A process for the extraction of seabed combustible ice

Abstract

The present application relates to the technical field of ocean oil and gas exploitation, and particularly relates to a kind of subsea combustible ice exploitation process method, comprising determining combustible ice deposit exploitation area and positioning sea surface exploitation ship;The capture box is placed above the combustible ice reservoir and fixed, and the skirt is sealed;Multiple high-voltage pulse electrodes are inserted into the reservoir in an array form;Then, high-voltage power supply on the exploitation ship is used to continuously input high-voltage electric pulse into the electrodes, and the electric field, joule heating effect and stress wave generated by the high-voltage electric pulse are used to cooperate to destroy the crystalline structure of combustible ice and make it decompose and release methane;After the methane gas escapes in the subsea silt and is collected by the box, it is transported to the purification device of the sea surface exploitation ship through the transport pipeline connected to the capture box, and the methane product is obtained after purification treatment and storage. The present application has the advantages of simple process, low energy consumption, small disturbance to geological environment, and is suitable for large-scale subsea combustible ice exploitation.

Description

Technical Field

[0001] This invention relates to the field of marine oil and gas extraction technology, specifically a process for extracting combustible ice from the seabed. Background Technology

[0002] Currently, the mainstream methods for extracting combustible ice from the seabed mainly include depressurization extraction, thermal activation extraction, chemical reagent injection, and carbon dioxide replacement. However, existing methods generally have certain drawbacks: depressurization relies on the natural pressure difference of the reservoir for decomposition, resulting in low extraction efficiency, and large-scale depressurization can easily induce instability in the seabed geological structure, causing environmental disasters such as landslides and collapses; thermal activation requires continuous input of heat into the reservoir, and a large amount of heat is dissipated during the transportation process, resulting in low energy utilization and high overall extraction energy consumption; chemical reagent injection requires the injection of a large amount of inhibitors to change the stability conditions of the hydrate, which not only results in high extraction costs but also easily causes marine environmental pollution; carbon dioxide replacement has a slow reaction rate, long extraction cycle, and low single-well production, making it difficult to meet the production capacity requirements of industrial-scale extraction.

[0003] In recent years, some researchers have proposed using high-voltage pulsed discharge to generate shock waves or thermal effects to break rocks. However, no one has yet proposed a scheme to use high-voltage pulsed discharge for the extraction of combustible ice, nor has the synergistic mechanism between various physical effects been clearly explained, nor has the basis for electrode layout and parameter adjustment for different reservoir conditions been given. Therefore, in view of the above-mentioned shortcomings of the current combustible ice extraction, it is urgent to develop a process for the extraction of combustible ice on the seabed to overcome the deficiencies in the current practical application. Summary of the Invention

[0004] The purpose of this invention is to provide a process for mining combustible ice on the seabed, so as to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a process for mining combustible ice on the seabed, comprising the following steps: S1, determining the mining area of ​​combustible ice on the seabed, and navigating the surface mining vessel to the mining area to complete the positioning.

[0006] S2. Place the trapping box above the mining area so that the trapping box completely covers the target mining area and is sealed and fixed to the surface of the seabed sediment.

[0007] S3. Insert multiple high-voltage pulse electrodes in an array into the combustible ice reservoir in the mining area, and electrically connect each of the high-voltage pulse electrodes to the high-voltage pulse power supply of the offshore mining vessel. The output voltage of the high-voltage pulse power supply is 50kV~200kV, the pulse frequency is 1Hz~10Hz, and the pulse width is 100ns~1000ns.

[0008] S4. A high-voltage pulse is introduced into the high-voltage pulse electrode. The high-voltage pulse simultaneously generates a pulse electric field, Joule heating effect and stress wave in the combustible ice reservoir. The generated pulse electric field, Joule heating effect and stress wave work together to destroy the crystal structure of combustible ice, causing combustible ice to decompose and release methane gas.

[0009] S5. The methane released from the decomposition escapes upward into the collection tank and is collected. The collected methane is then transported to the offshore mining vessel via a pipeline connected to the collection tank, relying on its own buoyancy.

[0010] S6. Methane transported to the sea surface is processed and then extracted to obtain methane products.

[0011] As a further aspect of the present invention, the spacing between adjacent high-voltage pulse electrodes is 0.5 to 2.0 meters.

[0012] As a further aspect of the present invention, the insertion depth of the high-voltage pulse electrode is 60% to 90% of the thickness of the combustible ice reservoir.

[0013] As a further aspect of the present invention: the array arrangement is a rectangular array or a ring array, and each high-voltage pulse electrode is inserted into the combustible ice reservoir through a directional drilling tool via a guide channel reserved on the collection box.

[0014] As a further aspect of the present invention: the collection box is a rigid sealed steel structure, with counterweights provided on the side walls of the box and a flexible sealing skirt provided at the bottom of the box.

[0015] As a further aspect of the present invention: the conveying pipeline is a flexible sealed pipeline with anti-corrosion and anti-weather treatment on the inner wall of the pipeline, the bottom end of the pipeline is sealed and connected to the air outlet at the top of the collection box, and the top end of the pipeline extends to the gas-liquid separation device of the offshore mining vessel.

[0016] Compared with the prior art, the beneficial effects of the present invention are: (1) Simple process and low construction difficulty: The present invention only needs to complete a few core steps such as placing the capture box, inserting the electrode and pulse excitation, without the need for complex downhole operation equipment, with a short construction cycle and easy to realize large-scale industrial mining.

[0017] (2) Low energy consumption and low mining cost: This invention uses high-pressure pulse excitation to decompose combustible ice, which does not require continuous input of a large amount of heat or injection of expensive chemical reagents. The energy consumption per unit of methane mining is much lower than that of the traditional thermal excitation method, and the mining cost can be greatly reduced.

[0018] (3) Environmentally friendly and safe: This mining process causes less disturbance to the overall structure of the seabed reservoir. Compared with the traditional depressurization mining method, it greatly reduces the risk of seabed landslides and geological instability. At the same time, it does not introduce foreign chemical reagents and will not cause marine environmental pollution.

[0019] (4) Low energy consumption for transportation: This invention utilizes the buoyancy of methane to transport gas from the seabed to the sea surface, eliminating the need for additional high-power gas transmission equipment, thus further reducing energy consumption and equipment investment costs in the mining process. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the process for extracting combustible ice from the seabed.

[0021] In the diagram: 1. Offshore mining vessel; 2. High-voltage pulse power supply; 3. Conveying pipeline; 4. Armored cable; 5. High-voltage pulse electrode; 6. Collection box; 7. Counterweight; 8. Combustible ice reservoir; 9. Flexible sealing skirt; 10. Gas-liquid separation device. Detailed Implementation

[0022] The technical solution of this application will be further described in detail below with reference to specific embodiments.

[0023] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0024] A process for mining combustible ice on the seabed includes the following steps: S1, determining the mining area of ​​combustible ice on the seabed, and navigating the surface mining vessel 1 to the mining area to complete the positioning.

[0025] Specifically, by using marine seismic exploration and multibeam echo sounding equipment, the distribution range and burial depth of the seabed combustible ice reservoir 8 in the target sea area are determined, and mining sites with stable geological conditions and concentrated reserves are selected. The surface mining vessel 1 is then sailed to the mining area and anchored for positioning.

[0026] S2. Place the trapping box 6 above the mining area so that the trapping box 6 completely covers the target mining area and is sealed and fixed to the surface of the seabed sediment.

[0027] Specifically, the prefabricated sealed collection box 6 is lowered to the seabed using a lifting device, so that the collection box 6 completely covers the target mining area of ​​combustible ice. After adjusting the position of the box, the bottom edge of the box is inserted into the seabed surface sediment to complete the sealing and fixation of the collection box 6, so as to prevent the subsequent gas leakage.

[0028] S3. Insert multiple high-voltage pulse electrodes 5 in an array into the combustible ice reservoir 8 in the mining area, and electrically connect each of the high-voltage pulse electrodes 5 to the high-voltage pulse power supply 2 of the offshore mining vessel 1.

[0029] The high-voltage pulse electrodes 5 are arranged with opposite polarities in adjacent columns. The high-voltage pulse power supply 2 is switched on and off by the controller to achieve high-voltage pulse output between adjacent electrodes, thereby achieving efficient extraction of the combustible ice reservoir 8 in adjacent electrode areas. Specifically, odd-numbered columns of electrodes are connected to the positive output terminal of the high-voltage pulse power supply 2, and even-numbered columns are connected to the negative output terminal, so that a potential difference sufficient to break down the reservoir medium and generate pulse current is formed between any two adjacent electrodes. The switching power supply is done by column or row, and each group of electrodes is switched on and off alternately under the timing control of the controller, ensuring that only a portion of the electrode groups are powered at any given time, while the remaining electrode groups are in a high-blocking open state, thereby avoiding mutual interference of electric fields and reducing instantaneous power supply requirements.

[0030] Specifically, multiple high-voltage pulse electrodes 5 are mounted on a directional drilling tool and lowered by the hoisting equipment of the offshore mining vessel 1. They are inserted into the combustible ice reservoir 8 in the target mining area through the guide channel reserved in the collection box 6. The multiple electrodes are arranged in an array with an electrode spacing of 0.5 to 2.0 meters. The electrode insertion depth is 60% to 90% of the reservoir thickness. The drilling tool is used to position and fix each electrode. Each high-voltage pulse electrode 5 is electrically connected to the high-voltage pulse power supply 2 of the offshore mining vessel 1. The output voltage of the high-voltage pulse power supply 2 is 50kV to 200kV, the pulse frequency is 1Hz to 10Hz, and the pulse width is 100ns to 1000ns.

[0031] As a further optimization of the present invention, in order to reduce the number of electrodes required for a single operation, reduce equipment investment costs, and improve the flexibility of mining operations, the array arrangement of the high-voltage pulse electrodes 5 adopts a zoned, sequential mining method. In specific implementation, based on the total area of ​​the target mining area and the coverage range of the collection box 6, the box-covered area is divided into several mining zones of equal area; an electrode array is independently arranged in each mining zone; after the combustible ice decomposition and gas collection of the zone are completed, the electrode array is pulled out as a whole and moved to the next unmined zone for re-insertion, and the mining operation of the entire box-covered area is completed zone by zone.

[0032] Taking a square trap box with sides of 25m covering an 18m×18m target mining area as an example, a 3×3 rectangular electrode array (9 electrodes in total) is used, with a single electrode array covering an area of ​​2.4m×2.4m. Pulse excitation is performed on only a single mining area at a time. After the combustible ice in that area has been fully decomposed and degassed, the electrode array is removed and moved to an adjacent unmined area, then reinserted and mining continues. Using this method, approximately 60 electrode array relocations are required to complete the mining of the entire 18m×18m area. This zonal sequential mining method significantly reduces the number of electrodes required per operation (only 9 electrodes are needed to cover a large mining area), avoiding the high equipment costs and deployment difficulties associated with deploying hundreds of electrodes at once within the trap's coverage area. Furthermore, after each round of mining, the electrode spacing, insertion depth, and pulse parameters can be flexibly adjusted based on the actual decomposition effect, facilitating targeted control of differences in reservoir properties and improving overall mining efficiency and process adaptability.

[0033] S4. A high-voltage pulse is introduced into the high-voltage pulse electrode 5. The high-voltage pulse simultaneously generates a pulse electric field, Joule heating effect and stress wave in the combustible ice reservoir 8. The generated pulse electric field, Joule heating effect and stress wave work together to destroy the crystal structure of combustible ice, causing combustible ice to decompose and release methane gas.

[0034] S5. The methane released from the decomposition escapes upward into the collection box 6 and is collected. The collected methane is then transported to the offshore mining vessel 1 via the transport pipe 3 connected to the collection box 6 by its own buoyancy.

[0035] S6. Methane transported to the sea surface is processed and then extracted to obtain methane products.

[0036] like Figure 1 As shown, the equipment involved in the mining process of the present invention mainly includes: a surface mining vessel 1, a high-voltage pulse power supply 2 installed on the surface mining vessel 1, a conveying pipeline 3 with one end connected to the surface mining vessel 1 and the other end extending to the seabed, an armored cable 4 connecting the high-voltage pulse power supply 2 and the high-voltage pulse electrode 5, a high-voltage pulse electrode 5 inserted into the combustible ice reservoir 8, an underwater collection box 6 covering the mining area, a counterweight 7 installed on the side wall of the collection box 6, a flexible sealing skirt 9 installed at the bottom of the collection box 6, and a gas-liquid separation device 10 installed on the surface mining vessel 1.

[0037] The high-voltage pulse electrode 5 simultaneously produces the following three physical effects in the combustible ice reservoir 8: (a) Pulse electric field effect: The high-voltage pulse forms an instantaneous strong electric field between the electrodes, which causes the polar water molecules and methane molecules in the combustible ice lattice to become polarized, and undergo rapid turning and dipole dissipation under the high-frequency alternating electric field. This process directly interferes with the hydrogen bond network of the hydrate cage structure, reduces the phase equilibrium temperature of the hydrate, and makes it tend to be unstable under the original temperature and pressure conditions.

[0038] (b) Joule heating effect: When the pulsed current flows through the water-bearing sediment with a certain resistivity, it is conducted along the interface with the strongest conductivity, generating instantaneous Joule heating near the electrode and in the current channel, causing the local temperature of the reservoir to rise instantaneously in the nanosecond to microsecond range, providing heat for hydrate decomposition and promoting the rapid transformation of the hydrate solid phase into the gas phase and liquid phase.

[0039] (c) Stress wave effect: The instantaneous large current of the high-voltage pulse generates electrostriction and expansion in liquid and solid media, radiating stress waves (including compression waves and tensile waves) outward. When the stress waves propagate in the reservoir, they generate tensile stress and shear stress, causing microcracks and particle interface separation inside the reservoir, which separates the combustible ice particles from the surrounding medium, and at the same time promotes the formation of pore water and gas discharge channels.

[0040] The three effects mentioned above occur simultaneously at the same moment the pulse is applied and work together in the same reservoir region: the pulsed electric field lowers the energy threshold required for decomposition, Joule heating provides the heat required for decomposition, and the stress wave expands the decomposition reaction interface between methane hydrate and sediment particles and improves the mass transfer channels. This spatiotemporal synergy enables methane hydrate to decompose rapidly and efficiently at energy inputs far lower than those of traditional thermal activation methods.

[0041] After the collection box 6 collects methane, as the amount of methane gas in the box increases, the methane gas is transported upward through the conveying pipe 3 under the action of buoyancy, pushing the methane to flow upward along the conveying pipe 3 connected to the top of the collection box 6, and relying on the buoyancy of the methane itself to achieve the transportation to the offshore mining vessel 1.

[0042] The methane transported to the sea surface enters the gas-liquid separation unit 10. After gas-liquid separation, dehydration and impurity removal and purification, the methane product that meets industrial standards is obtained and stored in the gas storage tank or transported out through the gas pipeline.

[0043] The parameters of the high-voltage pulse power supply 2 can be adjusted within a given range according to the burial depth and hydrate saturation of the combustible ice reservoir 8: for reservoirs that are buried deeper or have higher hydrate saturation, a higher voltage and a wider pulse width are selected to ensure sufficient intensity; for shallow or low-saturation reservoirs, a lower voltage and a narrower pulse width are selected to ensure the decomposition efficiency of combustible ice while avoiding excessive damage to the seabed geological structure caused by excessive pulse energy.

[0044] Example 1

[0045] This embodiment targets the exploitation of shallow combustible ice reservoirs with a burial depth of 100m to 200m in shallow sea areas. The specific steps are as follows: 1. Preliminary exploration and positioning: Through three-dimensional seismic exploration combined with drilling and sampling, the distribution range of combustible ice reservoir 8 in the target sea area is determined. A mining block with concentrated reserves and stable geological structure is selected. The offshore mining vessel 1 is sailed to the top of the mining block and positioned and fixed by the anchoring system.

[0046] 2. Installation of the capture box 6: In this embodiment, a square, rigid, sealed capture box 6 with a side length of 25m is used. Four counterweights 7 are pre-installed on the side walls of the box, and a flexible rubber sealing skirt 9 is installed at the bottom. The capture box 6 is slowly lowered to the seabed by the surface mining vessel 1. The position of the box is adjusted so that it completely covers the target mining area of ​​18m×18m. After the box is lowered, the flexible sealing skirt 9 is embedded into the seabed surface sediment by its own weight, thus completing the sealing and fixation.

[0047] 3. Electrode Lowering and Fixing: Nine high-voltage pulse electrodes 5 are set up and arranged in a 3×3 rectangular array. Each high-voltage pulse electrode 5 is connected to the high-voltage pulse power supply 2 of the offshore mining vessel 1 through armored cables 4. The electrodes are mounted on the directional drilling tool and lowered through the guide hole in the central area of ​​the collection box 6. The electrodes are inserted into the combustible ice reservoir 8 to a depth of 75% of the reservoir thickness (approximately 20m in actual measurement), i.e., 15m. The electrode spacing is set to 1.2m, thus completing the positioning and fixing of each electrode.

[0048] 4. High-voltage pulse excitation: The high-voltage pulse power supply 2 is activated. Based on the shallow reservoir burial (approximately 150m) and moderate hydrate saturation (approximately 55%) conditions in this embodiment, the output voltage is set to 100kV, the pulse frequency to 5Hz, and the pulse width to 500ns, continuously supplying a high-voltage pulse to the electrode. The high-voltage pulse simultaneously generates a pulsed electric field, Joule heating effect, and stress wave in the reservoir. The pulsed electric field lowers the hydrate phase equilibrium temperature, Joule heating causes a rapid increase in the local temperature around the electrode within microseconds, and the stress wave generates tensile stress in the reservoir, forming a microfracture network. These three effects work synergistically to promote the rapid decomposition of methane hydrate, releasing methane gas.

[0049] 5. Collection and Buoyancy Transport: The methane produced by decomposition naturally escapes upwards and is completely captured by the sealed collection box 6. As the amount of methane gas in the box increases, the pressure inside the box gradually rises to a level higher than the liquid column pressure in the transport pipe 3, pushing the methane into the flexible transport pipe 3 connected to the top of the box. Relying on its own buoyancy, the methane is transported upwards along the pipe and directly to the gas-liquid separation device 10 of the offshore mining vessel 1.

[0050] 6. Purification and storage: The mixed gas delivered to the gas-liquid separation device 10 is sequentially separated to remove liquid water, decarbonize, desulfurize and remove impurities. After purification, product gas with methane purity of not less than 96% is obtained and stored in the gas storage tank of the offshore mining vessel 1 to complete the mining operation.

[0051] In this embodiment, the energy consumption per unit volume of methane extraction is significantly lower than that of the traditional thermal activation method, resulting in a substantial reduction in extraction costs, a significant shortening of the construction period, and no obvious seabed geological deformation was detected during the extraction process, thus meeting the environmental safety requirements.

[0052] Example 2

[0053] The difference between this embodiment and Embodiment 1 is as follows: For high-density combustible ice reservoirs with a burial depth of 300m~500m (reservoir thickness approximately 35m, hydrate saturation approximately 72%), the number of high-voltage pulse electrodes 5 is set to 16, arranged in a 4×4 rectangular array. The electrode insertion depth is adjusted to 80% of the reservoir thickness (i.e., 28m), and the electrode spacing is adjusted to 1.8m. The parameters of the high-voltage pulse power supply 2 are adjusted to: output voltage 180kV, pulse frequency 2Hz, and pulse width 800ns. The larger voltage and pulse width provide higher electric field strength and single injection energy to meet the requirements of high-density reservoirs for higher decomposition energy thresholds; the lower frequency avoids excessive formation deformation caused by rapid heat accumulation in deep-buried reservoirs. Actual mining verification shows that the mining rate of this embodiment is significantly improved compared to the conventional parameters of Embodiment 1, making it suitable for large-scale mining of high-reserve, thick reservoirs. The remaining process steps are consistent with Embodiment 1.

[0054] Example 3

[0055] The difference between this embodiment and Embodiment 1 is that, for extremely shallow, low-saturation (approximately 40%) methane hydrate reservoirs with a burial depth of about 80m, the number of high-pressure pulse electrodes 5 is set to 4, arranged in a 2×2 rectangular array, with an electrode spacing of 0.8m and an electrode insertion depth of 65% of the reservoir thickness. Lower pulse parameters are used: output voltage 60kV, pulse frequency 8Hz, and pulse width 150ns. The lower voltage and pulse width avoid excessive impact damage to the extremely shallow reservoir, while the higher frequency compensates for the insufficient intensity of a single action, ensuring decomposition efficiency. In this embodiment, formation disturbance is minimal, making it particularly suitable for methane hydrate extraction operations near ecologically sensitive areas. The remaining process steps are consistent with Embodiment 1.

[0056] The above are merely preferred embodiments of the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, and these should also be considered within the scope of protection of the present invention. These will not affect the effectiveness of the implementation of the present invention or the practicality of the patent.

Claims

1. A method for extracting combustible ice from the seabed, characterized in that, Includes the following steps: S1. Determine the seabed combustible ice mining area and navigate the surface mining vessel to the mining area to complete the positioning. S2. Place the trapping box above the mining area so that the trapping box completely covers the target mining area and is sealed and fixed to the surface of the seabed sediment. S3. Insert multiple high-voltage pulse electrodes in an array into the combustible ice reservoir in the mining area, and electrically connect each of the high-voltage pulse electrodes to the high-voltage pulse power supply of the offshore mining vessel. The output voltage of the high-voltage pulse power supply is 50kV~200kV, the pulse frequency is 1Hz~10Hz, and the pulse width is 100ns~1000ns. S4. A high-voltage pulse is introduced into the high-voltage pulse electrode. The high-voltage pulse simultaneously generates a pulse electric field, Joule heating effect and stress wave in the combustible ice reservoir. The generated pulse electric field, Joule heating effect and stress wave work together to destroy the crystal structure of combustible ice, causing combustible ice to decompose and release methane gas. S5. The methane released by decomposition escapes upward into the collection box and is collected. The collected methane is transported to the offshore mining vessel by its own buoyancy through the conveying pipe connected to the collection box. S6. Methane transported to the sea surface is processed and then extracted to obtain methane products.

2. The method for extracting combustible ice from the seabed according to claim 1, characterized in that, In step S3, the spacing between adjacent high-voltage pulse electrodes is 0.5 to 2.0 meters.

3. The method for extracting combustible ice from the seabed according to claim 1, characterized in that, In step S3, the insertion depth of the high-voltage pulse electrode is 60% to 90% of the thickness of the combustible ice reservoir.

4. The method for extracting combustible ice from the seabed according to claim 1, characterized in that, In step S3, the array arrangement is a rectangular array or a ring array, and each high-voltage pulse electrode is inserted into the combustible ice reservoir through a guide channel reserved on the collection box via a directional drilling tool.

5. The method for extracting combustible ice from the seabed according to claim 1, characterized in that, In step S5, the collection box is a rigid sealed steel structure, with counterweights on the side walls and a flexible sealing skirt at the bottom.

6. The method for extracting combustible ice from the seabed according to claim 1, characterized in that, In step S5, the conveying pipeline is a flexible sealed pipeline with anti-corrosion and anti-weather treatment on the inner wall. The bottom end of the pipeline is sealed and connected to the air outlet at the top of the collection box, and the top end of the pipeline extends to the gas-liquid separation device of the offshore mining vessel.

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