This invention relates to a method for forming a window in a tubular and an apparatus for use in said method.

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 posi¬ tion. 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 tubu- lars they have two problems. Firstly, standard whipstocks cannot be used where the tubular passes around a tight radius. Secondly, it is normally neces¬ sary 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 even- tually cuts through the tubular. This relies on suffi¬ cient radial pressure being provided by the work string and can be a long and tedious process.

In order to help reduce this problem the present invention provides a method for forming a window in a tubular, characterised in that said method comprises the steps of: placing an apparatus having a bore therethrough at a desired location in said tubular; introducing a mill into said apparatus until said mill acts between said apparatus and said tubular; and

rotating said mill to form a window in said tubu¬ lar.

* * *

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 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.

In order to facilitate recovery the present inven¬ tion provides a whipstock assembly which comprises a whipstock, characterised in that said whipstock assembly further comprises a connection apparatus for releasably connecting the whipstock to an anchor.

* * *

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 (theor¬ etically nil). The present invention addresses this problem. According to the present invention there is provi¬ ded a mill for milling an opening in a tubular, which mill comprises a body having an upper end connectable to a drill string, a lower end provided with milling and/or cutting material, a first fluid flow bore extending generally axially of said body, a second fluid flow

bore extending from said first fluid flow bore and opening in the exterior of said body substantially on the rotational axis thereof, and a plurality of inclined fluid flow bores extending from said first fluid flow bore and opening in the exterior of said body around said second fluid flow bore, characterised in that said second fluid flow bore is lined with a layer of milling and/or cutting material.

* * *

The present invention also provides a modified watermelon mill having a plurality of blades provided with cutting or grinding material, characterised in that the radial extremity of at least one of said blades is coated with cutting or grinding material.

For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings, in which:-

Fig. IA 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;

Fig. IB 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;

Fig. 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;

Fig. 2B is a section taken on line 2B-2B of Fig. 2A;

Fig. 3 is a view similar to Fig. 2A but showing the apparatus in use;

Fig. 4 is a side view, in cross-section, showing a second embodiment of an apparatus in accordance with the present invention in use.

* * *

Fig. 5A is a perspective view of a whipstock as- sembly in accordance with the present invention;

Fig. 5B is a cross-section, on an enlarged scale, of a connection apparatus which forms part of the whip¬ stock assembly shown in Fig. 5A;

Fig. 5C is a section taken on line 5C-5C of Fig. 5A;

Fig. 5D is a side view of one component of the connection apparatus shown in Fig. 5A.

* * *

Fig. 6A is a side view of a first embodiment of a mill in accordance with the present invention;

Fig. 6B is a bottom plan view of the mill shown in Fig. 6A; Fig. 6C is a view similar to Fig. 6A but with part cut away;

Fig. 6D is a view taken on line 6D-6D of Fig. 6C; Fig. 7A is a side view of a second embodiment of a mill in accordance with the present invention with part cut away;

Fig. 7B is a bottom plan view of the mill shown in Fig. 7A.

* * *

Fig. 8 is a side elevation of a conventional water- melon mill;

Fig. 9 is a side elevation of a modified watermelon mill in accordance with the present invention;

Fig. 10A is a side view, partly in cross-section and partly in elevation showing the modifed watermelon mill of Fig. 9 in a first miling sytem immdiately prior to use;

Fig. 10B is a view similar to Fig. 10 but showing the modified watermelon mill during the formation of a window; and Fig. 11 is a side view, partly in cross-section and partly in elevation showing the modifed watermelon mill of Fig. 9 in a second milling system during the forma¬ tion of a window.

* * *

Referring to Fig. IA 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 significant- ly against the casing C and the formation of a window in an acceptable period of time is most unlikely.

Referring now to Fig. IB 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 Fig. IA. However, forming a window could still take a very long time.

Referring now to Figs. 2A, 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.

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 por¬ tion 1 of the casing 5 as shown.

In use, the apparatus 10 is conveniently lowered down the casing 5 on a work string or on coiled tubing and the anchor 4 can conveniently be mechanically actua¬ ted.

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

7, passes downwardly through the bore 8 and comes to rest on the curved portion of the casing 5. As shown in Fig. 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 Fig. 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 quar¬ ter, 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 bear¬ ing layer which can be replaced after the apparatus 10 is retrieved. Alternatively, the section 30 may be thickened or hardened if desired.

Various modifications to the apparatus described are envisaged, for example the shape of the lower sec¬ tion 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 Fig. 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 15.

In Fig. 4 the apparatus 15 is shown resting on the concave 24 of a whipstock 20.

In use, a mill 25 is lowered 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 15 which can thus be made of comparatively light material. However, the whipstock 20 could con¬ ceivably 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.

* * *

Referring now to Fig. 5A 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-5 341 873, co-owned with the present invention.

As shown in Fig. 5B, the connection apparatus 206 comprises an upper member 222 and a (lower) fishing

member 216 which are connected by a shear pin 210 de¬ signed to fail at about 43,200 kg (95000 lbs) and which extends through a hole 212 in the neck 214 of the fish¬ ing 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 down¬ wardly from the bottom of the whipstock 202. The whip¬ stock 202 is then welded to the upper member 222 circum- jacent 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.

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. )

* * *

Referring now to Figs. 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.

A first fluid flow bore 406 (Fig. 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 effi¬ ciency. It is suspected that by applying cutting and/or grinding material to the inside of the inclined fluid flow bores 16 a further small increase in efficiency may be obtained.

Referring now to Figs. 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 threa- ded 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 inter¬ nally with grinding material as shown.

* * *

Fig. 8 shows a conventional watermelon mill which is generally identified by the reference numeral 270. The watermelon mill 270 has a body 272 having an upper threaded end 274, a lower end 276 and a plurality of blades 278 the sides of which are covered with milling inserts and/or matrix milling material 279.

It will be noted that the outer surfaces 277 of the blades 278 are NOT covered with any form of abrasive material and are, in fact, ground down to a smooth

finish so as not to damage the surface of the whipstock which they will bear against in use.

Fig. 9 shows a modified watermelon mill according to the invention which is generally identified by the reference numeral 290. The watermelon mill 290 has a body 292 having an upper threaded end 294, a lower end 296 and a plurality of blades 298, the sides and radial extremities 297 of which are covered with milling in¬ serts and/or matrix milling material. The modified watermelon mill 290 has a special application in the formation of windows in tubulars as will hereinafter be described with reference to Figs. 10A, 10B and 11.

* * *

Fig. 10A shows a milling system 300 which comprises a starter or window mill 310 and a modified watermelon mill 312 which are connected by a neck 322. The top of the modified watermelon mill 312 is connected to a single length of drill pipe which is connected to the bottom of a drill collar. The drill collar adds weight to the milling system 300 whilst the drill pipe adds flexibility.

As shown in Fig. 10A the bottom of the mill 310 rests on the concave 304 of whipstock 306 and biases the mill 310 against the casing. The mill 310 and the modified watermelon mill 312 are sized and positioned so that as the mill 310 opens the initial window the mod¬ ified watermelon mill 312 engages the tubular but does NOT touch the concave 304 (Fig. 10B). Because the mod¬ ified watermelon mill 312 does not touch the concave 304 the cutting and/or milling material on the radial extre- mities of the blades does not damage the concave 304

which would otherwise be severely damaged. By using the modified watermelon mill 312 a window is produced with edges which reduce or even eliminate the need for ream¬ ing out the window once milling ceases. This can elimi- nate one complete "trip" and hence save a substantial amount of time and money. Furthermore, the rough outer surface of the modified watermelon mill 312 results in a reduced torque requirement and will smooth out any lip on the casing at the bottom of the window as it passes therethrough.

If a larger window is required the milling system 350 (Fig. 11) may be used. This comprises a window mill 360, a conventional watermelon mill 362 and a modified watermelon mill 364. In use, a starter mill (not shown) cuts an initial window in the casing 352 and is with¬ drawn. The milling system 350 is then lowered into position and rotated. The window mill 360 proceeds through the annulus of cement 354 surrounding the casing 352 into the formation to be bored. The conventional watermelon mill 362 bears on the concave 358 of the whipstock 359 and starts opening the window whilst the modified watermelon mill 364 further opens the windows and finishes the edges thereof. The arrangement is designed so that the modified watermelon mill 364 is supported with a minimum of clearance from the concave 358 of the whipstock 359.

Field trials have shown that the use of the mod¬ ified watermelon mill 364 can save a very substantial amount of time. * * *