A method for quantitatively evaluating the free gas conduction capacity in shale and coal matrix

Abstract

The present application provides a kind of quantitative evaluation shale, coal matrix free gas conduction ability test method, for the limitation of existing coal matrix diffusion coefficient test method, this method uses raw coal as experimental object, by monitoring the dynamic deformation evolution characteristics of coal matrix system in gas injection process, combined with the coal matrix effective diffusion coefficient model based on dynamic strain constructed, the diffusion coefficient of different characteristic directions of coal matrix system is back calculated, so as to achieve the goal of quickly and accurately testing the anisotropy of coal matrix system free gas diffusion coefficient.This method can simultaneously achieve the goal of testing the anisotropy of raw coal sample free gas diffusion coefficient, so as to provide more accurate reservoir physical parameter for the free gas migration characteristics in coalbed methane, shale gas exploitation, and provide reference basis for later reservoir reconstruction and development.

Description

Technical Field

[0001] This invention relates to the field of coal and shale gas resource development technology, and in particular to a method for quantitatively assessing the free gas conductivity within shale and coal matrices. Background Technology

[0002] my country's shale gas and coalbed methane resources have basically achieved industrialized development; however, the issues of increasing and stabilizing gas well production still constrain the efficient development of these resources. 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 primary seepage channel, while the matrix system serves as the gas storage space. It is usually assumed that seepage in the fracture system follows Darcy's law, while gas migration in the matrix system is controlled by diffusion. The matrix diffusion coefficient is an important parameter for evaluating the gas migration capacity within the porous coal matrix. Understanding the gas flow capacity within the coal matrix in a dual-porosity fracture system is of great significance for shale gas and coalbed methane development and is key to solving a series of engineering problems.

[0003] To study the gas flow characteristics within a matrix, previous researchers conducted a series of experiments to test the matrix diffusion coefficient and investigate the effects of different factors such as temperature and coal rank on diffusion characteristics. These experiments primarily involved breaking coal samples into particles of a certain mesh size (raw coal samples were crushed into pulverized coal particles with a diameter of 0.25–0.50 mm), then placing the particle samples in a sample container. The pressure change within the sample container after gas injection was monitored to infer the coal matrix diffusion coefficient—this is the coal particle testing method. The main advantages of this method are its fast experimental speed and low equipment requirements. However, this method has certain drawbacks: during the coal sample crushing process, due to the differences in macroscopic coal petrographic components, there is a certain degree of sorting required for samples of different particle sizes; moreover, the opening of closed pores after sample crushing disrupts the original pore system structure, making it difficult to reflect the actual gas transport path; the diffusion coefficient obtained is affected by adsorption and desorption processes, and the adsorption / desorption equilibrium process requires a long time; and the optimal particle diameter remains controversial. Influenced by these factors, some researchers have conducted experiments using columnar coal cores, processing the samples into… or The study of standard columnar raw coal samples revealed significant differences in diffusion behavior between coal particles and columnar coal cores. The raw coal samples retained more pore and fracture features, closely resembling the original coal body morphology. However, the presence of macroscopic fractures in the coal samples inevitably affects the analysis of gas migration paths, leading to inaccurate estimation of diffusion distance and consequently impacting the analysis and calculation of diffusion coefficients. Furthermore, only one direction can be tested at a time, placing high demands on sample processing precision and the airtightness of experimental equipment.

[0004] To fully explain the diffusion experiment results, previous researchers have studied the diffusion laws of coal samples using theoretical modeling and numerical simulation. Particulate coal sample diffusion models are mainly divided into single-pore, dual-pore, and multi-pore diffusion models. For example, in the single-pore model, it is assumed that the coal particle surface concentration remains zero under constant pressure, and then the coal particle diffusion coefficient is 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, researchers have established a coal adsorbed gas surface diffusion coefficient model using kinetic and thermodynamic methods, the random jumping mechanism of gas molecules on the surface, and the theory of adsorbed gas diffusion transition states. 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 this invention is to provide a test method for evaluating the anisotropic characteristics of free gas conductivity in in-situ shale and coal matrix. This method can directly use raw coal samples, preserve the original pore structure, improve the authenticity of the test, and achieve anisotropic analysis of conductivity in the 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] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0007] This invention provides a method for quantitatively evaluating the free gas conductivity within shale and coal matrix, comprising the following steps:

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

[0009] S2. Monitoring device arrangement: 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, and 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 container in a controlled temperature environment and inject helium gas at a certain rate gradient to the preset pressure. During the pressurization process, use a pressure monitoring system to monitor the pressure change in real time, record the strain data in each direction simultaneously, and control the experimental environment temperature 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 For linear strain in the stress direction, ε 22 For the stress direction, the strain is ε. 33 The value represents the strain along the stress direction (3-line). The subscripts of other parameters represent their respective application directions. K1 represents the bulk modulus along the stress direction (1-line). p m0 This represents the initial pore pressure in the matrix. Let be the compressive stress in stress direction 1, t be the diffusion time, and D1 be the diffusion coefficient in stress direction 1; c m Where φ is the shape factor, φ is the porosity of the matrix system, and p f Pore ​​pressure in the fractured system;

[0014] By using this anisotropic diffusion model to inversely deduce the effective diffusion coefficient of the coal matrix in different directions, the anisotropic characteristics of free gas conductivity can be analyzed.

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

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

[0017] Preferably, in step S2, when attaching the strain gauge, the strain gauge should be attached to an area on the sample surface where open cracks are not developed, in order to monitor the linear strain of the coal matrix in the sample. The attachment of the strain gauge should be firm and well-fitting to avoid loosening or falling off during the experiment, which would affect the accuracy of the strain data acquisition.

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

[0019] Preferably, in step S3, a pressure-resistant container is selected as the high-pressure container. After the strain gauge is attached to the selected area on the sample surface, the sample is placed in the pressure-resistant container. 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, until the pressure is gradually increased to 5.0 MPa.

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

[0021] (1) Coal matrix deformation control equation

[0022] Based on porosity elasticity, the constitutive relation for coal seam deformation is expressed as:

[0023]

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

[0025]

[0026] Where ε v =ε 11 +ε 22 +ε 33 It is the volumetric strain of the coal matrix. The average compressive stress and the effective stress are σ. eij =σ ij +αp m δ ij Therefore, the linear strain in the three stress directions is expressed as:

[0027]

[0028] (2) Gas transport control equations

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

[0030] J1 = Dc m (ρ m -ρ f (4)

[0031] In the formula, J1 is the gas diffusion flux per unit volume of coal matrix, kg / (m³). 3 ·s); D is the diffusion coefficient, m 2 / s;c m m is the shape factor of the coal matrix. -2;ρ m The density of gas in the coal matrix is ​​kg / m³. 3 ;ρ f The density of gas in the fracture is kg / m³. 3 The gas density in the fractures and coal matrix is ​​expressed as:

[0032]

[0033] Where: M g p is the molecular weight of the gas. f The pressure represents the pore pressure of the fractured 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 equation for gas diffusion and transport in the coal matrix, derived from the law of conservation of mass, is:

[0035]

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

[0037] 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 Represented as:

[0042]

[0043] Where L x L y The crack spacing;

[0044] Simultaneously removed by formula (8): get:

[0045]

[0046] Continue to simplify:

[0047]

[0048] make but:

[0049]

[0050] Under a specific pressure condition, pf It is a constant; from formula (12), the gas pressure p inside the coal matrix is ​​obtained. m for:

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

[0052] The solution yields:

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

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

[0055] Assume time 0, p m =p m0 From formula (14), we get:

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

[0057] In the formula: p m0 The initial value of the pore pressure of the coal matrix system under initial conditions;

[0058] Therefore, the gas pressure p inside the coal matrix m Represented as:

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

[0060] Solve for two conditions: the initial time t = 0, then p m =p m0 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] In the formula: 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 the two are compared and analyzed.

[0064] Preferably, after the experiment, statistical methods are used to process and analyze the collected strain data and the inferred diffusion coefficient to ensure the reliability and accuracy of the experimental results. Simultaneously, the experimental data are compared with previous similar experimental data to further verify the superiority and innovation of this testing method.

[0065] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0066] 1. Existing technologies require destroying the original pore structure of coal samples, and the samples do not have the sorting properties of coal components due to differences in the mechanical properties of different components. Furthermore, the adsorption / desorption process is too time-consuming. This method can directly use raw coal samples, preserve the original pore structure, and improve the authenticity of the test.

[0067] 2. Existing technologies cannot avoid macroscopic crack interference and can only test a single direction at a time, requiring high precision. This method achieves anisotropic analysis of transmission capacity in the vertical and horizontal directions through three-dimensional strain monitoring.

[0068] 3. Existing models mainly target coal particles and lack anisotropic diffusion models of free gas suitable for raw coal sample matrices. The diffusion model constructed based on dynamic strain data in this method is applicable to the complex pore and fracture system of raw coal samples. Attached Figure Description

[0069] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0070] Figure 1 The sample strain gauge arrangement diagram and sample crack region scanning diagram are provided for embodiments of the present invention;

[0071] Figure 2 This is a schematic diagram of the experimental apparatus provided in an embodiment of the present invention;

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

[0073] Figure 4 This is a diagram showing the strain evolution during the helium injection process provided in an embodiment of the present invention.

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

[0075] Figure 6 This is a comparison chart of strain simulation results and experimental results provided in the embodiments of the present invention;

[0076] Figure 7 The diagram shows the simulation results of the dynamic strain curve provided in the embodiment of the present invention. Detailed Implementation

[0077] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. Of course, the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0078] This embodiment provides a quantitative method for evaluating the free gas conductivity in shale and coal matrix. This method is applicable to both shale and coal matrix. This embodiment takes coal matrix as an example, and 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 No. 3 coal seam of Zhaozhuang Coal Mine in the Jincheng Mining Area of ​​the southern Qinshui Basin. Industrial analysis results showed a moisture content of 0.81%, ash content of 9.53%, volatile matter of 14.68%, and fixed carbon of 74.98%. Irregularly shaped large samples taken from the coal mine were cut into rectangular samples, and the development characteristics of the coal sample's pore and fracture system were obtained through X-ray CT imaging and high-pressure mercury intrusion porosimetry (MICP). The sample's height was 3.23 cm, length was 3.99 cm, and width was 3.22 cm. The samples were vacuum-dried at 80℃ for more than 72 hours, and repeatedly weighed until the sample mass remained stable to eliminate the influence of moisture on the experimental results.

[0082] 1.2 Experimental Methods and Procedures

[0083] 1.2.1 Characterization of the pore and fracture system:

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

[0085] High-pressure mercury intrusion spectroscopy involves injecting mercury into the pores of a sample for analysis to study the pore size distribution of coal samples. (Approximately 1×1×1 cm) 3The pore development characteristics of cubic samples were determined by high-pressure mercury intrusion porosimetry (MICP). The geometric characteristics of porosity distribution and connectivity determine the diffusion coefficient of coal. This MICP experiment used a Micromeritics AutoPore IV 9510 instrument. The samples were vacuum-dried at 60℃ for 72 hours to remove the influence of moisture before measurement. 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 formula.

[0086] 1.2.2 Dynamic Strain Experimental Apparatus and Procedure

[0087] First, strain gauges are attached to areas of the sample surface where open fractures are not well developed, in order to monitor the linear strain of the coal matrix portion of the sample (see strain gauge arrangement diagram). Figure 1 Helium was then injected into the coal sample under free expansion conditions, and the differences in the sample's diffusion capacity were analyzed by monitoring the dynamic evolution of strain at different locations in the sample.

[0088] The experimental setup in this embodiment uses a device for measuring the Biot coefficient of a dual-porosity coal matrix system, as disclosed in Chinese Invention Patent Publication No. CN115711798B. Its structure is as follows: Figure 2 As shown in the figure. 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 coal deformation, in order to study the relationship between sample deformation and dynamic gas injection. Subsequently, the entire sample was placed in a pressure vessel. After the system temperature reached equilibrium, helium gas 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, helium gas was gradually increased from 1.0 MPa to 2.0 MPa at a rate of 0.02 MPa / s, and so on, until the pressure was gradually increased to 5.0 MPa. The temperature in the constant-temperature chamber was maintained at 35 ± 0.1℃ during the experiment.

[0089] 2 Experimental Results

[0090] 2.1 Description of the pore fracture system

[0091] The results of the high-pressure mercury intrusion method experiment (see...) Figure 3 As the applied pressure increased, a large amount of mercury entered the interconnected pores of the coal sample, which mainly included mesopores and macropores. From the pore volume distribution of the coal sample, the pore volume distribution density function generally increased with decreasing pore size, with the increase starting rapidly when the pore size was <100 nm, reaching a maximum at around 3 nm. Pores with an average throat size of less than 10 nm accounted for more than 50% of the total volume.

[0092] Figure 1The distribution characteristics of fractures in the sample are shown. It can be seen that the open fractures only develop one large fracture and several small fractures in the Z-axis direction. In contrast, the filled fractures are widely developed inside the sample, forming a complex network structure. From the perspective of the field scale and considering the differences in their seepage capacity, this study treats the open fractures as the fracture system and the filled part as part of the coal sample matrix system.

[0093] 2.2 Dynamic Strain Test Results

[0094] Figure 4 The strain variation over time during helium injection is shown. Under each gas pressure condition, the strain evolution curve exhibits a characteristic of initially decreasing rapidly, then slowly rebounding until equilibrium is reached. Initially, after helium injection, the coal sample is rapidly compressed to its minimum value as the pressure increases rapidly. 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 in the horizontal direction, while the difference between the two horizontal directions is relatively small. The lowest strain observed in the experiment ranges from approximately -12 με to -352 με.

[0095] 3. Analysis and Discussion

[0096] 3.1 Model Construction

[0097] A fully coupled numerical model is constructed here to analyze the mechanism of strain evolution caused by the migration of free gas within the coal matrix. Using the constructed fluid-structure interaction model, the spatial distribution characteristics of the sample strain are simulated, the dynamic coupling process of the fluid-structure interaction field within the coal matrix is ​​explored, and the dynamic evolution of the dynamic linear strain of the coal matrix from initial equilibrium to final equilibrium is fitted. Based on this, the differences in the effective diffusion coefficient of free gas in different directions within the coal matrix of the coal sample are calculated.

[0098] (1) Coal matrix deformation control equation

[0099] Based on porosity elasticity, the constitutive relation for coal seam deformation is expressed as:

[0100]

[0101] Where ε ij It is a component of the total strain tensor, σ ij The components of the total stress tensor are G = E / 2(1+ν), K = E / 3(1-2ν), where G is the shear modulus, ν is Poisson's ratio, and σ is the shear modulus. kk =σ 11 +σ 22 +σ 33 α is the Biot coefficient, K is the bulk modulus, E is Young's modulus, and p m It is the pore pressure of the coal matrix system, δ ijIt is a Kronecker symbol; from formula (1), the mass strain of the coal matrix is:

[0102]

[0103] Where ε v =ε 11 +ε 22 +ε 33 It is the volumetric strain of the coal matrix. The average compressive stress and the effective stress are σ. eij =σ ij +αp m δ ij Therefore, the linear strain in the three stress directions can be expressed as:

[0104]

[0105] (2) Gas transport control equations

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

[0107]

[0108] In the formula, J1 is the gas diffusion flux per unit volume of coal matrix, kg / (m³). 3 ·s); D is the diffusion coefficient, m 2 / s;c m m is the shape factor of the coal matrix. -2 ;ρ m The density of gas in the coal matrix is ​​kg / m³. 3 ;ρ f The density of gas in the fracture is kg / m³. 3 .

[0109]

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

[0111]

[0112] 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 represented as:

[0117]

[0118] Where L x L y This represents the crack spacing.

[0119] 3.2 Analysis and Discussion of Simulation Results

[0120] like Figure 2 As shown, the sample is placed inside a high-pressure vessel. The attached strain gauges prevent direct gas exchange with the matrix, while the area around the strain gauges is in direct contact with the gas in the high-pressure vessel. Therefore, the area near the strain gauges is considered a fracture, and the strain gauges themselves are treated as part of the matrix. Thus, gas is injected from the perimeter of the model (e.g., Figure 5 As shown, the coal matrix controlled by the strain gauge is considered as a fractured region, and the mass exchange between the coal matrix and the fractures occurs at the connecting boundary. The strain gauge size 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. Taking He injection as an example, the evolution of the overall strain inside the coal matrix of the coal sample under experimental test conditions was simulated.

[0121] The applied confining pressure and pore pressure gradually increased to a constant value (1.0 MPa) over 20 seconds. The model was solved using the COMSOL multiphysics coupled solver, and the mesh generation during the solution process was as follows: Figure 5 As shown in Table 1, the relevant parameter values ​​are as follows.

[0122] Table 1

[0123]

[0124]

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

[0126] 3.3 Effective diffusion coefficient model of coal matrix based on dynamic strain;

[0127] Simultaneously removed by formula (8): get:

[0128]

[0129] Continue to simplify:

[0130]

[0131] make but:

[0132]

[0133] Under a specific pressure condition, p f It is a constant. From formula (12), the gas pressure p inside the coal matrix can be obtained. m for:

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

[0135] The solution yields:

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

[0137] Assume time 0, p m =p m0 From formula (14), we can obtain:

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

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

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

[0141] Solve for two conditions: the initial time t = 0, then p m =p m0 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] In the formula: 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 above formula establishes the relationship between strain and diffusion coefficient. 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 experimental dynamic strain results are fitted, as shown below. Figure 7 As 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 within the protection scope of the present invention.

Claims

1. A method for quantitatively evaluating the free gas conductivity within shale and coal matrix, characterized in that, Includes the following steps: S1. Sample preparation: Select blocky coal samples to make regular-shaped raw coal samples, perform water removal pretreatment on the samples to eliminate the influence of moisture, and comprehensively characterize the pore and fracture system of the samples. S2. Monitoring device arrangement: 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, and 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 container in a controlled temperature environment and inject helium gas at a certain rate gradient to the preset pressure. During the pressurization process, use a pressure monitoring system to monitor the pressure change in real time, record the strain data in each direction simultaneously, and control the experimental environment temperature 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 For linear strain in the stress direction, ε 22 For the stress direction, the strain is ε. 33 Let K1 be the strain in the stress direction 3, K1 be the bulk modulus in the stress direction 1, and p be the strain in the stress direction 3. m0 This represents the initial pore pressure in the matrix. Let be the compressive stress in stress direction 1, t be the diffusion time, and D1 be the diffusion coefficient in stress direction 1; c m Where φ is the shape factor, φ is the porosity of the matrix system, and p f Pore ​​pressure in the fractured system; By using this anisotropic diffusion model to inversely deduce the effective diffusion coefficient of the coal matrix in different directions, the anisotropic characteristics of the free gas conduction capacity can be analyzed.

2. The method according to claim 1, characterized in that, In step S1, the sample size is 5cm×5cm×5cm. During the dehydration pretreatment, the vacuum drying temperature is 80℃ 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 intrusion porosimetry are used to characterize the pore fracture system. The resolution of the X-CT scan is not less than 1 μm to ensure that the pore fracture distribution inside the sample can be clearly and accurately identified. High-pressure mercury intrusion porosimetry involves injecting mercury into the pores of the sample for analysis to study the pore size distribution of the coal sample and obtain accurate pore fracture structure parameters.

4. The method according to claim 1, characterized in that, In step S2, when attaching the strain gauge, the strain gauge is attached to an area on the sample surface where open cracks are not developed, in order to monitor the linear strain of the coal matrix in the sample. The attachment of the strain gauge should be firm and well-fitted to avoid loosening or falling off during the experiment, so as not to affect the accuracy of the strain data acquisition.

5. The method according to claim 1, characterized in that, In step S3, the experiment is conducted in a constant temperature chamber to control the experimental environment temperature. The temperature in the constant temperature chamber is maintained at 35±0.1℃. By monitoring the dynamic evolution of strain at different parts of the sample, the differences in the sample's diffusion capacity are analyzed.

6. The method according to claim 1, characterized in that, In step S3, a pressure-resistant container is selected as the high-pressure vessel. After the strain gauge is attached to the selected area on the sample surface, the sample is placed in the pressure-resistant container. 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, until 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 as follows: (1) Coal matrix deformation control equation Based on porosity elasticity, the constitutive relation for coal seam deformation is expressed as: Where ε ij It is a component of the total strain tensor, σ ij The components of the total stress tensor are G = E / 2(1+ν), K = E / 3(1-2ν), where G is the shear modulus, ν is Poisson's ratio, and σ is the shear modulus. kk =σ 11 +σ 22 +σ 33 α is the Biot coefficient, K is the bulk modulus, E is Young's modulus, and p m It is the pore pressure of the coal matrix system, δ ij It is a Kronecker symbol; from formula (1), the mass strain of the coal matrix is: Where ε v =ε 11 +ε 22 +ε 33 It is the volumetric strain of the coal matrix. The average compressive stress and the effective stress are σ. eij =σ ij +αp m δ ij Therefore, the linear strain in the three stress directions is expressed as: (2) Gas transport control equations After helium injection, the gas pressure in the coal matrix will be lower than the gas pressure in the fractures, allowing the gas in the fractures to diffuse into the coal matrix system. J1=Dc m (r m -r f ) (4) In the formula, J1 is the gas diffusion flux per unit volume of coal matrix, kg / (m³). 3 ·s); D is the diffusion coefficient, m 2 / s;c m m is the shape factor of the coal matrix. -2 ; ρ m The density of gas in the coal matrix is ​​kg / m³. 3 ; ρ f The density of gas in the fracture is kg / m³. 3 The gas density in the fractures and coal matrix is ​​expressed as: Where: M g p is the molecular weight of the gas. f The pressure represents the pore pressure of the fractured 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 equation for gas diffusion and transport in the coal matrix, derived from the law of conservation of mass, is: Where: m is the mass of free gas per unit volume of coal matrix, and t is the diffusion time; Mass of helium stored per unit volume of coal matrix: In the formula: φ Porosity of the matrix system; Substituting formulas (5)-(7) into formula (4) yields: Shape factor c m Represented as: Where L x L y The crack spacing; Simultaneously removed by formula (8): get: Continue to simplify: make but: Under a specific pressure condition, p f It is a constant; from formula (12), the gas pressure p inside the coal matrix is ​​obtained. m for: p m =What -∫(a)dt +e -∫(a)dt No f (it ∫(a)dt )dt (13) The solution yields: p m =What -at +p f (14) In the formula: c is a constant determined by the initial conditions, and a is a coefficient; Assume time 0, p m =p m0 From formula (14), we get: C=p m0 -p f (15) In the formula: p m0 The initial value of the pore pressure of the coal matrix system under initial conditions; Therefore, the gas pressure p inside the coal matrix m Represented as: p m =(p m0 -p f )e -at +p f (16) Solve for two conditions: the initial time t = 0, then p m =p m0 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: In the formula: 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 the anisotropic characteristics, the effective diffusion coefficients in the vertical and horizontal directions are calculated separately, 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 and analyze the collected strain data and the back-derived diffusion coefficient 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.

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