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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and is a continuation-in-part of U.S. patent application Ser. No. 13/350,030, filed on Jan. 13, 2012, the contents of which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     The present invention relates to systems and methods used in down hole applications. More particularly, the present invention relates to the setting of a down hole tool in various down hole applications using a pressure sensitive sleeve that moves when subjected to a predetermined threshold pressure. 
     In the course of treating and preparing a subterranean well for production, down hole tools, such as well packers, are commonly run into the well on a tubular conveyance such as a work string, casing string, or production tubing. The purpose of the well packer is not only to support production tubing and other completion equipment, such as sand control assemblies adjacent to a producing formation, but also to seal the annulus between the outside of the tubular conveyance and the inside of the well casing or the wellbore itself. As a result, the movement of fluids through the annulus and past the deployed location of the packer is substantially prevented. 
     Some well packers are designed to be set using complex electronics that often fail or may otherwise malfunction in the presence of corrosive and/or severe down hole environments. Other well packers require that a specialized plug or other wellbore device be sent down the well to set the packer. While reliable in some applications, these and other methods of setting well packers add additional and unnecessary complexity and cost to the pack off process. 
     SUMMARY 
     The present invention relates to systems and methods used in down hole applications. More particularly, the present invention relates to the setting of a down hole tool in various down hole applications using a pressure sensitive sleeve that moves when subjected to a predetermined threshold pressure. 
     In some embodiments, a system for activating a down hole tool in a wellbore can include a base pipe having an interior and a port extending between the interior and a chamber. A pressure sensitive trigger can be moveably positioned in the interior of the base pipe, the trigger including a first end having a first area, and a second end having a second area that is smaller than the first area. The trigger can be moveable between an unactivated position where the port is blocked and substantial fluid communication between the interior and the chamber is prevented, and an activated position where the port is open and fluid communication between the interior and the chamber is allowed. At least one latch member can prevent movement of the trigger from the unactivated position to the activated position until a predetermined force is applied to the trigger. Increasing a pressure in the interior can increase a force differential between the first and second end, and when the force differential is substantially equal to the predetermined force, the latch can release and allow the trigger to move from the unactivated position to the activated position, thereby opening the port and pressurizing the chamber to permit activation of the down hole tool. 
     In other embodiments, a method for controlling activation of a down hole tool in a wellbore can include advancing the down hole tool into the wellbore with the down hole tool being coupled to a base pipe positioned within the wellbore and the base pipe defining an interior. Pressure in the interior can be increased to create a force differential on a trigger located within the interior, the trigger having a first end with a first area and a second, opposite end with a second area that is smaller than the first area. The trigger can be moveable between an unactivated position whereby activation of the down hole tool is prevented and an activated position whereby activation of the down hole tool is permitted. Movement of the trigger from the unactivated position to the activated position can be prevented with at least one latch member until the force differential is substantially equal to a predetermined latch release force, at which point the latch member can release the trigger and the force differential can cause movement of the trigger from the unactivated position to the activated position to permit activation of the down hole tool. 
     In yet other embodiments, a wellbore system can include a base pipe moveable along the wellbore and defining an interior and a port extending between the interior and a chamber. A pressure sensitive trigger can be moveably positioned in the interior of the base pipe. The trigger can be moveable between an unactivated position where the port is blocked and substantial fluid communication between the interior and the chamber is prevented, and an activated position where the port is open and fluid communication between the interior and the chamber is allowed. At least one latch member can prevent movement of the trigger from the unactivated position to the activated position until a pressure in the interior is increased to a predetermined level, at which point the latch member releases the trigger and allows the trigger to move from the unactivated position to the activated position. A down hole tool can be coupled to the base pipe. An activation assembly can include a chamber in communication with the port and a piston having a first end exposed to the chamber and a second end coupled to the down hole tool. Movement of the trigger to the activated position can open the port to permit pressurization of the chamber to move the piston and activate the down hole tool. 
     Features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the preferred embodiments that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following figures are included to illustrate certain aspects of the present invention, and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the art and having the benefit of this disclosure. 
         FIG. 1  illustrates a cross-sectional view of a portion of a base pipe and accompanying activation system, according to one or more embodiments disclosed. 
         FIG. 2  illustrates an enlarged view of a portion of the activation system shown in  FIG. 1  in an unactivated position. 
         FIG. 3  illustrates the portion of the activation system shown in  FIG. 2  in an activated position. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates to systems and methods used in down hole applications. More particularly, the present invention relates to the setting of a down hole tool in various down hole applications using a pressure sensitive sleeve that moves when subjected to a predetermined threshold pressure. 
     Systems and methods disclosed herein can be configured to activate and set a down hole tool, such as a well packer, in order to isolate the annular space defined between a wellbore and a base pipe (e.g., production string), thereby helping to prevent the migration of fluids through a cement column and to the surface. Other applications will be readily apparent to those skilled in the art. Systems and methods are disclosed that permit the down hole tool to be hydraulically-set without the use of electronics, signaling, or mechanical means. The systems and methods take advantage of a sleeve positioned within the pressure differentials between, for example, the annular space between the wellbore and the base pipe and one or more chambers formed in or around the tool itself and/or the base pipe. Consequently, the disclosed systems and methods simplify the setting process and reduce potential problems that would otherwise prevent the packer or down hole tool from setting. To facilitate a better understanding of the present invention, the following examples are given. It should be noted that the examples provided are not to be read as limiting or defining the scope of the invention. 
     Referring to  FIG. 1 , illustrated is a cross-sectional view of an exemplary activation system  100 , according to one or more embodiments. The system  100  may include a base pipe  102  extending within a wellbore  104  that has been drilled into the Earth&#39;s surface to penetrate various earth strata containing, for example, hydrocarbon-bearing formations. It will be appreciated that the system  100  is not limited to use in any specific type of well, but may be used in all types, such as vertical wells, horizontal wells, multilateral (e.g., slanted) wells, combinations thereof, and the like. A casing  106  may be disposed within the wellbore  104  and thereby define an annulus  108  between the casing  106  and the base pipe  102 . The casing  106  forms a protective lining within the wellbore  104  and may be made from materials such as metals, plastics, composites, or the like. In some embodiments, the casing  106  may be expanded or unexpanded as part of an installation procedure and/or may be segmented or continuous. In at least one embodiment, the casing  106  may be omitted and the annulus  108  may instead be defined between the inner wall of the wellbore  104  and the base pipe  102 . In still other embodiments, the base pipe  102  may be run within another, previously set casing string. 
     The base pipe  102  may include one or more tubular joints, having metal-to-metal threaded connections or otherwise threadedly joined to form a tubing string. In other embodiments, the base pipe  102  may form a portion of a coiled tubing. The base pipe  102  may have a generally tubular shape, with an inner radial surface  102   a  and an outer radial surface  102   b  having substantially concentric and circular cross-sections. However, other configurations of the base pipe  102  may be suitable, depending on particular conditions and circumstances. For example, some configurations of the base pipe  102  may include offset bores, sidepockets, etc. Moreover, the base pipe  102  may include portions formed of a non-uniform construction, for example, a joint of tubing having compartments, cavities or other components therein or thereon. Even further, the base pipe  102  may be formed of various components, including, but not limited to, a joint casing, a coupling, a lower shoe, a crossover component, or any other component known to those skilled in the art. In some embodiments, various elements may be joined via metal-to-metal threaded connections, welded, or otherwise joined to form the base pipe  102 . When formed from casing threads with metal-to-metal seals, the base pipe  102  may omit elastomeric or other materials subject to aging, and/or attack by environmental chemicals or conditions. 
     The system  100  may further include at least one down hole tool  110  coupled to or otherwise disposed about the base pipe  102 . In some embodiments, the down hole tool  110  may be a packer element, such as a well packer. In other embodiments, however, the down hole tool  110  may be a casing annulus isolation tool, a stage cementing tool, a multistage tool, formation packer shoes or collars, combinations thereof, or any other down hole tool. As the base pipe  102  is run into the well, the system  100  may be adapted to substantially isolate the down hole tool  110  from any fluid actions from within the casing  106 , thereby effectively isolating the down hole tool  110  so that circulation within the annulus  108  is maintained until the down hole tool  110  is actuated. 
     In one or more embodiments, the down hole tool  110  may include a resilient expansion element that expands radially outward when moved over a ramped element. Alternatively, the down hole tool  110  may include a compression element that expands when subjected to compression, a compressible slip on a swellable element, a compression-set element that partially collapses, a cup-type element, a chevron-type seal, one or more inflatable elements, an epoxy or gel introduced into the annulus  108 , combinations thereof, or other sealing elements. 
     The down hole tool  110  may be disposed about the base pipe  102  in a number of ways. For example, in some embodiments the down hole tool  110  may directly or indirectly contact the outer radial surface  102   b  of the base pipe  102 . In other embodiments, however, the down hole tool  110  may be arranged about or otherwise radially-offset from another component of the base pipe  102 . 
     Referring also to  FIG. 2 , the system  100  includes a trigger  112  that may be in the form of a pressure sensitive sleeve. The trigger  112  may be arranged within the interior of base pipe  102  and, in the illustrated configuration, may be axially movable with respect thereto. As illustrated, the trigger  112  may include a first end  116  having a first area and an opposite second end  120  having a second area that is smaller than the first area. The first and second areas may be axially projected areas obtained by calculating the area of the apparent shape of the trigger  112  when viewed in the direction of arrow A 1  ( FIG. 1 ) for the first area and in the direction of arrow A 2  ( FIG. 1 ) for the second area. 
     In the illustrated embodiment, the trigger  112  is substantially annular and includes a substantially constant inner diameter  124  and a stepped outer diameter  128  such that a first portion  138  of the trigger  112  adjacent the first end  116  may have a greater outer diameter and wall thickness than a second portion  142  of the trigger  112  adjacent the second end  120 . Although other configurations are possible, the stepped outer diameter of the trigger  112  contributes to the resulting difference between the first area and the second area. 
     In the illustrated embodiment, the outer diameter of the first portion  138  of the trigger  112  may engage the inner radial surface  102   a  of the base pipe  102 , and may include one or more seals  146  (one shown) positioned therebetween. Also in the illustrated embodiment, the outer diameter of the second portion  142  of the trigger  112  may engage a substantially annular collar  150  that may be fixed with respect to the base pipe  102  such that the trigger  112  is received by and axially slidable within the collar  150 . As shown, the collar  150  is located in an annular space between the second portion  142  of the trigger  112  and the inner radial surface  102   a  of the base pipe  102 . One or both of the collar  150  and the trigger  112  may include one or more seals  154  for sealing the engaging surfaces of the collar  150 , the trigger  112 , and the base pipe  102 . 
     The system  100  may also include a force-sensitive and releasable latch for preventing substantial movement of the trigger  112  with respect to the base pipe  102  until a predetermined force is applied to the trigger  112 . For example, the system  100  may include one or more shear pins  158  having a first end that is fixed with respect to the base pipe  102  and a second end that is fixed with respect to the trigger  112 . In the illustrated embodiment, the pins  158  include a first end that extends into the collar  150  and a second end that extends into the trigger  112 . In other embodiments, the pins  158  may extend into the base pipe  102  and the trigger  112 . In still other embodiments, a shear lip, a friction fit, or another force-sensitive and releasable securement may also or alternatively be provided to prevent substantial movement of the trigger  112  with respect to the base pipe  102  until a predetermined force is applied to the trigger  112 . 
     The system  100  may also include one or more ports  162  extending through or otherwise defined by or in the base pipe  102  and/or other system components for providing fluid communication between the interior of the base pipe  102  and a tool activation assembly  164 . In the illustrated embodiment the activation assembly  164  includes an activation chamber  166  located on the exterior of the base pipe  102  and defined in part by one or more external sleeves  170  disposed about the base pipe  102 . The activation assembly  164  can also include a movable element in the form of a piston  174  having a first end  178  exposed to the activation chamber  166  and a second end  182  operatively coupled to the down hole tool  110  such that movement of the piston  174  causes the down hole tool  110  to activate and set. Although the illustrated system  100  shows the piston  174  directly engaging the down hole tool  110 , various sleeves, guides, and other intermediate structures can also be provided between the piston  174  and the down hole tool  110  depending on the configuration or needs of a particular application. In some embodiments, a ratchet assembly  180  can be coupled to the piston  174  and configured to permit only one-way movement of the piston in the direction that sets the down hole tool  110 . In this way, the ratchet assembly  180  can secure the down hole tool  110  in the activated or set configuration. 
     In operation, the system  100  is advanced in the wellbore  104  until the tool  110  is at a desired location in the wellbore  104 . A shutoff plug (not shown), or other type of blanking device (e.g., dart, ball, etc.), may be landed down hole of the system  100  such that a pressure increase can be observed in the interior of the base pipe  102 . Pressure in the interior creates a force differential on the trigger  112  that tends to move the trigger  112  axially toward the second end  120 . More specifically, because the second end  120  has a smaller area than the first end  116 , the pressure in the interior creates a greater force on the first end  116  than the second end. The resulting force acting on the trigger  112  is an axial force that is substantially equal to the pressure in the interior multiplied by the difference between the first area and the second area. Accordingly, the force on the trigger  112  is proportional to the pressure in the interior, and as the pressure in the interior increases, so does the force on the trigger  112 . 
     As discussed above, the releasable latch, which in the illustrated embodiment includes shear pins  158 , prevents substantial axial movement of the trigger  112 . The latch is configured to release, e.g., the pins  158  are configured to shear, in response to application of a predetermined force to the trigger  112 . Thus, when the force on the trigger  112  caused by the increased pressure in the interior of the base pipe  102  becomes substantially equal to the predetermined force, the pins  158  will shear and the trigger  112  will be released for axial movement along the base pipe  102  in the direction A 1 . 
       FIGS. 1 and 2  show the trigger  112  in an unactivated position before the shear pins  158  have sheared. When in the unactivated position the trigger  112  blocks or otherwise substantially occludes the port  162  and thereby prevents substantial fluid communication between the interior of the base pipe  102  and the activation chamber  166 , which in turn prevents activation of the down hole tool  110 . After the pins  158  have sheared and the trigger  112  is released for axial movement along the base pipe  102 , the force differential caused by pressure acting on the different first and second areas of the first and second ends  116 ,  120  moves the trigger  112  axially in the direction A 1  to the activated position shown in  FIG. 3 . Axial movement of the trigger  112  may be halted when the larger-outer diameter first portion  138  of the trigger  112  engages and otherwise abuts the collar  150 . With the trigger  112  in the activated position, the port  162  provides fluid communication between the interior of the base pipe  102  and the activation chamber  166 . 
     With the trigger  112  in the activated position, pressure from the interior of the base pipe  102  is communicated to the activation chamber  166 . In some embodiments, the pressure required to move the trigger may be sufficient to move the piston  174 . In other embodiments, after the trigger  112  has moved to the activated position, pressure in the interior of the base pipe  102  may need to be further increased to cause movement of the piston  174 . In either case, as pressure from the interior of the base pipe  102  is communicated to the activation chamber  166 , the pressure acts on the first end  178  of the piston  174  until a force sufficient to move the piston  174  is reached. Because the second end  182  of the piston is operatively coupled to the down hole tool  110 , movement of the piston  174  causes the down hole tool  110  to activate and set. 
     Accordingly, the disclosed systems  100  and related methods may be used to remotely set the down hole tool  110 . The trigger  112  activates the setting action of the down hole tool  110  without the need for electronic devices, magnets, or mechanical actuators, but instead relies on elevating the pressure in the interior of the base pipe  102 . 
     In the foregoing description of the representative embodiments of the invention, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used for convenience in referring to the accompanying drawings. In general, “above”, “upper”, “upward” and similar terms refer to a direction toward the earth&#39;s surface along a wellbore, and “below”, “lower”, “downward” and similar terms refer to a direction away from the earth&#39;s surface along the wellbore. 
     Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended due to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the present invention. In addition, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the elements that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted.

Summary:
Systems and methods for activating a down hole tool in a wellbore. A trigger is moveably positioned in the interior of a base pipe and includes a first end and a second, smaller end. The trigger is moveable between an unactivated position where a port in the base pipe is blocked and an activated position where the port is open. At least one latch member prevents movement of the trigger from the unactivated position to the activated position until a predetermined force is applied to the trigger. Increasing pressure in the interior increases a force differential between the first end and the second end. When the force differential is substantially equal to the predetermined force, the latch releases and allows the trigger to move from the unactivated position to the activated position, thereby opening the port to permit activation of the down hole tool.