Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR FORMING A WINDOW IN A TUBULAR AND APPARATUS
FOR USE IN SAID METHOD
This invention relates to a method for forming a window in a tubular and an
apparatus
for use in said method. This application is a division of Canadian Patent
Application
Serial Number 2,242,341, filed January 24, 1997.
Conventionally, when it is desired to form a window in a tubular, for example
a length
of casing, a whipstock is lowered down the tubular and set in position. A mill
is then
lowered down the tubular on a work string and rotated. The whipstock has a
long
tapered concave so that part of the weight of the work string biases the mill
against the
tubular to enable the mill to cut into the tubular and form the desired
window.
Whilst whipstocks work well in long straight tubulars they have two problems.
Firstly,
standard whipstocks cannot be used where the tubular passes around a tight
radius.
Secondly, it is normally necessary to provide the tapered surface of the
whipstock with
a sacrificial layer of material, for example brass, which is eroded by the
mill and which
has to be replaced each time the whipstock is used.
One solution to forming a window in a tubular which passes around a tight
radius is to
position the mill in the desired position and rotate it until the mill
eventually cuts
through the tubular. This relies on sufficient radial pressure being provided
by the work
string and can be a long and tedious process.
According to a first aspect of the present invention there is provided a
method for
milling an opening in a tubular in a wellbore, the method comprising:
installing a mill
guide in the tubular at a desired milling location, the mill guide comprising
a. hollow
straight cylindrical body having a.n axial bore therethrough, an upper end
with an upper
end opening and a lower end with a lower end opening, the lower end having a
first
inside surface and a second inside surface extending along straight lines
parallel to one
another and to the axis of the bore and diametrically opposite one another,
the first
inside surface being shorter than the second inside surface, inserting a mill
through the
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tubular and the bore of the mill guide so that the mill is in contact with the
second
inside surface and is directed by said contact against the tubular at the
desired milling
location adjacent the lower end opening, and milling an opening in the
tubular.
According to a second aspect of the present invention there is provided a
method for
milling an opening in a tubular in a wellbore, the method comprising:
installing a mill
guide in the tubular at a desired milling location, the tubular having a
whipstock
installed therein, the whipstock having a concave member with a slanted
portion for
diverting a mill in a desired direction, and the mill guide comprising: a
hollow
cylindrical body having a bore therethrough, an upper end with an upper end
opening
and a lower end with a lower end opening, the lower end having a diverting
portion
shaped to correspond to the shape of the concave member of the whipstock, the
lower
end of the mill guide being movable to contact the concave member and protect
it,
inserting a mill through the tubular and bore of the mill guide so that the
mill contacts
the tubular at the desired milling location while the mill contacts and is
directed toward
the tubular by the mill guide, and milling an opening in the tubular.
According to a third aspect of the present invention there is provided a mill
guide for
use within a tubular in a wellbore, the mill guide comprising: a hollow
straight
cylindrical body having an axial bore therethrough, an upper end with an upper
end
opening and a lower end with a lower end opening, the lower end having a first
inside
surface and a second inside surface extending along straight lines parallel to
one another
and to the axis of the bore and diametrically opposite one another, the first
inside
surface being shorter than the second inside surface so that a mill inserted
through the
bore and disposed at the lower end is free on one side thereof to mill the
tubular whilst
simultaneously being in contact with the second inside surface on the side
opposite said
one side.
According to a fourth aspect of the present invention there is provided a mill
guide for
use within a tubular in a wellbore in association with a whipstock having a
concave
member with a slanted portion for diverting the mill to mill the tubular on
one side
thereof, the mill guide comprising: a hollow cylindrical body having a bore
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therethrough, an upper end with an upper end opening and a lower end with a
lower end
opening, the lower end having a diverting portion shaped to correspond to the
shape of
the slanted portion of the whipstock and positioned to overlie the slanted
portion so that
the mill contacts the diverting portion whilst milling the tubular on said one
side and the
slanted portion is protected.
For a better understanding of the present invention reference will now be
made.. by way
of example, to the accompanying drawings, in which:
Figure 1A is a schematic side view, partially in cross-section, showing a mill
attempting
to cut a window in a length of casing using one prior art method;
Figure 1B is a schematic side view, partially in cross-section, showing a mill
attempting
to cut a window in a different length of casing using the same prior art
method;
Figure 2A is a side view, in cross-section, showing a first embodiment of an
apparatus
in accordance with the present invention anchored in a length of casing;
Figure 2B is a section taken on line 2B-2B of Figure 2A;
Figure 3 is a view similar to Figure ZA but showing the apparatus in use;
Figure 4 is a side view, in cross-section, showing a second embodiment of an
apparatus
in accordance with the present invention in use;
Figure SA is a perspective view of a whipstock assembly for use with an
embodiment of
the present invention;
Figure SB is a cross-section, on an enlarged scale, of a connection apparatus
which
forms part of the whipstock assembly shown in Figure SA;
Figure SC is a section taken on line SC-SC of Figure SA;
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Figure SD is a side view of one component of the connection apparatus shown in
Figure
SA;
Figure 6A is a side view of a first example of a mill for use with an
embodiment of the
present invention;
Figure 6B is a bottom plan view of the mill shown in Figure 6A;
Figure 6C is a view similar to Figure 6A but with part cut away;
Figure 6D is a view taken on line 6D-6D of Figure 6C;
Figure 7A is a side view of a second example of a mill for use with an
embodiment of
the present invention with part cut away; and
Figure 7B is a bottom plan view of the mill shown in Figure 7A.
Referring to Figure 1 A of the drawings there is shown a length of casing C. A
mill M is
mounted on the bottom of a drill string P and abuts the casing C at a point T.
When the drill string P is rotated the mill M will rub against the inside of
the casing C.
However, it will be appreciated that because of the inherent flexibility of
the drill string
P the mill M is not biased significantly against the casing C and the
formation of a
window in an acceptable period of time is most unlikely.
Referring now to Figure 1B of the drawings there is shown a length of casing
S. A mill
L is mounted on the bottom of the drill string R and abuts the casing S at a
point N in a
curved portion V of the casing S. When the drill string R is rotated the mill
L will rub
against the inside of the casing S. Because of the curvature of the casing S
the force
exerted by the mill L on the casing S will be greater than that of the mill M
against the
casing C in Figure 1A. However, forming a window could still take a very long
time.
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Referring now to Figures ZA, 2B and 3 there is shown a first embodiment of an
apparatus in accordance with the present invention which is generally
identified by the
reference numeral 10.
5 The apparatus 10 comprises a hollow cylindrical body 9 having a bore 8 which
extends
therethrough from an open top end 7 to an open bottom end 6.
The apparatus 10 is positioned in a length of casing 5 and retained therein by
an anchor
4.
The lower section 3 of the apparatus 10 is shaped so that the open bottom end
6 overlies
the curved portion 1 of the casing S as shown.
In use, the apparatus 10 is conveniently lowered down the casing 5 on a work
string or
1 S on coiled tubing and the anchor 4 can conveniently be mechanically
actuated.
Once the apparatus 10 is in position a mill 11 is lowered down the casing 5 on
a drill
string 12. The mill 11 enters the apparatus 10 through the open top end '~,
passes
downwardly through the bore 8 and comes to rest on the curved portion of the
casing 5.
As shown in Figure 3 the mill 11 is trapped between the side 30 of the
apparatus 10 and
the casing 5 and consequently part of the weight of the drill string 12 biases
the mill 11
against the casing 5.
When the mill 11 is rotated it cuts into the casing 5 forming a window therein
as shown
in Figure 3. The mill 11 is provided with an elongate body 13 which remains in
contact
with the side 30 of the apparatus 10 whilst at least the initial portion,
preferably at least
a quarter, of the axial length of the first window is formed.
It will be appreciated that as the window is cut the mill 11 rotates against
the section 30.
If desired the section 30 may be provided with a sacrificial bearing layer
which can be
replaced after the apparatus 10 is retrieved. Alternatively, the section 30
may be
thickened or hardened if desired.
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Various modifications to the apparatus described are envisaged, for example
the shape
of the lower section 3 of the apparatus 10 (and hence the shape of the open
bottom end
6) could be varied to facilitate the formation of the window in the general
shape desired.
Turning now to Figure 4, there is shown a second embodiment of an apparatus in
accordance with the present invention. The apparatus, which is generally
identified by
the reference numeral 15, has a hollow cylindrical body 16 with a bore 19
therethrough
which extends from an open top end 17 to an open bottom end 18 which is
generally
perpendicular to the plane of the open top end 17. The apparatus 15 has a
slanted side
wall 21 which terminates at the bottom of the apparatus 1 S.
In Figure 4 the apparatus 15 is shown resting on the concave 24 of the
whipstock 20.
In use, a mill 25 is towered through the apparatus 15 on a drill string 26 and
is deflected
into contact with the wall of the casing 22 by the slanted side wall 21 which
acts as a
sacrificial bearing for the concave 24 of the whipstock 20. The weight of the
drill string
26 acting downwardly on the mill 25 biases the mill 25 into engagement with
the wall
of the casing 22 and subsequent rotation of the mill 25 forms the window 27.
In this embodiment the whipstock 20 supports the apparatus 1 S which can thus
be made
of comparatively light material. However, the whipstock 20 could conceivably
be
dispensed with if the apparatus 15 were made sufficiently strong.
If desired the apparatus 15 could be removably attached to the whipstock 20
and, if
desired, could be lowered into position with the whipstock 20 before use.
It should also be appreciated that, whilst an anchor similar to the anchor 4
is highly
desirable, it may not be essential in all applications, for example where the
apparatus is
attached to a whipstock.
If a window is formed with the use of a whipstock then eventually it becomes
necessary
to remove the whipstock and the anchor to which it is attached. At one extreme
this can
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be effected by simply drilling out both the whipstock and the anchor. However,
whipstocks are relatively expensive to construct and recovery of the whipstock
is
desirable.
Referring now to Figure SA there is shown a whipstock assembly which is
generally
identified by the reference numeral 200.
The whipstock assembly 200 comprises a whipstock 202 having a concave 204, an
anchor 208 and a connection apparatus 206.
The whipstock 202 and the anchor 208 are of essentially conventional
construction, the
anchor 208 being described in US A S 341 873, co-owned with the present
invention.
As shown in Figure SB, the connection apparatus 206 comprises an upper member
222
and a (lower) fishing member 216 which are connected by a shear pin 210
designed to
fail at about 43,200kg (950001bs) and which extends through a hole 212 in the
neck 214
of the fishing member 216 and the holes 226 in the lower portion of the upper'
member
222.
The top of the upper member 222 is provided with a recess 228 which receives a
stub
which projects downwardly from the bottom of the whipstock 202. The whipstock
202
is then welded to the upper member 222 circumjacent the stub.
The lower end of the fishing member 216 is provided with a stub 218 which is
welded
to the anchor 208.
It will be noted that the fishing member 216 is provided with a fluid relief
channel 211
which extends along the fishing member 216 and opens into the cavity 224 which
is
formed in the upper member 222. The upper member 222 is also provided with a
fluid
relief channel 230 which communicates with the cavity 224.
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In use, when it is desired to remove the whipstock assembly 200 a fishing tool
having a
hook is lowered until it reaches the concave 204. The hook is then manipulated
until it
enters the rectangular slot in the concave 204. The fishing tool is then
lifted. This
causes the shear pin 210 to fail and the whipstock 202 can then be recovered,
separation
of the upper member 222 from the fishing member 216 being facilitated by the
fluid
relief channel 230.
Once the whipstock 202 and the upper member 222 have been recovered another
fishing
tool can be lowered to clamp onto the fishing member 216 for retrieval of the
anchor
208. If desired the fluid relief channel 211 may be connected to a mechanism
to release
the anchor 208 although the anchor 208 could be provided with a variety of
mechanical
or hydraulic release devices.
Once the anchor 208 is released it can be lifted to the surface and recovered.
(As used herein the term "fishing member" refers to any member which can be
gripped
for removal of the anchor and is not limited to members having flanges and/or
collars
which are particularly adapted to be retrieved by grapples.)
Mills tend to be judged by the speed at which they cut. When cutting a window
it is not
unusual for the performance of most mills to suddenly drop and later recover.
This has
been attributed to various reasons including "coring". Coring occurs when the
centre of
the mill is over the wall of the casing and the relative speed between the
mill and the
wall is minimal (theoretically nil).
Refernng now to Figures 6A to 6D there is shown a mill which is generally
identified
by the reference numeral 400. The mill 400 comprises a body 402 having an
upper
threaded end 404 and a plurality of (optional) blades 408 on the lower end 412
thereof.
The leading faces of the blades 408 and the bottom of the mill 400 are covered
with
cutting and/or grinding material, for example milling inserts with or without
chipbreakers and/or tungsten carbide chips.
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A first fluid flow bore 406 (Figure 6C) extends from the top of the body 402
and
divides into a single second fluid flow bore which is effectively an extension
of the first
fluid flow bore 406 (but of smaller diameter than the first fluid flow bore
406), and a
plurality of inclined flow bores 16 which are inclined downwardly and
outwardly from
the first fluid flow bore 406 and are of smaller diameter than both the first
fluid flow
bore 406 and the second fluid flow bore.
The second fluid flow bore opens on the rotational axis of the mill 400 and is
provided
internally with cutting and/or grinding material similar to the bottom of the
mill 400.
In use, drilling mud is pumped down the first fluid flow bore 406 whilst the
mill 400 is
rotated. It has been found that the presence of the cutting and/or grinding
material on
the inside of the second fluid flow bore produces a significant increase in
drilling
efficiency. It is suspected that by applying cutting and/or grinding material
to t:he inside
I S of the inclined fluid flow bores 16 a further small increase in efficiency
may be
obtained.
Referring now to Figures 7A and 7B there is shown a mill which is generally
identified
by the reference numeral 420. The mill 420 is generally similar to the mill
400 and
comprises a body 422 having an upper threaded end 424. However, the mill 420
does
not have any blades. The bottom of the mill and the lower part of the side
thereof are
covered with grinding material in the form of tungsten carbide chips which are
brazed
thereto.
A first fluid flow bore 426 extends from the top of the body 422 and divides
into a
single second fluid flow bore 438 which is effectively an extension of the
first fluid
flow bore 426 (but smaller in diameter), and a plurality of inclined flow
bores 428 that
are of smaller diameter than both the first fluid flow bore 426 and the second
fluid flow
bore 438.
The second fluid flow bore 438 is provided internally with grinding material
as shown.
