CN108344675A - Coal body adopts the test method of permeation fluid mechanics rule under the conditions of simulation protective coat extracted - Google Patents

Abstract

The invention relates to a test method for simulating the mechanical behavior law of coal mining seepage under the mining condition of a protective layer, which belongs to the field of geotechnical engineering. It includes the following steps: select the coal-rock test piece; record the basic data of the test piece, install the coal-rock test piece in the system for testing the permeability of the unloaded coal-rock mass in the mining process, and debug all the equipment of the system; Carry out the simulation operation of in-situ stress recovery and pressure relief expansion deformation of coal and rock specimens under the mining conditions of the simulated protective layer. This stage includes four stages: in-situ stress recovery, axial compression, pressure relief expansion and stress recovery, and simulate the protection of the specimens The mining stress evolution simulation operation of coal and rock specimens under the condition of layer mining, the operation includes hydrostatic pressure, the first unloading and the second unloading stages; according to the compressible gas horizontal linear steady seepage Darcy formula, the permeability in different periods is calculated It is suitable for the study of the law of seepage mechanics behavior in coal mining under the mining condition of the protective layer.

Description

Experimental method for simulating the law of seepage flow mechanics in coal mining under the condition of mining in protective layer

technical field

The invention belongs to the field of geotechnical engineering, and relates to an indoor test method for simulating the law of seepage flow mechanics behavior in coal body mining under the mining condition of a protective layer.

Background technique

Under the mining condition of the protective layer, the gas migration in the coal seam is obviously different with the differential development of mining stress and mining fracture, which mainly involves the related research in the field of seepage mechanics. Seepage mechanics was originally applied in the fields of hydraulic engineering, water purification and groundwater resources development. Since the 1920s, seepage mechanics has gradually become the theoretical basis of the oil and gas development industry. In the late 1940s, scholars from the former Soviet Union applied Darcy's law - the law of linear seepage to describe the flow of gas in coal seams, and pioneered the study of gas seepage considering the nature of gas adsorption. In the 1960s, from the perspective of seepage mechanics, Zhou Shining and others assumed that the flow of gas basically complied with Darcy’s law, regarded the porous coal seam as a virtual continuum with uniform distribution on a large scale, and proposed the "linear gas flow theory". The proposal of this theory has a very profound impact on the study of gas flow theory in my country. In the 1980s, the research on gas flow theory tended to be active, mainly to revise and perfect the mathematical model of gas flow, and the focus was on the modification of the gas flow equation. Guo Yongyi studied the complete solution of the gas flow equation for the one-dimensional situation combined with the similarity theory, and pointed out the approximation of the parabolic relationship between the gas content and the pore pressure in the research of Zhou Shining et al., and used the Langmuir equation to describe the gas isothermal Adsorption capacity, a revised gas flow equation is proposed. Tan Xuexue studied the gas state equation of gas, thought that it is more realistic to apply the real gas state equation of gas, and proposed a revised mine coal seam real gas seepage equation. On the basis of summarizing the previous research results, Sun Peide further revised and improved the mathematical model of gas flow in homogeneous coal seams, and at the same time developed the mathematical model of gas flow in heterogeneous coal seams, and on this basis, he applied computer to carry out numerical simulation and comparison The results show that the new linear gas flow model is closer to reality than the three typical models at home and abroad. Yu Chuxin et al. believed that the amount of gas involved in seepage in coal seams was only a part of the amount that could be desorbed, and established the governing equation of gas seepage under the condition that the process of gas adsorption and desorption in coal was completely reversible.

With the development of computers, it has become possible to use computer simulation to study the distribution and evolution of gas flow field, which is also one of the main research methods of gas seepage mechanics. As early as the 1980s, Wei Xiaolin and Li Yingjun respectively reported the achievements of the Guangdong Provincial Coal Research Institute and the Fushun Coal Research Institute in the application of computer research on gas flow. Combined with the actual problems of coal mines, the finite difference method (DEM) was used to analyze the gas flow field for the first time. Numerical simulation of medium pressure distribution and flow change has been realized, and the gas pressure change law in the gas flow field has been predicted successfully. After entering the 21st century, researchers have carried out combined macro and micro analysis of gas seepage, visualization research and study of seepage field characteristics under multi-field coupling conditions. However, in the prior art, no research has considered the huge influence of the real mining stress environment on the change of coal permeability under the influence of different mining methods (disturbances) of the coal seam.

In addition, the pertinence and effectiveness of gas drainage in underground mining are the key issues of coal and gas mining, the core of which is to define and analyze the permeability enhancement formed by the fracture network caused by mining in theory and technology . The high-density and high-connectivity mining fractures caused by mining have fundamentally changed the permeability of the coal seam. However, there is no suitable theory to quantitatively describe the permeability enhancement mechanism and effect, let alone guide the coal seam permeability. The evaluation method and system of joint mining with gas can provide quantitative indicators and scientific methods for the evaluation of coal seam permeability enhancement effect in coal and gas joint mining projects.

Therefore, in order to fill the gaps in the existing technology and accurately reveal the law of coal mining seepage mechanics under the mining conditions of the protective layer, the effects of the mining stress environment evolution of different mining methods and the coupling effect of gas adsorption and expansion on the damaged fractured coal body should be considered comprehensively. Based on the theory of seepage mechanics, it is necessary to use the theory of permeability enhancement to quantitatively analyze the distribution and evolution of permeability enhancement in the overlying rock and coal seam during the mining process, and propose a law of seepage mechanics behavior in coal mining under the condition of simulating the mining of protective layers. On the basis of the above research, this method can further consider the real stress environment of coal and rock mining under the mining conditions of the protective layer, and link it with engineering activities to carry out coal mining seepage The study of the laws of mechanical behavior.

Contents of the invention

The technical problem to be solved by the present invention is to fill in the gaps in the prior art and provide a test method for indoor simulation of the law of seepage flow mechanics in coal mining under the mining conditions of the protective layer. The real stress environment during mining is used to study the law of seepage mechanics behavior of coal mining.

The technical solution adopted by the present invention to solve the technical problems is: a test method for the law of coal mining seepage mechanics under the condition of simulating the mining of the protective layer, comprising the following steps:

A. Select coal and rock specimens;

B. Record the basic data of the test piece, install the coal-rock test piece in the system used to test the permeability of the unloaded coal-rock mass during the mining process, and debug all the equipment of the system;

C. Carry out the simulation operation of the stress recovery and pressure relief expansion deformation of the coal and rock specimens under the mining conditions of the protective layer, which specifically includes the following four stages:

C1. Ground stress recovery stage: set the initial ground stress and vertical stress gradient, load the axial pressure and confining pressure at a certain loading rate according to the preset axial pressure and confining pressure ratio, until the confining pressure and axial pressure respectively reach the initial confining pressure value, initial axial pressure value;

C2. Axial compression stage: Loading is carried out by keeping the confining pressure constant and increasing the axial pressure until the axial stress reaches the preset value;

C3. Unloading and expansion stage: Unloading is carried out by keeping the difference between the axial stress and the confining pressure σ ₁ -σ ₃ constant and reducing the confining pressure, and unloading at an appropriate unloading rate of the confining pressure until the specimen deforms and enters the yield stage , where σ ₁ is the axial pressure, σ ₃ is the confining pressure;

C4. Stress recovery stage: reduce the difference between the axial stress and the confining pressure σ ₁ - σ ₃ while increasing the confining pressure until the confining pressure and axial pressure respectively reach the initial confining pressure value and initial axial pressure value;

D. Carry out the simulation operation of the mining stress evolution of the coal and rock test piece under the condition of simulating the mining of the protective layer on the test piece:

D1. Hydrostatic pressure stage: set the initial vertical stress gradient and the corresponding design embedding depth, and obtain the initial axial pressure according to the initial vertical stress gradient and the corresponding design embedding depth, apply confining pressure at an appropriate loading rate, so that When the preset confining pressure and initial axial pressure are reached, the entire gas pipeline is evacuated, and then methane gas with the same gas pressure is applied to the upper and lower ends of the test piece until the volume of the test piece remains unchanged;

D2. The first unloading stage: Unloading at a certain confining pressure unloading rate makes the specimen rock gradually change from the hydrostatic pressure state until the axial stress concentration coefficient K is equal to the first coefficient value, that is, the difference between the axial stress and the confining pressure σ ₁ The ratio between the increase of -σ ₃ and the unloading of the confining pressure σ ₃ is the set ratio, until the preset first unloading axial pressure and the preset first unloading confining pressure are reached, and the gas outlet end of the gas pipeline is connected to the outside world before loading. During the loading process, the pressure level at the inlet end is kept constant, and the change of gas pressure and flow rate is continuously recorded;

D3. The second unloading stage: set the confining pressure of the coal and rock near the mining face to be linearly distributed as the working face advances, keep the unloading rate of the confining pressure constant, and unload in the way that the axial stress increases at an increasing rate, so that the axial stress is concentrated The coefficient K is from the first coefficient value to the unloading failure of the specimen. When the unloading failure of the specimen occurs, the axial stress concentration coefficient K is the second coefficient value. The second coefficient value is greater than the first coefficient value, that is, the axial deviatoric stress σ ₁ - The ratio between the increase of σ ₃ and the unloading of lateral stress σ ₃ is a set ratio, and the pressure level of the gas inlet is kept constant during the whole process of loading, and the change of gas pressure and flow rate is continuously recorded;

E. According to the Darcy's formula for horizontal linear steady seepage of compressible gas, the permeability in different periods is calculated, and the calculation formula is as follows:

In the formula, k represents the permeability, the unit is m ² ;, q is the gas flow rate, the unit is m ³ /s; p ₀ is the atmospheric pressure at the measurement point, which is 0.101325MPa; A is the cross-sectional area of the test piece, the unit is m ² ; μ is the dynamic viscosity coefficient of gas, which is 1.087×10 ^-5 Pa·s at 20°C; L is the length of the test piece, in m; p ₁ and p ₂ are the gas pressure and The gas pressure at the gas outlet, in MPa.

Further, in step A, the coal sample with a smooth surface is selected as a cylinder with a diameter of 47-51mm and a height-to-diameter ratio of (2±0.2). The upper and lower end diameter deviation is not more than 0.3mm.

Specifically, Step B specifically includes the following steps:

B1. Record the basic data of the test piece and connect to other equipment;

B2. Install the test piece in the triaxial chamber of the MTS rock mechanics testing system. After spraying the heat shrinkable film on the test piece, apply a load to fix the test piece on the loading platform, and perform oil filling and exhaust operations on the triaxial chamber;

B3. Gradually apply the confining pressure to a specific value and keep the confining pressure constant, and then fill the methane with stable air pressure until the volume of the specimen remains unchanged and start loading;

B4. Use axial load control, and then use circumferential deformation control until the residual strength appears, and then fill the triaxial chamber with mineral silicone oil.

Further, the loading and unloading operations are carried out at a constant rate, the loading rate is 3MPa/min, and the unloading rate is 1MPa/min.

Further, step D3 also includes that after the test reaches the peak load, in order to ensure the safety of the test equipment, the confining pressure is no longer reduced, and the test is stopped after continuing to load until the residual strength of the specimen appears.

Specifically, the preset ratio of confining pressure and axial pressure in step C1 is 1:1, and the initial confining pressure value is γH, where γ represents bulk density in KN/m ³ , and H represents depth in m; step D1 The ratio of confining pressure and axial pressure is set to 1:1 during the loading process in .

The beneficial effect of the present invention is: use the system for testing the permeability of unloaded coal and rock mass in the mining process to carry out the test operation, the system is small in size, can be placed indoors to realize the corresponding test operation, and there is no need to coat the side wall of the test piece with silica gel , the test procedure is effectively simplified, the test method is easy to operate, and can be carried out indoors. The selected coal and rock specimens are placed in the rock mechanics test system, and the real stress environment of coal and rock mining under the mining conditions of the protective layer is simulated. The rock is loaded to obtain the corresponding seepage data, so as to realize the research on the law of seepage mechanics behavior of coal mining. The invention is applicable to the research on the law of seepage mechanics behavior in coal body mining under the mining condition of the protective layer.

Description of drawings

Fig. 1 is the system structure schematic diagram used for testing mining process unloading coal rock mass permeability used in the present invention;

Fig. 2 is the schematic diagram of the system used in the present invention for testing the permeability of unloaded coal rock mass in the mining process;

Fig. 3 is the simulated stress path figure of protected layer stress before mining in the present invention;

Fig. 4 is unloading test stress path among the present invention;

Among them, 1 is the gas source tank, 2-1 is the first pressure reducing valve, 2-2 is the second pressure reducing valve, 3-1 is the first valve, 3-2 is the second valve, 3-3 is the third valve Valves, 3-4 is the fourth valve, 3-5 is the fifth valve, 3-6 is the sixth valve, 3-7 is the seventh valve, 4 is the vacuum pump, 5 is the gas stabilizing temperature increasing control device, 6 is the MTS confining pressure chamber, 6-1 is the test piece, 7-1 is the first pressure gauge, 7-2 is the second pressure gauge, 8-1 is the first flowmeter, 8-2 is the second flowmeter, 9 is Gas booster pump, 10 is the gas heating controller, 11-1 is the first gas heating chamber, 11-2 is the second gas heating chamber, 12-high pressure gas storage reactor, 13 is the air compressor, 14 is the pressure gauge .

Detailed ways

The technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings.

The test method of the law of coal mining seepage mechanics under the simulated protection layer mining conditions of the application is based on the publication number CN104374684A, titled "System and Application for Testing the Permeability of Unloaded Coal and Rock Mass in the Mining Process" The system for testing the permeability of unloaded coal and rock mass during the mining process in the patent application, the reference signs used are those in the patent application, and the accompanying drawing 1 in this technical solution is drawn based on the patent application.

Use the system whose publication number is CN104374684A, titled "system and its application for testing the permeability of unloaded coal rock mass in the mining process" in the patent application for testing the permeability of unloaded coal rock mass in the mining process to carry out the protection layer Laboratory simulation of seepage mechanics behavior law in coal mining under mining conditions. The system for testing the permeability of unloaded coal and rock mass during the mining process is also the mining pressure relief and anti-permeability test platform. The MTS confining pressure chamber in the system for testing the permeability of unloaded coal and rock mass during mining is a part of the MTS815 Flex Text GT rock mechanics testing system.

As shown in Fig. 1, the existing system for testing the permeability of unloaded coal and rock mass during the mining process includes the MTS815Flex TextGT rock mechanics test system and the gas seepage control unit matched with it. The gas seepage control unit includes a gas supply system, a gas stabilizer Pressure and temperature control system, measurement system and pipeline vacuum control system. The gas supply system includes a gas source tank and a first pressure reducing valve, and the gas source tank is filled with high-pressure gas; the vacuum control system of the pipeline includes a vacuum pump, the first valve, the second valve and corresponding pipelines; the measurement system includes a flow meter, a pressure Considering the corresponding valves, it can also include a computer and a data acquisition and recording device. In addition, it can be connected with manual data recording and data processing.

The MTS815Flex Text GT rock mechanics testing system used in this test includes an acoustic emission three-dimensional positioning acquisition unit, a program-controlled acquisition unit, an ultrasonic excitation acquisition unit, a master control unit, a high-temperature control unit and a loading unit. The loading unit is used to load axial pressure and confining pressure and changing osmotic pressure. The test loading technical indicators of the MTS815 rock mechanics comprehensive experiment system selected in this example are as follows:

(1) Static test: maximum axial load: 4600kN (compression), 2300kN (tension); axial travel: 100mm; confining pressure: 140Mpa; pore pressure: 140MPa; pore pressure difference 30Mpa; temperature: room temperature to 200 ℃;

(2) Dynamic test: vibration frequency: up to 5Hz or more; vibration waveform: sine wave, triangle wave, square wave, ramp wave, random wave; phase difference: 0～2π can be set arbitrarily;

The measurement technical indicators are:

(1) Axial load: 0~4600kN (compression), 0~250kN (tension);

(2) Axial displacement: 0~100mm (±50mm);

(3) Axial strain: (uniaxial and triaxial high temperature and high pressure);

(4) Transverse strain: (uniaxial and triaxial high temperature and high pressure);

(5) Confining pressure: 0~140MPa;

(6) Temperature: room temperature to 200°C;

(7) Pore pressure: 0~140MPa;

(8) Pore pressure difference: 0~30MPa;

(9) Permeability: 10-4～5×10-8DC;

(10) Body change: >100ml.

The main technical indicators of the gas seepage control unit are as follows: gas pore pressure application range: 0.1~20MPa; external air source heating range: room temperature~70℃; gas pressure gauge: 0.1~30MPa; gas flow meter: 10mL/min~5000mL/ min.

The mining pressure relief and anti-permeability test platform can complete various mechanical tests and multiple acoustic tests related to gas seepage. During the working process, the MTS815 test system provides axial and hoop loads, measures and records deformation, acoustic emission and other data, and the gas seepage control system provides a constant temperature and stable pressure gas seepage environment, and the seepage gas pressure and flow data are detected simultaneously with the deformation data Record. The main working principle of the test platform is shown in Figure 2.

The test method for simulating the mechanical behavior law of coal mining seepage under the mining conditions of the protective layer, specifically includes the following steps:

1. Select coal and rock specimens:

The selected coal and rock test pieces are coal samples, also known as samples. The coal samples used in the test are all based on the provisions of the national standard "Measurement Methods of Physical and Mechanical Properties of Coal and Rock—Part 1: General Regulations on Sampling" and the on-site mining area. The actual working conditions were selected. The coal samples selected in this example were collected from the Furong mining area in Sichuan Province and the Tongmei Tashan Mine in Shanxi Province. The coal samples in the Furong mining area were taken from the 1 ^# ~ 4 ^# coal seams of the Baijiao Coal Mine. The coal quality is sulfur-rich anthracite, and the buried _depth is 300 ~ _450m. , the mine has relatively prominent gas control problems in production practice. The coal samples of Tashan Mine are taken from the 8202 working face of the mine. The coal seam structure of the working face is complex, and the sampling point is buried at a depth of about 500m. The average thickness of the coal seam is 11.17m.

According to the provisions of Chapter 5 of Part 7 of the national standard "Measurement Methods of Physical and Mechanical Properties of Coal and Rocks", the coal samples used in the test are cylinders with a diameter of 47-51mm, and the height-to-diameter ratio is (2±0.2), and the height-to-diameter ratio is less than 2, the impact on the test results should be considered; the non-parallelism of the two ends of the test piece should not be greater than 0.05mm; the diameter deviation of the upper and lower ends of the test piece should not be greater than 0.3mm; the surface of the test piece should be smooth and avoid irregularities. of stress concentration. See Table 1 and Table 2 for the basic information of the samples. Industrial analysis, elemental and mineral analysis of coal samples are all collected from the coal body powder. In this paper, all standard coal samples are uniformly numbered, and the initial letter indicates the information of the simulated mining method of the test, that is, the protective layer mining (PCM) corresponds to P, the middle number is the gas pressure level, and the last number is the sequence number of the sample, for example , P-2-2 represents the second sample of simulating 2MPa air pressure difference in protective layer mining.

Table 1 Statistical summary of physical properties of coal samples in Baijiao Mine

Table 2 Statistical summary table of physical properties of coal samples in Tashan Mine

In order to compare and study the influence mechanism of the protective layer mining method on the evolution of coal mechanical properties and permeability before the pressure relief expansion of the protected seam, the P-0-1 and P-2-1 coal samples can be designed without pre-mining pressure relief test , directly carry out mining dynamics test simulation, as a control group, provide test data for comparative analysis of test results. Due to the large differences in coal samples from different mining areas, they were differentiated during analysis.

2. Installation and commissioning:

(1) Record the basic data of the test piece, including the size data of the test piece, such as the height and radius of the test piece, and connect to other equipment;

(2) Install the specimen in the triaxial chamber of the MTS rock mechanics testing system, and spray heat-shrinkable film on it. The triaxial chamber is the MTS confining pressure chamber 6 in Figure 1. In order to enhance the spraying effect, two layers of heat shrinkable film were sprayed, and the specimen was fixed on the press of the loading platform by applying a 2kN load, and then the triaxial chamber was filled with oil and exhaust operation. Gradually apply confining pressure at a rate of 3MPa/min to 12MPa (corresponding to the stress level of the sample sampling depth of 500m); during the process of oil filling and applying confining pressure, vacuumize the intake pipe for about 60min. Apply confining pressure and keep the confining pressure constant, then fill the intake pipeline with methane gas at a pressure of 2 MPa through the gas booster pump, and stabilize the air pressure until the volume of the specimen remains constant before loading. Finally, the axial load control is adopted, and the loading rate is 30kN/min, and then the circumferential deformation control is adopted, and the load is loaded at a speed of 0.08mm/min for the circumferential deformation until the residual strength appears. Then lower the triaxial chamber completely, fix the bolts, and fill the triaxial chamber with mineral silicone oil for subsequent application of confining pressure.

During the whole loading process, high-precision flow meters and pressure gauges can also be used to test the data of inlet and outlet flow and pore pressure. In order to ensure the accuracy of the measured flow data, a mechanical flowmeter can be used to check the electronic flowmeter in the system used to test the permeability of unloaded coal and rock mass during mining. After the peak load, the test can be stopped by continuing to release the pressure until it is greater than the set test gas pressure value such as 3MPa, and the flow data will be continuously tested during this period. Due to the high confining pressure applied to the test piece, the heat shrinkable film is in good contact with the side of the test piece, and no gas overflow occurs on the side wall, so whether to apply a silica gel layer on the side of the test piece has an effect on the permeability characteristics of coal samples under different gas pressure conditions Therefore, during the test, no silica gel was applied to the side of the test piece, which effectively simplified the test steps.

The test air pressure is divided into 3 levels, namely 0.5MPa, 1MPa and 2MPa. Two test pieces are tested for each level of air pressure. The sample numbers and basic information are shown in Table 1.

3. The indoor simulation test operation of the law of seepage flow mechanics behavior in coal mining under the condition of protective layer mining includes the following steps:

(1) Simulation operation of in-situ stress recovery and pressure relief expansion deformation of coal and rock specimens under the mining conditions of the protective layer

In view of the mining conditions of the protective layer, it is first necessary to carry out in-situ stress recovery and pressure relief expansion deformation simulation operations on coal samples. The test process uses axial stress and circumferential deformation as control indicators. The coal sample stress recovery and pressure relief expansion loading scheme is shown in Figure 3 . The loading and unloading process simulation can be divided into the following four stages for description:

(a) In-situ stress recovery stage: the function of this stage is to restore the original stress state of the coal body. Assuming that the initial in-situ stress is hydrostatic pressure, the vertical stress gradient can be adjusted according to the actual test situation. In this example, the vertical stress gradient is 25kPa/m (vertical stress Gradient size is equal to bulk density), corresponding to the OA' section in Fig. 3, the load is carried out at an appropriate loading rate according to the loading method with the ratio of axial pressure and confining pressure being 1, until the confining pressure and axial pressure are γH respectively, that is, A' (γH,γH), wherein, γ represents bulk density in KN/m ³ , and H represents depth in m. In order to ensure the loading effect, the loading rate is selected here as 3MPa/min;

(b) Axial compression stage: this stage is the axial compression and expansion stage of the protected layer under simulated protection layer mining, corresponding to the A'B' section in Fig. 3, in order to keep the confining pressure constant and the axial pressure increase The loading rate can take other values. Generally speaking, the slower the loading rate, the better, but the longer the experimental time required, so the loading rate is generally the faster the better without affecting the experimental results. In this example, the preferred axial loading rate is 30kN/ min, loaded until the axial stress reaches 1.5γH, that is, B'(1.5γH,γH);

(c) Pressure relief and expansion stage: this stage is to simulate the circumferential pressure _relief and expansion stage of the protected layer, corresponding to the _B'C ' section in Fig. Change, reduce the unloading method of confining pressure to unload, unload at an appropriate unloading rate of confining pressure, such as 3MPa/min, unload until the coal sample deforms and enters the yield stage, the coordinate of C' cannot be quantified, and there will be slight changes in different samples;

(d) Stress recovery stage: this stage simulates the recompression process of the deformed rock layer after unloading and expansion of the protected layer, corresponding to the C'A' section in Figure 3, when the difference between axial stress and confining pressure σ ₁ -σ ₃ is reduced At the same time, increase the confining pressure to reach the stress level of point A in Figure 3. During the loading process, the confining pressure loading rate is determined according to different test conditions. In this example, the confining pressure loading rate is 3MPa/min.

(2) Simulation test of coal mining stress evolution under the condition of protective layer mining

During the mining process, the coal body generally undergoes a complete stress change process from the stress of the original rock, to the difference between the axial stress and the confining pressure σ ₁ -σ ₃ rising and the confining pressure σ ₃ decreasing (unloading), until failure. Therefore, in this test, under the conditions of different mining methods in the simulated working face, the stress of the specimen increases from the original rock stress to the difference σ ₁ -σ ₃ between the axial stress and the confining pressure, while the confining pressure σ ₃ decreases (unloading) until The complete stress change process of failure. Considering the occurrence of gas in the coal seam, in view of the actual working conditions, a test scheme as shown in Figure 4 is drawn up to explore the influence of mining methods on the mechanical properties, failure mechanism and seepage capacity evolution of coal and rock. The simulation process can be divided into the following three The stages are described:

(a) Hydrostatic pressure stage: apply hydrostatic confining pressure to 25MPa at an appropriate loading rate such as 3MPa/min (assuming that the vertical stress gradient is 25kPa/m, corresponding to a design buried depth of about 1000m), that is, section OA in Figure 4, load During the process, the ratio of confining pressure and axial pressure is set to 1:1; for the simulated specimen considering gas occurrence conditions, after reaching the preset confining pressure, the upstream and downstream pipelines of the coal sample are vacuumed, and the vacuuming time is for 30 minutes, and then apply the same gas pressure on the upper and lower ends of the coal sample to allow the coal sample to absorb methane saturated. The adsorption time depends on the adsorption deformation of the coal sample until the volume of the coal sample remains unchanged;

(b) The first unloading stage: Simulating the initial stage of mining impact, the coal rock gradually changes from the hydrostatic pressure state to the axial stress concentration factor K equal to 1.5, so as to realize the stress environment of the simulated rock in the real mining environment, that is, the coal sample axis The ratio between the increase of the difference between the stress and the confining pressure σ ₁ - _{σ 3} and the unloading of the confining pressure σ ₃ is 2.25:1, which is the section AB in Fig. 4. The medium confining pressure unloading rate is 1MPa/min, that is, rock samples are unloaded from point A, the confining pressure unloading rate is 1MPa/min, and at the same time, the deviatoric stress loading rate is 2.25MPa/min loaded to point B. For the simulated coal samples considering the gas occurrence conditions, before the start of the test, the gas outlet of the coal samples was connected to the atmosphere and then loaded, that is, the impact of coal mining on gas penetration was simulated, and the pressure level at the inlet was kept constant during the loading process. And continuously record the change of gas pressure and flow rate.

(c) The second unloading stage: As the working face advances, assuming that the coal and rock confining pressure near the mining face is linearly distributed, the unloading rate of the confining pressure remains unchanged, and the speed of the roof pressure (that is, the axial stress) increases, the corresponding The stress concentration factor is from 1.5 to the unloading failure of coal and rock, and different axial loading rates are preset. The corresponding stress concentration factor of coal and rock is equal to 2.0 when the coal is damaged, that is, the axial deviatoric stress (σ ₁ -σ ₃ ) increases and the transverse stress (σ ₃ ) The unloading ratio is 2.25:1, which corresponds to the BC section in Figure 4, and corresponds to the mining stress evolution simulation path of the coal body near the working face under the three mining conditions of the protective layer; the pressure level at the inlet end is kept constant throughout the loading process, and Continuously record the change of gas pressure and flow, that is, record the gas pressure at the gas inlet and the gas pressure p ₁ and p ₂ at the gas outlet.

4. Data processing and test result analysis

Set the test conditions according to the test, record the relevant data, and use the compressible gas horizontal linear steady seepage Darcy formula to calculate the permeability in different periods. The calculation formula is as follows:

In the formula, k represents the permeability, m ² ; p ₀ is the atmospheric pressure at the measuring point, which is 0.101325 MPa; A is the cross-sectional area of the test piece, m ² ; μ is the viscosity coefficient of gas, which is 1.087×10 at 20°C ^-5 Pa·s; L is the length of the test piece, m; p ₁ and p ₂ are the gas pressure at the inlet and outlet, MPa.

Based on the above data, the mechanical behavior law of coal mining seepage flow under the simulated protection layer mining conditions is analyzed and obtained.

In addition, steps 2-4 can be repeated for multiple times to improve the accuracy of the results.

After the test is completed, the individual equipment is disassembled and installed.

Claims (6)

1. The test method of coal body mining seepage mechanics law under the condition of simulating protective layer mining is characterized in that, comprises the following steps: A. Select coal and rock specimens; B. Record the basic data of the test piece, install the coal-rock test piece in the system used to test the permeability of the unloaded coal-rock mass during the mining process, and debug all the equipment of the system; C. Carry out the simulation operation of the stress recovery and pressure relief expansion deformation of the coal and rock specimens under the mining conditions of the protective layer, which specifically includes the following four stages: C1. Ground stress recovery stage: set the initial ground stress and vertical stress gradient, load the axial pressure and confining pressure at a certain loading rate according to the preset axial pressure and confining pressure ratio, until the confining pressure and axial pressure respectively reach the initial confining pressure value, initial axial pressure value; C2. Axial compression stage: Loading is carried out by keeping the confining pressure constant and increasing the axial pressure until the axial stress reaches the preset value; C3. Unloading and expansion stage: Unloading is carried out by keeping the difference between the axial stress and the confining pressure σ ₁ -σ ₃ constant and reducing the confining pressure, and unloading at an appropriate unloading rate of the confining pressure until the specimen deforms and enters the yield stage , where σ ₁ is the axial pressure, σ ₃ is the confining pressure; C4. Stress recovery stage: reduce the difference between the axial stress and the confining pressure σ ₁ - σ ₃ while increasing the confining pressure until the confining pressure and axial pressure respectively reach the initial confining pressure value and initial axial pressure value; D. Carry out the simulation operation of the mining stress evolution of the coal and rock test piece under the condition of simulating the mining of the protective layer on the test piece: D1. Hydrostatic pressure stage: set the initial vertical stress gradient and the corresponding design embedding depth, and obtain the initial axial pressure according to the initial vertical stress gradient and the corresponding design embedding depth, apply confining pressure at an appropriate loading rate, so that When the preset confining pressure and initial axial pressure are reached, the entire gas pipeline is evacuated, and then methane gas with the same gas pressure is applied to the upper and lower ends of the test piece until the volume of the test piece remains unchanged; D2. The first unloading stage: Unloading at a certain confining pressure unloading rate makes the specimen rock gradually change from the hydrostatic pressure state until the axial stress concentration coefficient K is equal to the first coefficient value, that is, the difference between the axial stress and the confining pressure σ ₁ The ratio between the increase of -σ ₃ and the unloading of the confining pressure σ ₃ is the set ratio, until the preset first unloading axial pressure and the preset first unloading confining pressure are reached, and the gas outlet end of the gas pipeline is connected to the outside world before loading. During the loading process, the pressure level at the inlet end is kept constant, and the change of gas pressure and flow rate is continuously recorded; D3. The second unloading stage: set the confining pressure of the coal and rock near the mining face to be linearly distributed as the working face advances, keep the unloading rate of the confining pressure constant, and unload in the way that the axial stress increases at an increasing rate, so that the axial stress is concentrated The coefficient K is from the first coefficient value to the unloading failure of the specimen. When the unloading failure of the specimen occurs, the axial stress concentration coefficient K is the second coefficient value. The second coefficient value is greater than the first coefficient value, that is, the axial deviatoric stress σ ₁ - The ratio between the increase of σ ₃ and the unloading of lateral stress σ ₃ is a set ratio, and the pressure level of the gas inlet is kept constant during the whole process of loading, and the change of gas pressure and flow rate is continuously recorded; E. According to the Darcy's formula for horizontal linear steady seepage of compressible gas, the permeability in different periods is calculated, and the calculation formula is as follows: In the formula, k represents the permeability, the unit is m ² ;, q is the gas flow rate, the unit is m ³ /s; p ₀ is the atmospheric pressure at the measurement point, which is 0.101325MPa; A is the cross-sectional area of the test piece, the unit is m ² ; μ is the dynamic viscosity coefficient of gas, which is 1.087×10 ^-5 Pa·s at 20°C; L is the length of the test piece, in m; p ₁ and p ₂ are the gas pressure and The gas pressure at the gas outlet, in MPa. 2. The method according to claim 1, characterized in that, in the step A, the coal rock sample is a coal sample with a smooth surface, a diameter of 47 to 51mm, and a cylinder with a height-to-diameter ratio of (2 ± 0.2). The non-parallelism of the two ends is not more than 0.05mm, and the diameter deviation of the upper and lower ends of the test piece is not more than 0.3mm. 3. The method according to claim 1, wherein step B specifically comprises the following steps: B1. Record the basic data of the test piece and connect to other equipment; B2. Install the test piece in the triaxial chamber of the MTS rock mechanics testing system. After spraying the heat shrinkable film on the test piece, apply a load to fix the test piece on the loading platform, and perform oil filling and exhaust operations on the triaxial chamber; B3. Gradually apply the confining pressure to a specific value and keep the confining pressure constant, and then fill the methane with stable air pressure until the volume of the specimen remains unchanged and start loading; B4. Use axial load control, and then use circumferential deformation control until the residual strength appears, and then fill the triaxial chamber with mineral silicone oil. 4. The method according to claim 1, wherein the loading and unloading operations are carried out at a constant rate, the loading rate is 3MPa/min, and the unloading rate is 1MPa/min. 5. The method according to claim 1, characterized in that, step D3 further comprises that after the test reaches the peak load, in order to ensure the safety of the test equipment, the confining pressure is no longer reduced, and the test is stopped after continuing to load until the residual strength of the test piece appears. 6. The method according to claim 1, characterized in that the preset ratio of confining pressure and axial pressure in step C1 is 1:1, and the initial confining pressure value is γH, where γ represents the bulk density and the unit is KN/m ^3. H represents the depth in m; during the loading process in step D1, the ratio of confining pressure and axial pressure is set to 1:1.