MILLING TOOL This invention relates to a milling tool, a milling apparatus comprising two or more such milling tools, a whipstock for use with said milling tool or said milling apparatus, a milling system comprising such a whipstock and a milling tool or a milling apparatus in accordance with the invention, and two methods for milling a window in a casing using the aforesaid milling tool or milling apparatus. In oil and gas exploration it is often desirable to be able to drill a new hole which extends away from an existing hole. In order to achieve this an inflatable packer is first lowered down the hole and inflated at the desired depth to form a platform. A device known as a whipstock is then bolted to the tip of a special starting mill and the whipstock and starting mill are lowered down the hole on the end of a workstring. When the whipstock approaches the packer it is correctly orientated and then lowered onto the inflatable packer. The engagement of the whipstock on the packer sets the whipstock, i.e. locks it in place.

After the whipstock is set the workstring is low¬ ered and the weight shears the bolts attaching the special starting mill to the whipstock. The special starting mill is then rotated to cut an initial window in the casing. The disadvantage with this arrangement, which is shown in Figure 8, is that the starting mill is inhibited when the tip engages the casing. At this point the entire workstring has to be withdrawn and the starting mill replaced with a different type of mill to open the window and cut into the formation.

The aim of the present invention, at least in its preferred embodiments, is to provide an apparatus in which the initial penetration of the starting mill is not as limited as previously described.

According to the present invention there is provi¬ ded a milling tool characterized in that it comprises a passageway for receiving a whipstock orientation member or for accommodating a core. Preferably the milling tool comprises retaining means for retaining the whipstock orientation member within the passageway.

Advantageously, the milling tool comprises catching means within the milling means for catching and holding said core.

The present invention also provides a milling apparatus comprising two or more milling tools in accor¬ dance with the present invention arranged in series.

Preferably, the milling tools comprise a window mill and a finishing mill.

Advantageously the milling tool further comprising a hollow tubular member to allow upward passage of said whipstock orientation member.

The present invention also provides a whipstock comprising a body member having a concave face, charac¬ terized by a whipstock orientation member secured to the body member for receipt within the passageway of a milling tool or a milling apparatus in accordance with the present invention. Preferably, the whipstock further comprising a stop bar extending through a hole in the whipstock orienta¬ tion member.

Advantageously, the whipstock orientation member is secured to a pilot block secured to the concave portion of the body member.

There is also provided a milling system comprising a whipstock in accordance with the invention and either a milling tool or a milling apparatus in accordance with the present invention. The present invention also provides a method for

milling a window in a casing in a wellbore, which method comprises the steps of: installing a platform in the wellbore at a selec¬ ted point beneath a proposed location for the window; lowering a milling system down said wellbore; orientating said whipstock and setting it on said platform; lowering said milling apparatus with respect to said whipstock orientation member and accommodating said whipstock orientation member in said passageway in said milling tool(s); rotating and lowering the milling tool along the whipstock to cut a window in said casing.

There is also provided a two-trip milling operation for milling a window in a casing in a cased wellbore, said method comprising a first equipment trip into the wellbore and a second equipment trip into the wellbore after equipment inserted into the wellbore on the first trip has been removed, the first equipment trip compris- ing lowering a whipstock bolted onto a starting mill down the casing of said wellbore; setting said whipstock at a desired position in said casing; shearing the shear stud to free the starting mill for milling; creating an initial window through the casing with the starting mill; removing the starting mill from the wellbore; the second equipment trip comprising running a milling tool or a milling apparatus in accordance with the invention into the casing to further mill the in¬ itial window to form a finished window through the casing.

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

Fig. 1A is a side view, partly in section, of one embodiment of a milling system in accordance with the invention;

Figs. IB to 1H are views, to an enlarged scale, of parts of the milling system shown in Fig. 1A, Figs. IB and IC being shown in side elevation and Figs. ID to 1H in cross-section;

Figs. 2A and 2B show steps in the operation of the milling system shown in Fig. 1A;

Fig. 3 is an enlarged view of part of the milling system shown in Fig. 2A; Fig. 4 is an enlarged view of part of the milling system shown in Fig. 2B;

Fig. 5 is an enlarged view of another part of the milling system shown in Fig 2A;

Fig. 6 is a side view of the part shown in Fig. 5 with the whipstock orientation member removed;

Fig. 7 is a side view of the part shown in Fig. IB;

Fig. 8 is a side view of a known milling system;

Fig. 9A is a side view of the milling system of Fig. 8 used in the first stage of a method according to the present invention; and

Fig. 9B is a side view of a second embodiment of a milling tool in accordance with the present invention being used in the second stage of a method in accordance with the present invention. Referring to Figs. 1A - 1H and 2A and 2B there is shown a milling system which is generally identified by reference numeral 10. The milling system 10 comprises a whipstock 20, a milling apparatus 30 and a whipstock orientation member 60. The whipstock 20 has a concave face 22 to which is

welded a pilot block 24 provided with two threaded bolt holes 28.

The whipstock orientation member 60 has a body 62 the lower end of which is secured to the whipstock 20 by bolts 69 which pass through holes 63 and engage the threaded bolt holes 28.

The body 62 is provided with a groove 64 and a stop bar 29 which extends through a stop bar hole 66.

The top of the whipstock orientation member 60 is provided with an enlarged portion 65.

The milling apparatus 30 comprises a starting mill 40 connected to and below a finishing mill 50. Interior threads 48 of the starting mill 40 engage exterior threads 58 of the finishing mill 50. As shown in Fig. ID, the starting mill 40 has a passageway 44 which extends axially therethrough and a cutting end with carbide cutters 42. The starting mill 40 also has a cavity 45 with a shoulder 47 on which rests a splined bearing 91 and a core catcher 14 which comprises two wedge shaped members (Fig.IE). The radial¬ ly inner surface of the core catcher 14 is castellated to provide a multiplicity of axially extending channels.

The finishing mill 50 (Fig. IF) has a plurality of milling blades 52 and a passageway 54 which extends axially therethrough.

A retainer 12 is disposed within the passageway 54 and rests on a shoulder 57 of the finishing mill 50.

The retainer 12, as shown in Fig. 1G, comprises a spring with a plurality of fingers 55 which protrude radially inwardly.

A pup joint 80 (Fig. 1H) is mounted on top of the finishing mill 50. External threads 86 on the lower end of the pup joint 80 engage upper internal threads 56 of the finishing mill 50. Upper internal threads 88 of the

pup joint 80 engage a part of a drill string (not shown) e.g. a crossover sub below a mud motor.

A passageway 84 extends axially through the pup joint 80 and is sized and configured to receive a por- tion of the whipstock orientation member 60.

Figs. 2A and 2B illustrate steps in the use of the milling system shown in Figs. 1A to 1H.

As shown in Fig. 2A, the passageway 44 of the starting mill 40 accommodates the upper portion 65 of the whipstock orientation member 60 which is bolted to the whipstock 20.

As a first step a platform, for example an inflat¬ able packer (not shown), is lowered down casing in a wellbore and set at a predetermined distance below the intended position of the window in the casing. The milling system shown in Fig. 1A is then lowered down the casing and the whipstock 20 set on the platform when the desired orientation is obtained using conventional techniques. The starting mill 40 is then pushed down on the stop bar 29 breaking it. As the starting mill 40 is lowered further the whipstock orientation member 60 slides upwardly through the passageway 44 until the starting mill 40 engages the pilot block 24 (Fig. 2B). Milling now commences and the starting mill 40 mills through the pilot block 24. As the starting mill 40 moves down the concave face 22 of the whipstock 20 it is displaced sideways in the casing and starts cutting a window in the casing. As shown in Fig. 4 once the groove 64 is at or above the level of the retainer 12, the fingers 55 will inhibit the whipstock orientation member 60 exiting the milling apparatus 30.

The whipstock orientation member 60 has an indented end 71 to facilitate entry of a core into the starting

mill 40.

Circulation fluid is preferably circulated down¬ wardly through the drill pipe and passageways 84, 54 and 44, around the bottom of the starting mill 40, and upwardly past the splined bearing 91, finishing mill 50 and drill pipe to the surface. In this connection it should be noted that the circulation fluid passes through a small annular passageway formed between the circumference of the whipstock orientation member 60 and the passageways and also between the circumference of the core and the passageways 84, 54 and 44.

The splined bearing 91 inhibits rotation of the whipstock 21 relative to the tool string whilst the whipstock 20 is being orientated. After the starting mill 40 has cut an initial hole it is followed by the finishing mill 50 which completes the milling operation.

When the milling apparatus 30 is removed from the wellbore the whipstock orientation member 60, and core are all held within the passageways 44, 54 and 84.

As shown in Figs. 9A and 9B, in a two-trip milling operation according to the present invention, a conven¬ tional milling system including a whipstock 120 and a starting mill 125 secured thereto with a sheer bolt 126 is run into a cased wellbore in which some type of anchoring-orientation device, e.g. a keyed packer (not shown), has been installed. Upon emplacement and orien¬ tation of the whipstock 120, the sheer stud 126 is sheared by pushing down on the starting mill 125 and milling is commenced producing an initial window in the casing. The milling apparatus is removed leaving the whipstock 120 in place and then a milling apparatus (like the milling apparatus 30 shown in Fig. 1A) is run into the hole to continue milling at the location of the initial window or pocket. This milling system includes

the items above the whipstock orientation member 60 in Fig. 1A, but not the whipstock orientation member 60. The milling apparatus, as shown in Fig. 9B, is used as previously described but without the whipstock orienta- tion member 60. This two-trip operation results in a finished window through the casing.