A method of establishing a carbon dioxide hydrate blanket on the sea floor

Abstract

This invention discloses a method for establishing a carbon dioxide hydrate caprock on the seabed, belonging to the field of carbon dioxide sequestration technology. The method includes: establishing an injection well and a production well, both located in the seabed sedimentary layer, with the production well's opening above the injection well's opening; injecting liquid CO2 through the injection well; a portion of the injected liquid CO2 forms CO2 hydrate, while a portion migrates to the area where the production well's opening is located; extracting fluid through the production well at a pressure of 85%–115% of the CO2 gas-liquid equilibrium pressure under the temperature conditions in the production well's area, thus converting the liquid CO2 into gaseous CO2, enhancing CO2 hydrate formation, and obtaining a carbon dioxide hydrate caprock in the area where the production well's opening is located. This invention can enhance CO2 hydrate formation, obtain a carbon dioxide hydrate caprock, increase CO2 sequestration capacity, and reduce the risk of CO2 leakage.

Description

Technical Field

[0001] This invention belongs to the field of carbon dioxide sequestration technology, specifically relating to a method for establishing a carbon dioxide hydrate cap layer on the seabed. Background Technology

[0002] With rapid industrialization, the burning of fossil fuels such as coal and oil, along with human activities like deforestation and land-use changes, has led to a surge in carbon dioxide emissions, far exceeding nature's absorption capacity. The continuous accumulation of carbon dioxide in the atmosphere forms a greenhouse gas barrier, triggering global warming and causing numerous environmental problems such as glacial melting and extreme weather events. This poses extremely serious challenges to the stability of ecosystems and human survival and development. Faced with the immense pressure of global warming, reducing greenhouse gas emissions has become an urgent priority, and carbon dioxide sequestration technology has emerged as a highly effective emission reduction approach.

[0003] Carbon dioxide sequestration via hydrates involves directly injecting liquid CO2 into seabed sediments. Because liquid CO2 is denser than the overlying pore fluid, it can remain stably for a short period under gravity. The pristine low-temperature, high-pressure conditions of the seabed are conducive to CO2 hydrate formation. While this method offers advantages such as minimal impact on the marine environment and high sequestration density, leakage risks remain. To ensure no leakage occurs during injection and formation, a sealing cap or a very low-permeability overlying layer is required above the injection area to prevent CO2 diffusion and leakage.

[0004] In addition, after liquid CO2 is injected into the seabed sedimentary layer, it reacts with water to generate a large amount of CO2 hydrates that fill the pores, greatly reducing the porosity of the sedimentary layer and even causing regional blockage, which affects the diffusion of subsequently injected liquid CO2 and thus reduces the amount of CO2 sequestered.

[0005] However, in actual seabed environments, the upper layer of the liquid CO2 injection area does not always have a capping layer, and the amount of liquid CO2 that can be sealed in the injection still needs to be improved. Therefore, it is of great significance to develop a method for establishing a carbon dioxide hydrate capping layer on the seabed. Summary of the Invention

[0006] To address the aforementioned problems, the present invention aims to provide a method for establishing a carbon dioxide hydrate cap layer on the seabed. This invention can enhance CO2 hydrate formation, obtain a carbon dioxide hydrate cap layer, increase CO2 sequestration capacity, and reduce the risk of CO2 leakage.

[0007] To achieve the above objectives, the present invention provides a method for establishing a carbon dioxide hydrate cap layer on the seabed, comprising the following steps: S1: Establish an injection well, the wellhead of which is located in the seabed sedimentary layer; S2: Establish a production well, the wellhead of which is located above the wellhead of the injection well and is located in the seabed sedimentary layer; S3: Inject liquid CO2 into the seabed sedimentary layer through the injection well; S4: Part of the liquid CO2 injected into the injection well forms CO2 hydrate, and the liquid CO2 that does not form CO2 hydrate migrates to the wellhead area of ​​the production well. The fluid in the seabed sedimentary layer is extracted through the production well. The production pressure of the production well is 85% to 115% of the CO2 gas-liquid equilibrium pressure under the temperature conditions in the wellhead area of ​​the production well. This causes at least a portion of the liquid CO2 in the wellhead area of ​​the production well to be converted into gaseous CO2, enhancing the formation of CO2 hydrate, and obtaining a carbon dioxide hydrate caprock in the wellhead area of ​​the production well.

[0008] According to a specific embodiment of the present invention, preferably, the injection well is a horizontal injection well.

[0009] According to a specific embodiment of the present invention, preferably, the production well is a horizontal production well.

[0010] According to a specific embodiment of the present invention, preferably, the seabed sedimentary layer includes a stable region of seabed CO2 hydrate and a region below the stable region of seabed CO2 hydrate.

[0011] According to a specific embodiment of the present invention, preferably, the wellhead of the injection well is located in the stable zone of the seabed CO2 hydrate and / or in the region below the stable zone of the seabed CO2 hydrate.

[0012] According to a specific embodiment of the present invention, preferably, the wellhead of the production well is located in the stable zone of seabed CO2 hydrate.

[0013] According to a specific embodiment of the present invention, preferably, the depth of the wellhead of the injection well is 200-300 meters below the seabed.

[0014] According to a specific embodiment of the present invention, preferably, the vertical distance between the wellhead of the production well and the wellhead of the injection well is 80 to 120 meters.

[0015] According to a specific embodiment of the present invention, preferably, the pressure of the liquid CO2 injected into the injection well is 3~15MPa.

[0016] According to a specific embodiment of the present invention, preferably, the injection rate of liquid CO2 in the injection well is 100~500m. 3 / d.

[0017] According to a specific embodiment of the present invention, preferably, the temperature of the seabed sediment layer is above -1°C. More preferably, the temperature of the stable zone of the seabed CO2 hydrate is 2~8°C, and the temperature of the region below the stable zone of the seabed CO2 hydrate is above 12°C.

[0018] According to a specific embodiment of the present invention, preferably, the pressure of the seabed sediment layer is 3-15 MPa. More preferably, the pressure of the stable zone of the seabed CO2 hydrate is 3-7 MPa, and the pressure of the region below the stable zone of the seabed CO2 hydrate is 7-15 MPa.

[0019] According to a specific embodiment of the present invention, preferably, the permeability of the seabed sedimentary layer is 5-1000 mD. More preferably, the permeability of the stable zone of the seabed CO2 hydrate is 5-60 mD, and the permeability of the region below the stable zone of the seabed CO2 hydrate is 5-500 mD.

[0020] According to a specific embodiment of the present invention, preferably, the production pressure of the production well is 88% to 115% of the CO2 gas-liquid equilibrium pressure under the temperature conditions of the area where the wellhead of the production well is located.

[0021] According to a specific embodiment of the present invention, preferably, when the fluid extracted from the seabed sedimentary layer through the extraction well contains more than 0.1% CO2 by mass, the extraction well is closed, the extraction of fluid is stopped, and the injection of liquid CO2 is stopped.

[0022] According to a specific embodiment of the present invention, preferably, the permeability of the carbon dioxide hydrate caprock is 0~60 mD. More preferably, the carbon dioxide hydrate caprock reduces the permeability of the wellhead area of ​​the produced well by more than 70%.

[0023] According to a specific embodiment of the present invention, preferably, the conversion rate of CO2 hydrate generated by the method is 55% or higher.

[0024] The present invention has at least the following beneficial effects: The method of this invention injects liquid CO2 into a seabed sedimentary layer. Part of the liquid CO2 reacts with water to form CO2 hydrates, while the remaining liquid CO2 migrates upwards. This invention controls the pressure of the seabed sedimentary layer at the wellhead of the well to be near the CO2 gas-liquid equilibrium pressure, causing the liquid CO2 to vaporize upon reaching this point. In other words, controlling the production pressure induces the conversion of unreacted CO2 into gaseous CO2. On one hand, gaseous CO2 has a lower density and moves violently within the pores of the seabed sedimentary layer. The gas expansion and migration increase fluid disturbance within the sedimentary layer, increasing the gas-liquid contact area, enhancing diffusion, and strengthening heat and mass transfer processes, thereby enhancing the formation of solid CO2 hydrates. On the other hand, during injection, the liquid CO2 reacts with seawater to form a hydrate film, isolating the reactants and causing a sharp drop in the reaction rate. The large amount of heat released during CO2 hydrate formation can vaporize a larger area of ​​liquid CO2. The vaporized CO2 can, to some extent, break through the hydrate film, restarting the reaction and further enhancing CO2 hydrate formation. Therefore, this invention enhances CO2 hydrate formation, creating a CO2 hydrate cap layer above the liquid CO2 injection area, thereby increasing the CO2 hydrate conversion rate and CO2 sequestration capacity, and thus improving CO2 sequestration effectiveness. Furthermore, the artificial CO2 hydrate cap layer established by this invention reduces the permeability of the wellhead area in the seafloor sedimentary layer, inhibiting further CO2 buoyancy and reducing the risk of CO2 leakage, thus achieving stable CO2 sequestration and benefiting the geological framework of the seafloor sedimentary layer. In addition, this invention does not add any chemical reagents, relying solely on CO2 phase transition and reaction heat to enhance CO2 hydrate formation and sequestration, avoiding potential pollution from chemical additives, reducing potential harm to the marine ecosystem, exhibiting higher long-term stability, and being more economical. Therefore, this invention is of great significance for CO2 sequestration in seafloor sedimentary layers. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of a method for establishing a carbon dioxide hydrate cap layer on the seabed according to a specific embodiment of the present invention.

[0026] Figure 2 This is a schematic diagram of the physical simulation experimental apparatus used in the embodiments and comparative examples.

[0027] Figure 3 This is an optical image of the phenomenon of liquid CO2 vaporizing and floating through the sediment in Example 1.

[0028] Figure 4 This is a comparison graph showing the conversion rate and saturation of CO2 hydrate in Example 1 and Comparative Examples 1 and 2. Detailed Implementation

[0029] To provide a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the present invention will now be described in detail below, but this should not be construed as limiting the scope of the invention.

[0030] It should be noted that, unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0031] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0032] It should be understood that the terms “comprising,” “including,” and / or “containing” as used herein specify the presence of the stated features, integers, steps, components, or combinations thereof, but do not exclude the presence or addition of one or more other features, integers, steps, components, or combinations thereof.

[0033] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0034] According to specific embodiments of the present invention, the present invention provides a method for establishing a carbon dioxide hydrate cap layer on the seabed, such as... Figure 1 As shown, it includes the following steps: S1: Establish an injection well, the wellhead of which is located in the seabed sedimentary layer; S2: Establish a production well, the wellhead of which is located above the wellhead of the injection well and is located in the seabed sedimentary layer; S3: Inject liquid CO2 into the seabed sedimentary layer through the injection well; S4: Part of the liquid CO2 injected into the injection well forms CO2 hydrate, and the liquid CO2 that does not form CO2 hydrate migrates to the wellhead area of ​​the production well. The fluid in the seabed sedimentary layer is extracted through the production well. The production pressure of the production well is 85% to 115% of the CO2 gas-liquid equilibrium pressure under the temperature conditions in the wellhead area of ​​the production well. This causes at least part (i.e., part or all) of the liquid CO2 in the wellhead area of ​​the production well to be converted into gaseous CO2, thereby enhancing the formation of CO2 hydrate and obtaining a carbon dioxide hydrate caprock in the wellhead area of ​​the production well.

[0035] like Figure 1 As shown, the injection well is connected to a liquid CO2 storage tank, and the production well is connected to a CO2 collection tank.

[0036] In some embodiments, the injection well is a horizontal injection well.

[0037] In some embodiments, the production well is a horizontal production well.

[0038] In this invention, by selecting horizontal wells as both injection and production wells, it is beneficial to increase the horizontal sweep range, thereby further enhancing the formation of carbon dioxide hydrate caprock and further increasing CO2 sequestration.

[0039] In some embodiments, the seabed sedimentary layer includes a stable region of seabed CO2 hydrates and a region below the stable region of seabed CO2 hydrates. Those skilled in the art can determine the stable region of seabed CO2 hydrates using conventional techniques; this invention does not impose any special limitations on how to determine the stable region of seabed CO2 hydrates.

[0040] In some embodiments, the wellhead of the injection well is located in the stable zone of the seabed CO2 hydrate and / or in the area below the stable zone of the seabed CO2 hydrate.

[0041] In some embodiments, the wellhead of the production well is located in the stable zone of seabed CO2 hydrates. In this invention, by setting the wellhead of the production well in the stable zone of seabed CO2 hydrates and coordinating with the aforementioned production pressure control, it is beneficial for CO2 to be rapidly and massively converted into solid CO2 hydrates after entering the hydrate stable zone. This sealed state is more stable and more favorable to the geological framework structure of the seabed sedimentary layer.

[0042] In some embodiments, the wellhead of the injection well is 200-300 meters below the seabed. Under laboratory conditions, the simulated wellhead depth can be 1-2 meters below the seabed. By controlling the wellhead depth within the above range, it is beneficial to achieve a larger horizontal sweep range for liquid CO2, thereby increasing the CO2 sequestration capacity of a single well.

[0043] In some embodiments, the vertical distance (i.e., vertical elevation difference) between the wellhead of the production well and the wellhead of the injection well is 80-120 meters. Under laboratory conditions, the simulated vertical distance between the wellhead of the production well and the wellhead of the injection well can be 80-100 cm. By controlling the wellhead position of the production well within the above range, the wellhead can be placed within the range of migrating liquid CO2, while also being located where liquid CO2 is more prone to leakage, which is beneficial for further improving the CO2 sequestration capacity and sequestration stability.

[0044] In some embodiments, the pressure of the liquid CO2 injected into the injection well is 3~15MPa.

[0045] In some embodiments, the injection rate of liquid CO2 into the injection well is 100~500 m / s. 3 / d. Under laboratory conditions, the simulated injection rate of liquid CO2 into the injection well can be 0.5~1 mL / min.

[0046] In some embodiments, the temperature of the seabed sedimentary layer is above -1°C, preferably above 2°C. Preferably, the temperature of the stable zone of the seabed CO2 hydrate is 2-8°C, and the temperature of the region below the stable zone of the seabed CO2 hydrate is above 12°C. More preferably, the temperature of the stable zone of the seabed CO2 hydrate is 2-8°C, and the temperature of the region below the stable zone of the seabed CO2 hydrate is 12-30°C. It is understood that the temperatures of the areas where the injection wellhead and the production wellhead are located are within the above ranges.

[0047] In some embodiments, the pressure of the seafloor sedimentary layer is 3-15 MPa. Preferably, the pressure of the stable zone of the seafloor CO2 hydrate is 3-7 MPa, and the pressure of the region below the stable zone of the seafloor CO2 hydrate is 7-15 MPa. It is understood that the pressure in the area where the wellhead of the injection well and the wellhead of the production well are located are within the above range.

[0048] In some embodiments, the permeability of the seafloor sedimentary layer is 5-1000 mD. Preferably, the permeability of the stable zone of the seafloor CO2 hydrate is 5-60 mD, and the permeability of the region below the stable zone of the seafloor CO2 hydrate is 5-500 mD. It is understood that the permeability of the area where the injection wellhead and the area where the production wellhead are located are within the above range.

[0049] In some embodiments, the production pressure of the production well is 88% to 115% of the CO2 gas-liquid equilibrium pressure under the temperature conditions of the area where the wellhead of the production well is located.

[0050] In some embodiments, when the fluid extracted from the seabed sedimentary layer through the extraction well contains more than 0.1% CO2 by mass, the extraction well is closed, fluid extraction is stopped, and liquid CO2 injection is stopped. The CO2 content in the extracted fluid can be determined by gas chromatography.

[0051] In some embodiments, the permeability of the carbon dioxide hydrate caprock is 0-60 mD, preferably 0-18 mD. Preferably, the carbon dioxide hydrate caprock reduces the permeability of the wellhead area of ​​the produced well by more than 70%, more preferably by more than 80%.

[0052] In some embodiments, the conversion rate of CO2 hydrate generated by the method is 55% or more, preferably 60% or more.

[0053] In this invention, it should be noted that after CO2 enters the stable zone of CO2 hydrate on the seabed, it will generate CO2 solid hydrate. However, due to the difference in the ratio of CO2 to seawater in the pores, CO2 may not be completely converted, and a portion will be sealed in liquid form.

[0054] The technical solutions of the present invention are specifically illustrated below through embodiments, but the present invention is not limited to these embodiments. Of course, various modifications can be made within the scope of the key points of the present invention.

[0055] Testing and calculation methods: The conversion rate of CO2 hydrate is calculated as: (number of moles of CO2 hydrate generated / number of moles of injected liquid CO2) × 100%. The number of moles of CO2 hydrate generated is determined using the following method:

[0056] in, The number of moles of CO2 hydrate generated; The number of moles of water consumed; N is the hydration number, which is calculated as N=6; The volume of water consumed; The density of seawater is given. The volume of water consumed is determined by the following method: During the experiment, the volume of seawater added to the reactor to maintain a constant system pressure is recorded in real time through a constant pressure pipeline connected to the bottom of the reactor and a high-precision metering pump (or a graduated constant pressure container). This added volume is the volume of water consumed in the reaction.

[0057] CO2 sequestration amount: Amount of injected CO2 + Amount of existing CO2 in the formation. In the examples and comparative experiments below, CO2 sequestration amount refers to the amount of liquid CO2 injected.

[0058] Hydrate saturation: The method for calculating hydrate saturation is briefly described below: CO2 hydrate formation satisfies the following relationship:

[0059] The volumes of each phase in the system satisfy the following relationship:

[0060] in, V R This refers to the effective volume of the reactor. V S This represents the actual volume of the sediment. , , Let t represent the volume of liquid CO2, the volume of CO2 hydrate, and the volume of seawater, respectively.

[0061] The volumes of each phase in the pores of the above sediments satisfy the following relationship:

[0062] in, The mass of sediments in the system; This represents the true density of the sediment. The density of liquid CO2 at time t is calculated using the BWRS equation of state. This refers to the amount of CO2 injected. Let be the CO2 hydrate density at time t; Let be the density of seawater at time t; This represents the proportion of liquid CO2 converted into CO2 hydrate at time t; The degree of hydration; This is the initial water volume; This refers to the amount of water to be replenished. To determine the amount of water displaced, the molar volume of H2O is taken as 18.0 cm³. 3 / mol, CO2 hydrate density is taken as 1.117 g / cm³. 3 .

[0063] Penetration rate: The permeability of the sedimentary layer and hydrate caprock was measured using the pressure pulse method. The permeability can be calculated using the following formula:

[0064] By recording the pressure difference between the upstream and downstream containers, and fitting a dimensionless pressure difference-time semi-logarithmic curve, the above equation can be transformed into:

[0065] in, The pressure difference between the upstream and downstream containers at time t is expressed in MPa. The initial pressure difference is expressed in MPa.V 1 , V 2 These are the volumes of the upstream and downstream containers, in cm³. 3 ; t Elapsed time, in seconds (s); k Permeability, in mD; A The cross-sectional area of ​​the reactor is expressed in cm². 2 ; μ The fluid viscosity is 0.00101 N. s / m 2 ; β The fluid compressibility coefficient is 5.1E. -10 m 2 / N; L The length of sediment fill is expressed in cm. This represents the slope of the dimensionless pressure difference-time curve. It should be noted that... V 1 , V 2 The two containers are only used for measuring permeability and will not be connected to the experimental setup described below in the simulation experiment.

[0066] Example 1

[0067] This embodiment provides a method for establishing a carbon dioxide hydrate cap layer on the seabed. A physical simulation experiment was conducted according to this method, and the structure of the apparatus used is as follows. Figure 2 As shown, the device includes: a reaction vessel, a liquid CO2 storage tank, an air bath, a collection tank, a back pressure valve, a constant speed and constant pressure dual-cylinder pump, pressure pipelines, temperature and pressure sensors, a data acquisition system, an electronic scale, and a high-speed camera.

[0068] The method in this embodiment includes the following steps: (1) A sapphire reactor was used as the reactor. A seawater reservoir was prepared in advance in the reactor to simulate the stable zone of seabed CO2 hydrate in the seabed sediment layer. Its permeability was 50 mD. The reactor was connected to temperature and pressure sensors, which were connected to the data acquisition system. Horizontal injection wells and horizontal production wells were set up in the seabed sediment layer. The wellhead of the horizontal production well was located above the wellhead of the horizontal injection well. The horizontal injection well was connected to a liquid CO2 storage tank, and the horizontal production well was connected to a collection tank. The depth of the wellhead of the injection well was 1 m below the seabed. The vertical distance between the wellhead of the production well and the wellhead of the injection well was 1 m (due to laboratory limitations, the actual distance could not be fully simulated). Back pressure valves were installed on the pipelines connecting the horizontal production well and the collection tank. The reactor was placed in an air bath to control the temperature of the seabed sediment layer at 4 °C to simulate the stable zone of seabed CO2 hydrate. That is to say, the wellheads of the horizontal injection well and the horizontal production well were both located in the stable zone of seabed CO2 hydrate. (2) Open the horizontal injection well and ensure that the pressure in the reactor is constant at 4 MPa by using the back pressure valve. Continuously inject liquid CO2 into the seabed sediment layer at a constant rate of 4 MPa and 0.5 mL / min. At the same time, open the horizontal production well to extract the fluid from the seabed sediment layer at a pressure of 4 MPa (i.e., 103.39% of the CO2 gas-liquid equilibrium pressure at 4℃). The extracted fluid enters the collection tank. (3) When the fluid used in the horizontal production well contains more than 0.1% CO2 by mass (i.e. when CO2 leakage is observed), in this experiment, when the droplet discharge rate at the wellhead of the production well decreases and obvious bubbles appear, it is considered that CO2 leakage has been observed. The horizontal production well is closed, the production fluid is stopped, and the horizontal injection well is closed to stop the injection of liquid CO2. (4) Open the constant pressure pipeline (located on the side of the bottom of the sapphire reactor) to keep the pressure of the seabed sediment layer constant at 4MPa, and take a picture of the vaporization and floating of liquid CO2 with a high-speed camera. CO2 hydrate continues to be generated. When the water flow in the constant pressure pipeline is less than 0.1mL in 2 hours, the experiment ends.

[0069] In the above process, a portion of the injected liquid CO2 is converted into CO2 hydrate, and the liquid CO2 that is not converted into CO2 hydrate migrates to the wellhead area of ​​the horizontal production well. At least part (i.e., part or all) of the liquid CO2 in the wellhead area of ​​the horizontal production well is converted into gaseous CO2, which enhances the formation of CO2 hydrate and a CO2 hydrate caprock is obtained in the wellhead area of ​​the horizontal production well. Figure 3 This is an optical image of the phenomenon in this embodiment where liquid CO2 vaporizes and floats up through sediments in the seabed sedimentary layer.

[0070] like Figure 4 As shown, under the condition that the temperature in the wellhead area of ​​the horizontal production well is 4℃, and the production pressure is controlled at 4MPa, the injected amount of liquid CO2 is 0.3358mol, the CO2 hydrate conversion amount (i.e., the number of moles of CO2 hydrate generated) is 0.234mol, the CO2 hydrate conversion rate is 69.68%, and the CO2 sequestration amount is 0.3358mol. The permeability of the CO2 hydrate caprock is 9mD, and the CO2 hydrate caprock reduces the permeability of the wellhead area of ​​the horizontal production well by 82%.

[0071] Example 2

[0072] This embodiment is basically the same as Embodiment 1, except that the temperature of the seabed sediment layer is controlled at 5°C and the production pressure of the production well is 4MPa (i.e., 103.39% of the CO2 gas-liquid equilibrium pressure under 5°C conditions).

[0073] The experimental results are as follows: Under the condition that the temperature in the wellhead area of ​​the horizontal production well is 5℃, and the production pressure is controlled at 4MPa, the injected amount of liquid CO2 is 0.3411 mol, the CO2 hydrate conversion amount is 0.215 mol, the CO2 hydrate conversion rate is 63.03%, and the CO2 sequestration amount is 0.3411 mol. The permeability of the CO2 hydrate caprock is 10.5 mD, and the CO2 hydrate caprock reduces the permeability of the wellhead area of ​​the horizontal production well by 79%.

[0074] Example 3

[0075] This embodiment is basically the same as Embodiment 1, except that the pressure of the injected liquid CO2 in the injection well is 4 MPa and the injection rate of liquid CO2 is 1 mL / min.

[0076] The experimental results are as follows: Under the condition that the temperature in the wellhead area of ​​the horizontal production well is 4℃, and the production pressure is controlled at 4MPa, the injected amount of liquid CO2 is 0.3326 mol, the CO2 hydrate conversion amount is 0.197 mol, the CO2 hydrate conversion rate is 59.23%, and the CO2 sequestration amount is 0.3326 mol. The permeability of the CO2 hydrate caprock is 13 mD, and the CO2 hydrate caprock reduces the permeability of the wellhead area of ​​the horizontal production well by 74%.

[0077] Comparative Example 1

[0078] This comparative example is compared with Example 1, and is basically the same as Example 1, except that the production pressure of the production well is 6 MPa.

[0079] like Figure 4As shown, under the condition that the temperature in the wellhead area of ​​the horizontal production well is 4℃, and the production pressure is controlled at 6MPa (i.e., 155.1% of the CO2 gas-liquid equilibrium pressure at 4℃), the injected amount of liquid CO2 is 0.342mol, the CO2 hydrate conversion amount is 0.1117mol, the CO2 hydrate conversion rate is 32.65%, and the CO2 sequestration amount is 0.342mol. The permeability of the CO2 hydrate caprock is 19.5mD, and the CO2 hydrate caprock reduces the permeability of the wellhead area of ​​the horizontal production well by 41%.

[0080] Comparative Example 2

[0081] This comparative example is compared with Example 1, and is basically the same as Example 1, except that the production pressure of the production well is 8 MPa.

[0082] like Figure 4 As shown, under the condition that the temperature in the wellhead area of ​​the horizontal production well is 4℃, and the production pressure is controlled at 8MPa (i.e., 206.8% of the CO2 gas-liquid equilibrium pressure at 4℃), the injected amount of liquid CO2 is 0.3472mol, the CO2 hydrate conversion amount is 0.101mol, the CO2 hydrate conversion rate is 29.08%, and the CO2 sequestration amount is 0.3472mol. The permeability of the CO2 hydrate caprock is 21mD, and the CO2 hydrate caprock reduces the permeability of the wellhead area of ​​the horizontal production well by 38%.

[0083] Comparative Example 3

[0084] This comparative example is compared with Example 1, and is basically the same as Example 1, except that the production pressure of the production well is 3.5 MPa.

[0085] The experimental results are as follows: Under the condition that the temperature in the wellhead area of ​​the horizontal production well is 4℃, and the production pressure is controlled at 3.5MPa (i.e., 78.1% of the CO2 gas-liquid equilibrium pressure at 4℃), the injected amount of liquid CO2 is 0.3357mol, the CO2 hydrate conversion amount is 0.0645mol, the CO2 hydrate conversion rate is 19.21%, and the CO2 sequestration amount is 0.3357mol. The permeability of the CO2 hydrate caprock is 37.5mD, and the CO2 hydrate caprock reduces the permeability of the wellhead area of ​​the horizontal production well by 25%.

[0086] As can be seen from Example 1 and Comparative Examples 1, 2, and 3, the present invention, by controlling the production pressure of the production well to 85%~115% of the CO2 gas-liquid equilibrium pressure under the temperature conditions of the wellhead area, can induce unreacted CO2 to be converted into gaseous CO2, enhance the formation of CO2 hydrate, and obtain a CO2 hydrate caprock. Compared with Comparative Examples 1 to 3, the CO2 hydrate conversion rate and CO2 sequestration amount of Example 1 are significantly improved. Furthermore, the present invention also reduces the permeability of the wellhead area in the seafloor sedimentary layer, thereby reducing the risk of CO2 leakage.

[0087] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for establishing a carbon dioxide hydrate cap layer on the seabed, comprising the following steps: S1: Establish an injection well, the wellhead of which is located in the seabed sedimentary layer; S2: Establish a production well, the wellhead of which is located above the wellhead of the injection well and is located in the seabed sedimentary layer; S3: Inject liquid CO2 into the seabed sedimentary layer through the injection well; S4: Part of the liquid CO2 injected into the injection well forms CO2 hydrate, and the liquid CO2 that does not form CO2 hydrate migrates to the wellhead area of ​​the production well. The fluid in the seabed sedimentary layer is extracted through the production well. The production pressure of the production well is 85% to 115% of the CO2 gas-liquid equilibrium pressure under the temperature conditions in the wellhead area of ​​the production well. This causes at least a portion of the liquid CO2 in the wellhead area of ​​the production well to be converted into gaseous CO2, enhancing the formation of CO2 hydrate, and obtaining a carbon dioxide hydrate caprock in the wellhead area of ​​the production well.

2. The method for establishing a carbon dioxide hydrate cap layer on the seabed according to claim 1, wherein, The injection well is a horizontal injection well; And / or, the production well is a horizontal production well.

3. The method for establishing a carbon dioxide hydrate cap layer on the seabed according to claim 1, wherein, The seabed sedimentary layer includes a stable region of seabed CO2 hydrates and the region below the stable region of seabed CO2 hydrates; And / or, the wellhead of the injection well is located in the stable zone of the seabed CO2 hydrate and / or in the area below the stable zone of the seabed CO2 hydrate; And / or, the wellhead of the produced well is located in the stable zone of seafloor CO2 hydrates.

4. The method for establishing a carbon dioxide hydrate cap layer on the seabed according to claim 1, wherein, The depth of the injection wellhead is 200-300 meters below the seabed; And / or, the vertical distance between the wellhead of the production well and the wellhead of the injection well is 80 to 120 meters.

5. The method for establishing a carbon dioxide hydrate cap layer on the seabed according to claim 1, wherein, The pressure of the liquid CO2 injected into the injection well is 3~15MPa; And / or, the injection rate of liquid CO2 into the injection well is 100~500m³ / h. 3 / d.

6. The method for establishing a carbon dioxide hydrate cap layer on the seabed according to claim 1, wherein, The temperature of the seabed sedimentary layer is above -1℃; And / or, the pressure of the seafloor sedimentary layer is 3~15MPa; And / or, the permeability of the seabed sedimentary layer is 5~1000mD.

7. The method for establishing a carbon dioxide hydrate cap layer on the seabed according to claim 3, wherein, The temperature of the stable zone of the seabed CO2 hydrate is 2~8℃, and the temperature of the region below the stable zone of the seabed CO2 hydrate is above 12℃. And / or, the pressure in the stable zone of the seabed CO2 hydrate is 3~7 MPa, and the pressure in the region below the stable zone of the seabed CO2 hydrate is 7~15 MPa; And / or, the permeability of the stable zone of the seabed CO2 hydrate is 5-60 mD, and the permeability of the region below the stable zone of the seabed CO2 hydrate is 5-500 mD.

8. The method for establishing a carbon dioxide hydrate cap layer on the seabed according to claim 1, wherein, The production pressure of the production well is 88% to 115% of the CO2 gas-liquid equilibrium pressure under the temperature conditions of the area where the wellhead is located.

9. The method for establishing a carbon dioxide hydrate cap layer on the seabed according to claim 1, wherein, When the fluid extracted from the seabed sediment layer through the extraction well contains more than 0.1% CO2 by mass, the extraction well is closed, the extraction of fluid is stopped, and the injection of liquid CO2 is stopped.

10. The method for establishing a carbon dioxide hydrate cap layer on the seabed according to claim 1, wherein, The permeability of the carbon dioxide hydrate capping layer is 0~60 mD; And / or, the carbon dioxide hydrate caprock reduces the permeability of the wellhead area of ​​the produced well by more than 70%; And / or, the conversion rate of CO2 hydrate generated by the method is above 55%.

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