During the construction of oil and gas wells a hole is bored in the ground. The hole is then lined with a string of casing which is often cemented in place by an annulus of cement.

In order to extend the catchment area of a well it is becoming increasingly common to provide laterals.

This involves forming a window in the casing and then drilling a new hole in the formation surrounding the window. The new hole is often provided with its own liner which itself may be provided with a lateral at a later date.

At the present time laterals are formed by insert- ing a whip stock in the casing and using this to bias a rotating mill against the casing to form a window there- in. The edges of the window are then usually enlarged and shaped by another mill. Typically, the window will be formed by a tool string which has a starting mill and a water melon mill. After the window has been formed the starting mill and the water melon mill are brought to the surface and replaced by a formation drill for cut- ting into the formation. The formation drill is then lowered and the lateral drilled.

Until the present time it has always been consid- ered necessary to make a separate trip to replace the mill(s) with the formation drill. This is a time consum- ing and expensive operation, particularly if the lateral is being constructed at a significant distance from the wellhead.

It should perhaps be emphasised that cutting tools for penetrating casing have a very limited life in

formation whilst cutting tools for penetrating formation will often not cut through casing at all. A simple analogy is a domestic masonry drill which will not cut through metal and a domestic twist drill which dulls after being used to cut a small hole in masonry.

The present invention is based on the concept of using a formation drill which is provided with a protec- tive layer which will protect the formation drill whilst the window is formed but which can subsequently expose the formation drill ideally immediately after the window has been formed. Ideally the protective layer will take an active part in the formation of the window although this is not absolutely essential.

According to the present invention there is provi- ded a tool for use in forming a lateral, which tool comprises a formation drill which is provided with a protective layer, and a mill for forming a window in casing, the arrangement being such that, in use, said protective layer will inhibit damage to said formation drill whilst said mill forms a window in said casing.

As used herein the term "formation drill" is inten- ded to mean any form of cutting device which is primari- ly intended for cutting through formation, for example limestone or sandstone, whilst the term "mill" is inten- ded to mean any form of device which is primarily inten- ded for making a window in casing, for example a steel tubular or an alloy tubular.

The formation drill may be exposed in a variety of ways, for example by advancing the formation drill relative to what remains of the mill after the window has been opened, disengaging the remains of the mill from the formation drill, or by erosion of the protec- tive layer, preferably during the formation of the window. Examples of all three alternatives are described with reference to the accompanying drawings. However,

exposure by erosion is currently felt to be the most promising alternative.

With regard to erosion the design of the protective layer is not too critical. Ideally, the protective layer should be eroded at the time the window is completed.

However, little is lost if the final traces of the protective layer are stripped off in the formation.

Similarly, no great harm will occur if the protective layer is stripped off whilst the window is nearing completion. However, it would be most undesirable for the protective layer to be stripped before the initial break through of the casing by the mill since this would almost certainly damage the formation drill.

The protective layer preferably forms part of the mill and could conceivably form the entire mill.

Further features of the invention are set out in Claims 2 et seq

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. 1 is a side view of a first embodiment of a tool according to the present invention; Fig. 2 is a side view of a second embodiment of a tool according to the present invention; Fig. 3 is a side view, partially in cross-section of a third embodiment of a tool according to the present invention; Fig. 4A is a view partly in elevation and partly in section of a fourth embodiment of a tool according to the present invention; Fig. 4B is a view mainly in side elevation of an element which forms part of the tool shown in Fig. 4A; Fig. 4C is a view similar to Fig. 4B but showing parts cut away; Fig. 5A is a perspective view of a prior art drill collar; Fig. SB is a perspective view of a fifth embodiment of a tool in accordance with the present invention; Fig. 6A is a perspective view of a sixth embodiment of a tool according to the present invention; Fig. 6B is a view in cross-section of the tool of Fig. 6A; Fig. 6C is an enlarged view of a blade of the tool of Fig. 6A; Fig. 7A is a schematic perspective view of a se- venth embodiment of a tool according to the present invention in a first stage of operation; Fig. 7B is a schematic perspective view of the tool of Fig. 7A in a second stage of operation; Fig. 8 is a side schematic view of an eighth embo- diment of a tool according to the present invention; Fig. 9A is a side view of a ninth embodiment of a

tool according to the present invention; and Fig. 9B is a bottom view of the tool of Fig. 9A.

Fig. 10A shows a tenth embodiment of a tool in accordance with the present invention in a tool string in a casing; Fig. lOB shows, to an enlarged scale, part of the tool string shown in Fig. 10A; Fig. lOC shows certain parts of the apparatus during the formation of a lateral; Fig. 11A is a side view of the tool of Fig. 10A without its protective cover; Fig. 11B is an end view of the tool shown in Fig.

11A; Fig. llC is a view, to an enlarged scale, of the detail encircled in Fig. 11B; Fig. 12A is an end view of the tool shown in Fig.

11A with its protective cover; Fig. 12B shows, to an enlarged scale, part of the tool of Fig. 12A; Fig. 12C is a view, to an enlarged scale, of the detail shown in Fig. 12B; Fig. 13 is a perspective view of an eleventh embo- diment of a tool in accordance with the present inven- tion; Fig. 14 is a perspective view of a twelfth embodi- ment of a tool in accordance with the present invention; and Fig. 15 is a perspective view of a thirteenth embodiment of a tool in accordance with the present invention.

Referring to Fig. 1 a tool 10 is shown schematical- ly which has a tubular body 12 with an upper threaded portion 14 for connection to a tubular string (not shown) for use in a tubular in a wellbore extending from the earth's surface down through an earth formation.

The tubular string may be rotated by a conventional rotary table or by a downhole motor in the tubular string. The tubular string may be of drill pipe or a coiled tubing string.

The tool 10 has a lower milling section 20 with milling material 22 thereon for milling a tubular, such as a liner, casing or other tubular, which lines a well- bore and/or extends down through a wellbore or through a wellbore and a lateral wellbore communicating therewith.

The milling section 20 may be configured as any known mill, for example a typical window mill with milling material on a front lower face 24 thereof and on sides 26 thereof. "Milling material" means any known milling material, such as crushed carbide, matrix milling mater- ial, and/or milling elements or inserts of any suitable size, shape and configuration applied in any suitable known array, arrangement, or pattern. The milling material is suitable for milling an opening through a downhole tubular so that then a formation drill section 30 may subsequently drill a borehole from the opening away from the tubular into a formation through which the wellbore is to be extended. The milling section 20 is worn down as it mills the opening and/or as it enters the formation so that a formation drill section 30 is exposed or freed to drill the borehole. The formation drill section 30 may be any suitable known drill or drill bit employing drilling material 32 which may be any suitable known drilling material and/or drilling insert(s) or element(s) suitable for drilling into a formation, for example limestone or sandstone. Known

appropriate fluid passageways may be provided through the body 12 to the formation drilling section 30 and lower milling section 20 (and also for any tool de- scribed herein).

Fig. 2 shows schematically a tool 40 according to the present invention with a tubular body 42, a forma- tion drill section 44 (like the formation drill section 30, Fig. 1), a milling section 46 (like the milling section 20, Fig. 1), and a release section 50 connected between the formation drill section 44 and milling section 46.

The release section 50 is either releasably connec- ted to the formation drill section 44 or the milling section 46 is releasably connected to the release sec- tion 50 so that upon the completion of an opening through a tubular the formation drill section 44 and milling section 46 may be separated so that the forma- tion drill section 44 is freed to drill a borehole in a formation adjacent a window.

In the aspect in which the milling section 46 is released from the release section 50, the release sec- tion 50 is, preferably made of an easily drillable material. In one aspect the release section is shear pinned to the formation drill section 44 and the shear pins are sheared by pushing or pulling on the tool or by using a mechanical selectively operable shearing mechan- ism. In one aspect the release section 50 is made of a chemically responsive material, e.g. a metal (e.g.

aluminium (which is erodable e.g. by ammonia), plastic, or composite which is weakened and/or eroded by the introduction of a particular chemical specific to the material used to which the release section is chemically responsive, thereby releasing the release section from the drill section or releasing the mill section from the release section. Alternatively, chemically responsive

connectors, e.g. pins, screws, bolts, are used to con- nect the sections together. In another aspect only enough of a chemical (e.g. an acid) is introduced in the area of and/or applied to the tool to erode, corrode, or weaken a mill section so that, upon contacting a forma- tion, the mill section separates from the drill section.

In one aspect timed deteriorating connectors are used, e.g. bolts with erosive chemicals therein that erode in a time sufficient to allow the introduction of the tool into the wellbore and the milling of the desired window in the tubular.

In one aspect the release section 50 is made of suitable material (e.g. but not limited to ceramics, cermets, glass, or metal members of appropriate dimen- sions) which is responsive to heat or sonic waves gener- ated and directed at the release section by a heat (or sonic) generator to weaken or sever the release section 50 to effect its separation from the drill section 44.

Fig 3 shows a tool 60 which, in some respects, is similar to the bit disclosed in US Patent 5,289,889 (incorporated fully herein for all purposes), but which, according to the present invention, is useful not only for drilling a borehole but also for milling a tubular window prior to drilling.

The tool 60 has an internally threaded top end 61 of a body 62. A plurality of milling blades 63 project from and surround the body 62 and are covered with milling material 64. Milling material 65 is also on journal segment arms 66 which are affixed to a lower end of the body 62. Each of a plurality of roller cone cutters 67 with a cutting structure of inserts 68 is rotatably mounted on one of the arms 66. The inserts 68 may be any of the inserts or elements described herein or they may be known drilling inserts. In one aspect the roller cones are selectively held immobile until milling

is completed.

Figs. 4A - 4D show a tool 70 according to the present invention which, in some aspects, is similar to the bits disclosed in US Patent 4,244,432 (incorporated fully herein for all purposes), but which is useful not only for drilling a borehole but for milling a tubular window in a wellbore tubular prior to drilling the borehole. This tool and the others disclosed herein may be used to produce a borehole from a main wellbore in a single trip into the main wellbore in which both tubular milling and borehole drilling are accomplished.

The tool 70 has a body 72 connected to a typical drill collar 74 which is part of a tubular string (not shown). Milling blades 76 project from the body 72 and are covered with milling material 78 for milling an opening through a wellbore tubular. A plurality of cutters 80 are disposed in sockets 82 in a lower end of the body 72. The cutters 80 may be typical drilling cutters or they may be as shown in Fig. 4B with a body 84 with a milling material portion 86 and a drilling material portion 88. Additional cutters 80 may be added to the lower end of the body 72 to enhance milling of a tubular. In one aspect the lower end is substantially covered with cutters. Fig. 4D is a cross-section view of the cutter of Fig. 4B.

Fig. 4D shows an alternative element 90 with a body 92 mounted in a socket 93 in an end of a tool 94. The element 90 has a milling portion 95 made of milling material for milling a wellbore tubular and a drilling portion 96 made of drilling material for drilling a formation. The milling portion 95 is sized so that it does not wear away until a tubular is milled through.

Alternatively a milling insert or element may be em- placed on a tool and/or in a socket thereof on top of (in front of) a drilling element. It is within the

scope of this invention to emplace and secure in the sockets 93 one or more drilling elements behind one or more milling elements and/or to use PCBN elements.

Referring now to Fig. SA, there is shown a drill roller 100. Drill collars with a cutting surface are known per se in the prior art; see, e.g. U.S. Patent 3,343,615 incorporated fully herein for all purposes.

Fig. 5A shows a drill collar 100 with a body 102 having a flow bore 103 extending therethrough from top to bottom. Threaded ends 104, 105 make it possible to connect the drill collar 100 to other tools, tubulars, and devices. In one aspect two, three, or more such collars are connected together. The collar(s) may be used in the system according to the present invention.

The drill collar 100 has projections 106 each with an outer layer of milling material 107 and an inner layer of drilling material 108. Although four projec- tions 106 are shown, the drill collar 100 may have two, three, five, six or more such projections.

Fig. 5B shows a tool 110 including a drill collar 111 (like that of U.S. Patent 3,343,615) with a body 112 having a fluid flow bore 113 therethrough from a top end 114 to a bottom end 115. Drilling material 116 is on an exterior of each projection 117. Shown schematically is a mill 118 connected to the bottom of the drill collar 111. Two, three or more drill collars 111 may be con- nected end-to-end above the mill 118. The mill 118 may be releasably connected to the drill collar 111 and a spacer member (not shown) may be used between the mill 118 and the drill collar 111. The drill collar 111 may also have an outer layer of milling material on each of its projections (like the drill collar 100, Fig. 29A).

A flow bore 119 communicates with the bore 113.

Fig. 6A - 6B show a tool 120 which, in some aspects, is like the bit of U.S. Patent 4,719,979

(incorporated fully herein for all purposes) but which can also mill through a wellbore tubular prior to dril- ling a borehole.

The tool 120 has a body 122 with a flow bore 123 therethrough from top to bottom in communication with one or more jets 124 which have exit ports 125 adjacent each of a plurality of blades 126. The upper end of each blade 126 is dressed with milling material 127 and has mounted therein a plurality of milling inserts 128.

The lower end of each blade also has drilling material 129 thereon and a plurality of drilling elements 121 therein.

Figs. 7A and 7B show a tool 200 schematically with a plurality of milling members 202 each having an end or face milling surface 204 and a plurality of drilling members 206 each with an end or face drilling surface 208. A mechanism 210 interconnected with the drilling members 206 selectively moves the drilling members 206 from a first non-drilling position (Fig. 7A) to a second drilling position (Fig. 7B).

The milling and drilling members may be enclosed in any suitable housing, sleeve, or tubular member and any known fluid flow system or structure may be used to provide fluid flow to the milling and drilling surfaces.

The entire length of the milling members 202 may be made of milling material and the entire length of the dril- ling members 206 may be made of drilling material.

Alternatively, an upper portion of the milling members may be made of drilling material to coincide with and be disposed adjacent the drilling members to facilitate drilling.

Fig. 8 shows schematically a tool 240 with a body 242 having an upper threaded end 244. Milling and drilling material 246, e.g. polycrystalline cubic boron nitride, in a matrix and/or with cutting elements made

therefrom covers a plurality of cutting blades 252 (two shown; three, four, five, six, seven, eight, nine or more within the scope of this invention) disposed on and around the body 242. For each blade 252 there is a corresponding buffer member 254 adjacent thereto and, in one aspect, in contact therewith, each buffer member interconnected with a movement mechanism 256 (shown schematically). The buffer members 254 are selectively movable both radially (arrows R) and axially (arrows A) so that a selected amount of the milling and drilling material is provided so that the cutting depth of the tool 200 is controlled. For the continuous milling of a metal tubular a certain amount of the milling and dril- ling material is "freed," i.e., with no buffer member portion adjacent thereto. For drilling, the buffer members are retracted axially and radially to "free" more of the milling and drilling material so that a bigger "bite" can be taken into a formation.

Fig. 9A shows a tool 260 according to the present invention connected to the lower end of a string 262.

The tool 260 has a body 264 with a plurality of members 266 that extend within the body 264 from top to bottom and which, at the lower end, extend across part of the end face of the body 262. Up to a certain level L, each member 266 is made of milling material. Above the level L, each member 266 is made of drilling material. The level L is chosen, in one aspect, so that there is sufficient milling material to mill through a tubular.

Interspersed between the members 266 may be milling matrix material up to the level L and drilling matrix material above the level L. Alternatively, the space between these members may be void and empty, or filled with any suitable metal or other filler. A central flow bore 267 provides fluid to the end of the tool from the surface pumped through the string 262. As desired sub-

channels may be provided to effect jetting fluid action at the end of and/or on the sides of the tool.

As shown in Fig. 10A a system 500 has a top water- melon mill 501 (shown schematically in Fig. 10A) which is connected to a flexible member, flexible pipe, or flex sub 502. The flex sub 502 is connected to a second watermelon mill 503 which is connected to a second flex sub 504. The flex sub 504 is connected to a tool 520.

The tool 520 is releasably connected to a sacrificial face element 510. The sacrificial face element is connected to a whipstock 505. The whipstock 505 is anchored in a tubular, e.g. casing C of a casing string in a wellbore, by an anchor A which is any known anchor, anchor-packer, packer, or setting apparatus.

As shown in Fig. lOB, the tool 520 (shown without the material 527) has side blades 521 dressed with matrix milling material 522. In one aspect the exterior blade surfaces of the side blades 521 are smooth (e.g.

ground smooth with a grinder). The matrix milling material may be any known mill dressing material applied in any known manner.

Matrix milling material 523 covers lower ends 524 of the side blades 521 (see, e.g. Fig. 11A and Figs. 12A - 12C). Blades 525 form part of a formation drill and are arranged (see Fig. 11A) on the nose 526 of the tool 520 are initially laterally protected with a material 527 (e.g. but not limited to bearing material such as brass) and, the end of which is optionally partially or wholly covered with wear away material and with matrix milling material 523 (see Fig. 12C). Fluid under pres- sure, pumped from the surface, exits through ports 528 at the lower ends 524 of the side blades 521. The wear away material may act like a bearing or bearing material may be used in its place so that the side portion of tool acts as a bearing.

Fig. lOB shows the system 500 in a cased wellbore with various positions of the tool 520 shown in dotted lines. Initially (as shown) the tool 520 has not been released from the fingers 511. Following release from the fingers 511 and downward movement, the lower ends 524 of the blades 521 have milled away a portion of the sacrificial element 510 including the fingers 511 and the outer blade surfaces have moved to contact at point A an inner surface S of a casing C in a wellbore. A distance d is, preferably, of sufficient extent that the lower blade surface along the distance d is wider than the casing thickness t. The blades mill down the sacri- ficial element 510, leaving "chunks" thereof behind as the tool 520 moves onto the whipstock 505 and blades reach the outer surface of the casing at point B. The outer blade surfaces which contact the whipstock are, preferably, smooth to facilitate movement of the mill- drill tool 520 down the whipstock 505 and to minimize milling of the whipstock 505 itself. The tool 520 continues downwardly (e.g. rotated all the while by a surface rotary or by a downhole motor in the string at some point above the tool), milling away the sacrificial element 510, moving down the whipstock 505, milling through the casing C, to a point C at which outer sur- face of the material 527 of the nose 526 contacts the inner surface of the casing C. At this point the mater- ial 527 begins to be worn away. The tool 520 continues to mill down the casing to a point D at which the nose 526 begins to exit the casing C and the tool 520 begins to cut the formation outside the casing C. At this point the blades 525 of the formation drill start to be ex- posed. The tool moves down the whipstock 505 forming the beginning of a lateral wellbore. A lateral wellbore L thus formed is shown in Fig. 10C. Such a wellbore may be any desired length including, but not limited to:

about one foot long; two feet long or less; five feet long or less; between five feet and fifty feet long; one hundred feet long or less; between about one hundred and about two hundred feet long; two hundred feet long; five hundred feet long; a thousand feet long; or several thousand feet long.

When a full gauge body is used for the tool 520, the resulting window and lateral wellbore are full gauge, i.e. a desired diameter and no further milling is required.

Fig. 13 shows a tool 650 with a cylindrical body 651 (shown partially), a plurality of milling blades 652 dressed with matrix milling material, and two rotatable drill bit roller cones 653. (One, three, four, or more such cones may be used.) As viewed in Fig. 14B, the drill bit roller cones 653 may be disposed to project beyond (upwardly in Fig. 14B) a top surface 654 of the milling blades 652. Alternatively, the cones may be at a similar level as or below the top surfaces 654.

Fig. 14C shows a drill bit roller cone 660 with a rotatable cone 664 on a body 661 (which is mountable or formable in known manner as part of a drill bit), the cone having thereon stubs of drilling material 662 and a projecting body 663 of milling material, e.g. welded to the body 661. Such a cone may replace the one or more of the cones of the tool 650. Alternatively a blade body may be formed on the body 661 which is then dressed with matrix milling material.

Fig. 15 shows schematically a tool 670 with a cylindrical body 671 having a fluid flow bore 672 there- through, a milling surface 673 and a rotatable drill bit roller cone 674 rotatably mounted to the body. Option- ally lateral milling blades may be provided on the vertical sides of the body 671.