WO2000017484A1

WO2000017484A1 - Method for dissolution, storage and transportation of gas hydrates

Info

Publication number: WO2000017484A1
Application number: PCT/NO1999/000289
Authority: WO
Inventors: Erlend O. Straume; Stig M. Bakkeng
Original assignee: Petreco As
Priority date: 1998-09-21
Filing date: 1999-09-17
Publication date: 2000-03-30
Also published as: AU5885799A

Abstract

Method for dissolving hydrates and ice, for example in wells, pipelines or other processing equipment, or in naturally occurring hydrate reservoirs in the crust of the earth. Hydrates are formed when sufficient amounts of water and a gas comprising natural gas and possible oil or condensate, is present at a pressure and a temperature which provides conditions for hydrate or ice formation. The gas or liquid phase adjacent to the hydrates is exchanged with a gas or liquid which does not form hydrates at the given pressure and temperature. If it is necessary, at temperatures below 0 °C, the pressure may be raised, so that both hydrate equilibrium temperature and freezing temperature are lower than the surrounding temperature. The method may also be used to provide stable storage and transport of gas or hydrates with high gas contents, at temperatures preferably between +5 and -20 °C and absolute pressure preferably up to 5 bar. The gas or liquid phase adjacent to the hydrate is completely or partly exchanged with propane and/or isobutane where the sum of the mol fractions of isobutane and propane is more than 40 %.

Description

Method for dissolution, storage and transportation of gas hydrates „

The present invention relates to a method for dissolution of hydrates and ice, for example in wells, pipelines or other processing equipment, and naturally occurring hydrate reservoirs, according to the preamble of patent claim 1 and 4. The invention relates further to a method for stable storage and transport of gas hydrates at temperatures between +5 and -20 °C, and absolute pressure up to 5 bar, according to the preamble of patent claim 8.

Background Gas hydrates are solids resembling ice, that are built up of water-molecules organized in crystal lattices, and with gas molecules which stabilize the lattice structure by filling the cavities of the crystal lattice. As opposed to ice, gas hydrates may be stable at temperatures above 0°C at high pressure. At pressures above 100 bar, natural gas hydrates will be stable at temperatures between 10 and 20 °C. Gas hydrates are well known as a huge problem in the oil and gas industry. So-called "hydrate plugs" are accumulations of hydrates that block the fluid transport in wells, pipelines or other processing equipment. When enough water is present in the pipe, at a sufficiently high pressure and temperature, hydrates may be formed. Naturally occurring hydrates exists in reservoirs both above, and below, the seabed, and in areas with permafrost in arctic regions. Huge amounts of natural gas is trapped in these reservoirs, and the gas may be obtained when the hydrate melts.

Ice plugs are accumulations of ice that block the transport of fluid in wells, pipelines and other processing equipment. Ice plugs may be made of hydrates txirning into ice, when a hydrate plug is depressurized, or by water freezing to ice when the surroundings are cooling the fluid in wells, pipelines or other processing equipment to a temperature below 0°C. Several methods may be used to melt ice plugs and hydrates. Depressurizing is mostly used but the hydrate may, in certain situations be melted by heating, use of inhibitors or a combination between one or more of these methods. By depressurizing one of the sides of a hydrate plug, problems concerning freezing of the hydrate may arise, as some gas leaks through and cools due to the pressure drop over the plug. Another problem is security. If the plug loosens when there is considerably higher pressure on one side than the other, it can move at high speed through the pipe, and cause great damage to processing equipment. In order to avoid these problems, the pressure must be as equal as possible on both sides of the plug. However, it is often difficult or impossible to depressurize both sides.

The methods for removal of ice plugs are somewhat limited. Heating, and addition of inhibitors are possible options for removal. However, if the temperature in the surroundings of a pipeline is below the freezing point of the water in the pipe, the plug can not be melted with heat from the surroundings. Examples of this are pipelines exposed to melting water from salt-water ice. These ocean streams can have temperatures down to -2°C.

To avoid fresh water freezing at these temperatures, the pressure must be high. When the pressure is substantially raised in a system, where natural gas and water is present, hydrates are made.

Gas hydrates may also be used to store and transport natural gas, or other gasses and gas- mixtures. A i m³ hydrate may store up to 180 Sm³ gas. Different methods for storing hydrates are also described in other patents. It is especially referred to Norwegian Patent Applications 19964544 and 19975373, and US patent publications 5,536,893 and 3,514,274.

Norwegian Patent Application 19964544 relates to methods for producing, storing and transporting hydrocarbon products. According to this patent application, the hydrates should be stored at atmospheric pressure and the temperature should be lower than the equilibrium temperature of the hydrate. Application 19964544 assumes that the equilibrium pressure of the hydrate is dependent of the temperature and the composition of the hydrate, which recently has proven to be an imprecise assumption. Experiments which are performed in connection with the present invention show that the stability of a hydrate is more dependent of the composition of the gas surrounding the hydrate, and less the gas which is trapped in the hydrate. Application 19964544 states that the temperature zone for stable hydrates at atmospheric pressure is lower than -30 °C for hydrates produced from a gas which contains more than 80 volume% methane, and lower than -20 °C for hydrates produced from a gas which contains up to 35 volume% ethane and propane. These temperature zones are lower than the zones described in the present application. A disadvantage with the storage method of application 19964544, is thus that the hydrate must be cooled down to very low temperatures. This is avoided with the present invention, because the storage temperature is substantially higher.

US patent publication 5,536,893 asserts that hydrates are stable at atmospheric pressure or slight overpressure at temperatures below 0°C, preferably at -10 to -15 °C. In patent application 19964544 this is rejected, and it is referred to the fact that hydrates behave so- called "metastable" at temperatures below the freezing point for water. It is assumed that the gas stays inside the hydrate even if it is not thermodynamically stable, independent of the gas surrounding the hydrate. This might be correct for hydrates which are frozen in ice, where there is a composition of about 50 % ice and 50 % hydrate. Experiments with hydrates containing a large volume of gas (with a large volume of gas it is in this connection meant that the hydrates preferably contain between 120 and 180 Sm³) show, however, that huge amounts of the gas is lost from the hydrate, when it is stored in equilibrium. This is avoided by the present invention where the hydrate is stored thermodynamically stable, considering the adjacent gas or liquid. Norwegian patent application 19975373 relates to a method for transporting and storing oil and gas, where the gas is trapped in hydrates, and is transported as a slurry together with oil. The slurry is preferably transported at atmospheric pressure and temperatures below 0°C. Hydrates of natural gas are as mentioned above, not stable at atmospheric pressure and temperatures above about -20 °C, and the problem is therefore the same as US-patent 5,536,893.

US-patent 3,514,274 describes how natural gas may be transported in a "alluvion" with liquid propane. Liquid propane evaporates at -43 °C at atmospheric pressure, and the composition must thus be transported and stored at this temperature or lower. Propane is accordingly not added to store the hydrate stable in the temperature zone where propane- hydrate is stable. The disadvantage with this method is, again, the low temperature, which demands extra energy to cool the hydrate product.

The object

The main object of the invention is thus to provide an alternative method for dissolving hydrates and/or ice, without the above said problems. Another object is to provide an alternative method for storing and transporting hydrates with high gas content at temperatures preferably between +5 and -20 °C and absolute pressure preferably approximately at atmospheric pressure, but also up to 5 bar. A further object is to provide a method which utilizes simpler equipment, is easier to use than known methods, and whereby the expenses for operation are reasonable. The invention

The object of the invention is fulfilled with methods according to the characterizing parts of the independent patent claims. Further features appears from the additional dependent claims. The method according to the present invention concerns exchange of an adjacent gas or liquid phase of the hydrates/ice, in order to melt or stabilize them.

The stability of hydrates is mainly dependent on the composition of the adjacent gas or liquid phase, and not so much the composition of the hydrate, as assumed in Norwegian patent application 19964544. When the hydrate is in contact with a gas or liquid at a pressure and a temperature where this gas or liquid is able to make hydrates, the crystal lattice on the surface will not be broken, and the hydrate will stay stable. The cause of this is that the surface of the hydrate will be exposed to a gas or liquid which makes stable hydrates at given pressure and temperature. Thereby the bonds on the surface of the crystal lattice will not be broken, and the molecules which fill the cavities of the crystal lattice inside the hydrate, will still be trapped, and the hydrate stays stable.

When the adjacent gas or liquid is replaced with a gas or liquid which does not make hydrates at the given temperature and pressure, the hydrate melts. The cause of this is that the surface of the hydrate is exposed to a gas or liquid which does not make stable hydrates at the given pressure and temperature. Thereby the bonds on the surface of the crystal lattice will be broken, the gas molecules in the cavities in the crystal lattice will be released and the hydrate will melt.

This is proven in the two following examples.

Nitrogen has a hydrate equilibrium pressure of 162 bar at 0°C, according to calculations from the Hydfls software program from Calsep. Hydfls is a "hydrate flash" program which calculates the hydrate equilibrium condition for gas-oil-water compositions. Experiments show that hydrates of natural gas at 65 bar and 4°C melt, unless the adjacent gas is replaced by Nitrogen, or stabilized condensate when pressure and temperature are held constant.

A mixture of propane and fresh water has a hydrate equilibrium temperature of -10,4° C at atmospheric pressure, while a composition of isobutane and fresh water has a hydrate equilibrium temperature of -0,3 °C at atmospheric pressure, according to calculations by Hydfls. Experiments show that hydrates of natural gas, with equilibrium temperature of - 24 °C at 1,7 bar, can, according to calculations by Hydfls from Calsep, be stored stable at 1,7 bar and -10°C, if the adjacent gas or liquid to the hydrate during storage contains a substantial part isobutane. Experiments have been performed with other gases and liquids, and the results are the same. For melting hydrates are for example Argon, air, Nitrogen, oil or a condensate which is saturated with some of these gases, or a stabilized oil or condensate, well-suited. For stable storage and transport are an adjacent gas or liquid phase containing, for example, isobutane, or propane, a mixture of isobutane and propane, a mixture gas of isobutane and/or propane and other gas-components where the total mol fraction of isobutane and propane are between 40 and 100 %, a gas which is supplied with a large part isobutane and/or propane to stabilize the hydrate, another gas, gas mixture or liquid, having a hydrate equilibrium temperature at atmospheric pressure, above -20 °C, or hydrate equilibrium pressure at +5 °C below 5 bar, a hydrocarbon liquid which is saturated with one of the aforementioned gases or gas mixtures, or an aqueous solution or a salt water solution which is saturated with one of the said gases or gas-mixtures, well-suited.

Nitrogen and water have a triple junction between hydrate, water and ice at -1,06° C and 145,8 bar, according to calculations by Hydfls from Calsep. Experiments show that ice melts at 1 ,5 ° C and 197 bar in Nitrogen atmosphere.

The first step of the method for stable storage and transport of gas and hydrates with high gas content, preferably between 120 and 180 Sm³ gas per m³ hydrate, at temperatures preferably between +5 and -20 °C and absolute pressure preferably up to 5 bar, more preferably between 1-2 bar, is the formation of hydrates from the gas. Hydrates are formed when sufficient amounts of water or salt water, and a hydrate forming gas, which may contain for example methane, propane, isobutane, normal butane, carbon dioxide, Nitrogen, Oxygen, hydrogen sulphide and eventually oil or condensate, are present at a pressure and a temperature which give conditions for hydrate formation.

Hydrates of natural gas are not stable at the given pressure and temperature for storage and transportation with the formation gas and/or condensate as adjacent gas or liquid phase. Prior to storage and transportation, the adjacent gas shall therefore be exchanged with, or added larger amounts of a gas, or liquid that forms stable hydrates at the given storage and transportation temperature and pressure. The method may for example be used for transportation of, for example natural gas. The adjacent gas or liquid phase may be exchanged with, or added a gas mixture with larger amounts of propane and/or isobutane, where the sum of the mol fractions of isobutane and propane are more than 40 %, preferably more than 90 %, and most preferably 100 %. The adjacent gas or liquid phase may also be exchanged with, or added larger amounts of, isobutane or propane, or a hydrocarbon liquid, water, or salt water which is saturated with one of the previously mentioned gases or gas mixtures.

The hydrate may be produced of salt water leading to the concentration of salt in the water, and thus the freezing point of the water is lowered and the hydrate may thereby be stored in a salt water slurry which is saturated with one of the gases or liquid phases which is mentioned above.

Example The invention will be described hereinafter with reference to an example of performance and the accompanying figures, wherein:

Fig. 1 shows a flow-simulator to test the melting of hydrates, according to the present invention, Fig. 2 shows an axial section through a pressure cell, for testing the freezing point of water, and hydrate equilibrium pressure and temperature,

Fig. 3 shows a graphical representation of hydrate equilibrium curves for hydrate and ice melting according to the present invention,

Fig. 4 shows a pressure tank for testing hydrate melting by exchanging the adjacent liquid phase with stabilized condensate according to the present invention, Fig. 5 shows a graphical representation of hydrate equilibrium curves for some gas mixtures,

Fig. 6 shows a graphical representation of hydrate equilibrium curves for a normal natural gas and different water solutions, and Fig. 7 shows hydrate equilibrium curves for natural gas and different mixtures of methane and isobutane.

Fig. 1 shows a torus shaped flow simulator 1 comprising a pipe 2 which extends in a closed circle, only interrupted by two diametrically opposite windows 3 a, 3b which are used for video monitoring the contents of the pipe 2. The pipe 2 is arranged to rotate around its own axis, as a shaft extension 4 in the centre is connected to the pipe 2 with radially extending bars 5, equipped with adjusters 6. In experiments related to the present invention, a flow simulator with a diameter of 2 metres was used with a pipe diameter of 2".

A gas inlet 7 and a gas outlet 8 are attached to the pipe, the inlet and outlet are preferably diametrically opposite each other. The pipe 2 is further equipped with a temperature transmitter 9, and a pressure transmitter 10. An experiment was performed to prove that hydrates melt when the gas which is in contact with the hydrate, is exchanged with Nitrogen. The hydrate was made within the flow simulator 1.

The total volume of the system is 13,4 L, and 1 L water was added. Further added was 3,0 L Exxsol D60 (a low aromatic solvent in the groups of aliphates). The pipe 2 in the flow simulator 1 was filled with natural gas to about 70 bar at 25 °C. The natural gas was a mixture of 85,5 % methane, 7,5 % ethane, 6 % propane and 1 % isobutane.

The flow simulator 1 was cooled down to 4°C during rotation. Hydrates were formed at about 7°C. After cooling and formation of hydrates, the pressure was about 65 bar. The wheel was stopped after formation of the hydrates, and one of the windows 3 a, 3b was placed at a lowest point so that a hydrate was visible in front of the window. The wheel was then left standing still for about 3 hours.

Thereafter the gas atmosphere in the wheel was exchanged by adding Nitrogen via the inlet-valve 7 from one side, and draining the existing gas atmosphere in the wheel, through the outlet valve 8 on the other side. The pressure and the temperature after exchange of the gas atmosphere was the same as before the exchange. After the first exchange was observed that a free water phase was precipitated. This was probably water from the hydrate which melted on the walls in the gas atmosphere. Four exchanges were carried out the first day of the experiment, with about an hour between each. Large differences concerning the consistency of the hydrates, beyond observed water phase after the first exchange, was not observed.

After the fourth exchange, the wheel stood still for 18 hours before the fifth exchange. During this time, the pressure in the pipe had raised from 65 to 67 bar. One may assume that the increase in pressure was due to gas which was released by the hydrate. One hour later, the gas atmosphere was changed for the sixth time. Three hours later the wheel was started, and was operated at a speed of lm/s. Over a 20 minutes period, all of the hydrate was melted.

Figure 2 shows a test cell 11 for testing the freezing point of water at different pressures, and the equilibrium pressure for hydrate at different temperatures. The test cell comprises a PMMA cylinder 12 of the same type as the windows 3a, 3b of the flow simulator 1, shown in figure 1. The cylinder 12 has a length of 150 mm, exterior diameter of 150 mm and interior diameter of 52,5 mm. It is placed between two flanges 13a, 13b with O-ring sealing. At the larger flange 13a there is a filling and draining valve 14 for gas, and a pressure sensor 15 for measuring the pressure in the test cell 11. The temperature in the test cell 11 is measured with a temperature sensor 16 which extends through the other flange 13b. During experiments the test cell 11 is placed in a freezer (not shown) with temperature control. An experiment, for measuring the freezing temperature of water at different pressures was performed. The test cell was cooled to about 0°C prior to the experiment, and filled half full with a mixture of ice and water. Thereafter it was placed in the freezer, and the gas outlet 14 was connected to a 200 bar Nitrogen flask. The temperature in the freezer was held at about 0°C. The pressure in the test cell 11 was gradually raised up to about 200 bar. Both the pressure and temperature were logged during the experiment. The experiment proves that the freezing point of water may be displaced by pressurizing with Nitrogen gas. The freezing temperature of water was measured to about -1,5 °C at 197 bar.

An experiment was also performed to measure the stability pressure of a hydrate with an atmosphere rich in isobutane around the hydrate, at 10°C. The test cell was filled with hydrate which was cooled to -50°C. Then isobutane was introduced, and the temperature was raised to -10°C. The pressure was then lowered to atmospheric level again. Subsequently the pressure raised to about 1,6 - 1,7 bar and stabilized at this level. It is assumed that this increase is due to the fact that the boiling point for the gas is in the same region as this temperature and pressure. The temperature was held at 10°C, and the pressure was between 1,6 and 1,7 bar for 5 days. Thereafter the hydrate was melted, and the gas- content measured. The measurements of gas content proved that the hydrate had the same gas content as it did previous to storage.

Fig. 3 shows curves for hydrate equilibrium and freezing point for water, calculated by Hydfls, for different mixtures of natural gas and Nitrogen. The mixture of the natural gas is the same as described above.

The apparatus in Fig. 4 was used for melting hydrates by exchanging the adjacent liquid phase with stabilized condensate. The pressure tank 21 was filled with a mixture of hydrate and condensate at 65 bar and 2°C, and thereafter cooled to -50°C. After the cooling, the pressure tank was connected to a pump 18 by valve 19, a piston accumulator 25 by valve 20 and a separator 29 by valve 23. Cooled condensate, which did not contain gas, was pumped from container 17 through valve 20 into the pressure tank 21, while the condensate that was in the pressure tank was drained through safety valve 28 which opens at 65 bar. The accumulator 25 stabilized the pressure with 65 bar Nitrogen gas that was added to the cylinder 27 at the right side of the piston 26, prior to the opening of the valve 24 towards the storage tank. The filter 22 filtered the hydrate from the condensate during exchange of the condensate. The separator 29 was used to separate gas from liquid in the bottom of the separator, when needed. The gas was drained through the top of the separator. The gas and the liquid volume was measured. When the condensate in the pressure tank was exchanged with stabilized condensate, the temperature was raised to 4°C. The pressure was held at 65 bar. Then the condensate was exchanged with stabilized condensate at this temperature. After the exchange of the condensate, all of the hydrates were melted. Fig. 5 shows hydrate equilibrium curves for different mixtures of methane, propane, and isobutane, as calculated by Hydfls, for different mixtures of natural gas and Nitrogen. Hydfls is as previous mentioned, a "hydrate flash" program that calculates the state of hydrate equilibrium for gas-oil-water mixtures. When pressure and temperature for the gas- mixture surrounding the hydrate is above and to the left of the hydrate equilibrium curve in Figure 5, the hydrate is stable, and the gas will stay trapped in the crystal lattice, and when the pressure and temperature is below and to the right of the hydrate equilibrium curve, the hydrate is unstable, and gas which is trapped, will be released.

The curves show that hydrates may be stored stable with an adjacent gas atmosphere which contains substantial amounts of propane and/or isobutane.

Fig. 6 shows hydrate equilibrium curves for a representative natural gas mixture and water with different amount of salt, calculated by Hydfls. If the hydrate should be produced from saltwater, the salt in the water will be concentrated in the water that does not transfer into hydrates. The hydrate equilibrium temperature and the freezing point will then be lowered. If high enough salt concentration is reached, the freezing point for water may be below -5°C, and the hydrate may be stored at low pressure, preferably atmospheric pressure and with a gas atmosphere which mainly comprises propane and/or isobutane.

Figure 7 shows hydrate equilibrium curves for natural gas, different mixtures of methane and isobutane, and the freezing point curve for 8 weight% salt water. The curve shows that the hydrate is stable at atmospheric pressure and -5,3 °C, in 8 weight% salt-solution with an adjacent gas atmosphere which contains 90 % isobutane and the rest methane. The gas or liquid phase may be exchanged or enriched with a gas or liquid phase when the hydrates are transferred from a production reactor to a storage or transportation tank. This may, for example, be carried out in one or several tanks where the hydrate-slurry of liquid and hydrate or hydrate particles, and an adjacent gas phase, is transferred from a reactor, and the gas or liquid phase is exchanged with a gas or liquid phase with higher hydrate equilibrium temperature at atmospheric pressure before the pressure and temperature are lowered to storage conditions. This may also be carried out in several steps where the pressure and the temperature are lowered gradually for each step.

If the hydrate is produced of natural gas and salt water, hydrates may be produced until a sufficient amount of water is transferred to hydrate. An example of volume sharing is 70 volume% hydrate and 30 volume% salt water; which gives a salt concentration in the remaining water of about 8 weight%. After production the pressure may be lowered to the hydrate equilibrium pressure, and the gas atmosphere around the hydrate may be exchanged gradually while the pressure is further lowered simultaneously with the hydrate equilibrium curve of the adjacent gas or liquid phase of the hydrate. When the gas exchange is finished, pressure and temperature may be lowered to storage pressure and temperature. During this process, the pressure and temperature must be above, but close to the hydrate equilibrium curve for the adjacent gas or liquid phase at the time. When wells, pipelines and other processing equipment are clogged by a hydrate-plug or an ice-plug, the plug may be removed by exchanging the gas which is in contact with the hydrate. Pressure and temperature conditions must be outside both the freezing point curve for water and the hydrate equilibrium curve for the gas atmosphere surrounding the hydrate. If the temperature is above 0°C, the hydrate will be melted by the new adjacent gas or liquid phase, at constant pressure. If the temperature is below 0°C, it may be necessary to raise the pressure to melt possible ice, and to avoid ice being formed from the melted hydrates.

The gas in the system may be exchanged by repeated depressurization and filling with the new gas or liquid. The gas exchange may, if possible, be carried out by filling the gas or liquid through an inlet in the system, and letting it out through a distinct outlet. Stabilized oil or condensate, which do not contain an insignificant amount of hydrate forming compounds, may be used for melting hydrate by, for example, filling the oil or condensate on to one side of the hydrate plug, possibly in connection with the exchange of the gas atmosphere. When melting the hydrate in a reservoir according to this method, Nitrogen gas, for example, may be injected into a well, and a mixture of Nitrogen gas and natural gas, which is released from the hydrate, may be produced from a well close to the injecting well. The Nitrogen gas, which is injected, will then flow through the channels in the hydrate formation and to the producing well. As the hydrate melts, these channels will expand. The gas flow may also transport water which is released when the hydrate melts. When the gas flow reaches the surface, the Nitrogen and natural gas may be separated and the Nitrogen gas can be reinjected.

In the same way, stabilized oil or condensate which does not contain, or contains an insignificant amount of hydrate forming components, may be reinjected in a well. When the condensate or oil flows through the reservoir, gas will be released from the hydrate. The oil or condensate may be produced from a well close to the injection well, and the gas may be separated on the surface, before the oil or condensate is reinjected.

From the experiments that have been performed, it has been shown that gas hydrates and ice may be stabilized or melted by exchanging the adjacent gas or liquid phase to the hydrate, with a gas or liquid that makes, or respectively does not make, hydrates at the pressure and temperature of the new surroundings. Natural gas hydrates may be melted by exchanging the gas or liquid phase with Nitrogen, at a pressure and a temperature where Nitrogen gas hydrates are not stable. Ice may be melted by raising the pressure, by pressurization with Nitrogen gas, to a pressure where ice melts at a given temperature. In the same way hydrates of natural gas may be melted in contact with air or Argon. It is also possible to use another gas, preferably a gas with molecule diameter less than 3,8 A, or stabilized oil/ condensate which does not make hydrates at given pressure and temperature.

Natural gas hydrates may in the opposite way be stabilized at absolute pressure up to 5 bar, and temperatures between +5 and -20 °C, by exchanging the adjacent gas or liquid phase with a gas or liquid which, for example, contains isobutane, propane, a mixture of isobutane and propane, a gas which is a mixture of isobutane and/or propane and other gas compounds where the sum of the mole fractions of isobutane and propane are between 40 and 100 %, a gas which is added a larger portion of isobutane and/or propane so that the hydrate will be stable, another gas, gas mixture or liquid with hydrate equilibrium temperature at atmospheric pressure above -20°C, or hydrate equilibrium pressure at +5 °C below 5 bar, a hydrocarbon liquid saturated with one of the said gases or gas mixtures, or an aqueous solution or salt water solution which is saturated with one of the said gases or gas mixtures.

Claims

Claims:

1. Method for dissolving hydrates and ice in, for example, wells, pipelines and other processing equipment, or naturally occurring hydrate reservoirs in the crust of the earth, the hydrates being formed when sufficient amounts of water and a gas comprising natural gas and possible oil or condensate are present at a pressure and a temperature which provide conditions for hydrate or ice- formation, characterized in that the gas- or liquid phase adjacent to the hydrates is completely or partly exchanged with a liquid which lacks the ability to form hydrates at said pressure and temperature, and that the pressure if necessary, is raised so that both the hydrate equilibrium temperature and the melting temperature for ice at a give pressure is lower than the temperature in the surroundings.

2. Method according to claim 1, characterized in that the adjacent gas or liquid phase is exchanged with a liquid which is saturated with air, Nitrogen, Argon or a gas with molecule diameter less than 3,8 A, or a mixture of these.

3. Method according to claim 1, characterized in that the adjacent gas or liquid phase is exchanged with stabilized condensate or oil.

4. Method for dissolving hydrates and ice in, for example wells, pipelines and other processing equipment, the hydrates being formed when sufficient amounts of water and a gas comprising natural gas and possible oil or condensate are present at a pressure and a temperature which provide conditions for hydrate or ice- formation, characterized in that the gas- or liquid phase adjacent to the hydrate is completely or partly exchanged with a gas which lacks the ability to form hydrates at said pressure and temperature, and that the pressure if necessary, is raised so that both the hydrate equilibrium temperature and the melting temperature for ice at a give pressure is lower than the temperature in the surroundings.

5. Method according to claim 4, characterized in that the adjacent gas or liquid phase is exchanged with Nitrogen.

6. Method according to claim 4, characterized in that the adjacent gas or liquid phase is exchanged with air.

7. Method according to claim 4,

5 characterized in that the adjacent gas or liquid phase is exchanged with Argon.

8. Method according to claim 4, characterized in that the adjacent gas or liquid phase is exchanged with a gas with a molecule diameter which is less than 3,8 A. 10

9. Method for stable storage and transport of gas or hydrates with high gas content, preferably a content between 120 and 180 Sm³ gas per m³ hydrate, at temperatures preferably between +5 and -20 ┬░C and absolute pressure preferably up to 5 bar, by forming hydrates of the gas when there is sufficient amounts of water or salt water present at a

15 pressure and temperature, different from the storing pressure and temperature, which provide conditions for hydrate formation, characterized in that the gas or liquid phase adjacent to the hydrates, prior to storage and transport is completely or partly exchanged with water or saltwater which is saturated with

- a gas mixture including propane and/or isobutane wherein the sum of the mole fractions 20 of isobutane and propane are more than 40%, preferably between 90 and 100 %, more preferred 100 %,

- pure or larger amount of isobutane, or

- pure or larger amounts of propane.

25 10. Method according to claim 9, characterized in that the hydrate will be produced of saltwater, whereby the salt will be concentrated in the water phase, so that the hydrate may be stored in a salt water slurry which is saturated with a gas or a gas phase according to claim 9.