Patent Publication Number: US-10315846-B2

Title: Methods for storing hydrogen in a salt cavern

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/182,582, which is a continuation-in-part of U.S. Pat. No. 8,690,476, filed on May 25, 2012, each of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a novel method for storing high purity hydrogen into a salt cavern. Particularly, the storage process involves storing high purity hydrogen into a salt cavern without seepage of the stored hydrogen across or through the walls of the salt cavern. 
     BACKGROUND OF THE INVENTION 
     Hydrogen is supplied to customers connected to a hydrogen pipeline system. Typically, the hydrogen is manufactured by steam methane reforming in which a hydrocarbon and steam are reacted at high temperature in order to produce a synthesis gas containing hydrogen and carbon monoxide. Hydrogen is separated from the synthesis gas to produce a hydrogen product stream that is introduced into the pipeline system for distribution to customers that are connected to the pipeline system. Alternatively, hydrogen produced from the partial oxidation of a hydrocarbon can be recovered from a hydrogen rich stream. Typically, hydrogen is supplied to customers under agreements that require availability and on stream times for the steam methane reformer or hydrogen recovery plant. When a steam methane reformer is taken off-line for unplanned or extended maintenance, the result could be a violation of such agreements. Additionally, there are instances in which customer demand can exceed hydrogen production capacity of existing plants. Having a storage facility to supply back-up hydrogen to the pipeline supply is therefore desirable in connection with hydrogen pipeline operations. Considering that hydrogen production plants on average have production capacities that are roughly 50 million standard cubic feet per day or greater, a storage facility for hydrogen that would allow a plant to be taken off-line, to be effective, would need to have storage capacity in the order of 1 billion standard cubic feet or greater. 
     The large storage capacity can be met by means of salt caverns to store the hydrogen underground. Low purity grades of hydrogen (i.e., below 95% purity) as well as other gases have been stored in salt caverns. Salt caverns are large underground voids that are formed by adding fresh water to the underground salt, thus creating brine, which is often referred to as solution mining. Caverns are common in the gulf states of the United States where demand for hydrogen is particularly high. Such hydrogen storage has taken place where there are no purity requirements or less stringent (&lt;96% pure) requirements placed upon the hydrogen product. In such case, the stored hydrogen from the salt cavern is simply removed from the salt cavern without further processing. 
     High purity (e.g., 99.99%) hydrogen storage within salt caverns presents several challenges. For example, storing large quantities (e.g., greater than 100 million standard cubic feet) of pure (e.g., 99.99%) gaseous hydrogen in underground salt caverns consisting of a minimum salt purity of 75% halite (NaCl) or greater without measurable losses of the stored hydrogen is difficult based on the properties of hydrogen. Hydrogen is the smallest and lightest element within the periodic table of elements, having an atomic radius measuring 25 pm+/−5 pm. Further, hydrogen is flammable, and therefore a very dangerous chemical if not handled properly. Salt caverns consist of salt walls that have various ranges of permeability (e.g., 0-23×10^-6 Darcy) that if not controlled properly could easily allow gaseous hydrogen to permeate through the salt walls and escape to the surface of the formation. If the stored hydrogen within an underground salt formation was to escape and permeate through the salt formation to the surface, a dangerous situation could arise with fatality potential or immense structural damage potential. Consequently, high purity hydrogen is typically considered one of the most difficult elements to contain within underground salt formations. 
     As will be discussed, among other advantages of the present invention, an improved method and system for storing hydrogen in a salt cavern is disclosed. 
     SUMMARY OF THE INVENTION 
     The invention relates, in part, to a method and system for storing high purity hydrogen into a salt cavern. The cavern pressure has been found to affect the ability to form a leak-tight cavern not susceptible to hydrogen leakage. It has been found that maintaining the cavern pressure within a specific pressure range improves the structural integrity of the salt cavern. The method and system for storage as will be explained below is capable of storing high purity hydrogen without detection of substantial seepage through the salt cavern. The storage process is conducive for the storage of hydrogen having purity levels from at least 95% up to about 99.999% or greater. 
     In a first aspect, a method for storing hydrogen product in a salt cavern is provided. Hydrogen product is removed from a hydrogen pipeline. The hydrogen product is compressed to produce a compressed hydrogen product. The compressed hydrogen product is introduced into the salt cavern to produce stored hydrogen within the salt cavern. A substantially impermeable permeation barrier is created to the stored hydrogen along at least a portion of the walls of the salt cavern by storing the stored hydrogen at a pressure that is between a predetermined lower limit and a predetermined upper limit. 
     In a second aspect, a method for improved confinement of stored hydrogen within a substantially impermeable permeation barrier of a salt cavern is provided. Hydrogen product is removed from a hydrogen pipeline. The hydrogen product is compressed to produce a compressed hydrogen product. The cooling rate of an after cooler situated downstream of a compressor is regulated to manipulate a heat of compression within the compressed hydrogen product. The compressed hydrogen product is introduced through a well casing extending into the salt cavern. At least a portion of the heat of compression is transferred from the compressed hydrogen product to the well casing. The temperature of the well casing is increased by the transfer of heat from the compressed hydrogen product to increase the ductility of the casing and suppress the onset of hydrogen embrittlement along the casing, thereby maintaining a seal between the casing and the cavern to improve confinement of the stored hydrogen. The compressed hydrogen product from the casing is introduced into the salt cavern to produce stored hydrogen within the salt cavern at a pressure between a predetermined lower limit and predetermined upper limit. A substantially impermeable permeation barrier to the stored hydrogen is created along at least a portion of the walls of the salt cavern by storing the stored hydrogen at a pressure that is between a predetermined lower limit and a predetermined upper limit. 
     In a third aspect, a method for maintaining a substantially impermeable permeation barrier in a salt cavern contained a stored gas is provided. A substantial portion of the stored gas from the salt cavern is withdrawn to a level at which a pressure of the remaining portion of the stored hydrogen is reduced below a predetermined lower limit. A sufficient amount of compressed fluid is introduced into the salt cavern to reduce an effective volume of the salt cavern and pressurize the remaining portion of the stored gas to increase the pressure above the predetermined lower limit. A substantially impermeable permeation barrier to the stored gas at the pressure above the predetermined lower limit is created along at least a portion of the walls of the salt cavern. 
     In a fourth aspect, a method for storing hydrogen product in a salt cavern is provided. Hydrogen product is removed from a hydrogen source. The hydrogen product is introduced into the salt cavern to produce stored hydrogen within the salt cavern. The seepage of the stored hydrogen is reduced through walls of the salt cavern by maintaining the stored hydrogen at a pressure between a predetermined lower limit and a predetermined upper limit, thereby creating a substantially impermeable permeation barrier to the stored hydrogen along at least a portion of the walls of the salt cavern. 
     In a fifth aspect, an underground hydrogen storage cavern is provided. The cavern is formed by solution mining an underground salt formation to form a cavity defined by the salt walls and filling the cavity with a compressed hydrogen gas. The salt cavern has been modified to include a substantially impermeable permeation barrier to stored hydrogen within the storage cavern along at least a portion of the walls. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objectives and advantages of the invention will be better understood from the following detailed description of the preferred embodiments thereof in connection with the accompanying figures wherein like numbers denote same features throughout and wherein: 
         FIG. 1  shows a fragmentary schematic of a protocol for introducing and storing hydrogen to a hydrogen pipeline incorporating the principles of the invention; 
         FIG. 2  shows a salt cavern having a substantially impermeable permeation barrier; and 
         FIG. 3  shows a brine pond reservoir for providing brine into the salt cavern of  FIG. 2  as needed to increase the cavern pressure to a predetermined pressure threshold for maintaining the substantially impermeable permeation barrier; and 
         FIG. 4 a    shows a salt cavern wall containing stored hydrogen at a pressure below a lower limit in which hydrogen seepage across the salt cavern occurs; 
         FIG. 4 b    shows a salt cavern wall containing stored hydrogen at a pressure within the pressure threshold limits so as to form a substantially impermeable permeation barrier in accordance with the principles of the present invention; 
         FIG. 4 c    shows a salt cavern wall containing stored hydrogen at a pressure above the upper limit in which hydrogen leakage occurs; 
         FIG. 5 a    shows a geothermal temperature profile generated during a mechanical integrity test; and 
         FIG. 5 b    shows a leakage detection system that can be employed during the mechanical integrity verification of the salt cavern. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used herein, all concentrations are expressed as volumetric percentages. With reference to  FIG. 1 , a hydrogen storage and processing facility  1  is illustrated that is designed to remove hydrogen product from a hydrogen pipeline  2  during periods of low customer demand and store such hydrogen product within a salt cavern  3  as stored hydrogen  4 . During periods at which demand for the hydrogen product exceeds the capabilities of the hydrogen pipeline  2  to supply hydrogen product to customers, stored hydrogen  4  can be removed from the salt cavern  3  and reintroduced into the hydrogen pipeline  2 . In a manner that will be discussed, a substantially impermeable permeation barrier within the salt cavern  3  can be formed and continuously maintained for storing hydrogen product. The ability to store hydrogen product within such a salt cavern  3  having a substantially impermeable permeation barrier can advantageously produce a substantially leak-tight salt cavern not prone to seepage of stored hydrogen product through the salt cavern walls, as typically occurs with conventional salt caverns. The term “permeation barrier” as used herein is intended to refer to the walls of the salt cavern that when pressurized and/or thermally activated, restricts the passage of hydrogen flow. As a result, there is a reduction in the size and/or the quantity of interconnected pores or voids within the walls of the salt cavern that allow the permeation of hydrogen gas molecules into the wall or contaminant gas molecules through the wall. Accordingly, this reduction in the size and/or quantity of interconnected pores or voids substantially minimizes or prevents the availability of flow paths for high purity hydrogen molecules, preferably of at least 95% purity and above, from escaping therein and thereafter through the surrounding rock salt of the salt cavern. 
     As known in the art, salt cavern  3  is formed by solution mining in which water is injected through a pipe known as a brine string  10 . The water dissolves the salt, and the resulting brine during the mining operation is returned through the annular space (not shown) formed in the final well casing  12  or other conduit between the inner wall of the final well casing  12  and the outer wall of the brine string  10 . After the solution mining operation is complete, the residual brine in the salt cavern  3  can be removed through the brine string  10  by pressure displacement resulting from injection of hydrogen through the final casing  12  or other conduit. Once the brine level reaches the bottom of the brine string  10 , a top section of the brine string  10  is sealed off by valve  216  and a residual brine layer  20 , also known as a brine sump, may remain in the salt cavern  3  at the bottom portion  207 . 
     The rock salt walls of the salt cavern deposit into thick layers over time. The deposited layers are gradually covered and buried by overburden sediments. The weight or pressure of such overburden sediments causes the salt formations to form densified structures, which tend to undergo viscoplastic slippage or deformation to create densified mircrocracks. Such slippage causes the grains of the salt to possess an inherent porosity, which is generally somewhat pervious to hydrogen. While the salt generally has low permeability and porosity rendering it impermeable to hydrocarbons, the salt is significantly more prone to hydrogen permeation by virtue of hydrogen&#39;s small atomic radius. 
     When customer demand for the hydrogen, supplied by hydrogen pipeline  2  is low or for any reason, production exceeds demand, a hydrogen stream  13  can be removed from the hydrogen pipeline  2  or alternatively received from another hydrogen production or storage source (not shown) and introduced into the salt cavern  3 . “Hydrogen pipeline” or “hydrogen product pipeline” as used herein is intended to refer any conduit or passageway extending between the salt cavern  3  and the hydrogen production or storage source. In this regard, referring to  FIG. 1 , valve  24  is open to allow a portion of the product hydrogen in pipeline  2  to enter leg “A” of flow network  5 . As used herein and in the claims, the term “legs” means flow paths within the flow network  5  that are formed by suitable conduits. Such conduits would be positioned to conduct the flow of the hydrogen streams within the flow network  5  as illustrated. Bypass valve  14  is set in a closed position, and valve  15  is set in an open position and valve  20  and valve  303  are set in a closed position to allow hydrogen stream  13  to be compressed in a hydrogen compressor  7  to produce a compressed hydrogen stream  16 . Hydrogen compressor  7  can be any known compressor as used in the art, and is typically a compressor having a reciprocating piston. Hydrogen compressor  7  incorporates a first stage  8  and a second stage  9  in series with interstage cooling between stages and an aftercooler  10  which can be employed to remove the heat of compression. Alternatively, and as will be explained in greater detail below, the heat of compression can be transferred to the compressed hydrogen stream  16  when entering the salt cavern  3 , as part of a thermal activation of the salt cavern  3 . The compressor  7  is conventionally controlled to maintain the inlet pressure at a target suction pressure to maintain energy efficient operation of the compressor  7 . 
     The compressed hydrogen stream  16  is introduced into the salt cavern  3  to form the stored hydrogen  4 . The compressed hydrogen stream  16  continues to flow through the first leg “A”. The compressed hydrogen stream  16  thereafter enters well-casing or conduit  12  ( FIG. 2 ), which is connected to a transfer well head assembly  202 , and thereafter into an annular flow area (not shown) within final well casing  12  (between the inside of final well casing  12  and brine string  10 ) from which the compressed hydrogen feed stream  16  enters salt cavern  3 . Flow orifice meter  17 , pressure transmitter  18  and temperature transmitter  19  can be used to determine the quantity of compressed hydrogen stream  16  that is introduced into the salt cavern  3 . 
       FIG. 2  shows the cavern  3  of  FIG. 1  in isolation. The pressure of the stored hydrogen  4  exerts a pressure, denoted as “P”, against the walls  203  of the salt cavern  3 . The cavern depth that starts at the top of the salt and ends at the bottom of the salt cavity is denoted as “d” and is defined as the vertical distance spanning from the top-most portion  204  to the bottom-most portion  207  of the salt cavern  3 . The pressure exerted by the stored hydrogen  4  against the salt cavern walls  203  is maintained above a lower threshold limit and below an upper threshold limit such that there is a reduction in the size and the quantity of interconnected pores or voids within the walls  203  of the salt to form a substantially impermeable permeation barrier  206 .  FIG. 2  shows that the substantially impermeable permeation barrier  206  extends along the entire edge or boundary of the cavern  3 . The substantially impermeable permeation barrier  206  formed along the salt cavern walls has a reduced amount of interconnected porosity such that there are few or virtually no pathways for hydrogen to diffuse therethrough. The substantially impermeable permeation barrier  206  as defined herein substantially prevents all of the molecules of the stored hydrogen  4  from passing therethrough and seeping into the rock salt  205 , as shown in  FIG. 2 . It should be understood that the substantially impermeable permeation barrier  206  in  FIGS. 1 and 2  is shown as having a finite thickness only for purposes of illustrating the principles of the present invention. In addition, not maintaining the pressure between a lower threshold limit and an upper threshold limit for short periods of time in commercial operation is not believed to have deleterious effects on the maintenance of the barrier, and as such is contemplated by the present invention. 
     The lower limit has been found by the inventors to be greater than about 0.2 psi per liner foot of cavern depth. In this example, at a cavern depth of about 2500 feet as shown in  FIG. 2 , the minimum pressure must be maintained at greater than about 500 psig to allow formation of the substantially impermeable permeation barrier  206 . In a preferred embodiment, the minimum pressure may be regulated so as to counteract the tendency of the salt cavern  3  to undergo creep closure, which occurs when the overburden cavern pressures are greater than the pressure within the cavern which causes the salt cavern  3  to close in and reduce the overall physical storage volume. Unlike the prior art, the present invention eliminates permeability of the salt cavern but still allows for counterbalancing of the creep closure. 
     Although maintaining the cavern  3  at a pressure exceeding the lower limit is advantageous, the inventors have also discovered an upper limit for pressure which cannot be exceeded. The upper limit has been found to be less than about 1 psi per liner foot of cavern depth. In this example, at a cavern depth of about 2500 feet as shown in  FIG. 2 , the maximum pressure must be maintained at a pressure lower than 2500 psig to allow for proper maintenance of the substantially impermeable permeation barrier  206 . Exceeding the upper limit can cause the salt walls  203  to fracture, thereby causing the stored hydrogen  4  to flow upward through the fractures into the rock salt  205  and eventually to the surface, which could cause a potential safety hazard if the proper conditions existed such that the released hydrogen ignited. 
     Exploded views of a portion of the substantially impermeable permeation barrier  206  in  FIG. 2  that is circumscribed by the rectangular dotted region is illustrated in  FIGS. 4A-4C  under three different cavern pressure scenarios.  FIG. 4A  shows that when the stored hydrogen  4  is maintained in the cavern  3  at a pressure substantially less than 0.2 psi per foot of cavern depth, hydrogen seepage occurs across the salt walls  203 .  FIG. 4A  indicates that the porosity or voids along at least certain portions of the salt walls  203  are marginally large enough to allow availability of hydrogen molecules to flow therethrough. Such a scenario is representative of the stored hydrogen  4  being stored below the lower limit. 
       FIG. 4C , on the other hand, is indicative of one or more cracks or fractures along the salt walls  203  which can potentially form when the stored hydrogen  4  is maintained in the cavern  3  at a pressure substantially greater than about 1 psi per foot of cavern depth. The cracks are sufficiently large to allow hydrogen to leak therethrough. By way of comparison, the hydrogen leakage across the salt walls  203  occurs at a higher flow rate than the hydrogen seepage in  FIG. 4A  by virtue of the cracks creating larger flow paths. The scenario of  FIG. 4C  is representative of the stored hydrogen  4  being stored above the upper limit. 
       FIG. 4B  shows successful formation of the substantially impermeable permeation barrier  206  in which molecules of the stored hydrogen  4  remain entirely confined within the interior volume of the salt cavern  3 .  FIG. 4B  shows that the stored hydrogen  4  is maintained in the cavern  3  at a pressure greater than 0.2 psi per foot of cavern depth but less than 1 psi per foot of cavern depth. The substantially impermeable permeation barrier  206  creates a reduction in the size and quantity of interconnected pores or voids within the salt walls  203 , thereby reducing or preventing the availability of flow paths for high purity hydrogen molecules from escaping the interior volume of the salt cavern  3 . 
     In a preferred embodiment, the stored hydrogen  4  can be maintained in a pressure range that can vary between 0.4 to 0.8 psi per liner foot of cavern depth to form and maintain a substantially impermeable permeation barrier  206  that can confine the stored hydrogen  4  within the walls  203  of the salt cavern  3 . The substantially impermeable permeation barrier  206  is formed by reducing the porosity of the cavern walls sufficiently enough to prevent the passage of high purity hydrogen molecules. In a preferred embodiment, the salt cavern  3  can be stored with 99.99% pure hydrogen gas without detectable seepage through the barrier  206 . 
     Effectiveness of the substantially impermeable permeation barrier  206  can be assessed with pressure and temperature measurements. For instance, a pressure measurement is made in the cavern  3  by a downhole pressure transducer  208  to ensure the proper pressure range is maintained. Alternatively, a local cavern wellhead surface pressure measurement device (not shown), which may be located within the cavern wellhead assembly  202 , can be employed for measuring pressure. The pressure transducer  208  extends through the well-casing or conduit  12  of the cavern well head assembly  202 . 
     Alternatively or in addition to pressure gauges, one or more temperature gauges are placed at various locations within the interior volume of the salt cavern  3  to monitor the integrity of the substantially impermeable permeation barrier  206 . For example, a downhole temperature gauge can extend through the conduit  12  and be positioned at a predetermined location within the stored hydrogen  4 . Because hydrogen displays a negative Joule-Thompson coefficient, any seepage of the stored hydrogen  4  through walls  203  will manifest itself as a localized temperature excursion. Estimated temperature excursions as a result of seepage of hydrogen leakage have been observed to be on the order of 4° F. for pressure loss of and about 1200 psi in the cavern  3 . 
     Generally speaking, if it is determined that leakage is occurring through the cavern  3 , the pressure of the stored hydrogen  4  can be adjusted as needed to form a substantially impermeable permeation barrier  206  that is less prone to leakage of the stored hydrogen  4  therethrough. For example, if the downhole pressure transducer  208  shows that the pressure in the cavern  3  has exceeded the upper limit, a portion of the stored hydrogen  4  can be withdrawn from cavern  3  until the pressure falls to below the upper limit. Alternatively, if the downhole pressure transducer  208  indicates that the pressure in the cavern  3  has fallen below the lower limit, a portion of the hydrogen product from the pipeline  2  can be introduced into the cavern  3  until the pressure of the cavern  3  increases to at least slightly above the lower limit. As a result, a critical mass of stored hydrogen  4  is always kept in cavern  3  to maintain the substantially impermeable permeation barrier  206 . 
     Large swings in customer demand for hydrogen product from the hydrogen pipeline  2  can significantly alter the amount of stored hydrogen  4  required to be withdrawn from the cavern  3  and directed to the hydrogen pipeline  2 . This in turn increases the variation in pressure in the cavern  3 . The pressure in the cavern  3  may vary to the extent that the pressure falls outside the predetermined upper and lower limits for short periods of time. As a result, real-time adjustments are performed to ensure that the substantially impermeable permeation barrier  206  to the stored hydrogen  4  is restored during operation of the cavern  3 . 
     For instance, when customer demand is below production capabilities, or, for any reason, demand may fall below production, the cavern pressure may exceed the upper limit as a result of an increased amount of hydrogen product from the pipeline  2  being introduced into the cavern  3 . The present invention may utilize one or more downhole temperature and pressure gauges  501  and  208  ( FIGS. 2 and 5 ) to detect that the pressure of the stored hydrogen  4  has exceeded an upper limit for short periods of time. After measuring and detecting the elevated pressure, at least a portion of the stored hydrogen  4  from the cavern  3  can be withdrawn and returned to the hydrogen product pipeline  2 . Referring to  FIG. 1 , valve  20  would be opened to allow a portion of the stored hydrogen  4  to be discharged from the cavern  3  as a crude hydrogen stream  21 . Because the pressure of the stored hydrogen  4  is higher than that of the pipeline  2  in this particular example, the crude hydrogen stream  21  readily flows through the second leg “B” of flow network  5 . A sufficient amount of stored hydrogen  4  can be withdrawn so that the pressure of the stored hydrogen  4  remaining in the cavern  3  falls below the upper limit. As the pressure decreases below the upper limit, the pressure in the cavern is insufficient to have a tendency to fracture the walls  203  of the salt cavern  3 , and a substantially impermeable permeation barrier  206  is re-established along the walls  203  of the cavern  3 . When the appropriate amount of stored hydrogen  4  has been removed, valve  20  is closed to isolate the cavern  3 . Cavern pressure and temperature can be monitored to ensure that the substantially impermeable permeation barrier  206  remains within the predetermined lower and upper limits. 
     In another scenario, customer demand may fall below production capabilities, or, for any reason, demand may fall below production, such that a real-time analysis of the cavern pressure indicates that the pressure is trending upwards and approaching the upper limit. After detecting the upward trend and in response thereto, at least a portion of the stored hydrogen  4  can be withdrawn to the hydrogen product pipeline  2 . Withdrawal of stored hydrogen  4  continues until the pressure of the stored hydrogen  4  is considered to fall below the upper limit. In one example, withdrawal of the stored hydrogen  4  continues until the pressure is determined to be sufficiently below the upper limit, such that the inherent pressure fluctuation of the stored hydrogen  4  in the cavern  3  does not exceed the upper limit. This downward adjustment in pressure allows the substantially impermeable permeation barrier  206  to the stored hydrogen  4  to be maintained. 
     During the operation of the salt cavern  3 , there may be instances when customer demand may be greater than the supply of the hydrogen in the product pipeline  2 , or, for any reason, demand exceeds production of hydrogen. As a result, the pressure of the stored hydrogen  4  may fall below the lower limit for short periods of time. In response to such measurement and detection, a portion of hydrogen product from the hydrogen product pipeline  2  can be introduced from the hydrogen pipeline  2  into the salt cavern  3  to increase the pressure of the stored hydrogen  3  above the lower limit. Referring to  FIG. 1 , valve  24  is set in the open position and bypass valve  14  is set in the open position, as the pressure in the pipeline  2  in this particular example is sufficient for the hydrogen product to free flow along leg “A” without requiring pressurization by compressor  7 . Valve  15  is opened and valve  20  is closed. The hydrogen product introduced into the cavern  3  increases the pressure in the cavern  3  that is exerted against the walls  203  of the cavern  3 . The porosity of the walls  203  of salt cavern  3  is partially compressed, which in turn reduces the voids and/or grain boundaries of the salt walls  203  to a size substantially small enough to prevent the stored hydrogen  4  from passing therethrough. Cavern pressure and temperature can be monitored to ensure that the substantially impermeable permeation barrier  206  is adequate. When the appropriate amount of hydrogen product from the pipeline  2  has been introduced into the cavern  3 , valve  15  is closed to isolate the cavern  3 . As the pressure in the cavern  3  increases above the lower limit, the pressure is sufficient to have a tendency to counteract creep closure of the walls  203  of the salt cavern  3 , and a substantially impermeable permeation barrier  206  to the stored hydrogen  4  is re-established along the walls  203  of the cavern  3 . 
     Still further, customer demand can be greater than the supply of the hydrogen in the product pipeline  2 , or, for any reason, demand exceeds production of hydrogen such that a real-time analysis of the pressure indicates that the pressure is trending downwards and approaching the lower limit. After detecting the downward trend and in response thereto, a portion of the hydrogen product from the hydrogen pipeline  2  can be introduced into the salt cavern  3  until the pressure of the stored hydrogen  4  within the cavern  3  is determined to increase above the lower limit. In one example, hydrogen product from the hydrogen pipeline  2  continues to be introduced into the cavern  3  until the pressure in the cavern  3  increases sufficiently above the lower limit, such that inherent pressure fluctuation in the stored hydrogen  4  does not fall below the lower limit. This upward adjustment in pressure therefore allows the substantially impermeable permeation barrier  206  to the stored hydrogen  4  to be maintained. 
     In an alternative embodiment, there will be instances when a hydrogen generation facility is taken off-line or when demand for hydrogen by customers otherwise exceeds the available production capabilities, either of which necessitates removal of substantially all of the stored hydrogen  4  from the salt cavern  3 . In such a case, the cavern  3  can approach a hydrogen depleted state for a sustained period of time during operation of the cavern  3 . A cavern in a “hydrogen depleted state” as defined herein refers to a cavern containing an insufficient quantity of hydrogen such that the cavern pressure is below the lower limit as defined herein for a sustained period of time. In one example, the hydrogen depleted state may be about 50-90% below the lower limit. In order to maintain the substantially impermeable permeation barrier  206 , fluid can be temporarily introduced into the salt cavern  3  to maintain the pressure in the cavern  3  necessary for the substantially impermeable permeation barrier  206 . The term “fluid” as used herein is intended to cover either a gas phase, liquid phase or a combination thereof.  FIG. 3  describes one possible embodiment of a brine pond system  300  for achieving continued maintenance of the substantially impermeable permeation barrier  206 . Brine  315  from a brine pond reservoir  301  can be introduced into the salt cavern  3  so as to occupy the depleted cavern  3 . The brine pond system  300  includes a reservoir  301  and sump pump  302  for transporting brine  302  into the salt cavern  3  as needed to increase the cavern pressure beyond the lower limit for maintaining the substantially impermeable permeation barrier  206 . The brine pond  300  also includes a flow network  318  consisting of a discharge leg “C”, a return leg “D”, valve  303  and valve  304 . The flow network  318  allows the brine  315  to be transported to the salt cavern  3  through leg “C” and returned thereafter back into the brine pond  301  through leg “D”. 
     In operation, brine  315  exits from the bottom of the brine pond reservoir  301  utilizing sump pump  302 , which pressurizes and transports the brine  315  along flow leg “C” as brine stream  316 . Valve  303  is closed, and valve  304  is set in the open position to allow the brine stream  316  to flow through a conduit  319  connected to the transfer well head valve  202  and thereafter into an annular flow area (not shown) within final well casing  12  (between the inside of final well casing  12  and brine string  10 ) from which the brine stream  316  enters salt cavern  3 . 
     The brine  315  occupies the bottom portion  207  of the cavern  3 . As a result, the usable volume of the cavern  3  is reduced. The reduction in volume of the salt cavern  3  allows for the remaining stored hydrogen  4  contained in the interior volume of the cavern  3  to occupy a smaller storage volume, thereby increasing the pressure of the cavern  3 . Brine  315  continues to enter salt cavern  3  through brine string  10  until downhole pressure transducer  208  detects that the cavern pressure has reached above the lower limit. Alternatively, wellhead pressure measuring devices (not shown), which may be located within the cavern wellhead assembly  202 , can be utilized to detect cavern pressure. When the desired caver pressure is detected, valve  304  can be closed to isolate the cavern  3 . In the manner described herein, the substantially impermeable permeation barrier  206  can be maintained, even though the cavern  3  has been depleted of hydrogen. 
     Other variations are possible and are within the scope of the present invention. For instance, the brine  315  may be introduced into the cavern  3  so as to displace the stored hydrogen  4  therein. As pressurized brine  315  enters brine string  315 , the stored hydrogen  4  can be displaced upwards through the annular space of the well casing  12  as a crude hydrogen stream. In other words, the driving force for displacing the stored hydrogen  4  is provided by the pressurized brine  315  entering downwards into the cavern  3  through the brine string  10 . Well head valve  227  is set in an open position to enable the crude hydrogen stream to enter the hydrogen storage and processing facility  1  shown in  FIG. 1 . The crude hydrogen is discharged as a crude hydrogen stream  21 . The crude hydrogen stream  21  readily flows through the second leg “B” of flow network  5  and is exported into the product pipeline  2  with valve  23  set in the open position. In still another variation, not all of the stored hydrogen  4  need be removed. Brine  315  can be injected into the cavern  3  as described above so as to establish a specific cavern pressure in between the lower pressure threshold and the upper pressure threshold to maintain the substantially impermeable permeation barrier. The addition of brine  315  occupies the bottom portion  207  of cavern  3  ( FIG. 3 ) and reduces the effective volume of the stored hydrogen  4  contained therein. The reduction in volume compresses the stored hydrogen  4 , thereby increasing the cavern pressure to the desired pressure level that is within the lower and upper limits. In this manner, substantially impermeable permeation barrier  206  can be effectively formed and maintained. 
     When a sufficient amount of product hydrogen from the product pipeline  2  is to be stored in hydrogen cavern  3 , as may occur, for example, as a result of hydrogen production exceeding customer demand, the brine  315  within the salt cavern  3  can be returned to the brine pond  301 . In a preferred embodiment, product hydrogen is drawn off from the pipeline  2 , compressed and injected into the cavern  3 . Compression by compressor  7  ensures that the product hydrogen stream has sufficient driving force to displace the brine  315  from out of the cavern  3  into return leg “D” of flow network  318  ( FIG. 3 ). Valve  24  ( FIG. 1 ) is open to allow a portion of the product hydrogen in pipeline  2  to enter leg “A” of flow network  5  as hydrogen product stream  13 . Valve  20  is closed, and bypass valve  14  is set in a closed position to allow the hydrogen product stream  13  to be compressed by compressor  7  to form a compressed hydrogen product stream  16 . Valve  15  is open to allow the compressed hydrogen product stream to flow through well casing  12  and thereafter enter cavern  3 . 
     The compressed hydrogen stream  16  is introduced into the salt cavern  3  to form the stored hydrogen  4 . The compressed hydrogen stream  16  continues to flow through the first leg “A”. The compressed hydrogen stream  16  thereafter enters conduit  12  ( FIG. 2 ), which is connected to a transfer well head assembly  202 , and thereafter into an annular flow area (not shown) within final well casing  12  (between the inside of final well casing  12  and brine string  10 ) from which the compressed hydrogen feed stream  16  enters salt cavern  3 . As the compressed hydrogen product enters salt cavern  3 , the brine  315  stored therein is displaced upwards through the well casing  12 . Valve  303  is opened, and valve  304  is closed to allow the brine  315  to flow as stream  317  through the return leg “D” of flow network  318  of the brine pond system  300  into the brine pond  301 . A pump may be employed to pressurize brine stream  317 , if necessary. In this manner, the return of the brine  315  to the brine pond  301  is possible. 
     As an alternative to the above described brine pond system  300 , it should be understood that the present invention also contemplates permanently retaining a minimal amount of brine  315  along the bottom portion  207  of the cavern  3  so that brine  315  does not need to be transported to and from a brine pond  301 . The preferred amount of brine  315  to be permanently retained at the bottom portion  207  of the cavern  3  would be that amount which is equivalent to reduce the effective hydrogen storage volume of the cavern  3  such that the compression of the cavern volume is always pressurized slightly above the lower limit pressure threshold. In such an embodiment of the present invention, only the upper limit for the cavern pressure threshold would need to be regulated to ensure formation of the substantially impermeable permeation barrier  206  without fracture of the salt walls  203 . Cavern pressure and temperature can be monitored with suitable instrumentation as has been described to ensure that the substantially impermeable permeation barrier  206  at the lower limit is being maintained by the brine  315  and at the upper limit by introduction of hydrogen from the pipeline  2 . 
     Other techniques for forming and maintaining the substantially impermeable permeation barrier  206  of the salt cavern  3  are contemplated. For example, a controlled amount of heat can be imparted to the salt walls  203  to cause the walls  203  to attain a state of plasticity in which a portion of the walls  203  begin to move, thus closing and sealing any pores, voids and/or microfractures within the salt walls  203 . The amount of heat that gets transferred from the compressed hydrogen stream  16  to the walls  203  can vary, depending upon the crystal and grain structure of the salt walls  203 , the composition of the salt itself and other operating factors, such as the quantity of residual brine remaining in the cavern  3  and from the throttling of the aftercooler  10 . Preferably, the amount of heat needed to sufficiently produce a rise in the temperature in the cavern  3  at a particular depth of cavern  3  should be greater than the natural geothermal temperature gradient of the earth that corresponds to the particular depth of cavern  3 . The amount of temperature rise needed to create this sealing mechanism and drive the salt to a more plastic physical state may be about 0.1° F./linear foot of depth of the cavern  3 , denoted as “d” of  FIG. 2 , or 250° F. for the cavern being referenced in this embodiment. The portions of the walls  203  can become fluid-like when heated by the hydrogen stream  16  to fill in at least a portion of the porous walls  203 , thereby creating a more densified wall  203  that is less permeable to the flow of stored hydrogen  4  therethrough. The temperature of compressed hydrogen stream  16  can be modulated by controlling the rate of cooling from the aftercoolers  10  situated downstream of the compressor  7 . In one example, the temperature of the compressed hydrogen stream  16  can be controlled to be greater than 200° F., but less than about 400° F., by momentarily shutting off the aftercoolers  10  for a predetermined time. As the hotter hydrogen stream  16  is introduced into the cavern  3 , it contacts the cooler walls  203 , thereby heating the cooler walls  203 . A portion of the walls  203  is heated and can become sufficiently plastic-like to enable filling in of some of the porous material, thereby altering the microstructure of the salt walls  203 . In particular, the temperature of the salt walls  203  increases as heat diffuses therein. The heating causes grains to combine with each other. As a result, the grain sizes are increased, and the number of grain boundaries decrease. The reduction in grain boundaries creates fewer diffusion pathways for the stored hydrogen  4 . A less porous structure is thereby created in which the number of pores and size of the pores of the walls  203  can both be decreased. This transfer of heat from the compressed hot hydrogen stream  16  to the salt walls  203  can potentially reduce the impact of stress dilation and micro fracturing of the salt  203 , thereby strengthening and improving the properties of the substantially impermeable permeation barrier  206 . Such a temperature treatment to the cavern  3  can be conducted one or more times as needed to suitably alter the grain microstructure of the salt walls  203 . 
     In an alternative embodiment, a method for enhancing confinement of the stored hydrogen within the substantially impermeable permeation barrier  206  of the salt cavern  3  can be achieved by ensuring an adequate seal between the casing  12  and cavern  3  is maintained during the service life of the salt cavern  3 . The casing  12  is generally made of steel or steel alloy such as carbon steel. The casing  12  is typically cemented into position and forms a seal with the top portion of  204  of the cavern  3 . However, exposure to hydrogen can cause the steel or steel alloyed casing  12  to undergo hydrogen embrittlement. Mechanical stress in casing  12  due to hydrogen embrittlement can reduce the ductility of the casing  12 , which can lead to brittle failure of the casing  12 , thus creating residual stresses. The residual stresses within the casing  12  can be high enough to push against the top portion  204  and potentially create gaps that destroy the seal between casing  12  and the top portion  204  of the cavern  3 . The gaps create a flow path for the stored hydrogen  4  to exit therethrough. As such, the structural integrity of the cavern  3  is compromised and the cavern  3  can become inoperable. 
     To counteract hydrogen embrittlement, hydrogen product that is removed from the pipeline  2  for storage in the cavern  3  can be compressed by compressor  7  to produce compressed hydrogen stream  16 . However, rather than remove the heat of compression, all of the heat of compression or a substantial portion of the heat of compression is maintained within the compressed hydrogen stream  16  by regulating the after cooling rate of the compressed stream  16  through various heat exchange equipment (i.e., after coolers, fin fans, or other suitable means as known in the art). For example, an after cooler  10  situated downstream of the compressor  7  as shown in  FIG. 1  can be throttled to manipulate the temperature of the compressed hydrogen stream  16  so that all of the heat of compression or a substantial portion of the heat of compression is maintained within stream  16 . 
     As the compressed hydrogen stream  16  flows through the well casing  12 , a sufficient amount of heat is transferred to the steel or steel alloyed casing  12  and increases the temperature of the well casing  12  a sufficient amount to both increase the ductility of the casing  12  and suppress the onset of hydrogen embrittlement along the casing. It should be understood that the term “suppress” as used herein means a reduction or delay of hydrogen embrittlement. The heated temperature of the well casing  12  can vary depending on the particular steel alloys utilized. In a preferred embodiment, the well casing is carbon steel which is heated to a temperature from about 200° F. to about 400° F. 
     In this manner, the seal between the casing  12  and the cavern  3  remains structurally in-tact during the service life of the cavern  3  to improve confinement of the stored hydrogen  4  by the elimination or substantial reduction of hydrogen leakage through gaps between the casing  12  and the top portion  204  of the cavern  3 . 
     It should be understood that various modifications to the present invention are contemplated without departing from the spirit and scope of the present invention. For example, a portion of the hydrogen product that is removed from a pipeline  2  may be sufficiently pressurized, thereby eliminating a need for its compression. Particularly, a portion of the hydrogen product that is removed from the hydrogen pipeline  2  can bypass the compressor  7 , and thereafter be introduced into the salt cavern  3 . As the pressure in the cavern  3  increases and begins to deviate from predetermined compression requirements (e.g., pressure in the cavern  3  approaches or exceeds the pressure in the hydrogen pipeline  2 ), additional hydrogen product which is removed from the hydrogen pipeline  2  may require pressurization and can therefore be compressed by the compressor  7  prior to its introduction into the salt cavern  3 . Additionally, it should be understood that stored hydrogen  4  can be withdrawn from the cavern  3  and re-routed to the compressor  7 , if required to pressurize the withdrawn hydrogen to a sufficient pressure equal to or greater than the pressure of the pipeline  2 . In this manner, the compressor  7  can be selectively utilized to introduce hydrogen product into the cavern  3  and withdraw stored hydrogen  4  from the cavern  3 , as necessary during operation of the cavern  3 . Additionally, the introduction and withdrawal of brine or other gases into the cavern  3  for preferably reducing the effective volume of the cavern  3  may also occur in a similar manner. 
     While the present invention has been described in relation to the ability to store hydrogen at purity grades of preferably about 95% and higher, it should be understood that the principles of the present invention also are applicable to storage of lower purity grades of hydrogen (i.e., below 95% purity) as well as other non-hydrogen containing gases, including inert gases, and any combination thereof. In addition to confinement, the present invention can substantially reduce permeation of contaminants from the walls of the salt cavern  3  into the stored gas, thereby mitigating concerns of contamination typically encountered when maintaining stored gas within conventional salt caverns. The ability to reduce contamination of the stored gas may in some instances eliminate the need for implementation of purification equipment upon withdrawal of the stored gas from the salt cavern  3 . As a result, the present invention offers a process benefit of substantial reduction in cost and complexity of operating the salt cavern  3 . Still further, hydrogen product may be removed from various hydrogen sources. By way of example, hydrogen product may be removed from a hydrogen production source, such as one or more stream methane reformers, and thereafter introduced by a hydrogen pipeline into the salt cavern  3 . In another example, the hydrogen product may be removed from a hydrogen recovery plant and directed by a hydrogen pipeline into the salt cavern  3 . Alternatively, hydrogen product may be removed from any type of hydrogen storage source and routed by a hydrogen pipeline into the salt cavern  3 . 
     EXAMPLE 
     A mechanical integrity test was conducted to evaluate and verify the structural integrity of a salt cavern for storing hydrogen in accordance with the principles of the present invention.  FIG. 5 b    shows the test-setup. A gaseous hydrogen stream was withdrawn from hydrogen pipeline, compressed and then injected into the cavern  3  as a compressed stored hydrogen  504 , in a similar manner as described in  FIG. 1 . A pressure transducer (e.g., as shown in  FIG. 2 ) was used to monitor and regulate the pressure of the stored hydrogen  504  to ensure that the pressure was maintained above the minimum limit but below the upper limit. As a result, a substantially impermeable permeation barrier  506  was formed and maintained throughout the mechanical integrity test.  FIG. 5  shows that the substantially impermeable permeation barrier  506  extended continuously along the walls of the cavern  500 . 
     Downhole retractable temperature gauges and instrumentation were inserted into the cavern  3  at various depths to generate a temperature profile gradient as a function of cavern depth, “d” (shown in  FIG. 5 a   ). The temperature gauges were also utilized to detect leakage of hydrogen on the basis of any temperature excursions in the cavern  500 . The temperature excursions occur because hydrogen has a negative Joule-Thompson coefficient upon volume expansion.  FIG. 5  shows one of the temperature gauges  501  that were inserted through well casing  502  within the interior volume of salt cavern  500 . The temperature gauge  501  and the other gauges (not shown, for purposes of clarity) were calibrated to accurately and precisely detect temperature excursions on the order of 0.01° F. or more. Placement of the temperature gauges at different depths within the cavern  3  allowed for the capability to detect localized leaks, including those from the well casing  502 . The output signal  507  from the temperature gauge  501  and others (not shown) were coupled to an active control system  508  configured to close the cavern&#39;s emergency shutdown valves located within cavern wellhead assembly  202  ( FIG. 2 ), if necessary, as a result of leakage detection. Accompanying high temperature alarms were programmed into the control system  507 . Similarly, alarms for low and high pressure limits were programmed into control system  508 . The temperature gauges utilized in the test provided accuracy within plus/minus 0.1° F., and the pressure gauge utilized in the test provided accuracy to within plus/minus 0.05%. 
     The pressure and temperature readings were compiled for  72  continuous hours. The downhole pressure and temperature readings obtained at a depth of 50% of the cavern depth, d, were employed to calculate the volume of stored hydrogen  504  within the salt cavern  500  at the start of the test and at the conclusion of the test. Any measurement error, as indicated by the inherent accuracy and precision of the temperature pressure gauges and instrumentation, were factored into the gas volume computation. The results indicated that the volume of stored hydrogen  504  at the start of the test was equal to the total volume of stored hydrogen  504  at the end of the test. Further, the temperature profile gradient shown in  FIG. 5  was linear and did not exhibit any discernible temperature excursions. The results supported the conclusion that the substantially impermeable permeation barrier  506  was formed and maintained during the 72 hours test window. 
     The ability of the present invention to store ultrahigh purity hydrogen without volume losses of the stored product is an improvement over conventional storage methods. Furthermore, the substantially impermeable permeation barrier reduces seepage and leakage of the hydrocarbon contaminants from the salt walls  203  into the stored hydrogen volume, potentially reducing the costs associated with implementing suitable purification equipment for the subsequent withdrawal of the stored hydrogen  4 . For example, the required sizing of adsorption beds in the present invention would be potentially smaller than of conventional storage caverns, as less contaminants would be required to be removed upon withdrawal of the stored hydrogen  4  from salt cavern  3  to achieve a product purity specification. In conventional storage caverns, the inherent porosity of the salt walls  203  may contribute to introduction of a large amount of contaminants from the salt walls  203  into the interior of the cavern  3 , thereby requiring larger purification units (e.g., adsorption units). Accordingly, the present invention offers a unique process benefit with respect to the amount of purification required when the stored hydrogen gas  4  is withdrawn from the cavern  3 . Such a process benefit translates into a more cost effective hydrogen storage processing facility relative to conventional salt cavern hydrogen storage facilities. 
     While it has been shown and described what is considered to be certain embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail can readily be made without departing from the spirit and scope of the invention. It is, therefore, intended that this invention not be limited to the exact form and detail herein shown and described, nor to anything less than the whole of the invention herein disclosed and hereinafter claimed.