Patent Publication Number: US-7896077-B2

Title: Providing dynamic transient pressure conditions to improve perforation characteristics

Description:
TECHNICAL FIELD 
     The invention relates generally to providing dynamic transient pressure conditions in a wellbore to improve characteristics of perforations formed in reservoirs. 
     BACKGROUND 
     To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun string may be lowered into the well and the guns fired to create openings in casings and to extend perforations into the surrounding formation. 
     The explosive nature of the formation of perforation tunnels shatters sand grains of the formation. A layer of “shock damaged region” having a permeability lower than that of the virgin formation matrix may be formed around each perforation tunnel. The process may also generate a tunnel full of rock debris mixed in with the perforator charge debris. The extent of the damage, and the amount of loose debris in the tunnels may impair the productivity of production wells or the injectivity of injector wells. 
     To obtain clean perforations and to remove perforation damage, underbalanced perforating can be performed, where the perforation is carried out with lower wellbore pressure than the formation pressure. Schlumberger&#39;s PURE (Perforating for Ultimate Reservoir Exploitation) technology has been used to provide a transient underbalance just after creating perforations to minimize or eliminate perforation damage and to enhance productivity or infectivity. 
     However, it has been determined that using just a transient underbalance does not provide optimal perforations in some scenarios. 
     SUMMARY 
     In general, according to an embodiment, a method for use in a well includes creating a transient overbalance condition in a wellbore interval such that a pressure of the wellbore interval is greater than a reservoir pressure in surrounding formation, where creating the transient overbalance condition causes a near-wellbore region of the formation to increase in pressure. The pressure in the wellbore interval is reduced at a rate that produces a relative underbalance condition in which the pressure in the wellbore interval is less than the pressure of the near-wellbore region of the formation, but the pressure in the wellbore interval is greater than the reservoir pressure. 
     In general, according to another embodiment, a method for use in a well includes generating a pressure overbalance condition in a wellbore interval using a device having an inflatable element, where the inflatable element is inflated to generate the transient pressure overbalance condition. After generation of the pressure overbalance condition, the device is used to drop the pressure in the wellbore interval to create a pressure differential between the wellbore interval and surrounding near-wellbore region of the formation. 
     Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example arrangement of a portion of a tool string used to form perforations in a formation surrounding a wellbore interval, according to an embodiment. 
         FIG. 2  illustrates generation of pressure pulses using a pressure-controlling device in the tool string of  FIG. 1 . 
         FIGS. 3-5  illustrate an example of a dynamic overbalance chamber device for generating a transient overbalance condition according to an embodiment. 
         FIG. 6  is a graph depicting wellbore pressure and near-wellbore formation pressure as a function of time, generated using the tool string according to an embodiment. 
         FIG. 7  illustrates a perforating gun having a surge chamber. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments are possible. 
     As used here, the terms “above” and “below”; “up” and “down”; “upper” and “lower”; “upwardly” and “downwardly”; or other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments of the invention. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or diagonal relationship as appropriate. 
     In accordance with some embodiments, a transient pressure overbalance condition is generated in a wellbore interval using a dynamic overbalance chamber (DOBC) device that has an inflatable element that is inflated to generate the pressure overbalance condition. In some implementations, the transient pressure overbalance condition can be created prior to initiation of shaped charges in a perforating gun such that during formation of perforation tunnels in surrounding formation, wellbore fluid is forced into the perforations resulting in an increase in pore pressure adjacent to the perforations. 
     The DOBC device can also be used to create a pressure differential between the wellbore interval and the surrounding formation by deflating or abruptly halting the inflation of the inflatable element of the DOBC device. In some embodiments, deflation of the inflatable element in the DOBC device allows the pressure in the wellbore interval to drop faster than the surrounding formation pressure. As a result, there is some period of time during which the wellbore interval has a lower pressure than the surrounding formation pressure, effectively providing a relative underbalance condition in which the pressure in the wellbore interval is less than the pressure of the surrounding formation, at least in the near-wellbore region of the formation. The near-wellbore region of a formation refers to the region of the formation adjacent the wellbore. The ability to create the pressure differential between the wellbore interval and at least the near-wellbore region of the formation addresses issues in which a true underbalance condition cannot easily be created, such as when reservoir pressure is relatively low. 
     Effectively, a technique according to some embodiments allows for super-charging of the near-wellbore region of the formation to a higher pressure, using the DOBC device, such that the subsequent drop in the wellbore interval at a faster rate than the near-wellbore region of the formation allows for the creation of the relative underbalance condition in which the wellbore pressure is less than the pressure of the formation in the near-wellbore region. A true underbalance condition is a condition where the wellbore interval pressure is lower than the surrounding reservoir pressure. The relative underbalance condition created using the DOBC device provides an underbalance of the wellbore interval relative to the super-charged near-wellbore region—the reservoir pressure may actually be at or lower than the wellbore interval pressure. 
       FIG. 1  illustrates an example arrangement that shows a portion of a perforating tool that includes a perforating gun  102 , a first DOBC device  104  above the perforating gun  102 , and a second DOBC device  106  below the perforating gun  102 . In alternative implementations, just one DOBC device (or more than two DOBC devices) can be used. 
     The perforating gun  102  includes shaped charges  103  that when fired creates perforating jets that extend into the formation  108  that surrounds wellbore interval  110 . In the example arrangement of  FIG. 1 , the DOBC devices  104  and  106  are initiated prior to initiation of the perforating gun  102 . In one example implementation, the DOBC devices  104 ,  106  can be activated simultaneously, or substantially simultaneously (within some predefined amount of time of each other that is less than the amount of time between activation of a DOBC device and activation of the perforating gun  102 ). 
     Activation of the DOBC devices  104 ,  106  (which inflates inflatable elements in the DOBC devices  104 ,  106 ) causes a transient overbalance pressure condition to be created in the wellbore interval  110 . After a predetermined delay time, the perforating gun  102  is fired (in the presence of the transient pressure overbalance condition). The effect of the transient overbalance condition created by the DOBC devices  104 ,  106  is that a near-wellbore region  112  of the formation  108  is super-charged (in other words, the pressure of the near-wellbore region  112  is increased relative to the reservoir pressure). Following activation of the perforating gun  102 , the pressure of the wellbore interval  110  is dropped (such as by deflating or abruptly halting inflation of the inflatable elements in the DOBC devices  104 ,  106 ) to create a pressure differential between the wellbore interval  110  and at least the near-wellbore region  112  of the surrounding formation  108 . This effectively provides a dynamic underbalance condition to allow for perforations formed by the perforating gun  102  to be cleaned, and perforation damage to be removed or reduced. 
     In some implementations, to enhance the relative underbalance condition in the wellbore interval  110 , the perforating gun  102  can be a gun that is able to create a pressure drop (in the form of a surge) after the perforating operation. In such implementations, the pressure drop can be accomplished by using a surge chamber in the perforating gun  102 , where the surge chamber is initially sealed from the wellbore environment. The surge chamber can include an atmospheric chamber. Activation of the perforating gun  102  and firing of shape charges  103  in the perforating gun  102  causes one or more ports of the surge chamber to be opened such that surrounding wellbore fluids can rapidly flow into the surge chamber to create the dynamic underbalance condition in the wellbore interval  110 . 
     In other implementations, the perforating gun  102  can be a standard perforating gun without a surge chamber. In such implementations, the DOBC devices  104 ,  106  are relied upon to provide the relative underbalance condition in the wellbore interval  110 . 
     In some implementations, each of the DOBC devices  104 ,  106  and perforating gun  102  can be activated by using a respective initiating device  120 ,  122 , and  124 . The initiating devices  120 ,  122 ,  124  can be exploding foil initiator (EFI) devices or exploding bridge wire (EBW) devices, in which provision of an input activation voltage causes a portion (e.g., a metallic foil) to explode or vaporize, which causes a small flyer to shear from a surface and to travel in a direction towards an explosive element. The flyer, upon impact with the explosive element, causes detonation of the explosive element. 
     The EFI device can be a triggered EFI device, where a trigger input is provided to allow easier and more reliable activation of the EFI device. 
     The EFI devices  120 ,  122 , and  124  can be associated with delay mechanisms to allow for one of the EFI devices (e.g., EFI device  124  associated with the perforating gun  102 ) to be delayed with respect to at least another EFI device (e.g., EFI device  120  and/or EFI device  122 ). The delay mechanism allows for a delay of several milliseconds, for example, between activation of the DOBC devices and the perforating gun, such that the perforating gun can be fired in the presence of the transient overbalance condition created by the DOBC devices. 
       FIG. 2  illustrates how a DOBC device  104  or  106  is able to create a transient overbalance condition. Activation of the DOBC device  104  or  106  causes two pressure pulses  200  and  202  to be created, one moving in a first direction  204  along the wellbore  208 , and the second pressure pulse  202  traveling in the second direction  206  that is opposite the first direction  204  along the wellbore  208 . Thus, going back to the example of  FIG. 1 , activation of the DOBC device  106  would cause a first pressure pulse to travel upwardly, and a second pressure pulse to travel downwardly. Activation of the DOBC device  104  would also cause a first pressure pulse to travel upwardly, and a second pressure pulse to travel downwardly. In the region adjacent the perforating gun  102 , the two pressure pulses (the downwardly traveling pressure pulse from DOBC device  104  and the upwardly traveling pressure pulse from the DOBC device  106 ) combine to generate the transient overbalance condition. Note that use of just one DOBC device (instead of two as depicted in  FIG. 1 ) would also be sufficient to generate the transient overbalance condition. 
     An example DOBC device  104  or  106  is depicted in  FIG. 3 , where the DOBC device  104  or  106  includes an inflatable element  300  (which can be an inflatable bladder) contained in a housing  302  of the DOBC device. The inflatable bladder  300  can be formed of a polymer or other flexible material that allows for inflation of the bladder  300 . Alternatively, the bladder  300  can be formed of a high strength textile material which can be deployed similar in manner to an automotive air bag. The housing  302  has ports  304  that allow fluid communication between an inner cavity  306  of the DOBC device and the outside of the DOBC device. These ports can be holes of controlled diameter or permeable barriers. 
     Another example of an inflatable element can be a moving metal boundary, such as a metallic canister containing an energetic material. This example would create a wellbore pressure overbalance condition of shorter duration but larger amplitude than the inflatable bladder example. 
     The DOBC device  104  or  106  also includes pressure source  308  that is positioned in the housing  302  next to the inflatable bladder  300 . The pressure source  308  can be a propellant or a pressurized gas cylinder, according to some examples. 
     A pressure communication mechanism  310  is provided between the pressure source  308  and the inflatable bladder  300 . The other end of the inflatable bladder  300  is connected to an end plug  318 . The pressure communication mechanism  310 , when activated, allows for pressure from the pressure source  308  to be communicated into an inner chamber  312  of the inflatable bladder  300  to cause the inflatable bladder  300  to expand radially outwardly. For example, if the pressure source  308  is a pressurized gas cylinder, then the pressure communication mechanism  310  can include a pierce valve  314  that pierces an opening in the pressurized gas cylinder  308  to allow pressure in the pressurized gas cylinder  308  to flow through the pierce valve  314  and a flow path  316  into the inner chamber  312  of the inflatable bladder  300 . Piercing of the pressurized gas cylinder  308  can be accomplished by moving the pressurized gas cylinder longitudinally toward the pierce valve  314  such that a seal of the pressurized gas cylinder is broken. Alternatively, the pierce valve  314  can have a moveable piercing element that when actuated can pierce a seal of the pressurized gas cylinder, or alternatively, a seal of the inflatable bladder  300 . 
     If the pressure source  308  is a propellant, then the pierce valve  314  can be omitted, as the propellant would be ignited to burn to cause creation of the pressurized gas that is communicated through the pressure communication mechanism  310  into the inner chamber  312  of the inflatable bladder  300 . 
       FIG. 4  shows engagement of a pressurized gas cylinder  308 , which has been moved longitudinally along the longitudinal axis of the DOBC device  104 ,  106  to engage the pierce valve  314  such that the pressurized gas inside the pressurized gas cylinder  308  communicates through the pressure communication mechanism  310  into the inner chamber  312  of the inflatable bladder  300 . As depicted in  FIG. 4 , the inflatable bladder  300  is in its inflated state. 
       FIG. 5  is an outer view of the DOBC device that shows the external housing  302  along with the ports  304  of the housing  302 . 
       FIG. 6  is a graph that shows wellbore pressure and near-wellbore pressure as a function of time, where the pressures are generated by operation of a DOBC device. The wellbore pressure is initially at a relatively low level ( 600 ), which corresponds to a time period where the DOBC device has not yet been activated. At some point, the DOBC device is activated, such as by igniting a propellant or by communicating the pressurized gas of a pressurized gas cylinder into the inner chamber of the inflatable bladder. Inflation of the inflatable bladder of the DOBC device causes the wellbore pressure to increase (as indicated at  602 ). Although a step  602  is illustrated to show the pressure increase, it is noted that the rise in pressure is likely to be more gradual, as indicated by the dashed ramp indicated as  604 . 
     The wellbore pressure reaches a high level ( 606 ) which corresponds to the pulse created by the DOBC device. As further shown in  FIG. 6 , in response to the transient overbalance condition in the wellbore interval, the near-wellbore region of the surrounding formation is super-charged (as represented by the gradual increase in pressure represented as  608 ). 
     At some point, pressurized gas is removed from the inner chamber of the inflatable bladder, which can occur by moving the pressurized gas cylinder away from the inflatable bladder, or due to the propellant burnout. Alternatively, the inflation of the bladder can be abruptly halted. As a result, as further depicted in  FIG. 6 , the wellbore pressure drops relatively rapidly (as indicated by  610 ). The pressure drop in the near-wellbore region of the formation is more gradual, as depicted by  1612 . Thus, there is some time duration (represented as  614 ) where the pressure in the wellbore interval is lower than the pressure of the near-wellbore region of the formation, which effectively provides a relative underbalance condition to allow perforations to be cleaned and damage in perforations to be reduced or removed. 
     Referring to  FIG. 7 , creating an underbalance condition during a perforating a perforating gun  102  includes a gun housing  702 . In one embodiment, the perforating gun  102  is a hollow carrier gun having shaped charges  103  inside a chamber  718  of the sealed housing  702 . 
     During detonation of the shaped charges  103 , perforating ports  720  are formed in the housing  702  as a result of perforating jets produced by the shaped charges  103 . During detonation of the shaped charges  103 , hot gas fills the internal chamber  718  of the gun  102 . If the resultant detonation gas pressure is less than the wellbore pressure by a given amount, then the cooler wellbore fluids will be drawn into the chamber  718  of the gun  102 . The rapid acceleration of well fluids through the perforation ports  720  will break the fluid up into droplets, which results in rapid cooling of the gas within the chamber  718 . The resultant rapid gun pressure loss and even more rapid wellbore fluid drainage into the chamber  718  causes the wellbore pressure to be reduced. 
     In some implementations, a treating fluid can be provided in the vicinity of the perforating gun  102 . The treating fluid can be provided in the wellbore interval  110 , in the perforating gun  102  itself, or in some other container. The treating fluid is driven into perforations by the transient overbalance condition created by the DOBC devices. 
     One type of treating fluid is a consolidation fluid that can be used to strengthen the perforations and near-wellbore region of the formation to prevent formation movement or movement of fine particles. One example type of consolidation fluid includes an epoxy fluid that is embedded with micro-capsules, where the micro-capsules have inner cavities that contain a hardener or catalyst fluid. Initially the hardener fluid inside the micro-capsules is isolated from the epoxy fluid. Initially, the wellbore interval can have a modest overbalance condition with the consolidation fluid covering the wellbore interval to be perforated. The creation of a large dynamic overbalance condition by the DOBC devices results in a shock wave moving through the wellbore fluid to fracture the micro-capsules such that the hardener fluid inside the micro-capsules are mixed with the epoxy. During this time period, the large dynamic overbalance condition forces the epoxy mixture into the near-wellbore region of the formation. Hardening of the epoxy helps to consolidate weak and unconsolidated rock in the near-wellbore region. A benefit of performing consolidation as discussed above is that a one-trip screen-less sand control operation is possible. 
     Another technique of delivering a hardener or catalyst fluid into the formations is to pre-deliver the hardener or catalyst fluid into the perforations, such as with drilling fluid used during the drilling of the wellbore. 
     Additionally, fluid above the DOBC device can be a post-wash fluid that is injected by application of continuous wellhead pressure. For applying the post-wash fluid, guns with big hole charges can be used. Such guns do not need to have surge chambers. 
     In another application, the treating fluid can be an acid, such as HCl, to treat a carbonate reservoir. The application of a large transient dynamic overbalance condition would inject a relatively large amount of acid into the perforations to provide stimulation. Perforating in the presence of the transient overbalance condition created by the DOBC device(s), with acid, enables perforating plus acidizing. Acidizing helps remove or reduce perforation damage. 
     Another type of treating fluid that can be used is proppant-laden fracturing fluid provided in the wellbore interval  110 . Proppant refers to particles mixed with fracturing fluid, which can be used in a fracturing operation to hold fractures open. 
     In another application, multiple treating fluids can be provided in the presence of the transient overbalance condition created by the DOBC device(s). Activation of the perforating gun to perform perforating can then cause the multiple treating fluids to be mixed. In some implementations, mixing of multiple fluids can cause activation of the fluids. This may be useful with resin consolidation, for example. 
     In another implementation, sequential application of multiple treating fluids can be performed. A first treating fluid can be applied in the presence of the transient overbalance condition created by the DOBC device(s). After waiting a predetermined delay, another transient overbalance condition can be created, such as by release of a pressurized gas (e.g., nitrogen). A second treating fluid can be applied to the wellbore interval in the presence of the second transient overbalance condition. 
     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations there from. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.