FROM A MAIN WELLBORE

This invention relates to a method and an apparatus for producing a sidetrack bore from a main wellbore. In a typical milling operation, such as disclosed in US-A-2 147 537, a section of casing cemented in a cased wellbore is milled out and then a whipstock is positioned adjacent a portion of the milled out section with the top of the whipstock spanning the milled out section.

A drill bit is then lowered down the casing on a drill string and is deflected through the milled out section by the whipstock.

This technique has been in use for many years. Whilst it has often been noted that the mill is subject to rapid wear during this operation this has always been accepted as a natural consequence of the sidetracking process.

We have discovered that a surprising amount of wear can be reduced by removing a small length of casing opposite the whipstock and above the section prior to introducing the drill bit.

According to one aspect of the present invention there is provided a method for producing a sidetrack bore from a main wellbore, the main wellbore being lined with a string of hollow tubular members, said method comprising the steps of: lowering milling apparatus into said main wellbore to a predetermined depth; using said milling apparatus to remove a section of at least one of said hollow tubular members; removing said milling apparatus; lowering a whipstock using whipstock lowering means in said main wellbore to approximately said predeter- mined depth;

anchoring said whipstock below said milled section; removing said whipstock lowering means; and lowering a drilling system comprising a drill bit into said main wellbore so that said drill bit is de- fleeted by said whipstock through said milled section; characterised in that prior to lowering said dril¬ ling bit a second mill is used to remove a portion of at least one of said hollow tubular members opposite said whipstock and above said section. According to another aspect of the present inven¬ tion there is provided a side track bore from a main wellbore, said apparatus comprising a whipstock provided with a separate and distinct body extension member.

Further features are set out in the dependent claims.

A further advantage of the method of the present invention is that if a retrievable whipstock is used recovery is facilitated since there is less chance of the recovery grapple catching onto the bottom of the casing above the milled out section.

For a better understanding of the invention refer¬ ence will now be made, by way of example, to the accom¬ panying drawings, in which:-

Fig. IA - IF are side views of a first embodiment of an apparatus according to the present invention at various stages of operations;

Fig. 2A is a side view of a tapered mill prior to the application of cutting or milling elements;

Fig. 2B is a side view of the tapered mill of Fig. 2A with cutting or milling elements;

Fig. 2C is an end view of the tapered mill of Fig. 2B;

Fig. 3 is a side view, partially in cross-section, of the apparatus according to the present invention; Fig. 4A is a side view partly in cross-section of the apparatus emplaced across a milled-out casing sec¬ tion in a wellbore;

Fig. 4B is a side view partly in cross section of a tapered mill in the wellbore of Fig. 4A. * * *

The first step of the operation is shown in Fig. IA and the final step in Fig. IF.

Fig. IA shows a wellbore W through a formation F cased with casing C cemented in place by cement T with a bridge plug B set in the casing C.

Fig. IB shows a typical section mill on a drill string L (shown partially, but extends up to surface equipment) which has milled out a section S from the casing C. This milling has also resulted in the milling of some of the cement T adjacent the section S. A top stub 6 and a bottom stub 8 of the casing remain.

Fig. IC shows an apparatus according to the present invention comprising a whipstock with a concave guide section 12 releasably secured to a body extension 14 which is itself releasably secured to a setting section

16. A setting tool N is releasably secured (e.g. by a shear pin, not shown) to the concave guide section 12. Alternatively a starting mill releasably secured to the concave guide section by a shear pin or shear bolt may be used instead of the setting tool. Anchor apparatus P anchors the whipstock 10 which is actuated by a mechan¬ ism triggered by the bridge plug B and in the casing C. In other aspects instead of a bridge plug a packer or other "false bottom" device is used, or the whipstock is set on the bottom of the wellbore. The anchor apparatus P includes slips 15 and a pivot slip 17 which provides a fulcrum point about which the whipstock pivots. As shown in Fig. IC the anchor apparatus is disposed on a part of the setting section 16 in the casing C beneath the section S. It may be advantageous to anchor the whipstock 10 (or other deflection device used instead of the whipstock 10) within the section S; and, in certain embodiments, to anchor it on the top of the bottom stub and to use the bottom stub as a "trigger" to actuate setting or anchoring devices. Alternatively, anchoring both within the section S and within the casing C is within the scope of this invention. Stabilizers 19 (one shown) protect the slips while the whipstock is run into the wellbore. The whipstock 10 is sized and disposed so that a top end of the concave guide section 12 abuts the top stub 6 of the casing C. The setting section 16 abuts the bottom stub 8. It is within the scope of this invention for the concave guide section 12 to be of sufficient length to abut both stubs. In the embodiment shown in Fig. IC the body extension 14 is of such length that the concave guide section 12 does not contact the bottom stub 8. Also, the body extension is of such a length that a mill or drill bit is deflected sufficient- ly that it preferably will not contact the bottom stub 8

or parts of the whipstock within the bottom stub 8 (or will contact them only incidentally). As shown the whipstock 10 bridges the sections S from the top stub 6 to the bottom stub 8. In certain embodiments the sec- tion S is 1.2m to 1.5m (four to five feet) long (but could be up to 15m (fif y feet) long) and the whipstock is long enough to bridge the milled out section.

Fig. ID shows the setting tool N removed and a tapered mill 50 on a drill string L (or a coil tubing drilling system may be used) which has been inserted into the casing C and has contacted the top 18 of the concave guide section 12 at which point milling of the top stub 6 has commenced.

Fig. IE shows the mill 50 as it has milled down past the end of the top stub 6 opposite the whipstock to contact the cement T (and, possibly, mill some of the cement T) .

Fig. IF shows that the mill 50 has been removed and a drill system 40 on the drill string L has been intro- duced into the casing C, has been deflected toward the section S by the concave guide section 12, and has drilled a new bore R into the formation F. A drill bit 42 of the drill system 40 did not contact the top stub 6 in the drilling of the bore R. Also, the bit 42 has been deflected in such a way that it has not contacted the bottom stub 8 or the lower portion of the whipstock 10.

Figs. 2A - 2C show various views of mill 50. The mill 50 has a body 52 with a bottom nose 53, a top threaded end 54 and a bottom mill end 56. The mill end 56 has six blades, three blades 57 and three blades 58 extending outwardly and downwardly therefrom. As shown in Figs. 2B and 2C, each blade may be dressed with tungsten carbide material 51 and/or milling inserts 52. It is within the scope of this invention to use any

known section mill for the step shown in Fig. IB.

Each blade 58 extends from a blade top 59 to the bottom nose 53 of the mill 50. Each blade 58 has four milling surfaces 61, 62, 63, and 64. These milling surfaces are sized, configured, and disposed so that the mill 50 avoids or minimizes contact with the formation F, yet adequately mills away the bottom stub 6. The milling surface 62 is at an angle of about 23° to a central longitudinal axis X of the mill 50. The milling surface 63 is at an angle y to the horizontal. The angle y for the mill 50 as shown is about 45°. The milling surface 64 is at an angle of about 15° to the horizontal. The tops 59 of the blades 58 are at an angle of about 45° to the horizontal. Each blade 57 has three milling surfaces 71, 72, and 73. The milling surfaces 71 on the blades 57 corre¬ spond to the milling surfaces 61 on the blades 58. The milling surfaces 72 correspond to the milling surfaces 62 on the blades 58. The milling surfaces 72 are also angled as are the milling surfaces 62 so that milling of the formation F is avoided (or reduced), (as are the milling surfaces 63 and 73). The mill end 56 is tapered to accommodate the various angled milling surfaces of the blades. A plurality of fluid flow bores extend down through the mill 50 for the flow of circulating fluid through the mill to facilitate the evacuation of milled mater¬ ial. Fluid exits from these bores through exit ports 67 in the bottom nose 53 and then flows back up past the blades. It is within the scope of this invention to provide a mill without blades, but with angled milling surfaces which effect avoidance of formation contact or reduced formation contact.

Fig. 3 shows a whipstock 80 with an upper concave guide section 82; a body extension 84 connected to the

upper concave guide section 82; and a lower whipstock portion 86 connected to the body extension 84. These connections may be permanent, e.g. welded, or releas¬ able, e.g. shear-pinned or threaded. A retrievable whipstock such as that disclosed in U.S. Patent 5,341,873 may be used.

Fig. 4A illustrates a retrievable whipstock 100 in a wellbore 102 in which is cemented casing 104 with cement 106. A formation 107 surrounds the wellbore 102. The whipstock rests on a bridge plug 103. The whipstock has a concave guide section 110 which has a top 112 that rests against a top stub 114 of the casing 104. A lower portion of the whipstock body 116 rests against a bottom casing part 118. Slips 122 and 124 secure the whipstock 100 in the lower casing. It is essential to mill off the part of the top stub 114 indicated by the bracket and numeral 130 to facilitate entry of a bit into the formation.

As shown in Fig. 4B the part 130 has been milled out by a mill 150 according to the present invention and the mill 150 has not milled past the cement 106. The mill 150 has an angled mill surface 152 which is sub¬ stantially parallel to a formation surface 126 and a nose 154 of the mill 150 is blunt so that it does not contact the formation when the mill is in the position shown in Fig. 4B.

By employing a mill with a blunt nose and inwardly tapered sides and/or inwardly tapered blades (see Figs. 2A and 4B) (tapered inward from top to bottom) , contact with the formation is reduced or avoided completely (see Figs. IE and 4B). Preferred methods according to this invention are useful in producing sidetracked bores at relatively abrupt angles to the axis of a main wellbore, e.g. an angle of at most about thirty degrees and as small as about one degree. By using such a taper mill

milling is effected to an extent equal to the total width of the mill and no undesirable unmilled casing portion or sliver is produced.

CLAIMS :

1. A method for producing a sidetrack bore from a main wellbore, the main wellbore being lined with a string of hollow tubular members, said method comprising the steps of: lowering milling apparatus into said main wellbore to a predetermined depth; using said milling apparatus to remove a section of at least one of said hollow tubular members; removing said milling apparatus; lowering a whipstock using whipstock lowering means in said main wellbore to approximately said predeter¬ mined depth; anchoring said whipstock below said milled section; removing said whipstock lowering means; and lowering a drilling system comprising a drill bit into said main wellbore so that said drill bit is de¬ flected by said whipstock through said milled section; characterised in that prior to lowering said dril- ling bit a second mill is used to remove a portion of at least one of said hollow tubular members opposite said whipstock and above said section.

2. A method according to Claim 1, wherein said second mill is a tapered mill. 3. A method according to Claim 1 or 2, wherein said second mill is incorporated into said whipstock lowering means.

4. A method according to Claim 1 or 2, wherein said whipstock lowering means is removed before said second mill is lowered into said main wellbore.

5. A method according to Claim 1, 2, 3 or 4, wherein said whipstock comprises a guide section and a setting section wherein said setting section anchors said whip¬ stock to said hollow tubular member below said milled out section and biases said guide section against said

hollow tubular member above said milled out section.

6. A method according to any preceding claim, wherein said whipstock is retrievable and said method further comprises the step of removing said whipstock from said main wellbore.

7. A method according to any preceding claim, wherein said tapered mill does not mill the formation external to said hollow tubular member.

8. A method according to any preceding claim, wherein said drill bit is rotated by a downhole motor.

9. A method according to Claim 8, wherein said dril¬ ling system comprises coil tubing reeled from a coil tubing unit through a coil tubing injector.

10. A method according to any of Claims 1 to 7, wherein said drill bit is rotated from the top of said wellbore.

11. A method according to any preceding claim, wherein said whipstock is lengthened prior to lowering in said wellbore.

12. A method according to Claim 11, when appended directly or indirectly to Claim 5, wherein said whip¬ stock is lengthened by arranging an extension member between said guide section and said setting section.

13. A method according to any preceding claim, wherein said hollow tubular member comprises casing. 14. An apparatus for producing a side track bore from a main wellbore, said apparatus comprising a whipstock provided with a separate and distinct body extension member (14, 84). 15. An apparatus as claimed in Claim 14, wherein said body extension member (14, 84) is arranged between said guide section (12, 82) and said setting section (16, 86).