Apparatus and methods for shaped charge tubing cutters

Abstract

A shaped charge pipe cutter is constructed with the cutter explosive material packed intimately around an axially elongated void space that is continued through a heavy wall boss portion of the upper thrust disc. The boss wall is continued to within a critical initiation distance of a half-cuter junction plane. An explosive detonator is positioned along the void space axis proximate of the outer plane of the upper thrust disc. Geometric configurations of the charge thrust disc and end-plate concentrate the detonation energy at the critical initiation zone.

Claims

1. A shaped charge tubing cutter comprising: first and second explosive units, wherein each explosive unit comprises a primary explosive material between inner surfaces of a conical metallic liner and a metallic backing plate, wherein a truncated apex of the conical metallic liner of the first explosive unit and a truncated apex of the conical metallic liner of the second explosive unit are joined coaxially along a common juncture plane; an aperture extending along an axis of revolution through the metallic backing plate of the first explosive unit and the primary explosive material of the first explosive unit; a pellet of a secondary explosive material positioned entirely within the aperture between the metallic backing plates; and an explosive detonator positioned along the axis of revolution adjacent to and externally of the first and second explosive units, wherein initiation of the explosive detonator propagates a shock wave through the aperture, initiating the first and second explosive units.

2. The shaped charge tubing cutter as described by claim 1, wherein the aperture extends along the axis of revolution through the metallic backing plate of the second explosive unit.

3. The shaped charge tubing cutter as described by claim 2, wherein the portion of the aperture extending along the axis of revolution through the second metallic backing plate is plugged.

4. The shaped charge tubing cutter as described by claim 1, wherein the aperture comprises a first diameter adjacent to the metallic backing plate of the first explosive unit, and a second diameter adjacent to the common juncture plane, and wherein the first diameter is greater than the second diameter.

5. The shaped charge tubing cutter as described by claim 1, wherein the metallic backing plates of the respective first and second explosive units comprise brass.

6. An explosive well tool assembly comprising: a housing secured to a top sub, wherein the top sub comprises a planar, distal end-face aligned normal to an axis of revolution when secured to the housing; and an explosive shaped charge within the housing, wherein the shaped charge comprises first and second matched explosive units, wherein the first matched explosive unit comprises a first conical metallic liner and a first metallic backing plate, wherein the second matched explosive unit comprises a second conical metallic liner and a second metallic backing plate, wherein each matched explosive unit is a singular element developed symmetrically about the axis of revolution and comprises an explosive material intimately formed between the respective first and second conical metallic liners and the respective first and second metallic backing plates, wherein a truncated apex of the first conical metallic liner and a truncated apex of the second conical metallic liner are joined coaxially along a common juncture plane, wherein an external surface of the first and second backing plates is located opposite from the explosive material and substantially normal to the axis of revolution, wherein the external surface of at least one metallic backing plate comprises a plurality of empty pockets distributed in a prescribed pattern about the axis of revolution, and wherein a plane of the external surface of said at least one metallic backing plate is adjacent to and parallel with a plane of the distal end-face of the top sub.

7. The explosive well tool assembly as described by claim 6, wherein the plurality of empty pockets comprises a plurality of blind borings into the external surface, and wherein the prescribed pattern is a circular distribution about the axis of revolution.

8. The explosive well tool assembly as described by claim 6, wherein the plurality of empty pockets comprises a plurality of slots within the external surface, and wherein the prescribed pattern extends radially from the axis of revolution in regular arcuate increments.

9. A shaped charge assembly comprising: first and second matched explosive units, each unit being a singular element developed symmetrically about an axis of revolution and comprising an explosive material intimately formed between a conical metallic liner and a metallic backing plate, wherein truncated apices of the conical metallic liners are joined coaxially along a common juncture plane, wherein the metallic backing plates each comprise an external surface opposite from their respective explosive materials and substantially normal to the axis of revolution, and wherein the external surface of at least one of the metallic backing plates comprising a plurality of empty pockets distributed in a prescribed pattern about the axis of revolution.

10. The shaped charge assembly as described by claim 9, wherein the empty pockets of the at least one metallic backing plate comprise a plurality of blind borings into the external surface of the at least one metallic backing plate, and the prescribed pattern comprises a circle about the axis of revolution.

11. The shaped charge assembly as described by claim 9, wherein the empty pockets of the at least one metallic backing plate comprise a plurality of slots into the external surface of the at least one metallic backing plate extending radially from the axis of revolution, and the prescribed pattern is a distribution thereabout in uniform arcuate increments.

12. A method of detonating a shaped charge tubing cutter comprising the steps of: providing a shaped charge tubing cutter having explosive materials between respective pairs of metallic liners and end plates aligned about a common axis, wherein the end plates are aligned normal to the common axis, wherein an aperture extends along the common axis from an exterior surface of one of the pair of end plates to an interior surface of the other of the pair of end plates, wherein the aperture houses a pellet of secondary explosive material entirely between the backing plates; positioning a detonator along the common axis, external of the aperture and adjacent an exterior surface opening of one of the pair of end plates; positioning the tubing cutter within a tubing bore; and actuating the detonator.

13. The method of detonating a shaped charge tubing cutter as described by claim 12, wherein the aperture is continued through the other of the pair of end plates.

14. The method of detonating a shaped charge tubing cutter as described by claim 13, further comprising plugging an end of the portion of the aperture extending through the other of the pair of end plates.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The invention is hereafter described in detail and with reference to the drawings wherein like reference characters designate like or similar elements throughout the several figures and views that collectively comprise the drawings. Respective to each drawing figure:

(2) FIG. 1 is a cross-section of a first embodiment of the invention in assembly with the housing, centralizer and connecting sub.

(3) FIG. 2 is a cross-section of a second embodiment of the invention in assembly with the housing, centralizer and connecting sub.

(4) FIG. 3 is a cross-section of a third embodiment of the invention in assembly with the housing, centralizer and connecting sub.

(5) FIG. 4 is a cross-section of a fourth embodiment of the invention in assembly with the housing, centralizer and connecting sub.

(6) FIG. 5 is a plan view of an end plate showing marker pocket borings.

(7) FIG. 6 is a cross-section view of an end plate along cutting plane 6-6 of FIG. 5.

(8) FIG. 7 is a bottom plan view of a top sub after detonation of the cutter.

(9) FIG. 8 is a plan view of a backing plate showing an alternative marker pocket pattern of slots.

(10) FIG. 9 is a bottom plan view of the cutter assembly with the invention centralizer.

(11) FIG. 10 is a side view of the invention centralizer.

(12) FIG. 11 is an operational plan view of the invention centralizer.

(13) FIG. 12 is a cross-section of a fifth embodiment of the invention showing all essential elements of the firing head assembly.

DETAILED DESCRIPTION OF THE INVENTION

(14) As used herein, the terms up and down, upper and lower, upwardly and downwardly, upstream and downstream; above and below; and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments of the invention. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or other relationship as appropriate. Moreover, in the specification and appended claims, the terms pipe, tube, tubular, casing, liner and/or other tubular goods are to be interpreted and defined generically to mean any and all of such elements without limitation of industry usage.

(15) Referring initially to the invention embodiment of FIG. 1, the cutter assembly 10 comprises a top sub 12 having a threaded internal socket 14 that axially penetrates the upper end of the top sub. The socket thread 14 provides a secure mechanism for attaching the cutter assembly with an appropriate wire line or tubing suspension string not shown. In general, the cutter assembly has a substantially circular cross-section. Consequentially, the outer configuration of the cutter assembly is substantially cylindrical. The lower end of the top sub includes a substantially flat end face 15. The end face perimeter is delineated by a housing assembly thread 16 and an O-ring seal 18. The axial center 13 of the top sub is bored between the assembly socket 14 and the end face 15 to provide a socket 30 for an explosive detonator 31.

(16) The cutter housing 20 is secured to the top sub 12 by an internally threaded sleeve 22. The O-ring 18 seals the interface from fluid invasion of the interior housing volume. A jet window section 24 of the housing interior is that inside wall portion of the housing 20 that bounds the jet cavity 25 around the shaped charge between the outer or base perimeters 52 and 54 of the liners 50. Preferably, the upper and lower limits of the jet window 25 are coordinated with the shaped charge dimensions to place the window sills at the approximate mid-line between the inner and outer surfaces of the liner 50. Representatively, the shaped charge housing 20 may be a frangible steel material of approximately 55-60 Rockwell C hardness.

(17) Below the jet window 25, the cutter housing cavity is internally terminated by an integral end wall 32 having a substantially flat internal end-face 33. The external end-face 34 of the end wall may be frusto-conical about a central end boss 36. A hardened steel centralizer assembly 38 may be secured to the end boss by an assembly bolt 39.

(18) With respect of FIGS. 9, 10 and 11, a preferred centralizer assembly comprises a plurality of blade plates 82, 83, and 84. For example, a set of three blade plates may be used in a 1.50 inch tubing bore. Typically, the blades may be fabricated of Rockwell C60 hardness spring steel of approximately 0.004 inch (0.101 mm) thickness, having, for example, four, 0.250 inch (6.35 mm) wide blades with a 0.765 inch (19.431 mm) radius length. These blades are loosely stacked, serially, along the cylindrical, axially centralized, journal surface of shoulder screw 86. An axially centralized aperture through each of the blade plates is dimensioned to allow substantially free rotation of the plates about the shoulder screw journal surface. In the presently preferred embodiment, the shoulder screw head confines the several blade plates to the length of the shoulder screw journal surface.

(19) Relative to prior art centralizer blade plates of about 0.015 inch (0.381 mm) thickness, approximately 0.765 inch (19.431 mm) radius length and approximately 0.250 inch (6.35 mm) width for a 1.50 inch (38.1 mm) tubing bore, the present invention provides a much lower bending strength for each blade and freedom to angularly reorient about the tool axis as it traverses the tubing bore length as represented by FIG. 11. The blade plate 82 is shown as rotated from angular symmetry by the internal tube seam weld 88 without compromise of a central radial alignment with the tube bore axis.

(20) Substantially free rotation of the centralizer blade plates about the cutter assembly axis 13 has additional advantages in a wireline operation. Wirelines for downhole tool control and tethering typically comprise a double helix winding of high tensile strength wire with the outer layer winding turned in the opposite hand direction from the first, inner layer. These steel wire windings are laid around one or more insulated signal or electrical power conduits. Although the radial difference between the inner and outer windings is minute, this small difference imposes substantial torsional force over several miles of wireline length. To relieve the wireline of this internal torsional stress as the suspended tool descends into a well, the tool must be allowed to rotate about the tool/wireline axis. However, the frictional bearing of traditional centralizers on the internal bore wall of well tubing and the internal standing tube assembly seam of the well tubing inhibit any rotation of the tool as it descends into the well. Consequently, the wireline is restrained from relieving internal torsional stress. Resultantly, the two wound wire strength layers of the wireline may separate, forming a bulbous bird cage as it is known in the art. By permitting the centralizer blades to freely rotate about the tool axis, the wireline is allowed to rotate about its own axis to relieve this internal torsional stress.

(21) The shaped charge assembly 40 is preferably spaced between the top sub end face 15 and the internal end-face 33 of the cutter housing 20 by a pair of resilient, electrically non-conductive, ring spacers 56 and 58. An air space of at least 0.100 (2.54 mm) is preferred between the top sub end face 15 and the adjacent face of the cutter assembly thrust disc 46. Similarly, a resilient, non-conductive lower ring spacer 58 provides an air space that is preferably at least 0.100 (2.54 mm) between the internal end-face 33 and the adjacent cutter assembly lower end plate 48.

(22) Loose explosive particles can be ignited by impact or friction in handling, bumping or dropping the assembly. Ignition that is capable of propagating a premature explosion may occur at contact points between a steel, shaped charge thrust disc 46 or end plate 48 and a steel housing 20. To minimize such ignition opportunities, the thrust disc 46 and lower end plate 48, for the present invention, are preferably fabricated of non-sparking brass.

(23) The outer faces 91 and 93 of end plates 46 (upper thrust disc) and 48, as respectively shown by FIG. 1, are blind bored with marker pockets 95 in a prescribed pattern such as a circle with uniform arcuate spacing between adjacent pockets as illustrated by FIGS. 5 and 6. These pockets 95 in the outer face 91, 93 are selectively weakened areas of the end plates. When the explosive material 60 detonates, the marker pocket walls are converted to jet material in a development similar to a V-shaped charge cutting liner. These cutting jets of fluidized end plate material scar the lower end face 15 of the top sub 12 with impression marks 99 in a pattern corresponding to the original pockets as shown by FIG. 7. When the top sub 12 is retrieved after detonation, the uniformity and distribution of these impression marks 99 reveal the quality and uniformity of the detonation and hence, the quality of the cut. For example, if the top sub face 15 is marked with only a half section the end plate pocket pattern, it may be reliability concluded that only half of the cutter explosive correctly detonated.

(24) FIG. 8 illustrates an alternative pattern of marker pockets shown as radial slots 97 distributed about the plate axis in substantially uniform arcuate segments.

(25) The explosive material 60 traditionally used in the composition of shaped charge tubing cutters comprises a precisely measured quantity of powdered, high explosive material such as RDX or HMX. The FIG. 1 invention embodiment includes a liner 50 that is formed into a truncated cone. The liner 50 substance may be an alloy of copper and lead, for example. In some cases, a thin sheet, 0.050 (1.27 mm), for example, of the alloy is mechanically formed to the frusto-conical configuration. Other methods of liner fabrication may provide a mixture of metal powders that is pressed or sintered to the frusto-conical form. In either case, the frusto-conical liner 50 is formed with open circular zones for the apex and base.

(26) This frusto-conical liner 50 is placed in a press mold fixture with a portion of the fixture wall bridging the liner apex opening as an annulus around a central core post. A precisely measured quantity of powdered explosive material such as RDX or HMX is distributed within the internal cavity of the mold intimately against the interior liner surface and the fixture wall bridging the liner apex opening around the core post. Using a central core post as a guide mandrel through an axial aperture 47 in the upper thrust disc 46, the thrust disc is placed over the explosive powder and the assembly subjected to a specified compression pressure. This pressed lamination comprises a half section of the cutter assembly 40.

(27) The lower half section of the charge assembly 40 is formed in the same manner as described above, each having a central aperture 62 of about 0.125 (3.18 mm) diameter in axial alignment with thrust disc aperture 47 and the end plate aperture 49. A complete cutter assembly comprises the contiguous union of the apex zone half sections respective to the lower and upper half sections along the juncture plane 64. Notably, the thrust disc 46 and end plate 48 are each fabricated around respective annular boss sections 70 and 72 that provide a protective material mass between the respective apertures 47 and 49 and the explosive material 60. These bosses are terminated by distal end faces 71 and 73 within a critical initiation distance of about 0.050 (1.27 mm) to about 0.100 (2.54 mm) from the assembly juncture plane 64 for a 2.50 (63.5 mm) cutter. The critical initiation distance may be increased or decreased proportionally for other sizes. Hence, the explosive material 60 is insulated from an ignition wave issued by the detonator 31 until the wave arrives in the proximity of the juncture plane 64.

(28) Distinctively, the apertures 47, 49 and 62 for the FIG. 1 embodiment remain open and free of boosters or other explosive materials. Although an original explosive initiation point for the cutting charge 40 only occurs between the boss end faces 71 and 73, the original detonation event is generated by the detonator 31 outside of the thrust disc aperture 47. The detonation wave is channeled along the empty thrust disc aperture 47 to the empty central aperture 62 in the cutter explosive material. Typically, an explosive load quantity of 1.36 oz (38.6 gms) of HMX compressed to a loading pressure of 3,000 psi (20,000 kPa) may require a moderately large detonator 31 of 0.015 oz (420 mg) HMX for detonation

(29) The FIG. 1 embodiment obviates any possibility of orientation error in the field while loading a cutter housing. A detonation wave may be channeled along either boss aperture 47 or 49 to the explosive 60 around the central aperture 62. Regardless of which orientation the shaped charge assembly is given when inserted in the housing 20, the detonator 31 will initiate the cutter explosive 60.

(30) A modification of the invention is represented by FIG. 2 showing the axial aperture 80 in the thrust disc 46 to be tapered with a conically convergent diameter from the disc face proximate of the detonator 31 to the central aperture 62. Typical of this embodiment, the thrust disc aperture 80 may have a taper angle of about 10 between an approximately 0.080 (2.03 mm) inner diameter to an approximately 0.125 (3.18 mm) diameter outer diameter. The taper angle, also characterized as the included angle, is the angle measured between diametrically opposite conical surfaces in a plane that includes the conical axis 13.

(31) Original initiation of the FIG. 2 cutter charge 60 occurs at the outer plane of the tapered aperture 80 having initiation proximity with a detonator 31. The initiation shock wave propagates inwardly along the tapered aperture 80 toward the explosive junction plane 64. As the shock wave progresses axially along the aperture 80, the concentration of shock wave energy intensifies due to the progressively increased confinement and concentration of the explosive energy. Consequently, the detonator shock wave strikes the cutter charge 60 at the inner juncture plane 64 with an amplified impact.

(32) Comparatively, the same explosive charge 60 as suggested for FIG. 1 comprising, for example, approximately 1.36 oz (38.6 gms) of HMX compressed under a loading pressure of about 3,000 psi (20,000 kPa), when placed in the FIG. 2 embodiment may require only a relatively small detonator 31 of HMX for detonation. Significantly, the conically tapered aperture 80 of FIG. 2 appears to focus the detonator energy to the central aperture 62 thereby igniting a given charge with much less source energy.

(33) Although the FIG. 3 invention embodiment relies upon an open, substantially cylindrical aperture 47 in the upper thrust disc 46 as shown in the FIG. 1 embodiment, either no aperture is provided in the end plate boss 72 of FIG. 3 or the aperture 49 in the lower end plate 48 is filled with a dense, metallic plug 76. The plug 76 may be inserted in the aperture 49 upon final assembly or pressed into place beforehand. As in the case of the FIG. 2 embodiment, a FIG. 3 cutter comprising, for example, approximately 1.36 oz (38.6 gms) of HMX compressed under a loading pressure of about 3,000 psi (20,000 kPa) also may require only a relatively small detonator 31 of HMX for detonation. Apparently, the detonation wave emitted by the detonator 31 is reflected back upon itself in the central aperture 62 by the plug 76 thereby amplifying a focused concentration of detonation energy in the critical zone 62.

(34) The FIG. 4 invention embodiment combines the energy concentrating features of FIG. 2 and FIG. 3 but further adds a relatively small, explosive initiation pellet 66 in the central aperture 62. Of course, the explosive initiation pellet 66 concept may also be applied to the FIG. 1 embodiment.

(35) The FIG. 12 invention embodiment is distinguished by the thin, 0.0097-0.010 in. (0.2464-0.2540 mm), material vessel shaped as a sealing cup 100 that separates the detonator 31 from the outer face of the thrust disc 46. Sealing cup 100 encloses the detonator 31 as a receptacle and includes a fluid tight rim or sidewall fit to the internal bore wall 35 of the top sub 12. This fluid tight fit between the cup 100 wall and the top sub bore wall 35 may be, for a few examples, an interference press fit, a threaded fit, a soldered fit or an integrally machined portion of the top sub 12 material. In any case, the distal end face of cup 100 is positioned from the lower end face 15 of the top sub as to assemble within about 0.032 in. (0.812 mm) of juxtaposition with the thrust disc 46 outer face 91 when the top sub shoulder 27 engages the distal edge of the cutter housing thread sleeve 23. This cup 100 provides an absolute barrier to any moisture that may penetrate any assembly seals 102 above the seal 18.

(36) The cutter housing 20 is destroyed upon a single use by detonation of the explosive material 60. Hence, the interior sealing surfaces of the threaded sleeve 23 are normally new and highly polished to assure a fluid seal of the O-ring 18 across the low pressure transitional zone of a well bore. Also, the top sub 12, however, is not often reused. However, tubing or pipe string units above the top sub 12 having fluid paths through tool joints into the top sub cavity 108 frequently are subject to corruption, contamination and scarring due to repeated assembly and disassembly. For this reason, the seals 102 between the firing head housing 110 for the capacitance discharge unit 112 and the top sub 12 are more likely to leak as the tool descends the well bore through the low fluid pressure zone. Such leaks allow well bore fluid, mostly water, to migrate past the sub assembly threads 106 into the internal cavity 108. Once in the cavity 108, migrating fluid continues past the detonator retainer 114 into the cutter housing 20. This fluid flow path along the top sub cavity 108 is reliably blocked by the cup 100.

(37) Operationally, the assembly is dimensioned to place the distal end of the detonator 31 against the interior bottom of the cup 100 when all assembly joints are tight. Since the detonator 31 is external of the charge aperture 47, it may be as large as need be to rupture the thin film of the cup 100 bottom and detonate the cutter explosive material 60.

(38) Although several preferred embodiments of the invention have been illustrated in the accompanying drawings and describe in the foregoing specification, it will be understood by those of skill in the art that additional embodiments, modifications and alterations may be constructed from the invention principles disclosed herein. These various embodiments have been described herein with respect to cutting a pipe. Clearly, other embodiments of the cutter of the present invention may be employed for cutting any tubular good including, but not limited to, pipe, tubing, production/casing liner and/or casing. Accordingly, use of the term tubular in the following claims is defined to include and encompass all forms of pipe, tube, tubing, casing, liner, and similar mechanical elements.