A method of exploiting natural gas hydrates and sequestering carbon dioxide

Abstract

The application provides a method for exploiting natural gas hydrate and sealing carbon dioxide. The method comprises the following steps: injecting nitrogen into a natural gas hydrate reservoir of an area to be exploited, displacing reservoir pore fluid, making natural gas hydrate decompose to generate natural gas, collecting output fluid of a production well of the area to be exploited; detecting the composition of the output fluid, and when the natural gas content in the output fluid is less than a set value, stopping the injection of nitrogen, injecting carbon dioxide into the natural gas hydrate reservoir of the area to be exploited, forming carbon dioxide hydrate in a natural gas hydrate cavity, and displacing reservoir pore fluid, and collecting output fluid of the production well of the area to be exploited; the above steps can be repeated. The method provided by the application integrates natural gas hydrate exploitation and carbon dioxide sealing, and has the advantages of safety and reliability, wide action range, high working efficiency and the like.

Description

Technical Field

[0001] This invention relates to a method for extracting natural gas hydrates and storing carbon dioxide, belonging to the field of natural gas hydrate extraction technology. Background Technology

[0002] Natural gas hydrates are a clean energy source with vast reserves, wide distribution, and high energy density, representing a highly promising alternative energy source for the future. The development and utilization of natural gas hydrates are receiving increasing attention. Simultaneously, breakthroughs are urgently needed in the development and utilization of new energy sources and carbon dioxide sequestration technologies.

[0003] Natural gas hydrate extraction is more complex than traditional oil and gas development, involving multiple disciplines such as geology, fluid mechanics, thermodynamics, and engineering. Currently, the main extraction methods for natural gas hydrates include thermal shock, depressurization, and chemical inhibitor injection. Their mechanisms can be simply summarized as the thermodynamic process of hydrate decomposition and the kinetic process of gas / liquid / solid multiphase flow and migration. While traditional depressurization is simple to operate, it has low extraction efficiency, a very limited effective range, a short single-well lifespan, and can easily trigger geological disasters. Thermal shock and inhibitor injection are usually used as auxiliary methods to depressurization, which can improve extraction efficiency to some extent, but have little impact on other issues and suffer from low thermal efficiency, high cost, and impact on the sedimentary ecological environment. Carbon dioxide sequestration mainly relies on deep-sea burial and chemical conversion, but its sequestration efficiency and capacity are low, it is prone to instability, and mature sequestration technology has not yet been developed.

[0004] The CO2-CH4 replacement method has unique advantages in the field of natural gas hydrate development, achieving the dual functions of natural gas production and carbon dioxide sequestration. However, this technology still suffers from unresolved problems such as low replacement efficiency, small single-well effective area, and susceptibility to clogging. While the mixed gas injection technology based on the CO2-CH4 replacement method improves replacement efficiency to some extent, it suffers from low natural gas content in the produced gas, low carbon dioxide sequestration efficiency, and uneven distribution of carbon dioxide hydrates. Summary of the Invention

[0005] To address the aforementioned technical problems, the present invention aims to provide a method for extracting natural gas hydrates and storing carbon dioxide. The method provided by this invention integrates natural gas hydrate development and carbon dioxide storage, and has advantages such as safety, reliability, wide applicability, and high efficiency.

[0006] To achieve the above objectives, the present invention provides a method for extracting natural gas hydrates and storing carbon dioxide, comprising the following steps:

[0007] S1: Nitrogen gas is injected into the natural gas hydrate reservoir in the area to be exploited through the injection well, displacing the pore fluid in the reservoir and causing the natural gas hydrate to decompose and produce natural gas. The produced fluid from the production well in the area to be exploited is then collected.

[0008] S2: When the content of natural gas (i.e., methane) in the produced fluid of step S1 is less than the set value, stop injecting nitrogen and replace the injected gas with carbon dioxide. Through the injection well of the natural gas hydrate to be mined area, inject carbon dioxide into the natural gas hydrate reservoir of the to-be-mined area to form carbon dioxide hydrate and displace the pore fluid of the reservoir. Collect the produced fluid of the production well of the natural gas hydrate to-be-mined area.

[0009] In the above method, preferably, in step S1, the injection well pressure is higher than the production well pressure when injecting nitrogen. More preferably, in step S1, the injection pressure driving force of the injection well is 1-5 MPa when injecting nitrogen. This pressure driving force refers to the value by which the injection pressure is higher than the initial reservoir pressure. That is, the injected nitrogen is high-pressure nitrogen. The delivery and injection of high-pressure nitrogen can be carried out using conventional high-pressure delivery systems and high-pressure injection systems in the art. The initial reservoir pressure refers to the initial pressure of the natural gas hydrate reservoir in the area to be exploited before carrying out the method of the present invention. The calculation method for the initial reservoir pressure can be conventional in the art.

[0010] In the above method, preferably, in step S1, the pressure difference between the injection well and the production well is 2-10 MPa.

[0011] In the above method, preferably, in step S1, the nitrogen injection rate is 10000-70000 s m. 3 / day. The gas injection rate refers to an injection rate of 10,000-70,000 m³ / day under conditions of 20°C and 1 standard atmosphere. 3 / sky.

[0012] In the above method, preferably, step S1 further includes: after collecting the produced fluid from the production well in the area to be exploited for natural gas hydrate, detecting the composition of the produced fluid and detecting the amount of natural gas produced.

[0013] In step S1 of the above method, the present invention first injects high-pressure nitrogen into the natural gas hydrate reservoir through an injection well. The pressure of the injection well is higher than that of the production well. The nitrogen displaces the reservoir pore fluid, reduces the partial pressure of methane gas in the reservoir pores, and promotes the decomposition of natural gas hydrate. The high-pressure nitrogen maintains the reservoir pressure and inhibits the inflow of peripheral fluids. Under the pressure difference between the injection well and the production well, the natural gas produced by the decomposition of natural gas hydrate is quickly carried out, produced in the production well, and collected.

[0014] In the above method, preferably, in step S2, the set value is 40-70 mol.%, that is, the natural gas content in the produced fluid is set to 40-70 mol.%.

[0015] In the above method, preferably, in step S2, when injecting carbon dioxide, the injection pressure driving force of the injection well is 1-5 MPa. More preferably, in step S2, the injection pressure of carbon dioxide is the same as the injection pressure of nitrogen in step S1.

[0016] In the above method, preferably, in step S2, the pressure difference between the injection well and the production well is 2-10 MPa.

[0017] In the above method, preferably, in step S2, the amount of carbon dioxide injected is the amount of natural gas extracted in step S1.

[0018] In the above method, preferably, in step S2, the carbon dioxide injection rate is 10,000-70,000 sm. 3 / day.

[0019] In the above method, preferably, step S2 further includes: after collecting the produced fluid from the production well in the area to be exploited for natural gas hydrate, detecting the composition of the produced fluid and detecting the amount of natural gas produced.

[0020] In step S2 of the above method, the injected carbon dioxide mainly reacts with free water in the goaf of natural gas hydrate generated in step S1 to form carbon dioxide hydrate, and a small amount participates in the natural gas hydrate replacement reaction. The pore fluid with high nitrogen content flows to the production well under the displacement of carbon dioxide, which triggers the decomposition of natural gas hydrate, and the mixed gas containing natural gas is produced from the production well.

[0021] According to a specific embodiment of the present invention, preferably, the above-described method for extracting natural gas hydrates and sequestering carbon dioxide further includes the following steps:

[0022] S3: When the amount of carbon dioxide injected in step S2 reaches the amount of natural gas (i.e., methane) produced in step S1 or the carbon dioxide storage reaches saturation, stop injecting carbon dioxide and replace the injected gas with nitrogen. Inject nitrogen into the natural gas hydrate reservoir in the area to be exploited through the injection well, causing the carbon dioxide hydrate formed in step S2 near the injection well to decompose and migrate from the area near the injection well to the area away from the injection well. At the same time, it promotes the decomposition of natural gas hydrate in the area far from the injection well to produce natural gas, and collects the produced fluid from the production well in the area to be exploited.

[0023] In the above method, preferably, in step S3, the method for determining whether carbon dioxide sequestration has reached saturation is: when the CO2 extraction concentration in the produced fluid is >10 mol.%, then carbon dioxide sequestration has reached saturation.

[0024] In the above method, preferably, in step S3, the injection well pressure is higher than the production well pressure when injecting nitrogen. More preferably, in step S3, the injection pressure driving force of the injection well is generally 1-5 MPa when injecting nitrogen. That is, the injected nitrogen is high-pressure nitrogen. The delivery and injection of high-pressure nitrogen can be carried out using conventional high-pressure delivery systems and high-pressure injection systems in the art.

[0025] In the above method, preferably, in step S3, the pressure difference between the injection well and the production well is 2-10 MPa.

[0026] In the above method, preferably, in step S3, the nitrogen injection rate is 10000-70000 s m. 3 / sky.

[0027] In the above method, preferably, step S3 further includes: after collecting the produced fluid from the production well in the area to be exploited for natural gas hydrate, detecting the composition of the produced fluid and detecting the amount of natural gas produced.

[0028] In step S3 of the above method, when the amount of carbon dioxide injected in step S2 reaches the predetermined value (i.e., the amount of natural gas produced in step S1) or the carbon dioxide storage reaches saturation, the injected gas is replaced with nitrogen. The pressure of the injection well is higher than that of the production well. The injection of nitrogen causes the carbon dioxide hydrate formed in step S2 near the injection well to decompose, and promotes the decomposition of natural gas hydrate in the area far from the injection well to produce natural gas. At the same time, the carbon dioxide hydrate migrates from the area near the injection well to the area far from the injection well, and the mixed gas containing natural gas is produced from the production well.

[0029] According to a specific embodiment of the present invention, preferably, the above-described method for extracting natural gas hydrates and sequestering carbon dioxide further includes the following steps:

[0030] S4: When the natural gas content in the produced fluid of step S3 is less than the set value, stop injecting nitrogen and replace the injected gas with carbon dioxide. Through the injection well in the area to be exploited by natural gas hydrate, inject carbon dioxide into the natural gas hydrate reservoir in the area to be exploited to form carbon dioxide hydrate and displace the pore fluid in the reservoir. Collect the produced fluid from the production well in the area to be exploited by natural gas hydrate.

[0031] In the above method, preferably, in step S4, the set value is 40-70 mol.%, that is, the natural gas content in the produced fluid is set to 40-70 mol.%.

[0032] In the above method, preferably, in step S4, when injecting carbon dioxide, the injection pressure driving force of the injection well is 1-5 MPa. More preferably, in step S4, the injection pressure of carbon dioxide is the same as the injection pressure of nitrogen in step S3.

[0033] In the above method, preferably, in step S4, the pressure difference between the injection well and the production well is 2-10 MPa.

[0034] In the above method, preferably, in step S4, the amount of carbon dioxide injected is the amount of natural gas produced in step S3.

[0035] In the above method, preferably, in step S4, the carbon dioxide injection rate is 10,000-70,000 sm. 3 / day.

[0036] In the above method, preferably, step S4 further includes: after collecting the produced fluid from the production well in the area to be exploited for natural gas hydrate, detecting the composition of the produced fluid and detecting the amount of natural gas produced.

[0037] In step S4 of the above method, the injected carbon dioxide mainly reacts with free water in the goaf of natural gas hydrate generated in step S3 to form carbon dioxide hydrate, and a small amount participates in the natural gas hydrate replacement reaction. The pore fluid with high nitrogen content flows to the production well under the displacement of carbon dioxide, which triggers the decomposition of natural gas hydrate, and the mixed gas containing natural gas is produced from the production well.

[0038] In the above method, preferably, steps S1, S2, S3, and S4 further include: collecting the produced fluid from the production well in the area to be exploited for natural gas hydrate, performing gas-liquid separation on the produced fluid, and collecting the separated gas. The gas-liquid separation device and the gas collection device used can be conventional devices or systems in the art, and the present invention does not specifically limit them.

[0039] According to a specific embodiment of the present invention, preferably, the above-described method for extracting natural gas hydrates and storing carbon dioxide further includes the following steps before step S1:

[0040] S0: The pressure reduction method is used to extract pore fluids from the natural gas hydrate reservoir.

[0041] More specifically, step S0 may include: under extraction pressure, extracting pore fluid from the natural gas hydrate reservoir through a production well in the area to be extracted, thereby establishing a pressure gradient in the reservoir. The depressurization method is a conventional method for extracting natural gas hydrates in this field, and the extraction can be performed using a conventional pump. Before step S1, the present invention uses a depressurization method to extract pore fluid. Its main function is not to extract natural gas hydrates through depressurization, but rather to establish a pressure gradient in the reservoir by extracting pore fluid, guiding the subsequent injected gas towards the production well. However, it can also produce fluid containing natural gas hydrates. More preferably, the extraction pressure can be the natural gas hydrate phase equilibrium pressure corresponding to the temperature of the natural gas hydrate reservoir in the area to be extracted, or higher. Under this pressure, during the depressurization extraction process (i.e., before step 1 of the present invention), only a small amount of natural gas hydrate decomposes. More preferably, in the subsequent extraction process (i.e., steps S1-S2 or steps S1-S4 and their repetition), the depressurization method is continuously used to extract pore fluid in the natural gas hydrate reservoir so that the production well is always kept in a low-pressure state to ensure the flow and production of reservoir fluid.

[0042] According to a specific embodiment of the present invention, preferably, the above-described method for extracting natural gas hydrates and sequestering carbon dioxide further includes the following steps:

[0043] Repeat steps S1 and S2 several times, or repeat steps S1-S4 several times, alternating between nitrogen and carbon dioxide injection, until the natural gas content in the produced fluid is less than 30 mol.% after a certain round of carbon dioxide injection. Then, stop switching the injection gas and continue injecting carbon dioxide until the natural gas content in the produced fluid is less than 10 mol.%, at which point mining ends and carbon dioxide sequestration is completed.

[0044] In the above method, preferably, the distance between the injection well and the production well is 50-100m.

[0045] The method for extracting natural gas hydrates and storing carbon dioxide provided by this invention has at least the following superior technical effects:

[0046] 1. This invention differs from the traditional depressurization and displacement extraction concepts. Instead, it provides a method for extracting natural gas hydrate resources by alternating nitrogen and carbon dioxide injection and for solid-state carbon dioxide sequestration. This method achieves efficient development of natural gas hydrates and efficient, uniform, and stable sequestration of carbon dioxide while maintaining reservoir pressure.

[0047] 2. The method of the present invention mainly induces the decomposition of natural gas hydrate by injecting nitrogen to displace pore fluid. The extraction pressure is relatively freely controlled, which can prevent geological disasters caused by a large pressure drop.

[0048] 3. The method of the present invention is a propulsion-type natural gas hydrate extraction-carbon dioxide storage technology, which avoids the gas short-circuiting behavior that is prone to occur in continuous injection of mixed gas replacement. Each propulsion evolution process of carbon dioxide hydrate will re-establish the flow channel, which is beneficial to improving the natural gas recovery rate.

[0049] 4. In the application of the method of the present invention, the fluid phase in the reservoir pores is mainly gas phase, with a small sand carrying capacity. High-pressure nitrogen can limit the intrusion of external fluids, increase the gas-water ratio of the produced gas, and the carbon dioxide sequestration process can timely strengthen the goaf of natural gas hydrate, stabilize the sedimentary layer, and achieve the effect of effectively preventing sand blockage in the reservoir and wellbore.

[0050] 5. The method of the present invention can control the volume of the decomposition area by adjusting the alternation frequency, thereby reducing the risk of reservoir instability during the mining process.

[0051] In summary, the method for extracting natural gas hydrates and storing carbon dioxide provided by this invention is an alternating gas injection method for extracting natural gas hydrates and storing carbon dioxide. This method, through the alternating injection of nitrogen and carbon dioxide, efficiently utilizes both the N2-induced decomposition of natural gas hydrates and the ease with which carbon dioxide combines with decomposed water to form hydrates. This effectively improves the extraction efficiency of natural gas hydrates while simultaneously enabling the rapid, uniform, and stable storage of large amounts of carbon dioxide in solid hydrate form, providing significant protection for reservoir stability. The method provided by this invention integrates natural gas hydrate development and carbon dioxide storage, offering advantages such as safety, reliability, wide applicability, and high efficiency. It enables the safe and efficient development of natural gas hydrates, achieving both propulsive extraction of natural gas resources and in-situ carbon dioxide storage. Attached Figure Description

[0052] Figure 1 This is a schematic diagram of the method and equipment for extracting natural gas hydrates and storing carbon dioxide provided in Example 1.

[0053] Figure 2 The results are numerical simulation test results of the method for extracting natural gas hydrates and storing carbon dioxide in Example 2.

[0054] Explanation of key component symbols: Gas injection device 1, injection well 2, production well 3, gas-liquid separation device 4, and gas collection device 5. Detailed Implementation

[0055] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0056] According to a specific embodiment of the present invention, preferably, the method for extracting natural gas hydrates and storing carbon dioxide of the present invention includes the following steps:

[0057] S0 (Conventional Depressurization Method for Pore Fluid Extraction): The production well in the area to be exploited for natural gas hydrate is opened. Using a conventional depressurization method, pore fluid within the natural gas hydrate reservoir is extracted through the production well at the exploitation pressure, establishing a pressure gradient within the reservoir. The exploitation pressure is the natural gas hydrate phase equilibrium pressure corresponding to the temperature of the natural gas hydrate reservoir in the area to be exploited, or higher. During the depressurization extraction process at this pressure (i.e., before step 1 of this invention is performed), only a small amount of natural gas hydrate decomposition occurs. The extraction can be performed using a conventional pump. In subsequent exploitation processes, the conventional depressurization method is continuously used to extract pore fluid from the natural gas hydrate reservoir to maintain a low-pressure state in the production well, ensuring the flow and production of reservoir fluid.

[0058] S1 (Continuous Nitrogen Injection Displacement): High-pressure nitrogen is injected into the natural gas hydrate reservoir of the area to be exploited through an injection well. The injection well pressure is higher than the production well pressure, the injection pressure driving force of the injection well is 1-5 MPa, the pressure difference between the injection well and the production well is 2-10 MPa, and the nitrogen injection rate is 10,000-70,000 Sm. 3 / day, nitrogen is used to displace the reservoir pore fluid, reducing the partial pressure of methane gas in the reservoir pores and promoting the decomposition of natural gas hydrates. High-pressure nitrogen is used to maintain the reservoir pressure and inhibit the inflow of external fluids. Under the pressure difference between the injection well and the production well, the natural gas produced by the decomposition of natural gas hydrates is rapidly carried out. The produced fluid of the production well in the area to be exploited for natural gas hydrates is collected, and the produced fluid is subjected to gas-liquid separation. The gas obtained after separation is collected, and the composition of the produced fluid and the amount of natural gas produced are detected.

[0059] S2 (Carbon Dioxide Displacement Injection): When the natural gas content in the produced fluid is less than a set value, nitrogen injection is stopped, and carbon dioxide is injected into the natural gas hydrate reservoir in the area to be exploited through an injection well. The set value is 40-70 mol.%, meaning the natural gas content in the produced fluid is set to 40-70 mol.%. The injection pressure of the injection well is 1-5 MPa. Preferably, the carbon dioxide injection pressure is the same as the nitrogen injection pressure in step S1. The pressure difference between the injection well and the production well is 2-10 MPa. The carbon dioxide injection rate can be the natural gas production rate in step S1, and the carbon dioxide injection rate is 10,000-70,000 s / m. 3 / day, the injected carbon dioxide mainly reacts with free water in the natural gas hydrate goaf generated in step S1 to form carbon dioxide hydrate, and a small amount participates in the natural gas hydrate replacement reaction. The pore fluid with high nitrogen content flows to the production well under the displacement of carbon dioxide, which triggers the decomposition of natural gas hydrate. The mixed gas containing natural gas is produced from the production well. The produced fluid of the production well in the area to be exploited by natural gas hydrate is collected. After the produced fluid is separated into gas and liquid, the gas obtained after separation is collected, and the composition of the produced fluid and the amount of natural gas produced are detected.

[0060] S3 (Continuous Nitrogen Displacement): When the carbon dioxide injection rate in step S2 reaches the natural gas production rate in step S1 or carbon dioxide sequestration reaches saturation, carbon dioxide injection is stopped, and nitrogen is replaced with nitrogen gas. Nitrogen is injected into the natural gas hydrate reservoir in the area to be exploited through an injection well. The method for determining carbon dioxide sequestration saturation is: when the CO2 production concentration in the produced fluid is >10 mol.%, carbon dioxide sequestration is considered saturated. The injection well pressure is higher than the production well pressure, the injection pressure driving force of the injection well is 1-5 MPa, the pressure difference between the injection well and the production well is 2-10 MPa, and the nitrogen injection rate is 10000-70000 sm. 3 / day; decompose the carbon dioxide hydrate formed in step S2 near the injection well area and cause the carbon dioxide hydrate to migrate from the area near the injection well area to the area away from the injection well area. At the same time, promote the decomposition of natural gas hydrate in the area far from the injection well to produce natural gas. The natural gas-containing mixture is produced from the production well. Collect the produced fluid from the production well in the area where natural gas hydrate is to be exploited. After the produced fluid is separated into gas and liquid, collect the gas obtained after separation and detect the composition of the produced fluid and the amount of natural gas produced.

[0061] S4 (Carbon Dioxide Displacement Injection): When the natural gas content in the produced fluid of step S3 is less than a set value, nitrogen injection is stopped, and the injected gas is replaced with carbon dioxide. Carbon dioxide is injected into the natural gas hydrate reservoir of the area to be exploited through an injection well. The set value is 40-70 mol.%, meaning the natural gas content in the produced fluid is set to 40-70 mol.%. The injection pressure of the injection well is 1-5 MPa. Preferably, the carbon dioxide injection pressure is the same as the nitrogen injection pressure in step S3. The pressure difference between the injection well and the production well is 2-10 MPa. The carbon dioxide injection rate can be the natural gas production rate in step S3, and the carbon dioxide injection rate is 10,000-70,000 s m / s. 3 / day, the injected carbon dioxide mainly reacts with free water in the natural gas hydrate goaf generated in step S3 to form carbon dioxide hydrate, and a small amount participates in the natural gas hydrate replacement reaction. The pore fluid with high nitrogen content flows to the production well under the displacement of carbon dioxide, which triggers the decomposition of natural gas hydrate. The mixed gas containing natural gas is produced from the production well. The produced fluid of the production well in the area to be exploited by natural gas hydrate is collected. After the produced fluid is separated into gas and liquid, the gas obtained after separation is collected, and the composition of the produced fluid and the amount of natural gas produced are detected.

[0062] Repeat steps S1 and S2 several times, or repeat steps S1-S4 several times, alternately injecting nitrogen and carbon dioxide until the natural gas content in the produced fluid is less than 30 mol.% after a certain round of carbon dioxide injection. Then stop switching the injected gas and continue injecting carbon dioxide until the natural gas content in the produced fluid is less than 10 mol.% to produce fluid, end the mining and complete the carbon dioxide sequestration, and shut down the production well.

[0063] More preferably, the distance between the injection well and the production well is 50-100m.

[0064] Example 1

[0065] This embodiment provides a method for extracting natural gas hydrates and storing carbon dioxide.

[0066] This method uses, for example Figure 1 The equipment shown includes a gas injection device 1, a gas-liquid separation device 4, and a gas collection device 5. The gas injection device 1 is connected to an injection well 2 via conventional methods. The injection well 2 is connected to a production well 3 via conventional methods. The production well 3 is connected to the gas-liquid separation device 4 via conventional methods. The gas-liquid separation device 4 is connected to the gas collection device 5 via conventional methods. The injection well 2 and the production well 3 are conventional natural gas hydrate extraction wells. The distance between the injection well 2 and the production well 3 is 50 meters.

[0067] like Figure 1 As shown, the method includes the following steps:

[0068] S0 (Conventional Depressurization Method for Pore Fluid Extraction): Production well 3 in the area to be exploited for natural gas hydrate is opened. Using a conventional depressurization method, pore fluid within the natural gas hydrate reservoir is extracted through production well 3 at the exploitation pressure, establishing a pressure gradient within the reservoir. The exploitation pressure is the phase equilibrium pressure of the natural gas hydrate corresponding to the temperature of the natural gas hydrate reservoir in the area to be exploited. During depressurization extraction at this pressure (i.e., before step 1 of this invention is performed), only a small amount of natural gas hydrate decomposes. The extraction can be performed using a conventional pump. The produced fluid is separated into gas and liquid phases by a gas-liquid separator 4, with the gas phase entering a gas collection device 5. In subsequent exploitation processes, the conventional depressurization method is continuously used to extract pore fluid from the natural gas hydrate reservoir, ensuring that production well 3 remains under low pressure to guarantee the flow and production of reservoir fluid.

[0069] S1 (Continuous Nitrogen Displacement): High-pressure nitrogen is injected into the natural gas hydrate reservoir through injection well 2 in the area to be exploited using gas injection device 1. The pressure of injection well 2 is set to 5.5 MPa, the pressure of production well 3 is set to 3.5 MPa, the pressure difference between injection well 2 and production well 3 is 2 MPa, and the nitrogen injection rate is 10000 Sm. 3 / day, under the action of reservoir pressure gradient, nitrogen flows between injection well 2 and production well 3 and displaces pore fluid in sediment, reducing the partial pressure of methane gas in reservoir pores, stimulating rapid decomposition of natural gas hydrate, and maintaining reservoir pressure through high-pressure nitrogen to inhibit the inflow of peripheral fluid. Under the action of pressure difference between injection well 2 and production well 3, natural gas produced by the decomposition of natural gas hydrate is rapidly carried out. The produced fluid of production well 3 in the area to be exploited by natural gas hydrate is collected, and the produced fluid is separated into gas and liquid phases by gas-liquid separator 4. The gas phase enters gas collection device 5. During this period, the composition of the produced fluid is detected every 1 hour, and the methane concentration and methane production are recorded.

[0070] S2 (Carbon Dioxide Displacement Injection): When the methane concentration in the produced fluid is <70 mol.%, nitrogen injection is stopped, and the injected gas is replaced with carbon dioxide. Carbon dioxide is injected into the natural gas hydrate reservoir of the area to be exploited through injection well 2 using gas injection device 1. The injection pressure of injection well 2 is 5.5 MPa, the pressure difference between injection well 2 and production well 3 is 2 MPa, the injected carbon dioxide rate is the same as the methane production rate in step S1, and the carbon dioxide injection rate is 10000 s m / s. 3 / day, the injected carbon dioxide mainly reacts with free water in the goaf of natural gas hydrate generated in step S1 to form carbon dioxide hydrate, a small amount participates in the natural gas hydrate replacement reaction, and enhances the production of natural gas at the front end. Under the displacement of carbon dioxide, the pore fluid with high nitrogen content flows to the production well 3, triggering the decomposition of natural gas hydrate. The mixed gas containing natural gas is produced from the production well 3. The produced fluid of the production well 3 in the area to be exploited by natural gas hydrate is collected, and the produced fluid is separated into gas and liquid phases by the gas-liquid separation device 4, wherein the gas phase enters the gas collection device 5.

[0071] S3 (Continuous Nitrogen Displacement): When the carbon dioxide injection rate in step S2 reaches the methane production rate in step S1, carbon dioxide injection is stopped, and the injected gas is replaced with nitrogen. High-pressure nitrogen is injected into the natural gas hydrate reservoir through injection well 2 in the area to be exploited using gas injection device 1. The pressure of injection well 2 is higher than that of production well 3. The pressure of injection well 2 is set to 5.5 MPa, and the pressure of production well 3 is set to 3.5 MPa. The pressure difference between injection well 2 and production well 3 is 2 MPa, and the nitrogen injection rate is 10000 s m / s. 3 / day, causing the carbon dioxide hydrate formed in step S2 near injection well 2 to decompose and migrate from the area near injection well 2 to the area away from injection well 2. At the same time, it promotes the decomposition of natural gas hydrate in the area far from injection well 2 to produce natural gas. The natural gas hydrate mining area expands towards production well 3, and the carbon dioxide hydrate migrates towards production well 3. The mixed gas containing natural gas is produced from production well 3. The produced fluid of production well 3 in the area to be mined by natural gas hydrate is collected, and the produced fluid is separated into gas and liquid phases by gas-liquid separation device 4. The gas phase enters gas collection device 5. During this period, the composition of the produced fluid is detected every 1 hour, and the methane concentration and methane production amount are recorded.

[0072] S4 (Carbon Dioxide Displacement Injection): When the methane concentration in the produced fluid of step S3 is <70 mol.%, nitrogen injection is stopped, and the injected gas is replaced with carbon dioxide. Carbon dioxide is injected into the natural gas hydrate reservoir of the area to be exploited through injection well 2 using gas injection device 1. The injection pressure of injection well 2 is 5.5 MPa, the pressure difference between injection well 2 and production well 3 is 2 MPa, the injected carbon dioxide rate is the same as the methane produced in step S3, and the carbon dioxide injection rate is 10000 sm. 3 / day, the injected carbon dioxide mainly reacts with free water in the goaf of natural gas hydrate produced in step S3 to form carbon dioxide hydrate, a small amount participates in the natural gas hydrate replacement reaction, and enhances natural gas production. Under the displacement of carbon dioxide, the pore fluid with high nitrogen content flows to the production well 3, triggering the decomposition of natural gas hydrate. The mixed gas containing natural gas is produced from the production well 3. The produced fluid of the production well 3 in the area to be exploited by natural gas hydrate is collected, and the produced fluid is separated into gas and liquid phases by the gas-liquid separation device 4. The gas phase enters the gas collection device 5 to detect the composition of the produced fluid and the amount of methane produced.

[0073] Repeat steps S1-S4 twice, alternating between injecting nitrogen and carbon dioxide, until the methane concentration in the produced fluid after carbon dioxide injection is below 30 mol.%. Then stop switching the injected gas and continue injecting carbon dioxide until the methane concentration in the produced fluid is below 10 mol.%. End mining and complete carbon dioxide sequestration, stop production and shut down production well 3.

[0074] The analysis and calculation of the produced fluid and the gas collected in the gas collection device 5 yielded the following results. In this embodiment, the methane recovery rate reached over 95%, the methane concentration of the gas obtained in the gas collection device 5 was >85%, the proportion of methane obtained in the three alternating injection processes was 45%, 31%, and 24%, respectively, and the methane concentration of the obtained gas reached 96%, 87%, and 63%, respectively. The amount of carbon dioxide solid-state sequestration was similar to the total amount of methane produced.

[0075] Compared to traditional depressurization extraction, the method in this embodiment increases the methane recovery rate by more than three times, and the produced gas-water ratio is reduced from <50m³ / h. 3 CH4 / m 3 H2O increased to >200m 3 CH4 / m 3 H2O.

[0076] Example 2

[0077] This embodiment presents a numerical simulation study of the method for extracting natural gas hydrates and storing carbon dioxide provided by the present invention, which specifically includes the following steps:

[0078] (1) Based on the numerical simulation software TOUGH+HYDRATE, a one-dimensional geological model with similar properties to marine natural gas hydrate sediments was established. The total length was 100m, the diameter was 1.65cm, and the number of grids was 100 (1×100).

[0079] (2) Nitrogen and carbon dioxide are alternately injected from the right end of the model, and the injected gas is switched every two days. Gas is produced from the left end of the model. The total extraction cycle is 24 days, that is, a total of 12 rounds of alternating injection of nitrogen and carbon dioxide (that is, the injection of nitrogen is counted as one round, the injection of carbon dioxide is counted as one round, and so on, for a total of 12 rounds).

[0080] like Figure 2 As shown, the numerical simulation results reveal the spatiotemporal evolution of hydrate distribution in the reservoir. After the first round of nitrogen injection, hydrate saturation dropped to 0 in the 86-100 meter section near the injection end, indicating complete decomposition of methane hydrates in this area. After the first round of carbon dioxide injection, hydrate saturation in the methane hydrate goaf area created during the first round of nitrogen injection increased to over 30%, indicating the reformation of new CO2 hydrates in this area. Simultaneously, hydrate saturation dropped to 0 in the 75-82 meter section, meaning that CO2 propelled nitrogen forward during this stage, stimulating methane hydrate decomposition and repairing the goaf area created during nitrogen injection. Similarly, after each subsequent round of gas injection, only a small portion of the sedimentary layer had hydrate saturation below 20%, and this low-saturation area continuously advanced from the injection end towards the production end. Ultimately, almost all methane hydrates in the reservoir were converted to carbon dioxide hydrates, and the hydrate saturation in the reservoir remained consistent before and after extraction.

[0081] The results demonstrate that the method provided by this invention can effectively maintain the overall hydrate saturation in the reservoir at a near-initial level, thereby effectively ensuring the safe extraction process of the reservoir. The results of Example 2 are highly consistent with those of Example 1.

Claims

1. A method for extracting natural gas hydrates and sequestering carbon dioxide, comprising the following steps: S1: Nitrogen gas is injected into the natural gas hydrate reservoir in the area to be exploited through the injection well. When injecting nitrogen gas, the injection well pressure is higher than the production well pressure, which displaces the pore fluid in the reservoir, causing the natural gas hydrate to decompose and produce natural gas. The produced fluid from the production well in the area to be exploited is collected. S2: When the natural gas content in the produced fluid of step S1 is less than the set value, which is 40-70 mol.%, stop injecting nitrogen and replace the injected gas with carbon dioxide. Through the injection well in the area to be exploited by natural gas hydrate, inject carbon dioxide into the natural gas hydrate reservoir in the area to be exploited to form carbon dioxide hydrate and displace the pore fluid in the reservoir. Collect the produced fluid from the production well in the area to be exploited by natural gas hydrate. S3: When the amount of carbon dioxide injected in step S2 reaches the amount of natural gas produced in step S1 or the carbon dioxide storage reaches saturation, stop injecting carbon dioxide and replace the injected gas with nitrogen. Inject nitrogen into the natural gas hydrate reservoir in the area to be exploited through the injection well. When injecting nitrogen, the pressure of the injection well is higher than that of the production well, causing the carbon dioxide hydrate formed in step S2 near the injection well to decompose and migrate from the area near the injection well to the area away from the injection well. At the same time, it promotes the decomposition of natural gas hydrate in the area far from the injection well to produce natural gas. Collect the produced fluid from the production well in the area to be exploited. S4: When the natural gas content in the produced fluid of step S3 is less than the set value, which is 40-70 mol.%, stop injecting nitrogen and replace the injected gas with carbon dioxide. Through the injection well in the area to be exploited by natural gas hydrate, inject carbon dioxide into the natural gas hydrate reservoir in the area to be exploited to form carbon dioxide hydrate and displace the pore fluid in the reservoir. Collect the produced fluid from the production well in the area to be exploited by natural gas hydrate. Repeat steps S1-S4 several times, alternating between nitrogen and carbon dioxide, until the natural gas content in the produced fluid is less than 30 mol.% after a certain round of carbon dioxide injection. At this point, stop switching the injected gas and continue injecting carbon dioxide until the natural gas content in the produced fluid is less than 10 mol.%, then end the mining and complete the carbon dioxide sequestration.

2. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, In step S1, when nitrogen is injected, the injection pressure driving force of the injection well is 1-5 MPa.

3. The method for extracting natural gas hydrates and sequestering carbon dioxide according to claim 1, wherein, In step S1, the pressure difference between the injection well and the production well is 2-10 MPa.

4. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, In step S1, the nitrogen injection rate is 10,000-70,000 S m. 3 / day.

5. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, Step S1 further includes: after collecting the produced fluid from the production well in the area to be exploited for natural gas hydrate, detecting the composition of the produced fluid and detecting the amount of natural gas produced.

6. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, In step S2, when injecting carbon dioxide, the injection pressure driving force of the injection well is 1-5 MPa.

7. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, In step S2, the pressure difference between the injection well and the production well is 2-10 MPa.

8. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, In step S2, the amount of carbon dioxide injected is the same as the amount of natural gas produced in step S1.

9. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, In step S2, the carbon dioxide injection rate is 10,000-70,000 S m. 3 / day.

10. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, Step S2 further includes: after collecting the produced fluid from the production well in the area to be exploited for natural gas hydrate, detecting the composition of the produced fluid and detecting the amount of natural gas produced.

11. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, In step S3, the method for determining whether carbon dioxide sequestration has reached saturation is: when the CO2 extraction concentration in the produced fluid is > 10 mol.%, then carbon dioxide sequestration has reached saturation.

12. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, In step S3, when nitrogen is injected, the injection pressure driving force of the injection well is 1-5 MPa.

13. The method for extracting natural gas hydrates and sequestering carbon dioxide according to claim 1, wherein, In step S3, the pressure difference between the injection well and the production well is 2-10 MPa.

14. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, In step S3, the nitrogen injection rate is 10,000-70,000 S m. 3 / day.

15. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, Step S3 further includes: after collecting the produced fluid from the production well in the area to be exploited for natural gas hydrate, detecting the composition of the produced fluid and detecting the amount of natural gas produced.

16. The method for extracting natural gas hydrates and sequestering carbon dioxide according to claim 1, wherein, In step S4, when injecting carbon dioxide, the injection pressure driving force of the injection well is 1-5 MPa.

17. The method for extracting natural gas hydrates and sequestering carbon dioxide according to claim 1, wherein, In step S4, the pressure difference between the injection well and the production well is 2-10 MPa.

18. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, In step S4, the amount of carbon dioxide injected is the same as the amount of natural gas extracted in step S3.

19. The method for extracting natural gas hydrates and sequestering carbon dioxide according to claim 1, wherein, In step S4, the carbon dioxide injection rate is 10,000-70,000 S m. 3 / day.

20. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, Step S4 further includes: after collecting the produced fluid from the production well in the area to be exploited for natural gas hydrate, detecting the composition of the produced fluid and detecting the amount of natural gas produced.

21. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, Steps S1, S2, S3 and S4 further include: after collecting the produced fluid from the production well in the area to be exploited for natural gas hydrate, the produced fluid is subjected to gas-liquid separation, and the gas obtained after separation is collected.

22. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 1, wherein, The method, prior to step S1, further includes the following steps: S0: The pressure reduction method is used to extract pore fluids from the natural gas hydrate reservoir.

23. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 22, wherein, Step S0 includes: under extraction pressure, extracting pore fluid from the natural gas hydrate reservoir through a production well in the area to be exploited, and establishing a pressure gradient in the reservoir.

24. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 23, wherein, The extraction pressure is the natural gas hydrate phase equilibrium pressure corresponding to the temperature of the natural gas hydrate reservoir in the area to be extracted, or higher.

25. The method for extracting natural gas hydrates and storing carbon dioxide according to claim 23, wherein, In subsequent extraction processes, the depressurization method is continuously used to extract pore fluids from the natural gas hydrate reservoir to keep the production well under low pressure, thereby ensuring the flow and production of reservoir fluids.

26. The method for extracting natural gas hydrates and sequestering carbon dioxide according to claim 1, wherein, The distance between the injection well and the production well is 50-100 m.

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