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BACKGROUND  
         [0001]    The present invention relates generally to gas storage in subterranean formations and, in an embodiment described herein, more particularly provides a gas storage and production system.  
           [0002]    Natural gas stored underground is typically stored in leached out salt dome caverns or in depleted hydrocarbon-bearing formations. Where depleted formations are utilized, the formations are generally unconsolidated or poorly consolidated sandstones, which makes it possible to flow gas into and out of pores of the formations at high flow rates.  
           [0003]    To prevent production of formation sand when gas is withdrawn from the formations, gravel packing is typically used. In a gravel packing operation, gravel (e.g., sand, ceramic or bauxite proppant, etc.) is placed in an annulus between a sand screen and a wellbore intersecting a formation. The gravel provides structure against which the formation sand bridges off, thereby preventing migration of the formation sand through the gravel, while permitting gas to flow therethrough.  
           [0004]    In a common method of injecting gas into, and withdrawing gas from, a storage formation, a single tubing string is used for both the injecting and withdrawing operations. That is, the same tubing string is used to store the gas in the formation as is used to produce the stored gas from the formation. Thus, gas is alternately flowed from the surface through the tubing string into the formation, and from the formation through the tubing string to the surface.  
           [0005]    Unfortunately, several problems are associated with this method. One problem is that only a single wellbore is available for both storage and production operations. Another problem is that when operations shift between storage and production, a flow reversal is experienced at the gravel pack in the wellbore. This flow reversal disturbs the gravel and the formation sand bridges therein, thereby escalating the migration of formation sand through the gravel.  
           [0006]    Yet another problem with gravel packs in gas storage wells has to do with the high flow rates generally used in these wells. Typical gravel packs have an open upper end, and so the gravel is not fully contained. High gas flow rates through these gravel packs cause the gravel to move about, “fluffing” the gravel so that it has more open space between its grains. This makes it easier for formation sand to migrate through the spaces between the grains of gravel.  
           [0007]    When formation sand migrates through a gravel pack, it enters the production flowpath and erodes equipment, plugs passages and must be separated from the produced gas. Each of these undermines the profitability of the operation. Therefore, it may be seen that it would be highly advantageous to provide a gas storage and production system which addresses some or all of the above problems.  
         SUMMARY  
         [0008]    In carrying out the principles of the present invention, in accordance with an embodiment thereof, a gas storage and production system is provided which enhances the profitability of subterranean gas storage by preventing or at least substantially decreasing migration of formation sand through a gravel pack.  
           [0009]    In one aspect of the invention, a gas storage and production system is provided. The system includes a gas storage formation, a production and storage wellbores and a junction between the storage and production wellbores. The system is of the type wherein gas is stored within pores of formation rock, such as in a depleted hydrocarbon-bearing formation.  
           [0010]    The production wellbore extends into the formation for withdrawing gas from the formation. The storage wellbore also extends into the same formation for injecting gas into the formation. In this manner, it is not necessary for a single wellbore to be used for both injecting and producing the gas.  
           [0011]    In another aspect of the invention, a gas storage and production system is provided wherein production and storage wellbores extend from a wellbore junction at a main wellbore. The main wellbore extends from the earth&#39;s surface to the wellbore junction. The storage and production wellbores each extend from the wellbore junction into a gas storage formation. Gas is injected from the main wellbore into the formation via the storage wellbore, and gas is withdrawn from the formation into the main wellbore via the production wellbore.  
           [0012]    In yet another aspect of the invention, various means may be utilized for delivering gas to the storage wellbore for injection into the formation, and for delivering gas from the production wellbore to the earth&#39;s surface. For example, a single tubular string may be used to deliver the gas to the storage wellbore, and the gas may be received from the production wellbore into an annulus between the tubular string and the main wellbore for flowing to the earth&#39;s surface. As another example, a single tubular string may be used for alternately delivering gas to the storage wellbore and receiving gas from the production wellbore. As yet another example, separate tubular strings may be used for delivering gas to the storage wellbore and receiving gas from the production wellbore.  
           [0013]    Also provided is a method of gravel packing a wellbore, which is particularly useful in high flow rate gas production of the type typically experienced in gas storage and production systems. The method includes the steps of positioning a sand control device in the wellbore, placing gravel in an annulus formed between the sand control device and the wellbore, and flowing a retainer material into the annulus. The retainer material prevents displacement of the gravel in the annulus.  
           [0014]    These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic view of a gas storage and production system embodying principles of the present invention, wherein main and storage wellbores have been drilled, and the storage wellbore has been gravel packed;  
         [0016]    [0016]FIG. 2 is a schematic view of the system of FIG. 1, wherein a production wellbore has been drilled and gravel packed;  
         [0017]    [0017]FIG. 3 is a schematic view of the system of FIG. 1, wherein cement has been placed above the storage wellbore gravel pack;  
         [0018]    [0018]FIG. 4 is a schematic view of the system of FIG. 1, wherein a first method of storing and producing the gas has been implemented;  
         [0019]    [0019]FIG. 5 is a schematic view of the system of FIG. 1, wherein a second method of storing and producing the gas has been implemented;  
         [0020]    [0020]FIG. 6 is a schematic view of the system of FIG. 1, wherein a third method of storing and producing the gas has been implemented; and  
         [0021]    [0021]FIG. 7 is a schematic view of the system of FIG. 1, wherein a fourth method of storing and producing the gas has been implemented. 
     
    
     DETAILED DESCRIPTION  
       [0022]    Representatively illustrated in FIG. 1 is a gas storage and production system  10  which embodies principles of the present invention. In the following description of the system  10  and other apparatus and methods described herein, directional terms, such as “above”, “below”, “upper”, “lower”, etc., are used only for convenience in referring to the accompanying drawings. Additionally, it is to be understood that the various embodiments of the present invention described herein may be utilized in various orientations, such as inclined, inverted, horizontal, vertical, etc., and in various configurations, without departing from the principles of the present invention.  
         [0023]    As depicted in FIG. 1, initial steps of a method used to practice the system have been performed. A main wellbore  12  has been drilled, cased and cemented, so that it extends from the earth&#39;s surface into a formation  14  in which it is desired to store gas. It is not necessary, however, for the main wellbore  12  to extend into the formation  14 .  
         [0024]    A casing string  16  cemented in the main wellbore  12  includes an orienting latch coupling  18  of the type well known to those skilled in the art. The latch coupling  18  is positioned below a desired exit window  20  through the casing  16 , so that, when a whipstock  22  is latched into the coupling  18 , a window mill (not shown) will be directed to mill through the casing at the desired position and in the desired direction. Note that the window  20  may be preformed, or at least provided for, in the casing string  16  when installed, for example, by including an item of equipment known to those skilled in the art as a window bushing or a window joint in the casing string.  
         [0025]    After the casing string  16  is cemented in the main wellbore  12 , a storage wellbore  24  is drilled as an extension of the main wellbore. Alternatively, the storage wellbore  24  could be drilled as a lateral or branch wellbore from the main wellbore  12 . As shown in FIG. 1, the storage wellbore  24  is deviated, so that it extends substantially horizontally in the formation  14 . This maximizes the surface area of the formation  14  exposed to the storage wellbore  24  to increase the flow rate at which gas may be flowed from the storage wellbore into the formation. However, it is to be clearly understood that it is not necessary for the storage wellbore  24  to be horizontal or deviated in the formation  14 .  
         [0026]    After the storage wellbore  24  is drilled, a sand control assembly  26  is installed in the storage wellbore. The sand control assembly  26  may be conventional and may include a gravel pack packer  28  (which is preferably set in the casing  16  above the storage wellbore), a tubular string  30  and a sand control device  32 . Of course, if the formation  14  is well consolidated, or there is otherwise no need for controlling influx of formation sand into the storage wellbore  24 , then the sand control assembly  26  may not be used.  
         [0027]    The sand control device  32  is representatively illustrated in FIG. 1 as a tubular screen of the kind well known to those skilled in the art. The screen  32  may be any type of well screen, including a wire-wrapped screen, a sintered metal screen, a wire mesh screen, etc. Other types of sand control devices may also be used in the system  10 , such as slotted or perforated liners, etc. Therefore, the terms “sand control device” and “sand control screen” as used herein are to be taken as including any apparatus or device which excludes particulate matter, but permits liquid or gas to flow therethrough.  
         [0028]    After the sand control assembly  26  is positioned in the storage wellbore  24 , the wellbore is gravel packed. That is, gravel  34  is placed in an annulus  36  formed between the sand control assembly  26  and the wellbore  24 . Placement of the gravel  34  is accomplished using techniques well known to those skilled in the art. For example, a workstring (not shown) may be used to flow a gravel slurry from the workstring outward through a crossover tool (not shown) below the packer  28 . Of course, other methods of gravel packing the storage wellbore  24  may be used without departing from the principles of the present invention.  
         [0029]    After the storage wellbore  24  is gravel packed, a plug  38  is installed in the packer  28 . The plug  38  prevents debris from the window milling and cementing operations described below from passing into the sand control assembly  26 . Otherwise, this debris could fully or partially plug the screen  32 , thereby preventing or decreasing the flow of gas therethrough.  
         [0030]    A whipstock  22 , or other deflection device, is then installed in the main wellbore  12 . The latch coupling  18  secures the whipstock  22  longitudinally in the casing  16  and orients the whipstock so that it faces in the desired direction for milling the window  20  through the casing. A window mill (not shown) or other cutting device is then deflected off of the whipstock  22 , so that it cuts the window  20  through the casing  16 .  
         [0031]    At this point, or after passing additional cutting tools, such as one or more drills, through the window  20 , an initial recess  40  is cut into the formation  14  beyond the cemented casing  16 . Preferably, a permeability reducing material  42  is then forced outwardly into the formation  14  surrounding the recess  40 . The material  42  may be, for example, a plastic resin, a polymer, a cementitious material, a material known as PermaSeal™, etc. The main purpose of using the material  42  is to prevent gas in the formation  14  surrounding the window  20  from passing through the window into the casing  16 . However, use of the material  42  is not necessary in keeping with the principles of the present invention.  
         [0032]    Referring additionally now to FIG. 2, the system  10  is depicted with further steps having been performed. The recess  40  has been extended outward into the formation  14 , for example, by deflecting one or more drill bits off of the whipstock  22  and through the window  20 , thereby forming a production wellbore  44 . The production wellbore  44  is preferably deviated or substantially horizontal in the formation  14  to expose a greater surface area of the formation to the wellbore, but this is not necessary in keeping with the principles of the invention.  
         [0033]    Another sand control assembly  46  is installed in the production wellbore  44 . A packer  48  of the sand control assembly  46  is set in the casing  16  above the window  20 , a sand control screen  50  is installed in the production wellbore  44 , and a tubular string  52  extends between the packer and the screen. The sand control assembly  46  is similar to the sand control assembly  26  described above, but may differ in some respects.  
         [0034]    In particular, the sand control assembly  46  may include ported collars  54 ,  56  of the type used in cementing operations, interconnected in the tubular string  52  between the packer  48  and the screen  50 . Preferably, the ported collar  54  is positioned between the window  20  and the screen  50 , and the ported collar  56  is positioned between the packer  48  and the window. The use of the ported collars  54 ,  56  in the system  10  is described in more detail below.  
         [0035]    After installing the sand control assembly  46 , the production wellbore  44  is gravel packed using techniques well known to those skilled in the art. Gravel  58  is placed in an annulus  60  between the sand control assembly  46  and the production wellbore  44  about the screen  50 . Preferably, the gravel  58  extends somewhat beyond the ports in the lower ported collar  54 .  
         [0036]    One of the inventive aspects of the system  10  is a manner in which the gravel  58  is retained in the wellbore  44  about the screen  50 . Due to high flow rates of gas from a storage formation into a screen through a conventional gravel pack, the gravel is typically made to move about, disturbing any sand bridging that had previously developed, and permitting increased migration of sand through the gravel pack.  
         [0037]    One reason the gravel in a conventional gravel pack is able to move about due to high gas flow rates therethrough is that the annulus above the gravel pack is typically open. That is, the upper level of a conventional gravel pack is typically spaced apart from the packer, leaving the annulus therebetween available for the gravel to displace into.  
         [0038]    An example of this is shown in the accompanying figures wherein the storage wellbore  24  is gravel packed. The gravel  34  spaced apart from the packer  28 , leaving the annulus  36  open therebetween. This does not present a problem of sand migration in the system lo, however, since gas preferably flows outward from the sand control assembly  26  into the formation  14 , and not in the other direction, which is another significant advantage of the system.  
         [0039]    For the production wellbore  44 , wherein gas flows from the formation  14  into the sand control assembly  46 , the problem of gravel movement is reduced or eliminated by retaining the gravel  58  in the annulus  60  about the screen  50 , so that it cannot displace upward in the annulus  60 .  
         [0040]    Referring additionally now to FIG. 3, the system  10  is depicted wherein additional steps have been performed. Specifically, a retainer material  62  has been flowed into the annulus  60  above the gravel  58 . The retainer material  62  is flowed outward into the annulus  60  through the lower ported collar  54 , and is flowed upward through the annulus, until it extends through the window  20 . During this process, returns are taken from the annulus  60  through the upper ported collar  56 .  
         [0041]    Preferably, the retainer material  62  is cement or another cementitious material. In that case, conventional cementing techniques may be used to place the cement  62  in the annulus  60  above the gravel  58 . For example, a workstring, such as a coiled tubing string (not shown), may be inserted into the sand control assembly  46  and used to open the ported collars  54 ,  56  prior to pumping the cement through the workstring into the annulus  60 . Withdrawal of the workstring may cause the ported collars  54 ,  56  to close.  
         [0042]    Any of the gravel  58  above the ports in the ported collar  54  will be displaced along with the cement  62  as it is flowed into the annulus  60 . This procedure will ensure intimate contact between the cement  62  and the top of the gravel  58  in the annulus  60 . Thus, when the cement  62  sets or hardens in the annulus  60 , it will prevent the gravel  58  from displacing when gas flows therethrough at a high rate. Note that the gravel  34  in the storage wellbore  24  could similarly be retained in keeping with the principles of the invention.  
         [0043]    Of course, materials other than cement may be used for the retainer material  62 . For example, a polymer material may be flowed into the annulus  60  above the gravel  58 . Such a material may gel instead of harden when set. A gelatinous material may be used. In short, any material which may serve to prevent displacement of the gravel  58  in the annulus  60  can be used for the retainer material  62 .  
         [0044]    After the retainer material  62  is permitted to set in the annulus  60 , the packer  48  is retrieved from the main wellbore  12 . Alternatively, the packer  48  could be retrieved before placing the retainer material  62  in the annulus  60 , in which case there would be no need to include the upper ported collar  56  in the tubular string  52 .  
         [0045]    Referring additionally now to FIG. 4, the system  10  is depicted wherein further steps have been performed. The sand control assembly  46  extending inwardly through the window  20  has been milled away, so that the tubular string  52  terminates at the window. Any retainer material  62  left in the casing string  16  has also been removed. The whipstock  22  has been retrieved, for example, by using a washover tool well known to those skilled in the art. The plug  38  has been retrieved from the packer  28 .  
         [0046]    A tubing string  64  having a seal assembly  66  proximate a lower end thereof is installed in the main wellbore  12 . The seal assembly  66  is stabbed into the packer  28  or an associated seal bore extension. The tubing string  64  now provides a conduit for injecting gas from the earth&#39;s surface, into the sand control assembly  26  in the storage wellbore  24 , and outward into the formation  14 . The direction of gas flow is indicated by the arrow  68 .  
         [0047]    Another conduit for gas flow is provided by an annulus  70  formed between the tubing string  64  and the wellbore  12 . Gas is received into the annulus  70  from the sand control assembly  46 , which in turn receives the gas from the formation  14 . The gas may be flowed to the earth&#39;s surface in the annulus  70 , in the direction indicated by arrows  72 .  
         [0048]    Preferably, the directions of gas flow indicated by arrows  68 ,  72  are not reversed in normal gas storage and production operations. Thus, the problems of flow reversal are substantially, if not totally, eliminated. In the storage wellbore  24 , gas is preferably only flowed into the formation  14 . In the production wellbore  44 , gas is preferably only flowed out of the formation  14 . Of course, these flow directions could be reversed if conditions warrant.  
         [0049]    It should also be clearly understood that it is not necessary for the gas to be injected via the tubing string  64  and the gas to be produced via the annulus  70 . The gas could instead be injected via the annulus  70  and produced via the tubing string  64 . For example, the tubing string  64  could extend into the production wellbore  44 , where the seal assembly  66  could be stabbed into a seal bore (not shown) of the tubular string  52 .  
         [0050]    Referring additionally now to FIG. 5, the system  10  is depicted wherein an alternate method of storing and producing the gas in the formation  14  is used. In this version, the tubing string  64  is installed in the main wellbore  12  and a seal assembly  66  is stabbed into the packer  28 , or a seal bore associated therewith, as described above for the version depicted in FIG. 4. However, another tubing string  74  is installed in the main wellbore  12 , and a packer  76  on the tubing string is set in the casing  16  above the window  20 .  
         [0051]    As with the version depicted in FIG. 4, gas is preferably injected into the formation  14  via the tubing string  64 . However, the gas is produced via an annulus  78  formed between the tubing strings  64 ,  74 . This method may be more desirable in jurisdictions where an annulus extending to the earth&#39;s surface, such as the annulus  80  between the tubing string  74  and the wellbore  12 , must be available for well control and monitoring, and cannot be used for production. Use of the tubing string  74  provides the additional annulus  78  for production of the gas, leaving the annulus  80  available for well control and monitoring.  
         [0052]    As shown in FIG. 5, the tubing strings  64 ,  74  are concentric or coaxial, and the flow of gas is as indicated by the arrows  68 ,  72 . However, it is to be clearly understood that the tubing strings  64 ,  74  could be otherwise positioned, and the gas flow could be otherwise directed, in keeping with the principles of the invention. For example, the tubing strings  64 ,  74  could be positioned side-by-side in the main wellbore  12 , the gas could be produced through the interior bore of the tubing string  64 , the gas could be injected through the interior bore of the tubing string  74 , etc.  
         [0053]    Referring additionally now to FIG. 6, the system  10  is depicted wherein another alternate method of producing and storing gas in the formation  14  is used. As with the previously described versions, the tubing string  64  is installed in the main wellbore  12  and the seal assembly  66  is stabbed into the packer  28 . However, in this version, the tubing string  64  includes a packer  82 , which is set in the casing  16  above the window  20 , and a valve  84 , which is positioned between the packers  28 ,  82 .  
         [0054]    The valve  84  is of the type well known to those skilled in the art which alternately permits flow through its sidewall and its internal longitudinal bore. That is, the valve  84  has two positions—in the first position the valve permits flow through its sidewall but prevents flow through its internal bore, and in the second position the valve prevents flow through its sidewall and permits flow through its internal bore. Such valves are used in several oilfield operations, including drill stem testing, where the valves are known as “tester” valves. An example is the Omni™ valve available from Halliburton Energy Services, Inc.  
         [0055]    The valve  84  may be of the type which uses pressure in a control line  86  to control its operation, as is commonly used in subsea operations. However, other actuation means may be used, such as acoustic, electromagnetic, etc., telemetry from a remote location, pressure or pressure pulses in the tubing string  64  or annulus  70 , etc.  
         [0056]    When the valve  84  is in its first position, gas is produced from the production wellbore  44 , through the sidewall of the valve, and to the earth&#39;s surface via the tubing string  64  above the valve. Flow between the storage wellbore  24  and the tubing string  64  above the valve  84  is prevented by the valve. Thus, when it is desired to produce gas from the formation  14 , the valve  84  is operated to its first position.  
         [0057]    When the valve  84  is in its second position, gas is injected through the tubing string  64 , through the internal bore of the valve, and into the storage wellbore  24 . Flow between the production wellbore  44  and the tubing string  64  is prevented by the valve  84 . Thus, when it is desired to store gas in the formation  14 , the valve is operated to its second position.  
         [0058]    An advantage of this method shown in FIG. 6 is that only a single tubing string  64  is needed to both store and produce gas via the multiple wellbores  24 ,  44 , while leaving an annulus  88  extending to the earth&#39;s surface above the packer  82  available for well control. No flow reversal occurs in any gravel pack of the system  10 . The valve  84  is merely alternated between its first and second positions as needed to store or produce the gas.  
         [0059]    Referring additionally now to FIG. 7, the system  10  is depicted wherein yet another method of storing and producing the gas is used. This method is similar to the method shown in FIG. 6 except that, instead of the valve  84 , two check valves  90 ,  92  are used to control flow between the tubing string  64  and each of the storage and production wellbores  24 ,  44 .  
         [0060]    The check valve  90  prevents flow from the interior of the tubing string  64  to the production wellbore  44 , but permits flow from the production wellbore to the interior of the tubing string. The check valve  92  prevents flow from the storage wellbore  24  to the interior of the tubing string  64 , but permits flow from the interior of the tubing string to the storage wellbore.  
         [0061]    When it is desired to produce gas from the formation  14 , pressure in the tubing string  64  is decreased below that in the production wellbore  44 . This pressure differential opens the check valve  90  and gas flows from the production wellbore  44 , through the check valve  90 , into the tubing string  64 , and to the earth&#39;s surface. The pressure in the tubing string  64  is also less than pressure in the storage wellbore  24 , which maintains the check valve  92  in its closed position.  
         [0062]    When it is desired to inject gas into the formation  14 , pressure in the tubing string  64  is increased above that in the storage wellbore  24 . This pressure differential opens the check valve  92 , and gas flows from the tubing string  64 , through the check valve, and into the storage wellbore  24 . The pressure in the tubing string  64  is also greater than pressure in the production wellbore  44 , which maintains the check valve  90  in its closed position.  
         [0063]    Biasing devices, such as springs, may be added to the check valves  90 ,  92 , so that predetermined pressure differentials are needed to open the valves. This may also ensure more positive closing of the valves  90 ,  92  and/or allow greater latitude in the pressures which may be applied to the tubing string  64  to open or close the valves as desired.  
         [0064]    The check valves  90 ,  92  are shown schematically in FIG. 7 as being separate valves spaced apart in the tubing string  64 . However, these valves  90 ,  92  could be otherwise configured and positioned in keeping with the principles of the present invention. For example, the valves  90 ,  92  could be combined into a single assembly, the valves could be retrievable by slickline or coiled tubing, etc.  
         [0065]    Note that the system  10  as depicted in FIG. 7 also has the advantage of using only a single tubing string  64  to inject and produce gas in the multiple wellbores  24 ,  44 , while leaving the annulus  88  available for well control. This storage and production of gas through the tubing string  64  is accomplished without requiring flow reversal in any gravel pack of the system  10 .  
         [0066]    In the accompanying FIGS.  1 - 7  depicting several embodiments of the invention, the production wellbore  44  is shown as intersecting the main wellbore  12  at a wellbore junction, and the storage wellbore  24  is shown as being an extension of the main wellbore. The main wellbore  16  is cased, while the production and storage wellbores  24 ,  44  are uncased. The production wellbore  44  is above the storage wellbore  24 . However, it is to be clearly understood that these examples of embodiments of the invention are merely used for illustration purposes. The main wellbore  12  could be uncased at its junction with the production and/or storage wellbores  24 ,  44 , the storage and/or production wellbores could be cased, the storage wellbore could be above the production wellbore, the storage wellbore could intersect the main wellbore at a wellbore junction, the production wellbore could be an extension of the main wellbore, etc.  
         [0067]    The junction between the main wellbore  12  and the production wellbore  44  has been depicted in the drawings and described above as one in which the tubular string  52  in the production wellbore extends into the main wellbore and is cemented at least up to the window  20 . However, it is to be understood that other types of wellbore junctions may be utilized, without departing from the principles of the present invention. For example, any of the wellbore junctions known to those skilled in the art as Levels  1 - 6  may be used, as well as any other type of wellbore junction.  
         [0068]    Thus, a person skilled in the art would, upon a careful consideration of the above description of representative embodiments of the invention, readily appreciate that many modifications, additions, substitutions, deletions, and other changes may be made to these specific embodiments, and such changes are contemplated by the principles of the present invention. Accordingly, the foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims and their equivalents.

Summary:
A gas storage and production system decreases production of formation sand and permits high gas flow rates in storing and producing operations. In a described embodiment, different flowpaths are used for injecting and withdrawing gas from a subterranean formation. In another embodiment, a gravel pack is confined to a set volume, so that it is not expanded when gas flows at a relatively high rate therethrough.