EP0377405B1 - Artificial subterranean cavern for the storage of natural gas in the gaseous state at an elevated pressure and a low temperature, and method for its production - Google Patents

Description

The invention relates to an artificial, underground cavern for storing gaseous natural gas according to the preamble of claim 1 and a method for its production.

It is known to store natural gas as an energy source in times when there is little or no consumption. If you save the e.g. Natural gas generated in a constant amount throughout the year can be covered with the help of the stored gas during the period of increased demand.

It is known to store natural gas under high pressure and low temperature in natural caverns. As is known from the specialist literature, these natural caverns consist, for example, of salt caverns, exhausted gas fields, so-called aquifer caverns or porous rocks at greater depths in the order of several hundred or even several thousand meters.

In the event that such natural caverns are not available for geographical reasons, it is necessary to build artificial storage facilities.

It is known to store liquefied natural gas at atmospheric pressure in above-ground isolation tanks ("peakshaving"). Both the liquefaction of the natural gas and the tanks are extremely expensive and therefore not of great economic interest.

The invention is based on the formation of an artificial, underground cavern for the storage of natural gas under high pressure, in the order of 100 to 200 bar at a low temperature in the order of approximately -50 to -70 ° C, which gas-tight against surrounding rock and which can be created in an economical manner.

This object is achieved with the features specified in the characterizing part of claim 1 and with the aid of a method according to the measures specified in the characterizing part of claim 3.

An advantageous further development of a cavern according to the invention is described in claim 2, while claims 4 to 12 contain advantageous embodiments for the manufacturing process.

The invention is described below with reference to exemplary embodiments shown in the drawing.

Fig. 1 shows a schematic representation of an embodiment of an underground, horizontally arranged cavern, while the

1a to 1f show the various stages in the production of the gas-tight cover layer of the cavern, including the cavern in the state of natural gas storage.

FIG. 2 shows a schematic representation of an embodiment of a subterranean, horizontally arranged cavern modified compared to FIG. 1, while the

2a to 2g show the various stages in the production of the gas-tight cover layer of the cavern, including the cavern in the state of natural gas storage.

The cavern 1 shown in Fig. 1 is tunnel-shaped and, for example, at a depth of 150 to 200 m in the surrounding rock, e.g. Granite blasted out. Although this horizontal arrangement of the cavern is particularly advantageous, the invention also encompasses e.g. vertically formed caverns.

In general, it should be noted that the cavern is not arranged at a depth that ensures that the storage pressure withstands the hydrostatic head of the water contained in the surrounding rock.

In the cavern 1, a cladding 2 is introduced at a short distance from the cavern wall of, for example, approximately 2 to 10 cm, and ends at a short distance from the cavern base area of, for example, 10 centimeters.

This formwork 2 can be made of steel, aluminum or plastic, for example. Its wall thickness is determined in accordance with the hydrostatic water pressure in method step 2 described later (see FIG. 1b). The formwork itself, which in most cases is composed of several pieces, in particular is welded together, cannot be produced for the prints mentioned.

It is a serious advantage of the invention that this formwork does not have to be gas-tight, but instead can have cracks and smaller holes, for example, which, as described below, are closed with the measures according to the invention, so that when natural gas is stored, the required gas tightness is secured against the surrounding rock.

The cavern 1 is closed off from the outside atmosphere by means of a pin 4 inserted into a bore 3, which e.g. can consist of metal or possibly also of plastic. Depending on the design of the cavern, in principle several pins can also be arranged.

In the present embodiment, the pin 4 is penetrated by three lines 5, 6 and 7.

The line 5, which is firmly fixed in the journal, ends in the gap space 8 formed by the cavern 1 and casing 2. It serves to supply water or to vent the gap space 8.

The line 6, which is also fixed in the pin 4, penetrates the casing 2 and ends in the head part of the storage space 9. It serves to lead gas into the cavern 1 or out of it.

The line 7 is movable, ie slidably mounted in the pin 4 and penetrates the formwork 2. This line serves either water or gas or remove liquid refrigerant from cavern 1 or introduce water or liquid refrigerant into it.

The individual process steps are explained below with the aid of FIGS. 1a to 1f, with the aid of which a gas-tight lining of the cavern against the surrounding rock is achieved.

To vent the cavern, the line 7 is pushed into the lower region of the storage space 9 in a first process step (see FIG. 1a) and water is introduced through it until the storage space 9 and the surrounding gap space 8 are completely filled with water.

The air escapes through lines 5 and 6. In this process step, the pressure in the cavern corresponds to the sum of the atmospheric pressure and the hydrostatic height of the water in the cavern. In the present exemplary embodiment, the temperature in the cavern space is approximately 4 ° C.

In the second process step (see FIG. 1b), water is pressed out of cavern 1 with the aid of natural gas, the cavern being under a pressure of approximately 20 bar. The natural gas is introduced through line 6 and extracts water from the cavern through line 7, the end of which is in its lowest position. Line 5 is closed, so that the gap space 8 remains filled with water. During this process step, the interior of the cavern is at approximately ambient temperature. It depends on the temperature of the natural gas injected. At the end of this step, the Formwork 2 withstand the hydrostatic pressure of the water accumulated in the gap space 8.

In the third method step (cf. FIG. 1c), the line 7 is moved upwards so far that its end lies above the water level of the water w located above the base area of the cavern, but still occupies a position in the lower region of the storage space 9.

Now a predetermined amount of liquid refrigerant k, e.g. Propane, introduced through line 7 into the storage space 9, such that the deepest area of the storage space 9 above the water level is filled with the liquid refrigerant k. Other hydrocarbons such as e.g. Ethane or ethylene, while refrigerants such as Ammonia, freons and the like are not suitable for environmental or cost reasons.

The liquid refrigerant is introduced at ambient temperature, and since a pressure of approximately 20 bar is maintained in the cavern, the refrigerant will not start to boil.

The fourth method step can now begin (see FIG. 1d).

Natural gas is drawn off from the storage space 9 through line 6, and the refrigerant k in the lower storage space 9 begins to boil, as a result of which the temperature in all parts flooded by the refrigerant drops below 0 ° C., so that the water level is above the base area of the Cavern 1 and the corresponding water w located in the lower part of the gap space 9 freezes to ice e.

During this process step, care must be taken to ensure that the pressure in the gap space 8 is higher than that in the storage space 9, so that small amounts of water can penetrate into the storage space 9 through cracks or small openings present in the casing 2 and trickle down into the liquid refrigerant k .

In the subsequent fifth method step (see FIG. 1e), the storage space 9 is flooded with liquid refrigerant k.

The liquid refrigerant k, which is continuously introduced through line 7, should boil immediately when it exits into the storage space 9, in order to avoid reheating at any point within the cavern 1.

In order to make this possible, the line 7 is continuously shifted upwards during the flooding in such a way that the outlet of the line 7 is always above the level of the boiling refrigerant k. Refrigerant vapor and some natural gas generated during the boiling process flow out through line 6.

The level of the refrigerant rises continuously until the storage space 9 is completely filled with boiling refrigerant. When carrying out this method step, it must be taken into account that the boiling temperature of the refrigerant k rises in the bottom space of the storage space 9, even if the pressure on the surface of the boiling refrigerant is kept constant. The reason for this there is the level of the hydrostatic pressure of the liquid refrigerant in the floor space.

If the boiling point of the deepest refrigerant layer is above 0 ° C, the ice will still not melt unless the heat from the rock surrounding the cavern is so great that the ice begins to melt.

In order to avoid such a melting process with certainty, an advantageous measure consists in reducing the pressure at the level of the boiling refrigerant while the refrigerant is rising and also not dimensioning the cavern depth too great.

In the sixth process step (cf. FIG. 1f), the refrigerant k is removed from the storage space 9 of the cavern 1, namely by the fact that the refrigerant is pressed through line 7 with cold natural gas, for which purpose the cold natural gas of, for example, approx. -20 to - 70 ° C is introduced through line 6. During this process the pressure at the refrigerant level k is kept low, e.g. to 1 to 2 bar in order to ensure that some refrigerant k constantly boils.

It should also be mentioned that, of course, in all process steps in which compressed gas is used, the temperature increases associated with the pressure increase are compensated for by additional cooling.

It should be noted that the formation of the ice jacket in the gap space 8 also, for example, instead of the procedure described above can only be done with cold gas. In this case, however, very large amounts of gas have to be circulated and recooled because of the relatively poor heat transfer between gas and water.

In a seventh process step, the process of storing natural gas at a pressure of approximately 60 to 150 bar and a temperature of approximately -70 ° C is explained (see FIG. 1g).

This natural gas can originate, for example, from a pipeline or from a plant as described in Swiss Patent Application No. 827 / 88-3.

After the sixth process step has been completed, the cavern is ready for storage (cf. FIG. 1g).

Cold natural gas is initially introduced through the line 7 pushed into the lower part of the storage space 9, for example with pipeline pressure at approximately 60 bar and approximately -70 ° C. Lines 5 and 6 are closed.

At these pressure and temperature conditions, the methane in the natural gas is hydrated at the "open" locations of the formwork 2, ie in its cracks and small openings upon contact with the ice from the gap space 8. This methane hydrate thus formed has elastic properties and seals the previously open positions gas-tight to the outside. The ice in the gap space 8 is brittle at the pressure and temperature conditions mentioned. By adding additives, such as methanol, to the water filled in during the second process step, the ice layer can be given a desired elasticity.

After this process is completed, the natural gas to be stored is compressed to the desired storage pressure of, for example, 150 bar and introduced into the storage space 9 through line 7. If necessary, stored natural gas is withdrawn from cavern 1 through line 6.

If no natural gas for consumption is to be removed from the filled storage space 9 for a long time, it is advisable to circulate a small amount of cold natural gas through the storage space. For this purpose, cold natural gas is taken from a refrigeration system, not shown, installed above ground and introduced through line 7 into the storage space 9, and a corresponding amount of natural gas heated therein is taken through line 6 and recirculated into the refrigeration system.

This measure is advantageous because it ensures that the cavern always stays cold and prevents ice from melting at any point in the "ice jacket" surrounding the storage space 9.

FIG. 2 shows a variant embodiment of a cavern designed according to the invention compared to the exemplary embodiment described above (see FIG. 1).

This embodiment offers certain advantages over the first embodiment, which mainly consist in the fact that considerably less refrigerant is required to produce a gas-tight cover for the storage space of the cavern with respect to the surrounding rock.

In the embodiment shown in FIG. 2, all elements that correspond to FIG. 1 are identified by the same reference numerals, which are provided with an apostrophe.

In addition to the first formwork 2 ', a second formwork 10 is arranged within this at a distance of a few centimeters, the ends of which are set back by a distance from the first formwork 2'. This formwork corresponds in its training in terms of material and method of manufacture with the formwork 2 '.

In addition, two lines 11 and 12 are passed through the pin 4 ', which open into the gap space 13 formed by the two casings 2' and 10. These two lines, which are fixed in the pin 4 ', serve to fill the gap 13 with gas or liquid or to remove gas from it.

The individual process steps are explained below with the aid of FIGS. 2a to 2g, with the aid of which a gas-tight lining of the cavern against the surrounding rock is achieved.

During the first process step (see FIG. 2a), the cavern 1 'is vented analogously to the first exemplary embodiment (see FIG. 1a) and filled with water for this purpose. All four lines 5 ', 6', 11 and 12 are used to displace the cavern air.

In the second process step (see FIG. 2b) natural gas is pressed through the lines 6 'and 11 and through the gap 13 between the casings 2' and 10, so that no water can penetrate into the gap 13. During this process step, the lines 5 'and 12 are closed and line 7' is used to drain the water.

In the third process step (see FIG. 2c), liquid refrigerant k is introduced into the gap space 13, namely until the water level above the base of the cavern 1 'and the lowermost part of the casing 10 are completely flooded with refrigerant.

During this process step, natural gas escapes from the gap space 13 through line 12 and from the storage space 9 'through line 6'. The gas discharge through the lines 12 and 6 'is controlled such that a slightly lower pressure is maintained in the gap space 13, so that the level of the liquid refrigerant in this space is higher than in the storage space 9'.

During the fourth process step (cf. Fig. 2d), the lines 5 ', 7' and 11 are closed and the lines 6 'and 12 are open to lower the pressure in the cavern 1', so that the refrigerant k boils.

The pressure between the gap space 13 and the storage space 9 'is controlled in such a way that a higher level of the liquid refrigerant k in the gap space 13 than in the storage space 9' is maintained. In this case, however, the lower part of the casing 10 remains flooded with refrigerant k.

Due to the boiling process of the refrigerant, the water located in the bottom of the cavern 1 'freezes the water in the lower part of the outer gap space 8 'to ice e.

In the fifth process step (cf. FIG. 2e), liquid refrigerant k is introduced into the lower gap space 13 through line 12 at room temperature. Gas and refrigerant vapor escape through line 12 from the gap space 13 and through line 6 'from the storage space 9'.

The amounts of gas or vapor discharged are controlled such that the liquid level in the storage space 9 'is lower than in the gap space 13, the level of the boiling refrigerant k in the gap space 13 constantly increasing until this gap space is completely filled with boiling refrigerant. When this is reached, the water w in the outer gap space 8 'is completely frozen to ice e.

At the end of this process step, the pressure on the uppermost surface of the refrigerant in the gap 13 is greater than in the interior 9 ', so that the refrigerant boils in the gap 13, but not in the interior 9'.

A comparison between FIGS. 2e and 1e shows the savings in refrigerant when using a double casing 2 ', 10 compared to a single casing 2, since with simple casing the interior 9 of the cavern 1 must be completely filled with refrigerant.

In the sixth process step (see. Fig. 2f), the liquid refrigerant from the gap space 13 and from the storage space 9 'with the help of cold natural gas at about -70 ° C through the lines 12 and 6' is pressed. The Movable line 7 'is lowered and used to suck off the refrigerant located in the bottom of the cavern 1'.

The storage of the cavern with cold natural gas is explained in a seventh method step (see FIG. 2g). This method step essentially corresponds to the corresponding one of the first exemplary embodiment.

First, cold, compressed natural gas is pressed through the line 12 and through the gap space 13 into the storage space 9 '. Analogous to the first embodiment, in the cracks and small openings formed during the manufacture of the formwork 2 'or during the manufacture of the seal through contact with the ice in the gap space 8', methane hydrate, which closes the open areas in a gas-tight manner with respect to the surrounding rock.

Analogously to FIG. 1g, it is advantageous to carry out a corresponding circulation of a small amount of natural gas, which is recooled in an above-ground refrigeration system, during times when there is no consumption of natural gas. In this way it is ensured that melting of ice by the incidence of heat from the surrounding rock is avoided between the casing 2 'and the rock.

A comparison between FIGS. 1g and 2g further shows that more effective cooling can be achieved in a cavern with a double formwork than in a cavern with a simple formwork, since the gas velocities on the surface of the formwork 1 'are higher and therefore a better one Heat transfer is achieved.

Finally, some special advantages of the invention are pointed out.

As already mentioned, the formwork can be produced in an extremely cost-effective manner, since it does not have to be made absolutely gas-tight. Even if during the time in which the reservoir is filled, due to pressure fluctuations, e.g. may be caused by earth movements or due to thermal expansion of the ice layer, e.g. As a result of heat from the surrounding rock, cracks in the casing or in the ice layer should develop, methane hydrate immediately forms again in the "open areas" due to the low storage temperature and the high storage pressure, so that gas tightness to the surrounding area also occurs during storage Rock is guaranteed.

In order to absorb the expansion of the water during ice formation in the gap space, it is advisable to produce the formwork from a material with elastic properties, or to bring about a sufficient elasticity of the formwork by appropriate shaping of the wall. For example, the wall can be made from corrugated sheets.

In order to be able to store large amounts of natural gas, it makes sense to create a storage system from several caverns designed according to the invention.

Claims (12)

1. An artificial subterranean cavern for the storage of natural gas, the main component of which consists of methane, at elevated pressure and low temperature, characterised in that a lining is provided inside the cavern wall at a slight distance therefrom and terminates a slight distance above the base of the cavern and in that a gas-tight seal from the cavern wall in an elastic layer consists of ice and methane hydrate, the unsealed zones of the lining being sealed so as to be gas-tight by methane hydrate formed during the storage, and in that the cavern is also sealed off from the atmosphere by means of a plug through which supply and discharge lines are passed.

2. A cavern according to claim 1, characterised in that another lining is provided inside the lining at a slight distance therefrom and terminates a slight distance above the end of the first lining, two lines which pass through the plug leading into the gap formed by the two linings.

3. A method of making an artificial subterranean cavern according to claim 1, characterised in that the air is first removed from the cavern by means of water, then water is forced, by means of compressed natural gas, into the gap formed between the lining and the cavern wall, and then this water and a layer of water still present above the base of the cavern is frozen by means of a coolant circulating through the interior of the cavern.

4. A method according to claim 3, characterised in that the coolant consists of cold natural gas.

5. A method according to claim 3, characterised in that the coolant used is a vaporising refrigerant.

6. A method according to claim 5, characterised in that the refrigerant consists of a hydrocarbon.

7. A method according to claim 6, characterised in that the hydrocarbon is propane, ethyl or ethylene.

8. A method according to claim 3, characterised in that there is added to the water an additive which at the storage pressure and storage temperature increases the elastic properties of the ice produced.

9. A method according to claim 8, characterised in that the additive used is methanol.

10. A method according to claim 3, characterised in that for the storage of natural gas in open zones of the lining the ice forming between it and the surrounding rock is brought into contact with the methane of the natural gas, methane hydrate forming and sealing the open zones of the lining so that they are gas-tight.

11. A method according to claim 10, characterised in that during the periods when there is no use of stored natural gas a small quantity of natural gas is removed from the storage space, re-cooled outside the cavern, and recycled to the storage space.

12. A method according to claim 5 for the production of a cavern according to claim 2, characterised in that the pressures in the storage space and in the gap formed by the two linings are so adjusted that the gap is completely flooded with refrigerant, and in that only the bottom part of the storage space is filled with refrigerant, the bottom end of the inner lining being flooded.

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