A molecular dynamics simulation method and system for evaluating the miscibility of carbon dioxide and crude oil

Abstract

The present invention belongs to the technical field of oil and gas exploration and development, and discloses a molecular dynamics simulation method and system for evaluating the miscibility of carbon dioxide and crude oil, including: constructing an oil and gas component model based on molecular dynamics software and conducting molecular dynamics simulation, recording the total energy of the system and the energy change inside the molecules during the simulation process, and calculating the solubility parameters of each component under different conditions in combination with the calculation formulas of cohesive energy density and solubility parameter. The miscibility between two substances is reflected by the difference in solubility parameters between the components. The present invention can quickly and accurately quantify the oil and gas miscibility under different conditions from the perspective of the energy between components, provide important parameters for evaluating the oil and gas miscibility ability, and has certain guiding significance for the efficient development of oil reservoirs.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas exploration and development, and in particular relates to a molecular dynamics simulation method and system for evaluating the degree of mutual solubility of carbon dioxide and crude oil. Background Art

[0002] For my country's highly heterogeneous and water-sensitive reservoirs, water injection development will cause clay minerals to swell, resulting in extremely low recovery rates. Therefore, it is urgent to find a more effective development method. CO2 can rely on its low viscosity and strong diffusivity to effectively penetrate into tiny pores and dissolve with crude oil, reducing the interfacial tension between oil and gas and the viscosity of crude oil, thereby enhancing the fluidity of crude oil and achieving the effect of improving crude oil recovery.

[0003] Oil-gas miscibility is an important mechanism in the process of CO2 enhanced oil recovery, and its quantitative characterization is of great significance for optimizing CO2 flooding technology. However, in the prior art, there is no method that can effectively quantify and characterize the degree of miscibility between oil and gas. Molecular dynamics method, as a technology that can simulate the process of oil-gas miscibility at the molecular level, can make up for the shortcomings of experiments in nanoscale research, reduce experimental costs, and reveal the dynamic behavior and interaction mechanism of oil and gas in a confined space based on the interaction between atoms. Therefore, based on the above considerations, the present invention provides a molecular dynamics simulation method for evaluating the miscibility of carbon dioxide and crude oil. Summary of the invention

[0004] In order to solve the problems existing in the prior art, the present invention provides a molecular dynamics simulation method and system for evaluating the miscibility of carbon dioxide and crude oil. The molecular dynamics simulation method is used to quantitatively characterize the miscibility between CO2 and crude oil. The miscibility of oil and gas under different conditions can be quickly and accurately quantified from the perspective of energy between components, providing important parameters for evaluating the miscibility of oil and gas, which has certain guiding significance for the efficient development of oil reservoirs.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] A molecular dynamics simulation method for evaluating the miscibility of carbon dioxide and crude oil, the method comprising:

[0007] S1. Construct oil and gas component model based on molecular dynamics simulation software and carry out molecular dynamics simulation;

[0008] S2. Based on the results of molecular dynamics simulation, the total energy of all molecules in the model system and the internal energy of the molecules are calculated;

[0009] S3. Combine cohesive energy density and solubility parameters to analyze the miscibility of CO2 and crude oil under different conditions.

[0010] Preferably, in the step S1, constructing an oil-gas component model based on molecular dynamics simulation software includes:

[0011] S11. Sequentially construct the component structures of CO2, crude oil, and pore channels;

[0012] S12. Combine the component structures of CO2, crude oil, and pore channels to form a complete displacement model;

[0013] S13. Assign a force field and charges to the displacement model.

[0014] Preferably, in the step S1, conducting molecular dynamics simulation includes:

[0015] S14. Conduct equilibrium molecular dynamics simulation to simulate the initial formation environment;

[0016] S15. Conduct non-equilibrium molecular dynamics simulation, apply initial kinetic energy to CO2, and simulate the process of CO2 displacing crude oil.

[0017] Preferably, in the step S2, based on the molecular dynamics simulation results, statistically calculate the total energy of all molecules within the model system and the internal energy of molecules, including:

[0018] S21. Conduct the oil-gas miscibility process and record the change of the total energy of the model system with the simulation time, which is E total ;

[0019] S22. Shield the interaction between CO2 and crude oil components, conduct the oil-gas miscibility process, and record the internal energy of molecules, which is E intra .

[0020] Preferably, the cohesive energy calculation formula is:

[0021]

[0022] In the formula: E coh represents the cohesive energy of the molecular system; E inter represents the intermolecular interaction energy; <…> represents the average value of the numerical value; △H v represents the molar heat of vaporization; RT is the expansion work required for the liquid to be converted into gas;

[0023] The intermolecular interaction calculation formula is:

[0024]

[0025] In the formula: E inter represents the intermolecular interaction energy; E intra represents the internal energy of molecules; E total represents the total energy of all molecules in the simulation system; E vanrepresents the energy obtained through van der Waals forces between molecules; E elect represents the energy obtained through electrostatic interactions between molecules; E other represents the energy obtained through hydrogen bond interactions between molecules.

[0026] Preferably, in the step S3, analyzing the miscibility of CO2 - crude oil under different conditions by combining the cohesive energy density and solubility parameter includes:

[0027] The cohesive energy density represents the cohesive energy per unit volume, and the calculation formula is:

[0028]

[0029] In the formula: CED is the cohesive energy per unit volume; E coh represents the cohesive energy of the molecular system; V is the volume of the oil - gas system;

[0030] The solubility parameter calculation formula is:

[0031]

[0032] In the formula: CED is the cohesive energy per unit volume; SP is the solubility parameter.

[0033] The present invention also provides a molecular dynamics simulation system for evaluating the miscibility of carbon dioxide - crude oil. The system is used to implement any one of the above - mentioned methods, and the system includes: a model construction module, an energy statistics module, and a miscibility analysis module;

[0034] The model construction module is used to construct an oil - gas component model based on molecular dynamics simulation software and carry out molecular dynamics simulation;

[0035] The energy statistics module is used to statistically analyze the total energy of all molecules inside the model system and the internal energy of molecules based on the molecular dynamics simulation results;

[0036] The miscibility analysis module is used to analyze the miscibility of CO2 - crude oil under different conditions by combining the cohesive energy density and solubility parameter.

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] The present invention provides a molecular dynamics simulation method for evaluating the miscibility of carbon dioxide - crude oil. By using the molecular dynamics simulation method to construct an oil - gas system model to carry out the oil - gas miscibility process, recording the changes in the total energy of the system and the internal energy of molecules during the simulation process, and combining the calculation formulas of cohesive energy, cohesive energy density, and solubility parameter to characterize the miscibility of CO2 - crude oil under different conditions, it provides a theoretical guidance for the efficient development of oil reservoirs. Description of the Drawings

[0039] To more clearly illustrate the technical solutions of the present invention, the accompanying drawings required for use in the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0040] Figure 1 It is a flow chart of a molecular dynamics simulation method for evaluating the miscibility of carbon dioxide and crude oil in an embodiment of the present invention;

[0041] Figure 2 It is the molecular structure diagram of crude oil C8H in an embodiment of the present invention 18 ;

[0042] Figure 3 It is the molecular structure diagram of CO2 in an embodiment of the present invention;

[0043] Figure 4 It is the system structure diagram of the hydroxylated pore wall surface in an embodiment of the present invention;

[0044] Figure 5 It is the system structure diagram of the methylated pore wall surface in an embodiment of the present invention;

[0045] Figure 6 It is the system structure diagram of the hydroxylated pore model in an embodiment of the present invention;

[0046] Figure 7 It is the system structure diagram of the methylated pore model in an embodiment of the present invention. Detailed implementation manners

[0047] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

[0048] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners.

[0049] Embodiment 1

[0050] The embodiment of the present invention provides a molecular dynamics simulation method for evaluating the miscibility of carbon dioxide and crude oil, and the method includes:

[0051] S1. Construct an oil-gas component model based on molecular dynamics simulation software and carry out molecular dynamics simulation;

[0052] S2. Based on the results of molecular dynamics simulation, calculate the total energy of all molecules in the model system and the internal energy of the molecules.

[0053] S3. Analyze the miscibility of CO2 - crude oil under different conditions by combining the cohesive energy density and solubility parameter.

[0054] In this embodiment, in S1, constructing the oil - gas component model based on the molecular dynamics simulation software includes:

[0055] S11. Sequentially construct the component structures of CO2, crude oil (C8H 18 ), and pores (hydroxylated, methylated).

[0056] S12. Combine the crude oil and pore structures to form a sandwich structure of pore - crude oil - pore, and then combine it with CO2 to form a complete displacement model.

[0057] S13. Assign the correct force field and charge to the displacement model, that is, use the OPLS - AA force field to describe the crude oil component model, use the ClayFF force field to describe the pore model, and use the EPM2 force field to describe the CO2 model. The final model charge remains in an electrically neutral state.

[0058] In this embodiment, in S1, carrying out the molecular dynamics simulation includes:

[0059] S14. Carry out equilibrium molecular dynamics simulation, assign temperature to the CO2 molecules and crude oil molecules in the model to make them freely diffuse within the system relying on intermolecular interactions, thereby simulating the initial formation environment.

[0060] S15. Carry out non - equilibrium molecular dynamics simulation, that is, further apply a certain external force to CO2 to simulate the process of CO2 displacing crude oil.

[0061] In this embodiment, the internal energy of the molecule is the total energy of the remaining atoms after shielding the CO2 molecules and crude oil molecules in the system.

[0062] In this embodiment, in S2, based on the results of molecular dynamics simulation, calculating the total energy of all molecules in the model system and the internal energy of the molecules includes:

[0063] S21. Carry out the oil - gas miscibility process and record the change of the total energy of the model system with the simulation time, which is E total ;

[0064]

[0065] where: E coh represents the total energy of the system; E CO2 represents the internal energy within the CO2 system; EOil Represents the energy within the crude oil system; E channel Represents the energy of the pore system;

[0066] S22 shields the interaction between CO2 and crude oil components, conducts the oil-gas miscibility process, and records the internal energy of the molecules, which is E intra .

[0067] In this embodiment, the cohesive energy is used to describe the energy required to eliminate the intermolecular forces of a substance in the aggregated state. It is the average value of the energy required to separate all molecules from each other to an infinite distance. The formula for calculating the cohesive energy is:

[0068]

[0069] In the formula: E coh Represents the cohesive energy of the molecular system; E inter Represents the intermolecular interaction energy; <…> represents the average value of the numerical value; △H v Represents the molar heat of vaporization; RT is the expansion work required for the liquid to be converted into a gas;

[0070] The intermolecular interaction energy can evaluate the intermolecular interaction and miscibility from a microscopic perspective. The formula for calculating the intermolecular interaction is:

[0071]

[0072]

[0073] In the formula: E inter Represents the intermolecular interaction energy; E intra Represents the internal energy of the molecule; E total Represents the total energy of all molecules in the simulation system; E van Represents the energy obtained by the molecules through van der Waals forces; E elect Represents the energy obtained by the molecules through electrostatic interactions; E other Represents the energy obtained by the molecules through hydrogen bond interactions.

[0074] In this embodiment, in S3, analyzing the miscibility degree of CO2-crude oil under different conditions by combining the cohesive energy density and solubility parameter includes:

[0075] The cohesive energy density is equal to the opposite of the intermolecular interaction energy. The intermolecular interaction energy is the total energy of all molecules in the simulation system minus the internal energy of the molecule. The cohesive energy density represents the cohesive energy per unit volume. The calculation formula is:

[0076]

[0077] In the formula: CED is the cohesive energy per unit volume; Ecoh represents the cohesive energy of the molecular system; V is the volume of the oil-gas system;

[0078] The solubility parameter is a physical quantity used to measure the compatibility of fluids and is the square root of the cohesive energy density. The solubility parameter calculation formula is:

[0079]

[0080] In the formula: CED is the cohesive energy per unit volume; SP is the solubility parameter.

[0081] As the square root of the cohesive energy, the difference in solubility parameters between substances can reflect the mutual solubility between two substances. The smaller the difference, the stronger the mutual solubility of the two substances.

[0082] Example Two

[0083] The embodiment of the present invention provides a molecular dynamics simulation method for evaluating the mutual solubility of carbon dioxide and crude oil. The specific steps are as follows:

[0084] S1. In this embodiment, C8H 18 is used as the crude oil component. The pore wall surfaces with hydroxylated quartz and methylated quartz as the basic unit cells are established respectively. Through the molecular dynamics simulation software, the system structures of crude oil, CO2, hydroxylated pore, and methylated pore are constructed in sequence, as shown in Figure 2 , Figure 3 , Figure 4 , Figure 5 ;

[0085] S2. The above structures are combined to form a complete initial model, as shown in Figure 6 , Figure 7 . The pore space size is 24.6 Å × 157 Å × 38 Å, and the correct force field and charges are assigned to the initial model;

[0086] S3. Initial kinetic energy is assigned to the CO2 molecules and crude oil molecules in the model, and equilibrium molecular dynamics simulation is carried out under the NVT ensemble to simulate the real formation environment;

[0087] S4. Non-equilibrium molecular dynamics simulation is carried out, and an external force is applied to the CO2 molecules to simulate the process of CO2 displacing crude oil inside the pore;

[0088] S5. Record the total energy E total ;

[0089] S6. Shield the interaction between CO2 and crude oil components, and count the total energy E intra ;

[0090] S7. The cohesive energy is used to describe the energy required to eliminate the intermolecular forces in the aggregated state of matter. It is the average value of the energy required to separate all molecules from each other to an infinite distance. The cohesive energy density is often the opposite of the intermolecular energy, and the specific expression is as follows:

[0091] (1)

[0092] In the formula: E coh represents the cohesive energy of the molecular system; E inter represents the intermolecular interaction energy; <…> represents the average value of the numerical value; △H v represents the molar heat of vaporization; RT is the expansion work required for the liquid to be converted into a gas.

[0093] S8. The intermolecular interaction energy can evaluate the intermolecular interaction and solubility from a microscopic perspective. The intermolecular interaction energy usually includes the energy obtained by the intermolecular van der Waals force, the energy obtained by the intermolecular electrostatic interaction, and the energy obtained by the intermolecular hydrogen bond interaction. The intermolecular interaction energy is usually calculated by subtracting the internal energy of the molecule from the total energy of the molecules in the system. The specific calculation formula is as follows:

[0094] (2)

[0095] (3)

[0096] In the formula: E inter represents the intermolecular interaction energy; E intra represents the internal energy of the molecule; E total represents the total energy of all molecules in the simulation system; E van represents the energy obtained by the intermolecular van der Waals force; E elect represents the energy obtained by the intermolecular electrostatic interaction; E other represents the energy obtained by the intermolecular hydrogen bond interaction.

[0097] S9. The cohesive energy density represents the cohesive energy per unit system, and its calculation form is as follows:

[0098] (4)

[0099] In the formula: CED is the cohesive energy per unit volume; E coh represents the cohesive energy of the molecular system; V is the volume of the oil and gas system;

[0100] S10. The solubility parameter is a physical quantity used to measure the fluid compatibility, usually in the form of the square root of the cohesive energy density, and its expression is as follows:

[0101] (5)

[0102] Wherein: SP represents the solubility parameter; CED is the cohesive energy per unit volume;

[0103] The differences in solubility parameters between the internal shale oil and CO2 in the hydroxylation model and methylation model systems are 0.096×10 8 J / cm 3 and 0.136×10 8 J / cm 3 respectively, as shown in Table 1 (Table 1 shows the cohesive energy density and solubility parameter in the hydroxylation and methylation systems). The difference in solubility parameters can reflect the mutual solubility between two substances to a certain extent. The smaller the difference in solubility parameters, the stronger the mutual solubility between the two substances. Therefore, by comparing the differences in solubility parameters within the hydroxylation model and methylation model systems, it can be seen that: 0.096×10 8 J / cm 3 <0.136×10 8 J / cm 3 , and the mutual solubility of oil and gas within the hydroxylation system is stronger.

[0104] Table 1

[0105]

[0106] Example Three

[0107] The embodiment of the present invention also provides a molecular dynamics simulation system for evaluating the mutual solubility of carbon dioxide - crude oil, and the system is used to implement any one of the methods described above. The system includes: a model construction module, an energy statistics module, and a mutual solubility analysis module;

[0108] The model construction module is used to construct an oil and gas component model based on molecular dynamics simulation software and carry out molecular dynamics simulation;

[0109] The energy statistics module is used to statistically analyze the total energy of all molecules and the internal energy of molecules within the model system based on the molecular dynamics simulation results;

[0110] The mutual solubility analysis module is used to analyze the mutual solubility of CO2 - crude oil under different conditions by combining the cohesive energy density and solubility parameter.

[0111] The above - described embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A molecular dynamics simulation method for evaluating the miscibility of carbon dioxide and crude oil, characterized in that, The method includes: S1. Construct an oil-gas component model based on molecular dynamics simulation software and conduct molecular dynamics simulation; S2. Based on the molecular dynamics simulation results, statistically analyze the total energy of all molecules within the model system and the internal energy of the molecules; S3. Analyze the mutual solubility of CO2-crude oil under different conditions by combining the cohesive energy density and solubility parameter; In S2, based on the molecular dynamics simulation results, statistically analyzing the total energy of all molecules within the model system and the internal energy of the molecules includes: S21. Carry out the oil-gas mutual dissolution process and record the change of the total energy of the model system with the simulation time, which is E total ; S22. Shield the interaction between CO2 and crude oil components, carry out the oil-gas miscibility process, and record the internal energy of molecules, which is E intra ; The formula for calculating the cohesive energy is: ； Where: E coh represents the cohesive energy of the molecular system; E inter represents the intermolecular interaction energy; <…> represents the average value of the numerical value; △H v represents the molar heat of vaporization; RT is the expansion work required for the liquid to be converted into gas; The formula for calculating the intermolecular interaction is: ； ； Where: E inter represents the intermolecular interaction energy; E intra represents the internal energy of the molecule; E total represents the total energy of all molecules in the simulation system; E van represents the energy obtained by intermolecular van der Waals forces; E elect represents the energy obtained by intermolecular electrostatic interactions; E other represents the energy obtained by intermolecular hydrogen bond interactions; In S3, analyzing the mutual solubility of CO2-crude oil under different conditions by combining the cohesive energy density and solubility parameter includes: The cohesive energy density represents the cohesive energy per unit volume, and the calculation formula is: ； Where: CED is the cohesive energy per unit volume; E coh represents the cohesive energy of the molecular system; V is the volume of the oil and gas system; The formula for calculating the solubility parameter is: ； In the formula: CED is the cohesive energy per unit volume; SP is the solubility parameter; In S1, constructing the oil-gas component model based on molecular dynamics simulation software includes: S11. Sequentially construct the component structures of CO2, C8H 18 , hydroxylated or methylated pore channels; S12. Combine crude oil and pore structures to form a sandwich structure of pore-crude oil-pore, and then combine it with CO2 to form a complete displacement model; S13. Assign force fields and charges to the displacement model. Use the OPLS-AA force field to describe the crude oil component model, use the ClayFF force field to describe the pore model, and use the EPM2 force field to describe the CO2 model. The final model charge maintains an electrically neutral state; In S1, conducting molecular dynamics simulation includes: S14. Conduct equilibrium molecular dynamics simulation, assign temperatures to the CO2 molecules and crude oil molecules in the model so that they freely diffuse within the system relying on intermolecular interactions, thereby simulating the initial formation environment; S15. Conduct non-equilibrium molecular dynamics simulation, that is, further apply a certain external force to CO2 to simulate the process of CO2 displacing crude oil; The energy inside the molecule is the total energy of the remaining atoms after shielding the CO2 molecules and crude oil molecules in the system; Specifically: Using C8H 18 as the crude oil component, pore walls with hydroxylated quartz and methylated quartz as the basic unit cells are established respectively, and the crude oil, CO2, hydroxylated pore or methylated pore system structures are constructed in turn through molecular dynamics simulation software; Combine the structures to form a complete initial model. The pore space size is 24.6 Å×157 Å×38 Å, and assign force fields and charges to the initial model; Assign initial kinetic energy to the CO2 molecules and crude oil molecules in the model, and conduct equilibrium molecular dynamics simulation under the NVT ensemble to simulate the real formation environment; Conduct non-equilibrium molecular dynamics simulation, apply an external force to the CO2 molecules, and simulate the process of CO2 displacing crude oil inside the pore.

2. A molecular dynamics simulation system for evaluating the miscibility of carbon dioxide and crude oil, the system being used to implement the method described in claim 1, characterized in that, The system includes: a model construction module, an energy statistics module, and a mutual solubility analysis module; The model construction module is used to construct an oil-gas component model based on molecular dynamics simulation software and conduct molecular dynamics simulation; The energy statistics module is used to statistically analyze the total energy of all molecules within the model system and the internal energy of the molecules based on the molecular dynamics simulation results; The mutual solubility analysis module is used to analyze the mutual solubility of CO2-crude oil under different conditions by combining the cohesive energy density and solubility parameter.

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