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BACKGROUND  
         [0001]    The present invention relates generally to equipment utilized and operations performed in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides a system and method for damping fluid pressure waves in a subterranean well.  
           [0002]    It is well known that detonation of perforating guns in a well can cause damage to equipment in the well. It has generally been considered that this damage is due primarily to forces generated by detonation of the perforating guns. These forces are transmitted to other equipment via a tubing string in which the perforating guns and the other equipment are interconnected.  
           [0003]    For this reason, previous attempts to protect the equipment from damage have focused on isolating the equipment from the forces generated by the perforating guns&#39; detonation. For example, shock absorbers have been interconnected in the tubing string between the equipment and the perforating guns. As another example, methods have been developed wherein the equipment is physically separated from the perforating guns prior to detonating the perforating guns.  
           [0004]    However, damage to equipment may actually, or additionally, be caused by pressure waves generated by the perforating guns when they are detonated. Shock absorbers do not isolate the equipment from damage due to these pressure waves. Furthermore, separating the equipment from the perforating guns may not be necessary if damage to the equipment may be prevented, or at least substantially reduced, by damping the pressure waves.  
           [0005]    Damping pressure waves may also be beneficial in other operations performed in wells. For example, fracturing operations, propellant-driven packer setting, casing repair, etc.  
         SUMMARY  
         [0006]    In carrying out the principles of the present invention, in accordance with embodiments thereof, a system and method of damping fluid pressure waves in a subterranean well is provided. In a described embodiment, pressure waves are damped by positioning a dampener in the well during a perforating operation. The dampener may attenuate the pressure waves by absorbing the pressure waves, flowing the pressure waves through viscously damping material, generating complementary pressure waves, changing a material phase, or by a combination of these methods.  
           [0007]    In one aspect of the invention, a perforating system for a subterranean well is provided. The system includes a perforating gun positioned in the well, and a fluid pressure wave dampener positioned in the well, The dampener damps pressure waves generated by detonation of the perforating gun.  
           [0008]    In another aspect of the invention, a method of damping pressure waves in a subterranean well is provided. The method includes the steps of: providing a fluid pressure wave dampener; positioning the dampener in the well; generating the pressure waves in the well; and damping the pressure waves with the dampener.  
           [0009]    These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a schematic cross-sectional view of a first method embodying principles of the present invention;  
         [0011]    [0011]FIG. 2 is a perspective view of a first pressure wave dampener embodying principles of the invention;  
         [0012]    [0012]FIG. 3 is a schematic cross-sectional view of the first pressure wave dampener;  
         [0013]    [0013]FIG. 4 is a schematic cross-sectional view of a first alternate construction of the first pressure wave dampener;  
         [0014]    [0014]FIG. 5 is a schematic cross-sectional view of a second alternate construction of the first pressure wave dampener;  
         [0015]    [0015]FIG. 6 is a schematic cross-sectional view of a second pressure wave dampener embodying principles of the invention;  
         [0016]    [0016]FIG. 7 is a schematic cross-sectional view of a third pressure wave dampener embodying principles of the invention;  
         [0017]    [0017]FIG. 8 is a schematic cross-sectional view of a fourth pressure wave dampener embodying principles of the invention;  
         [0018]    [0018]FIG. 9 is a perspective view of a fifth pressure wave dampener embodying principles of the invention;  
         [0019]    [0019]FIG. 10 is a side elevational view of the fifth pressure wave dampener.  
         [0020]    [0020]FIG. 11 is a schematic cross-sectional view of a second method embodying principles of the present invention; and  
         [0021]    [0021]FIG. 12 is a schematic cross-sectional view of a third method embodying principles of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]    Representatively illustrated in FIG. 1 is a method  10  which embodies principles of the present invention. In the following description of the method  10  and other apparatus and methods described herein, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used only for convenience in referring to the accompanying drawings. Additionally, it is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention.  
         [0023]    In the method  10 , a tubing string  12  is conveyed into a wellbore  14 . The tubing string  12  includes a packer  16 , a production valve  18 , a perforating gun  20  and a firing head  22 . The packer  16  is set in casing  24  lining the wellbore  14 , and the perforating gun  20  is detonated to form perforations  26  extending outwardly through the casing.  
         [0024]    A bridge plug or sump packer  28  may be set in the casing  24  below the perforating gun  20  prior to, or in conjunction with, running the tubing string  12  into the well. Alternatively, the wellbore  14  below the perforating gun  20  may be open to the casing shoe (not shown) or the bottom of the well.  
         [0025]    Any number of perforating guns, firing heads, etc. may be used in the method  10  in keeping with the principles of the invention. It should also be clearly understood that, although the method  10  as described herein is a method wherein a perforating operation is performed, the principles of the invention are not limited to any particular details of the method described herein, and are not limited to perforating operations at all. The principles of the invention have application in any operation wherein it is desired to dampen pressure waves in a well, for example, formation fracturing operations, casing repair operations, packer setting, etc., each of which may generate damaging pressure waves in the well.  
         [0026]    It has been found that pressure waves generated by detonation of a perforating gun, such as the perforating gun  20 , travel through fluid in the well and create pressure differentials across equipment in the well. For example, a pressure wave generated at the perforating gun  20  will travel both upward and downward in the wellbore  14 . Upwardly traveling pressure waves will reflect off of the packer  16  and begin to travel downward. Downwardly traveling pressure waves will reflect off of the plug  28 , or the bottom of the well, and begin to travel upward.  
         [0027]    Where coinciding in-phase, or approximately in-phase, pressure waves are at their maximum pressure amplitude, a relatively high pressure is experienced by the tubing string  12 . This condition is believed to occur typically just below the packer  16 , at the top end of the perforating gun  20 , and just above the plug  28  or bottom of the well.  
         [0028]    Where coinciding in-phase, or approximately in-phase, pressure waves are at their minimum pressure amplitude, a relatively low pressure is experienced by the tubing string  12 . This condition is believed to occur typically one-fourth wavelength above the plug  28  or bottom of the well, one-fourth of the distance from the top end of the guns to the plug or bottom of the well, and one-fourth of the distance from the packer to the plug or bottom of the well.  
         [0029]    When the relatively high and low pressures are applied to the tubing string  12 , the differential between the high and low pressures produces very high stresses in the tubing string, leading to significant damage to the equipment interconnected therein. Therefore, in the method  10 , a pressure wave dampener  30  is interconnected in the tubing string  12 . The dampener  30  acts to reduce the amplitude of the pressure waves generated in the well, thereby decreasing the pressure differential produced across the tubing string  12 .  
         [0030]    The dampener  30  may operate by absorbing or viscously damping the pressure waves, or by generating a resonant frequency which complements that of the pressure waves in the well. If the dampener  30  operates by absorbing or viscously damping the pressure waves, it should preferably be positioned at one or more locations where the highest fluid velocity is found, which is where the pressure wave amplitude is at its minimum, as described above. If the dampener  30  operates by generating complementary pressure waves, it should preferably be positioned at one or more locations where the lowest fluid velocity is found, which is where the pressure wave amplitude is at its maximum, as described above.  
         [0031]    Referring additionally now to FIG. 2, a pressure wave dampener  32  is representatively illustrated. The dampener  32  may be used for the dampener  30  in the method  10 . However, it should be understood that the dampener  32  may be used in other methods, without departing from the principles of the invention.  
         [0032]    The dampener  32  includes a pressure wave absorbent material  34  enclosed in a protective outer cage  36 . The pressure wave absorbent material  34  is preferably a porous or fibrous material, such as steel wool, mineral wool, open-cell foam, etc. The material  34  viscously dampens pressure waves by forcing the fluid to flow through its many small passages in order to transmit pressure therethrough.  
         [0033]    Referring additionally now to FIG. 3, a cross-sectional view of the dampener  32  is representatively illustrated. In this view it may be seen that a hollow cavity  38  is formed within the material  34 . The cavity  38  is hollow in that it has none of the material  34  therein. The size (height, diameter, volume, etc.), shape and position of the cavity  38  may be adjusted as desired to “tune” the dampener  32  so that it attenuates a particular pressure wave frequency. For example, it may be found through experimentation or practical observation that a particular frequency band causes a substantial portion of damage to the tubular string  12 . In that case, the size of the cavity  38 , or other parts of the dampener  32 , may be adjusted to target that frequency band.  
         [0034]    Note that interior and exterior surfaces  37 ,  39  of the material  34  may be smooth, and/or may be provided with scallops, crenellations, fingers, peaks and valleys, other recesses, other projections etc., as depicted in FIG. 3. These various surfaces may be used to target a particular pressure wave frequency and/or increase the overall attenuation provided by the dampener  32 .  
         [0035]    Referring additionally now to FIG. 4, another alternate construction of the dampener  32  is representatively illustrated. In this construction, a flow passage  40  of the tubing string  12  extends axially through the dampener  32 . The material  34  is isolated from the flow passage  40 . This construction enables production flow, equipment, circulation, etc., to pass through the dampener  32 .  
         [0036]    An annular cavity  42  may be provided in the material  34 . As with the cavity  38  described above, the size, shape and position of this cavity  42  may be adjusted as desired to target a particular frequency band for damping. As with the construction depicted in FIG. 3, the interior and/or exterior surfaces  37 ,  39  of the material  34  may be smooth, and/or may be provided with scallops, crenellations, fingers, peaks and valleys, recesses, projections, etc.  
         [0037]    Referring additionally now to FIG. 5, another alternate construction of the dampener  32  is representatively illustrated. In this alternate construction, the material  34  is isolated from the cavity  38  by a flexible impermeable membrane  44 . The membrane  44  could, for example, be made of an elastomer material, such as rubber, nitrile, viton, etc., or it could be made of a non-elastomer.  
         [0038]    Preferably, the cavity  38  is filled with a liquid, such as silicone oil, etc. Alternatively, the cavity  38  could be in fluid communication with the wellbore  14  external to the dampener  32 , so that well fluid is in the cavity. Thus, the cavity  38  could be pressure balanced with the wellbore  14  surrounding the dampener  32 . Again, the size, shape and position of the cavity  38  may be adjusted to target a particular pressure wave frequency band. As with the construction depicted in FIG. 3, the interior and/or exterior surfaces  37 ,  39  of the material  34  may be smooth, and/or may be provided with scallops, crenellations, fingers, peaks and valleys, recesses, projections, etc.  
         [0039]    Referring additionally now to FIG. 6, another pressure wave dampener  46  is representatively illustrated. The dampener  46  may be used for the dampener  30  in the method  10 . However, it should be understood that the dampener  46  may be used in other methods, without departing from the principles of the invention.  
         [0040]    The dampener  46  includes an enclosed volume  48  within a housing  50  having multiple openings  52  through a sidewall thereof. Flowpaths  54  provide fluid communication between the volume  48  and the openings  52 . When the dampener  46  is positioned in a well, such as that depicted in FIG. 1, the openings  52  and flowpaths  54  provide fluid communication between the volume  48  and the wellbore  14  external to the dampener.  
         [0041]    The dampener  46  is similar in many respects to a device known to those skilled in the acoustic damping art as a Helmholtz resonator. A Helmholtz resonator cancels sound waves by generating sound waves out of phase. The sound waves enter the resonator openings, travel through the flowpaths to the volume, and are reflected back out of phase.  
         [0042]    The Helmholtz resonator is particularly useful in targeting a relatively narrow frequency band of sound waves at which it resonates. The approximate resonant frequency of a Helmholtz resonator is given by the following formula: f=c/2π(A/LV) 1/2 , in which c is the speed of sound, A is the area of the openings, L is the length of the flowpaths and V is the internal volume. It is believed that the same formula would approximate the resonant frequency of the dampener  46  depicted in FIG. 6.  
         [0043]    Several modifications may be made to the dampener  46  to increase the frequency band at which it is effective to dampen the pressure waves. For example, the flowpaths  54  may be perforated as shown at  56  to thereby provide multiple flowpath lengths between the openings  52  and the volume  48 , and to add viscous damping. As another example, a pressure wave absorbent material  58  may be positioned in the volume  48  to add viscous damping.  
         [0044]    Referring additionally now to FIG. 7, another pressure wave dampener  60  is representatively illustrated. The dampener  60  may be used for the dampener  30  in the method  10 . However, it should be understood that the dampener  60  may be used in other methods, without departing from the principles of the invention.  
         [0045]    The dampener  60  is somewhat similar to the dampener  46  described above, in that it includes an internal chamber  62  and multiple openings  64  providing fluid communication between the internal chamber and the well exterior to the dampener. The openings  64  are formed through a sidewall  66  separating the chamber  62  from the well exterior to the dampener  60 . However, the dampener  60  does not have elongated flowpaths between the openings  64  and the chamber  62 .  
         [0046]    Preferably, the openings  64  have a combined area which is approximately 30% to 60% of the surface area of the sidewall  66 . This configuration uses viscous damping of the pressure waves traveling through the sidewall  66  to damp the pressure waves. By adjusting the size, shape, number and positioning of the openings  64 , and the size and shape of the chamber  62 , the frequency band at which maximum pressure wave attenuation is achieved may be altered as desired. In addition, pressure wave absorbent material  68  may be positioned in the chamber  62 .  
         [0047]    Referring additionally now to FIG. 8, another pressure wave dampener  70  is representatively illustrated. The dampener  70  may be used for the dampener  30  in the method  10 , except that the dampener  70  is combined with a perforating gun  72 . Of course, the dampener  70  may be used in other methods, without departing from the principles of the invention.  
         [0048]    An internal volume  74  is formed in the gun  72 . Flowpaths  76  extend into the volume  74  from a sidewall  78  of the gun  72 . It will be readily appreciated that, when the gun  72  is detonated, openings (not shown) will be formed by perforators  80  (explosive shaped charges) through the sidewall  78 . At that point, the gun  72  will be very similar to the dampener  46  depicted in FIG. 6, in that the openings and flowpaths  76  will provide fluid communication between the volume  74  and the wellbore external to the dampener  70 .  
         [0049]    Referring additionally now to FIG. 9, another pressure wave dampener  82  is representatively illustrated. The dampener  82  may be used for the dampener  30  in the method  10 . However, it should be understood that the dampener  82  may be used in other methods, without departing from the principles of the invention.  
         [0050]    The dampener  82  acts by viscously damping the pressure waves traveling through an annulus  84  formed between the wellbore  14  and the tubing string  12 . The dampener  82  includes whiskers or fibers  86  extending outwardly from a central axially extending mandrel  88 . Preferably, the fibers  86  contact the wellbore  14 , in which case the fibers may be deployed after the dampener  82  is conveyed into the well, for example, by removing a shroud (not shown) initially constraining the fibers. Removal of the shroud enables the fibers  86  to extend outward into contact with the wellbore  14 .  
         [0051]    The fibers  86  may be made of any material, including steel, other metals, plastics, composites, etc. The fibers  86  may be made of a phase change alloy, in which case the pressure waves traveling through the fibers induce strain in the fibers, which causes the fibers to change phase and thereby absorb increased energy from the pressure waves.  
         [0052]    In FIG. 10, the dampener  82  is depicted from a side view apart from the wellbore  14 . In this view it may be clearly seen that the fibers  86  have a density which increases in the downward direction. It will be readily appreciated that the fibers  86  also have a density which increases in the radially inward direction as well. This varied density aids in impedance matching to the fluid in the well, decreasing the amplitude of pressure waves reflected from the dampener  82 .  
         [0053]    Referring additionally now to FIG. 11, another method  90  embodying principles of the invention is representatively illustrated. Elements depicted in FIG. 11 which are similar to elements previously described are indicated in FIG. 11 using the same reference numbers.  
         [0054]    In the method  90 , the perforating gun  20  is separated from the equipment, such as a well screen  92  and packer  16 , for which protection is desired. For example, the perforating gun  20  may be separately conveyed into the wellbore  14  (such as by wireline or tubing conveyance) and anchored therein using a gun hanger  94 . Alternatively, the perforating gun  20 , hanger  94  and the remainder of a tubing string  96  may be conveyed together into the wellbore  14 , the hanger  94  set in the casing  24 , the tubing string  96  above the hanger disconnected and raised in the wellbore  14 , and the packer  16  set in the casing to anchor the tubing string.  
         [0055]    Although the packer  16  and screen  92  are physically separated from the perforating gun  20 , they are still subject to damage due to pressure waves generated by detonation of the perforating gun  20 . Any of the dampeners  32 ,  46 ,  60 ,  70 ,  82  described above may be used in the method  90  to dampen these pressure waves. However, the method  90  uses another pressure wave dampener  98 .  
         [0056]    The dampener  98  is constructed with a relatively thin outer wall or shroud  100  which is intentionally designed to deform when it encounters the pressure waves generated by the perforating gun  20 . This deformation of the shroud  100  absorbs energy from the pressure waves. The shroud  100  may deform plastically and/or elastically in response to the pressure waves. It is preferred that the shroud  100  deform plastically in order to absorb a greater amount of energy.  
         [0057]    Referring additionally now to FIG. 12, another method  102  embodying principles of the invention is representatively illustrated. Elements depicted in FIG. 12 which are similar to elements previously described are indicated in FIG. 12 using the same reference numbers.  
         [0058]    The method  102  is substantially similar to the method  90  described above. However, instead of the dampener  98 , the method  102  uses a pressure wave dampener  104  which has whiskers or fibers  106  extending inwardly from an outer shroud  108 . The fibers  106  may be similar to the fibers  86  described above.  
         [0059]    The dampener  104  viscously dampens the pressure waves as they travel through the fibers  106 . This reduces the transmission and reflection of the pressure waves in the wellbore  14 , thereby protecting the packer  16  and screen  92  from damage due to pressure differentials created by the pressure waves.  
         [0060]    Of course, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the invention, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to these specific embodiments, and such changes are contemplated by the principles of the present invention. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims and their equivalents.

Summary:
A system and method of damping fluid pressure waves in a subterranean well. In a described embodiment, pressure waves are damped by positioning a dampener in the well during a perforating operation. The dampener may attenuate the pressure waves by absorbing the pressure waves, flowing the pressure waves through viscously damping material, generating complementary pressure waves, changing a material phase, or by a combination of these methods.