A method for coal seam permeability improvement and gas extraction by high-temperature nitrogen and liquid carbon dioxide

Abstract

This application provides a method for enhancing coal seam permeability and promoting gas extraction using high-temperature nitrogen and liquid carbon dioxide. Multiple extraction holes and injection holes are arranged to connect the target working face coal seam according to a preset extraction radius. A guide pipe is installed along the axial direction in each extraction hole and injection hole, and the gap between the outer wall of the guide pipe and the inner wall of the hole is sealed by pressure. A target value for the injection pressure of liquid carbon dioxide is determined based on the injection volume gradient method. After pre-cooling the guide pipe installed in the injection hole, liquid carbon dioxide is injected into the injection hole through the pre-cooled guide pipe. When the injection pressure reaches the target value, the injection of liquid carbon dioxide is stopped, and high-temperature nitrogen is injected into the injection hole to fully vaporize the liquid carbon dioxide and enhance the permeability of the target working face coal seam. The above steps are repeated until the gas in the target working face coal seam is completely extracted. This technical solution can improve coal seam permeability and gas extraction efficiency.

Description

Technical Field

[0001] This application relates to the field of coal mining technology, and in particular to a method for enhancing the permeability of coal seams and promoting gas extraction using high-temperature nitrogen and liquid carbon dioxide. Background Technology

[0002] Coal, as a dual medium with a complex pore-fracture structure, is an ideal location for gas accumulation. In my country, over 95% of the coal seams mined in outburst-prone and high-gas mines are low-permeability seams, with permeability mostly around 10%. -6 ~10 -7 μ / m 2 In order to improve the extraction rate of coal seam gas and shorten the pre-extraction time, artificial permeability enhancement needs to be implemented.

[0003] Among related technologies, coal seam permeability enhancement can be achieved through liquid carbon dioxide phase change technology. Essentially, this involves using the volume expansion of the liquid-gas phase change to create impact fracturing of the coal seam, thereby increasing its permeability. However, the low-temperature effect generated during the phase change process causes the coal matrix to shrink, increasing the micropore closure rate within the coal seam. This results in a short-lived permeability enhancement effect and poor overall permeability improvement. Summary of the Invention

[0004] This application provides a method for enhancing coal seam permeability and promoting gas extraction using a high-temperature nitrogen-coordinated liquid carbon dioxide approach. This method addresses the shortcomings of existing technologies, such as short permeability enhancement time and poor permeability improvement, thereby increasing permeability during the coal seam enhancement process and ultimately improving the efficiency of gas extraction.

[0005] To achieve the above objectives, this application proposes the following technical solution:

[0006] This application provides a method for enhancing coal seam permeability and promoting gas extraction using a combination of high-temperature nitrogen and liquid carbon dioxide, characterized in that the method includes:

[0007] Step 1: Arrange multiple extraction holes and injection holes connecting the coal seam of the target working face according to the preset extraction radius. The multiple extraction holes and injection holes are arranged linearly, and an injection hole is set every two extraction holes.

[0008] Step 2: Install guide pipes along their axial direction in each extraction hole and injection hole, and pressurize and seal the gap between the outer wall of the guide pipe and the inner wall of the hole.

[0009] Step 3: Determine the target value of the injection pressure for liquid carbon dioxide based on the injection volume gradient method;

[0010] Step 4: After precooling the guide pipe installed in the injection hole, liquid carbon dioxide is injected into the injection hole through the precooled guide pipe. When the injection pressure reaches the target injection pressure value, the injection of liquid carbon dioxide is stopped and high-temperature nitrogen is injected into the injection hole to fully vaporize the liquid carbon dioxide to enhance the permeability of the coal seam in the target working face.

[0011] Repeat step 4 until the gas in the coal seam of the target working face has been completely extracted.

[0012] In one possible implementation, determining the target injection pressure value for liquid carbon dioxide based on the injection rate gradient method includes:

[0013] Liquid carbon dioxide of varying injection volume is injected into the injection orifice to obtain the pressure value corresponding to each injection volume gradient.

[0014] Based on the injection volume gradient and the pressure value corresponding to the injection volume gradient, fit an injection volume gradient-pressure curve;

[0015] The initiation pressure is obtained from the injection volume gradient-pressure curve, which is the target value of the injection pressure.

[0016] In one possible implementation, the injection displacement gradients are 5 L / min, 10 L / min, 15 L / min and 20 L / min in sequence, and each injection displacement gradient lasts for 5-15 minutes.

[0017] In one possible implementation, the injection rate of the liquid carbon dioxide is 1-5 m / s.

[0018] In one possible implementation, the temperature of the high-temperature nitrogen gas is 100-120°C, and the injection flow rate is 40-60 m³ / h. 3 / h, the injection pressure is 70-95% of the target injection pressure value.

[0019] In one possible implementation, the pressurized plugging includes:

[0020] The gap between the outer wall of the guide tube and the inner wall of the hole is sealed by multiple pressurized grouting using a sealing bag. The grouting depth is less than the length of the guide tube extending into the coal seam.

[0021] In one possible implementation, the outer wall of the guide tube disposed in the injection hole is further provided with a sleeve;

[0022] The gap between the sleeve and the outer wall of the guide tube is sealed by pressure.

[0023] In one possible implementation, a control valve is provided at the end of the guide pipe located in the injection hole that is away from the coal seam of the target working face, and the control valve is also connected to the high-temperature nitrogen injection pipeline and the liquid carbon dioxide injection pipeline respectively.

[0024] In one possible implementation, the length of the guide pipe laid in the extraction hole is less than the length of the guide pipe laid in the injection hole.

[0025] In one possible implementation, a gas concentration sensor is installed in the extraction pipeline connected to the extraction hole;

[0026] The gas concentration sensor is used to monitor the gas concentration in the extraction pipeline.

[0027] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages:

[0028] This application embodiment uses liquid carbon dioxide and high-temperature nitrogen in synergy to enhance coal seam permeability, avoiding the matrix shrinkage effect associated with low-temperature fracturing caused by injecting liquid carbon dioxide alone. Furthermore, the timing of high-temperature nitrogen injection is determined based on the pressure value at the injection hole when liquid carbon dioxide is injected, ensuring that high-temperature nitrogen is injected at the appropriate time and achieving precise control of the injected gas. This effectively solves the problems of low timeliness and poor effect in coal seam permeability enhancement, improves the timeliness of permeability enhancement during the coal seam permeability enhancement process, and thus improves the efficiency of gas extraction. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments of this application or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 A flowchart of a high-temperature nitrogen-coordinated liquid carbon dioxide coal seam permeability enhancement and gas extraction method provided in this application embodiment;

[0031] Figure 2 A coal seam profile and borehole layout diagram provided for an embodiment of this application;

[0032] Figure 3 A schematic diagram of a high-temperature nitrogen-coordinated liquid carbon dioxide coal seam permeability enhancement and gas extraction method provided for embodiments of this application;

[0033] Figure 4 A flowchart of another high-temperature nitrogen-coordinated liquid carbon dioxide coal seam permeability enhancement and gas extraction method provided in this application embodiment.

[0034] Figure label:

[0035] 1-Coal seam; 2-Left side injection hole; 3-Extraction hole; 4-Extraction pipe; 5-Ball valve; 6-Injection pipe; 7-First sealing section; 8-Second sealing section; 9-Casing; 10-Liquid carbon dioxide injection pipe; 11-High temperature nitrogen injection pipe; 12-Right side injection hole. Detailed Implementation

[0036] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0037] In the description of the embodiments of this application, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the embodiments of this application and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application. The terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; a mechanical connection or an electrical connection; a direct connection or an indirect connection through an intermediate medium; or a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of this application according to the specific circumstances.

[0038] First, the application scenario of this application is introduced. This application is applied to the scenario of replacing gas from coal seams. Coal, as a dual medium with a complex pore-fracture structure, is a good place for gas to occur. More than 95% of the coal seams mined in my country's outburst and high-gas mines are low-permeability coal seams, and the permeability of these low-permeability coal seams is mostly around 10%. -6 ~10 -7 μ / m 2 In order to improve the extraction rate of coal seam gas and shorten the pre-extraction time, artificial permeability enhancement needs to be implemented.

[0039] Among related technologies, coal seam permeability enhancement can be achieved through liquid carbon dioxide phase change technology. Essentially, this involves using the volume expansion of the liquid-gas phase change to create impact fracturing of the coal seam, thereby increasing its permeability. However, the low-temperature effect generated during the phase change process causes the coal matrix to shrink, increasing the micropore closure rate within the coal seam. This results in a short-lived permeability enhancement effect and poor overall permeability improvement.

[0040] Therefore, this application provides a method for enhancing coal seam permeability and promoting gas extraction using high-temperature nitrogen combined with liquid carbon dioxide. Multiple extraction holes and injection holes are arranged in a linear arrangement, connecting the coal seam of the target working face, according to a preset extraction radius. An injection hole is placed every two extraction holes. A guide pipe is installed along the axial direction in each extraction hole and injection hole, and the gap between the outer wall of the guide pipe and the inner wall of the hole is sealed with pressure. The target value of the liquid carbon dioxide injection pressure is determined based on the injection volume gradient method. After pre-cooling the guide pipe installed in the injection hole, liquid carbon dioxide is injected into the injection hole through the pre-cooled guide pipe. When the injection pressure reaches the target value, the injection of liquid carbon dioxide is stopped and... High-temperature nitrogen is injected into the injection hole, allowing liquid carbon dioxide to fully vaporize and enhance the permeability of the coal seam in the target working face. The above steps are repeated until the gas in the coal seam in the target working face is completely extracted. In this embodiment, the coal seam permeability is enhanced by the synergistic use of liquid carbon dioxide and high-temperature nitrogen, avoiding the matrix shrinkage effect associated with low-temperature fracturing caused by injecting liquid carbon dioxide alone. Furthermore, the timing of high-temperature nitrogen injection is determined based on the pressure value when liquid carbon dioxide is injected into the injection hole, ensuring that high-temperature nitrogen is injected at the appropriate time and achieving precise control of the injected gas. This effectively solves the problems of low timeliness and poor effect in coal seam permeability enhancement, improves the timeliness of permeability enhancement during the coal seam permeability enhancement process, and thus improves the efficiency of gas extraction.

[0041] Figure 1 This is a flowchart illustrating a high-temperature nitrogen-coordinated liquid carbon dioxide method for enhancing coal seam permeability and promoting gas extraction, as provided in an embodiment of this application. Figure 1 As shown, the method may include the following steps.

[0042] Step 1: Arrange multiple extraction holes and injection holes connecting the coal seam of the target working face according to the preset extraction radius. The multiple extraction holes and injection holes are arranged linearly, and an injection hole is set every two extraction holes.

[0043] For example, the target working face coal seam is the layer in which coal seam permeability enhancement and gas extraction are carried out. The height of the target working face coal seam above the ground can be set by the user and is not limited here. The extraction hole can be used to extract coal seam gas, such as methane, from the target working face coal seam, and the injection hole can be used to inject fluids for coal seam permeability enhancement, such as liquid carbon dioxide and high-temperature nitrogen.

[0044] For example, the preset extraction radius can be set based on the actual situation of the coal seam in the target working face, and is not limited here. The distribution of extraction holes and injection holes can also be set according to the actual situation of the working face, and is not limited here.

[0045] For example, you can refer to Figure 2 The coal seam profile and borehole layout diagram show that 6 extraction holes and 2 injection holes are set in the coal seam of the target working face. Among them, 1 injection hole is set every 2 extraction holes. Hole 1 and Hole 2 are extraction holes, and Hole 3 is an injection hole.

[0046] In one possible implementation, the distribution pattern of extraction and injection holes can be determined through numerical modeling and simulation. This distribution pattern may include parameters such as borehole spacing, borehole inclination angle, or borehole azimuth angle, which are not limited here. Thus, a preferred distribution pattern can be obtained through numerical modeling, and distributing the extraction and injection holes according to this pattern can improve the permeability enhancement and extraction effect.

[0047] Step 2: Install a guide pipe along its axial direction in each extraction hole and injection hole, and pressurize and seal the gap between the outer wall of the guide pipe and the inner wall of the hole.

[0048] For example, the guide pipe of the extraction hole and the guide pipe of the injection hole can be the same or different, and there is no limitation here. The guide pipe built into the extraction hole can be called the extraction pipe, and the guide pipe built into the injection hole can be called the injection pipe. The injection pipe can be a 4-point galvanized iron pipe.

[0049] Step 3: Determine the target value of the injection pressure for liquid carbon dioxide based on the injection volume gradient method.

[0050] In one possible implementation, step 3 may include: injecting liquid carbon dioxide of varying injection volume into the injection hole to obtain a pressure value corresponding to each injection volume gradient; fitting an injection volume gradient-pressure curve based on the injection volume gradient and the pressure value corresponding to the injection volume gradient; and obtaining the crack initiation pressure from the injection volume gradient-pressure curve, which is the target value of the injection pressure.

[0051] Among them, the initiation pressure can be the pressure value that causes the coal seam in the target working face to generate a fracture for the first time.

[0052] In one possible implementation, the injection displacement gradient is successively 5 L / min, 10 L / min, 15 L / min and 20 L / min, and each injection displacement gradient is performed for 5-15 minutes.

[0053] For example, liquid carbon dioxide can be injected into the target working face coal seam in sequence according to the injection volume; for each injection volume of liquid carbon dioxide, the liquid carbon dioxide can be continuously injected into the injection hole according to a preset time threshold, and then the target value of the injection pressure corresponding to the liquid carbon dioxide can be determined.

[0054] For example, liquid carbon dioxide can be injected into the coal seam of the working face in the order of 5L / min, 10L / min, 15L / min, and 20L / min. The preset time threshold can be in the range of 5min-15min, such as 5min, 10min, or 15min, etc., and is not limited here.

[0055] Step 4: After precooling the guide pipe installed in the injection hole, inject liquid carbon dioxide into the injection hole through the precooled guide pipe. When the injection pressure reaches the target value, stop injecting liquid carbon dioxide and inject high-temperature nitrogen into the injection hole to fully vaporize the liquid carbon dioxide and enhance the permeability of the coal seam in the target working face. Repeat Step 4 until the gas in the coal seam of the target working face is completely extracted.

[0056] According to the above technical means, by pre-cooling the guide tube, the phase change of liquid carbon dioxide at the inlet of the guide tube can be prevented, so that the liquid carbon dioxide can maintain a turbulent state.

[0057] This application embodiment uses liquid carbon dioxide and high-temperature nitrogen in synergy to enhance coal seam permeability, avoiding the matrix shrinkage effect associated with low-temperature fracturing caused by injecting liquid carbon dioxide alone. Furthermore, the timing of high-temperature nitrogen injection is determined based on the pressure value at the injection hole when liquid carbon dioxide is injected, ensuring that high-temperature nitrogen is injected at the appropriate time and achieving precise control of the injected gas. This effectively solves the problems of low timeliness and poor effect in coal seam permeability enhancement, improves the timeliness of permeability enhancement during the coal seam permeability enhancement process, and thus improves the efficiency of gas extraction.

[0058] In one possible implementation, the injection rate of the liquid carbon dioxide is 1-5 m / s.

[0059] For example, the injection rate could be 1 m / s, 2 m / s, 3 m / s, or 5 m / s, etc., and is not limited here. Injecting liquid carbon dioxide at this rate ensures that the liquid carbon dioxide inside the pipe remains in a turbulent state.

[0060] In one possible implementation, the temperature of the high-temperature nitrogen gas is 100-120°C, and the injection flow rate is 40-60 m³ / h. 3 / h, the injection pressure is 70-95% of the target injection pressure value.

[0061] For example, the injected flow is 40m.3 / h, 50m 3 / h or 60m 3 The pressure (e.g., / h) is not limited here. The temperature of the high-temperature nitrogen gas can be 100℃, 110℃, or 120℃, etc., not limited here. For example, the injection pressure of the high-temperature nitrogen gas can be 70%, 80%, 85%, or 90%, etc., not limited here. In this way, injecting high-temperature nitrogen gas based on this injection pressure value can maintain the opening of fractures around the coal seam in the target working face and prevent excessive expansion.

[0062] In one possible implementation, the pressurized sealing includes: using a sealing bag to repeatedly inject grout into the gap between the outer wall of the guide pipe and the inner wall of the hole to seal the hole, wherein the grouting depth is less than the length of the guide pipe extending into the coal seam. In this way, pressurized grouting sealing ensures the airtightness of the guide pipe, prevents air leakage, and increases the extraction concentration.

[0063] In one possible implementation, the outer wall of the guide tube disposed in the injection hole is further provided with a sleeve; the gap between the sleeve and the outer wall of the guide tube is sealed by pressure.

[0064] In one possible implementation, a control valve is provided at the end of the guide pipe located within the injection hole, away from the target working face coal seam. This control valve is also connected to both a high-temperature nitrogen injection pipeline and a liquid carbon dioxide injection pipeline. For example, the control valve could be a ball valve, a tee valve, etc., and is not limited here.

[0065] For example, after the guide pipe installed in the injection hole is pre-cooled, the control valve can be opened, and liquid carbon dioxide can be injected into the injection hole through the liquid carbon dioxide injection pipe and the pre-cooling guide pipe according to the injection rate. When the injection pressure reaches the target injection pressure value, the control valve can be closed to stop the injection of liquid carbon dioxide. After stopping the injection of liquid carbon dioxide, the control valve can be opened, and high-temperature nitrogen can be injected into the injection hole through the high-temperature nitrogen injection pipe and the pre-cooling guide pipe according to the injection flow rate.

[0066] In one possible implementation, the length of the guide pipe installed in the extraction hole is less than the length of the guide pipe installed in the injection hole.

[0067] In one possible implementation, the method may further include: after enhancing the permeability of the coal seam in the target working face, displacing the coal seam gas around the injection hole; collecting the coal seam gas through a guide pipe in the extraction hole; performing gas separation on the coal seam gas to obtain one or more of methane, carbon dioxide, and nitrogen; liquefying the carbon dioxide gas to obtain target liquid carbon dioxide; heating the nitrogen gas to obtain target high-temperature nitrogen; and repeatedly injecting the target liquid carbon dioxide and the target high-temperature nitrogen into the guide pipe of the injection hole in the coal seam of the target working face to displac all the coal seam gas around the injection hole.

[0068] For example, coal seam gas can be separated at a gas extraction pumping station.

[0069] Through the above-mentioned technical means, carbon dioxide and nitrogen can be separated from coalbed methane, and the carbon dioxide and nitrogen can be processed to obtain liquid carbon dioxide and high-temperature nitrogen. The processed liquid carbon dioxide and high-temperature nitrogen can then be injected into other injection holes for subsequent coalbed permeability enhancement and gas extraction, thereby realizing the recycling of coalbed methane and improving resource utilization efficiency.

[0070] It should be noted that this turbulent displacement refers to the turbulent displacement of coal seam gas in the target working face using liquid carbon dioxide and high-temperature nitrogen. The high-temperature nitrogen promotes the full vaporization of liquid carbon dioxide, lowering the temperature around the coal seam. This causes the embrittled zone of the coal seam matrix to fracture under temperature stress, and carries the gaseous carbon dioxide deep into the coal seam fractures, displacing the coal seam gas. This accelerates the desorption, diffusion, and migration of the coal seam gas, improving the extraction efficiency.

[0071] The following is combined with Figure 3 A schematic diagram of a high-temperature nitrogen-coordinated liquid carbon dioxide method for enhancing coal seam permeability and promoting gas extraction is provided, and the extraction holes and injection holes in the coal seam of the target working face are introduced.

[0072] For example, multiple extraction and injection holes can be installed in coal seam 1, including a left injection hole 2, a right injection hole 12, and an extraction hole 3. An extraction pipe 4 is installed in extraction hole 3, and the extraction pipe 4 can be sealed for 12m to ensure a tight seal. A casing 9 can be installed in the right injection hole 12, with a length of 35m. An injection pipe 6 can be inserted into the casing 9 before sealing. A 30m sealing bag can be used to perform multiple pressurized grouting seals on the casing 9 to obtain the first sealing section 7. Then, multiple pressurized grouting seals can be performed on the injection pipe 6 to obtain the second sealing section 8, with a sealing depth of 30m. At this point, the right injection hole 12 can be connected to a coal seam gas extraction pipeline for extraction, stabilizing the gas concentration inside the right injection hole 12. The bottom opening of the injection pipe 6 can be sealed by a ball valve 5. The ball valve 5 can be connected to a liquid carbon dioxide injection pipe 10 or a high-temperature nitrogen injection pipe 11. Figure 3 (Not connected in the middle).

[0073] In one possible implementation, a gas concentration sensor is installed in the extraction pipeline connected to the extraction port; the gas concentration sensor is used to monitor the gas concentration in the extraction pipeline. The gas concentration sensor can collect gas concentrations at different times; if the concentration difference between multiple gas concentrations is less than or equal to a preset difference threshold, the control valve is closed to stop the injection of high-temperature nitrogen.

[0074] For example, the gas concentration of the coal seam can be collected multiple times at different times within a preset time period to obtain the gas concentration at each time point. This preset difference threshold can be set by the user and is not limited here.

[0075] Figure 4 A flowchart illustrating another high-temperature nitrogen-coordinated liquid carbon dioxide method for enhancing coal seam permeability and promoting gas extraction, provided as an embodiment of this application. Figure 4 As shown, the method may include the following steps.

[0076] S401. Inject liquid carbon dioxide at different injection rate gradients into the coal seam of the target working face to obtain the pressure value corresponding to each injection rate gradient; take the pressure value that causes the coal seam of the target working face to generate a fracture for the first time as the target injection pressure value.

[0077] S402. Parallel drilling is carried out in the coal seam of the target working face according to the preset extraction radius to obtain multiple extraction holes and multiple injection holes.

[0078] S403. A sleeve is built into the target injection hole among multiple injection holes. The sleeve contains an injection pipe. The port of the injection pipe is sealed by a ball valve. The injection pipe is connected to a liquid carbon dioxide injection pipe or a high-temperature nitrogen injection pipe through the ball valve.

[0079] S404. Connect the ball valve to the liquid carbon dioxide injection pipe and pre-cool the liquid carbon dioxide injection pipe; control the ball valve to open and inject the liquid carbon dioxide into the injection pipe at the injection rate.

[0080] S405. When the injection pressure value corresponding to the liquid carbon dioxide reaches the injection pressure target value, control the ball valve to close; connect the ball valve to the high-temperature nitrogen injection pipe, control the ball valve to open, and inject the high-temperature nitrogen into the injection pipe according to the injection flow rate.

[0081] S406. Turbulent displacement is carried out using high-temperature nitrogen and liquid carbon dioxide to enhance the permeability of the coal seam in the target working face.

[0082] S407. Collect coal seam gas in the coal seam of the target working face through the extraction pipeline connected to the extraction hole.

[0083] S408. Obtain the gas concentration of coal seam gas at different times; if the concentration difference between multiple gas concentrations is less than or equal to a preset difference threshold, control the ball valve to close and stop injecting the high-temperature nitrogen into the target injection hole.

[0084] S409. Separate the coal seam gas to obtain one or more of methane, carbon dioxide, and nitrogen; process the carbon dioxide and nitrogen separately to obtain target liquid carbon dioxide and target high-temperature nitrogen; repeatedly inject the target liquid carbon dioxide and target high-temperature nitrogen into the injection pipe to displace all coal seam gas around the injection hole.

[0085] This application embodiment uses liquid carbon dioxide and high-temperature nitrogen in synergy to enhance coal seam permeability, avoiding the matrix shrinkage effect associated with low-temperature fracturing caused by injecting liquid carbon dioxide alone. Furthermore, the timing of high-temperature nitrogen injection is determined based on the pressure value at the injection hole when liquid carbon dioxide is injected, ensuring that high-temperature nitrogen is injected at the appropriate time and achieving precise control of the injected gas. This effectively solves the problems of low timeliness and poor effect in coal seam permeability enhancement, improves the timeliness of permeability enhancement during the coal seam permeability enhancement process, and thus improves the efficiency of gas extraction.

[0086] Furthermore, carbon dioxide and nitrogen can be separated from the extracted coalbed gas, and the carbon dioxide and nitrogen can be processed to obtain liquid carbon dioxide and high-temperature nitrogen. The processed liquid carbon dioxide and high-temperature nitrogen can then be injected into other injection holes besides the target injection hole for subsequent coalbed permeability enhancement and gas extraction, thereby realizing the recycling of coalbed gas and improving resource utilization efficiency.

[0087] The various embodiments in this specification are described in a progressive manner. For the same or similar parts between the various embodiments, please refer to each other. Each embodiment focuses on describing the differences from other embodiments.

[0088] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit this application. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of this application.

Claims

1. A method for enhancing coal seam permeability and promoting gas extraction using high-temperature nitrogen and liquid carbon dioxide synergistically, characterized in that, The method includes: Step 1: Arrange multiple extraction holes and injection holes connecting the coal seam of the target working face according to the preset extraction radius. The multiple extraction holes and injection holes are arranged linearly, and an injection hole is set every two extraction holes. Step 2: Install guide pipes along their axial direction in each extraction hole and injection hole, and pressurize and seal the gap between the outer wall of the guide pipe and the inner wall of the hole. Step 3: Determine the target value of the injection pressure for liquid carbon dioxide based on the injection volume gradient method; Step 4: After precooling the guide pipe installed in the injection hole, liquid carbon dioxide is injected into the injection hole through the precooled guide pipe. When the injection pressure reaches the target injection pressure value, the injection of liquid carbon dioxide is stopped and high-temperature nitrogen is injected into the injection hole to fully vaporize the liquid carbon dioxide to enhance the permeability of the coal seam in the target working face. Repeat step 4 until the gas in the coal seam of the target working face has been completely extracted; Step 3 includes: Liquid carbon dioxide of varying injection volume is injected into the injection orifice to obtain the pressure value corresponding to each injection volume gradient. Based on the injection volume gradient and the pressure value corresponding to the injection volume gradient, fit an injection volume gradient-pressure curve; The initiation pressure is obtained from the injection volume gradient-pressure curve, which is the target value of the injection pressure.

2. The method for enhancing coal seam permeability and promoting gas extraction using high-temperature nitrogen synergistically combined with liquid carbon dioxide as described in claim 1, characterized in that, The injection rate gradients are 5 L / min, 10 L / min, 15 L / min and 20 L / min, and each injection rate gradient is carried out for 5-15 minutes.

3. The method for enhancing coal seam permeability and promoting gas extraction using high-temperature nitrogen combined with liquid carbon dioxide according to claim 1, characterized in that, The injection rate of the liquid carbon dioxide is 1-5 m / s.

4. The method for enhancing coal seam permeability and promoting gas extraction using high-temperature nitrogen synergistically combined with liquid carbon dioxide as described in claim 1, characterized in that, The high-temperature nitrogen gas has a temperature of 100-120℃ and an injection flow rate of 40-60m³. 3 / h, the injection pressure is 70-95% of the target injection pressure value.

5. The method for enhancing coal seam permeability and promoting gas extraction using high-temperature nitrogen synergistically combined with liquid carbon dioxide as described in claim 1, characterized in that, The pressurized sealing includes: The gap between the outer wall of the guide tube and the inner wall of the hole is sealed by multiple pressurized grouting using a sealing bag. The grouting depth is less than the length of the guide tube extending into the coal seam.

6. The method for enhancing coal seam permeability and promoting gas extraction using high-temperature nitrogen synergistically combined with liquid carbon dioxide as described in claim 1, characterized in that, The outer wall of the guide tube disposed in the injection hole is also provided with a sleeve; The gap between the sleeve and the outer wall of the guide tube is sealed by pressure.

7. The method for enhancing coal seam permeability and promoting gas extraction using high-temperature nitrogen and liquid carbon dioxide synergistically according to claim 1, characterized in that, The end of the guide pipe located in the injection hole that is far from the coal seam of the target working face is equipped with a control valve, and the control valve is also connected to the high-temperature nitrogen injection pipeline and the liquid carbon dioxide injection pipeline respectively.

8. The method for enhancing coal seam permeability and promoting gas extraction using high-temperature nitrogen synergistically combined with liquid carbon dioxide as described in claim 1, characterized in that, The length of the guide pipe installed in the extraction hole is less than the length of the guide pipe installed in the injection hole.

9. The method for enhancing coal seam permeability and promoting gas extraction using high-temperature nitrogen and liquid carbon dioxide synergistically according to claim 1, characterized in that, A gas concentration sensor is installed in the extraction pipeline connected to the extraction hole. The gas concentration sensor is used to monitor the gas concentration in the extraction pipeline.

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