A method for constructing an imonite mixed-layer kerogen composite molecular model of an oil shale reservoir

Abstract

This invention belongs to the field of digital rock physics and molecular simulation technology for oil shale reservoirs, specifically relating to a method for constructing a composite molecular model of an illite-montmorillonite mixed-layer kerogen structure in an oil shale reservoir. The invention determines the illite-montmorillonite mixed-layer ratio and the charge number of the montmorillonite layer by X-ray diffraction and cation exchange capacity determination of clay isolates from oil shale samples. Combining the temperature and pressure conditions of the target oil shale reservoir and the vitrinite reflectivity of the kerogen, molecular models of kerogen, illite, and montmorillonite, as well as mineral wall layer models, are constructed. The illite-montmorillonite mixed-layer-kerogen composite system is corrected using a ReaxFF reactive force field, and the initial model is assembled according to the layer sequence of montmorillonite wall layer – kerogen molecular layer – illite wall layer. The composite molecular model under the target oil shale reservoir conditions is obtained through molecular dynamics annealing. The results of the embodiments show that this invention can improve the realism, parameter constraints, and modeling repeatability of the mineral-organic interface characterization of oil shale reservoirs.

Description

Technical Field

[0001] This invention belongs to the field of digital rock physics and molecular simulation technology for oil shale reservoirs, specifically relating to a method for constructing a composite molecular model of an oil shale reservoir with mixed layers of montmorillonite and kerogen. Background Technology

[0002] The interaction between clay minerals and kerogen organic matter in oil shale reservoirs has a decisive influence on hydrocarbon generation, retention, adsorption, pore evolution, and reservoir mechanical properties. Illite-montmorillonite mixed-layer (I / S) minerals are one of the most important clay mineral types in oil shale reservoirs. Their illiteration degree varies systematically with increasing burial depth and temperature, directly affecting the surface charge density, interlayer structure, and interfacial interaction with kerogen.

[0003] Existing patents related to the molecular characterization of oil shale reservoirs can be mainly divided into three categories: The first category, represented by Ning Zhengfu et al. (CN108595778 A, 2018), proposes a method for constructing a three-dimensional shale composite organic matter molecular model based on multiple characterization methods such as FTIR, 13C NMR, and XPS. This method establishes a complete characterization process for the molecular structure of kerogen, but the constructed model is a pure organic matter three-dimensional bulk phase model, without introducing mineral walls, and cannot simulate the adsorption behavior of the mineral-organic matter interface. The second category, represented by Li Zheng et al. (CN 117953979 A, ​​2024), proposes to construct a composite flexible shale molecular model that considers the bonding connection between organic and inorganic matter by calculating the bonding reaction energy of mineral-kerogen functional groups. This method introduces the chemical bonding between mineral and organic matter, but uses a three-dimensional bulk phase model of a single mineral (quartz) particle and kerogen, without establishing the interfacial layered configuration of kerogen sandwiched by mineral slab walls, nor distinguishing the different component proportions of the illite-montmorillonite mixed layer minerals. The third category, represented by Qiu Xingdong et al. (CN 118190755 A, 2024) and Zhao Fei et al. (CN 116858630 A, 2023), focuses on the construction of kerogen pore models. They use methods such as graphene plates / carbon nanopores or RCF deep learning to extract pore shape information, but none of them involve the mineral-kerogen interface system.

[0004] The current kerogen model for oil shale reservoirs has the following defects: (1) There is a lack of an interface layer model with mineral walls holding kerogen, especially the mineral-kerogen sequence relationship is not determined based on the geological diagenetic time; (2) The measured mixing ratio of the mixed-layer minerals of Imbrium and Mnium is not included in the quantitative constraint system of the molecular model construction, resulting in a serious disconnect between the mineral composition and the actual oil shale reservoir; (3) The mineral-kerogen interface is mostly calculated using classical force fields (COMPASS, CVFF), which cannot simulate the possible bonding reactions between the mineral walls and the functional groups of kerogen.

[0005] Therefore, it is necessary to propose a systematic, parameterized, and geologically constrained method for constructing a composite molecular model of the Immon-Kerogen mixed layer in oil shale reservoirs, in order to improve the accuracy and reproducibility of the study of the micro-mechanisms of oil shale reservoirs. Summary of the Invention

[0006] The purpose of this invention is to provide a systematic, repeatable, and geologically constrained method for constructing a composite molecular model of Immon-Kerogen mixed-layer oil shale reservoirs. The method provided by this invention not only overcomes the shortcomings of existing studies, such as the disconnect between molecular models and actual formation conditions and the lack of standardized construction procedures, but also provides a standardized modeling foundation for molecular simulation studies of hydrocarbon generation, adsorption, migration, and occurrence mechanisms in oil shale reservoirs.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] This invention provides a method for constructing a molecular model of an illite-saturated mixed-layer kerogen complex in an oil shale reservoir, comprising the following steps:

[0009] Step S1: Perform X-ray diffraction tests on the clay separation of the oil shale sample to obtain the montmorillonite mixed layer ratio; calculate the montmorillonite layer charge by determining the cation exchange capacity; and obtain the formation temperature, pressure, and vitrinite reflectance of the target oil shale reservoir by combining the test data.

[0010] Step S2: Construct molecular models of kerogen, montmorillonite, and illite, and determine the molar ratio of illite to montmorillonite based on the illite-montmorillonite mixed layer ratio obtained in Step S1;

[0011] Step S3: Construct illite and montmorillonite supercells and truncate them to create a mineral wall panel model;

[0012] Step S4: Perform ReaxFF reaction force field correction on the Immon-Kerogen mixed-layer composite system of oil shale reservoirs, and correct specific parameters in the force field involving clay mineral framework, interlayer ion coordination and organic-mineral interface bonding.

[0013] Step S5: Assemble the stacked box. Assemble the initial box model according to the layer sequence of montmorillonite wall - kerogen molecular layer - illite wall, and set the initial configuration of the dominant adsorption orientation of kerogen molecules according to the interfacial interaction energy to form the illite-montmorillonite mixed layer-kerogen composite double wall sandwich structure of oil shale reservoir.

[0014] Step S6: Using the ReaxFF reactive force field modified in step S4, obtain the Immon-Kerogen mixed-layer molecular model under the target oil shale reservoir conditions based on molecular dynamics annealing.

[0015] Furthermore, step S1 above also includes the following specific steps:

[0016] The X-ray diffraction tests were performed sequentially on the clay fractions of the same oil shale sample under three treatment conditions: air-dried, ethylene glycol-saturated, and heated to 550°C. The differences between these three conditions separated the two mineral peaks, eliminating peak overlap interference. The illite-montmorillonite mixing ratio was calculated using the peak area ratio of the ethylene glycol-saturated state diffraction. Based on this mixing ratio, the molar ratio of illite to montmorillonite in the molecular model was determined. The montmorillonite layer charge was obtained by converting the cation exchange capacity measurement results. The number of monovalent compensating cations was determined by taking the nearest integer value of the total supercell layer charge, and this number was used for the interlayer structure setting in step S3.

[0017] Furthermore, step S3, which constructs the unit cells and walls of illite and montmorillonite, includes the following specific steps:

[0018] A1. Construct initial unit cells for illite and montmorillonite. Set interlayer potassium ions for illite according to standard crystal parameters. For montmorillonite, determine the number of compensating cations based on the layer charge number calculated in step S1 and the total layer charge of the supercell. Then, optimize the geometric structure of the unit cells of the two minerals respectively.

[0019] A2. Periodically expand the geometrically optimized unit cell along the ab plane to form a supercell of not less than 4×2×1 times. The dimensions of the expanded supercell in both the a and b directions should be greater than twice the ReaxFF force field cutoff radius to eliminate the interaction between periodic mirror images.

[0020] A3. Using the (001) crystal plane as the cut-off plane, the supercell is cut off to establish a mineral wall panel model, and a vacuum layer of not less than 20 Å is set in the direction perpendicular to the wall to fully eliminate the electrostatic and van der Waals interactions between the periodic mirror layers.

[0021] A4. The surface Si-O and Al-O dangling bonds generated by the truncation of A3 are unified with hydrogen atoms to end them, thus completing the establishment of the mineral wall panel model.

[0022] Furthermore, in step S4, the ReaxFF force field correction for the illite-montmorillon mixed-layer-kerogen composite system in oil shale reservoirs uses a publicly available ReaxFF parameter set containing Si, Al, O, H, C, N, S, K, and Na elements as the initial parameter set. The calibration targets are experimental or quantum chemical results regarding the Al-O-Si bond angle distribution in illite interlayers, K⁺ coordination structure, Na⁺ hydration coordination structure in montmorillonite interlayers, and the condensation reaction pathways of Si-OH and oxygen-containing functional groups on the mineral wall. The specific content obtained through iterative fitting is as follows:

[0023] B1. Set the equilibrium bond angle Theta0 of the aluminum-silicon bond (Al-O-Si) bond angle parameter to 142.5°;

[0024] B2. Set the dissociation energy Ediss of the coordination bond between K⁺ in the illite interlayer and the six-membered silicon-oxygen ring to 0.12 kcal / mol;

[0025] B3. Set the reference bond length Ro of the hydration coordination single bond of Na⁺ in the interlayer of montmorillonite to 1.62 Å; set the bond order decay parameter gamma to 12.2, which is a dimensionless parameter; set the reference bond length rsigma of the σ bond to 1.62 Å; and set the equilibrium bond angle Theta0 of HO-Na to 78.0°.

[0026] B4. Modify the Si-OC bond angle parameter at the organic-mineral interface, setting the equilibrium bond angle Theta0 to 116° to describe the reaction pathway in which Si-OH on the mineral wall forms a condensation bond (Si-OC type) with the hydroxyl (-OH) and carboxyl (-COOH) groups of the kerogen carbon chain.

[0027] Furthermore, in step S5, the stratigraphic sequence of the montmorillonite wall-kerogen molecular layer-illite wall is determined based on the geological diagenetic relationship of the oil shale reservoir: montmorillonite diagenesis occurred in the early stage of shallow burial, during which kerogen was rich in oxygen-containing functional groups, forming an interface with the montmorillonite wall mainly based on chemical adsorption. The contact between the two occurred before illite formation, and the montmorillonite wall was the outer contact surface of the kerogen; illite formation occurred in the late stage of deep burial, and illite contacted the kerogen through physical adsorption, forming the inner contact surface; thus, a mixed-layer montmorillonite-kerogen composite double-wall clamping model is constructed.

[0028] Furthermore, in step S5, the initial orientation of the kerogen molecules is set based on the interfacial interaction energy calculated in step S4 according to the modified force field. The carbon chain hydroxyl (-OH) and carboxyl (-COOH) groups are preferably oriented towards the montmorillonite wall, the carbonyl (-C=O) groups are preferably oriented towards the illite wall, and no orientation constraints are imposed on the aliphatic chains and aromatic fused rings.

[0029] Furthermore, in step S6, the molecular dynamics annealing all adopts the ReaxFF reaction force field modified in step S4, and the time step is uniformly set to 0.25 fs. The specific steps are as follows:

[0030] C1. Perform geometric optimization on the kerogen model constructed in step S2 to eliminate atomic overlap in the initial configuration, and relax it for 50 ps under the NVT ensemble to stabilize the kerogen molecules and provide an initial configuration for the subsequent assembly of the oil shale reservoir composite system.

[0031] C2. In a mineral double-wall system without kerogen, complete the interlayer K-line operation using an NVT ensemble for no less than 200 ps. + and Na + The structure is pre-balanced to avoid unreasonable and drastic readjustment of the interlayer structure due to organic molecule disturbances when kerogen is introduced later;

[0032] C3. Combine the kerogen model obtained in step C1 with the mineral interlayer structure obtained after pre-equilibrium in step C2, and assemble it as the initial input for the entire system according to the sequence and orientation constraints described in step S5. Then, sequentially perform NVT heating, high-temperature relaxation, NPT cooling, and NPT final equilibrium to obtain the Immon-Montene mixed-layer-kerogen complex molecular model.

[0033] Furthermore, step C3 specifically involves:

[0034] C31. The peak annealing temperature in step C3 is determined according to the kerogen maturity Ro. 300K is used when Ro < 0.5%, 400K is used when 0.5% ≤ Ro < 1.3%, and 500K is used when Ro ≥ 1.3%, to prevent the reactive force field from triggering thermal decomposition of low-maturity kerogen at excessively high temperatures, while ensuring sufficient relaxation of high-maturity kerogen.

[0035] C32. Using the NVT ensemble, the temperature is raised to the peak annealing temperature determined in step C31, the simulation time is 50 ps, ​​and the time step is set to 0.25 fs to simulate the relaxation process of kerogen molecules crossing the energy barrier from the equilibrium state.

[0036] C33. Maintain the peak annealing temperature and continue to run the NVT ensemble for 100 ps with a time step of 0.25 fs to allow the adsorption sites of organic molecules and mineral-organic interfaces to fully relax and escape the local energy minimum.

[0037] C34. Switch to the NPT ensemble and cool down from the peak annealing temperature to 300K under formation pressure. The simulation time is 100ps and the time step is set to 0.25fs to simulate the cooling and contraction process and eliminate thermal stress.

[0038] C35. Maintaining the NPT ensemble, continue relaxation for 100 ps under formation pressure and 300 K, with a time step of 0.25 fs, so that the system density and potential energy converge to a stable fluctuation range. The resulting configuration is the Immon-Kerogen mixed-layer molecular model under the target oil shale reservoir conditions.

[0039] This invention provides a mixed-layered illite-montmorillonite-kerogen composite molecular model of an oil shale reservoir constructed using the above-mentioned model construction method. Illite and montmorillonite models are stacked with kerogen models according to the corresponding mixing ratio to form a mixed-layered illite-montmorillonite-kerogen composite double-wall clamping model with montmorillonite-kerogen-illite as the layer sequence.

[0040] The beneficial effects of this invention are as follows:

[0041] (1) A quantitative process was established to directly map the measured data of three-state XRD of oil shale samples to the mineral molar ratio of the molecular model. The number of interlayer compensation cations was determined by CEC after converting the layer charge number and combined with the conservation of total charge of the supercell. This achieved full-process constraint from experimental parameters to model parameters and solved the problem of model distortion caused by arbitrary assumptions about mineral composition.

[0042] (2) Four parameters of the ReaxFF force field were modified specifically for the illite-saturated mixed-layer-kerogen composite system in oil shale reservoirs, so that the force field can accurately describe the bond angle of the layered mineral framework and the K-axis of illite interlayers. + Coordination and Na in montmorillonite interlayers + Hydration coordination structure improves the model's interface characterization ability;

[0043] (3) Based on the geological diagenetic sequence of oil shale reservoir, a double-walled sequence of montmorillonite-kerogen-illite is established, and the initial orientation setting of kerogen is driven by the interfacial interaction energy, so that the mineral-organic interface structure of the model is consistent with the actual geological conditions of oil shale reservoir.

[0044] (4) The annealing process links the peak temperature with the maturity of the kerogen to prevent the ReaxFF active force field from triggering artificial cracking at inappropriate high temperatures, and ensures that kerogens of all maturity levels can obtain a reasonable balanced configuration. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1 This is a schematic diagram of the overall process for constructing the molecular model of the Yimeng-Kerogen mixed layer in the oil shale reservoir according to the present invention.

[0047] Figure 2 This is a schematic diagram of the annealing procedure sequence for step S6.

[0048] Figure 3 This is a schematic diagram of the stacked box assembly for step S5.

[0049] Figure 4 This is a three-dimensional schematic diagram of the Yimeng-Kerogen mixed-layer molecular model of the oil shale reservoir in Example 1. Detailed Implementation

[0050] This invention provides a molecular model of an Imbrium-Monganese mixed-layer kerogen composite in an oil shale reservoir and its construction method. To further illustrate this invention, the technical solutions provided below are described in detail with reference to the accompanying drawings and embodiments, but these should not be construed as limiting the scope of protection of this invention.

[0051] Unless otherwise specified, the production processes, experimental methods, or testing methods involved in the embodiments of this invention are all conventional methods in the prior art, and their names and / or abbreviations are all conventional names in the field, which are very clear and distinct in the relevant application areas. Those skilled in the art can understand the conventional process steps based on the names and apply the corresponding equipment, and implement them according to conventional conditions or the conditions recommended by the manufacturer.

[0052] The various instruments, equipment, raw materials or reagents used in the embodiments of this invention are not subject to any special restrictions on their source. They are all conventional products that can be purchased through regular commercial channels and can be prepared according to conventional methods known to those skilled in the art.

[0053] like Figure 1 As shown, the method of the present invention includes the following steps:

[0054] Step S1: Perform X-ray diffraction tests on the clay separation of the oil shale sample to obtain the montmorillonite mixed layer ratio; calculate the montmorillonite layer charge by determining the cation exchange capacity; and obtain the formation temperature, pressure, and vitrinite reflectance of the target oil shale reservoir by combining the test data.

[0055] Specifically, in this step, X-ray diffraction tests were performed sequentially on the clay fractions of the same oil shale sample under three treatment conditions: air-dried, ethylene glycol-saturated, and heated to 550℃. The differences between the three conditions separated the two mineral peaks, eliminating peak overlap interference. The illite-montmorillonite mixing ratio was calculated using the peak area ratio of the ethylene glycol-saturated state diffraction. Based on the obtained mixing ratio, the molar ratio of illite to montmorillonite in the molecular model was determined. The montmorillonite layer charge was obtained by converting the cation exchange capacity measurement results. The number of monovalent compensating cations was determined by taking the nearest integer of the total supercell layer charge, which was used for the interlayer structure setting in step S3.

[0056] The formula for calculating the ratio of Imammoyne-Montene mixtures is as follows:

[0057]

[0058] In the formula, SI is the integrated peak area of ​​illite diffraction peak, and SS is the integrated peak area of ​​ethylene glycol saturated montmorillonite diffraction peak. Both are obtained by peak fitting of Gauss-Lorentz mixture function.

[0059] Based on the obtained RI / S value, the molar ratio of illite to montmorillonite in the molecular model was determined to be RI / S:(100-RI / S).

[0060] The formula for calculating the charge number of montmorillonite layers is as follows:

[0061]

[0062] In the formula, x is the layer charge per unit cell of montmorillonite, e / uc; CEC is the cation exchange capacity, mmol / 100g; Muc is the molar mass of a unit cell of montmorillonite, g / mol; ωs is the mass fraction of montmorillonite in the sample; 100000 is the unit conversion factor from mmol to mol based on a 100g sample.

[0063] The obtained x represents the layer charge number corresponding to the montmorillonite unit cell. The total layer charge Q of the supercell is calculated based on the supercell size selected in step S3. The number of compensating cations N is determined by taking the nearest integer from Q / z, where z is the cation valence. For Na… + For monovalent compensating cations, the number of compensating cations is determined by taking the nearest integer Q, and is directly used for the interlayer structure setting in step S3.

[0064] The calculation formula is as follows:

[0065]

[0066]

[0067] In the formula, Q is the total layer charge; n is the number of unit cells; N is the number of cations; round() is the floor function; z is the chemical valence of the cation, such as Na + The corresponding z value is 1.

[0068] Step S2: Construct molecular models of kerogen, montmorillonite, and illite, and determine the molar ratio of illite to montmorillonite based on the illite-montmorillonite mixed layer ratio obtained in Step S1;

[0069] Step S3: Construct illite and montmorillonite supercells and truncate them to create a mineral wall panel model; including the following specific steps:

[0070] A1. Construct initial unit cells for illite and montmorillonite. Set interlayer potassium ions for illite according to standard crystal parameters. For montmorillonite, determine the number of compensating cations based on the layer charge number calculated in step S1 and the total layer charge of the supercell. Then, optimize the geometric structure of the unit cells of the two minerals respectively.

[0071] A2. Periodically expand the geometrically optimized unit cell along the ab plane to form a supercell of not less than 4×2×1 times. The dimensions of the expanded supercell in both the a and b directions should be greater than twice the ReaxFF force field cutoff radius to eliminate the interaction between periodic mirror images.

[0072] A3. Using the (001) crystal plane as the cut-off plane, the supercell is cut off to establish a mineral wall panel model, and a vacuum layer of not less than 20 Å is set in the direction perpendicular to the wall to fully eliminate the electrostatic and van der Waals interactions between the periodic mirror layers.

[0073] A4. The surface Si-O and Al-O dangling bonds generated by the truncation of A3 are unified with hydrogen atoms to end them, thus completing the establishment of the mineral wall panel model.

[0074] Step S4: Perform ReaxFF reaction force field correction on the Immon-Kerogen mixed-layer composite system of oil shale reservoirs, and correct specific parameters in the force field involving clay mineral framework, interlayer ion coordination and organic-mineral interface bonding.

[0075] Specifically, in this step, the ReaxFF force field correction for the illite-montmorillon mixed-layer-kerogen composite system in oil shale reservoirs uses the publicly available ReaxFF parameter set containing Si, Al, O, H, C, N, S, K, and Na elements as the initial parameter set. The calibration targets are experimental or quantum chemical results regarding the Al-O-Si bond angle distribution in illite interlayers, K⁺ coordination structure, Na⁺ hydration coordination structure in montmorillonite interlayers, and the condensation reaction pathways of Si-OH with oxygen-containing functional groups on the mineral wall. The results obtained through iterative fitting are as follows:

[0076] B1. Set the equilibrium bond angle Theta0 of the aluminum-silicon bond (Al-O-Si) bond angle parameter to 142.5°;

[0077] B2. Set the dissociation energy Ediss of the coordination bond between K⁺ in the illite interlayer and the six-membered silicon-oxygen ring to 0.12 kcal / mol;

[0078] B3. Set the reference bond length Ro of the hydration coordination single bond of Na⁺ in the interlayer of montmorillonite to 1.62 Å; set the bond order decay parameter gamma to 12.2, which is a dimensionless parameter; set the reference bond length rsigma of the σ bond to 1.62 Å; and set the equilibrium bond angle Theta0 of HO-Na to 78.0°.

[0079] B4. Modify the Si-OC bond angle parameter at the organic-mineral interface, setting the equilibrium bond angle Theta0 to 116° to describe the reaction pathway in which Si-OH on the mineral wall forms a condensation bond (Si-OC type) with the hydroxyl (-OH) and carboxyl (-COOH) groups of the kerogen carbon chain.

[0080] Step S5: Assemble the stacked box. Assemble the initial box model according to the layer sequence of montmorillonite wall surface - kerogen molecular layer - illite wall surface, and set the initial configuration of the dominant adsorption orientation of kerogen molecules according to the interfacial interaction energy.

[0081] Specifically, in this step, the stratigraphic sequence of the montmorillonite wall, kerogen molecular layer, and illite wall is determined based on the geological and diagenetic relationship of the oil shale reservoir: montmorillonite diagenesis occurred in the early stage of shallow burial, during which kerogen was rich in oxygen-containing functional groups, forming an interface mainly based on chemical adsorption with the montmorillonite wall. The contact between the two occurred before illite formation, and the montmorillonite wall was the outer contact surface of the kerogen; illite formation occurred in the late stage of deep burial, and illite contacted the kerogen through physical adsorption, forming the inner contact surface; thus, a mixed-layer montmorillonite-kerogen composite double-wall clamping model is constructed.

[0082] Specifically, in this step, the initial orientation of the kerogen molecules during assembly is set based on the interfacial interaction energy obtained from the force field calculation in step S4. The carbon chain hydroxyl (-OH) and carboxyl (-COOH) groups are preferably oriented towards the montmorillonite wall, and the carbonyl (-C=O) groups are preferably oriented towards the illite wall. No orientation constraints are imposed on the aliphatic chains and aromatic fused rings.

[0083] The formula for calculating the interfacial interaction energy is as follows:

[0084]

[0085] In the formula, Etotal is the average potential energy of the mineral-organic interface composite system in the oil shale reservoir; Emineral is the average potential energy of the mineral subsystem obtained after removing kerogen under the same periodic box, the same mineral wall configuration, and the same boundary conditions; and Ekerogen is the average potential energy of the organic subsystem obtained after removing the mineral wall under the same periodic box, the same kerogen configuration, and the same boundary conditions.

[0086] The interface interaction energy was calculated using the ReaxFF force field corrected in step S4, a time step of 0.25 fs, the same periodic box and charge balance settings as the assembly model, and the average potential energy was taken after 20 ps of temperature equilibrium in the target formation under the NVT ensemble.

[0087] Step S6: Using the ReaxFF reactive force field modified in step S4, obtain the Immon-Kerogen mixed-layer molecular model under the target oil shale reservoir conditions based on molecular dynamics annealing.

[0088] Specifically, in this step, the molecular dynamics annealing process is as follows: Figure 2 As shown, the ReaxFF reactive force field corrected in step S4 is used, and the time step is uniformly set to 0.25 fs. The process includes the following steps:

[0089] C1. Perform geometric optimization on the kerogen model constructed in step S2 to eliminate atomic overlap in the initial configuration, and relax it for 50 ps under the NVT ensemble to bring the kerogen molecule to a stable state.

[0090] C2. In a mineral double-wall system without kerogen, complete the interlayer K-line operation using an NVT ensemble for no less than 200 ps. + and Na + The structure is pre-balanced to avoid unreasonable and drastic readjustment of the interlayer structure due to organic molecule disturbances when kerogen is introduced later;

[0091] C3. Combine the kerogen model obtained in step C1 with the mineral interlayer structure obtained after pre-equilibrium in step C2, and assemble it as the initial input for the entire system according to the sequence and orientation constraints described in step S5. Then, sequentially perform NVT heating, high-temperature relaxation, NPT cooling, and final NPT equilibrium to obtain the imonten mixed-layer-kerogen complex molecular model. This includes the following specific steps:

[0092] C31. The peak annealing temperature in step C3 is determined according to the kerogen maturity Ro. 300K is used when Ro < 0.5%, 400K is used when 0.5% ≤ Ro < 1.3%, and 500K is used when Ro ≥ 1.3%, to prevent the reactive force field from triggering thermal decomposition of low-maturity kerogen at excessively high temperatures, while ensuring sufficient relaxation of high-maturity kerogen.

[0093] C32. Using the NVT ensemble, the temperature is raised to the peak annealing temperature determined in step C31, the simulation time is 50 ps, ​​and the time step is set to 0.25 fs to simulate the relaxation process of kerogen molecules crossing the energy barrier from the equilibrium state.

[0094] C33. Maintain the peak annealing temperature and continue to run the NVT ensemble for 100 ps with a time step of 0.25 fs to allow the adsorption sites of organic molecules and mineral-organic interfaces to fully relax and escape the local energy minimum.

[0095] C34. Switch to the NPT ensemble and cool down from the peak annealing temperature to 300K under formation pressure. The simulation time is 100ps and the time step is set to 0.25fs to simulate the cooling and contraction process and eliminate thermal stress.

[0096] C35. Maintaining the NPT ensemble, continue relaxation for 100 ps under formation pressure and 300 K, with a time step of 0.25 fs, so that the system density and potential energy converge to a stable fluctuation range. The resulting configuration is the Immon-Kerogen mixed-layer molecular model under the target oil shale reservoir conditions.

[0097] Example 1

[0098] The construction method of this invention will be described in detail below with reference to an example from the Nenjiang Formation in the Songliao Basin. The research object is an oil shale sample from the lacustrine dark mudstone and shale section of the Nenjiang Formation in a block in the southern Songliao Basin. The sample is mainly composed of lacustrine algal organic matter and has a kerogen type of I.

[0099] Step S1: X-ray diffraction tests were performed on the Nenjiang Formation oil shale sample, and the illite-montmorillonite mixing ratio was found to be 49%. The montmorillonite layer charge was calculated to be 0.33e / uc by cation exchange capacity determination. Combined with the 4×2×1 supercell scale used in Step S3, the total supercell layer charge was found to be approximately 2.64e. Based on this, three Na⁺ cations were placed between the montmorillonite supercell layers as compensating cations. The formation temperature of the target oil shale reservoir was found to be 65℃, the pressure to be 18MPa, and the vitrinite reflectance of kerogen to be 0.65%.

[0100] Step S2: Construct molecular models of kerogen, montmorillonite, and illite. Based on the illite-montmorillonite mixed layer ratio obtained in Step S1, determine that the molar ratio of illite to montmorillonite is approximately 1:1.

[0101] Step S3: Construct illite and montmorillonite supercells and truncate them to create a mineral wall panel model; including the following specific steps:

[0102] A1. Constructing illite unit cells (KAl2(AlSi3)O) 10 (OH)2, space group C2 / c, standard crystal parameters a=5.19Å, b=9.00Å, c=10.22Å, β=101.6°, K⁺ set in the interlayer; montmorillonite initial unit cell was constructed, space group C2 / m, with 3 Na⁺ cells set in the interlayer, and the geometry was optimized to minimize energy.

[0103] A2. Both types of unit cells are extended along the ab plane to form 4×2×1 supercells. After extension, the dimensions in the ab direction are 20.76Å×18.00Å (illite) and 20.72Å×17.94Å (montmorillonite), respectively. Both are greater than twice the cutoff radius of the ReaxFF force field, thus satisfying the size requirements of A2.

[0104] A3. Two supercells were truncated with the (001) crystal plane, and a 25 Å vacuum layer was set in the c direction to establish a mineral wall panel model.

[0105] A4. The surface Si-O and Al-O dangling bonds generated by the truncation are uniformly capped with hydrogen atoms, with the capped OH bond length being 0.97 Å, thus completing the establishment of the mineral wall panel model.

[0106] Step S4: Perform ReaxFF reaction force field correction on the Immon-Kerogen mixed-layer composite system of oil shale reservoirs, and correct specific parameters in the force field involving clay mineral framework, interlayer ion coordination and organic-mineral interface bonding.

[0107] Based on the publicly available ReaxFF parameter set containing elements Si, Al, O, H, C, N, S, K, and Na, targeted modifications are made according to four items from B1 to B4:

[0108] B1. Set the equilibrium bond angle Theta0 of the aluminum-silicon bond (Al-O-Si) bond angle parameter to 142.5°;

[0109] B2. Set the dissociation energy Ediss of the coordination bond between K⁺ in the illite interlayer and the six-membered silicon-oxygen ring to 0.12 kcal / mol;

[0110] B3. Set the reference bond length Ro of the hydration coordination single bond of Na⁺ in the interlayer of montmorillonite to 1.62 Å; set the bond order decay parameter gamma to 12.2, which is a dimensionless parameter; set the reference bond length rsigma of the σ bond to 1.62 Å; and set the equilibrium bond angle Theta0 of HO-Na to 78.0°.

[0111] B4. Modify the Si-OC bond angle parameter at the organic-mineral interface, setting the equilibrium bond angle Theta0 to 116° to describe the reaction pathway in which Si-OH on the mineral wall forms a condensation bond (Si-OC type) with the hydroxyl (-OH) and carboxyl (-COOH) groups of the kerogen carbon chain.

[0112] The interaction energies of each functional group at the mineral-organic interface in the oil shale reservoir were calculated using the modified ReaxFF force field: carbon chain hydroxyl groups (-OH) have an interaction energy of approximately -8.6 kcal / mol on the montmorillonite wall, which is chemisorption and preferably faces the montmorillonite side; carboxyl groups (-COOH) have an interaction energy of approximately -7.3 kcal / mol on the same side; and carbonyl groups (-C=O) have an interaction energy of approximately -2.4 kcal / mol on the illite wall, which is weak hydrogen bonding and preferably faces the illite side.

[0113] Step S5, Assemble the stacked boxes as follows Figure 3 As shown, the initial box model is assembled according to the layer sequence of montmorillonite wall-kerogen molecular layer-illite wall, and the initial configuration of the dominant adsorption orientation of kerogen molecules is set according to the interfacial interaction energy to form an illite-montmorillonite mixed layer-kerogen composite double wall sandwich structure in oil shale reservoir.

[0114] Based on the above orientation constraints, six kerogen molecules were placed in the simulation box and positioned between the montmorillonite and illite walls using the Monte Carlo orientation filling algorithm. The initial interlayer spacing was set to 4.2 Å, thus completing the assembly of the montmorillonite-kerogen-illite sequence oil shale reservoir composite stack box.

[0115] Step S6: Using the ReaxFF reactive force field modified in step S4, obtain the Immon-Kerogen mixed-layer molecular model under the target oil shale reservoir conditions based on molecular dynamics annealing.

[0116] Step C1: Geometric optimization of 6 kerogen molecules to eliminate atomic overlap, followed by relaxation under the NVT ensemble for 50 ps, ​​using a modified ReaxFF force field with a time step of 0.25 fs to obtain a stable low-energy kerogen configuration.

[0117] Step C2: In an independent system containing only mineral double walls and interlayer structures, the NVT ensemble, T=338 K, time step 0.25 fs, runs for 200 ps to complete the structural pre-equilibrium of K⁺ and Na⁺.

[0118] Step C3: Combine the kerogen model obtained in Step C1 with the pre-equilibrium mineral interlayer structure obtained in Step C2, and assemble it as the initial input for the entire system according to the sequence and orientation constraints described in Step S5. In the kerogen-containing system, sequentially perform NVT heating, high-temperature relaxation, NPT cooling, and final NPT equilibrium to obtain the Iso-Montene mixed-layer-kerogen composite molecular model under the target oil shale reservoir conditions. This includes the following steps:

[0119] Step C31: The Ro content of this kerogen is 0.65%. Based on the selection rules, the peak annealing temperature is determined to be 400K.

[0120] Step C32, NVT ensemble, heating from 338K to 400K, simulation time 50ps, time step 0.25fs;

[0121] Step C33, NVT ensemble, maintain 400K, run for 100ps, time step 0.25fs, to allow the system to fully relax;

[0122] Step C34, NPT ensemble, cooling from 400K to 300K under formation pressure P=18MPa, simulation time 100ps, time step 0.25fs;

[0123] Step C35, NPT ensemble, maintaining 18 MPa and 300 K, continue relaxation for 100 ps, ​​time step 0.25 fs, until the system density and potential energy converge to a stable fluctuation range. The resulting configuration is the Nenjiang Formation Type I Immon-Kerogen mixed-layer molecular model under the target oil shale reservoir conditions, as shown below. Figure 4 As shown.

[0124] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for constructing an oil shale reservoir illite-smectite-macerals composite molecular model, characterized in that, Includes the following steps: S1. X-ray diffraction tests were performed on the clay separation of the oil shale sample to obtain the illite-montmorillonite mixed layer ratio; the montmorillonite layer charge was calculated by determining the cation exchange capacity; and the formation temperature, pressure, and vitrinite reflectance of the target oil shale reservoir were obtained by combining the test data. S2. Construct molecular models of kerogen, montmorillonite, and illite respectively, and determine the molar ratio of illite to montmorillonite based on the illite-montmorillonite mixed layer ratio obtained in step S1. S3. Construct illite and montmorillonite supercells and truncate them to establish a mineral wall panel model; S4. ReaxFF reaction force field correction was carried out for the Immon-Kerogen mixed-layer composite system of oil shale reservoirs, and specific parameters in the force field involving clay mineral framework, interlayer ion coordination and organic-mineral interface bonding were corrected. S5. Assemble the montmorillonite wall, kerogen molecular layer and illite wall in the order of montmorillonite wall-kerogen molecular layer-illite wall to form an initial box model, and set the initial adsorption orientation of kerogen molecules according to the interfacial interaction energy. S6. Using the ReaxFF reaction force field corrected in step S4, perform molecular dynamics annealing on the initial box model obtained in step S5 to obtain the Immon-Kerogen mixed-layer molecular model under the target oil shale reservoir conditions.

2. The construction method of claim 1, wherein, The X-ray diffraction tests were performed sequentially on the clay fraction of the same oil shale sample under three treatment conditions: air-dried, ethylene glycol-saturated, and heated to 550°C. The differences between the three conditions separated the peaks of the two minerals, eliminating peak overlap interference. The ethylene glycol-saturated diffraction peak area ratio was calculated using the ratio of the peak areas in the ethylene glycol-saturated state. The montmorillonite layer charge was obtained by converting the cation exchange capacity measurement results. For monovalent compensating cations, the number of compensating cations was determined by taking the nearest integer of the total layer charge of the supercell.

3. The construction method of claim 1, wherein, Step S3 involves constructing illite and montmorillonite supercells and truncating them to create a mineral wall panel model, which includes the following specific steps: A1. Construct initial unit cells for illite and montmorillonite. Set interlayer potassium ions for illite according to standard crystal parameters. For montmorillonite, determine the number of compensating cations based on the layer charge number calculated in step S1 and the total layer charge of the supercell. Then, optimize the geometric structure of the unit cells of the two minerals respectively. A2. Periodically expand the geometrically optimized unit cell along the ab plane to form a supercell of not less than 4×2×1 times. The dimensions of the expanded supercell in both the a and b directions should be greater than twice the ReaxFF force field cutoff radius to eliminate the interaction between periodic mirror images. A3. Using the (001) crystal plane as the cut-off plane, the supercell is cut off to establish a mineral wall panel model, and a vacuum layer of not less than 20 Å is set in the direction perpendicular to the wall to fully eliminate the electrostatic and van der Waals interactions between the periodic mirror layers. A4. The surface Si-O and Al-O dangling bonds generated by the truncation of A3 are unified with hydrogen atoms to end them, thus completing the establishment of the mineral wall panel model.

4. The construction method of claim 1, wherein, In step S4, the ReaxFF reaction force field correction for the Immon-Kerogen mixed-layer composite system in oil shale reservoirs uses a publicly available ReaxFF parameter set containing Si, Al, O, H, C, N, S, K, and Na elements as the initial parameter set. The calibration targets are experimental or quantum chemical results regarding Al-O-Si bond angle distribution, K⁺ coordination structure, Na⁺ hydration coordination structure, and the condensation reaction pathway between Si-OH and oxygen-containing functional groups on the mineral wall. The specific content obtained through iterative fitting is as follows: B1. Set the equilibrium bond angle Theta0 of the aluminum-silicon bond (Al-O-Si) bond angle parameter to 142.5°; B2. Set the dissociation energy Ediss of the coordination bond between K⁺ in the illite interlayer and the six-membered silicon-oxygen ring to 0.12 kcal / mol; B3. Set the reference bond length Ro of the hydration coordination single bond of Na⁺ in the interlayer of montmorillonite to 1.62 Å; set the bond order decay parameter gamma to 12.2, which is a dimensionless parameter; set the reference bond length rsigma of the σ bond to 1.62 Å; and set the equilibrium bond angle Theta0 of HO-Na to 78.0°. B4. Modify the Si-OC bond angle parameter at the organic-mineral interface, setting the equilibrium bond angle Theta0 to 116° to describe the reaction pathway in which Si-OH on the mineral wall forms a condensation bond (Si-OC type) with the hydroxyl (-OH) and carboxyl (-COOH) groups of the kerogen carbon chain.

5. The construction method of claim 1, wherein, In step S5, the layer sequence of the montmorillonite wall, the kerogen molecular layer, and the illite wall is: a double-wall clamping model of montmorillonite-kerogen-illite.

6. The construction method of claim 1, wherein, In step S5, the initial orientation of the kerogen molecules is set according to the interfacial interaction energy obtained from the force field calculation in step S4 during assembly.

7. The construction method of claim 6, wherein, In step S5, the initial molecular orientation is as follows: the carbon chain hydroxyl (-OH) and carboxyl (-COOH) groups face the montmorillonite wall, the carbonyl (-C=O) groups face the illite wall, and the aliphatic chain and aromatic fused rings are not subject to orientation constraints.

8. The construction method according to claim 1, characterized in that, In step S6, the molecular dynamics annealing uses the ReaxFF reaction force field modified in step S4, and the time step is uniformly set to 0.25 fs. The specific steps are as follows: C1. Perform geometric optimization on the kerogen model constructed in step S2 to eliminate atomic overlap in the initial configuration, and relax it for 50 ps under the NVT ensemble to bring the kerogen molecule to a stable state. C2. In mineral double-walled surface systems without kerogen, run NVT ensemble for at least 200 ps to complete interlayer K + and Na + pre-equilibration to avoid unreasonable and drastic readjustment of interlayer structure due to disturbance of organic molecules when kerogen is introduced later. C3. Combine the kerogen model obtained in step C1 with the mineral interlayer structure after pre-equilibrium in step C2, and assemble it as the initial input of the whole system according to the sequence and orientation described in step S5; then perform NVT heating, high-temperature relaxation, NPT cooling and NPT final equilibrium in sequence to obtain the Yimeng-Kerogen composite molecular model of the oil shale reservoir.

9. The construction method according to claim 8, characterized in that, Step C3 is as follows: C31. The peak annealing temperature in step C3 is determined according to the maturity Ro of the kerogen; 300K is taken when Ro < 0.5%, 400K is taken when 0.5% ≤ Ro < 1.3%, and 500K is taken when Ro ≥ 1.3%, to prevent the reaction force field from triggering thermal decomposition of low-maturity kerogen at excessively high temperatures, while ensuring sufficient relaxation of high-maturity kerogen. C32. Using the NVT ensemble, the temperature is raised to the peak annealing temperature determined in step C31, the simulation time is 50 ps, ​​and the time step is set to 0.25 fs to simulate the relaxation process of kerogen molecules crossing the energy barrier from the equilibrium state. C33. Maintain the peak annealing temperature and continue to run the NVT ensemble for 100 ps with a time step of 0.25 fs to allow the adsorption sites of organic molecules and mineral-organic interfaces to fully relax and escape the local energy minimum. C34. Switch to the NPT ensemble and cool down from the peak annealing temperature to 300K under formation pressure. The simulation time is 100ps and the time step is set to 0.25fs to simulate the cooling and contraction process and eliminate thermal stress. C35. Maintain the NPT ensemble and continue relaxation for 100 ps under formation pressure and 300 K, with a time step of 0.25 fs, so that the system density and potential energy converge to a stable fluctuation range. The resulting configuration is the molecular model of the Irmont-Morton mixed-kerogen composite oil shale reservoir under the target formation conditions.

10. A molecular model of an oil shale reservoir with mixed montmorillonite and kerogen layers constructed by the construction method according to any one of claims 1-9, characterized in that, The molecular model includes an illite-montmorillonite mixed-layered-kerogen composite double-walled clamping structure assembled according to the montmorillonite-kerogen-illite sequence stacking, and the component ratio of illite to montmorillonite corresponds to the illite-montmorillonite mixed-layer ratio of the target oil shale sample.

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