Deep coalbed methane desorption potential evaluation method based on methane carbon isotope fractionation
By constructing an atomic-scale coal molecular structure model and performing methane carbon isotope fractionation, the trajectory of methane molecules is simulated, solving the problem of high-precision quantitative evaluation of the desorption process in deep coalbed methane. This method is applicable to unconventional natural gas reservoirs and provides a quantitative basis for development strategies.
Patent Information
- Application Number
- CN202610757829.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-05-29
- Publication Date
- 2026-08-25
AI Technical Summary
Existing technologies cannot accurately simulate the desorption process of deep coalbed methane, nor can they quantitatively evaluate its potential. In particular, under high temperature and high pressure conditions, traditional methods cannot take into account isotope fractionation effects and dynamic changes.
A specific coal molecular structure model at the atomic scale was constructed. Combined with methane carbon isotope fractionation, the trajectory of methane molecules was recorded through molecular dynamics simulation. A quantitative functional relationship was established, the diffusion coefficient ratio and residual gas volume were inverted, and the desorption stage was divided.
It enables high-precision quantitative evaluation of deep coalbed methane, is applicable to unconventional natural gas reservoirs, reduces evaluation costs and time, and provides quantitative basis for development strategies.
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Figure CN122631478A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of coalbed methane exploration and development technology, specifically relating to a method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation. Background Technology
[0002] Deep coalbed methane (depth > 1000m) has enormous resource potential, but its complex occurrence mechanism, high desorption difficulty, and challenging resource evaluation are the core bottlenecks in current exploration and development. The desorption-diffusion process of methane in coal seam pores is controlled by the coupled effects of multiple factors, including reservoir temperature and pressure, coal molecular structure, moisture, and minerals. 12 CH4 and 13 CH4 undergoes significant isotopic fractionation during diffusion due to mass differences. This fractionation characteristic directly records the desorption process of the gas and the characteristics of the remaining gas. Currently, the evaluation methods for coalbed methane desorption potential mainly include: First, isothermal adsorption experiments, which fit the maximum adsorption amount using the Langmuir equation, but cannot simulate the dynamic process of depressurization desorption, and the experimental conditions are difficult to cover deep high-temperature (>150℃) and high-pressure (>20MPa) environments; Second, in-situ desorption tests, which can directly obtain the amount and composition of desorbed gas, but are time-consuming and costly, and can only obtain the final total amount of desorption, failing to reveal the desorption stages and the distribution of residual gas; Third, numerical simulation methods, such as dual-porosity media models, usually simplify the diffusion coefficient to a constant, ignoring the dynamic changes of the diffusion coefficient with the desorption process and isotope types, and lacking a bridge with measured geochemical parameters; Fourth, existing isotope studies are mostly qualitative descriptions, and have not established an "isotope signal" model. Desorption rate A quantitative model for "residual gas volume".
[0003] Due to the aforementioned shortcomings, existing technologies cannot achieve high-precision simulation of the deep coalbed methane desorption process and quantitative evaluation of its remaining potential. Therefore, it is necessary to design a method for evaluating the desorption potential of deep coalbed methane to improve upon these issues. Summary of the Invention
[0004] To address the problems of existing technologies, this invention provides a method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation, comprising the following steps: S1. Based on the measured geological and geochemical parameters of the target coal sample, construct an atomic-scale specific coal molecular structure model, and conduct methane adsorption simulation under reservoir temperature and pressure conditions to obtain initial methane occurrence state parameters, including the initial total number of methane molecules N. initial The number of adsorbed methane molecules N ads and the number of free methane molecules N free ; S2. Based on the aforementioned specific coal molecular structure model, perform methane depressurization desorption-diffusion molecular dynamics simulation and record... 13 CH4 and 12 The trajectory of CH4 molecules was calculated at different times t. 12 diffusion coefficient of CH4 and 13 diffusion coefficient of CH4 The diffusion coefficient was obtained as a ratio And calculate the cumulative desorption rate. Isotope values of produced gas and the number of remaining adsorbed molecules and the number of remaining free molecules ; S3. The diffusion coefficient obtained from the simulation in step S2 is compared to Cumulative desorption rate Isotope values of produced gas Number of remaining adsorbed molecules and the number of remaining free molecules The following quantitative functional relationship is established through mathematical fitting: diffusion coefficient ratio With cumulative desorption rate Functional relationship: ; diffusion coefficient ratio Isotopic values of produced gas Functional relationship: ; and the number of remaining adsorbed molecules Number of remaining free molecules Ratio to diffusion coefficient Functional relationship: ; Where c, h, p, and q are fitting constants. , This is the fitted function; S4. Obtain the measured methane carbon isotope values of the deep coalbed methane sample to be evaluated. Substitute it into the diffusion coefficient ratio in step S3 Isotopic values of produced gas In the functional relationship, the current diffusion coefficient of the sample is obtained by inversion. ; S5, compare the current diffusion coefficient with Substituting the diffusion coefficient ratio into step S3 With cumulative desorption rate In the functional relationship, the cumulative desorption rate is obtained by inversion. and the obtained diffusion coefficient is compared with Substitute the number of remaining adsorbed molecules from step S3 Number of remaining free molecules Ratio to diffusion coefficient In the functional relationship, the number of molecules in the current remaining adsorbed state is obtained by inversion. and the number of free molecules ; S6. Based on the cumulative desorption rate obtained from the inversion. Number of remaining adsorbed molecules Number of free molecules and the initial total number of methane molecules N initia The remaining gas volume, adsorbed gas ratio, and free gas ratio of the sample to be evaluated are calculated to complete the quantitative evaluation of the desorption potential of deep coalbed methane.
[0005] Furthermore, step S3 also includes: based on the diffusion coefficient ratio For cumulative desorption rate first derivative d / d The positive, negative, or zero value of the value divides the desorption process into three stages: the initial stage, the stable stage, and the final stage. This serves as a quantitative standard for determining the desorption stage of coalbed methane.
[0006] Furthermore, in step S1, the specific method for constructing a dedicated coal molecular structure model includes: using the elemental composition, functional group composition, pore structure characteristics, mineral and water content information of the target coal sample as constraints, determining the size of the aromatic cluster core, introducing heteroatoms and functional groups, constructing a three-dimensional cross-linked network of aromatic clusters and forming a pore system that matches the measured data, embedding kaolinite, montmorillonite clay minerals, and water molecules represented by a spherical trap model; then performing molecular dynamics equilibrium simulation under reservoir temperature and pressure conditions, iteratively optimizing until the calculated density and specific surface area of the model match the measured values of the sample.
[0007] Furthermore, in step S2, the molecular dynamics simulation employs a molecular force field suitable for organic matter-hydrocarbon-water multiphase systems to simulate the reservoir depressurization process, records molecular trajectories, and calculates the diffusion coefficient within a fixed time window. The calculation formula is as follows: ; in, represent 12 CH4 or 13 CH4; D is the diffusion coefficient; MSD is the mean square displacement; It is a fixed time window.
[0008] Furthermore, in step S3, the cumulative desorption rate The calculation formula is: ; in, The cumulative desorption rate; This represents the cumulative number of methane molecules desorbed up to time t. This represents the original number of methane molecules; Carbon isotopes of produced gas The calculation formula is: ; in, Deadline Hourly gas isotope values; of 13 The number of CH4 molecules was obtained statistically. For desorption at time t 12 The number of CH4 molecules was obtained statistically. For the international standard reference material VPDB 13 C / 12 C ratio.
[0009] Furthermore, in step S6, the remaining gas volume The calculation formula is: ; in, Let Avogadro's constant be 1. This represents the remaining volume of methane in the model at time t.
[0010] Furthermore, the volume of adsorbed gas With free gas volume The calculation formula is: ; in, , These represent the remaining adsorbed methane volume and free methane volume in the model at time t, respectively.
[0011] Furthermore, the method is applicable to coalbed methane reservoirs with a burial depth greater than 1000m, and can be extended to the evaluation of desorption potential in unconventional natural gas reservoirs such as shale gas and tight sandstone gas.
[0012] The beneficial effects that this invention can produce include: 1) The method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation provided by this invention is suitable for deep high temperature and high pressure conditions and can be extended to unconventional natural gas fields such as shale gas and tight sandstone gas. It has a wide range of applications. By constructing an atomic-scale exclusive coal molecular structure model to restore the real desorption behavior, and coupling the isotope fractionation effect to establish a quantitative correlation model, it realizes microscopic simulation, isotope diagnosis and quantitative assessment of remaining potential. 2) This invention establishes a quantitative mathematical model for the diffusion coefficient ratio, cumulative desorption rate, and produced gas isotopes, and incorporates measured data. The value is directly converted into a quantitative indicator of desorption potential, breaking through the limitations of qualitative description, with d / d The core quantitative classification standard for the desorption stage objectively distinguishes the initial, stable, and final stages of desorption, providing a direct basis for adjusting development strategies; it simultaneously enables quantitative calculation of remaining gas volume and the ratio of adsorbed gas to free gas, providing key parameters for reserve assessment and fracturing selection; the evaluation can be completed with only one gas sample isotope measurement value and a small amount of basic geochemical data, eliminating the need for time-consuming on-site desorption experiments, thus achieving high efficiency. Attached Figure Description
[0013] Figure 1 This is a flowchart of the method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation according to the present invention. Figure 2 As described in the embodiments of the present invention 12 CH4 / 13 CH4 diffusion coefficient ratio as a function of cumulative desorption rate and desorption stage division curve; Figure 3 The gas δ produced in this embodiment of the invention 13 C ratio to diffusion coefficient Quantitative relationship chart. Detailed Implementation
[0014] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0015] Please see Figure 1-3 This invention provides a method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation, comprising the following steps: S1. Based on the measured geological and geochemical parameters of the target coal sample, construct an atomic-scale specific coal molecular structure model, and conduct methane adsorption simulation under reservoir temperature and pressure conditions to obtain initial methane occurrence state parameters, including the initial total number of methane molecules N. initial The number of adsorbed methane molecules N ads and the number of free methane molecules N free ; S2. Based on the aforementioned specific coal molecular structure model, perform methane depressurization desorption-diffusion molecular dynamics simulation and record... 13 CH4 and 12The trajectory of CH4 molecules was calculated at different times t. 12 diffusion coefficient of CH4 and 13 diffusion coefficient of CH4 The diffusion coefficient was obtained as a ratio And calculate the cumulative desorption rate. Isotope values of produced gas and the number of remaining adsorbed molecules and the number of remaining free molecules ; S3. The diffusion coefficient obtained from the simulation in step S2 is compared to Cumulative desorption rate Isotope values of produced gas Number of remaining adsorbed molecules and the number of remaining free molecules The following quantitative functional relationship is established through mathematical fitting: diffusion coefficient ratio With cumulative desorption rate Functional relationship: ; diffusion coefficient ratio Isotopic values of produced gas Functional relationship: ; and the number of remaining adsorbed molecules Number of remaining free molecules Ratio to diffusion coefficient Functional relationship: ; Where c, h, p, and q are fitting constants. , This is the fitted function; S4. Obtain the measured methane carbon isotope values of the deep coalbed methane sample to be evaluated. Substitute it into the diffusion coefficient ratio in step S3 Isotopic values of produced gas In the functional relationship, the current diffusion coefficient of the sample is obtained by inversion. ; S5, compare the current diffusion coefficient with Substituting the diffusion coefficient ratio into step S3 With cumulative desorption rate In the functional relationship, the cumulative desorption rate is obtained by inversion. and the obtained diffusion coefficient is compared with Substitute the number of remaining adsorbed molecules from step S3 Number of remaining free molecules Ratio to diffusion coefficient In the functional relationship, the number of molecules in the current remaining adsorbed state is obtained by inversion. and the number of free molecules ; S6. Based on the cumulative desorption rate obtained from the inversion. Number of remaining adsorbed molecules Number of free molecules and the initial total number of methane molecules N initia The remaining gas volume, adsorbed gas ratio, and free gas ratio of the sample to be evaluated are calculated to complete the quantitative evaluation of the desorption potential of deep coalbed methane.
[0016] Furthermore, step S3 also includes: based on the diffusion coefficient ratio For cumulative desorption rate first derivative d / d The positive, negative, or zero value of the value divides the desorption process into three stages: the initial stage, the stable stage, and the final stage. This serves as a quantitative standard for determining the desorption stage of coalbed methane.
[0017] Furthermore, in step S1, the specific method for constructing a dedicated coal molecular structure model includes: using the elemental composition (obtained via XPS), functional group composition (obtained via 13CNMR), pore structure characteristics (obtained via low-temperature nitrogen / carbon dioxide adsorption), mineral and water-bearing information of the target coal sample as constraints, determining the size of the aromatic cluster core, introducing heteroatoms and functional groups, constructing a three-dimensional cross-linked network of aromatic clusters and forming a pore system that matches the measured data, embedding kaolinite, montmorillonite clay minerals, and water molecules represented by a spherical trap model; then performing molecular dynamics equilibrium simulations under reservoir temperature and pressure conditions, iteratively optimizing until the calculated density and specific surface area of the model match the measured values of the sample.
[0018] Furthermore, the molecular dynamics simulation employs molecular force fields suitable for organic-hydrocarbon-water multiphase systems (such as COMPASSIII, ClayFF, or combinations thereof), and fixes key calculation parameters such as cutoff radius and long-range force algorithm to simulate the reservoir depressurization desorption process and record... 12 CH4 and 13 The trajectory of CH4 molecules was analyzed to obtain their mean square displacement (MSD) time curves, and the diffusion coefficients at different stages were calculated using fixed time windows. The calculation formulas are as follows: ; in, represent 12 CH4 or 13 CH4; D is the diffusion coefficient; MSD is the mean square displacement; It is a fixed time window.
[0019] Furthermore, in step S3, the cumulative desorption rate The calculation formula is: ; in, The cumulative desorption rate; This represents the cumulative number of methane molecules desorbed up to time t. This represents the original (undesorbed) number of methane molecules; Carbon isotopes of produced gas The calculation formula is: ; in, The isotope values of the produced gas at time t are the cutoff times. of 13 The number of CH4 molecules was obtained statistically. For desorption at time t 12 The number of CH4 molecules was obtained statistically. For the international standard reference material VPDB 13 C / 12 C ratio; the unit of the result calculated by this formula is ‰; Furthermore, in step S6, the remaining gas volume The calculation formula is: ; in Avogadro's constant, with volume in m³. 3 , This represents the remaining volume of methane in the model at time t.
[0020] Furthermore, the volume of adsorbed gas Free gas volume The calculation formula is: ; in These represent the remaining adsorbed methane volume and free methane volume in the model at cutoff time t, respectively. Furthermore, the method is applicable to coalbed methane reservoirs with a burial depth greater than 1000m, and can be extended to the evaluation of desorption potential of unconventional natural gas reservoirs such as shale gas and tight sandstone gas.
[0021] It should be noted that the construction of the atomic-scale specific coal molecular structure model first involves constructing a molecular-scale structural model based on the measured geological and geochemical parameters of the target coal sample. Specifically, the elemental and functional group composition of the sample (obtained through XPS and 13CNMR), pore structure characteristics (obtained through low-temperature nitrogen / carbon dioxide adsorption), and mineral and water-bearing information are used as constraints to determine the size of the aromatic cluster core, introduce corresponding heteroatoms and functional groups, construct a three-dimensional cross-linked network of aromatic clusters, and form a pore system matching the measured data. Simultaneously, kaolinite, montmorillonite clay minerals, and water molecules represented using a spherical form trap model are embedded. Subsequently, molecular dynamics equilibrium simulations are performed on the initial model under reservoir temperature and pressure conditions, and iterative optimization is used to ensure that key properties such as calculated density and specific surface area match the measured values of the sample, thereby obtaining a high-fidelity coal molecular model representing the microstructure of the target reservoir, which serves as the sole starting structure for subsequent molecular dynamics desorption simulations. For the sample under evaluation, giant canonical Monte Carlo (GCMC) adsorption simulations are performed under the same temperature and pressure conditions to obtain initial methane occurrence state parameters. In the molecular dynamics simulation, the simulation system pressure was set to decrease linearly from the initial reservoir pressure (e.g., 20 MPa) to 0.1 MPa at a rate of 0.1 MPa / ps. The total simulation duration was 10 ns, and molecular trajectories were recorded every 50 ps. 13 CH4 and 12 The trajectory of CH4 molecules was analyzed to obtain their mean square displacement (MSD) time curves, and the diffusion coefficients at different stages were calculated using a fixed time window. The calculation formula is as follows: ; in, represent 12 CH4 or 13 CH4; D is the diffusion coefficient; MSD is the mean square displacement; For a fixed time window; Count the cumulative number of methane molecules desorbed at each time point t, and calculate the cumulative desorption rate. Established through fitting 13 CH4 and 12 CH4 diffusion coefficient ratio With cumulative desorption rate The functional relationship is calculated using the following formula: ; ; Where F is a function that changes with time; This represents the cumulative number of methane molecules desorbed up to time t. denoted as the original (undesorbed) number of methane molecules; c and h are constants obtained through molecular dynamics simulation fitting. Statistics on the desorbed output at each time point t 13 CH4 and 12 Calculate the instantaneous isotopic value of the produced gas based on the number of CH4 molecules. (Unit: ‰), the diffusion coefficient ratio was established through fitting. Isotopic values of produced gas The quantitative relationship between the ratios is calculated using the following formula: ; ; in, The isotope values of the produced gas at time t are the cutoff times. of 13 The number of CH4 molecules was obtained statistically. For desorption at time t 12 The number of CH4 molecules was obtained statistically. For the international standard reference material VPDB 13 C / 12 The C ratio is set to 0.011180; p and q are constants obtained through molecular dynamics simulation fitting. Further statistical analysis was conducted on the number of remaining adsorbed and free methane molecules in the model at different time points t, and their ratio with the diffusion coefficient was established. The proportional relationship is calculated using the following formula: ; in, These represent the number of adsorbed methane molecules and the number of free methane molecules remaining in the model at cutoff time t. , This is the fitted function; Based on the diffusion coefficient obtained from the molecular dynamics simulation in step S2 With cumulative desorption rate rate of change d / d Based on the changing relationship, the following quantitative criteria are defined for the three desorption stages:
[0022] Based on cumulative desorption rate The remaining number of molecules and the initial total number of methane molecules are used to calculate the remaining gas volume, adsorbed gas ratio, and free gas ratio of the sample to be evaluated. Remaining gas volume The calculation formula is: ; Adsorbed gas volume Free gas volume The calculation formula is: ; in Avogadro's constant, with volume in m³. 3 ; This represents the remaining methane volume in the model at time t, in cubic meters. 3 ; This represents the volume of adsorbed (free) methane remaining in the model at cutoff time t, in cubic meters. 3 ; Example 1: A method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation, comprising the following steps: Step 1: Coal Sample Collection and Basic Testing Collect coal core samples from deep coalbed methane reservoirs and perform XPS analysis. 13 CNMR analysis yielded elemental and functional group composition, while low-temperature nitrogen / carbon dioxide adsorption obtained pore structure parameters. Step 2: Construction of Coal Molecular Model and Characterization of Initial State A three-dimensional coal molecular model was constructed using measured parameters as constraints. After NPT ensemble equilibrium simulation, GCMC methane adsorption simulation was conducted to obtain the initial parameters: N initial 1.2×10 6 1, N ads 9.6×10 5 indivual, 2.4×10 5 indivual; Step 3: Desorption-Diffusion Kinetics Simulation Molecular dynamics simulations of depressurization desorption were conducted using the COMPASS III force field. The initial pressure was set at 20 MPa, and the pressure was linearly decreased to 0.1 MPa at a rate of 0.1 MPa / ps. The total simulation duration was 10 ns, and molecular trajectories were recorded every 50 ps. The diffusion coefficient was calculated using the following formula: ; Obtain the diffusion coefficient ratio The cumulative desorption rate is calculated by counting the cumulative number of desorbed molecules. The fitting yielded: ; draw Follow The change curve and the result are as follows Figure 2 As shown (the horizontal axis represents the cumulative desorption rate) The vertical axis is 13 CH4 and 12 CH4 diffusion coefficient ratio ), calculate the derivative d / d Early stage of desorption With cumulative desorption rate Increase rapidly, satisfying d / d >0, desorption plateau period Keep constant, satisfying d / d =0, end of desorption phase With cumulative desorption rate The curve shows a continuous decrease, and based on the characteristics of this curve, a quantitative standard for classifying the desorption stage is formulated to intuitively reflect the coalbed methane desorption process and clearly divide the three stages of desorption. Step 4: Fit the quantitative relationship of isotopic fractionation and calculate the carbon isotope value of the produced gas according to the formula: ; in, 0.011180 (VPDB standard); ; A quantitative relationship chart was drawn, and the results are as follows: Figure 3 As shown; can be verified by actual measurement. The corresponding diffusion coefficient ratio can be directly queried / calculated, providing core graphical support for desorption parameter inversion; Step 5: Construction of the Residual Gas Occurrence State Model The function obtained by fitting the data is the ratio of residual adsorbed gas, free gas, and diffusion coefficient: ; Step 6: On-site gas sample inversion evaluation Gas samples were collected at the site and measured. -38.5‰, substitute Figure 3 The corresponding formula is: 1.28; Substitute Figure 2 The cumulative desorption rate is obtained from the corresponding formula. 0.65, calculate the derivative The desorption is determined to be in a stable period, and the remaining gas volume V is calculated. rem =4862m 3 Adsorbed gas accounts for 78%, and free gas accounts for 22%. Evaluation results: The coalbed methane reservoir is in a stable desorption period with abundant remaining gas resources, mainly adsorbed gas, and has the potential for continuous development.
[0023] It should be noted that a high-fidelity three-dimensional coal molecular structure model was constructed, constrained by the measured elemental composition, functional groups, pore structure, minerals, and water content information of the target coal sample. This model not only considers the aromatic cluster cross-linking network of organic matter but also incorporates clay minerals and water molecules. It was iteratively optimized through molecular dynamics equilibrium simulations to ensure that the model density and specific surface area match the measured values. This technical feature directly solves the deficiency of traditional methods in reproducing the true microenvironment of deep coal seams, providing an accurate initial structure for subsequent dynamic simulations. Under reservoir temperature and pressure conditions, a molecular force field (such as COMPASS III) suitable for organic matter-hydrocarbon-water multiphase systems was used to simulate the complete depressurization process from the initial reservoir pressure to atmospheric pressure. (Separately, the model was tracked...) 13 CH4 and 12 The trajectory of CH4 molecules was analyzed, and its diffusion coefficient and diffusion coefficient ratio were calculated. This feature breaks through the limitation of traditional simulations that treat the diffusion coefficient as a constant, and for the first time considers the dynamic fractionation effect caused by the difference in isotope mass at the atomic scale, making the simulation results closer to the actual desorption behavior. By mathematical fitting, a diffusion coefficient ratio is constructed. With cumulative desorption rate The functional relationship and diffusion coefficient ratio Isotopic values of produced gas Functional relationship, number of remaining adsorbed molecules Number of remaining free molecules Ratio to diffusion coefficient The functional relationships between these relationships enable a quantitative bridge between microscopic simulation parameters (diffusion coefficient ratio) and macroscopic engineering parameters (desorption rate, isotope value), completely solving the problem that traditional isotope research can only provide qualitative descriptions. The inversion evaluation process based on measured isotope values only requires obtaining the measured methane carbon isotope values of the coalbed methane sample to be evaluated. This allows for the sequential inversion of the current diffusion coefficient ratio, cumulative desorption rate, and the number of remaining adsorbed and free molecules, thereby enabling the calculation of the remaining gas volume, adsorbed gas ratio, and free gas ratio. This process eliminates the need for time-consuming and expensive on-site desorption experiments, as well as complex numerical simulation parameter tuning, achieving rapid evaluation with low cost and high efficiency. Based on the diffusion coefficient ratio For cumulative desorption rate first derivative d / d The positive, negative, or zero value of the desorption process is used to divide the desorption process into three stages: the initial stage, the stable stage, and the final stage. This standard provides an objective and quantitative basis for the dynamic adjustment of development strategies (such as whether fracturing is needed and when to intensify the well network). By employing atomic-scale models and molecular dynamics simulations, the evaluation process can realistically reflect the non-equilibrium dynamic behavior of methane desorption-diffusion under deep, high-temperature, and high-pressure conditions. The simulation accuracy is significantly higher than that of traditional isothermal adsorption experiments and simplified numerical models. For the first time, methane carbon isotope fractionation signals are converted into quantitative parameters such as desorption rate, remaining gas volume, and adsorbed / free gas ratio, which can be directly used for reserve assessment and development decisions, achieving a leap from "looking at trends" to "calculating data." Desorption potential evaluation can be completed with only the measured carbon isotope values of a single gas sample and a small amount of basic geochemical data, eliminating the need for lengthy on-site desorption tests or complex experiments, significantly reducing evaluation costs and time. Based on diffusion coefficient ratios... For cumulative desorption rate The derivative criterion can objectively determine whether the current reservoir is in the early, stable or late stage of desorption, providing a scientific basis for formulating differentiated development plans (such as controlling pressure difference in the early stage, maintaining stable production in the stable stage, and considering production enhancement measures in the late stage). This method is not only applicable to deep coalbed methane reservoirs with a burial depth greater than 1000m, but can also be extended to other unconventional natural gas reservoirs such as shale gas and tight sandstone gas, and has strong industry versatility and technical extensibility.
[0024] The above description is merely a few embodiments of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any modifications or alterations made by those skilled in the art without departing from the scope of the technical solution of the present invention using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.
Claims
1. A method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation, characterized in that, Includes the following steps: S1. Based on the measured geological and geochemical parameters of the target coal sample, construct an atomic-scale specific coal molecular structure model, and conduct methane adsorption simulation under reservoir temperature and pressure conditions to obtain initial methane occurrence state parameters, including the initial total number of methane molecules N. initial The number of adsorbed methane molecules N ads and the number of free methane molecules N free ; S2. Based on the aforementioned specific coal molecular structure model, perform methane depressurization desorption-diffusion molecular dynamics simulation and record... 13 CH4 and 12 The trajectory of CH4 molecules was calculated at different times t. 12 diffusion coefficient of CH4 and 13 diffusion coefficient of CH4 The diffusion coefficient was obtained as a ratio And calculate the cumulative desorption rate. Isotope values of produced gas and the number of remaining adsorbed molecules and the number of remaining free molecules ; S3. The diffusion coefficient obtained from the simulation in step S2 is compared with Cumulative desorption rate Isotope values of produced gas Number of remaining adsorbed molecules and the number of remaining free molecules The following quantitative functional relationship is established through mathematical fitting: diffusion coefficient ratio With cumulative desorption rate Functional relationship: ； diffusion coefficient ratio Isotopic values of produced gas Functional relationship: ； and the number of remaining adsorbed molecules Number of remaining free molecules Ratio to diffusion coefficient Functional relationship: ； Where c, h, p, and q are fitting constants. , This is the fitted function; S4. Obtain the measured methane carbon isotope values of the deep coalbed methane sample to be evaluated. Substitute it into the diffusion coefficient ratio in step S3 Isotopic values of produced gas In the functional relationship, the current diffusion coefficient of the sample is obtained by inversion. ; S5, compare the current diffusion coefficient with Substituting the diffusion coefficient ratio into step S3 With cumulative desorption rate In the functional relationship, the cumulative desorption rate is obtained by inversion. and the obtained diffusion coefficient is compared with Substitute the number of remaining adsorbed molecules from step S3 Number of remaining free molecules Ratio to diffusion coefficient In the functional relationship, the number of molecules in the current remaining adsorbed state is obtained by inversion. and the number of free molecules ; S6. Based on the cumulative desorption rate obtained from the inversion. Number of remaining adsorbed molecules Number of free molecules and the initial total number of methane molecules N initia The remaining gas volume, adsorbed gas ratio, and free gas ratio of the sample to be evaluated are calculated to complete the quantitative evaluation of the desorption potential of deep coalbed methane.
2. The method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation according to claim 1, characterized in that, Step S3 also includes: based on the diffusion coefficient ratio For cumulative desorption rate first derivative d / d The positive, negative, or zero value of the value divides the desorption process into three stages: the initial stage, the stable stage, and the final stage. This serves as a quantitative standard for determining the desorption stage of coalbed methane.
3. The method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation according to claim 1, characterized in that, In step S1, the specific method for constructing a specific coal molecular structure model includes: using the elemental composition, functional group composition, pore structure characteristics, mineral and water-bearing information of the target coal sample as constraints, determining the size of the aromatic cluster core, introducing heteroatoms and functional groups, constructing a three-dimensional cross-linked network of aromatic clusters and forming a pore system that matches the measured data, embedding kaolinite, montmorillonite clay minerals and water molecules represented by a spherical trap model; then performing molecular dynamics equilibrium simulation under reservoir temperature and pressure conditions, iteratively optimizing until the calculated density and specific surface area of the model match the measured values of the sample.
4. The method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation according to claim 1, characterized in that, In step S2, the molecular dynamics simulation employs a molecular force field suitable for organic matter-hydrocarbon-water multiphase systems to simulate the reservoir depressurization process. Molecular trajectories are recorded, and the diffusion coefficient is calculated over a fixed time window. The calculation formula is as follows: ； in, represent 12 CH4 or 13 CH4; D is the diffusion coefficient; MSD is the mean square displacement; It is a fixed time window.
5. The method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation according to claim 1, characterized in that, In step S3, the cumulative desorption rate The calculation formula is: ； in, The cumulative desorption rate; This represents the cumulative number of methane molecules desorbed up to time t. This represents the original number of methane molecules; Carbon isotopes of produced gas The calculation formula is: ； in, Deadline Hourly gas isotope values; of 13 The number of CH4 molecules was obtained statistically. For desorption at time t 12 The number of CH4 molecules was obtained statistically. For the international standard reference material VPDB 13 C / 12 C ratio.
6. The method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation according to claim 1, characterized in that, In step S6, the remaining gas volume The calculation formula is: ； in, Let Avogadro's constant be 1. This represents the remaining volume of methane in the model at time t.
7. The method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation according to claim 1, characterized in that, Adsorbed gas volume With free gas volume The calculation formula is: ； in, , These represent the remaining adsorbed methane volume and free methane volume in the model at time t, respectively.
8. The method for evaluating the desorption potential of deep coalbed methane based on methane carbon isotope fractionation according to claim 1, characterized in that, The method is applicable to coalbed methane reservoirs with a burial depth greater than 1000m, and can be extended to the evaluation of desorption potential of unconventional natural gas reservoirs such as shale gas and tight sandstone gas.