During the construction of an oil or gas well a hole is bored in the ground. A string of casing is then lowered down the hole and the annular space between the casing and the hole filled with cement.

It is often desirable to provide one or more lat- eral bores which radiate outwardly from the casing.

This is typically achieved by anchoring a whipstock in the casing and then using the whipstock to divert a rotating starter mill against the casing and form an opening therein. The opening is then enlarged by a window mill and its edges smoothed by a water melon mill. The lateral bore is then drilled and cased as required.

Whilst techniques for providing lateral bores are continually improving there are still two noteworthy problems, in particular:- 1. It would be desirable to reduce the time taken to lower the whipstock to its desired depth; and 2. It would be desirable to increase the accuracy of the direction in which the lateral bore is drilled.

In order to address the first problem the present invention provides a whipstock having a fluid flow channel therethrough.

Preferably, a valve is provided in said fluid flow channel.

Advantageously, said body has set filler material therein defining a guide surface and said fluid flow channel extends through said filler material.

Preferably, said whipstock includes for retaining said filler material in said whipstock.

Advantageously, said plug has portions which are provided with ramps.

Before addressing the second problem it is impor- tant to understand that conventional whipstocks are typically provided with a single sacrificial surface which extends along the concave. In a conventional milling system the starting mill acts between the single sacrificial surface and the casing and biases the mill against the casing as it cuts/grinds its way there- through. The degree of wear on the sacrificial surface is unpredictable. After the starting mill has cut an opening in the casing the opening is usually enlarged by a window mill, and the edges cleaned up by a water melon mill. Drilling of the lateral bore then proceeds through the opening.

Over the years it has become increasingly important that the lateral bore extends as close as possible to that specified by the well operator. Many techniques are employed for this purpose. However, we have disco- vered that wear to the guide surface by the starting mill can result in the lateral bore being deflected from the intended direction.

In order to help reduce this problem the present invention provides a whipstock comprising a concave having a guide surface, characterised by a sacrificial element having a surface spaced from said guide surface for, in use, inhibiting a starter mill damaging said guide surface whilst opening a window in a casing.

Preferably, said surface of said sacrificial element extends for at least 40cm (18 inches).

Advantageously, said surface of said sacrificial element extends for at least 60cm (24 inches).

More advantageously, said surface of said sacrifi- cial element extends for at least 90cm (36 inches).

Preferably, said guide surface is defined, at least in part, by a set filler material.

The present invention also provides a method of

forming an opening in casing, which method comprises the steps of positioning a whipstock in accordance with the present invention in said casing, utilising said guide surface to deflect a rotating starter mill against said casing to penetrate said casing, and continuing forming an opening in said casing until the surface of said sacrificial element is substantially consumed.

* * * It should be noted that it has been proposed to lower a whipstock on a starter bar which extends from a starter mill and is shear pinned to a lug which pro- trudes from the concave. After the whipstock has been set downward pressure is applied to the starter mill to shear the shear pin. The mill is then rotated. It will be appreciated that the lug does serve to hold the starter bar, and thus the starter mill, away from the surface the concave. In the applicants' own equipment the lug would not support the starting mill clear of the guide surface of the whipstock. However, even if this had not been the case, the effect would have been very localized. This can be appreciated by considering that the axial length of a lug on a conventional whipstock is typically 2.5 inches whilst the new sacrificial surface of the present invention is typically at least 18 inches and preferably at least 24 inches. A length of at least 36 inches is recommended.

For a better understanding of the present inven- tion, reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1A is a cross-section of a whipstock according to the present invention; Fig. 1B is an enlargement of part of the whipstock of Fig. lA; Fig. 1C is a view on line lC-1C of Fig. lA; Fig. 1D is a front view of part of the whipstock of Fig. 1A; Fig. 1E is a view on line lE-lE of Fig. lB; Fig. 1F is a partial view of part of the whipstock as shown in Fig. 1B; Fig. 2A is a cross-section of part of the whipstock of Fig. 1A supported by a running tool; Fig. 2B and 2C show enlarged views of parts of the whipstock and running tool respectively of Fig. 2A; Fig. 3 is a side view of a starting mill which may be used with the whipstock shown in Fig. 1A; Fig. 4 is a side view of a window mill which may also be used with the whipstock shown in Fig. 1A; Fig. 5A is a front view, partly in cross-section of a retrieving tool approaching the whipstock shown in Fig. 1A; Fig. 5B is a side view in cross-section showing the retrieving tool of Fig. 5A engaging the whipstock; Fig. 5C is a view on line 5C-5C of Fig. 5A (with the whipstock omitted); Fig. 5D is a view on line 5D-5D of Fig. 5B; and Figs. 6A-6D and 7A-7E show the whipstock of Fig. 1A in use.

Referring to the drawings, Fig. 1A shows a whip- stock 10 according to the present invention having a body 12. A sacrificial element 20 with two guiding faces is secured to the body 12 with bolts 26. A guide

surface is formed by filler 28 which extends along a recess 30 of the body 12. A plug 40 is disposed in the bottom 34 of the body 12.

The top 14 of the body 12 extends above the sacri- ficial element 20 (preferably made of readily millable material, e.g. brass, bronze, composite material, iron, cast iron, typical relatively soft bearing materials, soft steels, fiberglass, aluminum, zinc, other suitable metals, or alloys or combinations thereof) and has a sloped ramp 38 (or a top shoulder 35 as shown in Fig.

2A). One-way teeth 16 are formed in the top 14 so that a member (not shown in Fig. 1A) with corresponding teeth may push down on the body 12 so that exerted force is transmitted from the corresponding teeth of the member to the body 12 and so that the teeth 16 and the corresponding teeth on the member slide apart when pulling up on the member with sufficient force. A hole 18 provides an opening for receiving a connector to connect the member to the body 12.

The first face 22 of the sacrificial element 20 is slanted so that a starting mill with an appropriate corresponding ramped lead portion contacts the first face 22 and is directed away from the body 12 (at an angle of between 5" to 25" and in one aspect about 15° from the central longitudinal axis of the body) e.g. to commence milling of a tubular (not shown), e.g. casing or tubing, in which the whipstock 10 is anchored. Any suitable known anchor device may be used. The second face 24 is configured, sized and disposed for further direction of the starting mill away from the body 12 as it mills the tubular.

A third face 32 includes sides or "rails" 12a, 12b (see Figs. 1C, 10 and 5A) of the body 12 which are sufficiently wide and strong to guide a window mill moving downwardly adjacent the whipstock. A fourth face

33 extends below the third face 32. In one aspect the fourth face 33 is straight and the third face 32 is a chord of a circle. The first, second, third, and fourth faces may each be straight or curved (e.g. a chord of a circle) as desired and either inclined at any desired angle in a straight line away from a longitudinal axis of the body or curved as a chord of any desired circle.

A plug 40 is secured in the bottom 34 of the body 12. The plug 40 retains the filler 28 within the recess 32. Via a channel 41 through a tube 42 (e.g. made of readily millable material), a channel 55 through a valve body 56 (e.g. made of readily millable material), a channel 72 through a body 62, and a sleeve 74 in a body 64, fluid flow through the plug 40 is possible when a valve member 58 rotates upwardly about a pivot 60.

As shown in Fig. 1B the valve member 58 is closing off fluid flow from above the plug 40 to beneath it, either due to the fact that there is little or no upward fluid flow and gravity holds the valve member 58 down or the force of fluid flow from below into the channel 72 is insufficient to overcome the weight of fluid on top of the valve member 58. Epoxy or some other suitable adhesive may be used to hold the body 62, body 64, and sleeve 74 together.

As shown in Fig. 1C, in one aspect a surface 20a of the sacrificial element 20 is shaped and configured as part of a curve to correspond to a curved outer shape of a nose of a starter mill to facilitate milling and guide the starter mill moving down the sacrificial element, e.g., the starter mill 200 shown in Fig. 3 which has a nose 240 with a cylindrical portion 244 that matches the curve of the surface 20a and a tapered portion 243 which is sized and configured to co-act effectively with the surface 20a. These corresponding curved shapes make possible line contact rather than point contact between

the starter mill and the surface 20a so that enhanced guiding of the starter mill is achieved.

Preferably the plug 40 is off center with respect to a central longitudinal axis from top to bottom of the body 12 to facilitate eventual milling out of the filler 28 (which may be, for example an epoxy resin or cement) and of the plug 40 from the recess 30.

To ensure proper positioning of the plug 40 upon installation in the recess 30 and to hold the plug 40 in position as filler 28 is fed into the recess 30, a rod 44 preferably made of readily millable material is secured at its bottom end in a hole 63 in a part 65 of the body 64 and at its top end 48 by nuts 50 and 52 in a hole 45 in a locating plate 46 which itself is secured in place by hardened filler 28 (see Fig. 1E). The tube 42 which can conveniently be made of glass fibre passes through a hole 51 in the locating plate 46.

Set screws 66 (made from, for example readily millable material) hold the part 65 of the body 64 in place. Set screws 67 also connect an adaptor 71 to the body 12. The adaptor 71 is connected to an anchor device (e.g. mechanical anchor, anchor packer, packer, etc). Additional bolts (not shown) extend through the holes 91, 92.

As shown in Fig. 1F, following milling out of the filler 28 and of the plug element 40, after the whip- stock 10 has served its purpose, a ring 90 remains which has as its lower part at one side a portion of a ramped part 70 of the body 64 and a portion of a ramped part 68 of the body 64. These remaining ramped portions (on the right side of the ring 90 as viewed in Fig. 1F) facili- tate the passage of other members, tools, or devices past the ring 90.

The ring 90 as shown in Fig. 1F results when the wellbore in which the system 10 is used is non-vertical

so that the body 12 is tilted to one side within the wellbore. The ring 90 results from milling when the "low side" of the wellbore is the left side of the apparatus as viewed in Fig. 1F. For this reason the portion of the bolts 66 initially projecting into the body 12 and into the adapter 71 are completely milled away since the mill is moving along this side of the apparatus - and it is for this reason that the mill, which must have some clearance to move in the apparatus, does not completely mill off the portion of the bolts projecting into the apparatus from the "high side" (right side) in Fig. 1F. So that such milling does not create a stop member within the apparatus, the remaining part of the ramped portions 68 and 70 are used along which a tool may move more easily as compared to a ring with portions projecting normal to the apparatus side wall. In a vertical or nearly vertical hole, milling produces a resulting ring with a ramped portion around all or around substantially all of the top and bottom of the ring. If desired, a ramp may be used on only one side (top or bottom, e.g. 68 or 70) of the original ring.

When the whipstock 10 is being inserted into a wellbore, fluid in the wellbore is permitted to flow up through the plug element 40 as the valve member 58 opens in response to the fluid. The fluid flows up and out from the whipstock body 12 through the channel 41 of the tube 42, thus buoyancy of the system 10 is not a problem while it enters and passes down through the wellbore.

Preferably parts of the plug 40 are made of brass, plastic, bronze, epoxy resin, aluminum, composite mater- ial, iron, cast iron, relatively soft bearing material, fiberglass, some other readily millable material, or a combination thereof. In certain aspects the locating plate 46, rod 44 and tube 42 are positioned so that the

plug 40 will be on the "high side" when the whipstock 10 is disposed in a non-vertical wellbore (with the rod 44 closer to the "low side" than the tube 42).

The plug 40 serves to maintain filler 28 in the recess 30 as the filler is initially fed into the recess 30 and prior to setting of the filler. The plug 40 maintains the filler 28 in the recess 30 when a mill is milling out the filler 28 thus preventing a mass of the filler 28 from exiting the body 12 and falling down into a wellbore. The plug 40 also inhibits the force of a hydrostatic head of fluid in the wellbore from pushing the filler 28 or part of it upwardly and out from the recess 30. Any known and appropriate valve device or apparatus may be used instead of the valve member 58.

To facilitate maintenance of the filler in the recess, interior indentations or threads may be provided on the recess and/or an initial coating of epoxy resin and/or fiberglass fibers is applied to the interior of the recess and allowed to set.

Fig. 2A shows a running tool 100 releasably atta- ched by a shear bolt 115 (shearable, e.g. in response to about 30000 lbs of force) to the top 14 of the body 12.

Fluid (e.g. working fluid, water, mud) pumped from the surface by a surface pumping unit, not shown) flows down a tubular string (not shown) to which the running tool 100 and the whipstock 10 are connected through a channel 108 through a fill-up sub 102, past a valve 120, and through a channel 110 of a body 104. This fluid then flows through holes in a centralizer 131 that centralizes a piston 134 and a rod 132 in a body 106.

An end 133 of the rod 132 is held in a recess 138 in the body 106. When the fluid is of sufficient force, shear screws or pins 137 holding a piston 134 to a holding member 135 are severed and the fluid pushes the piston 134 down on the rod 132. Fluid, e.g. oil, in a cavity

136 in the body 106 is thus forced out from the cavity 136, through a port 139, into an hydraulic line 114 (shown partially) which extends down along the whipstock 10 (and/or through the plug 40) to an hydraulically settable anchor device (not shown) for anchoring the whipstock 10 at a desired location in a wellbore or in a tubular member. To check anchor setting, weight is applied to the whipstock 10 through the running tool 100. The teeth 16 of the body 12 and corresponding teeth 116 of the running tool 100 transfer the load (e.g. about 80,000 pounds) to the whipstock and thus to the anchor device. These teeth also isolate the sacrificial element 20 and the shear bolt 115 from the downward load. In certain aspects this facilitates passage of the whipstock 10 through tight spots in a tubular string and permits a relatively large load to be applied without prematurely shearing the shear bolt 115 and ensures that the sacrificial element 20 is not inadvertently damaged or sheared off.

While the running tool is being introduced with the whipstock 10 into a wellbore, fluid in the wellbore flows from outside the running tool through a port 149, through a groove 151 surrounding the interior of the body 104, through a channel 152 in a body 141, up to and out through a port 161, out a channel 163, and up into the channel 108 of the sub 102 up into the working string. Thus buoyancy of the system and of the running tool is reduced or eliminated.

A valve member ball 127 as shown in Fig. 2A is seated against a valve seat surface 169, thereby pre- venting fluid flow out from the port 149 (e.g. when actuating an anchor device with fluid under pressure through a channel 140). A spring-loaded cylinder 122 is urged down by a spring 124 to hold the ball 127 against the valve seat surface 169. The spring 124 has its top

end biased against an inner top surface of a retainer 123 and its lower end biased against a shoulder on the exterior of the cylinder 122. The retainer 123 is secured to a top 126 of the body 141. A spacer 121 holds the body 141 in position.

A rupture disc (or discs) 145 is disposed across a channel 146 and is held in place against a seal 147 in a recess 143. Initially the rupture disc 145 prevents fluid flow through the channel 146. Once the running tool 100 has been separated from the whipstock body 12 by shearing the shear bolt 115 with an upward pulling force following correct positioning of the body 12 and setting of its anchor (using typical positioning de- vices, e.g. a gyro) and the running tool 100 is to be raised and removed from the wellbore, the force of fluid pumped from the surface under pressure to the running tool and in the string to which the running tool is attached ruptures the disc 145 and pumped fluid from within the string flows down through the running tool, through the channel 140 and out through the port 146 draining the workstring thereby facilitating removal thereof. Thus the fluid in the string is drained there- from into the wellbore.

Fig. 3 shows a starting mill 200 useful with the whipstock 10 for forming an initial window, e.g. in casing in which the whipstock 10 is positioned. The starting mill 200 has a body 202 with a fluid flow channel 204 therethrough (shown in dotted lines). Three sets of cutting blades 210, 220 and 230 with, respect- ively, a plurality of blades 211, 221 and 231 are spaced apart on the body 202. Jet ports 239 are in fluid communication with the channel 204. A nose 240 projects down from the body 202 and has a tapered end 241, a tapered ramped portion 242, a tapered portion 243, and a cylindrical portion 244. In one aspect the nose is made

of readily millable material and is releasably secured to the body 202; e.g. so that it can be twisted off by shearing a shearable member that holds the nose to the body. Then the released nose may be milled by the mill.

The nose 240 may have a fluid flow channel and valve as shown, e.g., in the system of Fig. 13.

The nose 240 is sized, shaped and configured so that it contacts the sacrificial element 20 which is typically 38 inches long as the starting mill 200 initially moves down in a wellbore to mill and mill through a tubular, e.g. casing or tubing (not shown).

The nose 240 contacts and moves down along and adjacent the sacrificial element 20 as the blades first contact and begin milling into the casing to form the initial window at the desired location. The nose 240 and its co-action with the sacrificial element 20 keep the starting mill 200 from contacting and milling the body 12. The cylindrical portion 244 of the nose 240 acts like a bearing against the sacrificial element 20.

After the starting mill 200 has milled down the casing, e.g. for several inches, it has milled through the casing. For example, with casing approximately .5 inches thick, the starting mill 200 will have milled through the casing after milling down three or four inches. Then the starting mill 200 continues to move down and mill more casing to form the initial window.

After the starting mill 20 has moved downwardly to an extent greater than the length of the nose 240, the blades 231 are in position to mill the sacrificial element 20 in addition to milling the casing opposite the sacrificial element 20. Simultaneously the blades 221 and 211 are milling casing above the sacrificial element 20. At this point the sacrificial element 20 begins to be milled by the blades 231. The sacrificial element 20 as shown is sized and disposed to prevent

the blades 231 from milling the whipstock body 12. It is within the scope of this invention for the element 20 to be sized so that a minor milling of the whipstock body occurs.

In one aspect of the starter mill, the body, and the sacrificial element are sized, disposed, and confi- gured so that an initial window in the casing of desired length is milled out without the mill contacting the whipstock body or the filler therein. In one aspect such a window is completed with about two inches, one inch, or less of the lower part of the sacrificial element 20 remaining. At this point in the procedure the starting mill 200 is removed from the wellbore. It is preferred to stop the starter mill at this point rather than remove the whole surface of the sacrificial element 20 and risk damage if the starter mill advances further and rotates on the guide surface defined by the filler. In another aspect the nose 240 is sized, dis- posed, and configured, e.g. as shown in Fig. 3, so that at the bottom extent of milling there is some minimal clearance between the nose 240 and the interior casing wall so that the nose 240 is not held therebetween and so that damage to the nose 240 is reduced or eliminated.

In one aspect the angle of taper of the tapered portion 243 corresponds substantially to the angle of taper of the face 24 of the sacrificial element 20 so the contact between the two is effected to maximize the ability of the sacrificial element 20 to direct the mill away from the whipstock and against the casing. Also, in this embodiment the taper angle of the tapered por- tion 243 is such that when milling is finished (see Fig.

6D) the tapered portion 243 is substantially parallel to the interior casing surface adjacent the nose 240 inhibiting wedging contact of the two and reducing friction therebetween.

In one particular embodiment sacrificial element 20 is about 30 inches long (excluding the extending top part with teeth) and the blade sets of the mill 200 are spaced apart about two feet and the nose 240 is about 18 inches from its lower end to the first set of blades 231. With such a mill a completed initial window is about 60 inches long. It is within the scope of certain preferred embodiments of this invention for the initial window through the casing to be two, three, four, five, six, seven or more feet long.

Fig. 4 shows a window mill 250 for enlarging the window made by the starter mill. The window mill 250 has a body 252 with a fluid flow channel 254 from top to bottom and jet ports 255 to assist in the removal of cuttings and debris. A plurality of blades 256 present a smooth finished surface 258 which moves along what is left of the sacrificial element 20 (e.g. one, two, three up to about twelve to fourteen inches) and then on the filler 28 and the edges of body 12 that define the recess 30 with little or no milling of the filler 28 and of the edges of the body 12 which define the recess 30.

The lower ends of the blades 256 and a lower portion of the body 252 are dressed with milling material 260 (e.g.

but not limited to known milling matrix material and/or known milling/cutting inserts applied in any known way, in any known combination, and in any known pattern or array).

In one aspect the lower end of the body 252 tapers inwardly an angle C to inhibit or prevent the lower end of the window mill from contacting and milling the filler 28 and body 12 (i.e. the angle C is preferably greater than the angle a in Fig. 1A).

In one aspect the surface 258 is about fourteen inches long and, when used with the starter mill 200 having blades about two feet apart as described above,

an opening of about five feet in length is formed in the casing when the sacrificial element 20 has been comple- tely milled down. In this embodiment the window mill 250 is then used to mill down another ten to fifteen feet so that a completed opening of fifteen to twenty feet is formed, which includes a window in the casing of about eleven to fifteen feet and a milled bore into formation adjacent the casing of about five to nine feet.

In one embodiment the lower ends of the blades of the window mill body 252 taper upwardly from the outer surface toward the body center an angle d (Fig. 4).

This taper part tends to pull the body 252 outwardly in a direction away from the filler 28, and away from the whipstock body 12 into the formation adjacent the cas- ing, acting like a mill-directing wedge ring. Also this presents a ramp to the casing which is so inclined that the mill end tends to move down and radially outward (to the right in Fig. 7E) rather than toward the whipstock.

In one method a mill (such as the window mill 250) mills down the whipstock, milling a window. Following completion of the desired window in the casing and removal of the window mill, a variety of sidetracking operations may be conducted through the resulting window (and, in some aspects, in and through the partial lateral wellbore milled out by the mill as it progressed out from the casing). In such a method the remaining portion of the whipstock is left in place and may, if desired be milled out so that the main original wellbore is again opened. In one aspect the filler 28 and plug 40 are milled out to provide an open passage through the whipstock.

In another aspect, in the event there is a problem in the milling operation prior to completion of the window, the whipstock is removed. As shown in Figs. 5A

and 5B, a retrieving tool 270 with a body 272 has a barrel 280 threadedly connected to the body 272. A fluid flow channel 268 extends down into the body 272 from a top end thereof and is in fluid communication with a top channel 273 and a side channel 274 so that fluid may be pumped through or flow through the retriev- ing tool 270. As shown in Fig. 5A, the tool 270 has been inserted into the wellbore and has contacted the body 12 of the whipstock 10. Preferably the threads 281 are positioned on the barrel 280 interior so that the corresponding threads on the whipstock body are not engaged until the barrel has moved down over a signif- icant portion of the body 12 so that threaded engagement does not occur at a relatively thin portion of the top of the whipstock. Interior threads 281 of the barrel 280 have threadedly mated with exterior threads 282 of the body 12. A nose 278 of the body 272 has entered a space between the casing and the top of the whipstock body 12. The body 272 may be connected to a string of hollow tubular members, e.g. but not limited to a drill string or workstring.

Fig. 5B illustrates the tool 270 as it first con- tacts the whipstock top 14 before any milling has been done. To retrieve a whipstock from the position shown in Fig. 5B, the tool 270 (e.g. on a drill string) after engaging the whipstock is pulled upwardly (e.g. with 30,000 to 80,000 or more pounds of force). A tapered surface 277 of the nose 278 contacts the top 14 and (when the whipstock 10 is in a non-vertical hole with the whipstock on the "low" side of the hole) pushes down on it thereby leveraging and lifting the body 12 away from the "low" side of the casing facilitating the engagement of the threads 281 with the threads 282.

Upon correct engagement of the whipstock by the tool 270, the whipstock is removed from the wellbore by

removing the drill string from the wellbore (e.g. by pulling with about 100,000 lbs force which, in certain aspects releases the whipstock from the anchor e.g. by shearing a shearable whipstock stinger from an anchor device). The sacrificial element, although present, is not shown in Fig. 5A. The tool 270 may also be used following milling.

Filler 28 may be cermet, cement, brass, fiberglass, bronze, wood, bearing material, cast iron, polymer, epoxy resin mixed with fiberglass fibers, resin, plas- tic, or some combination thereof.

Figs. 6A-6D illustrate steps in a method using the whipstock 10 and starting mill 200. The starting mill 200 is connected to a working string D that extends to the surface. As shown in Fig. 6A, the whipstock 10 has been located, positioned, and anchored in a tubular string of casing G that extends down from the earth's surface (not shown) in a wellbore W through an earth formation F. The tapered end 241 of the nose 240 of the starting mill 200 has contacted the first face 22 of the sacrificial element 20. Preferably the blades 211, 221, 231, do not touch the casing on the whipstock side (left side, Fig. 6A) and are held against the casing on the opposite side (right side, Fig. 6A) both by the co- action of the tapered end 241 with the first face 22 and by a stabilizer S (any known stabilizer or smooth faced or smooth bladed mill, e.g. a starting mill with smooth outer surfaces). At this point milling is started by rotating the starting mill 200 (e.g. by rotating with the surface rotary the string D to which the starting mill 200 is attached that extends to the surface; or by using a downhole motor positioned in the string above the starting mill.

As shown in Fig. 6B the three sets of blades of the starting mill 200 have begun to mill into the casing G;

the tapered portion 243 of the nose 240 has moved down to contact the sacrificial element 20; and the blades are held away from the whipstock side (left side, Fig.

6B) of the casing G.

As shown in Fig. 6C, the tapered portion 243 of the nose 240 has continued to move down and co-act with the second face 24 of the sacrificial element 20; the blades 231 have milled through the casing G; the blades 231 have milled away part of the sacrificial element 20; the three sets of blades have been directed away from the whipstock side of the casing G; the blades 221 have milled through the casing G; the blades 221 have milled and are about to mill through the casing G; the nose 240 is not caught or wedged in between the sacrificial element 20 and the inner wall of the casing G; part of the top bolt 26 has been milled away; and the body 12 of the whipstock and filler 28 are not milled by the start- ing mill 200.

As shown in Fig. 6D an initial casing window I has been completed; the surface 244 acts as a bearing sur- face against the second face 24; portions of bolts 26 have been milled away; parts of the formation F has been milled away; the majority of the sacrificial element 20 has been milled away and a portion of the sacrificial element 20 remains; the body 12 of the whipstock and filler 28 have not been milled (or in any other aspects only a minor portion of the top of the whipstock body 12 has been milled); the nose 240 has moved freely or with minimal contact of the casing G to the position shown; the cylindrical portion 244 is wedged between the sacri- ficial element 20 and the casing G indicating at the surface that there is no more progression of the mill; and the mill 200 is ready to be removed from the well- bore so that further milling with additional mill(s) can be done to complete the desired window. Preferably the

nose 240 (other than portion 244) is not touching the casing G or only has incidental contact therewith.

If the initial window as shown in Fig. 6D is suit- able, no other milling is done. If the window in Fig.

6D is to be enlarged and/or lengthened, another mill or series of mills is introduced into the wellbore. As shown in Fig. 7A, the window mill 250 (Fig. 4A) has been run into the wellbore (e.g. on a tubular string N of, e.g. a drill string of drill pipe to be rotated from above or to be rotated with a downhole motor as de- scribed above). The inwardly tapered portion 260 of the body 252 of the window mill 250 preferably does not mill the top of the body 12 of the whipstock or mills it minimally.

As shown in Fig. 7B the window mill 250 proceeds down along the remainder of the sacrificial element 20 with the mill surface 258 holding the milling end away from the sacrificial element 20 and directing the mill 250 away from the body 12 toward the casing G. The inwardly tapered portion of the mill 250 (tapered at angle d, Fig. 4) encounters a ledge L created by the starting mill 200, and due to the inwardly tapered portion, the mill moves outwardly with respect to the ledge L, begins the mill the casing G, and also begins to mill the remainder of the sacrificial element 20.

The surface 258 will continue to co-act with the result- ing milled surface on the sacrificial element 20 until the surface 258 is no longer in contact with the sacri- ficial element 258 as the window mill 250 mills down the casing G. Thus the window, (at the point at which the window mill 250 ceases contact with the sacrificial element 20) that includes the initial window formed by the starting mill 200 and the additional portion milled by the window mill 250 is created without the mills contacting the whipstock body 12 or the filler 28. The

tubular string N is present, but not shown, in Figs. 7B- 7F.

As shown in Fig. 7C, the mill 250 has continued to mill out the window in the casing G and has both contac- ted the body 12 of the whipstock and begun to mill a bore B into the formation F (e.g. a bore suitable for sidetracking operations). Since the window mill is operating in the opening the pressure on the guide surface of the whipstock is comparatively small and hence damage thereto is comparatively minimal. Prefer- ably the surface 258 of the mill 250 is contoured, configured and shaped to correspond to the curved shape presented by the rails 12a and 12b (see Fig. 1C) so that these parts of the body 12 have more than point contact and effectively direct the window mill 250 away from the whipstock. The radiused face 32 of the body 12 and filler 28 also assists in directing the mill 250 at a desired angle away from the whipstock. Eventually the window mill 250 contacts a straight (non-radiused) face 17 of the body of the whipstock and filler material 28.

As shown in Fig. 7D the window mill 250 has milled completely through the casing G and has extended the bore down beyond the plug 40 and the sub 71. Further milling may be conducted with the window mill 250 or other mills, or the window mill 250 may be withdrawn from the wellbore.

An additional mill or mills as desired may be used above the window mill 250. As shown in Fig. 7F a water- melon mill 280 is used above the window mill 250 to facilitate milling, window formation, and smoothing of milled surfaces.

The filler 28 may have a metal sheath or shield covering exposed portions thereof. The filler 28 may be one or more containers of filler material positioned in the originally hollow portion of the whipstock. These

containers may be relatively rigid, e.g. steel plate, or relatively flexible, e.g. metal foil or plastic of sufficient thickness, yet puncturable, ruptureable by pressure and/or chemicals, or tearable so that at a desired time their contents (e.g. sand, rocks, liquid, balls of material, granular material, or a mixture thereof) flows out and down away from the whipstock. In one aspect spacers (solid, containers, spoked wheels, etc) are used so that there is a series of filler masses or filler containers and spacers in the hollow portion of the whipstock. In another aspect the spacers are hollow and empty or hollow with liquid or granular material there which easily flows out and down through the tool on breaking or rupture of the spacer body or wall. In one aspect the sheath, shield, and/or spacers are made of bearing material for contact by a mill or mills.