US3106824A

US3106824A - Method of underground fluid storage

Info

Publication number: US3106824A
Application number: US1495A
Authority: US
Inventors: James N Gregory
Original assignee: Individual
Current assignee: Individual
Priority date: 1960-01-11
Filing date: 1960-01-11
Publication date: 1963-10-15
Anticipated expiration: 1980-10-15

Description

Oct. 15, 1963 J. N. GREGORY METHOD OF UNDERGROUND FLUID STORAGE 3 Sheets-Sheet l Filed Jan. ll, 1960 INVENTOR. Jam es Gregory BY Fulwder, Mafl'inglg 8 Hunrleg ATTO l2 N EYS Oct. 15, 1963 J. N. GREGORY 3,106,824

METHOD OF UNDERGROUND FLUID STORAGE Filed Jan. ll, 1960 3 Sheets-Sheet 2 Fi g. 3 \F E G P H X A B INI/EMDR.

James Gregory"` BY Fulwider, MaH'inglg 8 Hunleg ATTOR NEYS Oct. 15, 1963 J. N. GREGORY METHOD OF UNDERGROUND FLUID STORAGE I:s sheets-sheet s Filed Jan. ll, 1960 IN V EN TOR.

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G9 N m um wm, ma Y hwm IM ATTORNEYS United States Patent O 3,106,824 METHOD GF UNDERGROUND FLUID STORAGE James N. Gregory, 5651 Sorrento Drive, Long Beach, Calif. Filed Jan. ll, 1966, Ser. No. 1,495 4 Claims. (Cl. 61.5)

The present invention relates generally to the underground storage of fluids and more particularly to new and novel arrangements of underground storage tanks particularly useful in storing fluids such as oil, chemicals and liquiiied petroleum gases.

The underground storage of iiuids offers many advantages over above ground storage tanks particularly in congested, residential or hazardous areas. In residential areas above ground storage tanks are a constant remainder of the existence of oil wells, refineries, chemical plants or distribution Icenters and their attendant hazards, unsightliness and odor even if the latter are actually nonexistent. With an underground storage tank no suggestion of hazards is presented and the area may be more attractively landscaped. Underground storage tanks may be made stronger than above ground storage tanks since the earth itself may be utilized as walls of the tank. Underground storage tanks may be made to provide more capacity for a given diameter. This is true since design considerations limit the height and diameter of above ground storage tanks. Below ground only the characteristics of the earth need limit the depth of an underground tank. There can be no `doubt but that underground storage is far safer than above ground storage. This will be apparent when the various rigid requirements of the various building codes are considered, Ii.e. tire walls, tire iighting equipment and the like. When an above ground storage tank ruptures, the contents of the tank are free to flow over the ground. Should an underground storage tank rupture, the contents thereof would seep slowly into the earth with little danger of explosion or re. Additionally, an underground storage tank would not be aected `by tires or explosions taking place in its vicinity. With particular regard to petroleum products, the earth acts as an excellent thermal insulator for Van underground storage tank, and accordingly the contents of the tank may be kept at a suiiiciently high ternperature as to remain free flowing. This is a consider'- able problem in above ground storage where the contents of the tank are in heat-transfer relationship with the atmosphere. Where an underground storage tank is substituted for an above ground tank, a reinforced deck may be provided so as to permit the installation of other facilities above the underground tank. This can be of considerable importance in an area of high land values.

Although the above set forth advantages of underground storage ltanks are generally known, such tanks have not met with appreciable widespread acceptance to date because of the many problems inherent to heretofore proposed underground storage tanks. It is a major object ot the present invention to provide new and irnvproved underground storage tank arrangements.

The underground storage tanks embodying the present invention utilize a liner formed of a flexible synthetic plastic which is inert relative to the stored iluid and to the elements of the surrounding earth. 'I'he use of a synthetic plastic liner in place of the conventional metal tanks heretofore employed makes possible considerable economies of construction. Moreover, such synthetic plastics are not subject to corrosion. Accordingly, the underground storage tanks of the present invention can provide a long and trouble-free service life.

In the construction of the underground storage tanks embodying the present invention, a vertical bore is first sunk into the earth. Thereafter, a flexible synthetic plastic liner is positioned within this bore. The liner serves to contain the stored fluid. The pressure within the tank is always maintained above the external pressure of the fluids in the earth surrounding the bore tending to cause the ybore walls to collapse or slough. The existence of this internal pressure will 'also keep the liner plastered against the walls of the borre. In this manner the liner` is restrained against folding inwardly and thereby undergoing abrasion. It will be appreciate-d that such abrasion could damage the lines and lead to leakage.

It is another aspect of the present invention that the internal pressure within the underground storage tank is maintained sufficiently high enough to prevent collapse of the bore, and yet such pressure is not permitted to rise above a magnitude which would cause the earthen walls of the bore to fracture. Under these pressure conditions the synthetic plastic liner need not he dependent upon for providing any appreciable structural strength. Instead, the liner will tbe maintained in a condition of balance, and the walls of the bore will provide the strength for the tank.

It is accordingly an object of the present invention to provide an underground storage tank utilizing a vertical bore -formed in the earth wherein is positioned a flexible synthetic plastic liner, with the fluid pressure within the liner being maintained between the limits required to prevent collapse of the bore and keep the lliner plastered against the walls of the `bore and a magnitude which would tend to eifect fraoturing of the bore walls.

It will be understood that where the uid stored in an underground storage tank embodying the teachings of the present invention is lighter than water, .there will exist a tendency for the tank to become buoyant. Accordingly, it is another object of the present invention to provide an underground storage tank of the aforedescribed nature which utilizes ballast to provide sutiicient overburden to balance the upward thrust of the tanks contents.

It is yet a further object of the present invention to provide an underground storage tank that is particularly adapted for the storage of liquified petroleum gases. Presently one of the most practical and economical methods of storing liquiiied petroleum gases is to store such ilui-d in underground caverns washed out of salt beds. Suitable salt layers, however, do not exist uniformly nor throughout the country. In areas of the country where salt Abed structures do not exist, caverns have been mined out of shale and limestone, and additionally depleted oil and gas sands and salt water sands have been utilized. With all of the-se larrangements there is considerable likelihood of leakage of the stored liquied petroleum gas. The underground storage tank embodying the present invention, however, is not subject to leakage since it utilizes an impervious synthetic plastic liner.

Yet a further object of the present invention is to provide an underground storage tank utilizing an outer synthetic plastic liner which is constantly maintained plastered against the earths bore as by means of -air pressure and an inner synthetic plastic liner which may collapse as a iluid stored therewithin is withdrawn. This form of tank maintains the stored fluid isolated from the atmosphere with vapors from the stored iiuid.

These and other objects and advantages of the present invention will become apparent when taken in conjunction with the appended drawings wherein:

yFIGURE 1 is a vertical cross-sectional view showing a first form of Iunderground storage tank embodying the present invention;

'FIGURES 2 and 3 are depth-pressure graphs showing the required internal and external pressure conditions within the underground storage tank of FIGURE l; and

FIGURE 4 is a vertical cross-sectional view showing a second form of underground storage tank embodying the present invention;

-FIGURE 5 is a fragmentary vertical cross-sectional view in enlarged scale showing a detail of the tank of FIGURE l; and

`FIGURE 6 is a fragmentary vertical cross-sectional view in enlarged scale showing a detail of the tank of FIGURE 4.

Referring to the drawings and particularly FIGURE 1 thereof, the first form of underground storage tank embodying the present invention utilizes a vertical bore 10 formed in the earth. Disposed within this bore is a cylindrical liner 12 formed of a suitable llexible synthetic plastic, such as polyvinyl chloride or the like. It is necessary that the material from which the liner l12 is formed is not subject to damage from the iluid to be stored within the tank, nor from the `chemical constituents within the earth formation 14 wherein the bore 10 is formed. The dimensions of the liner 12 should be such that with the side walls of the liner expanded into contact with the side walls of the bore 110 a relatively snug llt will be obtained.

Above the bore 10 there is formed a ballast cavity 16. In the embodiment shown in FIGURE l the diameter of the cavity 16 exceeds the diameter of the bore 10. A horizontal deck 18 is disposed within the lower portion of the ballast cavity 16 and the upper wall 20 of the liner abuts the underside of the deck 18. The deck 18 may be formed of a rigid material such as reinforced concrete and is supported by the shoulder 19 defined between the upper end of the bore 10 and the lower end of the ballast cavity 16. The space within the ballast cavity 16 above the upper surface of the deck 18 is lled with a suitable ballast material 22, as for example wet gravel, having a density greater than that of the lluid 24 being stored within the liner `10. A rigid ballast retaining pipe 25 extends between the central portion of the deck `18 and the earths surface. This pipe 25 may be formed of concrete and, as indicated in FIGURE 5, its lower end is received within an annular recess 29 formed at the inner periphery of the deck 18.

A vertical casing pipe 26 of smaller diameter than the ballast retaining pipe 25 is aillxed to and extends upwardly from the central portion of the upper wall 20 of the liner to a point above the earths surface. At this point the upper end of the casing 26 is suitably alllxed to a conventional casing head 28. The casing 26 may be formed of a rigid synthetic plastic and, as indicated in FIGURE 5, its lower end may be secured to the upper central portion of the upper Wall 2t) of liner 12. Conveniently, the upper wall 20 will be bent upwardly to define a collar 27 that is bonded to the casing 26 as by heat. The casing head 28 is in turn connected in a conventional manner to a pressure fluid conduit 30. A conventional pressure indicating gage 32` is provided for the pipe 30. The pressure lluid conduit 30 is connected with a tluid pressure producing means 33 such as a conventional compressor, with such compressor having its intake in communication with a source of suitable fluid, such as natural gas. A stored lluid conduit 34 extends concentrically downwardly through the casing head 28 and the casing 26 to a point spaced above the bottom wall 36 of the liner 12. A conventional pressure gage 38 is provided for this conduit 34.

In the operation of the underground storage tank of FIGURE 1, the lluid 24 to be stored is pumped into and removed from the interior of the liner 12 by means of the stored tluid conduit 34. The interior of the liner 12 above the level of the stored fluid 24 is pressurized by means of the pressure lluid conduit 30 and the annulus 35 between the conduit 34 and the casing 26. Conventional pumping means (not shown) may be utilized in conjunction with the stored fluid pipe 34.

Referring now to FIGURE 2, it will be understood that Awhere a bore or shaft is drilled into the earth the tendency of the walls of the bore to collapse is dependent upon the rigidity of the earth formation through which the bore is formed as well as upon the pressure of the lluids contained within the interstices of such earth formation. Generally, even in soft unconsolidated sands and clays the pressure of the formation lluids will not exceed the head of a column of water standing in the hole and extending to the earths surface. Accordingly, if a fluid pressure is maintained within the bore which is greater than the hydrostatic pressure of the formation fluids the hole will be stable and its walls resistant to collapse. In the depth pressure chart of FIGURE 2, the vertical line OX indicates the depth from the earths surface to the lower end of the bore 10. The line OA represents the maximum hydrostatic pressure likely to be exerted by the formation lluids. The line OB represents the breakdown or fracturing pressure at which the walls of the bore 10 are likely to be fractured by a pressurized fluid disposed with the bore. This breakdown pressure if not known may be determined by conventional means while the bore -10 is being drilled. Under these circumstances, if the fluid pressure within the liner 12 is always maintained between the lines OA and OB, the walls of the bore 10 will neither collapse inwardly nor undergo fracture. Additionally, the liner 12 will be maintained plastered against the walls of the bore ill).

Under the conditions indicated in FIGURE 2 should the walls of the "core 16 be impermeable a lluid could be stored therewithin without requiring the plastic liner 12. Such impermeability is seldom found, however, and accordingly it is necessary to provide the plastic liner 12. This liner, however, need not contribute any strength to the tank so long as the pressures are correctly maintained.

Assuming the stored fluid 24 has a density lower than that of the formation fluids adjacent the bore 10, such stored fluid will exert a pressure indicated by the line KC. Since this line KC lies to the left of the line OA, the stored fluid 24 would not exert sullcient outward pressure against the liner 12 so as to maintain the liner plastered against the sides of the well bore 10 with sufficient force to prevent inward collapse of such walls. Accordingly, it is necessary to exert additional internal pressure within the liner 12. This may be accomplished by applying a lluid under pressure through the pressure lluid conduit 30. When this has been accomplished, the depth-pressure curve will be shifted to the right, as indicated by the line PD. When this has been accomplished, it will be apparent that Ibelow the depth indicated by the horizontal line E, the line PD falls between the lines OA and OB. Accordingly, below this depth the pressure within the liner 12 will be sullicient to prevent inward collapse of the bore walls and yet not so great as to cause the liner 12 to be bulged outwardly and rupture the walls of the bore.

Above the depth E, however, the pressure within the liner 12 would be of a magnitude sufficient to effect outward bulging of the liner 12 with the consequent likelihood of fracturing the walls of the bore 10. The liner I.12 and its contents 24 would also become buoyant and tend to lloat out of the hole 10. It is accordingly necessary to terminate the upper end of the bore 10 at the depth E. An overburden such as the deck 18 and the ballast material 22 is then positioned above the upper end of the bore 10. In this manner a depth-pressure gradient equal to or greater than that represented by the line OB will be provided and the conditions of stability will have been met. This is indicated by the line OF in FIGURE 3. With continued reference to FIGURE 3, so long as the depth-pressure gradient is kept within the area GHBA the plastic liner 12 will be maintained plastered against the walls with suflicient force to restrain any tendency of the bore walls to collapse. The outward force exerted against .'well bore walls.

Referring again to FIGURE 3, if the liner l2 is filled with a liquid its pressure gradient may be represented by the line P1A. The pressure applied through the pressure iluid conduit 30 must be of a magnitude indicated by the value not less than that indicated by P1. If the stored liquid is displaced by a gas, however, the pressure applied to such gas must be increased to the value P2. The pressure gradient of the gas would then be represented by the line PZA. The interior of the liner 12 is of course sealed relative to the atmosphere. So long as these conditions are met the desired conditions of stability will exist within the liner 12. The computations required for establishing these conditions will of course vary with the nature of the lluid being stored and the type of earth formation involved. These computations, however, may be readily made by those skilled in the art. It should be noted that under certain conditions where the bore walls are suillciently resistant to fluid movement it may not be necessary to utilize the liner. Alternatively, the bore walls may be lined with a material other than synthetic plastic, as for example, Gunnite or a metal.

It should be particularly noted that the aforedescribed underground storage tank may oe utilized successfully in storing liquiiled petroleum gases. In order to store liquilled petroleum gases the conduit 39 is employed to admit the liquifled petroleum gas to the upper portion of the tank and similarly to remove it therefrom. The conduit 34 is employed to admit water to the lower portion of the tank and likewise to withdraw such water therefrom. The liquied petroleum gas admitted through the conduit 3h is maintained under suilicient pressure that it will remain liquid while it is being stored. This liquified petroleum gas will remain above the surface of the Water disposed within the lower portion of the tank inasmuch as water has a higher density than liquied petroleum gas. As the liquiled petroleum gas is drawn from the upper portion of the tank it will be replaced with water admitted through conduit 34. Suillcient water must be provided to equal the capacity of the storage tank. The liquiiled petroleum gas should be maintained at sufllcient pressure to overcome the hydrostatic pressure of the formation fluids in the portion of the bore 1t) outwardly of the stored liquiied petroleum gas. The pressure of the liquified petroleum gas, however, should not be of a magnitude to cause the liner 12 to be bulged outwardly and rupture the walls of the bore l0.

If desired, the liquied petroleum gas may be drawn oif as a gas and not replaced with another liquid such as water. The conduit 34 could therefor be sealed olf. In this case, however, the depth of the bore lil must be limited in order that the Vapor pressure of the stored liquii'led petroleum gas will never be less than the hydrostatic pressure of the formation fluids surrounding the bore lil. Such pressure may be assumed as being equal to the head of a column of water disposed in the bore l0 and extending to the earths surface.

Should it become desirable to replace or repair the aforedescribed liner 12, the stored iluid 24 is rst withdrawn therefrom. Next, the conduit 34 is removed. The column of ballast material 22a disposed between the ballast retaining pipe 25 and the casing 26 is then removed, as by a washing operation. Thereafter, the liner l2 may be collapsed by forcing water downwardly through the annulus formerly occupied by the ballast material 22a. Finally, the liner 12 is pulled upwardly through the ballast retaining pipe 25 by means of the casing 26. The liner 12 may be replaced by a reversal of this operation.

Referring now to FIGURE 4, there is shown a second form of underground storage tank em'bodying the present invention. This form is particularly adapted for storing a petroleum product, such as gasoline or other fuel, where it is desirable to prevent vapors of the stored fuel from being discharged into the atmosphere, and, whene an ample supply of natural gas for pressurizing the portion of the tank above the stored liquid is not available. As an example of such use, the underground storage tanks of FIGURE 4 may be satisfactorily employed to store gasoline at a service station or the like.

This second form of underground storage tank utilizes a vertical bore 50 `formed in an earth formation S2. Disposed within this bore 50 is an outer cylindrical liner 54 formed of a material similar to that described in conjunction with the liner 12 shown in FIGURE l. The dimensions of the outer liner 54 should be such that with its side walls expanded into Contact with the side walls of the bore 50 a relatively snug fit will be obtained. Above the upper wall 56 of the liner 54 there is disposed a rigid horizontal deck 58 having the same `diameter as the bor-e Sil. This deck 58 may be formed of a material such as reinforced concrete. The remainder yof the bore 5t) above the deck 58 is lilled with a suitable lballast material 6@ having a density greater than that of the iluid 62 'being stored within the tank. The upper end of the bore 5t) merges into a counterbore 64- of larger diameter. Disposed within this counterbore 64 in a rigid foundation ring 66 which may conveniently be formed of reinforced concrete. The upper portion of the counterbore 64 is bridged by a plurality of rigid beams 68. These beams are ail'lxed to the upper end of a plurality of vertical tie rods 70. The lower ends of the tie rods 70 are axed to the deck 5S. This arrangement serves to support the deck 58 above the outer liner 54.

A rigid ballast retaining pipe 71 extends upwardly from the 'central portion of the deck 58 to the earths surface. A vertical casing '72 of smaller diameter than the ballast retaining pipe 71 extends downwardly concentrically of the latter from a conventional head 74 mounted on the beams 68 centrally of the b'ore 50. This lcasing 72 may `he of nigid synthetic plastic and its lower end is afllxed to the upper central portion of an inner liner 76, as shown in FGURE 6. This inner liner 76 is preferably formed of the same material as the outer liner 54. A stored fluid conduit '78 of lesser diameter than the casing 72 is concentrically supported :by the casing head 74. This stored fluid pipe 78 terminates at its lower end at a point somewhat above the lower wall 80 of the inner liner 76. A conventional pressure indicating gage 82 is provided for the stored fluid conduit 78. An outer casing 83 of greater diameter than casing 72 but of similar material extends downwardly `from the casing head 74 and is alllxed at its lower end to the central portion of the upper wall 56 of the outer liner 54. The interior of the easing 72 is connected to a conventional air compressor 84 by means of a horizontal pipe 86. A conventional pressure indicating gage 8S is positioned within the horizontal pipe 86. With this arrangement, compressed air may be forced downwardly between the annulus existing between the inner casing 72 and the outer casing 83. A conventional vent and relief valve 91 for the compressed air may be provided in the casing head 74 or the horizontal pipe 86.

In the operation of the underground storage tank shown in FIGURE 4, the stored fuel 62, such as gasoline, is pumped into and removed from the interior of the inner liner 76 by means of the stored fluid conduit 78. As the stored gasoline 62 is withdrawn from the inner liner 76, the walls vof the inner liner will collapse so as to conform to the remaining gasoline disposed within the inne-r liner. The outer liner 54, however, is constantly kept plastered against the walls of the bore 50 by compressed air forced downwardly through the annulus 90 from the air compresser S4. The pressure of such compressed air should constantly be maintained at a Value which is greater than the hydrostatic pressure of the fluids within the for-mation 52 adjacent the bore 5t). This will serve to prevent collapse of the walls of the bore 50. This pressure, however, should not exceed the breakdown yor Ifracturing pressure at which the walls of the bore S0 are likely to be fractured. The requisite balancing of pressures should be computed in the same manner described hereinbefore in connection with the form of underground storage tank shown in FIGURE 1.

It will be apparent that the underground storage tank shown in FIGURE 4 will be particularly useful in storing gasoline since the stored gasoline never cornes in contact with the atmosphere. The

liners

54 and 76 may be removed in a manner similar to that described hereinbefore in connection with the tank of FIGURE l.

Various modifications and changes may he made with respect to the foregoing description without departing from the spirit of the present invention or the scope of the following claims.

I claim:

l. A method of storing fluids underground, that includes: forming a `cavity in the earth; lining said cavity with a material that is not subject to damage from the fluid to :be stored; admitting the fluid to be stored into the interior of said liner; maintaining the interior of said liner at a selected fluid pressure greater than the hydrostatic pressure of the fluids present in the earth formation surrounding said cavity, with said selected fluid pressure being less than that which would effect fracturing of said earth formation surrounding said cavity; and maintaining the upper portion of said cavity ballasted above the depth at which said selected fluid pressure would tend to effect fracturing `of the earth formation surrounding said cavity, with maintaining said upper portion of said cavity ballasted also resisting the buoyancy of said liner and fluid contents.

2. A method of storing fluids underground, that includes: forming a cavity in the earth; lining said cavity with a material that is not subject to damage from the fluid to be stored; admitting the fluid to be stored into the interior of said liner; sealing the interior of said liner from the atmosphere; maintaining the interior of said liner at a selected fluid pressure greater than the hydrostatic pressure of the fluids present in the earth formation surrounding said cavity, with said selected fluid pressure being less than that which would effect fracturing of said earth formation surrounding said cavity; and maintaining the upper portion of said cavity ballasted above the depth at which said selected fluid pressure would tend to eect fracturing of the earth formation surrounding said cavity, with maintaining said upper portion of said cavity ballasted also resisting the buoyance of said liner and fluid contents.

3. A method of storing fluids underground, that includes: forming a cavity in the earth; lining said cavity with a fluid-tight flexible material that is not subject to damage from the fluid being stored within its interior; admitting the fluid to be stored into the interior of said liner; maintaining the interior of said liner at a selected fluid pressure greater than the hydrostatic pressure of the fiuids present in the earth formation surrounding said cavity, with said selected fluid pressure being less than that which would effect fracturing of said earth formation surrounding `said cavity, said liner remaining plastered against the walls of said cavity; and maintaining the upper portion of said cavity ballasted above the depth at which said selected fluid pressure would tend to effect fracturing of the earth formation surrounding said cavity, with maintaining said upper portion of said cavity ballasted also resisting the buoyance of said liner and fluid contents.

4. A method of storing fluids underground, that includes: forming a cavity in the earth; lining said cavity with a fluid-tight flexible material that is not subject to damage from the fluid being stored within its interior; admitting the fluid to be stored into the interior of said liner; sealing `the interior of said liner from the atmosphere; maintaining the interior of said liner at a Selected fluid pressure greater than the hydrostatic pressure of the fluids present in the earth formation surrounding said cavity, with said selected fluid pressure being less than that which would effect fracturing of said earth formation surrounding said cavity, said liner remaining plastered against the walls of said cavity; and maintaining the upper portion of said cavity ballasted above the depth at which said selected fluid pressure would tend to efect fracturing of the earth formation surrounding said cavity, with maintaining said upper portion of said cavity ballasted also resisting the buoyance of said liner and fluid contents.

References Cited in the file of this patent UNITED STATES PATENTS 703,824 Quinn July l, 1902 2,211,958 Mahaffey Aug. 20, 1940 2,398,828 Gray Apr. 23, 1946 2,459,227 Kerr Jan. 18, 1949 2,661,062 Edholm Dec. 1, 1953 2,748,739 Monti et al. June 5, 1956 2,749,714 Hunter June 12, 1956 2,947,147 Johnson Aug. 2, 196() FOREIGN PATENTS 738,917 Great Britain 1955 777,644 Great Britain 1957

Claims

1. A METHOD OF STORING FLUIDS UNDERGROUND, THAT INCLUDES: FORMING A CAVITY IN THE EARTH; LINING SAID CAVITY WITH A MATERIAL THAT IS NOT SUBJECT TO DAMAGE FROM THE FLUID TO BE STORED; ADMITTING THE FLUID TO BE STORED INTO THE INTERIOR OF SAID LINEAR; MAINTAINING THE INTERIOR OF SAID LINER AT A SELECTED FLUID PRESSURE GREATER THAN THE HYDROSTATIC PRESSURE OF THE FLUIDS PRESENT IN THE EARTH FORMATION SURROUNDING SAID CAVITY, WITH SAID SELECTED FLUID PRESSURE BEING LESS THAN THAT WHICH WOULD EFFECT FRACTURING OF SAID EARTH FORMATION SURROUNDING SAID CAVITY; AND MAINTAINING THE UPPER PORTION OF SAID CAVITY BALLASTED ABOVE THE DEPTH AT WHICH SAID SELECTED FLUID PRESSURE WOULD TEND TO EFFECT FRACTURING OF THE EARTH FORMATION SURROUNDING SAID CAVITY, WITH MAINTAINING SAID UPPER PORTION OF SAID CAVITY BALLASTED ALSO RESISTING THE BUOYANCY OF SAID LINEAR AND FLUID CONTENTS.