WO1998050678A1

WO1998050678A1 - Perforating apparatus and method

Info

Publication number: WO1998050678A1
Application number: PCT/GB1998/001143
Authority: WO
Inventors: Clive John French
Original assignee: Ocre (Scotland) Limited
Priority date: 1997-05-03
Filing date: 1998-05-05
Publication date: 1998-11-12
Also published as: AU7341998A; GB9709007D0

Abstract

A downhole perforating apparatus comprises a length of tubing (78) having a wall defining a through bore for fluid communication with a tubular string. Perforating charges (84) are located internally of the tubing wall. The tubing is mounted on the lower end of an elongate support such as a production string and run into a borehole and located adjacent a production formation. The charges (84) are then detonated to create flow ports in the tubing and to perforate the borehole wall, such that formation fluid may flow from the formation into the tubing through the flow ports.

Description

PERFORATING APPARATUS AND METHOD

This invention relates to a downhole perforating apparatus and a downhole perforating method.

Boreholes drilled to gain access to underground hydrocarbon-bearing formations are typically lined over most of their length by steel casing or liner. The liner which intersects the hydrocarbon-bearing or production formation initially has a solid wall, to prevent production fluid flowing into the bore until production tubing has been located in the bore and all of the associated apparatus and systems have been installed. The liner is then perforated to allow production fluid to flow into the wellbore by detonation of explosive charges, typically spaced individual charges which are lowered into the bore and detonated at appropriate locations adjacent the liner. The charges, in the form of perforating "guns", may be lowered into the bore on electric wireline, slickline, coiled tubing or some other suitable support. However, the length of the perforating guns is limited by the depth of the safety valve in the wellbore, and the length of liner to be perforated is generally longer than this depth, such that a perforating operation will tend to involve a number of runs, and this is relatively time consuming. Further, it is desirable to carry out "underbalanced" perforating, in which the pressure within the wellbore is lower than the formation pressure such that, following perforation, the debris produced by the perforating operation is washed out of the wellbore by the formation fluid. In the absence of such a pressure differential debris may be pushed into the perforations, restricting the flow of production fluid into the wellbore. When carrying out a perforating operation using wireline, slickline or coiled tubing and requiring a number of runs, only the first perforating operation may be underbalanced .

Perforating guns have been mounted on the lower end of production tubing, thus reducing the need for separate runs and separate perforating operations. However, the remains of the charge-carrying tubing, charges and firing arrangements which occupy the perforated liner reduce the flow area of the liner, reducing the production capability of the well; typically, the flow area will' be reduced to the area of the annulus between the gun tubing and the liner. Further, the presence of the debris in the wellbore restricts the ability of the operator to lower tools, test apparatus and logging equipment into the production zone. If necessary, this debris may be milled out, such that it falls to the bottom of the well. However, to accommodate the debris from several thousand metres of perforating guns requires the drilling of a substantial extra section of bore, which may take several weeks' drilling, adding substantially to the drilling costs for the wellbore. It is among the objectives of embodiments of the present invention to obviate or mitigate these difficulties .

According to the present invention there is provided a downhole perforating apparatus comprising: a length of tubing having a wall defining a through bore for fluid communication with a tubular string; and perforating charges located internally of the tubing wall. According to a further aspect of the present invention there is provided a method of perforating a section of a bore wall, the method comprising the steps: providing a length of tubing having a wall defining a through bore with perforating charges located internally of the tubing wall; mounting the tubing on the lower end of an elongate support ; running the tubing into a borehole and locating the tubing in the borehole adjacent a production'.formation; and detonating the charges to create flow ports in the tubing and to perforate the borehole wall, such that formation fluid may flow from the formation into the tubing through said flow ports.

Perforating apparatus is generally known as "guns" by those of skill in the art, and for brevity this term may be used herein when referring to the apparatus.

Preferably, the apparatus is mounted on the lower end of a tubular string, such as a string of production tubing, with the through bore in fluid communication with the string. Thus, production fluid may flow directly from the gun tubing through bore into the production tubing.

The use of the bore of the gun tubing as a conduit for production fluid provides numerous significant advantages over conventional tubing-mounted guns, in which the gun body does not provide fluid communication with the production tubing and production fluid is constrained to flow upwardly in the annulus between the gun body and the perforated casing. In the present invention, the production fluid will flow into the gun body and then flow directly from the gun body bore into the production tubing.

The gun may be used to perforate casing-lined borehole walls or unlined borehole walls. In the latter case the perforated gun tubing may serve as borehole liner.

Preferably, the charges are mounted in the tubing such that, following detonation of the charges, the tubing defines a substantially unobstructed internal bore.

Preferably also, one or both of the detonation of the charges and the flow of production fluid through the tubing clears debris from the tubing such that the tubing defines a substantially unobstructed internal bore.

The tubing may have a continuous wall which is perforated by the charges on detonation. In such embodiments, it is preferred that at least some of the changes are adapted to produce a relatively large area perforation in the tubing; the so-called "Big Hole" charges may be utilised for this purpose. Preferably, the tubing wall is weakened or thinned adjacent the charges, to facilitate perforation of the tubing. Conveniently, the outer surface of the tubing is scalloped. Alternatively, the tubing wall may define apertures, and the charges may be aligned with the apertures. The apertures may be initially closed by covers or caps which are perforated or otherwise displaced or destroyed on detonation of the charges . The covers may seal the apertures .

In the preferred apparatus, the gun wall is pressure- tight, allowing the charges to be isolated from high temperature well fluid and maintained at atmospheric pressure if desired; this generally improves the reliability of the apparatus. Further, the perforation operation may be carried out "underbalanced" , that is with the pressure within the tubing lower than the formation pressure, such that the debris created by the detonation is washed out of the fissures created in the formation by the pressurised formation fluid, and then washed out of the tubing and to the surface. The charges may be mounted directly on the tubing wall, for example the charges including profiles for engaging corresponding profiles defined in the tubing wall, which profiles may be dovetails. Alternatively, the charges may be mounted on supports which disintegrate on detonation or shortly thereafter: in one embodiment, the charges are mounted on an inner support which is hydrocarbon-soluble such that the flow of formation fluid into and through the tubing following perforation dissolves the support; in another embodiment the charges are mounted on a combustible support, for example a support formed of balsa wood or American oak, which wood may be impregnated with oxygen-containing combustible material. Other materials such as zinc alloys may be utilised to form supports, which materials combust and\or disintegrate into small fragments on detonation.

Preferably also, closures such as plugs are provided in the ends of the tubing, which plugs may themselves contain charges which are detonated with the perforating charges to remove or destroy the plugs.

Preferably also, the outer diameter of the tubing is selected to be only slightly smaller than the diameter of the borehole, casing or liner to be perforated. In this way, the perforating charges are located in close proximity to the surface to be perforated, and thus are more effective in perforating the surface. As noted above, in conventional tubing-mounted guns, the tubing external diameter must be considerably smaller than the casing internal diameter, as the production fluid flows upwardly through the annulus between the tubing and the casing, and using large diameter tubing would provide only a small flow area .

Preferably also, the charges are linked by explosive transfer means for communicating a detonation signal to each charge. Most preferably, the explosive transfer means extends through the interior of the tubing. The transfer means will typically be in the form of one or more tracks of detonation cord. The detonation cord may be positioned and retained within the tubing by appropriate locating members, which may include locating plates.

Preferably also, the tubing is provided in separable tubing or gun sections, each including a number of charges. The gun sections may be connected by any suitable means, but are preferably connected by threaded collars which are rotatable on the end of one gun section for engaging a corresponding threaded portion on the end of an adjacent gun section. Preferably also, the gun sections are provided with connectors for explosive transfer means for linking the charges in adjacent guns. Most preferably, the connectors include boosters or transfer detonators .

Preferably also, the apparatus includes firing means for initiating detonation of the charges. The firing means may be activated by one or more of electrical, hydraulic or mechanical means .

Preferably, the firing means is provided in combination with a valve, such as our full "bore isolation valve (FBIV) as described in PCT\GB97\00308 , the disclosure of which is incorporated herein by reference. The valve is utilised to selectively isolate the gun body bore from internal string pressure. In one embodiment, the valve includes a valve portion, preferably a valve seat, which is movable on pressure being bled off above the valve and the valve opening, which movement of the valve seat releases a firing pin actuating arrangement. The firing pin actuating arrangement preferably incorporates a spring tending to bias the firing pin to a firing position, which spring is released by upward movement of the valve seat. Alternatively, the firing pin itself may be actuated by hydraulic pressure, and may be initially remain or be retained in a primed position by means of a rupture disc or retainer which permits releases the firing pin on application of a predetermined fluid pressure thereto, which pressure may be the hydrostatic pressure at the desired depth in a wellbore. In another embodiment, a further valve is provided between the isolation valve and a hydraulic pressure-actuated firing pin. The further valve opens following opening of the isolation valve, to provide a short delay between opening the isolation valve and detonating the guns. These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic part-sectional view of an upper portion of perforating apparatus in accordance with a preferred embodiment of the present invention;

Figure 2 is a schematic part-sectional view of an intermediate portion of perforating apparatus in accordance with a preferred embodiment of the present invention;

Figure 3 is a schematic part-sectional view of a lower portion of perforating apparatus in accordance with a preferred embodiment of the present invention; and

Figure 4 is a sectional view of a perforating system in accordance with a further embodiment of the present invention . Figures 1, 2 and 3 of the drawings illustrate parts of perforating apparatus in accordance with a preferred embodiment of the present invention. Reference is first made to Figure 1 of the drawings, which illustrates an upper portion of the apparatus which extends downwardly from a retrieval sleeve 10 forming part of a packer- like arrangement as more fully described in our International Patent Application No. PCT\GB97\00495 , the disclosure of which is incorporated herein by reference. The retrieval sleeve 10 engages the lower end of the wellbore casing (not shown) , the perforating apparatus extending into the section of the wellbore which intersects the hydrocarbon- bearing or production formation. This section of the wellbore is provided with tubular steel "liner" which initially has a solid wall and which will be perforated using the apparatus, as described below. In other embodiments of the invention the apparatus may be utilised to perforate unlined sections of borehole. The apparatus is mounted on the lower end of a production tubing string which, in this preferred embodiment, incorporates a Multi -Function Tool 14 as supplied by the applicant, and examples of which are described in PCT\GB95\02046 and PCT\GB96\01907 , the disclosures of which are incorporated herein by reference. The tool operates in response to pressure cycles and is utilised to control the operation of a pressure-actuated isolation valve 16 mounted below the tool 14. The valve in this example is a Full Bore Isolation Valve (FBIV) as supplied by the applicant and as described in PCT\GB97\00308 , the disclosure of which is incorporated herein by reference. The tool 14 and valve 16 allow completion testing, packer setting and the like to be carried out by application of fluid pressure above the valve 16, and for the valve 16 to be opened after the conclusion of the completion operations. The valve 16 incorporates firing heads 18,19, as described in our earlier application PCT\GB97\00495 , the disclosure of which is incorporated herein by reference, the firing heads 18, 19 being activated by opening of the valve 16.

The firing heads 18, 19 are linked to detonation cord 20 which extends through the apparatus, linking the perforating charges 22. The upper end of the detonation cord 20 is set in a plug 24 which seals the upper end of the perforating system. A retainer snap ring 26 below the plug 24 supports the detonation cord 20 in the lower end of the valve 16. The perforating apparatus comprises a plurality of gun sections 28, the uppermost gun section 28a being illustrated in Figure 1 and an intermediate gun section 28b being illustrated in Figure 2. To facilitate making up of the gun sections 28 on the surface, the tubing ends define bevels 30 to facilitate stabbing-in. Adjacent gun sections are then connected by an internally threaded inter-gun coupling 32. To facilitate connection of the sections of detonating cord 20 between gun sections 28, the cord is mounted within a support tube 34 and the ends of the cord sections are provided with transfer detonators 36 located within detonator alignment tubes 38.

The gun sections 28 define scallops 40 on the outer surfaces thereof and the perforating charges 22 are located -lion the inside of the tubing sections adjacent a respective scallop 40. In this embodiment the perforating charges 22 are mounted on an inner tube 44 formed of a hydrocarbon- soluble material, such as polystyrene. Each gun section 28 accommodates a respective inner tube 44, the tubes 44 being located top and bottom by respective orientation plates 46 47 fixed relative to the gun sections 38 by respective orientation pins 48.

Reference is now made in particular to Figure 3 of the drawings, which illustrates the lowermost gun section 28c of the perforating apparatus. The lower end of the section 28c is provided with an internally threaded coupling 52, somewhat like the inter-gun couplings 32 described above, however the coupling 52 mounts a bull plug 54 on the lower end of the gun section 28c. The plug 54 accommodates an explosive cartridge 56 and a detonator 58 linked to the detonation cord 20. The plug 54 is fixed relative to the coupling 52 by a shear pin 55.

In use, the gun sections 28 are made up on the surface and are assembled to provide a perforating gun of the desired length, which may be several thousand metres long. The gun is mounted on the valve 16 on the lower end of the production string 12 and is lowered into the wellbore as the string 12 is made up. As described above, the gun is positioned in the lined portion of the wellbore which intersects the hydrocarbon-bearing formation, and completion testing operations and the like are then carried out on the string 12. Once the string 12 has been tested and is installed, the valve 16 is opened to activate the firing heads 18, 19. This results in the perforating charges 22 and the explosive cartridge 56 detonating. Detonation of the charges 22 perforates the tubing 28 at the scalloped locations 40 and also perforates the surrounding liner and a portion of the formation behind the liner. The detonation of the cartridge 56 blows the plug 54 from the lower end of the gun section 28c, and the detonation of the various charges breaks up the upper plug 24.

As the plugs 24, 54 seal the upper and lower ends of the gun, the interior of the gun remains at atmospheric pressure until detonation has occurred. Accordingly, on detonation, the pressure within the gun will be lower than the formation pressure, such that formation fluid will immediately flow through the perforated liner and tubing and into the gun, and then flow upwardly through the gun and the production string 12. This flow of fluid will wash away the debris remaining within the gun following detonation, and will also dissolve the polystyrene inner tubes 44. Thus, shortly after detonation, the gun will present an unobstructed throughbore, providing a large flow area, maximising the production capabilities of the bore, and also allowing test and logging apparatus and the like to be lowered into the gun.

Reference is now^' made to Figure 4 of the drawings, which illustrates a perforating apparatus 70 in accordance with a further embodiment of the present invention. The system shares a number of features in common with the first described embodiment and in the interest of brevity features common to both systems will not be described again in any detail. In a similar manner to the first described embodiment, the system 70 is mounted to the lower end of a production or test tubing string and is isolated from the string bore

72 by an isolation valve 74, similar to the full bore isolation valve (FBIV) as supplied by the applicant and as described in PCT\GB97\00308. The valve 74 forms part of an isolation and firing system 76 provided within a tubular body 78. Depending from the body 78 are a number of perforating gun sections 80 (only two illustrated) , each comprising a tubular body 82 containing a number of perforating charges 84. Each gun body 82 has a solid outer wall 86, the outer surface of the wall 86 being scalloped

(not shown) adjacent the charge locations. The ends of each body 82 define an external thread 88 for engaging a corresponding internal thread 90 defined on a connecting collar 92 which provides a pressure-tight connection between adjacent gun bodies 82.

The perforating charges 84 comprise an explosive charge contained within a zinc alloy body which, on detonation, is consumed and reduced to powder form. The charges 84 in each gun are mounted on a central cylindrical support 94 formed of balsa wood impregnated with an oxygen- containing self -combustible material; on detonation the supports 94 are consumed leaving minimal residue. The supports 94 each define a central bore 96 which accommodates three detonation cords 98 (only one shown) , each of which is linked to a respective helical row of charges 84. Appropriate connections, including booster charges 100, are provided between the detonation cords in adjacent guns 80.

As noted above, the guns 80 are provided below an isolation and firing system 76, which isolates the hydraulic pressure activated guns from the elevated fluid pressures experienced within the production string during completion testing, the setting of packers and the like. The system 76 includes a second isolation valve 102 provided between the valve 74 and the guns 80. The valve 102 is initially closed to isolate the gun firing system, which is located below the valve 102 in an atmospheric volume 103. However, the volume 104 between the valves 74, 102 is open to the wellbore annulus and therefore is at full hydrostatic pressure.

The lower isolation valve 102 has a concave valve member 106 which engages a valve seat 108 initially locked relative to the body 78 by a snap-ring 110 located in a profile 112 defined by a valve seat supporting sleeve 114. The snap-ring 110 abuts a collar 116 which is threaded to the body 78 and is held in the profile 112 by a piston sleeve 118 located between the sleeve 114 and the body 78. The upper face of the piston sleeve 118 is in communication with a pressure communicating bore 120 extending axially through the wall of the body 78. The upper end of the bore 120 is closed by a rupture disc 122 initially isolated from the hydrostatic pressure present in the bore annulus and the volume 104 by the lower end of a rod 124 coupled to the lower valve seat 126 of the upper isolation valve 74. With the valve 74 locked closed, the valve seat 126 is fixed in position relative to the body 78, however the seat 126 is biassed upwardly relative to the body by a spring 128 which tends to lift the seat 126 when the valve 74 is unlocked, as will be described.

The valve seat 108 of the second isolation valve 102 is also biassed upwardly by a spring 130, though as mentioned above the valve seat 108 is initially locked in the closed position by the snap-ring 110.

The lower portion of the isolation and firing system 76 accommodates three hydraulic pressure actuated firing guns 134, the location of the firing guns 134 being illustrated in Figure 4 where a section of the drawing has been rotated through 90° to illustrate the section on line 4a - 4a. The guns 134 are located in axial bores 136 in the body 78, the upper ends of the bores 136 intersecting cross bores for communication with the volume 103. However, initially the lower end of a sleeve 138 threaded to the valve seat supporting sleeve 114 isolates the bores 136 from the volume 103. The sleeve 138 is initially retained in its isolating position by shear pins 140.

In use, the system 70 is made up on surface and run into a cased wellbore on the lower end of a production tubing string. Once the string is fully assembled, and the guns 80 are at the desired location within the wellbore, completion testing, packer setting operations and the like are carried out on the string. These operations will include a setting of the packer between the string and the wellbore casing in the annulus above the system 70, such that the wellbore sump is then isolated from the upper annulus. The string is then filled with low density fluid, to reduce the hydrostatic pressure above the valve 74. The pressure above the valve 74 is then cycled to unlock the valve 74 and as pressure is bled off above the unlocked valve 74 the valve seat 126 will tend to move upwardly under the influence of the spring 128. As the seat 126 moves upwardly, the rod 124 is lifted, exposing the rupture disc 122 to hydrostatic pressure. The disc 122 is selected such that it will rupture at hydrostatic pressure, and when this occurs the hydrostatic pressure is applied to the piston sleeve 118, pushing the sleeve 118 downwardly and removing support from the snap-ring 110. The sleeve 114 is now released from the body 78, such that the hydrostatic pressure above the valve 102 will push the valve 102, valve seat 108, sleeve 114 and sleeve 138 downwardly into the atmospheric chamber 103 below the valve 102, against the action of the spring 130. This downward movement continues until the valve member 106 engages with a pair of sprung pins 144 mounted on the body 78.

The unlocking and downward movement of the valve 102 occurs while the upper isolation valve 74 remains in the closed position. However, as the pressure in the production string is further reduced, the valve 74 will open, leading to a reduction of pressure in the volume 104 above the valve 102. The spring 130 is then able to lift the valve seat 108, and as the valve member 106 is now engaged with the pins 144, upward movement of the seat 108 causes the valve member 106 to pivot about the pins 144 into a valve member receiving profile 146 formed in the inner wall of the body 78. The valve seat 108 continues to move upwardly to isolate the valve member 106 in the profile 146, and provide a smooth bore past open valve 102.

As the valve 102 is opened, the sleeve 138 is raised above its original position, exposing the firing gun bores

136 to hydrostatic pressure. The pressure activates the firing guns 134, commencing detonation of the perforating charges 84.

On the opening of the valve 74, the pressure within the volume 104 and in the sump will fall, due to the lower hydrostatic pressure above the valve 74. Accordingly, when the perforating charges are detonated the bore will be underbalanced, the advantages of which are described above.

On detonation, each charge 84 will perforate the adjacent portion of gun body 82 and will also perforate the casing located radially outwardly of the charge. At least some of the charges 84 are "Big Hole" charges, intended to produce relatively large perforations in the body 82 and assure a clear flow of production fluid into the body 82.

As the detonation cord 98, the zinc alloy charge bodies and the balsa wood charge supports 94 are combustible, the gun bodies 82 define an unobstructed bore following detonation, corresponding to the body wall internal diameter. Thus, production fluid is free to flow through the perforated casing and the perforated gun bodies 82 and then flow upwardly through the gun bodies 82 and the open valves 74, 102 into the production tubing.

It will be clear to those of skill in the art that the above-described embodiments are merely exemplary of the present invention, and that various modifications and improvements may be made thereto, without departing from the scope of the present invention. For example, in other embodiments, the perforating charges may be provided with profiles for engaging corresponding profiles formed in the tubing sections, obviating the need to provide an inner tube 44 or support 94 for supporting the charges. In other embodiments the gun body may define apertures prior to the charges being detonated, with the charges being located internally of the gun body wall and aligned with the apertures .

Claims

1. A downhole perforating apparatus comprising: a length of tubing having a wall defining a through bore for fluid communication with a tubular string; and perforating charges located internally of the tubing wall.

2. The perforating apparatus of claim 1, wherein the apparatus is mounted on the lower end of a tubular string with the through bore in fluid communication with the string bore .

3. The perforating apparatus of claim 1 or_^ 2 , wherein the charges are arranged in the tubing such that, following detonation of the charges, the tubing wall defines a substantially unobstructed bore.

4. The perforating apparatus of any of the preceding claims, wherein the tubing has a continuous wall which is perforated by the charges on detonation.

5. The perforating apparatus of claim 4, wherein at least some of the changes are adapted to produce large area perforation in the tubing.

6. The apparatus of claim 4 or 5 , wherein the tubing wall is relatively weak adjacent the charges, to facilitate perforation of the tubing.

7. The apparatus of any of claims 1 to 3 , wherein the tubing wall defines apertures and the charges are aligned with respective apertures.

8. The apparatus of claim 7, wherein the apertures are initially closed by covers.

9. The apparatus of any of the preceding claims, wherein the tubing wall is pressure-tight.

10. The apparatus of any of the preceding claims, wherein the charges are mounted on the tubing wall.

11. The apparatus of any, of the preceding claims, wherein the charges are mounted on supports arranged to disintegrate on detonation or shortly thereafter.

12. The apparatus of any of the preceding claims, wherein closures are provided in the ends of the tubing.

13. The apparatus of claim 12, wherein at least one of the closures is associated with a charge for detonation with the perforating charges to remove or destroy the closure.

14. The apparatus of any of the preceding claims, wherein the charges are linked by explosive transfer means extending through the tubing bore.

15. The apparatus of any of the preceding claims, wherein the tubing is formed of a plurality of separable tubing sections .

16. The apparatus of any of the preceding claims, wherein the apparatus includes firing means for initiating detonation of the charges.

17. The apparatus of claim 16, wherein the firing means is provided in combination with a valve for selectively isolating the body bore from internal string pressure.

18. The apparatus of claim 17, wherein the valve includes a valve portion movable on pressure being bled off above the valve and the valve opening, which movement of the valve portion releases a firing pin actuating arrangement .

19. The apparatus of claim 18, wherein the firing means includes a pin actuating arrangement incorporates a spring tending to bias the firing pin to a firing position, which spring is released by upward movement of the valve portion.

20. The apparatus of claim 18, wherein the firing means includes a firing pin actuatable by hydraulic pressure, and means for retaining the pin in a primed position which permits release of the firing pin on application of a predetermined fluid pressure thereto.

21. The apparatus of claim 17 or 20, wherein a further valve is provided between the isolating valve and a hydraulic pressure-actuated firing pin, the further valve being arranged to open following opening of the isolation valve, to provide a delay between opening the isolation valve and detonating the charges.

22. A downhole perforating apparatus comprising: a pressure tight length of tubing having a wall defining a through bore for fluid communication with a tubular string; and perforating charges within tubing.

23. A method of perforating a section of a bore wall, the method comprising the steps: providing a length of tubing having a wall defining a through bore with perforating charges located internally of the tubing wall; mounting the tubing on the lower end of an elongate support; running the tubing into a borehole and locating the tubing in the borehole adjacent a production formation; and detonating the charges to create flow ports in the tubing and to perforate the borehole wall, such that formation fluid may flow from the formation into the tubing through said flow ports.

24. The method of claim 23, wherein the tubing is mounted on the^' lower end of a tubular string with the through bore in fluid communication with the string, and following detonation of the charges production fluid flows directly from the tubing through bore into the production tubing.

25. The method of claim 23 or 24, wherein the detonating charges perforate a casing-lined borehole wall.

26. The method of claim 23 or 24, wherein the detonating charges perforate an unlined borehole wall.

27. The method of claim 26, wherein the tubing serves as borehole liner.

28. The method of any of claims 23 to 27, wherein, following detonation of the charges, the tubing defines a substantially unobstructed internal bore.

29. The method of any of claims 23 to 28, wherein one or both of the detonation of the charges and the flow of production fluid through the tubing clears debris from the tubing such that the tubing defines a substantially unobstructed internal bore .

30. The method of claim 29, wherein the tubing wall is perforated by the charges on detonation.

31. The method of any of claims 23 to 30, wherein the tubing wall is pressure- tight .

32. The method of claim 31, wherein the perforation operation is carried out underbalanced.

33. The method of any of claims 23 to 32, wherein the outer diameter of the tubing is selected to be only slightly smaller than the diameter of the hole wall to be perforated.

34. A method of perforating a section of a bore wall, the method comprising the steps: providing a length of tubing having a pressure-tight wall defining a through bore with perforating charges located in the tubing; mounting the tubing on the lower end of an elongate support; running the tubing into a borehole and locating the tubing in the borehole adjacent a production formation; and detonating the charges to create flow ports in the tubing and to perforate the borehole wall, such that formation fluid may flow from the formation into the tubing through said flow ports.