Method for improving efficiency of exploiting combustible ice through gas carbon dioxide replacement

Abstract

The invention aims to provide a method for improving the efficiency of exploiting combustible ice through gaseous carbon dioxide replacement, and the method specifically comprises the following steps: firstly, determining the critical pressure of carbon dioxide liquefaction and the phase equilibrium pressure of methane hydrate according to the specific environment of a combustible ice target reservoir; then, the environment pressure in the target reservoir is controlled to be a preset first pressure value; then, a certain amount of gaseous carbon dioxide (first volume) is injected into the target reservoir, so that the pressure intensity of the target reservoir is increased to a second pressure intensity value; the combustible ice in the target reservoir is exploited on the basis of carbon dioxide in the target reservoir in a replacement mode, the pressure intensity of the target reservoir reaches the third pressure intensity, and if the methane gas partial pressure of the third pressure intensity reaches the methane hydrate phase balance pressure intensity, the step of controlling the environment pressure intensity in the target reservoir to be the first pressure intensity value is executed again; in addition, according to the proportion of natural gas in produced gas, the amount (second volume value) of carbon dioxide which needs to be injected into the target reservoir subsequently is judged; and judging whether mining is completed or not according to the natural gas production rate of the target reservoir. The combustible ice replacement mining method based on gaseous carbon dioxide is used for mining combustible ice with low cost and high efficiency.

Description

Technical Field

[0001] This application relates to the field of combustible ice extraction technology, and in particular to a method for improving the efficiency of combustible ice extraction by gaseous carbon dioxide replacement. Background Technology

[0002] Against the backdrop of continuously growing global energy demand and increasing pressure on environmental protection, combustible ice, as an abundant, clean, and efficient energy resource, has attracted much attention for its extraction and utilization. Combustible ice, also known as natural gas hydrate, is a crystalline substance formed from natural gas and water under low temperature and high pressure conditions. Its carbon content is far greater than other fossil fuels, earning it the reputation of a "future energy treasure trove." Therefore, the extraction of combustible ice has become a key focus in current fuel development.

[0003] Currently, the extraction technologies for combustible ice are mainly divided into two categories: decomposition extraction and non-decomposition extraction. Decomposition extraction methods include hot water injection, hot steam injection, and thermodynamic inhibitor injection. However, methods such as hot water injection and inhibitor injection have high extraction costs, while bottom-layer heating and combustion methods are technically challenging and unsuitable for practical combustible ice extraction. Furthermore, non-decomposition extraction involves injecting liquid carbon dioxide. This method yields a low proportion of natural gas, significantly increases subsequent separation costs, and the liquid carbon dioxide cannot effectively contact the combustible ice during extraction, resulting in low sequestration rates and inefficient replacement extraction, failing to meet practical requirements. Therefore, developing a combustible ice extraction technology that can improve extraction efficiency, reduce subsequent separation costs, and achieve efficient carbon dioxide sequestration has become an urgent technical challenge. For example, China has successfully achieved commercial extraction of combustible ice in the South China Sea using its independently developed water, sand, and gas separation core technology, marking a significant breakthrough in the energy sector. However, the extraction cost of methane hydrate is relatively high, and it faces environmental risks, such as the greenhouse effect that methane gas leaks may trigger. Therefore, how to safely and economically extract methane hydrate and separate methane gas from it remains a core challenge for countries researching and utilizing methane hydrate. Summary of the Invention

[0004] In view of this, this application provides a method for improving the efficiency of gaseous carbon dioxide replacement mining of combustible ice, so as to mine combustible ice at low cost and high efficiency.

[0005] Specifically, this application is implemented through the following technical solution: Firstly, the combustible ice is mined by decompression method, and part of water, silt and natural gas is mined out, the porosity of the target reservoir of the combustible ice is controlled at about 10% to 30%, the permeability of the target reservoir is improved, in the mining process, the combustible ice is decomposed and absorbs heat, the temperature of the reservoir is reduced to 0-5 DEG C, the pressure of the target reservoir is reduced to 5% to 10% below the phase equilibrium pressure of the combustible ice corresponding to the temperature, about 2-3 MPa, that is, the first pressure.

[0006] Gaseous carbon dioxide is injected into the target reservoir, and the temperature and pressure of the target reservoir are detected, in the injection process, the temperature of the target reservoir is increased to 3-8 DEG C, and the pressure is also increased to the second pressure, about 2.5-3.5 MPa, the second pressure is 5% to 10% lower than the critical pressure of liquid carbon dioxide, the volume of the injected carbon dioxide is the first volume.

[0007] The carbon dioxide in the target reservoir replaces the methane in the combustible ice, and the carbon dioxide is stored into hydrate, the heat generated by the carbon dioxide hydrate is about equal to the heat absorbed by the decomposition of the methane hydrate, the temperature of the target reservoir is slightly increased under the influence of geothermal heat, about 7-9 DEG C, and the pressure is increased to the third pressure, about 6-7 MPa, wherein the partial pressure of methane reaches the phase equilibrium pressure of the methane hydrate, about 5.5-6.5 MPa, at this time, the environmental pressure in the target reservoir is returned to the first pressure value in the step.

[0008] The gaseous mixed gas in the target reservoir is mined out, and whether the proportion of methane in the mixed gas is lower than the first proportion of the preset natural gas is analyzed, the second volume of the gaseous carbon dioxide injected into the target reservoir is determined, if yes, the second volume is less than the first volume, otherwise, the second volume is equal to the first volume.

[0009] Whether the natural gas mining rate is lower than the first preset natural gas mining rate is analyzed, if yes, the collection is completed, otherwise, the natural gas in the target reservoir is continuously mined until the environmental pressure is reduced to the first pressure value, then the second volume of gaseous carbon dioxide is injected, and the replacement mining process is repeated.

[0010] Advantages of the present application: 1. A small amount of gaseous carbon dioxide is injected in batches, the compression energy required by carbon dioxide is reduced, the proportion of methane in the mined gas is increased, the subsequent separation cost is reduced, the pressure required for the generation of carbon dioxide hydrate is reduced, and the deep-sea storage rate of carbon dioxide is improved. BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1 A flow chart of a method for improving the efficiency of gaseous carbon dioxide replacement mining of combustible ice according to the present application is provided. Figure 2A temperature, pressure and injected carbon dioxide volume variation diagram for improving gaseous carbon dioxide displacement exploitation of combustible ice efficiency is provided for the present application. DETAILED DESCRIPTION

[0012] The exemplary embodiments will be described in detail herein with reference to the attached drawings. In the following description, unless otherwise indicated, like numbers refer to like elements throughout the description and drawings. The following description of exemplary embodiments is not intended to represent all embodiments in accordance with the present application.

[0013] The terminology used in the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. As used in the present application, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0014] It is to be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. It is to be understood that the term "and / or" as used herein encompasses all possible combinations of one or more of the associated listed items and can be abbreviated as "or". It is to be understood that the terms "including," "comprising," "consisting" and "consisting essentially of" when used herein, specify the presence of stated features, integers, steps, operations, elements, and / or components but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0015] The following specific embodiments are given to introduce the technical solutions of the present application in detail.

[0016] Figure 1 A flow chart of a method embodiment one for improving gaseous carbon dioxide displacement exploitation of combustible ice efficiency is provided for the present application. Please refer to Figure 1 The method provided by the present embodiment can include: First, combustible ice is exploited by decompression method, and part of water, silt and natural gas are extracted, the porosity of the target reservoir of combustible ice is controlled to be about 10% to 30%, the permeability of the target reservoir is improved, in the exploitation process, combustible ice is decomposed and absorbs heat, the temperature of the reservoir is reduced to 0-5℃, the pressure of the target reservoir is reduced to 5% to 10% below the combustible ice phase equilibrium pressure corresponding to the temperature, about 2-3MPa, which is the first pressure.

[0017] The gaseous carbon dioxide is injected into the target reservoir, and the temperature and pressure of the target reservoir are detected. During the injection, the temperature of the target reservoir increases to 3-8℃, and the pressure also increases to a second pressure, about 2.5-3.5 MPa, which is 5%-10% lower than the critical pressure of the liquefaction of carbon dioxide. The volume of the injected carbon dioxide is the first volume of the injected carbon dioxide.

[0018] The carbon dioxide in the target reservoir replaces the methane in the combustible ice, and the carbon dioxide is stored in the hydrate. The heat generated by the carbon dioxide hydrate is about equal to the heat absorbed by the decomposition of the methane hydrate. Due to the geothermal environment, the temperature of the target reservoir slightly increases, about 7-9℃, and the pressure of the target reservoir increases to a third pressure, about 6-7 MPa, which makes the partial pressure of the methane in the target reservoir reach the phase equilibrium pressure of the methane hydrate, about 5.5-6.5 MPa. At this time, the environmental pressure in the target reservoir is returned to the first pressure value.

[0019] The gaseous carbon dioxide is injected into the target reservoir, and the temperature and pressure of the target reservoir are detected. During the injection, the temperature of the target reservoir increases to 3-8℃, and the pressure also increases to a second pressure, about 2.5-3.5 MPa, which is 5%-10% lower than the critical pressure of the liquefaction of carbon dioxide. The volume of the injected carbon dioxide is the first volume of the injected carbon dioxide.

[0020] The gaseous carbon dioxide is injected into the target reservoir, and the temperature and pressure of the target reservoir are detected. During the injection, the temperature of the target reservoir increases to 3-8℃, and the pressure also increases to a second pressure, about 2.5-3.5 MPa, which is 5%-10% lower than the critical pressure of the liquefaction of carbon dioxide. The volume of the injected carbon dioxide is the first volume of the injected carbon dioxide.

[0021] A specific embodiment is given below to introduce the processing of the target reservoir before obtaining the environmental pressure of the target reservoir in detail: (1) According to the replacement characteristics of gaseous carbon dioxide, we determine the conditions required for replacing natural gas, which reflects the influence of different porosities and permeabilities of the target reservoir on the difficulty of replacing natural gas.

[0022] Specifically, gaseous carbon dioxide is carbon dioxide in gaseous form. It should be noted that gaseous carbon dioxide has good flowability and easy diffusion.

[0023] In specific implementation, when natural gas is extracted, the temperature and pressure conditions of the target reservoir should be ensured to maintain the gaseous state of gaseous carbon dioxide.

[0024] Further, the replacement capacity of gaseous carbon dioxide is determined according to the properties of gaseous carbon dioxide itself. In specific implementation, the replacement capacity of gaseous carbon dioxide for replacing natural gas can be obtained in the laboratory.

[0025] Further, the replacement condition refers to the elements that must be met when using gaseous carbon dioxide to replace natural gas, and these conditions have a direct impact on the replacement efficiency and effect.

[0026] In a specific implementation, the replacement condition at least includes the relationship between the replacement difficulty and the target reservoir porosity and the relationship between the replacement difficulty and the target reservoir permeability. For example, in an embodiment, the replacement condition is that the porosity of the target reservoir is at least a%, and the permeability of the target reservoir is at least b%.

[0027] Further, the porosity is the percentage of the pore volume to the total volume, and the porosity reflects the ability of the target reservoir to accommodate objects. It should be noted that in the exploitation of combustible ice, the higher the porosity, the more space can be used to accommodate carbon dioxide, thereby improving the replacement efficiency.

[0028] Further, the permeability reflects the ability of a fluid to pass through a porous medium, specifically the speed and smoothness of the fluid flowing inside the medium. It should be noted that in the exploitation of combustible ice, the higher the permeability, the easier it is for gaseous carbon dioxide to penetrate into combustible ice and replace natural gas.

[0029] In a specific implementation, the relationship between different porosities and permeabilities and the difficulty of replacing natural gas can be determined according to the replacement condition.

[0030] (2) Based on the reservoir reconstruction technology, the porosity and permeability are increased in combination with the replacement condition.

[0031] Specifically, the reservoir reconstruction technology is to change the physical properties (such as porosity, permeability, etc.) of the reservoir through artificial methods to improve its flowability and increase the efficiency of natural gas exploitation. It should be noted that the specific reconstruction method of the reservoir reconstruction technology is determined according to actual needs, and in this embodiment, it is not limited.

[0032] In a specific implementation, water and silt in the target reservoir can be removed through exploitation operations, and the flow effect is used to improve the porosity and permeability until the requirements of the replacement condition are met; another method is to inject oxygen into the target reservoir, use the combustion reaction to improve the porosity and permeability until the conditions required for replacement are met; and the porosity and permeability can also be improved by explosion operation in the target reservoir until the requirements of the replacement condition are met.

[0033] The method for improving the efficiency of gaseous carbon dioxide displacement exploitation of combustible ice provided by the embodiment utilizes the physical and chemical properties of gaseous carbon dioxide, and the optimal displacement conditions are carefully determined to maximize the diffusion efficiency of gaseous carbon dioxide in the target reservoir, thereby accelerating the displacement process of natural gas. Further, the reservoir reconstruction technology improves the porosity and permeability of the target reservoir to open up more diffusion paths and storage sites for gaseous carbon dioxide, thereby enabling gaseous carbon dioxide to more effectively penetrate into combustible ice and efficiently displace natural gas. Further, by increasing the porosity and permeability, gaseous carbon dioxide and combustible ice can be brought into sufficient contact, thereby ensuring the sealing rate of displacement.

[0034] A specific embodiment is given below to introduce in detail the setting process of the first pressure of the target reservoir: Step one: obtaining the real-time temperature of the target reservoir. Step two: calculating the phase equilibrium pressure of combustible ice based on the real-time temperature, and calculating the first pressure value equal to 90% to 95% of the phase equilibrium pressure.

[0035] Specifically, the phase equilibrium pressure of combustible ice is lower than the critical pressure of carbon dioxide, and the calculated first pressure is lower than the critical pressure.

[0036] Further, the specific value of the first pressure value can be set according to actual needs, or can be calculated from the exploitation data. It should be noted that when natural gas is first exploited, a preset first pressure value can be used; during continuous exploitation, combustible ice absorbs heat during decomposition, and the real-time temperature of the target reservoir continuously decreases. According to the real-time temperature of the target reservoir, the phase equilibrium pressure of combustible ice can be calculated, which is lower than the critical pressure, and the first pressure value is slightly lower than the phase equilibrium pressure of combustible ice by about 5% to 10%.

[0037] In specific implementation, the critical pressure can be determined according to the properties of gaseous carbon dioxide, and the first pressure value is set to be lower than the phase equilibrium pressure. In addition, the average pressure value of combustible ice in historical exploitation data can also be referred to, and the first pressure value is set to be slightly lower than the average value. For example, in an embodiment, the first pressure value can be set to 2 MPa according to the real-time temperature of the target reservoir.

[0038] Further, the critical pressure refers to the pressure value corresponding to the transition of a substance from one state to another. In specific implementation, the pressure corresponding to the phase transition process from liquid carbon dioxide to gaseous carbon dioxide is determined as the critical pressure. For example, in an embodiment, the critical pressure is used to determine the pressure when carbon dioxide is injected, and the pressure value in the target reservoir and the proportion of natural gas in the produced gas are measured by the exploitation well to prevent the partial pressure of carbon dioxide in the target reservoir from being too high, thereby inhibiting the escape of methane gas from combustible ice and reducing the proportion of methane in the produced gas.

[0039] In an alternative implementation, the mixture in the target reservoir is extracted, and the ambient pressure is detected; When the ambient pressure falls to the first pressure value, the extraction is stopped.

[0040] It can be understood that, in the process of extracting combustible ice, due to the endothermic effect of decomposition, the replacement rate of carbon dioxide and natural gas extraction will gradually slow down, and when the extraction is stopped, both rates will reach the lowest point.

[0041] The method provided by the embodiment can ensure that the replacement process is carried out under the best pressure condition, and by reducing the pressure of the target reservoir to the first pressure value, it can ensure that the injected carbon dioxide is in a gaseous state, and ensure that the methane is fully reacted.

[0042] A specific embodiment is given below to introduce the process of obtaining the second pressure value in detail. Step 1: Obtain the real-time temperature of the target storage. Step 2: Calculate the critical pressure of carbon dioxide based on the real-time temperature, which is 90% to 95% of the critical pressure, and calculate the second pressure value.

[0043] Specifically, the second pressure value is set according to actual needs, and the embodiment does not make specific limitations. In actual operation, the second pressure value can be flexibly set to between 2.5 and 3.5 MPa.

[0044] Further, the second pressure is 90% to 95% of the critical pressure, which can ensure that the injected carbon dioxide is in a gaseous state.

[0045] In a specific implementation, by injecting gaseous carbon dioxide into the target reservoir, the real-time pressure of the target reservoir can be increased to the second pressure value. In this way, by setting the second pressure value to 90% to 95% of the critical pressure, it can be ensured that the combustible ice in the target reservoir can be replaced with gaseous carbon dioxide at the best efficiency.

[0046] It should be noted that after injecting an initial volume of gaseous carbon dioxide into the target reservoir, the reservoir pressure will gradually increase, so the second pressure value must be greater than the initial pressure value. In addition, in order to ensure that gaseous carbon dioxide is used throughout the natural gas extraction process, the second pressure value must be lower than the critical pressure.

[0047] Further, according to the historical mining data, it is determined that the pressure needs to be increased by 10% to 20% during mining, the corresponding increase ratio when the mining rate is the fastest is obtained, and the first pressure is calculated to obtain the second pressure. For example, in an embodiment, the fastest mining is obtained by increasing 15%, and the second pressure is determined as 115% of the first pressure.

[0048] Further, the specific value of the first volume is set according to actual needs, and in this embodiment, no limitation is made. In specific implementation, the average volume of gaseous carbon dioxide injected into the mining well in the historical mining data can be determined as the first volume. For example, the average volume of gaseous carbon dioxide injected into the mining well for the first time, and the corresponding average temperature and pressure are extracted from the historical mining data. The average temperature is compared with the current temperature of the target reservoir to determine the parameter T1; similarly, the average pressure is compared with the current pressure of the target reservoir to determine the parameter P1. Then, the average volume is weighted based on T1 and P1, and combined with the real-time temperature and pressure data measured by the mining well, and finally the first volume at this time is calculated.

[0049] The method for improving the efficiency of gaseous carbon dioxide displacement mining of combustible ice provided in this embodiment can more accurately grasp the stable boundary of combustible ice in the mining process by using the phase equilibrium pressure of combustible ice calculated at the environmental temperature and the critical pressure of carbon dioxide, which helps to avoid uncontrollable decomposition of combustible ice or reduce the mining efficiency due to the change of pressure during the mining process. In this way, the timing and volume of gaseous carbon dioxide injection can be determined by combining the phase equilibrium pressure and the critical pressure, the pressure of the target reservoir is increased to the second pressure value, the energy loss and raw material waste during the mining process are reduced, and the overall mining efficiency is improved.

[0050] A specific embodiment is given below to introduce the calculation process of the first volume in detail. Step one: detecting the real-time pressure and real-time temperature in the target reservoir. Step two: calculating the phase equilibrium pressure of combustible ice according to the real-time temperature, and setting 95% of the phase equilibrium pressure of combustible ice as the first pressure. At the same time, referring to the critical pressure value of carbon dioxide, 95% of the critical pressure of carbon dioxide is set as the second pressure. Step three: calculating the amount of gaseous carbon dioxide required to be injected based on the difference between the second pressure and the first pressure, and calculating the first volume based on the amount of gaseous carbon dioxide injection.

[0051] It can be understood that during the mining process, since the combustible ice in the target reservoir has been partially mined, the specific settings of the first volume and the second pressure value need to be flexibly adjusted to ensure that the mining efficiency is not affected.

[0052] Specifically, the real-time temperature and the real-time pressure of the target reservoir change over time. In a specific implementation, the real-time temperature and the real-time pressure of the target reservoir can be determined based on a temperature sensor and a pressure sensor inside the production well, respectively.

[0053] In a specific implementation, the difference between the real-time pressure and the first pressure is calculated. If the difference between the real-time pressure and the first pressure is less than 5%, the real-time pressure is regarded as a new first pressure, and gaseous carbon dioxide is injected into the target reservoir. Then, the difference between the real-time pressure and the second pressure is calculated. If the difference is less than 5%, the real-time pressure is regarded as a new second pressure.

[0054] Further, the first volume of injected carbon dioxide is measured by a flow meter. The first volume of injected carbon dioxide required is calculated based on the gas components before and after the injection, the real-time pressure, the hydrate porosity, and a gas state equation. When the difference between the first volume of injected carbon dioxide measured by the flow meter and the first volume of injected carbon dioxide required is less than 10%, the calculation process can be used to calculate the second volume of injected carbon dioxide, i.e., the new first volume of injected carbon dioxide.

[0055] Further, in another possible implementation, the first volume required can be accurately calculated by combining the real-time pressure with a preset increase ratio. The preset ratio is intended to reflect the correlation between the pressure before the injection of carbon dioxide and the current real-time pressure. It should be noted that the specific value of the preset increase ratio is set according to actual needs, and in this embodiment, it is not limited. In a specific implementation, the preset increase ratio is set according to the specific size of the first pressure value. For example, if the first pressure value is 2 MPa and it is desired to increase the pressure in the target reservoir to 6 MPa, the preset increase ratio should be set to 3.

[0056] For another example, in another embodiment, referring to the foregoing example, the second pressure is set to 115% of the first pressure. By monitoring the change in the pressure in the target reservoir in real time, the volume of carbon dioxide injected from the first pressure to 115% of the first pressure is defined as the first volume.

[0057] Further, the first volume required is determined by combining the preset increase ratio and the volume of the bottom of the production well.

[0058] The method for improving the efficiency of gaseous carbon dioxide displacement for mining hydrates provided by the embodiment can ensure that the injection amount is neither too much nor too little, so as to ensure that the pressure in the target reservoir can be stabilized at the most suitable pressure for mining natural gas.

[0059] A specific embodiment is given below to introduce in detail the acquisition process of the third pressure value: Specifically, in the process of displacement mining of combustible ice, the real-time pressure needs to be monitored in real time, and the methane partial pressure under the pressure is accurately compared with the real-time methane hydrate phase equilibrium pressure. Once the methane partial pressure approaches the phase equilibrium pressure, the real-time pressure of the target reservoir is recorded as the third pressure.

[0060] It can be understood that if the methane partial pressure under the real-time pressure of the target reservoir is higher than the real-time methane hydrate partial pressure, the rate of displacement of methane hydrate by carbon dioxide will decrease significantly.

[0061] Specifically, after the injection of gaseous carbon dioxide, the temperature of the target reservoir gradually rises, causing the decomposition of natural gas hydrate, the increase of natural gas partial pressure, the entry of carbon dioxide into the hydrate cage, and the decrease of carbon dioxide partial pressure.

[0062] Further, when the increase value of natural gas partial pressure exceeds the decrease value of carbon dioxide partial pressure, the pressure of the target reservoir will rise, and when the displacement rate decreases sharply, the pressure change rate of the target reservoir will also reach a minimum.

[0063] The method for improving the efficiency of gaseous carbon dioxide displacement mining of combustible ice provided by the embodiment is based on the combustible ice phase equilibrium pressure and the methane partial pressure calculated based on the environmental temperature, pressure and natural gas components, which can more accurately grasp the stable boundary of combustible ice in the mining process, and help to avoid the decrease of mining efficiency caused by the inhibition of methane hydrate decomposition by methane partial pressure in the mining process. Thus, in combination with the phase equilibrium pressure and the methane partial pressure, the timing and rate of mining mixed gas can be determined, the pressure of the target reservoir is increased to the third pressure value, the carbon dioxide storage rate is increased, and the overall mining efficiency is improved.

[0064] A specific embodiment is given below to introduce in detail the adjustment process of the first volume, i.e. the calculation process of the second volume: It can be understood that when the proportion of methane in the produced gas is low, the separation cost in the later stage will increase. At this time, by reducing the second volume of injected carbon dioxide, the proportion of natural gas in the produced gas can be effectively improved.

[0065] Specifically, when the pressure of the target reservoir reaches the preset third pressure, the proportion of methane in the produced gas needs to be at least higher than the first preset proportion. With the mining of mixed gas, combustible ice continues to decompose, thereby increasing the proportion of natural gas, and at the same time, carbon dioxide is stored in the hydrate, and its proportion decreases accordingly, thereby increasing the overall proportion of natural gas in the produced gas.

[0066] It should be noted that the first preset proportion is not a fixed value recognized by the industry. The first preset proportion is related to the gas separation cost. The lower the separation cost, the lower the first preset proportion.

[0067] Further, if the proportion of natural gas in the produced gas is lower than the first preset proportion, the second volume of injected carbon dioxide needs to be further reduced to effectively increase the proportion of natural gas in the produced gas.

[0068] In a specific implementation, in the process of mining combustible ice, the proportion of natural gas in the produced gas from the reservoir is monitored in real time through a sensor or a sampling device installed near the mining well. When the proportion of natural gas in the produced gas is lower than the first preset proportion, the second volume of injected carbon dioxide is reduced to ensure that the proportion of natural gas in the produced gas is higher than the first preset proportion. Until the second volume of injected carbon dioxide reaches a minimum value (0), it can be determined that the collection is completed.

[0069] Further, for example, in an embodiment, the pressure of the target reservoir is reduced to a first pressure value 2 MPa, at this time, the temperature of the target reservoir is reduced from 10℃ to 5℃, and by batch injection of gaseous carbon dioxide, the target reservoir can reach a second pressure. In the process of cyclic mining, the ratio of the second pressure to the first pressure gradually decreases, and the second volume gradually decreases. In the future, until the proportion of the produced natural gas (main component is methane) is reduced to 50%.

[0070] It should be noted that the specific value of the first preset proportion is set according to actual needs, and in this embodiment, it is not limited. In a specific implementation, the first preset proportion can be set to 50%.

[0071] The method for improving the efficiency of gaseous carbon dioxide displacement mining of combustible ice provided in this embodiment can ensure that the proportion of methane in the produced gas is higher than the first preset proportion by monitoring the natural gas component of the produced gas from the target reservoir in real time and gradually reducing the volume of injected gaseous carbon dioxide according to the gradually decreasing ratio of the second pressure to the first pressure, thereby reducing the late separation cost.

[0072] A specific embodiment is given below to introduce the specific judgment process of gas production completion in detail. It can be understood that the pressure gradually decreases in the process of mining combustible ice. Since carbon dioxide can only displace part of the combustible ice, the heat released by the formation of carbon dioxide hydrate cannot meet the heat required for the decomposition of all combustible ice. This leads to a gradual decrease in temperature, and thus the decomposition rate of combustible ice gradually slows down.

[0073] Specifically, when the second stage volume of injected gaseous carbon dioxide is adjusted to a minimum value, the mining rate of natural gas will be lower than the first stage natural gas mining rate, at this time, it is considered that the gas production process is completed.

[0074] It should be noted that the first preset natural gas production rate is not a fixed value recognized by the industry, and the first preset natural gas production rate is related to the sales price of natural gas. The higher the sales price of natural gas, the lower the first preset proportion of natural gas production rate.

[0075] The combustible ice mining method based on gaseous carbon dioxide provided in the embodiment can significantly improve the carbon dioxide storage ratio in the target reservoir under the same injection amount compared with the traditional one-time injection of carbon dioxide method, and the increase can be more than 50%, or even up to 100%. This helps to store carbon dioxide for a long time and reduce greenhouse gas emissions. At the same time, by real-time regulation of the pressure and temperature conditions of the target reservoir at different stages of mining, the natural gas mining rate can be further improved. Further, by real-time adjustment of the volume of injected carbon dioxide, we can ensure a high proportion of natural gas in the produced gas, which not only improves the mining efficiency of combustible ice, but also effectively reduces the cost required for gas separation in the later stage. In this way, through multiple injections of gaseous carbon dioxide, efficient and adjustable combustible ice mining can be achieved.

[0076] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A method for improving the efficiency of gas hydrate recovery by replacement with gaseous carbon dioxide, characterized in that, The method includes: Determine the critical pressure for carbon dioxide liquefaction and the equilibrium pressure for methane hydrate phase based on the environment of the target methane hydrate reservoir. The environmental pressure within the target reservoir is controlled to a first pressure value, which is less than the critical pressure and the phase equilibrium pressure. A first volume of gaseous carbon dioxide is injected into the target reservoir, increasing the pressure of the target reservoir to a second pressure value, which is less than the critical pressure and greater than the first pressure value. By using gaseous carbon dioxide to replace the combustible ice in the target reservoir, the reservoir pressure is increased to the third pressure. If the partial pressure of methane gas at the third pressure in the target reservoir reaches the real-time methane hydrate phase equilibrium pressure, return to the step of controlling the ambient pressure in the target reservoir to the first pressure value. The second volume of gaseous carbon dioxide injected into the target reservoir is determined based on whether the proportion of methane in the produced gas is lower than the preset first proportion of methane. If so, the second volume is less than the first volume; otherwise, the second volume is equal to the first volume. If the natural gas extraction rate is lower than the preset first natural gas extraction rate, the extraction is completed; otherwise, the process returns to the step of controlling the environmental pressure of the target reservoir to the first pressure value.

2. The method of claim 1, wherein, The extraction of combustible ice from the target reservoir based on the replacement of gaseous carbon dioxide in the target reservoir includes: When the partial pressure of methane gas at the third pressure in the target reservoir reaches the real-time methane hydrate phase equilibrium pressure, gas is extracted from the target reservoir in real time, and the pressure of the target reservoir decreases, reaching the first pressure value.

3. The method of claim 1, wherein, Before the step of returning the environmental pressure within the target reservoir to the first pressure value, the method further includes: Based on the real-time gas collection rate of combustible ice and the proportion of natural gas in the extracted gas, the first volume and the second pressure are adjusted accordingly. The first volume is positively correlated with the proportion of natural gas and negatively correlated with the collection rate. At the same time, the first volume and the first pressure together determine the magnitude of the second pressure. The step involves using the adjusted first volume value as the second volume value and returning the environmental pressure within the target reservoir as the first pressure value.

4. The method of claim 1, wherein, Before controlling the environmental pressure within the target reservoir to a first pressure value, the method further includes: Obtain the real-time temperature of the target storage; The phase equilibrium pressure of natural gas hydrate is calculated based on the real-time temperature, and the first pressure value is calculated based on the phase equilibrium pressure.

5. The method of claim 1, wherein, After injecting a first volume of gaseous carbon dioxide into the target reservoir, and the pressure of the target reservoir reaches a second pressure value, the method further includes: Immediately after injecting carbon dioxide into the target reservoir, its real-time temperature is obtained, and the critical pressure of carbon dioxide is calculated accordingly. It should be noted that the second pressure should be slightly lower than this critical pressure.

6. The method of claim 1, wherein, The method further includes: The methane gas partial pressure at the third pressure in the target storage reaches the real-time methane hydrate phase equilibrium pressure. The real-time temperature change process of the target storage is obtained during the process, with the temperature gradually increasing; calculating a phase equilibrium pressure of methane hydrate based on a temperature of the target reservoir, the phase equilibrium pressure gradually increasing, calculating a third pressure based on a partial pressure of the natural gas and a proportion of the natural gas, the partial pressure of the natural gas gradually increasing; the increasing speed of the partial pressure of the natural gas is greater than the increasing speed of the phase equilibrium pressure.

7. The method of claim 1, wherein, The method further comprises, before determining the first pressure of the target reservoir according to the environment of the target reservoir of the combustible ice: determining a displacement condition of displacing the natural gas based on a displacement capacity of the gaseous carbon dioxide, the displacement condition representing different porosities and permeabilities of the target reservoir and degrees of difficulty of the natural gas being displaced out; increasing the porosities and permeabilities based on a reservoir reconstruction technology in combination with the displacement condition.

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