Device and method for exploiting natural gas hydrate through offshore wind-wave-light multi-energy combined power generation and collaborative depressurization

Abstract

The invention relates to a device and method for exploiting natural gas hydrate through offshore wind-wave-light multi-energy combined power generation and depressurization cooperation, and belongs to the field of ocean natural gas hydrate resource development. The device comprises an underground wind-wave-light multi-energy combined power generation device, an energy storage heating device and a central control device; the underground wind-wave-light multi-energy combined power generation device comprises a photovoltaic panel, a vertical axis fan and an oscillating water column type power generator; the energy storage heating device comprises an electric energy storage device, a voltage converter and an underground heater; the central control device comprises a computer, a wellhead pressure adjusting signal executing mechanism, an electric energy adjusting signal executing mechanism and a wellhead. The computer transmits wellhead pressure and electric energy adjusting information through the real-time signal optical cable, and real-time adjustment of the wellhead pressure and power supply of the underground heater is achieved. Heat absorbed by hydrate decomposition is supplemented to a hydrate stratum in an electric heating mode, the temperature of the hydrate stratum is increased, and guarantee is provided for efficient development of marine natural gas hydrates.

Description

Technical Field

[0001] The present invention relates to a device and method for jointly reducing pressure and exploiting natural gas hydrate by combining offshore wind, wave and solar energy, and belongs to the technical field of development of marine natural gas hydrate resources. Background Art

[0002] As a potential clean energy, marine natural gas hydrate is an energy source that countries around the world are competing to seize. The deep-water natural gas hydrate reservoirs in the South China Sea of our country are characterized by high shale content, weak cementation strength, low permeability, etc., and face the problem of low production during the exploitation process of hydrates. The current main hydrate exploitation methods include pressure reduction method, heating method, chemical agent injection method, CO 2 replacement method, solid fluidization method, dissolution method, etc. Up to now, our country has carried out 3 offshore natural gas hydrate trial production projects using the pressure reduction method. The highest daily average gas volume has reached 28,700 cubic meters per day, and the highest total gas production has reached 861,400 cubic meters. However, there is still a certain gap from the production threshold for commercial exploitation of hydrates. Therefore, exploring a new method for realizing commercial development of marine natural gas hydrate is a bottleneck that needs to be urgently broken through. In order to increase the production of hydrates during exploitation, combining different exploitation methods is an important way. Among them, combining the pressure reduction method and the heating method is a relatively promising combined exploitation method, but it faces difficulties such as large heating power demand, small heating range, and high cost.

[0003] Offshore wind energy, wave energy and solar energy resources are rich, and they are all marine renewable energy sources with considerable reserves. Wind energy has advantages such as large energy flux density and good stability. Wave energy has advantages such as good quality, good stability, relatively large energy flux density, and wide distribution. Solar energy has advantages such as large energy flux density and wide distribution. If any one of the above-mentioned energies is developed alone, it will face the problem of relatively high operation and maintenance costs. Therefore, by reasonably allocating and jointly developing the three energies, the synergistic effect of the three can be exerted, which can not only share the infrastructure construction, operation and maintenance costs, but also improve the stability of the overall power output and optimize the power quality. This can provide a stable source of heat required for heating during the exploitation of marine natural gas hydrates and promote the development of marine natural gas hydrate resource development technology.

[0004] In summary, there is currently a lack of a high-yield exploitation method for natural gas hydrates, which is the key difficulty restricting the commercial exploitation of natural gas hydrates. Therefore, the present invention is proposed. Summary of the Invention

[0005] In view of the deficiencies of the prior art, especially the problems of low production and short duration existing in the existing hydrate extraction methods, by combining wind-wave-photovoltaic multi-energy combined power generation, the heat absorbed by the decomposition of hydrates in the hydrate formation is supplemented by electric heating to increase the temperature of the hydrate formation, and the pressure reduction extraction method of natural gas hydrates is synergistically used, and a corresponding device for offshore wind-wave-photovoltaic multi-energy combined power generation and synergistic pressure reduction extraction of natural gas hydrates is invented to provide a guarantee for the efficient development of marine natural gas hydrates.

[0006] The present invention adopts the following technical solutions:

[0007] A device for offshore wind-wave-photovoltaic multi-energy combined power generation and synergistic pressure reduction extraction of natural gas hydrates, comprising an underground wind-wave-photovoltaic multi-energy combined power generation device, an energy storage heating device, and a central control device;

[0008] The underground wind-wave-photovoltaic multi-energy combined power generation device includes photovoltaic panels, vertical-axis wind turbines, and oscillating water column generators; the photovoltaic panels are installed on the top of the living area and around the platform of the semi-submersible drilling platform, and generate electricity by absorbing solar energy; the vertical-axis wind turbines are installed around the semi-submersible drilling platform, and the wind turbines rotate under the action of wind to generate electricity; the oscillating water column generator is installed on the sea surface around the semi-submersible drilling platform, and the water column in the air chamber oscillates driven by the wave motion, and the reciprocating airflow drives the air turbine to rotate and generate electricity;

[0009] The energy storage heating device includes an electric energy storage device, a voltage converter, and an underground heater; the electric energy storage device on the semi-submersible drilling platform is connected to the power generation devices of the photovoltaic panels, vertical-axis wind turbines, and oscillating water column generators, and the electric energy generated by the three is first stored in the electric energy storage device; the electric energy storage device is successively connected to an electric energy regulation signal execution mechanism and a computer. On the one hand, the electric energy regulation signal execution mechanism receives the execution instructions of the computer, and on the other hand, according to the instructions, it transmits the electric energy in the electric energy storage device to the underground heater; the underground heater generates heat under the supply of electric energy, and then heats the surrounding formation to supplement the temperature loss caused by the absorption of heat by the decomposition of hydrates, realizing the increase in production of natural gas hydrates;

[0010] The central control device includes a computer, a wellhead pressure regulation signal execution mechanism, an electric energy regulation signal execution mechanism, and a wellhead; the computer is respectively connected to the wellhead pressure regulation signal execution mechanism and the electric energy regulation signal execution mechanism, the wellhead pressure regulation signal execution mechanism is connected to the wellhead, the electric energy regulation signal execution mechanism is connected to the voltage converter, the voltage converter is connected to the underground heater through a cable, and the computer transmits the wellhead pressure and electric energy regulation information through a real-time signal optical cable to realize the real-time regulation of the wellhead pressure and the power supply of the underground heater.

[0011] A working method of the above device for combined offshore wind-wave-solar power generation and synergistic pressure-reducing exploitation of natural gas hydrates, comprising:

[0012] The photovoltaic panels installed on the semi-submersible drilling platform absorb sunlight for power generation, the vertical-axis wind turbines rotate under the action of wind for power generation, and the oscillating water column generator on the sea surface generates electricity under the action of waves; the electricity generated by wind, waves and sunlight is on the one hand directly transmitted to the voltage converter through the cable, and after voltage conversion, it is transmitted to the downhole heater at the bottom of the well through the cable, and the formation near the bottom of the well is heated by electric heating to make up for the heat loss caused by the decomposition of hydrates; on the other hand, during the peak power generation season, when the electricity generated by light, waves and wind is more than the heating demand of the downhole heater, it can be stored in the electric energy storage device on the platform through cable transmission. In this way, the power generation demand of the downhole heater in the off-peak power generation season can be supplemented, and the excess electric energy can also be used for lighting and other needs on the platform; the computer is connected to the wellhead pressure regulating signal actuator and the voltage converter through the signal transmission optical cable, and can control the heating power of the downhole heater in real time, and at the same time regulate the pressure of the wellhead in real time, so as to control the decomposition rate of hydrates in the formation (i.e., the production of hydrates), and then increase the production of natural gas hydrates.

[0013] Preferably, the specific steps are as follows:

[0014] (1) Combined wind-wave-solar power generation

[0015] Install an oscillating water column generator in the waters near the semi-submersible drilling platform to make the most of the incoming sea waves and adapt to the tidal difference change, and the power generation device oscillates under the action of waves to generate electricity; install vertical-axis wind turbines around the semi-submersible drilling platform to improve the platform utilization rate in a limited space, and the wind turbines rotate under the action of wind to generate electricity; install photovoltaic panels on the top of the living area and around the semi-submersible drilling platform to maximize the solar energy absorption area, and the solar panels generate electricity under sunlight irradiation;

[0016] (2) Conversion and storage of combined wind-wave-solar energy

[0017] The electric energy generated by the oscillation of the oscillating water column generator, the rotation of the vertical-axis wind turbine and the irradiation of the photovoltaic panel are respectively transmitted to the electric energy storage device through the transmission cable, and then converted into electric energy with a stable voltage through the voltage converter and transmitted to the downhole heater, and the remaining electric energy is stored in the electric energy storage device and used for the platform;

[0018] (3) Synergistic exploitation of hydrates by pressure reduction and electric heating

[0019] By controlling the wellhead pressure to adjust the bottomhole pressure, the hydrate in the reservoir is decomposed into gas and water for production (during the hydrate production process, the bottomhole pressure is affected by the change of the wellhead pressure. The bottomhole pressure = wellhead pressure + the gravity of the fluid in the wellbore + the fluid friction in the wellbore. Once the wellhead pressure changes, the bottomhole pressure will change accordingly; the basis for adjustment is to control the bottomhole pressure to be lower than the decomposition pressure of the hydrate in the reservoir by adjusting the wellhead pressure, so as to ensure that the hydrate can be decomposed). At the same time, downhole heaters are installed on the wall of the production string in the bottomhole section of the well. A stable voltage is provided to the bottomhole through the electric energy storage device on the platform, so as to promote the downhole heater to heat up and provide a stable heat source for the hydrate production layer. On the one hand, it eliminates the problem of secondary hydrate formation caused by the decrease of the reservoir temperature due to heat absorption during depressurization production. On the other hand, by increasing the reservoir temperature, it improves the decomposition and production rate of hydrates, and thus increases the production of natural gas hydrates in the sea area.

[0020] Preferably, in step (1), vertical axis wind turbines are installed around the semi-submersible drilling platform to improve the platform utilization rate in a limited space and generate electricity under the action of wind. The rotation axis of the vertical axis wind turbine is parallel to the blade and perpendicular to the ground. Compared with the horizontal axis generator, its advantages are that there is no requirement for the wind direction (that is, it can receive wind from multiple directions), the blade rotation space is small, the wind resistance ability is strong (it can resist typhoons of level 12-14), the starting wind speed is small, it is convenient to maintain and is less affected by the wake effect, and no additional yaw device is required, and the maintenance cost is low, which is suitable for offshore installation. The relationship between the output power of the vertical axis wind turbine generator set and the actual wind speed can be expressed as:

[0021]

[0022] Among them, P w is the wind energy of the air flowing through the cross-sectional area of the wind turbine impeller per second, that is, the power of the wind turbine, W; C p is the wind energy utilization rate of the impeller, %; η m is the power coefficient of the transmission system (generally 0.90-0.95); η e is the power coefficient of the generator (generally 0.95-0.98); ρ g is the air density, kg / m 3 ; A is the area swept by the wind turbine impeller rotating one week, m 2 ; V is the wind speed, m / s.

[0023] Preferably, in step (1), the photovoltaic panel is a semiconductor device that converts light energy into electrical energy. When sunlight shines on the photovoltaic panel, the energy contained in the photons is absorbed by the photovoltaic panel, generating the photovoltaic effect. After connecting the two poles of the photovoltaic panel with a wire to form a loop, a photocurrent is formed. The present invention proposes to install photovoltaic panels on the top of the living area and around the platform of a semi-submersible drilling rig to maximize the solar energy absorption area, and the photovoltaic panels generate electricity under sunlight irradiation; the total power output by the photovoltaic panel power generation array can be calculated by the following formula:

[0024] P a =S·cosθ·η·F·A a (2)

[0025] In the formula, P a is the total power output by the photovoltaic panel, W; S is the light intensity, with the unit of W / m 2 ; θ is the incident angle of the photovoltaic panel, °; η is the efficiency of the photovoltaic panel, dimensionless; F is the sum of the design and attenuation coefficients of the photovoltaic panel power generation array, dimensionless; A a is the total area of the photovoltaic panel power generation array, m 2 .

[0026] Preferably, in step (1), the wave energy in the sea has the advantages of large reserves, high energy flux density, high reliability, and small environmental impact, and has great development potential. According to the wave energy capture principle, wave energy utilization devices can be divided into three categories: oscillating water column type, concentrating wave overtopping type, and oscillating body type. In view of the natural gas hydrate mining scenario in the sea area, the present invention selects the oscillating water column type device. This type of device drives the water column in the air chamber to oscillate through wave motion, and the reciprocating air flow generated drives the air turbine to rotate and generate electricity. This type of device has the advantages of simple structure and high reliability. The power generation power of the oscillating water column type generator can be expressed as the superposition of the time series single wave power within the wave energy capture width:

[0027]

[0028] In the formula, P wal is the power generation power of the oscillating water column type generator, W; b is the capture width of the oscillating water column type generator; n is the total number of waves; P is the single wave power, and P i is the single wave power of the i-th sea wave, W;

[0029] Since the natural gas hydrate mining sea area is mainly located in deep water areas, and the sea water depth h is greater than 0.5 times the wave length L (h≥0.5L), the single wave power of the oscillating water column type generator can be expressed as:

[0030]

[0031] In the formula, g is the acceleration of gravity, generally taken as 9.8m / s 2; H is the wave height of the ocean wave, in m; T is the wave period of the ocean wave.

[0032] Preferably, in step (2), the electric energy generated by the rotation of the vertical-axis wind turbine, the irradiation of the photovoltaic panel, and the oscillation of the oscillating water column type power generator is respectively transmitted to the electric energy storage device on the platform through transmission cables. The electric energy therein is converted into electric energy with a stable voltage through a voltage converter and then transmitted to the respective power consumption areas on the platform. Since the wind-wave-light power generation varies under different seasons and sea conditions, it is necessary to adjust the power generation of each device in real time according to the actual situation. When the output power of each power generation device is higher than the power required by the downhole heater, the electric energy storage device absorbs the excess power and stores it. In the case of the saturation of the electric energy storage device, the remaining power can be used for platform lighting, etc.; when the output power of each power generation device is lower than the power required by the downhole heater, the electric energy storage device compensates for the shortage of the power required by the downhole heater, thereby providing a stable power source for hydrate development.

[0033] Preferably, in step (3), the wellhead pressure is controlled by a computer to adjust the bottomhole pressure so that the pressure in the reservoir is lower than the hydrate stability pressure, and then the hydrate in the reservoir decomposes into gas and water and is produced by the production string; the hydrate decomposition rate in the reservoir can be obtained from the following formula:

[0034] m g =k d M g A s (p e -p g ) (5)

[0035] In the formula, m g is the gas production rate, in kg·m -3 ·s -1 ; M g is the molar mass of methane, in kg·mol -1 ; k d is the hydrate decomposition rate constant, in mol·m -2 ·Pa -1 ·s -1 ; A s is the surface area of hydrate decomposition per unit volume (SRSA), in m -1 ; p e and p g are the three-phase equilibrium pressure and the gas pressure in the two-phase flow, respectively, in Pa, and p e -p g is the driving force for the hydrate decomposition reaction, in Pa;

[0036] The hydrate decomposition rate constant k d is a key parameter affecting hydrate decomposition and can be calculated from the following formula:

[0037]

[0038] In the formula, k 0 is the hydrate decomposition rate constant, taking 3.6×10 4 mol·m -2 ·Pa -1 ·s -1 ; R is the gas constant, J·mol -1 ·K -1 ; T is the temperature, K; ΔE a is the activation energy, J, and ΔE a / R = 9752.73K;

[0039] The decomposition of natural gas hydrate is an endothermic process and can only continue under the condition that the surrounding environment provides sufficient heat. The decomposition rate of natural gas hydrate depends on the local temperature. The higher the temperature, the faster the decomposition rate. Therefore, the present invention proposes to install an electric heater on the wall surface of the production string in the bottom hole section, and supply a stable voltage to the bottom hole through the electric energy storage device on the platform to promote the electric heater to heat up and provide a stable heat source for the hydrate reservoir. On the one hand, it can eliminate the problem of secondary formation of hydrate caused by the reduction of reservoir temperature due to endothermic during pressure reduction production. On the other hand, by increasing the reservoir temperature and cooperating with pressure reduction production, the decomposition and production rate of hydrate can be increased, and thus the production of natural gas hydrate in the sea area can be improved.

[0040] Due to the existence of the downhole heater, heat will be continuously provided to the surrounding formation and make up for the heat absorbed by the decomposition of hydrate, thereby ensuring that a relatively high hydrate decomposition rate can be maintained in the formation; the temperature distribution in the formation under the conditions of hydrate pressure reduction decomposition and downhole heating can be calculated by the following formula:

[0041]

[0042] In the formula, T is the temperature, K; v g and v w are the Darcy velocities of methane gas and water respectively, m·s -1 ; is the absolute porosity, dimensionless; S g , S w , S h are the saturations of methane gas, water and hydrate respectively, dimensionless; C g , C w , C h are the specific heat capacities of methane gas, water, hydrate and skeleton respectively, J·kg -1 ·K -1 ; ρ g , ρ w , ρ hare the densities of methane gas, water, hydrate, and the framework, kg·m -3 ; λ eq is the equivalent heat transfer coefficient, W·m -1 ·K -1 ; λ g , λ w , λ h are the heat transfer coefficients of methane, water, hydrate, and the framework, respectively, W·m -1 ·K -1 ; σ g is the Joule - Thomson coefficient, J / (kg·Pa); ΔH D is the endothermic rate of hydrate decomposition, J·kg -3 ; Q a is the heat provided by the downhole heater, W;

[0043] is a general mathematical symbol representing the gradient of a vector. For example, represents the gradient of the temperature field; Δp g is the gradient of the gas pore pressure, MPa; Q is the heat exchange rate, W; (ρC) eq is the average value of the product of density and specific heat capacity, J·m -3 .K -3 ; m h is the hydrate decomposition rate, kg / s;

[0044] In addition, during the production process of natural gas hydrates, the computer analyzes the decomposition situation of natural gas hydrates in real - time through the monitoring of well - head flow data (temperature, pressure, flow rate, etc.), and accordingly adjusts the well - head pressure and the heating power of the downhole heater to maintain the production of hydrates at a relatively high level for a long time.

[0045] Preferably, the well - head flow data such as temperature, pressure, and flow rate monitored by the well - head monitoring module can reflect the decomposition situation of hydrates at the bottom of the well. That is, the higher the gas flow rate, the greater the decomposition rate of hydrates at the bottom of the well; when the gas flow rate data monitored at the well - head starts to decrease, it indicates that the decomposition rate of hydrates at the bottom of the well decreases. Once the production demand cannot be met, it means that the current well - head pressure and the power of the downhole heater need to be adjusted. By reducing the well - head pressure and increasing the power of the downhole heater, the decomposition rate of hydrates can be increased together, thereby increasing the hydrate decomposition rate.

[0046] For details not elaborated in the present invention, reference can be made to the prior art.

[0047] The beneficial effects of the present invention are:

[0048] In view of the low production characteristics faced in the exploitation of deep - water natural gas hydrates, the present invention proposes to jointly generate electricity through offshore wind - wave - solar energy and synergistically reduce pressure to exploit natural gas hydrates. On the premise of using the pressure - reduction method to exploit hydrates, by combining wind - wave - solar multi - energy combined power generation, the heat absorbed by the decomposition of hydrates in the hydrate formation is supplemented in the form of electric heating, the temperature of the hydrate formation is increased, and thus the production of natural gas hydrates is increased. At the same time, wind - wave - solar multi - energy combined power generation has the advantages of environmental protection, and through energy storage devices, it can store electricity when there is a surplus and supplement electricity when there is a shortage. Moreover, the excess electricity can also be used for platform lighting, etc., which can ensure the long - term stable supply of heat required for downhole heating, thus providing guarantee for the long - term and efficient development of marine natural gas hydrates. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The schematic diagrams of the specification forming a part of this application are used to provide a further understanding of this application. The illustrative embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation of this application.

[0050] Figure 1 It is a schematic diagram of the device for jointly generating electricity through offshore wind - wave - solar energy and synergistically reducing pressure to exploit natural gas hydrates according to the present invention;

[0051] In the figure, 1. computer; 2. wellhead pressure regulation signal actuator; 3. wellhead; 4. electric energy regulation signal actuator; 5. voltage converter; 6. wellhead monitoring module; 7. photovoltaic panel; 8. vertical - axis wind turbine; 9. electric energy energy storage device; 10. oscillating - water - column power generator; 11. semi - submersible drilling platform; 12. sea level; 13. production string; 14. blowout preventer; 15. seawater; 16. cement sheath; 17. formation; 18. downhole heater. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0052] In order to enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of this invention are clearly and completely described below in conjunction with the accompanying drawings in the embodiments of this specification. However, it is not limited to this. For those parts not elaborated in detail in the present invention, they are all conventional techniques in the art.

[0053] Embodiment 1

[0054] A device for jointly generating electricity through offshore wind - wave - solar energy and synergistically reducing pressure to exploit natural gas hydrates, as Figure 1 shown, includes a downhole wind - wave - solar multi - energy combined power generation device, an energy storage heating device, and a central control device. Below the sea level 12 is seawater 15, and at the bottom of the seawater 15 is the formation 17. The production string 13 extends from above the sea level to the formation, and a cement sheath 16 and a blowout preventer 14 are provided on the production string 13.

[0055] The downhole wind-wave-light multi-energy combined power generation device includes a photovoltaic panel 7, a vertical-axis wind turbine 8, and an oscillating water column power generator 10; the photovoltaic panel 7 is installed on the top of the living area and around the platform of the semi-submersible drilling platform 11, and generates electricity by absorbing solar energy; the vertical-axis wind turbine 8 is installed around the semi-submersible drilling platform 11, and the wind turbine rotates under the action of wind to generate electricity; the oscillating water column power generator 10 is installed on the sea surface around the semi-submersible drilling platform 11, and the water column in the air chamber is oscillated by the wave motion, and the reciprocating airflow drives the air turbine to rotate and then generate electricity;

[0056] The energy storage heating device includes an electric energy storage device 9, a voltage converter 5, and a downhole heater 18; the electric energy storage device 9 on the semi-submersible drilling platform 11 is connected to the power generation devices of the photovoltaic panel 7, the vertical-axis wind turbine 8, and the oscillating water column power generator 9, and the electric energy generated by the three is first stored in the electric energy storage device 9; the electric energy storage device 9 is successively connected to the electric energy regulation signal actuator 4 and the computer 1. On the one hand, the electric energy regulation signal actuator 4 receives the execution instruction of the computer 1, and on the other hand, according to the instruction, it transmits the electric energy in the electric energy storage device 9 to the downhole heater 18; the downhole heater 18 generates heat under the power supply, and then heats the surrounding formation to supplement the temperature lost due to the absorption of heat by the hydrate decomposition, so as to increase the production of natural gas hydrate;

[0057] The central control device includes a computer 1, a wellhead pressure regulation signal actuator 2, an electric energy regulation signal actuator 4, and a wellhead 3; the computer 1 is respectively connected to the wellhead pressure regulation signal actuator 2 and the electric energy regulation signal actuator 4, the wellhead pressure regulation signal actuator 2 is connected to the wellhead 3, the electric energy regulation signal actuator 4 is connected to the voltage converter 5, the voltage converter 5 is connected to the downhole heater 18 through a cable, and the computer 1 transmits the wellhead pressure and electric energy regulation information through a real-time signal optical cable to realize the real-time regulation of the wellhead pressure and the power supply of the downhole heater.

[0058] Embodiment 2

[0059] A working method of a device for jointly generating electricity with wind-wave-light multi-energy and synergistically reducing pressure to exploit natural gas hydrate in the sea includes:

[0060] The photovoltaic panels 7 installed on the semi-submersible drilling platform 11 absorb sunlight for power generation. The vertical-axis wind turbines 8 rotate under the action of wind to generate electricity. The oscillating water column power generator 10 on the sea surface generates electricity under the action of waves. The electricity generated by wind, waves, and light is, on the one hand, directly transmitted to the voltage converter 5 through a cable, and after voltage conversion, it is transmitted to the downhole heater 18 at the bottom of the well through a cable. The formation near the bottom of the well is heated by electric heating to supplement the heat loss due to the decomposition of hydrates. On the other hand, during the peak power generation season, when the electricity generated by light, waves, and wind is more than the demand for heating by the downhole heater, it can be transmitted through a cable and stored in the electric energy storage device 9 on the platform. This can supplement the power generation demand of the downhole heater during the off-peak power generation season, and at the same time, the excess electricity can also be used for lighting and other needs on the platform. The computer 1 is connected to the wellhead pressure regulation signal actuator 2 and the voltage converter 5 through a signal transmission optical cable, and can control the heating power of the downhole heater 18 in real time, and at the same time regulate the pressure at the wellhead in real time, so as to control the decomposition rate of hydrates in the formation (i.e., the production of hydrates), and then increase the production of natural gas hydrates.

[0061] The specific steps are as follows:

[0062] (1) Wind-wave-light multi-energy combined power generation

[0063] Install the oscillating water column power generator 10 in the waters near the semi-submersible drilling platform 11 to make the most of the incoming sea waves and adapt to the tidal difference changes. The power generation device oscillates under the action of waves to generate electricity. Install the vertical-axis wind turbines 8 around the semi-submersible drilling platform 11 to improve the platform utilization rate in a limited space. The wind turbines rotate under the action of wind to generate electricity. Install the photovoltaic panels 7 on the top of the living area and around the semi-submersible drilling platform 11 to maximize the solar energy absorption area. The solar panels generate electricity under sunlight irradiation;

[0064] (2) Wind-wave-light multi-energy conversion and storage

[0065] The electric energy generated by the oscillation of the oscillating water column power generator 10, the rotation of the vertical-axis wind turbines 8, and the irradiation of the photovoltaic panels 7 are respectively transmitted to the electric energy storage device 9 through transmission cables, and then are converted into electric energy with a stable voltage by the voltage converter 5 and transmitted to the downhole heater 18. The remaining electric energy is stored in the electric energy storage device 9 and used for the platform;

[0066] (3) Synergistic exploitation of hydrates by step-down and electric heating

[0067] By controlling the wellhead pressure to adjust the bottomhole pressure, the hydrate in the reservoir is decomposed into gas and water for production (during the hydrate production process, the bottomhole pressure is affected by the change of the wellhead pressure. Bottomhole pressure = wellhead pressure + weight of the fluid in the wellbore + frictional resistance of the fluid in the wellbore. Once the wellhead pressure changes, the bottomhole pressure will change accordingly; the basis for adjustment is to control the bottomhole pressure to be lower than the decomposition pressure of the hydrate in the reservoir by adjusting the wellhead pressure, so as to ensure that the hydrate can be decomposed). At the same time, downhole heaters are installed on the wall of the production string in the bottomhole section of the well. A stable voltage is provided to the bottomhole through the electric energy storage device on the platform, promoting the downhole heaters to heat up to provide a stable heat source for the hydrate production layer. On the one hand, it eliminates the problem of secondary hydrate formation caused by the decrease in reservoir temperature due to heat absorption during pressure reduction production. On the other hand, by increasing the reservoir temperature, it improves the decomposition and production rate of hydrates, thereby increasing the production of marine natural gas hydrates.

[0068] Example 3

[0069] A working method of a device for collaborative pressure reduction production of natural gas hydrates by combined wind-wave-solar multi-energy power generation at sea is as described in Example 2. The difference is that in step (1), vertical axis wind turbines are installed around the semi-submersible drilling platform to improve the platform utilization rate in a limited space, and the wind turbines rotate to generate electricity under the action of wind. The rotation axis of the vertical axis wind turbine is parallel to the blade and perpendicular to the ground. Compared with the horizontal axis generator, its advantages are that it has no requirement for the wind direction (i.e., it can receive wind from multiple directions), the blade rotation space is small, the wind resistance ability is strong (it can resist typhoons of level 12 - 14), the starting wind speed is small, it is convenient to maintain and is less affected by the wake effect, and no additional yaw device is required, with low maintenance cost and is suitable for offshore installation. The relationship between the output power of the vertical axis wind turbine generator set and the actual wind speed can be expressed as:

[0070]

[0071] where, P w is the wind energy of the air flowing through the cross-sectional area of the wind turbine impeller per second, that is, the power of the wind turbine, W; C p is the wind energy utilization rate of the impeller, %; η m is the power coefficient of the transmission system (generally 0.90 - 0.95); η e is the power coefficient of the generator (generally 0.95 - 0.98); ρ g is the air density, kg / m 3 ; A is the area swept by the wind turbine impeller rotating one week, m 2 ; V is the wind speed, m / s.

[0072] Preferably, in step (1), the photovoltaic panel is a semiconductor device that converts light energy into electrical energy. When sunlight shines on the photovoltaic panel, the energy contained in the photons is absorbed by the photovoltaic panel, generating the photovoltaic effect. After connecting the two poles of the photovoltaic panel with a wire to form a loop, a photocurrent is formed. The present invention proposes to install photovoltaic panels on the top of the living area and around the platform of the semi-submersible drilling platform to maximize the solar energy absorption area, and the photovoltaic panels generate electricity under sunlight irradiation; the total power output by the photovoltaic panel power generation array can be calculated by the following formula:

[0073] P a =S·cosθ·η·F·A a (2)

[0074] In the formula, P a is the total power output by the photovoltaic panel, W; S is the light intensity, with the unit of W / m 2 ; θ is the incident angle of the photovoltaic panel, °; η is the efficiency of the photovoltaic panel, dimensionless; F is the sum of the design and attenuation coefficients of the photovoltaic panel power generation array, dimensionless; A a is the total area of the photovoltaic panel power generation array, m 2 .

[0075] Preferably, in step (1), the wave energy in the sea has the advantages of large reserves, large energy flow density, high reliability, and small environmental impact, and has great development potential. According to the wave energy capture principle, wave energy utilization devices can be divided into three categories: oscillating water column type, concentrating wave overtopping type, and oscillating body type. In view of the natural gas hydrate mining scenario in the sea area, the present invention selects the oscillating water column type device. This type of device drives the water column in the air chamber to oscillate through wave motion, and the reciprocating air flow generated drives the air turbine to rotate and generate electricity. This type of device has the advantages of simple structure and high reliability. The power generation power of the oscillating water column type generator can be expressed as the superposition of the time series single wave power within the wave energy capture width:

[0076]

[0077] In the formula, P wal is the power generation power of the oscillating water column type generator, W; b is the capture width of the oscillating water column type generator; n is the total number of waves; P is the single wave power, and P i is the single wave power of the i-th sea wave, W;

[0078] Since the natural gas hydrate mining sea area is mainly located in deep water areas, the seawater depth h is greater than 0.5 times the wave length L (h≥0.5L), and the single wave power of the oscillating water column type generator can be expressed as:

[0079]

[0080] In the formula, g is the acceleration of gravity, generally taken as 9.8m / s 2; H is the wave height of the sea wave, in m; T is the wave period of the sea wave.

[0081] Example 4

[0082] A working method of a device for combined offshore wind-wave-solar multi-energy power generation and collaborative pressure reduction for natural gas hydrate exploitation is as described in Example 3. The difference is that in step (2), the electric energy generated by the rotation of the vertical-axis wind turbine, the irradiation of the photovoltaic panel, and the oscillation of the oscillating water column type generator is respectively transmitted to the electric energy energy storage device on the platform through transmission cables. The electric energy is converted into electric energy with a stable voltage by a voltage converter and then transmitted to the respective power consumption areas on the platform. Since the wind-wave-solar power generation power is different under different seasons and sea conditions, it is necessary to adjust the power generation of each device in real time according to the actual situation. When the output power of each power generation device is higher than the power required by the downhole heater, the electric energy energy storage device absorbs the excess power and stores it. In the case of the saturation of the electric energy energy storage device, the remaining power can be used for platform lighting, etc.; when the output power of each power generation device is higher than the power required by the downhole heater, the electric energy energy storage device compensates for the shortage of the power required by the downhole heater, so as to provide a stable power source for hydrate development.

[0083] Example 5

[0084] A working method of a device for combined offshore wind-wave-solar multi-energy power generation and collaborative pressure reduction for natural gas hydrate exploitation is as described in Example 4. The difference is that in step (3), the wellhead pressure is controlled by a computer to adjust the bottom hole pressure so that the pressure in the reservoir is lower than the hydrate stability pressure, and then the hydrate in the reservoir decomposes into gas and water and is produced by the production string; the hydrate decomposition rate in the reservoir can be obtained by the following formula:

[0085] m g = k d M g A s (p e - p g ) (5)

[0086] In the formula, m g is the gas production rate, in kg·m -3 ·s -1 ; M g is the molar mass of methane, in kg·mol -1 ; k d is the hydrate decomposition rate constant, in mol·m -2 ·Pa -1 ·s -1 ; A s is the hydrate decomposition surface area per unit volume (SRSA), in m -1 ; p e and p gThey are the three-phase equilibrium pressure and the gas pressure in two-phase flow, Pa, p e -p g is the driving force for the hydrate decomposition reaction, Pa;

[0087] The hydrate decomposition rate constant k d is a key parameter affecting hydrate decomposition and can be calculated by the following formula:

[0088]

[0089] In the formula, k 0 is the hydrate decomposition rate constant, taking 3.6×10 4 mol·m -2 ·Pa -1 ·s -1 ; R is the gas constant, J·mol -1 ·K -1 ; T is the temperature, K; ΔE a is the activation energy, J, and ΔE a / R = 9752.73K;

[0090] The decomposition of natural gas hydrate is an endothermic process and can only continue under the condition that the surrounding environment provides sufficient heat. The decomposition rate of natural gas hydrate depends on the local temperature. The higher the temperature, the faster the decomposition rate. Therefore, the present invention proposes to install an electric heater on the wall of the production string in the bottom well section, and supply a stable voltage to the bottom of the well through the electric energy storage device on the platform to promote the electric heater to heat up to provide a stable heat source for the hydrate reservoir. On the one hand, it can eliminate the problem of secondary hydrate formation caused by the reduction of reservoir temperature due to endothermic decompression exploitation. On the other hand, it can improve the hydrate decomposition and production rate by increasing the reservoir temperature and cooperating with decompression exploitation, thereby increasing the production of natural gas hydrate in the sea area.

[0091] Due to the presence of the downhole heater, it will continuously supply heat to the surrounding formation and make up for the heat absorbed by hydrate decomposition, thereby ensuring that a relatively high hydrate decomposition rate can be maintained in the formation; the temperature distribution in the formation under the conditions of hydrate pressure reduction decomposition and downhole heating can be calculated by the following formula:

[0092]

[0093] In the formula, T is the temperature, K; v g and v w are the Darcy velocities of methane gas and water respectively, m·s -1 ; is the absolute porosity, dimensionless; S g , S w , S hare the saturations of methane gas, water, and hydrate, dimensionless; C g , C w , C h are the specific heat capacities of methane gas, water, hydrate, and the framework, J·kg -1 ·K -1 ; ρ g , ρ w , ρ h are the densities of methane gas, water, hydrate, and the framework, kg·m -3 ; λ eq is the equivalent heat transfer coefficient, W·m -1 ·K -1 ; λ g , λ w , λ h are the heat transfer coefficients of methane, water, hydrate, and the framework, W·m -1 ·K -1 ; σ g is the Joule-Thomson coefficient, J / (kg·Pa); ΔH D is the endothermic rate of hydrate dissociation, J·kg -3 ; Q a is the heat provided by the downhole heater, W;

[0094] is a general mathematical symbol representing the gradient of a vector. For example, represents the gradient of the temperature field; Δp g is the gradient of the gas pore pressure, MPa; Q is the heat exchange rate, W; (ρC) eq is the average value of the product of density and specific heat capacity, J·m -3 .K -3 ; m h is the hydrate dissociation rate, kg / s;

[0095] In addition, during the production process of natural gas hydrate extraction, the computer analyzes the decomposition situation of natural gas hydrate in real time through the monitoring of wellhead flow data (temperature, pressure, flow rate, etc.), and accordingly adjusts the wellhead pressure and the heating power of the downhole heater to maintain the production of hydrate at a relatively high level for a long time.

[0096] Preferably, the wellhead flow data such as temperature, pressure, and flow rate monitored by the wellhead monitoring module 6 can reflect the decomposition situation of hydrate at the bottom of the well. That is, the higher the gas flow rate, the greater the decomposition rate of hydrate at the bottom of the well; when the gas flow rate data monitored at the wellhead starts to decrease, it indicates that the decomposition rate of hydrate at the bottom of the well decreases. Once the production demand cannot be met, it means that the current wellhead pressure and the power of the downhole heater need to be adjusted. By reducing the wellhead pressure and increasing the power of the downhole heater, the decomposition rate of hydrate can be increased together, thereby increasing the hydrate decomposition rate.

[0097] The above are the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. An offshore wind-wave-light multi-energy combined power generation device for coordinated pressure reduction and exploitation of natural gas hydrates, characterized in that: It includes an underground wind-wave-light multi-energy combined power generation device, an energy storage heating device and a central control device; The underground wind-wave-light multi-energy combined power generation device comprises a photovoltaic panel, a vertical axis fan, and an oscillating water column type generator; the photovoltaic panel is installed on the top of the living area and around the platform on the semi-submersible drilling platform, and generates electricity by absorbing solar energy; the vertical axis fan is installed around the semi-submersible drilling platform, and generates electricity by rotating the fan under the action of wind; the oscillating water column type generator is installed on the sea surface around the semi-submersible drilling platform, and the water column in the air chamber is driven to oscillate by the wave movement, and the reciprocating airflow generated drives the air turbine to rotate and generate electricity; The energy storage and heating device includes an electric energy storage device, a voltage converter, and a downhole heater; the electric energy storage device on the semi-submersible drilling platform is connected to the power generation device of the photovoltaic panel, the vertical axis fan, and the oscillating water column generator, and the electric energy generated by the three is first stored in the electric energy storage device; the electric energy storage device is connected to the electric energy regulation signal execution mechanism and the computer in sequence, and the electric energy regulation signal execution mechanism receives the execution instructions of the computer on the one hand, and transmits the electric energy in the electric energy storage device to the downhole heater according to the instructions on the other hand; the downhole heater generates heat under the supply of electric energy, and then heats the surrounding formation to make up for the temperature lost due to the decomposition of hydrates and the absorption of heat, so as to achieve the increase in the production of natural gas hydrates; The central control device includes a computer, a wellhead pressure regulating signal actuator, an electric energy regulating signal actuator and a wellhead; the computer is connected to the wellhead pressure regulating signal actuator and the electric energy regulating signal actuator respectively, the wellhead pressure regulating signal actuator is connected to the wellhead, the electric energy regulating signal actuator is connected to the voltage converter, the voltage converter is connected to the downhole heater through a cable, the computer transmits the wellhead pressure and electric energy regulation information through the real-time signal optical cable, and realizes the real-time regulation of the wellhead pressure and the power supply of the downhole heater.

2. A working method of the offshore wind-wave-light multi-energy combined power generation and coordinated pressure reduction device for exploiting natural gas hydrates as claimed in claim 1, characterized in that: include: Photovoltaic panels installed on the semi-submersible drilling platform absorb sunlight to generate electricity, vertical axis fans rotate under the action of wind to generate electricity, and oscillating water column generators on the sea surface generate electricity under the action of waves; on the one hand, the electricity generated by wind, waves and light is directly transmitted to the voltage converter through cables, and after voltage conversion, it is transmitted to the downhole heater at the bottom of the well through cables, and the formation near the bottom of the well is heated by electric heating to make up for the problem of heat loss due to hydrate decomposition and absorption; on the other hand, in the peak season of power generation, when the electricity generated by light, waves and wind is more than the heating demand of the downhole heater, it can be transmitted through cables and stored in the electric energy storage device on the platform to make up for the power generation demand of the downhole heater during the off-season of power generation, and the excess electricity is used for lighting and other needs on the platform; the computer is connected to the wellhead pressure regulation signal actuator and the voltage converter through the signal transmission optical cable, and the heating power of the downhole heater is controlled in real time, and the pressure of the wellhead is adjusted in real time, so as to realize the regulation of the hydrate decomposition rate in the formation, thereby increasing the production of natural gas hydrate.

3. The working method of the offshore wind-wave-light multi-energy combined power generation and coordinated pressure reduction device for exploiting natural gas hydrates according to claim 2 is characterized in that: The specific steps are as follows: (1) Wind-wave-solar multi-energy power generation Install oscillating water column generators in the waters near the semi-submersible drilling platform to maximize the use of incoming waves and adapt to tidal changes. The power generation device generates electricity by oscillating motion under the action of waves; install vertical axis fans around the semi-submersible drilling platform to improve the platform utilization rate in a limited space. The fans rotate under the action of wind to generate electricity; install photovoltaic panels on the top of the living area of ​​the semi-submersible drilling platform and around the platform to maximize the solar energy absorption area. The solar panels generate electricity under the sunlight; (2) Wind-wave-light multi-energy conversion and storage The electric energy generated by the oscillation of the oscillating water column generator, the rotation of the vertical axis fan and the irradiation of the photovoltaic panel is respectively transmitted to the electric energy storage device through the transmission cable, and then converted into electric energy with a stable voltage by the voltage converter and transmitted to the downhole heater. The remaining electric energy is stored in the electric energy storage device and used by the platform; (3) Hydrate extraction by depressurization and electric heating By controlling the wellhead pressure to adjust the bottom hole pressure, the hydrate in the reservoir is decomposed into gas and water for production. At the same time, the downhole heater on the wall of the production tubing in the bottom well section provides a stable voltage to the bottom of the well through the electric energy storage device on the platform, prompting the downhole heater to heat up and provide a stable heat source for the hydrate production layer. On the one hand, it eliminates the problem of secondary hydrate generation caused by the decrease in reservoir temperature due to heat absorption caused by pressure reduction production. On the other hand, by increasing the reservoir temperature, the hydrate decomposition and production rate is increased, thereby increasing the production of natural gas hydrate in the sea area.

4. The working method of the offshore wind-wave-light multi-energy combined power generation and coordinated pressure reduction device for exploiting natural gas hydrates according to claim 3 is characterized in that: In step (1), the rotation axis of the vertical axis wind turbine is parallel to the blades and perpendicular to the ground. The relationship between the output power of the vertical axis wind turbine and the actual wind speed is expressed as: Among them, P w is the wind energy of air flowing through the cross-sectional area of ​​the wind turbine impeller per second, that is, the wind turbine power, W; C p is the wind energy utilization rate of the impeller, %; η m is the power coefficient of the transmission system; η e is the generator power factor; ρ g is the air density, kg / m 3 ; A is the area swept by the wind turbine impeller during one rotation, m 2 ; V is wind speed, m / s.

5. The working method of the offshore wind-wave-light multi-energy combined power generation and coordinated pressure reduction device for exploiting natural gas hydrates according to claim 4 is characterized in that: In step (1), when sunlight shines on the photovoltaic panel, the energy contained in the photons is absorbed by the photovoltaic panel, generating a photovoltaic effect. The two electrodes of the photovoltaic panel are connected into a loop with a wire to form a photocurrent. The total power output of the photovoltaic panel power generation array is calculated by the following formula: P a =S·cosθ·η·F·A a (2) Where P a is the total power output by the photovoltaic panel, W; S is the light intensity, in W / m 2 ; θ is the incident angle of the photovoltaic panel, °; η is the efficiency of the photovoltaic panel, dimensionless; F is the sum of the photovoltaic panel array design and attenuation coefficient, dimensionless; A a is the total area of ​​the photovoltaic panel array, m 2 .

6. The working method of the offshore wind-wave-light multi-energy combined power generation and coordinated pressure reduction device for exploiting natural gas hydrates according to claim 5 is characterized in that: In step (1), the power generation of the oscillating water column generator is expressed as the superposition of the time series single wave power within the wave energy capture width: Where P wal is the power generation power of the oscillating water column generator, W; b is the capture width of the oscillating water column generator; n is the total number of waves; P is the single wave power, P i is the single wave power of the ith wave, W; Since the natural gas hydrate exploitation area is located in deep water, the seawater depth h is greater than 0.5 times the wave wavelength L, and the single wave power of the oscillating water column generator is expressed as: In the formula, g is the acceleration due to gravity, which is 9.8 m / s 2 ; H is the wave height, m; T is the wave period.

7. The working method of the offshore wind-wave-light multi-energy combined power generation and coordinated pressure reduction device for exploiting natural gas hydrates according to claim 6 is characterized in that: In step (2), the electric energy generated by the rotation of the vertical axis fan, the irradiation of the photovoltaic panel, and the oscillation of the oscillating water column generator is respectively transmitted to the electric energy storage device on the platform through the transmission cable. When the output power of each power generation device is higher than the power required by the downhole heater, the electric energy storage device absorbs the excess power and stores it therein. When the electric energy storage device is saturated, the remaining power will be used for platform lighting; when the output power of each power generation device is higher than the power required by the downhole heater, the electric energy storage device will compensate for the shortfall in power required by the downhole heater, thereby providing a stable source of electricity for hydrate development.

8. The working method of the offshore wind-wave-light multi-energy combined power generation and coordinated pressure reduction device for exploiting natural gas hydrates according to claim 7 is characterized in that: In step (3), the bottom hole pressure is adjusted by computer-controlled wellhead pressure so that the pressure in the reservoir is lower than the hydrate stability pressure, thereby causing the hydrate in the reservoir to decompose into gas and water and then be produced from the production string; the hydrate decomposition rate in the reservoir is obtained by the following formula: m g =k d M g A s (p e -p g ) (5) In the formula, m g is the gas production rate, kg·m -3 ·s -1 ;M g is the molar mass of methane, kg·mol -1 ;k d is the hydrate decomposition rate constant, mol·m -2 ·Pa -1 ·s -1 ; A s is the hydrate decomposition surface area per unit volume, m -1 ; p e and p g are the three-phase equilibrium pressure and the gas pressure in the two-phase flow, Pa, p e -p g is the driving force for the hydrate decomposition reaction, Pa; Hydrate decomposition rate constant k d It is the key parameter affecting the decomposition of hydrates and is calculated by the following formula: Where k0 is the hydrate decomposition rate constant, which is 3.6×10 4 mol·m -2 ·Pa -1 ·s -1 ; R is the gas constant, J·mol -1 ·K -1 ; T is temperature, K; ΔE a is the activation energy, J, and ΔE a / R = 9752.73K; Due to the presence of the downhole heater, heat is continuously provided to the surrounding formations and the heat absorbed by the hydrate decomposition is compensated, thereby ensuring that a high hydrate decomposition rate can be maintained in the formation; the temperature distribution in the formation under the conditions of hydrate decomposition and downhole heating is calculated by the following formula: Where, T is temperature, K; v g and v w are the Darcy speeds of methane gas and water, m·s -1 ; is the absolute porosity, dimensionless; S g , S w , S h are the saturations of methane gas, water and hydrate respectively, dimensionless; C g , C w , C h are the specific heat capacities of methane gas, water, hydrate and skeleton, J·kg -1 ·K -1 ; ρ g , w , h are the densities of methane gas, water, hydrate and skeleton, kg·m -3 ; eq is the equivalent heat transfer coefficient, W·m -1 ·K -1 ; g , w , h are the heat transfer coefficients of methane, water, hydrate and skeleton, W·m -1 ·K -1 ; σ g is the Joule-Thomson coefficient, J / (kg.Pa); ΔH D is the endothermic rate of hydrate decomposition, J·kg -3 ;Q a Provides heat for downhole heaters, W; It represents the gradient of the temperature field; Δp g is the gradient of gas pore pressure, MPa; Q is the heat exchange rate, W; (ρC) eq is the average value of the product of density and specific heat capacity, Jm -3 .K -3 ;m h is the hydrate decomposition rate, kg / s; In addition, during the natural gas hydrate production process, the computer analyzes the decomposition of natural gas hydrates in real time by monitoring the wellhead flow data, and adjusts the wellhead pressure and the heating power of the downhole heater accordingly to keep the hydrate production at a high level for a long time.

9. The working method of the offshore wind-wave-light multi-energy combined power generation and coordinated pressure reduction device for exploiting natural gas hydrates according to claim 8 is characterized in that: The wellhead flow data monitored by the wellhead monitoring module reflects the hydrate decomposition situation at the bottom of the well, that is, the higher the gas flow rate, the greater the hydrate decomposition rate at the bottom of the well; when the gas flow data monitored at the wellhead begins to decrease, it means that the hydrate decomposition rate at the bottom of the well decreases. Once it cannot meet the production demand, it means that the current wellhead pressure and the power of the bottom hole heater need to be adjusted. The hydrate decomposition rate can be increased by reducing the wellhead pressure and increasing the power of the bottom hole heater, thereby improving the hydrate decomposition rate.

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