Abstract:
Apparatus for perforating a section of liner intersecting a hydrocarbon-bearing formation comprises a length of tubing, the wall of the tubing defining a plurality of apertures, and perforating charges being located in the apertures. The tubing is adapted for mounting on the lower end of a length of production or test tubing such that the formation fluid may flow into the tubing and then directly into the production or test tubing. The charges disintegrate on detonation to leave the apertures unobstructed and to form light or small parts which may be swept out of the well by the formation fluid. Following detonation of the charges, the flow area of the tubing corresponds to the tubing internal diameter.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The application is a divisional of Ser. No. 09/125,583, filed Feb. 17, 1999 and presently pending which is a 371 of PCT/GB97/00495 Feb. 21, 1997. 
    
    
     This invention relates to downhole apparatus, and in particular but not exclusively to apparatus for use in sealing and locating a length of tubing within a casing-lined borehole. The invention also relates to a perforating system. 
     BACKGROUND OF THE INVENTION 
     Boreholes drilled to gain access to underground hydrocarbon-bearing formations are typically lined over most of their length by steel casing. If tests are to be carried out on a hydrocarbon-bearing formation, or oil or gas is to be extracted from the formation, test or production tubing is lowered into the borehole, and fluid communication with the surface is achieved through the tubing. Conventionally, the tubing is located relative to the casing, and the annulus between the casing and the tubing sealed, using one or more expandable or inflatable packers. Such packers are set when the tubing is in position in the borehole by, for example, inflating the packers with pressurised well fluid. Such setting operations may be time-consuming and often encounter difficulties. Further, the tubing consists of a plurality of threaded sections and the tubing must be tested for pressure integrity as the sections are made up and lowered into the borehole. Such “completion” testing is achieved by pressurising the tubing using well fluid, which may result in inflation and premature setting of the packers. 
     It is among the objects of embodiments of the present invention to provide an apparatus and a method for sealing and locating tubing in casing which obviate or mitigate the above-mentioned disadvantages. 
     The section of casing or liner which intersects the hydrocarbon-bearing formation is initially solid, to prevent the production fluid from flowing into the bore until the production tubing is in place and all of the associated apparatus and systems have been prepared. The liner is perforated by explosive charges or guns, typically spaced individual charges which are lowered into the bore and detonated at an appropriate location. The charges may be lowered into the bore on electric wireline, slickline or coiled tubing. However, as the length of the perforating guns which may be used 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, a perforating operation will tend to involve a number of runs and thus 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 higher pressure formation fluid. In the absence of such a pressure differential the 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 which requires a number of runs only the first perforating operation may be underbalanced. 
     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 charges and firing arrangements which occupy the wellbore following the perforating operation reduces the internal area of the tubing, thus reducing the production capability of the well. This debris may be milled out, such that it falls to the bottom of the well. However, to accommodate the debris from several thousand meters of perforating guns requires the drilling of a substantial extra section of bore, which may take several weeks&#39; drilling, adding substantially to the drilling cost for the wellbore. 
     It is among the objectives of embodiments of the present invention to obviate or mitigate these difficulties. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is providing perforating apparatus comprising a length of tubing, the wall of the tubing defining a plurality of apertures, and perforating charges being located in the apertures. 
     The invention also relates to a perforating method utilising such apparatus. 
     In use, when the charges are detonated, the charges disintegrate to leave the apertures clear and to permit formation fluid to flow through the apertures into the tubing. The use of tubing as a mounting for the charges allows a perforating “gun” of considerable length (typically 4000 to 7000 m) to be provided, such that a wellbore may be perforated in a single operation, facilitating underbalance perforating. 
     The tubing is preferably mounted on the lower end of a length of production or test tubing such that the formation fluid may flow into the tubing and then directly into the production or test tubing. 
     Preferably, the charges disintegrate on detonation to form light or small parts which may be swept out of the well by the formation fluid. 
     Preferably also, following detonation of the charges the flow area of the tubing corresponds to the tubing internal diameter. 
     Preferably also, each charge is locatable in a respective aperture from the tubing exterior. Each charge may include a cap adapted to engage with the respective aperture. 
     Preferably also, the charges are linked by explosive transfer means for communicating a detonation signal to each charge. Most preferably, the explosived 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 cored. 
     Preferably also, the tubing is provided in separable tubing sections, each section carrying a number of charges. The sections may be connected by any suitable means, but are preferably connected by threaded collars rotatably mounted on the end of one section for engaging a corresponding threaded portion on the end of an adjacent section. Preferably also, the sections are provided with connectors for explosive transfer means for linking the charges in adjacent guns. Most preferably, the connectors include booster and may define female booster connection and receive a respective end of a central male booster connection portion. Most preferably, the male booster connection portion may be located in the female booster connections after the tubing sections have been placed end-to-end. 
     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. Most preferably, 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 may itself by hydraulic pressure actuated, and may be initially retained in a primed position by a rupture disc or retainer which is releases the firing pin on application of a predetermined fluid pressure thereto. 
     Preferably also, a plug is provided at the end of the tubing, which plug is blown from the tube when the charges are detonated. 
     According to the another aspect of the present invention there is provided apparatus for locating and sealing tubing in a casing-lined borehole, the apparatus comprising: 
     a length of tubing; 
     a sleeve mounted on the tubing; and 
     the sleeve carrying landing means for engaging a restriction in the casing, locking means for locking the sleeve relative to the casing, and sealing means for sealing the sleeve relative to the casing. 
     In use, the sleeve may serve an equivalent function to a conventional packer, that is locating and sealing the tubing relative to the casing (as used herein, the term “casing” is intended to encompass any liner provided in a borehole). The tubing may be in the form of test tubing or production tubing. 
     Preferably, the sleeve is formed of a rigid material, typically steel. Thus, it is relatively easy to provide fluid communication passages, or control lines, through the sleeve. 
     Preferably also, the sleeve is releasable retained on the tubing, such that the tubing may be move relative to the “set” sleeve and may be retrieved from the borehole while the sleeve remains fixed in the casing. 
     Preferably also, at least one of the sleeve and tubing carries a seal for slidably engaging the other of the sleeve and tubing. 
     Preferably also, the sleeve defines means for engaging a retrieval tool: such means may be in the form of a J-slot, such that a tool may be lowered and manipulated to engage the sleeve, further manipulated to release the locking means, and then pulled to retrieve the sleeve. 
     Preferably also, the landing means is defined by a landing sleeve. Most preferably, the sealing means and locking means are carried by the landing sleeve. In the preferred embodiment, the sealing means and locking means are activated by upward longitudinal movement of the landing sleeve relative to the sleeve on the landing sleeve engaging and being restrained against further longitudinal movement by its engagement with a casing restriction. The landing sleeve may be initially releasably retained relative to the sleeve by, for example, a shear pin or bolt. Preferably, ratchet means are provided between the landing sleeve and the sleeve for maintaining the relative longitudinal positioning therebetween. The ratchet means may be releasable by rotation of the sleeve relative to the landing sleeve; on releasing the ratchet means the landing sleeve is free to move relative to the sleeve and the locking means and the sealing means may be de-activated, releasing the sleeve from the casing. 
     Preferably also, the casing defines the restriction, and further may define profiles for receiving and cooperating with the locking means and sealing means. 
     In one embodiment of the invention perforating guns may be mounted on the lower end of the tubing. Preferably, the guns are mounted on hollow tubing of the same or similar internal diameter to the tubing. Most preferably, the guns are full-bore, with strip gun-type charges embedded into hollow tubing. Alternatively, the guns may be mounted on the sleeve itself; the sleeve is capable of supporting a large amount of weight, and the guns will not then restrict the bore diameter and will permit tubing to be run into the bottom of the sump. The perforating guns may be made in accordance with the first aspect of the present invention. 
     According to another aspect of the present invention there is provided a method of sealing and locating tubing in a casing-lined borehole, the method comprising: 
     locating a sleeve on a length of tubing with a seal therebetween; 
     running the tubing into a borehole lined with casing until the sleeve engages a restriction in the casing, the engagement with the casing activating sealing means and locking means on the sleeve to sealingly locate the sleeve in the casing; and 
     releasing the sleeve from the tubing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic representation of the lower portion of a borehole including apparatus in accordance with an embodiment of one aspect the present invention; 
     FIG. 2 is an enlarged sectional view of the apparatus of FIG. 1, during run-in; 
     FIG. 3 is a further enlarged sectional view of a portion of the apparatus of FIG. 1; 
     FIG. 4 is a representation of a retrieval J-slot defined on the apparatus of FIG. 1; 
     FIG. 5 is a side view of the lower end of a perforating gun section of perforating apparatus in accordance with another aspect of the present invention; 
     FIG. 6 is a sectional side view of the upper end of a perforating gun section; 
     FIG. 7 is a sectional side view of two connected gun sections; 
     FIGS. 8 a ,  8   b  and  8   c  are half sectional views of a part of the firing system for the gun sections of FIGS. 5 and 6; and 
     FIGS. 9 a  and  9   b  are half sectional views of a further part of the firing system for the gun sections of FIGS.  5  and  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is first made to FIG. 1 of the drawings, which illustrates the lower portion of a borehole  10  and including apparatus  12  in accordance with an embodiment of one aspect of the present invention. The Figure shows the lower end of the borehole casing  14  which lines the borehole  10  over the majority of its length and is set in the borehole using concrete. The casing  14  stops short of the end of the borehole  10  which is initially sealed by a liner  16  located relative to the casing  14  by a liner hanger and seal  18 . The liner  16  extends into the oil-bearing formation and is perforated, as will be described, to allow oil to flow from the formation into the borehole  10 . The oil is carried to the surface through production tubing  20  which, in this embodiment, extends to the lower end of the borehole  10 . The production tubing is located relative to the casing  14  by a retrieval sleeve  22  forming part of the present invention. The retrieval sleeve  22  is located and sealed relative to the casing  14  by locking means  24  and sealing means  26 , as illustrated schematically in FIG.  1 . 
     Reference is now also made to FIGS. 2 and 3 of the drawings, which illustrate the sleeve  22  in greater detail. The sleeve  22  is initially carried by a section of the tubing  20  and is run into the borehole  10  on the tubing  20 ; FIGS. 2 and 3 illustrate the sleeve still fixed relative to the tubing  20 . 
     In this particular embodiment the tubing  20  carries perforating guns for perforating the liner  16 , the guns forming the lower end of the tubing  20  and including a large number of strip gun type charges (not shown) located in corresponding apertures  32  in the tube  20 . The charges disintegrate following detonation, leaving the apertures  32  as illustrated. The guns are in accordance with one embodiment of another aspect of the present invention, a further embodiment of this aspect of the invention being described separately, with reference to FIGS. 5 to  8  of the drawings. 
     The sleeve  22  is initially retained on the tubing  20  by a shear pin  34  and a seal is provided between the tubing  20  and the sleeve  22  by completion seals  28 . 
     The sleeve  22  itself carries a landing sleeve  36  which is initially fixed to the sleeve  22  by a shear pin  38 . The landing sleeve  36  defines a shoulder  40  for engaging a corresponding shoulder  42  defined by a casing restriction  44 . Above the restriction  44  the casing defines two profiles  46 ,  47  for receiving the sleeve lock means and seal means in the form of a split lock ring  24  and a radially expandable seal  26 . The inner face of the landing sleeve  36  defines a ratchet thread  50  for engaging a ratch ring  52  mounted on the sleeve  22 . 
     The upper end of the sleeve  22  defines a retrieval J-slot  54 , shown in section in FIG.  3  and also illustrated in FIG. 4 of the drawings. 
     The sleeve  22  is set in the casing  14  simply by running the tubing  20  and sleeve  22  into the borehole until the landing sleeve shoulder  40  engages the casing shoulder  42 . The landing sleeve  36  is thus restrained against further downward movement. Following shearing of the pin  38 , the tubing  20  and sleeve  22  continue to move downward relative to the landing sleeve  36  and this relative movement energises the split lock ring  24  and the seal  26 . The relative positioning of the sleeves  36 ,  22  is maintained by the engagement of the ratchet thread  50  and ratch ring  52 . Application of further weight to the tubing  20  results in the pin  34  shearing, such that the tubing  20  may now be moved longitudinally relative to the set sleeve  22 . 
     In this particular embodiment, once the sleeve  22  is set, the perforating guns may be located in the liner  16  and the charges detonated to perforate the liner  16  . As noted above, the charges will disintegrate following detonating, such that production fluid may then flow through the perforated liner  16  and the apertures  32  into the bore of the tubing  20  and then the surface. 
     If the tubing  20  and guns are to be retrieved from the borehole  10 , it is merely necessary to pull the tubing  20  upwardly, through the sleeve  22 . If the sleeve  22  is to be retrieved, a retrieval tool is lowered into the borehole  10  and manipulated to engage the J-slot  54 . The sleeve  22  may then be rotated relative to the landing sleeve to disengage the ratchet thread  50  and ratch ring  52 . This de-energises the split lock ring  24  and seal  26  such that the sleeve  22  may be pulled from the borehole. 
     Reference is now made to FIGS. 5 to  9  of the drawings, which illustrate elements of a perforating system in accordance with an embodiment of a further aspect of the present invention. The apparatus comprises a tubular perforating gun, made up of a plurality of gun sections  60 ,  61 , comprising a section of tubing  62 ,  63 . Apertures  64  are formed in each section of tubing  62 ,  63 , the apertures  64  being arranged in six longitudinal rows (only three rows being visible in FIG.  5 ). Each aperture  64  accommodates a perforating charge  66  located in the respective apertures  64  by an internally and externally threaded plastic cap  68 . The charges  66  in each row are connected by a respective detonation cord  72 . As may be seen in FIG. 6, each tubing end  63  is provided with a sleeve  74  which supports a pair of explosive transfer boosters or connectors  76 ,  77 , each of which communicates with three detonation cords  72 . The boosters  76 ,  77  are adapted to co-operate with corresponding boosters provided in the lower end of the adjacent tubing section  62 , a male booster in the form of a shaped charge  79  being provided to link the adjacent boosters. 
     To minimise the effect of poor or faulty links between the boosters in adjacent gun sections the boosters in each section connect to different cords  72 . In the absence of this feature it would be possible for a single faulty link to prevent detonation of half of the charges below the fault. However, by varying the cord connections the effects of a faulty link will be minimised as the detonation signal will travel back up the cords from a link below the fault. 
     The adjacent ends of the tubing sections  62 ,  63  are connected by means of a threaded sleeve  78  which is initially rotably mounted on the upper end of the tubing section  63  and defines windows through which the shaped charges  79  may be passed for location in the slotted boosters  76 ,  77 . The sleeve  78  defines an internal thread  80  which is made up to a corresponding external thread  81  on the tubing section  62 . Once the threads are made up, grub screws are inserted in threaded bores  82  in the sleeve  78  to lock the sleeve against rotation. 
     Reference is now made to FIGS. 8 a ,  8   b ,  8   c  and  9   a  and  9   b  of the drawings which illustrate details of the firing system for the gun sections  60 ,  61 . This embodiment of the invention is intended for use with the applicant&#39;s full bore isolation valve (FBIV) as described in PCT\GB97\00308. Part of the valve is illustrated in the upper portion of the Figures, the valve including a closure member  84  which is initially held against a lower valve seat  87  by a locked retaining sleeve  88 . The valve remains closed while completion testing is carried out on the tubing above the perforating apparatus, and after a predetermined number of pressure cycles the retaining sleeve  88  is unlocked so that it may be retracted by application of bore pressure. As the sleeve  88  is retracted the closure member  84  remains in contact with the valve seat  87  due to the pressure differential across the closure member  84 . However, once pressure is bled off above the valve, the closure member  84  opens, and the sleeve  88  is then extended into contact with the valve seat  86 , to provide a slick bore. Once the closure member  84  has opened the valve seat  86  may move axially upwardly relative to the valve body  90  under the influence of a spring  92 , to allow initiation the firing heads of the perforating system, as described below. 
     FIG. 8 a  illustrates the valve in the closed position, with the valve seat  86  being held axially relative to the valve body  90  by the locked retaining sleeve  88 . The valve seat  86  is formed on the upper end of the sleeve  94 , the lower end of which engages the upper end of the spring  92 . The lower end of the spring  92  engages a shoulder formed on fingers  98  which extends upwardly between the valve body  80  and the valve seat sleeve  94 . The upper end of the fingers  98  are held relative to the valve body  90  by a split ring  100  which is radially supported by the valve seat sleeve  94  to engage with a profile  102  in the valve body  90 . The lower end of the fingers  98  provide mounting for a firing pin  104  which extends through a portion of the valve body and is positioned above a firing head  106 . The firing head connects to the detonation cord. 
     On the valve opening, and the valve seat  87  and valve seat sleeve  94  moving upwardly relative to the body  90 , an external profile on the sleeve  94  is positioned at the rear of the split ring  100 , allowing the ring  100  to collapse inwardly and the fingers  98  to move downwardly under the influence of the spring  92 . The downward movement of the finger  98  and firing pin  104  brings the end of the firing pin  104  into contact with the firing head  106 . This contact initiates detonation of the charges  66 , which will normally occur two to three minutes after the contact taking place. 
     FIG. 8 a ,  8   b  and  8   c  illustrates a mechanical firing arrangement, and a somewhat similar firing arrangement is also provided on the apparatus, where movement of a firing pin completes an electrical connection to initiate electrical firing of the charges. Further, the apparatus also includes a hydraulically initiated firing system, as illustrated in FIGS. 9 a  and  9   b  of the drawings. A hydraulic firing pin  108  is provided in the valve body  90  and is initially fixed to the valve body  90  by a rupture disk  110 . The upper face of the rupture disk  110  is in communication with the valve bore  112  via a port  114  and a longitudinal passage  115 . Thus, if the mechanical or hydraulic firing system should fail, an increase in bore pressure will rupture the disk  110  allowing the firing pin  108  to be pushed downwardly by fluid pressure to engage the respective firing head  118 . All of the firing systems may be operated simultaneously, or the systems may be arranged such that they operate individually. 
     It is preferred that when the charges  66  are detonated the system is underbalanced, that is the fluid pressure within the gun section  60 ,  61  is lower than the formation pressure, such that the production fluid will tend to wash the debris of the detonated charges  66 , cord  72 , and boosters  76 ,  77 ,  79  upwardly and out of the tubing. Accordingly, if the hydraulically initiated firing system is utilised, the two to three minute delay between the contact of the firing pin  108  with the firing head  118  and the detonation of the charge is utilised to bleed off pressure from the tubing. 
     As noted above, when detonation of the charges  66  occurs, the individual charges  66  break up to leave the apertures  64  clear, and the detonation also breaks up the cord  72  and the boosters  76 ,  77 ,  79 . The resulting debris is made up of small, relatively light parts, which may then be washed from the tubing by the formation fluid which flows into the gun sections  60 ,  61 . 
     It will be clear to those of skill in the art that the above-described embodiments of the present invention provides a cost effective and safe means of perforating a large interval of liner. Mounting the perforating guns on tubing as described above obviates the requirements to run perforating guns separately on wireline, coil tubing or the like and thus saves considerable time. Further, the sleeve  22  of the first described embodiment has a relatively small radial dimension when compared to a conventional packer, such that the gun charges are located close to the liner  16  and thus act more effectively when detonated. Further, in the first described embodiment, the tubing  20  and guns may be released from the sleeve without difficulty, which option is generally not available with conventional packers. In addition, the set sleeve  22  may be retrieved from the borehole  10  without difficultly, in contrast to conventional packers which generally have to be milled out. 
     It will be clear to those of skill in the art that the apparatus of the embodiments of the present invention may be employed in may other applications in which some or all of the advantages outlined above may be usefully applied. 
     Further, the use of a solid metal sleeve  22  facilitates provision of fluid communication lines through the sleeve. It will also 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 invention, for example the perforating guns may be mounted on an extended sleeve  22 , rather than on the tubing  20 , and the perforating guns may be utilised in combination with conventional tubing and inflatable packers.