Mini-severing and back-off tool with pressure balanced explosives

Abstract

A mini-severing and back-off tool includes a tubular magazine for a column of explosive pellets that are sized in diameter to provide a predetermined weight of explosive per unit column length. The tubular wall is vented at prescribed intervals for fluid pressure equalization between the internal bore of the tubular and the fluid pressure around the outside of the tubular. A fluid barrier is positioned between one end of the explosive pellet tubular and a detonator or an explosive booster provided to detonate the explosive pellet tubular. The explosive pellet tubular is positioned within the flow bore of a pipe string and adjacent to an intended pipe joint, and the explosive pellet tubular is detonated simultaneously with an application of torque on the pipe string, wherein the torque is applied in the thread release direction to disassemble the intended pipe joint.

Claims

1. A downhole mini-severing and back-off tool apparatus comprising: a firing head secured to an initiation housing, wherein the firing head comprises a detonator therein positioned in ignition proximity to a booster explosive within the initiation housing; a pellet tube secured in the initiation housing, wherein the pellet tube comprises an enclosure wall; a plurality of explosive pellets in contiguous alignment within the enclosure wall of the pellet tube; a bias at a first end of the plurality of explosive pellets, wherein the bias urges a second end of the plurality of explosive pellets in detonation proximity with the booster explosive; and at least one aperture in the enclosure wall of the pellet tube, wherein the at least one aperture substantially equalizes a pressure within the enclosure wall of the pellet tube with a pressure external of the enclosure wall of the pellet tube prior to detonation.

2. The apparatus as described by claim 1, wherein the bias at the first end of the plurality of explosive pellets is mechanical.

3. The apparatus as described by claim 2, wherein the mechanical bias at the first end of the plurality of explosive pellets comprises a coil spring.

4. The apparatus as described by claim 1, wherein the at least one aperture comprises a plurality of apertures distributed along a length of the enclosure wall.

5. The apparatus as described by claim 4, wherein the plurality of apertures are distributed at approximately two-foot intervals.

6. The apparatus as described by claim 1, further comprising a fluid barrier disposed between the booster explosive and the plurality of explosive pellets.

7. The back-off tool as described by claim 1, further comprising an O-ring seal between the firing head and the initiation housing.

8. A method of releasing a threaded pipe joint within a pipe string comprising the steps of: providing a tube having a bore space enclosed by a tube wall; providing an initiation housing; placing a booster explosive within said initiation housing; securing a first end of said tube to said initiation housing; penetrating said tube wall with at least one aperture to equalize pressure internal and external to the tube wall; inserting a plurality of explosive pellets into the bore space; biasing against one end of the plurality of explosive pellets to urge an opposite end of the plurality of explosive pellets into detonation proximity with the booster explosive; positioning the tube within a pipe string flow bore adjacent to a threaded pipe joint; applying a torque on the pipe string for disassembling the pipe string; and detonating at least one of the plurality of explosive pellets.

9. The method of claim 8, wherein the step of penetrating said tube wall with at least one aperture comprises penetrating said tube wall with a plurality of apertures at approximately two-foot intervals.

10. The method of claim 8, wherein the step of biasing against one end of the plurality of explosive pellets is accomplished with a coil spring.

11. The method of claim 8, further comprising the step of providing a fluid bather between the booster explosive and the plurality of explosive pellets.

12. The method of claim 8, wherein the step of inserting a plurality of explosive pellets into the bore space comprises contiguously aligning and inserting each of the plurality of explosive pellets into the bore space.

13. A method of releasing an intended pipe joint threaded within a pipe string, wherein said method comprises the steps of: tabulating values representing a weight of explosive distributed over a unit length corresponding to a type of pipe, a size of pipe, a well depth location of the intended pipe joint, a density of fluid within a well, or combinations thereof, such that when the explosive is detonated adjacent to the intended pipe joint and under moderate torque, the detonation will initiate the release of the intended pipe joint; contiguously aligning a plurality of explosive pellets having a concentration of explosive corresponding to the tabulated value adjacent to the intended pipe joint within a tubular bore; venting a wall of the tubular bore with at least one aperture to equalize pressure within the tubular bore to pressure external to the tubular bore; positioning the tubular bore within the pipe string proximate to the intended pipe joint; applying a moderate torque to the pipe string; and detonating the plurality of explosive pellets simultaneously with the application of the moderate torque to disassemble the pipe string.

14. The method of claim 13, wherein the step of venting a wall of the tubular bore with at least one aperture comprises venting said wall with a plurality of apertures at approximately two-foot intervals.

15. The method of claim 14, wherein the step of detonating the plurality of explosive pellets is accomplished by a booster explosive positioned at one end of, and in detonation proximity with, the plurality of explosive pellets.

16. The method of claim 15, further comprising the step of providing a fluid barrier between the booster explosive and the plurality of explosive pellets.

17. A downhole mini-severing and back-off tool apparatus comprising: a firing head secured to an initiation housing, wherein the firing head comprises a detonator therein; a pellet tube secured in the initiation housing, wherein the pellet tube comprises an enclosure wall; a plurality of explosive pellets in contiguous alignment within the enclosure wall of the pellet tube; at least one of said plurality of explosive pellets positioned within ignition proximity of said detonator; a bias at a first end of the plurality of explosive pellets, wherein the bias urges a second end of the plurality of explosive pellets in detonation proximity with the booster explosive; and at least one aperture in the enclosure wall of the pellet tube, wherein the at least one aperture substantially equalizes a pressure within the enclosure wall of the pellet tube with a pressure external of the enclosure wall of the pellet tube prior to detonation.

18. The apparatus as described by claim 17, wherein the bias at the first end of the plurality of explosive pellets is mechanical.

19. The apparatus as described by claim 18, wherein the mechanical bias at the first end of the plurality of explosive pellets comprises a coil spring.

20. The apparatus as described by claim 17, wherein the at least one aperture comprises a plurality of apertures distributed along a length of the enclosure wall.

21. The apparatus as described by claim 20, wherein the plurality of apertures are distributed at approximately two-foot intervals.

22. The apparatus as described by claim 17, further comprising a fluid barrier disposed between the detonator and at least one of the plurality of explosive pellets.

23. The apparatus as described by claim 17, further comprising an O-ring seal between the firing head and the initiation housing.

24. A method of releasing a threaded pipe joint within a pipe string comprising the steps of: providing a tube having a bore space enclosed by a tube wall; providing an initiation housing; securing a first end of said tube to said initiation housing; providing a firing head having an explosive detonator therein; securing said initiation housing to said firing head; penetrating said tube wall with at least one aperture to equalize pressure internal and external to the tube wall; inserting a plurality of explosive pellets into the bore space; biasing against one end of the plurality of explosive pellets to urge an opposite end of the plurality of explosive pellets into detonation proximity with said detonator; positioning the be within a pipe string flow bore adjacent to a threaded pipe joint; and applying a torque on the pipe string and simultaneously detonating at least one of the plurality of explosive pellets for disassembling the pipe string.

25. The method of claim 24, wherein the step of penetrating said tube wall with at least one aperture comprises penetrating said tube wall with a plurality of apertures at approximately two-foot intervals.

26. The method of claim 24, wherein the step of biasing against one end of the plurality of explosive pellets is accomplished with a coil spring.

27. The method of claim 24, further comprising the step of providing a fluid barrier between the explosive detonator and the plurality of explosive pellets.

28. The method of claim 24, wherein the step of inserting a plurality of explosive pellets into the bore space comprises contiguously aligning and inserting each of the plurality of explosive pellets into the bore space.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Relative to the drawings wherein like reference characters designate like or similar elements or steps through the several figures of the drawings:

(2) FIG. 1 represents a section of raw borehole having a drill string seized therein by a collapsed borehole wall.

(3) FIG. 2 is an embodiment of an enlarged detail of a firing head and pellet tube assembly.

(4) FIG. 3 is an alternate embodiment of a firing head and pellet tube assembly.

DETAILED DESCRIPTION OF THE INVENTION

(5) Before explaining selected embodiments of the present invention in detail, it is to be understood that the present invention is not limited to the particular embodiments described herein and that the present invention can be practiced or carried out in various ways. 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.

(6) To illustrate the operational environment of the invention, reference is given to the sectional view of FIG. 1 showing a drill collar portion of a drill pipe string 20 suspended in a raw borehole 10. Below the box joint 22, the drill pipe string 20 is immovably seized by a borehole wall collapse (i.e., seizure point) 12. Following the drill pipe seizure, an immediate operational objective, of the well drilling management, normally is to locate the seizure point 12 and to de-couple the threaded tool joint assembly 26, between the first box 22 and pin 24 assembly, above the seizure point 12.

(7) After having located a threaded tool joint assembly 26 that is above the seizure point 12, preferably the first threaded joint above the seizure, the present mini-severing and back-off tool 30 is suspended within the drill collar flow bore 29 by an appropriate suspension string, such as a wire line, slick line or, as illustrated, from a length of coiled tubing 31. As shown in FIG. 1, the mini-severing and back-off tool 30 comprises an initiation housing 35 and a firing head 33. A suitable connection mechanism, such as a bail or threads 39, as shown in FIG. 2, can be used to secure the back-off tool 30 to the end of a coiled tubing string 31, for example. The back-off tool 30 can be positioned to locate the explosive pellet tube 32 in bridging opposition of the specifically identified threaded tool joint assembly 26. As shown in FIG. 1, the explosive pellet tube 32 can include at least one centralizer 50 fastened to the distal end of the nose piece 48 of the mini-severing and back-off tool 30.

(8) Referring to FIG. 2, the firing head 33 houses an electrical ignition circuit 36, which can be used for igniting an electrically initiated detonator 37. The detonator 37 can project from the end of the firing head into an ignition proximity with a pellet of booster explosive 40 (i.e., booster explosive) that can be encapsulated within a booster cavity 41 of the initiation housing 35. Ignition proximity is a spatial separation between a donor or detonator explosive and a receptor explosive, and within which area or vicinity the ignition of the detonator will result in the detonation of the receptor explosive. An O-ring seal 38 is positioned between the firing head 33 and the initiation housing 35 for isolating the booster explosive 40, located in the booster cavity 41, from potential well fluid contamination. Additionally, the firing head 33 and the initiation housing 35 can be threadably connected 43 and can be set off by at least one washer 34 which, in an embodiment, can be constructed of a flexible polymer material, such as silicone. This offset creates an empty chamber 49 between the two materials which allows the ignition reaction, between the booster explosive 40 and the detonator 37, to take place at atmospheric pressure.

(9) The lower end of the initiation housing 35 can include a threaded socket 42 for securing a pellet tube 32, for example, a 3.1 m (10 ft.) long pellet tube 32. The lower distal end of the pellet tube 32 can be terminated by a nose piece 48. Explosive pellets 45 can fill the length of the pellet tube 32 in contiguous serial alignment to form the explosive pellet tube 32. The first of such explosive pellets 45 in the contiguous serial alignment should be positioned within detonation proximity of the booster explosive 40. A mechanical bias 47, such as coil spring, can be compressed between the nose piece 48 and the lowermost explosive pellet 45.

(10) Along the length of the explosive pellet tube 32, at approximately 2 ft. intervals, for example, vent apertures 44 can be formed through the wall of the explosive pellet tube 32. The vent apertures 44 allow the fluid pressure within the bore of the explosive pellet tube 32 to equalize with the fluid pressure outside the explosive pellet tube 32. Although it is conventional prior art wisdom to isolate explosive materials from direct contact with well fluids, it has been found that such concerns are greatly overstated. Due to the brief time periods of exposure, usually less than 1 hour, and the high degree of explosive compaction, it has been discovered that back-off tool explosives are largely unaffected by well fluid exposure. Consequently, the explosive pellet tube 32 may be constructed, for example, of thin wall, mild steel. At the upper end of the explosive pellet tube 32, between the booster cavity 41 and an uppermost explosive pellet of the plurality of explosive pellets 45, a fluid barrier 46 can be formed within the initiation housing 35 structure to isolate the booster explosive 40 from any well fluid contamination arriving from the bore of the pellet tube 32.

(11) In conducting the selection process for the size of the explosive pellet tube 32 and the consequent explosive weight distribution, a few parameters can be determined and considered, including the nominal size of the pipe joint to be un-screwed, the well depth of the seizure, and the fluid density of the in situ well fluid. From these determined parameters, an explosive weight distribution value per unit of length can be suggested for shocking a pipe (i.e., tubing) coupling. Notably, the suggested explosive weight distribution value can be a distributed explosive value in grains per foot. When the explosive pellets 45 within the pellet tube 32 detonate, the resulting shock is a relatively low grade expansion within the tubing bore, along the length of pellet tube 32 and across the intended coupled or threaded pipe joint assembly 26 (See FIG. 1). In addition, the explosive pellet tube 32 can include at least one centralizer 50, embodied in FIG. 2 as two blades of sheet metal fastened to the distal end of the nose piece 48. Alternatively, embodiments of the present invention may utilize any centralizer, usable for centralizing a downhole tool, or no centralizer.

(12) Moderate torque, as applied herein, is a highly subjective value determined in each case by the driller. Although most, if not all, modern drilling rigs have reasonably precise torque measuring capacity, which can be highly variable; however, the torque measuring capacity can also be very specific to a particular type of pipe, e.g. casing, drill pipe or tubing, and can be sufficient to unscrew a particular tool joint under back-off shock, but not unscrew any other tool joint in the string. Hence, the value of moderate torque is a subjective operational value that is recognized by those of skill in the art for the particular equipment with which they are working.

(13) An alternative embodiment of the mini-severing and back-off tool is shown in FIG. 3 which omits the explosive booster 40 and positions the detonator 37 in ignition proximity of an uppermost explosive pellet of the plurality of explosive pellets 45. As shown, the firing head 33 can be threadably connected, at a first end, to a suspension string, such as a wire line, slick line or, as illustrated, from a length of coiled tubing 31, for lowering the mini-severing and back-off tool 30 into a bore, for example, a drill collar flow bore 29 (shown in FIG. 1). At the second or opposite end, the firing head 33 can be threadably connected to an initiation housing 35, wherein O-ring seals or other sealing members can be positioned, between the initiation housing 35 and firing head 33, to prevent contamination from well fluids and other potential contaminating sources.

(14) The firing head 33 can house an electrical ignition circuit 36, which can be used for igniting an electrically initiated detonator 37, which can project through the firing head 33 and into ignition proximity with an uppermost explosive pellet 45 that is aligned within an explosive pellet tube 32 (i.e., pellet tube 32 comprising explosive pellets 45). At the upper end of the explosive pellet tube 32, between the detonator 37 and the uppermost explosive pellet 45, a fluid barrier 46 can be formed within the initiation housing 35 structure to isolate the detonator 37 from any well fluid contamination arriving from the bore of the pellet tube 32.

(15) Along the length of the explosive pellet tube 32 and at regular intervals, for example, at approximately 0.6 m (2 ft.) intervals, one or more vent apertures 44 can be formed through the wall of the explosive pellet tube 32. The vent apertures 44 allow the fluid pressure within the bore of the explosive pellet tube 32 to equalize with the fluid pressure outside the explosive pellet tube 32.

(16) As shown in FIG. 3, the lower end of the initiation housing 35 can include a threaded socket 42 for securing the pellet tube 32. Explosive pellets 45 can fill the length of the pellet tube 32 in contiguous serial alignment to form the explosive pellet tube 32, with the first or uppermost of such explosive pellets 45, in the contiguous serial alignment, positioned within detonation proximity of the detonator 37. A mechanical bias 47, such as coil spring, can be compressed between a nose piece 48, located at a distal end of the pellet tube 32, and the lowermost explosive pellet 45. In an embodiment, the explosive pellet tube 32 can include at least one centralizer 50, embodied in FIG. 3 as two blades of sheet metal fastened to the distal end of the nose piece 48. Alternatively, embodiments of the present invention may utilize any centralizer, usable for centralizing a downhole tool, or no centralizer.

(17) Although the invention disclosed herein has been described in terms of specified and presently preferred embodiments which are set forth in detail, it should be understood that this is by illustration only and that the invention is not necessarily limited thereto. Alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the present disclosure. Accordingly, modifications of the invention are contemplated which may be made without departing from the spirit of the claimed invention.