Abstract:
A technique facilitates a perforation operation. A perforating gun carrier is combined with a pressure enhancement mechanism. The pressure enhancement mechanism provides a controlled increase in pressure within the perforating gun carrier as the perforating gun carrier is delivered into a higher pressure environment. The increase in internal pressure counters the buildup of a pressure differential to the degree desired for a given perforating gun carrier.

Description:
BACKGROUND 
       [0001]    In a well perforating operation, a perforating gun string is used to carry a perforating gun downhole into a wellbore to a desired region. The perforating gun comprises a carrier tube designed to carry a plurality of charges which are detonated to form perforations that extend outwardly in a radial direction into a surrounding formation. As the carrier tube is conveyed deeper into the wellbore, a substantial pressure differential is established between the high pressure external well environment and the interior of the carrier tube. The high differential pressure increases both the collapse tendency and the leak potential of the carrier. Following perforation, the differential pressure also can drive well fluid into the perforating gun and cause a detrimental pressure pulse which propagates through the wellbore fluid. 
       SUMMARY 
       [0002]    In general, the present disclosure provides a methodology and system which facilitate a perforation operation. A perforating gun carrier is combined with a pressure enhancement mechanism. The pressure enhancement mechanism enables a controlled increase in pressure within the perforating gun as the perforating gun carrier is delivered into a higher pressure environment. The increase in internal pressure counters the buildup of a pressure differential to the degree desired for a given perforating gun carrier. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Certain embodiments will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements. It should be understood, however, that the accompanying figures illustrate only the various implementations described herein and are not meant to limit the scope of various technologies described herein, and: 
           [0004]      FIG. 1  is a schematic illustration of an example of a perforating gun string, according to an embodiment of the disclosure; 
           [0005]      FIG. 2  is an illustration of a perforating gun carrier, according to an embodiment of the disclosure; 
           [0006]      FIG. 3  is an illustration of another example of a perforating gun carrier, according to an embodiment of the disclosure; 
           [0007]      FIG. 4  is a schematic illustration of a perforating operation, according to an embodiment of the disclosure; 
           [0008]      FIG. 5  is an illustration of a perforating gun carrier on a perforating gun string during an initial stage of conveyance downhole, according to an embodiment of the disclosure; 
           [0009]      FIG. 6  is an illustration of a perforating gun carrier on a perforating gun string similar to that of  FIG. 5  but during a subsequent stage of conveyance downhole, according to an embodiment of the disclosure; and 
           [0010]      FIG. 7  is an illustration of a perforating gun carrier on a perforating gun string similar to that of  FIG. 5  but positioned at a perforating region, according to an embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In the following description, numerous details are set forth to provide an understanding of some illustrative embodiments of the present disclosure. However, it will be understood by those of ordinary skill in the art that the system and/or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
         [0012]    The disclosure herein generally relates to a system and methodology which can be employed to alleviate the detrimental effects of differential pressures acting on a hollow body during a perforating operation. In downhole perforating operations, for example, the perforating gun carrier is subjected to high downhole wellbore pressures which can create detrimental differential pressures between the exterior and interior of the perforating gun carrier. According to an embodiment of the present system and methodology the static pressure differential in a perforating gun carrier is reduced prior to shooting, thus reducing the collapse tendency of the carrier and also reducing the leak potential of sealing elements. The increased in-gun pressure also reduces the influx of wellbore fluid which would otherwise enter into the perforating gun carrier due to the pressure differential. Consequently, perforating gun filling and the resulting pressure pulse propagating through the wellbore fluid are eliminated or adequately reduced. Eliminating or adequately reducing the pressure pulse removes a variety of detrimental effects, e.g. excessive stresses, which would otherwise act against the well equipment. 
         [0013]    In perforating applications, the interior pressure within the perforating gun carrier can be increased in a controlled manner to reduce or eliminate the pressure differential between the interior and the exterior of the gun carrier. By way of example, pressure in the interior of the perforating gun carrier may be increased by a pressure enhancement mechanism carried by the perforating gun string. An example of a pressure enhancement mechanism comprises an internal gas generator, such as a propellant charge. In another example of a pressure enhancement mechanism, the interior pressure may be increased through activation of a subcritical fluid, e.g. CO2, at the downhole temperature. Additionally, the interior pressure may be controlled by a pressure enhancement mechanism which releases compressed gas from a compressed gas chamber working in cooperation with the gun carrier. 
         [0014]    In many applications, the pressurization occurs after the gun carrier is placed in a wellbore. For example, the controlled pressurization can be executed downhole on a continuous basis as the perforating gun carrier is lowered to a desired perforating region along a surrounding formation. The pressurization also may be performed within the perforating gun carrier in discrete steps, e.g. at sequential, discrete locations along the wellbore, as the perforating gun is conveyed downhole to the desired perforating region. 
         [0015]    Perforating operations can be performed in many types of downhole applications and in other applications via several types of perforating guns. For example, some perforating guns comprise a perforating gun carrier, such as a perforating gun carrier tube, which is designed to hold charges that are selectively detonated to form perforations in the surrounding structures. According to an embodiment, a perforating gun string is provided with a perforating gun carrier and the carrier is conveyed downhole into a wellbore. During conveyance, pressure is increased within the perforating gun carrier via the pressure enhancement mechanism. The pressure enhancement mechanism may be carried by the perforating gun string and is designed to provide a controlled increase in pressure during the conveyance downhole. 
         [0016]    Referring generally to  FIG. 1 , an example of one type of application for facilitating a perforating operation is illustrated. The example is provided to facilitate explanation, and it should be understood that a variety of perforating gun strings and systems may be used in a variety of well related applications as well as in many types of non-well related applications in which perforations are to be formed. The perforating gun string and other structures described herein may comprise many types of components arranged in various configurations depending on the parameters of a specific perforating application. 
         [0017]    In  FIG. 1 , an embodiment of a perforating system  20  is illustrated as comprising a perforating gun carrier string  22  positioned in a wellbore  24  extending from a surface location  26 . In some applications, the wellbore  24  is cased with a well casing  28 . The perforating gun carrier string  22  comprises a perforating gun  30  having a perforating gun carrier assembly  32 . The perforating gun carrier assembly  32  comprises a perforating gun carrier  34 , e.g. a perforating gun carrier tube, designed to hold a plurality of charges  36 . Depending on the specific application, the charges  36  may comprise shaped charges constructed and oriented to form precise perforations that extend radially outward through the casing  28  and into a surrounding formation  38 . In the example illustrated, the perforating gun carrier assembly  32  also comprises a pressure enhancement mechanism  40  which may be carried by perforating gun carrier string  22  at a location within the perforating gun carrier  34  and/or at a position external to perforating gun carrier  34 . It should be noted that in well related applications, wellbore  24  may comprise many types of wellbores, including deviated, e.g. horizontal, single bore, multilateral, cased, and uncased (open bore) wellbores. 
         [0018]    Referring generally to  FIG. 2 , an embodiment of a perforating gun carrier  34  is illustrated. In this embodiment, perforating gun carrier  34  comprises an interior  42  separated from an exterior environment, e.g. a wellbore environment, by a gun carrier wall  44 . In at least some applications, the carrier wall  44  is arranged in a tubular form with charges  36  mounted to orient the perforations in a radially outward direction. The pressure enhancement mechanism  40  is mounted to enable a controlled increase in pressurization of interior  42 , as indicated by arrows  46 . The increase in pressurization of interior  42  is selectively controlled to counter or to eliminate the differential in pressure between the internal pressure  46  and an external pressure represented by arrows  48 . 
         [0019]    For example, pressure enhancement mechanism  40  may be designed to enable selective release of gas into interior  42  to provide control over the pressure differential, e.g. to provide a reduction of the pressure differential between internal pressure  46  and external pressure  48 . In a variety of well applications, the internal pressure represented by arrows  46  can be increased while the perforating gun carrier  34  is in wellbore  24 . By way of example, the internal pressure may be increased gradually and continuously as the perforating gun carrier  34  is deployed downhole along wellbore  24 . In another example, the internal pressure may be increased periodically in discrete steps during conveyance of perforating gun carrier  34  downhole. The amount of pressure increase may be determined based on the collapse resistance of the perforating gun carrier  34  and/or based on other application related parameters. 
         [0020]    Referring again to  FIG. 2 , the illustrated example of a pressure enhancement mechanism  40  comprises a chamber  50  containing a pressurized gas  52 . The chamber  50  may be placed in operative cooperation with interior  42  of perforating gun carrier  34  to selectively release the high pressure gas  52  into interior  42  to decrease or eliminate the differential pressure acting on perforating gun carrier  34 . The chamber  50  may be carried by perforating gun string  22  and may be placed proximate, e.g. adjacent, the perforating gun carrier  34 . In this example, the pressure enhancement mechanism  40  also comprises a gas release member  54  which may be selectively activated to provide a controlled release of pressurized gas  52  from chamber  50  and into interior  42  of perforating gun carrier  34 . By way of example, the gas release member  54  comprises a valve or other actuatable member which may be actuated, for example, electrically or hydraulically via input from a control line  56 . However, the gas release member  54  may comprise other types of mechanisms, such as passive release mechanisms in the form of spring-loaded members and/or a series of rupture discs. In other embodiments, the gas release member  54  may comprise a timed release mechanism, a pressure activated mechanism, or another suitable gas release mechanism to provide for controlled increase of pressure within interior  42 . 
         [0021]    In another example, the pressure enhancement mechanism  40  comprises a gas generator  58 , as illustrated in  FIG. 3 . The gas generator  58  may be selectively activated to release gas into interior  42  and to thus raise the internal pressure, thereby reducing the pressure differential between the internally acting pressure  46  and the externally acting pressure  48 . In some applications, the gas generator  58  may be located at an internal location within perforating gun carrier  34 . By way of example, the gas generator  58  may comprise a propellant charge which is selectively activated to release gas and to increase the pressure within interior  42 . In another example, the gas generator  58  may comprise a subcritical fluid, e.g. CO2, which is activated at downhole temperature. Depending on the specific type of gas generator  58 , a corresponding gas release member  54  may be used to selectively initiate activation of the gas generator  58  for release of the gas within interior  42 . 
         [0022]    Referring generally to  FIG. 4 , an illustration is provided of the perforating gun carrier  34  following detonation of charges  36  to form a plurality of perforations  60 . As illustrated, the perforations  60  may be formed in a radially outward direction through casing  28  and into the surrounding formation  38 . By increasing the pressure within interior  42  as the perforating gun carrier  34  is moved downhole, the collapse tendency of the perforating gun carrier  34  is reduced and the potential for a detrimental post-shot pressure pulse is reduced or eliminated. The amount of pressure increase may be determined according to collapse resistance, leak resistance, and/or susceptibility to damage from the post-shot pressure pulse. Depending on the parameters of a specific application and environment, the internal pressure, represented by arrows  46  in  FIGS. 2 and 3 , may be sufficiently increased to reduce an underbalance pressure situation; to equalize internal and external pressures; or to create an overbalance pressure situation in which the internal pressure is greater than the external pressure. 
         [0023]    Internal perforating gun carrier post-shot pressures also are affected by the explosive detonation gas density and temperature resulting from detonation of charges  36 . The addition of gas  52  and the resulting increase of internal pressure via activation of pressure enhancement mechanism  40  further increase the post-shot gas density and thus further increase the post-shot pressure acting against the influx of well fluid (see arrows  62 ) and against the resultant detrimental pressure pulse. In  FIG. 4 , a pressure pulse is illustrated by arrows  64  as propagating away from perforating gun carrier  34 . Similarly, a corresponding decompression wave is illustrated by arrows  66 . The introduction of additional gas  52  and higher internal pressures via pressure enhancement mechanism  40  enables better control over or even elimination of these effects caused by detonation of charges  36 . 
         [0024]    In operation, the pressure level in interior  42  of perforating gun carrier  34  (and thus the pressure differential acting on the perforating gun carrier  34 ) may be selectively controlled during conveyance of the perforating gun  30  downhole or to another desired perforating region. As illustrated in  FIGS. 5-7 , a controlled increase in pressure within perforating gun carrier  34  is provided during conveyance of the perforating gun carrier  34  downhole into wellbore  24  via perforating gun string  22 . Referring to  FIG. 5 , the pressure enhancement mechanism  40  may be initially activated once the perforating gun carrier  44  is moved down to a desired position within wellbore  24 , as indicated by arrow  68 . 
         [0025]    During conveyance to greater depths downhole, additional gas  52  is released to increase the pressure within perforating gun carrier  34 , as illustrated in  FIG. 6 . As discussed above, the release of gas may be conducted continually or periodically at discrete locations as the perforating gun carrier  34  is lowered downhole. The increased internal pressure within interior  42  reduces the pressure differential acting on perforating gun carrier  34 , thus enhancing collapse survivability while also inhibiting leaks into the perforating gun carrier  34 . 
         [0026]    Once the perforating gun  30  is at a desired perforating region along formation  38  and once the internal pressure created via pressure enhancement mechanism  40  is at a desired level, the charges  36  are detonated to create perforations  60  as illustrated in  FIG. 7 . When the perforating gun  30  is fired, in-gun pressure is increased above what it otherwise would be due to the post-detonation gas pressure created by the explosion/heat of the detonated charges  36 . This increase in pressure plus the pre-shot static pressure established by the controlled release of gas  52  via pressure enhancement mechanism  40  eliminates or minimizes the severity of perforating gun filling and thus eliminates or minimizes the magnitude of the resultant pressure pulse. 
         [0027]    The system and methodology described herein may be employed in non-well related perforation applications which subject the perforating gun to pressure differentials. The type of perforating gun and charges employed may vary depending on the specific application and environment in which the perforating application is carried out. In some applications, the explosive charges  36  can be replaced with other types of perforating devices or techniques, such as high pressure jet perforating tools. 
         [0028]    Additionally, the system and methodology may be employed in many types of well applications, including many types of single zone or multi-zone perforating applications. Single gas generating devices or a plurality of gas generating devices may be used in cooperation with each perforating gun carrier. Additionally, the size and construction of the perforating gun carrier can vary depending on the specific parameters of a given application and/or environment. Furthermore, the perforating gun may be combined with several types of additional devices and systems to carry out other functions at the perforating region. For example, a variety of chemical treatment devices or other well treatment related devices may be combined with the perforating string to carry out desired service operations in the well environment or in another perforating environment. 
         [0029]    Although only a few embodiments of the system and methodology have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims.