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BACKGROUND 
   The invention generally relates to a gas lift valve. 
   For purposes of communicating well fluid to a surface of a well, the well may include a production tubing. More specifically, the production tubing typically extends downhole into a wellbore of the well for purposes of communicating well fluid from one or more subterranean formations through a central passageway of the production tubing to the surface of the well. Due to its weight, the column of well fluid that is present in the production tubing may suppress the rate at which the well fluid is produced from the formation. More specifically, the column of well fluid inside the production tubing exerts a hydrostatic pressure that increases with well depth. Thus, near a particular producing formation, the hydrostatic pressure may be significant enough to substantially slow down the rate at which the well fluid is produced from the formation. 
   For purposes of reducing the hydrostatic pressure and thus, enhancing the rate at which fluid is produced, an artificial-lift technique may be employed. One such technique involves injecting gas into the production tubing to displace some of the well fluid in the tubing with lighter gas. The displacement of the well fluid with the lighter gas reduces the hydrostatic pressure inside the production tubing and allows reservoir fluids to enter the wellbore at a higher flow rate. The gas to be injected into the production tubing typically is conveyed downhole via the annulus (the annular space surrounding the production tubing) and enters the production tubing through one or more gas lift valves. 
   As an example,  FIG. 1  depicts a gas lift system  10  that includes a production tubing  14  that extends into a wellbore. For purposes of gas injection, the system  10  includes a gas compressor  12  that is located at the surface of the well for purposes of introducing pressurized gas into an annulus  15  of the well. To control the communication of gas between the annulus  15  and a central passageway  17  of the production tubing  14 , the system  10  may include several gas lift mandrels  16  (gas lift mandrels  16   a ,  16   b  and  16   c , depicted as examples). Each one of these gas lift mandrels  16  includes an associated gas lift valve  18  (gas lift valves  18   a ,  18   b  and  18   c , depicted as examples) that responds to the annulus pressure. More specifically, when the annulus pressure at the gas lift valve  18  exceeds a predefined threshold, the gas lift valve  18  opens to allow communication between the annulus  15  and the central passageway  17 . For an annulus pressure below this threshold, the gas lift valve  16  closes and thus, prevents communication between the annulus  15  and the central passageway  17 . 
   It is typically desirable to maximize the number of cycles in which each gas lift valve  18  may be opened and closed, as the cost of the gas lift valves  18  may be a significant component of the overall production costs. The number of times that a gas lift valve may be opened and closed may be a function of the loading that is experienced by the various seals of the gas lift valve  18 . 
   SUMMARY 
   In an embodiment of the invention, a gas lift valve that is usable with a subterranean well includes a housing, a valve stem and at least one bellows. The housing has a port that is in communication with a first fluid, and the valve stem is responsive to the first fluid to establish a predefined threshold to open the valve. The bellow(s) form a seal between the valve stem and the housing. The bellow(s) are subject to a force that is exerted by the first fluid; and a second fluid contained in the bellow(s) opposes the force that is exerted by the first fluid. 
   Advantages and other features of the invention will become apparent from the following description, drawing and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a schematic diagram of a gas lift system according to the prior art. 
       FIG. 2  is a schematic diagram of a portion of a gas lift mandrel according to an embodiment of the invention. 
       FIG. 3  is a schematic diagram of a middle portion of a gas lift valve according to an embodiment of the invention. 
       FIG. 4  is a schematic diagram of a lower portion of the gas lift valve according to an embodiment of the invention. 
       FIGS. 5 and 6  are schematic diagrams of gas lift valves according to other embodiments of the invention. 
       FIG. 7  is a schematic diagram of a bellows assembly in accordance with another embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 2 , an embodiment  20  of a gas lift mandrel in accordance with the invention is constructed to be installed in a production tubing (not shown) for purposes of controlling the introduction of gas into a central passageway of the production tubing. As shown, the gas lift mandrel  20  includes two generally cylindrical passageways  22  and  24 , each of which has a longitudinal axis that is parallel to the longitudinal axis of the production tubing. More particularly, the passageway  24  is coaxial with the longitudinal axis of the production tubing, as the passageway  24  forms part of the central passageway of the production tubing. The passageway  22  is eccentric to the passageway  24  and houses a gas lift valve  30 . 
   The purpose of the gas lift valve  30  is to selectively control fluid communication between an annulus of the well and the central passageway of the production tubing so that gas may be introduced into the production tubing at the location of the gas lift valve  30 . The term “annulus” refers to the annular region that surrounds the exterior of the production tubing. For a cased wellbore, the “annulus” may include the annular space, or region, between the interior surface of the casing string and the exterior surface of the production tubing. The gas lift valve  30  may be part of a gas lift system. In such a system, a gas may be introduced into the well annulus so that one or more of the gas lift valves  30  (that are installed in the production tubing) may be operated for purposes of introducing the gas into the central passageway of the production tubing, as can be appreciated by one skilled in the art. 
   More specifically, the function of the gas lift valve  30  is to control communication between its one or more inlet ports  108  and its one or more output ports  120 . The gas lift mandrel  20  includes one or more inlet ports  28  that are in communication with the annulus; and the gas lift valve  30  includes seals (O-rings, MSE seals, or T-seals, for example)  110  that straddle the inlet port(s)  28  and inlet ports  108  for purposes creating a sealed region for the gas lift valve  30  to receive fluid from the annulus. The outlet port(s)  120  are in communication with one or more outlet ports  26  formed in the mandrel  20  between the passageways  22  and  24 . Thus, due to this arrangement, when the gas lift valve  30  is open, gas flows from the annulus, through the ports  28 ,  108 ,  120  and  26  (in the listed order) and into the passageway  24 . When the gas lift valve  30  is closed, the gas lift valve  30  blocks communication between the ports  108  and  120  to isolate the passageway  24  from the annulus. 
   In general, the gas lift valve  30  transitions between its open and closed states in response to annulus or tubing pressure. Typically, if the gas lift valve  30  is an injection pressure operated (IPO) valve it is responsive to annulus pressure. If the gas lift valve  30  is a production pressure operated (PPO) valve, it is typically responsive to tubing pressure. When the annulus or tubing pressure exceeds a predefined threshold, the gas lift valve  30  opens; and otherwise, the gas lift valve  30  closes. In some embodiments of the invention, this predefined threshold may be established by the presence of a gas charge in the gas lift valve  30 , as further described below. 
   A more specific embodiment of the gas lift valve  30  is illustrated in  FIGS. 3 and 4 . In this manner,  FIG. 3  depicts a middle section  30 A of the gas lift valve  30 , and  FIG. 4  depicts a lower section  30 B of the gas lift valve. 
   Referring to  FIG. 3 , in some embodiments of the invention, the gas lift valve  30  includes a pressure or reservoir  60  that forms part of a gas charge section of the gas lift valve  30 , a section that establishes a bias to keep the gas lift valve  30  closed and a predefined annulus threshold that must be overcome to open the valve  30 . More specifically, in some embodiments of the invention, the reservoir  60  may be filled with an inert gas, such as Nitrogen, that exists in the reservoir  60  for purposes of exerting a closing force on a gas stem  70  of the gas lift valve  30 . 
   The gas stem  70  and a fluid stem  80  (of the valve  30 ) collectively form a valve stem for the gas lift valve  30 . Assuming the gas lift valve  30  is closed, the valve stem moves in an upward direction to open the gas lift valve  30 ; and assuming the gas lift valve  30  is open, the valve stem moves in a downward direction to close the gas lift valve  30 . More specifically, the gas stem  70  is coaxial with the longitudinal axis  40  of the gas lift valve  30  and is connected at its lower end  70   a  to the upper end  80   b  of the fluid stem  80 . The fluid stem  80  is also coaxial with the longitudinal axis  40  of the gas lift valve  30 . It is noted that the cross-sectional diameters of the gas  70  and fluid  80  stems are different. This relationship permits a lower pressure to be used in the reservoir  60 , as further described below. 
   It is important to note that although the embodiment shown in  FIG. 3  shows the gas stem  70  affixed to the fluid stem  80 , in alternate embodiments, the gas stem  70  and fluid stem  80  are separated parts that are coupled together by pressure during activation. In further alternate embodiment, the gas stem  70  and the fluid stem  80  are manufactured as a single part. Referring also to  FIG. 4 , near its lower end  80   a , the fluid stem  80  has a ball-type tip  104  that, when the gas lift valve  30  is closed, forms a seal with a valve seat  103  for purposes of closing off communication through a port  102  of the gas lift valve  30 . Because all communication between the inlet  108  and outlet  120  ports occurs through the port  102 , the gas lift valve  30  is closed when the tip  104  is seated in the valve seat  103 . This condition occurs when the valve stem is at its farthest point of downward travel. Conversely, the gas lift valve  30  is open when the valve stem is raised and the tip  104  is not seated in the valve seat  103 . 
   Referring to  FIG. 3 , the gas pressure inside the reservoir  60  acts on a top surface  75  of the gas stem  70  to create a downward force on the valve stem. This downward force, in turn, tends to keep the gas lift valve  30  closed in the absence of a greater opposing force that may be developed by the annulus or tubing pressure on the valve stem (as described below). 
   The gas reservoir  60  is formed from an upper housing section  79  that contains a chamber  78  (of the gas lift valve  30 ) for storing the gas in the reservoir  60 . The chamber  78  may also house the gas stem  70  and an upper bellows assembly, described below. The upper housing section  79  is connected to a middle housing section  50  of the gas lift valve  30 . 
   The gas lift valve  30  includes an upper bellows assembly that forms a flexible seal between the gas stem  70  and the middle housing section  50  to accommodate movement of the valve stem. In some embodiments of the invention, the upper bellows assembly may include a seal bellows  52  and a compensation bellows  54 , both of which are coaxial with and circumscribe the gas stem  70 . The seal  52  and compensation  54  bellows are located inside the chamber  78 , as depicted in FIG.  3 . 
   As shown, the seal bellows  52  is located closer to the upper end  70   b  of the gas stem  70  than to the lower end  70   a  of the gas stem  70 ; and the seal bellows  52  circumscribes this upper portion of the gas stem  70 . The upper end of the seal bellows  52  is connected to the upper end  70   b  of the gas stem  70 , and the lower end of the seal bellows  52  is connected to an annular plate  56 . 
   The compensation bellows  54  circumscribes the lower part of gas stem  70  and has a larger diameter than the seal bellows  52 . The upper end of the compensation bellows  54  is connected to the annular plate  56 , as the plate  56  radially extends between the upper end of the compensation bellows  54  and the lower end of the seal bellows  52 . The lower end of the compensation bellows  54  is attached to the middle housing section  50 . 
   It should be understood that in alternate embodiments, the relative location of the seal bellows  52  and the compensation bellows  54  along the gas stem  70  can be inverted. For example, the compensation bellows  54  can be located closer to the upper end  70   b  of the gas stem  70 , while the seal bellows circumscribes the lower part of the gas stem  70 . 
   In the embodiment shown, when the gas stem  70  (and thus, the valve stem) moves in a downward direction, the compensation bellows  54  longitudinally expands and the seal bellows  52  longitudinally compresses. Conversely, when the gas stem  70  moves in an upward direction, the compensation bellows  54  longitudinally compresses and the seal bellows  52  longitudinally expands. 
   The pressure that is exerted on the bellows  52  and  54  by the gas inside the reservoir  60  may cause a significant pressure differential across the walls of the seal bellows  52  and across the walls of the compensation bellows  54 , if not for the pressure balancing features of the gas lift valve  30 . In some embodiments of the invention, the pressure balancing features include an incompressible fluid that is contained inside the bellows  52  and  54 . 
   More specifically, in some embodiments of the invention, the incompressible fluid is contained within annular spaces  62  and  63 . The walls of the seal bellows  52  define the annular region  62 , a region that is located between the interior surface of the seal bellows  52  and the adjacent exterior surface of the gas stem  70 . The walls of the compensation bellows  54  define the annular region  63 , a region that is located between the interior surface of the seal bellows  54  and the adjacent exterior surface of the gas stem  70 . The two regions  62  and  63  are isolated by the bellows  52  and  54  from the gas in the reservoir  60  and are in communication so that the incompressible fluid may move between the regions  62  and  63  when the bellows  52  and  54  are compressed/decompressed. 
   The incompressible fluid serves to remove any pressure differential that otherwise exists across the walls of the bellows  52  and  54  due to the pressure that is exerted by the gas in the reservoir  60 . More specifically, the incompressible fluid is a non-compressible fluid that exerts forces (on the interior surface of the walls of the bellows  52  and  54 ) that are equal and opposed to the forces on the outer surfaces of the walls of the bellows  52  and  54  (exerted by the gas in the reservoir  60 ). 
   In operation, when the gas stem  70  moves in a downward direction, the compensation bellows  54  expands and the seal bellows  52  compresses. Therefore, some of the incompressible fluid contained within the seal bellows  52  is displaced into the compensation bellows  54 , as the volume of incompressible fluid remains constant. When the gas stem  70  moves in an upward direction, the compensation bellows  54  compresses and the seals bellows  52  expands. Some of the incompressible fluid contained within the compensation bellows  54  is displaced into the seal bellows  52 , as the volume of the incompressible fluid remains constant. Thus, regardless of the positions of the bellows  52  and  54 , the incompressible fluid remains inside the bellows  52  and  54  to compensate forces that are exerted by the gas inside the reservoir  60 . 
   To summarize, the bellows  52  and  54  and the incompressible fluid establish a pressure compensation system to equalize the pressure difference across the walls of the bellows  52  and  54 . The result is that the bellows  52  and  54  transfer a more uniform load to the incompressible fluid, and consequently to the seal  76 . 
   Among the other features of the gas charge section of the gas lift valve  30 , the gas lift valve  30  may include, in some embodiments of the invention, a fluid fill port  74  for purposes of introducing the incompressible fluid into the annular regions  62  and  63 . The fill port  74  may be located, for example, in the top surface of the gas stem  70  and may be in communication with the annular regions  62  and  63  via one or more passageways  77  that are formed in the gas stem  70 . The gas lift valve  30  also includes an annular seal  76  that closely circumscribes the exterior surface of the gas stem  70  to form a seal between the annular regions  62  and  63  and the middle housing section  50  for purposes of sealing the incompressible fluid inside the bellows  52  and  54 . The gas lift valve  30  also includes another annular seal  82  for purposes of forming a seal between the exterior surface of the fluid stem  80  and the incompressible fluid used for purposes of equalizing, or balancing, pressures that are exerted on bellows on the well fluid section part of the gas lift valve, described below. 
   Turning to the well fluid section of the gas lift valve  30 , in some embodiments of the invention, this section includes a lower bellows assembly. This lower bellows assembly includes an upper seal bellows  84  and a lower compensation bellows  86 , both of which are coaxial with the longitudinal axis  40  of the gas lift valve  30 . The seal bellows  84  has a top end  84   a  that is connected to the fluid stem  80 . A radially extending annular plate  88  connects the lower end  84   b  of the seal bellows  84  to the upper end  86   a  of the compensation bellows  86 . The lower end  86   b  of the compensation bellows  86 , in turn, is connected to the middle housing section  50 . As discussed above with regard to the upper bellows assembly, in alternate embodiments, the orientation of the upper seal bellows  84  and the lower compensation bellows  86  can be reversed. 
   As depicted in  FIG. 3 , the seal bellows  84  circumscribes part of the fluid stem  80  and has a smaller diameter than the diameter of the compensation bellows  86 . The compensation bellow  86  circumscribes a lower portion of the fluid stem  80 . 
   Fluid from the well annulus is in communication with an annular region  90  that exists between the exterior surface of the fluid stem  80  and the interior wall surfaces of the bellows  84  and  86 . This annular region  90  is in communication with a fluid chamber  83  formed in a lower housing section  81  of the gas lift valve  30 . The lower housing section  81  is connected to the middle housing section  50 , and in addition to establishing the fluid chamber  83 , the lower housing section  81  contains the lower bellows assembly and fluid stem  80 . 
   An annular region  92  exists between the outer surface of the wall of the seal bellows  84  and the inner surface of the middle housing  50 ; and an annular region  91  exists between the outer surface of the wall of the compensation bellows  86  and the inner surface of the middle housing  50 . Both regions  91  and  92  contain the incompressible fluid for purposes of equalizing the pressure across the walls of the bellows  84  and  86 , in a similar arrangement to that described for the bellows  52  and  54  with the exception that here, the incompressible fluid is located outside of the bellows walls and the fluid that exerts the forces on the bellows walls is located inside of the bellows walls. 
   In operation, when the fluid stem  80  moves in a downward direction, the bellows  84  compresses, thereby evacuating the incompressible fluid from the annular region  91  into the annular region  92 . During the compression of the bellows  84 , the bellows  86  expands to compensate the incompressible fluid that is displaced from the compressed annular region  91 . Conversely, when the fluid stem  80  moves in an upward direction, the bellows  86  compresses, and fluid that is displaced from the region  92  enters the region  91  as the bellows  84  expands. By maintaining a constant volume of the incompressible fluid, the differential pressure across the walls of the bellows  84  and  86  is eliminated. 
   As described above, the pressure of the gas in the reservoir  60  tends to force the valve stem (i.e., the gas  70  and fluid  80  stems) in a downward direction. However, the pressure that is exerted by fluid in the annulus of the well exerts an upward force on the gas  70  and fluid  80  stems, tending to push the stems  70  and  80  in an upward direction. 
   Therefore, the pressure inside the reservoir  60  establishes a predefined threshold that must be overcome for the gas stem  70  and the fluid stem  80  to move in an upward direction to open the gas lift valve  30 . 
   In some embodiments of the invention, the diameter of the seal  76  of the gas stem  70  is larger than the diameter of the seal  82  of the fluid stem  80 . This means that for a given pressure level for the reservoir  60 , more downward force is developed on the valve stem than the upward force that is developed on the valve stem for the same pressure level for the annulus fluid. Thus, the above-described relationship of seal diameters between the gas  70  and fluid  80  stems intensifies the pressure that is exerted by the gas in the reservoir  60  with respect to the pressure that is exerted by the annulus or tubing fluid. Such intensifier relationship enables the use of lower charge pressure based on a given annulus or tubing pressure. 
   Referring to  FIG. 4 , among its other features, in some embodiments of the invention, the gas lift valve  30  includes the radial ports  108  (see also  FIG. 2 ) that are formed in the lower housing section  81  for purposes of establishing fluid communication between the annulus and the fluid chamber  83 . The bottom end of the valve stem, i.e., the tip  104 , controls communication of the annulus fluid through the port  102 , a port that establishes communication between the fluid chamber  83  and an intermediate chamber  106 . Thus, when the gas  70  and fluid  80  stems are retracted in an upward direction, the tip  104  is moved off of the valve seat  103  to permit fluid communication between the chambers  83  and  106 . 
   A one-way communication path exists between the intermediate chamber  106  and an exit chamber  105 , a chamber  105  in which the outlet ports  120  (see also  FIG. 2 ) are formed. In this manner the one-way communication path is effectively established by a check valve, a valve that ensures that annulus fluid flows from the chamber  106  into the production tubing and does not flow from the production tubing into the annulus. 
   The check valve opens in response to annulus pressure so that fluid flows from the annulus through a port  119  that exists between the chambers  106  and  105 . In some embodiments of the invention, the check valve may include a valve stem  118  that has a tip  121  that seats in a valve seat  123  for purposes of preventing fluid from flowing in the reverse direction through the port  119 . Thus, a differential force that would cause fluid to flow from the production tubing into the annulus forces the tip  121  into the valve seat  123  to block communication through the port  119 . Conversely, a differential force that would cause fluid to flow from the annulus into the production tubing removes the tip  121  from the valve seat  123  to permit communication through the port  119 . 
   Referring to  FIG. 5 , in some embodiments of the invention, the gas lift valve  30  may be replaced by a gas lift valve  200 . Components (of the gas lift valve  200 ) that are similar to components of the gas lift valve  30  are denoted by similar reference numerals. 
   Unlike the gas lift valve  30 , the gas lift valve  200  includes a tubing pressure assist mechanism for purposes of using pressure in the central passageway of the production tubing to assist in opening the gas lift valve  200 . Such a system may be beneficial when a relatively lower pressure is used in the annulus for purposes of opening the gas lift valve. 
   More specifically, in some embodiments of the invention, the gas lift valve  200  includes a tubing assist bellows  202  that is in communication with the central passageway of the production tubing so that the tubing pressure compresses the bellows  202 . The exterior of the bellows  202  is in communication with a port  201  that, in turn, communicates with the tubing fluid. 
   The bellows  202  contains a fluid (an incompressible fluid, for example) that is in communication (via a communication line  209 ) to an interior space of another bellows  210 . The bellows  210 , in turn, is connected to a valve stem  212  so that when the bellows  202  compresses (due to the force exerted due to the tubing pressure), the fluid enters the bellows  210  to expand the bellows  210 . This expansion, in turn, lifts the stem  212  to open the gas lift valve  200  to allow communication between the well annulus and the production tubing. 
   The tendency of the bellows  210  to expand and open the gas lift valve  30  in response to the tubing pressure is countered by a charge pressure that exists inside an internal charge reservoir  206  of the valve  200 . In this manner, the bellows  210  is contained inside the reservoir  206  so that the gas inside the reservoir  206  exerts a force on the exterior surface of the bellows  210 . Thus, the predefined threshold established by the charge  206  must be overcome to allow the bellows  210  to expand by a sufficient amount to limit the stem  212  to lift the stem  212  to open the gas lift valve  200 . 
   In some embodiments of the invention, the charge reservoir  206  is in communication (via a pressure line  215 ) to a space inside another bellows  220 . In this manner, gas from the reservoir  206  may work to expand the bellows  220 . When expanded, the bellows  220  tends to move the stem  212  in a downward direction to close the gas lift valve  200 . However, the tendency of the bellows  220  to expand is countered by pressure in the well annulus. In this regard, the exterior of the bellows  220  is in communication with the well annulus via radial inlet ports  108 . 
   In some embodiments of the invention, the gas lift valves  30  and  200  may be replaced by a gas lift valve  300  that is depicted in FIG.  6 . Components (of the gas lift valve  300 ) that are similar to components of the gas lift valves  30  and  200  are denoted by similar reference numerals. 
   Unlike the gas lift valves described above, the gas lift valve  300  includes a venturi orifice  326  between the ports  102  and  119  for purposes of minimizing the pressure drop and the turbulence in the flow of gas from the well annulus to the central passageway of the production tubing. 
   Other embodiments are within the scope of the following claims. For example, in the embodiments described above, for each set of seal and compensation bellows, a seal (seals  76  and  82 , for example) was located in the body, or housing, of the gas lift valve assembly to form a seal between a rod, or stem (stems  70  and  80 , for example) and the housing. This arrangement kept the volume of incompressible fluid contained within the bellows constant. However, in other embodiments of the invention, the seal may be located in, or secured to, the rod so that the seal moves with the rod. 
   As a more specific example,  FIG. 7  depicts an exemplary bellows assembly  350  according to another embodiment of the invention. The assembly  350  includes a seal bellows  354 , a compensation bellows  356  and a stem, or rod  352 , to expand and compress the bellows  354  and  356 , as described above in the other embodiments described herein. However, unlike these other embodiments, a seal  360  (an O-ring seal, for example) is attached to, or located in, the rod  352  so that the seal  360  moves with the rod  352 . 
   More particularly, the seal  360  is located inside an annular groove  362  of the rod  352  and forms a seal between the exterior surface of the rod and an interior surface of a housing  370 . This interior surface of the housing  370  defines a passageway  364  through which the rod  352  slides. The seal  360  maintains an incompressible fluid  380  within the interior regions defined by the seal  354  and compensation  356  bellows. 
   Unlike the embodiments in which the seal is located in the housing, the seal  360  in the assembly  350  moves with the rod  352 . This arrangement affects the movement of the bellows  354  and  356 , since the movement of the seal  360  with the rod  352  forces the volume of fluid  380  into the interior regions that are defined by the bellows  354  and  356 . In response to the rod  352  moving in an upward direction, the seal  354  and compensation  356  bellows move in upward directions. The rates at which the seal  354  and compensation  356  bellows move is different. 
   Thus, by placing the seal  360  on the rod  352 , the movement of the bellows  352  and  354  follows the movement of the rod  352 . The internal regions that are defined by the seal  354  and compensation  356  bellows is still filled with the incompressible fluid  380  that transfers the pressure loads to the seal  360 , allowing the bellows to see no differential loading. 
   In the preceding description, directional terms, such as “upper,” “lower,” “vertical,” “horizontal,” etc. may have been used for reasons of convenience to describe the gas lift valve and its associated components. However, such orientations are not needed to practice the invention, and thus, other orientations are possible in other embodiments of the invention. For example, the gas lift valve and its associated components, in some embodiments in some embodiments of the invention, may be tilted by approximately 90° to the orientations depicted in the figures. 
   While the present invention has been described 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 therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.

Summary:
A gas lift valve that is usable with a subterranean well includes a housing, a valve stem and at least one bellows. The housing has a port that is in communication with a first fluid, and the valve stem is responsive to the first fluid to establish a predefined threshold to open the valve. The bellow(s) form a seal between the valve stem and the housing. The bellow(s) are subject to a force that is exerted by the first fluid; and a second fluid contained in the bellow(s) opposes the force that is exerted by the first fluid.