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BACKGROUND 
     1. Field of Invention 
     The invention is directed to pressure relief devices for compensating for pressure changes within sealed or isolated zones of an annulus of an oil or gas wellbore. 
     2. Description of Art 
     Sealing or isolating zones or areas of an annulus of wellbores is well known in the art. In general, one or more wellbore barriers such as packers or bridge plugs are disposed with in a wellbore above and below a “zone” or area of the wellbore in which production, or other wellbore intervention operations are performed. In some instances, the isolated zone is not being produced or intervention operations are not being performed, however, tubing, e.g., an inner casing, is disposed through this zone so that oil or gas production or other downhole operations can be performed below the isolated zone. In these instances, the fluid trapped or sealed in this isolated zone can expand or contract depending on the temperature of the fluid trapped in the isolated zone. When the temperature increases, such as during production from other zones within in the wellbore, the fluid expands and can cause damage to the inner casing of the wellbore, the outer casing of the wellbore, other components within the wellbore, or the formation itself. When the temperature decreases, such as when fluid is pumped or injected into the wellbore, the fluid contracts and can cause damage to the inner casing of the wellbore, the outer casing of the wellbore, other components within the wellbore, or the formation itself. 
     SUMMARY OF INVENTION 
     In situations where wells are designed with multiple barriers, such as packers, bridge plugs and the like, in the annular space, fluid becomes trapped in the space between these barriers. If the temperature of this trapped fluid increases, such as during production from the well, pressure within this isolated annular space increases. If the temperature of this trapped fluid decreases, such as during injection of fluids into the well, pressure within this isolated annular space decreases. In some situations, these pressure changes can be substantial and may cause failure of critical well components, including damage to the formation itself. 
     The pressure relief devices disclosed herein facilitate compensation of the pressure within the isolated wellbore annulus. Broadly, the pressure relief devices disclosed herein comprise a tubular member having a housing disposed on an outer wall surface of the tubular member. The housing includes a housing chamber and one or more ports disposed through the housing. An expandable member is disposed within the housing chamber. An interior portion of the expandable member is in fluid communication with the one or more ports. An outer wall surface of the expandable member isolates the remaining volume of the housing chamber to provide a sealed chamber. The sealed chamber can be maintained at atmospheric pressure or at a charged pressure. 
     The pressure relief devices can be disposed on a tubular string and located within a wellbore. As pressure in an environment located outside the pressure relief device, referred to herein as an “outside environment,” such as within an isolated wellbore annulus, increases such as due to an increase in temperature within the outside environment, the resultant increase in pressure is distributed through the port and into the interior of the expandable member causing expansion of the expandable member. As pressure within the outside environment decreases, such as due to a decrease in temperature within that environment, the resultant decrease in pressure is compensated by pressure moving from the interior of the expandable member, through the port, and into the outside environment. As a result, the likelihood that the change in pressure within the outside environment will cause damage to the wellbore or the tubing disposed within the wellbore or any other wellbore component within the outside environment is decreased. 
     During expansion of the expandable member due to the increased pressure within the outside environment exerting force on the hydrostatic side of the expandable member, the volume of the interior of the expandable member is increased and the volume of the sealed chamber becomes decreased. Decreasing the volume of the sealed chamber energizes the fluid or gas contained in the sealed chamber. Conversely, when the hydrostatic pressure is decreased, the compressed fluid or gas in the sealed chamber exerts a force on the sealed side of the expandable member to force the expandable member back until equilibrium of pressure on both sides of the expandable member is established, or until the expandable member can no longer move, such as due to all of the fluid within the interior of the expandable member being forced out by the pressure of the fluid within the sealed chamber. In other words, the atmospheric pressure or gas pressure within the sealed chamber acts as a return mechanism for the piston. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  comprises a cross-sectional view of one specific embodiment of a pressure relief device disclosed herein having an expandable member,  FIG. 1  showing the expandable member in a contracted position. 
         FIG. 2  comprises a cross-sectional view of the pressure relief device of  FIG. 1  showing the expandable member in a expanded position. 
     
    
    
     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF INVENTION 
     Referring now to  FIGS. 1-2 , one specific embodiment of a pressure relief device  10  is shown. This embodiment of pressure relief device  10  comprises tubular member  20  having outer wall surface  22  and inner wall surface  24  defining bore  26  having axis  28 . Disposed on outer wall surface  22  is housing  30 . 
     Housing  30  comprises upper end  31  and lower end  32 , and inner wall surface  33  for connecting housing  30  to outer wall surface  22  of tubular member  20 . Housing  30  also comprises housing chamber  34  and outer wall surface  35 . One or more ports  36  are disposed through one or both of upper and lower ends  31 ,  32 . As shown in  FIGS. 1-2 , housing  30  comprises four ports  36 . Ports  36  are in fluid communication with an environment outside of pressure relief device  10  and, as discussed in greater detail below, with an interior of an expandable member. 
     In the embodiment of  FIGS. 1-2 , each of ports  36  comprise filter  38  disposed within ports  36  to restrict flow of certain sized particles through ports  36 . Filter  38  may be a foam or meshed material formed by a polymer, ceramic, or metal. Alternatively, filter  38  can be glass or sintered metallic beads or other aggregate materials. 
     Expandable member  40  is disposed within housing chamber  34 . Expandable member  40  comprises upper end  41 , lower end  42 , interior  44  defined by inner wall surface  45 , and outer wall surface  46 . Interior  44  is in fluid communication with each of ports  36  so that inner wall surface  45  of expandable member  40  which defines interior  44  is referred to herein as the hydrostatic side of expandable member  40 . Outer wall surface  46  of expandable member  40  is also referred to herein as the sealed side of expandable member  40  because sealed chamber  50  is defined by outer wall surface  46  of expandable member  40  and upper end  31 , lower end  32 , and inner wall surface  33  of housing  30 . Thus, sealed chamber  50  comprises a portion of housing chamber  34 . 
     Expandable member  40  can be formed out of any material known or desired that permits expansion of expandable member  40 . Suitable materials include elastomers such as rubbers, ethylene-propylene terpolymers (EDPM), and the like. 
     In one particular embodiment, sealed chamber  50  comprises a pressure disposed therein. The pressure within sealed chamber can be atmospheric pressure or can be a charged pressure. A charged pressure means that a fluid such as nitrogen or some other gas or fluid is pumped into sealed chamber  50  to a desired pressure. For example the pressure within sealed chamber  50  can be charged to the operational pressure of pressure relief device  10 . Operational pressure is defined herein as the pressure anticipated at the location within the wellbore where pressure relief device  10  will be disposed. As noted above, the charged pressure within sealed chamber  50  can be established using air, nitrogen, or any other gas or fluid desired or necessary to provide the desired pressure within sealed chamber  50 . The charged pressure can be established by pumping the gas or other fluid through charge port  39 . Charge port  39  can include a one-way check valve  18  or other device known in the art to facilitate injection of the gas or other fluid so that the charged pressure remains within sealed chamber  50 . 
     In the embodiment of  FIGS. 1-2 , anti-extrusion devices  60  are disposed along outer wall surface  22  of tubular member at upper and lower ends  41 ,  42  of expandable member  40  so as to prevent expandable member  40  from extruding upward and downward. In embodiments comprising anti-extrusion devices  60 , ports  36  pass through anti-extrusion devices  60  so that interior  44  of expandable member  40  is in fluid communication with the environment outside of pressure relief device  10 . Anti-extrusion devices  60  can comprise rings or other devices secured to outer wall surface  22  of tubular member  20 . 
     In one specific operation of pressure relief device  10 , pressure relief device  10 , disposed in the contracted position (shown in  FIG. 1 ), is placed in a work string such as production string or other string of tubing (not shown) and run-into a cased wellbore (not shown). Pressure relief device  10  is then disposed within the cased wellbore at a location where the annulus of the wellbore is isolated from other parts of the wellbore. The isolation of the wellbore can be established by any method or device known in the art such as by use of one or more wellbore barriers such as packers, bridge plugs, valves, wellheads, the bottom of the wellbore, and the like. In so doing, interior  44  of expandable member  40  is placed in fluid communication with the isolated wellbore annulus through ports  36 . In the event that the fluid contained within the isolated wellbore annulus expands, or the pressure within the isolated wellbore annulus increases, such as due to production operations being performed through the work string, the increased pressure enters interior  44  of expandable member  40  and exerts a force on inner wall surface  45  causing expansion of expandable member  40  toward the expanded position (shown in  FIG. 2 ). Expansion of expandable member  40  causes the volume of sealed chamber  50  to decrease. As a result, the atmospheric pressure or gas pressure within sealed chamber  50  becomes compressed or “energized.” In addition, in certain embodiments, a portion of outer wall surface  35  of housing  30  inflects inwardly as shown in  FIG. 2  due to hydrostatic pressure also acting on outer wall surface  35 . 
     Expandable member  40  continues to expand within sealed chamber  50  until the pressure on both inner wall surface  45  and outer wall surface  46  reach equilibrium, or until expandable member  40  can no longer expand due to the size of sealed chamber  50 . In so doing, the pressure being exerted on the inner wall of the casing, or the inner wall of the formation, or the outer wall surface of the work string, is spread out and lessened, which decreases the likelihood of failure of any of the casing, the formation, or the work string, or any other wellbore component disposed in the isolated wellbore annulus. 
     Thereafter, if the pressure within the isolated wellbore annulus decreases, such as due to a temperature decrease due to cessation of production operations through the work string, the compressed atmospheric pressure or compressed fluid pressure within sealed chamber  50  exerts a force against outer wall surface  46  of expandable member  40  that is greater than the hydrostatic pressure within interior  44 , i.e., the hydrostatic pressure acting on inner wall surface  45 . Accordingly, expandable member  40  contracts from the expanded position ( FIG. 2 ) toward the contracted position ( FIG. 1 ) causing the volume in interior  44  to decrease and the volume of sealed chamber  50  to increase. Expandable member  40  continues to move toward the contracted position, reducing the volume of interior  44  and increasing the volume of sealed chamber  50 , until the pressure acting on inner wall surface  45  and outer wall surface  46  reach equilibrium, or until the volume within interior  44  can no longer decrease. Thereafter, expandable member  40  is in a position such that it can again expand in response to a pressure increase within the isolated wellbore annulus. 
     In another particular embodiment, one or more ports  36  is disposed only through lower end  32 . Location of the one or more port  36  through lower end  32  facilitates retaining gas within housing chamber  34  in the event that expandable member  40  fails. For example, in an embodiment in which sealed chamber  50  contains a gas, such as nitrogen, in the event that expandable member  40  fails, the gas will not be allowed to flow out of housing chamber  34 . Instead, it would be trapped above any fluid that previously flowed through the one or more ports  36  into interior  44  of expandable member  40 . Thus, failure of expandable member  40  will not result in loss of the gas from housing chamber  34 . 
     It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. For example, the sealed chamber of the pressure relief devices are not required to be charged with a gas or other fluid before use. Instead, sealed chamber may be an atmospheric chamber such that no charging of the sealed chamber required. In addition, the pressure relief devices disclosed herein can be used in circumstances in which the pressure within the wellbore annulus increases or decreases. Moreover, of the use of “upper” and “lower” in describing the embodiments is not intended to limit the direction of the pressure relief devices when in operation. In other words, the pressure relief devices are not required to be disposed in a wellbore where the “upper” structures are toward the top of the wellbore and the “lower” structures are toward the bottom of the wellbore. Accordingly, the use of “upper” and “lower” herein is not intended to limit the orientation of the pressure relief devices within a wellbore. Moreover, a rupture disk or other device can be disposed within the port(s) so that fluid is not permitted to flow through the port(s) until the pressure relief device is located within the well at the desired depth. Accordingly, the invention is therefore to be limited only by the scope of the appended claims.

Summary:
Downhole tools comprise a housing chamber with an expandable member disposed therein. An interior of the expandable member is in fluid communication with an outside environment so that hydrostatic pressure can act on an inner wall surface of the expandable member. The outer wall surface of the expandable member partially defines a sealed chamber within the housing chamber such that expansion of the expandable member due to an increase in hydrostatic pressure causes the volume within the sealed chamber to decrease, thereby energizing the sealed chamber. Thus, an increase in hydrostatic pressure within an outside environment is compensated. Further, when the hydrostatic pressure within the outside environment decreases, the energized sealed chamber causes contraction of the expandable member, thereby compensating for the decrease in hydrostatic pressure.