A method for in-situ sequestration of complex carbon dioxide hydrates in the ocean

Abstract

This invention relates to a method for in-situ sequestration of composite carbon dioxide hydrates in the ocean. The sequestration method includes the following steps: injecting liquid CO2 into a CO2 sequestration area in the ocean to impact sediments; the impacted sediments mix with CO2 hydrates to form composite carbon dioxide hydrates, thus achieving CO2 sequestration. This method utilizes the impact of high-velocity liquid CO2 on native marine sediments in situ, creating locally rich sediment-rich suspended spaces due to the strong disturbance. Relying on the small sediment particles that promote hydrate nucleation and the high density of sediment particles, the rapidly injected liquid CO2 solidifies into hydrates and highly mixes with the sediments to form composite CO2 hydrates with autonomous settling characteristics, which are finally deposited on the seabed surface, achieving CO2 solidification and sequestration in the ocean.

Description

Technical Field

[0001] This invention relates to a method for in-situ sequestration of composite carbon dioxide hydrates in the ocean, belonging to the field of carbon dioxide sequestration technology. Background Technology

[0002] As the largest active carbon sink on Earth, the ocean possesses enormous carbon sequestration potential. Sequestering CO2 into the ocean is expected to be an effective measure to reduce short-term peak CO2 concentrations in the atmosphere. Currently, in addition to natural carbon sequestration models such as marine ecology and biological processes, artificial carbon sequestration models mainly include marine geological carbon sequestration and deep-sea CO2 hydrate carbon sequestration.

[0003] Marine geological carbon sequestration mainly includes oil reservoir geological structure sequestration and saline water layer sequestration, which are highly dependent on special geological structures and have drawbacks such as limited sequestration areas and high sequestration costs.

[0004] Deep-sea CO2 hydrate carbon sequestration has unique natural conditions, and CO2 hydrates can spontaneously form in areas with a water depth of about 500m. Therefore, this method has advantages such as a wider storage area and potentially lower storage costs in the future.

[0005] In addition, the mainstream approaches to CO2 sequestration based on hydrates currently include sedimentary layer sequestration, displacement methane hydrate sequestration, and direct seawater sequestration. Sedimentary layer sequestration relies heavily on the self-sealing effect, significantly improving the stability of hydrates stored in sediments. However, the challenge lies in CO2 injection; hydrate formation in sedimentary pores easily clogs the injection port and seepage channels, hindering large-scale, rapid sequestration. The mechanism of displacement methane hydrate sequestration has been verified, but due to the extremely low abundance of methane hydrates on the seabed, its economic viability and CO2 sequestration scale are far from meeting expectations, thus the technology is currently in the early research stages. Direct seawater sequestration is characterized by lower requirements for the sequestration area, a wider range, and potentially better economic viability. However, key challenges include slow hydrate formation rates, low conversion rates, low density, and poor stability. In particular, CO2 released within the positive buoyancy zone of the ocean will float and dissipate due to low conversion rates and density differences, thus failing to achieve the goal of CO2 sequestration.

[0006] To address the aforementioned challenges in CO2 sequestration in seawater, some existing patents and literature have proposed several strategies, such as: directly releasing dry ice (https: / / doi.org / 10.1007 / 978-94-009-9111-8_28), CO2 hydrates (https: / / doi.org / 10.1016 / 0196-8904(92)90067-7), dry ice and liquid CO2 mixtures (https: / / doi.org / 10.1115 / OMAE2003-37211), and liquid CO2-seawater emulsions mixed with calcium carbonate particles (https: / / doi.org / 10.1021 / es062137g). These methods all increase the initial density of the released substances, allowing them to sink autonomously to deeper seabeds. However, these methods have several major drawbacks: they all involve long settling distances, resulting in greater impact on the marine environment from the dissolved CO2; and they all require additional pre-preparation processes for the released material, which translates to higher energy consumption and increased costs. Furthermore, a device (UN20030070435A1) has been developed that can directly release negatively buoyant liquid CO2-seawater-CO2 hydrate composite particles, but its release pipe diameter is limited, making it unable to rapidly dispose of large quantities of CO2, and the manufacturing cost of the device is high, making practical operation in the ocean difficult. Summary of the Invention

[0007] To address the aforementioned technical problems, the present invention aims to provide an in-situ method for the sequestration of composite carbon dioxide hydrates formed by seabed sediments, which enables stable in-situ sequestration of carbon dioxide.

[0008] To achieve the above objectives, the present invention provides a method for in-situ sequestration of complex carbon dioxide hydrates in the ocean, comprising the following steps:

[0009] In CO2 sequestration zones in the ocean, liquid CO2 is injected to impact sediments;

[0010] The stirred-up sediments mix with CO2 hydrates to form composite carbon dioxide hydrates, thus achieving CO2 sequestration.

[0011] The sediments in this invention refer to in-situ primary sediments of the seabed, specifically substances naturally formed in the ocean and existing on the surface of the seabed. Depending on their origin, the sediments include one or more combinations of terrigenous clastic sediments (mainly composed of detrital silicate minerals and clay minerals), biogenic sediments (mainly composed of marine organism remains and shells), authigenic sediments (minerals formed directly in the ocean through chemical or biochemical processes, mainly composed of manganese nodules, polymetallic sulfides, and glauconite), cosmic sediments (cosmic dust and meteorites, mainly composed of micrometeorites and cosmic chondrules), and volcanic sediments (volcanic eruption material, mainly composed of volcanic glass and volcanic minerals).

[0012] The composition, proportion, and grain size of sediments within the target sequestration area vary depending on the specific marine environment and conditions, with terrigenous clastic sediments generally dominating. The sequestration method provided by this invention is applicable to sequestration areas where the sediments preferably include one or more combinations of detrital silicate minerals (quartz, feldspar, and mica, etc.), clay minerals (illite, montmorillonite, kaolinite, and chlorite, etc.), and biogenic sediments (calcareous biogenic sediments and siliceous biogenic sediments, etc.). The method provided by this invention is particularly suitable for geological scenarios where the sediments contain both detrital silicate minerals and clay minerals, and the proportion of both in the total mass of the sediments is equal to or greater than 50%.

[0013] According to a specific embodiment of the present invention, preferably, the median particle size of the sediment is -1Φ to 14Φ; where Φ = -log2D, D is the particle size of the sediment in mm; Φ is a particle size scale; conforming to the Udden-Wentworth particle size classification standard. Controlling the sediment particle size within the above range allows for easier agitation (disruption) of the sediment during liquid CO2 impaction, and the CO2 hydrate formed by the liquid CO2 and seawater can mix relatively quickly with the agitated sediment to form a composite carbon dioxide hydrate. More preferably, the median particle size of the sediment is 4Φ to 12Φ, and even more preferably 6Φ to 10Φ. The particle size of the sediment can be obtained by sampling sediment from the intended CO2 sequestration area and then performing particle size testing.

[0014] According to a specific embodiment of the present invention, the CO2 sequestration method provided by the present invention is suitable for nearshore or offshore CO2 sequestration scenarios. Preferably, the sequestration area meets the following conditions: water depth (ocean depth, seawater depth) greater than or equal to 750 m, temperature (ambient temperature) less than or equal to 10 °C, and pressure (ambient pressure) greater than or equal to 7.5 MPa. When the above conditions are met, the location of the sequestration area (including location, etc.) is within the stable formation zone of marine CO2 hydrates, which helps to achieve good sequestration results. Specifically, the sequestration area meets the following conditions: water depth (ocean depth, seawater depth) greater than or equal to 750 m, temperature (ambient temperature) approximately 2-7 °C, and pressure (ambient pressure) greater than or equal to 7.5 MPa. More preferably, the sequestration area meets the following conditions: water depth greater than or equal to 1500 m, temperature approximately 2-4 °C, and pressure greater than or equal to 15 MPa. This can further reduce the density difference between CO2 and seawater and improve the stability of CO2 hydrates. More preferably, the storage area meets the following conditions: water depth greater than or equal to 2800m, temperature of 2-4℃, and pressure greater than or equal to 28MPa. This allows access to the CO2 negative buoyancy zone, where the CO2 density is greater than that of seawater, which is beneficial for CO2 storage.

[0015] In this invention, the impact of injected liquid CO2 on sediments refers to the high-speed injection of liquid CO2, which impacts the sediments through its own action. High-speed injected liquid CO2 refers to the initial state of the CO2 injected into the ocean, encompassing two important factors: liquid CO2 and high-speed injection.

[0016] According to a specific embodiment of the present invention, preferably, the temperature of the liquid CO2 is approximately -56.6°C to 10°C. By controlling the temperature of the liquid CO2 within the above range, it can be ensured that CO2 entering the ocean is ejected in liquid form, and that the phase equilibrium conditions for the formation of CO2 and its hydrates can be met in the storage area. More preferably, the temperature of the liquid CO2 is -30°C to 7°C, and even more preferably -15°C to 5°C.

[0017] According to a specific embodiment of the present invention, preferably, when injecting liquid CO2, a continuous injection method or a pulse injection method (i.e., injection is performed at certain time intervals) is adopted.

[0018] According to a specific embodiment of the present invention, preferably, the We of the liquid CO2 at the outlet of the injection device is equal to or greater than 1 × 10⁻⁶. 2 We is the dimensionless number in fluid dynamics (Weber number), We = ρυ 2 L / σ, ρ is the fluid density (unit: kg / m³). 3υ is the fluid velocity (i.e., the CO2 injection velocity, in m / s), L is the characteristic length (e.g., the nozzle outlet diameter of the injection head of the injection device, in m), and σ is the fluid surface tension coefficient (i.e., the CO2 surface tension coefficient, in N / m). By controlling the We of the liquid CO2 at the outlet of the injection device within the above range, the injected CO2 can have sufficient impact capability. More preferably, the We of the liquid CO2 at the outlet of the injection device is equal to or greater than 1 × 10⁻⁶. 6 .

[0019] In this invention, the injected liquid CO2 immediately forms hydrates upon impacting the sediments. Since the sediment particles are small, they do not all immediately settle after the impact. On one hand, the rapid formation of CO2 hydrates can encapsulate the sediment particles; on the other hand, the sediment particles can adhere to the hydrate surface. Thus, a highly mixed composite CO2 hydrate of sediments can be formed. Furthermore, in the early stages of mixing, some liquid CO2 may not completely convert to hydrates, but it can gradually transform into hydrates in subsequent processes.

[0020] In this invention, the CO2 injection area is located near the seabed, which can be seawater above the seabed or surface sediments below the seabed. Using the seabed mudline as the boundary, the distance between the CO2 injection port and the mudline is defined as h, i.e., the distance h between the outlet of the injection device and the seabed mudline. Preferably, h is less than or equal to 3m, thus ensuring sufficient disturbance to the sediments under CO2 impact, guaranteeing that the high-speed ejected CO2 can stir up the seabed sediments and achieve thorough mixing. More preferably, h satisfies the following condition: -10m ≤ h ≤ 1m; even more preferably, h satisfies the following condition: -5m ≤ h ≤ 0.5m; where a negative number indicates that the CO2 injection port of the injection device is inserted into the sediments.

[0021] In this invention, the agitated sediment mixes with CO2 hydrate to form a composite carbon dioxide hydrate. This composite carbon dioxide hydrate can form a "composite CO2 hydrate with autonomous settling characteristics," specifically referring to the composite hydrate morphology formed after the sediment and CO2 hydrate are mixed and cemented, and its density is greater than that of seawater. Under high Weber number injection conditions, liquid CO2 will atomize, meaning that the ejected droplets are almost all in the sub-millimeter range. When mixed with sub-millimeter to nano-sized sediments, it easily induces hydrate nucleation at the droplet interface. When the conversion rate of the formed composite hydrate is greater than or equal to 18%, it can possess autonomous settling properties. Preferably, the composite carbon dioxide hydrate contains carbon dioxide hydrate, sediment, and liquid carbon dioxide (present when the initial liquid CO2 has not been completely converted into hydrate), or the composite carbon dioxide hydrate contains carbon dioxide hydrate and sediment, or the composite carbon dioxide hydrate contains one or a combination of two of the following: carbon dioxide hydrate and liquid carbon dioxide; wherein the mass percentage of carbon dioxide hydrate is greater than or equal to 18%.

[0022] According to a specific embodiment of the present invention, the source of the CO2 pre-sequestered by the present invention can be marine or land-based. For example, marine carbon sources can be associated gas from offshore oil fields, while land-based carbon sources can be industrial flue gas. Depending on the location of the carbon source, the CO2 transport route to the target sequestration area is diverse, and can include long-distance transport pipelines connecting to onshore carbon source bases or vertical injection pipes connecting to moving vessels or offshore platforms.

[0023] According to a specific embodiment of the present invention, the injection device used in the present invention can be an injection device commonly used in the field for CO2 storage. For example, liquid CO2 can be injected through a CO2 injection pipe, which can be a horizontal pipe, a vertical pipe, a branch pipe, or a complex pipe network system. The injection pipe can be a single pipe or multiple pipes. The injection pipe can be equipped with one or more nozzles, and the nozzle orientation can be horizontal, vertically downward / upward, or obliquely downward / upward at a certain angle.

[0024] In the sequestration method of this invention, after the marine sediments are agitated by injected CO2, a disturbed space filled with suspended micro-solid particles is formed, which can effectively provide nucleation sites for hydrates and promote their nucleation and growth. Based on this, the sequestration method provided by this invention can promote a high degree of mixing between the seabed sediments and the injected CO2, promote the rapid nucleation and growth of CO2 hydrates, and form cemented clusters of CO2 hydrate and sediments, i.e., composite carbon dioxide hydrates. In this way, the injected CO2 is rapidly converted into composite carbon dioxide hydrates with a significantly increased density, which have autonomous settling characteristics, significantly reducing CO2 escape and increasing the effective solidification and sequestration of CO2. Furthermore, the composite carbon dioxide hydrates of highly mixed sediments can be similar to the state of natural gas hydrates stored in porous sediments, and the capillary effect and self-sealing effect can significantly improve the sequestration stability of hydrates.

[0025] Furthermore, liquid CO2 can generate strong impacts and atomize under high flow velocities. Therefore, when high-speed CO2 impacts seabed surface sediments, it causes strong disturbance to the sediments, and the atomized CO2 droplets can rapidly nucleate and grow into hydrates. This invention utilizes the impact of liquid CO2 itself on seabed surface sediments to achieve a high degree of mixing between CO2 and sediments. This not only fully achieves the purpose of two-phase mixing, but also features a simple, environmentally friendly, and low-cost injection process. This method can effectively overcome the two main difficulties in CO2 sequestration in seawater layers: slow hydrate growth rate and low density. It is applicable to all depths of the ocean, including the CO2 positive buoyancy zone (meeting the phase equilibrium conditions for CO2 hydrate formation), achieving the solidification and in-situ sequestration of CO2 in the form of composite hydrates with autonomous settling characteristics.

[0026] This invention provides a method for rapidly solidifying injected CO2 into hydrates and stably storing them in situ in the ocean. High-velocity liquid CO2 impacts native marine sediments, creating a sediment-rich suspended space under intense disturbance. The small sediment particles promote hydrate nucleation, and the high density of the sediment particles further enhances the process. During this rapid solidification, the injected liquid CO2 mixes extensively with the sediments to form a composite CO2 hydrate with autonomous settling characteristics. Finally, the composite CO2 hydrate is deposited on the seabed surface, achieving CO2 solidification and storage in the ocean.

[0027] The sealing method provided by this invention has the following advantages:

[0028] 1. This invention utilizes only the original sediments on the seabed as a medium to promote the nucleation and growth of hydrates and increase the density of composite hydrates. While improving the hydrate conversion rate, it also reduces the non-solidification and escape of CO2. In particular, it is applicable to the positive buoyancy zone of CO2 in the ocean (where the density of CO2 is less than that of seawater) at depths of less than about 2800m. The rapid nucleation, growth and weight gain of hydrates can achieve the solidification and sequestration of CO2 in a wider space in the ocean.

[0029] 2. This invention expands the application of hydrate-based CO2 sequestration in seawater, and in particular provides a new strategy for achieving carbon sequestration in the shallow CO2 positive buoyancy region of the ocean.

[0030] 3. This invention does not introduce additional complex equipment, devices and materials, and has the advantages of simple process, economy and environmental protection. Attached Figure Description

[0031] Figure 1 This is a flowchart of a specific implementation scheme of the sealing method provided by the present invention.

[0032] Figure 2 The diagrams are schematic representations of the processes in Embodiments 1 and 2 of the present invention.

[0033] Figure 3 This is a graph showing the pressure changes during the experiment in Example 1.

[0034] Figure 4 This is a comparison chart of the results of Experiment Example 1.

[0035] Figure 5 This is a comparison chart of the results from Experiment Example 2. Detailed Implementation

[0036] To make the objectives, technical solutions, and technical advantages of the embodiments of the present invention clearer, the specific technical solutions of the embodiments of the present invention will be described in detail and completely below with reference to the accompanying drawings. It should be particularly noted that the described embodiments are only a part of the embodiments that can be covered by the present invention, and not all of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained without creative effort beyond the scope of the technical solutions covered by the present invention should fall within the protection scope of the present invention.

[0037] When sealing using the method provided by this invention, it can be done according to Figure 1 The specific implementation steps are as follows:

[0038] Step S1: Target storage area selection and CO2 injection method determination:

[0039] First, the phase equilibrium conditions of the target sequestration area should be in the liquid CO2 region and the stable CO2 hydrate formation region, specifically meeting the following requirements: the water depth of the target sequestration area is greater than or equal to 750m, the temperature is less than or equal to 10℃, and the pressure is greater than or equal to 7.5MPa; the main components of the seabed sediments in the target sequestration area are clay minerals and detrital silicate minerals, accounting for more than or equal to 50%, and the median grain size should be between -1Φ and 14Φ.

[0040] Secondly, the CO2 injection method is mainly determined by the location of the CO2 source and the amount of CO2 stored, which mainly includes nearshore storage and offshore storage. Nearshore storage can be achieved directly through shore-connected pipelines, while offshore storage generally requires CO2 injection through CO2 tanks combined with offshore operation platforms.

[0041] Step S2, CO2 injection pipeline layout and injection conditions determination:

[0042] First, the CO2 injection pipeline is laid out according to the injection method determined in step S1. The transport pipeline should fully consider the corrosion of seawater and CO2, and select pipelines made of suitable materials (this can be done in the usual way). The CO2 injection port should be located near the seabed, either in the seawater above the seabed or in the surface sediments below the seabed, with the mudline of the seabed as the dividing line. The distance h between the CO2 injection port and the mudline should be less than or equal to 3m.

[0043] Secondly, the CO2 injection conditions mainly involve determining the CO2 temperature and velocity. Specifically, the CO2 temperature is controlled between -56.6℃ and 10℃; the We of the CO2 at the injection equipment outlet is ≥1×10⁻⁶. 2 .

[0044] Step S3, CO2 injection and formation of composite CO2 hydrate:

[0045] CO2 itself acts as an impact medium, causing strong disturbance to sediments. Submillimeter to nanometer-sized sediments serve as a dual-function medium for promoting hydrate nucleation and increasing composite weight, prompting CO2 to rapidly form hydrates upon injection into the ocean and cement with surrounding sediments to form highly mixed composite CO2 hydrate morphologies.

[0046] Step S4: The composite CO2 hydrate settles and is deposited on the seabed surface.

[0047] Under high Weber number conditions, the increased conversion rate of CO2 hydrates and the additional weight provided by sediment cementation enable composite CO2 hydrates to have autonomous settling characteristics. Their overall density can be significantly greater than that of seawater, thus enabling rapid solidification and settling of injected CO2, and stable existence of the sediment-CO2 hydrate composite on the seabed surface.

[0048] Figure 1The process shown is a specific implementation of the sealing method provided by the present invention, and does not mean that all sealing methods provided by the present invention need to be carried out in the above manner.

[0049] Example 1

[0050] This embodiment targets the positive buoyancy zone of marine CO2 as the storage area, and the storage method is as follows: Figure 2 As shown:

[0051] The carbon source is the land-based plant 1. Liquid CO2 is transported to the target storage area through the shore transport pipeline 2, impacting the sediment particles 3 in the storage area. The CO2 droplets 4 and the sediment particles 3 combine to form a composite CO2 hydrate 5 (sediment-CO2 hydrate cemented state).

[0052] The method includes the following specific steps:

[0053] The CO2 source is from the land-based plant 1, and CO2 is injected directly into the target storage area through the onshore transport pipeline 2. The selected target storage area is about 1500m above sea level, the CO2 density is less than the seawater density, and it is located in the CO2 positive buoyancy zone. The temperature and pressure in the area are about 4℃ and 15MPa, respectively. Under this condition, CO2 is in liquid state and meets the phase equilibrium conditions for the formation of CO2 hydrate.

[0054] The main components of seabed sediments are: terrigenous clastics greater than 90% (clay minerals 40-60%, mainly illite and other clays; clastic silicate minerals 30-60%, mainly quartz and feldspar silts), and a small amount of biogenic sediments; the median grain size of the sediments is 6-8Φ.

[0055] The CO2 release location is near the seabed, with h = -1m (inserted into the surface layer of seabed sediments).

[0056] Injection is performed using a horizontal multi-pipe, multi-nozzle, multi-directional nozzle method;

[0057] The initial temperature of CO2 is -5℃, and We is taken as 1×10⁻⁶. 3 .

[0058] The high-speed ejection of CO2 droplets causes intense disturbance to the sediments, creating a sediment-CO2 droplet mixing field in localized areas. This promotes the rapid nucleation and growth of CO2 hydrates, which then highly mix and cement with the sediments, forming composite CO2 hydrates with autonomous settling characteristics. Ultimately, the composite CO2 hydrates settle to the seabed and achieve long-term stable existence.

[0059] Example 2:

[0060] This embodiment targets the negative buoyancy zone of ocean CO2 as the storage area, and the storage method is as follows: Figure 2 As shown:

[0061] On the offshore oil and gas production platform 6, liquid CO2 is transported to the target storage area through CO2 injection well 7, impacting the sediment particles 8 in the storage area. The CO2 droplets 9 and the sediment particles 8 combine to form a composite CO2 hydrate 10 (sediment-CO2 hydrate cemented state).

[0062] The method includes the following specific steps:

[0063] The CO2 source is associated gas from offshore oil fields, and CO2 reinjection and marine storage are achieved directly through the offshore oil and gas extraction platform 6. The selected target storage area is about 3000m above sea level, where the CO2 density is greater than that of seawater and it is located in the CO2 negative buoyancy zone. The temperature and pressure in the area are 3℃ and 30MPa, respectively. Under these conditions, the CO2 is in liquid state and meets the phase equilibrium conditions for the formation of CO2 hydrates.

[0064] The main components of seabed sediments are: clay minerals 50-70%, siliceous biofilm 20-40%; the median grain size of the sediments is 8.5-10Φ.

[0065] The CO2 release location is near the seabed, with h = 0m (close to the surface of seabed sediments).

[0066] Injection is performed using a vertical single pipe with multiple nozzles, with the nozzles pointing downwards (angle with the vertical direction 0°≤θ≤45°).

[0067] The initial temperature of CO2 is -20℃, and We is taken as 1×10⁻⁶. 5 .

[0068] The high-speed ejection of liquid CO2 creates a strong impact on the sediments, forming a sediment-CO2 droplet mixing field in localized areas. CO2 hydrates rapidly nucleate and grow, then highly mix and cement with the sediments, forming composite CO2 hydrates with autonomous settling characteristics. Ultimately, these composite CO2 hydrates settle to the seabed and achieve long-term stable existence.

[0069] The method provided by this invention can solve several major problems in the CO2 solidification method using seawater hydrates, such as slow hydrate formation rate, low conversion rate, low density, and poor stability.

[0070] The method provided by this invention utilizes high-velocity CO2 to impact the soft sediments on the seabed surface. By leveraging the dual functions of sediments in promoting hydrate nucleation and cementation for weight gain, CO2 is rapidly converted into complex hydrates with autonomous settling characteristics. This method offers the following advantages:

[0071] 1) This method can achieve rapid nucleation and solidification of injected CO2 into hydrates, and the density of composite CO2 hydrates in the highly mixed sediments formed can be significantly improved, effectively overcoming the two difficulties of slow hydrate formation rate and low density in the seawater hydrate solidification and sequestration method.

[0072] 2) The composite CO2 hydrate formed by this method has autonomous settling characteristics and can solve the problem of CO2 floating and dissipating in the positive buoyancy zone of the ocean (water depth less than 2800m). That is, this method can be applied to CO2 sequestration in the full depth range of the ocean (water depth greater than 750m) (the phase equilibrium conditions for CO2 hydrate formation must be met), which greatly expands the potential space for marine CO2 sequestration.

[0073] 3) This method utilizes only native marine sediments as a medium to promote hydrate nucleation and weight gain, and relies solely on CO2 as a disturbance medium, thus implementing the green concept of "utilizing nature and learning from nature." It does not introduce other reagents or devices and has significant advantages in terms of ecology and economy compared with other technologies.

[0074] Furthermore, the method of this invention has undergone extensive and rigorous experimental testing and verification. Verification experiments were conducted using a small-scale sapphire reactor and a pilot-scale large reactor. Through positive and negative experimental phenomena, it was clearly demonstrated that high-speed liquid CO2 impacting sediments can rapidly form composite CO2 hydrates with autonomous settling characteristics. Two specific experimental examples are provided, and the specific experimental schemes and results are described below:

[0075] Experimental Example 1

[0076] The reaction experiment was conducted using a fully transparent sapphire reactor. High-speed liquid CO2 was injected into the sedimentary layer under simulated marine conditions (pressure 8 MPa, temperature 3 °C, 3.5 wt% NaCl solution).

[0077] The experimental steps are as follows:

[0078] (1) Add 800 mesh (approximately 5.72Φ) bentonite with a thickness of about 4cm into a high-pressure sapphire autoclave (inner diameter approximately 72mm, height 100mm);

[0079] (2) Pour about 300 mL of 3.5 wt% sodium chloride solution into the high-pressure sapphire reactor, maintain normal pressure, and control the temperature to 3 °C;

[0080] (3) Prepare liquid CO2 into the piston container and pressurize it to 15 MPa, while controlling the temperature at 3°C;

[0081] (4) Rapidly inject liquid CO2 into the reactor (Weber number approximately 1×10⁻⁶). 7 The impact on bentonite, and the pressure changes during the experiment are as follows: Figure 3 As shown.

[0082] Experimental results are as follows Figure 4 As shown.

[0083] Under the same aquatic environment conditions:

[0084] Figure 4 The left figure shows that in the case where no sedimentation system is added (i.e. step (1) is omitted), all the hydrates formed by the injected CO2 accumulate at the top of the vessel. This is because the density of CO2 is less than that of simulated seawater at this time. After the CO2 is injected, it does not immediately form hydrates but floats up and accumulates at the top of the vessel. Although the density of the hydrates formed later is greater than that of simulated seawater, they still cannot sink to the bottom due to insufficient conversion rate and wall adhesion effect.

[0085] Figure 4 The right figure shows the experimental results of the impact sedimentary system. In the gem-quality reactor, hydrates are observed to be primarily located within and on the surface of the bentonite, with only a small number of vacuolated composite hydrate particles adhering to the top. This indicates that hydrates form immediately during the impact of liquid CO2 on the sediment, and that the density of the resulting composite CO2 hydrates is greater than that of simulated seawater.

[0086] The comparative experimental results described above demonstrate that the method provided by this invention can achieve stable storage of hydrates.

[0087] Experiment Example 2

[0088] A 20L reactor with a viewing window was used for the reaction experiment. High-speed liquid CO2 impact on the sedimentary layer was injected under simulated open ocean conditions (pressure 8MPa, temperature 3℃, 3.5wt% NaCl solution).

[0089] Experimental steps:

[0090] (1) Prepare a 3.5 wt% sodium chloride solution and inject it into the reactor. After the reactor is full, continue to inject the solution to bring the back pressure to 10 MPa and cool it down to 3 °C.

[0091] (2) Prepare a clay slurry with a mass fraction of 15% (clay particle size of 800 mesh, about 5.72Φ) and inject it into a small container (inner diameter of about 80mm and height of 100mm) in the reactor. First fill the small container, and then continue to inject, allowing the clay slurry to overflow the small container.

[0092] (3) Liquid CO2 at 3℃ and 12MPa was prepared by pulsed valve opening (valve opening time was 1s, and the interval between two openings was 5min; Weber number was approximately 1×10⁻⁶). 6 Liquid CO2 is injected into a container containing clay slurry, creating an impact on the clay slurry.

[0093] Experimental results are as follows Figure 5 As shown.

[0094] Under the same working conditions, Figure 5 The left figure shows the injection of liquid CO2 without the addition of clay slurry (i.e., step (2) is omitted). The results show that the injected CO2 quickly forms granular clusters and floats to the surface. This indicates that a hydrate shell is formed at the interface of the CO2 droplets, but the conversion rate is too low to exceed the density of the water. Figure 5 The right figure shows that CO2 and sediment formed a cemented composite state and sank, indicating that the CO2 droplet interface also rapidly formed a hydrate shell and formed a good cement with the sediment. This further shows that the participation of sediment significantly increased the density of the composite CO2 hydrate, which was greater than the density of water and had autonomous settling characteristics.

[0095] The above comparative experimental results further demonstrate that the method provided by this invention can achieve stable sequestration of hydrates in an open environment close to that of the actual ocean.

[0096] The foregoing description, with reference to the accompanying drawings, outlines two typical implementation scenarios and preferred embodiments of the present invention, aiming to more clearly illustrate the core technology of the invention. However, the actual embodiments covered by the present invention are more comprehensive and diverse. Furthermore, since many modifications and changes are readily apparent to those skilled in the art, all embodiments of the present invention are not limited to the precise data, structures, layouts, and operations described in the embodiments. All other modifications and equivalents falling within the technical scope of the present invention should be considered protected by the present invention.

Claims

1. A method for in-situ sequestration of complex carbon dioxide hydrates in the ocean, comprising the following steps: In CO2 sequestration zones in the ocean, liquid CO2 is injected to impact sediments; The stirred-up sediments mix with CO2 hydrates to form composite carbon dioxide hydrates with autonomous settling properties, thus achieving CO2 sequestration. The median grain size of the sediments is -1Φ to 14Φ; where Φ = -log2D, and D is the grain size of the sediments in mm. The We of liquid CO2 at the outlet of the injection device is equal to or greater than 1 × 10⁻⁶. 2 ; The distance h between the outlet of the injection equipment and the seabed mud line is less than or equal to 3m.

2. The sealing method according to claim 1, wherein, The sediments include one or more of the following: terrigenous clastic sediments, biogenic sediments, authigenic sediments, cosmogenic sediments, and volcanic sediments.

3. The sealing method according to claim 2, wherein, The sediments include one or more of detrital silicate minerals, clay minerals, and biogenic sediments.

4. The sealing method according to claim 3, wherein, The sediments contain detrital silicate minerals and clay minerals, and the two together account for 50% or more of the total mass of the sediments.

5. The sealing method according to claim 1, wherein, The median grain size of the sediments is 4Φ to 12Φ.

6. The sealing method according to claim 1, wherein, The sealed area meets the following conditions: The water depth is greater than or equal to 750m, the temperature is less than or equal to 10℃, and the pressure is greater than or equal to 7.5MPa.

7. The sealing method according to claim 6, wherein, The sealed area meets the following conditions: The water depth is greater than or equal to 1500m, the temperature is 2-4℃, and the pressure is greater than or equal to 15MPa.

8. The sealing method according to claim 1, wherein, The temperature of the liquid CO2 is between -56.6°C and 10°C.

9. The sealing method according to claim 8, wherein, The temperature of the liquid CO2 is between -30°C and 7°C.

10. The sealing method according to claim 1, wherein, The We of liquid CO2 at the outlet of the injection device is equal to or greater than 1 × 10⁻⁶. 6 .

11. The sealing method according to claim 1, wherein, -10m≤h≤1m.

12. The sealing method according to claim 1, wherein, The composite carbon dioxide hydrate contains carbon dioxide hydrate, sediment, and liquid carbon dioxide; or, the composite carbon dioxide hydrate contains carbon dioxide hydrate and sediment; or, the composite carbon dioxide hydrate contains one or a combination of two of the following: carbon dioxide hydrate and liquid carbon dioxide. Of these, carbon dioxide hydrates account for 18% or more by mass.

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