. The bottom hole assembly of claim 1 wherein the bottom hole assembly length has a midpoint and wherein the first bearing mill reaches an upper end of a whipstock during a milling operation before the midpoint of the bottom hole assembly length reaches the upper end of the whipstock.
10. The bottom hole assembly of claim 1 wherein the window mill contacts the whipstock to experience a contact force which gradually increases until the window mill reaches halfway across the whipstock ramp and then gradually declines as the first bearing mill passes a top of the ramp.
11. The bottom hole assembly of claim 1 wherein the shaft is formed of first and second shaft sections that are threaded together and wherein:
- the first bearing mill is carried by the first shaft section; and
- the second bearing mill is carried by the second shaft section.
12. The bottom hole assembly of claim 1 wherein:
- the second distance is less than half the length of the ramp; and
- the first distance is greater than half the length of the ramp.
13. A window cutting arrangement for forming a window within a wellbore casing, the window cutting arrangement comprising:
- a whipstock to be disposed within the wellbore casing, the whipstock presenting an angled ramp and having a whipstock ramp length;
- a bottom hole assembly for contacting the angled ramp and cutting a window within the wellbore casing, the bottom hole assembly having a bottom hole assembly length and comprising: a shaft; a window mill proximate a lower end of the shaft and operable for cutting a window in the wellbore casing; a first bearing mill upon the shaft; a second bearing mill upon the shaft; the window mill and the second bearing mill being spaced from each other by a first distance; the first bearing mill being spaced from the window mill at a second distance that is from about one-fifth to one-half of the first distance; and the first distance is from about 75% to about 90% of the bottom hole assembly length.
14. The window cutting arrangement of claim 13 wherein the first distance is from about 80% to about 85% of the bottom hole assembly length.
15. The window cutting arrangement of claim 13 wherein the second distance is about one-third of the first distance.
16. The window cutting arrangement of claim 13 wherein the first and second bearing mills are separated from each other by a third distance that is at least as great as the second distance.
17. The window cutting arrangement of claim 13 wherein the first distance is equal to or greater than the whipstock ramp length.
18. The window cutting arrangement of claim 13 wherein the first bearing mill has an arrowhead-shaped configuration.
19. The window cutting arrangement of claim 13 wherein the second bearing mill presents a cross-section having a substantially flat bearing surface.
20. The window cutting arrangement of claim 13 wherein the bottom hole assembly length has a midpoint and wherein the first bearing mill reaches an upper end of the whipstock during a milling operation before the midpoint of the bottom hole assembly length reaches the upper end of the whipstock.
21. The window cutting arrangement of claim 13 wherein the first distance is from about 1.0 to about 1.25 times the whipstock ramp length.
22. The window cutting arrangement of claim 13 wherein the first distance is from about 1.15 to about 1.20 times the whipstock ramp length.
23. The window cutting arrangement of claim 13 wherein:
- the angled ramp presents a whipstock scoop angle;
- the first bearing mill includes a lower portion which presents a mill blade taper angle; and
- the mill blade taper angle is from 1.5 times to 3 times the whipstock scoop angle.
Type: Application
Filed: Feb 5, 2011
Publication Date: Jul 21, 2011
Applicant: Baker Hughes Incorporated (Houston, TX)
Inventors: Suhas S. Verma (Austin, TX), James S. Trahan (Magnolia, TX), Daniel R. Hart (Sugar Land, TX), Christopher W. Guidry (Spring, TX), Reena Thomas (Houston, TX)
Application Number: 13/021,726
International Classification: E21B 29/06 (20060101); E21B 29/00 (20060101);