Deep coal bed gas adsorption state proportion calculation method and system
By establishing coal matrix wall and coalbed methane molecular models, conducting molecular dynamics simulation and data fitting, the problem of accurately calculating the proportion of adsorbed coalbed methane in deep layers was solved, the visualization and quantitative description of the coalbed methane occurrence state was achieved, and the research efficiency and accuracy were improved.
Patent Information
- Application Number
- CN202510771007.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-06-10
- Publication Date
- 2025-09-19
AI Technical Summary
The existing technology lacks a method to accurately measure the thickness of the deep coalbed methane adsorption layer in nano-scale pores, and cannot intuitively and effectively determine the occurrence state of deep coalbed methane. In particular, the absolute adsorption amount of deep coalbed methane is not accurately quantified under high-pressure conditions. Moreover, the experimental method will destroy the coal sample, making it impossible to conduct multi-condition experiments.
By establishing a coal matrix wall molecular model and a coalbed methane molecular model, molecular dynamics simulation was performed to obtain the coalbed methane adsorption equilibrium trajectory. Combined with density distribution calculation and NIST database, the coalbed methane isotherm adsorption curve and free state density equation were fitted, and the mass ratio of adsorbed and free states was calculated.
It has achieved accurate determination of adsorption layer thickness and absolute adsorption amount at the molecular scale, avoided coal sample destruction, improved research efficiency and accuracy, and provided a quantitative description of the occurrence state of deep coalbed methane and the scientific nature of the development plan.
Smart Images
Figure CN120673868A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of coal gas layer resource evaluation, and in particular to a method and system for calculating the adsorption state ratio of deep coal bed methane. Background Art
[0002] As a clean energy resource, the development and utilization of coalbed methane (CBM) can help offset the shortage of conventional oil and gas resources to a certain extent. Deep coalbed methane differs significantly from shallow and medium-layer coalbed methane in terms of its genesis, enrichment patterns, occurrence state, and development patterns. Deep coal reservoirs are rich in free gas, which is a prerequisite for high gas well production.
[0003] During coal formation, as organic matter decomposes under specific temperature and pressure conditions, methane gas gradually accumulates and is stored within coal seams. Coalbed methane can be divided into two forms: adsorbed gas, attached to the surface of coal particles through forces such as van der Waals forces; and free gas, freely flowing within the cracks of the coal seam. Because coal seams are primarily composed of organic matter and contain numerous nanoscale pores, the vast majority of coalbed methane exists in an adsorbed state. However, the extraction process inevitably involves the conversion of coalbed methane from an adsorbed state to a free state. Therefore, understanding these two distinct forms of coalbed methane is crucial for assessing potential resources and formulating effective development strategies.
[0004] Currently, the commonly used experimental methods for determining the adsorption capacity of coalbed methane include the volumetric method and the gravimetric method. However, the main problems with the current methods for determining the adsorption capacity and adsorption proportion of deep coalbed methane in coal seam nanopores include: there is still a lack of a method that can accurately measure the thickness of the deep coalbed methane adsorption layer in nano-scale pores, making it impossible to intuitively and effectively determine the occurrence state of deep coalbed methane; due to scale limitations, common experimental methods have technical flaws when studying the occurrence of deep coalbed methane in nanopores, especially under high-pressure conditions, and the absolute adsorption capacity of deep coalbed methane is not accurately quantified. It is urgent to consider the existence of the adsorption phase volume of deep coalbed methane or the accurate adsorption phase density to accurately calculate the adsorption capacity of deep coalbed methane and determine the adsorption and free state proportions of deep coalbed methane; the use of experimental methods will damage the coal sample, making it impossible to conduct multiple or multiple experiments on a single coal sample, and it is impossible to conduct other research on the same coal sample. Summary of the Invention
[0005] The present invention provides a method and system for calculating the adsorption ratio of deep coalbed methane, which are used to solve the defects of the prior art.
[0006] The present invention provides a method for calculating the adsorption ratio of deep coalbed methane, comprising: S1: Establish the coal matrix wall molecular model and coalbed methane molecular model to obtain the adsorption molecular model; S2: Based on the adsorption molecular model, a molecular dynamics simulation is performed on the coalbed methane molecules and the coal matrix wall molecular model to obtain the coalbed methane adsorption equilibrium trajectory; S3: performing statistical analysis on the coalbed methane adsorption equilibrium trajectory by density distribution calculation to obtain the coalbed methane adsorption amount; S4: Fitting the coalbed methane adsorption amount under different pressures to obtain the coalbed methane adsorption isotherm curve; S5: Fit the physical property parameters of free-state coalbed methane by querying the NIST database to obtain the free-state density equation of coalbed methane; S6: Calculate the mass of adsorbed coalbed methane and the mass of free coalbed methane according to the coalbed methane isotherm adsorption curve and the coalbed methane free state density equation to obtain the adsorbed coalbed methane ratio.
[0007] According to a method for calculating the adsorption percentage of deep coalbed methane provided by the present invention, step S1 further includes: S11: Establishment of molecular model of coal matrix wall; S12: Establish a molecular model of coalbed methane; S13: combining the coal matrix wall molecular model and the coalbed methane molecular model, and controlling the coalbed methane adsorption pressure to obtain an adsorption molecular model.
[0008] According to a method for calculating the adsorption percentage of deep coalbed methane provided by the present invention, step S1 further includes: S111: Selecting a monomer coal molecule and drawing a molecular model of the monomer coal molecule to obtain a first basic molecular model; S112: Introducing CVFF force field parameters into the first basic molecular model and converting it into a readable format to obtain a force field molecular model; S113: compressing the force field molecular models corresponding to the plurality of monomeric coal molecules through a simulation system to obtain a compressed molecular model; S114: performing energy minimization and coal molecule extension on the compressed molecular model to obtain a coal matrix wall molecular model.
[0009] According to a method for calculating the adsorption ratio of deep coalbed methane provided by the present invention, step S12 further includes: S121: creating a model file of a monatomic methane molecule and obtaining a molecular model file; S122: placing multiple monatomic methane molecules in the simulation system to obtain a second basic molecular model; S123: Adding the TraPPe-UA force field to the second basic molecular model to obtain a coalbed methane molecular model.
[0010] According to a method for calculating the adsorption percentage of deep coalbed methane provided by the present invention, step S2 further includes: S21: Set the force field cutoff radius and set the periodic boundary to obtain the basic adsorption system; S22: Minimizing the energy of the basic adsorption system to obtain a minimized adsorption system; S23: performing molecular dynamics simulation on the minimized adsorption system to output the adsorption molecular configuration of the equilibrium coalbed methane on the surface of the coal matrix, and obtaining the coalbed methane adsorption equilibrium trajectory.
[0011] According to a method for calculating the adsorption percentage of deep coalbed methane provided by the present invention, step S3 further includes: S31: performing molecular dynamics simulation on the adsorption molecular configuration of the equilibrium coalbed methane on the coal matrix surface corresponding to the coalbed methane adsorption equilibrium trajectory, and outputting a density distribution curve of the coalbed methane near the coal matrix wall; S32: selecting an adsorption state position from the density distribution curve, and calculating the thickness of the coalbed methane adsorption layer according to the adsorption state position; S33: Calculating the coalbed methane adsorption amount according to the thickness of the coalbed methane adsorption layer.
[0012] According to a method for calculating the adsorption ratio of deep coalbed methane provided by the present invention, step S5 further includes: S51: Query coalbed methane standard data; S52: Obtaining a coalbed methane free state density equation related to pressure based on the coalbed methane standard data through a linear fitting method.
[0013] According to a method for calculating the adsorption percentage of deep coalbed methane provided by the present invention, step S6 further includes: S61: Calculating the adsorbed mass of coalbed methane per unit length of pores based on the coalbed methane isotherm adsorption curve; S62: Calculating the free-state mass of coalbed methane per unit length of pores according to the free-state density equation of coalbed methane; S63: Calculate the adsorbed coalbed methane ratio based on the adsorbed coalbed methane mass and the free coalbed methane mass.
[0014] According to a method for calculating the adsorption ratio of deep coalbed methane provided by the present invention, the expression of the adsorption mass of the coalbed methane in step S61 is:
[0015] in, To calculate the adsorbed mass of coalbed methane, is the surface roughness of the coal matrix, is the YOZ area of the system, is the amount of coalbed methane adsorbed on the unit coal matrix wall; The expression of the free-state mass of the coalbed methane in step S62 is:
[0016] in, To calculate the free mass of coalbed methane, is the pore size, is the thickness of the coalbed methane adsorption layer, is the density of coalbed methane; The expression of the adsorbed state ratio of the coalbed methane in step S63 is:
[0017] in, is the mass ratio of adsorbed coalbed methane in the pores under preset conditions.
[0018] The present invention also provides a system for calculating the adsorption ratio of deep coalbed methane, comprising: Construction module: used to establish the coal matrix wall molecular model and coalbed methane molecular model, and obtain the adsorption molecular model; Simulation module: used to perform molecular dynamics simulation on the coalbed methane molecules and the coal matrix wall molecular model based on the adsorption molecular model to obtain the coalbed methane adsorption equilibrium trajectory; Statistics module: used for performing statistical analysis on the coalbed methane adsorption equilibrium trajectory through density distribution calculation to obtain the coalbed methane adsorption amount; The first fitting module is used to fit the coalbed methane adsorption amount under different pressures to obtain the coalbed methane isothermal adsorption curve; The second fitting module is used to fit the physical property parameters of free-state coalbed methane by querying the NIST database to obtain the free-state density equation of coalbed methane; Calculation module: used to calculate the mass of adsorbed coalbed methane and the mass of free coalbed methane according to the coalbed methane isotherm adsorption curve and the coalbed methane free state density equation, and obtain the adsorbed coalbed methane ratio.
[0019] The present invention provides a method and system for calculating the adsorbed state proportion of deep coalbed methane. The method compresses coal matrix molecules through molecular modeling software to construct a coal matrix wall molecular model with surface roughness. The method can truly restore the microstructural characteristics of deep coal reservoirs at the molecular scale. Compared with the limitation of traditional experimental methods that require coal samples to be crushed and destroyed, the present invention completely avoids the physical destruction of coal samples, so that the same coal sample can be repeatedly used for research under different conditions, greatly reducing experimental costs and improving research efficiency. At the same time, by precisely controlling the compression density of coal matrix molecules, the model is highly consistent with the actual physical properties of deep coal reservoirs, laying a reliable physical foundation for subsequent adsorption behavior analysis.
[0020] The present invention obtains the coalbed methane adsorption equilibrium trajectory by performing adsorption equilibrium processing on the coalbed methane molecules and the coal matrix wall molecular model through molecular dynamics simulation, which can visually observe the adsorption process and equilibrium state of the coalbed methane molecules on the molecular scale, solving the key problem that the experimental method cannot visualize the coalbed methane occurrence state. Moreover, through precise pressure control and temperature regulation, it realizes the accurate simulation of the real geological conditions of deep coal reservoirs, ensuring the geological applicability and engineering guidance value of the adsorption behavior research.
[0021] The present invention also performs statistical analysis on the coalbed methane adsorption equilibrium trajectory through density distribution calculation to obtain the coalbed methane density distribution curve and adsorption layer thickness, thereby realizing the precise measurement of the coalbed methane adsorption layer thickness in nano-scale pores, and solving the core technical problem that the traditional method cannot accurately determine the adsorption layer boundary due to scale limitations. Through high-precision spatial division and multiple statistical sampling, the statistical reliability and calculation accuracy of the density distribution data are ensured, providing a key geometric parameter basis for the precise calculation of the absolute adsorption amount of deep coalbed methane. Compared with the traditional method of estimating the approximate adsorption phase density, the present invention can directly obtain the real adsorption layer geometric information, significantly improving the accuracy of the adsorption amount calculation.
[0022] In addition, the present invention fits the coalbed methane adsorption amount under different pressures through the Langmuir isotherm adsorption equation to obtain the coalbed methane isotherm adsorption curve, and establishes a quantitative description model for the adsorption behavior of deep coalbed methane. The high-precision results of the fitting determination coefficient can prove the scientific nature and reliability of the present invention, and provide an accurate theoretical basis for the evaluation of the gas content of deep coal reservoirs and the design of development plans. Compared with the limitations of traditional experimental methods that require a large number of coal samples and long-term testing, the present invention can quickly obtain adsorption characteristic parameters within the entire pressure range, greatly improving the work efficiency of coalbed methane resource evaluation.
[0023] The present invention obtains the free-state density equation of coalbed methane by fitting the physical property parameters of free-state coalbed methane through NIST database query, ensuring the standardization and authority of the physical property parameters of free-state coalbed methane, avoiding possible systematic errors in experimental measurements, and providing reliable basic data support for the calculation of the proportion of adsorbed and free states.
[0024] Finally, the mass of adsorbed and free coalbed methane was calculated using the mass ratio calculation formula to obtain the adsorbed coalbed methane ratio in different pore sizes, realizing the quantitative distinction of the coalbed methane occurrence state in deep coal reservoirs, solving the key technical bottleneck that traditional methods cannot determine the specific ratio of adsorbed and free states, and providing important guarantees for the accuracy of deep coalbed methane reserve calculations. At the same time, by establishing a ratio map covering different pore sizes and pressure conditions, it provides on-site engineering and technical personnel with a simple and fast query tool, which significantly improves the scientific nature and pertinence of deep coalbed methane development plan formulation. BRIEF DESCRIPTION OF THE DRAWINGS
[0025] In order to more clearly illustrate the technical solutions in the present invention or the prior art, a brief introduction will be given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.
[0026] Figure 1 A schematic flow chart of a method for calculating the adsorption percentage of deep coalbed methane provided by the present invention; Figure 2 A schematic diagram of the molecular model of coalbed methane adsorbed on the surface of a coal matrix provided by the present invention; Figure 3 A schematic diagram of the density distribution curve of coalbed methane near the coal matrix wall provided by the present invention; Figure 4 A schematic diagram of the isothermal adsorption curve of coalbed methane on the surface of a coal matrix provided by the present invention; Figure 5 A schematic diagram of a curve showing the change of coalbed methane density with pressure at 323K and a fitted free-state density equation provided by the present invention; Figure 6 A chart showing the mass ratio of coalbed methane adsorption in different pores at 323K provided by the present invention; Figure 7 This is a schematic diagram of the structure of a deep coalbed methane adsorption state ratio calculation system provided by the present invention. DETAILED DESCRIPTION
[0027] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the drawings in the present invention. Obviously, the embodiments described are part of the embodiments of the present invention, not all of the embodiments, and they should not be understood as limitations on the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention. In the description of the present invention, it should be understood that the terms used are only for descriptive purposes and cannot be understood as indicating or implying relative importance.
[0028] In order to better understand the present invention, the research background of the present invention is first explained below.
[0029] As a clean energy resource, the development and utilization of coalbed methane (CBM) can help offset the shortage of conventional oil and gas resources to a certain extent. Currently, the vast industrial and residential gas market, coupled with abundant CBM resources, holds a promising future for its development. Deep CBM differs significantly from shallow and medium CBM in terms of its genesis, accumulation patterns, occurrence state, and development strategies. Deep coal reservoirs are rich in free gas, a prerequisite for high gas well production.
[0030] During coal formation, as organic matter decomposes under specific temperature and pressure conditions, methane gas gradually accumulates and is stored within coal seams. Based on its state of existence, coalbed methane can be divided into two forms: adsorbed gas, attached to the surface of coal particles through forces such as van der Waals forces; and free gas, freely flowing within coal seam fractures. Because coal seams are primarily organic and rich in nanoscale pores, the vast majority of coalbed methane exists in the adsorbed form. However, during extraction, the conversion of coalbed methane from the adsorbed to the free form is inevitable. Understanding these two distinct forms of coalbed methane is crucial for assessing potential resources and formulating effective development strategies. For example, when estimating reserves, the specific distribution and recoverability of methane under different occurrence states must be considered. Furthermore, when designing production plans, appropriate technical measures must be implemented based on the characteristics of the adsorbed and free forms. Therefore, accurately understanding the adsorbed content characteristics of coalbed methane is essential for efficient and safe extraction.
[0031] At present, the commonly used experimental methods to determine the adsorption capacity of coalbed methane are the volumetric method and the gravimetric method. The volumetric method is a method that measures the volume and pressure changes before and after coalbed methane adsorption and combines it with the state equation to calculate the adsorption amount. In the process of using the volumetric method, the free space volume is a necessary parameter for the adsorption amount calculation. Helium is usually used to calibrate the free space volume because helium is hardly adsorbed by any solid material and can accurately fill the free space in the container. However, methane will be strongly adsorbed by coal samples, especially under high pressure conditions. As the methane adsorption amount increases, the volume occupied by the adsorbed phase in the coal sample will reduce the actual free space volume. This change will also affect the final adsorption amount calculation results.
[0032] The gravimetric method calculates the adsorption capacity by measuring the mass and buoyancy changes of a coal sample before and after adsorption. This method can accurately measure the excess adsorption capacity of coalbed methane (the excess adsorption capacity is the portion of the adsorbed phase that exceeds the gas phase density). However, under high-pressure testing conditions, the excess adsorption capacity decreases as pressure increases. Therefore, using excess adsorption capacity when calculating coalbed methane reserves can significantly underestimate the gas content of deep coal seams. Furthermore, due to scale limitations, experimental methods like the gravimetric method cannot determine the thickness of the adsorption layer. Consequently, methods that derive absolute adsorption capacity from excess adsorption capacity often rely on approximate calculations of the density of the adsorbed phase. Therefore, gravimetric methods have limitations in providing accurate absolute adsorption capacity. Furthermore, laboratory experimental methods require crushing the coal rock sample into fine particles, which is destructive and can affect subsequent testing. In addition, both experimental methods cannot visualize the adsorption state of coalbed methane, let alone determine the proportion of the adsorbed state in deep coalbed methane, which has obvious limitations in deeply understanding the adsorption behavior of deep coalbed methane.
[0033] The main problems with the current methods for determining the adsorption amount and adsorption state ratio of deep coalbed methane in nanopores of coal seams include: there is still a lack of a method that can accurately measure the thickness of the deep coalbed methane adsorption layer in nano-scale pores, and it is impossible to intuitively and effectively determine the occurrence state of deep coalbed methane. Due to scale limitations, common experimental methods have technical flaws when studying the occurrence of deep coalbed methane in nanopores, especially under high-pressure conditions, and the absolute adsorption amount of deep coalbed methane is not accurately quantified. It is urgent to consider the existence of the adsorption phase volume of deep coalbed methane or the accurate density of the adsorption phase to accurately calculate the adsorption amount of deep coalbed methane and determine the adsorption state and free state ratio of deep coalbed methane. The use of experimental methods will damage the coal sample, and it is impossible to conduct experiments under multiple or multiple conditions on a coal sample, and it is impossible to conduct other research on the same coal sample.
[0034] like Figure 1As shown, the present invention provides a method for calculating the adsorption ratio of deep coalbed methane, including: S1: Establish the coal matrix wall molecular model and coalbed methane molecular model to obtain the adsorption molecular model.
[0035] Wherein, step S1 further includes: S11: Establish a molecular model of the coal matrix wall.
[0036] Wherein, step S1 further includes: S111: Select a monomer coal molecule and draw a molecular model of the monomer coal molecule to obtain a first basic molecular model; S112: Introduce CVFF force field parameters to the first basic molecular model and convert it into a readable format to obtain a force field molecular model; S113: Through a simulation system, compress the force field molecular models corresponding to multiple monomer coal molecules to obtain a compressed molecular model; S114: Minimize the energy of the compressed molecular model and extend the coal molecules to obtain a coal matrix wall molecular model.
[0037] In step S11, this embodiment uses a monomeric Shenfu coal molecule (SFC) as the monomeric coal molecule. In the specific implementation, Materials Studio software is first called to draw a molecular model of the SFC. Subsequently, the Amorphous Cell Calculation module of Materials Studio is called to add CVFF force field parameters to the single SFC molecular model and export it as SFC.car and SFC.mdf files.
[0038] Subsequently, SFC.car and SFC.mdf were imported into the msi2lmp package of Lammps and the file format was converted using . / msi2lmp.exe SFC -class I -frc cvff -i>data.SFC to obtain the monomer SFC.data file that can be read by the subsequent Lammps molecular simulation program.
[0039] Subsequently, 205,074 individual coal molecules were placed in a 5 nm × 5 nm × 15 nm simulation system in the software package PACKMOL. A virtual moving wall was set at the maximum value of X. The virtual wall was moved in the negative X direction at a speed of 0.001 nm / ps to gradually compress the coal molecules until the coal matrix density with 24 coal molecules reached the reference density of 1.3 g / cm3 in the article. During the above process, a canonical ensemble was applied to the SFC molecules, the temperature was 323 K, and the calculation step size was 1 fs.
[0040] After obtaining the compressed coal molecules, the compressed virtual wall was removed and a 1000-step energy minimization operation was applied to the entire system to eliminate the high stress on the coal matrix wall and stretch the coal molecules at the wall, so that the coal matrix wall has a certain degree of roughness. The Construct surface mesh function was used in OVITO to calculate the area of the rough coal matrix surface. , the YOZ area of the simulation box is , roughness is defined according to the following formula, and the calculated roughness is .
[0041]
[0042] in, is the roughness of the coal matrix surface, is the surface area of the rough coal matrix, is the YOZ area of the simulation box.
[0043] S12: Establish a molecular model of coalbed methane.
[0044] Wherein, step S12 further includes: S121: Create a model file of a monatomic methane molecule to obtain a molecular model file; S122: Place multiple monatomic methane molecules in a simulation system to obtain a second basic molecular model; S123: Add a TraPPe-UA force field to the second basic molecular model to obtain a coalbed methane molecular model.
[0045] In step S12, the model file CH4.xyz of the coalbed methane monatomic methane molecule is first written, and then 1000 methane molecules are placed in a 5nm×5nm×5nm simulation system in the software package PACKMOL. The TraPPe-UA force field is added and exported as a CH4.data file that can be recognized by Lammps to complete the construction of the coalbed methane molecular model.
[0046] Then, the CBM molecular model and the coal matrix wall molecular model are combined, and a movable piston is added in the positive direction of X of the CBM. The adsorption pressure of the CBM is controlled according to the following formula. Finally, the molecular model of CBM adsorption on the coal matrix surface is completed.
[0047]
[0048] in, is the system pressure, is the force applied to the piston.
[0049] S13: combining the coal matrix wall molecular model and the coalbed methane molecular model, and controlling the coalbed methane adsorption pressure to obtain an adsorption molecular model.
[0050] Finally, in step S13, the present invention combines the coal matrix wall molecular model and the coalbed methane molecular model and controls the coalbed methane adsorption pressure to obtain the adsorption molecular model as the spatial splicing of two independent systems, such as Figure 2 As shown in the figure, in the obtained molecular model, the coal matrix wall molecular model is kept in the left area of the simulation box, and the coalbed methane molecular model is placed in the right area. There is an appropriate gap between the two for the diffusion and adsorption of methane molecules. The movable piston is set at the right end boundary of the coalbed methane area. The piston is moved by applying an external force. The system pressure is controlled, and the piston position is dynamically adjusted through force balance conditions until the system pressure stabilizes at the set value. The entire adsorption molecular model contains three main components: coal matrix molecules, methane molecules, and pistons, laying the structural foundation for subsequent molecular dynamics adsorption simulations.
[0051] S2: Based on the adsorption molecular model, a molecular dynamics simulation is performed on the coalbed methane molecules and the coal matrix wall molecular model to obtain the coalbed methane adsorption equilibrium trajectory.
[0052] Wherein, step S2 further includes: S21: Set the force field cutoff radius and set the periodic boundary to obtain the basic adsorption system.
[0053] In step S21, the force field cutoff radius is first set, that is, the interaction between two atoms outside the cutoff radius can be ignored. The purpose is to reduce the calculation time, and periodic boundaries are taken in the xyz directions of the model. That is, when molecules move out of the model boundary, the same number of molecules must return to the model from the opposite interface, thereby ensuring that the number of particles in the simulation system is constant, which can effectively reduce the boundary effect caused by the scale limitation of the simulation system.
[0054] S22: Minimizing the energy of the basic adsorption system to obtain a minimized adsorption system.
[0055] Furthermore, in step S32, overlapping atomic configurations in the coalbed methane adsorption system are eliminated by energy minimization, aiming to eliminate atomic overlap and unreasonable geometric configurations in the initial configuration. The main steps are to find the local energy minimum of the system through the gradient descent algorithm or the conjugate gradient algorithm. Finally, after the system energy is stabilized, the molecular configuration of the coalbed methane adsorption system with minimized structural energy is obtained.
[0056] S23: performing molecular dynamics simulation on the minimized adsorption system to output the adsorption molecular configuration of the equilibrium coalbed methane on the surface of the coal matrix, and obtaining the coalbed methane adsorption equilibrium trajectory.
[0057] The molecular dynamics simulation stage requires solving Newton's equations of motion to obtain the position and velocity changes of each atom in each time step. In each time step, the program needs to calculate the interaction forces between all atomic pairs, update the position and velocity of each atom, and record macroscopic physical quantities such as the temperature, pressure, and energy of the system. When the total energy, temperature, and pressure of the system remain stable in a statistical sense, it is considered to have reached a thermodynamic equilibrium state. At this time, the output molecular configuration trajectory file contains the coordinate information of all atoms in each time step.
[0058] Specifically, in step S13, the present invention fixes the coal matrix wall surface based on the molecular configuration of the CBM adsorption system, which was minimized in the previous step, to prevent wall drift from affecting the accuracy of the calculation. Specifically, the system is placed in the NVT canonical ensemble and run for 3 nanoseconds at 323K with a step size of 1 fs. During this process, pressure is applied to the piston according to the formula in step S12, maintaining the CBM adsorption pressure at 10 MPa. After 3 ns, the equilibrium position of the piston is determined, ultimately obtaining the equilibrium molecular configuration of the CBM adsorbed on the coal matrix surface.
[0059] S3: Statistically analyzing the coalbed methane adsorption equilibrium trajectory through density distribution calculation to obtain the coalbed methane adsorption amount.
[0060] Wherein, step S3 further includes: S31: performing molecular dynamics simulation on the adsorption molecular configuration of the equilibrium coalbed methane on the coal matrix surface corresponding to the coalbed methane adsorption equilibrium trajectory, and outputting a density distribution curve of the coalbed methane near the coal matrix wall.
[0061] In step S31, the spatial density distribution of the CBM adsorption equilibrium trajectory file is first calculated. This density distribution calculation is performed by dividing the simulation box into several thin layers perpendicular to the coal matrix wall, and then counting the number density of CBM molecules within each thin layer. During the statistical process, all equilibrium trajectory frames are time-averaged. This means that the number of molecules in each grid layer across all frames is added together and divided by the total number of frames to obtain the average molecular number density for that grid layer. The molecular number density is then converted to mass density by dividing by the grid volume. The final output is a density distribution curve with the distance from the coal matrix surface as the horizontal axis and the density value as the vertical axis.
[0062] S32: selecting an adsorption state position from the density distribution curve, and calculating the thickness of the coalbed methane adsorption layer according to the adsorption state position.
[0063] The characteristic of the density distribution curve is that there will be obvious density peaks near the surface of the coal matrix. These peaks correspond to the adsorption layer positions of the coalbed methane molecules. The adsorption state position is determined by identifying the local maximum point on the density curve. Specifically, the present invention adopts a peak detection method. First, the density distribution curve is smoothed to eliminate statistical noise, and then the first-order and second-order derivatives of each point on the curve are calculated. When the first-order derivative is zero and the second-order derivative is negative, the point is identified as a local maximum point. After determining the adsorption state position, it is necessary to calculate the thickness of the coalbed methane adsorption layer. The calculation method of the adsorption layer thickness is to find the half-peak width of the first peak on the density curve or to find the position where the density value drops to the bulk density. The bulk density refers to the density of the coalbed methane away from the wall. Usually, the average density value far enough away from the wall is taken as a reference.
[0064] S33: Calculating the coalbed methane adsorption amount according to the thickness of the coalbed methane adsorption layer.
[0065] The calculation of the coalbed methane adsorption capacity in step S33 of the present invention is based on the adsorption layer thickness and the average density within the adsorption layer. First, the boundary range of the adsorption layer needs to be determined. The lower boundary is set as the position on the surface of the coal matrix, and the upper boundary is set as the position where the density curve drops to the bulk density or the position of the first density valley. After the adsorption layer range is determined, the excess density of all grid layers within the range is integrated and summed. The excess density refers to the difference between the actual density of a point in the adsorption layer and the bulk density. The integral calculation formula is that the adsorption capacity is equal to the excess density in the range from the lower boundary to the upper boundary multiplied by the integral of the distance element. The numerical integration divides the integral interval into several small intervals. In each small interval, the excess density curve is approximated by a straight line or parabola. The total adsorption capacity is then calculated by summing the areas of each small interval.
[0066] In the specific implementation, the equilibrium coalbed methane adsorption molecular system on the coal matrix surface is simulated for another 2 nanoseconds under the original simulation conditions. At this time, the piston is fixed, and a data statistical box is established at intervals of 0.005 nm along the X direction in the space where the coalbed methane is located. The average value of 2,000,000 groups of coalbed methane mass distribution is taken with a step size of 1 femtosecond according to the following formula to obtain the density distribution curve of coalbed methane near the coal matrix wall, as shown in the following figure: Figure 3 shown.
[0067]
[0068] in, For coalbed methane The average density in the data box, For coalbed methane The number of data in the statistics box, is the relative molecular mass of coalbed methane, is Avogadro's constant, The volume of the statistical box.
[0069] Depend on Figure 3 The density distribution curve shows that an obvious adsorption layer of coalbed methane appears on the wall of the coal matrix. The position where the free state far away from the wall fluctuates according to the change of coalbed methane density is defined as the adsorption state. The calculated thickness of the coalbed methane adsorption layer is 1.406 nm. The following formula is used to calculate the coalbed methane adsorption amount per unit area. The adsorption amount calculation result is: .
[0070]
[0071] Where, is the amount of coalbed methane adsorbed on the unit coal matrix wall surface, and the sum symbol represents the sum of the product of the coalbed methane density within the adsorption layer and the volume of the statistical box.
[0072] S4: Fit the coalbed methane adsorption amount under different pressures to obtain the coalbed methane adsorption isotherm curve.
[0073] Subsequently, step S3 was repeated to obtain the adsorption equilibrium trajectory pressure of coalbed methane on the coal matrix surface through molecular simulation, and the pressure added to the piston was changed. The coalbed methane adsorption study was carried out with adsorption pressures of 10 MPa, 12 MPa, 14 MPa, 16 MPa, 20 MPa, 22 MPa, 24 MPa, 26 MPa, 28 MPa, and 30 MPa, respectively. The adsorption layer thickness and coalbed methane adsorption amount were accurately calculated as shown in Table 1.
[0074] Table 1 Adsorption parameters of coalbed methane in Shenfu coal (SFC) at 323K and 10MPa-30MPa
[0075] According to the data in Table 1, the isothermal adsorption curve of coalbed methane on the coal matrix surface at 323K is fitted in the form of Langmuir isothermal adsorption curve. Figure 4 As shown in the following formula and the corresponding calculated value, the results show that the fitting accuracy , proving that the fitting curve can well characterize the adsorption characteristics of coalbed methane.
[0076]
[0077] Where, is the Langmuir maximum adsorption capacity, is the Langmuir adsorption constant, is the system pressure.
[0078] S5: Fit the physical property parameters of free coalbed methane by querying the NIST database to obtain the free coalbed methane density equation.
[0079] Wherein, step S5 further includes: S51: Query coalbed methane standard data.
[0080] Since the main components of coalbed methane are , so the present invention searches the NIST database The query range corresponds to the calculation conditions of fitting the coalbed methane isothermal adsorption curve, that is, the temperature is 323K and the pressure is 10MPa-30MPa. The final query result of CH4 density is shown in Table 2.
[0081] Table 2 Density variation parameters of free CH4 at 323K with respect to pressure
[0082] S52: Obtaining a coalbed methane free state density equation related to pressure based on the coalbed methane standard data through a linear fitting method.
[0083] Specifically, based on the data in Table 2, with pressure as the horizontal axis and coalbed methane density as the vertical axis, the present invention plots the change of coalbed methane density with pressure under 323K conditions, and then uses a linear fitting method to fit the coalbed methane free state density equation with respect to pressure. The final equation diagram is shown as follows: Figure 5 As shown, the corresponding fitting equation is the following expression, and the fitting determination coefficient is 0.994, indicating that the fitting effect is good.
[0084]
[0085] Where, is the density of coalbed methane.
[0086] S6: Calculate the mass of adsorbed coalbed methane and the mass of free coalbed methane according to the coalbed methane isotherm adsorption curve and the coalbed methane free state density equation to obtain the adsorbed coalbed methane ratio.
[0087] Wherein, step S6 further includes: S61: Calculate the adsorbed mass of coalbed methane in unit length of pores based on the coalbed methane isotherm adsorption curve; S62: Calculate the free mass of coalbed methane in unit length of pores based on the coalbed methane free state density equation; S63: Calculate the adsorbed proportion of coalbed methane based on the adsorbed mass of coalbed methane and the free mass of coalbed methane.
[0088] The expression of the adsorbed mass of coalbed methane in step S61 is:
[0089] in, To calculate the adsorbed mass of coalbed methane, is the surface roughness of the coal matrix, is the YOZ area of the system, is the amount of coalbed methane adsorbed on the unit coal matrix wall; The expression of the free-state mass of the coalbed methane in step S62 is:
[0090] in, To calculate the free mass of coalbed methane, is the pore size, is the thickness of the coalbed methane adsorption layer, is the density of coalbed methane; The expression of the adsorbed state ratio of the coalbed methane in step S63 is:
[0091] in, is the mass ratio of adsorbed coalbed methane in the pores under preset conditions.
[0092] In step S6, according to the above-mentioned formula for the proportion of coalbed methane adsorbed in unit length of pores, combined with the fitted coalbed methane isotherm adsorption curve, the adsorbed mass of coalbed methane in unit length of pores is calculated according to the above formula, combined with the coalbed methane free state density equation, the free mass of coalbed methane in unit length of pores is calculated according to the above formula, and finally the proportion of the adsorbed mass of coalbed methane in nanopores of different sizes is calculated.
[0093] In the embodiment of the present invention, the calculated parameter values are as follows.
[0094]
[0095]
[0096]
[0097] Where, is the mass ratio of adsorbed coalbed methane in pores at 323K.
[0098] After step S6, the present invention also draws a graph showing the proportion of coalbed methane adsorption and free states. Specifically, according to the formula obtained in step S6, the roughness is taken as =1.1, different The value can be obtained by plotting the mass ratio of coalbed methane adsorption in different pores at 323K, as shown in Figure 6 As shown, the occurrence status of coalbed methane can be quickly queried based on the map.
[0099] like Figure 7 As shown, the present invention also provides a deep coalbed methane adsorption state ratio calculation system, comprising: Construction module 100: used to establish a coal matrix wall molecular model and a coalbed methane molecular model, and obtain an adsorption molecular model; Simulation module 200: for performing molecular dynamics simulation on the coalbed methane molecules and the coal matrix wall molecular model based on the adsorption molecular model to obtain the coalbed methane adsorption equilibrium trajectory; Statistics module 300: for performing statistical analysis on the coalbed methane adsorption equilibrium trajectory through density distribution calculation to obtain the coalbed methane adsorption amount; The first fitting module 400 is used to fit the coalbed methane adsorption amount under different pressures to obtain the coalbed methane adsorption isotherm curve; The second fitting module 500 is used to fit the physical property parameters of free-state coalbed methane by querying the NIST database to obtain the free-state density equation of coalbed methane; Calculation module 600: used to calculate the mass of adsorbed coalbed methane and the mass of free coalbed methane according to the coalbed methane isotherm adsorption curve and the coalbed methane free state density equation, and obtain the adsorbed coalbed methane ratio.
[0100] The device embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate, and the components shown as units may or may not be physical units, i.e., they may be located in one location or distributed across multiple network units. Some or all of the modules may be selected based on actual needs to achieve the objectives of the present embodiment. Persons of ordinary skill in the art will be able to understand and implement the present invention without inventive effort.
[0101] Through the above description of the embodiments, those skilled in the art will clearly understand that each embodiment can be implemented using software plus a necessary general-purpose hardware platform, or of course, hardware. Based on this understanding, the essence of the above technical solution, or the portion that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as ROM / RAM, a magnetic disk, or an optical disk, and includes a number of instructions for causing a computer device (such as a personal computer, server, or network device) to execute the methods described in each embodiment or certain portions of the embodiments.
[0102] The present invention provides a method and system for calculating the adsorption state ratio of deep coalbed methane, which can visualize the occurrence state of deep coalbed methane, define the adsorption layer thickness of deep coalbed methane from the molecular scale, and accurately determine the absolute adsorption amount of deep coalbed methane on the coal rock surface; secondly, by combining the NIST database of coalbed methane, the free state density equation of deep coalbed methane is calculated, and the mass ratio of adsorption state and free state of deep coalbed methane in nanopores is determined; it also makes full use of the advantages of molecular simulation, avoids the destruction of coal samples caused by the use of volumetric and gravimetric methods, and is conducive to the reuse of coal samples; in addition, a chart for calculating the adsorption amount of deep coalbed methane is formed, which is simple, convenient, fast and has high prediction accuracy.
[0103] 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 make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.
Claims
1. A method for calculating the adsorption ratio of deep coalbed methane, characterized in that: include: S1: Establish the coal matrix wall molecular model and coalbed methane molecular model to obtain the adsorption molecular model; S2: Based on the adsorption molecular model, a molecular dynamics simulation is performed on the coalbed methane molecules and the coal matrix wall molecular model to obtain the coalbed methane adsorption equilibrium trajectory; S3: performing statistical analysis on the coalbed methane adsorption equilibrium trajectory by density distribution calculation to obtain the coalbed methane adsorption amount; S4: Fitting the coalbed methane adsorption amount under different pressures to obtain the coalbed methane adsorption isotherm curve; S5: Fit the physical property parameters of free-state coalbed methane by querying the NIST database to obtain the free-state density equation of coalbed methane; S6: Calculate the mass of adsorbed coalbed methane and the mass of free coalbed methane according to the coalbed methane isotherm adsorption curve and the coalbed methane free state density equation to obtain the adsorbed coalbed methane ratio.
2. The method for calculating the adsorption ratio of deep coalbed methane according to claim 1, characterized in that: Step S1 further comprises: S11: Establishment of molecular model of coal matrix wall; S12: Establish a molecular model of coalbed methane; S13: combining the coal matrix wall molecular model and the coalbed methane molecular model, and controlling the coalbed methane adsorption pressure to obtain an adsorption molecular model.
3. The method for calculating the adsorption ratio of deep coalbed methane according to claim 2, characterized in that: Step S1 further comprises: S111: Selecting a monomer coal molecule and drawing a molecular model of the monomer coal molecule to obtain a first basic molecular model; S112: Introducing CVFF force field parameters into the first basic molecular model and converting it into a readable format to obtain a force field molecular model; S113: compressing the force field molecular models corresponding to the plurality of monomeric coal molecules through a simulation system to obtain a compressed molecular model; S114: performing energy minimization and coal molecule extension on the compressed molecular model to obtain a coal matrix wall molecular model.
4. The method for calculating the adsorption ratio of deep coalbed methane according to claim 2, characterized in that: Step S12 further includes: S121: creating a model file of a monatomic methane molecule and obtaining a molecular model file; S122: placing multiple monatomic methane molecules in the simulation system to obtain a second basic molecular model; S123: Adding the TraPPe-UA force field to the second basic molecular model to obtain a coalbed methane molecular model.
5. The method for calculating the adsorption ratio of deep coalbed methane according to claim 1, characterized in that: Step S2 further comprises: S21: Set the force field cutoff radius and set the periodic boundary to obtain the basic adsorption system; S22: Minimizing the energy of the basic adsorption system to obtain a minimized adsorption system; S23: performing molecular dynamics simulation on the minimized adsorption system to output the adsorption molecular configuration of the equilibrium coalbed methane on the surface of the coal matrix, and obtaining the coalbed methane adsorption equilibrium trajectory.
6. The method for calculating the adsorption ratio of deep coalbed methane according to claim 1, characterized in that: Step S3 further comprises: S31: performing molecular dynamics simulation on the adsorption molecular configuration of the equilibrium coalbed methane on the coal matrix surface corresponding to the coalbed methane adsorption equilibrium trajectory, and outputting a density distribution curve of the coalbed methane near the coal matrix wall; S32: selecting an adsorption state position from the density distribution curve, and calculating the thickness of the coalbed methane adsorption layer according to the adsorption state position; S33: Calculating the coalbed methane adsorption amount according to the thickness of the coalbed methane adsorption layer.
7. The method for calculating the adsorption ratio of deep coalbed methane according to claim 1, characterized in that: Step S5 further comprises: S51: Query coalbed methane standard data; S52: Obtaining a coalbed methane free state density equation related to pressure based on the coalbed methane standard data through a linear fitting method.
8. The method for calculating the adsorption ratio of deep coalbed methane according to claim 1, characterized in that: Step S6 further comprises: S61: Calculating the adsorbed mass of coalbed methane per unit length of pores based on the coalbed methane isotherm adsorption curve; S62: Calculating the free-state mass of coalbed methane per unit length of pores according to the free-state density equation of coalbed methane; S63: Calculate the adsorbed coalbed methane ratio based on the adsorbed coalbed methane mass and the free coalbed methane mass.
9. The method for calculating the adsorption ratio of deep coalbed methane according to claim 1, characterized in that: The expression of the adsorbed mass of the coalbed methane in step S61 is: in, To calculate the adsorbed mass of coalbed methane, is the surface roughness of the coal matrix, is the YOZ area of the system, is the amount of coalbed methane adsorbed on the unit coal matrix wall; The expression of the free-state mass of the coalbed methane in step S62 is: in, To calculate the free mass of coalbed methane, is the pore size, is the thickness of the coalbed methane adsorption layer, is the density of coalbed methane; The expression of the adsorbed state ratio of the coalbed methane in step S63 is: in, is the mass ratio of adsorbed coalbed methane in the pores under preset conditions.
10. A system for calculating the adsorption ratio of deep coalbed methane, characterized in that: include: Construction module: used to establish the coal matrix wall molecular model and coalbed methane molecular model, and obtain the adsorption molecular model; Simulation module: used to perform molecular dynamics simulation on the coalbed methane molecules and the coal matrix wall molecular model based on the adsorption molecular model to obtain the coalbed methane adsorption equilibrium trajectory; Statistics module: used for performing statistical analysis on the coalbed methane adsorption equilibrium trajectory through density distribution calculation to obtain the coalbed methane adsorption amount; The first fitting module is used to fit the coalbed methane adsorption amount under different pressures to obtain the coalbed methane isothermal adsorption curve; The second fitting module is used to fit the physical property parameters of free-state coalbed methane by querying the NIST database to obtain the free-state density equation of coalbed methane; Calculation module: used to calculate the mass of adsorbed coalbed methane and the mass of free coalbed methane according to the coalbed methane isotherm adsorption curve and the coalbed methane free state density equation, and obtain the adsorbed coalbed methane ratio.