CA2004000A1

CA2004000A1 - Artificial underground cavern for storing natural gas in gas form and a process for the preparation of the cavern

Info

Publication number: CA2004000A1
Application number: CA 2004000
Authority: CA
Inventors: Charles Mandrin
Original assignee: Gebrueder Sulzer AG
Current assignee: Sulzer AG
Priority date: 1988-12-06
Filing date: 1989-11-28
Publication date: 1990-06-06
Also published as: NO171329C; SE8904075L; NO171329B; DE58901092D1; EP0377405B1; NO894870L; SE8904075D0; EP0377405A1; FI895818A0; NO894870D0

Abstract

ABSTRACT OF THE DISCLOSURE

The artificial underground cavern for high-pressure low-temperature storage of natural gas has a casing which is disposed a few centimeters away from the cavern wall and which terminates above the cavern floor. The cavern is sealed off from the atmosphere by a plug through which supply and discharge lines extend, the lines extending to the gap between the casing and the cavern wall and into the storage chamber. After venting of the cavern, water is forced into the gap by means of a pressure gas. This water, and a layer of water which is still present above the cavern floor, is then frozen by means of a liquid refrigerant circulating through the cavern interior, the refrigerant boiling and evaporating. When the cavern is filled with high-pressure low-temperature natural gas, methane hydrates are formed in the open places of the casing by contact with the ice and seal the casing in gas-tight manner relative to the surrounding rock.

Description

~oo~ooo p.6237 AN AR~IFICIAL UNDERGROUND CAV~RN FOR ~TORING
NATURAL GA~ IN GA8 FORM AND A PROCE~

The invention relates to an artificial underground cavern for storing natural gas in gas form and to a process for the preparation of the cavern.
As is known, various types of storage facilities have been known for the storing of natural gas.
Furthermore, it has also been known to store natural gas in times of little or no consumption. If the natural gas which accrues, for example, in a constant quantity throughout the year is stored during these slack periods, the stored gas can be used to cover peak demands.
Natural gas can, of course, be stored as an energy source at high pressure and low temperature in natural caverns. As the technical literature shows, these natural caverns are, for example, salt mines, exhausted gas fields, aquifer caverns or porous rocks at substantial depths of the order of several hundred or even several thousand meters.
If for geographical reasons no such natural caverns are available, artificial stores must be devised. For example, it is known to store liquefied natural gas at 200~1~00 atmospheric pressure in above-ground insulated tanks.
However, the use of tanks and the liquefaction of natural gas are very costly and, therefore, of reduced economic interest.
Accordingly, it is an object of the invention to provide an artificial underground gas-tight cavern for the storage of natural gas at an elevated pressure of the order from 100 to 200 bar and at a low temperature in the order of from approximately -50 to -70c.
It is another object of the invention to provide an economic process for constructing an artificial;
underground cavern for the storing of natural gas in gas form.
It is another object of the invention to be able to store natural gas in gaseous form and in large quantities.
It is another object of the invention to provide an economic process for the storing of a gaseous form of natural gas.
Briefly, the invention provides for the storage of gaseous natural gas in an underground cavern having a floor and a wall enclosing a sealed chamber.
In accordance with the invention, at least one casing is disposed in tha sealed chamber of the underground cavern in spaced relation to the wall in order to define a gap therebetween and in spaced relation to the floor. In addition, a plurality of lines extend into the cavern with at least one line extending through the casing for the conveyance of various materials therethrough. In addition, an elastic layer of ice and methane hydrate is provided in the gap between the casing and cavern wall in order to form a gas-tight seal relative to the cavern wall.
At least one plug may also be provided for closing an opening into the cavern. In this case, the lines may also extend through the plug.
The casing may be made of any suitable material, for example of plastic or a metal such as aluminum. In addition, the casing is spaced from the cavern wall at a distance of from 2 to lO centimeters while being spaced from the cavern floor at a distance of about lO
centimeters.
Once constructed, natural gas in gaseous form may be pumped into the sealed chamber defined within the casing and stored therein at a suitable storage pressure, for example, of 150 bar and at cold temperatures, for example of approximately -70C.
The invention also provides a process of fabricating the underground storage facility. In accordance with this process, a casing is positioned within an underground cavern which has been prepared to receive the casing and to provide a sealed chamber for the storage of cold, natural gas. This, casing is positioned in spaced relation to a wall of the cavern in order to define a gap therebetween as well as in spaced relation to a floor of the cavern. Thereafter, the interior of the casing is flooded with water while air is vented from within the casing and the cavern. During this time, not only is the space within the casing flooded with water, but also the gap between the casing and the cavern wall. Next, compressed natural gas is passed into the casing through one of the lines in order to force the water from within the casing into the gap between the casing and cavern wall and out through one of the other lines. During this time, a layer of water is maintained on the cavern floor. Next, a refrigerant is passed into the casing in order to freeze the water remaining in the gap between the casing and the cavern wall as well as in the layer on the floor of the cavern.
Thereafter, cold natural gas is delivered into the chamber defined by the casing through one line while the refrigerant is removed through another line.
In a second embodiment, a second casing may be Z0040~)0 placed within the first casing in space relation to define a second gap. In this embodiment, the second gap is used to receive the refrigerant while water is disposed in the gap between the outer casing and the cavern wall. In this way, a reduced amount of refrigerant is required in order to freeze the water in the gap between the outer casing and the cavern wall.
These and other objects and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings wherein:
Fig. 1 illustrates a cross-sectional view of an underground storage facility constructed in accordance with the invention;
Fig. la illustrates a view similar to Fig. 1 during filling of a prepared cavern with water in accordance with the invention;
Fig. lb illustrates a view similar to Fig. la during filling of the facility with natural gas to displace the water therein;
Fig. lc illustrates a view similar to Fig. la during filling of the facility with liquid refrigerant:
Fig. ld illustrates a view similar to Fig. la during freezing of the water along the floor of the cavern and in the gap between the casing and cavern wall in accordance with the invention;
Fig. le illustrates a view similar to Fig. la during flooding of the sealed chamber with liquid refrigerant;
Fig. lf illustrates a view similar to Fig. la during filling of the facility with natural gas and displacement of the liquid refrigerant;
Fig. lg illustrates a view similar to Fig. la during filling of the chamber with gaseous natural gas under pressure;
Fig. 2 illustrates a cross sectional view of a storage facility employing at least two casings in accordance with the invention;
Fig. 2a illustrates a cross sectional view of the storage facility of Fig. 2 during filling with water in accordance with the invention;
Fig. 2b illustrates a view similar to Fig. 2a during filling of the chamber with natural gas to displace water from within the gap between the two casings in accordance with the invention;
Fig. 2c illustrates a view similar to Fig. 2a during filling of the chamber and gap between the two casings with refrigerant;
Fig. 2d illustrates a view similar to Fig. 2a during freezing of the water in the layer along the cavern floor and in the outermost gap;
Fig. 2e illustrates a view similar to Fig. 2a during filling of the gap between the two casings with liquid refrigerant;
Fig. 2f illustrates a view similar to Fig. 2a during freezing of the water in the gap between the outermost casing and the cavern wall; and Fig. 2g illustrates a view similar to Fig. 2a during delivery of natural gas into the gap between the two casings while displacing refrigerant from within the sealed chamber.
Referring to Fig. 1, a horizontally disposed tunnel-like cavern l is blasted out of the surrounding rock, for example, granite, at a depth of from 150 to 200 meters. The horizontal arrangement of the cavern is particularly advantageous but may also be vertical. In general, the cavern need not be disposed at such a depth as to ensure that the storage pressure withstands the hydrostatic pressure of the water in the surrounding rock.
A casing 2 is disposed in the cavern 1 a short distance away, for example, approximately from 2 to 10 centimeters away, from the cavern wall and ends a short distance, for example, 10 centimeters above the cavern floor.
The casing 2 can be made, for example, of steel or aluminum or plastics. The wall thickness of the casing 2 is determined in accordance with the hydrostatic water pressure in the second process step (of Fig. lb) rather than for the storage pressures of the natural gas. The casing may also be built up from a number of elements, more particularly by welding the elements together.
The casing 2 need not initially be gas-tight but lo can contain, for example, cracks and minor apertures which, as will be described hereinafter, become closed, thus ensuring the required gas-tightness relative to the surrounding rock when natural gas is stored.
The cavern 1 is sealed off from the outside atmosphere by means of a plug 4 which is inserted into a bore hole 3 and which can be made of metal or possibly even of plastics. Basically, and depending upon cavern construction, a plurality of plugs can be provided.
In the present embodiment, three lines 5, 6, 7 extend through the plug 4 for ~onveying materials therethrough.
The line 5, which is fixed in the plug 4, terminates in a gap or cavity 8 between the wall of the cavern 1 and the casing 2 and is effective to supply water or vent the gap 8.
The line 6, which is also fixed in the plug 4, extends through the casing 2 and terminates in the top of a storage chamber within the casing 2. The line 6 is effective to convey gas into and from the cavern 1.
The line 7 is movingly mounted in the plug 4 and extends through the casing 2. This line 7 is effective to remove either water or gas or liquid refrigerant from the cavern 1 or to introduce water or liquid refrigerant thereinto.
The various process steps for producing a gas-tight lining of the cavern 1 relative to the surrounding rock will be described hereinafter ~ith reference to Figs. la ZOOAOOO

to lf.
To vent the cavern, in a first process step (cf Fig. la), the line 7 is moved into the bottom part of the storage chamber 9 and water is introduced through the line 7 until the chamber 9 and the surrounding gap 8 are completely full of water the air escaping through the lines 5, 6. In this process step, the pressure in the cavern corresponds to the sum of atmospheric pressure and the hydrostatic head of the water in the cavern. In the present embodiment, the temperature in the cavern interior is, for example, approximately 4C.
In the second process step (cf Fig. lb), the water is expelled from the cavern 1 by means of natural gas, the cavern 1 being at a pressure of approximately 20 bar. The natural gas is introduced through the line 6 and expels water from the cavern 1 through the line 7, whose end is in its lowest position. The line 5 is in the closed state so that the gap 8 stays full of water.
During this step, the cavern interior is approximately at ambient temperature, which depends upon the temperature of the injected gas. At the end of this step, the casing 2 must withstand the hydrostatic pressure of the water confined in the gap 8.
In the third process step (cf Fig. lc), the line 7 is raised so far that its end is disposed above the level of the water _ present above the cavern floor but is still in the lower part of the storage chamber 9.
A predetermined quantity of liquid refrigerant, such as propane, is injected through the line 7 into the storage chamber 9 so that the lowest part thereof is filled with the liquid refrigerant k above the water level. As refrigerants, other hydrocarbons, such as ethane or ethylene can be considered; however, refrigerants such as ammonia, freon or the like are unsuitable for environmental and cost reasons.
The liquid refrigerant is introduced at ambient temperature and since a pressure of approximately 20 bar is maintained in the cavern 1, the refrigerant does not start to boil.
The fourth process step can now begin (cf Fiy. ld).
At this time, natural gas is withdrawn through ths line 6 from the storage chamber 9 and the refrigerant k in the bottom of the chamber 9 starts to boil so that the temperature in all the refrigerant-flooded parts drops below 0C. Thus, the layer of water above the cavern floor and the corresponding water w present in the bottom part of the storage chamber 9 freeze into ice e.
During this step, care must be taken to ensure that the pressure in the gap 8 is higher than the pressure in the storage chamber 9 so that small quantities of water can enter the same through cracks or minor apertures in the casing 2 and trickle down into the liquid refrigerant k.
In a subsequent fifth process step (cf Fig. le), the storage chamber 9 is flooded with liquid refrigerant k while the natural gas is expelled through the natural gas line 6.
The liquid refrigerant k which is introduced continuously through the line 7 should boil immediately upon exiting into the storage chamber 9 to obviate re-heating anywhere inside the cavern 1. To this end, the line 7 is moved upwards continuously during the flooding so that the outlet of the line 7 is always above the level of the boiling refrigerant k. Some natural gas and refrigerant vapor is evolving during boiling discharge through the natural gas line 6.
The refrigerant level rises continuously until the storage chamber 9 is completely full of boiling refrigerant. In performing this process step, care must be taken to ensure that the boiling point of the refrigerant k near the floor of the storage chamber 9 rises even when the pressure at the surface of the boiling refrigerant remains constant. The reason for ~his is the hydrostatic pressure of the liquid refrigerant near the floor.
Should the boiling point of the lowest layer of refrigerant be abo~e 0C, the ice e does not melt unless the input of heat from the cavern-surrounding rock is so great that the ice e starts to melt.
To reliably preclude any such melting, an advantageous step is to reduce the pressure at the level of the boiling refrigerant while the refrigerant is rising and also not to make cavern depth excessive.
In a sixth process step (cf Fig. lf), the refrigerant k is removed from the cavern storage chamber 9 by being forced out through the line 7, by cold natural gas which is introduced through the line 6 at a temperature, for example, of approximately from -20 to -70C. During this step, the pressure at the level of the refrigerant k remains low, for example, 1 to 2 bar, to ensure that there is always some refrigerant k boiling. Upon the termination of the sixth process step, the cavern i~ ready for storage (cf Fig. lg).
In all the process steps involving the use of compressed gas, the temperature increases associated with the pressure increase are of course compensated for by extra cooling.
The ice layer or jacket in the gap 8 can be formed, for example, not by the procedure hereinbefore described but solely by means of cold gas. In this event, however, very large quantities of gas must be circulated and recooled because of the relatively poor heat transfer between gas and water.
The seventh step, in which natural gas is stored at a pressure of approximately from 60 to 150 bar and a temperature of approximately -70, will be described with reference to Fig. lg. The natural gas for this purpose can come, for example, from a pipeline or from a plant such as has been described in Swiss Patent Application No. 827/88-3.
Cold natural gas is initially introduced, for example, at a pipeline pressure of approximately 60 bar and at a temperature of approximately -70c, through the line 7 which has been moved into the bottom part of the chamber 9. The lines 5, 6 are in the closed state.
In these conditions of pressure and temperature, the methane in the natural gas is hydrated at the "open"
places of the casing 2 - i.e., in the cracks and minor apertures thereof - upon contact with the ice from the gap 8. The methane hydrate thus formed has elastic properties and seals the previously open places in a gas-tight manner with respect to the exterior. In the conditions of pressure and temperature specified, the ice in the gap 8 is brittle. A required elasticity can be imparted to the ice layer by an addition of additives, such as methanol, to the water introduced during the second step of the process.
After this step has finished, the natural gas to be stored is compressed to the required storage pressure of, for example, 150 bar and introduced through the line 7 into the chamber 9. Stored natural gas is removed from the cavern 1 on demand by way of the line 6.
If no natural gas is required to be removed from the filled chamber 9 for consumption for a prolonged time, it is advisable to circulate a small quantity of cold natural gas through the storage chamber 9. To this end, cold natural gas is taken from an above-ground refrigerating plant (not hown) and introduced through the line 7 into the storage chamber 9 and a corresponding quantity of natural gas which has warmed up in the chamber 9 is removed through the line 6 and recirculated into the above-ground refrigeration plant.
This step ensures that the cavern always remains cold and that there is no risk of ice melting anywhere in the "ice jacket" surrounding the storage chamber 9.
Referring to Fig. 2, wherein like reference characters indicate like parts as above, a second casing 10 is disposed in the first casing 2 and at a distance 200~000 of a few centimeters therefrom, the ends of the second casing 10 being set back from the ends of the first casing 2'. The material and production parameters for the second casing 10 are as for the first casing 2'.
Also, as compared with the embodiment of Fig. 1, two extra lines 11, 12 extend through the plug 4' and terminate in the gap 13 between the two casings 2', 10.
The two lines 11, 12 are fixed in the plug 4' and are effective to fill the gap 13 with gas or liquid and to lo remove gas from the gap 13.
The embodiment shown in Fig. 2 has advantages over the embodiment shown in Fig. 1 mainly residing in that substantially less refrigerant is required to seal off the cavern storage chamber from the surrounding rock in gas-tight manner.
A description will be given hereinafter with reference to Figs. 2a to 2g of the various process steps for producing a gas-tight lining of the cavern relative to the surrounding rock.
In the first process step (cf Fig. 2a), the cavern 1' is vented as in the first embodiment (cf Fig. la) and for this purpose filled with water. All four lines 5', 6', 11 and 12 are used for removal of air from the cavern.
In the second process step (cf Fig.2b), natural gas is injected through the lines 6', 11 and through the gap 13 between the casings 2', 10 so that no water can enter the gap 13. During this step, the lines 5', 12 are in the closed state and the line 7' is used to remove the water.
In the third process step (cf Fig. 2c), liquid refrigerant k is introduced into the gap 13 through the line 11 until the water level above the cavern floor and the bottom part of the casing 10 are completely flood~d with refrigerant.
During this step, natural gas escapes from the gap 13 through the line 12 and from the storage chamber 9' through the line 6'. The removal of gas through the lines 12, 6' is so controlled that a lower pressure is maintained in the gap 13 so that the level of liquid refrigerant therein is higher than in the storage chamber 9'.
In the fourth process step (cf ig. 2d), the lines 5', 7', 11 are in the closed state and the lines 6', 12 are open in order to reduce the pressure in the cavern 1' so that the refrigerant k boils.
The pressure between the gap 13 and the storage chamber 9' is controlled so that the level of liquid refrigerant k in the qap 13 is always above the level of liquid refrigerant k in the storage chamber 9' although the bottom part of the casing 10 always remains flooded by refrigerant k.
Due to the refrigerant boiling, the water near the floor of the cavern 1' and the water in the bottom part of the outer gap 8' freeze to form a layer of ice e.
In the fifth process step (c Fig. 2e), the liquid refrigerant k is injected at room temperature through the line 12 into the bottom part of the gap 13. Coolant vapor and gas escape through the line 11 from the gap 13 and through the line 6' from the storage chamber 9'.
The quantities of gas and vapor removed are so controlled that the level of liquid in the storage chamber 9' is lower than in the gap 13, the level of boiling refrigerant k continung to rise in the gap 13 until the same is completely filled with boiling refrigerant. When this has occurred, the water _ in the outer gap 8' has completely frozen into ice e.
At the end of this process step, the pressure at the top surface of the refrigerant in the gap 13 is greater than in the inner chamber 9' so that the refrigerant in the gap 13, but not the refrigerant in the chamber 9', boils.
A comparison between Figs. 2e and le will show the saving of refrigerant obtained when a double casing 2', 10 is used as compared with when a single casing 2 is used, for when a single casing is used the cavern interior 9 must be completely filled with refrigerant.
In the sixth process step (c Fig. 2f), cold natural gas at a temperature of approximately -70C is introduced via the line 11 into the gap 13. The liquid refrigerant is thus displaced from the gap 13 and storage chamber 9' through the moving line 7' which is lowered and used to extract the refrigerant at the bottom of the cavern 1'.
A description will be given with reference to Fig.
2g of the storage of cold natural gas in the cavern in a seventh process step. The same is very similar to the corresponding step of the first embodiment.
First, cold compressed natural gas is forced into the storage chamber 9' through the line 12 and through the gap 13. As in the first embodiment,methane hydrate is formed by contact with the ice in the gap 8' in the cracks and minor apertures produced in the production of the casing 2' or in the preparation of the seal and seals off the open places in gas-tight manner from the surrounding rock.
As in the case of Fig. lg, it is advantageous to have a corresponding circulation of a small quantity of Z5 natural gas, which is recooled in an above-ground refrigeration plant, during periodsof nil gas consumption. This obviates the melting of ice between the casing 2' and the surrounding rock due to heating thereby.
As a comparison between Figs. lg and 2g will also show, cooling can be more effective in a double-casing cavern than in a single-casing cavern since the gas speeds on the surface of the casing 2'are higher and so heat transfer is better.
Finally, the storage facility has a number of special advantages.
For example, the casings can be produced very cheaply since they do not have to be completely gas-tight. Even should cracking occur in the casing or in the ice layer as a result of pressure variations when the store is full, for example, because of earth movements or because of heat expansion of the ice layer, for example, due to heating by the surrounding rock, because of the low storage temperature and high storage pressure, methane hydrate immediately reforms in the "open places", and so gas sealing tightness relative to the surrounding rock is ensured during storage.
Conveniently, to take up expansion of the water during icing-up in the gap, the casing is made of a substance having elastic properties or else the casing is given adequate elasticity by an appropriate shaping of the wall. For example, the wall can be prepared from corrugated panels.
So that large quantities of natural gas can be stored, it is a logical step for a storage plant to be embodied by a number of caverns constructed in the above manner.
The invention thus provides anunderground storage facility for storing gaseous natural gas in an economic manner.
Further, the invention provides a storage facility for natural gas wherein the natural gas can be stored at relatively low temperature and elevated pressures.

Claims

1. In combination, an underground cavern having a floor and a wall enclosing a sealed chamber;
at least one casing disposed in said chamber in spaced relation to said wall to define a gap therebetween and in spaced relation to said floor;
a plurality of lines extending into said cavern, at least one of said lines extending through said casing; and a elastic layer of ice and methane hydrate in said gap to form a gas-tight seal relative to said cavern wall.

2. The combination as set forth in claim 1 which further comprises a second casing disposed within and in spaced relation to said one casing, said casing having lower ends disposed above corresponding lower ends of said one casing, and at least one of said lines extending through said second casing,

3. The combination as set forth in claim 1 which further comprises at least one plug for closing an opening into said cavern and having said lines extending therethrough.

4. The combination as set forth in claim 1 wherein said casing is made of plastic.

5. The combination as set forth in claim 1 wherein said casing is made of metal.

6. The combination as set forth in claim 1 wherein said casing is spaced from said cavern wall at a distance of from 2 to 10 centimeters.

7. The combination as set forth in claim 1 wherein said casing is spaced from said cavern floor at a distance of 10 centimeters.

8. A process of fabricating an underground storage facility for natural gas comprising the steps of positioning a first casing within an underground cavern in spaced relation to a wall of the cavern to define a first gap therebetween and in spaced relation to a floor of the cavern;
flooding the interior of the casing with water while venting air from within the casing and cavern;
thereafter passing compressed natural gas into the casing to force water from within the casing into said gap while maintaining a layer of water on the cavern floor, and thereafter passing a refrigerant into said casing to freeze the water in said gap and in said layer on said floor.

9. A process as set forth in claim 8 wherein the refrigerant is cold natural gas.

10. A process as set forth in claim 8 which further comprises the steps of delivering natural gas within the casing while displacing the refrigerant therefrom.

11. A process as set forth in claim 10 wherein the natural gas is delivered at a temperature of about -70°C.

12. A process as set froth in claim 10 wherein the natural gas is stored at a pressure of from 60 to 160 bar and a temperature of about -70°C.

13. A process as set forth in claim 8 wherein the refrigerant is an evaporating refrigerant

14. A process as set forth in claim 13 wherein the refrigerant is a hydrocarbon.

15. A process as set forth in claim 14 wherein the refrigerant is one of propane, ethyl and ethylene.

16. A process as set forth in claim 8 which further comprises the steps of positioning a second casing within and spaced from the first casing to define a second gap therebetween, and wherein the refrigerant is passed into the second gap from within the second casing to fill the second gap to freeze the water in the first gap and said layer on said cavern floor.

17. A process as set forth in claim 8 which further comprises the step of adding an additive to the water to increase the elastic properties of the ice at the storage pressure and storage temperature.

18. A process as set forth in claim 17 wherein said additive is methanol.

19. A process of storing natural gas comprising the steps of positioning a first casing within an underground cavern in spaced relation to a wall of the cavern to define a first gap therebetween and in spaced relation to a floor of the cavern;
forming a layer of ice between the casing and the cavern wall and along the floor of the cavern; and thereafter pumping natural gas into the casing for storage therein.

20. A process as set forth in claim 19 wherein ice from said layer coming into contact with methane of the natural gas at open places in the casing forms methane hydrate at said open places to form a gas-tight seal thereat.

21. A process as set forth in claim 19 which further comprises the steps of removing a small quantity of natural gas from the casing, cooling the removed gas and re-circulating the cooled gas into the casing.