WO2015002544A2

WO2015002544A2 - Method and system for natural gas production

Info

Publication number: WO2015002544A2
Application number: PCT/NO2014/050121
Authority: WO
Inventors: Øyvind BØNES; Arild Vik
Original assignee: Energy Research Group As
Priority date: 2013-07-05
Filing date: 2014-07-04
Publication date: 2015-01-08
Also published as: WO2015002544A3

Abstract

A method and system for producing gas from a gas hydrate formation are disclosed. Also a method for stabilization of a gas depleted hydrate formation and/or for enhancing the formation of a gas hydrate formation is described.

Description

Title: Method and system for natural gas production

Field of the invention

The present invention relates to a method and system for production of hydrocarbons from subterranean formations, and especially to a system for producing of natural gas and/or energy from a gas hydrate formation. More specifically, the invention relates to an improved method for the production of methane gas from a methane hydrate formation.

Background of the invention

Methane clathrate (CH₄ ^«5.75H₂0), also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate is a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice.

Methane hydrate deposits are abundant throughout the word and have been estimated to represent the greatest portion of the world's fossil energy reserve.

In general, there are two primary geologic/geographic environments for hydrate

accumulation: 1) areas with deep water in close proximity to land, and 2) continents in polar regions. Deep-water settings can be further classified as: 1) inland deep-water seas and lakes, 2) stable passive continental margins, 3) unstable passive continental margins, and 4) active tectonic boundaries.

In conventional gas reservoirs, natural gas migrates to the recovery point via pressure gradients. For these gas reservoirs, the recovery rate is a function of the formation permeability and pressure gradients established between the reservoir and extraction well(s).

Natural gas recovery from hydrate-bearing deposits requires additional energy to dissociate the crystalline water lattice that forms the gas hydrate structure. A variety of methods have been proposed for producing natural gas from hydrate deposits:

1) thermal stimulation, where the temperature is increased above the hydrate stability region;

2) depressurization, where the pressure is decreased below the hydrate stability region; 3) chemical injection of inhibitors, where the temperature and pressure conditions for hydrate stability are shifted; and 4) C0₂ or mixed C0₂ and N₂ exchange, where C0₂ and N₂ replace CH₄ in the hydrate structure.

Thermal stimulation requires large amounts of energy. Dissociation of methane hydrate at 15°C requires approximately 10% of the energy in the produced methane, but to transfer the heating fluid downhole to the hydrate formation results in huge heat loss. A method for thermal stimulation is described in US2008/0268300 (Pfefferle). Heated water is transferred via an injection well to a hydrate formation and methane gas is liberated via a production well. The methane is fed to an anode chamber of a fuel cell and an oxidant (air or oxygen) is fed to the cathode chamber. Heat and C0₂ are produced, and the heat is used to heat water and this heated water is fed to the injection well. A method for releasing gas from a gas hydrate without melting the gas hydrate is disclosed in WO06/036575 (Graue). C0₂ is used as a releasing agent and is spontaneously replaced with methane in the hydrate structure. The combination of C0₂ injection and methane dissociation is an energy efficient method, however as both reactions are close to

equilibrium the resulting yield is limited. JP2005139825 describes a method and system where electromagnetic waves and ultrasonic waves are used to defrost a hydrate formation by transfer of heat from said waves to the hydrate formation. The system contains a cracking unit which contains an irradiation means arranged in or near the hydrate formation. A drilling device cuts the gas-hydrate layer, and the cracking unit irradiates the stratum cut material (termed "T"), and a recovery system collects the gas decomposed from the hydrate formation.

It is thus desirable to provide a more efficient and effective method and system for recovering of gas from natural gas hydrates. More specifically, it is desirable to provide a method and system using a combination of ultrasonic waves and heated water to dissociate and liberate gas from a gas hydrate formation. It is the objective of the invention to provide energy and cost efficient method for production of hydrocarbons and/or electricity from methane (gas) hydrate reservoirs. Such a desire to establish a more efficient system and method can be accomplished by the method and system of the present invention.

Summary of the invention

A first aspect of the present invention relates to a method of producing gas from a gas hydrate wherein gas is dissociated and liberated from the hydrate formation by ultrasound and heating, characterized in that the method comprises the following features;

- a heating agent is added through a tube in at least one injection well and contacted to the gas hydrate formation,

- the heating agent dissociates the gas from the gas hydrate formation, and the dissociated gas is led to at least one production well,

- an ultrasonic means applies ultrasonic waves to said gas hydrate formation in an area between the at least one injection well and the at least one production well,

- a pressure gradient is established between the injection well and the production well by the injection of said heating agent in the at least one injection well, and

- gas dissociated and liberated from the hydrate formation is captured in the production well. Preferable, the process steps indicated above define a production phase, and wherein the method thereafter comprises a post-production phase where another gas is injected into the gas-depleted hydrate formation via an injection well for stabilizing the hydrate formation.

Preferable, said heating agent is water, and wherein said water injected to the gas hydrate formation via the injection well is heated to a temperature of at least 10 to 150°C, more preferable 40°C to 150°C, and more preferable 50°C to 120°C, and more preferable 50°C to 70°C, in order to enhance the dissociation and liberation of sad gas from the gas hydrate formation. Preferable, said ultrasonic wave has a frequency in the range of 20 kHz to 200 MHz, more preferable 20 kHz to 200 kHz, and more preferable 20 kHz to 100 kHz, and more preferable 20 kHz to 50 kHz. Preferable, said ultrasonic wave has an intensity in the range of 1 watt/cm² to 10 watts/cm², and more preferable 10 watt/cm² to 100 watts/cm², and more preferable 100 watt/cm² to 1000 watts/cm², and preferable also above 1000 watts/cm².

In a preferred embodiment are the ultrasonic means arranged in one or more injection wells, and the ultrasonic waves are directed to the gas hydrate formation between an injection well and a production well. In a preferred embodiment is the ultrasonic means arranged on top of a sedimentary rock in communication with a well bore which extends into the gas hydrate formation, and wherein the well bore is filled with a liquid or gel, preferable water.

In a preferred embodiment are one or more ultrasonic means arranged in a horizontal well bore drilled between an injection well and a production well, and the ultrasonic means are adapted to be moved through the horizontal well bore.

In a preferred embodiment are one or more ultrasonic means arranged in one or more well bores, and the ultrasonic waves are directed to the area below the wells in order to improve the thermal conductance and convection in the gas hydrate formation and in the area below the gas hydrate formation.

In a preferred embodiment are the ultrasonic waves provided to the layer below the gas hydrate formation.

Preferable, the ultrasonic means is arranged for irradiation of the volume below the gas hydrate formation.

Preferable, the ultrasound is concentrated in selected volumes by a shaped reflector, such as a parabolic reflector, directing the output of the ultrasonic processor means.

Preferable, the ultrasonic wave is adapted to provide ultrasonic cavitation to an area of the gas hydrate formation. Preferable, the ultrasonic wave is adapted to provide heat transfer enhancement by ultrasonic irradiation and cavitation to an area below the gas hydrate formation.

Preferable, the method further comprises;

i) a production phase where the gas of the gas hydrate formation is dissociated and liberated, and wherein said liberated gas is captured via the production well, and ii) a post-production phase where another gas is injected into the gas-depleted hydrate formation via an injection well for stabilizing the hydrate formation. Preferable, said heating agent is hot water or steam, preferable heated sea water.

Preferable, the gas of said gas hydrate formation is methane.

Preferable, a portion of said hydrate formation is heated to a temperature above the methane hydrate equilibrium conditions.

Preferable, the gas injected into the gas-depleted hydrate formation is C0₂.

Preferable, at least 50% of the gases, preferable methane, of the gas hydrate formation is removed in the production phase before the post-production phase is initiated.

Preferable, the C0₂ injection is combined with an active cooling of the C0₂ hydrate formation (44), and wherein the active cooling is preferably provided by adding cold water, and more preferable where the addition of C0₂ and water is in a stoichiometric relation of about 1 to 6.

Preferable, said gas is fed to a fuel cell where the gas is oxidized and produces heat and C0_2.

A second aspect of the invention relates to a method for stabilizing a gas depleted hydrate formation by power ultrasound and/or for enhancing the formation of a gas hydrate formation, wherein ultrasonic waves are applied to, and C0₂ is injected into said gas depleted hydrate formation. Preferable, said ultrasonic waves are provided as power ultrasound, and wherein said power ultrasound is capable of reducing the size of ice crystals in said gas depleted hydrate formation. Preferable, said ultrasonic wave has a frequency in the range of 20 kHz to 200 MHz, more preferable 20 kHz to 200 kHz, and more preferable 20 kHz to 100 kHz, and more preferable 20 kHz to 50 kHz, and/or an intensity in the range of 1 watt/cm² to 10 watts/cm², and more preferable 10 watt/cm² to 100 watts/cm², and more preferable 100 watt/cm² to 1000 watts/cm², and preferable also above 1000 watts/cm².

Preferable, the method further comprising the addition of water to the gas depleted formation.

Preferable, said water has a temperature of less than 10 °C, more preferable less than 5 °C, and more preferable in the range of 0 to 5 °C.

Preferable, wherein the method is used in combination with a method according to the first aspect indicated above.

A third aspect of the invention relates to a system for producing natural gas from a hydrate formation, wherein the system comprises;

- at least one injection well and at least one production well in a predetermined distance extending into a gas hydrate formation,

- at least one tube in one of said wells for provision of a heating agent into said gas hydrate formation,

- at least one ultrasonic means for provision of ultrasonic waves to the hydrate formation in the area between the at least one injection well and the at least one production well

- wherein the heating and irradiation of the gas hydrate formation dissociates the gas from the gas hydrate formation,

- wherein the addition of heating agent via the at least one injection well establishes a pressure gradient between the at least one injection well and the at least one production well, and

- wherein the dissociated gas is captured via the production well. In a preferred embodiment comprises the system at least one injection well for injection of a second gas for stabilization of the hydrate formation.

Preferable, the ultrasonic means is arranged in a distance from the gas hydrate formation.

Preferable, the ultrasonic means is arranged in or above the sedimentary rock above the gas hydrate formation.

Preferable, the ultrasonic means is arranged in a bore or pipe line in the sedimentary rock extending partly towards the gas hydrate formation.

Preferable, the ultrasonic means is arranged in a well bore in the sedimentary rock, wherein the well bore extend into the gas hydrate formation and wherein the well bore is sealed at the top with a cap.

Preferable, the ultrasonic means is arranged in or near the gas hydrate formation.

In a preferred embodiment is the ultrasonic means is arranged for irradiation of the volume below the gas hydrate formation.

In a preferred embodiment are the one or more ultrasonic means arranged in one or more well bores, wherein the ultrasonic waves are directed to the area below the wells in order to improve the thermal conductance and convection in the gas hydrate formation and in the area below the gas hydrate formation.

Preferable, the system comprises a shaped reflector, such as a parabolic reflector, for directing the output of the ultrasonic processor means in selected volumes.

Preferable, said heating agent is water, and wherein said water has a temperature of at least 10 to 150°C, more preferable 40°C to 150°C, and more preferable 50°C to 120°C, and more preferable 50°C to 70°C, in order to enhance the dissociation and liberation of sad gas from the gas hydrate formation. Preferable, the first gas is methane. Preferable, the second gas is C0₂.

Preferable, the system comprises a heat exchanger.

Preferable, the injection well comprises perforations in the region penetrating the hydrate formation.

Preferable, the system comprises a gas capturing means for retrieval of said first gas from the hydrate formation. Preferable, said system is provided by pressure providing means in the injection well and suction means in the production well.

Preferable, the system comprises a fuel cell, and wherein said first gas from the production well is fed to anode chamber of the fuel cell, and wherein air (or preferable oxygen) is fed to the cathode chamber of the fuel cell, and wherein oxygen is transferred across the membrane and oxidizes the gas (methane) into C0₂ and water.

Preferable, C0₂ from the fuel cell, preferable cooled or liquefied, is fed via the injection well to the hydrate formation in in the post-production phase.

Preferable, the C0₂ injection is combined with an active cooling of the hydrate formation, and wherein the active cooling is preferably provided by adding cold water, and more preferable where the addition of C0₂ and water is in a stoichiometric relation of about 1 to 6. Preferable, in a given area, the two phases are run simultaneously, i.e. that some wells are in the production phase and other wells in formations that have been depleted for gas are in the post-production phase.

Preferable, at least two well bores are drilled horizontally, or in an angle with respect to the hydrate formation. Preferable, said ultrasonic wave has a frequency in the range of 20 kHz to 200 MHz, more preferable 20 kHz to 200 kHz, and more preferable 20 kHz to 100 kHz, and more preferable 20 kHz to 50 kHz, and an intensity in the range of 1 watt/cm² to 10 watts/cm², and more preferable 10 watt/cm² to 100 watts/cm², and more preferable 100 watt/cm² to 1000 watts/cm², and preferable also above 1000 watts/cm².

A fourth aspect of the present invention relates to a method of producing gas from a gas hydrate formation, wherein the method comprises;

i) a production phase where the gas of the gas hydrate formation is dissociated and liberated, and wherein said liberated gas is captured via a production well, and ii) a post-production phase where another gas is injected into the gas-depleted hydrate formation via an injection well for stabilizing the hydrate formation. A fifth aspect of the present invention relates to a system for producing gas from a gas hydrate formation, wherein the system comprises;

i) a production phase where the gas of the gas hydrate formation is dissociated and liberated, and wherein said liberated gas is captured via a production well, and ii) a post-production phase where another gas is injected into the gas-depleted hydrate formation via an injection well for stabilizing the gas-depleted hydrate formation.

Brief description of the drawings

Preferred embodiments of the invention are described in detail below with reference to the attached drawings, wherein:

Figure 1 is a schematic drawing of a production system according to the present invention showing ultrasonic means and injection wells and a production well for capturing of methane gas from a methane hydrate formation.

Figure 2 shows a schematic of a well. Figure 3 shows an alternative embodiment where the ultrasonic means are positioned downhole near the methane hydrate formation. Figure 4 shows an alternative embodiment where the ultrasonic means is arranged topsite but where the ultrasonic waves are provided to the gas hydrate formation via a pipeline or well bore filled with liquid.

Figure 5 shows an alternative embodiment where the ultrasonic means are displaced, preferable horizontally, within the well bore in the gas hydrate formation.

Figure 6 shows an embodiment where a number of ultrasonic means are positioned in a number of wells in order to improve the thermal conductance and convection in the gas hydrate formation and the layer below.

Figure 7 shows a system schematic drawing of a preferred embodiment of the invention showing an ultrasonic means and two wells for production of methane gas from a methane hydrate formation, and one well for stabilizing of a gas depleted hydrate formation.

Figure 8 shows a pattern of wells.

Figure 9 shows dissociated area in the initial production phase.

Figure 10 shows heat recovery before the next production phase. Figure 1 1 shows dissociated area in the second production phase. Figure 12 shows dissociated area and C0₂ filled area before the third production phase. Figure 13 shows dissociated area and C0₂ filled area after the third production phase. Description of embodiments of the invention

The method and system according to the invention contains at least two wells and an ultrasonic means for applying ultrasonic waves to a gas hydrate formation between said two wells. Figure 1 shows a schematic overview of a preferred embodiment of a system according to the invention. The system in figure 1 contains at least one injection well 12 and a production well 14, and at least one ultrasonic means 50 for providing ultrasonic waves to the area between the wells 12,14.

The term "heating agent" means an agent, such as a gel or liquid with a temperature higher than the gas hydrate temperature. Preferable is the heating agent a liquid, and preferable is this liquid water or seawater. The temperature of the heating agent is preferable at least 10°C, more preferable at least 15 °C, more preferable at least 20°C.

Figure 2 shows a schematic arrangement of an embodiment of a well 12,14. The wellbore 30 is sealed at the top in the rock formation 40 with a cap 32. The wellbore extends through the impermeable layer 42 into the hydrate formation 44 and optionally into the underlying layer 46. The wellbore has perforations 30a in the hydrate formation 44 and optionally in the underlying layer 46. A tube 34 is inserted into the wellbore 30 and sealed at the top to the cap 32. The tube 34 can be used for water injection, methane production or C0₂ injection. Optionally the well may be equipped with a tube 36 and a pump 38 for pressure control of the water phase.

As explained in more details below, heated water is provided to the hydrate formation, and an ultrasonic means 50 irradiates the hydrate formation between at least one injection well 12 and one production well 14. The combined treatment with a heating agent and ultrasonic waves will heat and crack the gas hydrate formation and provide an enhanced dissociation of gas from the gas hydrate formation 44. The present invention thus relates the combined and/or synergistic action of heating and ultrasonic irradiation of a gas hydrate formation 44 to improve the dissociation and liberation of gas from the hydrate formation 44.

The figures 3-6 show alternative embodiments of the invention, i.e. where the ultrasonic means 50 are arranged at various positions with respect to the gas hydrate formation 44 and the wells 12,14. Figure 3 shows that each injection well 12 can contain an ultrasonic means 50. The ultrasonic means 50 is lowered downhole into the well bore and positioned near the gas hydrate formation 44. The ultrasound is directed towards the volume of the methane hydrate formation where enhanced production rate or yield is desired, thus increasing the production rate in the production well 14 and the total amount of gas that practically can be produced from the well. Figure 4 shows an embodiment where a well bore or a pipe line 51 extends from an ultrasonic means 50 until the gas hydrate formation 33. The well bore or pipe line 51 is filled with a liquid or gel, preferable water. This embodiment allows ultrasonic stimulation of volumes in the methane hydrate formation with the ultrasonic means on top of the solid formation.

Figure 5 shows an embodiment where a horizontal well bore is drilled between an injection well 12 and a production well 14, and where at least one ultrasonic means is arranged within the methane hydrate formation. The ultrasonic means can travel through the well bore and stimulate selected volumes by ultrasonic irradiation.

Figure 6 shows an embodiment where a number of ultrasonic means are positioned in a number of wells in order to improve the thermal conductance and convection in the volume below the gas hydrate formation. The temperature generally increases with increasing depth, and the heat transfer enhancement effect of the ultrasonic irradiation will cause a heating of the methane hydrate layer, reducing the need for heating agent injected in the injection well 12. The ultrasound emitted from the ultrasonic means 50 is in one embodiment of the invention used to transfer heat directly to the hydrate formation 44. The intensity of a plane sound wave travelling in a medium with an absorption constant a is attenuated with increasing distance l(x)=IO * exp(-2ax)

The heat supplied to the medium per unit time and volume is

-dl/dx=Q=2*a*l

The transmission loss (attenuation) of an ultrasonic wave is relatively small, and the wave has sufficient energy to cause an internal heating of the solid (ice) gas hydrate formation and thus to locally defrost portions of the hydrate formation. The defrosting, i.e. melting will enhance the effect of the ultrasonic wave since the capacity to heat is enhanced if water is present. Further irradiating of the gas hydrate 44, preferable in the presents of some water, will decompose the hydrate formation and the entrapped gas will dissociate and liberate from the hydrate formation 44. The ultrasound will also cause local cracking of the hydrate formation 44, and as some of the solid formation has melted, the transport of liquid and gas through the formation 44 will be increased. The flow of liquid and gas will be directed by the pressure gradient established between the at least two well 12,14, and the gas dissociated and liberated from the gas hydrate formation 44 is thus led to the second well bore 14 which is equipped with gas capturing means for capturing of the released gas, preferable methane.

The frequency of the ultrasound wave is adjusted to a range where they are effectively absorbed by the gas hydrate 44, but less effectively absorbed by the water. Such a range of frequencies are preferable 20 kHz to 200 MHz, more preferable 20 kHz to 200 kHz, and more preferable 20 kHz to 100 kHz, and more preferable 20 kHz to 50 kHz. The intensity is preferable in the range of 1 watt/cm² to 10 watts/cm², and more preferable 10 watt/cm² to 100 watts/cm², and more preferable 100 watt/cm² to 1000 watts/cm², and preferable also above 1000 watts/cm².

It is preferred that the ultrasound has a frequency and intensity which enables formation of cavitation air bobbles near the gas hydrate formation. These air bubbles will collapse near the hydrate formation and induce a high local heating (-5000K), high pressures (~1000atm), and liquid jet streams (~400km/hr). The intense energy input, high pressure and temperature transients results in an increase in reaction rate and more complete dissociation of the hydrate otherwise trapped within the hydrate containing particles. Further, ultrasonic cavitation results in significantly improved heat transfer due to improved convection. The transport of micro bubbles within particles as well as the transport of macro bubbles is intensified by the cavitation.

The acoustic field in ice and gas hydrate formations is more complex than in air. Ice is an elastic solid and support two types of waves: a longitudinal or compression wave (which is akin to an acoustic wave) and transverse or shear waves, where the motion of the vibration is transverse to the direction of propagation.

In a shear wave, the transverse vibration does not result in the molecules being compressed and rarefied, but rather they oscillate in a manner analogous to the wave motion of a rope excited by a snap of the wrist. Longitudinal waves and shear waves travel at different speed and the longitudinal wave speed (cL) is always faster than the transverse wave speed (cT). When a shock wave passes from water into ice the transmitted energy is divided between the longitudinal and transverse waves in the ice. The proportion of the energy that each wave gets depends on the material properties of the ice and the angle of incidence. If the wave is normally incident on the ice surface then all the energy is converted into a longitudinal wave in the ice and no energy is available for transverse waves. As the angle of incidence increases less energy is converted into a longitudinal wave and more is converted into transverse waves. The complex shape of natural ice results in a non-trivial partition of energy between the two types of wave. However, it is expected that shock waves will fragment the solid ice or hydrate particles and several mechanisms for this fragmentation have been described. These include spallation and shear stresses, which in combination with the melting of the crystalline phase, will contribute to the fracture of the solid hydrate formation. Also cavitation will enhance the fragmentation of the hydrate formation.

The ultrasonic waves applied to the hydrate formation 44 will thus induce perforations of the solid hydrate formation, and will also enhance the dissociation of the gas from the hydrate formation 44.

A pressure gradient between the well bores 12,14 is established by adding water to the gas hydrate formation 44 via one of the well bores. In the embodiment shown in figure 1 is the well bore 12 used for injection of water, preferable sea water, and the pressure gradient between the well bores moves the water and gas liberated from the hydrate formation 44 to the well bore 14 where the gas is captured. The injection well 12 is equipped with a pipeline 34 for feeding of water to (and from) the hydrate formation 44. The injection well 12 is equipped with perforations 34a in the region penetrating the hydrate formation 44. The perforations can be provided in the complete circumference of the wellbore 30, and the vertical extension of the perforation section can be adapted to the hydrate formation 44. In an alternative embodiment are the perforations 34a only provided in one or more section of the circumference of the wellbore 30. The perforations 34a can be provided in the wellbore 30 after installation of the wellbore 30 in the hydrate formation 44.

The direction of the movement of the water between the wells 12,14 , i.e. from a water injection well 12 to a gas production well 14, is regulated by pressure gradients established by the water injection, and preferable also, optionally, by provision of a pressure pump 26 in the injection well 12 and a suction pump 38 in the production well 14. In a preferred embodiment is the water provided in the injection well 12 heated in order to enhance the dissociation of gas from the hydrate layer, i.e. the liberation of gas is effected by a combination of both thermal heating and irradiation. Thus, a thermal communication is established between the injection well 12 and the production well 14. The thermal connection will transfer heat to the hydrate formation 44 and increase the dissociation of methane from the formation 44.

The combined irradiation and heating of the methane hydrate liberates gaseous methane and/or methane solved in water. By using an inlet temperature of 50°C, in contrast to 15°C as known from the prior art, the yield of CH₄ is increased by three to four times, from about 13% to 57%. In addition, the ultrasonic wave will contribute to the heating, but the ultrasonic waves will also locally crack the hydrate formation 44 and increase the dissociation of methane and the flow of liquid through the formation. The dissociation of gaseous CH₄ from the methane hydrates is an endothermic process. The equilibrium is given by pressure and temperature so that for a given pressure the methane hydrate will dissociate when temperature is raised above the equilibrium temperature.

However, as the methane hydrate may be trapped in small pores the pressure may be locally higher than outside the external pressure. Increasing the temperature significantly above equilibrium temperature for the bulk pressure will therefore increase the yield as well as the dissociation rate.

A combination of ultrasonic waves, temperature and depressurization will reduce the energy needs, but the level of depressurization is limited as it will create instabilities in the sediments. However, it is envisaged that a combination of ultrasonic waves, heating and mild depressurization will enhance the method by reducing the total energy needs.

Another aspects of the present invention relate to a method and system for dissociation and liberation of gas from a gas hydrate formation combined with a stabilization of the gas depleted hydrate formation. The method and system will be elaborated below, but the essential feature is that the method and system consists of two phases, i.e. i) a production phase where the methane is dissociated and liberated from the methane hydrate formation, and methane is captured in high yield, and ii) a post-production phase where the methane depleted hydrate is injected with C0₂ and preferably cooled to produce a C0₂ stabilized hydrate. These aspects four and five are defined in the claims 52 and 53, respectively. A preferred embodiment of the aspect one, two and three of the present invention relates to a combination of this two-phase method and system (aspects four and five) combined with the ultrasonic irradiation of the area between an injection well and a production well (aspects one, two and three).

The method and system according to this preferred embodiment will be explained by the application of a combination of heating and ultrasonic waves for the dissociation and liberation of methane from the hydrate formation (first phase, i.e. production phase), and with the addition of C02 and preferable water for stabilization of the gas depleted hydrate formation (second phase, i.e. the post-production phase).

An embodiment of the system 10 comprises an injection well 12 for injection of hot water, and a production well 14 for recovery of methane and cooled water and an ultrasonic means 50. The water introduced into the injection well is preferably heated in a heat exchanger 16.

In a preferred embodiment, as shown in figure 7, the various wells are connected to a fuel cell 18, and methane gas is fed from the production well 14, preferably via a methane tank (not shown) to an anode chamber 18a, and air (or purified oxygen) is fed to the cathode chamber 18b of the fuel cell 18. The fuel (methane) is oxidized in the anode chamber by oxygen transported through the fuel cell membrane 18c producing C0₂ and H₂0. The produced C0₂ is preferably transported to an intermediate storage tank 20 and injected via the C0₂ injection well 22 into the hydrate formation after completion of the production phase. The C0₂ can preferably be liquefied in a gas liquefication apparatus 24, and supplied to the hydrate formation in either gaseous or liquefied form.

As shown in figure 7, the production phase comprises at least two wells, i.e. a heating agent injection well 12 and a production well 14, and a ultrasonic means for providing of ultrasonic waves to the hydrate formation 44 . Heating agent, preferable heated water, preferable sea water, at a temperature of about 10 to 150°C, more preferable 40°C to 150°C, and more preferable 50°C to 120°C, and more preferable 50°C to 70°C is injected into the hydrate formation 44 via the injection well 12. The injection well 12 is equipped with a pipeline 34 for feeding of heating agent, preferable water to (and from) the hydrate formation 44. The injection well 12 is equipped with perforations 34a in the region penetrating the hydrate formation 44. The perforations can be provided in the complete circumference of the wellbore 30, and the vertical extension of the perforation section can be adapted to the hydrate formation 44. In an alternative embodiment are the perforations 34a only provided in one or more section of the circumference of the wellbore 30. The perforations 34a can be provided in the wellbore 30 after installation of the wellbore 30 in the hydrate formation 44.

As indicated above, the direction of the movement of the water between the various wells 12,14 , i.e. from an injection well 12 to a production well 14, is regulated by pressure gradients provided by injection of water through the injection well 12, and optionally provided by pressure pump 26 in the injection well 12 and a suction pump 38 in the production well 14 so that a thermal communication is established between the injection well 12 and the production well 14. The thermal connection will transfer heat to the hydrate formation 44 and dissociate methane from the formation 44.

The heating and the ultrasonic treatment of the methane hydrate liberates gaseous methane and/or methane solved in water. By using an inlet temperature of 50°C, in contrast to 15°C as known from the prior art, the yield of CH₄ is increased by three to four times, from about 13% to 57%. The dissociation of gaseous CH₄ from the methane hydrates is an endothermic process. The equilibrium is given by pressure and temperature so that for a given pressure the methane hydrate will dissociate when temperature is raised above the equilibrium temperature as given in figure 9. However, as the methane hydrate may be trapped in small pores the pressure may be locally higher than outside the external pressure. Increasing the temperature significantly above equilibrium temperature for the bulk pressure will therefore increase the yield as well as the dissociation rate. A combination of ultrasonic waves, temperature and depressurization will reduce the energy needs, but the level of depressurization is limited as it will create instabilities in the sediments. However, it is envisaged that a combination of heating and mild depressurization will enhance the method by reducing the total energy needs. The second phase of the method, i.e. the post-production phase is initiated when the production of methane from the hydrate is complete, for instance when 50-75 % of the methane has been removed from the hydrate formation 44.

This post-production phase consists of an injection of C0₂ into the hydrate formation 44 via the C0₂ injection well 22. The C0₂ injection can optionally be combined with an active cooling of the hydrate formation 44 by addition of cold water (and C0₂). The C0₂ can be provided from any source but is preferable provided from the bleed stream of the anode fuel cell chamber 18a of the optional fuel cell 18. The C0₂ hydrate will form when the

temperature is lowered below the equilibrium temperature as given in figure 10.

In a preferred embodiment of this post-production phase is the stabilizing of the gas hydrate 44 enhanced by provision of ultrasonic waves to the gas depleted hydrate formation (further explained below). It is important to note that the injection 12 and/or production well 14, after completion of the capturing of methane, can later be used as C0₂ injection wells 22.

A described above, the method and system according to this embodiment of the invention comprises at least two wells (injection and production) in the production phase, and a further well (which can be a former injection well) for stabilizing of the hydrate formation 44 in the post-production phase. Preferable, the system and method contains several wells, and figure 8 shows a possible pattern of wells 30. The pattern may be arranged in different ways, but the arrangement shown in figure 8 is used to illustrate the invention. One injection well 12 is surrounded by a plurality of production wells 14, and one production well 14 is surrounded by a plurality of injection wells 12. The possible arrangement or pattern shown in figure 8 indicates that one injection well 12 is surrounded by 4 production well 14, and vice versa. Such an arrangement gives rows of respective wells 12 and 14, preferable arranged with a displacement between the well in the rows in order to obtain similar distance between the various wells.

Figure 9 shows a pattern of wells 12, 14 and dissociated area 12a in an initial production phase. Hot water is injected into injection wells 12 at a higher pressure than in the production wells 14. Methane hydrate starts to dissociate from the hydrate formation 44 and is forced to the production wells 14 by the established pressure gradient. The ultrasonic waves provided by the ultrasonic means 50 enhance the gas dissociation. The dissociation starts at the injection wells 12 and the dissociated zone 12a extends towards the production wells 14.

Figure 10 shows an optional method for heat recovery. When sufficient amount of hot water has been provided via the injection wells 12 into the hydrate formation 44, it is possible to inject cold water into the injection wells 12. This will force the hot water in the dissociated area to flow towards the production wells 14 and enhance the methane dissociation from the hydrate formation 44.

When the first production phase is ended and the heat recovery is completed, the C0₂ injection phase, i.e. the post-production phase, can start. C0₂ is injected into the C0₂ injection wells 22 and the C0₂ is fed to the methane depleted hydrate formation and will stabilize the hydrate formation 44. As explained above, the injection of C0₂ can use wells that formerly have been uses as productions wells 14 or injection wells 12 in the production phase. In a preferred embodiment are the gas depleted formation irradiated with ultrasonic waves to enhance the stabilization of the solid gas hydrate.

Figure 11 shows dissociated area in the second production phase. A new dissociation zone 12a' extends towards the "new" production wells 14'.

Figure 12 shows dissociated area and C0₂ stabilized hydrate formation of an embodiment where with a first and second production phase and where a post-production phase has been initiated. The injected C0₂, either in liquid or gaseous form, and optionally provided with cooling, will be transferred to the methane depleted hydrate formation and C0₂ will form C0₂ hydrates and stabilize the hydrate formation.

It is important to note that the present invention does not relate to passive replacement of C0₂ for CH₄ in the hydrate formation, but the method is actually an active two phase process where methane is first removed from the hydrate formation 44 and thereafter the C0₂ is injected to re-stabilize the methane depleted hydrate formation 44.

C0₂ is injected at a higher pressure than water. This ensures that all directional flows are in the correct direction driven by the pressure gradient. The pressure gradient is established by the pressures of the water to and from the wells. In preferred embodiments of the invention the pressure gradient is additionally enhanced by the optional provision of pumping 16 and suction means 38 in the wells 12, 14, 22, and/or by provision of perforations in only selected segments of the wellbore 30.

Figure 13 shows dissociated area and C0₂ filled area after the third production phase.

Further injection wells 12" and production wells 14" are activated, and also a larger hydrate formation area has been stabilized by injection of C0₂ through further C0₂ injection wells 22. Please note that the former hot water injection wells 12 are now used as C0₂ injection well 22.

A production process is initiated by drilling at least two wells. At least one of these wells 12 is for injection of hot water, and at least one well 14 is for production of methane. The area between the wells is irradiated with ultrasonic waves. After completion of the production phase, i.e. sufficient methane is captured, new wells are drilled. These new wells can be used as new water injection and production wells (production phase), whereas the former wells can be used as C0₂ injection wells (post-production wells). A further aspect of the present invention relates to stabilization of a gas depleted hydrate formation by applying ultrasound to the formation. The provision of power ultrasound is used to reduce the size of ice crystals on the frozen solid formation. This leads to finest ice crystals and shortens the time between the onset of crystallization and the complete formation of the solid ice formation, mainly due to acoustic cavitation. This results in a more homogenous formation of C0₂ hydrate and improved structural stability of the hydrate layer. Suitable frequencies and intensities are used. The ultrasonic waves can be provided by the ultrasonic means 50 shown in figure 1 , and 3-6, or the method and system can comprises several ultrasonic means 50. In a preferred embodiment of the invention are the at least two well bores drilled horizontally, or in an angle with respect to the hydrate formation.

Claims

1. A method of producing gas from a gas hydrate formation (44), wherein the method comprises the following features;

- a heating agent is added through a tube (34) in at least one injection well (12) and contacted to the gas hydrate formation (44),

- the heating agent dissociates the gas from the gas hydrate formation, and the dissociated gas is led to at least one production well (14)

- an ultrasonic means (50) applies ultrasonic waves to said gas hydrate formation (44) in an area between the at least one injection well (12) and the at least one production well (14,

- a pressure gradient is established between the injection well (12) and the production well (14) by the injection of said heating agent in the at least one injection well (12), and

- gas dissociated and liberated from the hydrate formation (44) is captured in the production well (14).

2. A method according to claim 1 , wherein the process steps of claim 1 defines a production phase, and wherein the method thereafter comprises a post-production phase where another gas is injected into the gas-depleted hydrate formation (44) via an injection well (22) for stabilizing the hydrate formation (44).

3. A method according to claim 1 , wherein said heating agent is water, and wherein said water injected to the gas hydrate formation (44) via the injection well (12) is heated to a temperature of at least 10 to 150°C, more preferable 40°C to 150°C, and more preferable 50°C to 120°C, and more preferable 50°C to 70°C, in order to enhance the dissociation and liberation of sad gas from the gas hydrate formation (44).

4. A method according to claim 1 , wherein said ultrasonic wave has a frequency in the range of 20 kHz to 200 MHz, more preferable 20 kHz to 200 kHz, and more preferable 20 kHz to 100 kHz, and more preferable 20 kHz to 50 kHz.

5. A method according to claim 1 , wherein said ultrasonic wave has an intensity in the range of 1 watt/cm² to 10 watts/cm², and more preferable 10 watt/cm² to 100 watts/cm², and more preferable 100 watt/cm² to 1000 watts/cm², and preferable also above 1000 watts/cm².

6. A method according to claim 1 , wherein the ultrasonic means (50) is arranged in one or more injection wells (12), and wherein the ultrasonic waves are directed to the gas hydrate formation (44) between an injection well (12) and a production well (14).

7. A method according to claim 1 , wherein the ultrasonic means (50) is arranged on top of a sedimentary rock in communication with a well bore (51) which extends into the gas hydrate formation (44), and wherein the well bore (51) is filled with a liquid or gel, preferable water.

8. A method according to claim 1 , wherein one or more ultrasonic means (50) are arranged in a horizontal well bore drilled between an injection well (12) and a production well (14), and wherein the ultrasonic means (50) are adapted to be moved through the horizontal well bore.

9. A method according to claim 1 , wherein one or more ultrasonic means (50) are arranged in one or more well bores (12, 14), wherein the ultrasonic waves are directed to the area below the wells in order to improve the thermal conductance and convection in the gas hydrate formation (44) and in the area below the gas hydrate formation (44).

10. A method according to claim 9, wherein the ultrasonic waves (44) are provided to the layer below the gas hydrate formation (44).

11. A method in accordance with claim 1 , wherein the ultrasound is concentrated in selected volumes by a shaped reflector, such as a parabolic reflector, directing the output of the ultrasonic processor means (50).

12. A method in accordance with claim 1 , wherein the ultrasonic wave is adapted to provide ultrasonic cavitation to an area of the gas hydrate formation (44).

13. A method in accordance with claim 1 , wherein the ultrasonic wave is adapted to provide heat transfer enhancement by ultrasonic irradiation and cavitation to an area below the gas hydrate formation.

14. A method in accordance with claim 1 , wherein the method further comprises;

i) a production phase where the gas of the gas hydrate formation (44) is dissociated and liberated, and wherein said liberated gas is captured via the production well (14), and

ii) a post-production phase where another gas is injected into the gas-depleted hydrate formation (44) via an injection well (22) for stabilizing the hydrate formation (44).

15. A method according to claim 1 , wherein said heating agent is hot water or steam, preferable heated sea water.

16. A method according to claim 1 , wherein the gas of said gas hydrate formation (44) is methane.

17. A method according to claim 16, wherein a portion of said hydrate formation (44) is heated to a temperature above the methane hydrate equilibrium conditions.

18. A method according to claim 14, wherein the gas injected into the gas-depleted hydrate formation (44) is C0₂.

19. A method in accordance with any of the preceding claims, wherein at least 50% of the gases, preferable methane, of the gas hydrate formation is removed in the production phase before the post-production phase is initiated.

20. A method in accordance with claim 18, wherein the C0₂ injection is combined with an active cooling of the C0₂ hydrate formation (44), and wherein the active cooling is preferably provided by adding cold water, and more preferable where the addition of C0₂ and water is in a stoichiometric relation of about 1 to 6.

21. Method in accordance with any of the preceding claims, wherein said gas is fed to a fuel cell (18) where the gas is oxidized and produces heat and C0_2.

22. A method for stabilization of a gas depleted hydrate formation and/or for enhancing the formation of a gas hydrate formation, wherein ultrasonic waves are applied to, and C0₂ is injected into said gas depleted hydrate formation.

23. A method according to claim 22, wherein said ultrasonic waves are provided as power ultrasound, and wherein said power ultrasound is capable of reducing the size of ice crystals in said gas depleted hydrate formation.

24. A method according to claim 22, wherein said ultrasonic wave has a frequency in the range of 20 kHz to 200 MHz, more preferable 20 kHz to 200 kHz, and more preferable 20 kHz to 100 kHz, and more preferable 20 kHz to 50 kHz, and/or an intensity in the range of 1 watt/cm² to 10 watts/cm², and more preferable 10 watt/cm² to 100 watts/cm², and more preferable 100 watt/cm² to 1000 watts/cm², and preferable also above 1000 watts/cm².

25. A method according to claim 22, wherein the method further comprising the addition of water to the gas depleted formation (44).

26. A method according to claim 22, wherein said water has a temperature of less than 10 °C, more preferable less than 5 °C, and more preferable in the range of 0 to 5 °C.

27. A method according to any of the claim 22 to 26, wherein the method is used in a method in accordance with any of the claims 1-21.

28. A system for producing natural gas from a hydrate formation (44), wherein the system comprises;

- at least one injection well (12) and at least one production well (14) in a predetermined distance extending into a gas hydrate formation (44),

- at least one tube (34) in one of said wells (12) for provision of a heating agent into said gas hydrate formation (44),

- at least one ultrasonic means (50) for provision of ultrasonic waves to the hydrate formation (44) in the area between the at least one injection well (12) and the at least one production well (14)

- wherein the heating and irradiation of the gas hydrate formation (44) dissociates the gas from the gas hydrate formation,

- wherein the addition of heating agent via the at least one injection well (12) establishes a pressure gradient between the at least one injection well (12) and the at least one production well (14), and

- wherein the dissociated gas is captured via the production well (14).

29. A system in accordance with claim 28, wherein the system further comprises at least one injection well (22) for injection of a second gas for stabilization of the hydrate formation (44).

30. A system in accordance with claim 28, wherein the ultrasonic means (50) is arranged in a distance from the gas hydrate formation (44).

31. A system in accordance with claim 30, wherein the ultrasonic means (50) is arranged in or above the sedimentary rock above the gas hydrate formation (44).

32. A system in accordance with claim 31 , wherein the ultrasonic means (50) is arranged in a bore or pipe line (51) in the sedimentary rock extending partly towards the gas hydrate formation (44).

33. A system in accordance with claim 31 , wherein the ultrasonic means (50) is arranged in a well bore (12, 14) in the sedimentary rock, wherein the well bore (12, 14) extend into the gas hydrate formation (44) and wherein the well bore (12, 14) is sealed at the top with a cap (32).

34. A system in accordance with claim 28, wherein the ultrasonic means (50) is arranged in or near the gas hydrate formation (44).

35. A system according to claim 28, wherein the ultrasonic means (50) is arranged for irradiation of the volume below the gas hydrate formation (44).

36. A system according to claim 28, wherein one or more ultrasonic means (50) are arranged in one or more well bores (12, 14), wherein the ultrasonic waves are directed to the area below the wells in order to improve the thermal conductance and convection in the gas hydrate formation (44) and in the area below the gas hydrate formation (44).

37. A system according to claim 36, wherein the ultrasonic waves are provided to the layer below the gas hydrate formation (44).

38. A system according to claim 28, wherein the system comprises a shaped reflector, such as a parabolic reflector, for directing the output of the ultrasonic processor means (50) in selected volumes.

39. A system according to claim 28, wherein said heating agent is water, and wherein said water has a temperature of at least 10 to 150°C, more preferable 40°C to 150°C, and more preferable 50°C to 120°C, and more preferable 50°C to 70°C, in order to enhance the dissociation and liberation of sad gas from the gas hydrate formation (44).

40. A system in accordance with claim 28, wherein the first gas is methane.

41. A system in accordance with claim 29, wherein the second gas is C0₂.

42. A system in accordance with claim28 or 29, wherein the system comprises a heat exchanger (16).

43. A system in accordance with claim 28, wherein the injection well (12) comprises perforations (34a) in the region penetrating the hydrate formation (44).

44. A system in accordance with claim 28, wherein the system comprises a gas capturing means for retrieval of said first gas from the hydrate formation (44).

45. A system in accordance with claim 28, wherein said system is provided by pressure providing means (26) in the injection well (12) and suction means (38) in the production well (14).

46. System in accordance with claim 28 or 29, wherein the system comprises a fuel cell (18), and wherein said first gas from the production well (14) is fed to anode chamber (18a) of the fuel cell (18), and wherein air (or preferable oxygen) is fed to the cathode chamber (18b) of the fuel cell (18), and wherein oxygen is transferred across the membrane (18c) and oxidizes the gas (methane) into C0₂ and water.

47. System in accordance with claim 28, wherein C0₂ from the fuel cell (18), preferable cooled or liquefied, is fed via the injection well (22) to the hydrate formation (44) in in the post-production phase.

48. System in accordance with claim 47, wherein the C0₂ injection is combined with an active cooling of the hydrate formation (44), and wherein the active cooling is preferably provided by adding cold water, and more preferable where the addition of C0₂ and water is in a stoichiometric relation of about 1 to 6.

49. System in accordance with claim 29, wherein, in a given area, the two phases are run simultaneously, i.e. that some wells are in the production phase and other wells in formations that have been depleted for gas are in the post-production phase.

50. System in accordance with claim 28, wherein at least two well bores are drilled horizontally, or in an angle with respect to the hydrate formation (44).

51. System in accordance with any of the claims 28-50, wherein said ultrasonic wave has a frequency in the range of 20 kHz to 200 MHz, more preferable 20 kHz to 200 kHz, and more preferable 20 kHz to 100 kHz, and more preferable 20 kHz to 50 kHz, and an intensity in the range of 1 watt/cm² to 10 watts/cm², and more preferable 10 watt/cm² to 100 watts/cm², and more preferable 100 watt/cm² to 1000 watts/cm², and preferable also above 1000 watts/cm².

52. A method of producing gas from a gas hydrate formation (44), wherein the method comprises;

i) a production phase where the gas of the gas hydrate formation is dissociated and liberated, and wherein said liberated gas is captured via a production well (14), and ii) a post-production phase where another gas is injected into the gas-depleted hydrate formation (44) via an injection well (22) for stabilizing the hydrate formation (44).

53. A system for producing gas from a gas hydrate formation (44), wherein the system comprises;

i) a production phase where the gas of the gas hydrate formation is dissociated and liberated, and wherein said liberated gas is captured via a production well (14), and ii) a post-production phase where another gas is injected into the gas-depleted hydrate formation (44) via an injection well (22) for stabilizing the gas-depleted hydrate formation (44).