Coal bed methane interdiffusion experiment method based on in-situ optical monitoring

Abstract

The application discloses a coalbed methane mutual diffusion experiment method based on in-situ optical monitoring, which is implemented based on a special experiment device, and the device comprises five systems of gas supply pressurization, mutual diffusion reaction, in-situ optical monitoring, environment control and vacuum exhaust; the method realizes accurate determination of a coalbed methane mutual diffusion coefficient through device debugging, environment setting, system vacuumizing, spectrum baseline establishment, double-chamber accurate gas charging, diffusion starting and in-situ high-frequency monitoring, concentration inversion and model fitting, and continuous testing of multiple working conditions of one-time charging can also be realized. The application adopts non-contact in-situ optical monitoring, completely eliminates the measurement error caused by traditional sampling, and improves the data accuracy; the second-level high-frequency acquisition can capture the concentration change characteristics in the initial diffusion stage, and improves the model fitting accuracy; the automatic operation reduces the safety hidden danger, the multiple gradient continuous testing greatly improves the experiment efficiency, and can provide accurate core parameters for optimization of a coalbed methane injection-production scheme.

Description

Technical Field

[0001] This invention belongs to the field of coalbed methane development and unconventional natural gas reservoir physical property experimental technology, and particularly relates to an experimental method for coalbed methane interdiffusion based on in-situ optical monitoring. Background Technology

[0002] Against the backdrop of global climate change and my country's "dual carbon" goals, the efficient development of unconventional natural gas (coalbed methane and shale gas) is a core path to ensure national energy security and achieve a clean energy transition. The interdiffusion coefficient of multi-component gases under high-temperature and high-pressure reservoir conditions is a core physical parameter for evaluating the efficiency of coalbed methane injection and replacement, predicting the CO2 storage potential of coal seams, and optimizing injection and production process parameters.

[0003] Current coalbed methane interdiffusion coefficient testing technology relies primarily on a "physical sampling + offline gas chromatography analysis" model, which suffers from three major technical bottlenecks: First, the sampling process requires opening high-pressure valves to extract the mixed gas, inevitably leading to a sudden drop in pressure and flow field disturbance within the sample chamber. Furthermore, the dead volume of the sampling pipeline and the ambient temperature difference introduce significant measurement errors, resulting in a large deviation between the measured diffusion kinetics data and the actual reservoir environment. Second, offline sampling cannot achieve high-frequency continuous monitoring, resulting in sparse data points and difficulty in capturing the key characteristics of rapid gas concentration changes in the early stages of diffusion. This leads to low accuracy in the diffusion model fitting and an inability to accurately reconstruct the entire interdiffusion process. Third, frequent opening and closing of high-pressure valves not only increases the operational safety risks for experimental personnel but also easily triggers micro-leakage of high-pressure flammable gases, posing safety hazards. Moreover, it cannot achieve continuous testing under multiple operating conditions with a single loading, resulting in extremely low experimental efficiency. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned defects of the prior art and provide an experimental method for coalbed methane interdiffusion based on in-situ optical monitoring. By replacing traditional physical sampling with non-contact in-situ optical monitoring, the measurement error caused by sampling is eliminated at the source. At the same time, it realizes high-frequency continuous data acquisition at the second level, accurately captures the characteristics of the initial diffusion stage, and combined with standardized device operation procedures throughout the process, it ensures the accuracy and repeatability of the interdiffusion coefficient measurement. It also enables continuous testing under multiple operating conditions with a single loading, improving experimental efficiency and reducing operational safety risks.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: The experimental method for coalbed methane interdiffusion based on in-situ optical monitoring is implemented using a dedicated coalbed methane interdiffusion experimental device, which includes a gas supply and pressurization system, an interdiffusion reaction system, an in-situ optical monitoring system, an environmental control system, and a vacuum exhaust system. The gas supply and pressurization system includes a methane gas source unit, a nitrogen gas source unit, a main gas source filter (5), a precision booster pump (6), and a high-pressure buffer tank (7); the methane gas source unit includes a methane cylinder (31), and a methane cylinder valve (1) and a methane pressure reducing valve (2) are connected sequentially to the gas supply line of the methane cylinder (31); the nitrogen gas source unit includes a nitrogen cylinder (32), and a nitrogen cylinder valve (3), a nitrogen pressure reducing valve (4), and a methane pressure reducing valve are connected sequentially to the gas supply line of the nitrogen cylinder (32). (2) The outlet of the nitrogen pressure reducing valve (4) is connected to the inlet of the main gas source filter (5). The outlet of the main gas source filter (5) is connected to the precision booster pump (6) and the high pressure buffer tank (7) in sequence. The outlet of the high pressure buffer tank (7) is connected to the methane injection branch and the nitrogen injection branch. The methane injection branch is equipped with the methane injection branch switch valve (8) and the methane side air intake fine adjustment valve (10) in sequence. The nitrogen injection branch is equipped with the nitrogen injection branch switch valve (9) and the nitrogen side air intake fine adjustment valve (11) in sequence. The interdiffusion reaction system includes an interdiffusion reactor (16). The internal cavity of the interdiffusion reactor (16) is divided into a methane diffusion chamber (16-1) on the left and a nitrogen diffusion chamber (16-2) on the right by a diffusion start valve (17). The methane diffusion chamber (16-1) and the nitrogen diffusion chamber (16-2) are independent of each other and have the same volume. The inlet of the methane diffusion chamber (16-1) is connected to the outlet of the methane injection branch, and the inlet of the nitrogen diffusion chamber (16-2) is connected to the outlet of the nitrogen injection branch. A methane-side pressure sensor (14) is installed in the methane diffusion chamber (16-1), and a nitrogen-side pressure sensor (15) is installed in the nitrogen diffusion chamber (16-2). The in-situ optical monitoring system includes a left sapphire window assembly (18), a right sapphire window assembly (19), a left optical probe (20), a right optical probe (21), a spectrometer (22), and a data acquisition computer (23); the left sapphire window assembly (18) is sealed and installed at one end of the interdiffusion reactor (16) corresponding to the methane diffusion chamber (16-1), and the right sapphire window assembly (19) is sealed and installed at one end of the interdiffusion reactor (16) corresponding to the nitrogen diffusion chamber (16-2); the left optical... The optical probe (20) is coaxially positioned outside the left sapphire window assembly (18), and the right optical probe (21) is coaxially positioned outside the right sapphire window assembly (19). The detection optical paths of the left optical probe (20) and the right optical probe (21) both pass through the central axis of the corresponding diffusion chamber. The left optical probe (20) and the right optical probe (21) are both connected to the signal input end of the spectrometer (22) via optical fiber. The signal output end of the spectrometer (22) is connected to the data acquisition computer (23) for communication. The environmental control system includes a constant temperature chamber (24) and a confining pressure loading unit; the interdiffusion reactor (16) is placed inside the internal cavity of the constant temperature chamber (24), and a constant temperature chamber temperature sensor (25) is installed inside the constant temperature chamber (24) for real-time temperature monitoring; the confining pressure loading unit includes a confining pressure pump (26), a confining pressure gauge (27) and a confining pressure loading valve (28), the outlet of the confining pressure pump (26) is connected to the confining pressure layer of the interdiffusion reactor (16) through the confining pressure loading valve (28), and the confining pressure gauge (27) is installed on the pipeline between the confining pressure loading valve (28) and the confining pressure layer; The vacuum exhaust system includes a vacuum pump (12), a system vacuum valve (13), an exhaust valve (29), and a safety relief valve (30); the vacuum pump (12) is connected in parallel with the methane injection branch and the nitrogen injection branch through the system vacuum valve (13), and the connection point is located between the inlet fine-tuning valve and the injection branch switch valve; the exhaust valve (29) and the safety relief valve (30) are connected to the end pipeline of the interdiffusion reactor (16); The experimental method for coalbed methane interdiffusion includes the following steps: Step S1: Debugging and setting environmental parameters of the dedicated coalbed methane interdiffusion experimental device before the experiment; Step S2: Vacuuming and airtightness test of the dedicated coalbed methane interdiffusion experimental device; Step S3: Establishing spectral baseline and calibrating concentration; Step S4: Filling the methane diffusion chamber (16-1) and nitrogen diffusion chamber (16-2) with gas and setting experimental conditions; Step S5: Diffusion initiation and in-situ continuous optical monitoring; Step S6: Concentration inversion and interdiffusion coefficient calculation.

[0006] Furthermore, step S1 specifically includes the following sub-steps: S1.1 Check the cleanliness of the interior of the interdiffusion reactor (16), confirm that the left sapphire window assembly (18) and the right sapphire window assembly (19) are installed securely and sealed well, install the left optical probe (20) and the right optical probe (21) coaxially with the corresponding sapphire window assembly, adjust the focal length of the optical probe so that the detection light path passes through the central area of ​​the corresponding diffusion chamber. S1.2 Turn on the constant temperature chamber (24), set the constant temperature according to the experimental target, monitor the temperature inside the chamber in real time through the temperature sensor (25), and after the temperature stabilizes to the set value and remains constant, proceed to the next step; S1.3 Start the confining pressure pump (26), open the confining pressure loading valve (28), apply the preset confining pressure to the confining pressure layer of the interdiffusion reactor (16), monitor the confining pressure value in real time through the confining pressure gauge (27), and close the confining pressure loading valve (28) after the confining pressure stabilizes to the preset value to keep the confining pressure constant. In step S1.3, the applied confining pressure Pc is always higher than the highest gas pressure Pg during the experiment, ensuring that the cavity of the interdiffusion reactor (16) is always under effective confining pressure protection.

[0007] Furthermore, step S2 specifically includes the following sub-steps: S2.1 Confirm that the methane injection branch switch valve (8), nitrogen injection branch switch valve (9), diffusion start valve (17), and exhaust valve (29) are all closed. Turn on the vacuum pump (12), slowly open the system vacuum valve (13), and then open the methane side inlet fine adjustment valve (10) and nitrogen side inlet fine adjustment valve (11) in sequence to connect the methane injection branch, nitrogen injection branch, methane diffusion chamber (16-1), and nitrogen diffusion chamber (16-2) to the vacuum pump (12) and perform full system vacuuming. S2.2 Continue evacuating until the readings of the methane-side pressure sensor (14) and the nitrogen-side pressure sensor (15) drop to the preset vacuum threshold and no longer change. Close the system vacuum valve (13), maintain the pressure and observe for the preset time. If the readings of the two pressure sensors do not rise significantly, the system is deemed to be airtight. In step S2.2, the preset vacuum threshold is not higher than 10Pa, the preset duration of pressure holding observation is not less than 1h, and the air tightness is determined to be qualified if the readings of the methane side pressure sensor (14) and the nitrogen side pressure sensor (15) do not rise by more than 50Pa during the pressure holding period.

[0008] Further, step S3 specifically involves the following steps: Under system vacuum conditions, the spectrometer (22) and data acquisition computer (23) are activated, and the left optical probe (20) and right optical probe (21) are controlled to perform cavity spectral scanning on the methane diffusion chamber (16-1) and nitrogen diffusion chamber (16-2) respectively. The baseline transmission signal I0 of the characteristic absorption peak of the target component is collected and stored in the data acquisition computer (23) as the reference for subsequent absorbance calculation. At the same time, the concentration-absorbance calibration is completed using a standard gas to obtain the calibration relationship A. t =lg(I0 / I t )=kC t +b, where At is the absorbance at time t, C t Let I be the concentration of the target component at time t, and k and b be the calibration fitting coefficients; t The optical probe continuously acquires the transmitted signals from both diffusion chambers through a sapphire window.

[0009] Furthermore, step S4 specifically includes the following sub-steps: S4.1 Confirm that the diffusion start valve (17), system vacuum valve (13), and exhaust valve (29) are all closed. Open the methane cylinder valve (1) and methane pressure reducing valve (2), start the precision booster pump (6), and sequentially open the methane injection branch switch valve (8) and methane side inlet fine adjustment valve (10) to fill the methane diffusion chamber (16-1) with high-purity methane gas. Monitor the chamber pressure in real time through the methane side pressure sensor (14) until the pressure reaches the preset experimental gas pressure Pg. Then close the methane side inlet fine adjustment valve (10) and methane injection branch switch valve (8). S4.2 Open the nitrogen cylinder valve (3) and nitrogen pressure reducing valve (4). After pressurizing by the precision booster pump (6), open the nitrogen injection branch switch valve (9) and nitrogen side air inlet fine adjustment valve (11) in sequence to fill the nitrogen diffusion chamber (16-2) with high-purity nitrogen. Monitor the chamber pressure in real time by the nitrogen side pressure sensor (15). Fine adjust the filling flow rate so that the pressure of the nitrogen diffusion chamber (16-2) is completely consistent with the pressure of the methane diffusion chamber (16-1). Close the nitrogen side air inlet fine adjustment valve (11) and nitrogen injection branch switch valve (9). S4.3 After the pressure in the dual chamber and the temperature in the constant temperature chamber (24) have stabilized, record the confining pressure Pc, gas pressure Pg, and temperature T of the current working condition, and calculate the effective stress σe under the working condition according to the formula σe=Pc-Pg. When filling the methane diffusion chamber (16-1) and nitrogen diffusion chamber (16-2) with gas, the methane-side inlet fine adjustment valve (10) and the nitrogen-side inlet fine adjustment valve (11) are filled with gas slowly with a small opening to avoid over-adjustment of chamber pressure during the filling process. If the filling pressure of one chamber is over-adjusted, the pressure is adjusted by a small amount of pressure relief through the exhaust valve (29) so that the pressure difference between the two chambers does not exceed 0.05MPa throughout the process. After filling is completed, the chamber is left to stand and maintain pressure. After confirming that there is no pressure fluctuation, the next step is carried out.

[0010] Further, step S5 specifically includes the following process: quickly and completely opening the diffusion start valve (17) to connect the methane diffusion chamber (16-1) and the nitrogen diffusion chamber (16-2), and the gas inter-diffusion process is officially started; at the same time, the preset automatic acquisition program is started by the data acquisition computer (23) to control the left optical probe (20) and the right optical probe (21) to continuously collect the gas in the dual chamber through the corresponding sapphire window components according to the preset sampling frequency; the spectrometer (22) records the transmission signal It at each moment in real time and transmits the data synchronously to the data acquisition computer (23) for storage; In step S5, the preset sampling frequency is on the order of seconds, the sampling interval is no more than 10 seconds, and in the first 10 minutes after the diffusion starts, the highest sampling frequency is used for high-frequency acquisition to capture the rapid concentration change characteristics in the early stage of diffusion.

[0011] Furthermore, step S6 specifically includes the following sub-steps: S6.1, Data Acquisition Computer (23) According to Lambert-Beer's law, the transmission signal It at each time moment is compared with the baseline transmission signal I0 to calculate the absorbance A of the target component at time t. t Then, based on the calibration relationship obtained in step S3, the real-time concentration C of the target component in the dual-chamber chamber at each time point is obtained by inversion. t ; S6.2, Real-time concentration C t Normalization is performed according to formula Y. t =(C t -C0) / (C ∞ The normalized concentration Y is calculated using -C0). t Where C0 is the initial concentration of the target component at the initial diffusion moment, C ∞ The equilibrium concentration of the target component when diffusion reaches equilibrium; Y at the initial stage of diffusion. t =0, when diffusion reaches equilibrium, Y = 0. t =1; S6.3. Based on Fick's second law of one-dimensional unsteady diffusion, establish the normalized concentration Y. t The diffusion kinetics model with time t was fitted by substituting the measured Yt-t data into the analytical formula of the diffusion model, and the interdiffusion coefficient D of methane and nitrogen under the experimental conditions was obtained by inversion. The interdiffusion coefficient D was then verified by linear regression in the linear segment of the later diffusion stage.

[0012] Furthermore, in step S6.3, the expression for Fick's second law of one-dimensional unsteady diffusion is: Where C is the component concentration, t is the diffusion time, x is the axial position of the diffusion direction, and D is the interdiffusion coefficient; the analytical formula for the diffusion model is: Y t = f(D,t,L) Where L is the characteristic diffusion length, f(D,t,L) This is an analytical function expression; for data in the approximately linear region of the later diffusion stage, the formula can be further used. Perform linear regression to evaluate Y. t = f(D,t,L) The obtained interdiffusion coefficient D Perform verification.

[0013] Furthermore, the coalbed methane interdiffusion experimental method also includes a multi-condition continuous testing step S7 under a single charge, which specifically includes the following sub-steps: S7.1 After the gas diffusion in the dual chambers reaches equilibrium, the interdiffusion test under the current operating condition is completed. The diffusion start valve (17) is closed. The methane diffusion chamber (16-1) and the nitrogen diffusion chamber (16-2) are evacuated to the preset threshold through the vacuum exhaust system. Then, pure methane and pure nitrogen are refilled into the methane diffusion chamber (16-1) and the nitrogen diffusion chamber (16-2) through the gas supply pressurization system to establish a new initial concentration gradient. The operating condition parameters are adjusted to the next gradient, and the diffusion and monitoring process is started. S7.2 During the adjustment of the operating conditions, the in-situ optical monitoring system continuously acquires spectra. After the operating conditions parameters stabilize, the concentration inversion and interdiffusion coefficient calculation process of step S6 is repeated to obtain the interdiffusion coefficient D under the gradient operating conditions. S7.3 Repeat steps S7.1 to S7.2 until all preset gradient conditions are tested, and obtain multiple sets of interdiffusion coefficients D under different conditions. Establish a quantitative relationship model between interdiffusion coefficient D and effective stress σe, gas pressure Pg, and temperature T.

[0014] Furthermore, in step S7.1, the methods for adjusting the experimental conditions include: keeping the confining pressure Pc and temperature T constant, and slowly reducing the total system pressure through the exhaust valve (29) to achieve continuous testing of different gas pressure Pg gradients; keeping the gas pressure Pg and temperature T constant, and adjusting the confining pressure Pc through the confining pressure pump (26) to achieve continuous testing of different effective stress σe gradients; keeping the confining pressure Pc and gas pressure Pg constant, and adjusting the temperature T through the constant temperature chamber (24) to achieve continuous testing of different temperature gradients.

[0015] Using the above technical solution, the dedicated experimental device upon which this invention is based is divided into five major systems, and the specific functions of each system are as follows: (1) Gas supply and pressurization system: provides stable and controllable high-pressure experimental gas for the experiment. Among them, the methane / nitrogen cylinder valve and pressure reducing valve realize the on / off of the gas source and the primary pressure regulation; the main gas source filter filters solid impurities in the gas to protect subsequent precision equipment; the precision booster pump realizes high-pressure boosting of the gas to meet the high-pressure experimental conditions of the reservoir; the high-pressure buffer tank stabilizes the gas pressure after pressurization to avoid pressure fluctuations during the gas filling process; the injection branch switch valve realizes the on / off control of the two gas paths, and the inlet fine-tuning valve realizes the precise adjustment of the filling gas flow rate to ensure precise matching of the pressure in the two chambers.

[0016] (2) Interdiffusion reaction system: provides a closed and controllable reaction space for the interdiffusion process of coalbed methane. Among them, the interdiffusion reactor is a cavity body that is resistant to high pressure and high temperature, which can simulate the reservoir confining pressure environment; the diffusion start valve divides the cavity into two independent chambers, realizing the precise start and stop of the diffusion process and avoiding premature contact of gases during the gas filling process; the methane / nitrogen diffusion chambers respectively contain pure methane and pure nitrogen, providing initial conditions for interdiffusion; the pressure sensors on both sides monitor the chamber pressure in real time, providing data support for operating condition setting and airtightness testing.

[0017] (3) In-situ optical monitoring system: realizes non-contact, continuous online monitoring of the interdiffusion process. Among them, the sapphire window component has ultra-high light transmittance and high pressure resistance, which not only ensures the sealing and pressure resistance of the cavity, but also provides a light transmission channel for optical detection; the optical probe realizes the transmission spectrum acquisition of the gas in the cavity. The coaxial installation ensures that the light path passes through the center of the cavity and the acquired data is representative. The left and right optical probes are both transceiver type near-infrared fiber optic probes with built-in broadband light source module and photoelectric detection module. The detection light emitted by the probe passes through the sapphire window and the corresponding cavity, and is received by the probe after being reflected by the inner wall of the cavity. It is converted into an electrical signal and transmitted to the spectrometer; the spectrometer converts the optical signal into an electrical signal, extracts the characteristic absorption peak of the target component, and realizes the quantitative inversion of concentration; the data acquisition computer realizes the automatic control of the acquisition program, real-time storage of data and subsequent processing.

[0018] (4) Environmental control system: simulates the temperature and stress environment of a real coal seam reservoir. The constant temperature chamber provides a constant temperature environment for the experiment. The temperature sensor monitors and feeds back the temperature inside the chamber in real time to ensure that the temperature remains constant during the experiment. The confining pressure pump, confining pressure loading valve and confining pressure gauge work together to achieve precise application and real-time monitoring of the confining pressure, simulate the reservoir stress environment, and ensure that the experimental conditions are consistent with the actual reservoir.

[0019] (5) Vacuum exhaust system: realizes system vacuuming, airtightness testing and safe discharge of exhaust gas. Among them, the vacuum pump provides a vacuum environment for the system to eliminate the interference of residual air in the cavity on the experimental results; the system vacuum valve realizes the on and off control of the vacuum pipeline; the exhaust gas venting valve realizes the safe discharge of exhaust gas after the experiment, and is also used for pressure gradient adjustment in multi-condition testing; the safety pressure relief valve provides overpressure protection for the system to avoid equipment damage and safety accidents under high pressure conditions.

[0020] The technical solution of this invention is based on the aforementioned dedicated experimental apparatus. Through standardized operating procedures, it achieves in-situ optical monitoring of the interdiffusion process of coalbed methane and precise determination of the interdiffusion coefficient. The innovative logic of this invention lies in: achieving non-contact online monitoring of gas concentration within a closed cavity through sapphire windows and fiber optic spectroscopy, completely eliminating traditional physical sampling operations and eradicating pressure fluctuations and measurement errors caused by sampling; ensuring precise control and repeatability of experimental conditions through standardized valve operation and parameter control throughout the entire process; and significantly improving experimental efficiency through a continuous multi-condition testing process with a single loading, while simultaneously establishing a quantitative relationship model between the interdiffusion coefficient and key reservoir parameters.

[0021] The overall technical effects of this invention are as follows: 1) Significantly improved data accuracy: In-situ non-contact optical monitoring is adopted, and the entire experimental process does not require opening the cavity to extract gas. The reactor is always in a closed constant pressure state, which completely avoids measurement errors such as pressure drop, flow field disturbance and pipeline temperature difference caused by sampling. The measured diffusion kinetic data are highly consistent with the real reservoir environment, and the accuracy of interdiffusion coefficient measurement is greatly improved.

[0022] 2) Breakthrough in monitoring continuity and model accuracy: It achieves second-level high-frequency continuous spectral acquisition, which can accurately capture the characteristics of rapid concentration changes in the early stage of diffusion at the millisecond level. It provides a high-density, full-time dataset for diffusion model fitting, which solves the problems of sparse data points and low fitting accuracy of traditional methods. It can accurately restore the dynamic law of the entire process of interdiffusion.

[0023] 3) Significantly improved experimental efficiency: It supports continuous testing of multiple pressures, temperatures, and effective stress gradients with a single loading, eliminating the need for repeated disassembly and assembly of the chamber, vacuuming, and airtightness testing. The experimental cycle is shortened from several days to several hours, and the experimental efficiency is improved by more than 80%.

[0024] 4) Comprehensive upgrade in operational safety: The entire process is automated for data acquisition, requiring only a few valve operations before and after the experiment. This eliminates the need for frequent manual operation of high-pressure valves in traditional methods, significantly reducing the risk of micro-leakage of high-pressure flammable gases, and avoiding the safety hazards of long-term contact between experimental personnel and high-pressure equipment.

[0025] 5) Outstanding engineering application value: It can accurately simulate the interdiffusion process of coalbed methane under different reservoir conditions, establish a quantitative relationship model between the interdiffusion coefficient and effective stress, gas pressure and temperature, provide accurate core physical parameter support for the optimization of coalbed methane injection and replacement mining schemes and the assessment of CO2 coal seam sealing potential, and has extremely strong on-site engineering guidance significance. Attached Figure Description

[0026] Figure 1This is a schematic diagram of the overall structure of the coalbed methane interdiffusion experimental apparatus on which the method of the present invention is based. The corresponding codes and names of the components in the attached diagram are as follows: 1-Methane cylinder valve, 2-Methane pressure reducing valve, 3-Nitrogen cylinder valve, 4-Nitrogen pressure reducing valve, 5-Main gas source filter, 6-Precision booster pump, 7-High pressure buffer tank, 8-Methane injection branch switch valve, 9-Nitrogen injection branch switch valve, 10-Methane side inlet fine-tuning valve, 11-Nitrogen side inlet fine-tuning valve, 12-Vacuum pump, 13-System vacuum valve, 14-Methane side pressure sensor, 15-Nitrogen side pressure sensor, 16-Interdiffusion reactor, 16-1-Methane diffusion Chamber, 16-2-Nitrogen diffusion chamber, 17-Diffusion start valve, 18-Left sapphire window assembly, 19-Right sapphire window assembly, 20-Left optical probe, 21-Right optical probe, 22-Spectrometer, 23-Data acquisition computer, 24-Thermostat, 25-Thermostat temperature sensor, 26-Containing pressure pump, 27-Containing pressure gauge, 28-Containing pressure loading valve, 29-Exhaust gas vent valve, 30-Safety relief valve, 31-Methane cylinder, 32-Nitrogen cylinder. Detailed Implementation

[0027] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The described embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0028] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings, Table 1 (test dataset of coalbed methane interdiffusion coefficient), and Table 2 (parameter table of interdiffusion coefficient quantification model). This embodiment is implemented based on the dedicated coalbed methane interdiffusion experimental apparatus described in claims 1-10. All component reference numerals correspond completely to the accompanying drawings and claims. The described embodiments are only for explaining the present invention and are not intended to limit the scope of protection of the present invention. All test results of this embodiment are summarized in Table 1, and the interdiffusion coefficient quantification relationship model and parameters obtained based on the test data are summarized in Table 2.

[0029] Table 1 Test Dataset

[0030] Test data obtained from a dedicated coalbed methane interdiffusion experimental device show that the interdiffusion coefficient has a quantifiable functional relationship with effective stress, gas pressure, and temperature (Table 2).

[0031] Table 2. Model parameters for interdiffusion coefficient

[0032] Example 1: System preparation, airtightness test and spectral calibration before the experiment This embodiment is a prerequisite for all interdiffusion tests, corresponding to steps S1-S3 in claim 1, and the specific operation is as follows: I. Cavity and Optical System Debugging: Disassemble the interdiffusion reactor (16), clean the inside of the cavity with anhydrous ethanol, and confirm that there are no residual impurities, oil stains and solid particles; install the left sapphire window assembly (18) and the right sapphire window assembly (19) at both ends of the reactor through high-pressure metal sealing gaskets, and tighten the bolts diagonally and evenly to ensure that the window and the cavity are sealed properly; install the left optical probe (20) and the right optical probe (21) coaxially with the corresponding sapphire window assembly, adjust the probe focal length and optical path so that the detection optical path passes through the central axis of the corresponding diffusion chamber along the cavity axis to ensure the representativeness of the spectral acquisition data; connect the two optical probes to the signal input end of the spectrometer (22) through single-mode optical fiber, and complete the Ethernet communication connection between the signal output end of the spectrometer (22) and the data acquisition computer (23), and debug the equipment to normal working state.

[0033] II. Environmental Parameter Preset: Place the assembled interdiffusion reactor (16) horizontally in the internal cavity of the constant temperature chamber (24), and connect all high-pressure pipelines and signal lines; turn on the constant temperature chamber (24), set the target temperature according to the subsequent experimental conditions, and monitor the temperature inside the chamber in real time through the temperature sensor (25). After the temperature stabilizes to the set value ±0.5℃ and remains stable for 30 minutes without fluctuation, proceed to the next step; start the confining pressure pump (26), slowly open the confining pressure loading valve (28), and apply the preset confining pressure to the confining pressure layer of the interdiffusion reactor (16). Monitor the confining pressure value in real time through the confining pressure gauge (27). After the confining pressure stabilizes to the preset value, close the confining pressure loading valve (28). Ensure that the applied confining pressure Pc is always higher than the highest gas pressure Pg during the experiment, and ensure that the cavity is always in an effective confining pressure protection state.

[0034] III. System Vacuuming and Airtightness Test: Confirm that the methane injection branch switch valve (8), nitrogen injection branch switch valve (9), diffusion start valve (17), and exhaust valve (29) are all completely closed; turn on the vacuum pump (12), slowly open the system vacuum valve (13), and then slowly open the methane side inlet fine adjustment valve (10) and nitrogen side inlet fine adjustment valve (11) in sequence, so that the methane injection branch, nitrogen injection branch, methane diffusion chamber (16-1), and nitrogen diffusion chamber (16-2) are all connected to the vacuum pump (12) to perform system vacuuming; continue vacuuming for more than 2 hours until the readings of the methane side pressure sensor (14) and nitrogen side pressure sensor (15) drop below 10 Pa and no longer change; close the system vacuum valve (13), maintain pressure and observe for 1 hour. If the readings of the two pressure sensors rise by no more than 50 Pa, the system airtightness is determined to meet the experimental requirements.

[0035] IV. Establishment of Spectral Baseline and Concentration Calibration: Under system vacuum, start the spectrometer (22) and data acquisition computer (23), and set the spectral acquisition parameters: near-infrared scanning range 1600nm-1800nm ​​(methane characteristic absorption peak range), spectral resolution 0.5nm, average number of scans 10, integration time 100ms; control the left optical probe (20) and the right optical probe (21) to perform cavity spectral scanning on the methane diffusion chamber (16-1) and the nitrogen diffusion chamber (16-2) respectively, and continuously acquire the baseline transmission signal I0 3 times, take the average value and store it in the data acquisition computer (23) as the reference signal for subsequent absorbance calculation.

[0036] Simultaneous concentration-absorbance calibration was performed: Six sets of methane-nitrogen standard gas mixtures with different concentration gradients were sequentially introduced into the vacuum chamber. After the pressure of each gas set stabilized, the transmission spectrum was collected, the corresponding absorbance was calculated, and a calibration curve of absorbance At versus methane concentration Ct was established. The calibration relationship At=kCt+b was obtained, where the fitting coefficients k and b were obtained by the least squares method. The goodness of fit R²≥0.999, which meets the accuracy requirements of quantitative inversion.

[0037] Example 2: Continuous testing of interdiffusion coefficient under different effective stress gradients (corresponding to PX1 data groups in Tables 1 and 2) This embodiment corresponds to steps S4-S6 of claim 1 and the multi-condition continuous test of claims 9-10, and is used to investigate the influence of effective stress on the methane-nitrogen interdiffusion coefficient under fixed gas pressure and temperature conditions. The specific operation is as follows: Ⅰ. Initial operating condition setting: After completing the system preparation and calibration of Example 1, confirm that the diffusion start valve (17), system vacuum valve (13), and exhaust valve (29) are all in the closed state; set the temperature of the constant temperature chamber (24) to 45℃ and keep it constant, and set the initial confining pressure Pc=20.0MPa and keep it stable.

[0038] II. Precise Dual-Chamber Gas Filling: Open the methane cylinder valve (1) and methane pressure reducing valve (2), adjust the outlet pressure of the pressure reducing valve to 2MPa, start the precision booster pump (6), and sequentially open the methane injection branch switch valve (8) and methane side inlet fine-tuning valve (10) to slowly fill the methane diffusion chamber (16-1) with a small opening. Monitor the chamber pressure in real time through the methane side pressure sensor (14) until the pressure stabilizes at 15.0MPa, and immediately close the methane side inlet fine-tuning valve (10) and methane injection branch switch valve (8); open the nitrogen cylinder valve (3) and nitrogen pressure reducing valve (4), and after pressurization by the precision booster pump (6), sequentially open the nitrogen injection branch switch valve. (9) Use the nitrogen-side inlet fine-tuning valve (11) to slowly fill the nitrogen diffusion chamber (16-2) with high-purity nitrogen (purity ≥99.999%). Fine-tune the valve opening to make the reading of the nitrogen-side pressure sensor (15) completely consistent with the reading of the methane-side pressure sensor (14). Control the pressure difference between the two chambers within 0.02MPa. Close the nitrogen-side inlet fine-tuning valve (11) and the nitrogen injection branch switch valve (9). Let it stand for 30 minutes until the pressure of the two chambers and the temperature of the constant temperature chamber are completely stable. Record the current working conditions: confining pressure Pc = 20.0MPa, gas pressure Pg = 15.0MPa, temperature T = 45℃. According to the formula σe = Pc - Pg, the effective stress σe = 5.0MPa is calculated.

[0039] III. Diffusion Initiation and In-situ Monitoring: The diffusion initiation valve (17) is quickly and completely opened, and the mutual diffusion process is officially started. At the same time, the automatic acquisition program is started through the data acquisition computer (23), the basic sampling interval is set to 5s, and the frequency is increased to 1s / time for the first 10 minutes after diffusion starts. The left optical probe (20) and the right optical probe (21) are controlled to continuously acquire the spectrum of the dual chambers in in-situ. The spectrum analyzer (22) records the transmission signal It at each moment in real time and transmits it synchronously to the data acquisition computer (23) for storage until the concentration does not change significantly within 30 minutes, and the diffusion is judged to have reached equilibrium.

[0040] IV. Data Processing and Coefficient Calculation: The data acquisition computer (23) calculates the absorbance at each time step At = lg(I0 / I) according to the Lambert-Beer law. t Substituting the values ​​into the calibration relation, the real-time methane concentration Ct at each time point is obtained. Ct is normalized according to the formula Yt=(Ct-C0) / (C∞-C0) to obtain a dataset of normalized concentration Yt versus time t. Based on Fick's second law of one-dimensional unsteady diffusion, the Yt-t dataset is substituted into the analytical formula of the diffusion model for nonlinear fitting, and the interdiffusion coefficient D=9.26×10⁻⁶ under this condition is obtained. -7 cm² / s, the results are shown in the first row of Group PX1 in Table 1.

[0041] V. Multi-gradient continuous test: After completing the test under the current working condition, keep the diffusion start valve (17) fully open, the gas pressure Pg=15.0MPa, and the temperature T=45℃ constant. Increase the confining pressure Pc step by step through the confining pressure pump (26) and the confining pressure loading valve (28), and set them to 25.0MPa, 30.0MPa, 35.0MPa, 40.0MPa, 45.0MPa, and 50.0MPa in sequence. The corresponding effective stresses σe are 10.0MPa, 15.0MPa, 20.0MPa, 25.0MPa, 30.0MPa, and 35.0MPa in sequence. After each confining pressure level stabilizes, repeat the above monitoring and data processing process to continuously obtain the interdiffusion coefficient D under each effective stress. The results are summarized in Table 1PX1 group.

[0042] VI. Quantitative Model Fitting: Based on the test data of group PX1 in Table 1, with the effective stress σe as the independent variable and the interdiffusion coefficient D as the dependent variable, an exponential model was fitted to obtain the model expression D=a·exp(-bσe). e ), where the fitting parameter a = 1.222 × 10 -6 cm² / s, b=0.05268MPa -1 The goodness of fit R² = 0.9859, and the results are shown in Table 2 for group PX1, which clarifies that the interdiffusion coefficient decreases exponentially with increasing effective stress.

[0043] Example 3: Continuous testing of interdiffusion coefficient under different gas pressure gradients (corresponding to the PX2 data sets in Tables 1 and 2) This embodiment corresponds to steps S4-S6 of claim 1 and the multi-condition continuous test of claims 9-10, used to investigate the influence of gas pressure on the methane-nitrogen interdiffusion coefficient under fixed confining pressure and temperature conditions. The specific operation is as follows: I. Operating Condition Setting and Gas Injection: After completing the system preparation and calibration of Example 1, set the temperature of the constant temperature chamber (24) to 70℃ and keep it constant, set the confining pressure Pc=45.0MPa and keep it stable; confirm that the diffusion start valve (17) is closed, fill the methane diffusion chamber (16-1) with high-purity methane to the initial pressure of 15.0MPa, fill the nitrogen diffusion chamber (16-2) with high-purity nitrogen to the pressure of the methane chamber, and the pressure difference between the two chambers is ≤0.02MPa. Let it stand until the pressure and temperature are completely stable, and record the initial operating conditions: confining pressure Pc=45.0MPa, gas pressure Pg=15.0MPa, temperature T=70℃, effective stress σe=30.0MPa.

[0044] II. Diffusion Testing and Data Processing: The interdiffusion process was initiated by quickly opening the diffusion start valve (17). In-situ spectral acquisition was performed using the same sampling parameters as in Example 2. Data inversion and model fitting were completed, and the interdiffusion coefficient D = 2.30 × 10⁻⁶ under this condition was obtained.-7 cm² / s, the results are shown in the first row of the PX2 group in Table 1.

[0045] III. Multi-gradient continuous test: After completing the initial working condition test, keep the diffusion start valve (17) fully open, the confining pressure Pc=45.0MPa, and the temperature T=70℃ constant. The total system pressure Pg is gradually increased through the gas supply boosting system and set to 17.5MPa, 20.0MPa, 22.5MPa, 25.0MPa, 27.5MPa, and 30.0MPa in sequence. The corresponding effective stress σe is 27.5MPa, 25.0MPa, 22.5MPa, 20.0MPa, 17.5MPa, and 15.0MPa in sequence. After each pressure level stabilizes, repeat the monitoring and data processing process to continuously obtain the interdiffusion coefficient D under each gas pressure. The results are summarized in Table 1PX2 group.

[0046] IV. Quantitative Model Fitting: Based on the test data of group PX2 in Table 1, a linear model was fitted with gas pressure Pg as the independent variable and interdiffusion coefficient D as the dependent variable, yielding the model expression D = α·Pg + β, where the fitting parameter α = 3.691 × 10⁻⁶. - 8 cm² / (s·MPa), β=-2.989×10 -7 cm² / s, goodness of fit R²=0.9465, results are shown in Table 2 for group PX2, which clarifies that the interdiffusion coefficient increases linearly with increasing gas pressure.

[0047] Example 4: Continuous testing of interdiffusion coefficient under different temperature gradients (corresponding to PX3 data sets in Tables 1 and 2) This embodiment corresponds to steps S4-S6 of claim 1 and the multi-condition continuous test of claims 5-6, used to investigate the influence of temperature on the methane-nitrogen interdiffusion coefficient under fixed confining pressure, fixed gas pressure, and fixed effective stress. The specific operation is as follows: I. Operating Condition Setting and Gas Induction: After completing the system preparation and calibration of Example 1, set the initial temperature of the constant temperature chamber (24) to 45℃, set the confining pressure Pc=35.0MPa and keep it stable; confirm that the diffusion start valve (17) is closed, fill the methane diffusion chamber (16-1) with high-purity methane to 22.5MPa, fill the nitrogen diffusion chamber (16-2) with high-purity nitrogen to the same pressure, the pressure difference between the two chambers is ≤0.02MPa, let it stand until the pressure and temperature are completely stable, and record the initial operating conditions: confining pressure Pc=35.0MPa, gas pressure Pg=22.5MPa, temperature T=45℃, effective stress σe=12.5MPa.

[0048] II. Diffusion Testing and Data Processing: The interdiffusion process was initiated by quickly opening the diffusion start valve (17). In-situ spectral acquisition was performed using the same sampling parameters as in Example 2. Data inversion and model fitting were completed, and the interdiffusion coefficient D = 5.81 × 10⁻⁶ under this condition was obtained. -7 cm² / s, the results are shown in the first row of Group PX3 in Table 1.

[0049] III. Multi-gradient continuous test: After completing the initial working condition test, keep the diffusion start valve (17) fully open, the confining pressure Pc=35.0MPa, and the gas pressure Pg=22.5MPa constant. The temperature is gradually increased through the constant temperature chamber (24) and set to 50℃, 55℃, 60℃, 65℃, 70℃, and 75℃ in sequence. After each temperature level stabilizes, keep it at that temperature for 30 minutes. Repeat the monitoring and data processing process to continuously obtain the interdiffusion coefficient D at each temperature. The results are summarized in Table 1PX3 group.

[0050] IV. Quantitative Model Fitting: Based on the test data of group PX3 in Table 1, the model expression D = D0·exp(-a / RT) was obtained by fitting the model according to the Arrhenius formula, where the pre-exponential factor D0 = 1.077 × 10⁻⁶. -4 cm² / s, diffusion activation energy Ea = 13.88 kJ / mol, R is the universal gas constant, goodness of fit R² = 0.9750, the results are shown in Table 2 for group PX3, which clarifies that the interdiffusion coefficient increases exponentially with increasing temperature.

[0051] Example 5: Continuous test of interdiffusion coefficient under multi-parameter coupling conditions (corresponding to PX4 data sets in Tables 1 and 2) This embodiment corresponds to the continuous multi-condition test with a single charge as described in claims 5-6. It is used to simulate the influence of the coupled changes of multiple parameters such as confining pressure, gas pressure, and temperature on the interdiffusion coefficient during the change of coal seam depth. It verifies the advantage of this method in completing a full gradient continuous test with a single charge. The specific operation is as follows: I. Initial operating condition setting: After completing the system preparation and calibration of Example 1, the initial operating conditions are set as follows: constant temperature chamber temperature T=45℃, confining pressure Pc=20.0MPa, methane and nitrogen are filled into the two chambers respectively until Pg=15.0MPa, the pressure of the two chambers is consistent, and the system is allowed to stand until the parameters are stable. The effective stress σe=5.0MPa is calculated.

[0052] II. Coupled Gradient Continuous Test: Quickly open the diffusion start valve (17) to complete the mutual diffusion test under the initial operating conditions, obtaining D=9.26×10 -7cm² / s; Keep the diffusion start valve (17) fully open, and adjust the confining pressure Pc, gas pressure Pg, and temperature T in sync according to the working parameters of Group PX4 in Table 1. Complete the continuous test of 7 sets of coupled working conditions in sequence. After the parameters of each working condition are stable, repeat the monitoring and data processing process to continuously obtain the interdiffusion coefficient D under each coupled working condition. The results are summarized in Group PX4 in Table 1. Only one loading is required throughout the process. There is no need to repeatedly disassemble and reassemble the cavity, vacuum and airtightness test. The experimental cycle is shortened by more than 85% compared with the traditional method.

[0053] III. Quantitative Model Fitting: Based on the test data of group PX4 in Table 1, with the effective stress σe as the independent variable and the interdiffusion coefficient D as the dependent variable, an exponential model was fitted, yielding the model expression D = a·exp(-bσe), where the fitting parameter a = 9.865 × 10⁻⁶. -7 cm² / s, b = 0.01921 MPa -1 Goodness of fit R 2 =0.9230, the results are shown in Table 2 for group PX4, which verifies that under multi-parameter coupling conditions, the interdiffusion coefficient still shows a significant exponential decay relationship with the effective stress.

[0054] The above embodiments fully verify the feasibility and accuracy of the method of the present invention. Through in-situ optical non-contact monitoring, the measurement error of traditional sampling methods is completely eliminated. The interdiffusion coefficient of coalbed methane under different reservoir conditions can be accurately measured. At the same time, it supports continuous testing under multiple conditions with one loading. A quantitative relationship model between the interdiffusion coefficient and key reservoir parameters can be established, providing accurate experimental methods and core data support for coalbed methane injection replacement mining and CO2 coal seam sealing.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention 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 of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. An experimental method for coalbed methane interdiffusion based on in-situ optical monitoring, characterized in that: The aforementioned coalbed methane interdiffusion experimental method is implemented based on a dedicated coalbed methane interdiffusion experimental device, which includes a gas supply and pressurization system, an interdiffusion reaction system, an in-situ optical monitoring system, an environmental control system, and a vacuum exhaust system. The gas supply and pressurization system includes a methane gas source unit, a nitrogen gas source unit, a main gas source filter (5), a precision booster pump (6), and a high-pressure buffer tank (7); the methane gas source unit includes a methane cylinder (31), and a methane cylinder valve (1) and a methane pressure reducing valve (2) are connected sequentially to the gas supply line of the methane cylinder (31); the nitrogen gas source unit includes a nitrogen cylinder (32), and a nitrogen cylinder valve (3), a nitrogen pressure reducing valve (4), and a methane pressure reducing valve (2) are connected sequentially to the gas supply line of the nitrogen cylinder (32). The outlets of the nitrogen pressure reducing valve (4) are connected to the inlet of the main gas source filter (5), and the outlets of the main gas source filter (5) are connected in sequence to the precision booster pump (6) and the high-pressure buffer tank (7); the outlet of the high-pressure buffer tank (7) is connected to the methane injection branch and the nitrogen injection branch. The methane injection branch is equipped with the methane injection branch switch valve (8) and the methane side intake fine adjustment valve (10) in sequence, and the nitrogen injection branch is equipped with the nitrogen injection branch switch valve (9) and the nitrogen side intake fine adjustment valve (11) in sequence. The interdiffusion reaction system includes an interdiffusion reactor (16). The internal cavity of the interdiffusion reactor (16) is divided into a methane diffusion chamber (16-1) on the left and a nitrogen diffusion chamber (16-2) on the right by a diffusion start valve (17). The methane diffusion chamber (16-1) and the nitrogen diffusion chamber (16-2) are independent of each other and have the same volume. The inlet of the methane diffusion chamber (16-1) is connected to the outlet of the methane injection branch, and the inlet of the nitrogen diffusion chamber (16-2) is connected to the outlet of the nitrogen injection branch. A methane-side pressure sensor (14) is installed in the methane diffusion chamber (16-1), and a nitrogen-side pressure sensor (15) is installed in the nitrogen diffusion chamber (16-2). The in-situ optical monitoring system includes a left sapphire window assembly (18), a right sapphire window assembly (19), a left optical probe (20), a right optical probe (21), a spectrometer (22), and a data acquisition computer (23); the left sapphire window assembly (18) is sealed and installed at one end of the interdiffusion reactor (16) corresponding to the methane diffusion chamber (16-1), and the right sapphire window assembly (19) is sealed and installed at one end of the interdiffusion reactor (16) corresponding to the nitrogen diffusion chamber (16-2); the left optical... The optical probe (20) is coaxially positioned outside the left sapphire window assembly (18), and the right optical probe (21) is coaxially positioned outside the right sapphire window assembly (19). The detection optical paths of the left optical probe (20) and the right optical probe (21) both pass through the central axis of the corresponding diffusion chamber. The left optical probe (20) and the right optical probe (21) are both connected to the signal input end of the spectrometer (22) via optical fiber. The signal output end of the spectrometer (22) is connected to the data acquisition computer (23) for communication. The environmental control system includes a constant temperature chamber (24) and a confining pressure loading unit; the interdiffusion reactor (16) is placed inside the internal cavity of the constant temperature chamber (24), and a constant temperature chamber temperature sensor (25) is installed inside the constant temperature chamber (24) for real-time temperature monitoring; the confining pressure loading unit includes a confining pressure pump (26), a confining pressure gauge (27) and a confining pressure loading valve (28), the outlet of the confining pressure pump (26) is connected to the confining pressure layer of the interdiffusion reactor (16) through the confining pressure loading valve (28), and the confining pressure gauge (27) is installed on the pipeline between the confining pressure loading valve (28) and the confining pressure layer; The vacuum exhaust system includes a vacuum pump (12), a system vacuum valve (13), an exhaust valve (29), and a safety relief valve (30); the vacuum pump (12) is connected in parallel with the methane injection branch and the nitrogen injection branch through the system vacuum valve (13), and the connection point is located between the inlet fine-tuning valve and the injection branch switch valve; the exhaust valve (29) and the safety relief valve (30) are connected to the end pipeline of the interdiffusion reactor (16); The experimental method for coalbed methane interdiffusion includes the following steps: Step S1: Debugging and setting environmental parameters of the dedicated coalbed methane interdiffusion experimental device before the experiment; Step S2: Vacuuming and airtightness test of the dedicated coalbed methane interdiffusion experimental device; Step S3: Establishing spectral baseline and calibrating concentration; Step S4: Filling the methane diffusion chamber (16-1) and nitrogen diffusion chamber (16-2) with gas and setting experimental conditions; Step S5: Diffusion initiation and in-situ continuous optical monitoring; Step S6: Concentration inversion and interdiffusion coefficient calculation.

2. The experimental method for coalbed methane interdiffusion based on in-situ optical monitoring according to claim 1, characterized in that: Step S1 specifically includes the following sub-steps: S1.1 Check the cleanliness of the interior of the interdiffusion reactor (16), confirm that the left sapphire window assembly (18) and the right sapphire window assembly (19) are installed securely and sealed well, install the left optical probe (20) and the right optical probe (21) coaxially with the corresponding sapphire window assembly, adjust the focal length of the optical probe so that the detection light path passes through the central area of ​​the corresponding diffusion chamber. S1.2 Turn on the constant temperature chamber (24), set the constant temperature according to the experimental target, monitor the temperature inside the chamber in real time through the temperature sensor (25), and after the temperature stabilizes to the set value and remains constant, proceed to the next step; S1.3 Start the confining pressure pump (26), open the confining pressure loading valve (28), apply the preset confining pressure to the confining pressure layer of the interdiffusion reactor (16), monitor the confining pressure value in real time through the confining pressure gauge (27), and after the confining pressure stabilizes to the preset value, keep the confining pressure loading valve (28) open, and compensate the pressure in real time through the confining pressure pump (26) so that the confining pressure is stable at the preset value throughout the process and is always higher than the highest gas pressure during the experiment; In step S1.3, the applied confining pressure Pc is always higher than the highest gas pressure Pg during the experiment, ensuring that the cavity of the interdiffusion reactor (16) is always under effective confining pressure protection.

3. The experimental method for coalbed methane interdiffusion based on in-situ optical monitoring according to claim 1, characterized in that: Step S2 specifically includes the following sub-steps: S2.1 Confirm that the methane injection branch switch valve (8), nitrogen injection branch switch valve (9), diffusion start valve (17), and exhaust valve (29) are all closed. Turn on the vacuum pump (12), slowly open the system vacuum valve (13), and then open the methane side inlet fine adjustment valve (10) and nitrogen side inlet fine adjustment valve (11) in sequence to connect the methane injection branch, nitrogen injection branch, methane diffusion chamber (16-1), and nitrogen diffusion chamber (16-2) to the vacuum pump (12) and perform full system vacuuming. S2.2 Continue evacuating until the readings of the methane-side pressure sensor (14) and the nitrogen-side pressure sensor (15) drop to the preset vacuum threshold and no longer change. Close the system vacuum valve (13), maintain the pressure and observe for the preset time. If the readings of the two pressure sensors do not rise significantly, the system is deemed to be airtight. In step S2.2, the preset vacuum threshold is not higher than 10Pa, the preset duration of pressure holding observation is not less than 1h, and the air tightness is determined to be qualified if the readings of the methane side pressure sensor (14) and the nitrogen side pressure sensor (15) do not rise by more than 50Pa during the pressure holding period.

4. The experimental method for coalbed methane interdiffusion based on in-situ optical monitoring according to claim 1, characterized in that: Step S3 is as follows: Under system vacuum, start the spectrometer (22) and data acquisition computer (23), control the left optical probe (20) and right optical probe (21) to perform cavity spectral scanning on the methane diffusion chamber (16-1) and nitrogen diffusion chamber (16-2) respectively, collect the baseline transmission signal I0 of the characteristic absorption peak of the target component and store it in the data acquisition computer (23) as the reference for subsequent absorbance calculation; at the same time, complete the concentration-absorbance calibration through standard gas to obtain the calibration relationship A. t =lg(I0 / I t )=kC t +b, where At is the absorbance at time t, C t Let I be the concentration of the target component at time t, and k and b be the calibration fitting coefficients; t The optical probe continuously acquires the transmitted signals from both diffusion chambers through a sapphire window.

5. The experimental method for coalbed methane interdiffusion based on in-situ optical monitoring according to claim 1, characterized in that: Step S4 specifically includes the following sub-steps: S4.1 Confirm that the diffusion start valve (17), system vacuum valve (13), and exhaust valve (29) are all closed. Open the methane cylinder valve (1) and methane pressure reducing valve (2), start the precision booster pump (6), and sequentially open the methane injection branch switch valve (8) and methane side inlet fine adjustment valve (10) to fill the methane diffusion chamber (16-1) with high-purity methane gas. Monitor the chamber pressure in real time through the methane side pressure sensor (14) until the pressure reaches the preset experimental gas pressure Pg. Then close the methane side inlet fine adjustment valve (10) and methane injection branch switch valve (8). S4.2 Open the nitrogen cylinder valve (3) and nitrogen pressure reducing valve (4). After pressurizing by the precision booster pump (6), open the nitrogen injection branch switch valve (9) and nitrogen side air inlet fine adjustment valve (11) in sequence to fill the nitrogen diffusion chamber (16-2) with high-purity nitrogen. Monitor the chamber pressure in real time by the nitrogen side pressure sensor (15). Fine adjust the filling flow rate so that the pressure of the nitrogen diffusion chamber (16-2) is completely consistent with the pressure of the methane diffusion chamber (16-1). Close the nitrogen side air inlet fine adjustment valve (11) and nitrogen injection branch switch valve (9). S4.3 After the pressure in the dual chamber and the temperature in the constant temperature chamber (24) have stabilized, record the confining pressure Pc, gas pressure Pg, and temperature T of the current working condition, and calculate the effective stress σe under the working condition according to the formula σe=Pc-Pg. When filling the methane diffusion chamber (16-1) and nitrogen diffusion chamber (16-2) with gas, the methane-side inlet fine adjustment valve (10) and the nitrogen-side inlet fine adjustment valve (11) are filled with gas slowly with a small opening to avoid over-adjustment of chamber pressure during the filling process. If the filling pressure of one chamber is over-adjusted, the pressure is adjusted by a small amount of pressure relief through the exhaust valve (29) so that the pressure difference between the two chambers does not exceed 0.05MPa throughout the process. After filling is completed, the chamber is left to stand and maintain pressure. After confirming that there is no pressure fluctuation, the next step is carried out.

6. The experimental method for coalbed methane interdiffusion based on in-situ optical monitoring according to claim 1, characterized in that: Step S5 details The process includes the following steps: the diffusion start valve (17) is opened quickly and completely to connect the methane diffusion chamber (16-1) and the nitrogen diffusion chamber (16-2), and the gas interdiffusion process is officially started; at the same time, the preset automatic acquisition program is started by the data acquisition computer (23), which controls the left optical probe (20) and the right optical probe (21) to continuously collect the in-situ spectrum of the gas in the dual chambers through the corresponding sapphire window components according to the preset sampling frequency; the spectrometer (22) records the transmission signal It at each moment in real time and transmits the data synchronously to the data acquisition computer (23) for storage; In step S5, the preset sampling frequency is on the order of seconds, the sampling interval is no more than 10 seconds, and in the first 10 minutes after the diffusion starts, the highest sampling frequency is used for high-frequency acquisition to capture the rapid concentration change characteristics in the early stage of diffusion.

7. The experimental method for coalbed methane interdiffusion based on in-situ optical monitoring according to claim 1, characterized in that: Step S6 specifically includes the following sub-steps: S6.1, Data Acquisition Computer (23) According to Lambert-Beer's law, the transmission signal It at each time moment is compared with the baseline transmission signal I0 to calculate the absorbance A of the target component at time t. t Then, based on the calibration relationship obtained in step S3, the real-time concentration C of the target component in the dual-chamber chamber at each time point is obtained by inversion. t ; S6.2, Real-time concentration C t Normalization is performed according to formula Y. t =(C t -C0) / (C ∞ The normalized concentration Y is calculated using -C0). t Where C0 is the initial concentration of the target component at the initial diffusion moment, C ∞ The equilibrium concentration of the target component when diffusion reaches equilibrium; Y at the initial stage of diffusion. t =0, when diffusion reaches equilibrium, Y = 0. t =1; S6.

3. Based on Fick's second law of one-dimensional unsteady diffusion, establish the normalized concentration Y. t The diffusion kinetics model with time t was fitted by substituting the measured Yt-t data into the analytical formula of the diffusion model, and the interdiffusion coefficient D of methane and nitrogen under the experimental conditions was obtained by inversion. The interdiffusion coefficient D was then verified by linear regression in the linear segment of the later diffusion stage.

8. The experimental method for coalbed methane interdiffusion based on in-situ optical monitoring according to claim 7, characterized in that: In step S6.3, the expression for Fick's second law of one-dimensional unsteady diffusion is: Where C is the component concentration, t is the diffusion time, x is the axial position of the diffusion direction, and D is the interdiffusion coefficient; the analytical formula for the diffusion model is: Y t = f(D,t,L) Where L is the characteristic diffusion length, f(D,t,L) This is an analytical function expression; for data in the approximately linear region of the later diffusion stage, the formula can be further used. Perform linear regression to evaluate Y. t = f(D,t, L) The obtained interdiffusion coefficient D Perform verification.

9. The experimental method for coalbed methane interdiffusion based on in-situ optical monitoring according to claim 1, characterized in that: The coalbed methane interdiffusion experimental method further includes a multi-condition continuous testing step S7 under a single charge, which specifically includes the following sub-steps: S7.1 After the gas diffusion in the dual chambers reaches equilibrium, the interdiffusion test under the current operating condition is completed. The diffusion start valve (17) is closed. The methane diffusion chamber (16-1) and the nitrogen diffusion chamber (16-2) are evacuated to the preset threshold through the vacuum exhaust system. Then, pure methane and pure nitrogen are refilled into the methane diffusion chamber (16-1) and the nitrogen diffusion chamber (16-2) through the gas supply pressurization system to establish a new initial concentration gradient. The operating condition parameters are adjusted to the next gradient, and the diffusion and monitoring process is started. S7.2 During the adjustment of the operating conditions, the in-situ optical monitoring system continuously acquires spectra. After the operating conditions parameters stabilize, the concentration inversion and interdiffusion coefficient calculation process of step S6 is repeated to obtain the interdiffusion coefficient D under the gradient operating conditions. S7.3 Repeat steps S7.1 to S7.2 until all preset gradient conditions are tested, and obtain multiple sets of interdiffusion coefficients D under different conditions. Establish a quantitative relationship model between interdiffusion coefficient D and effective stress σe, gas pressure Pg, and temperature T.

10. The experimental method for coalbed methane interdiffusion based on in-situ optical monitoring according to claim 9, characterized in that: In step S7.1, the methods for adjusting the experimental conditions include: keeping the confining pressure Pc and temperature T constant, and slowly reducing the total system pressure through the exhaust valve (29) to achieve continuous testing of different gas pressure Pg gradients; keeping the gas pressure Pg and temperature T constant, and adjusting the confining pressure Pc through the confining pressure pump (26) to achieve continuous testing of different effective stress σe gradients; keeping the confining pressure Pc and gas pressure Pg constant, and adjusting the temperature T through the constant temperature chamber (24) to achieve continuous testing of different temperature gradients.

Images