Testing method for quantitatively evaluating conductivity of free gas in shale and coal matrix

Abstract

The invention provides a test method for quantitatively evaluating the conductivity of free gas in shale and coal matrix, aiming at the limitation of the existing coal matrix diffusion coefficient test method, the method adopts raw coal as an experimental object, and the conductivity of the free gas in the shale and coal matrix is evaluated by monitoring the dynamic deformation evolution characteristics of a coal matrix system in the gas injection process. And reversely deducing the diffusion coefficients of the coal matrix system in different characteristic directions in combination with a constructed coal matrix effective diffusion coefficient model based on dynamic strain, thereby achieving the target of rapidly and accurately testing the anisotropy of the free gas diffusion coefficient of the coal matrix system. According to the method, the target of testing the anisotropy of the free gas diffusion coefficient of the raw coal sample can be achieved at the same time, more accurate reservoir physical property parameters are provided for free gas migration characteristics in coal bed gas and shale gas exploitation, and a reference basis is provided for later reservoir transformation and development.

Description

Technical Field

[0001] The present invention relates to the technical field of coal and shale gas resource development, and in particular to a test method for quantitatively evaluating the conductivity of free gas in shale and coal matrices. Background Art

[0002] my country's shale gas and coalbed methane resources have basically achieved industrial development, but the problems of increasing and stabilizing gas well production still restrict the efficient development of shale gas and coalbed methane in my country. Shale and coal reservoirs are generally considered to be dual-porosity media, consisting of a porous coal matrix and surrounding fractures. The fracture system is the main seepage channel, while the matrix system is the gas storage space. It is generally assumed that the seepage in the fracture system conforms to Darcy's law, while the gas migration in the matrix system is controlled by diffusion. The matrix diffusion coefficient is an important parameter for evaluating the ability of gas to migrate within a porous coal matrix. Understanding the gas flow capacity within the coal matrix in a dual-porosity fracture system is of great significance to the development of shale gas and coalbed methane, and is the key to solving a series of engineering problems.

[0003] In order to study the gas flow characteristics within the matrix, a series of experiments were conducted to test the matrix diffusion coefficient and study the effects of different factors such as temperature and coal rank on the diffusion characteristics. These experiments were mainly conducted by breaking the coal sample into particles of a certain mesh size (the original coal sample was crushed into pulverized coal particles with a diameter of 0.25 to 0.50 mm), and then placing the particle sample in a sample tank. By monitoring the pressure change in the sample tank after gas injection, the diffusion coefficient of the coal matrix was inferred, which is the coal particle test method. The main advantages of this method are fast experimental speed and low experimental equipment requirements. However, this method has certain defects: in the process of coal sample crushing, due to the certain degree of difference in the macroscopic coal rock components of the coal sample, the selection of samples of different particle sizes has a certain degree of sorting; and because the closed pores are opened after the sample is crushed, the original pore system structure is destroyed, making it difficult to reflect the real gas migration path; due to the influence of adsorption, the obtained diffusion coefficient is affected by the adsorption and desorption processes, and the adsorption / desorption equilibrium process takes a long time; the size of the particles to be used is still controversial. Influenced by this, some scholars used cylindrical coal cores to conduct experiments, processing the samples into or The study used standard columnar raw coal samples and found that there were obvious differences in the diffusion behavior between coal particles and columnar coal cores. The raw coal samples retained more porous crack characteristics and were close to the original coal body morphology. However, the inevitable presence of macro cracks in the coal samples would affect the analysis of gas migration paths and lead to the problem that the diffusion distance could not be accurately estimated, thereby affecting the analysis and calculation of the diffusion coefficient. Moreover, only one direction could be tested at a time, which placed high requirements on sample processing accuracy and air tightness of experimental equipment.

[0004] To fully explain the results of diffusion experiments, previous researchers have studied the diffusion laws of coal samples through theoretical modeling and numerical simulation. Diffusion models for granular coal samples are mainly divided into single-pore, double-pore, and multi-pore diffusion models. For example, in the single-pore model, it is assumed that the surface concentration of coal particles remains zero under constant pressure conditions, and the diffusion coefficient of coal particles is then solved. This model is consistent with the boundary conditions of conventional desorption experiments and is therefore widely used to fit the desorption diffusion coefficient. Since the diffusion of adsorbed gases is mainly surface diffusion, scholars have established a surface diffusion coefficient model for coal adsorbed gases using kinetic and thermodynamic methods based on the random hopping mechanism of surface diffusion gas molecules and the transition state theory of adsorbed gas diffusion. However, the above models are mainly diffusion coefficient calculation models established for coal particles, and a diffusion coefficient calculation model for raw coal samples has not yet been constructed. Summary of the Invention

[0005] The purpose of the present invention is to provide a test method for evaluating the anisotropic characteristics of free gas conductivity in shale and coal matrices in situ. The method can directly use raw coal samples, retain the original pore structure, improve the authenticity of the test, and realize anisotropic analysis of conductivity in vertical and horizontal directions through three-dimensional strain monitoring. The diffusion model constructed based on dynamic strain data is applicable to the complex pore and fracture system of raw coal samples.

[0006] In order to achieve the above-mentioned purpose of the invention, the technical solution adopted by the present invention is as follows:

[0007] The present invention provides a test method for quantitatively evaluating the free gas conductivity in shale and coal matrices, comprising the following steps:

[0008] S1. Sample preparation: Select lump coal samples to make regular-shaped raw coal samples, perform water removal pretreatment on the samples to eliminate the influence of moisture, and fully characterize the pore and fracture system of the samples;

[0009] S2. Monitoring device layout: Multiple strain gauges are attached to the surface of the prepared sample, with at least three strain gauges distributed in the vertical and horizontal directions. These strain gauges are connected to an external data acquisition system to monitor and record the dynamic strain of the sample under different pressures in real time.

[0010] S3. Experimental Procedure: Place the sample in a high-pressure vessel in a controlled temperature environment and inject helium at a constant rate gradient to a preset pressure. During the pressurization process, a pressure monitoring system is used to monitor pressure changes in real time, and strain data in all directions is simultaneously recorded. The experimental environment temperature is controlled to eliminate the interference of thermal deformation on the experimental results.

[0011] S4. Data analysis: Based on the law of conservation of mass and the collected strain data, an anisotropic diffusion model is constructed:

[0012]

[0013] Where: ε 11 is the linear strain in the stress direction, ε 22 is the linear strain in the stress direction, ε 33 is the linear strain in stress direction 3, the subscripts of other parameters represent the corresponding different application directions, K1 is the bulk modulus in stress direction 1, p m0 is the initial pore pressure in the matrix, is the compressive stress in stress direction 1, t is the diffusion time, D1 is the diffusion coefficient in stress direction 1; c m is the shape factor, φ is the porosity of the matrix system, p f is the pore pressure of the fracture system;

[0014] The anisotropic diffusion model is used to infer the effective diffusion coefficient of the coal matrix in different directions, and then the anisotropic characteristics of the free gas conductivity are analyzed.

[0015] Preferably, in step S1, the sample size is 5 cm×5 cm×5 cm, and during the water removal pretreatment, the vacuum drying temperature is 80° C. and the time is ≥72 hours.

[0016] Preferably, in step S1, X-CT imaging and high-pressure mercury injection are used to characterize the pore and fracture system, wherein the resolution of the X-CT scan should be no less than 1 μm to ensure that the pore and fracture distribution inside the sample can be clearly and accurately identified. The high-pressure mercury injection method injects mercury into the sample pores for analysis to study the pore size distribution of the coal sample and obtain accurate pore and fracture structure parameters.

[0017] Preferably, in step S2, when pasting the strain gauge, the strain gauge should be pasted on the area of ​​the sample surface where cracks are not developed to monitor the linear strain of the sample coal matrix part, and the strain gauge should be pasted firmly and tightly to avoid loosening or falling off during the experiment, which affects the accuracy of strain data collection.

[0018] Preferably, in step S3, the experiment is carried out in a constant temperature chamber to control the experimental environment temperature. The temperature in the constant temperature chamber is maintained at 35±0.1°C. By monitoring the dynamic evolution of strain at different parts of the sample, the difference in the diffusion capacity of the sample is analyzed.

[0019] Preferably, in step S3, the high-pressure container is a pressure tank. After the strain gauge is attached to the selected area on the surface of the sample, the sample is placed in the pressure tank. After the system temperature is balanced, helium is injected into the coal sample under free expansion conditions. The helium is gradually increased to 1.0 MPa at a rate of 0.02 MPa / s, and then kept constant at 1.0 MPa for about 2 hours. Subsequently, the helium is gradually increased from 1.0 MPa to 2.0 MPa at a rate of 0.02 MPa / s, and so on, and the pressure is gradually increased to 5.0 MPa.

[0020] Preferably, in step S4, the method for constructing the anisotropic diffusion model is:

[0021] (1) Coal matrix deformation control equation

[0022] Based on poroelasticity, the constitutive relationship of coal seam deformation is expressed as:

[0023]

[0024] where ε ij are the components of the total strain tensor, σ ij Represents the components of the total stress tensor, G=E / 2(1+ν), K=E / 3(1-2ν), G is the shear modulus, ν is Poisson's ratio, σ kk =σ 11 +σ 22 +σ 33 , α is the Biot coefficient, K is the bulk modulus, E is the Young's modulus, p m is the pore pressure of the coal matrix system, δ ij is the Kronecker symbol; according to formula (1), the coal matrix volume strain is:

[0025]

[0026] where ε v =ε 11 +ε 22 +ε 33 is the volume strain of the coal matrix, Average compressive stress, effective stress is σ eij =σ ij +αp m δ ij , so the linear strains in the three stress directions are expressed as:

[0027]

[0028] (2) Gas migration control equation

[0029] After helium injection, the coal matrix gas pressure will be lower than the fracture gas pressure, and the fracture gas will diffuse into the coal matrix system:

[0030] J1=Dc m (ρ m -ρ f ) (4)

[0031] Where J1 is the gas diffusion flux per unit volume of coal matrix, kg / (m 3 ·s); D is the diffusion coefficient, m 2 / s;c m is the coal matrix shape factor, m -2ρ m is the gas density in the coal matrix, kg / m 3 ρ f is the gas density in the crack, kg / m 3 ; The gas density in the cracks and coal matrix is ​​expressed as:

[0032]

[0033] Where: M g is the gas molecular weight, p f represents the pore pressure of the fracture system, R is the universal gas constant (J / (mol·K)), and T is the absolute temperature of the gas (K);

[0034] For a unit volume of coal matrix, the coal matrix gas diffusion transport equation is obtained from the law of conservation of mass:

[0035]

[0036] Where: m is the mass of free gas per unit volume of coal matrix, t is the diffusion time;

[0037] The mass of helium stored per unit volume of coal matrix:

[0038]

[0039] Substituting formulas (5)-(7) into formula (4) yields:

[0040]

[0041] Shape factor c m Expressed as:

[0042]

[0043] Among them L x , L y is the crack spacing;

[0044] By formula (8), we can simultaneously remove: get:

[0045]

[0046] Continue to simplify:

[0047]

[0048] make but:

[0049]

[0050] Under a certain pressure condition, pf is a constant; from formula (12), the gas pressure p in the coal matrix is m for:

[0051] p m =Ce -∫(a)dt +e -∫(a)dt ∫ap f (e ∫(a)dt )dt (13)

[0052] The solution is:

[0053] p m =Ce -at +p f (14)

[0054] Where: c is a constant determined by the initial conditions, a is a coefficient;

[0055] Assume that at time 0 p m =p m0 , according to formula (14):

[0056] C=p m0 -p f (15)

[0057] Where: p m0 is the initial value of the pore pressure of the coal matrix system under the initial state;

[0058] Therefore, the gas pressure in the coal matrix p m Expressed as:

[0059] p m =(p m0 -p f )e -at +p f (16)

[0060] Solve the two conditions, that is, the initial time t=0, then p m =p m0 ; and as time gradually increases, p m ≈p f The pressure inside the sample reached equilibrium, verifying the reliability of the model;

[0061] Substituting formula (16) into formula (3) yields:

[0062]

[0063] Where: D1 is the diffusion coefficient in stress direction 1, D2 is the diffusion coefficient in stress direction 2, and D3 is the diffusion coefficient in stress direction 3. When analyzing anisotropic characteristics, the effective diffusion coefficients in the vertical and horizontal directions are calculated separately and compared.

[0064] Preferably, after the experiment, statistical methods are used to process the collected strain data and the diffusion coefficient obtained by inversion to ensure the reliability and accuracy of the experimental results. At the same time, the experimental data are compared with the data of previous similar experiments to further verify the superiority and innovation of this test method.

[0065] Compared with the prior art, the present invention has the following beneficial effects:

[0066] 1. The existing technology needs to destroy the original pore structure of the coal sample. The sample does not have the coal component sorting caused by the difference in mechanical properties of different components, and the adsorption / desorption process takes too long. This method can directly use the raw coal sample, retain the original pore structure, and improve the authenticity of the test.

[0067] 2. The existing technology cannot avoid macro crack interference and can only test a single direction at a time, which requires high precision. This method realizes anisotropic analysis of vertical and horizontal conductivity through three-dimensional strain monitoring.

[0068] 3. Existing models are mainly targeted at coal particles and lack anisotropic free gas diffusion models applicable to raw coal sample matrices. This method, based on a diffusion model constructed from dynamic strain data, is applicable to the complex pore and fracture systems of raw coal samples. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The accompanying drawings are used to provide further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention.

[0070] Figure 1 The sample strain gauge layout diagram and sample crack area scanning diagram provided by the embodiment of the present invention;

[0071] Figure 2 A schematic diagram of the structure of an experimental device provided in an embodiment of the present invention;

[0072] Figure 3 A graph showing the relationship between the logarithmic differential mercury intrusion volume and pore diameter of a coal sample provided in an embodiment of the present invention;

[0073] Figure 4 A graph showing the strain evolution during the helium injection process provided by an embodiment of the present invention;

[0074] Figure 5 The geometric shape and boundary condition diagram of the two-dimensional symmetric model provided by the embodiment of the present invention;

[0075] Figure 6 A comparison chart of strain simulation results and experimental results provided by an embodiment of the present invention;

[0076] Figure 7 This is a diagram of the dynamic strain curve simulation results provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0077] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. Of course, the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0078] This embodiment provides a test method for quantitatively evaluating the free gas conductivity in shale and coal matrices. This method is applicable to shale and coal matrices. This embodiment takes coal as an example. The specific steps are as follows:

[0079] 1. Samples and experimental methods

[0080] 1.1 Sample

[0081] This study selected lean coal samples from the 3# coal seam of the Zhaozhuang Coal Mine in the Jincheng Mining District in the southern Qinshui Basin. Industrial analysis revealed moisture content of 0.81%, ash content of 9.53%, volatile matter content of 14.68%, and fixed carbon content of 74.98%. The irregularly shaped bulk sample removed from the mine was cut into rectangular pieces. The pore and fracture system characteristics of the coal sample were determined using X-ray CT imaging and high-pressure mercury intrusion (MICP) testing. The sample measured 3.23 cm in height, 3.99 cm in length, and 3.22 cm in width. The sample was vacuum-dried at 80°C for at least 72 hours and repeatedly weighed until the sample mass remained stable to eliminate the effects of moisture on the experimental measurements.

[0082] 1.2 Experimental methods and procedures

[0083] 1.2.1 Characterization of pore and fracture systems:

[0084] X-ray computed tomography (CT) is a non-destructive technique that can quantitatively examine the internal three-dimensional structure of a sample. First, X-CT imaging was used to determine the distribution of cracks throughout the sample. The sample was scanned using a Nanotom X-ray computed tomography scanner (GE Phoenix) with a resolution of 1 μm.

[0085] High-pressure mercury injection is used to analyze the pore size distribution of coal samples by injecting mercury into the pores of the sample. 3Cubic samples were tested using high-pressure mercury intrusion (MICP) to determine the pore development characteristics of the samples. The geometric characteristics of porosity distribution and connectivity determine the diffusion coefficient of the coal. This high-pressure mercury injection experiment was conducted using a Micromeritics AutoPore IV 9510 instrument. The samples were vacuum-dried at 60°C for 72 hours to remove moisture. The surface tension of mercury was 485 mN / m, and the contact angle was 130°. The corresponding pore throat distribution was calculated using the Washburn equation.

[0086] 1.2.2 Dynamic strain experimental apparatus and experimental process

[0087] First, the strain gauges were pasted on the surface of the sample where cracks were not developed to monitor the linear strain of the coal matrix (see the strain gauge arrangement diagram for details). Figure 1 Helium was then injected into the coal sample under free expansion conditions, and the differences in the diffusion capacity of the samples were analyzed by monitoring the dynamic evolution of strain at different locations of the samples.

[0088] The experimental device of this embodiment adopts a device for measuring the Biot coefficient of a dual-porosity coal-rock-coal matrix system disclosed in the Chinese invention patent publication number CN115711798B. Its structure is as follows: Figure 2 As shown. During the experiment, the high-pressure vessel was placed in a constant-temperature chamber to eliminate the influence of temperature changes on the deformation of the coal sample. Three strain gauges were attached to selected areas on the sample surface to measure the spatial distribution in different directions during the coal deformation process, in order to study the relationship between the sample deformation and the dynamic injection of gas. The entire sample was then placed in a pressure-resistant tank. After the system temperature was balanced, the helium pressure was gradually increased to 1.0 MPa at a rate of 0.02 MPa / s, and then kept constant at 1.0 MPa for about 2 hours. Subsequently, the helium pressure was gradually increased from 1.0 MPa to 2.0 MPa at a rate of 0.02 MPa / s, and the pressure was gradually increased to 5.0 MPa. During the experiment, the temperature in the constant-temperature chamber was maintained at 35±0.1°C.

[0089] 2 Experimental results

[0090] 2.1 Description of pore-fracture system

[0091] The results of high-pressure mercury injection experiments show that (see Figure 3 As applied pressure increases, a large amount of mercury enters the interconnected pores of the coal sample, primarily mesopores and macropores. The pore volume distribution of the coal sample shows that the pore volume density function generally increases with decreasing pore diameter. The increase begins to rise rapidly when the pore diameter is less than 100 nm, reaching a maximum value around 3 nm. Pores with an average pore throat size of less than 10 nm account for more than 50% of the total volume.

[0092] Figure 1The distribution characteristics of fractures within the sample are shown. The image shows that the open fractures develop only one large fracture and several smaller fractures along the Z-axis. In contrast, filled fractures develop extensively within the sample, forming a complex network structure. Based on the field scale and considering the differences in permeability, this study considers the open fractures as the fracture system, while the filled fractures are considered part of the coal matrix system.

[0093] 2.2 Dynamic strain test results

[0094] Figure 4 The figure shows the temporal evolution of strain during helium injection. Under each gas pressure condition, the strain evolution curve exhibits a characteristic of initially rapidly decreasing, then slowly rebounding, until equilibrium is reached. First, after helium injection, the coal sample is rapidly compressed to its minimum value as the pressure rapidly increases. Subsequently, as the gas gradually diffuses into the sample, the measured strain begins to gradually rebound until equilibrium is reached. The equilibrium time in the vertical direction is much longer than that in the horizontal direction, and the difference between the two horizontal directions is relatively small. The minimum strain observed experimentally ranges from approximately -12με to -352με.

[0095] 3 Analysis and Discussion

[0096] 3.1 Model construction

[0097] A fully coupled numerical model was constructed to analyze the strain evolution mechanism caused by free gas migration within the coal matrix. The constructed fluid-structure coupling model simulated the spatial distribution of sample strain and explored the dynamic coupling process of the fluid-structure coupling field within the coal matrix. The dynamic evolution of the dynamic linear strain of the sample coal matrix from initial equilibrium to final equilibrium was fitted, allowing the differential characteristics of the effective diffusion coefficient of free gas in different directions within the coal matrix of the sample to be calculated.

[0098] (1) Coal matrix deformation control equation

[0099] Based on poroelasticity, the constitutive relationship of coal seam deformation is expressed as:

[0100]

[0101] where ε ij are the components of the total strain tensor, σ ij Represents the components of the total stress tensor, G=E / 2(1+ν), K=E / 3(1-2ν), G is the shear modulus, ν is Poisson's ratio, σ kk =σ 11 +σ 22 +σ 33 , α is the Biot coefficient, K is the bulk modulus, E is the Young's modulus, p m is the pore pressure of the coal matrix system, δ ijis the Kronecker symbol; according to formula (1), the coal matrix volume strain is:

[0102]

[0103] where ε v =ε 11 +ε 22 +ε 33 is the volume strain of the coal matrix, Average compressive stress, effective stress is σ eij =σ ij +αp m δ ij , so the linear strains in the three stress directions can be expressed as:

[0104]

[0105] (2) Gas migration control equation

[0106] After helium injection, the coal matrix gas pressure will be lower than the fracture gas pressure, and the fracture gas will diffuse into the coal matrix system:

[0107]

[0108] Where J1 is the gas diffusion flux per unit volume of coal matrix, kg / (m 3 ·s); D is the diffusion coefficient, m 2 / s;c m is the coal matrix shape factor, m -2 ρ m is the gas density in the coal matrix, kg / m 3 ρ f is the gas density in the crack, kg / m 3 .

[0109]

[0110] For a unit volume of coal matrix, the coal matrix gas diffusion transport equation can be obtained from the law of conservation of mass as follows:

[0111]

[0112] The mass of helium stored per unit volume of coal matrix:

[0113]

[0114] Substituting formulas (5)-(7) into formula (4) yields:

[0115]

[0116] Shape factor c mIt can be expressed as:

[0117]

[0118] Among them L x , L y is the crack spacing.

[0119] 3.2 Analysis and discussion of simulation results

[0120] like Figure 2 As shown in Figure 1, the sample is placed in a high-pressure tank. The attached strain gauge blocks the direct exchange of gas with the matrix, while the area around the strain gauge is in direct contact with the gas in the high-pressure tank. Therefore, the area near the strain gauge is considered as a crack, while the strain gauge itself is considered as part of the matrix. Therefore, gas is injected from all sides of the model (e.g. Figure 5 (as shown in the figure), the coal matrix surrounding the strain gauge is considered to be a fractured region. Mass exchange between the coal matrix and the fractures occurs at the connecting boundaries. The strain gauge dimensions can be considered as the fracture spacing (Lx and Ly). Therefore, the sample is simplified into a two-dimensional model with a width and height of 15 mm. Using He injection as an example, the evolution of the overall strain within the coal matrix of the coal sample under experimental test conditions is simulated.

[0121] The applied confining pressure and pore pressure gradually increase to a constant value (1.0 MPa) within 20 seconds. The model is solved by COMSOL multi-physics coupling solver. The mesh division in the solution process is as follows: Figure 5 The relevant parameter values ​​are shown in Table 1.

[0122] Table 1

[0123]

[0124]

[0125] Linear strain simulation Figure 6 As shown in the figure, the simulation results in the three directions show a good fit with the experimental results.

[0126] 3.3 Coal matrix effective diffusion coefficient model based on dynamic strain;

[0127] By formula (8), we can simultaneously remove: get:

[0128]

[0129] Continue to simplify:

[0130]

[0131] make but:

[0132]

[0133] Under a certain pressure condition, p f is a constant. According to formula (12), the gas pressure p in the coal matrix is m for:

[0134] p m =Ce -∫(a)dt +e -∫(a)dt ∫ap f (e ∫(a)dt )dt (13)

[0135] The solution is:

[0136] p m =Ce -at +p f (14)

[0137] Assume that at time 0 p m =p m0 , from formula (14):

[0138] C=p m0 -p f (15)

[0139] Therefore, the gas pressure in the coal matrix p m It can be expressed as:

[0140] p m =(p m0 -p f )e -at +p f (16)

[0141] Solve the two conditions, that is, the initial time t=0, then p m =p m0 ; and as time gradually increases, p m ≈p f The pressure inside the sample reached equilibrium, verifying the reliability of the model.

[0142] Substituting formula (16) into formula (3) yields:

[0143]

[0144] Where: D1 is the diffusion coefficient in stress direction 1, D2 is the diffusion coefficient in stress direction 2, and D3 is the diffusion coefficient in stress direction 3. The relationship between strain and diffusion coefficient is established in the above formula. From this model, it can be seen that the effective diffusion coefficient of the coal matrix system can be further inferred by testing the dynamic deformation of the coal sample. The dynamic strain results of the experimental test are fitted, as shown in Figure 7 shown.

[0145] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A test method for quantitatively evaluating the free gas conductivity in shale and coal matrices, characterized in that: The following steps are involved: S1. Sample preparation: Select lump coal samples to make regular-shaped raw coal samples, perform water removal pretreatment on the samples to eliminate the influence of moisture, and fully characterize the pore and fracture system of the samples; S2. Monitoring device layout: Multiple strain gauges are attached to the surface of the prepared sample, with at least three strain gauges distributed in the vertical and horizontal directions. These strain gauges are connected to an external data acquisition system to monitor and record the dynamic strain of the sample under different pressures in real time. S3. Experimental Procedure: Place the sample in a high-pressure vessel in a controlled temperature environment and inject helium at a constant rate gradient to a preset pressure. During the pressurization process, a pressure monitoring system is used to monitor pressure changes in real time, and strain data in all directions is simultaneously recorded. The experimental environment temperature is controlled to eliminate the interference of thermal deformation on the experimental results. S4. Data analysis: Based on the law of conservation of mass and the collected strain data, an anisotropic diffusion model is constructed: Where: ε 11 is the linear strain in the stress direction, ε 22 is the linear strain in the stress direction, ε 33 is the linear strain in stress direction 3, K1 is the bulk modulus in stress direction 1, p m0 is the initial pore pressure in the matrix, is the compressive stress in stress direction 1, t is the diffusion time, D1 is the diffusion coefficient in stress direction 1; c m is the shape factor, φ is the porosity of the matrix system, p f is the pore pressure of the fracture system; The anisotropic diffusion model is used to infer the effective diffusion coefficient of the coal matrix in different directions, and then the anisotropic characteristics of the free gas conductivity are analyzed.

2. The method according to claim 1, characterized in that In step S1, the sample size is 5 cm×5 cm×5 cm, and during the water removal pretreatment, the vacuum drying temperature is 80° C. and the time is ≥72 hours.

3. The method according to claim 1, characterized in that In step S1, X-CT imaging and high-pressure mercury injection are used to characterize the pore and fracture system, wherein the resolution of the X-CT scan is not less than 1 μm to ensure that the pore and fracture distribution inside the sample can be clearly and accurately identified. The high-pressure mercury injection method injects mercury into the sample pores for analysis to study the pore size distribution of the coal sample and obtain accurate pore and fracture structure parameters.

4. The method according to claim 1, wherein In step S2, when pasting the strain gauge, the strain gauge is pasted on the area of ​​the sample surface where open cracks are not developed to monitor the linear strain of the sample coal matrix. The strain gauge should be pasted firmly and tightly to avoid loosening or falling off during the experiment, so as not to affect the accuracy of strain data collection.

5. The method according to claim 1, wherein In step S3, the experiment is carried out in a constant temperature chamber to control the experimental environment temperature. The temperature in the constant temperature chamber is maintained at 35±0.1°C. By monitoring the dynamic evolution of strain at different parts of the sample, the differences in the diffusion capacity of the sample are analyzed.

6. The method according to claim 1, wherein In step S3, a pressure tank is selected as the high-pressure container. After the strain gauge is pasted on the selected area of ​​the sample surface, the sample is placed in the pressure tank. After the system temperature is balanced, helium is injected into the coal sample under free expansion conditions. The helium is gradually increased to 1.0 MPa at a rate of 0.02 MPa / s, and then kept constant at 1.0 MPa for about 2 hours. Subsequently, the helium is gradually increased from 1.0 MPa to 2.0 MPa at a rate of 0.02 MPa / s, and so on, and the pressure is gradually increased to 5.0 MPa.

7. The method according to claim 1, characterized in that In step S4, the method for constructing the anisotropic diffusion model is: (1) Coal matrix deformation control equation Based on poroelasticity, the constitutive relationship of coal seam deformation is expressed as: where ε ij are the components of the total strain tensor, σ ij Represents the components of the total stress tensor, G=E / 2(1+ν), K=E / 3(1-2ν), G is the shear modulus, ν is Poisson's ratio, σ kk =σ 11 +σ 22 +σ 33 , α is the Biot coefficient, K is the bulk modulus, E is the Young's modulus, p m is the pore pressure of the coal matrix system, δ ij is the Kronecker symbol; according to formula (1), the coal matrix volume strain is: where ε v =ε 11 +ε 22 +ε 33 is the volume strain of the coal matrix, Average compressive stress, effective stress is σ eij =σ ij +αp m δ ij , so the linear strains in the three stress directions are expressed as: (2) Gas migration control equation After helium injection, the coal matrix gas pressure will be lower than the gas pressure in the fractures, and the gas in the fractures will diffuse into the coal matrix system: J1=Dc m (r m -r f ) (4) Where J1 is the gas diffusion flux per unit volume of coal matrix, kg / (m 3 ·s); D is the diffusion coefficient, m 2 / s;c m is the coal matrix shape factor, m -2 ; ρ m is the gas density in the coal matrix, kg / m 3 ; ρ f is the gas density in the crack, kg / m 3 ; The gas density in the cracks and coal matrix is ​​expressed as: Where: M g is the gas molecular weight, p f represents the pore pressure of the fracture system, R is the universal gas constant (J / (mol·K)), and T is the absolute temperature of the gas (K); For a unit volume of coal matrix, the coal matrix gas diffusion transport equation is obtained from the law of conservation of mass: Where: m is the mass of free gas per unit volume of coal matrix, t is the diffusion time; The mass of helium stored per unit volume of coal matrix: Where: φ is the porosity of the matrix system; Substituting formulas (5)-(7) into formula (4) yields: Shape factor c m Expressed as: Among them L x , L y is the crack spacing; By formula (8), we can simultaneously remove: get: Continue to simplify: make but: Under a certain pressure condition, p f is a constant; from formula (12), the gas pressure p in the coal matrix is m for: p m =What -∫(a)dt +e -∫(a)dt No f (it ∫(a)dt )dt (13) The solution is: p m =What -at +p f (14) Where: c is a constant determined by the initial conditions, a is a coefficient; Assume that at time 0 p m =p m0 , according to formula (14): C=p m0 -p f (15) Where: p m0 is the initial value of the pore pressure of the coal matrix system under the initial state; Therefore, the gas pressure in the coal matrix p m Expressed as: p m =(p m0 -p f )e -at +p f (16) Solve the two conditions, that is, the initial time t=0, then p m =p m0 ; and as time gradually increases, p m ≈p f The pressure inside the sample reached equilibrium, verifying the reliability of the model; Substituting formula (16) into formula (3) yields: Where: D1 is the diffusion coefficient in stress direction 1, D2 is the diffusion coefficient in stress direction 2, and D3 is the diffusion coefficient in stress direction 3. When analyzing anisotropic characteristics, the effective diffusion coefficients in the vertical and horizontal directions are calculated respectively, and the two are compared and analyzed.

8. The method according to claim 1, characterized in that After the experiment, statistical methods were used to process the collected strain data and the diffusion coefficient obtained by inverse calculation and perform error analysis to ensure the reliability and accuracy of the experimental results. At the same time, the experimental data were compared with previous similar experimental data to further verify the superiority and innovation of this test method.

Images