Wellbore milling system and method

A method for milling an opening, slot, hole, or window in a tubular of a tubular string in a wellbore, has been invented which includes installing a mill system in the tubular at a desired milling location, and milling an opening in the tubular. In one aspect the method includes using a mill which has a weight member below the mill, above the mill, or both. In one aspect additional weight is added to the weight member once it has been introduced into the tubular string.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is directed to milling tubulars in a wellbore and, in one 
aspect, to methods for directing a mill in a wellbore and apparatus useful 
in such methods. 
2. Description of Related Art 
An opening, slot, or a window is formed in a tubular, e.g. casing, in a 
wellbore with a milling tool with one or more mills. Typically the tool is 
threadably attached to a section of drill pipe or other heavy tubular 
components comprising a bottom hole assembly that is in a well to cut a 
window through the side of a piece of casing. In certain methods the 
milling tool is assisted in generating a window by a device known as a 
whipstock, a wedge shaped object, anchored in the casing wellbore which 
serves to support the milling tool and forcibly direct it outward through 
the side of the casing, the facilitating formation of the window. 
In certain methods a whipstock is not utilized, and the ability of the 
milling tool to generate the window without the wedge shape forcing it 
through the casing is severely inhibited and often practically impossible. 
This is primarily due to the fact that, without the whipstock, the only 
force available to urge the mill sideways into the casing is the inherent 
stiffness of the milling tool and associated drill pipe transversing a 
curve in the casing (see FIGS. 1A-1B). In many cases, side loading on the 
milling tool is not sufficient to initiate and maintain cutting action. 
In some cases a whipstock utilized in a downhole application for generating 
a window in casing is susceptible to damage from the aggressive cutting 
surface of a mill and, if not protected, is inadvertantly damaged or cut 
away. 
There has long been a need for an efficient and effective milling system 
and method of its use. There has long been a need for a method for milling 
a window in a tubular at a desired location without the use of a 
whipstock. There has long been a need for a milling method which does not 
result in damage to a whipstock. 
SUMMARY OF THE PRESENT INVENTION 
The present invention, in one embodiment, discloses a method for milling a 
tubular of a string of tubulars in a wellbore, e.g. a piece of casing in a 
cased wellbore, the method including: installing a milling system in a 
tubular at a desired location at which a slot, opening, or window is to be 
milled out of the tubular; and milling the window through the tubular. In 
one aspect the milling system includes a mill (e.g., but not limited to, a 
starting mill or a section mill with two, three, four, five, six, seven or 
eight pivotable milling blades), and at least one weight member 
interconnected with or connected to the mill, e.g. a weight member below 
the mill or above the mill or both. In one embodiment centralizing 
apparatus is used above the mill, below the mill, or in both locations. 
In one aspect the weight member is one or more joints (e.g. thirty feet 
long each) of drill pipe or drill collars. In another aspect the weight 
member(s) are solid tubulars made of metal, metal and plastic, or 
plastic-coated metal. In another aspect the weight member(s) are hollow so 
that additional weights and/or fluid is introducible therein either at the 
surface or when the system has been lowered to some desired location in a 
wellbore. 
The centralizing apparatus may be any known centralizer, stabilizer, or 
reamer-stabilizer. In one aspect the centralizing apparatus has hollow 
blades or hollow centralizing members in fluid communication with a hollow 
bore through the centralizer so that additional weights and/or fluid is 
introducible into the centralizer either at the surface at some desired 
location in a wellbore. 
In one aspect a measurement-while-drilling ("MWD") device is interconnected 
with or connected to the mill so that the well's location and orientation 
are known at the surface and can be adjusted as desired. 
In one aspect a weight apparatus is employed that is flexible or that has a 
flexible or jointed part so that it will tend to lie against a bottom side 
of a downhole curved casing. In one particular embodiment an expansible 
tube or bladder is employed above or below (or both) the mill into which 
are introducible weights and/or fluid either at the surface or at a 
desired location downhole in a wellbore. 
In one aspect in which a hollow weight member is used, a selective release 
apparatus is employed (e.g. but not limited to a burstable diaphragm, a 
rupture disc, or a bursting tube as described in U.S. application Ser. No. 
08/429,763 filed Apr. 26, 1995 entitled "Cementing System" now U.S. Pat. 
No. 5,553,667 or the U.S. application Ser. No. 08/632,927 which is the 
offspring of U.S. application Ser. No. 08/429,763 filed on Apr. 16, 1996 
entitled "Wellbore Cementing System," both co-owned with the present 
invention, both applications incorporated fully herein by reference for 
all purposes) so that either during milling or when milling is completed 
the weight apparatus is opened so that weights and/or fluid flow out 
therefrom and into the casing or wellbore. 
In one aspect of the present invention, a selectively operable biasing 
member is positioned above the mill, below the mill, or in both locations. 
Such a biasing member may be any known device which is hydraulically, 
pneumatically, and/or mechanically selectively activated from the surface 
for moving a tubular to one side in another tubular or in a wellbore. Such 
biasing member(s) may be used with or instead of a whipstock. In one 
aspect the biasing member is a selectively fillable or inflatable bladder 
or packer which expands to one side of the mill thereby forcing the mill 
against one side of a tubular or casing. Such a biasing member is useful 
to direct a mill toward the bottom side of a curved or horizontal casing 
or toward the top side of a curved or horizontal casing, i.e., it can 
facilitate milling on a bottom side or on a top side, and is also useful 
to facilitate milling in one location (side, top, or bottom) and then, 
after deactivation and movement, is activated again for milling in another 
location (top, side, or bottom). 
In certain embodiments the present invention discloses tapered members for 
use with or connection to blades of a mill to facilitate and ease 
insertion of the mill through a tubular string, e.g. past upsets and/or 
indentations of casing. In another aspect retractable leaf springs or 
spring bows are used below the mill blades to inhibit or prevent the 
bottom surfaces of the blades or the blade points from catching on items 
or surfaces in the casing. 
In certain embodiments the weight member is cylindrical, hollow or solid, 
and extends downwardly from a mill and is either secured thereto, 
removably connected thereto, or formed integrally thereof. 
It is, therefore, an object of at least certain preferred embodiments of 
the present invention to provide: 
New, useful, unique, efficient, nonobvious wellbore milling methods, 
methods for directing a milling tool to mill a hole, slot, or window in a 
tubular in a wellbore and tools useful in such methods; 
Such a method useful with or without a whipstock; 
Such a method in which a milling tool is directed by one or more biasing 
members and/or one or more selectively activatable biasing devices against 
a tubular; 
Such a method or apparatus in which one or more weight members direct a 
mill against a tubular; and 
Apparatus useful in such methods. 
Certain embodiments of this invention are not limited to any particular 
individual feature disclosed here, but include combinations of them 
distinguished from the prior art in their structures and functions. 
Features of the invention have been broadly described so that the detailed 
descriptions that follow may be better understood, and in order that the 
contributions of this invention to the arts may be better appreciated. 
There are, of course, additional aspects of the invention described below 
and which may be included in the subject matter of the claims to this 
invention. Those skilled in the art who have the benefit of this 
invention, its teachings, and suggestions will appreciate that the 
conceptions of this disclosure may be used as a creative basis for 
designing other structures, methods and systems for carrying out and 
practicing the present invention. The claims of this invention are to be 
read to include any legally equivalent devices or methods which do not 
depart from the spirit and scope of the present invention. 
The present invention recognizes and addresses the previously-mentioned 
problems and long-felt needs and provides a solution to those problems and 
a satisfactory meeting of those needs in its various possible embodiments 
and equivalents thereof. To one of skill in this art who has the benefits 
of this invention's realizations, teachings, disclosures, and suggestions, 
other purposes and advantages will be appreciated from the following 
description of preferred embodiments, given for the purpose of disclosure, 
when taken in conjunction with the accompanying drawings. The detail in 
these descriptions is not intended to thwart this patent's object to claim 
this invention no matter how others may later disguise it by variations in 
form or additions of further improvements.

DESCRIPTION OF EMBODIMENTS PREFERRED AT THE TIME OF FILING FOR THIS PATENT 
FIG. 1A shows a prior art mill M attached to drill pipe P in a casing C. 
When the mill M contacts the interior of the casing C at a point T, the 
mill M tends to be deflected away from the point T. The stiffness of the 
drill pipe P (and other drill pipe and items in a string above and 
connected to the drill pipe P, not shown) is, in most cases, insufficient 
to prevent the mill M from deflecting away from the interior of the casing 
C. 
FIG. 1B shows a prior art mill L attached to a drill pipe R in a casing S 
with a curved portion V. When the mill L contacts the interior of the 
casing S at a point N of the curved portion V, the mill L is deflected 
away from the point N. 
FIGS. 2A and 2B show a mill guide 10 according to the present invention 
with a hollow cylindrical body 9 having a bore 8 therethrough, an open top 
end 7 and an open bottom end 6. The mill guide 10 is disposed in a piece 
of casing 5 which is part of a string of casing (not shown) in a wellbore 
in the earth. An anchor 4 (or anchors) holds the mill guide 10 in place at 
a desired location in the casing with an opening 3 of the mill guide's 
bottom end 6 disposed and oriented so that a mill passing through the mill 
guide 10 will mill a desired area of the casing, creating a desired hole, 
slot, opening, or window. The bottom end 6 of the mill guide 10 is formed 
or cut to have a desired shape 2. This shape 2 may be made to correspond 
to a curved portion 1 of the casing 5. 
As shown in FIG. 3, a mill 11 on a string of drill pipe 12 has been 
introduced through the casing 5 and the mill guide 10 to contact the 
casing 5 and begin to mill a hole therethrough. A body 13 of the mill 11 
has a length such that at least about a fourth of the desired opening is 
milled (and in other aspects substantially all of the desired opening) 
while the mill body 13 remains in contact with a side 30 of the bottom end 
6 of the mill guide 10, thus providing a continuous reaction support 
during part or substantially all of the milling. The side 30 may be the 
same thickness as a side 32 which is shorter than the side 30; or the side 
30 may be thicker than the side 32. The interior of the side 30 may one or 
more additional layers of material thereon. Such material may also inhibit 
the mill from milling the side 30. This additional material may be any 
desired practical thickness and may be any known suitable material, 
including, but not limited to, steel, carbide steel, stainless steel, 
known alloys, and hardfacing material. Such a layer or layers may be added 
by any known method (e.g., welding or hardfacing) or may be formed 
integrally of the side 30. 
FIG. 4 shows a mill guide 15 with a hollow body 16, a top open end 17, a 
bottom end point 18, a side opening 19, and a slanted side member 21. A 
whipstock 20 disposed in a casing 22 in a wellbore 23 has a concave 
surface 24 which corresponds to the shape of the slanted side member 21. 
The mill guide 15 is made of a strong metal, e.g. steel, so that the 
slanted side member 21 protects the concave surface 24 from the effects of 
a mill 25 on flexible pipe 26. The whipstock 20 and the side opening 19 
are positioned so that a window 27 is cut at a desired location on the 
casing 22. As shown in FIG. 4 the window 27 has only been partially milled 
and will be completed as the mill 25 moves down the slanted side member 
21. It is within the scope of this invention for the mill guide 15 and the 
whipstock 20 to be connected together; to be formed integrally as one 
member; or for the mill guide 15 to be releasably connected to the 
whipstock (e.g. but not limited to, by one or more shear studs or shear 
lugs). In another aspect the mill guide and the whipstock are installed 
separately. 
Referring now to FIGS. 5A-5D, a system 100 according to the present 
invention has an upper hollow connector or sub 102 interconnected with a 
tubular string 104 that extends up to the surface in a wellbore 106 cased 
with casing 108. A stabilizer 110 is connected to one (or more) pieces of 
drill pipe (or drill collars) 112. A mill 114 is connected to the drill 
pipe 112. A weight member 116 (optionally with centralizing blades, two or 
more, not shown) is connected to the mill 112. 
As shown in FIG. 5A the system 100 has been lowered in the casing 108 to a 
desired location. As shown in FIG. 5B the mill 114 has begun to mill the 
casing 108. As shown in FIG. 5C the mill 114 has milled through the casing 
108 and the weight member 116 has moved off center due, inter alia, to the 
force of gravity thereby directing the mill 114 against the casing (to the 
left in FIG. 5C). As shown in FIG. 5D, the mill 114 has milled out an 
opening or window 120 in the casing 108. 
As shown in FIGS. 5A-5D the wellbore 106 is canted from the vertical. It is 
to be understood that the system 100 is useful in any wellbore in which 
gravity will act on the weight member 116 to facilitate the directing of 
the mill against the casing, including, but not limited to, a horizontal 
wellbore. FIG. 5F shows all the blades on the mill 114, but FIGS. 5A-5D 
show slots where some of the blades are located. 
FIG. 6 shows a system 150 according to the present invention with a mill 
152 connected to a tubular string 154 which extends to the surface (not 
shown) in a wellbore 156 cased with casing 158. A weight member 160 is 
connected to the mill 154. Fluid and/or weights are introducible into an 
interior 162 of the weight member 160 and into an interior 164 of blades 
166 of the weight member 160 via a flow bore 168 through the mill 152 and 
a flow bore 170 of the tubular string 154. If there is a fluid in the 
casing, e.g. water or drilling fluid, the fluid used to add weight to the 
weight member is denser than the fluid already present. Additional weights 
may be used with or instead of fluid. In certain aspects these weights are 
rocks or spheres of metal, or plastic-coated or Teflon.TM.--coated iron, 
steel, aluminum, aluminum alloy, zinc, zinc alloy, plastic or some 
combination thereof. These spheres may range in size from microspheres to 
spheres several inches (1/2 inch, 1, 2, 3, or more inches) in diameter. 
A rupture disc or diaphragm 172 is positioned in a lower end 174 of the 
weight member 160. By increasing fluid pressure to a desired pressure 
level on the rupture disc 172, the disc bursts and the contents of the 
weight member (and string) flow out and into the casing. 
FIG. 7A shows a system 180 according to the present invention with a mill 
182 connected to a tubular string 184 which extends to the surface (not 
shown) in a wellbore 186 cased with casing 188. A weight member 190 is 
connected to the mill 182 and via a flow bore 192 through the mill and a 
flow bore 194 through the tubular string, additional weights and/or fluid 
is introducible into an extendable hollow member 196 of the weight member 
190. The extendable hollow member 196 is shown as made of a tough yet 
flexible material, e.g. Kevlar.TM. material, Aramid.TM. material, 
plasticized rubber, etc.; however, it could be made of plastic or metal 
telescoping pieces. As shown in FIG. 7B, the extendable hollow member 196 
has extended due to the weight of spherical weights 197 and fluid 199. The 
fluid 199 could extend further up to the mill, to the tubular string, and 
to the surface as desired. 
FIG. 8 shows a system 200 according to the present invention with a mill 
202 connected to measurement-while-drilling apparatus 204 which is 
connected to a joint of drill pipe 206. A stabilizer 208 is connected to 
the drill pipe 206 and a tubular string 210 (shown partially) is connected 
to the stabilizer 208. The tubular string extends to the surface in a 
wellbore (not shown) cased with casing 212. A weight member 214 (like any 
weight member shown and/or described herein) is connected beneath the mill 
202 and the drill pipe 206 acts as a weight member above the mill 202. As 
shown a window 216 has been milled by the mill 202 in the casing 208 and 
the weight members have directed the mill against the bottom side of the 
casing. 
FIG. 9 shows a system 230 according to the present invention with a mill 
232 connected to a joint of drill pipe 236. A stabilizer 238 is connected 
to the drill pipe 236 and a tubular string 234 (shown partially) is 
connected to the stabilizer 238. The tubular string extends to the surface 
in a wellbore (not shown) cased with casing 235. A weight member 237 (like 
any weight member shown and/or described herein) is connected beneath the 
mill 232 and the drill pipe 236 acts as a weight member above the mill 
232. As shown a window 239 has been milled by the mill 232 in the casing 
235 and the weight members have directed the mill against the bottom side 
of the casing. 
A selectively operable (from the surface or by introducing an object into 
the tubular string to go down to it) biasing apparatus 240 is disposed 
above the mill 232 which is activated when the mill 232 is in the desired 
location for forming the window 236. Another selectively operable biasing 
apparatus 241 is disposed on the weight member 237 and, as shown in FIG. 
9, has been activated to bias the system 230 against one side of the 
casing. Alternatively, the biasing apparatus or apparatuses can be 
oriented so that the system 230 is pushed upwardly so that milling on the 
top side of the casing is facilitated. Systems according to this invention 
may use only one of the biasing apparatuses describe herein. 
FIG. 10 shows a mill 300 according to the present invention with a body 302 
and a plurality of blades 304. Associated with each blade 304 is a taper 
member 306 which is secured to the body 302, or to the blade 304, or to 
both, either with an adhesive such as epoxy, with connectors such as 
screws, bolts, or Velcro .TM. straps or pieces, or by a mating fit of 
parts such as tongue-and-groove. The taper members may be made of any 
suitable wood, plastic, composite, foam, metal, ceramic or cermet. In 
certain embodiments the taper members are affixed to the mill so that upon 
contact of the lower point of the mill blades with the casing to be 
milled, the taper members break away so that milling is not impeded. 
FIG. 11 shows a mill 330 according to the present invention with a body 332 
and a plurality of blades 334. A taper device 336 is secured around the 
mill 330 or formed integrally thereon. The taper device 336 extends around 
the entire circumference of the mill 330 beneath the blades 334 and 
facilitates movement of the mill 330 through tubulars. The taper device 
336 may be a two-piece snap-on or bolt-on device and may be made of the 
same material as the taper member 306. 
FIG. 12 shows a blade-taper member combination with a blade 340 having a 
groove 342 and a taper member 344 with a tongue 346. The tongue 346 is 
received in the groove 342 to facilitate securement of the taper member 
344 to the blade 340. Optionally, an epoxy or other adhesive may be used 
to glue the taper member to the blade, to a mill body, or to both. The 
tongue and groove may be dovetail shaped. 
FIG. 13 shows a blade-taper member combination with a blade 350 and a taper 
member 352 with a recess 354. The blade 350 is received in and held in the 
recess 354. Optionally an adhesive may be used to enhance securement of 
the taper member 352 to the blade, to the mill, or to both. 
FIG. 14 shows a mill body 370 like the bodies of the mills shown in FIG. 
5A, 10, and 11, but with a series of grooves 372 therein which extend 
longitudinally on the mill body and are sized, configured, and disposed to 
receive and hold a taper member as shown in FIG. 10, FIG. 12, or FIG. 13. 
Such a mill body may be used instead of or in combination with any 
previously-described taper securement means. 
FIG. 15 shows a mill body 380 like the bodies of the mills shown in FIGS. 
5A, 10, and 11, but with a series of dovetail grooves 382 therein which 
extend longitudinally on the mill body and are sized, configured, and 
disposed to receive and hold a taper member as shown in FIG. 10, FIG. 12, 
or FIG. 13. Such a mill body may be used instead of or in combination with 
any previously-described taper securement means. 
FIG. 16 shows a system 400 like previously described systems, but with a 
flexible connection 402 between a mill 404 and a weight member 406. The 
flexible connection permits pivoting of the weight member 406 with respect 
to the mill 404 in response to the force of gravity. The mill 404 is 
connected to a tubular string 408 which extends up to the surface (not 
shown) in a casing string 410 in a wellbore 412. Flexible connections are 
well known, see e.g. U.S. Pat. No. 4,699,224. Alternatively a 
ball-and-socket joint may be used or a knuckle-joint, see also U.S. Pat. 
No. 4,699,224. 
FIG. 17 shows a system 420 with a mill 422 connected to a tubular string 
424 which extends to the surface (not shown) through a casing string 426 
in a wellbore 428. A weight member 430 connected to the mill 422 has a 
body 432 with a flexible neck 434 which permits the weight member to move 
toward a bottom side of the casing in response to the force of gravity. 
Additional weights or fluid may be added to the weight member 430 as 
described for previous embodiments. 
In conclusion, therefore, it is seen that the present invention and the 
embodiments disclosed herein and those covered by the appended claims are 
well adapted to carry out the objectives and obtain the ends set forth. 
Certain changes can be made in the described and in the claimed subject 
matter without departing from the spirit and the scope of this invention. 
It is realized that changes are possible within the scope of this 
invention and it is further intended that each element or step recited in 
any of the following claims is to be understood as referring to all 
equivalent elements or steps. The following claims are intended to cover 
the invention as broadly as legally possible in whatever form its 
principles may be utilized. The invention claimed herein is new and novel 
in accordance with 35 U.S.C. .sctn. 102 and satisfies the conditions for 
patentability in .sctn. 102. The invention claimed herein is not obvious 
in accordance with 35 U.S.C. .sctn. 103 and satisfies the conditions for 
patentability in .sctn. 103. This specification and the claims that follow 
are in accordance with all of the requirements of 35 U.S.C. .sctn. 112.