Method for promoting carbon dioxide to replace natural gas hydrate in argillaceous silt by using urea

Abstract

The invention discloses a method for utilizing urea to promote carbon dioxide replacement to exploit natural gas hydrate in argillaceous silt, and belongs to the crossing field of natural gas hydrate exploitation and carbon dioxide storage. The method comprises the following steps: preparing an argillaceous silt sediment sample containing urea; methane hydrate is generated in a low-temperature and high-pressure environment by using a soaking method; releasing residual methane gas and injecting carbon dioxide for replacement; the replacement effect is investigated under different urea concentrations. Researches find that when the concentration of urea is in a range of 1.0-10wt%, the promotion effect is obvious. The method has the advantages that raw materials are easy to obtain, non-toxic and environment-friendly, the effect of promoting methane hydrate replacement with carbon dioxide is remarkable, and the method is suitable for natural gas hydrate replacement exploitation research under laboratory and field conditions and has good application prospects.

Description

Technical Field

[0001] This invention belongs to the interdisciplinary field of natural gas hydrate extraction and carbon dioxide storage, specifically relating to a technical method for extracting natural gas hydrates in silty mud using urea to promote carbon dioxide replacement. Background Technology

[0002] Natural gas hydrates are crystalline clathrates formed by water molecules forming cage-like structures through hydrogen bonds, encapsulating small gas molecules. They are mainly distributed in marine sediments and permafrost. As a clean energy source, the extraction and utilization of natural gas holds promise for reducing carbon emissions and global warming. However, a key challenge in its development and utilization lies in how to efficiently and safely release methane gas from hydrates. Several traditional methods for extracting natural gas from natural gas hydrates have been proposed and tested: the depressurization method and the thermal stimulation method promote hydrate decomposition and obtain natural gas by reducing reservoir pressure and increasing temperature. However, these methods reduce the stability and mechanical strength of hydrate-bearing sediments and can lead to geological hazards such as seafloor subsidence. Among these methods, the CO2 replacement of CH4 hydrates method can not only achieve natural gas extraction but also realize CO2 geological sequestration, which has significant environmental and energy implications.

[0003] However, in actual displacement processes, problems such as limited interfacial mass transfer and low displacement efficiency exist. Introducing an additive into the system that can improve the interfacial state or promote gas transfer can significantly improve the displacement efficiency of CO2 to CH4. Urea, as a common, low-cost, and water-soluble compound, has a significant promoting effect on hydrate formation, but its displacement effect on CH4 / CO2 has not been reported in the current literature. Summary of the Invention

[0004] To overcome the problems of high energy consumption, low efficiency, and geological disasters in traditional natural gas hydrate replacement mining processes, the present invention aims to provide an efficient and environmentally friendly auxiliary method for CO2 replacement of natural gas hydrates.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for promoting the replacement of natural gas hydrates in silty mudstone with carbon dioxide using urea: urea is introduced into silty mudstone sediments, and the replacement of CH4 hydrates with CO2 is promoted by adjusting the urea concentration.

[0007] Urea was mixed evenly with 3.5 wt% NaCl solution and then added to the silty mud sediment. The concentration of urea in the reaction system was 1 wt% to 10 wt%.

[0008] Includes the following steps:

[0009] (1) Preparation of silty mudstone sediments: Three kinds of quartz sand with different mesh sizes were mixed with montmorillonite in a mass ratio of 3:1:3:3. The average particle size of the mixed minerals was 6.618 μm. Urea was dissolved in 3.5 wt% NaCl solution and then fully mixed with the mixed minerals to form silty mudstone sediments. The sediments were filled into a high-pressure reactor at a temperature of 293.15 K and compacted.

[0010] (2) Formation of natural gas hydrate: Methane gas is introduced and the temperature is lowered to 274.15K. The temperature in the reactor suddenly rises, indicating that hydrate has begun to form. When the system pressure and temperature tend to stabilize, the natural gas hydrate formation experiment is considered to be complete.

[0011] (3) Carbon dioxide replacement: Cool down to 268.15K, introduce carbon dioxide, raise the temperature to 274.15K and maintain the reaction;

[0012] (4) Results determination: The contents of CH4 and CO2 in the gas phase were determined by gas chromatography, and the CH4 replacement rate and CO2 sequestration rate were calculated.

[0013] In step (1), the quartz sand is 100-200 mesh, 325 mesh and 600 mesh respectively; the montmorillonite is 1250 mesh.

[0014] In step (2), the pressure of methane introduced is 4 MPa to 10 MPa; in step (3), the pressure of carbon dioxide introduced is 3 MPa to 3.5 MPa.

[0015] In step (3), the time for CO2 to replace CH4 is 120h.

[0016] This invention investigates the displacement characteristics of carbon dioxide and methane hydrate in silty mud under different urea concentrations, revealing that urea plays a significant promoting role in the gas displacement reaction. The mechanisms of urea's action include: (1) enhancing the mass transfer capacity of CO2 molecules; and (2) catalyzing the formation of the cage-like structure of the hydrate. Experimental results show that when the mass fraction of urea reaches 10.0 wt%, the methane displacement rate increases to 75.51%, and the CO2 sequestration rate reaches 34.87%, representing increases of approximately 40.03% and 17.93% respectively compared to the urea-free system, with the overall displacement efficiency more than doubling. These results indicate that urea plays a crucial kinetic promoting role in the CO2 displacement mining of natural gas hydrate in silty mud, providing new technical insights and experimental basis for the efficient mining of natural gas hydrate and the geological sequestration of CO2.

[0017] Compared with existing methods, the present invention has the following advantages:

[0018] (1) High methane replacement efficiency: The introduction of urea can increase the CH4 replacement rate. The CH4 replacement rate in silty mudstone sediments containing 10 wt% urea reached 75.51%. Compared with the silty mudstone sediment system without added urea, the replacement efficiency was more than doubled, demonstrating an excellent promoting effect.

[0019] (2) High CO2 sequestration rate: The introduction of urea can significantly promote the transport of CO2 molecules between interfaces during the replacement process, thereby improving the CO2 sequestration efficiency. In a silty mudstone sedimentary system containing 10 wt% urea, the CO2 sequestration rate reached 34.87%. Compared with the traditional natural gas hydrate replacement extraction method, this method not only improves the CO2 sequestration efficiency, but also effectively reduces energy consumption and maintains the stability of the formation structure.

[0020] (3) Environmentally friendly: Urea is non-toxic and biodegradable, and will not pollute the geological environment;

[0021] (4) Wide range of applications: Urea is abundant and inexpensive, and has the potential for laboratory and field applications. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the experimental setup;

[0023] Figure 2 Pressure-temperature curves for hydrate formation and replacement processes;

[0024] Figure 3 The trend of CH4 replacement rate over time under different urea concentrations;

[0025] Figure 4 The trend of CO2 sequestration rate over time under different urea concentrations is shown. Detailed Implementation

[0026] The technical solution and implementation effects of the present invention will be further described in detail below with reference to the embodiments, but this should not be construed as limiting the scope of implementation of the present invention.

[0027] To investigate the method for extracting natural gas hydrates from silty clay using urea-promoted carbon dioxide replacement, as provided in this invention, the experimental equipment employed is a high-pressure reactor and its supporting system. The main components include: a high-pressure reactor, a balancing vessel, a constant-temperature water bath, a temperature and pressure data monitoring and acquisition system, and a gas chromatograph. The high-pressure reactor is the core component of the entire experiment; it is made of 316L stainless steel, has a maximum working pressure of 32.0 MPa, and an effective volume of 205.0 mL. Pressure sensors and Pt100 precision platinum resistance temperature sensors are installed on the top of both the high-pressure reactor and the balancing vessel. The pressure sensor has a measurement range of 0.0–25.0 MPa with an uncertainty of ±0.05 MPa, and the temperature sensor has a measurement range of 263.15–373.15 K with an uncertainty of ±0.1 K. A constant-temperature water bath (model DC-2006, temperature control range -20~100℃, total power 2kW, external circulation pump flow rate 8.0L / min, temperature control accuracy ±0.05℃) is used to control the temperature of the high-pressure reactor and the equilibrium vessel. It is connected to the jackets of the reactor and equilibrium vessel. The coolant used in the constant-temperature water bath is a mixture of ethylene glycol and deionized water at a volume ratio of 1:2. The equilibrium vessel is mainly made of 316L stainless steel, with a maximum working pressure of 32.0MPa and an effective volume of 1000.0mL, used for storing and pre-cooling gases. The data acquisition system automatically collects temperature and pressure data every 1 second. The gas chromatograph used in the experiment is a GC-950, which can be used to determine the content of CH4 and CO2 in the gas phase during the reaction process. The high-pressure reactor and supporting system used are commonly used devices for studying hydrate extraction and replacement technology. See [link to relevant documentation]. Figure 1 .

[0028] Before the reaction began, the entire experimental system was cleaned with deionized water and kept vacuum dry. Then, a prepared urea-containing muddy silt deposit was filled into the reactor. After sealing the system, the system temperature was maintained stable for 2 hours. Methane gas was then introduced into the reactor to replace the air inside 2-3 times. Methane gas was introduced from the equilibrium vessel to raise the system pressure to the target pressure. The inlet valve was closed, and the temperature of the constant-temperature water bath was adjusted to the experimental temperature. As the reaction proceeded, the gas was continuously consumed, and relevant data were obtained from the reactor using pressure and temperature measuring instruments. Subsequently, after the reactor pressure was maintained constant for 12 hours, the CH4 hydrate synthesis ended, and the experimental temperature dropped to 268.15 K. At this temperature, CH4 hydrate exhibits the best self-preservation effect. After the temperature stabilized, CH4 was released, and pre-cooled CO2 was rapidly injected to the required pressure. Then, the temperature was raised to the target value to begin replacement. Gas samples were collected periodically during the experiment, and the changes in the molar fractions of CH4 and CO2 in the gas phase at each time point were analyzed using gas chromatography. The methane replacement rate and carbon dioxide sequestration rate were calculated based on the gas composition data.

[0029] Comparative Example 1

[0030] Sample preparation: 90g of the three types of quartz sand and montmorillonite were mixed in a mass ratio of 3:1:3:3 to form a muddy silt sediment. 14g of a solution containing 3.5wt% NaCl was thoroughly mixed with the sediment, filled into a high-pressure reactor at a temperature of 293.15K, and compacted.

[0031] Hydrate synthesis: Methane gas was introduced into the reactor to 8.5 MPa, and the constant temperature water bath was adjusted to lower the experimental temperature to 274.15 K. When the temperature in the reactor suddenly rose, it indicated that hydrates began to form. When the temperature and pressure in the reactor remained stable for a long time, the CH4 hydrate formation experiment was considered complete. The hydrate saturation was 53.11%, and the amount of methane hydrate was calculated to be 0.1102 mol.

[0032] Gas displacement: The temperature was lowered to 268.15 K, where CH4 hydrates have the best self-protection effect. CH4 gas was released, and pre-cooled CO2 was injected to 3.5 MPa. The temperature was then raised to 274.15 K and the reaction was maintained for 120 hours.

[0033] Results: Gas samples were collected from the reactor at different time points after the replacement (0h, 1h, 2h, 4h, 8h, 12h, 24h, 48h, 72h, 96h, and 120h). The content of each component in the gas samples after the reaction was completed was determined by gas chromatography, and the CH4 replacement rate and CO2 sequestration rate were calculated. The final CH4 replacement rate and CO2 sequestration rate were 35.48% and 16.94%, respectively, as shown in Table 1.

[0034] Example 1 (1.0 wt% urea)

[0035] Sample preparation: 90g of three types of quartz sand and montmorillonite were mixed in a mass ratio of 3:1:3:3 to form a muddy silt sediment. 14g of a solution containing 3.5wt% NaCl and 1.0wt% urea was thoroughly mixed with the sediment and filled into a high-pressure reactor at a temperature of 293.15K and compacted.

[0036] Hydrate synthesis: Methane gas was introduced into the reactor to 8.5 MPa, and the constant temperature water bath was adjusted to lower the experimental temperature to 274.15 K. When the temperature in the reactor suddenly rose, it indicated that hydrates began to form. When the temperature and pressure in the reactor remained stable for a long time, the CH4 hydrate formation experiment was considered complete. The hydrate saturation was 53.11%, and the amount of methane hydrate was calculated to be 0.1102 mol.

[0037] Gas displacement: The temperature was lowered to 268.15 K, where CH4 hydrates have the best self-protection effect. CH4 gas was released, and pre-cooled CO2 was injected to 3.5 MPa. The temperature was then raised to 274.15 K and the reaction was maintained for 120 hours.

[0038] Results: Gas samples were collected from the reactor at different time points after the replacement (0h, 1h, 2h, 4h, 8h, 12h, 24h, 48h, 72h, 96h, and 120h). The content of each component in the gas samples after the reaction was completed was determined by gas chromatography, and the CH4 replacement rate and CO2 sequestration rate were calculated. The final CH4 replacement rate and CO2 sequestration rate were 51.99% and 24.44%, respectively, as shown in Table 1. Compared with Comparative Example 1, the replacement and sequestration effects were better, with the CH4 replacement rate and CO2 sequestration rate increasing by 16.51% and 7.5%, respectively. Relevant data are as follows: Figure 3 and Figure 4 As shown.

[0039] Example 2 (3.0 wt% urea)

[0040] Sample preparation: 90g of three types of quartz sand and montmorillonite were mixed in a mass ratio of 3:1:3:3 to form a sediment matrix. 14g of a solution containing 3.5wt% NaCl and 3.0wt% urea was added and thoroughly mixed with the sediment. The mixture was then filled into a high-pressure reactor at a temperature of 293.15K and compacted.

[0041] Hydrate synthesis: Methane gas was introduced into the reactor to 8.5 MPa, and the constant temperature water bath was adjusted to lower the experimental temperature to 274.15 K. When the temperature in the reactor suddenly rose, it indicated that hydrates began to form. When the temperature and pressure in the reactor remained stable for a long time, the CH4 hydrate formation experiment was considered complete. The hydrate saturation was 53.11%, and the amount of methane hydrate was calculated to be 0.1102 mol.

[0042] Gas displacement: The temperature was lowered to 268.15 K, where CH4 hydrates have the best self-protection effect. CH4 gas was released, and pre-cooled CO2 was injected to 3.5 MPa. The temperature was then raised to 274.15 K and the reaction was maintained for 120 hours.

[0043] Results: Gas samples were collected from the reactor at different time points after the replacement (0h, 1h, 2h, 4h, 8h, 12h, 24h, 48h, 72h, 96h, and 120h). The content of each component in the gas samples after the reaction was completed was determined by gas chromatography, and the CH4 replacement rate and CO2 sequestration rate were calculated. The final CH4 replacement rate and CO2 sequestration rate were 56.71% and 26.59%, respectively, as shown in Table 1. Compared with Comparative Example 1, the replacement and sequestration effects were better, with the CH4 replacement rate and CO2 sequestration rate increasing by 21.23% and 9.65%, respectively. Relevant data are as follows: Figure 3 and Figure 4 As shown.

[0044] Example 3 (5.0 wt% urea)

[0045] Sample preparation: 90g of three types of quartz sand and montmorillonite were mixed in a mass ratio of 3:1:3:3 to form a sediment matrix. 14g of a solution containing 3.5wt% NaCl and 5.0wt% urea was added and thoroughly mixed with the sediment. The mixture was then filled into a high-pressure reactor at a temperature of 293.15K and compacted.

[0046] Hydrate synthesis: Methane gas was introduced into the reactor to 8.5 MPa, and the constant temperature water bath was adjusted to lower the experimental temperature to 274.15 K. When the temperature in the reactor suddenly rose, it indicated that hydrates began to form. When the temperature and pressure in the reactor remained stable for a long time, the CH4 hydrate formation experiment was considered complete. The hydrate saturation was 53.11%, and the amount of methane hydrate was calculated to be 0.1102 mol.

[0047] Gas displacement: The temperature was lowered to 268.15 K, where CH4 hydrates have the best self-protection effect. CH4 gas was released, and pre-cooled CO2 was injected to 3.5 MPa. The temperature was then raised to 274.15 K and the reaction was maintained for 120 hours.

[0048] Results: Gas samples were collected from the reactor at different time points after the replacement (0h, 1h, 2h, 4h, 8h, 12h, 24h, 48h, 72h, 96h, and 120h). The content of each component in the gas samples after the reaction was completed was determined by gas chromatography, and the CH4 replacement rate and CO2 sequestration rate were calculated. The final CH4 replacement rate and CO2 sequestration rate were 66.61% and 31.02%, respectively, as shown in Table 1. Compared with Comparative Example 1, the replacement and sequestration effects were better, with the CH4 replacement rate and CO2 sequestration rate increasing by 31.13% and 14.08%, respectively. Relevant data are as follows: Figure 3 and Figure 4 As shown.

[0049] Example 4 (10.0 wt% urea)

[0050] Sample preparation: 90g of three types of quartz sand and montmorillonite were mixed in a mass ratio of 3:1:3:3 to form a sediment matrix. 14g of a solution containing 3.5wt% NaCl and 10.0wt% urea was added and thoroughly mixed with the sediment. The mixture was then filled into a high-pressure reactor at a temperature of 293.15K and compacted.

[0051] Hydrate synthesis: Methane gas was introduced into the reactor to 8.5 MPa, and the constant temperature water bath was adjusted to lower the experimental temperature to 274.15 K. When the temperature in the reactor suddenly rose, it indicated that hydrates began to form. When the temperature and pressure in the reactor remained stable for a long time, the CH4 hydrate formation experiment was considered complete. The hydrate saturation was 53.11%, and the amount of methane hydrate was calculated to be 0.1102 mol.

[0052] Gas displacement: The temperature was lowered to 268.15 K, where CH4 hydrates have the best self-protection effect. CH4 gas was released, and pre-cooled CO2 was injected to 3.5 MPa. The temperature was then raised to 274.15 K and the reaction was maintained for 120 hours.

[0053] Results: Gas samples were collected from the reactor at different time points after the replacement (0h, 1h, 2h, 4h, 8h, 12h, 24h, 48h, 72h, 96h, and 120h). The content of each component in the gas samples after the reaction was completed was determined by gas chromatography, and the CH4 replacement rate and CO2 sequestration rate were calculated. The final CH4 replacement rate and CO2 sequestration rate were 75.51% and 34.87%, respectively, as shown in Table 1. Compared with Comparative Example 1, the replacement and sequestration effects were better, with the CH4 replacement rate and CO2 sequestration rate increasing by 40.03% and 17.93%, respectively, more than doubling. Relevant data are as follows: Figure 3 and Figure 4 As shown.

[0054] Table 1. Methane replacement rate and carbon dioxide sequestration rate under different urea concentrations.

[0055]

[0056] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A method for using urea to promote the replacement of natural gas hydrates in silty clay with carbon dioxide, characterized in that: Introducing urea into silty mud sediments and adjusting the urea concentration promotes the replacement of CH4 hydrates by CO2.

2. The method according to claim 1, characterized in that: Urea was mixed with 3.5 wt% NaCl solution and then added to the silty mud sediment. The concentration of urea in the reaction system was 1 wt% to 10 wt%.

3. The method according to claim 1, characterized in that: Includes the following steps: (1) Preparation of silty mudstone sediments: Three kinds of quartz sand with different mesh sizes were mixed with montmorillonite in a mass ratio of 3:1:3:

3. The average particle size of the mixed minerals was 6.618 μm. Urea was dissolved in 3.5 wt% NaCl solution and then fully mixed with the mixed minerals to form silty mudstone sediments. The sediments were filled into a high-pressure reactor at a temperature of 293.15 K and compacted. (2) Formation of natural gas hydrate: Methane gas is introduced and the temperature is lowered to 274.15 K. The temperature in the reactor suddenly rises, indicating that hydrate has begun to form. When the system pressure and temperature tend to stabilize, the natural gas hydrate formation experiment is considered to be complete. (3) Carbon dioxide replacement: Cool down to 268.15 K, introduce carbon dioxide, raise the temperature to 274.15 K and maintain the reaction; (4) Results determination: The contents of CH4 and CO2 in the gas phase were determined by gas chromatography, and the CH4 replacement rate and CO2 sequestration rate were calculated.

4. The method according to claim 3, characterized in that: In step (1), the quartz sand is 100~200 mesh, 325 mesh and 600 mesh respectively; the montmorillonite is 1250 mesh.

5. The method according to claim 3, characterized in that: The pressure of methane introduced in step (2) is 4 MPa to 10 MPa; the pressure of carbon dioxide introduced in step (3) is 3 MPa to 3.5 MPa.

6. The method according to claim 3, characterized in that: In step (3), the time for CO2 to replace CH4 is 120 h.

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