N2-co2 alternate injection displacement flow boosting gas extraction system and gas injection method thereof

Abstract

The present application belongs to the technical field of coal mine gas extraction, and particularly relates to a N2-CO2 alternate injection displacement flow-increasing gas extraction system and a gas injection method thereof. The gas injection method comprises: a first stage of injecting N2, ending when the methane pure flow rate decreases to a first threshold value, or the N2 backflow concentration reaches a second threshold value, or the CH4 concentration fluctuation rate is lower than a third threshold value; a second stage of switching to injecting CO2, ending when the methane pure flow rate gain decreases to a fourth threshold value, or the CO2 backflow concentration reaches a fifth threshold value, or the CH4 concentration fluctuation rate is lower than a sixth threshold value; and a third stage of again injecting N2, repeating the three stages until the methane pure flow rate gain is lower than a termination threshold value or a design target is reached. The system comprises a gas injection system, an extraction system, a monitoring system and a control system, the monitoring system collects gas injection pressure, flow rate, extraction negative pressure and mixed gas components in real time, the control system calculates methane pure flow rate change rate, N2 backflow concentration, CO2 backflow concentration and CH4 concentration fluctuation rate based on the above, and controls gas injection stage switching.

Description

Technical Field

[0001] This invention belongs to the field of coal mine gas extraction technology, specifically relating to an N2-CO2 alternating injection displacement flow-increasing gas extraction system and its gas injection method. Background Technology

[0002] Patent document CN117514095A discloses a gas injection displacement coal seam gas extraction test device and method. The device includes a sealed box, gas injection components (nitrogen cylinder, carbon dioxide cylinder, booster, and gas storage cylinder), extraction components (extraction pump and extraction pipeline), and methane adsorption components. The sample is placed in the sealed box. A gas injection screen is installed in the gas injection hole and connected to the gas storage cylinder through a gas supply pipeline. An extraction screen is installed in the extraction hole and connected to the extraction pump through an extraction pipeline. It can simulate three gas injection displacement operation modes: injection before extraction, simultaneous injection and extraction, and extraction before injection and extraction. By recording the total amount and composition of injected and extracted gas, it is used to study the relationship between key parameters such as different gas injection methods, gas component ratio, gas injection flow rate, and gas injection pressure and the gas extraction volume.

[0003] This technical solution has the following technical defects: First, in this scheme, nitrogen and carbon dioxide are first mixed in a set ratio and then pressurized before being injected into the coal seam as a mixed gas. When the two gases are injected at the same time, they compete and interfere with each other in adsorption. It is impossible to utilize the carrying and displacement effect of nitrogen on free gas and the displacement and desorption effect of carbon dioxide on adsorbed gas in stages, resulting in the synergistic effect of the two gases not being fully utilized and the extraction efficiency being limited.

[0004] Second, this scheme is a laboratory test device that adopts gas injection displacement operation methods such as injection followed by extraction, simultaneous injection and extraction, and extraction followed by injection and extraction. The switching of this operation method is based on preset time or fixed flow threshold. It lacks dynamic stage identification and intelligent switching mechanism based on real-time monitoring data downhole, including changes in methane pure flow rate, backflow gas concentration, and gas concentration fluctuation rate, and cannot adapt to changes in downhole coal seam conditions.

[0005] Third, the scheme uses mixed gas injection and does not set up a post-processing stage for independent nitrogen injection. When carbon dioxide in the mixed gas is injected into the coal seam, the coal seam permeability gradually decreases due to the strong adsorption of carbon dioxide in the coal matrix, which affects the long-term extraction effect. The scheme does not provide technical means to solve the problem of carbon dioxide retention. Summary of the Invention

[0006] The purpose of this invention is to provide an N2-CO2 alternating injection displacement-enhancing gas extraction system and its injection method, in order to solve the problems in the prior art where the extraction efficiency is limited due to the lack of staged alternating injection based on the different mechanisms of action of different gases in the coal seam, and the poor synergy between gas injection and extraction due to the reliance on a fixed attenuation threshold for switching. Furthermore, it solves the problems in the prior art where strong carbon dioxide adsorption leads to a decrease in coal seam permeability and gas retention.

[0007] The technical solution of the present invention is as follows: On the one hand, it provides a method for N2-CO2 alternating injection displacement-boosting gas extraction and injection, comprising the following steps: S1: Air tightness test: Inert gas is introduced into the gas extraction system and kept for a certain period of time. The pressure change is observed to confirm the airtightness. S2: Baseline extraction, gas extraction is performed under non-injection conditions to obtain basic parameters such as extraction mixed flow rate, CH4 concentration, gas injection orifice pressure and extraction negative pressure. S3: First stage of gas injection: Open the gas injection switching valve to connect the N2 gas source with the gas injection main pipe and inject N2 into the coal seam, while keeping the extraction system running; when the methane pure flow rate drops to the first threshold, the N2 backflow concentration rises to the second threshold, or the CH4 concentration fluctuation rate is lower than the third threshold, the first stage ends. S4: Second stage of gas injection, switch the gas injection switching valve to connect the CO2 gas source with the gas injection main pipe, inject CO2 into the coal seam, and keep the extraction system running at the same time; when the methane pure flow gain is monitored to decrease to the fourth threshold, the CO2 backflow concentration increases to the fifth threshold, or the CH4 concentration fluctuation rate is lower than the sixth threshold, the second stage ends. S5: Third stage gas injection, switch the gas injection switching valve again to connect the N2 gas source with the gas injection main pipe, inject N2 into the coal seam, and keep the extraction system running at the same time; Repeat steps S3 through S5 until the methane pure flow gain in the continuous cycle is below the termination threshold or the designed extraction target is reached.

[0008] Preferably, in step S3, the first threshold is that the pure methane flow rate drops below 0.5 L / min, the second threshold is that the N2 backflow concentration reaches 10%, and the third threshold is that the CH4 concentration fluctuation rate is less than 5%; in step S4, the fourth threshold is that the pure methane flow rate gain drops below 0.5 L / min, the fifth threshold is that the CO2 backflow concentration reaches 10%, and the sixth threshold is that the CH4 concentration fluctuation rate is less than 5%.

[0009] Preferably, in step S3, the duration of the first stage of gas injection is dynamically adjusted according to real-time monitoring data, and the N2 pre-injection time ranges from 6 to 24 hours; in step S4, the duration of the second stage of gas injection is dynamically adjusted, and the CO2 displacement time ranges from 4 to 18 hours; in step S5, the duration of the third stage of gas injection is dynamically adjusted, and the N2 re-injection time ranges from 4 to 20 hours.

[0010] Preferably, it also includes: during each stage of gas injection, dynamically adjusting the injection pressure and injection flow rate according to the real-time change rate of CH4 concentration in the extracted mixed gas; when the CO2 concentration in the extracted mixed gas exceeds a preset safety threshold, ending the second stage in advance and entering the third stage.

[0011] On the other hand, an N2-CO2 alternating injection displacement-boosting gas extraction system is provided, comprising: The gas injection system includes an N2 gas source, a CO2 gas source, and a gas injection switching valve connecting the N2 gas source and the CO2 gas source. The output end of the gas injection switching valve is connected to the gas injection main pipe, and the gas injection main pipe is connected to multiple gas injection ports through multiple gas injection branch pipes. The extraction system includes multiple extraction holes, an extraction main pipe, a gas-liquid separator, and an extraction pump station. The extraction holes are connected to the extraction main pipe through extraction branch pipes, and the extraction main pipe is connected to the gas-liquid separator and the extraction pump station in sequence. The monitoring system includes pressure sensors, temperature sensors, CH4 concentration sensors and flow meters installed on the injection side and extraction side, for real-time acquisition of injection pressure, injection flow rate, extraction negative pressure, extraction flow rate and CH4 concentration, N2 concentration and CO2 concentration in the extracted mixed gas. The control system includes a data acquisition unit, a data processing unit, and an execution unit. The data acquisition unit is connected to each sensor of the monitoring system. The data processing unit calculates the methane pure flow rate change rate, N2 backflow concentration, CO2 backflow concentration, and CH4 concentration fluctuation rate based on the acquired data, and generates a gas injection stage switching command based on a preset threshold. The execution unit is connected to the gas injection switching valve, as well as the N2 branch regulating valve and the CO2 branch regulating valve, and is used to execute the switching command.

[0012] Preferably, the gas injection system further includes: The N2 injection pipeline is equipped with an N2 storage tank, a primary pressure reducing valve, an N2 flow controller, and a secondary pressure regulating valve in sequence. The CO2 injection pipeline is equipped with a CO2 storage tank, a primary pressure reducing valve, a CO2 flow controller, and a secondary pressure regulating valve in sequence. The gas injection switching valve is a three-way switching valve. The two inlets of the three-way switching valve are connected to the N2 gas injection line and the CO2 gas injection line, respectively, and the outlet of the three-way switching valve is connected to the main gas injection line.

[0013] Preferably, the extraction system further includes: an online gas component analyzer installed on the extraction main pipe for continuously detecting the volume fractions of CH4, CO2 and N2 in the extracted mixed gas; and extraction branch flow meters and extraction branch regulating valves installed on each extraction branch pipe.

[0014] Preferably, the monitoring system includes: The gas injection side monitoring unit includes a pressure sensor, a temperature sensor, and a flow meter installed on the N2 gas injection line and the CO2 gas injection line; The orifice monitoring unit includes a pressure sensor, a temperature sensor, and a CH4 concentration sensor installed on the main gas injection pipe; The sampling monitoring unit includes a pressure sensor, a temperature sensor, a CH4 concentration sensor, and a flow meter installed on the sampling manifold.

[0015] Preferably, the control system further includes: The data acquisition module is connected to the signals from each sensor. Signal conditioner, which is connected to the data acquisition module; The communication module is connected to the signal conditioner. The monitoring host connects to the communication module and stores the monitoring data. The data processing unit is built into the monitoring host and is used for stage-specific identification calculations. The execution terminal receives control decisions from the data processing unit and outputs digital or analog signals to control the N2 branch regulating valve, the CO2 branch regulating valve, and the gas injection switching valve.

[0016] Preferably, it also includes a safety interlocking unit, the safety interlocking unit comprising: Pressure relief devices, alarm devices, and emergency shut-off devices are installed on the main injection pipe and the main extraction pipe; The alarm device is connected to the CH4 concentration sensor, CO2 concentration sensor and pressure sensor of the monitoring system to issue an alarm signal when the concentration or pressure exceeds the set threshold. The emergency shut-off device is controlled by the control system and is used to shut down the gas injection source and extraction pump station when monitoring parameters exceed limits or equipment malfunctions.

[0017] Compared with the prior art, the advantages of the present invention are: (1) By injecting in three stages, the first stage uses the carrying and displacement effect of N2 to improve the flowability of free gas and promote initial desorption. The second stage uses the displacement and desorption effect of CO2 to efficiently release adsorbed gas. The third stage uses the carrying and displacement effect of N2 to clean up residual CO2 and carry the remaining gas. Thus, the dominant effects of the two gases are utilized in stages to improve extraction efficiency.

[0018] (2) The methane pure flow rate change rate, backflow gas concentration and gas concentration fluctuation rate are collected in real time by the monitoring system, and the gas injection stage is dynamically switched by the control system based on the preset threshold of multiple parameters to realize intelligent coordination between gas injection and extraction, and avoid switching lag or prematurely.

[0019] (3) Through the third stage of N2 re-injection, the residual CO2 is carried out by the carrying and displacement effect of N2, thereby reducing the amount of residual CO2 adsorbed in the coal seam, maintaining the coal seam permeability, and extending the effective extraction cycle. Attached Figure Description

[0020] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram of the overall structure of the N2-CO2 alternating injection displacement flow enhancement gas extraction system described in this invention; Figure 2 This is a schematic diagram of the internal structure of the control system described in this invention.

[0021] 1. N2 gas storage tank; 2. N2 primary flow controller; 3. N2 primary pressure reducing valve; 4. N2 pipeline pressure sensor; 5. N2 pipeline temperature sensor; 6. N2 secondary flow controller; 7. N2 secondary pressure reducing valve; 8. CO2 gas storage tank; 9. CO2 primary flow controller; 10. N2 injection pipeline; 11. CO2 primary pressure reducing valve; 12. CO2 pipeline pressure sensor; 13. CO2 pipeline temperature sensor; 14. CO2 secondary flow controller; 15. CO2 secondary pressure reducing valve; 16. Injection switching valve; 17. Injection branch flow meter; 18. N2 branch regulating valve; 19. CO2 branch regulating valve; 20. CO2 injection pipeline; 21. Injection main pipe flow meter; 22. Monitoring port pressure sensor; 23. Monitoring port temperature sensor; 24. Monitoring port CH4 concentration sensor; 25. 10. Injection branch pipe regulating valve; 26. Injection branch pipe flow meter; 27. Injection main pipe pressure sensor; 28. Injection main pipe temperature sensor; 29. ​​Injection main pipe CH4 concentration sensor; 30. Injection main pipe; 31. Extraction branch pipe regulating valve; 32. Extraction main pipe flow meter; 33. Extraction main pipe pressure sensor; 34. Extraction main pipe temperature sensor; 35. Extraction main pipe CH4 concentration sensor; 36. Gas-liquid separator; 37. Extraction branch pipe flow meter; 38. Extraction pump station; 40. Monitoring port; 50. Injection port; 60. Extraction port; 70. Extraction main pipe; 71. Data acquisition module; 72. Signal conditioner; 73. Communication module; 74. Monitoring host; 75. Data processing unit; 76. Execution terminal; 101. Injection system; 201. Extraction system; 301. Monitoring system; 401. Control system. Detailed Implementation

[0022] The present invention will be further described in detail below with reference to specific embodiments.

[0023] Example 1

[0024] System Construction This embodiment provides an N2-CO2 alternating injection displacement-boosting gas extraction system 201, the structure of which is as follows: Figure 1 As shown. This system was installed in a low-permeability coal seam working face in a coal mine, with an average coal seam thickness of 5.2m and a gas content of 12.6m³. 3 / t, air permeability coefficient 0.032mD.

[0025] The gas injection system 101 includes an N2 storage tank 1, a CO2 storage tank 8, and a gas injection switching valve 16. The outlet of the N2 storage tank 1 is sequentially connected to an N2 primary pressure reducing valve 3, an N2 primary flow controller 2, an N2 secondary flow controller 6, an N2 secondary pressure reducing valve 7, and an N2 injection pipeline 10. The outlet of the CO2 storage tank 8 is sequentially connected to a CO2 primary pressure reducing valve 11, a CO2 primary flow controller 9, a CO2 secondary flow controller 14, a CO2 secondary pressure reducing valve 15, and a CO2 injection pipeline 20. The gas injection switching valve 16 is an electromagnetically driven three-way switching valve. Its two inlets are connected to the N2 injection pipeline 10 and the CO2 injection pipeline 20, respectively, and its outlet is connected to the main gas injection pipe 30. The main gas injection pipe 30 is connected to four injection ports 50 via four injection branch pipes. Each injection branch pipe is equipped with an injection branch pipe regulating valve 25 and an injection branch pipe flow meter 26. The 50 air injection holes are arranged along the coal seam, with a depth of 80m, a diameter of 113mm, and a sealing length of 15m.

[0026] The extraction system 201 includes twelve extraction holes 60, arranged in three parallel rows on both sides of the injection holes 50. Each extraction hole 60 has a depth of 70m, a diameter of 94mm, and a sealing length of 12m. Each extraction hole 60 is connected to the main extraction pipe 70 via an extraction branch pipe. The extraction branch pipe is equipped with an extraction branch pipe flow meter 37 and an extraction branch pipe regulating valve 31. The main extraction pipe 70 is sequentially connected to a gas-liquid separator 36 and an extraction pump station 38. An online gas component analyzer is also installed on the main extraction pipe 70 for real-time detection of the volume fractions of CH4, CO2, and N2 in the extracted mixed gas.

[0027] Monitoring system 301 includes: On the gas injection side: N2 gas injection line 10 is equipped with N2 gas injection line pressure sensor 4, N2 gas injection line temperature sensor 5, and gas injection branch flow meter 17; CO2 gas injection line 20 is equipped with CO2 gas injection line pressure sensor 12, CO2 gas injection line temperature sensor 13, and gas injection branch flow meter 17 (the same flow meter is located before the gas injection switching valve 16 and is shared); gas injection main line 30 is equipped with gas injection main line flow meter 21, gas injection main line pressure sensor 27, gas injection main line temperature sensor 28, and gas injection main line CH4 concentration sensor 29.

[0028] Orifice monitoring: A monitoring orifice pressure sensor 22, a monitoring orifice temperature sensor 23, and a monitoring orifice CH4 concentration sensor 24 are installed in the monitoring orifice 40 to monitor the changes in gas pressure, temperature, and concentration during the coal seam gas injection and displacement process.

[0029] On the extraction side: the extraction main pipe 70 is equipped with an extraction main pipe pressure sensor 33, an extraction main pipe temperature sensor 34, an extraction main pipe CH4 concentration sensor 35, and an extraction main pipe flow meter 32.

[0030] All sensors are intrinsically safe for mining use, and the output signal is 4-20mA or a frequency signal.

[0031] Control System 401 Figure 2 As shown, the system includes a data acquisition module 71, a signal conditioner 72, a communication module 73, a monitoring host 74, a data processing unit 75, and an execution terminal 76. The data acquisition module 71 uses an NI cDAQ-9185 and connects to each sensor via shielded cables. The signal conditioner 72 uses an SCXI-1125 to filter and linearize analog signals. The communication module 73 uses a fiber optic Ethernet switch to upload data to the monitoring host 74. The monitoring host 74 is an industrial computer configured with an Intel Core i7 processor and 16GB of RAM. The data processing unit 75 is a software module embedded in the monitoring host 74, developed based on LabVIEW, used to calculate the pure methane flow rate (the product of the extracted mixed flow rate and the CH4 concentration), the daily growth rate of the pure methane flow rate, the N2 backflow concentration (the volume fraction of N2 in the extracted mixed gas), the CO2 backflow concentration (the volume fraction of CO2 in the extracted mixed gas), and the CH4 concentration fluctuation rate (the standard deviation of five consecutive CH4 concentration samples divided by the average). The execution terminal 76 is a Siemens S7-1200 PLC controller. The execution terminal 76 receives the control decision from the data processing unit 75 and outputs a 24V digital signal to control the solenoid valves of the N2 branch regulating valve 18, the CO2 branch regulating valve 19, the gas injection branch regulating valve 25, the extraction branch regulating valve 31, and the gas injection switching valve 16.

[0032] The safety interlock unit includes: a safety valve (set pressure 2.5MPa) installed on the main injection pipe 30, a pressure relief valve (set pressure -60kPa) installed on the main extraction pipe 70, and a gas over-limit alarm (set to sound and light alarm when CH4 concentration ≥1%). The emergency shut-off device is a pneumatic shut-off valve, which is automatically closed by the actuator 76 when the injection pressure exceeds 2.2MPa or the CH4 concentration in the extraction pipeline is >1.5%.

[0033] Gas injection method The specific steps for gas extraction using the above system are as follows: S1: Air tightness test. Close all exhaust ports of the system, fill the system with helium to a pressure of 1.2 MPa, turn off the gas source and let it stand for 6 hours. If the pressure gauge reading drops by less than 0.02 MPa, the system is confirmed to be airtight.

[0034] S2: Baseline extraction. Start extraction pump station 38, adjust the extraction negative pressure to 30 kPa, and extract continuously for 72 hours. Record the basic parameters: the average extraction mixed flow rate is 45.2 L / min, the average CH4 concentration is 56.8%, and the average reading of the injection main pipe pressure sensor 27 is -25 kPa.

[0035] S3: First Stage Gas Injection (N2 Pre-injection). The gas injection switching valve 16 is opened to the N2 side via the control system 401, and the N2 branch regulating valve 18 is opened to adjust the injection pressure to 0.8 MPa and the flow rate to 120 L / min, injecting N2 into the coal seam. Data is collected every 10 minutes during continuous gas injection. By the 8th hour of injection, the pure methane flow rate decreased from the initial 25.7 L / min to 0.48 L / min, below 0.5 L / min; simultaneously, the N2 concentration in the extraction main reached 10.2%, exceeding 10%; and the CH4 concentration fluctuation rate was 4.2%, below 5%. The data processing unit 75 determines that the first stage termination conditions are met and sends a command to close the N2 injection valve.

[0036] During N2 injection, N2 has weak adsorption capacity and preferentially enters the coal seam fractures and pores, carrying and displacing free gas to migrate towards extraction hole 60. Simultaneously, it reduces the partial pressure of CH4, promoting the initial desorption of adsorbed gas. When the pure methane flow rate decreases to a set threshold, it indicates that the carrying and displacing effect has significantly weakened. When the N2 backflow concentration increases to 10%, it indicates that the injected N2 has penetrated the coal seam and reached extraction hole 60, reducing the marginal benefit of continuing N2 injection. When the CH4 concentration fluctuation rate is below 5%, it indicates that gas desorption is stabilizing. Therefore, at this point, CO2 injection is switched to injection.

[0037] S4: Second Stage Gas Injection (CO2 Displacement). Control system 401 switches injection switching valve 16 to the CO2 side, opens CO2 branch regulating valve 19, adjusts the injection pressure to 1.0 MPa, and sets the flow rate to 80 L / min, injecting CO2 into the coal seam. After 5 hours of injection, the methane pure flow gain (current pure flow rate minus the pure flow rate before injection) drops to 0.4 L / min, below 0.5 L / min; the CO2 backflow concentration reaches 10.5%, exceeding 10%; and the CH4 concentration fluctuation rate is 3.8%, below 5%. The second stage termination conditions are met, and the CO2 injection valve is closed.

[0038] During CO2 injection, CO2 has a strong adsorption potential and competes with adsorption sites on the coal surface for adsorption, displacing adsorbed CH4 into free CH4; this is displacement desorption. When the methane pure flow rate gain drops below 0.5 L / min, it indicates that most of the displaced free CH4 has been extracted. When the CO2 backflow concentration reaches 10%, it indicates that the injected CO2 has exceeded the extraction orifice 60, and continued CO2 injection will lead to a large amount of CO2 retention. When the CH4 concentration fluctuation rate is below 5%, it indicates that the displacement rate is stabilizing. At this point, the process should switch to the third stage.

[0039] S5: Third stage gas injection (N2 re-injection). Switch the gas injection switching valve 16 to the N2 side again, injecting N2 at an injection pressure of 0.6 MPa and a flow rate of 100 L / min for 6 hours. During this stage, the CO2 concentration in the extracted mixed gas gradually decreased from 7.2% to 1.8%, while the CH4 concentration rose back to 42.5%. This indicates that the carrying and displacement effect of N2 carried away the residual CO2 and the displaced CH4.

[0040] The S3 to S5 cycles were then repeated three times. After each cycle, the peak methane pure flow rate was 22.1 L / min, 18.6 L / min, and 15.2 L / min, respectively. After the third cycle, the methane pure flow rate gain was less than 0.3 L / min for two consecutive cycles, which met the termination condition. Gas injection was stopped, and extraction was maintained until the end.

[0041] To verify the extraction effect of the present invention, comparative experiments were conducted using different gas injection methods, and the results are shown in Table 1.

[0042] Table 1: Comparison of extraction effects of different gas injection methods (under the same coal seam conditions, continuous extraction for 30 days)

[0043] As shown in Table 1, compared with continuous CO2 injection, the present invention reduces residual CO2 adsorption by 83.2%, increases the average pure CH4 flow rate by 22.7%, and improves extraction efficiency by 106.5% compared with extraction alone; compared with N2+CO2 mixed injection, residual CO2 adsorption is reduced by 72.3%, and extraction efficiency is improved by approximately 50 percentage points. This indicates that the three-stage alternating injection of the present invention can significantly improve gas extraction efficiency and reduce CO2 retention.

[0044] Example 2 This embodiment is basically the same as Embodiment 1, except that different threshold combinations are used for switching the gas injection stage. Under the condition of lower coal seam permeability (permeability coefficient 0.012mD), the first threshold methane pure flow rate is reduced to 0.3L / min, the second threshold N2 backflow concentration is adjusted to 8%, and the third threshold CH4 concentration fluctuation rate is adjusted to 6%; the fourth threshold methane pure flow rate gain is reduced to 0.3L / min, the fifth threshold CO2 backflow concentration is adjusted to 8%, and the sixth threshold CH4 concentration fluctuation rate is adjusted to 6%. The first stage of gas injection lasts for 14 hours, the second stage for 9 hours, and the third stage for 7 hours. The final extraction effect: the average CH4 pure flow rate reaches 22.8L / min, and the residual CO2 adsorption is 0.28m³. 3 / t, the extraction efficiency is improved by 92.7%. This shows that the method of the present invention has good adaptability to coal seams with different permeability, and the coal seam conditions can be matched by adjusting the threshold parameter.

[0045] Comparative Example This comparative example uses the same coal seam conditions as Example 1, but the gas injection method is changed to the mixed injection method disclosed in CN117514095A in the background art: first, N2 and CO2 are mixed at a volume ratio of 2:1 and pressurized to 1.0 MPa, then continuously injected into the coal seam at a flow rate of 100 L / min, while maintaining extraction operation, using the "inject first, then extract" mode for continuous gas injection and displacement for 30 days. This comparative example does not involve staged switching or an N2 re-injection stage.

[0046] The results showed that the average pure CH4 flow rate was 18.5 L / min, the peak CH4 concentration was 44.7%, and the residual CO2 adsorption capacity reached 1.56 m³. 3 / t, the extraction efficiency is improved by 50.4% compared to extraction alone. Compared to Example 1, the average pure CH4 flow rate of the comparative example is reduced by 27.2%, the residual CO2 adsorption is 5.03 times that of Example 1, and the CH4 concentration decreases significantly faster in the later stage of extraction, i.e. after day 18, indicating that the continuous retention of CO2 in the coal seam during mixed injection leads to a decrease in permeability. This invention improves the long-term extraction effect by alternating injection of N2 and CO2, especially the third-stage N2 re-injection to remove residual CO2.

[0047] The above embodiments are merely illustrative of the technical concept and features of the present invention, intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and should not be construed as limiting the scope of protection of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the present invention is defined by the appended claims rather than the foregoing description, and thus all changes falling within the meaning and scope of the equivalents of the claims are intended to be included within the present invention.

Claims

1. A method for N2-CO2 alternating injection displacement-boosting gas extraction and injection, characterized in that, Includes the following steps: S1: Air tightness test: Inert gas is introduced into the gas extraction system and kept for a certain period of time. The pressure change is observed to confirm the airtightness. S2: Baseline extraction, gas extraction is performed under non-injection conditions to obtain basic parameters such as extraction mixed flow rate, CH4 concentration, gas injection orifice pressure and extraction negative pressure. S3: First stage of gas injection: Open the gas injection switching valve to connect the N2 gas source with the gas injection main pipe and inject N2 into the coal seam, while keeping the extraction system running; when the methane pure flow rate drops to the first threshold, the N2 backflow concentration rises to the second threshold, or the CH4 concentration fluctuation rate is lower than the third threshold, the first stage ends. S4: Second stage of gas injection, switch the gas injection switching valve to connect the CO2 gas source with the gas injection main pipe, inject CO2 into the coal seam, and keep the extraction system running at the same time; when the methane pure flow gain is monitored to decrease to the fourth threshold, the CO2 backflow concentration increases to the fifth threshold, or the CH4 concentration fluctuation rate is lower than the sixth threshold, the second stage ends. S5: Third stage gas injection, switch the gas injection switching valve again to connect the N2 gas source with the gas injection main pipe, inject N2 into the coal seam, and keep the extraction system running at the same time; Repeat steps S3 through S5 until the methane pure flow gain in the continuous cycle is below the termination threshold or the designed extraction target is reached.

2. The N2-CO2 alternating injection displacement and flow-increasing gas extraction and injection method according to claim 1, characterized in that, In step S3, the first threshold is when the pure methane flow rate drops below 0.5 L / min, the second threshold is when the N2 backflow concentration reaches 10%, and the third threshold is when the CH4 concentration fluctuation rate is below 5%. In step S4, the fourth threshold is when the pure methane flow rate gain drops below 0.5 L / min, the fifth threshold is when the CO2 backflow concentration reaches 10%, and the sixth threshold is when the CH4 concentration fluctuation rate is below 5%.

3. The N2-CO2 alternating injection displacement-enhancing gas extraction and injection method according to claim 1, characterized in that, In step S3, the duration of the first stage of gas injection is dynamically adjusted based on real-time monitoring data, with the N2 pre-injection time ranging from 6 to 24 hours; in step S4, the duration of the second stage of gas injection is dynamically adjusted, with the CO2 displacement time ranging from 4 to 18 hours; in step S5, the duration of the third stage of gas injection is dynamically adjusted, with the N2 re-injection time ranging from 4 to 20 hours.

4. The N2-CO2 alternating injection displacement-enhancing gas extraction and injection method according to claim 1, characterized in that, Also includes: During each stage of gas injection, the injection pressure and flow rate are dynamically adjusted based on the real-time change rate of CH4 concentration in the extracted mixed gas. When the CO2 concentration in the extracted mixed gas exceeds the preset safety threshold, the second stage ends early and the third stage begins.

5. A gas extraction system with alternating N2-CO2 injection displacement and flow enhancement, characterized in that, The method for performing the N2-CO2 alternating injection displacement-boosting gas extraction and injection method as described in any one of claims 1-4 includes: The gas injection system includes an N2 gas source, a CO2 gas source, and a gas injection switching valve connecting the N2 gas source and the CO2 gas source. The output end of the gas injection switching valve is connected to the gas injection main pipe, and the gas injection main pipe is connected to multiple gas injection ports through multiple gas injection branch pipes. The extraction system includes multiple extraction holes, an extraction main pipe, a gas-liquid separator, and an extraction pump station. The extraction holes are connected to the extraction main pipe through extraction branch pipes, and the extraction main pipe is connected to the gas-liquid separator and the extraction pump station in sequence. The monitoring system includes pressure sensors, temperature sensors, CH4 concentration sensors and flow meters installed on the injection side and extraction side, for real-time acquisition of injection pressure, injection flow rate, extraction negative pressure, extraction flow rate and CH4 concentration, N2 concentration and CO2 concentration in the extracted mixed gas. The control system includes a data acquisition unit, a data processing unit, and an execution unit. The data acquisition unit is connected to each sensor of the monitoring system. The data processing unit calculates the methane pure flow rate change rate, N2 backflow concentration, CO2 backflow concentration, and CH4 concentration fluctuation rate based on the acquired data, and generates a gas injection stage switching command based on a preset threshold. The execution unit is connected to the gas injection switching valve, as well as the N2 branch regulating valve and the CO2 branch regulating valve, and is used to execute the switching command.

6. The N2-CO2 alternating injection displacement-boosting gas extraction system according to claim 5, characterized in that, The gas injection system also includes: The N2 injection pipeline is equipped with an N2 storage tank, a primary pressure reducing valve, an N2 flow controller, and a secondary pressure regulating valve in sequence. The CO2 injection pipeline is equipped with a CO2 storage tank, a primary pressure reducing valve, a CO2 flow controller, and a secondary pressure regulating valve in sequence. The gas injection switching valve is a three-way switching valve. The two inlets of the three-way switching valve are connected to the N2 gas injection line and the CO2 gas injection line, respectively, and the outlet of the three-way switching valve is connected to the main gas injection line.

7. The N2-CO2 alternating injection displacement-boosting gas extraction system according to claim 5, characterized in that, The extraction system also includes: an online gas component analyzer installed on the extraction main pipe for continuously detecting the volume fractions of CH4, CO2 and N2 in the extracted mixed gas; and extraction branch flow meters and extraction branch regulating valves installed on each extraction branch pipe.

8. The N2-CO2 alternating injection displacement-boosting gas extraction system according to claim 6, characterized in that, The monitoring system includes: The gas injection side monitoring unit includes a pressure sensor, a temperature sensor, and a flow meter installed on the N2 gas injection line and the CO2 gas injection line; The orifice monitoring unit includes a pressure sensor, a temperature sensor, and a CH4 concentration sensor installed on the main gas injection pipe; The sampling monitoring unit includes a pressure sensor, a temperature sensor, a CH4 concentration sensor, and a flow meter installed on the sampling manifold.

9. The N2-CO2 alternating injection displacement-boosting gas extraction system according to claim 5, characterized in that, The control system further includes: The data acquisition module is connected to the signals from each sensor. Signal conditioner, which is connected to the data acquisition module; The communication module is connected to the signal conditioner. The monitoring host connects to the communication module and stores the monitoring data. The data processing unit is built into the monitoring host and is used for stage-specific identification calculations. The execution terminal receives control decisions from the data processing unit and outputs digital or analog signals to control the N2 branch regulating valve, the CO2 branch regulating valve, and the gas injection switching valve.

10. The N2-CO2 alternating injection displacement-boosting gas extraction system according to claim 5, characterized in that, It also includes a safety interlocking unit, which comprises: Pressure relief devices, alarm devices, and emergency shut-off devices are installed on the main injection pipe and the main extraction pipe; The alarm device is connected to the CH4 concentration sensor, CO2 concentration sensor and pressure sensor of the monitoring system to issue an alarm signal when the concentration or pressure exceeds the set threshold. The emergency shut-off device is controlled by the control system and is used to shut down the gas injection source and extraction pump station when monitoring parameters exceed limits or equipment malfunctions.

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