GB2242379A - Junk mill - Google Patents

Abstract

The mill has a body (3) with a lower end having approximately sectorially shaped lands, the lands being dressed with tungsten carbide (5). An enlarged internal diameter bore of the body is provided with a reciprocal fluid driven piston (20), the piston being sealingly engaged to the internal bore and biassed away from the lands by a compression spring (30). interspaced between each of the lands is a pivotal arm (40) having a pivotal axis perpendicular to the body longitudinal axis, each arm having at least a lower end portion dressed with tungsten carbide, the lower end of each arm being substantially in the same plane as that of the dressed lands. The piston is arranged to be driven by fluid pressure toward the arms to rotate the arms radially outwardly so that the arms may expand the cutting surface of the mill. The junk mill is therefore afforded the capability of being radially expanded. <IMAGE>

Description

JUNK MILL This invention relates to a junk mill for use downhole in energy exploration.

A known junk mill comprises a body with an end portion, downhole in use, which has three, for example, sectorial lands located in a relieved end surface. Each of the lands is dressed with tungsten carbide so that the mill acts as an end mill for cutting casing, detritus, etc., downhole.

Some wells which require milling operations have restrictions which prevent full well bore diameter mills being used. For example, wells drilled from a semisubmersible rig have subsea wellheads with a 44.6 cm (179/16 inches) internal restriction. However, the wellheads are connected to 50.8 cm (20 inches) casing with an inside diameter of 47.3 to 47.63 cm (18-5/8 to 18-3/4 inches).

Due to the 44.6 cm (17-9/16 inches) restriction, the largest diameter tool that can be used inside the casing is 44.45 cm (17-1/2 inches).

This in fact causes severe problems during recovery milling operations when a 33.97 cm (13-3/8 inches) casing that is conventionally employed is eccentric to the 50.8 cm (20 inches) casing. The 44.45 cm (17-1/2 inches) junk mill will try to center on the path of least resistance and because it cannot cut the full inside diameter of 47.3 cm (18-5/8 inches), it will leave a sliver of the 33.97 cm (13-3/8 inches) casing protruding above the milled area and against the 50.8 cm (20 inches) casing wall. When the next 33.97 cm (13-3/8 inches) casing mill is used in the tungsten carbide cutting elements tend to suffer extensive damage from the remaining sliver and thus poor mill performance occurs.

It is an object of this invention to provide a junk mill which is capable of overcoming the foregoing disadvantage.

According to this invention there is provided a junk mill for use downhole in energy exploration comprising a body member having cutting means at a downhole end thereof and a plurality of radially movable arms, each arm being pivotable about an axis which is perpendicular to a longitudinal axis of said body member and each arm being radially movable under the force of a bias means, each arm having a cutting surface on at least a downhole end face thereof, the cutting surface on each said arm being located substantially in the plane of said cutting means.

Preferably, said bias means is a reciprocal piston arranged to drive each said arm against the force of a compression spring.

Conveniently, said body member has an axial aperture for passing fluid to ports adjacent said cutting means and said piston is positioned in said axial aperture to be driven by said fluid.

Advantageously, said piston has an annular crosssection with an internal bore and the outer periphery of said piston is adapted to sealingly engage an internal wall of said body member. Preferably, said piston has an orifice located in the internal bore thereof for determining the force exerted by said piston on said arms.

Advantageously, the piston acts upon an abutment of each arm to rotate the arm associated therewith radially outwardly from said body member, and preferably limiting means are provided for limiting the radial outward movement of each said arm. Preferably, said limiting means is a shoulder adjacent said abutment on each arm, each shoulder being arranged to rotate with said arm to contact said piston and thereby prevent further radial outward rotation of said arm.

Advantageously, each arm is dressed with tungsten carbide along a downhole end face thereof and partially along an adjacent side edge thereof.

Preferably, the downhole end surface of the body is relieved to form lands which comprise the cutting means and the downhole surface of each land is dressed with tungsten carbide. Conveniently, three approximately sectorially shaped lands are provided and the arms are interspaced between the lands.

The invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a partial longitudinal cross-section of a junk mill in accordance with this invention, Figure 2 shows an end view in the direction of arrowheaded line II, Figure 3 shows a partial view of a cross-section of the uphole end, in use, of the piston, and Figure 4 shows a partial view of the piston fully radially opening an arm to show the manner in which expansion of the arms is limited.

In the Figures like reference numerals denote like parts.

Referring to Figures 1 and 2 the junk mill has a top sub 1 with an upper, in use, tapered external screw thread 2 for connection to a drill string. Threadably connected to the top sub 1 is a body 3, the downhole end of which is relieved to form three approximately sectorial shaped lands 4 which each have a lower end surface dressed with tungsten carbide 5. The lands 4 are so shaped that upon rotation of the mill in the direction of arrow-headed line 50 the lands are capable of milling away the entire surface of a tubular to be cut.

An axial passage 6 extends through the top sub 1 and body 4 and through orifices 7 to permit fluid to flow therethrough to exit at ports 8 in the lands 4 and an axial port 9.

The body 3 is locked to the top sub 1 by a threaded connection 51.

The body 3 has an enlarged internal diameter within which is located an annular piston 20 having a diverging downhole end portion 21 for permitting fluid flowing through the passage 6 and the internal bore 22 of the piston to exit through the ports 8 and 9. The piston outer periphery at an upstream end thereof has a seal 23 for sealing against the internal wall of the enlarged cavity of the body. The upstream end of the piston has a counter bore in which is located a member 25 defining an orifice 26, the member 25 being sealed to the piston by a seal 30 and located in the counter bore by a spacer 27 and a retaining ring 28, the retaining ring being secured in an undercut within the counter bore.The member 25 is selected so that the orifice 26 in use produces a differential pressure across the length of the piston so that fluid flowing through the passage 6 is able to force the piston downwardly. A lower end outside diameter of the piston is reduced in size and a compression spring 30 is located in the annular gap between the piston and the internal bore of the body, the spring being located between an edge 31 of the piston and an edge 32 of the body bore.

Axial movement of the piston away from the spring 30 is limited by a retaining ring 10.

The lower end of the body 3 is provided with three milled slots intermediate each of the lands 4, each slot being for receiving a respective pivotable arm 40 located on an axle 41 the lower end surface 45 and the adjacent side surface 46 of each arm is dressed with crushed tungsten carbide and the lower end of each arm is arranged to be substantially in the plane of the tungsten carbide 5 cutting surface. Each of the arms has an abutment 42 which is in abutting relationship with a lower end of the piston 20 and adjacent to abutment 42 is a shoulder 43 located nearer to the axle 41 than the abutment 42. As shown particularly in Figure 4 when the arm 40 is rotated radially outwardly by reciprocal motion of the piston 20 acting against abutment 42 so radial movement of the arm is limited by shoulder 43 contacting against a side of the piston 20.

In operation, the mill is secured to the lower end of a work string and lowered through the restriction to be positioned close to the proposed milling area. Pumps are engaged to pump fluid through the passage 6 and differential pressure across the piston forces the piston downwardly against the compression force of spring 30 to force the lower end of the piston against abutment 42.

Continued downward movement of piston 20 causes the arms 40 to rotate about their respective axles so that the arms radially expand until further expansion is limited by shoulder 43 on the arms abutting the side of the piston 20.

Pumping pressure is maintained throughout the milling operation during which the mill is rotated in the direction of arrow-headed line 50. When the milling operation is completed the pumps are stopped and the pressure differential across the piston is relinguished whereupon the spring 30 returns the piston upwardly and the arms radially contract.

During the drilling operation drilling fluid is circulated through the passage 6 and the orifices 7 to emerge from ports 8 and 9, the ports being located behind the arms to prevent cuttings from building up so that not only does the drilling fluid keep the arms in an open position but also acts to cool the cutting surfaces and circulate cuttings.

The piston and arm portion adjacent the piston is arranged to ensure that the arms do not wedge in the open position. It will, however, be realised that by appropriate balance of the drilling fluid pressure and spring 30 force so the arms 40 may be opened to any required intermediate position between fully open and fully closed.

The arms 40 are intended to be tied or taped into the closed position when the tool is located into a hole, but the arms will close upon meeting a restriction when the tool is lifted out of a hole and the drilling fluid pressure is released.

In a currently preferred embodiment the arms 40 have a radial width of 5 cm (2 inches) and it is to be understood that all of the dimensions given herein are exemplary and not intended to be limiting. It will also be appreciated by those skilled in the art that a greater or lesser number of radial arms and lands may be used than those described in the exemplary embodiment.

Claims (12)

CLAIMS:

1. A junk mill for use downhole in energy exploration comprising a body member having cutting means at a downhole end thereof and a plurality of radially movable arms, each arm being pivotable about an axis which is perpendicular to a longitudinal axis of said body member and each arm being radially movable under the force of a bias means, each arm having a cutting surface on at least a downhole end face thereof, the cutting surface on each said arm being located substantially in the plane of said cutting means.

2. A junk mill as claimed in claim 1 wherein said bias means is a reciprocal piston arranged to drive each said arm against the force of a compression spring.

3. A junk mill as claimed in claim 2 wherein said body member has an axial aperture for passing fluid to ports adjacent said cutting means and said piston is positioned in said axial aperture to be driven by said fluid.

4. A junk mill as claimed in claim 3 wherein said piston has an annular cross-section with an internal bore and the outer periphery of said piston is adapted to sealingly engage an internal wall of said body member.

5. A junk mill as claimed in claim 4 wherein said piston has an orifice located in the internal bore thereof for determining the force exerted by said piston on said arms.

6. A junk mill as claimed in any preceding claim wherein the piston acts upon an abutment of each arm to rotate the arm associated therewith radially outwardly from said body member.

7. A junk mill as claimed in claim 6 wherein limiting means are provided for limiting the radial outward movement of each said arm.

8. A junk mill as claimed in claim 7 wherein said limiting means is a shoulder adjacent said abutment on each arm, each shoulder being arranged to rotate with said arm to contact said piston and thereby prevent further radial outward rotation of said arm.

9. A junk mill as claimed in any preceding claim wherein each arm is dressed with tungsten carbide along a downhole end face thereof and partially along an adjacent side edge thereof.

10. A junk mill as claimed in any preceding claim wherein the downhole end surface of the body is relieved to form lands which comprise the cutting means and the downhole surface of each land is dressed with tungsten carbide.

11. A junk mill as claimed in claim 10 wherein three approximately sectorially shaped lands are provided and the arms are interspaced between the lands.

12. A junk mill substantially as herein described with reference to, and as shown in, the accompanying drawings.