Method for detecting micro-nano cross-scale pore gas permeability of deep coal seam

Abstract

The invention provides a micro-nano cross-scale pore gas permeability detection method of a deep coal seam, and belongs to the technical field of pore gas permeability detection of the deep coal seam. The technical problem to be solved is to provide the micro-nano cross-scale pore gas permeability detection method for the deep coal seam. According to the technical scheme, the method comprises the following steps: analyzing a gas transmission mechanism of micro-nano scale pores, and respectively establishing calculation models of a surface diffusion transmission mechanism, a Knudsen diffusion transmission mechanism and a slippage or viscous flow transmission mechanism; respectively establishing weighting factors of a Knudsen diffusion transmission mechanism and a slippage or viscous flow transmission mechanism; establishing a calculation model of additional pressure drop caused by the end effect, and finally establishing a calculation model of the micro-nano-scale pore gas logging permeability of the deep coal seam through the coupling weighting factor and the end effect. The method is applied to detection of the deep coal seam pore gas permeability.

Description

Technical Field

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

[0002] Deep coalbed methane resources are a clean energy source, and deep coalbeds are ideal places for geological storage of carbon dioxide. Unlike conventional natural gas reservoirs, deep coalbeds are rich in nanoscale pores, with micropores smaller than 2nm accounting for 78% of the volume, mesopores between 2nm and 50nm accounting for 6.8% of the volume, and macropores between 50nm and 10μm accounting for 2.1% of the volume. For the exploitation of deep coalbeds, it is necessary to construct a high-permeability diversion channel of matrix-crack-wellbore through fracturing transformation to have production capacity. Specifically, after the deep coalbeds are fractured, during the migration of gas, the matrix nanopores are desorbed into free gas, which seeps into the wellbore through microcracks and artificial cracks and is produced; therefore, the gas migration and transmission mechanism process in deep coalbeds spans nanoscale pores and micrometer-scale pores. Detecting the permeability characteristics of gas in micro-nano cross-scale pores in deep coalbeds is of great significance for the efficient development of deep coalbed methane.

[0003] At present, relevant research work has been carried out on the gas transport process in nanoscale pores, but the detection and calculation model for studying cross-scale migration processes is still in its infancy. Since the gas migration and transmission mechanism in nanoscale pores is affected by the nano-confinement effect and exhibits unconventional characteristics, the classical fluid mechanics theory will no longer be applicable. However, the gas flow and transmission characteristics in micron-scale pores are relatively consistent with the calculation results of classical fluid mechanics theory. Due to the different gas flow patterns in pores of different scales, the correct pore gas permeability can be detected after clarifying the gas flow state of micro-nano cross-scale pores. Summary of the invention

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

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

[0006] Step S1: Analyze the gas transport mechanism of micro-nanoscale pores, and establish the calculation models of surface diffusion transport mechanism, Knudsen diffusion transport mechanism, and slip or viscous flow transport mechanism respectively;

[0007] Step S2: According to the theory of molecular collision, the weighting factors of Knudsen diffusion transport mechanism, slip or viscous flow transport mechanism are established respectively;

[0008] Step S3: establishing a calculation model for the additional pressure drop caused by the end effect, and at the same time establishing calculation models for characterizing stress deformation, adsorption expansion, real gas effect, and nano-confinement effect;

[0009] Step S4: A calculation model for gas permeability of micro-nanoscale pores in deep coal seams is established by coupling weighting factors and terminal effects, and the gas permeability of deep coal seams is detected by the calculation model.

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

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

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

[0013]

[0014] Where D s represents the surface diffusion coefficient, C represents the adsorbed gas concentration, p represents the pressure, θ represents the solid surface adsorbed gas coverage, M represents the molar mass of the gas molecule, and d m Represents the diameter of gas molecules, N A represents Avogadro's constant, T represents temperature, ΔH represents adsorption enthalpy, κ represents the discriminant parameter of gas molecule surface diffusion, and Rg represents the gas constant;

[0015] The specific method for establishing the Knudsen diffusion transport mechanism calculation model in step S1 is:

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

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

[0018]

[0019] Where Z represents the real gas factor;

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

[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] Where ρ represents gas density, R represents pore radius, μ represents viscosity, Kn represents Knudsen number; α represents tangential momentum adjustment coefficient, and its value varies in the range of 0 to 1.

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

[0026] Define the free gas mass flow rate Q in the pore center area free It is the weighted superposition of Knudsen diffusion transport mechanism and slip or viscous flow transport mechanism, and the calculation formula is:

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

[0028] Where δ represents the thickness of the adsorption layer; ω vis ,ω Kn 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, which is 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 an arbitrary unit area dxdy on the pore wall and introduce a coordinate system with the z axis perpendicular to the unit area;

[0031] Calculate the velocity of the wall per unit area and the gas molecules with velocity components at a given interval dv per unit time. x , dv y , dv z The number of collisions within dN g-w , the calculation formula is:

[0032]

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

[0034] Thus, the total number of collisions between gas molecules and the wall per unit area per unit time is obtained. g-w , the calculation formula is:

[0035]

[0036] Analyze the relative speed of a gas molecule relative to other gas molecules as v', define all gas molecules to have the same mass, and the relative kinetic energy of two gas molecules with mass m1 and mass m2 is v' 2 (m1m2 / (m1+m2)) / 2, calculate the number of molecules in a unit volume whose relative velocity is between v' and v'+dv', the calculation formula is:

[0037]

[0038] Calculate the number of times a gas molecule collides with other gas molecules per unit time. The calculation formula is:

[0039]

[0040] The total number of collisions between gas molecules per unit volume and per unit time is calculated using the following formula:

[0041]

[0042] Calculate the total number of collisions of gas molecules per unit length of pores using the following formula:

[0043]

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

[0045] The weighting factor of 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 of 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. The calculation formulas for the weighting factors of the two transport mechanisms are:

[0046]

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

[0048] Calculate the additional pressure drop caused by the end effect using the formula:

[0049]

[0050] Since there is also an additional pressure drop at the inlet section of the finite channel, the additional pressure drop for gas transport in the pores is 2Δp end ;

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

[0052] But under the capillary end effect, the pressure gradient becomes:

[0053] Δp pore =Δp-2Δp end (13);

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

[0055] Here we get the terminal effect influence factor ξ, which is calculated as follows:

[0056]

[0057] The specific method for establishing the calculation model for characterizing stress and deformation in step S3 is:

[0058] Define the pore diameter under stress-free conditions as R initial , the aperture after effective stress is R0, then the calculation formula of stress deformation is:

[0059]

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

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

[0062] Calculate the change in pore size under the action of adsorption expansion, the calculation formula is:

[0063]

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

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

[0066] The specific method of establishing the calculation model characterizing the real gas effect in step S3 is:

[0067] Calculate the gas deviation factor that represents the real gas effect. The calculation formula is:

[0068]

[0069] Where P r Indicates relative pressure, Pc represents the critical pressure, T r Relative temperature, T c represents the critical temperature;

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

[0071] Considering the confinement effect of nanopores, the critical pressure and critical temperature of the gas are calculated respectively. The calculation formula is:

[0072]

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

[0074] The specific method for establishing the calculation model of the gas permeability of the micro-nanoscale pores in the deep coal seam in step S4 is:

[0075] Considering the capillary end effect, real gas effect, nano-confinement effect, stress effect, and adsorption expansion factor, the calculation model of micro-nanoscale pore gas permeability is obtained:

[0076]

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

[0078]

[0079] The beneficial effects of the present invention compared with the prior art are as follows: the present invention detects the gas permeability of micro-nano cross-scale pores in deep coal seams, and by analyzing the gas transmission mechanism of micro-nano scale pores, provides calculation models of surface diffusion transmission mechanism, Knudsen diffusion transmission mechanism, and slip / viscous flow transmission mechanism, and according to the theory of mutual collision between molecules, establishes weighting factors of Knudsen diffusion and slip / viscous flow transmission mechanism, provides calculation models of additional pressure drop caused by end effect, and characterization equations of factors such as adsorption expansion, stress deformation, nano-confinement effect, and real gas effect, and finally couples weighting factors, end effect and other factors to establish a calculation model of gas permeability of micro-nano cross-scale pores in deep coal seams considering end effect; the gas permeability calculation model established by the present invention takes into account factors such as end effect, and establishes a calculation method of gas permeability of micro-nano cross-scale pores suitable for deep coal seams, which provides a more accurate permeability calculation method for natural gas low-carbon mining technology in deep coal seams, and facilitates the theoretical development and technical optimization of low-carbon mining of unconventional natural gas. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0081] Figure 1 is a flow chart of the steps of the detection method of the present invention;

[0082] Figure 2 Schematic diagram of gas molecule collision in deep coal seam pores in an embodiment of the present invention;

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

[0084] Figure 4 is a curve diagram showing the change of gas apparent permeability of nanopores in deep coal seams with pressure in an embodiment of the present invention;

[0085] Figure 5 This is a curve diagram showing the change of gas apparent permeability of micron pores in deep coal seams with pressure in an embodiment of the present invention. DETAILED DESCRIPTION

[0086] like Figure 1 As shown, the present invention provides a method for detecting gas permeability of micro-nano cross-scale pores in deep coal seams, which specifically relates to a calculation scheme for gas permeability of deep coal seam pores taking into account the end effect, and solves the problems existing in the gas cross-scale seepage theory in micro-nano pores under in-situ conditions in deep coal seams. The detection method mainly includes:

[0087] The gas transport mechanism of micro-nano scale pores is analyzed, and the calculation models of surface diffusion transport mechanism, Knudsen diffusion transport mechanism, and slip or viscous flow transport mechanism are given; based on the theory of intermolecular collision, the weighting factors of Knudsen diffusion and slip or viscous flow transport mechanism are established; the calculation formula for the additional pressure drop caused by the end effect, as well as the characterization equations of factors such as adsorption expansion, stress deformation, nano-confinement effect, and real gas effect are given; finally, the weighting factor, end effect and other factors are coupled to establish a calculation model for the gas permeability of micro-nano scale pores in deep coal seams; the above-mentioned gas permeability calculation model takes into account factors such as the end effect, and establishes a calculation method for gas permeability of micro-nano cross-scale pores in deep coal seams, which provides a more accurate permeability calculation method for low-carbon natural gas extraction technology in deep coal seams, and facilitates the theoretical development and technical optimization of low-carbon extraction of unconventional natural gas.

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

[0089] Step S1: Study of gas transport mechanism:

[0090] There are three gas transport mechanisms in micro-nano pores: surface diffusion, Knudsen diffusion, and slip / viscous flow, among which:

[0091] Surface diffusion is the flow mechanism of adsorbed gas near the wall. The mass flux Q of surface diffusion is surf The control equation is expressed as:

[0092]

[0093] Where D s represents the surface diffusion coefficient, m 2 ·s -1 ; C represents the adsorbed gas concentration; 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 Indicates the diameter of gas molecules, m; N A represents Avogadro's constant, 6.02214076×10 23 mol -1 ; T is temperature, K; ΔH is adsorption enthalpy, J·mol -1 ; κ represents the discriminant parameter of gas molecule surface diffusion, Rg represents the gas constant, 8.314 J·mol -1 ·K -1 .

[0094] Knudsen diffusion transport mechanism and slip flow transport mechanism are the flow mechanisms of free gas in the central area of ​​pores. The mass flow rate Q of slip flow is slip The control equation can be expressed as:

[0095]

[0096] Where ρ represents the gas density, kg·m -3 ; R represents the pore radius, m; μ represents the viscosity, Pa·s; Kn represents the Knudsen number; α represents the tangential momentum adjustment coefficient, and its value varies in the range of 0 to 1.

[0097] The mass flow control equation Q for Knudsen diffusion transport mechanism Kn for:

[0098]

[0099] Where Z represents the real gas factor.

[0100] Step S2: Study weighting factors:

[0101] Free gas mass flow rate Q in the pore center area freeis the weighted superposition of Knudsen diffusion transport mechanism and slip flow transport mechanism:

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

[0103] Where, δ represents the thickness of the adsorption layer, m; ω vis ,ω Kn 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 mechanism of the number of collisions between gas molecules and the number of collisions with the wall in a unit length of pore per unit time. The contribution of the Knudsen diffusion transport mechanism and the slip flow transport mechanism to the total free gas flow 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 state of gas molecules in the pore is as follows: Figure 2 shown.

[0105] The present invention selects a unit area dxdy on the pore wall at random and introduces a coordinate system with the z axis perpendicular to the unit area. Among the numerous gas molecules, only the molecules that can reach the wall in unit time, i.e., the molecules that collide with the wall, have coordinates z no greater than their velocity component v along the z axis. x ; Therefore, the unit area of ​​the wall and the gas molecules with velocity components in a given interval dv per unit time x , dv y , dv z The number of collisions within 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, kg.

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

[0109]

[0110] The present 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' relative to other molecules is considered; the relative motion 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 molecules with a relative velocity of gas molecules per unit volume between v' and v'+dv' is:

[0111]

[0112] There are many different processes of molecular collision, such as head-on collision, collision at an angle, etc. These processes occurring during collision are usually characterized by the effective cross section, which can be described by the projected area of ​​a spherical molecule; therefore, the number of collisions between a gas molecule and other molecules 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 wall) is:

[0117]

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

[0119] The flow resistance of the Knudsen diffusion transport mechanism is the collision between gas molecules and the solid wall, while the flow resistance of the slip flow is the collision between gas molecules. The free gas flow in the pores is a coupling process of the Knudsen diffusion process and the slip flow process; therefore, the weighting factor of 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 of 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 of the weighting factor is:

[0120]

[0121] Step S3: Study end effects and other key factors:

[0122] (1) Study the terminal effects:

[0123] For gas flow in finite-length pores, the apparent permeability caused by the capillary end effect is less than that of infinite-length pores. This is because the streamline bending at the end of the channel produces more viscous dissipation and additional hydrodynamic resistance. It should be noted that the streamlines diffusing on the surface of the adsorption layer are affected by the solid wall and are close to the solid wall, so 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] There is also an additional pressure drop at the inlet section of the limited channel, so the additional pressure drop of gas transport in the pore is 2Δp end , assuming the channel length is L and the inlet and outlet pressures are p in , p out , then the pore pressure gradient when calculating the gas flow rate 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 Indicates that the pore pressure drop at the end is not considered.

[0128] Here we propose the terminal effect influencing factor ξ:

[0129]

[0130] (2) Study other key factors:

[0131] Stress effect and adsorption expansion have a great influence on the permeability of deep coal pores by changing the pore size. The pore size under stress-free conditions is set to R initial , the aperture after effective stress is R0, and the stress effect can be expressed as:

[0132]

[0133] Wherein, σ represents effective stress, MPa; v represents Poisson's ratio; E represents Young's modulus, GPa; φ0 represents initial coal porosity.

[0134] Adsorption is the key mechanism for the storage of gas in coal seams and an important factor inducing coal seam expansion. The change in pore size under adsorption and expansion is:

[0135]

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

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

[0138] The real gas effect and nano-confinement effect will affect the gas density, viscosity and other properties, and thus affect the gas flow process; the gas deviation factor that characterizes the actual gas effect can be expressed as:

[0139]

[0140] Where P r Indicates relative pressure; P c Indicates critical pressure, Pa; T r Relative temperature; T c represents the critical temperature, K.

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

[0142]

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

[0144] Step S4: Obtaining a calculation model for the gas permeability of micro-nano-scale pores in deep coal seams:

[0145] The calculation model of micro-nanoscale pore gas permeability considering the capillary end effect, real gas effect, nano-confinement effect, stress effect, adsorption expansion and other factors is:

[0146]

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

[0148]

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

[0150] In the embodiment of the present invention, a coalbed methane field is taken as an example, the coal seam depth is 1240 meters, the pressure is 12.5 MPa, and the temperature is 63.4° C. The pressure gradient is set to 2 MPa / m, and other parameter values ​​required for permeability calculation are shown in the following table:

[0151]

[0152] Table 1 Parameter values

[0153] Substituting the above data into formula (20), we can draw the methane permeability curve of nanopores under different pressure conditions in deep coal seams, as follows: Figure 4 As shown, the ordinate in the figure represents the methane permeability.

[0154] Substituting the above data into formula (20), we can draw the methane permeability curve of micron pores under different pressure conditions in deep coal seams, as shown in Figure 5 As shown, the ordinate in the figure represents the methane permeability.

[0155] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements 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 of micro-nano cross-scale pores in deep coal seams, characterized by: The detection steps include the following: Step S1: Analyze the gas transport mechanism of micro-nanoscale pores, and establish the calculation models of surface diffusion transport mechanism, Knudsen diffusion transport mechanism, and slip or viscous flow transport mechanism respectively; Step S2: According to the theory of molecular collision, the weighting factors of Knudsen diffusion transport mechanism, slip or viscous flow transport mechanism are established respectively; Step S3: establishing a calculation model for the additional pressure drop caused by the end effect, and at the same time establishing calculation models for characterizing stress deformation, adsorption expansion, real gas effect, and nano-confinement effect; Step S4: A calculation model for gas permeability of micro-nanoscale pores in deep coal seams is established by coupling weighting factors and terminal effects, and the gas permeability of deep coal seams is detected by the calculation model.

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

3. The method for detecting micro-nano cross-scale pore gas permeability of deep coal seams according to claim 2 is characterized in that: The specific method for establishing the weighting factors of the Knudsen diffusion transport mechanism, the slip or viscous flow transport mechanism in step S2 is: Define the free gas mass flow rate Q in the pore center area free It is the weighted superposition of Knudsen diffusion transport mechanism and slip or viscous flow transport mechanism, and the calculation formula is: Q free =π(R-δ) 2 (oh vis Q slip +oh Kn Q Kn )(4); Where δ represents the thickness of the adsorption layer; ω vis ,ω Kn 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, which is 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 an arbitrary unit area dxdy on the pore wall and introduce a coordinate system with the z axis perpendicular to the unit area; Calculate the velocity of the wall per unit area and the gas molecules with velocity components at a given interval dv per unit time. x , dv y , dv z The number of collisions within dN g-w , the calculation formula is: In the formula, n represents the molecular number density; m represents the mass of a single molecule; Thus, the total number of collisions between gas molecules and the wall per unit area per unit time is obtained. g-w , the calculation formula is: Analyze the relative speed of a gas molecule relative to other gas molecules as v', define all gas molecules to have the same mass, and the relative kinetic energy of two gas molecules with mass m1 and mass m2 is v' 2 (m1m2 / (m1+m2)) / 2, calculate the number of molecules in a unit volume whose relative velocity is between v' and v'+dv', the calculation formula is: Calculate the number of times a gas molecule collides with other gas molecules per unit time. The calculation formula is: The total number of collisions between gas molecules per unit volume and per unit time is calculated using the following formula: Calculate the total number of collisions of gas molecules per unit length of pores using the following formula: In the formula, A represents the wall area of ​​the pore per unit length; V represents the spatial volume of the pore per unit length; The weighting factor of 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 of 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. The calculation formulas for the weighting factors of the two transport mechanisms are:

4. The method for detecting micro-nano cross-scale pore gas permeability of deep coal seams according to claim 3 is characterized by: The specific method for establishing the additional pressure drop calculation model caused by the end effect in step S3 is: Calculate the additional pressure drop caused by the end effect using the formula: Since there is also an additional pressure drop at the inlet section of the finite channel, the additional pressure drop for gas transport in the pores is 2Δp end ; Define the channel length as L and the inlet and outlet pressure as p in , p out , then the pore pressure gradient when calculating the gas flow rate is (p in -p out ) / L; but under the capillary end effect, the pressure gradient becomes: Δp pore =Δp-2Δp end (13); In the formula, Δp pore It means that the pore pressure drop at the end is not considered; Here we get the terminal effect influence factor ξ, which is calculated as follows:

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

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

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