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CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    NONE 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to devices and method for perforating a subterranean formation in an underbalanced condition. 
       BACKGROUND 
       [0003]    Hydrocarbons, such as oil and gas, are produced from cased wellbores intersecting one or more hydrocarbon reservoirs in a formation. These hydrocarbons flow into the wellbore through perforations in the cased wellbore. Perforations are usually made using a perforating gun that is generally comprised of a steel tube “carrier,” a charge tube riding on the inside of the carrier, and with shaped charges positioned in the charge tube. The gun is lowered into the wellbore on electric wireline, slickline, tubing, coiled tubing, or other conveyance device until it is adjacent to the hydrocarbon producing formation. Thereafter, a surface signal actuates a firing head associated with the perforating gun, which then detonates the shaped charges. Projectiles or jets formed by the explosion of the shaped charges penetrate the casing to thereby allow formation fluids to flow through the perforations and into a production string. 
         [0004]    In certain instances, it may be desirable to perforate the formation while the wellbore pressure is less than the formation pressure. This condition is known as an “underbalanced” condition. In an underbalanced condition, the fluid from the formation flows out of a newly formed perforation. This flow can clean the perforation of debris and improve production of resident hydrocarbons. The present disclosure addresses the need for perforating guns that can generate an underbalanced condition during a perforating activity. 
       SUMMARY 
       [0005]    In aspects, the present disclosure provides a perforating gun that has: at least one shaped charge that generates a high-pressure gas when detonated, a valve sub connected to the perforating gun, and a reservoir sub connected to the valve sub. The valve sub may have an enclosure having at least one port providing fluid communication between an exterior and an interior of the valve sub, a mandrel disposed in the enclosure, the mandrel having a piston head and a fluid path extending at least partially through the mandrel, a sleeve slidably mounted on the mandrel, the sleeve selectively blocking fluid flow through the at least one port, and a pressure chamber defined by an inner surface of the sleeve and an outer surface of the piston head, the pressure chamber receiving the generated high-pressure gas via the fluid path, wherein the sleeve slides toward the perforating gun after a predetermined pressure is created by the generated high-pressure gas in the pressure chamber. The reservoir sub may have at least one chamber in fluid communication with the interior of the valve sub. 
         [0006]    In further aspects, the present disclosure provides a dynamic underbalanced sub for use with a perforating gun having at least one shaped charge that generates a high-pressure gas when detonated. The dynamic underbalance sub may include a valve sub and a reservoir sub. The valve sub connects to the perforating gun and includes an enclosure having a longitudinal cavity and at least one port providing fluid communication between an exterior and the cavity, a mandrel disposed in the cavity and fixed to the enclosure, the mandrel having a piston head and a fluid path extending at least partially through the mandrel, the fluid path being in communication with an interior of the perforating gun, a tubular sleeve slidably mounted on the mandrel, the sleeve shifting from a first position and a second position inside the cavity, wherein the sleeve blocks fluid flow through the at least one port in the first position, and a pressure chamber defined by an inner surface of the sleeve and an outer surface of the piston head, the pressure chamber receiving the generated high-pressure gas via the fluid path, wherein the generated high-pressure gas in the pressure chamber displaces the sleeve to a second position wherein the at least one port is at least partially uncovered. The reservoir sub is coupled to the valve sub and may include at least one chamber in fluid communication with the cavity, wherein the sleeve slides away from the reservoir sub when shifting from the first position to the second position. 
         [0007]    Still further aspects of the present disclosure relate to methods for perforating a formation using the disclosed perforating gun systems. 
         [0008]    It should be understood that certain features of the disclosure have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will in some cases form the subject of the claims appended thereto. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    For detailed understanding of the present disclosure, references should be made to the following detailed description taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein: 
           [0010]      FIG. 1  schematically illustrates a side sectional view of a perforating gun with an underbalanced perforating sub according to one embodiment of the present disclosure; 
           [0011]      FIG. 2A  schematically illustrates a sectional view of a portion of an underbalanced perforating sub according to one embodiment of the present disclosure; 
           [0012]      FIG. 2B  schematically illustrates the  FIG. 2A  embodiment in an activated state; and 
           [0013]      FIG. 3  schematically illustrates a well in which embodiments of the present disclosure may be deployed. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The present disclosure relates to devices and methods for perforating a formation intersected by a wellbore. The present disclosure is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present disclosure with the understanding that the present disclosure is to be considered an exemplification of the principles of the disclosure, and is not intended to limit the disclosure to that illustrated and described herein. 
         [0015]    Referring now to  FIG. 1 , there is shown one embodiment of a perforating gun  100  in accordance with the present disclosure. For ease of discussion, devices such as boosters, electrical wiring, connectors, fasteners and detonating cords have been omitted. The perforating gun system  100  may include a gun  102  that perforates a section of a formation and a dynamic underbalance sub  104  (hereafter ‘sub  104 ’) that generates an underbalanced condition after the gun  102  fires. The gun  102  may include a carrier  106  that is shaped to receive a charge tube  108  and one or more shaped charges  110  that create jets for perforating a surrounding formation. 
         [0016]    The sub  104  generates a temporary pressure drop in the wellbore immediately after the gun  102  fires. This temporary pressure drop allows formation fluid to flow through and clean the newly formed perforations. In one embodiment, the sub  104  includes a valve sub  120  and a reservoir sub  122 . As used herein, the term “sub” refers to an assembly of components configured to perform one or more tasks and residing within a common structure such as a housing, frame, or enclosure. As discussed in greater detail below, the high pressure gas generated by the gun  102  actuates the valve sub  120 , which then allows wellbore fluid to flow into the reservoir sub  122 . The sudden inrush of fluid causes a pressure drop and the temporary (dynamic) underbalanced condition in the surrounding wellbore fluid. As noted previously, an underbalanced condition refers to a pressure environment wherein the wellbore pressure is less than the formation pressure. 
         [0017]    Referring to  FIG. 2A , the valve sub  120  may include an enclosure  124  in which are disposed a mandrel  126  and a sleeve  128 . The enclosure  124  may include a longitudinal cavity  130  having a passage  132  at an upper end  134  and a mouth  136  at a lower end  137 . The upper end  134  may be configured to connect with the gun  102  ( FIG. 1 ) and the lower end  136  may be configured to connect with the reservoir sub  122  ( FIG. 1 ). One or more ports  138  formed on a circumferential wall  140  allow fluid communication between an exterior of the valve sub  120  and the cavity  130 . Fluid flow through ports  138  is controlled by moving the sleeve  128  axially along the mandrel  126 . 
         [0018]    The mandrel  126  may be a cylindrical member having a shaft  142  that terminates at a diametrically larger piston head  144 . The shaft  142  may be fixed to the enclosure  124  and the piston head  144  has a surface that includes a pressure face  148  and an outer circumferential surface  150 . The mandrel  126  also includes a fluid passage  152  that include a bore  154  that extends from an upper end  156  to one or more transverse openings  158  that are positioned to communicate with the pressure face  148 . For instances, the bore  154  may be longitudinally aligned and the opening(s)  158  may radiate from the longitudinal bore  154 . 
         [0019]    The sleeve  128  may be a tubular member having a length sufficient to completely cover and thereby block flow through the ports  138  when in a pre-activated position. In the activated position, the sleeve  128  is axially spaced apart from and at least partially uncovers the ports  138 . The sleeve  128  may have a first bore  160  formed complementary to the shaft  142  and a larger second bore  162  in which the piston head  144  is disposed. An annular pressure chamber  164  is formed at a shoulder  166  defining a juncture between the first bore  160  and the second bore  162 . The pressure chamber  164  is defined by the pressure face  148  and an inner surface  170  of the sleeve  128 . In some embodiments, a retaining member  176  may be used to selectively lock the sleeve  128  to the mandrel  126 . For example, the retaining member  176  may be a shear pin that is configured to break when subjected to a known force. 
         [0020]    In some embodiments, seals may be used to form fluid barriers within the enclosure  124 . For example, seals  172  between the mandrel  126  and the sleeve  128  may be used to hydraulically isolate the pressure chamber  164  and seals  174  may be used to form fluid tight barriers between the sleeve  128  and the enclosure  124  to isolate the ports  138 . 
         [0021]    Additionally, in some embodiments, the shaft  142  and the passage  132  may be configured to provide a locking function. For instance, some or all of the passage  132  may be sized to be diametrically smaller than the shaft  142 . Thus, when the shaft  142  is forced under pressure to slide through the passage  132 , an interfering contact is formed, which can lock the shaft  142  to the enclosure  124 . 
         [0022]    Referring to  FIG. 1 , the reservoir sub  122  includes one or more interior chambers  180  for receiving wellbore fluids after the valve sub  120  is in the activated position. The chamber(s)  180  may be defined within one or more housings  182 . In some arrangements, the reservoir sub  122  may have an adjustable volumetric capacity by using modular housings. For instance, the housings  182  may interconnect with one another. Thus, adding two housings will double the volumetric capacity and increase the available pressure drop. 
         [0023]    Referring to  FIG. 3 , there is shown a well construction and/or hydrocarbon production facility  30  positioned over subterranean formations of interest  32 . The facility  30  can be a land-based or offshore rig adapted to drill, complete, or service the wellbore  12 . The facility  30  can include known equipment and structures such as a platform  40  at the earth&#39;s surface  42 , a wellhead  44 , and casing  46 . A work string  48  suspended within the well bore  12  is used to convey a perforating gun  100  into and out of the wellbore  12 . The work string  48  can include coiled tubing  50  injected by a coiled tubing injector (not shown). Other work strings can include tubing, drill pipe, wire line, slick line, or any other known conveyance means. A surface control unit (e.g., a power source and/or firing panel)  54  can be used to monitor and/or operate tooling connected to the work string  48 . 
         [0024]    Referring to  FIGS. 1-3 , in one illustrative method of use, the gun  100  is first positioned at a desired location in the wellbore  12 . In the pre-activated state, the sleeve  128  blocks the openings  138  and the interior of the reservoir sub  122  is empty of liquids and at a pressure lower than the ambient formation pressure (e.g., substantially atmospheric pressure). When fired, the shaped charges  110  create jets that form perforations or tunnels  60  into the adjacent formation  32 . Immediately thereafter, high pressure gas generated by the detonation of the shaped charges  110  flows from the interior of the gun  100  through the fluid passage  152  and into the pressure chamber  164 . After it reaches a predetermined value, the pressure in the pressure chamber  164  breaks the retaining member  176  and pushes the sleeve  128  axially upward, which uncovers the openings  138  as shown in  FIG. 2B . It should be appreciated that the sliding motion of the sleeve  128  is axially upward toward the perforating gun  100  and away from the reservoir sub  122 . Moreover, the shoulder  166  prevents the sleeve  128  from sliding toward the reservoir sub  122 . Thus, the sleeve  128  is retained within the valve sub  120 . 
         [0025]    Now that the valve sub  120  has been activated, wellbore fluid surrounding the perforating gun  100  can flow through the openings  138  and into the chambers of the reservoir sub  122 . The seals  172  and  174  prevent this flow from flowing upward to the perforating gun  100 . This inflow of fluid causes a transient reduction in surrounding wellbore pressure and an underbalanced condition. This underbalanced condition promotes the flow of formation fluid out of the newly formed perforation tunnels  60 . 
         [0026]    The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope of the invention. It is intended that the following claims be interpreted to embrace all such modifications and changes.

Summary:
A perforating gun has shaped charges that can generate a high-pressure gas. A valve sub connects to the perforating gun and a reservoir sub connects to the valve sub. The valve sub has an enclosure with a port. A mandrel in the enclosure has a piston head and a fluid path extending at least partially through the mandrel. A sleeve is slidably mounted on the mandrel and selectively blocks fluid flow through the port. A pressure chamber in the sleeve receives the generated high-pressure gas via the fluid path. The sleeve slides toward the perforating gun after a predetermined pressure is created by the generated high-pressure gas in the pressure chamber. The reservoir sub may have at least one chamber in fluid communication with the interior of the valve sub.