U.S. patent application number 11/691645 was filed with the patent office on 2007-08-16 for one trip milling system.
Invention is credited to Praful C. Desai, Charles H. Dewey.
Application Number | 20070187085 11/691645 |
Document ID | / |
Family ID | 26694938 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070187085 |
Kind Code |
A1 |
Dewey; Charles H. ; et
al. |
August 16, 2007 |
ONE TRIP MILLING SYSTEM
Abstract
The side tracking system includes a window mill having a full
diameter cutting surface and a reduced diameter tapered cutting
surface and a whipstock having a ramp engaging the reduced diameter
cutting surface. The cutting tool has a body with diamond cutters
initiating cutting of a steel casing, milling a window through the
steel casing and drilling a secondary borehole. The materials of
the whipstock have a first cutability and the materials of the
casing have a second cutability. The reduced diameter cutting
surface contacts the whipstock ramp at a first contact area and the
full diameter cutting surface contacts the wall of the casing at a
second contact area. As weight is applied to the mill, there is a
first contact stress at the first contact area and a second contact
stress at the second contact area. A cutability ratio is the first
cutability divided by the second cutability and a contact stress
ratio is the first contact stress divided by the second contact
stress. The mill cuts the casing rather than the whipstock by
maintaining the product of the cutability ratio and the contact
stress ratio less than one.
Inventors: |
Dewey; Charles H.; (US)
; Desai; Praful C.; (Kingwood, TX) |
Correspondence
Address: |
CONLEY ROSE, P.C.;David A. Rose
P.O. BOX 3267
HOUSTON
TX
77253-3267
US
|
Family ID: |
26694938 |
Appl. No.: |
11/691645 |
Filed: |
March 27, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10684629 |
Oct 14, 2003 |
7207401 |
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11691645 |
Mar 27, 2007 |
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|
09303049 |
Apr 30, 1999 |
6648068 |
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|
10684629 |
Oct 14, 2003 |
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|
09021630 |
Feb 10, 1998 |
6102123 |
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09303049 |
Apr 30, 1999 |
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08642829 |
May 3, 1996 |
5771972 |
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09021630 |
Feb 10, 1998 |
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Current U.S.
Class: |
166/55 |
Current CPC
Class: |
E21B 7/061 20130101;
E21B 29/06 20130101 |
Class at
Publication: |
166/055 |
International
Class: |
E21B 29/06 20060101
E21B029/06 |
Claims
1. A cutting tool for milling a window through a steel casing and
drilling a secondary borehole, comprising: a body having an outer
surface and a plurality of diamond cutters extending from said
outer surface; said diamond cutters initiating cutting of the steel
casing, milling said window through the steel casing and drilling
the secondary borehole.
2. The cutting tool of claim 1 further comprising: a whipstock
having a first cutability; wherein said steel casing has a second
cutability greater than said first cutability.
3. The cutting tool of claim 2 wherein said whipstock has a first
contact stress and said steel casing has a second contact stress,
said first cutability divided by said second cutability is a first
ratio, said first contact stress divided by said second contact
stress is a second ratio, and said first ratio times said second
ratio is less than one.
4. The cutting tool of claim 1 wherein said diamond cutters are
polycrystalline diamond cutters.
5. The cutting tool of claim 1 wherein said body has a longitudinal
axis and said body rotates on-center about said longitudinal
axis.
6. The cutting tool of claim 1 wherein said body has a longitudinal
axis and said outer surface has a taper with respect to said
longitudinal axis.
7. The cutting tool of claim 1 wherein said body has a longitudinal
axis and said outer surface comprises multiple surfaces having
different angled tapers with respect to said longitudinal axis.
8. The cutting tool of claim 1 wherein said diamond cutters mill a
full gauge window through the steel casing.
9. The cutting tool of claim 8 wherein said diamond cutters drill a
full gauge secondary borehole.
10. The cutting tool of claim 1 wherein said body has a
longitudinal axis and said outer surface is substantially parallel
to said longitudinal axis.
11. The cutting tool of claim 1 wherein said outer surface has a
back tapered surface.
12. The cutting tool of claim 1 wherein said body further includes
a generally circular bottom cutting surface.
13. The cutting tool of claim 1 wherein said outer surface further
comprises a tungsten carbide material.
14. A casing mill for milling a window through a steel casing and
drilling a secondary borehole, comprising: a body having an outer
surface and a multiplicity of cutting elements extending from said
outer surface, said cutting elements including tungsten carbide
material and diamond cutters; said diamonds initiating cutting of
the steel casing, milling said window through the steel casing and
drilling the secondary borehole.
15. The casing mill of claim 14 further comprising: a whipstock
having a first cutability; wherein said steel casing has a second
cutability greater than said first cutability.
16. The casing mill of claim 15 wherein said whipstock has a first
contact stress and said steel casing has a second contact stress,
said first cutability divided by said second cutability is a first
ratio, said first contact stress divided by said second contact
stress is a second ratio, and said first ratio times said second
ratio is less than one.
17. The casing mill of claim 14 wherein said diamond cutters
include natural or polycrystalline diamond cutters.
18. The casing mill of claim 14 wherein said body has an axis and
said cutting elements collectively form an external cutter profile
comprising: an upper cylindrical gage portion; and a lower conical
portion extending downwardly from said gage portion to an angle to
the axis of said body.
19. A cutting tool for milling a window through steel casing in a
well bore and being adapted to cooperate with a whipstock having a
whipstock axis and ramp surface disposed at a ramp angle to the
whipstock axis, the cutting tool comprising: a tool body having a
body axis and an outer surface; and a plurality of cutting faces
extending from said outer surface and having diamond material to
initiate and mill the window through the steel casing and drill
borehole; said cutting faces collectively forming an external
profile having a gage portion with a diameter corresponding to the
window to be milled through the casing, and a conical portion
having a length and extending from said gage portion at an
angle.
20. The cutting tool of claim 19 wherein said whipstock has a first
cutability and said steel casing has a second cutability greater
than said first cutability, and wherein said whipstock has a first
contact stress and said steel casing has a second contact stress,
said first cutability divided by said second cutability is a first
ratio, said first contact stress divided by said second contact
stress is a second ratio, and said first ratio times said second
ratio is less than one.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/684,629 filed Oct. 14, 2003, hereby
incorporated herein by reference, which is a continuation of U.S.
application Ser. No. 09/303,049 filed Apr. 30, 1999, now U.S. Pat.
No. 6,648,068, hereby incorporated herein by reference, which is a
continuation-in-part of U.S. patent application Ser. No. 09/021,630
filed Feb. 10, 1998, now U.S. Pat. No. 6,102,123, hereby
incorporated herein by reference, which is a continuation-in-part
of U.S. patent application Ser. No. 08/642,829 filed May 3, 1996,
now U.S. Pat. No. 5,771,972, hereby incorporated herein by
reference, and is related to U.S. patent application Ser. No.
08/572,592 entitled Two Trip Window Cutting System, filed Dec. 14,
1995, now U.S. Pat. No. 5,657,820, hereby incorporated herein by
reference, and U.S. patent application Ser. No. 08/916,932 filed
Aug. 21, 1997, now U.S. Pat. No. 5,894,889, hereby incorporated
herein by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This invention relates to a method and apparatus for
drilling a secondary borehole from an existing borehole in geologic
formations and more particularly, to a tapered window mill and
whipstock combination that in one trip, can drill a deviated
borehole from an existing earth borehole or complete a side
tracking window in a cased borehole.
[0005] 2. Background
[0006] Traditionally, whipstocks have been used to drill a deviated
borehole from an existing earth borehole. The whipstock has a ramp
surface which is set in a predetermined position to guide the drill
bit on the drill string in a deviated manner to drill into the side
of the earth borehole. In operation, the whipstock is set on the
bottom of the existing earth borehole, the set position of the
whipstock is surveyed, the whipstock is properly oriented for
directing the drill string in the proper direction, and the
drilling string is lowered into the well into engagement with the
whipstock causing the whipstock to orient the drill string to drill
a deviated borehole into the wall of the existing earth
borehole.
[0007] Previously drilled and cased wellbores, for one reason or
another, may become non-productive. When a wellbore becomes
unusable, a new borehole may be drilled in the vicinity of the
existing cased borehole or alternatively, a new borehole may be
sidetracked from or near the bottom of a serviceable portion of the
cased borehole. Sidetracking from a cased borehole is also useful
for developing multiple production zones.
[0008] Sidetracking is often preferred because drilling, casing and
cementing the borehole is avoided. This drilling procedure is
generally accomplished by either milling out an entire section of
casing followed by drilling through the side of the now exposed
borehole, or by milling through the side of the casing with a mill
that is guided by a wedge or "whipstock" component.
[0009] Drilling a side tracked hole through casing made of steel is
difficult and often results in unsuccessful penetration of the
casing and destruction of the whipstock. In addition, if the window
is improperly cut, a severely deviated dog leg may result rendering
the sidetracking operation unusable.
[0010] Several patents relate to methods and apparatus to sidetrack
through a cased borehole. U.S. Pat. No. 4,266,621 describes a
diamond milling cutter for elongating a laterally directed opening
window in a well casing that is set in a borehole in an earthen
formation. The mill has one or more eccentric lobes that engage the
angled surface of a whipstock and cause the mill to revolve on a
gyrating or non-fixed axis and effect oscillation of the cutter
center laterally of the edge thus enhancing the pipe cutting
action.
[0011] The foregoing system normally requires at least three trips
into the well in the sidetracking operation. A first stage begins a
window in the casing, a second stage extends the window through use
of a diamond milling cutter and a third stage with multiple mills
elongates and extends the window. While the window mill is
aggressive in opening a window in the casing, the number of trips,
such as three, to accomplish the task is expensive and time
consuming.
[0012] Typically window mills are designed with a square bottom,
i.e. a square cross-section. As is shown in FIG. 14, a prior art
square bottomed, cross-sectioned mill provides a point of contact
between the mill and the whipstock and a large axial surface
contact between the mill and the casing. As can be appreciated from
FIG. 14, the contact area between the square bottomed mill and
whipstock is substantially a line contact while the contact area
between the mill and casing is much greater. The applied force, due
to the weight on bit, per contact area determines the contact
stress between the members. Because the contact stress between the
mill and the casing is much greater than the contact stress between
the mill and whipstock, the mill tends to cut into the whipstock
rather than into the casing even where the cutability of the
whipstock has been reduced because of hardfacing.
[0013] U.S. Pat. Nos. 2,216,963; 3,908,759; and 4,397,355 disclose
mills having a taper or tapered nose. A starter mill with a tapered
nose will eventually wedge and cannot complete the window or drill
the lateral borehole. U.S. Pat. No. 3,908,759 appears to disclose a
taper on the mill. U.S. Pat. No. 2,216,963 discloses a tapered mill
which is used in a second trip into the well to increase the window
after a square bottomed mill opened the window in a previous trip
into the borehole. These patents do not teach guiding and moving
these tapered mills laterally through the casing so that at least
the center of the downwardly facing cutting surface of these mills
passes outside the exterior wall of the casing in one trip into the
borehole. At least two trips are required into the well, typically
using a starter mill in the first trip to begin cutting a window in
the casing and then a second mill in a second trip to increase the
window. Further, tapered mills are typically less than full gauge
requiring additional trips into the borehole to complete the
window.
[0014] Weatherford Enterra offers a mill which has a taper
extending upwardly and inwardly from a full diameter cutting base.
The mill also includes a support shoulder on the cutting face of
the mill. However, the reduced diameter taper extends above the
full diameter cutting gage of the mill which therefore tends to cut
the whipstock rather than the casing.
[0015] U.S. Pat. No. 5,109,924 teaches a one trip window cutting
operation to sidetrack a wellbore. A deflection wedge guide is
positioned behind the pilot mill cutter and spaced from the end of
a whipstock component. The shaft of the mill cutter is retained
against the deflection wedge guide such that the milling tool
frontal cutting surface does not come into contact with the ramped
face of the whipstock. In theory, the deflection wedge guide
surface takes over the guidance of the window cutting tool without
the angled ramp surface of the whipstock being destroyed.
[0016] However, when a second and third milling tool attached to
the same shaft as the window milling cutter and spaced, one from
the other on the support shaft contacts the whipstock ramped
surface, they mill away the deflection guide projection from the
ramp surface. This inhibits or interferes with the leading pilot
mill window cutter from sidetracking at a proper angle with respect
to an axis of the cased borehole and may cause the pilot window
cutting mill to contact the ramp surface of the whipstock before
the pilot window cutter mill clears the casing. The reamers or
mills aligned behind the pilot window mill, having the same or
larger diameter than the diameter of the pilot window mill,
prevents or at least inhibits the window pilot mill from easily
exiting from the steel casing. This difficulty is due to the lack
of clearance space and flexibility of the drill pipe assembly
making up the one trip window cutting tool when each of the
commonly supported reamer mills spaced along the shaft,
sequentially contact the window in the steel casing. Hence, the
sidetracking apparatus tends to go straight rather than be properly
angled through the steel pipe casing.
[0017] U.S. Pat. No. 5,445,222 teaches a combination whipstock and
staged sidetrack mill. A tapered, cone-shaped mill is located on
the end of a common shaft and has an outer diameter of about 50 to
75 percent of the maximum diameter to which the final sidetracked
hole will be completed. Three stages of cutting mills are disposed
above the tapered mill on the common shaft. Each successive stage
increases in diameter. A surface of a second stage cutter is, at
its smallest diameter, about the diameter of the maximum diameter
of the tapered mill, and is, at its largest diameter, at least 5
percent greater in diameter than the diameter of the tapered mill.
A surface of a final stage cutter mill is, at its largest diameter,
about the final diameter dimension, and at the smallest cutting
surface diameter, is a diameter of at least about 5 percent smaller
than the final diameter dimension. The whipstock guide is made of a
material that is harder than the casing but not as hard as the
cutting elements of the mill whereby the mill is to cut the casing
rather than the whipstock.
[0018] The sidetracking mill is designed to accomplish the milling
operation in one trip. The mill however, tends to go straight and
penetrate the ramped surface of the whipstock. Substantial damage
to the whipstock occurs and sidetracking may not occur as a
result.
[0019] While the intent is to perform a sidetracking operation in
one trip, difficulties often arise when attempting to deviate the
drill string from its original path to an off line sidetracking
path. Progressively larger in diameter reaming stages to enlarge
the window in the steel casing inhibits the drill shaft from
deviating or flexing sufficiently to direct the drill pipe in a
proper direction resulting in damage to the whipstock and
misdirected sidetracked boreholes. In other words, the sidetracking
assembly tends to go straight rather than deviating through the
steel casing.
[0020] The present invention overcomes these deficiencies in the
prior art.
SUMMARY OF THE INVENTION
[0021] The side tracking system of the present invention includes a
window mill having a tapered cutting surface which allows the mill
to initiate the cutting of a window into the casing and to move the
center of the downwardly facing cutting surface of the mill
laterally through the window and past the exterior wall of the
casing in one trip into the well without substantially cutting up
the whipstock. The tapered cutting surface of the window mill
includes taper from a full diameter cutting surface to a reduced
diameter cutting surface adjacent the downwardly facing bottom
cutting surface of the mill. The mill preferably is used in
combination with a whipstock having a ramp which engages the
tapered cutting surface of the mill forming a large contact area
between the mill and whipstock. The materials of the casing have a
first cutablity and the materials of the whipstock have a second
cutability.
[0022] The tapered cutting surface contacts the whipstock ramp at a
first contact area and the full diameter cutting surface of the
mill contacts the wall of the casing at a second contact area. As
weight is applied to the mill, there is a first contact stress at
the first contact area and a second contact stress at the second
contact area. The ratio of cutability of the mill with the
whipstock and casing is the first cutability divided by the second
cutability and the ratio of the contact stress of the mill with the
whipstock and casing is the first contact stress divided by the
second contact stress. The mill of the present invention cuts the
casing rather than the whipstock by maintaining the product of the
cutability ratio and the contact stress ratio less than one. This
also causes the height of the tapered cutting surface to be at
least 50% of the total height, the total height being the distance
from the top of the largest diameter cutting surface on the mill to
the bottom of the mill.
[0023] An object of the present invention is to achieve a
cutability ratio times the contact stress ratio of the mill with
the whipstock and casing which is less than one such that the mill
tends to cut the casing rather than the whipstock. Thus it is a
further objective to maximize the contact area between the mill and
the whipstock such as by having a tapered cutting surface on the
mill and a ramp on the whipstock which has angle substantially the
same as the taper of the tapered cutting surface on the mill.
Additionally, the contact area is maximized by causing the height
of the tapered cutting surface to be at least 50% of the total
height of the mill which is the height of the tapered cutting
surface and the full diameter cutting surface.
[0024] It is an object of this invention to provide a side tracking
system which will deflect and move the tapered mill laterally
through the casing so that at least the center of the downwardly
facing cutting surface of the mill passes outside the exterior wall
of the casing in one trip into the borehole. Further it is an
object to provide a side tracking system in two trips or less and
preferably a one trip cutting system for cutting a deviated hole in
an existing earth borehole.
[0025] It is another object of this invention to provide a one trip
window cutting system for cutting an opening in a pipe casing for
subsequent side tracking drilling operations.
[0026] More specifically, it is an object of this invention to
provide a mill with a tapered cutting end which matches the ramp
angle of the whipstock face such that in operation, as the drill
string is rotated downwardly, the face of the whipstock forces the
tapered cutting end of the window mill out through the pipe casing.
The angled face of the whipstock adjacent to the window cutting
mill and the cutter mill itself is hardfaced to minimize damage to
both the whipstock and the cuter mill.
[0027] A one trip side track window cutting apparatus for cutting
sidetracking windows in a casing positioned in previously drilled
boreholes consist of a window cutting mill affixed to an end of a
shaft, a body of the mill forming a tapered cutting end.
[0028] A whipstock forms a ramp, the angle of which substantially
parallels an angle of the tapered cutting end of the window mill.
The ramp acts as a bearing surface for laterally forcing the window
mill into the pipe casing. The face of the whipstock changes the
rate of deflection of the window mill into the pipe casing.
[0029] The whipstock upstream end is ramped about 15.degree. to
match a 15.degree. taper at the end of the window mill cutter. The
whipstock upper end is attached to the end of the window mill
cutter at the 15.degree. interface through a shear bolt extending
from a blade of the window mill for installation of the whipstock
in a cased borehole. The end of the whipstock is heavily hardfaced,
especially adjacent the interface with the window cutter mill.
Another mill is positioned upstream of the window mill on the same
supporting shaft and is preferably the same diameter as the window
mill. When the shear bolt is sheared through an upward force on the
drilling string after the whipstock is anchored and properly
oriented in the cased borehole, the hardfaced ramp formed by the
end of the whipstock forces the window mill immediately into the
wall of the casing. Simultaneously, the second mill spaced from the
window mill is forced into the casing thus starting two openings in
the casing. The whipstock face below the 15.degree. ramp parallel
the walls of the casing for a distance to allow both the window
mill and the second mill to cut the window started by the initial
15.degree. ramp. As the window cutting process proceeds, the ramp
surface of the whipstock transitions into a "normal" 3.degree. ramp
for a sufficient distance for the window mill to extend about half
way out of the casing where the ramped surface of the whipstock
transitions again to a more aggressive angle to further urge the
window mill out of the casing.
[0030] Once the window mill is centered on the wall of the casing,
further cutting becomes difficult because of the reduced rotation
of the cutting edges at the center of the tapered window mill. At
the exact center of the tapered window mill, there is essentially
zero rotation. Thus, in the prior art, it took a long cutting time
to have the window mill move and cut past its center line. On a
standard 3.degree. whip face, it often took a drilling length of
plus or minus ten inches to have the center line of the window mill
cross the wall of the casing. Very slow drilling progress is made
during this period of time because the window mill is attempting to
cut the wall of the casing with essentially zero rotation at the
center of the window mill.
[0031] It is advantageous for all of the mills to be full gage. One
advantage is that with your window mill being full gage, the window
hole will also be full gage when drilling is stopped with the
assembly. If the window mill is undergauged, then when the drilling
bit is run into the well, the full gage drilling bit is going to
slow down as it cuts the under gage borehole to full gage. This
then slows down the operator's ability to kick off and drill the
new borehole with the drilling bit. The drilling bit must remount
the bottom section of the borehole cut by the window mill. If the
hole is full gage, they will be able to use the whip to help build
an angle faster and apply weight to the drilling bit to drill
laterally the new borehole. If they have to go down and remount the
hole, then they are much further down in the hole before they can
kick out for their lateral drilling.
[0032] The window mill tapers conform to most of the ramp angles
formed by the whipstock. For example, the largest diameter of the
window mill forms a 3.degree. cutting section matching the
3.degree. section of the whipstock below the cylindrical portion of
the whipstock. Of course, the 15.degree. angle of the window mill
is parallel to the 15.degree. formed at the top of the whipstock.
These matching angulations minimize damage to the whipstock face
during the window cutting process thereby assuring a successfully
cut window in the casing of the borehole.
[0033] After both the window mill and the second mill cut
completely through the casing, the window mill is tripped out of
the borehole. The sidetracking drilling operation then
commences.
[0034] An advantage then of the present invention over the prior
art is the use of a tapered window mill with a surface contour
matching the ramp angle formed at the upstream end of the whipstock
such that the mill is forced into the casing immediately after the
window mill is released from the whipstock without damage to the
whipstock.
[0035] Another advantage of the present invention over the prior
art is the formation of angled and parallel ramp surfaces formed on
the whipstock to facilitate and enhance the cutting action of both
the window mill and the second mill, upstream of and spaced from
the window mill.
[0036] Still another advantage of the present invention over the
prior art is the use of an acutely angled ramp section at a point
along the ramped whipstock surface when the center of the window
mill reaches the inside diameter of the wall of the casing
resulting in a slowdown in the window cutting operation. The "kick
out" ramp more quickly moves the tapered window mill past this
phase of the window cutting process thus speeding up the completion
of the sidetrack window.
[0037] Other objects and advantages of the present invention will
appear from the following description.
DESCRIPTION OF THE DRAWINGS
[0038] For a detailed description of a preferred embodiment of the
invention, reference will now be made to the accompanying drawings
wherein:
[0039] FIG. 1 is a partial cross-sectional view of a prior art
sidetracking operation depicting setting an anchor for a typical
whipstock sidetracking system in a cased borehole.
[0040] FIG. 2 is a partial cross-sectional view of a first stage of
the prior art sidetracking operation illustrating cutting a window
section in a pipe casing with a typical starter mill.
[0041] FIGS. 3A and B are a partial cross-section of a preferred
embodiment of the invention whereby the top of the whipstock
matches the taper of the window mill.
[0042] FIG. 4 is an enlarged partial cross-section of the tapered
window mill illustrating the hollow shear pin attaching the tapered
window mill to the parallel ramped surface formed adjacent the top
of the whipstock.
[0043] FIG. 4A is an enlargement of the tapered window mill of FIG.
4 showing contact areas between the mill, casing, and
whipstock.
[0044] FIG. 4B is a free body force diagram showing the forces
applied to the assembly of FIG. 4.
[0045] FIG. 5 is a perspective view of the tapered window mill with
chip breaking cutter elements attached to the cutting face of each
blade of the window mill.
[0046] FIG. 6 is a partial cross-section of the one trip sidetrack
window cutting apparatus wherein the mill is sheared from the top
of the whipstock and is moved laterally through the casing by
15.degree. ramp angle formed in the top of the whipstock.
[0047] FIG. 7 is a partial cross-section of the window mill and
upstream "tear drop" cutter cutting the window in the pipe casing.
The ramp section immediately below the 15.degree. ramp formed in
the whipstock is parallel to the axis of the pipe casing while the
tear drop cutter completes its initial cut in the window from its
entry into the casing to its intersection with the cut made by the
tapered window mill.
[0048] FIG. 8 is a partial cross-section of the window mill
contacting a second "kick out" ramp formed in the 3.degree. ramp
portion of the whipstock, the kick out ramp serves to force the
window mill out of the casing so that it will complete the window
more efficiently.
[0049] FIGS. 9A and B are a partial cross-section of an alternative
window cutting apparatus identical to the apparatus shown with
respect to FIGS. 6 through 8 with the exception of a "watermelon"
mill positioned upstream of the tear drop mill.
[0050] FIGS. 10A and B are a partial cross-section of the
alternative apparatus illustrating the watermelon mill starting its
cut into the pipe casing above the window started by the downstream
mills.
[0051] FIGS. 11A and B are a partial cross-section of the
alternative apparatus after the window, tear drop and watermelon
mills have cut an elongated window in the casing.
[0052] FIG. 12 is a partial cross-section of an alternative
whipstock with a "kick out" ramp in the 3.degree. ramp portion.
[0053] FIG. 13 is a view taken through 13-13 of FIG. 12.
[0054] FIG. 14 is a diagrammatical representation of a prior art
square bottom mill showing contact areas.
[0055] FIG. 15 is a diagrammatical representation of an alternative
side tracking system of the present invention with a mill having a
rounded profile.
[0056] FIG. 16A is a diagrammatical representation of the mill of
the present invention with a prior art whipstock having no ramp at
its upper end.
[0057] FIG. 16B is a diagrammatical representation of the mill of
FIG. 16A with the tapered mill having cut a taper in the face of
the prior art whipstock.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0058] Referring now to the prior art of FIG. 1, the casing
sidetrack system generally designated as 10 consists of a drill
collar 12 attached to a starter mill 14. The starter mill 14 is
affixed to the end of the whipstock 16 through a shear bolt block
15. The whipstock 16 has an anchor 18 attached to the down hole end
of the whipstock. The entire assembly 10 is tripped into a borehole
9 cased with steel pipe casing 11. The casing 11 has an interior
annular wall having an inside diameter D.sub.I and an exterior
annular wall having an outside diameter D.sub.O. After the
sidetracking system reaches a desired depth in the borehole, the
whipstock 16 is oriented to a desired sidetrack angulation and set
or anchored in the steel pipe casing 11. Casing 11 generally is
made of steel but may be made of various other materials such as
fiberglass for example.
[0059] With reference to the prior art of FIG. 2, once the system
10 is properly oriented and set in the casing 11, the starter mill
14 is released from the end of the whipstock 16 by breaking the
solid shear pin 22 secured to the bolt block 15. The starter mill
14 is subsequently directed into casing 11 by shear bolt block 15
along ramped surface 17 formed by whipstock 16. The starter mill 14
then mills a window 20 through the wall of the casing 11. After the
starter mill 14 begins the window 20, it is tripped out of the
cased borehole 9.
[0060] Turning now to the preferred embodiments represented in
FIGS. 3 through 8, FIGS. 3A and B illustrate a one trip mill
assembly generally designated as 30 and a whipstock assembly
generally designated as 60 that includes a whipstock 44. The mill
assembly 30 includes a tapered window mill generally designated as
32. The mill 32 is attached to the bottom end of a shank or shaft
31. Upstream and spaced from the window mill is, for example, a
second mill 33 also mounted to the shaft 31. The upstream end of
the shaft 31 is either threadably connected to a drill string or
threaded to another subassembly (see FIGS. 9 through 11). A tubular
member 27 may form the shaft 31 on which mills 32 and 33 are
mounted. Tubular member 27 may include a lower reduced diameter
portion on which mill 32 is disposed with mill 33 being disposed on
the full diameter of tubular member 27. This reduction in diameter
provides flexibility between mills 32 and 33 during the milling
process.
[0061] A third mill may be mounted to a shaft upstream of second
mill 33. The third mill is desirable in some circumstances and will
be discussed in detail with respect to FIGS. 9, 10 and 11.
[0062] Referring now to FIGS. 3 through 5, the window mill 32
includes a plurality of blades, such as blade 34, having a
particular cutting profile. Each blade 34 has, for example, a
multiplicity of cutting elements such as tungsten carbide cutters
42 with "chip breakers" formed on the face of the cutters. The chip
breakers on the face of each cutter serves to break up the curled
cuttings resulting from the window mill 32 cutting through the pipe
casing 11 so that the cuttings may be transported up the drill
string annulus by the mud circulated through the drill string.
Without the chip breaker, the continuous cuttings create a "rats
nest" downhole and cannot be easily removed. These highly effective
cutters are manufactured by Rogers Tool Works, Rogers, Ark. and are
known as Millmaster. It would be obvious to utilize natural or
polycrystalline diamond cutters (not shown) on the cutting blades
34 of the tapered window mill 32 without departing from the spirit
of this invention.
[0063] Blade 38 immediately adjacent the parallel surface 45 of
whipstock 44 is preferably wider to accommodate the shear bolt 39
threaded into the blade 38. The head of the shear bolt 63 is seated
in the end of the whipstock 61 and the threaded shank 54 is
threaded into blade 38. The shank 54 of the shear bolt is
preferably 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 conduit 37 that directs
fluid to the whipstock anchor (not shown). It would be obvious
however to utilize a shear bolt with a solid shank without
departing from the scope of this invention.
[0064] The blades 34 of window mill 32 form a radial or lateral
cutting surface which includes the profile of three cutting
surfaces, namely a lower tapered cutting surface 52, a medial
cutting surface 43, and a full diameter cutting surface 53. As
defined, the radial cutting surface does not include the back
tapered surface 55 above full diameter cutting surface 53. The
tapered cutting surface of mill 32 is defined as that portion of
the radial cutting surface which forms an angle with the axis 29 of
mill 32 and as shown in the preferred embodiment, includes lower
tapered cutting surface 52 and medial tapered cutting surface 43.
It should be appreciated that although mill 32 is shown as having
two tapered cutting surfaces 43 and 52, mill 32 may have a common
taper or may have three or more different tapers.
[0065] The blades 34 also form a downwardly facing bottom cutting
surface 57. Bottom cutting surface 57 is generally flat and
circular having a diameter which is at least 30% and preferably 65%
of the diameter of the full diameter cutting surface 53. This sized
bottom cutting surface 57 provides stability to cutting operation
of the mill 32.
[0066] The lower tapered cutting surface 52 of the window mill 32
is tapered, for example, 15.degree. with respect to the axis 29 of
the window mill 32 and the casing 11 in the borehole. The taper may
be in the range of an angle A from 1 to 45.degree. with respect to
the axis 29. The height of tapered cutting surface 52 measured
along the axis 29 is L.sub.3. A shear pin 39 anchors the tapered
window mill 32 through a connection in blade 38 of the mill 32 to
profiled end surface 45 of whipstock 44. The end surface 45 of the
whipstock 44 is profiled (angle 15.degree.) to match the angle of
the lower tapered end 52 of the window mill (15.degree.) as
hereinafter described.
[0067] The medial cutting surface 43 has a reduced taper of
3.degree. which conforms to the 3.degree. tapers on the profiled
ramp surface 28 of the whipstock 44. The taper of surface 43 may be
in the range of 1 to 15.degree. with the axis 29. The height of
medial taper 43 measured along the axis 29 is L.sub.2.
[0068] The final full diameter cutting surface 53 extends
vertically above medial cutting surface 43 and is parallel to the
axis 29. The height of full diameter cutting surface 53 measured
along the axis 29 is L.sub.1. Full diameter cutting surface 53 is
the full diameter of the mill 32, i.e. it is the major (largest)
diameter of mill 32. It should be appreciated that the full
diameter of mill 32 is preferably at least 75% or greater of the
full diameter of casing 11 or of the maximum diameter to which the
final sidetracked borehole will be completed and still more
preferably is substantially full gauge. Full gauge is defined as
the maximum diameter of a mill which can pass down through the
casing 11.
[0069] The full diameter cutting surface begins at the first full
diameter of the mill 32 as one moves down the profile of the mill
32 from top to bottom. This is the first point where the mill 32
reaches its full diameter. In the preferred embodiment, the full
diameter is below tapered back surface 55. The height of the radial
cutting surface is the distance from the top of the full diameter
cutting surface 53, i.e. the top of the largest diameter surface of
mill 32, to the bottom of the tapered cutting surface adjacent
downwardly facing bottom cutting surface 57. This height equals
L.sub.1+L.sub.2+L.sub.3.
[0070] The tapered cutting surface, i.e. lower tapered end 52 and
medial cutting surface 43, are under full diameter since their
diameter is less than that of full diameter cutting surface 53. It
is preferred that the height of the full diameter cutting surface
53 of the mill 32 be at least 3% and no more than 70% of the radial
cutting surface of mill 32. Thus, L.sub.1 is less than 70% of the
sum of L.sub.1+L.sub.2+L.sub.3. It is even more preferred that the
height of the tapered cutting surface be greater than the height of
the full diameter cutting surface of mill 32. Stated differently,
the tapered cutting surface, i.e. L.sub.2+L.sub.3, be at least 50%
of the total radial cutting surface height, i.e.
L.sub.1+L.sub.2+L.sub.3. Preferably the full diameter cutting
surface 53 have a sufficient height so as to allow some wear on the
full diameter blades 34 and still maintain full diameter cutting.
Such sufficient height is approximately 3 to 20% of the total
radial cutting height.
[0071] Referring now to FIGS. 3A and 3B, the whipstock 44 has a
diameter D.sub.W which approximates the inside diameter D.sub.I 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.degree., a vertical surface 46 preferably parallel to the axis
26, an initial ramp surface 47 having a standard angle preferably
3.degree., a "kick out" surface 48 having a steep angle preferably
15.degree., and a subsequent ramp surface 49 having a standard
angle preferably 3.degree.. 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.degree., and preferably in the
range of 2 to 30.degree., and still more preferably in the range of
3 to 15.degree., and most preferably 15.degree.. The vertical
surface 46 has a length approximately equal to or greater than the
distance between mills 32 and 33.
[0072] Surface 45 may be heavily hardfaced with, for example, a
composite tungsten carbide material 51 metallurgically applied to
the ramp surface. Moreover, the entire profiled ramp surface 28 of
the whipstock 44, exposed to the cutting action of the mills, may
be hardfaced.
[0073] When the window mill 32 is full gage, the "kick out" ramp
surface 48 begins at that point on the initial 3.degree. ramp
surface 47 where the thickness of the ramp surface 47 is
approximately equal to the radius of the whipstock 44. In other
words, the radial distance between that point on surface 47 and the
inside diameter D.sub.I of the wall of the casing 11 should be
approximately the same or slightly greater than the radius of the
window mill 32. This ensures that "kick out" ramp surface 48 will
increase the rate of deflection of the window mill 32 just before
the center 25 of the bottom cutting surface 57 of window mill 32
reaches the inside diameter D.sub.I of the wall of the casing 11.
The "kick out" ramp surface 48 forms an accelerator ramp which
exerts a lateral force to the window mill 32 and greatly increases
the rate of deflection of the window mill 32 into the wall of the
casing 11. Although the preferred angle of "kick out" surface 48 is
15.degree., the angle may be from 10 to 45.degree.. It should be
appreciated that the kick out ramp surface 48 may be used in
constant angle whipstocks such as a whipstock having a standard
ramp surface of, for example, 2 to 3.degree., with the "kick out"
ramp surface having a substantially greater ramp angle located at
approximately the mid-whip position of the whipstock thereby
creating a jog or deviation in the otherwise constant angle of the
whipstock. The use of the "kick out" ramp surface 48 allows the
design of the window mill 32 to incorporate a lighter dressing
which will increase formation ROP.
[0074] The backside 62 of the whipstock 44, especially adjacent the
upper end 61 of the whipstock 44, is contoured to conform to the
inside diameter D.sub.I 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 a, for example, hydraulically
actuated anchor (not shown). A typical anchor is shown in U.S.
patent application Ser. No. 572,592 filed Dec. 14, 1995,
incorporated herein by reference.
[0075] The mill 32 and whipstock 44 of the present invention 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. Various ones of the physical
properties of the materials of the casing 11 and whipstock 44
determine their cutability, i.e. their resistance to cutting.
Cutability is not a particular property such as hardness but is a
combination of properties. Cutability is developed through the test
cutting of the materials for the whip 44 and for the casing 11. The
lower the cutability number the harder the material is to cut.
[0076] To insure that the mill 32 cuts the casing 11 rather than
the whipstock 44, the assembly must achieve the following formula:
C*(AF.sub.W/CA.sub.W)=AF.sub.C/A.sub.C
[0077] Where CA.sub.W is the contact area between the whipstock 44
and mill 32;
[0078] AF.sub.W is the applied force on the contact area CA.sub.W
of the whipstock 44;
[0079] CA.sub.C is the contact area between the casing 11 and mill
32;
[0080] AF.sub.C is the applied force on the contact area CA.sub.C
of the casing 11; and
[0081] C is the ratio of the cutability of the whipstock 44 to the
cutability of the casing 11.
[0082] Since contact stress CS is the applied force AF divided by
the contact area CA, CS=AF/CA, and therefore
CS.sub.W=AF.sub.W/CA.sub.W and CS.sub.C=AF.sub.C/CA.sub.C
Substituting: C*(CS.sub.W/CS.sub.C)<1 Thus, the mill 32 will
more easily cut the casing 11 before the whipstock 44 so long as
the cutability ratio times the contact stress of the whipstock 44
divided by the contact stress of the casing 11 is less than one.
One result of the contact stress equation is that it is preferred
that the height of the full diameter of the mill 32 be less than
the height of the under full diameter of the mill 32. As indicated
previously, being full diameter does not mean the mill necessarily
is full gauge.
[0083] Referring now to FIG. 4B, making some simple assumptions, a
free body force diagram is shown for the milling assembly of FIG.
4A. W.O.B. is the weight applied to the mill 32. The operator
controls the weight on bit force. The applied force AF.sub.C of the
casing 11 is shown applied to the full diameter cutting area 53.
The applied force AF.sub.W of the whipstock 44 is shown applied to
the lower tapered end 52 and is a component of the W.O.B.
determined by the angle A. It can be seen that the contact stress
is geometry dependent.
[0084] The smaller the ratio C of the cutability of the whipstock
44 to the cutability of the casing 11, the larger the ratio of the
contact stresses can be between the mill 32, casing 11 and
whipstock 44 and have the mill 32 cut the casing 11 better than the
whipstock 44. Thus, it is preferred that the material of the
whipstock 44 have a low cutability. An ideal situation would be to
have the whipstock made of a material such as tungsten carbide
while the casing 11 is made of steel to reduce the ratio C.
Further, a lower cutability ratio allows the height of the full
diameter cutting surface to be increased such that the height of
the full diameter cutting surface may be greater than the height of
the under gauge cutting surface. A higher cutability ratio will
require a lower contact stress ratio to insure that the product of
the ratios is less than one.
[0085] The tapered contact between the mill 32 and whipstock 44
provides a horizontal side component force which is applied to the
casing 11. The angle of contact A between the whipstock 44 and the
mill 32 determines this side component which equates to the
horizontal component of the applied force on the contact area.
Setting the sum of all forces to zero and assuming no resistance to
bending, AF.sub.C=W.O.B.*(1/Tan A) and AF.sub.W=W.O.B.*(1/Sin A).
The smaller the angle A, the larger the side load components
AF.sub.C and AF.sub.W. The object is to keep the contact area
CA.sub.C between the casing 11 and the mill 32 to a minimum. As the
milling progresses, CA.sub.C increases until the mill 32 reaches
the outside wall of the casing 11. Once the mill 32 breaks through
the casing 11, the contact area CA.sub.C begins to reduce.
[0086] Referring again to FIG. 4A, the equation may be applied to
the preferred embodiment. If both the materials of the whipstock 44
and the casing 11 are assumed to be the same, then the cutability
ratio C is 1 and no longer is a factor in the equation. If C is 1,
then the contact stress CS.sub.W of the whipstock 44 must be less
than the contact stress CS.sub.C on the casing 11 to prevent the
mill 32 from cutting away the whipstock 44.
[0087] Applying the equation to FIG. 4A, and assuming a W.O.B. of
5000 lbs and an angle A of 15.degree., then AF.sub.C=18,660 lbs and
AF.sub.W=19,319 lbs. If CA.sub.W=10 in.sup.2 and CA.sub.C=5
in.sup.2, then CS.sub.C=3732 psi and CS.sub.W=1932 psi. Inserting
these into the equation, then
C*(CS.sub.W/CS.sub.C)=1*(1932/3732)=0.5<1.
[0088] Referring to FIG. 14, there is shown a prior art mill. Again
assuming W.O.B. is 5000 lbs but with a square bottom mill and a
whipstock with a taper of 3.degree.. Calculating the applied
forces, AF.sub.C=95,406 lbs and AF.sub.W=95,537 lbs. With
CA.sub.C=10 in 2 and CA.sub.W=1 in.sup.2, then CS.sub.C=9,541 psi
and CS.sub.W=95,537 psi. Inserting these into the equation, then
C*(CS.sub.W/CS.sub.C)=1*(95,537/9,541)=10>1. With the ratio of
the contact stresses being greater than 1, the prior art square
bottom mill will cut the whipstock rather than the casing.
[0089] The preferred angle A will vary depending upon various
factors including the cutability of the casing 11 and whipstock 44.
By making the contact area between the mill 32 and the whipstock 44
large, the contact stress between the mill 32 and whipstock 44 is
low. The objective is to achieve a contact stress ratio which is as
low as possible. Any ratio less than 1 will accomplish the
objective of cutting the casing 11 over the whipstock 44.
[0090] The present application is directed to the interaction of
the mill 32, whipstock 44, and casing 11. One objective is to
maximize the contact area between the mill 32 and the whipstock 44
and to minimize the contact area between the mill 32 and the casing
11 during critical stages of the milling operation. It was intended
that the contract stresses on the casing 11 be higher so that the
casing 11 would be cut by the mill 32 rather than the mill 32
cutting away the whipstock 44. Thus, the objective is to have
sufficient contact area between the mill 32 and whipstock 44 to
ensure that the contact stresses between the mill 32 and the casing
11 are greater causing the casing 11 to be cut rather than the
whipstock 44.
[0091] The mill 32 of the present invention may have various cross
sectional cutting profiles so long as the contact areas with the
casing 11 and whipstock 44 produce the preferred contact stresses.
The objective is to configure the contact stresses between the mill
32, casing 11, and whipstock 44 so that the casing 11 will be cut
away. Referring now to FIG. 15, there is shown a mill 70 having a
rounded cutting surface 72. Assuming the cutability ratio to be
one, so long as the contact stress between the mill 70 and
whipstock 74 is greater than the contact stress between the mill 70
and casing 11, the casing 11 will be cut more than the whipstock
74.
[0092] In operation, the assembly 30 is lowered into cased borehole
9 to a predetermined depth. The whipstock 44 is then rotated to a
desired sidetrack direction followed by hydraulically actuating the
anchor (not shown) by directing drilling fluid or "mud" down the
drill string 12 under high pressure through flex conduit 37
connected to a coupling 35 on the end of the window mill 32.
Coupling 35 includes a weakened area therearound such as a reduced
diameter portion allowing coupling 35 to break cleanly from the
mill 32. The pressurized fluid then enters conduit 50 formed in the
whipstock 44 and from there to a connecting member 19 and then to
the anchor to extend the pipe gripping elements within the anchor
(not shown).
[0093] Referring particularly to the enlarged FIG. 4A, once the
anchor is set, weight/tension is applied to the drill string 27
imparting sufficient forces to break the shear pin 39 freeing the
tapered window mill 32. The mill 32 is then rotated and lowered to
make contact with the whipstock 44 and casing 11. The relatively
steep profiled angle A (15.degree.), formed in surface 45 of the
whipstock 44, immediately provides a lateral force to the tapered
end 52 of the mill 32 thus forcing the rotating mill 32 into the
interior of the wall of the pipe casing 11 to start forming a first
window 20A in the pipe casing 11.
[0094] The upstream second mill 33, which may be tear drop in
shape, is also forced into the wall of the pipe casing 11 thereby
simultaneously cutting a second window 20B above the first window
20A formed by the window mill 32. The surface 46 formed by the
whipstock 44 below angled surface 45 is preferably parallel to the
axis of the pipe casing 11 while the window mill 32 and the second
mill 33 cut simultaneous windows 20A and B (FIG. 6).
[0095] With specific reference to FIG. 7, once the upstream window
20B (cut by the second mill 33) merges with the downstream window
20A started by the window mill 32, cutting forces are lessened. The
ramp surface 47 formed by the whipstock 44 below the parallel
surface 46 then transitions into a ramp with a 3.degree. angle.
[0096] Referring now to FIG. 8, when the center 25 of the bottom
cutting surface 57 of the window mill 32 starts cutting at the
inside diameter of the wall of the casing 11 as the window milling
apparatus progresses down the whipstock 44 and out through the
window 20 cut into the pipe casing 11, the cutting or pipe milling
action is slowed considerably. At this point the "kick out" ramp 48
(15.degree. as compared to the 3.degree. ramp surface 47) "kicks"
the window mill 32 out through the casing 11 for more efficient
milling of the casing 11. Once the center 25 of mill 32 passes from
the interior to the exterior of the casing 11 and this part of the
window milling process is overcome, the ramp 49 below the kick out
ramp 48 reverts back to the standard 3.degree. ramp angle surface
49.
[0097] An alternative embodiment is illustrated in FIGS. 9 through
12. A second subassembly generally designated as 56 is positioned
intermediate mill assembly 30 and the drill string 12. A third mill
58, such as a watermelon mill, is spaced between the male and
female ends of the shank or shaft 59 (FIG. 9).
[0098] FIG. 10 illustrates the third mill 58 having generally the
same diameter as the window mill 32 and second mill 33 and serves
to both lengthen the window 20 penetrating the casing 12 above the
window 20 cut by the window and second mills 32, 33. It is
preferred that all three mills 32, 33 and 58 be full gage.
[0099] The third mill 58 also serves to dress the window opening 20
as shown in FIG. 11 for easy transition of the following side track
drill bit assembly.
[0100] The elongation of the window 20 by the watermelon mill 58 is
desirable to facilitate sidetracking drill bit assemblies that are
relatively stiff and the angle of the side track borehole is
slight. A longer window then would be necessary.
[0101] Where the side track angle is more severe and the drill bit
side track assembly is relatively limber, a shorter window will
suffice and the watermelon assembly 56 is omitted from the window
cutting apparatus as is shown with respect to FIGS. 3 through
8.
[0102] Upon assembly, mill assembly 30 is connected to whipstock
assembly 60 by shear bolt 39 with the lower tapered end 52 of
window mill 32 being engagingly disposed against starter surface
45. Further, hydraulic hose 37 is connected to assemblies 20,
30.
[0103] In operation, the whipstock assembly 20 and mill assembly 30
are connected to the lower end of a drill string 12 and lowered
into cased borehole 9 as shown in FIGS. 9A and B. Once the desired
depth is reached for the secondary or deflection bore, the
whipstock assembly 20 is aligned and oriented within the cased
borehole 9 and the anchor is set thereby anchoring the whipstock
assembly 20 within the cased borehole 9 at the desired location and
orientation. Tension is then pulled on drill string 12 to shear
shear bolt 39.
[0104] The mill assembly 30 is then rotated and lowered on the
drill string 12. The complimentary lower tapered end 52 on the
rotating window mill 32 cammingly and wedgingly engages starter
surface 45 on whipstock 44 thereby causing the window mill 32 to
kick out and engage the wall of the casing 11 thereby forcing the
cutting elements 34 into milling engagement. As the window mill 32
rotates and moves downwardly, the window mill 32 continues to be
deflected out against the wall of the casing 11 and eventually
punches through the wall of the casing 11. It is important that the
starter surface 45 and its center line match that of the initial
surface 52 on the window mill 32. The angle of tapered end 52 and
starter surface 45 may be up to 45.degree..
[0105] Once initial punch out has been achieved, weight on the
drill string 12 is required to push the window mill 32. It is the
"punch through" of the window mill 32 that is the most important
cutting. Once the window mill 32 punches through the wall of the
casing 11, a ledge is created allowing the whipstock 44 to then
guide the mill assembly 30 through the window 20 cut in the wall of
the casing 11.
[0106] This initial guidance of the starter surface 45, the large
contact area, and the hard facing 51 ensures that the whipstock 44
is not badly damaged by the window mill 32 and that the window mill
32 properly initiates the required window cut. It is important to
deflect the window mill 32 away from the ramp surface 20 of the
whipstock 44 to avoid the window mill 32 from milling the whipstock
44.
[0107] Referring now to FIGS. 10A and B, once the initial punch out
is made through the wall of the casing 11 by the window mill 32,
the window mill 32 has past the starter surface 45 and is adjacent
the straight surface 46 which allows the mill 32 to run along a
straight track. Once the window mill 32 moves past the starter
surface 45, window mill 32 continues to mill the wall of the casing
11 while the second mill 33 expands the window in the wall of the
casing 11 previously cut by the window mill 32. As the second mill
33 follows behind the window mill 32 and begins to cut into the
wall of the casing 11, there is formed an uncut portion of the
casing 11 between the two mills 32, 33 which has not yet been
milled. As the window mill 32 is lowered downwardly adjacent to
straight surface 42, the second mill 33 cuts the unmilled portion
of casing 11 which extends between mills 32, 33.
[0108] If the second mill 33 is deflected into the casing 11, then
that portion of tubular member 27 between the window mill 32 and
pilot mill 33 may engage the uncut portion of the casing wall which
has not yet been milled out. If the window mill 32 maintains the
steep angle of the starter surface 45, it is possible that that
portion will engage the uncut portion of the wall of the casing 11
and prevent the mills 32, 33 from cutting the wall of the casing
11. It is possible that the mill assembly 30 could bind and hinder
further milling. This is prevented by straight surface 46 which has
a height substantially equal to or greater than the distance
between mills 32 and 33.
[0109] Upon the window mill 32 moving past the straight surface 46,
any uncut portion of the casing wall between the mills 32, 33 has
now been cut by the second mill 33. At this point, the medial
surface 43 of window mill 32 engages the ramp surface 47 and the
window mill 32 is again deflected outwardly against the wall of
casing 11 to enlarge the window 20 and is guided by the surface 47
into the wall of the casing 11 without causing any damage to the
whipstock 44. Now that the window mill 32 has punched through the
wall of the casing 11, it begins cutting into the cement. The
second mill 33 is now passing along the straight surface 46 and
cutting the window 20 that has already been started by the window
mill 32 to make the window wider. As can be appreciated, watermelon
mill 58, following the second mill 33, also begins cutting and
widening the window 20 through casing 11. There may be one or more
additional watermelon mills above the first watermelon mill 58. The
purpose of the watermelon mills is to elongate the top of the
window 20 in the casing 11 and clean up the window 20 particularly
if there has been a ledge created.
[0110] Referring now to FIGS. 11A and B, upon completing the
milling along the surface 47, the casing wall will be underneath
the window mill 32 and the center 25 of the window mill 32 is
approaching the inside diameter of casing 11. At this point, the
window mill 32 engages kick out surface 48 to assist the crossing
of the wall of the casing 11. The steeper angle on surface 48
causes the center 25 of window mill 32 to more quickly kick out and
radially pass from the inside diameter to the outside diameter of
the wall of casing 11. The second mill 33 and watermelon mill 58
are following and expanding and clearing the window in the wall of
the casing 11. The mill assembly 30 drills faster into the
formation once the window mill 32 completely passes the cased wall
and into the formation.
[0111] The kick out wedge surface 48 is a second steep surface to
assist in moving the window mill 32 from the inside diameter to the
outside diameter of the wall of the casing 11. When the center line
25 of the window mill 32 is sitting on the wall of the casing 11,
the window mill 32 is essentially at zero rotation. The purpose for
the kick out surface 48 is to reduce the drilling time required to
cross the wall of the casing 11. The increased angle of surface 48
allows the window mill 32 to move quickly across the wall of casing
11. By increasing the angle between window mill 32 and whipstock
44, the cutting distance of the window mill 32 is shortened for the
center line 25 of the window mill 32 to cross the wall of the
casing 11.
[0112] Further, additional weight can be applied to the drill
string 12 to increase the force on the window mill 32 and to cause
the center 25 of the bottom cutting surface 57 of the window mill
32 to cross the casing wall more quickly. Once the center 25 of the
window mill 32 crosses the wall of the casing 11, the window mill
32 goes back to the final three degree surface 49 departure to
exit. This reduced drilling time and distance allows significant
savings.
[0113] Upon the window mill 32 moving past the kick out surface 48,
the center 25 of window mill 32 has passed outside of the wall of
the casing 11 and is creating a diverted path to form a side track
through the wall of the casing 11 and a window borehole in the
formation. At this point, the medial surface 43 of window mill 32
engages the lower surface 49 of ramp surface 20 and the window mill
32 is deflected laterally to drill the window borehole. The window
mill 32 is now being guided by the lower surface 49 into the
formation. The window mill 32 in effect drills the window borehole
for the drill bit so that the drill bit can get a faster start in
drilling the new borehole.
[0114] The window 20 is cut substantially the entire length of the
whipstock 44. Once the milling or cutting of the window is
completed, the drill string 12 and mill assembly 30 are replaced by
a standard drilling apparatus for drilling the new borehole.
[0115] Turning now to the alternative embodiments of FIGS. 12 and
13, a whipstock generally designated as 144 has, formed on its
3.degree. ramp surface 147, a kick out ramp 148.
[0116] The aggressive angle of the ramp 148 formed in the whipstock
guide surface 147 enables the conventional window mill cutter 132
to quickly move beyond that part of the milling process which
occurs when the center 25 of the mill 132 is passing over the wall
of the casing 109 as heretofore described.
[0117] FIG. 13 illustrates the window mill 132 passing over the
wall of the casing 109 as it progresses through window 120. The
window mill 132 need not have a tapered end as does mill 32 in the
embodiment of FIGS. 1-11. This mill 132 may have a leading end with
an angle in the range of 0 to 45.degree..
[0118] The ramp angles for ramps 45, 48 and 148 may be from 1 to
45.degree. with respect to the axis of the whipstocks 44 and 144
without departing from the scope of this invention.
[0119] Moreover, where parallel surfaces are mentioned such as
blade surface 52 formed by tapered mill 32 and ramp surfaces 45, 48
and 148 formed by whipstock 44, these surfaces are considered
"substantially" parallel when such surfaces are less than 3.degree.
from being exactly parallel.
[0120] It should also be noted that the pipe casing 11 lining the
borehole 9 may be other than steel.
[0121] Moreover, there may not be any casing lining the borehole 9.
Many of the unique features of this invention set forth above will
still be advantageous in successfully drilling a deviated borehole
in an existing earth borehole.
[0122] Referring now to FIGS. 16A and 16B, the tapered mill of the
present invention may be used with practically any whipstock.
Although it is preferred that the whipstock have a ramp which has
substantially the same angle as the taper of the tapered cutting
surface of the mill and that the ramp be of sufficient duration or
length that it deflects the mill 32 through the casing 11, the
tapered mill will cut its own contact area in the upper end of the
whipstock so as to achieve a contact area as it progresses down the
borehole that will cause the cutability ratio times the contact
stress ratio to be less than one.
[0123] It should be noted that the contact area of the whipstock
can be created by the mill itself even though there is no tapered
surface on the whipstock. It suffices to say that the mill must be
of a geometry such that it can in fact create the necessary
surfaces on the whipstock. For example, the whipstock must have a
sufficient thickness so as to allow the mill to cut the necessary
contact area.
[0124] FIG. 16A illustrates a tapered mill 80, substantially
identical to mill 32, in contact with the upper terminal end 82 of
prior art whipstock 84. Although the upper terminal end of many
prior art whipstocks has a small chamfer or taper, whipstock 84 is
shown with a blunt upper terminal end 82 for purposes of
illustration. It can be seen that there is only line contact
between mill 80 and whipstock 84 such that the contact area 86
between the mill 80 and casing 11 is substantially greater than the
line contact 88 between the mill 80 and whipstock 84. Thus, the
contact stress ratio of the contact stress between the mill 80 and
whipstock 84 and between the mill 80 and casing 11 will be over one
and therefore the mill 80 will cut the whipstock 84 rather than the
casing 11.
[0125] Since the upper terminal end 82 of the whipstock 84 is
squared off when the mill 80 is brought into contact with the top
of the whipstock 84, the mill 80 will mill the whipstock 84 as mill
80 progresses downwardly thereby increasing the contact area
between the mill 80 and the whipstock 84. Initially, the mill 80
only contacts the whipstock 84 at a very small contact area.
Therefore, the mill 80 will cut the whipstock 84 rather than the
casing 11. The mill 80 will continue to cut the top of the
whipstock 84 until the cutting of the whipstock progresses a
sufficient amount to increase its contact area such that the mill
80 initiates the cutting of the casing 11. Eventually the mill 80
will cut a taper into the whipstock 84 as shown in FIG. 16B. It
should be appreciated that the contact stresses, and thus the
contact stress ratio, will change as the mill 80 progresses
downwardly in the borehole 9. The contact stress ratio will
decrease as the mill 80 enlarges its contact area with the
whipstock 84. The mill 80 always mills the casing 11 to some degree
while in engagement with the casing 11, but as the contact area of
the mill 80 and whipstock 84 increases, the cutting of the casing
11 by the mill 80 is increased and the cutting of the whipstock 84
is reduced.
[0126] Referring now to FIG. 16B, the mill 80 is shown having cut a
taper or ramp 90 in the surface of whipstock 84 such that the
contact area has now increased and the contact stress ratio is less
than one whereby the mill 80 will begin to cut the casing 11 rather
than the whipstock 84. The previous position of the upper terminal
end of the whipstock 84 is shown in dotted lines. As mill 80
progresses downwardly and is deflecting outwardly by whipstock 84,
the window is cut in casing 11.
[0127] There are many configurations and profiles which will
achieve the objectives of the present invention, not just those
shown in the present application. See, for example, U.S. patent
application Ser. No. 10/684,629 filed Oct. 14, 2003, hereby
incorporated herein by reference; U.S. application Ser. No.
09/303,049 filed Apr. 30, 1999, now U.S. Pat. No. 6,648,068, hereby
incorporated herein by reference; U.S. patent application Ser. No.
09/021,630 filed Feb. 10, 1998, now U.S. Pat. No. 6,102,123, hereby
incorporated herein by reference; U.S. patent application Ser. No.
08/642,829 filed May 3, 1996, now U.S. Pat. No. 5,771,972, hereby
incorporated herein by reference; U.S. patent application Ser. No.
08/572,592 entitled Two Trip Window Cutting System, filed Dec. 14,
1995, now U.S. Pat. No. 5,657,820, hereby incorporated herein by
reference; and U.S. patent application Ser. No. 08/916,932 filed
Aug. 21, 1997, now U.S. Pat. No. 5,894,889, hereby incorporated
herein by reference.
[0128] It will of course be realized that various modifications can
be made in the design and operation of the present invention
without departing from the spirit of the spirit thereof. Thus,
while the principal preferred construction and mode of operation of
the invention have been explained in what is now considered to
represent its best embodiments, which have been illustrated and
described, it should be understood that within the scope of the
appended claims, the invention may be practiced otherwise than as
specifically illustrated and described.
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