A method for detecting the methane adsorption capacity of coal rock wall surface based on molecular simulation

Abstract

This application provides a molecular simulation-based method for detecting methane adsorption capacity on coal and rock walls. This method unifies model construction, water content definition, molecular dynamics (MD) wetting equilibrium, GCMC adsorption calculation, and evaluation index extraction. It can be used to evaluate the changes in methane adsorption capacity on different coal and rock walls, with different pore sizes and under different water content conditions. The detection method provided in this application first obtains a stable water-bearing state through molecular dynamics (MD) simulation, and then performs methane giant normal Monte Carlo (GCMC) adsorption simulation. This avoids the bias caused by directly calculating adsorption on unstable water-bearing structures. Therefore, by using indicators such as water coverage, effective pore size, methane quantity in the adsorption zone, adsorption capacity per unit area, and water-bearing adsorption inhibition coefficient, the influence pathway of water on methane adsorption can be quantitatively distinguished.

Description

Technical Field

[0001] This application relates to the fields of coalbed methane development, coal and rock gas adsorption evaluation, and molecular simulation technology, specifically to a method for detecting the methane adsorption capacity of coal and rock walls based on molecular simulation. Background Technology

[0002] Methane in coalbed methane exists primarily in free and adsorbed states within the pores of coal seams. The adsorbed methane state is controlled by the properties of the coal seam wall, pore size, water content, and temperature and pressure conditions. When water molecules enter the nanopores of coal seams, they not only occupy the pore space but may also preferentially distribute near the coal seam wall, forming adherent water films, water clusters, or localized water bridges. This reduces the contact opportunity between methane and the coal seam wall, thus lowering the methane adsorption capacity.

[0003] Existing methods for evaluating methane adsorption mostly rely on macroscopic isothermal adsorption experiments or empirical models. While these methods can obtain the total adsorption capacity, they struggle to directly explain how water molecules are distributed near the wall surface, which wall sites are blocked by water, how the effective pore size changes, and how these changes collectively affect the methane adsorption capacity. For coal and rock walls with different degrees of compaction, roughness, or functional group exposure states, traditional experiments also suffer from difficulties in individually controlling variables and a lack of consistent model comparisons.

[0004] Therefore, there is a need to provide a detection method that can be used to evaluate the changes in methane adsorption capacity under different coal and rock wall surfaces, different pore sizes, and different water content conditions. Summary of the Invention

[0005] In view of this, the technical problem to be solved by this application is to provide a method for detecting the methane adsorption capacity of coal and rock walls based on molecular simulation. The detection method provided by this application can avoid the problem of incomparability of water content states caused by the fixed number of water molecules in different models, and can quantitatively characterize the inhibitory effect of increased water content on methane adsorption capacity.

[0006] This application provides a method for detecting the methane adsorption capacity of coal and rock walls based on molecular simulation, comprising the following steps:

[0007] Step S1: Construct a molecular model of the characteristic coal and rock wall, wherein the molecular model of the characteristic coal and rock wall includes a slit hole formed by two opposing walls;

[0008] Step S2: Calculate the number of water molecules based on the pore volume of the slit and the target water saturation, and add the water molecules to the pore region of the slit to obtain a water-bearing slit.

[0009] Step S3: Perform molecular dynamics simulation to achieve wetting equilibrium of the water-containing slit hole, so that the water molecules reach a stable storage state;

[0010] Step S4: Under set conditions, perform a giant canonical Monte Carlo adsorption simulation on the water-containing slit pores obtained in step S3 to obtain the amount of methane in the adsorption zone.

[0011] Step S5: Obtain the amount of methane adsorbed per unit area based on the amount of methane in the adsorption zone obtained in step S4.

[0012] In some specific implementations, in step S2, the pore volume of the slit hole is calculated according to equation (1):

[0013] Equation (1)

[0014] In equation (1), Pore ​​volume, in nm 3 H represents the distance between the inner surfaces of the two coal and rock walls, in nm. The length of the slit hole in the direction parallel to the coal and rock wall is expressed in nm. The width of the slit hole in the direction parallel to the coal and rock wall is expressed in nm.

[0015] In some specific implementations, in step S2, the number of water molecules is calculated according to equation (2):

[0016] Equation (2)

[0017] In equation (2), This represents the number of water molecules, expressed in units of molecules. The target water saturation level is expressed in % (%). Water molecule number density, in units of molecules / nm 3 ; Pore ​​volume, unit: nm 3 .

[0018] In some specific implementations, the target water saturation is 0-60%.

[0019] In some specific implementations, after the water molecules reach a stable state in step S3, the method further includes obtaining the water density distribution, near-wall water film thickness, water coverage, and effective pore size.

[0020] In some specific implementations, the water density distribution is obtained according to the following method:

[0021] The slit orifice is divided into several statistical layers along a direction perpendicular to the coal and rock wall. The average number of water molecules in each statistical layer is counted, and the water density distribution is calculated according to equation (3):

[0022] Equation (3);

[0023] in, This represents the water density distribution in the kth statistical layer, in units of cells / nm³. This represents the average number of water molecules located in the k-th statistical layer within the statistical time window, expressed in units of molecules. The length of the slit hole in the direction parallel to the coal and rock wall is expressed in nm. The width of the slit hole in the direction parallel to the coal and rock wall, in nm; The thickness of the k-th statistical layer is expressed in nm.

[0024] The effective aperture is calculated according to equation (4):

[0025] Equation (4)

[0026] In equation (4), The effective aperture is in nm; H is the distance between the inner surfaces of the two coal and rock walls, in nm. The near-wall water film thickness is expressed in nm.

[0027] In some specific implementations, in step S5, the amount of methane adsorbed per unit area is calculated according to equation (5):

[0028] Equation (5)

[0029] In equation (5), This represents the amount of methane adsorbed per unit area, expressed as cells / nm. 2 , The number of methane molecules in the adsorption zone is expressed in units of molecule; The effective area of ​​the coal and rock wall is expressed in nm. 2 .

[0030] In some specific implementations, the detection method further includes:

[0031] Step S6: Obtain the water-containing adsorption inhibition coefficient based on the methane adsorption per unit area obtained in step S5.

[0032] In some specific implementations, the water-containing adsorption inhibition coefficient is calculated according to equation (6):

[0033] Equation (6)

[0034] In equation (6), This represents the amount of methane adsorbed per unit area under aqueous conditions, expressed in cells / nm. 2 ; The adsorption capacity of methane per unit area under dry conditions is expressed in units per nm. 2 ; This represents the water adsorption inhibition coefficient.

[0035] In some specific implementations, step S4 also includes obtaining the total number of methane molecules and the amount of methane in the free zone.

[0036] This application provides a method for detecting the methane adsorption capacity of coal and rock walls based on molecular simulation, comprising the following steps: Step S1, constructing a molecular model of a characteristic coal and rock wall, wherein the characteristic coal and rock wall molecular model includes a slit hole formed by two opposing walls; Step S2, calculating the number of water molecules based on the pore volume of the slit hole and the target water saturation, and adding water molecules to the pore region of the slit hole to obtain a water-bearing slit hole; Step S3, performing molecular dynamics simulation wetting equilibrium on the water-bearing slit hole to achieve a stable storage state for water molecules; Step S4, performing a giant canonical Monte Carlo adsorption simulation on the water-bearing slit hole obtained in Step S3 under set conditions to obtain the amount of methane in the adsorption zone; Step S5, obtaining the methane adsorption capacity per unit area based on the amount of methane in the adsorption zone obtained in Step S4. This application combines molecular dynamics simulation and grand canonical Monte Carlo simulation to form a detection route of "first stabilizing water content, then calculating methane adsorption". By using the calculation method of "pore volume - target water saturation - number of water molecules", the water content of models with different pore sizes and wall sizes is comparable. This can be used to compare the differences in methane adsorption capacity under different pore sizes and water saturation conditions, thereby quantitatively explaining the influence of water on the methane adsorption capacity of coal and rock walls. Attached Figure Description

[0037] Figure 1 A molecular model of the characteristic coal and rock wall surface constructed for the embodiments of this application;

[0038] Figure 2 Schematic diagram of water occurrence states under different water content conditions;

[0039] Figure 3 This is a GCMC adsorption equilibrium determination diagram for methane.

[0040] Figure 4 A flowchart illustrating the detection method provided in this application;

[0041] Figure 5 A schematic diagram illustrating the evaluation indicators for the influence of water on methane adsorption.

[0042] Figure 6 The variation of methane adsorption per unit area and water adsorption inhibition coefficient under different water contents. Detailed Implementation

[0043] This application provides a method for detecting the methane adsorption capacity of coal and rock walls based on molecular simulation. Those skilled in the art can refer to the content of this application and appropriately modify the process parameters to achieve the desired result. The method and application of this application have been described through preferred embodiments. Those skilled in the art can obviously modify or appropriately change and combine the method and application described herein without departing from the content, spirit, and scope of this application to realize and apply the technology of this application.

[0044] This application provides a method for detecting the methane adsorption capacity of coal and rock walls based on molecular simulation, comprising the following steps:

[0045] Step S1: Construct a molecular model of the characteristic coal and rock wall, wherein the molecular model of the characteristic coal and rock wall includes a slit hole formed by two opposing walls;

[0046] Step S2: Calculate the number of water molecules based on the pore volume of the slit and the target water saturation, and add the water molecules to the pore region of the slit to obtain a water-bearing slit.

[0047] Step S3: Perform molecular dynamics simulation to achieve wetting equilibrium of the water-containing slit hole, so that the water molecules reach a stable storage state;

[0048] Step S4: Under set conditions, perform a giant canonical Monte Carlo adsorption simulation on the water-containing slit pores obtained in step S3 to obtain the amount of methane in the adsorption zone.

[0049] Step S5: Obtain the amount of methane adsorbed per unit area based on the amount of methane in the adsorption zone obtained in step S4.

[0050] The detection method provided in this application is a molecular simulation method that unifies model construction, water content definition, molecular dynamics (MD) wetting equilibrium, GCMC adsorption calculation, and evaluation index extraction. It can be used to evaluate the changes in methane adsorption capacity under different coal and rock surfaces, pore sizes, and water content conditions. The detection method provided in this application first obtains a stable water-bearing state through molecular dynamics (MD) simulation, and then performs methane giant canonical Monte Carlo (GCMC) adsorption simulation. This avoids the result bias caused by directly calculating adsorption on unstable water-bearing structures. Therefore, by using indicators such as water coverage, effective pore size, methane quantity in the adsorption zone, adsorption capacity per unit area, and water-bearing adsorption inhibition coefficient, the influence pathway of water on methane adsorption can be quantitatively distinguished.

[0051] This application first constructs a molecular model of a characteristic coal and rock wall, which includes a slit hole formed by two opposing walls. Specifically, this application constructs two identical coal and rock walls and positions them opposite each other to form the slit hole. In some specific implementations, the coal and rock wall includes parameters such as coal and rock molecular structure, wall functional groups, and surface roughness; this application does not impose any special limitations on these parameters. See also Figure 1 , Figure 1 The characteristic coal and rock wall molecular model constructed for the embodiments of this application is shown, wherein B1 is the first coal and rock wall and B2 is the second coal and rock wall. The first coal and rock wall B1 and the second coal and rock wall B2 are arranged opposite each other in the vertical direction to form a slit hole.

[0052] After constructing a molecular model of the characteristic coal and rock wall, the characteristic parameters of the slit hole are obtained, including its geometric aperture H, length Lx parallel to the coal and rock wall direction, width Ly parallel to the coal and rock wall direction, and pore volume Vpore. Simultaneously, the effective area Awall of the coal and rock wall is obtained. Specifically, the geometric aperture H is the distance between the inner walls of two coal and rock walls, such as... Figure 1 As shown, the geometric aperture H is the distance between the inner wall surface of the first coal and rock wall B1 and the inner wall surface of the second coal and rock wall B2.

[0053] In some specific implementations, the geometric aperture H is the average distance between the inner surfaces of the two coal and rock walls. For an ideal parallel slit hole, H is the vertical distance between the inner surfaces of the two coal and rock walls; for walls with roughness, the average distance between the inner surfaces of the two coal and rock walls can be calculated using the following formula:

[0054]

[0055] Where A is the effective area of ​​the coal and rock wall, in nm. 2 h(x,y) is the local aperture at position (x,y), in nm. Here, x represents the length of the coal / rock wall, and y represents the width of the coal / rock wall.

[0056] In some specific implementations, the effective area A of the coal and rock wall is the sum of the areas of the two coal and rock walls, calculated according to the following formula:

[0057] A = 2 × Lx × Ly;

[0058] Where A is the effective area of ​​the coal and rock wall, in nm. 2 ; The length of the slit hole in the direction parallel to the coal and rock wall is expressed in nm. The width of the slit hole in the direction parallel to the coal and rock wall is expressed in nm.

[0059] Those skilled in the art will understand that the inner wall surface refers to the wall surface adjacent to the opposite coal and rock wall surface. In some specific implementations, the geometric aperture H can be 1nm~5nm, preferably 2nm~4nm, for example, 2nm, 3nm or 4nm, etc.

[0060] The length Lx parallel to the coal and rock wall can be freely defined. Those skilled in the art will understand that Lx can be the length of the coal and rock wall forming the slit, used to calculate the pore volume of the slit. In some specific implementations, the length Lx parallel to the coal and rock wall can be 1 nm to 15 nm, preferably 4 nm to 12 nm, more preferably 5 nm to 10 nm, for example, 5 nm, 5.5 nm, 5.764 nm, 6 nm, etc.

[0061] The width Ly parallel to the coal and rock wall can be freely defined. Those skilled in the art will understand that the width Ly parallel to the coal and rock wall can be the width of the coal and rock wall forming the slit, used to calculate the pore volume of the slit. In some specific implementations, the width Ly parallel to the coal and rock wall can be 1nm to 15nm, preferably 4nm to 12nm, more preferably 5nm to 10nm, such as 5nm, 6nm, 6.012nm, 6.5nm, etc.

[0062] In some specific implementations, the pore volume Vpore is calculated according to equation (1):

[0063] Equation (1)

[0064] In equation (1), Pore ​​volume, unit: nm 3 H represents the distance between the inner surfaces of the two coal and rock walls, in nm. The length of the slit hole in the direction parallel to the coal and rock wall is expressed in nm. The width of the slit hole in the direction parallel to the coal and rock wall is expressed in nm.

[0065] After obtaining a molecular model of the characteristic coal and rock wall surface including slit pores, the number of water molecules is calculated based on the pore volume of the slit pores and the target water saturation. The water molecules are then added to the pore region of the slit pores to obtain a water-bearing slit pore. The method provided in this application can detect the methane adsorption capacity of a coal and rock wall surface in a water-bearing state. The target water saturation is used to simulate the water content of the coal and rock wall surface, and can be 0~100%, preferably 0~80%, and more preferably 0~60%. When the target water saturation is 0, a dry baseline model can be formed; when the target water saturation is greater than 0, models with different water contents can be formed.

[0066] In some specific implementations, the number of water molecules is calculated according to equation (2):

[0067] Equation (2)

[0068] In equation (2), This represents the number of water molecules, expressed in units of molecules. The target water saturation level is expressed in % (%). Water molecule number density, in units of molecules / nm 3 ; Pore ​​volume, unit: nm 3 .

[0069] In equation (2), It is a constant, 33.4 units / nm. 3 .

[0070] After obtaining the number of water molecules, add them to the pore region of the slit hole according to the obtained number of water molecules to obtain a water-containing slit hole.

[0071] After obtaining the water-bearing slit pore, molecular dynamics (MD) simulations were performed to induce wetting equilibrium, allowing water molecules to reach a stable state. (See also...) Figure 2 , Figure 2 This diagram illustrates the different water content states of water. After wetting equilibrium is achieved through molecular dynamics (MD) simulations, water may reach a stable state by adhering to walls, forming clusters, or creating local water bridges.

[0072] In some specific implementations, the temperature for molecular dynamics (MD) simulation of wetting equilibrium is 200K~400K, preferably 250K~350K, and more preferably 313K; the time step is 0.01fs~10fs, preferably 0.1fs~5fs, and more preferably 0.5fs~1fs, at which time step the water-bearing system can be stably integrated; the equilibrium time is 1ns~10ns, preferably 2ns~5ns, at which equilibrium time the water distribution can enter a stable state. In some specific implementations, when performing molecular dynamics (MD) simulation of wetting equilibrium, the coal and rock wall is kept fixed or weakly constrained to maintain the stability of pore size and wall structure.

[0073] In some specific implementations, after the water molecules reach a stable state through molecular dynamics simulation of wetting equilibrium, the following are also included: obtaining the water density distribution, near-wall water film thickness, water coverage, and effective pore size.

[0074] In some specific implementations, the water density distribution is the number density distribution of water molecules along the pore diameter direction of the slit, which can be obtained using the following method:

[0075] The slit orifice is divided into several statistical layers along a direction perpendicular to the coal and rock wall. The average number of water molecules in each statistical layer is counted, and the water density distribution is calculated according to equation (3):

[0076] Equation (3);

[0077] in, This represents the water density distribution in the kth statistical layer, in units of cells / nm³. This represents the average number of water molecules located in the k-th statistical layer within the statistical time window, expressed in units of molecules. The length of the slit hole in the direction parallel to the coal and rock wall is expressed in nm. The width of the slit hole in the direction parallel to the coal and rock wall, in nm; The thickness of the k-th statistical layer is given in nm.

[0078] In some specific implementations, the thickness of each statistical layer is the same. In some specific implementations, when calculating the average number of water molecules within each statistical layer, the oxygen atom or the center of mass of the water molecule is used as the standard.

[0079] In some specific implementations, the near-wall water film thickness is determined based on the water density distribution, specifically including the following steps: The water molecule enrichment region near the inner wall of the coal and rock wall is designated as the near-wall water film region. When the water density distribution value is higher than a preset threshold, the location is considered to be covered by the near-wall water film. In some specific implementations, the preset threshold can be 10% to 50% of the water phase number density, preferably 50%. In some specific implementations, the near-wall water film thickness is calculated according to the following formula:

[0080]

[0081] in, The near-wall water film thickness is expressed in nm. The near-wall water film thickness is the inner wall thickness of the first coal and rock wall, in nm. The near-wall water film thickness is on the inner wall of another coal and rock wall, in nm.

[0082] In some specific implementations, the thickness of the near-wall water film can be 0~1nm, preferably 0.01nm~0.85nm, more preferably 0.05nm~0.80nm, such as 0.08nm, 0.14nm, 0.27nm, 0.49nm, 0.72nm, etc.

[0083] In some specific implementations, the water coverage rate is the ratio of the area of ​​the coal and rock wall covered by water molecules to the effective area of ​​the coal and rock wall. Specifically, the water coverage rate can be obtained by the following method:

[0084] The inner wall of the coal and rock wall is divided into multiple grids of the same area. In the statistical window of the molecular dynamics simulation of the wetting equilibrium trajectory, if water molecules, oxygen atoms, or water molecule centroids appear within a preset near-wall distance dw above a certain grid, then the grid is recorded as a water-covered grid.

[0085] Calculate water coverage using the following formula:

[0086]

[0087] in, Water coverage; The number of water cover grid cells, expressed in units. This represents the total number of grid cells on the coal and rock face, expressed in units of cells.

[0088] In some specific implementations, the near-wall distance dw can be 0.3nm~0.6nm, preferably 0.5nm.

[0089] In some specific implementations, the water coverage rate can be 0~1, preferably 0.01~0.90, more preferably 0.1~0.85, such as 0.12, 0.23, 0.41, 0.64, 0.82, etc.

[0090] In some specific implementations, the effective aperture is calculated according to equation (4):

[0091] Equation (4)

[0092] In equation (4), The effective aperture is in nm; H is the distance between the inner surfaces of the two coal and rock walls, in nm. The near-wall water film thickness is expressed in nm.

[0093] After water molecules reach a stable state, this application performs a giant canonical Monte Carlo (GCMC) adsorption simulation on the water-containing slit pores obtained in step S3 under set conditions to obtain the number of methane molecules in the adsorption region. In some specific implementations, the adsorbed component in the GCMC adsorption simulation is methane; the adsorption temperature is consistent with the MD wetting equilibrium, ranging from 200K to 400K, preferably from 250K to 350K, and more preferably from 313K; the adsorption pressure is from 1MPa to 20MPa, preferably from 5MPa to 15MPa, and more preferably from 10MPa; the adsorption region is the pore region of the slit pore, and insertion into the wall or external vacuum is not allowed; the equilibrium criterion is that the continuous statistical window fluctuation of the methane molecule number is less than 2% to 3%, indicating that adsorption has reached equilibrium. See [link to relevant documentation]. Figure 3 , Figure 3 This is a GCMC adsorption equilibrium determination diagram for methane, from... Figure 3 It can be seen that the number of methane molecules or the amount of adsorption reaches the plateau with the number of GCMC cycles.

[0094] After the giant canonical Monte Carlo (GCMC) adsorption simulation is completed, the number of methane molecules in the adsorption region is obtained, and preferably, the total number of methane molecules and the number of methane molecules in the free region are also obtained. In some specific implementations, this application uses the near-wall distance... To divide the adsorption region and the free region, for example, the area within 0.1 nm to 1 nm near the wall, preferably 0.5 nm, is used as the boundary. The area within 0.5 nm near the wall is the adsorption region, and the remaining core area is the free region.

[0095] After the GCMC adsorption simulation reaches equilibrium, for the first... For each CH4 molecule, calculate the distance from its molecular centroid to the reference plane of the nearest inner wall of the coal face. .when When, the CH4 molecule is included in the adsorption region; when When this CH4 molecule is included in the free region, calculate the number of methane molecules in the adsorbed region, the number of methane molecules in the free region, and the total number of methane molecules using the following formulas:

[0096]

[0097]

[0098]

[0099] in, The number of methane molecules in the adsorption zone is expressed in units of molecule; This represents the number of free methane molecules, expressed in units. This represents the total number of methane molecules, expressed in units. Near-wall distance, in nm; This is an indicator function that takes the value 1 if the condition is met, and 0 otherwise. This indicates the configuration average or time average during the GCMC balancing platform phase.

[0100] In some specific implementations, the The near-wall distance, which can also be considered as the thickness for determining the adsorption region, is 0.1 nm to 1.0 nm, preferably 0.5 nm. It should be noted that the adsorption region in this application is a near-wall statistical region defined based on the distance from the centroid of the CH4 molecule to the reference plane of the nearest inner wall surface of the coal wall, used to characterize the distribution state of methane near the coal wall surface; it is not limited to a single-molecule adsorption state in a strictly thermodynamic sense.

[0101] In some specific implementations, the number of methane molecules in the adsorption zone is 10 to 300, preferably 20 to 200, and more preferably 30 to 180.

[0102] In some specific implementations, the number of methane molecules in the free region is 10 to 300, preferably 30 to 200, and more preferably 50 to 150.

[0103] In some specific implementations, after obtaining the number of methane molecules in the adsorption zone, the total number of methane molecules, and the number of methane molecules in the free zone, the adsorption zone ratio can also be obtained to characterize the distribution ratio of CH4 in the near-wall adsorption zone. In some specific implementations, the adsorption zone ratio is calculated according to the following formula:

[0104]

[0105] in, This represents the proportion of the adsorption region. The number of methane molecules in the adsorption zone is expressed in units of molecule; This represents the total number of methane molecules, expressed in units.

[0106] After obtaining the amount of methane in the adsorption zone, the amount of methane adsorbed per unit area can be obtained based on the amount of methane in the adsorption zone.

[0107] In some specific implementations, the amount of methane adsorbed per unit area is calculated according to equation (5):

[0108] Equation (5)

[0109] In equation (5), This represents the amount of methane adsorbed per unit area, expressed as cells / nm. 2 , The number of methane molecules in the adsorption zone is expressed in units of molecule; The effective area of ​​the coal and rock wall is expressed in nm. 2 .

[0110] After obtaining the methane adsorption capacity per unit area, the water adsorption inhibition coefficient can also be obtained.

[0111] In some specific implementations, the water-containing adsorption inhibition coefficient is calculated according to equation (6):

[0112] Equation (6)

[0113] In equation (6), This represents the amount of methane adsorbed per unit area under aqueous conditions, expressed in cells / nm. 2 ; The adsorption capacity of methane per unit area under dry conditions is expressed in units per nm. 2 ; This represents the water adsorption inhibition coefficient.

[0114] In equation (6), This refers to the amount of methane adsorbed per unit area under conditions where the water saturation is 0%. The larger the value, the stronger the inhibition of water's ability to adsorb methane.

[0115] In some specific implementation methods, parameters such as water coverage, effective pore size, methane quantity in the adsorption zone, and adsorption capacity per unit area under different water saturation conditions can be obtained by adjusting different water saturation levels. This allows for the quantitative evaluation of the inhibitory effect of water on the methane adsorption capacity of coal and rock walls, or the evaluation of methane adsorption results under different water content states, different wall characteristics, or different pore sizes.

[0116] See Figure 4 , Figure 4 This is a schematic diagram of the detection method provided in this application. A typical process of this application is as follows: First, a characteristic coal and rock wall is established based on parameters such as the molecular structure of coal and rock, functional groups of the wall, and surface roughness, and two identical coal and rock walls are used to form a slit with a set pore size; then, the number of water molecules is calculated based on the pore volume of the slit and the target water saturation, and water molecules are added to the pore region to obtain a water-bearing slit; molecular dynamics equilibrium is achieved in the water-bearing slit, so that water molecules form a stable water film, water cluster, or local water bridge; then, under set conditions, including temperature, pressure, or fugacity, methane GCMC adsorption simulation is performed on the stable water-bearing model to obtain the methane adsorption equilibrium state; based on the trajectory statistics, parameters such as water coverage, effective pore size, number of methane molecules in the adsorption zone, number of methane molecules in the free zone, adsorption capacity per unit area, and water adsorption inhibition coefficient are calculated; finally, the influence of water on methane adsorption on the coal and rock wall is obtained based on each evaluation parameter, and the differences in methane adsorption capacity under different water contents, different pore sizes, or different wall characteristics are compared.

[0117] This application combines molecular dynamics simulation and grand canonical Monte Carlo simulation to form a detection route of "first stabilizing water content, then calculating methane adsorption". By using the calculation method of "pore volume - target water saturation - number of water molecules", the water content of models with different pore sizes and wall sizes is comparable. This can be used to compare the differences in methane adsorption capacity under different pore sizes and water saturation conditions, thereby quantitatively explaining the influence of water on the methane adsorption capacity of coal and rock walls.

[0118] The following examples further illustrate the method for detecting methane adsorption capacity of coal and rock walls based on molecular simulation provided in this application.

[0119] Example 1

[0120] Taking the 3nm slit pores formed by the wall of organic coal as an example, the influence of different water content states on methane adsorption capacity is evaluated, including the following steps:

[0121] S1. First, construct two identical organic coal rock walls and place them opposite each other in a vertical direction to form a slit with a geometric diameter of H=3nm (i.e., the distance between the inner walls of the two coal rock walls).

[0122] S2. Define the length of the slit hole as Lx = 6.012 nm in the direction parallel to the length of the coal and rock wall, and define the width of the slit hole as Ly = 5.764 nm in the direction parallel to the width of the coal and rock wall.

[0123] The pore volume Vpore of the slit hole was calculated to be 103.96 nm according to equation (1). 3 :

[0124] Equation (1).

[0125] The effective area of ​​the wall is A = 2 × Lx × Ly = 69.30 nm 2 .

[0126] The parameters of the slit aperture are shown in Table 1:

[0127] Table 1 Parameters of the slit orifice

[0128]

[0129] The number of water molecules under different water saturation conditions is calculated according to formula (2), and the water molecules are added to the pore region of the slit hole to obtain the water-bearing slit hole. See Table 2, which shows the water occurrence parameters under different water conditions.

[0130] Equation (2)

[0131] In equation (2), This represents the number of water molecules, expressed in units of molecules. The target water saturation level is expressed in % (%). Water molecule number density, in units of molecules / nm 3 ; Pore ​​volume, unit: nm 3 .

[0132] S3. Perform molecular dynamics simulation wetting equilibrium on the water-bearing slit hole according to the parameters shown in Table 3 to make the water molecules reach a stable state of existence. The water existence parameters under different water-bearing conditions are shown in Table 2.

[0133] Table 2 Water occurrence parameters under different water content conditions

[0134]

[0135] In Table 2, the water coverage rate was obtained using the following method:

[0136] The inner wall of the coal and rock wall is divided into multiple grids of equal area. Within the statistical window of the molecular dynamics simulation of the wetting equilibrium trajectory, if a water molecule, oxygen atom, or water molecule centroid appears within a preset near-wall distance dw above a certain grid, then that grid is recorded as a water-covered grid. The water coverage rate is calculated according to the following formula:

[0137]

[0138] in, Water coverage; The number of water cover grid cells, expressed in units. This represents the total number of grid cells on the coal and rock face, expressed in units of cells.

[0139] The thickness of the near-wall water film is calculated as follows: the area where water molecules are concentrated near the inner wall of the coal and rock wall is taken as the near-wall water film area. When the water density distribution value is higher than 50% of the water phase number density, the location is considered to be covered by the near-wall water film. The water density distribution is obtained as follows: the slit hole is divided into several statistical layers of the same thickness along the direction perpendicular to the coal and rock wall. The average number of water molecules in each statistical layer is counted, and the water density distribution is calculated according to formula (3):

[0140] Equation (3);

[0141] in, This represents the water density distribution in the kth statistical layer, in units of cells / nm³. This represents the average number of water molecules located in the k-th statistical layer within the statistical time window, expressed in units of molecules. The length of the slit hole in the direction parallel to the coal and rock wall is expressed in nm. The width of the slit hole in the direction parallel to the coal and rock wall, in nm; The thickness of the k-th statistical layer is given in nm.

[0142] After obtaining the near-wall water film thickness on the inner wall of the first coal and rock wall and the near-wall water film thickness on the inner wall of the other coal and rock wall, the near-wall water film thickness is calculated according to the following formula:

[0143]

[0144] in, The near-wall water film thickness is expressed in nm. The near-wall water film thickness is the inner wall thickness of the first coal and rock wall, in nm. The near-wall water film thickness is on the inner wall of another coal and rock wall, in nm.

[0145] The effective aperture is calculated according to equation (4):

[0146] Equation (4)

[0147] In equation (4), The effective aperture is in nm; H is the distance between the inner surfaces of the two coal and rock walls, in nm. The near-wall water film thickness is expressed in nm.

[0148] Table 3 MD wetting balance parameters

[0149]

[0150] S4. Perform a giant canonical Monte Carlo adsorption simulation on the water-bearing slit pore according to the parameters described in Table 4 to obtain the methane content in the adsorption zone, the methane content in the free zone, and the total methane content. The results are shown in Table 5, which presents the methane adsorption evaluation results under different water content conditions.

[0151] Table 4. Parameters of giant canonical Monte Carlo adsorption simulation

[0152]

[0153] After the GCMC adsorption simulation reaches equilibrium, for the first... For each CH4 molecule, calculate the distance from its molecular centroid to the reference plane of the nearest inner wall of the coal face. .when When, the CH4 molecule is included in the adsorption region; when When this CH4 molecule is included in the free region, calculate the number of methane molecules in the adsorbed region, the number of methane molecules in the free region, and the total number of methane molecules using the following formulas:

[0154]

[0155]

[0156]

[0157] in, The number of methane molecules in the adsorption zone is expressed in units of molecule; This represents the number of free methane molecules, expressed in units. This represents the total number of methane molecules, expressed in units. This is the near-wall distance, in nm; in this embodiment, it is taken as 0.5 nm. This is an indicator function that takes the value 1 if the condition is met, and 0 otherwise. This indicates the configuration average or time average during the GCMC balancing platform phase.

[0158] S5. The amount of methane adsorbed per unit area was calculated according to formula (5). The results are shown in Table 5. Table 5 shows the methane adsorption evaluation results under different water content conditions:

[0159] Equation (5);

[0160] In equation (5), This represents the amount of methane adsorbed per unit area, expressed as cells / nm. 2 , The number of methane molecules in the adsorption zone is expressed in units of molecule; The effective area of ​​the coal and rock wall is expressed in nm. 2 .

[0161] The adsorption zone ratio was calculated using the following formula, and the results are shown in Table 5. Table 5 presents the methane adsorption evaluation results under different water content conditions:

[0162]

[0163] in, This represents the proportion of the adsorption region. The number of methane molecules in the adsorption zone is expressed in units of molecule; This represents the total number of methane molecules, expressed in units.

[0164] S6. The water-containing adsorption inhibition coefficient is calculated according to formula (6). The results are shown in Table 5. Table 5 shows the methane adsorption evaluation results under different water-containing conditions.

[0165] Equation (6)

[0166] In equation (6), This represents the amount of methane adsorbed per unit area under aqueous conditions, expressed in units of adsorption cells. The adsorption capacity of methane per unit area under dry conditions is expressed in units of adsorption cells. This represents the water adsorption inhibition coefficient.

[0167] Table 5. Evaluation results of methane adsorption under different water content conditions

[0168]

[0169] As can be seen from the above data, as the water saturation increases from 0% to 60%, the water coverage of the coal and rock wall increases from 0 to 0.82, the effective pore size decreases from 3.00 nm to 1.56 nm, the number of methane adsorption molecules decreases from 170 to 42, and the adsorption capacity per unit area increases from 2.45 molecules / nm. 2 Reduced to 0.61 per nm 2 The water adsorption inhibition coefficient increased from 0 to 0.75. (See also...) Figure 5 , Figure 5This diagram illustrates the impact of water on methane adsorption. Increased water coverage leads to a decrease in effective pore size; a decrease in effective pore size reduces the number of methane molecules in the adsorption zone, resulting in a decrease in the adsorption capacity per unit area, and ultimately an increase in the water-containing adsorption inhibition coefficient. These results demonstrate that the method provided in this application can quantitatively evaluate the inhibitory effect of water on the methane adsorption capacity of coal and rock walls using parameters such as water coverage, effective pore size, methane quantity in the adsorption zone, and adsorption capacity per unit area.

[0170] The changes in methane adsorption per unit area and water adsorption inhibition coefficient under different water contents were statistically analyzed. (See attached data.) Figure 6 , Figure 6 The variation of methane adsorption per unit area and water adsorption inhibition coefficient under different water contents. Figure 6 The results show that the methane adsorption capacity per unit area decreases overall with increasing water content, while the water content adsorption inhibition coefficient increases overall with increasing water content. This indicates that water coverage and pore space compression weaken the methane adsorption capacity.

[0171] Example 2

[0172] Compared with Example 1, the difference is that the pressure point of the giant canonical Monte Carlo adsorption simulation is adjusted to 5 MPa, while other conditions are the same as in Example 1. The results of the number of methane molecules in the adsorption zone are shown in Table 6, which shows the number of methane molecules in the adsorption zone under different pressures.

[0173] Example 3

[0174] Compared with Example 1, the difference is that the pressure point of the giant canonical Monte Carlo adsorption simulation is adjusted to 15 MPa, while other conditions are the same as in Example 1. The results of the number of methane molecules in the adsorption zone are shown in Table 6, which shows the number of methane molecules in the adsorption zone under different pressures.

[0175] Table 6. Number of methane molecules in the adsorption zone under different pressures

[0176]

[0177] As can be seen from the pressure point data, under the same water content conditions, the number of molecules in the methane adsorption zone increases with increasing pressure; conversely, under the same pressure conditions, the number of molecules in the methane adsorption zone decreases with increasing water content. This trend indicates that the method provided in this application can evaluate both the effect of pressure on methane adsorption and identify the weakening effect of water content on methane adsorption capacity.

[0178] The embodiments described above merely illustrate specific implementation methods of this application, and while the descriptions are detailed and specific, they should not be construed as limiting the scope of protection of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the technical solution of this application, and these modifications and improvements all fall within the scope of protection of this application.

Claims

1. A method for detecting the methane adsorption capacity of coal and rock walls based on molecular simulation, characterized in that, Includes the following steps: Step S1: Construct a molecular model of the characteristic coal and rock wall, wherein the molecular model of the characteristic coal and rock wall includes a slit hole formed by two opposing walls; Step S2: Calculate the number of water molecules based on the pore volume of the slit and the target water saturation, and add the corresponding number of water molecules to the pore region of the slit to obtain a water-bearing slit. Step S3: Perform molecular dynamics simulation to achieve wetting equilibrium of the water-containing slit hole, so that the water molecules reach a stable storage state; Step S4: Under set conditions, perform a giant canonical Monte Carlo adsorption simulation on the water-containing slit pores obtained in step S3 to obtain the amount of methane in the adsorption zone. Step S5: Obtain the amount of methane adsorbed per unit area based on the amount of methane in the adsorption zone obtained in step S4.

2. The detection method according to claim 1, characterized in that, In step S2, the pore volume of the slit hole is calculated according to equation (1): Equation (1) In equation (1), Pore ​​volume, unit: nm 3 H represents the distance between the inner surfaces of the two coal and rock walls, in nm. The length of the slit hole in the direction parallel to the coal and rock wall is expressed in nm. The width of the slit hole in the direction parallel to the coal and rock wall is expressed in nm.

3. The detection method according to claim 2, characterized in that, In step S2, the number of water molecules is calculated according to equation (2): Equation (2) In equation (2), This represents the number of water molecules, expressed in units of molecules. The target water saturation level is expressed in % (%). Water molecule number density, in units of molecules / nm 3 ; Pore ​​volume, in nm 3 .

4. The detection method according to claim 3, characterized in that, The target water saturation is 0-60%.

5. The detection method according to claim 1, characterized in that, Step S3, after the water molecules reach a stable state, also includes obtaining the water density distribution, near-wall water film thickness, water coverage, and effective pore size.

6. The detection method according to claim 5, characterized in that, The water density distribution was obtained using the following method: The slit orifice is divided into several statistical layers along a direction perpendicular to the coal and rock wall. The average number of water molecules in each statistical layer is counted, and the water density distribution is calculated according to equation (3): Equation (3); in, This represents the water density distribution in the kth statistical layer, in units of cells / nm³. This represents the average number of water molecules located in the k-th statistical layer within the statistical time window, expressed in units of molecules. The length of the slit hole in the direction parallel to the coal and rock wall is expressed in nm. The width of the slit hole in the direction parallel to the coal and rock wall, in nm; The thickness of the k-th statistical layer is given in nm. The effective aperture is calculated according to equation (4): Equation (4) In equation (4), The effective aperture is in nm; H is the distance between the inner surfaces of the two coal and rock walls, in nm. The near-wall water film thickness is expressed in nm.

7. The detection method according to claim 1, characterized in that, In step S5, the amount of methane adsorbed per unit area is calculated according to equation (5): Equation (5) In equation (5), This represents the amount of methane adsorbed per unit area, expressed as cells / nm. 2 , The number of methane molecules in the adsorption zone is expressed in units of [number of molecules]. The effective area of ​​the coal and rock wall is expressed in nm. 2 .

8. The detection method according to claim 7, characterized in that, Also includes: Step S6: Obtain the water-containing adsorption inhibition coefficient based on the methane adsorption per unit area obtained in step S5.

9. The detection method according to claim 8, characterized in that, The water-containing adsorption inhibition coefficient is calculated according to equation (6): Equation (6) In equation (6), This represents the amount of methane adsorbed per unit area under aqueous conditions, expressed in cells / nm. 2 ; The adsorption capacity of methane per unit area under dry conditions is expressed in units per nm. 2 ; This represents the water adsorption inhibition coefficient.

10. The detection method according to any one of claims 1 to 9, characterized in that, Step S4 also includes obtaining the total number of methane molecules and the amount of methane in the free zone.

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