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CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application is a continuation of pending U.S. application Ser. No. 14/696,389 filed on Apr. 25, 2015. Applicant has concurrently filed an application entitled “Method for Detecting Leakage in An Underground Hydrocarbon Storage Cavern” which has Ser. No. 14/696,387 and was filed on Apr. 25, 2015. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention pertains to the field of underground storage caverns which are used for the bulk storage of liquid hydrocarbons, and in particular to the determination of the configuration of such caverns. 
         [0004]    2. Description of the Related Art 
         [0005]    In the use of underground storage caverns it is important to determine the approximate shape and volume of the cavern. This has heretofore been done by lowering a wireline device into the cavern and using sonic devices to measure distances from the device to the cavern wall. Another technique has been to pump a liquid into the annulus and determine cavern volume by measuring the liquid pressure and volume at the annulus and central tubing at the well surface. Wireline operations are complex, expensive and subject to leakage of gas or liquid from the wellhead or wireline connectors. Prior cavern survey techniques are shown in U.S. Pat. No. 2,792,708, issued May 21, 1957 entitled “Testing Underground Storage Cavities” and U.S. Pat. No. 3,049,920, issued Aug. 21, 1962 entitled “Method of Determining Amount of Fluid in Underground Storage”. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying original drawings in which: 
           [0007]      FIG. 1  is vertical section view of a hydrocarbon storage cavern together with casing, tubing and wellhead equipment, 
           [0008]      FIG. 2A  is a partial section view of the cavern well bore shown in  FIG. 1  showing a gas/liquid interface, 
           [0009]      FIG. 2B  is a waveform illustrating the signal monitored in the annulus at the surface as a result of an acoustic shot for the well illustration in  FIG. 2A , 
           [0010]      FIG. 3A  is a partial section view of the cavern well bore shown in  FIG. 1  showing a gas/liquid interface at a different level from that shown in  FIG. 2A , 
           [0011]      FIG. 3B  is a waveform illustrating the signal monitored in the annulus at the surface as a result of an acoustic shot for the well illustration in FIG. A, 
           [0012]      FIG. 4A  is a partial section view of the cavern well bore shown in  FIG. 1  showing a gas/liquid interface at just below the casing shoe level of the well, 
           [0013]      FIG. 4B  is a waveform illustrating the signal monitored in the annulus at the surface as a result of an acoustic shot for the well illustration in  FIG. 4A , 
           [0014]      FIG. 5  is a section view of the lower end of the casing in a cavern well bore and the upper portion of the cavern chimney wherein the gas/liquid interface is located at just below the casing shoe of the cavern, 
           [0015]      FIG. 6  is a section view of the lower end of the casing in a cavern well bore and the upper portion of the cavern chimney wherein the gas/liquid interface is located at a first depth below the casing shoe of the cavern, 
           [0016]      FIG. 7  is a section view of the lower end of the casing in a cavern well bore and the upper portion of the cavern chimney wherein the gas/liquid interface is located at a second depth below the casing shoe of the cavern, 
           [0017]      FIG. 8  is a section view of the lower end of the casing in a cavern well bore and the upper portion of the cavern chimney wherein the gas/liquid interface is located at a third depth below the casing shoe of the cavern, and 
           [0018]      FIG. 9  is a chart illustrating a chimney profile produced in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring to  FIG. 1 , there is shown a well bore  10  which extends from the earth surface  12  down to an underground storage cavern  14 . In this setting, the cavern  14  is used for the bulk storage of a liquid hydrocarbon  16  such as, for example, crude oil. Other liquid hydrocarbons include propane and butane. Within the well bore  10  there is a casing at the outer perimeter and a string of tubing  20  positioned in the interior of the casing  18 . Not all storage cavern bore holes include the tubing  20 . The tubing  20  comprises interconnected tubing joints. The space between the tubing  20  and the casing  18  is termed an annulus  22 . The annulus  22  is a generally circular open passageway which allows for the transmission of liquids and gasses. 
         [0020]    The storage cavern is also referred to as a storage well. 
         [0021]    The cavern  10  is typically produced in a salt formation by pumping water down through the tubing  20  to dissolve the salt and returning the resulting brine up through the annulus  22 . Additional equipment is used to shape the cavern. This process is continued until a cavern  14  of useful volume is produced. Caverns can be produced which have the capacity to hold millions of barrels of liquid product. The upper end of the cavern  14  typically has a region termed a chimney  24 . In this representation, the chimney can have a diameter of approximately 8 to 10 feet and a vertical length of 200 feet. The cavern  14  main chamber, as an example, can have a diameter of approximately two hundred feet and a vertical dimension of approximately 1,800 feet. The chimney could be located between 2,000 ft. and 2,200 ft. below the surface. 
         [0022]    Such caverns are cost efficient and safe storage for volatile liquids and gasses, usually bulk hydrocarbons. 
         [0023]    Further referring to  FIG. 1 , the cavern  14  has brine  30  at the bottom below the lighter liquid hydrocarbon  14 . Typically the liquid  16  fills the annulus  22 . An inert gas  32 , such as nitrogen, can be located above the liquid hydrocarbon  16  in the annulus  22  and chimney  24 . The annulus  22  is typically filled to the surface with the liquid hydrocarbon  16 . There can be a gas/liquid interface  34  at the surface of the liquid hydrocarbon  16  created by gas injection. Based on the volume of brine  30  and liquid hydrocarbon  16  in the annulus  22  and cavern  14 , the interface  34  can be located in any one of the cavern  14 , chimney  24  or annulus  22 . The tubing  20  is typically filled with brine  30 . The liquid hydrocarbon  16  is removed from the cavern  14  by pumping brine  30  down the tubing  20  thereby lifting the liquid hydrocarbon  16  and forcing it upward through the annulus  22  where it is directed through a pipeline  38  to storage tanks or a pipeline elsewhere, such as a refinery. 
         [0024]    Still referring to  FIG. 1 , there is located at the lower end of the casing  18  a structure termed a casing shoe  44  which provides a transition from the lower end of the casing  18  to the interior surface of the chimney  24 . 
         [0025]    The apparatus used in conjunction with the present invention is shown in the upper portion of  FIG. 1 . An Echometer Company well analyzer  50 , as shown at the site Echometer.com, is connected to activate a gas gun  52  which is mounted to pipe joints  54  for acoustic communication through the wellhead  56  to the annulus  22 . Representative gas guns are the models “Compact Gas Gun”, “Remote Fire Gas Gun”, Wireless Remote Gas Gun”, “5K PSI Gas Gun”, and 15K PSI Gas Gun”, all made and sold by Echometer Company. A representative gas gun is also shown in U.S. Pat. No. 4,408,676, entitled “Gas Gun Assembly” and issued Oct. 11, 1983, and which is incorporated by reference herein. The gas gun  52  can be manually activated or activated by the well analyzer  50  via a cable  58 , or equivalent wireless link. The gas gun  52  includes a microphone for recording the sounds carried through the annulus  22 . Sensors  64 , for pressure and/or temperature, are connected to the well analyzer  50  via cable  66 , or equivalent wireless link. 
         [0026]    Acoustic sounding operation and apparatus are described in U.S. Pat. No. 5,285,388 entitled “Detection of Liquid Reflection for Echo Sounding Operation” issued Feb. 8, 1994 and U.S. Pat. No. 5,117,399 entitled “Data Processing and Display for Echo Sounding Data” issued May 26, 1992, both of which are incorporated herein by reference. 
         [0027]    The gas gun  52  produces an acoustic (pressure) pulse which is transmitted through the pipe joints  54  to the annulus  22  and then downward toward the gas/liquid interface  34 . The acoustic pulse reflects from striking the interface  34  and returns up the annulus  22  and through the pipe joints  54  to the microphone in the gas gun  52 . The microphone receives acoustic energy of the reflected pulse and transmits it as a signal to the well analyzer  50 . A gas gun  52  can generate either a positive (increasing pressure) pulse by using an externally supplied source of compressed gas, or can vent gas from the pipe joints  54 , and thus from the annulus  22 , to produce a negative (decreasing pressure) pulse. 
         [0028]    For the present invention, the preferred acoustic pulse is a rarefaction pulse (negative) which creates a pressure pulse by a reduction in pressure. This is preferred because of the pressure that is present in the casing. The rarefaction pulse is produced by venting a small volume of gas from the casing. However, compression pulses can also be used. These compression pulses are produced by introducing gas at a higher pressure into the casing. This produces a pulse which has a pressure increase. Another type of pulse which can be used is a frequency shift pulse. This pulse has a changing frequency during the period of the pulse. A frequency shift pulse is detected by a process termed correlation, which produces a correlation pulse. The correlation pulse can have polarity that is a function of the relative area of the conduction path, such as the casing, and the interface surface, just as with rarefaction and compression pulses. 
         [0029]    There is further used in conjunction with an embodiment of the invention, a nitrogen gas tank  72  that is connected through piping to a valve  74 , a nitrogen gas mass flow meter assembly  76  and a valve  80  to the wellhead  56  into the annulus  22 . A model of the meter in assembly  76  is the Micro Motion product Elite Coriolis meter. When the valves  74  and  80  are opened, either manually or electrically, nitrogen gas from the tank  72  flows through the meter assembly  76  and wellhead  56  into the annulus  22 . A valve  78 , when opened with valve  74  closed and valve  80  open, will vent nitrogen gas from the annulus  22  into the atmosphere, or be recovered and compressed. Piping is included in the meter assembly  76  to direct the gas flow direction as needed. The meter in assembly  76  fundamentally measures mass for the gas transmitted through it, and the mass measurement can be converted to standard gas volume. 
         [0030]    The pressure in the tubing  20  is measured by a pressure meter  140 . The pressure in the annulus  22  is measured by the pressure meter  142 . 
         [0031]    The valves  74 ,  78 , and  80  and meter assembly  76  can optionally be electrically controlled by the well analyzer  50  through cables  82 . They can also be operated manually. 
         [0032]    Processes for operation of the invention are now described in reference to the Figures. Referring to  FIG. 1 , nitrogen gas is injected into the annulus  22 . One step that can be used in the invention involves establishing a reference location for the gas/liquid interface  34 . 
         [0033]    The first phase, injecting the nitrogen gas, is now described in reference to  FIGS. 1,2A, 2B, 3A, 3   b,    4 A,  4 B and  9 . The process begins with nitrogen gas injection into the annulus  22  from tank  72  via valve  74 , meter assembly  76  and valve  80 . During this first phase, the gas gun  52  is activated multiple times to produce acoustic shots. These can be at regular or irregular periods or at selected times. The initial shots are preferably rarefaction pulses produced by venting a small volume gas from the pressurized gas in the casing  18 . Interface  34  is typically located in the annulus  22  near the surface  12  after a small volume of gas has been injected. As more nitrogen gas is injected into the annulus  22 , the interface  34  is pushed downward. An acoustic shot signal  88  is shown in  FIG. 2B  taken when the interface  34  is located as shown in  FIG. 2A . An initial pulse  88 A is generated from the firing of the gas gun  52  and a reflection pulse  88 B is returned after the pulse travels down to the interface  34 , is reflected, and travels back to the microphone in the gas gun  52 . For this embodiment of the invention, a negative going initial pulse indicates a rarefaction pulse which encounters a reduced annulus area, that is, the interface  34  reduces the annulus area to near zero. In  FIG. 2B , the return is return has the same polarity as the initial pulse  88 A, that is, negative going. Pulse  88 B is not polarity inverted from pulse  88 A. 
         [0034]    Continuing to  FIGS. 3A and 3B , the interface  34  has been pushed further downward by an increasing amount of nitrogen gas injected into the well. An acoustic shot signal  94  is produced from a firing of the gas gun  52  to produce a rarefaction pulse  94 A. The interface  34  is still within the annulus  22  so the reflection pulse  94 B has the same polarity (negative going) as the initial pulse. 
         [0035]    In  FIG. 4A , the continuing flow, either steady or irregular, of nitrogen gas has pushed the interface  34  down to below the casing shoe  44  in the chimney  24 , and is slightly below the lower end of the casing  18  and the casing shoe  44 . This is typically in the range of 2 to 5 feet. The gas gun  52  produces a rarefaction shot which leads to the acoustic shot signal  98  ( FIG. 4B ) from the microphone in the gas gun  52 . The initial shot is shown as pulse  98 A. At this level, the surface area of the interface  34  is greater than that of the cross-sectional area of the annulus  22  within the casing  18 . When the pulse encounters a sudden increase in area, as compared to that of the annulus, an inverted reflection is created. This increase in area produces an inverted reflection pulse  98 B. This polarity inversion indicates that the gas/liquid interface  34  is located at a level in the chimney  24  just below the end of the casing  18 . This level is used as a reference point, and it is not necessary to make a measurement of the actual depth to this level. This depth can be determined approximately by reference to the well log. The depth to the end of the casing  18  for a particular bore hole to a cavern is usually recorded in the well log that describes the details of the particular site. Thus, this record can provide the depth to the end of the casing string. 
         [0036]    At the time that this polarity inversion is initially detected, the flow of nitrogen gas into the well is stopped. This location of the interface  34 , as shown in  FIG. 4B  is termed the “reference level” for the interface  34 . It is also referred to as a cross-sectional area transition level because the casing or annulus area is less than the cross-sectional area of the upper end of the chimney  24  immediately below the casing  18 . 
         [0037]    A parameter that is used in conjunction with the present invention is the gradient of the hydrocarbon liquid. This is expressed as psi/ft. The term “psi” is pounds per square inch. The gradient value can be determined by taking a sample of the hydrocarbon liquid, typically crude oil, at the surface from the annulus  22 . This can be measured with a density instrument, such as a hydrometer, and from that measurement, the gradient can be calculated. A typical value for the gradient of crude oil is 0.33 psi/ft. Alternately, the liquid gradient can be determined by experimentation at the cavern test site. The gas pressure can be increased by a known amount and the change in the level of the interface can be measured using a device, for example an acoustic instrument such as the Echometer Company well analyzer. As an example, if the gas pressure in the annulus at the test site is increased by 33 psi and the column is measured to move down 100 ft., then the gradient is calculated as: Gradient=33/100=0.33. 
         [0038]    Another technique for determining the gradient is to make a first pressure measurement when the interface  34  is at the top (the surface) of the casing  18 . Inert gas is then injected into the casing  18  and acoustic shots taken, as described above, until the interface  34  reaches the reference level, which is a short distance below the lower end of the casing, just inside the top of the chimney  24 . A second pressure measurement is taken with the interface  34  at the reference level. The depth of the reference is either known as a characteristic of the particular bore hole, or can be measured with acoustic sounding using the equipment described above. As an example, the casing pressure with the interface  34  at the surface could be 700 psi and when the interface  34  is pushed down to the reference level, the casing pressure, measured at the surface, could be 1,300 psi. The depth to the reference level is determined to be 2,100 ft. The gradient for this example is therefore: 600 psi/2,100 ft=0.29 psi/ft 
         [0039]    An embodiment of the invention is described in reference to  FIG. 1 , and the associated description, and with reference to  FIGS. 5, 6, 7 and 8 . The objective is to measure the approximate configuration of the chimney  24  and if needed, the configuration of the cavern  14 . This can be referred to as a survey of the cavern. First, the interface  34  is pushed downward to just below the casing shoe  44  as described above to set it at the reference level. This reference level is known to be at the top of the chimney  24 . In an example site, the nitrogen gas pressure, as measured at the surface by meter  142 , is 1,500 psi, which is the pressure needed to push the interface  34  down to the reference level. From prior experimentation, it has been determined that a fixed increase in gas pressure depresses the interface  34  a fixed distance. This is the gradient for the hydrocarbon liquid. For a test cavern, an increase in pressure of 0.33 psi depresses the interface  34  by approximately 1 foot. This is a gradient value of 0.33 psi/ft. The standard volume of gas required to increase the pressure at the surface by this amount is measured by the meter  142 . This volume of gas, adjusted for the pressure and temperature between the surface and at the level of the interface  34 , establishes the approximate volume contained in a one foot high section of the chimney  24 . 
         [0040]      FIG. 5  illustrates the location of interface  34  at the top (reference level) of the chimney  24  and with a surface gas pressure of 1,500 psi. Line  172  represents the position of the interface  34 . 
         [0041]    In  FIG. 6 , the interface  34  has been pushed down approximate 1 foot by increasing the gas pressure to 1,500.3 psi to produce a region  160  within the chimney  24 . Line  174  represents the position of interface  34  in this Figure. The space between lines  172  and  174  is a region  160 . The volume of this region is related to the incremental volume of injected gas, with the necessary pressure and temperature compensation. Given the height (1 foot) and measured volume (V) and assumed generally cylindrical configuration, the width of the chimney at this level can be calculated. If the measured and compensated volume is 28 cubic feet, the height (h) is 1 foot and the assumed shape is cylindrical, the diameter (2R) of the chimney at this depth is approximately 6 feet. This is determined by the formula: R 2 =V/hπ and D=2R Alternately, the volume can be divided by the incremental distance to determine an area for a given depth in the chimney  24 . For this section, the volume is 23 ft 3  and the incremental distance is 1 ft., therefore the area at this depth is 23 ft 2 . 
         [0042]    Next the surface gas pressure is increased to 1,500.6 psi to depress the interface  34  downward another foot, as shown in  FIG. 7 , to the level represented by line  176 . The space in the chimney  24  between lines  174  and  176  is a region  162 . The same calculations and compensations are done to determine the diameter of the chimney  24  for region  162 . This could be, for example, 6.5 feet. 
         [0043]    The gas pressure is further increased to 1,500.9 psi to push down the interface  34  as shown in  FIG. 8  to a level shown by line  178 . The space in chimney  24  between lines  176  and  178  is a region  164 . The diameter of the chimney in this region can be calculated as with the prior regions. It could be, for example, 6.7 feet. This process can be repeated for as deep into the chimney  24  as needed, and even going into the wider portion of cavern  14 . The same calculation process is used in the cavern below the chimney. The steps are not limited to only one foot increments, for example, the pressure could be increased by 3.3 psi for a 10 foot depression. 
         [0044]    The flow of inert gas into the casing described above can be continuous with continuous measurement of gas volume so that the parameters for calculating the profile can be produced, and plotted, at a continuous pace. For the processes using a release of gas, the flow can also be continuous with continuous monitoring of pressure and volume. 
         [0045]    The gas standard volume measurements taken at the surface must be compensated to determine the gas volume in the well, such as in the chimney  24 . This compensation includes changes in temperature and the weight of the gas column. Temperature profiles can be obtained from previously run temperature sampling for a specific cavern, be taken from standard temperature profiles for common fields or regions, or calculated based on models using the known physical aspects of a cavern, such as location, depth, earth strata and adjacent subsurface formations. 
         [0046]    An example of a profile, which was produced in accordance with the present invention, is shown in the chart of  FIG. 9 . This is the measured profile for a portion of a chimney of a storage cavern. The depth in the cavern is shown horizontally and the area in the chimney measured at various depths is shown vertically. The cavern data is plotted as line  180 . The chimney area begins to increase at the depth of approximate 1990 feet. The area increases rapidly at the depth of 1995, but mostly flattens out in the range of 1997 to 2005. Line  180  represents the area profile for the top of the chimney, starting narrow and widening out. If the chimney is assumed to be cylindrical, the diameter can be calculated to an approximate value. 
         [0047]    The above embodiments use a fixed pressure differential value to move the interface down and a measurement of gas volume to make a determination of a volume for a region of the chimney. Another embodiment is to use a fixed gas volume injection for each region with a measurement of the pressure change. This parameter exchange can be made in the above described embodiments. When these two parameters are known, the height and volume of an identified location region in the chimney are established. 
         [0048]    A further embodiment is a bottom up measurement process, which is essentially the reverse of the above described operation. In this further embodiment, the interface  34  is initially driven to a level in the chimney  24  or in the cavern to a level in the chimney or cavern which serves as a starting level. This can be, for example, the level at line  178  in  FIG. 8 . This level of the interface can be set by acoustic sounding using the well analyzer  50  and gas gun  52  as described above. At this level of the interface  34  ( FIG. 8 ), the gas pressure at the surface  12  is measured by pressure gauge  142 . The gas in the annulus is then released at the surface through valves  78  and  80  until the surface gas pressure is reduced by a predetermine amount, for example, 0.33 psi. This pressure reduction allows the interface  34  to rise to the level of line  176  as shown in  FIG. 7 . This establishes the region  164 . A further reduction of gas pressure by 0.33 psi allows the interface to rise to the level of line  174 , as shown in  FIG. 6  to form region  162 . A still further reduction in surface gas pressure by 0.33 psi raises the gas interface  34  to the level of line  172  as shown in  FIG. 5  to define region  160 . The diameters of each region are calculated as described above. 
         [0049]    Although several embodiments of the invention have been illustrated in the accompanying drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention.

Summary:
Underground storage caverns are used for the bulk storage of hydrocarbon liquids, such as crude oil and gases, such as natural gas. The cavern is accessed through a bore hole which has casing and, for some bore holes, internal tubing with an annulus between the casing and tubing. The upper end of the cavern has a roughly cylindrical region termed the chimney. In order to check it for physical integrity, it is necessary to measure the profile of the chimney. This is also referred to as conducting a survey of the cavern. The cavern typically has hydrocarbon liquid above brine up to the surface. An inert gas can be injected above the hydrocarbon liquid to form an interface. The profile is conducted by driving the gas/liquid interface downward with gas pressure to a reference level determined by sequentially transmitting acoustic pulses to locate the reference level. Gas is injected to increase the pressure by a predetermined value and thereby drive down the interface by a known distance. The volume of the gas injected is used together with the known distance to determine a profile of the chimney. The process of injection of gas to increase the pressure by the predetermined value and measurement of the volume is repeated sequentially to determine the chimney profile at progressively lower regions, thereby producing an extended profile of the chimney.