Perforating apparatus incorporating a shock absorber

Abstract: A shock absorber (38) is adapted to be disposed within a perforating gun string (32) or within the tubing string above the perforating gun (30) and includes an energy absorbing element (16,18) adapted to absorb and store mechanical energy during detonation of the perforating gun and to permanently deform in response to the storage of the mechanical energy, the stored energy being released in the form of heat, and not in the form of kinetic energy. Therefore, following absorption of the mechanical energy by the shock absorber, no further expansion of the shock absorber is experienced. The shock absorber includes an inner housing (12), an outer housing (10), a connection (14) for interconnecting the inner and outer housing, and a break up charge (20) for breaking the connection and releasing the inner housing from the outer housing when the perforating gun is detonated whereby the shock absorber is as strong as the tubing string before the connection is broken and is flexible after the connection is broken. The energy absorbing element (16,18) may be a damping coil or it may be a honeycomb. The damping coil and honeycomb energy absorbing elements permanently deform when mechanical energy is absorbed; therefore, the stored mechanical energy is subsequently released in the form of heat and not in the form of kinetic energy.

Description: BACKGROUND OF THE INVENTION The present invention relates to perforating apparatus incorporating a shock absorber, and more particularly, provides a shock absorber which is incorporated in a perforating gun string and which includes a collapsible energy absorbing element adapted to permanently deform when absorbing shock. Perforating guns are adapted to be disposed in a wellbore for perforating a formation. Well fluids flow from the perforated formation. When the perforating gun fires, a shock is received in the tubing string above the perforating gun. A shock absorber is usually incorporated in the tubing string above the perforating gun for absorbing the shock. The shock absorber usually includes a spring which stores mechanical energy by compression in response to the shock and releases the mechanical energy by expansion following compression over a longer period of time such that the force exerted is reduced. Although this configuration absorbs mechanical energy associated with the shock, attempts to improve this shock absorber have focussed on achieving a smoother release of the mechanical energy from the spring coil shock absorber system following storage of the mechanical energy. However, the problem associated with the release of the mechanical energy could be eliminated entirely if the absorbing element in the shock absosrber did not expand following compression but, instead, released the stored energy in a different form, such as heat. US-A-4 905 759 discloses a collapsible perforating gun assembly, comprising a spacer mandrel having two telescopically collapsible parts which are initially prevented from relative axial movement, but which are freed to telescope together during firing of the perforating gun. The purpose of this arrangement is to reduce the length of the assembly after firing, and no mechanism for absorbing the shock resulting from firing is disclosed. US-A-3 856 335 discloses a rolling diaphragm slip joint for inclusion in a casing string leading to an underground nuclear test site. The shock caused by the explosion is partly absorbed by plastic deformation of the rolling diaphragm. It is an object of the present invention to provide perforating apparatus having improved shock absorbing characteristics. SUMMARY OF THE INVENTION According to one aspect of the present invention, there is provided perforating apparatus for use at the lower end of a tubing string in a well bore, the apparatus comprising: a perforating gun for perforating the formation around the well bore; a detonating cord for firing said perforating gun; and shock absorbing means between the perforating gun and the tubing string for absorbing the shock produced by the firing of said perforating gun; characterised in that the shock absorbing means comprises: inner and outer housing members which are initially rigidly connected together; means responsive to the detonation wave which propagates along the detonating cord during the firing of the perforating gun to break the connection between the housing members and thereby render them relatively axially movable; and collapsible energy absorbing means arranged to be permanently deformed by the relative axial movement of the housing members, whereby to absorb said shock. According to another aspect of the present invention, there is provided a method practised by a shock absorber adapted to be connected to a well apparatus for absorbing shock, the shock absorber including an energy absorbing element, comprising the steps of: breaking a connection between an outer housing and an inner housing of said shock absorber immediately prior to said shock, the inner housing being released from the outer housing when the connection is broken; receiving said shock in said energy absorbing element of said shock absorber; storing mechanical energy associated with said shock in said energy absorbing element; and subsequently releasing the stored energy in the form of heat and not in the form of kinetic energy. Further scope of applicability of the present invention will become apparent from the detailed description presented hereinafter. It should be understood, however, that the detailed description and the specific examples, while representing a preferred embodiment of the present invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become obvious to one skilled in the art from a reading of the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS A full understanding of the present invention will be obtained from the detailed description of the preferred embodiment presented hereinbelow, and the accompanying drawings, which are given by way of illustration only and are not intended to be limitative of the present invention, and wherein: figure 1 illustrates an optimum theory associated with shock absorption in a perforating gun string; figure 2a illustrates a perforating gun including a firing head disposed on an end of a tubing string, and an energy absorbing element shock absorber disposed below the firing head within the perforating gun; figure 2a(1) illustrates the shock absorber of figure 2a in greater detail; figure 2b illustrates a perforating gun including a firing head disposed on an end of a tubing string, and an energy absorbing element shock absorber disposed above the firing head of the perforating gun; figure 3 illustrates an energy absorbing element adapted to be disposed within the shock absorber; figure 4 illustrates perforating apparatus in accordance with the present invention, comprising a novel shock absorber adapted to be incorporated below a perforating gun firing head and within a perforating gun string, the shock absorber including a damping coil mechanical energy absorbing element; figures 5a and 5b illustrate the shock absorber of figure 4 during and after detonation of the perforating gun; figure 6 illustrates perforating apparatus in accordance with another embodiment of the present invention and comprising a further novel shock absorber adapted to be incorporated above a perforating gun firing head and within a tubing string, the shock absorber including a honeycomb mechanical energy absorbing element; figure 6a is a cross-section of the shock absorber of figure 6, taken along section lines 6a-6a of figure 6; and figure 7 illustrates a plurality of graphs of force vs displacement for various types of energy absorbing shock absorbers. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Perforating guns are utilized in well logging for perforating a formation traversed by a borehole, well fluids being produced from the perforated formation. The perforating guns contain shape charges; when the shape charges detonate, the formation is perforated; however, a shock is generated from the gun, the shock propagating up the gun string. In order to reduce the severity of the shock, shock absorbers are usually incorporated within the tubing string above the perforating gun. All such shock absorbers to date absorb mechanical energy and subsequently release the mechanical energy in the form of kinetic energy. It has been important to carefully analyze the release of mechanical energy since an abrupt release of the mechanical energy may produce still another shock. Typical prior art shock absorbers store mechanical energy during absorption of a shock and subsequently release the mechanical energy in the form of kinetic energy. For example, in a standard spring shock absorber, the mechanical energy is stored during compression of the spring and is released in the form of kinetic energy during expansion of the spring. Shock severity may be reduced by storage of the input energy and its release in a "smoother" form over a longer period of time. For example, referring to figure 1, the energy input "IN" to a shock absorber system is shown by the first energy pulse, and the energy released "OUT" from the shock absorber system is shown by the second energy pulse. Note that the second energy pulse "OUT" illustrates a relatively flat amplitude pulse, the amplitude of the second pulse being smaller than the amplitude of the first pulse thereby indicating a release of the mechanical energy in a smoother form over a longer period of time. Shock absorbers of the prior art released their stored mechanical energy in the form of kinetic energy. Improvements to the shock absorbers of the prior art have primarily involved generating a smoother release of the stored mechanical energy in the form of kinetic energy. However, the shock absorber of the perforating apparatus of the present invention ultilizes a different principle of operation; that is, it is a "single event" shock absorber, one which receives mechanical energy during energy absorption but does not subsequently release the stored mechanical energy in the form of kinetic energy; instead, it releases the stored mechanical energy in the form of heat. This permits the shock absorber to be incorporated within the perforating gun string as well as within the tubing string above the perforating gun. Referring to figures 2a and 2a(1), a shock absorber in perforating apparatus in accordance with the present invention is disposed below a firing head of a perforating gun and within the perforating gun string. In figure 2a, a perforating gun 30 is connected to one end of a tubing string 32 in a borehole and an isolation packer 34 is disposed within the tubing string 32 above the perforating gun 30; when the packer 34 is set, an interval between the tubing string and a wall of the borehole above the packer is isolated from an interval between the tubing and the wall of the borehole below the packer. A gun release sub 50, a debris circulating sub 52, a drop bar firing head assembly 36, and a "single event" shock absorber assembly 38 are disposed between the perforating gun 30 and the isolation packer 34 on the tubing 32. The firing head assembly 36 is disposed above the perforating gun 30, and the "single event" shock absorber assembly is disposed below the firing head 36 and within the perforating gun 30 (and not within the tubing string above the firing head). In figure 2a(a), the shock absorber assembly 38 contains an energy absorbing element (not shown) disposed within a space 38a of the shock absorber 38, the energy absorbing element storing mechanical energy during shock absorption, and subsequently releasing the stored energy in the form of heat (not kinetic energy). As a result, since the shock absorber 38 is not located above the firing head 36 within the tubing string 32, fullbore access to the firing head 36 is available to a user at the well surface. Figure 2b shows perforating apparatus in accordance with the present invention, in which a shock absorber is disposed above a firing head of a perforating gun and within the tubing string. In figure 2b, a perforating gun 30 is connected to one end of a tubing string 32 in a borehole and an isolation packer 34 is disposed within the tubing string 32 above the perforating gun 30; when the packer 34 is set, an interval between the tubing string and a wall of the borehole above the packer is isolated from an interval between the tubing and the wall of the borehole below the packer. A "single event" shock absorber 38, a gun release sub 50, a debris circulating sub 52, and a drop bar firing head assembly 36 are disposed between the packer 34 and the perforating gun 30 on the tubing 32. The "single event" shock absorber assembly 38 is disposed above the firing head 36 of perforating gun 30 and between the gun release sub 50 and the packer 34 within the tubing 32. Since the shock absorber 38 is a "single event" type, it can be equally effective, relative to the shock absorber of figure 2a, in absorbing shock when disposed above the firing head 36 within the tubing string 32. The shock absorber of figure 2b also includes a space 38a in which a "single event" energy absorbing element is disposed. The term "single event" connotes the absorption of mechanical energy resultant from a shock produced during detonation of the perforating gun, but not the release of the stored mechanical energy in the form of kinetic energy. Referring to figure 3, one embodiment of a "single event" energy absorbing element, adapted to be disposed within space 38a of figure 2a(1), is illustrated. In figure 3, the energy absorbing element comprises a hollow damping coil 18. When a compressive force is applied to both of the ends of the hollow coil 18, the hollow coil 18 will permanently deform. The coil 18 will not expand following compression; therefore, the stored mechanical energy is not subsequently released in the form of kinetic energy; rather, the stored energy will be released in the form of heat. Referring to figure 4, a detailed construction of the shock absorber 38 of figure 2a, designed to be fit below the firing head assembly 36 and within the perforating gun 30, is illustrated. In figure 4, one embodiment of the shock absorber 38 of figure 2a is shown, and comprises an outer housing 10 having one end including a first inwardly disposed transverse member 10a, an inner housing 12 which includes a second transverse member 12a transversely disposed with respect to the inner housing 12 and having a surface in contact with an inner surface of the outer housing 10, and a joining member 14 which joins the outer housing 10 to the inner housing 12, the joining member 14 including an inner piece 14a forming an integral part of the inner housing 12, an outer piece 14b having one end integrally joined to the inner piece 14a, and a third transverse member 14c integrally joined to the other end of the other piece 14b, the third transerse member 14c contacting an inner surface of the outer housing 10. A first space is defined between the inner housing 12 and the outer housing 10 by the first inwardly disposed transverse member 10a of the outer housing 10 and the second transverse member 12a of the inner housing 12; a first energy absorbing element 16, otherwise termed a damping coil 16, is disposed within the first space. A second space is defined between the inner housing 12 and the outer housing 10 by the second transverse member 12a of the inner housing 12 and the third transverse member 14c of the joining member 14; a second energy absorbing element, or damping coil, 18 is disposed within the second space. The first and second damping coils 16 and 18 may each be made of aluminum or stainless steel. Each damping coil 16 and 18 has a hollow interior such that the damping coil will collapse and permanently deform when a compressive force of a predetermined magnitude is applied to the coil. A break up shape charge 20 is disposed within the inner housing 12, and a detonating cord 22 passes through the center of the break up charge 20. As will be more apparent with reference to figures 5a-5b, the break up shape charge 20 detonates when a detonation wave propagates along the detonating cord 22 and through the shape charge 20, the shape charge 20 severing the inner piece 14a of the joining member 14 into two parts thereby separating the inner housing 12 from the outer housing 10. Before the inner housing 12 is separated from the outer housing 10 by the shape charge 20, the shock absorber 38 is as strong as the tubing string 32; however, after the inner housing 12 is separated from the outer housing 10 by the break up shape charge 20, the shock absorber 38 is as flexible as any other shock absorber and therefore functions as a shock absorber. A functional description of the shock absorber 38 of figures 2a, 2a(1) and figure 4 will be set forth in the following paragraphs with reference to figures 4, 5a and 5b of the drawings. In figures 4, 5a, 5b, the shock absorber is incorporated below firing head 36 within a perforating gun string. The perforating gun 30 includes a plurality of shape charges. In figure 4, the shock absorber is shown before detonation of the shape charges disposed within the perforating gun; in figure 5a, the shock absorber is shown during detonation of the charges; and, in figure 5b, the shock absorber is shown after detonation of the perforating gun charges. In figure 4, the shock absorber is shown undisturbed, since a detonation wave has not yet propagated along detonating cord 22, and none of the shape charges of the perforating gun have detonated. In figure 5a, a detonation wave propagates along detonating cord 22 indicating that the plurality of shape charges in the perforating gun are either detonating or are about to detonate. when the detonation wave passes through the center of the break up charge 20 in figure 5a, the charge 20 cuts the joining member 14 into two pieces (e.g., severs the inner piece 14a into two pieces) thereby separating the inner housing 12 from the outer housing 10. In figure 5a, the breakup charge 20 is shown cutting the joining member 14 into two pieces, but the shock from the detonation of the perforation gun has not yet been received. In figure 5b, the joining member 14 has been cut, the inner piece 14a being shown as separated from the outer piece 14b of the joining member 14. As a result, inner housing 12 is separated from outer housing 10. In addition, a shock from the detonated perforating gun has been received, the shock causing the inner housing 12 to move upwardly in figure 5b relative to the outer housing 10. The second transverse member 12a of the inner housing 12 moves toward the third transverse member 14c of the joining member 14 thereby crushing the second damping coil 18 disposed within the second space. As a result, the second damping coil 18 has collapsed and is now permanently deformed. Although mechanical energy was stored in the damping coil 18 during compression, since the damping coil 18 has collapsed and is permanently deformed, no expansion of the coil 18 will occur; therefore, the mechanical energy is not released in the form of kinetic energy; rather, it is released in the form of heat. Referring to figures 6 and 6a, a detailed construction of the shock absorber 38 of figure 2b, designed to be fit above the firing head assembly 36 within the tubing string 32, is illustrated. While the shock absorber 38 of figures 4, 5a, 5b was designed to fit below the firing head 36 and within the perforating gun 30, the shock absorber 38 of figure 6 is designed to fit within the tubing string 32 above the firing head 36. The only other significant difference between the shock absorber 38 of figures 4, 5a and 5b and the shock absorber 38 of figure 6 is the specific structure of the energy absorbing element adapted to fit within space 38a of figure 2a(1). Whereas the damping coil 18 of figure 4 was the energy absorbing element used in connection with the shock absorber of figures 4, 5a and 5b, a corrugated honeycomb 40 is the energy absorbing element used in connection with the shock absorber of figure 6. Figure 6a illustrates the cross-sectional structure of the honeycomb 40 of figure 6, figure 6a being a cross section of the shock absorber 38 of figure 6, taken along section lines 6a-6a of figure 6. In figure 6a, note the "corrugated" structure of the honeycomb energy absorbing element 40 of figure 6. In fact, there are a plurality of layers of the corrugated structure 40 in figure 6a, each corrugated layer being disposed on top of its adjacent corrugated layer, the plurality of corrugated layers 40 collectively comprising the honeycomb energy absorbing element adapted to fit within space 38a of the shock absorber 38 of figure 2b. When the perforating gun charges detonate, the honeycomb 40 energy absorbing element absorbs mechanical energy and permanently deforms, the deformation being the same as that illustrated in figure 5b. Mechanical energy is absorbed and stored during the deformation of honeycomb 40; however, the stored energy is released in the form of heat, and not in the form of kinetic energy. Referring to figure 7, a plot of force vs. displacement for various types of energy absorbing elements disposed in a shock absorber is illustrated, the energy absorbed by a particular energy absorbing element being equal to the area under its curve. In figure 7, a prior art rubber elastomer energy absorbing element is illustrated as having the worst energy absorption, since the area under its curve is the least as compared to a spring element, a damping coil element and a honeycomb element. The honeycomb energy absorbing element 40 possesses the best energy absorption since it has the largest area under its curve and exhibits the lowest reaction force for a given energy absorption. The damping coil energy absorbing element 18 possesses the next best energy absorption.

Claims: Perforating apparatus for use at the lower end of a tubing string in a well bore, the apparatus comprising: a perforating gun (30) for perforating the formation around the well bore; a detonating cord (22) for firing said perforating gun (30); and shock absorbing means (38) between the perforating gun (30) and the tubing string (32) for absorbing the shock produced by the firing of said perforating gun; characterised in that the shock absorbing means (38) comprises: inner and outer housing members (12, 10) which are initially rigidly connected together; means (20) responsive to the detonation wave which propagates along the detonating cord (22) during the firing of the perforating gun (30) to break the connection between the housing members (12, 10) and thereby render them relatively axially movable; and collapsible energy absorbing means (16, 18 or 40) arranged to be permanently deformed by the relative axial movement of the housing members, thereby absorbing said shock and releasing energy in the form of heat rather than in the form of kinetic energy. Apparatus as claimed in claim 1, wherein the collapsible energy absorbing means comprises a coiled tube (16 or 18). Apparatus as claimed in claim 1, wherein the collapsible energy absorbing means comprises a honeycomb (40). Apparatus as claimed in any preceding claim, wherein respective collapsible energy absorbing means (16 or 18, or 40) is provided for both possible directions of relative movement between the inner and outer housing members (12, 10). Apparatus as claimed in any preceding claim, wherein the detonation wave responsive means comprises a shaped charge (20). A method practised by a shock absorber (38) adapted to be connected to a well apparatus for absorbing shock, the shock absorber including an energy absorbing element (16, 18 or 40), comprising the steps of: breaking a connection between an outer housing (10) and an inner housing (12) of said shock absorber (38) immediately prior to said shock, the inner housing being released from the outer housing when the connection is broken; receiving said shock in said energy absorbing element (16, 18 or 40) of said shock absorber (38); storing mechanical energy associated with said shock in said energy absorbing element (16, 18 or 40); and subsequently releasing the stored energy in the form of heat and not in the form of kinetic energy.

Inventors: HUBER KLAUS, MISZEWSKI ANTONI, HUBER, KLAUS, MISZEWSKI, ANTONI
Applicants: SCHLUMBERGER HOLDINGS, SCHLUMBERGER LTD, SCHLUMBERGER SERVICES PETROL, SCHLUMBERGER HOLDINGS LIMITED, SCHLUMBERGER LIMITED, SERVICES PETROLIERS SCHLUMBERGER
Application year: 1991
Main classification: E21B17
IPC codes: E21B17