A gas permeation measurement system and a gas permeability calculation model

Abstract

The present invention belongs to the technical fields of civil engineering and geological engineering, and provides a gas permeability measurement system and a gas permeability calculation model. The gas permeability measurement system includes an upstream end gas chamber (2), a downstream end gas chamber (3), and a vacuum pump (4). The vacuum pump (4) is connected to the upstream end gas chamber (2) and the downstream end gas chamber (3). The upstream end gas chamber (2) is connected to the bottom air hole input port of the main chamber (1), and the downstream end gas chamber (3) is connected to the top air hole output port of the main chamber (1). The gas permeability measurement system provided by the present invention and two gas permeability calculation methods based on the pressure change curves in the upstream end gas chamber or the downstream end gas chamber provide a sufficient theoretical basis for the research work of deep geological disposal repositories.

Description

Technical Field

[0001] The present invention relates to the field of civil engineering (geotechnical) and geological engineering technologies. Background Art

[0002] The development and utilization of nuclear energy will inevitably produce a large amount of high-level radioactive waste with strong radioactivity, high toxicity, high heat release, and long half-life (referred to as "high-level radioactive waste"), and it is necessary to dispose of it scientifically and effectively. At present, the disposal plan widely accepted by the international community is "deep geological disposal", that is, sealing high-level radioactive waste in a geological body 500-1000 meters deep underground, and achieving permanent isolation of high-level radioactive waste from the human living environment through a multi-barrier system (waste canister, bentonite buffer / backfill material, and surrounding rock); among them, highly compacted bentonite is considered the preferred buffer / backfill material in the industry due to its high expansibility, low permeability, and good thermal conductivity.

[0003] During the operation of a deep geological disposal repository:

[0004] Groundwater in the surrounding rock will scour and hydrate highly compacted bentonite, and at the same time, it will also corrode metal components (such as waste canisters); at the same time, due to the effects of groundwater radiolysis and degradation of organic matter, a large amount of gases such as carbon dioxide, methane, and hydrogen sulfide will be generated.

[0005] Because highly compacted bentonite after hydration saturation is an ultra-low permeability medium (permeability less than 1×10 -19 m 2) The generated gas will continuously accumulate in the bentonite, forming a high gas pressure (the peak pressure can reach 30 MPa). Research shows that the gas penetration process in bentonite has typical segmented characteristics (References: [1] Chinese Patent Application Publication No.: CN109655391 B, A Dual-Module Control Test System for Gas Breakthrough / Penetration Characteristics of Geotechnical Materials; [2] Research Progress on Gas Permeation of Buffer / Backfill Materials in High-Level Radioactive Waste Geological Disposal Repository, published by Ye Weimin, Liu Zhangrong, Cui Yujun, etc., Chinese Journal of Geotechnical Engineering, 2018, 40(06): 1125 - 1134.): When the gas pressure level is low, the gas mainly migrates in the soil in the form of discrete penetration. At this time, the gas penetration flow rate is so weak that it cannot be directly measured. However, due to the long-term low gas pressure state during the operation of the disposal repository, obtaining the gas permeability in this stage has important engineering value for the evaluation of the gas impermeability performance of the disposal repository; in addition, as the gas pressure continues to increase and reaches a certain critical value (gas breakthrough pressure), the gas will form a large number of preferential seepage channels in the highly compacted bentonite. Subsequently, the gas flow rate increases significantly and shows a highly non-linear growth. This phenomenon is also known as "gas breakthrough". Gas breakthrough will directly weaken the mechanical strength and buffering performance of the highly compacted bentonite. Therefore, determining this value of the gas breakthrough pressure is also crucial for the safety performance evaluation of the disposal repository.

[0006] Currently, the main research method for gas penetration tests of ultra-low permeability media such as saturated highly compacted bentonite is the steady-state method, that is, a constant gas injection pressure is applied to the bottom surface of the specimen, and at the same time, a flow meter is used to monitor the gas flow rate on the top surface of the specimen. This method can accurately obtain the gas breakthrough pressure (a significant increase in flow rate) of the highly compacted bentonite, but because it cannot accurately measure the gas flow rate before gas breakthrough, it is impossible to obtain the gas permeability index. In addition, due to the lack of relevant test devices, it is currently impossible to study the influence law of temperature on the gas full-process penetration characteristics of ultra-low permeability media such as saturated highly compacted bentonite by changing the temperature boundary. Summary of the Invention

[0007] In the research work of deep geological disposal repositories, since the gas penetration in highly compacted bentonite will inevitably be affected by the decay heat of radionuclides, it is of great practical significance to carry out research on the gas penetration problem of saturated highly compacted bentonite under temperature control conditions, and obtain two indexes, namely gas permeability and gas breakthrough pressure, for evaluating the gas full-process penetration characteristics, so as to achieve the safe disposal of high-level radioactive waste and ensure the sustainable development of the nuclear energy industry.

[0008] Based on the above background, the present invention discloses a gas permeability measurement system, calculation models for two gas permeabilities thereof, and a device for measuring the whole-process gas permeation parameters of a saturated ultra-low permeability medium, which can simultaneously obtain two indicators of gas permeability and gas breakthrough pressure. The present invention can be widely applied to the research on gas permeation problems in the fields of deep geological disposal of high-level radioactive waste, coalbed methane and shale gas exploitation, municipal solid waste landfill, and CO2 geological sequestration, accurately obtaining the two indicators of gas permeability and gas breakthrough pressure, and having important theoretical and engineering practical values.

[0009] A gas permeability measurement system includes an upstream end gas chamber 2, a downstream end gas chamber 3, and the vacuum pump 4, and the vacuum pump 4 is connected to the upstream end gas chamber 2 and the downstream end gas chamber 3.

[0010] The upstream end gas chamber 2 is connected to the bottom air hole input port of the main body chamber 1, and its downstream end gas chamber 3 is connected to the top air hole output port of the main body chamber 1.

[0011] One end of the upstream end gas chamber 2 is connected to an external pressure supply device, and the upstream end gas chamber 2 is also connected to the main body chamber 1 and the vacuum pump 4 respectively through a first two-way ball valve 16-1 and a second two-way ball valve 16-2.

[0012] A first safety valve 14-1 and a first high-precision air pressure sensor 15-1 are installed on the top of the upstream end gas chamber 2.

[0013] One end of the downstream end gas chamber 3 is connected to the top air hole output port of the main body chamber 1 for storing the gas permeation flow rate, and the other end is connected to the vacuum pump 4 through a third two-way ball valve 16-3.

[0014] The vacuum pump 4 is used to pump out the impurity gas in the upstream end gas chamber 2 and the downstream end gas chamber 3 before the gas permeation test starts.

[0015] During the test process, in the initial stage, the pressure in the downstream end gas chamber 3 is always less than the pressure in the upstream end gas chamber 2, so as to ensure that the gas permeation direction is always from the bottom surface of the specimen 1-6 to the top surface of the specimen. In addition, a second safety valve 14-2 and a second high-precision air pressure sensor 15-2 are also installed on the top of the downstream end gas chamber 3.

[0016] The vacuum pump 4 is a water-ring vacuum pump, and is connected to the upstream end gas chamber 2 and the downstream end gas chamber 3 respectively through the second two-way ball valve 16-2 and the third two-way ball valve 16-3. Before the test starts, the vacuum pump 4 is used to pump out the impurity gas in the whole set of test devices and pipelines to eliminate the test error.

[0017] The upstream end gas chamber 2 and the downstream end gas chamber 3 are made of stainless steel material, and the internal capacity is constantly 30 mL.

[0018] Calculation model 1 of gas permeability:

[0019] Calculation Model II of Gas Permeability:

[0020] Furthermore, based on the calculation model and device of permeability, software and hardware are further developed to disclose a test system for measuring the whole-process gas penetration parameters of saturated ultra-low permeability media.

[0021] Measurement and research method of the whole-process gas penetration parameters of saturated ultra-low permeability media based on the above test system. Carry out relevant gas penetration tests under different temperature conditions to provide a sufficient theoretical basis for the research work of deep geological disposal repositories.

[0022] Compared with the prior art, the present invention discloses a device and method for measuring the whole-process gas penetration parameters of saturated ultra-low permeability media. Under temperature control conditions, the gas breakthrough pressure is determined by gradually increasing the air pressure, and the real-time gas permeability is calculated based on the pressure change curve at each air pressure level. Moreover, the present invention provides two gas permeability calculation methods based on the pressure change curves in the upstream end gas chamber or the downstream end gas chamber, providing a sufficient theoretical basis for the research work of deep geological disposal repositories. Brief Description of the Drawings

[0023] Figure 1 Schematic diagram of the test system in the embodiment of the present invention;

[0024] Figure 2 is Figure 1 Schematic sectional structure diagram of the main chamber (a structure already existing in the field);

[0025] Figure 3 is Figure 1 Schematic composition diagram of the gas penetration measurement system in ;

[0026] Figure 4 Schematic electrical connection diagram of the embodiment of the present invention;

[0027] Figure 5 Schematic diagram of the software module flow of the test system of the present invention;

[0028] Figure 6 Calibration curve between the sample temperature and the set temperature of the temperature sensor in the embodiment of the present invention;

[0029] Figure 7 Gas permeability curve calculated based on the pressures in the upstream end and downstream end gas chambers in the embodiment of the present invention;

[0030] Figure 8 Range of gas breakthrough pressure determined in the embodiment of the present invention.

[0031] Marking Explanation:

[0032] Main chamber 1:

[0033] Base 1-1, side ring 1-2, top plate 1-4, "O" ring 1-7; piston 1-3, stainless steel perforated plate 1-8; specimen 1-6; load cell 1-5, third A / D converter 1-9;

[0034] Gas permeability measurement system:

[0035] Upstream end gas chamber 2, downstream end gas chamber 3, vacuum pump 4;

[0036] First safety valve 14-1, second safety valve 14-1;

[0037] First two-way ball valve 16-1, second two-way ball valve 16-2, third two-way ball valve 16-3, fourth two-way ball valve 16-4;

[0038] First A / D converter 17-1, second A / D sensor 17-2

[0039] Temperature control system:

[0040] Fiberglass heating tape 5, temperature sensor 6, temperature controller 7;

[0041] External pressure supply device:

[0042] Gas source 8, booster pump 9, air compressor 10, gas buffer tank 11, pressure regulating valve 12, fifth two-way ball valve 16-5;

[0043] Data acquisition and processing device 13:

[0044] First high-precision pressure sensor 15-1, second high-precision pressure sensor 15-2. Specific implementation method

[0045] Existing method (previous method):

[0046] When the gas pressure applied to the highly compacted bentonite is low (before "gas breakthrough" occurs), the flow rate is extremely weak and almost undetectable; when the applied gas pressure gradually increases to the gas breakthrough pressure, the gas breakthrough phenomenon appears, and the subsequent gas flow rate increases significantly. Therefore, conventional methods are mostly steady-state methods, and the gas permeability problem of highly compacted bentonite is studied by monitoring the flow rate, so only one index of "gas breakthrough pressure" can be measured (judged by the sudden increase in flow rate).

[0047] To address the technical bottlenecks existing in the prior art, the present invention discloses for the first time a test method (innovative idea), and its principle is:

[0048] On the one hand, the flow rate measurement is abandoned and the pressure measurement is adopted to bypass the technical problem that it is difficult to measure the flow rate before gas breakthrough, that is, a brand-new test method is proposed; compared with the previous flow rate measurement by predecessors, the pressure measured in the present invention can be used to calculate the gas permeability (the derivation process is shown below), and it can also be used to judge the occurrence of gas breakthrough according to its mutation situation (corresponding to the sudden increase in flow rate in the previous method).

[0049] On the other hand, the key innovative measures are further given, that is, "indirect flow rate measurement" is adopted, by utilizing and controlling the pressure sensitivity under a small volume and measuring its cumulative pressure. Through experimental verification, the gas chamber volume is controlled within the effective range of 10 mL to 50 mL. If the volume is too small, the error influence cannot be controlled. That is, in Figure 1 、 Figure 3 , a "gas permeability measurement system" is constituted by designing the upstream gas chamber 2, the downstream gas chamber 3, and the vacuum pump 4, that is, by installing the "gas permeability measurement system" to support the effective implementation of a brand-new test method.

[0050] In the embodiment, the recommended gas chamber volume is 30 mL.

[0051] On the premise of controlling the temperature of the "gas permeability measurement system" to be constant (set to a constant value), the calculation principle of gas permeability includes two calculation models of gas permeability, which are solved respectively based on the gas pressure change curves in the downstream gas chamber and the upstream gas chamber. The theoretical derivation is as follows:

[0052] Among them, the first calculation model of gas permeability:

[0053] The derivation process of calculating the whole-process gas permeability according to the pressure change in the downstream gas chamber 3 is as follows:

[0054] First of all, the volumetric flow rate of gas permeation can be described by Darcy's law considering gas compressibility,

[0055]

[0056] where Q g-out is the gas flow rate at the outlet end of the specimen, with the unit of m 3 / s, corresponding to the gas flow rate collected in the downstream gas chamber in the present invention, p1 and p2 are the gas pressures at the inlet end and the outlet end of the specimen respectively, corresponding to the gas pressures in the upstream and downstream gas chambers in the present invention, with the unit of Pa, A is the cross-sectional area of the specimen, with the unit of m 2 , L is the length of the specimen, with the unit of m, and μ is the dynamic viscosity of the gas, with the unit of Pa·s.

[0057] According to Boyle's law, the gas pressure p (Pa) and the gas volume V (m 3 ) in a closed container satisfy the ideal gas state equation:

[0058] pV = nRT or

[0059] wherein, n is the amount of substance of the gas, with the unit of mol, R is the ideal gas constant, 8.314 J / (K·mol), and T is the thermodynamic temperature, with the unit of K.

[0060] Therefore, by combining Equation (2), the volume form of the gas flow rate in Equation (1) can be converted into the mass form.

[0061]

[0062] Meanwhile, according to the ideal gas state equation (2), within a given time increment (dt), the differential form of the gas permeation flow rate is:

[0063]

[0064] It can be seen from Equation (4) that the right side of the equal sign contains two differential terms of volume and pressure, and the solution process is very complex. However, the test device provided by the present invention ingeniously sets the volumes of the upstream end and downstream end gas chambers as constant values, and the second term in Equation (2) can be omitted. The corresponding simplified form is

[0065]

[0066] Finally, by combining Equation (3) and Equation (5), the analytical form of the gas permeability at any injection pressure level can be solved.

[0067]

[0068] wherein, the calculation model of the gas permeability II:

[0069] The analytical form of the gas permeability provided by the present invention can also be calculated based on the pressure change of the upstream end gas chamber 2, and its solution process is similar to the calculation form based on the downstream end gas chamber. The analytical solution is directly given as:

[0070]

[0071] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0072] The present invention is a laboratory test device, not a test device directly applied to the field.

[0073] Example 1

[0074] As Figure 1As shown in the figure, the test system for measuring the whole-process gas permeation parameters of saturated ultra-low permeability media according to the present invention includes a main chamber 1, a gas permeation measurement system, an external pressure supply device, a temperature control system, and a data acquisition and processing device 13. The main chamber 1 is the main test part of the device for placing specimens to carry out gas permeation tests. The gas permeation measurement system includes an upstream gas chamber 2, a downstream gas chamber 3, a vacuum pump 4, and two safety valves (the first safety valve 14-1 and the second safety valve 14-2). The external pressure supply device includes a gas source 8, a booster pump 9, an air compressor 10, a gas buffer tank 11, and a pressure regulating valve 12. The temperature control system includes a fiberglass heating tape 5, a temperature sensor 6, and a temperature controller 7. When designing the application, five two-way ball valves (the first two-way ball valve 16-1, the second two-way ball valve 16-2, the third two-way ball valve 16-3, the fourth two-way ball valve 16-4, and the fifth two-way ball valve 16-5) need to be set.

[0075] Among them:

[0076] As Figure 2 shown, the main chamber 1 is the main part of the gas permeation device for placing the specimen 1-6 to carry out gas permeation tests.

[0077] The main chamber includes a base 1-1, a side ring 1-2, a piston 1-3, and a top plate 1-4. They are all made of stainless steel materials and are tightly connected together by external hexagon bolts to ensure that the specimen 1-6 is under constant volume conditions. Among them, an "O" ring 1-7 is arranged between the piston 1-3, the base 1-1, and the side ring 1-2 to ensure the airtightness of the test device. The specimen 1-6 is placed in the cavity between the top surface of the base 1-1 and the bottom surface of the piston 1-3. An air outlet pipeline is arranged inside the piston 1-3, and this pipeline leads to the downstream gas chamber 3 of the gas permeation measurement system. An air inlet pipeline is arranged inside the base 1-1, and this pipeline is connected to the upstream gas chamber 2 of the gas permeation measurement system. Further optimized, stainless steel porous plates 1-8 are embedded in the end faces of the base 1-1 and the piston 1-3 to ensure that the air flow uniformly flows into or out of the surface of the specimen 1-6.

[0078] As Figure 3 shown, the gas permeation measurement system:

[0079] As an example, by way of illustration and not limitation, in the gas permeation measurement system, the upstream gas chamber 2 and the downstream gas chamber 3 are made of stainless steel materials, and the internal volume is constantly 30 mL.

[0080] The upstream gas chamber 2 is connected to the bottom hole input port of the main chamber 1, and its downstream gas chamber 3 is connected to the top hole output port of the main chamber 1.

[0081] The upstream end gas chamber 2 has one end connected to the gas buffer tank 11 of the external pressure supply device, and a pressure regulating valve 12 is provided in between. The high-pressure gas in the gas buffer tank 11 is sent into the upstream end gas chamber 2 through the pressure regulating valve 12. The upstream end gas chamber 2 is also connected to the main body chamber 1 and the vacuum pump 4 through a first two-way ball valve 16-1 and a second two-way ball valve 16-2 respectively. The top of the upstream end gas chamber 2 is equipped with a first safety valve 14-1 and a first high-precision air pressure sensor 15-1. The first safety valve 14-1 automatically relieves pressure when the pressure in the gas chamber exceeds 14 MPa until the air pressure drops below MPa, ensuring the safety of the test.

[0082] The downstream end gas chamber 3 has one end connected to the top air hole output port of the main body chamber 1 for storing the gas permeation flow rate, and the other end is connected to the vacuum pump 4 through a third two-way ball valve 16-3; the vacuum pump 4 is used to pump out the impurity gas in the upstream end gas chamber 2 and the downstream end gas chamber 3 before the gas permeation test starts to ensure that both are balanced and there is no interference from impurity gas, so as to eliminate the error of the test system. Therefore, it is recommended to evacuate the upstream end gas chamber 2 and the downstream end gas chamber 3. During the test process, in the initial stage, the pressure in the downstream end gas chamber 3 is always less than the pressure in the upstream end gas chamber 2, so as to ensure that the gas permeation direction is always from the bottom surface of the specimen 1-6 to the top surface of the specimen. In addition, the top of the downstream end gas chamber 3 is also equipped with a second safety valve 14-2 and a second high-precision air pressure sensor 15-2.

[0083] The vacuum pump 4 is a water-ring vacuum pump with an ultimate vacuum of 3.3 kPa. It is connected to the upstream end gas chamber 2 and the downstream end gas chamber 3 through the second two-way ball valve 16-2 and the third two-way ball valve 16-3 respectively. Before the test starts, the vacuum pump 4 is used to pump out the impurity gas in the entire test device and pipeline to eliminate the test error.

[0084] This part of the external pressure supply device belongs to conventional technology and is used in conjunction with the gas permeation measurement system. In the external pressure supply device, the booster pump 9 is connected to the air compressor 10, the air source 8 and the gas buffer tank 11. The air pressure output by the air compressor 10 provides the driving force for the booster pump 9. The booster pump 9 boosts the gas in the air source 8 to a preset value and then sends it into the gas buffer tank 11. A fifth two-way ball valve 16-5 is provided between the air compressor 10 and the booster pump 9. Before the test, the preset pressure value in the gas buffer tank 11 needs to be higher than the estimated gas breakthrough pressure of the specimen 1-6, so as to ensure that the specimen 1-6 can have a gas breakthrough phenomenon. One end of the gas buffer tank 11 is connected to the booster pump 9, and the other end is connected to the upstream end gas chamber 2 through the pressure regulating valve 12, which is used to store the high-pressure gas sent by the booster pump 9 and gradually increase the gas pressure in the upstream end gas chamber 2 using the pressure regulating valve 12 during the test process.

[0085] This part of the temperature control system belongs to conventional technology. In the temperature control system, the temperature controller 7 is respectively connected to the fiberglass electric heating tape 5 and the temperature sensor 6. The fiberglass electric heating tape 5 is tightly wound around the surface of the main chamber outer shell to provide heat source, and the temperature sensor 6 is embedded in the outer shell of the main chamber 1 to collect the real-time temperature of the outer surface of the main chamber outer shell. The temperature controller 7 controls the on / off of the fiberglass electric heating tape 5 by comparing the temperature value of the outer surface of the main chamber outer shell measured by the temperature sensor 6 with the preset value. The three form a temperature closed-loop control to indirectly keep the temperature of the specimens 1-6 constant during the test. Since there is a temperature difference between the temperature set by the temperature controller 7 and the temperature of the specimens 1-6, the system needs to calibrate the two before the test starts and establish a calibration relationship curve, so as to achieve precise control of the temperature of the specimens 1-6 and further study the influence law of temperature on the gas permeability parameters of saturated ultra-low permeability media.

[0086] As Figure 4 shown, the data acquisition and processing device 13:

[0087] includes a PC, and the PC includes software modules and a human-machine interface;

[0088] and includes load cells 1-5 and a third A / D converter 1-9; the load cells 1-5 are arranged in contact with the bottom of the top plate 1-4 that plays a load-bearing role and are connected to the third A / D converter 1-9 and the PC in sequence, for real-time monitoring of the vertical pressure during the test, and then judging the occurrence of gas breakthrough;

[0089] and includes two high-precision pressure sensors (the first high-precision pressure sensor 15-1, the second high-precision pressure sensor 15-2) and two A / D converters (the first A / D converter 17-1 and the second A / D converter 17-2); the first high-precision pressure sensor 15-1 is used to measure the pressure change in the upstream end gas chamber 2 during the gas permeation process: before gas breakthrough, the pressure in the upstream end gas chamber 2 only shows a slight decrease, while it drops steeply when gas breakthrough occurs. Similarly, the second high-precision pressure sensor 15-2 is used to measure the pressure change in the downstream end gas chamber 3 during the gas permeation test: before the gas breakthrough phenomenon occurs, the pressure in the downstream end gas chamber 3 only shows a slight increase, while it increases rapidly when gas breakthrough occurs.

[0090] The software module as Figure 5 shown includes a gas permeability calculation module and a gas breakthrough pressure calculation module.

[0091] The software module is described as:

[0092] L1. Perform system initialization through the user interface;

[0093] L2. Record the currently applied first-stage gas pressure;

[0094] L3. Read the temperature change (T) provided by the temperature sensor 6 (the WZPT-035-GK-FY3PF type PT100 thermocouple is used in the embodiment);

[0095] L4. Read the pressure changes (p1 and p2) of the two end chambers provided by the high-precision pressure sensors of the upstream end chamber and the downstream end chamber (the SIN-P300 pressure sensor is used in the embodiment);

[0096] L5. Read the vertical pressure (F) provided by the load cells 1-5 (the LCZ-205A is used in the embodiment);

[0097] L6. According to two calculation models, output and display the real-time gas permeability (k g-in and k g-out );

[0098] L7. Judge the pressure changes of the upstream end chamber, the downstream end chamber and the vertical pressure change. If there is no obvious change, record the currently applied next-stage gas pressure and continue to execute L3-L6;

[0099] If an obvious mutation occurs, it is considered that the gas breakthrough phenomenon has occurred. Output and display the gas breakthrough pressure on the man-machine interface and end the process.

[0100] Taking the high-compacted bentonite material as an example, the judgment criteria are: dF / dt > 0.5 kg / h and d(p1 - p2) / dt < -40 kPa / h.

[0101] The gas breakthrough phenomenon corresponds to the sudden change of the pressures in the upstream end chamber and the downstream end chamber, and the sudden change of the vertical stress; correspondingly, the gas breakthrough pressure is between the upstream end chamber pressure at the time of the current-stage gas breakthrough and the range of the upstream end chamber pressure of the previous stage.

[0102] Example 2

[0103] Based on the above test system, the method for measuring and studying the whole-process gas permeation parameters of saturated ultra-low permeability media is as follows:

[0104] S1. Start the temperature controller 7 to heat the main chamber 1 and establish the calibration relationship between the internal temperature of the specimens 1-6 and the temperature set by the temperature controller 7. Based on this calibration relationship, on the one hand, the specimen temperature during the test can be accurately controlled; on the other hand, different control temperatures can be set according to this calibration relationship, and then the influence law of temperature on the gas permeation parameters of saturated ultra-low permeability media can be analyzed and studied.

[0105] S2. Close the pressure regulating valve 12 and the two-way ball valve 16-4 installed in the downstream end gas chamber 3. At the same time, open the first two-way ball valve 16-1, the second two-way ball valve 16-2 and the third two-way ball valve 16-3. Then start the vacuum pump 4 to evacuate the whole test device, ensuring that the upstream end gas chamber 2 and the downstream end gas chamber 3 are in a vacuum state after the operation. After the evacuation is completed, disconnect the first two-way ball valve 16-1, the second two-way ball valve 16-2 and the third two-way ball valve 16-3.

[0106] S3. Open the fifth two-way ball valve 16-5 and start the gas source 8 to supplement gas into the booster pump 10. At the same time, start the air compressor 9 to continuously output driving pressure to the booster pump 10, and slowly increase the pressure of the gas in the gas buffer tank 11 to the set pressure value, where the set pressure value is higher than the estimated specimen gas breakthrough pressure.

[0107] S4. Close the gas source 8, the air compressor 9 and the booster pump 10, and disconnect the two-way ball valve 16-5 at the same time. Then, use the pressure regulating valve 12 to output the gas in the gas buffer tank 11 to the upstream end gas chamber 2. When the gas pressure in the upstream end gas chamber 2 reaches the preset first-stage pressure, close the pressure regulating valve 12 to keep the upstream end gas chamber 2 disconnected from the gas buffer tank 11.

[0108] S5. Open the first two-way ball valve 16-1 between the upstream end gas chamber 2 and the main chamber 1 to start the gas permeation test. During the permeation process, the data acquisition and processing device 13 records the gas pressure data in the upstream end gas chamber 2 and the downstream end gas chamber 3 and the vertical stress data measured by the load cell 1-5 in real time. If there are no obvious mutations in the gas pressures in the upstream end gas chamber 2 and the downstream end gas chamber 3 and the vertical pressures measured by the load cell 1-5, open the pressure regulating valve 12 to increase the gas pressure in the upstream end gas chamber 2 to the second-stage gas pressure, and continue to monitor the above data for step-by-step tests. The second-stage gas pressure is higher than the first-stage gas pressure.

[0109] The purpose of this step is to determine the gas breakthrough pressure. The gas breakthrough pressure is a critical value, and it is determined by gradually increasing the gas pressure step by step. For example, apply the first-stage gas pressure of 1 MPa. If no gas breakthrough phenomenon is observed, then increase the gas pressure to 2 MPa. If there is still no gas breakthrough, continue to increase it to 3 MPa, and so on, for example, 4 MPa, 5 MPa, 6 MPa in sequence. If gas breakthrough occurs at 6 MPa, then it is considered that the gas breakthrough pressure is between 5 MPa and 6 MPa. If you want to improve the accuracy, just reduce the pressure difference between different gas pressures, such as increasing the gas pressure step by step with a pressure difference of 0.5 MPa or even 0.1 MPa.

[0110] S6. Repeat step S5 until there are sudden changes in the gas pressures in the upstream end chamber 2 and the downstream end chamber 3, and the vertical pressures measured by the load cells 1-5 (taking the highly compacted bentonite material as an example, the judgment criterion is: dF / dt > 0.5 kg / h and d(p1 - p2) / dt < -40 kPa / h), that is, the gas breakthrough phenomenon occurs, and then stop the test.

[0111] S7. According to the gas pressure time history curve and the vertical pressure curve obtained in steps S5 and S6, determine the gas permeability and the gas breakthrough pressure of the specimens 1-6. The gas breakthrough pressure is between the upstream end chamber pressure at the time of this gas breakthrough and the upstream end chamber pressure of the previous level.

[0112] The data acquisition and processing device 13 has a sampling frequency of 20 kHz.

[0113] The high-precision barometric pressure sensor is a SIN-P300 pressure sensor, with a barometric pressure range of 0 - 10 MPa and a resolution of 1 kPa.

[0114] The air compressor uses a silent oil-free air compressor, with an output pressure of 0 - 1.25 MPa.

[0115] The vacuum pump is a water-ring vacuum pump, with an ultimate vacuum of 3.3 kPa.

[0116] The temperature controller is a TE4-RC10W electronic temperature controller, with a measurement accuracy of ±0.5% F.S.

[0117] The temperature sensor uses a WZPT-035-GK-FY3PF type PT100 thermocouple, with a temperature range of -50 to 200 °C.

[0118] The load cell used is an LCZ-205A resistance strain type load cell, with a range of 0 - 3 t.

[0119] The analog sensor uses an LFAL-201 external single-channel current sensor, with a rated output of 4 - 20 mA signal.

[0120] The data acquisition and processing device uses an MLT16-SDAQ multi-channel synchronous data acquisition and processing device, with a sampling frequency of 20 kHz.

[0121] The method proposed by the present invention is verified below in combination with the specific test process and calculation data carried out by this device.

[0122] I. Test materials

[0123] The test material selected was Gaomiaozi bentonite. After controlling the water content of the bentonite powder by the gas phase method, the mass of the bentonite powder required for the target dry density specimen was weighed, and a cylindrical bentonite specimen to be tested was pressed based on the static compaction method. Subsequently, the bentonite specimen was saturated with water by the constant head method. Among them, the required mass of the bentonite powder was calculated according to the following formula:

[0124]

[0125] Where m f is the mass of the bentonite powder to be weighed, in g; d is the diameter of the specimen to be prepared, in cm; l is the height of the specimen to be prepared, in cm; ρ d is the dry density of the specimen, in g / cm 3 ; ω is the water content of the bentonite powder.

[0126] Table 1 shows the main physical property indexes of the bentonite specimen.

[0127] Table 1 Main Physical Properties of Bentonite Specimen

[0128]

[0129] II. Gas Selection

[0130] The main gas type in the repository is hydrogen. From the perspective of safety, helium with a relatively close molecular weight was selected as the test gas source. The gas sample used in the test was high-purity helium with a purity of 99.999%.

[0131] III. Test for Measuring the Whole Process Permeation Parameters of Gas

[0132] (1) The saturated bentonite specimen was loaded into the test device of the present invention, the temperature controller was started to heat the main chamber, and the preset temperature values of the temperature controller were set at 40 °C, 50 °C, 60 °C and 70 °C respectively. The temperature of the specimen under the corresponding working conditions was monitored, and the calibration relationship and curve graph between the two were as follows and Figure 6 .

[0133] T 试样 = 0.835·T 温控器 + 2.1, R 2 = 0.999

[0134] Where T 试样 is the temperature of the specimen, in °C; T 温控器 is the temperature set by the temperature controller, in °C.

[0135] The temperature of the outer shell of the main chamber was set at 45.4 °C by using the temperature controller, and the corresponding specimen temperature was 40 °C.

[0136] The vacuum pump was started to evacuate the whole set of test devices.

[0137] Increase the gas pressure in the gas buffer tank to 15 MPa, and use the gas pressure regulating valve to replenish the first-stage gas pressure in the upstream end gas chamber to about 0.5 MPa. If no gas breakthrough occurs under the current gas pressure conditions, gradually increase the gas pressure in the upstream end gas chamber in 0.5 MPa increments until gas breakthrough occurs and the test stops.

[0138] IV. Calculation of Gas Permeability

[0139] (1) The formula for calculating the gas permeability throughout the process based on the pressure change in the upstream end gas chamber is:

[0140]

[0141] (2) The formula for calculating the gas permeability throughout the process based on the pressure change in the downstream end gas chamber is:

[0142]

[0143] Among them, the dynamic viscosity of helium at 40 °C is 1.98×10 -5 Pa·s, the cross-sectional area of the specimen is 1.96×10 -4 m 2 , the length of the specimen is 0.01 m, and the volumes of the upstream and downstream end gas chambers are both 3×10 -5 m 3 . The calculation results are as shown in Figure 7 . It can be seen that before gas breakthrough, the calculation results of both gas permeabilities are at about the order of magnitude of 10 -20 m 2 , showing very typical ultra-low permeability characteristics.

[0144] V. Determination of Gas Breakthrough Pressure

[0145] According to the test results shown in Figure 8 , when the gas pressures in the upstream and downstream end gas chambers and the vertical pressure of the load cell change suddenly (since the test material in this example is a high-compacted bentonite specimen, the judgment criterion is: dF / dt > 0.5 kg / h and d(p1 - p2) / dt < -40 kPa / h), it is determined that a gas breakthrough phenomenon has occurred (i.e., Figure 8 the steep increase point A of the curve, which occurs around 598 hours). Correspondingly, the gas breakthrough pressure is between the current upstream end gas chamber pressure and the previous pressure level, that is: between 2.01 MPa and 2.53 MPa. It should be noted that the numerical range of the gas breakthrough pressure can be further narrowed by reducing the gas pressure difference. The above 2.01 MPa - 2.53 MPa is a case given in the present invention to illustrate the feasibility of the test method.

[0146] The above is only the preferred content of the present invention and is not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing content, those skilled in the art can still modify the technical solutions recorded in the foregoing content or perform equivalent replacements on some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An experimental system for measuring gas permeation parameters of saturated ultra-low permeability media, characterized in that, It includes a main chamber (1), a gas permeability measurement system, an external pressure supply device, and a data acquisition and processing device; the gas permeability measurement system includes an upstream end gas chamber (2), a downstream end gas chamber (3), a vacuum pump (4), and two safety valves; the volumes of the two gas chambers are kept constant, and the volume of each gas chamber is 30 mL; Wherein: The main chamber (1) is the main test part of the device, used to place specimens for gas permeability tests; The upstream end gas chamber (2) is connected to the external pressure supply device and is connected to the bottom air hole input of the main chamber (1); The downstream end gas chamber (3) is used to store the gas permeation flow rate and is connected to the top air hole output of the main chamber (1); The vacuum pump (4) is used to extract the impurity gases in the upstream end gas chamber (2) and the downstream end gas chamber (3) before the start of the gas permeability test; The data acquisition and processing device outputs and displays the real-time gas permeability according to two calculation models and : Calculation Model 1 of Gas Permeability , Calculation Model II of Gas Permeability ; wherein, and are the gas permeabilities at the inlet end and the outlet end, respectively μ is the dynamic viscosity of the gas, with the unit Pa·s, L is the specimen length, in m, A is the cross-sectional area of the specimen, unit: m 2 , p 1 and p 2 are the gas pressures at the sample inlet end and the outlet end respectively, with the unit of Pa. V1 and V2 are the volumes corresponding to the upstream end gas chamber (2) and the downstream end gas chamber (3) respectively; In differential form dt is Time increment dp 1 is the gas pressure increment at the inlet end dp 2 is the gas pressure increment at the outlet end

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