FIELD OF THE INVENTION The field of this invention relates to milling a window in casing and more particularly to guiding systems for mills to accomplish the cutting of a window.

BACKGROUND OF THE INVENTION

In the past, window-cutting systems have involved the use of whipstocks and multiple trips wim a starter mill and a window mill coming in behind it to cut out a window. Until recently, there has been no emphasis placed on retrievability of he whipstock in that the older designs and appUcations presuppose that the whipstock would remain in the wellbore after the window was milled.

More recently, the concept of retrievability of whipstocks has been intro- duced for a variety of applications. In some instances, the liner which is inserted through the window is cemented all the way back into the main wellbore. There¬ after, milling is required to remove uie section of liner extending into the main wellbore. This procedure is illustrated in U.S. Patent 5,301,760. Various attempts in the past have been made to retrieve whipstocks. One such tool is illustrated in U.S. Patent 5,341,873, assigned to Weatherford. These retrieving techniques employed in the past generally required that the stnictural integrity ofthe upper end ofthe whipstock be maintained so that the retrieving tool could get a firm grip on the whipstock to ensure its removal.

Milling techniques have also improved so that a one-trip system can be employed to create tiie window. U.S. Patent 5,109,924 illustrates a one-trip window-milling system where a started mill is followed by one or more watermel¬ on mills. The assembly is initially retained to the whipstock by a lug and a shear pin.

Figure 1 illustrates a lug of the type previously employed, with standard multi-trip window-milling systems as well as a one-trip system such as illustrated in Jurgens.

The initial contact wearing surface 10 was previously held at approximately an angle of 2-5° as represented by "a" in Figure 1. The starter mill 12 had a guide section 14 which was designed to contact the lug 16. In view of the speed of rotation ofthe starter mill 12 and the small angle "a" employed, the wear pattems on lug 16 were such hat it would be quickly ground away before the starter mill 12 could get much of a bite into the casing 18. When mis occurred, the mill would be driven away from the casing 18 so that it would retract from an initial window which had just started to open as the mill 12 is further advanced downwardly. If this was allowed to occur, eventually the starting mill 12 ground away the top of the whipstock 20 to a point represented by dashed line 22. This technique was somewhat hit or miss and frequently resulted in severe damage to the top of the whipstock 20. Such damage was generally sufficiendy extensive to prevent or at least make extremely difficult any attempt to recover the whipstock 20 from the wellbore. Tbis is because holes conveniently placed near the top of the whipstock for retrieval purposes would be one of the first things ground up if the blades of the starting mill 12 were allowed to progress into contact with the whipstock 20.

The shortcomings of the prior designs were due to the lug design and an effect called "whirl," which is best illustrated in Figure 2. Figure 2 schematically illustrates in a plan view a casing 18, along with a starter mill 12, which has a series of blades 24 thereon. The blades are designed to create the cutting action when engaged against the casing 18 due to a clockwise rotation ofthe starter mill 12, as illustrated by arrow 26. However, since the mill 12 is itself smaller than the opening in which it is disposed, the clockwise rotation imparted to the starter mill 12 as indicated by arrow 26 results in the entire mill 12 rotating in a counterclock- wise manner illustrated by arrow 28 within its surroundings. Since the imtial surroundings about the starting mill 12 are larger than the O.D. of the mill, the whirl effect creates contact between the blades 24 and the casing 18 such that an undesirable force in the direction of arrow 30 is applied to each ofthe blades as the starter mill 12 whirls in a counterclockwise direction indicated by arrow 28. In the

past, diis whirl effect has resulted in severe damage to the starter mill 12 and in many cases to the whipstock 20. The whirl action further exacerbated the wearing away ofthe lug 16.

While in past designs the objective of beginning a window may have been accomplished, diis achievement was at the cost of near complete destruction ofthe starter mill 12 as well as sufficient damage to the top end of the whipstock 20 to eliminate or at least make difficult subsequent attempts to retrieve it.

One of the many objectives of diis invention is to provide guidance and stabilization to the mill or mills dirough the use of me configuration of me lug to remove the effect of whirl and to spare the whipstock from damage during the process of milling the window in the casing 18. To diat end, a sacrificial lug including an imtial contact taper and a back-up shoulder has been developed. The taper allows applied weight on the mill during the window milling to more directiy orient the mill toward die casing where me window is to be cut Greater torque control is possible due to die improved guidance of the mill or mills which reduces stall-outs when the mill gets stuck. The lug configuration is directed to me objective of providing a wearing surface rather dian a surface that is milled during the creation of die window. Yet anomer objective is to preserve any retrieving slots or odier protrusions used for subsequent retrieval ofthe whipstock by ensuring diat the mill or mills do not destroy such features during die window-milling process.

SUMMARY OF THE INVENTION

A guide lug for a starting or window mill is provided at me upper end of die whipstock. A back-up shoulder is provided to act against any tendencies ofthe mill to whirl. A taper is provided for use in guiding the mill toward the casing for the cutting of die window. The lug is configured to be worn by a guide for die window mill or starter mill ratiier than being milled away. As a result ofthe use of die lug, the upper sections of die whipstock are protected from me mill. In turn, any holes or other projections provided for subsequent retrieval of me whipstock are

functional for such a retrieval at die conclusion of the milling process for the window.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional elevational view of me prior art lug used in conjunc¬ tion widi a whipstock and a starting mill.

Figure 2 is a schematic representation of die effect of whirl in prior designs of lugs for milling tools used in conjunction wim whipstocks.

Figure 3 is a plan view showing die lug ofthe present invention. Figure 4 is an elevational view of me lug illustrated in Figure 3.

Figure 5 is an illustration of a conventional or coiled tubing-supported starter mill on the lug ofthe present invention shown in sectional elevational view.

Figure 6 is a sectional elevational view of die lug of me present invention showing its use in a one-trip milling system such as that illustrated in U.S. Patent 5,109,924.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 3 illustrates a plan view of die lug L of the present invention. For clarity, the casing 18 is eliminated. A standard whipstock 20 can be used with me lug L, as shown in Figure 3. Lug L is preferably made from a bronze-aluminum alloy to facilitate the welded connection to die whipstock 20, which is preferred. However, other soft or wearable material, such as brass or bronze, can be used without departing from the spirit ofthe invention. The orientation of die mill, with respect to the lug L, is indicated by die centeiiines 32 and 34. Adjacent the top of lug L is a tapered surface 36 sloping downwardly toward centerlines 32 and 34.

Tapered surface 36 wraps around such diat a back-up shoulder 38 is formed as part of the lug L. It further acts as a guide surface for such items as guide 40 for the milling of a window. Taper 36 is preferably at an angle of >5° to <90° from the vertical, as illustrated by angle "b" of Figure 4. While some improvement over the

prior art designs is measured at angles "b" of as littie as >5°, significant improve¬ ment in die performance of die mill such as 12 is achieved when the taper angle is about 30° or more to about <90°. The back-up shoulder 38 eliminates the tendency of mill such as 12 to whirl. A portion of die guide 40 (see Figure 5) is illustrated in dashed lines in Figure 3. An arrow in Figure 3 illustrates me clockwise rotation of guide 40 which turns wim die mill such as 12 from rigid tubing extending to me surface or from a downhole motor such as in a coiled tubing application. The clockwise driving of die mill such as 12 tends to create an opposing turning motion on me mill itself in a counterclockwise direction since it is in an area ofthe casing where there is room around die mill for it to whirl in the absence of a shoulder 38.

The back-up shoulder 38 creates a smaller space around die mill as it begins to cut into the casing and firmly supports me mill 12 or, in me position shown in Figure 3, the guide 40, against a tendency to whirl in a counterclockwise direction. The sloping surface 36 can be used to provide a horizontal component to the guide 40 so diat the mill cutters 42 can be directed wim the horizontal component toward me casing wall opposite the whipstock 20. Many combinations of downward weights apphed to me mill 12 wim the taper angle 36 allow for better control of die milling process. The mill is less likely to advance overly rapidly to a jamming position between die casing diat has not yet been cut and me whipstock. The use of angles >5° also result in a gradual erosion or wearing down of the lug L as tiie mill 12 advances. Dashed line 44 in Figure 5 indicates how much of die lug L is worn away during normal operation.

It is desirable to configure me lug 40 so diat me transverse dimension from the imtial point of contact, shown schematically as 46, wim the lug L to die outer periphery of me guide 40 should exceed tiie thickness of the casing to be milled.

Stated differendy, dimension represented between die arrows 48 should exceed the tiiickness of the casing to be milled. The dimension 48 represents the amount of expected horizontal movement of the guide 40 as the wear pattern illustrated by dashed line 44 is accomplished during the milling operation. It should be noted

that die window-milling operation using the lug L as above described allows for successful lateral deflection of the guide 40 away from the whipstock 20 and toward die casing 18 (omitted from Figure 5 for clarity).

The operator at the surface curtails the milling operation with die mill 12 has advanced a predetermined amount. That amount is a distance generally about 3 feet which is sufficientiy smaller dian die imtial gap between the cutters 42 and the top end 50 of the whipstock 20. An opening such as 52 can effectively be used after milling for a retrieval operation widi known "fishing" tools such as shown in U.S. Patent 5,341,873. Such openings 52 or similar features to facilitate retrieval are preserved and not milled over as wim prior designs.

A lug L of the same design is adaptable for use in one-trip milling systems such as those described in U.S. Patent 5, 109,924. There, die lug L is secured in the same manner as previously described except that it conforms to a groove 52 above the window mill 54 (see Figure 6). In die preferred embodiment, mere is a slight clearance between die groove 52 and die vertically oriented surface 56 which arcs around groove 52 to obtain die desirable results described above. Just as in die embodiment of Figure 5, which illustrates conventional or coiled tubing-supported starting mills such as 12, the lug L is worn away as die window mill 54 progresses, all me while helping surface personnel to achieve a horizontal component force to direct die window mill 54 away from the whipstock 20 while at the same time eliminating its desire to whirl due to the provision ofthe back-up shoulder, such as 38' previously described. It should be noted diat to accommodate the one-trip system as described in the Jurgens patent No. 5,109,924, special features can be provided into e whipstock 20 without departing from tiie spirit ofthe invention. As a result of using die lug L of the present invention, several desirable features are achieved over prior art lug support systems. The cutters on die mills used are stabilized against the tendency to whirl. This provides a greater stability to the mill and a more reliable window cut. It also acts to protect the top of the whipstock which can be severely damaged from the whirling effect. Accordingly,

openings or protrusions or other devices used for subsequent recovery of the whipstock are not destroyed by the whirling mill as had occurred with prior de¬ signs. Window or starter mills are less likely to stall out due to jamming because a greater torque control is possible using the taper feature of die lug, as described above. Typically, widi an application of 500-3000 lbs. weight on the mill during the window-milling operation, a sufficient horizontal component is created to initiate die window and reduce jamming ofthe mill, such as between the casing and the whipstock, which had occurred in old designs with me lug milled away. Instead, widi the lug L ofthe present invention, die gradual planned for wearing, as indicated by dashed line 44, provides control throughout the window-cutting procedure and predictability of where die window will be cut. The problem of prior designs with the mill receding from an initial window when the lug was milled away is eliminated by the lug L of die present design. Similarly, with me back-up shoulder 38, the tendency to create a misaligned casing window wim respect to me whipstock face, also known in me art as "dog leg severity," is further eliminated due to the stabilizing effect on die mill from die design of the lug L. Additionally, die whipstock can now be easily retrieved witii confidence since the features for retrieving, such as slots or weldments, are preserved ratiier than being ground off widi d e lug, as in many ofthe past designs. The lug L of die present design can be used wim conventional window- cutting systems to improve performance. The lug L is even more important to coiled tubing applications for better control of die mill and for elimination of stall- outs. As shown in Figure 6, die lug L of tiie present invention has apphcation in one-trip milling systems where, although die lug is positioned behind me mill 54, its principle of operation and the benefits derived are the same as those for con¬ ventional or coiled tubing-supported mills described in Figure 5.

The lug L is simple to produce and secure by die preferred method of welding to a whipstock 20. Despite its economical construction, it retums signifi¬ cant benefits in preservation of the integrity of die equipment, such as the whip-

stock 20, as well as saving rig time in fishing by facihtating die integrity of retrieval features at the top of me whipstock.

The foregoing disclosure and description ofthe invention are illustrative and explanatory thereof, and various changes in tiie size, shape and materials, as well as in the details of the illustrated construction, may be made without departing from me spirit ofthe invention.