Apparatus for milling a window in well tubular

An apparatus for milling a window in a well tubular, e.g. a well casing, which is comprised of a whipstock and a cooperating mill. The mill has a nose portion depending from the bottom thereof and has cutting surfaces on both the bottom and the sides. The whipstock is comprised of a body which, in turn, has a tapered surface along at least a portion of its length. A guide, e.g. cylindrical pipe, is mounted on and is spaced from the tapered surface and is adapted to receive the nose portion on the mill to provide a guide path for the mill as it cuts or mills a window in the well casing. The guide is positioned so as to direct the side cutting surface of the mill into contact with the casing without contacting the tapered surface of the whipstock.

TECHNICAL FIELD 
The present invention relates to an apparatus for forming a window in a 
well tubular and in one of its aspects relates to an apparatus comprising 
a whipstock which, in turn, has a guide spaced from the tapered surface 
thereof to guide a mill into cutting contact with the inner wall of a well 
tubular, e.g. a well casing, while preventing the mill from engaging and 
damaging the surface of the whipstock. 
BACKGROUND ART 
Most wells of the type used in producing fluids (e.g. hydrocarbons) from 
subterranean formations are completed by cementing a string of well casing 
within the wellbore. The casing is then perforated adjacent the 
formation(s) of interest so that fluids can be injected/produced 
therethrough. As is well known in the art, there may be times when it 
becomes necessary to remove a section of this casing (i.e. cut or mill a 
"window" through the casing and cement) in order to carry out a required 
or desired operation. 
For example, it is common to mill a window(s) in a well tubular (e.g. well 
casing) in order to drill a curved or inclined "drain hole(s)" outward 
from the cased wellbore into a formation of interest. The drain hole, 
which aids in producing fluids from the formation, may consist of a 
horizontal wellbore which is drilled outward from a substantially 
vertical, cased wellbore or, as will be understood in this art, it may be 
a wellbore which extends upward/downward or otherwise radially outward 
from a substantially horizontal or inclined, cased wellbore. In addition 
to the drilling of drain holes, there are also several other operations 
which might require the "cutting of a window(s)" in a well casing, e.g. 
sidetracking, all of which are well known to those skilled in the art. 
There are several types of tools which have been developed for milling 
windows in well tubulars. For example, where a conventional drilling rig 
is used, a deflecting tool, commonly called a "whipstock" is lowered into 
the well casing and is supported at the desired depth by a concrete plug, 
an expanding anchor, or the like. The whipstock is properly oriented to 
insure that the window will be milled in the appropriate direction through 
the casing. A starting mill on the lower end of a rotating drill string is 
then lowered into engagement with the whipstock which, in turn, deflects 
the mill into contact with the casing to thereby mill a relatively short 
pilot hole in the casing. The drill string is retracted and the starting 
mill is replaced with a window mill or other speciality mill to complete 
the window-cutting operation. For a further description of this type of 
window-cutting operation, see U.S. Pat. No. 4,397,360, issued Aug. 9, 1983 
and also, the prior art processes disclosed and fully discussed in both 
U.S. Pat. Nos. 5,277,251 and 5,287,921, 
Due to the expense normally associated with conventional drilling rigs, 
there has recently been a trend towards using commercially-available 
"coiled tubing units" for the milling of windows and other related 
operations whenever and wherever practical; see U.S. Pat. Nos. 5,277,251 
and 5,287,921. As will be understood in the art, a typical coiled tubing 
unit is basically comprised of a continuous length of tubing which is 
wound on a large diameter drum and which can be fed into and out of a 
wellbore without having to "make up" or "break out" individual joints of 
the tubing. 
If rotation is needed for a particular operation, a downhole motor is 
connected onto the lower end of the coiled tubing string to drive the 
mill. Further, since most coiled tubing strings and related tools are 
designed to be run and operated through a string of well tubing (e.g. such 
as the production tubing normally present in a cased well), there is no 
need to remove and replace the tubing from a well when carrying out a 
desired operation with a coiled tubing unit. This feature alone can amount 
to substantial savings in both time and expense in most operations. 
In cutting a window in the well casing, either with conventional drilling 
rigs or with coiled tubing units, it is important that the cutting or 
milling surfaces of the mill be directed into contact with the casing 
without any substantial contact with the whipstock, itself. That is, the 
rotation of the mill has a tendency to draw the cutting surface of the 
mill into the first surface it contacts. It follows that if the mill 
contacts the casing first, the continued rotation of the mill will cause 
the mill to pull itself into the casing thereby cutting the desired window 
therein. However, if the cutting surfaces of the mill contacts the 
inclined or tapered surface of the whipstock first, then continued 
rotation will cause the mill to "dig in" into and cut the whipstock 
instead of the casing, thereby adversely affecting the window cutting 
operation. 
In typical prior art operations, a tapered, non-cutting surface on the 
pilot or starting mill cooperates with a wear pad or the like on the 
whipstock to direct the cutting surfaces of pilot mill into contact with 
the casing while preventing the cutting surfaces from contacting the 
whipstock. While successful, only a very short pilot hole is normally cut 
into the casing before the non-cutting surface of the starting mill 
contacts the casing thereby interferring with any further cutting of the 
window. This requires "tripping" the drill string to replace the starting 
mill with a window mill or the like in order to provide a window having an 
adequate length to allow the subsequent operation to be carried out. 
Accordingly, a need exists for improved whipstocks and mills which can 
initiate and mill a window of substantial length with a miminum of 
manipulated steps by insuring that the cutting surfaces of the mill will 
contact the casing rather than the tapered surface of the whipstock. 
DISCLOSURE OF THE INVENTION 
The present invention provides apparatus for use in milling a window in a 
well tubular, e.g. a casing in a cased, subterranean well. The apparatus 
is comprised of a whipstock and a cooperating mill. The mill has a nose 
portion depending from the bottom thereof and has cutting surfaces on both 
its bottom and its sides. The whipstock is comprised of an elongated body 
which, in turn, has a tapered surface along at least a portion of its 
length. A guide is mounted on and is spaced from the tapered surface and 
is adapted to receive the nose portion on the mill to guide the the mill 
into the casing and away from the tapered surface of the whipstock as the 
side surface of the mill cuts or mills a window in the well casing. 
More specifically, the whipstock of the present invention is run into a 
cased well on a workstring or the like and is positioned within the casing 
and is supported therein at a desired depth by any well known means; e.g. 
packer/anchor, expandable legs, etc. The whipstock is comprised of an 
elongated body which is substantially cylindrical at its lower portion and 
tapered along at least a portion of its length towards its upper end. An 
elongated guide (e.g. a length of conduit such as metal pipe) is spaced 
from and is mounted to the tapered portion of the whipstock by one or more 
supports. Preferably, the cylindrical guide is substantially parallel to 
the tapered surface of the whipstock and has its upper end of guide flared 
to provide a funnel-like entrance. 
The mill has a pilot or nose portion extending from its bottom end and has 
cutting surfaces both on its bottom and sides. As the mill is lowered 
within the wellbore, the nose portion on the mill is directed into the 
guide by the flared, upper end thereof. The cooperation between the nose 
portion and the guide directs or forces the mill into contact with the 
casing while preventing the mill from contacting the tapered surface of 
the whipstock. As the side cutting surface of the mill cuts a window in 
the casing, the cutting surface on the bottom thereof will mill away the 
guide, itself. 
Embodiments of the present whipstock can be used with either conventional 
drilling rigs or with coiled tubing units. When used with coiled tubing 
units, the whipstock and the mill are sized to be run and operated through 
well tubing which is normally present within most cased wellbores.

BEST KNOWN MODE FOR CARRYING OUT THE INVENTION 
Referring more particularly to the drawings, FIG. 1 illustrates a portion 
of a wellbore 10 which has been drilled and cased with casing 11. As will 
be understood, although not shown, casing 11 is cemented in place within 
the wellbore. While the present invention will be discribed in relation to 
a vertical wellbore, it should be understood that the invention is equally 
applicable for use in horizontal or inclined wellbores and accordingly, 
the terms "top and bottom" and "upper and lower", as used herein, are 
relative in nature when referring to respective positions within a 
wellbore. 
Whipstock 12 is run in on a workstring (not shown) or the like and is 
positioned within casing 11 at a desired depth where it is supported by 
any well known means; e.g. packer/anchor 13. The basic construction of 
whipstock 12 is similar to that of many prior art, conventional whipstocks 
in that it is comprised of an elongated body 14 which is substantially 
cylindrical at its lower portion and is inclined or tapered along a 
portion 15 (i.e. tapered portion 15) of its length towards its upper end 
16. The surface of tapered portion 15 may be somewhat concaved as 
represented by the dotted lines 17 (i.e. concaved surface 17) in FIG. 1. 
In accordance with the present invention, a guide 20 is spaced from and is 
mounted to tapered portion 15. As shown, guide 20 is an elongated member 
which lies substantially parallel to the tapered portion 15 and is 
preferably a length of substantially cylindrical conduit (i.e. metal pipe) 
which is attached to and is spaced from tapered portion 15 by one or more 
supports 21. Preferably, the upper end of guide 20 is flared to form a 
funnel-like entrance 22 or the like at its upper end. 
Mill 25 is mounted onto the lower end of drill string 26 and has a pilot or 
nose portion 27 extending from the bottom thereof. Mill 25 has cutting 
surfaces both on its bottom 25a and side 25b and is similar in 
construction to those commerically-available mills known as "pipe" mills. 
As drill string 26 is lowered into casing 11, nose portion 27 of the mill 
25 engages funnel 22 which, in turn, directs the nose portion 27 into 
guide 20. The cooperation between nose portion 27 and guide 20 provides a 
guide path for the mill by directing the mill 25 into contact with casing 
11 while preventing the mill from contacting concaved surface 17 on 
whipstock 12. 
As will be understood by those skilled in this art, whipstock 12 is 
properly oriented when set onto packer/anchor 13 to provide for the 
milling a window in the desired direction. Orientation of the whipstock 
may be carried out utilizing conventional and well known methods, e.g. see 
U.S. Pat. No. 5,287,921, which is incorporated herein by reference. 
As mill 25 is rotated by drill string 26, the cutting surfaces on the side 
25b of the mill will begin to cut a window W (FIG. 2) in casing 11 while 
the cutting surface on the bottom 25a will mill away the guide 20, itself, 
along with each support 21 as a support is reached by the mill. By forcing 
and maintaining the mill into cutting contact with the casing and away 
from inclined or tapered portion surface 15 of whipstock 12, a longer 
window W can be cut with the initial mill (i.e. starting mill) and will 
allow subsequent mills and/or drills to enter into the formation behind 
window W without having to mill a susbstantial amount of additional 
casing. 
Referring now to FIGS. 3A thru 4B, a further embodiment of the present 
invention is illustrated which is especially adapted to be run and 
operated through a tubular string (e.g. production tubing string 30 or the 
like) which is present within casing 111 of cased wellbore 110. Whipstock 
112 is similar to whipstock 12 in that it is comprised of an elongated 
body 114 which is substantially cylindrical at its lower lower end and is 
tapered along a portion 115 (i.e. tapered portion 115) of its length 
towards its upper end 116. The surface 117 (i.e. concaved surface 117) of 
tapered portion 115 may be increasingly concaved from the top to the 
bottom thereof as best seen in FIGS. 3B and 4B. In order for the whipstock 
112 to be run and operated through tubing string 30, the "envelope" or 
maximum diameter (i.e. that at the bottom of the whipstock) must be 
slightly less than the inner diameter of tubing 30. 
Guide 120 is spaced from and is mounted to tapered portion 115. As shown, 
guide 120 is a length of conduit (i.e. metal pipe) which is attached to 
and spaced from tapered portion 115 by one or more supports 121 (see FIG. 
3B) Preferably, the upper end of guide 120 is flared to form a funnel-like 
entrance 122 or the like while a portion of the otherwise cylindrical 
guide 120 (i.e. the front wall of the cylinder) is removed along 
approximately the lower half of its length. This is necessary to keep the 
overall diameter of the whipstock within the envelope required for 
lowering the whipstock through the tubing string 30. Again, while guide 
120 is shown basically as a cylindrical pipe with only a portion of its 
length being cut away, it should be understood that none of its length has 
to be an enclosed cylinder and could be open along the entire front of its 
length if desired; guide 120 may be a "trough". That is, the guide need 
only to be capable of receiving and "guiding" the nose 127 of mill 125 
along the assigned path while maintaining the mill away from concaved 
surface 117 of the whipstock. 
In operation, whipstock 112 is releasably mounted and lowered on a 
workstring (e.g. coiled tubing string, not shown). The workstring can be a 
separate setting string or, in some instances, may be the string which 
carries a downhole motor (not shown) and mill 125 (dotted lines in FIG. 
3A). In either case, the workstring is normally connected to whipstock 112 
by a releasable connection (e.g. shear pin) so that the workstring can be 
released after the whipstock is orientated and set. 
Again, whipstock 112 can be oriented and set in position by conventional 
means, e.g. set onto an expandable packer/anchor (not shown) which is 
lowered through tubing 30 and then expanded by a standard setting tool or, 
as illustrated, the whipstock can be lowered on a workstring which can be 
manipulated to deploy legs 50 which are retracted into the body of the 
whipstock as it passes through tubing 30. Legs 50, when properly 
orientated, engage casing 111 to force the lower of the whipstock towards 
one side of the casing and hold the whipstock in a desired oriented 
position. For a complete description of such a setting mechanism for a 
whipstock, see U.S. Pat. No. 5,222,554, issued Jun. 29, 1993, and which is 
incorporated herein by reference. 
Mill 125 (FIG. 3A) is basically the same as described above in that it has 
a pilot or nose portion 127 depending from the bottom thereof. Mill 125 
has cutting surfaces both on its bottom 125a and its side 125b. As drill 
string 26 is lowered into casing 111, nose portion 127 of the mill 125 
engages funnel 122 which, in turn, directs the nose portion 127 into guide 
120. The cooperation between nose portion 127 and guide 120 directs mill 
125 into contact with casing 111 while preventing the mill from contacting 
tapered surface 115 on whipstock 112. 
As mill 125 is rotated by a downhole motor (not shown), the cutting 
surfaces on the side 125b of the mill will begin to cut a window W.sub.1 
(FIGS. 3B and 4B) in casing 111 while the cutting surface on the bottom 
125a will mill away the guide 120, itself, and supports 121. The 
individual cross-sectional views shown in FIG. 3A and 3B illustrate the 
instantaneous relationship of (a) the concaved surface 117 on the 
whipstock 112, (b) mill 125, (c) guide 120; and (d) window W.sub.1 in 
casing 111 at respective levels a, b, c, etc. along the length of the 
whipstock. Only some of these levels and elements are labelled for the 
sake of clarity. Again, through the use of guide 120, the mill is forced 
and maintained into cutting contact with the casing and away from inclined 
or tapered surface 115 of whipstock 112 and a longer window W.sub.1 can be 
cut with the initially used mill 112.