Abstract:
The invention, which can be applied to both starting mills and window mills for use in sidetracking a cased borehole, makes use of carbide discs as cutting elements and the geometrical disposition of blades on which the discs are mounted to enhance cutting efficiency. The blades are angularly offset with respect to the tool centerline in a variety of configurations.

Description:
This invention relates to mills for use in sidetracking boreholes such as oil wells. 
     BACKGROUND OF THE INVENTION 
     Sidetracking of wells is a well-known procedure in which a new borehole is initiated at a small angle to an existing cased hole. The existing hole is closed by a packer below the intended sidetracking site, and a whipstock is secured on the packer to present an angled flat face towards the top of the well. A starting mill is then run into the well; the starting mill has a tapered nose to engage the angled face of the whipstock and side cutters which, when the string is rotated, cut an angled notch in the casing. The starting mill is then removed and replaced with a window mill. This is engaged in the notch and rotated to cut an elongate window through the casing and to form an initial length of the sidetrack bore. 
     The time taken to perform a sidetracking operation thus depends on the rate of penetration and reliability of the starting mill and the window mill. Mills used hitherto achieve only a modest performance. Starting mills require a high operating torque and tend to rotate unevenly or jam. Window mills have very poor penetration rate during part of the window-forming operation, to such an extent that they are frequently pulled because the operator suspects major damage to the tool but no such damage is found on recovery of the tool. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide improved mills for use in well sidetracking, in which the above problems are overcome or mitigated. 
     The invention provides a milling tool for use in sidetracking a borehole, comprising a generally tubular body, means at one end of the body for connecting the tool to the end of a drill string, and a plurality of cutting blades extending from and spaced around the body, each blade having a forward face in the direction of rotation provided with cutting elements in the form of carbide discs secured thereto, and each blade being angularly offset from the longitudinal axis of tubular body. 
     In one form, the milling tool is a starting mill, in which the end of the body opposite said one end is formed as a tapered nose, said blades are positioned on the body behind said tapered nose with the front surfaces of the carbide discs lying in planes which are radial of the body, and the blades have radially outer edges inclined at an angle to the longitudinal axis of the body such that the outside diameter of the tool across the blades is less towards said one end than towards said tapered nose. 
     In another form, the milling tool is a window mill, each blade having a first portion extending along the side of the body and a second portion extending across the end face of the body opposite said one end such that the front faces of the carbide discs on each blade lie in a plane displaced from a radial plane of the body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a side view, partially in section, of a starting mill forming one embodiment of the invention; 
     FIG. 2 is an end view of the mill of FIG. 1; 
     FIG. 3 is a partial view on the arrow III of FIG. 1; 
     FIG. 4 illustrates in more detail one blade forming part of the mill of FIG. 1; 
     FIG. 5 is a side view, partially in section, of a window mill forming another embodiment of the invention; and 
     FIG. 6 is an end, view of window mill of FIG. 5; and 
     FIG. 7 and 8 are similar views of a modified form of window mill. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 to 3, a starting mill comprises a generally cylindrical steel body 10 one end of which is formed with a standard threaded pin connector 12 for attaching the mill to the end of a drill string. The trailing portion 10a of the body 10 is parallel sided, while the leading portion 10b is tapered to form a nose, the extreme leading end being formed as a flat 14 with a through bore 16 for receiving a shear pin (not shown) by which the mill may be attached to a whipstock in a manner well known per se. 
     The cutting action of the mill is provided by eight equi-spaced ribs 18 each having an abrasive facing formed by tungsten carbide discs 20 secured in position by brassing. The discs 20 have their leading faces arranged in a plane which is radial with respect to the mill centreline. As seen in FIG. 1, the outer edge of each rib 18 is inclined at a small angle α in the range 1°-10°, such that the outside diameter across the ribs decreases in the upward direction. 
     This has the effect that in use, with the tapered nose portion 10b bearing on the angled face of the whipstock as indicated at 24, the ribs initially contact the casing indicated at 26 at their lower corners. Thus, the cutting action is initially effectively along a circumferential line. As cutting proceeds and as the tool wears, the area of contact will increase somewhat, but it will remain a restricted area approximating to a circumferential line. In prior art starting mills, in contrast, the outer edges of the cutting ribs are parallel to the tool centreline which produces a relatively large contact area, leading to high resisting torque. 
     The tapered nose portion 10b is provided with tungsten carbide pads 27 preferably, as shown, in the form of axially aligned strips, set in recesses. The pads 27 provide wear-resistant bearing surfaces for contact with the whipstock. 
     The trailing portion 10a of the body is hollow for receiving drilling fluid from the interior of the drill string. The drilling fluid is applied to lubricate the blades 18 and flush away cuttings via eight angled bores 28, each exiting in front and near the lower edge of a respective blade 18. 
     A typical starting mill in accordance with this embodiment has a maximum diameter over the blades of 8.504 inch (216 mm) and blade length of 6.793 inch (172.5 mm) on an overall body length of about 46.5 inch (1.18 m) and diameter of 5.5 inch (140 mm). 
     Referring particularly to FIG. 4 in which the downhole direction is shown by arrow D, the blade 18 comprises a steel body 21 with the carbide discs secured to its front face by a brass matrix as indicated at 22. The rear face is cut away at 30 to provide a welding angle for securing the blade to the body 10. The outer face 32 is at an angle β to the tangential to clear the casing when the front face is in cutting contact. The body 20 is cut away at the rear of the leading edge to accommodate a tungsten carbide wear pad 34; preferably the dimensions in this area are 
     A=1/4 inch (6.35 mm) 
     B=1 inch (25.4 mm) 
     C=2 inch (50.8 mm) 
     Turning to FIGS. 5 and 6, the window mill comprises a generally tubular steel body 50 provided at its upper end (not shown) with a standard pin connector for connection on the end of a drill string. The lower end of the mill is provided with six substantially L-shaped blades 52 each having a generally axial portion 52a and an inwardly-directed end portion 52b. In FIG. 5, for ease of illustration the blade appearing at the top of the figure is shown 30° out of position. Both portions are faced with tungsten carbide discs 54 brassed in place. The blades 52 are welded to the body 50, and are reinforced by a backing 56 of cemented carbide chip material such as &#34;Superloy&#34; by Tri-State. 
     Drilling fluid is passed from the interior of the body 50 to the blade region via angled bores 58 and pipe nozzles 60. 
     The generally axial portion 52a of the blade is set at a small angle to the axis, such as an angle of 1° increasing to 2° as shown. This has the effect that the end portion 52b is parallel to but offset rearwardly from a radius of the tool. Moreover, as seen in FIG. 6, the various end portions 52b are of unequal length, so as to overlap in the central zone of the end face. 
     It has been found with prior art window mills that the operator frequently withdraws the mill before the window is formed, because of penetration rate falling to such an extent that tool failure is suspected. 
     In such prior art mills, the cutting ribs are axial and radial and are faced with carbide chip compositions. It is believed that this drop in penetration rate occurs when the casing being cut is aligned across the centre of the mill, which produces coring of the tool. 
     The embodiment of FIG. 5 and 6 overcomes this problem in that the angled disposition of the blades produces a drag type cutting edge, and this is used effectively by the provision of disc-type cutting elements. 
     FIG. 7 and 8 are views similar to those of FIG. 5 and 6, illustrating a modified form of window mill, like parts being denoted by like references. In this embodiment, the side portion 52c of each rib is helically curved and is provided with carbide discs only along its outer edge. 
     In all of the foregoing embodiments, the carbide discs are suitably tungsten carbide about 3/8 inch (9.53 mm) diameter by 1/4 inch (6.35 mm) thick, for example those produced by Tri-State as &#34;Metal Muncher&#34; inserts.