WO1998039555A1

WO1998039555A1 - A method for providing gas-sealing around a rock chamber or rock storage chamber

Info

Publication number: WO1998039555A1
Application number: PCT/NO1998/000068
Authority: WO
Inventors: Ole Bernt Lile
Original assignee: Nyfotek As
Priority date: 1997-03-04
Filing date: 1998-03-03
Publication date: 1998-09-11
Also published as: AU6231998A; NO970985D0

Abstract

A method for obtaining gas-sealing around rock chambers or rock storage chambers. Hydrates of petroleum gas are made in pores and cracks in the rock (11) around the rock chamber (1) with the aid of cooled water and a suitable gas mixture of petroleum gases which are injected at suitable pressure into respective boreholes (3, 4, 5, 6, 7) in the rock around the rock chamber (1).

Description

A METHOD FOR PROVIDING GAS-SEALING AROUND A ROCK CHAMBER OR ROCK STORAGE CHAMBER

The present invention relates to a method for providing gas-sealing around a rock chamber or rock storage chamber, as disclosed in the preamble of the patent claims below.

The development of blasting techniques and methods for the control of rock pressure has made it possible to make large chambers in the rock which do not collapse and which therefore can be used for many different purposes. As examples, mention may be made of cold storage chambers, sports halls, shelters and store rooms of various kinds. Usually, the rock formations are permeated by cracks of varying sizes and pore space. Initially, no rock formation is completely dense.

Usually, there is an interest in sealing such rock chambers against water from outside leaking into the rock chamber. The vast majority of rock chambers will be below ground water level and therefore larger cracks especially will function as headers and leaders of the ground water into the rock chamber. If the volumes of water are problematic, it is usual to seal cracks of this kind by injecting different kinds of sealants into the rock formations around the rock chamber. Such injection takes place at high pressure. The most common sealing material is cement mixtures of various kinds which fill cavities and cracks and which after a short time harden and become a part of the rock formation. Such measures will usually be sufficient to prevent the water from entering the rock chamber.

Rock chambers are also used to store substances that must not leak out of the chamber. Here a distinction can be made between two forms: liquid and gas. It is common to use rock chambers for the storage of oil. Such chambers are built below the water table. When such chambers are filled with oil, the oil will float on top of a layer of water in the bottom of the chamber. Above the oil, the chamber will be filled with oil vapour and air, or possibly a gas which together with the oil vapour is not explosive. In such cases, the transport direction of ground water in the rock formations around the chamber will be towards the rock chamber, thus preventing leakage of oil from the rock chamber. Other types of liquid that are stored in rock chambers are cooled and liquefied gases, such as, e.g., liquid propane and butane. Liquid gas, whose temperature is below the freezing point of water, is favourable for storage when sealing of the rock chamber is involved. When the ground water freezes around the rock chamber, the ice forms a shield which seals against leakages both from inside and from outside. In cases where there is not sufficient water present, the leakages from such facilities will be so great that it is not possible to store liquid gases there. The reason for this is that new cracks will be formed in the rock formations around the storage chamber as these are cooled and contract. Such cracks are called thermal contraction cracks. In some liquid gas storage facilities, water injection is therefore used for sealing as described in the following paragraphs.

Rock chambers are also used for the storage of gas under pressure. In mining, rock chambers are used for storing compressed air for operating rock drilling and loading machines. The pressure in such stores is in the range of 0.7-1.2 MPa. Similar compressed air reservoirs are used in hydroelectric power stations as "buffers" or pressure compensation for pressure fluctuations in the water column which operates the turbines. In air chambers of this kind the pressure may rise to 8 MPa.

Generally, a rock chamber that is filled with gas will not be fully gas-proof in its natural state. All rocks contain cracks and pore space through which gas may escape. As pressure rises, gas leakage will increase.

It is essential to master the sealing of rock chambers which contain gas under pressure. In addition to the injection of cement mortar and possibly other chemical sealants, sealing with the aid of a so-called "water curtain" is one of the principal methods. In this case the method involves boring, above and around the rock chamber, fans having holes which are used for the continuous injection of water under pressure. The layer of water-saturated rock formation almost forms a sort of shield around the rock chamber, and in so doing prevents the gas from passing easily through this so-called "water curtain". In simple terms, the water curtain will function so that the water flow rate in towards the rock chamber is greater than the flow rate of the gas which leaks out of the rock chamber. Thus, any gas that might be on its way out will be swept along with the water and carried back to the rock chamber. In this way, there will be no net leakage of gas from the rock chamber. However, there will be a little gas in the water that has to be pumped out of the rock chamber continuously.

A more detailed description of the use of a water curtain can be found in the following references: a) Janelid, I (1972): US Patent No. 3670503 b) Kjδrholt H (1991): Dr. ing. Thesis, NTH 1991:34, NTH, University of Trondheim

Rock chambers can also be used for storing natural gas under pressure. The chief constituent of natural gas is methane. The rock chambers that have been used to date for storing natural gas are salt caverns which have been especially washed out for this purpose and closed-down salt and coal mines. When storing natural gas in chambers blasted out in other rock formations, such as granites, gneiss or shale formations, the use of a water curtain is considered for sealing the rock formations and maintaining the gas under pressure in place in the storage chamber.

US Patent 2,991,624 relates to the sealing of rock chambers in connection with the storage of hydrocarbons in the C2-C4 range. The patent discloses that the hydrocarbon is admixed with water in a sufficient amount to ensure substantial saturation of the hydrocarbon before it is stored. From this, it is evident that the hydrocarbons must be in liquid form and that water must be added until saturation is reached. It is generally known that hydrocarbons take up very little water in solution. The patent also discloses that when the hydrocarbon is diffused, the rock formation will be cooled so much that a hydrate is formed and an automatic sealing of the rock formation is obtained. The cooling will take place in that the gas, after it has vaporised from liquid form, expands in lower pressure regimes. Thus, there is no active cooling of the rock formations. However, it is unlikely that the water as liquid will accompany the leakages of hydrocarbons in gas form. Therefore, there will probably not be a sufficient amount of water present in the rock formations to allow the formation of a hydrate. In addition to this, it should be mentioned that the formation of hydrates is an exothermic process wherein heat is given off. If further hydrate formation is to take place, the emitted heat must be removed to enable the temperature to remain sufficiently low.

Today much is known about how hydrates are formed. The gas must be mixed with flowing water at the site where the hydrate is to be formed. Pre-mixing with water merely means that water vapour together with gas will leak out of the storage chamber above the water table. Water vapour and gas will not form hydrates in pores and cracks in the surrounding rock. Below the water table, water will leak out and there will probably be insufficient gas for the formation of a hydrate.

Thus, US Patent 2,991,624 does not describe an active maintenance of a hydrate layer around the rock chamber by means of continuous cooling by water injection, optionally water injection together with gas. However, it is believed that as soon as the gas begins to leak, the temperature will fall, that there will be sufficient water present, and that a hydrate will be formed which then seals the cracks (channels) in the rock around the store. It is unlikely that this will happen. Most of what vaporises of a hydrocarbon liquid with some water will be hydrocarbon gas. There will therefore not be sufficient water in the rock pores for the formation of hydrates in amounts sufficient to allow sealing to take place. The small amount of water vapour which will be present will be far from sufficient for the formation of hydrates. It is known that hydrocarbon gas and water (in liquid form) must be mixed well and for a long time if hydrates are to be formed.

Moreover, the presupposition in US Patent 2,991,624 is that as the gas leaks to progressively lower pressure ranges, it will expand and be cooled. Thus, it is presumed that the temperature will be sufficiently low to allow the formation of hydrates. It is well known that on the formation of hydrates, heat is emitted just as melting heat is given off when water freezes. This heat must be conducted away through the rock and this takes time. It is therefore probable that the melting heat emitted will prevent a further hydrate formation and that it will be very difficult to form hydrates under such conditions, both owing to insufficient water and owing to insufficient cooling. In the patent it is disclosed, moreover, that the distance from the storage chamber at which sealing will occur, will vary depending upon the particular hydrocarbon which is stored and the rate of pressure drop along the leakage channels of the hydrocarbon gas which leaks out. This means that even if hydrates were to be formed automatically as presumed, there is no control of where these hydrates are formed and consequently nor of where the sealing of the leakages takes place. US Patent 3,670,503 describes well-known use of a water curtain under pressure in order to prevent leakage around a rock gas chamber. This patent does not require the formation of hydrates to seal the rock formations.

GB 2 013 860 relates to the storage of liquids in rock chambers, in particular liquids at cryogenic temperatures, to be more precise, natural petroleum gases which have been liquefied by cooling (LNG = Liquefied Natural Gas). Methane, CH₄, which is the hydrocarbon gas requiring the lowest temperature to be liquefied, has its boiling point at -163° C at a pressure of 1 atmosphere. (The temperature given in the drawings in the British patent is -160°C.) The patent describes two main problems which arise when storing liquids having such low temperatures in rock chambers. One of the problems is cracking of the rock around the rock cavern because the rock formations contract on cooling. Such cracks are called contraction cracks or thermocracks. Gradually, as the rock around the chamber cools at ever greater distances, these contraction cracks propagate outwards. In the patent it is presumed that contraction cracks do not constitute a problem at temperatures above -50°C.

The second problem concerns the diffusion of water vapour (NB: water vapour) towards the storage chamber. This diffusion takes place from areas of high vapour pressure (high temperature, comparatively) towards areas of low vapour pressure (low temperature, comparatively). Since the rock chamber containing LNG has the lowest temperature, water vapour will diffuse through the rock towards the storage chamber where it will condense to ice. A diffusion of water vapour will also take place from areas where the water in the rock is frozen. The water will sublime, i.e., pass from ice to water vapour directly, and diffuse towards colder areas, i.e., towards the storage chamber.

The patent describes a system having an inner and an outer zone around the storage chamber where both temperature and pressure (pore pressure) in the rock are controlled by means of circulating gas (nitrogen, helium or petroleum gas). Innermost, closest to the storage chamber, the temperature is maintained at above -50°C in order to prevent the formation of contraction cracks. Outermost, the temperature is maintained at below 0°C to keep the water in the rock frozen so that ice in pores and cracks creates an ice sheath around both the storage chamber and the inner zone. Normally, water vapour will diffuse towards the inner zone and on towards the storage chamber. This is prevented by maintaining the pressure of the circulating gas in the outer zone so low that the diffusion of water vapour inwards is stopped. Moreover, the gas in the inner zone is used to remove water vapour continuously so that ice is not deposited around the storage chamber and in the inner zone. The water, i.e., the water vapour, must also be removed during this drying process.

The patent does not relate to the active use of hydrates for sealing, as it is improbable that hydrates will be formed because there is not sufficient free water present in the pores of the rock formation. All water will freeze, this is a prerequisite for the solution according to the patent. Then it will sublime and gradually be drawn out as water vapour through the drying process which is described.

Norwegian Published Patent Application 144 396 relates to the storage of liquids and gases at very low temperatures (as low as about -260°C). It is proposed to inject a sealant (e.g., organic liquids) which solidifies inside the rock formation. For instance, it is disclosed that the storage of liquefied natural gas can be carried out at -120°C. By injecting a liquid having a melting point in the range of -60 to -100°C, it is possible to ensure that this liquid freezes to a solid substance in cracks and pores in the rock formations around the storage chamber. The rock must be cooled to a very low temperature before the storage chamber is blasted out. The presence of water in rock cracks is mentioned and it is said that this is undesirable because the water will freeze into ice and thus prevent injection of the organic sealant. It is therefore proposed to remove the water by injecting a liquid which will not freeze and which will displace the water from the area concerned around the rock chamber. Thus, sealing with hydrates is not a part of this known solution.

In GB Patent No. 1 538 788 there is also a description of sealing of the rock formation around the rock storage chamber, but no mention is made of sealing with the aid of hydrates. The essence of the technique according to this patent is that liquids (condensed gases) are to be stored at low temperatures, usually far below 0°C, in rock chambers which are either unlined (just rock wall) or lined with insulation, concrete or both. In boreholes outside the chamber, a medium is to circulate for heat exchange and for absorption of water vapour which migrates towards the storage chamber. Furthermore, leakage from the storage chamber is to be caught by these boreholes and the liquid or the gas circulating therein. The leaked gas is passed out of the boreholes together with the medium and is to be removed before the medium is recirculated once more. Sealing of any cracks in the rock formation is to take place with the aid of substances which expand and solidify once they have come into place in cracks in the rock.

It has been generally known for many years that in nature hydrates of petroleum gases can be formed and have been formed where the pressure and temperature conditions allow it. In Siberia hydrate layers have been encountered when drilling for petroleum, and under the seabed in the Barents Sea and other places where the sea temperature is around 0°C, hydrates are formed in that petroleum gases, which migrate upwards from hydrocarbon-containing sediments below, come into contact with cold pore water in the uppermost layers beneath the seabed. The fact that hydrates can be formed in cracks and pores when gas and water are mixed at low temperatures and high pressure, is a consequence of a thermodynamic equilibrium, given by a law of nature.

The method relates to how the active formation of a hydrate as sealant can be achieved around a rock chamber and thus takes as its point of departure a law of nature relating to how hydrate is formed and describes a utilisation of this law of nature.

The characterising features of the inventive method are set forth in the appended patent claims, and in the description below with reference to the appended drawings.

The method according to the invention is especially, but not exclusively, intended for the storage of natural hydrocarbon gas, methane gas which consists chiefly of Cl, in a mixture with varying amounts of heavier petroleum gases, where hydrates of a suitable mixture of petroleum gases are made in pores and cracks in the rock around the rock chamber with the aid of cooled water and a suitable gas mixture of cooled petroleum gases which are injected at suitable pressure into the rock around the rock chamber.

The essential novelty of the invention is thus not that sealing of cracks in the rock is to take place with the aid of hydrates, but that the sealing is to take place in a manner whereby it is possible to control the formation and maintenance of hydrates inside the rock volume surrounding a rock chamber blasted out in a rock formation. Natural gas, i.e., Cl (CH₄ methane gas) in a natural mixture with C2 and C3, is to be stored inside the rock chamber. The temperature of the natural gas in the storage chamber does not have to be very low, but it is an advantage if the temperature is as close to 0°C as possible. The gas mixture that is injected into the rock volume around the storage chamber (together with water, either in the same holes or in separate holes) should be cooled to about 0°C to prevent heating of the rock chamber. Such cooling of the injected gas will promote the formation of a hydrate.

The invention is thus not linked to the way in which respectively water and gas in general can be passed into the rock, but the fact that water and gas are passed together into the rock in such manner and with such purpose that hydrates are formed to seal the rock formation in a certain area. This also means that water under pressure must be supplied to the rock formation around the storage chamber in order to cool the rock formation, so as to ensure sufficient water for the hydrate formation, and to create a sufficiently high pressure for hydrate formation at the specific temperature that is used.

The term "injection of cooled water and petroleum gas" is to be understood to mean that water and gas are forced into the boreholes by means of a known technique, and that water and gas are spread from the boreholes through cracks and pores into the rock formations surrounding the boreholes. "Known technique" is used here to mean that a steel pipe is passed into the borehole, and that the open hole around the pipe is sealed by means of, e.g., an expanding rubber bellows. This means that the annular space between the pipe and the borehole wall (rock formation) is sealed. When water is pumped into the borehole through the pipe, this water will be forced into the rock formations around the hole. In a similar way, gas can also be passed into the borehole and this gas will also be forced into the rock formations around the hole.

It is essential according to the invention that the sealing takes place in that hydrates are actively produced in the rock formations around the storage chamber when gas and water are supplied, so that the storage chamber can be used for storing, e.g., methane gas (natural gas in gas form under pressure). The hydrates should therefore preferably be made of a gas mixture which includes methane, although hydrates will also be capable of being made of, e.g., methane, ethane, propane or butane. However, gas mixtures including methane have in fact been found to be advantageous. In this connection reference is made to the following publications: E. Dendy Sloan, Jr: "Natural Gas Hydrates", JPT, Dec. 1991, p. 1414; E. Dendy Sloan, Jr: "Hydrate Nucleation from ICE", 69^th Annual GPA Convention, Phoenix, Arizona 3/12-13/90. Proceedings 1990, p. 52; and M. Motiee: "Estimate Possibility of Hydrates", Hydrocarbon Process, Vol. 70, 1991, No. 7, p.98.

Furthermore, it is essential that to maintain the hydrates there is an active supply of cooled water in the rock formations. This is to serve three purposes:

A) to cool the rock formations;

B) to supply water for hydrate formation; and

C) to create a high pore pressure in the rock by pumping water in at high pressure. Pore pressure is to be understood here as liquid pressure in pores, including small pores, cavities, cracks and the like.

For the storage of gas in rock, the most economical is to construct the storage chamber as close to the surface of the earth as possible. At a depth of 150 metres the water pressure in open cracks (corresponding to minimum pore pressure) will be equal to the pressure of a 150-metre water column, equal to 1.5 MPa. At this pressure, a hydrate of propane will be stable at temperatures of about 5°C and below. It thus possible to seal the rock formation with a stable hydrate of gas mixtures, optionally propane gas only, at a rock depth of not more than 150 metres in that the rock formations are kept cooled by circulating liquid coolant in cooling tubes which are mounted in the boreholes and in that cooled water is injected into the rock formation around the hydrate layer.

The invention will now be explained in more detail in the description below with reference to the appended drawings.

Figs. 1 and 2 show a rock storage chamber where holes have been bored around the storage chamber from access sites such as drifts or galleries, Fig. 2 showing a section along the line I-I in Fig. 1.

Fig. 3 shows a rock formation in which a shaft or pit has been made having cross galleries or drifts and where a rock chamber has been created. The present invention is based on the use of hydrates of hydrocarbons for sealing a rock chamber 1. Hydrates are to be regarded as bonds between water molecules and petroleum gases which are formed at low temperatures and high pressure. The hydrate resembles ice formed by the freezing of water and acts effectively as a sealant against gas flow. Hydrate formation in pipelines which transport hydrocarbons at low temperatures can be a major problem and therefore large amounts of methanol are often added to prevent hydrate formation. When drilling for hydrocarbons, the formation of hydrates may also be a problem if drilling operations come to a halt.

The stability range for hydrates varies greatly according to which petroleum gas is used. At temperatures in the range of 0-5°C, hydrates will be capable of being formed and will be stable from atmospheric pressure and up. Once hydrates have been formed, they will be capable of existing outside the stability range as metastable products if there is no intake of heat. When hydrates are stored under conditions with little influx of heat, they can remain in the form of hydrates for a long time.

By combining the technique of injecting water into boreholes around the rock chamber with the technique for producing hydrates, it is possible to obtain a layer or a shield of hydrates around the rock chamber 1. The method will thus be first to make boreholes at suitable intervals around the rock chamber. The boreholes are positioned in several layers suitably spaced around the rock chamber. The rock formations around the storage chamber are then cooled to about +0-+2°C with the aid of cooling tubes which are mounted in some of the boreholes. It is essential that the temperature does not fall so much that water freezes. Cooled water and cooled petroleum gas are then injected into the rock formation 9 by means of other boreholes. The petroleum gas may be a mixture of several gases which give stable hydrates at highest possible temperature and lowest possible pressure. One example of a gas mixture of this kind is propane in methane gas.

The invention discloses four conditions which must be met if hydrate formation is to be ensured:

1. Low temperature to promote the formation of hydrates. The temperature in the rock must be so low that the melting heat emitted when a hydrate is formed can be absorbed without the conditions for the formation of hydrates suffering further detriment. At the same time it is a vital point that the temperature must not be lower than 0°C minimum.

2. By pumping gas and water under pressure into the rock volume surrounding the storage chamber, a high pore pressure is obtained in the rock. This promotes the formation of hydrates.

3. Cooled water must be supplied, since water must be present to allow the formation of a hydrate. Hydrates will not be formed by water vapour and hydrocarbon gas.

4. The supply of a suitable mixture of cooled hydrocarbons in gas form, thereby obtaining the active production of hydrates in the rock volume around the storage chamber in that a mixture of hydrocarbon gases which form hydrates at highest possible temperature and lowest possible pressure are pumped into the rock volume through boreholes.

According to the method, the first important point is that the temperature in the rock around the storage chamber should not be below 0°C. Secondly, it also a point that all cracks, pores and cavities should be filled with water. Thirdly, hydrates should be formed in an active human-caused manner and under controlled conditions. These hydrates are to seal the rock volume around the storage chamber at a relatively high temperature, i.e., above 0°C. To be able to operate under reasonable pressure conditions, the temperature must however be below 10°C (approx.). By controlling the temperature in the way described below, it is possible to allow ice (i.e., hydrate) and water to co-exist in the rock. By cooling the rock continuously, gas under pressure may be pumped into the storage chamber without the temperature in the surrounding rock rising so much that the hydrate melts. By maintaining the rock at such a high temperature, the formation of contraction cracks is avoided. The spread of such cracks is very difficult to control or stop.

It is essential for the formation of hydrates that water and gas come into good, sustained contact with one another. This can be achieved in several ways.

a) Water and gas can be passed into the same borehole through two separate pipes. This means that it is first inside the borehole that the water and gas come into contact with one another. However, the water and the gas enjoy good, sustained contact only when they are mixed en route through pores and cracks in the rock formation. It is thus important that the contact between water and gas takes place over a greatest possible surface. It is also known that hydrate formation is promoted by turbulence, i.e., an intense agitation of water and gas. This is precisely what happens inside the rock formation.

b) Water and gas can be passed into separate boreholes. In a layer of boreholes, every second borehole may be used for water injection, and every second borehole for gas injection. When the water and the gas are forced into the rock formation around the boreholes, they will meet and mix inside the pores and cracks in the rock formation and a hydrate will be formed.

c) Water and gas can be passed into the same borehole, but not at the same time. First, water is pumped into the borehole and into the rock formation for a few minutes. Then the water injection is stopped. Gas is subsequently forced into the same borehole so that the gas passes into the water-filled pores and cracks around the borehole. When the gas meets the water in the rock formation, a hydrate will be formed. After some minutes the gas injection is stopped and water injection is recommenced. Thus water injection and gas injection alternate using the same boreholes. This technique can be combined with the technique described under b), so that every second hole is used for water and gas injection, and that the holes constantly go from being a water injection hole to being a gas injection hole and then revert to being a water injection hole again.

The techniques described under a), b) and c) are used until the rock formations around the boreholes are sealed by hydrates. This is noticed in that the pressure required to inject water and gas into the rock formation rises. The injection of water and gas must take place under pressure which is less than the splitting pressure of the rock formation. The splitting pressure is that pressure necessary to cause the rock formation to crack open in that water or gas presses the borehole wall out.

When the rock formation is saturated by hydrate and therefore gradually becomes sealed, it becomes difficult to inject more water and gas. Because of the intake of heat that takes place through the rock, the hydrate will gradually decompose and separate as water and gas. As mentioned, this will take a long time when the intake of heat is small, as it is in rock formations. This time can be increased by cooling a large volume of the rock formation in advance and also by keeping the rock formation cooled in a similar way after the hydrate has been formed. In this way, the rock formation can also be kept cooled to such an extent that the hydrate is stable. Through a continuous cooling of the rock formation, it is possible to retain a stable, sustained sealing of the rock formation by means of hydrates.

In practice, this can be done thus that at least three layers 3, 4, 5 of boreholes are bored around the storage chamber, one outside the other. In Figs. 1 and 2, by way of example, two further layers 6 and 7 are shown. Conveniently, the boring takes place from access sites such as galleries or drifts 2 provided via cross gallery 8 from a shaft 9. The shaft 9 extends from the surface 10 of the rock formation 11. Alternatively, it is also possible that the injection of water and gas mixture could take place at least in part via boreholes 12, 13 which are bored around the storage chamber from the surface 10. However, with today's technology this is an inefficient solution. By means of the middle layer 4 of boreholes, a hydrate layer is established which seals against leakages as described. In the innermost layer 3, coolant circulates in cooling tubes mounted in the boreholes. In this layer the temperature can be kept below 0°C so that heat from the storage chamber is intercepted. This will help to keep the hydrate layer sufficiently cooled to enable it to remain stable. Cooled water is injected continuously in the outermost layer 5 of boreholes. This will also help to keep the rock around and in the hydrate layer cooled to such an extent that the hydrates remain stable. When the hydrate layer is sealed, the injected water will not run into the storage chamber 1, but will disappear into the surrounding rock formations. At the same time this cooled excess water will ensure that gas which might leak out of the storage chamber 1 will bond to the water and form a new hydrate. In this way, an automatic sealing against leakages is obtained. If four or more layers of boreholes are used outside the storage chamber, the third layer 5 can be used for cooling with the aid of cooling tubes mounted in the boreholes, as described for the innermost layer 3. By having a cooling layer on either side of the hydrate layer, the hydrate layer is secured so that it remains stable. Injection of cooled water must then take place in a fourth layer of boreholes located outside layer 5.

The third layer 5 may also be used for both cooling with the aid of cooling tubes in the boreholes and injection of water, for example, by using every second borehole for cooling tubes and those in between for water injection. A double safeguard against leakages from the gas store can be achieved with the aid of a further two layers of boreholes. In a layer of boreholes outside layer 5, hydrate layer number two is made. Then, in a new layer of boreholes outside this layer, cooling and water injection can take place as described for the third layer 5. The sequence of functions in layers of boreholes starting from the storage chamber and moving outwards is then as follows: 1) cooling; 2) hydrate; 3) cooling plus water injection; 4) hydrate; 5) cooling plus water injection.

To ensure good cooling of the hydrate layer, some boreholes in the hydrate layers can be used for cooling by means of cooling tubes.

In order to have control of the temperature both inside and around the hydrate layers that are formed, temperature sensors must be mounted that monitor the cooling process around the storage chamber. This presupposes that the whole system can be controlled and monitored by many temperature sensors mounted in boreholes around the whole storage chamber.

Claims

P a t e n t c l a i m s

1.

A method for obtaining gas-sealing around a rock chamber or rock storage chamber, characterised in that hydrates of a suitable mixture of petroleum gases are made in pores and cracks in the rock (11) around the rock chamber (1) with the aid of cooled water and a suitable gas mixture of petroleum gases which are injected at suitable pressure into respective boreholes (3, 4, 5) in the rock around the rock chamber (1).

A method as disclosed in claim 1, characterised in that the injection of water and the gas mixture is carried out in boreholes which are bored from separate access galleries (2) outside the rock chamber (1) and which are bored around the rock chamber in one or more layers at different distances from the rock chamber (1).

A method as disclosed in claim 1, characterised in that at least some of the injection of water and the gas mixture takes place in boreholes (13, 11) which are bored around the storage chamber (1) from the earth surface.

A method as disclosed in claim 1, characterised in that the injection of water and the gas mixture is carried out at the same time and in the same boreholes around the rock chamber.

A method as disclosed in claim 1, characterised in that the injection of water and the gas mixture is carried out simultaneously, but in separate boreholes which are side by side, either in the same layer around the rock chamber or in different layers (3, 4, 5) which are at different distances from the rock chamber. A method as disclosed in claim 1, characterised in that the injection of water and the gas mixture is carried out in the same boreholes, but in succession, such that firstly cooled water is injected under pressure into the borehole until all pores and cracks are saturated with water, that the gas mixture is then injected under pressure into the same borehole for a certain time, and that subsequently cooled water is injected under pressure into the same borehole, and that the injections take place continuously in cycles until the rock formation is sealed by hydrates.

A method as disclosed in claims 4, 5 and 6, characterised in that the sealing of the rock formation with the aid of hydrates is detected in that the pressure required to inject water and gas mixture into the borehole rises and approaches the splitting pressure for the rock formation in question.

8.

A method as disclosed in one or more of the preceding claims, characterised in that the hydrate layer formed around the rock chamber (1) is maintained by cooling the rock around the rock chamber continuously with the aid of cooled water that is injected under pressure into one or more layers (3, 5) of boreholes which are made outside the hydrate layers (2, 4).

A method as disclosed in one or more of preceding claims 1-7, characterised in that the hydrate layer formed around the rock chamber (1) is maintained by injecting cooled water under pressure into one or more of the boreholes made within and outside the hydrate layer, so that there is always sufficient water present in pores and cracks to allow a new hydrate to be formed when free petroleum gas comes into contact with the water.

10.

A method as disclosed in one or more of the preceding claims, characterised in that the hydrate layer formed around the rock chamber (1) is maintained by cooling the rock around the rock chamber continuously by means of cooling tubes mounted in boreholes, either in the hydrate layer or in one or more layers within and outside the hydrate layer which have been bored in one or more layers within and outside the hydrate layer.

11.

A method as disclosed in one or more of the preceding claims, characterised in that the water which collects in the bottom of the rock chamber is reused for injection into the rock around the rock chamber, either alone for cooling of the rock formation or together with petroleum gases to make hydrates in cracks and pores in the rock formation.

12.

A method as disclosed in one or more of the preceding claims, characterised in that the gas mixture contains one or more of the gases methane, ethane, propane or butane.

13.

A method as disclosed in claims 8-10, characterised in that the temperature in the rock in and around the hydrate layers is monitored with the aid of temperature sensors which are mounted in many boreholes around the rock chamber.

14.

A method as disclosed in claim 10, characterised in that the cooling process is controlled automatically by means of temperature sensors which are mounted in many boreholes around the rock chamber.