Patent Publication Number: US-2023160279-A1

Title: Dynamic underbalance sub

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a national stage application of and claims priority to Patent Cooperation Treaty (PCT) Application No. PCT/EP2021/066167 filed Jun. 16, 2021, which claims priority to U.S. Provisional Patent Application No. 63/040,979 filed Jun. 18, 2020, U.S. Provisional Patent Application No. 63/079,699 filed Sep. 17, 2020, and U.S. Provisional Patent Application No. 63/079,705 filed Sep. 17, 2020, the contents of each of which are hereby incorporated by reference in their entirety. 
     BACKGROUND 
     In oil and gas wellbore completion operations, perforating guns with shaped charges are commonly used to puncture holes into a wellbore casing and to create a hydraulic connection between the oil, gas, or water bearing reservoir and the wellbore. The jet of a shaped charge punches a hole in the surrounding wellbore casing and travels into the rock formation of the reservoir. The grains of the formation are destroyed, and their remains are pushed radially away from the axial center of the jet, thereby forming an elongated cavity in the rock. This cavity is also referred as “tunnel” or “perforation tunnel.” The crushed grains and the debris from the perforation jet remain in a large portion in the perforation tunnel. These remains or crushed grains can reduce the permeability of the rock and thereby reduce or even block the flow path of fluid or gas towards the wellbore. The layer of crushed grains with reduced permeability is sometimes referred to as the “skin effect” or even perforating damage. 
     A local underpressure (i.e., a negative pressure) can be used to extract the remains from the perforation tunnel. In general, an empty container at ambient pressure is connected to the perforating gun and deployed to the wellbore. After initiation of the perforating gun a vent or valve rapidly opens the empty container in close proximity to the perforation tunnels. The wellbore fluid will flow into the container and create a local negative pressure for the time until the container is filled. This temporary pressure drop is called “dynamic underbalance.” The local pressure in the wellbore will drop for a short period of time under the pressure of the reservoir pressure in the rock formation. This effect causes a rapid flow from the perforation tunnel, which can flush a high amount of debris from the tunnel into the wellbore and causes a cleaning of the tunnel. The amount of the dynamic underbalance may increase with the size of the opening into the ambient pressure container and the speed at which the hole is opened. 
     Accordingly, it may be desirable to develop a dynamic underbalance mechanism with a fast opening valve or opening to the ambient pressure container. Further it may be desirable to develop a dynamic underbalance system that can be easily connected to a wellbore tool string and easily, quickly, and reliably actuated in a wellbore environment. 
     BRIEF SUMMARY 
     An exemplary embodiment of a dynamic underbalance sub for use in a wellbore may include a sub housing, a first chamber provided in an interior of the sub housing, an opening extending through the sub housing and configured such that the first chamber is in fluid communication with an exterior of the sub housing, a second chamber provided in the interior of the sub housing, and a pressure-isolating wall provided between the first chamber and the second chamber. 
     A wellbore tool string for use in a wellbore may include a first wellbore tool and a dynamic underbalance sub. The wellbore tool may include a tool housing and a tool explosive provided within the tool housing. The dynamic underbalance sub may include a sub housing, a first chamber provided in an interior of the sub housing, an opening extending through the sub housing and configured such that the first chamber is in fluid communication with an exterior of the sub housing, a second chamber provided in the interior of the sub housing, a pressure-isolating wall provided between the first chamber and the second chamber, and a shaped charge ballistically coupled to the tool explosive and positioned to break or perforate the pressure-isolating wall in response to detonation of the tool explosive. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       A more particular description will be rendered by reference to exemplary embodiments that are illustrated in the accompanying figures. Understanding that these drawings depict exemplary embodiments and do not limit the scope of this disclosure, the exemplary embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG.  1    illustrates a cross section of a dynamic underbalance sub according to an exemplary embodiment; 
         FIG.  2    illustrates a cross section of a wellbore tool string according to an exemplary embodiment; 
         FIG.  3    illustrates a cross section of a dynamic underbalance sub according to an exemplary embodiment; 
         FIG.  4    illustrates a cross section of a dynamic underbalance sub according to an exemplary embodiment; 
         FIG.  5    illustrates a cross section of a dynamic underbalance sub prior to actuation according to an exemplary embodiment; 
         FIG.  6    illustrates a cross section of a dynamic underbalance sub according to an exemplary embodiment; 
         FIG.  7    illustrates a cross section of a dynamic underbalance sub after actuation according to an exemplary embodiment; 
         FIG.  8    illustrates a cross section of a dynamic underbalance sub according to an exemplary embodiment; and 
         FIG.  9    illustrates a cross section of a dynamic underbalance sub according to an exemplary embodiment. 
     
    
    
     Various features, aspects, and advantages of the exemplary embodiments will become more apparent from the following detailed description, along with the accompanying drawings in which like numerals represent like components throughout the figures and detailed description. The various described features are not necessarily drawn to scale in the drawings but are drawn to aid in understanding the features of the exemplary embodiments. 
     The headings used herein are for organizational purposes only and are not meant to limit the scope of the disclosure or the claims. To facilitate understanding, reference numerals have been used, where possible, to designate like elements common to the figures. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various exemplary embodiments. Each example is provided by way of explanation and is not meant as a limitation and does not constitute a definition of all possible embodiments. It is understood that reference to a particular “exemplary embodiment” of, e.g., a structure, assembly, component, configuration, method, etc. includes exemplary embodiments of, e.g., the associated features, subcomponents, method steps, etc. forming a part of the “exemplary embodiment.” 
       FIG.  1    illustrates an exemplary embodiment of a dynamic underbalance sub  102 . The dynamic underbalance sub  102  may include a sub housing  104 . The sub housing  104  may be formed of steel. The dynamic underbalance sub  102  may further include a first chamber  106  provided in an interior of the sub housing  104 . An opening  108  may be provided in a side of the sub housing  104  such that the first chamber  106  is in fluid communication with an exterior  110  of the dynamic underbalance sub  102 . In other words, when the dynamic underbalance sub  102  is deployed in a well, a pressure within the first chamber  106  may be equal to a wellbore pressure. 
     The dynamic underbalance sub  102  may further include a second chamber  112  provided in the interior of the sub housing  104 . The second chamber  112  may be pressure-sealed from the first chamber  106  such that the pressure within the second chamber  112  may be maintained independently from the pressure within the first chamber  106 . For example, a pressure-isolating wall  114  may be provided between the first chamber  106  and the second chamber  112  at a second chamber first end  132  of the second chamber  112 . 
     As further seen in  FIG.  1   , the second chamber  112  may be open at a second chamber second end  134  that is spaced apart from the second chamber first end  132  in the axial direction. As explained in detail herein, a container  210  (see  FIG.  2   ) may be coupled at the second chamber second end  134  in order to seal the second chamber  112 . Alternatively, the dynamic underbalance sub  102  may be formed such that the second chamber  112  is closed at the second chamber second end  134 . 
     In an exemplary embodiment, the second chamber  112  may be sealed at a surface so as to set a pressure of the second chamber  112  approximately equal to a surface atmospheric pressure. Accordingly, in an exemplary embodiment, the pressure in the second chamber  112  may be lower than the wellbore pressure when the dynamic underbalance sub  102  is deployed in a wellbore. 
     The pressure-isolating wall  114  may be configured so as to be be breached, perforated, shattered, or otherwise broken by a shaped charge  140 . In an exemplary embodiment, the pressure-isolating wall  114  may include a brittle material such as glass or ceramic. In a further exemplary embodiment, the pressure-isolating wall  114  may include a borosilicate glass, a soda lime glass, or a soda lime silicate glass. In a further exemplary embodiment, the pressure-isolating wall  114  may be formed as a disc inserted into the second chamber  112 . Forming the pressure-isolating wall  114  of a brittle material helps to ensure that a substantial portion of the pressure-isolating wall  114  shatters, breaks, or disintegrates when the shaped charge  140  is initiated, thereby providing a larger hole between the first chamber  106  and the second chamber  112  in order to increase the dynamic underbalance. In comparison, if the pressure-isolating wall  114  was made by a malleable material such as a metal, the shaped charge  140  may only create a small perforation hole in the pressure-isolating wall  114 , thereby reducing the dynamic underbalance. 
       FIG.  1    further shows that the first chamber  106  may include a first interior surface  116  having a first diameter  118 , and the second chamber  112  may include a second interior surface  120  having a second diameter  122 . The second diameter  122  may be larger than the first diameter  118 . A shoulder  124  may extend between the first diameter  118  and the second chamber second diameter  122 . As noted above, the pressure-isolating wall  114  may be formed as a disc inserted into the second chamber  112 , in which case the pressure-isolating wall  114  may abut against shoulder  124 . A seal element  126  may be provided between the pressure-isolating wall  114  and the sub housing  104 . For example, as seen in  FIG.  1   , the seal element  126  may be provided in a groove formed in the shoulder  124 . Alternatively, the seal element  126  may be provided between an outer circumferential surface of the pressure-isolating wall  114  and the second interior surface  120 . In an exemplary embodiment, the seal element  126  may be an O-ring. However, it will be understood that the seal element  126  is not limited to an O-ring, and other seals including, but not limited to, liquid seals, foam seals, resins, polymers, coatings, or other suitable seal material. The seal element  126  may help to improve the seal between the first chamber  106  and the second chamber  112 , and may help to improve the pressure rating of the dynamic underbalance sub  102 . Additionally, the seal element  126  may help to improve reusability of the dynamic underbalance sub  102  (and the dynamic underbalance sub  302  discussed in detail herein with reference to  FIG.  3   ). 
     In an exemplary embodiment, the dynamic underbalance sub  102  may further include second chamber internal threads  128  formed on second interior surface  120 . A lock ring  130  may be threadedly engaged with the second chamber internal threads  128  so as to abut the pressure-isolating wall  114  and keep the pressure-isolating wall  114  pressed against the shoulder  124  so as to maintain the seal between the first chamber  106  and the second chamber  112 . 
     The dynamic underbalance sub  102  may further include sub housing external threads  136  formed on an outer surface  138  of the sub housing  104 . As explained in detail herein, a container  210  may be threadedly engaged with the sub housing external threads  136 . 
     As noted above, the dynamic underbalance sub  102  may include a shaped charge  140 . The shaped charge  140  may be positioned so as to break the pressure-isolating wall  114  in response to an initiation or detonation of the shaped charge. In other words, an aiming direction of the shaped charge  140  may be aligned so as to intersect with the pressure-isolating wall  114 . The shaped charge  140  may be provided in a shaped charge chamber  146  adjacent to the first chamber  106  and opposite the second chamber  112 . A separation wall  144  may be provided between the shaped charge chamber  146  and the first chamber  106 . When the shaped charge  140  is detonated, the resultant perforating jet will puncture the separation wall  144 , propagate through the first chamber  106 , and then shatter or otherwise break the pressure-isolating wall  114 . The shaped charge  140  may be held in place within the shaped charge chamber  146  via a charge retainer  148  that may be threadedly engaged with the sub housing  104 . The dynamic underbalance sub  102  may further include a booster  142  ballistically coupled to the shaped charge  140 . The booster  142  may be ballistically coupled to an upstream first wellbore tool such as a perforating gun such that when the perforating gun is initiated, the booster  142  is also initiated, thereby initiating the shaped charge  140 . 
       FIG.  2    further shows a gap  150  between the booster  142  and the shaped charge  140 . However, it will be understood that the gap  150  is not required, and that, in an exemplary embodiment, the gap  150  may be reduced or even eliminated. 
       FIG.  2    shows an exemplary embodiment of a wellbore tool string  202 . The wellbore tool string  202  may include a first wellbore tool  204  and the dynamic underbalance sub  102 . The dynamic underbalance sub  102  may be threadedly coupled to a tool housing  206  of the first wellbore tool  204 . 
     As seen in  FIG.  2   , the first wellbore tool  204  may include the tool housing  206  and a tool explosive  208  provided within the tool housing  206 . In an exemplary embodiment, the first wellbore tool  204  may be a perforating gun and the tool explosive  208  may be a detonating cord. The first wellbore tool  204  may include shaped charges  218  configured to be initiated by the tool explosive  208 . 
     As further seen in  FIG.  2   , a container  210  may be threadedly engaged with the sub housing external threads  136  such that the second chamber second end  134  is disposed within the container  210 . A seal element  212  may be provided between the container  210  and the sub housing  104 . In an exemplary embodiment, the seal element  212  may be an O-ring. However, it will be understood that the seal element  212  is not limited to an O-ring and other seals including, but not limited to, liquid seals, foam seals, resins, polymers, coatings, or other suitable seal material. 
     The container  210  may be configured as a cylinder with an open end and a closed end. In an exemplary embodiment, the container  210 , the seal element  212 , the pressure-isolating wall  114 , and the seal element  126  may combine to pressure-seal the second chamber  112  such that the pressure within the second chamber  112  may be maintained regardless of the pressure in the first chamber  106 . The container  210  may be manufactured in a variety of lengths so as to allow the second chamber  112  to be set to a variety of volumes depending on the specific application and amount of dynamic underbalance required. In other words, a longer container  210  would result in a larger volume of the second chamber  112 , thereby increasing the dynamic underbalance. 
     As further seen in  FIG.  2   , the booster  142  may include a booster first end  214  and a booster second end  216 . When the dynamic underbalance sub  102  is coupled to the first wellbore tool  204 , the booster first end  214  may be provided proximate to the tool explosive  208  such that the booster  142  is ballistically coupled to the tool explosive  208 . In other words, the booster  142  is positioned such that when the tool explosive  208  is initiated, the initiation of the tool explosive  208  will subsequently initiate that booster  142 . The booster second end  216  may be provided proximate to the shaped charge  140  such that the shaped charge  140  is ballistically coupled to the booster  142 . Thus, overall, the shaped charge  140  may be ballistically coupled to the tool explosive  208 . In other words, initiation of the tool explosive  208  may ultimately cause initiation of the shaped charge  140  to break the pressure-isolating wall  114  and create the dynamic underbalance. 
     Because the second chamber  112  is maintained at a pressure lower than the wellbore pressure, for example, at surface atmospheric pressure, the breaking of the pressure-isolating wall  114  caused by the initiation of the shaped charge  140  creates a significant pressure differential between the first chamber  106  and the second chamber  112 . The pressure differential causes the wellbore fluid to rapidly fill the second chamber  112 , thereby creating the desired dynamic underbalance. This causes a rapid inflow from the wellbore into the first chamber  106  through the opening  108 . 
     The exemplary embodiments described above may result in significant advantages over conventional dynamic underbalance systems. For example, the opening of the second chamber  112  has the velocity of a shaped charge explosion and by far exceeds the sonic velocity. In other words, opening of the second chamber  112  to create the dynamic underbalance may occur much faster than in a conventional gas pressure driven system. Additionally, the embodiments discussed above require comparatively fewer parts and a less complicated structure than conventional systems, thereby making manufacture and assembly more efficient and less expensive, as well as improving reliability in the generation of the dynamic underbalance. 
       FIG.  3    shows an exemplary embodiment of a dynamic underbalance sub  302  in which the separation wall  144  shown in  FIG.  1    is replaced with a spacer  304 . The spacer  304  may include a spacer wall  306  that is similar in function to the separation wall  144 . Spacer seals  308  may be provided between the spacer  304  and the sub housing  104  in a radial direction. Fasteners  310  may be inserted through the sub housing  104  in the radial direction to engage with the spacer  304  and maintain its position in the axial direction. The spacer  304  may be insertable and removable from the dynamic underbalance sub  302 . Use of the spacer  304  may provide an advantage in that it allows for the dynamic underbalance sub  302  to be reused in multiple well deployments. After initiation, the dynamic underbalance sub  302  can be removed from the well, the perforated spacer  304  can be removed from the dynamic underbalance sub  302 , and a new spacer  304  with an intact spacer wall  306  can be inserted into the dynamic underbalance sub  302  for another deployment. In this way, only the material for the material for the spacer  304  is spent for each deployment, instead of requiring an entirely new sub housing  104  due to a perforated separation wall  144 . 
     Additionally,  FIG.  3    shows an exemplary embodiment in which openings  312  are provided as approximately round holes through the sub housing  104 , as compared with the elongated opening  108  shown in  FIG.  1   . The use of round openings  312  may result in improved structural stability of the dynamic underbalance sub  302  during wellbore operations. In an exemplary embodiment, a diameter of the opening  312  is at least as large as a diameter  314  between the first chamber  106  and the second chamber  112 . 
       FIG.  4    shows an exemplary embodiment of a dynamic underbalance sub  402  which allows for ballistic transfer to downstream first wellbore tools. In the embodiments shown in  FIG.  1   ,  FIG.  2   , and  FIG.  3   , the dynamic underbalance sub  102  or the dynamic underbalance sub  302  is placed at a downhole end of the wellbore tool string  202 , as the ballistic continuity ends with the shaped charge  140 . In the dynamic underbalance sub  402  shown in  FIG.  4   , a receiver explosive  404  is provided in the second chamber  112 . In an exemplary embodiment, the receiver explosive  404  may be a receiver booster, but it will be understood that any explosive that can be initiated by the perforating jet of the shaped charge  140  may be used as the receiver explosive  404 . The receiver explosive  404  may be held in place by a booster holder  408 . The receiver explosive  404  may be ballistically coupled to a detonating cord  406 , and the detonating cord  406  may in turn be ballistically coupled to elements in a downstream first wellbore tool. Upon initiation of the shaped charge  140 , the perforating jet of the shaped charge  140  may initiate the receiver explosive  404 , in turn initiating the detonating cord  406 . This allows for ballistic transfer to further wellbore tools attached below the dynamic underbalance sub  402 . For example, a second wellbore tool may be coupled to the dynamic underbalance sub  402  opposite the first wellbore tool  204 , and an explosive within the second wellbore tool may be ballistically coupled to the receiver explosive  404  through the detonating cord  406 . Thus, one or more dynamic underbalance subs  402  may be provided at various positions along a wellbore tool string in order to provide dynamic underbalance throughout the tool string, and the receiver explosive  404  in each dynamic underbalance sub  402  may provide ballistic continuity throughout the wellbore tool string. 
       FIG.  5    show an exemplary embodiment of a dynamic underbalance sub  502  in a closed state, i.e., prior to actuation. As seen in  FIG.  1   , the sub has a sub body  504 , which may be made of steel, with threads  510 ,  512  and seal elements  514  on an upper end  506  and a lower end  508  of the sub body  504 . A hollow interior  516  may extend from the upper end  506  to the lower end  508 . The sub body  504  may have two different types of openings into the hollow interior  516 : one or more sub windows  702  (see  FIG.  7   ) and one or more inflow channels  518 . In the closed state shown in  FIG.  1   , the sub window  702  may be closed by a sliding sleeve piston  520  and its seals  522 , which are located between the sliding sleeve piston  520  and the sub body  504 . The sliding sleeve piston  520  may be held in place by one or more shear elements  524 . The sliding sleeve piston  520  may include a piston window  526  which is displaced from the sub window  702  when the dynamic underbalance sub  502  is in a closed position as shown in  FIG.  1   . 
     The inflow channels  518  may allow fluid communication between the hollow interior  516  of the dynamic underbalance sub  502  and an exterior environment, i.e., the wellbore. A valve sealing block  528  may be located inside the hollow interior  516  to close the inflow channels  518  by using seals  530  axially displaced from the inflow channels  518 . The sliding sleeve piston  520  may be axially displaced from the valve sealing block  528 . Interior sealing block walls  532  may be located centrally in the valve sealing block  528 . The interior sealing block wall  532  may have a small thickness so as to enable ballistic perforation thereof but are configured to withstand the surrounding wellbore pressure due to the relatively small surface area exposed to hydrostatic pressure. A valve actuating booster  534  may be axially displaced from the valve sealing block  528  in a direction opposite from the sliding sleeve piston  520 . The valve actuating booster  534  may be an explosive pellet with a focused output or a perforation industry standard percussion initiator. The direction of detonation energy output of the valve actuating booster  534  is aimed toward the valve sealing block  528 . On or more booster seals  536  may be configured to seal between the valve actuating booster  534  and the valve sealing block  528 . In other words, the booster seals  536  may be provided between the valve actuating booster  534  and the valve sealing block  528  in a radial direction. The valve actuating booster  534  may be held by a booster holder  538 . The booster holder  538  may be secured to the sub body  504  by using threads, as seen in  FIG.  5   , or by another suitable locking mechanism. 
       FIG.  6    shows an exemplary embodiment of a wellbore tool string  602  including the dynamic underbalance sub  502 . A first end of the dynamic underbalance sub  502  is attached to a perforating gun  604  and the connection may be sealed with O-rings  606  positioned between the dynamic underbalance sub  502  and the perforating gun  604  in a radial direction. A second end of the dynamic underbalance sub  502  may be attached to a container  608 . A container housing  610  of the container  608  may define a container interior  612 . The container interior  612  may be maintained at a pressure substantially lower than the wellbore pressure. For example, the container interior  612  may be maintained at surface atmospheric pressure. Additionally, a connection between the dynamic underbalance sub  502  and the container  608  may be sealed by O-rings  614 . 
     As further seen in  FIG.  6   , the perforating gun  604  may further include a gun housing  616 . A charge tube  618  housing shaped charges  620  may be provided in a gun interior  622  of the perforating gun  604 . An end plate  624  and a retainer ring  626  may be provided at each end of the charge tube  618  to fix a position of the charge tube  618  within gun housing  616 . The shaped charges  620  may face toward an outside of the gun housing  616 . A back side or apex of each shaped charge  620  may be attached to a detonating cord  628 . 
       FIG.  6    further shows that an end of the detonating cord  628  may be coupled to a ballistic booster  630 . The ballistic booster  630  may be held in place by a booster holder  632  in a fixed position relative to the perforating gun  604 . The booster holder  632  may be fixed on the end plate  624  by a locking mechanism and a spring  634 . The ballistic booster  630  may be located towards or proximate to the valve actuating booster  534  provided within the dynamic underbalance sub  502 . 
     The valve may be actuated to an open position ballistically by piercing or perforating the interior sealing block walls  532 . To activate the perforating gun  604 , the detonating cord  628  may be initiated. Initiating the detonating cord  628  detonates each shaped charge  620 , followed by the ballistic booster  630 , which detonates and ruptures the interior sealing block walls  532 . By rupturing the interior sealing block walls  532 , fluid from the wellbore can rapidly pass through the inflow channels  518  and into the hollow interior  516  between the sliding sleeve piston  520  and the valve sealing block  528 . With increasing pressure, the shear elements  524  may be sheared off and the sliding sleeve piston  520  may move towards the lower end  508  of the dynamic underbalance sub  502 . 
       FIG.  7    shows an exemplary embodiment of the dynamic underbalance sub  502  in an open state after actuation. The sliding sleeve piston  520  reaches a position where the piston window  526  and the sub window  702  are aligned in the axial direction. Once the piston window  526  and the sub window  702  are aligned, the container interior  612  (see  FIG.  6   ) is in fluid communication with the wellbore environment. Accordingly, because the pressure in the container interior  612  is substantially lower than the pressure in the wellbore environment, a negative pressure situation is created, and wellbore fluid will flow into the container interior  612 . This flow of the wellbore fluid will remove the debris from the perforations in the surrounding formation and may also reduce the skin-damage in the perforation tunnel. In an exemplary embodiment, an amount of dynamic underbalance (e.g., an amplitude and/or duration of the dynamic underbalance condition) may be varied by varying a size of the container interior  612 . For example, a variety of container sizes of container  608  may be manufactured, each having a different length, which consequently results in different sizes of the container interior  612 . In an application where a relatively low amount of dynamic underbalance is desired, a shorter container  608  may be used. Alternatively, in an application where a relatively higher amount of dynamic underbalance is required, a longer container  608  could be used. A user may select the size of the container  608  to be used at the time of assembling the wellbore tool string  602 . 
       FIG.  8    shows an exemplary embodiment of a dynamic underbalance sub  802  in a closed state. As shown in  FIG.  8   , the dynamic underbalance sub  802  has a sub body  804 , which may be made of steel, having a first end  806  and a second end  808 . The sub body  804  may have threads  810 ,  812  and seal elements  814 ,  816  provided at each end. The sub body  804  may have a hollow interior  818  and a sub window  902  (see  FIG.  9   ), which connects the hollow interior  818  with an exterior of the dynamic underbalance sub  802 . In an exemplary embodiment, the exterior of the dynamic underbalance sub  802  would be a wellbore filled with a fluid or a gas. 
     The dynamic underbalance sub  802  may further include a sliding sleeve  820 , a sleeve holding mechanism, and a valve actuating explosive element. The sliding sleeve  820  may be arranged inside the sub body  804  such that the sub window  902  is closed and sealed from the outside environment by sleeve seals  822 . One end of the sliding sleeve  820  may be configured as a plurality of hook arms  824 . The hook arms  824  may be coupled or engaged with a hook shoulder  826  inside the sub body  804 . Without any load the hook arms  824  are configured to have a diameter less than the inner diameter of the hook shoulder  826 . An arm retainer  828  may be attached to the hook arms  824 , and may be configured to spread or deflect the hook arms  824  away from a center axis and towards the hook shoulder  826 . The hook arms  824  may be resiliently or elastically biased so as to return to a position in which the hook arms  824  are not engaged with the hook shoulder  826  once the arm retainer  828  is removed. The arm retainer  828  may be sealed against an inner surface of the sub body  804  by arm retainer seals  830 . The arm retainer  828  may be held by a sleeve holder  832  and secured by one or more shear elements  834 . 
     As further seen in  FIG.  8   , a spring  836  may be provided at an end of the sliding sleeve  820  opposite the hook arms  824 . In the closed state shown in  FIG.  8   , the spring  836  is in a compressed state between the sub body  804  and the sliding sleeve  820 . A sleeve catcher  852  may be provided inside hollow interior  818  of the sub body  804  and attached to an inner surface of the sub body  804 , next to the second end  808  of the dynamic underbalance sub  802 . 
       FIG.  1    further shows that, in an exemplary embodiment, the sleeve holder  832  may have a bore  838  along its central axis, which may serve as a main gas channel. The outside of the sleeve holder  832  and the arm retainer  828  may form a cavity or pressure chamber  840 . The pressure chamber  840  may be fluidly connected to the bore  838  by one or more distribution channels  842 . 
     A valve actuating booster  844  may be positioned adjacent to the bore  838  and held in position by a booster holder  846 . A second booster  848 , for example, a tandem booster from a perforating gun, may be positioned separated from the valve actuating booster  844  by a booster gap  850 . The valve actuating booster  844  may be a percussion initiator, a bi-directional booster, or a detonating cord. 
     When the valve actuating booster  844  is initiated by detonating the second booster  848 , the valve actuating booster  844  produces a high amount of gas pressure. This gas pressure fills the bore  838 , the distribution channels  842 , and the pressure chamber  840 . The pressure buildup inside the pressure chamber  840  forces the pressure chamber  840  to expand until the shear elements  834  shear off, thus allowing the arm retainer  828  to move in a direction toward the first end  806  of the dynamic underbalance sub  802 . With the arm retainer  828  removed, the hook arms  824  are allowed to move towards the center axis due to the resilient/elastic bias, thereby disengaged from the hook shoulder  826 . With the hook arms  824  no longer engaged with the hook shoulder  826 , the sliding sleeve  820  is free to move in the axial direction. The bias force of the compressed spring  836  slides the sliding sleeve  820  toward the second end  808  of the dynamic underbalance sub  802 . The movement of the sliding sleeve  820  may be stopped by the sleeve catcher  852 . 
       FIG.  9    shows an exemplary embodiment of the dynamic underbalance sub  802  in an actuated state in which the sliding sleeve  820  is displaced toward the second end  808  of the dynamic underbalance sub  802 . Once the sliding sleeve  820  has been moved, the sub window  902  is exposed, thereby allowing fluid communication between the hollow interior  818  and an exterior of the dynamic underbalance sub  802 . 
     Similar to the embodiments shown in  FIG.  5   ,  FIG.  6   , and  FIG.  7   , a container may be coupled with the second end  808  of the dynamic underbalance sub  802 . The inner volume of the container, which is at ambient pressure, may be flooded through the dynamic underbalance sub  802  by the fluid or gas from the wellbore increasing the effect of dynamic underbalance during perforation. In an exemplary embodiment, an amount of dynamic underbalance (e.g., an amplitude and/or duration of the dynamic underbalance condition) may be varied by changing a size of the interior of the container. For example, a variety of containers may be provided, each having a different length, which consequently leads to different sized interiors. In an application where a relatively low amount of dynamic underbalance is desired, a shorter container could be used. Alternatively, in an application where a relatively higher amount of dynamic underbalance is desired, a longer container could be used. A user could select the size of the container to be used at the time of assembling the tool string. 
     This disclosure, in various embodiments, configurations and aspects, includes components, methods, processes, systems, and/or apparatuses as depicted and described herein, including various embodiments, sub-combinations, and subsets thereof. This disclosure contemplates, in various embodiments, configurations and aspects, the actual or optional use or inclusion of, e.g., components or processes as may be well-known or understood in the art and consistent with this disclosure though not depicted and/or described herein. 
     The phrases “at least one”, “one or more”, and “and/or” are open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B and C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “one or more of A, B, or C” and “A, B, and/or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together. 
     In this specification and the claims that follow, reference will be made to a number of terms that have the following meanings. The terms “a” (or “an”) and “the” refer to one or more of that entity, thereby including plural referents unless the context clearly dictates otherwise. As such, the terms “a” (or “an”), “one or more” and “at least one” can be used interchangeably herein. Furthermore, references to “one embodiment”, “some embodiments”, “an embodiment” and the like are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term such as “about” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Terms such as “first,” “second,” “upper,” “lower” etc. are used to identify one element from another, and unless otherwise specified are not meant to refer to a particular order or number of elements. 
     As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capabilityre, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while considering that in some circumstances the modified term may sometimes not be appropriate, capable, or suitable. For example, in some circumstances an event or capacity can be expected, while in other circumstances the event or capacity cannot occur—this distinction is captured by the terms “may” and “may be.” 
     As used in the claims, the word “comprises” and its grammatical variants logically also subtend and include phrases of varying and differing extent such as for example, but not limited thereto, “consisting essentially of” and “consisting of.” Where necessary, ranges have been supplied, and those ranges are inclusive of all sub-ranges therebetween. It is to be expected that the appended claims should cover variations in the ranges except where this disclosure makes clear the use of a particular range in certain embodiments. 
     The terms “determine”, “calculate” and “compute,” and variations thereof, as used herein, are used interchangeably and include any type of methodology, process, mathematical operation or technique. 
     This disclosure is presented for purposes of illustration and description. This disclosure is not limited to the form or forms disclosed herein. In the Detailed Description of this disclosure, for example, various features of some exemplary embodiments are grouped together to representatively describe those and other contemplated embodiments, configurations, and aspects, to the extent that including in this disclosure a description of every potential embodiment, variant, and combination of features is not feasible. Thus, the features of the disclosed embodiments, configurations, and aspects may be combined in alternate embodiments, configurations, and aspects not expressly discussed above. For example, the features recited in the following claims lie in less than all features of a single disclosed embodiment, configuration, or aspect. Thus, the following claims are hereby incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this disclosure. 
     Advances in science and technology may provide variations that are not necessarily express in the terminology of this disclosure although the claims would not necessarily exclude these variations.