An experimental method for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics

Abstract

The present application relates to a kind of experimental methods based on microfluidic test gas-liquid diffusion distance and diffusion coefficient.The experimental device is composed of microfluidic chip, microfluidic chip holder, 4 opening high temperature and high pressure micro reaction kettle, trace displacement pump, back pressure constant pressure pump, back pressure valve, intermediate container, temperature control box, image acquisition device and the like.The method comprises the following steps: (1) establishing corresponding simulation porous medium model; (2) configuring sample; (3) connecting experimental device and leak detection; (4) pre-saturation crude oil; (5) adjusting temperature to formation temperature; (6) establishing system pressure to formation pressure; (7) simulating micro diffusion experiment, and recording diffusion condition in real time; (8) after experiment, unloading pressure; (9) using image processing software to determine diffusion distance by observing color change of crude oil; (10) using diffusion distance to calculate diffusion coefficient.The present application is convenient and fast, and the result is accurate and reliable, which provides theoretical guidance and technical support for improving recovery efficiency.

Description

Technical Field

[0001] This invention relates to an experimental method for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics, belonging to the field of oil and gas field development engineering technology. Background Technology

[0002] Gas injection is one of the most important methods for improving oil recovery. Common injected gases include hydrocarbon gases, nitrogen, flue gas, and carbon dioxide. During gas injection in oil reservoirs, the different mobility ratios between the injected gas and crude oil, gravitational viscosity, and diffusion mixing all influence the dynamics of the injection process. Currently, molecular diffusion is considered a very important oil displacement mechanism, especially for matrix rocks with small pore throats, low permeability, and high capillary pressure, where gravitational gas expulsion is limited, and molecular diffusion becomes the primary mechanism. In recent years, in particular, after primary oil recovery, a large amount of crude oil remains as residual oil in the pores. Therefore, studying the gas-liquid diffusion coefficient at the microscopic pore scale has significant practical guiding significance for improving oil recovery.

[0003] In existing technologies, experimental methods for determining the gas-liquid diffusion coefficient can be divided into two categories: direct methods and indirect methods. Direct methods involve sampling the fluid at different times and diffusion distances, analyzing these samples to obtain gas concentration data, and then using a corresponding mathematical model to derive the diffusion coefficient. Indirect methods test parameters of the system changes due to diffusion, such as system pressure or fluid density caused by mass transfer, and finally use a corresponding mathematical model to determine the diffusion coefficient. Because the sampling process of direct methods is easily affected by subjective factors, leading to experimental errors, and with the development of modern testing technologies, indirect methods have become the main means of testing molecular diffusion coefficients. Currently, the two most widely used methods are as follows:

[0004] (1) NMR (Nuclear Magnetic Resonance) method: The diffusion coefficient is determined based on the principle that changes in the physical properties of a mixture during diffusion cause changes in the NMR spectrum. NMR equipment is expensive and costly. Representative achievements include: In 2005, Wen and Kantzas (JCPT) detected changes in the mobility of hydrogen-containing molecules in solvents and oils when solvents came into contact with heavy oil or asphalt samples by measuring changes in NMR relaxation characteristics. Then, based on Fick's second law, they calculated the concentration-independent diffusion coefficients for three oils and six solvents.

[0005] (2) PVT (Pressure-Volume-Temperature) Method: The testing principle is that gas molecules diffuse from the gas phase to the liquid phase until equilibrium is reached, during which the gas pressure changes continuously over time. The PVT method is widely used due to its convenience, simplicity, and accuracy. Representative achievements include: the constant volume diffusion test method established by Riazi, MR (SPEJ) in 1996; and the article "Calculation of Gas-Gas Diffusion Coefficient of Multicomponent Gases" published by Guo et al. in Volume 3 of Natural Gas Industry in 2010, which established a method for determining the molecular diffusion coefficient of high-temperature and high-pressure multicomponent oil and gas systems in a PVT cylinder based on Riazi's principle. However, none of the above achievements have considered the influence of porous media.

[0006] Currently, research on experimental methods for gas-liquid diffusion in porous media is still under exploration. Representative achievements include: Patent CN111239176A, which provides a testing device and method for determining the diffusion distance of injected gas in crude oil using nuclear magnetic resonance (NMR) testing and analysis; and Patent CN102644459B, which provides a device and method for determining the molecular diffusion coefficient of a multi-component gas-liquid system in a core sample under different reservoir temperature and pressure conditions, measuring the molecular diffusion coefficient of each component in the gas and oil phases of a multi-component gas-liquid system in a real core sample. However, all of the above research on gas-liquid diffusion experimental methods in porous media is conducted at the core scale, and there is still no research on the gas-liquid diffusion distance and diffusion coefficient at the pore scale in porous media. Furthermore, these methods still suffer from the drawbacks of high cost and long processing time associated with previous experimental methods.

[0007] Microfluidic technology, with its precise, quantitative, safe, and rapid visualization capabilities, plays an irreplaceable role in the study of oil, gas, and water flow patterns in pores and micro / nano matrices during oil and gas field development. Currently, micro-displacement experiments are widely used in seepage and micro-oil displacement experiments. Representative achievements include: the invention patent for a porous media micro-seepage simulation experimental device system (CN103792170A), which provides a device for intuitively observing the flow phenomena of multiphase fluids in porous media and the interaction between fluids and porous media; and the invention patent for a micro-oil displacement experimental method simulating reservoir conditions (CN109441414A), which provides an efficient experimental method and injection method for simulating micro-oil displacement under reservoir conditions. However, there is still no research on experimental methods for pore-scale gas-liquid diffusion experiments in porous media using microfluidic technology. Therefore, designing an experimental method based on microfluidics to test gas-liquid diffusion distance and diffusion coefficient can fill the technological gap in pore-scale gas-liquid diffusion experimental research in porous media, providing theoretical guidance and technical support for improving oil recovery. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides an experimental method for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics. By using microfluidics to test the gas-liquid diffusion distance and diffusion coefficient at the pore scale in porous media, the method evaluates and analyzes the gas-liquid diffusion effect at the pore scale in porous media, thereby providing theoretical guidance and technical support for improving oil recovery.

[0009] The first aspect of this invention provides a visualization experimental device for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics. The device comprises a microfluidic chip, a microfluidic chip holder, a four-opening high-temperature and high-pressure micro-reactor, a micro-displacement pump, a backpressure constant-pressure pump, a backpressure valve, an intermediate container, a high-magnification electron microscope, a heating mantle, a temperature control chamber, an image acquisition device, test tubes, and a three-way valve. The inlet and outlet ends of the microfluidic chip are fitted into corresponding positions in the grooves of the microfluidic chip holder within the four-opening high-temperature and high-pressure micro-reactor. The inlet end of the microfluidic chip holder is connected to the gas intermediate container, the crude oil intermediate container, and the micro-displacement pump via the three-way valve, while the outlet end is connected to the backpressure valve, the intermediate container, and the backpressure constant-pressure pump. Pressure gauges are provided at the inlet and outlet ends of the microfluidic chip holder. The entire microfluidic chip and the microfluidic chip holder are placed in a four-opening high-temperature and high-pressure micro-reactor. There is an observation window at the top of the four-opening high-temperature and high-pressure micro-reactor, on which an image acquisition device is placed. An illumination device is placed at the bottom of the four-opening high-temperature and high-pressure micro-reactor.

[0010] The four-opening high-temperature and high-pressure micro-reactor used in this invention is cylindrical in shape, consisting of an upper cover and a lower body. The upper cover and lower body are sealed with ten specially designed screws. Four microfluidic chip holders are embedded inside the four-opening high-temperature and high-pressure micro-reactor, holding the microfluidic chips in their corresponding positions and sealing them with rubber rings. An illumination device is placed directly below the microfluidic chips to improve image clarity, and an image acquisition device is placed directly above the microfluidic chips to observe the experimental process.

[0011] The microfluidic chip is connected to the drain port at its top and bottom, and to the injection port and outlet port at its left and right, respectively. Valves for establishing confining pressure are installed near the inlet and outlet on both sides of the four-opening high-temperature and high-pressure microreactor. During the experiment, pressure sensors are connected to the drain port, injection port, outlet port, and confining pressure inlet port to accurately monitor the internal force, injection pressure, back pressure, and confining pressure of the microfluidic chip. Pressure changes are detected in real time by pressure gauges to prevent the microfluidic chip from rupturing due to uneven pressure, ensuring the normal progress of the experiment.

[0012] Specifically, the fabrication method of microfluidic chips includes the following steps performed in sequence:

[0013] A first groove and a first inlet / outlet communicating with the first groove are etched on the right end of the microfluidic chip;

[0014] A second groove and a second inlet / outlet communicating with the second groove are etched on the left end of the microfluidic chip. The second outlet on the second groove matches and is positioned opposite to the first inlet / outlet on the first groove.

[0015] A third groove and a third outlet connected to the third groove are etched on the upper end of the microfluidic chip;

[0016] A fourth groove and a fourth outlet connected to the fourth groove are etched on the lower end of the microfluidic chip. The fourth outlet on the fourth groove matches and is positioned opposite to the third inlet on the second groove.

[0017] The second aspect of the present invention provides an experimental method for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics, comprising the following steps:

[0018] (1) Based on the optimization and combination of two-dimensional CT scan images of plunger rock samples, a corresponding simulated porous medium model is established.

[0019] (2) Prepare crude oil and conduct representative tests according to the research content requirements.

[0020] (3) Fit the glass sheet into the clamp, connect the entire experimental system, close the outlet valve, pressurize the system with air using the pump and intermediate container, close the inlet valve, check the airtightness of the entire experimental system, and restore the experimental device to its initial state after the leak detection is completed.

[0021] (4) Pre-saturate crude oil into the micro-diffusion chamber of the microfluidic chip at a rate of 0.0001 mL / min. When crude oil is observed to flow steadily out of the drain port, the pre-saturation is complete.

[0022] (5) According to the experimental plan, the temperature of the four-opening high-temperature and high-pressure micro-reactor was adjusted to the simulated formation temperature T using a heating jacket.

[0023] (6) According to the experimental scheme, the confining pressure and the internal pressure of the microfluidic chip are simultaneously established from the inlet end to the formation pressure P at a rate of 0.001 mL / min. During this process, the pressure difference between the confining pressure and the internal pressure should not exceed about 0.2 MPa. At the same time, a back pressure of about 3 MPa higher than the confining pressure and the internal pressure is applied at the outlet end.

[0024] (7) Using a micro-displacement pump, gas was injected into the micro-diffusion chamber of the microfluidic chip at a limited pressure P at a rate of 0.001 mL / min to simulate a micro-diffusion experiment. The initial time t of the diffusion was recorded in real time using an image acquisition device. o With effective oil production time t i The diffusion situation was observed, and the gas injection diffusion experiment ended when the pressure no longer changed and the pump speed was 0.

[0025] (8) After the experiment, the pressure of the entire system was released, the intermediate container and pipeline were removed, and the data were analyzed.

[0026] (9) Based on the real-time images captured by the image acquisition device, t i The image at time t was used to determine the corresponding t by observing the color change of the crude oil using image processing software. i Diffusion distance H at time i .

[0027] (10) The traditional equation for calculating the diffusion coefficient of gases in rock samples, based on the People's Republic of China Petroleum and Natural Gas Industry Standard SY / T6129-1995 "Determination of Diffusion Coefficient of Hydrocarbon Gases in Rocks", is as follows: This invention provides an experimental method for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics. The method extends the traditional calculation method for the gas diffusion coefficient in rock samples using the experimentally obtained diffusion distance. The extended method yields the following equation for calculating the gas-liquid diffusion coefficient at the pore scale in porous media:

[0028]

[0029] Where the intermediate variable E = A(1 / V) / L;

[0030] In the formula: D j —Diffusion coefficient of gas component j at the pore scale in porous media, cm 2 / s;

[0031] H i —The diffusion distance of gas component j at time i in the porous medium at the pore scale, in cm;

[0032] A—Cross-sectional area of ​​the microfluidic chip, in cm² 2 ;

[0033] L—Length of the microfluidic chip, in cm;

[0034] V—Volume of the micro-diffusion chamber, in cm³ 3 ;

[0035] t i —Effective oil recovery time, seconds;

[0036] t o —Initial time, s;

[0037] Transforming (a), we get: ln(H) ij ) = D j (t i -t o ), lnH ij ) and t i The relationship is linear. Using the least squares method for fitting, the slope S is obtained.j According to D j =S j The diffusion coefficient can be obtained from / E.

[0038] Compared with the prior art, the beneficial effects of the present invention are:

[0039] (1) The present invention provides a visualization experimental device for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics, which greatly shortens the experimental time and provides an accurate and efficient experimental method for testing gas-liquid diffusion distance and diffusion coefficient at the pore scale in porous media.

[0040] (2) The present invention can accurately and in real time monitor the internal pressure of the microfluidic chip by means of pressure sensors connected to the upper and lower ends of the microfluidic chip, so as to ensure the safe conduct of the experiment.

[0041] (3) This invention utilizes microscopic visualization experiments. By analyzing the changes in crude oil color through experimental images, the gas-liquid diffusion distance at the pore scale in porous media can be obtained intuitively and quickly, thereby achieving efficient calculation of the gas-liquid diffusion coefficient at the pore scale in porous media. Attached Figure Description

[0042] Figure 1 This is a partial structural schematic diagram of an experimental apparatus for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidic technology, provided in a specific embodiment of the present invention.

[0043] Figure 2 This is a schematic diagram of an experimental device for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidic technology, provided in a specific embodiment of the present invention.

[0044] Figure 3 This is a schematic diagram of the structure of a microfluidic chip provided in a specific embodiment of the present invention;

[0045] Figure 4 This is a schematic diagram of the gas injection diffusion process captured in a specific embodiment of the present invention;

[0046] Figure labeling: 1-High-power electron microscope; 2-Image acquisition device; 3-Deionized water intermediate container; 4-Gas intermediate container; 5-Crude oil intermediate container; 6-Micro displacement pump; 7, 8, 9, 10, 11, 12, 13, 14, 15-Three-way valves; 30, 31, 32, 33, 34, 35, 36, 37, 38-Valve; 16-N2 intermediate container; 17-Four-opening high-temperature and high-pressure micro-reactor. ; 18-Microfluidic chip; 19-Microfluidic chip holder; 20-Heating jacket; 39-Temperature control chamber; 21-Back pressure constant pressure pump; 22-Back pressure valve; 23-Test tube; 24, 25, 26, 27, 28, 29-Pressure sensors; 40-Illumination device; 41-Micro diffusion chamber; 42-Fluid channel; 43-Blind end; 44-First inlet / outlet; 47-Second inlet / outlet; 45-Third outlet; 46-Fourth outlet. Detailed Implementation

[0047] The present invention will be further described below with reference to the accompanying drawings and examples to enable those skilled in the art to understand the invention. However, it should be understood that the present invention is not limited to the specific embodiments described herein. For those skilled in the art, any variations that fall within the spirit and scope of the invention as defined and determined by the appended claims are all within the scope of protection.

[0048] A visualization experimental device for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics includes: 1-high-magnification electron microscope; 2-image acquisition device; 3-deionized water intermediate container; 4-gas intermediate container; 5-crude oil intermediate container; 6-micro displacement pump; 7, 8, 9, 10, 11, 12, 13, 14, 15-three-way valves; 16-N2 intermediate container; 17-four-opening high-temperature and high-pressure resistant micro-reactor; 18-microfluidic chip; 19-microfluidic chip Clamping device; 20-Heating jacket; 39-Temperature control box; 21-Back pressure constant pressure pump; 22-Back pressure valve; 23-Test tube; 24, 25, 26, 27, 28, 29-Pressure sensors; 30, 31, 32, 33, 34, 35, 36, 37, 38-Valve; 40-Lighting device; 41-Micro diffusion chamber; 42-Fluid channel; 43-Blind end; 44-First inlet / outlet; 47-Second inlet / outlet; 45-Third outlet; 46-Fourth outlet.

[0049] The aforementioned four-opening high-temperature and high-pressure resistant microscopic reactor 17 is cylindrical in appearance. The reactor consists of an upper cover and a lower body, which are sealed by 10 specially designed screws to prevent fluid leakage into the reactor during the experiment. The injection and sampling ends of the microfluidic chip 18 are embedded in the corresponding positions of the four microfluidic chip holders 19 inside the reactor and sealed with rubber rings. An illumination device 40 is placed directly below the microfluidic chip to supplement the light source; a high-powered microscope 1 and an image acquisition device 2 are placed directly above the microfluidic chip to observe the experimental process.

[0050] Example 1

[0051] The method for testing the gas-liquid diffusion distance and diffusion coefficient during the blind-end oil injection diffusion process in pores using the above-mentioned device includes the following steps:

[0052] (1) Preparation of simulated porous media model: The first inlet and outlet 44 and the second inlet and outlet 47 of the microfluidic chip (7.5cm×2.7cm×0.4cm)18, which are respectively connected to the rectangular micro-diffusion chambers, constitute the main fluid channel 42 through which the fluid passes; in the longitudinal direction of the fluid channel are four pairs of rectangular micro-diffusion chambers 41, each with a length of 3cm, a thickness of 0.1cm, and the same width of 0.085cm, 0.169cm, 0.247cm and 0.330cm respectively, and their blind end portions 43 connected to the third outlet and the fourth outlet respectively; the waste liquid in the pre-saturation stage is discharged through the third outlet 45 and the fourth outlet 46 respectively.

[0053] (2) Prepare crude oil and conduct representative tests according to the research content requirements.

[0054] (3) Connect the entire experimental system and check whether the airtightness of the entire experimental system is good. According to... Figure 1 and Figure 2 Connect the experimental setup. Using the micro-displacement pump 6, deionized water intermediate container 3, and gas intermediate container 4, pressurize the open-ended high-temperature and high-pressure microreactor 17 and microfluidic chip 18 to 5 MPa at a rate of 0.001 mL / min. Simultaneously, using the back pressure constant pressure pump 21 and N2 intermediate container 6, apply a back pressure that is always approximately 3 MPa higher than the confining pressure and internal pressure. First, close the inlet valves 37 and 35, then close the valve connected to the outlet three-way valve 13 and the back pressure valve 22. Then, check the airtightness of the entire experimental system. Throughout the entire process, the valves connected to the outside world by three-way valves 10 and 11 remain closed. After leak detection, restore the experimental setup to its initial state.

[0055] (4) Pre-saturated crude oil is introduced into the micro-diffusion chamber within the microfluidic chip. To achieve the experimental objective, a micro-displacement pump 6, a deionized water intermediate container 3, and a crude oil intermediate container 5 are used to pump the crude oil at a rate of 0.0001 mL / min. When stable crude oil flow is observed simultaneously in test tube 23, the pipeline connected to the outside via three-way valve 11, and the pipeline connected to the outside via three-way valve 10, the pre-saturated crude oil is introduced into the micro-diffusion chamber 41 within the microfluidic chip 18. Figure 4 As shown in Figure a. Close the valve connecting the three-way valve 11 to the outside, and the valve connecting the three-way valve 10 to the outside.

[0056] (5) Adjust the temperature to the simulated temperature of 75°C. The four-opening high-temperature and high-pressure micro-reactor is heated to the experimental temperature of 75°C by using the heating jacket 20 and the temperature control box 39 connected to it, and then allowed to stabilize.

[0057] (6) Adjust the pressure to the simulated formation pressure of 35 MPa. Using a micro-displacement pump 6, a deionized water intermediate container 3, and a crude oil intermediate container 5, simultaneously establish the confining pressure and internal pressure of the microfluidic chip at a pump speed of 0.001 ml / min. Since the pressure difference between the internal and confining pressures that the microfluidic chip can withstand has a certain pressure limit, exceeding the pressure limit will damage the microfluidic chip. Therefore, during the pressure establishment process, the difference between the readings of the confining pressure sensor 25 and the internal pressure sensor 26 should be maintained at approximately 0.2 MPa. At the same time as the pressure is established, a back pressure of approximately 3 MPa higher than the internal and confining pressures is continuously applied through the back pressure constant pressure pump 21 and the N2 intermediate container 16. When the confining pressure and internal pressure are established to the experimental pressure of 35 MPa and stabilized, close valves 31 and 34 and the corresponding valves of the three-way valves 14 and 18 connected to them, and close valve 33 and the corresponding valve of the three-way valve 15 connected to it.

[0058] (7) Conduct diffusion experiments in the micro-diffusion chamber. By opening valve 32 and the valve connected to the three-way valve 14, the pressure in the gas intermediate container 4 is increased to 35 MPa using the micro-displacement pump 6 at a pumping speed of 2 ml / min. After the pressure stabilizes, valve 33 and the corresponding valve of the three-way valve 15 connected to it are opened, as well as the corresponding valve of valve 35 and the corresponding valve of the three-way valve 8 connected to it are opened. CO2 is injected into the microfluidic chip 18 using the micro-displacement pump 6 and the gas (CO2) intermediate container 4 at a pumping speed of 0.001 mL / min under a pressure limit of 35 MPa. At the instant when CO2 diffuses into each pore in sequence, the initial time t of each pore is recorded sequentially based on the image observed by the image acquisition device 2. o Throughout the diffusion experiment, an image acquisition device (high-power electron microscope 1, image acquisition unit 2) was used to record the diffusion of CO2 within the microfluidic chip 18 in each micro-diffusion chamber in real time. The diffusion experiment ended when the pump speed of the micro-displacement pump 6 reached 0 and the readings of pressure sensors 24, 26, 27, and 28 were similar and no longer fluctuated. Figure 4 As shown in d. Based on the fact that the image observed by image acquisition device 2 remains unchanged, the effective oil production time t of each individual pore is recorded sequentially. i .

[0059] (8) After the experiment, the pressure of the entire system was relieved by the micro displacement pump 6 and the back pressure constant pressure pump 21, the intermediate container and pipeline were removed, and the images captured in real time during the experiment were analyzed to obtain experimental data on the blind end oil injection diffusion process in a single pore of the porous medium.

[0060] (9) Determine the diffusion distance based on the images obtained from the experiment. The diffusion distance is determined by the real-time images captured by the image acquisition device of t in each pore. i The image at time t was used to determine the corresponding time t from left to right by observing the color change of crude oil in the blind end pores using image processing software. i Gas diffusion distance H in each pore at any given time i The measured gas diffusion distances at the final moment for each pore were 0.532 cm, 0.854 cm, 1.233 cm, 1.381 cm, 1.364 cm, 1.225 cm, 0.983 cm, and 0.712 cm, respectively.

[0061] (10) Organize the data and calculate the diffusion coefficient. Substitute the diffusion distance of a single pore into the equation for calculating the gas diffusion coefficient in a single pore obtained after expansion, and calculate the gas diffusion coefficient in each pore sequentially. Take the average gas diffusion coefficient of each pair of identical pores with widths of 0.085cm, 0.169cm, 0.247cm, and 0.330cm as the gas diffusion coefficient of that pore. The final calculated gas diffusion coefficient for a single pore is 0.89 × 10⁻⁶. -8 m 2 ·s -1 1.02×10¹⁰ -8 m 2 ·s -1 1.37×10 -8 m 2 ·s -1 1.41×10 -8 m 2 ·s -1 .

Claims

1. An experimental method for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics, wherein the method utilizes a visualization experimental device to test the gas-liquid diffusion distance and diffusion coefficient at the pore scale in porous media using microfluidics technology, characterized in that... The visualization experimental setup includes: a high-power electron microscope (1), an image acquisition device (2), a deionized water intermediate container (3), a gas intermediate container (4), a crude oil intermediate container (5), a micro-displacement pump (6), three-way valves (7, 8, 9, 10, 11, 12, 13, 14, 15), an N2 intermediate container (16), a four-opening high-temperature and high-pressure micro-reactor (17), a microfluidic chip (18), a microfluidic chip holder (19), and a heating jacket (20). Temperature control chamber (39), back pressure constant pressure pump (21), back pressure valve (22), test tube (23), pressure sensor (24, 25, 26, 27, 28, 29), valve (30, 31, 32, 33, 34, 35, 36, 37, 38), lighting device (40), micro diffusion chamber (41), fluid channel (42), blind end (43), first inlet and outlet (44), second inlet and outlet (47), third outlet (45), fourth outlet (46); The first inlet and outlet (44) and the second inlet and outlet (47) of the microfluidic chip (18) are connected to the rectangular micro-diffusion chambers respectively to form a fluid channel (42) through which the fluid passes; in the fluid channel (42), there are four pairs of rectangular micro-diffusion chambers (41) with the same length and the same width in pairs, and blind end portions (43) that are connected to the third outlet (45) and the fourth outlet (46) respectively. The inlet and outlet ends of the microfluidic chip (18) are fitted into the corresponding positions of the grooves of the microfluidic chip holder (19) inside the four-opening high-temperature and high-pressure micro-reactor (17). The inlet end of the microfluidic chip holder (19) is connected to the gas intermediate container (4), the crude oil intermediate container (5), and the micro displacement pump (6) through three-way valves (7, 8, 9, 14, 15), and the outlet end is connected to the back pressure valve (22), the N2 intermediate container (16), and the back pressure constant pressure pump (21) through three-way valves (10, 11, 12, 13). Pressure sensors (24, 25, 26, 27, 28, 29) are provided at the inlet and outlet ends of the fluid control chip holder (19). The entire microfluidic chip (18) and the microfluidic chip holder (19) are placed in a four-opening high-temperature and high-pressure micro-reactor (17). There is an observation window at the upper end of the four-opening high-temperature and high-pressure micro-reactor (17). A high-power electron microscope (1) and an image acquisition device (2) are placed on the window. An illumination device (40) is placed at the lower end of the four-opening high-temperature and high-pressure micro-reactor (17). The experimental method for testing gas-liquid diffusion distance and diffusion coefficient includes the following steps: (1) Based on the optimization and combination of two-dimensional CT scan images of plunger rock samples, a corresponding simulated porous medium model is established; (2) Prepare crude oil and conduct representative tests according to the research requirements; (3) Fit the glass sheet into the holder, connect the entire experimental system, close the outlet valve, pressurize the system with air using the pump and intermediate container, close the inlet valve, check the air tightness of the entire experimental system, and restore the experimental device to its initial state after the leak detection is completed. (4) Pre-saturate crude oil into the micro-diffusion chamber of the microfluidic chip at a rate of 0.0001 mL / min. When crude oil is observed to flow steadily out of the drain port, the pre-saturation is complete. (5) According to the experimental plan, the temperature of the four-opening high-temperature and high-pressure micro-reactor was adjusted to the simulated formation temperature using a heating jacket; (6) According to the experimental scheme, the confining pressure and the internal pressure of the microfluidic chip were simultaneously established from the inlet end to the formation pressure at a rate of 0.001 mL / min, while a back pressure of about 3 MPa higher than the confining pressure and the internal pressure was applied at the outlet end. (7) Using a micro displacement pump, gas is injected into the micro diffusion chamber of the microfluidic chip at a pressure of 0.001 mL / min to simulate the micro diffusion experiment. The diffusion at the initial moment and the effective oil production moment is recorded in real time by a high-power electron microscope (1) and an image acquisition device (2). When the pressure no longer changes and the pump speed is 0, the gas injection diffusion experiment ends. (8) After the experiment, the pressure of the entire system was released, the intermediate container and pipelines were removed, and the data were analyzed. (9) Based on the high-power electron microscope (1) and image acquisition device (2) capturing images at each moment in real time, the diffusion distance at the corresponding moment is determined by observing the color change of crude oil using image processing software; (10) Calculate the gas-liquid diffusion coefficient at the pore scale in porous media by using the diffusion distance obtained from the experiment; The equation for calculating the gas-liquid diffusion coefficient at the pore size in the porous medium during step (9) is as follows: Where the intermediate variable E = A(1 / V) / L; In the formula: D j —Diffusion coefficient of gas component j at the pore scale in porous media, cm 2 / s; H i —The diffusion distance of gas component j at time i in the porous medium at the pore scale, in cm; A—Cross-sectional area of ​​the microfluidic chip, in cm² 2 ; L—Length of the microfluidic chip, in cm; V—Volume of the micro-diffusion chamber, in cm³ 3 ; t i —Effective oil recovery time, seconds; t o —Initial time, s; Transforming (a), we get: ln(H) ij ) = D j (t i -t o ), lnH ij ) and t i The relationship is linear. Using the least squares method for fitting, the slope S is obtained. j According to D j =S j The diffusion coefficient can be obtained from / E.

2. The experimental method for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics as described in claim 1, characterized in that, The four-opening high-temperature and high-pressure micro-reactor (17) is cylindrical in appearance. It consists of an upper cover and a lower body. The upper cover and the lower body are sealed by 10 special screws. The four-opening high-temperature and high-pressure micro-reactor (17) is fitted with four microfluidic chip holders (19) inside. The microfluidic chip (18) is fitted into the corresponding position in the microfluidic chip holder (19) and sealed with rubber rings. A lighting device (40) is placed directly below the microfluidic chip (18) and an image acquisition device is placed directly above the microfluidic chip (18) to observe the experimental process.

3. The experimental method for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics as described in claim 1, characterized in that, The microfluidic chip (18) is provided with 4 outlets. During the pre-saturation stage, the waste liquid is discharged from the third outlet (45) and the fourth outlet (46).

4. The experimental method for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics as described in claim 1, characterized in that, The third outlet (45) and the fourth outlet (46) are respectively connected to the pressure sensor (29) to monitor the internal pressure of the microfluidic chip (18) in real time and ensure the safe conduct of the experiment.

5. The experimental method for testing gas-liquid diffusion distance and diffusion coefficient based on microfluidics as described in claim 1, characterized in that, During steps (6) and (7), while simultaneously establishing the confining pressure and the internal pressure of the microfluidic chip to the formation pressure, the internal pressure of the microfluidic chip is monitored in real time by pressure sensors connected to the upper and lower ends of the microfluidic chip. The pressure difference between the confining pressure and the internal pressure is kept below approximately 0.2 MPa to ensure that the microfluidic chip is not damaged.

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