A method for detecting gas permeability of micro-nano cross-scale pores of deep coal seams

Abstract

The application provides a kind of deep coal seam micro-nano cross-scale pore gas permeability detection method, belongs to deep coal seam pore gas permeability detection technical field;The technical problem to be solved is to provide a kind of deep coal seam micro-nano cross-scale pore gas permeability detection method;The technical scheme adopted is: analyzing the gas transmission mechanism of micro-nano scale pore, respectively establishing: surface diffusion transmission mechanism, Knudsen diffusion transmission mechanism, slip or viscous flow transmission mechanism calculation model;And respectively establish the weighting factor of Knudsen diffusion transmission mechanism, slip or viscous flow transmission mechanism;Establish the additional pressure drop calculation model caused by end effect, finally through coupling weighting factor, end effect, establish the calculation model of deep coal seam micro-nano scale pore gas permeability;The application is applied to detect deep coal seam pore gas permeability.

Description

Technical Field

[0001] This invention provides a method for detecting micro-nano cross-scale pore gas permeability in deep coal seams, belonging to the field of pore gas permeability detection technology in deep coal seams. Background Technology

[0002] Deep coalbed methane (DBM) is a clean energy resource, and deep coal seams are ideal locations for geological carbon dioxide sequestration. Unlike conventional natural gas reservoirs, deep coal seams are rich in nanoscale pores, with micropores smaller than 2 nm accounting for 78% of the volume, mesopores between 2 nm and 50 nm accounting for 6.8%, and macropores between 50 nm and 10 μm accounting for 2.1%. For the exploitation of deep coal seams, fracturing is required to construct a high-permeability flow channel between the matrix, fractures, and wellbore to achieve production capacity. Specifically, after fracturing of the deep coal seam, during gas migration, the matrix nanopores desorb into free gas, which then seeps into the wellbore through microfractures and artificial fractures and is extracted. Therefore, the gas migration and transport mechanism in deep coal seams spans both nanoscale and microscale pores. Detecting the permeability characteristics of gas in the micro- and nanoscale pores of deep coal seams is of great significance for the efficient development of deep coalbed methane.

[0003] Current research has been conducted on gas transport processes in nanoscale pores, but the detection and calculation models for cross-scale transport processes are still in their infancy. Because the gas transport mechanism in nanoscale pores is affected by the nanoscale confinement effect, it exhibits unconventional characteristics, and classical fluid dynamics theory is no longer applicable. However, the gas flow and transport characteristics in micrometer-scale pores are more consistent with the calculation results of classical fluid dynamics theory. Since the gas flow patterns are different in pores of different scales, after clarifying the gas flow patterns in micro and nanoscale cross-scale pores, the correct pore gas permeability can be detected. Summary of the Invention

[0004] In order to overcome the shortcomings of the prior art, the present invention aims to solve the technical problem of providing a method for detecting gas permeability in micro-nano cross-scale pores of deep coal seams.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a method for detecting gas permeability in micro-nano cross-scale pores of deep coal seams, comprising the following detection steps:

[0006] Step S1: Analyze the gas transport mechanism in micro- and nano-scale pores, and establish computational models for surface diffusion transport mechanism, Knudsen diffusion transport mechanism, and slip or viscous flow transport mechanism respectively.

[0007] Step S2: Based on the theory of intermolecular collisions, establish the weighting factors for the Knudsen diffusion transport mechanism and the slip or viscous flow transport mechanism, respectively.

[0008] Step S3: Establish a calculation model for the additional pressure drop caused by the end effect, and at the same time establish calculation models to characterize stress deformation, adsorption expansion, real gas effect, and nanoconfinement effect respectively;

[0009] Step S4: Establish a calculation model for gas permeability measurement in deep coal seams at the micro-nano scale by coupling weighting factors and end effects, and detect the gas permeability of deep coal seams through the calculation model.

[0010] The specific method for establishing the surface diffusion transport mechanism calculation model in step S1 is as follows:

[0011] The surface diffusion transport mechanism is specifically the flow mechanism of adsorbed gas near the wall surface;

[0012] The mass flux Q of the surface diffusion transport mechanism surf The calculation formula is:

[0013]

[0014] In the formula, D s θ represents the surface diffusion coefficient, C represents the adsorbed gas concentration, p represents the pressure, θ represents the adsorbed gas coverage on the solid surface, M represents the molar mass of the gas molecule, and d represents the surface diffusion coefficient. m N represents the diameter of gas molecules. A denoted by Avogadro's constant, T represents temperature, ΔH represents the enthalpy of adsorption, κ represents the discrimination parameter for diffusion at the surface of gas molecules, and Rg represents the gas constant;

[0015] The specific method for establishing the computational model of the Knudsen diffusion transport mechanism in step S1 is as follows:

[0016] The Knudsen diffusion transport mechanism is the flow mechanism of free gas in the central region of the pores;

[0017] The mass flow rate Q of the Knudsen diffusion transport mechanism Kn The calculation formula is:

[0018]

[0019] In the formula, Z represents the real gas factor;

[0020] The specific method for establishing the calculation model of the slip or viscous flow transport mechanism in step S1 is as follows:

[0021] The slip or viscous flow transport mechanism is the flow mechanism of free gas in the central region of the pores;

[0022] The mass flow rate Q of the slip or viscous flow transport mechanism slipThe calculation formula is:

[0023]

[0024] In the formula, ρ represents gas density, R represents pore radius, μ represents viscosity, Kn represents Knudsen number, and α represents tangential momentum adjustment coefficient, which varies in the range of 0 to 1.

[0025] The specific method for establishing the weighting factors for the Knudsen diffusion transport mechanism, slip transport mechanism, or viscous flow transport mechanism in step S2 is as follows:

[0026] Define the free gas mass flow rate Q in the pore center region. free The weighted superposition of the Knudsen diffusion transport mechanism and the slip or viscous flow transport mechanism is calculated as follows:

[0027] Q free =π(R-δ) 2 (ω vis Q slip +ω Kn Q Kn (4);

[0028] In the formula, δ represents the thickness of the adsorption layer; ω vis ω Kn These are the weighting factors for the Knudsen diffusion transport mechanism and the slip or viscous flow transport mechanism, respectively.

[0029] The weighting factor represents the weight of the two transport mechanisms, specifically determined by the ratio of the number of gas molecule collisions and the number of gas molecule-wall collisions to the total number of collisions. The weighting factor is calculated as follows:

[0030] Choose any element area dxdy on the pore wall and introduce a coordinate system with the z-axis perpendicular to the element area;

[0031] Calculate the interaction between a unit area of ​​wall surface and gas molecules with velocity components within a given time interval dv. x dv y dv z Number of collisions dN g-w The calculation formula is:

[0032]

[0033] In the formula, n represents the number density of molecules; m represents the mass of a single molecule;

[0034] This gives us the total number of collisions N between gas molecules and the wall per unit area per unit time. g-w The calculation formula is:

[0035]

[0036] Let v' be the relative velocity of a gas molecule relative to other gas molecules. Define the relative kinetic energy of two gas molecules with masses m1 and m2 as v', where all gas molecules have the same mass. 2 (m1m2 / (m1+m2)) / 2, calculates the number of gas molecules per unit volume whose relative velocities are between v' and v'+dv'. The formula is:

[0037]

[0038] The formula for calculating the number of collisions a gas molecule undergoes with other gas molecules per unit time is:

[0039]

[0040] Therefore, the total number of collisions between gas molecules per unit volume and per unit time can be calculated using the following formula:

[0041]

[0042] The formula for calculating the total number of collisions of gas molecules per unit length of pore is as follows:

[0043]

[0044] In the formula, A represents the wall area per unit length of pore; V represents the spatial volume per unit length of pore.

[0045] The weighting factor for the Knudsen diffusion transport mechanism is the ratio of the number of collisions between gas molecules and the wall to the total number of collisions per unit time. The weighting factor for the slip or viscous flow transport mechanism is the ratio of the number of collisions between gas molecules to the total number of collisions per unit time. Therefore, the formulas for calculating the weighting factors for these two transport mechanisms are as follows:

[0046]

[0047] The specific method for establishing the calculation model of the additional pressure drop caused by the terminal effect in step S3 is as follows:

[0048] The additional pressure drop caused by the terminal effect is calculated using the following formula:

[0049]

[0050] Since an additional pressure drop also occurs at the inlet section of the limited channel, the additional pressure drop for gas transport within the pores is 2Δp. end ;

[0051] Define the channel length as L and the inlet and outlet pressures as p.in p out Then, when calculating the gas flow rate, the pore pressure gradient is (p in -p out ) / L;

[0052] However, due to the capillary end effect, the pressure gradient becomes:

[0053] Δp pore =Δp-2Δp end (13);

[0054] In the formula, Δp pore This indicates that the pore pressure drop at the end is not considered;

[0055] The terminal effect influence factor ξ is obtained here, and the calculation formula is:

[0056]

[0057] The specific method for establishing the computational model characterizing stress deformation in step S3 is as follows:

[0058] Define the aperture as R under stress-free conditions. initial If the aperture is R0 after the effective stress is applied, then the formula for calculating stress deformation is:

[0059]

[0060] In the formula, σ represents the effective stress, v represents Poisson's ratio, E represents Young's modulus, and φ0 represents the initial porosity of the coal body.

[0061] The specific method for establishing the computational model characterizing adsorption expansion in step S3 is as follows:

[0062] The formula for calculating the change in pore size under adsorption expansion is as follows:

[0063]

[0064] Therefore, the pore radius under in-situ coal seam conditions is obtained, and the calculation formula is:

[0065] R = R0 - R sw (17);

[0066] The specific method for establishing the computational model characterizing the real gas effect in step S3 is as follows:

[0067] The gas bias factor, which characterizes the real gas effect, is calculated using the following formula:

[0068]

[0069] In the formula, P r P represents relative pressure.c T represents the critical pressure. r T represents relative temperature. c Indicates the critical temperature;

[0070] The specific method for establishing the computational model characterizing the nanoconfinement effect in step S3 is as follows:

[0071] Considering the confinement effect of nanopores, the critical pressure and critical temperature of the gas are calculated using the following formulas:

[0072]

[0073] In the formula, T c0 denoted as the critical temperature without considering confinement effects, d represents the pore diameter, and a, b, and c represent the parameters of the gas equation of state, respectively.

[0074] The specific method for establishing the calculation model for micro-nano scale pore gas permeability of deep coal seams in step S4 is as follows:

[0075] Considering the effects of capillary effect, real gas effect, nano-confinement effect, stress effect, and adsorption expansion factor, the calculation model for micro / nano-scale pore gas permeability is obtained as follows:

[0076]

[0077] For deep coal seams, correction factors for the effects of porosity φ and tortuosity τ should also be considered. Therefore, the formula for calculating the gas permeability of deep coal seams is:

[0078]

[0079] The advantages of this invention compared to existing technologies are as follows: This invention detects the gas permeability of micro- and nano-scale pores in deep coal seams. By analyzing the gas transport mechanisms at the micro- and nano-scale pores, it provides calculation models for surface diffusion transport, Knudsen diffusion transport, and slip / viscous flow transport mechanisms. Based on the theory of intermolecular collisions, it establishes weighting factors for Knudsen diffusion and slip / viscous flow transport mechanisms, provides a calculation model for the additional pressure drop caused by the end effect, and characterization equations for factors such as adsorption expansion, stress deformation, nano-confinement effect, and real gas effect. Finally, by coupling weighting factors and end effects, it establishes a calculation model for the gas permeability of micro- and nano-scale pores in deep coal seams that considers the end effect. The gas permeability calculation model established by this invention considers factors such as the end effect and establishes a method for calculating the gas permeability of micro- and nano-scale pores in deep coal seams. This provides a more accurate permeability calculation method for low-carbon natural gas extraction technology in deep coal seams and facilitates the theoretical development and technological optimization of low-carbon extraction of unconventional natural gas. Attached Figure Description

[0080] The present invention will be further described below with reference to the accompanying drawings:

[0081] Figure 1 This is a flowchart of the detection method of the present invention;

[0082] Figure 2 This is a schematic diagram of gas molecule collisions in the pores of a deep coal seam in an embodiment of the present invention;

[0083] Figure 3 This is a graph showing the apparent permeability and flow rate of gas in the micro-nano scale pores of deep coal seams in an embodiment of the present invention.

[0084] Figure 4 This is a graph showing the variation of apparent gas permeability in deep coal seams with pressure in an embodiment of the present invention.

[0085] Figure 5 This is a graph showing the variation of apparent gas permeability in the micron-sized pores of a deep coal seam with pressure, as described in an embodiment of the present invention. Detailed Implementation

[0086] like Figure 1 As shown, this invention provides a method for detecting the gas permeability of micro-nano cross-scale pores in deep coal seams. Specifically, it involves a calculation scheme for the gas permeability of deep coal seam pores considering the end effect, solving the problems existing in the theory of gas cross-scale seepage in micro-nano pores under in-situ conditions in deep coal seams. The detection method mainly includes:

[0087] This paper analyzes the gas transport mechanism in micro- and nano-scale pores, and presents computational models for surface diffusion transport, Knudsen diffusion transport, and slip or viscous flow transport. Based on the theory of intermolecular collisions, weighting factors for Knudsen diffusion and slip or viscous flow transport are established. The paper also provides calculation formulas for the additional pressure drop caused by the end effect, as well as characterization equations for factors such as adsorption expansion, stress deformation, nano-confinement effect, and real gas effect. Finally, a calculation model for gas permeability in deep coal seams at the micro- and nano-scale pores is established by coupling weighting factors and end effects. This gas permeability calculation model, considering end effects and other factors, establishes a method suitable for calculating the permeability of gas in micro- and nano-scale pores in deep coal seams. This provides a more accurate permeability calculation method for low-carbon natural gas extraction technology in deep coal seams and facilitates the theoretical development and technological optimization of unconventional natural gas low-carbon extraction.

[0088] Furthermore, the method for detecting gas permeability in micro-nano cross-scale pores of deep coal seams provided by the present invention specifically includes the following steps:

[0089] Step S1: Investigate the gas transport mechanism:

[0090] Gas transport mechanisms in micro / nanopores include three types: surface diffusion, Knudsen diffusion, and slip / viscous flow, among which:

[0091] Surface diffusion is the flow mechanism of adsorbed gas near a wall surface; the mass flux of surface diffusion is Q. surf The governing equations are expressed as follows:

[0092]

[0093] In the formula, D s The surface diffusion coefficient, m 2 ·s -1 C represents the concentration of the adsorbed gas; p represents the pressure (Pa); θ represents the coverage of the adsorbed gas on the solid surface; M represents the molar mass of the gas molecule (kg·mol). -1 ;d m The diameter of a gas molecule is expressed in meters (m); N A This represents Avogadro's constant, 6.02214076 × 10⁻⁶. 23 mol -1 T represents temperature, K; ΔH represents adsorption enthalpy, J·mol⁻¹ -1 κ represents the discriminant parameter for diffusion across the surface of gas molecules, and Rg represents the gas constant, 8.314 J·mol⁻¹. -1 ·K -1 .

[0094] The Knudsen diffusion transport mechanism and the slip flow transport mechanism are the flow mechanisms of free gas in the pore center region, and the mass flow rate Q of slip flow is... slip The governing equations can be expressed as:

[0095]

[0096] In the formula, ρ represents the gas density, kg·m³. -3 R represents the pore radius, in meters; μ represents the viscosity, in Pa·s; Kn represents the Knudsen number; α represents the tangential momentum adjustment coefficient, which varies from 0 to 1.

[0097] The mass flow control equation of Knudsen's diffusion transport mechanism is Q. Kn for:

[0098]

[0099] In the formula, Z represents the real gas factor.

[0100] Step S2: Study the weighting factors:

[0101] The free gas mass flow rate Q in the pore center region freeA weighted superposition of the Knudsen diffusion transport mechanism and the slip flow transport mechanism:

[0102] Q free =π(R-δ) 2 (ω vis Q slip +ω Kn Q Kn (4);

[0103] In the formula, δ represents the thickness of the adsorption layer, in meters (m); ω vis ω Kn These are the weighting factors for the Knudsen diffusion transport mechanism and the slip flow transport mechanism, respectively.

[0104] The weighting factor represents the weight of the two gas flow mechanisms and should be derived from the physical mechanisms of the number of collisions between gas molecules and the wall per unit length of pore space per unit time. The contributions of the Knudsen diffusion transport mechanism and the slip flow transport mechanism to the total free gas flow rate can be determined by the ratio of the number of gas molecule collisions and the number of gas molecule-wall collisions to the total number of collisions. The collision states of gas molecules in the pore are as follows: Figure 2 As shown.

[0105] This invention arbitrarily selects a unit area dxdy on the pore wall and introduces a coordinate system with the z-axis perpendicular to the unit area. Among numerous gas molecules, only those molecules that can reach the wall within a unit time, i.e., those that collide with the wall, have a coordinate z that is no greater than their velocity component v along the z-axis. x Therefore, the interaction between a unit area of ​​the wall and gas molecules with velocity components at a given interval dv per unit time is... x dv y dv z Number of collisions dN g-w for:

[0106]

[0107] In the formula, n represents the molecular number density, m -3 m represents the mass of a single molecule, in kg.

[0108] Therefore, N is the total number of collisions N between gas molecules and the wall per unit area per unit time. g-w for:

[0109]

[0110] This invention selects a gas molecule and analyzes the motion of all other molecules relative to this molecule. That is, for each gas molecule, its absolute velocity v is not considered, but its relative velocity v' with respect to other molecules is considered; the relative kinetic energy of two gas molecules with mass m1 and mass m2 is: v' 2 (m1m2 / (m1+m2)) / 2; Considering that all molecules have the same mass, the number of gas molecules per unit volume whose relative velocities are between v' and v'+dv' is:

[0111]

[0112] Molecular collisions involve various processes, such as head-on collisions and collisions at an angle. These processes during collisions are typically characterized by an effective cross-section, which can be described by the projected area of ​​a spherical molecule. Therefore, the number of collisions a gas molecule undergoes per unit time is:

[0113]

[0114] Therefore, the total number of collisions between gas molecules per unit volume and per unit time is:

[0115]

[0116] The total number of collisions of gas molecules per unit length of pore (including collisions between gas molecules and collisions with the pore wall) is:

[0117]

[0118] In the above formula, A represents the wall area per unit length of pore; V represents the spatial volume per unit length of pore.

[0119] The flow resistance in the Knudsen diffusion transport mechanism is the collision between gas molecules and the solid wall, while the flow resistance in slip flow is the collision between gas molecules. The free gas flow in the pores is a coupling process of Knudsen diffusion and slip flow. Therefore, the weighting factor for the Knudsen diffusion transport mechanism is the ratio of the number of collisions between gas molecules and the wall to the total number of collisions per unit time, and the weighting factor for the slip flow transport mechanism is the ratio of the number of collisions between gas molecules to the total number of collisions per unit time. Therefore, the expression for the weighting factor is:

[0120]

[0121] Step S3: Investigate terminal effects and other key factors:

[0122] (1) Studying terminal effects:

[0123] For gas flow in finite-length pores, the apparent permeability due to the capillary end effect is lower than that in infinite-length pores. This is because the streamline bending at the channel end generates more viscous dissipation and additional hydrodynamic resistance. It's important to note that the streamlines diffusing from the adsorbed layer surface are influenced by and close to the solid wall; therefore, the capillary end effect only affects the free gas flow rate. The additional pressure drop caused by the end effect can be expressed as:

[0124]

[0125] An additional pressure drop also occurs at the inlet section of the limited channel; therefore, the additional pressure drop for gas transport within the pores is 2Δp. end Assume the channel length is L and the inlet and outlet pressures are p. in p out Then, when calculating the gas flow rate, the pore pressure gradient is (p in -p out ) / L, but under the capillary end effect, the pressure gradient becomes:

[0126] Δp pore =Δp-2Δp end (13);

[0127] In the formula, Δp pore This indicates that the pore pressure drop at the end is not considered.

[0128] Here, the terminal effect influencing factor ξ is proposed:

[0129]

[0130] (2) Study other key factors:

[0131] Stress effects and adsorption expansion significantly influence the permeability of deep coal pores by altering pore size. Let the pore size under stress-free conditions be R. initial After effective stress is applied, the aperture is R0, and the stress effect can be expressed as:

[0132]

[0133] In the formula, σ represents the effective stress (MPa); v represents Poisson's ratio; E represents Young's modulus (GPa); and φ0 represents the initial porosity of the coal body.

[0134] Adsorption is a key mechanism for gas occurrence in coal seams and an important factor inducing coal seam expansion; the change in pore size under adsorption expansion is as follows:

[0135]

[0136] Therefore, the pore radius under in-situ coal seam conditions is:

[0137] R = R0 - R sw (17);

[0138] The real gas effect and the nanoconfinement effect affect the density, viscosity, and other properties of a gas, thus influencing its flow process; the gas deviation factor characterizing the real gas effect can be expressed as:

[0139]

[0140] In the formula, P r P represents relative pressure. c Indicates critical pressure, Pa; T r T represents relative temperature. c This indicates the critical temperature, expressed in K.

[0141] Considering the confinement effect of nanopores, the critical pressure and critical temperature of the gas are as follows:

[0142]

[0143] In the formula, T c0 d represents the critical temperature without considering confinement effects, in K; d represents the pore diameter, in m; a, b, and c represent the parameters of the gas equation of state, respectively.

[0144] Step S4: Obtain the calculation model for micro-nano scale pore gas permeability in deep coal seams:

[0145] The micro / nano-scale pore gas permeability calculation model, considering factors such as capillary effect, real gas effect, nano-confinement effect, stress effect, and adsorption expansion, is as follows:

[0146]

[0147] For deep coal seams, correction factors for the effects of porosity φ and tortuosity τ should also be considered. Therefore, the gas permeability of deep coal seams is expressed as:

[0148]

[0149] Completing steps S1 to S4 above can accurately provide the apparent permeability and flow rate curves of gas in the micro-nano cross-scale pores of deep coal seams, such as... Figure 3 As shown.

[0150] In this embodiment of the invention, a coalbed methane field is used as an example. The coal seam depth is 1240 meters, the pressure is 12.5 MPa, and the temperature is 63.4℃. The pressure gradient is set to 2 MPa / m. Other parameters required for permeability calculation are shown in the table below:

[0151]

[0152] Table 1 Parameter Values

[0153] Substituting the above data into equation (20), we can plot the methane permeability curves of nanopores under different pressure conditions in deep coal seams, such as... Figure 4 As shown in the figure, the vertical axis represents methane permeability.

[0154] Substituting the above data into equation (20), we can plot the methane permeability curves of micron-sized pores under different pressure conditions in deep coal seams, such as... Figure 5 As shown in the figure, the vertical axis represents methane permeability.

[0155] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for detecting gas permeability in micro-nano cross-scale pores of deep coal seams, characterized in that: The testing steps include the following: Step S1: Analyze the gas transport mechanism in micro- and nano-scale pores, and establish computational models for surface diffusion transport mechanism, Knudsen diffusion transport mechanism, and slip or viscous flow transport mechanism respectively. Step S2: Based on the theory of intermolecular collisions, establish the weighting factors for the Knudsen diffusion transport mechanism and the slip or viscous flow transport mechanism, respectively. Step S3: Establish a calculation model for the additional pressure drop caused by the end effect, and at the same time establish calculation models to characterize stress deformation, adsorption expansion, real gas effect, and nanoconfinement effect respectively; The specific method for establishing a calculation model for the additional pressure drop caused by the terminal effect is as follows: The additional pressure drop caused by the terminal effect is calculated using the following formula: Since an additional pressure drop also occurs at the inlet section of the limited channel, the additional pressure drop for gas transport within the pores is 2Δp. end ; Define the channel length as L and the inlet and outlet pressures as p. in p out Then, when calculating the gas flow rate, the pore pressure gradient is (p in -p out ) / L; however, under the capillary end effect, the pressure gradient becomes: Δp pore =Δp-2Δp end (13); In the formula, Δp pore This indicates that the pore pressure drop at the end is not considered; The terminal effect influence factor ξ is obtained here, and the calculation formula is: Step S4: Establish a calculation model for gas permeability measurement in deep coal seams at the micro-nano scale by coupling weighting factors and end effects, and detect the gas permeability of deep coal seams through the calculation model.

2. The method for detecting gas permeability in micro-nano cross-scale pores of deep coal seams according to claim 1, characterized in that: The specific method for establishing the surface diffusion transport mechanism calculation model in step S1 is as follows: The surface diffusion transport mechanism is specifically the flow mechanism of adsorbed gas near the wall surface; The mass flux Q of the surface diffusion transport mechanism surf The calculation formula is: In the formula, D s θ represents the surface diffusion coefficient, C represents the adsorbed gas concentration, p represents the pressure, θ represents the adsorbed gas coverage on the solid surface, M represents the molar mass of the gas molecule, and d represents the surface diffusion coefficient. m N represents the diameter of gas molecules. A denoted by Avogadro's constant, T represents temperature, ΔH represents the enthalpy of adsorption, κ represents the discrimination parameter for diffusion at the surface of gas molecules, and Rg represents the gas constant; The specific method for establishing the computational model of the Knudsen diffusion transport mechanism in step S1 is as follows: The Knudsen diffusion transport mechanism is the flow mechanism of free gas in the central region of the pores; The mass flow rate Q of the Knudsen diffusion transport mechanism Kn The calculation formula is: In the formula, Z represents the real gas factor; The specific method for establishing the calculation model of the slip or viscous flow transport mechanism in step S1 is as follows: The slip or viscous flow transport mechanism is the flow mechanism of free gas in the central region of the pores; The mass flow rate Q of the slip or viscous flow transport mechanism slip The calculation formula is: In the formula, ρ represents gas density, R represents pore radius, μ represents viscosity, Kn represents Knudsen number, and α represents tangential momentum adjustment coefficient, which varies in the range of 0 to 1.

3. The method for detecting gas permeability in micro-nano cross-scale pores of deep coal seams according to claim 2, characterized in that: The specific method for establishing the weighting factors for the Knudsen diffusion transport mechanism, slip transport mechanism, or viscous flow transport mechanism in step S2 is as follows: Define the free gas mass flow rate Q in the pore center region. free The weighted superposition of the Knudsen diffusion transport mechanism and the slip or viscous flow transport mechanism is calculated as follows: Q free =π(R-δ) 2 (oh vis Q slip +oh Kn Q Kn )(4); In the formula, δ represents the thickness of the adsorption layer; ω vis ω Kn These are the weighting factors for the Knudsen diffusion transport mechanism and the slip or viscous flow transport mechanism, respectively. The weighting factor represents the weight of the two transport mechanisms, specifically determined by the ratio of the number of gas molecule collisions and the number of gas molecule-wall collisions to the total number of collisions. The weighting factor is calculated as follows: Choose any element area dxdy on the pore wall and introduce a coordinate system with the z-axis perpendicular to the element area; Calculate the interaction between a unit area of ​​wall surface and gas molecules with velocity components within a given time interval dv. x dv y dv z Number of collisions dN g-w The calculation formula is: In the formula, n represents the number density of molecules; m represents the mass of a single molecule; This gives us the total number of collisions N between gas molecules and the wall per unit area per unit time. g-w The calculation formula is: Let v' be the relative velocity of a gas molecule relative to other gas molecules. Define the relative kinetic energy of two gas molecules with masses m1 and m2 as v', where all gas molecules have the same mass. 2 (m1m2 / (m1+m2)) / 2, calculates the number of gas molecules per unit volume whose relative velocities are between v' and v'+dv'. The formula is: The formula for calculating the number of collisions a gas molecule undergoes with other gas molecules per unit time is: Therefore, the total number of collisions between gas molecules per unit volume and per unit time can be calculated using the following formula: The formula for calculating the total number of collisions of gas molecules per unit length of pore is as follows: In the formula, A represents the wall area per unit length of pore; V represents the spatial volume per unit length of pore. The weighting factor for the Knudsen diffusion transport mechanism is the ratio of the number of collisions between gas molecules and the wall to the total number of collisions per unit time. The weighting factor for the slip or viscous flow transport mechanism is the ratio of the number of collisions between gas molecules to the total number of collisions per unit time. Therefore, the formulas for calculating the weighting factors for these two transport mechanisms are as follows:

4. The method for detecting gas permeability in micro-nano cross-scale pores of deep coal seams according to claim 3, characterized in that: The specific method for establishing the computational model characterizing stress deformation in step S3 is as follows: Define the aperture as R under stress-free conditions. initial If the aperture is R0 after the effective stress is applied, then the formula for calculating stress deformation is: In the formula, σ represents the effective stress, v represents Poisson's ratio, E represents Young's modulus, and φ0 represents the initial porosity of the coal body. The specific method for establishing the computational model characterizing adsorption expansion in step S3 is as follows: The formula for calculating the change in pore size under adsorption expansion is as follows: Therefore, the pore radius under in-situ coal seam conditions is obtained, and the calculation formula is: R=R0-R sw (17); The specific method for establishing the computational model characterizing the real gas effect in step S3 is as follows: The gas bias factor, which characterizes the real gas effect, is calculated using the following formula: In the formula, P r P represents relative pressure. c T represents the critical pressure. r T represents relative temperature. c Indicates the critical temperature; The specific method for establishing the computational model characterizing the nanoconfinement effect in step S3 is as follows: Considering the confinement effect of nanopores, the critical pressure and critical temperature of the gas are calculated using the following formulas: In the formula, T c0 denoted as the critical temperature without considering confinement effects, d represents the pore diameter, and a, b, and c represent the parameters of the gas equation of state, respectively.

5. The method for detecting gas permeability in micro-nano cross-scale pores of deep coal seams according to claim 4, characterized in that: The specific method for establishing the calculation model for micro-nano scale pore gas permeability of deep coal seams in step S4 is as follows: Considering the effects of capillary effect, real gas effect, nano-confinement effect, stress effect, and adsorption expansion factor, the calculation model for micro / nano-scale pore gas permeability is obtained as follows: For deep coal seams, correction factors for the effects of porosity φ and tortuosity τ should also be considered. Therefore, the formula for calculating the gas permeability of deep coal seams is:

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