Molecular dynamics simulation method and system for evaluating carbon dioxide-crude oil mutual solubility degree

Abstract

The invention belongs to the technical field of oil-gas exploration and development, and discloses a molecular dynamics simulation method and system for evaluating the carbon dioxide-crude oil mutual solubility degree, and the method comprises the steps: building an oil-gas component model based on molecular dynamics software, and carrying out the molecular dynamics simulation; and recording the total energy of the system in the simulation process and the energy change condition in the molecule, and calculating the solubility parameters of each component under different conditions by combining the cohesive energy density and solubility parameter calculation formulas. And the intersolubility between the two substances is reflected through the difference of solubility parameters between the components. According to the method, the oil-gas intersolubility degree under different conditions can be rapidly and accurately quantified from the angle of energy among the components, important parameters are provided for evaluating the oil-gas intersolubility capacity, and the method has certain guiding significance for 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 expand, resulting in extremely low recovery rates. Therefore, it is urgent to find a more effective development method. 2 It can rely on its own low viscosity and strong diffusivity to effectively penetrate into the 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 as CO 2 An important mechanism in the process of enhanced oil recovery, its quantitative characterization is crucial for optimizing CO 2 Oil recovery technology is of great significance. However, in the prior art, there is no method that can effectively quantify the miscibility of oil and gas. Molecular dynamics method, as a technology that can simulate the miscibility of oil and gas 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, and uses the molecular dynamics simulation method to quantitatively characterize the CO 2 The miscibility between crude oil and crude oil can quickly and accurately quantify the miscibility of oil and gas under different conditions from the perspective of energy between components, provide important parameters for evaluating the miscibility of oil and gas, and have 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. Combining cohesive energy density and solubility parameters to analyze CO under different conditions 2 - Miscibility of crude oils.

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

[0011] S11, build CO in sequence 2 , crude oil, pore structure;

[0012] S12, CO 2 , crude oil, and pore structure are combined to form a complete displacement model;

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

[0014] Preferably, in S1, carrying out molecular dynamics simulation includes:

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

[0016] S15. Conduct nonequilibrium molecular dynamics simulations to analyze the CO 2 Apply initial kinetic energy to simulate CO 2 The process of displacing crude oil.

[0017] Preferably, in S2, based on the molecular dynamics simulation results, the total energy of all molecules in the statistical model system and the internal energy of the molecules include:

[0018] 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 ;

[0019] S22, Shielding CO 2 The interaction between the components of crude oil, the oil-gas miscibility process, and the internal energy of the molecule are recorded, which is E intra .

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

[0021]

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

[0023] The formula for calculating the intermolecular interaction is:

[0024]

[0025] 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 gained by the van der Waals force between molecules; E elect Indicates the energy gained by the electrostatic interaction between molecules; E other It represents the energy gained by the interaction between molecules through hydrogen bonds.

[0026] Preferably, in S3, the cohesive energy density and solubility parameter are combined to analyze the CO under different conditions. 2 -The miscibility of crude oils includes:

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

[0028]

[0029] 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;

[0030] The solubility parameter calculation formula is:

[0031]

[0032] Where: 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 and crude oil, the system is used to implement any one of the methods described above, the system comprising: a model building module, an energy statistics module and a miscibility analysis module;

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

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

[0036] The mutual solubility analysis module is used to analyze CO under different conditions by combining cohesive energy density and solubility parameters. 2 - Miscibility of crude oils.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] The present invention provides a molecular dynamics simulation method for evaluating the miscibility of carbon dioxide and crude oil. The molecular dynamics simulation method is used to construct an oil and gas system model to carry out the oil and gas miscibility process, record the total energy of the system and the change of the internal energy of the molecules during the simulation process, and combine the cohesive energy, cohesive energy density, and solubility parameter calculation formula to characterize the CO2 miscibility under different conditions. 2 -The miscibility of crude oil provides theoretical guidance for the efficient development of oil reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the technical solution of the present invention, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0040] Figure 1 This 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 Crude oil C in the embodiment of the present invention 8 H 18 Molecular structure diagram of

[0042] Figure 3 CO in the embodiment of the present invention 2 Molecular structure diagram of

[0043] Figure 4 This is a system structure diagram of the hydroxylated channel wall in an embodiment of the present invention;

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

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

[0046] Figure 7 This is a diagram of the methylation channel model system structure in an embodiment of the present invention. DETAILED DESCRIPTION

[0047] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0048] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[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, the method comprising:

[0051] S1. Construct oil and 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, the total energy of all molecules in the model system and the internal energy of the molecules are calculated;

[0053] S3. Combining cohesive energy density and solubility parameters to analyze CO under different conditions 2 - Miscibility of crude oils.

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

[0055] S11, build CO in sequence 2 、Crude oil(C 8 H 18 ), component structure of pores (hydroxylation, methylation);

[0056] S12, combining crude oil and pore structure to form a sandwich structure of pore-crude oil-pore, and then 2 Combined to form a complete displacement model;

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

[0058] In this embodiment, in S1, performing molecular dynamics simulation includes:

[0059] S14. Conduct equilibrium molecular dynamics simulations to simulate the CO 2 The molecules and crude oil molecules are given temperature so that they can diffuse freely within the system by relying on intermolecular interactions, thus simulating the initial formation environment;

[0060] S15. Conduct nonequilibrium molecular dynamics simulations to further analyze the CO 2 Apply a certain external force to simulate CO 2 The process of displacing crude oil.

[0061] In this embodiment, the energy inside the molecule is the energy of the system shielding CO 2 The total energy of the atoms remaining after the molecule, crude oil molecule.

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

[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 Indicates CO 2 The energy inside the system; E Oil Represents the energy inside the crude oil system; E channel represents the energy of the pore system;

[0066] S22, Shielding CO 2 The interaction between the components of crude oil, the oil-gas miscibility process, and the internal energy of the molecule are recorded, which is E intra .

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

[0068]

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

[0070] The intermolecular interaction energy can be used to evaluate the interaction and solubility between molecules from a microscopic perspective. The calculation formula for intermolecular interaction is:

[0071]

[0072]

[0073] 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; Evan Represents the energy gained by the van der Waals force between molecules; E elect Indicates the energy gained by the electrostatic interaction between molecules; E other It represents the energy gained by the interaction between molecules through hydrogen bonds.

[0074] In this embodiment, in S3, the cohesive energy density and solubility parameter are combined to analyze the CO under different conditions. 2 -The miscibility of crude oils 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 molecules. The cohesive energy density represents the cohesive energy per unit volume. The calculation formula is:

[0076]

[0077] 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;

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

[0079]

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

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

[0082] Embodiment 2

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

[0084] S1. This embodiment uses C 8 H 18 As the crude oil component, the pore walls with hydroxylated quartz and methylated quartz as the basic unit cells were established respectively, and the crude oil, CO 2 , hydroxylated pores, methylated pore architectures, such as Figure 2 , Figure 3 , Figure 4 , Figure 5 As shown;

[0085] S2, combine the above structures to form a complete initial model, such as Figure 6 , Figure 7 As shown, the pore space size is 24.6Å×157Å×38Å, and the correct force field and charge are assigned to the initial model;

[0086] S3 is the CO in the model 2 Initial kinetic energy is given to molecules and crude oil molecules, and equilibrium molecular dynamics simulation is carried out under the NVT ensemble to simulate the real formation environment;

[0087] S4. Conduct nonequilibrium molecular dynamics simulations to obtain CO 2 The molecules exert external forces to simulate the CO inside the pores. 2 The process of displacing crude oil;

[0088] S5. Record the total energy E inside the system during the simulation total ;

[0089] S6, Shielding CO 2 The interaction between the components of crude oil and the total energy E inside the molecule intra ;

[0090] S7. Cohesive energy is used to describe the energy required for aggregated substances to eliminate intermolecular forces. It is the average energy required to separate all molecules to an infinite distance. The cohesive energy density is often the opposite of the intermolecular energy. The specific expression is as follows:

[0091] (1)

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

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

[0094] (2)

[0095] (3)

[0096] Where: E inter represents the intermolecular interaction energy; E intra Represents the internal energy of the molecule; Etotal Represents the total energy of all molecules in the simulation system; E van Represents the energy gained by the van der Waals force between molecules; E elect Indicates the energy gained by the electrostatic interaction between molecules; E other It represents the energy gained by the interaction between molecules through hydrogen bonds.

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

[0098] (4)

[0099] 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;

[0100] S10. Solubility parameter is a physical quantity used to measure the solubility of fluids. It is usually expressed as the square root of the cohesive energy density. Its expression is as follows:

[0101] (5)

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

[0103] Shale oil and CO in the hydroxylation model and methylation model systems 2 The difference in solubility parameters between 8 J / cm 3 , 0.136×10 8 J / cm 3 , as shown in Table 1 (Table 1 shows the cohesive energy density and solubility parameters in the hydroxylation and methylation systems). The difference in solubility parameters can reflect the mutual solubility between the two substances to a certain extent. The smaller the difference in solubility parameters, the stronger the mutual solubility of the two substances. Therefore, by comparing the difference in solubility parameters within the hydroxylation model and the methylation model system, it can be seen that: 0.096×10 8 J / cm 3 <0.136×10 8 J / cm 3 , the degree of oil and gas miscibility within the hydroxylation system is relatively strong.

[0104] Table 1

[0105]

[0106] Embodiment 3

[0107] The embodiment of the present invention further provides a molecular dynamics simulation system for evaluating the miscibility of carbon dioxide and crude oil, the system being used to implement any one of the methods described above, the system comprising: a model building module, an energy statistics module and a miscibility analysis module;

[0108] Model building module, used to build oil and gas component models based on molecular dynamics simulation software and conduct molecular dynamics simulation;

[0109] Energy statistics module, which is used to calculate the total energy of all molecules in the model system and the internal energy of molecules based on the molecular dynamics simulation results;

[0110] The mutual solubility analysis module is used to analyze CO under different conditions by combining cohesive energy density and solubility parameters. 2 - Miscibility of crude oils.

[0111] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all 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 comprises: S1. Construct oil and gas component model based on molecular dynamics simulation software and carry out molecular dynamics simulation; 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; S3. Combining cohesive energy density and solubility parameters to analyze the miscibility of CO2 and crude oil under different conditions; In S2, based on the molecular dynamics simulation results, the total energy of all molecules in the statistical model system and the internal energy of the molecules include: 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 ; S22, shield the interaction between CO2 and crude oil components, carry out the oil-gas miscibility process, and record the internal energy of the molecule, which is E intra ; The formula for calculating 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; △H v represents the molar heat of vaporization; RT is the expansion work required to convert liquid 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 gained by the van der Waals force between molecules; E elect Indicates the energy gained by the electrostatic interaction between molecules; E other It represents the energy gained by the interaction between molecules through hydrogen bonds; In S3, the mutual solubility of CO2 and crude oil under different conditions is analyzed by combining cohesive energy density and solubility parameters, including: Cohesive energy density is 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 solubility parameter calculation formula is: Where: CED is the cohesive energy per unit volume; SP is the solubility parameter.

2. The method according to claim 1, characterized in that In S1, constructing an oil and gas component model based on molecular dynamics simulation software includes: S11, constructing the component structures of CO2, crude oil, and pores in sequence; S12, combining the component structures of CO2, crude oil, and pores to form a complete displacement model; S13. Assign force fields and charges to the displacement model.

3. The method according to claim 1, characterized in that In S1, carrying out molecular dynamics simulation includes: S14. Conduct equilibrium molecular dynamics simulation to simulate the initial formation environment; S15. Carry out non-equilibrium molecular dynamics simulation, apply initial kinetic energy to CO2, and simulate the process of CO2 displacing crude oil.

4. A molecular dynamics simulation system for evaluating the miscibility of carbon dioxide and crude oil, the system being used to implement the method according to any one of claims 1 to 3, characterized in that: The system comprises: a model building module, an energy statistics module and a miscibility analysis module; The model building module is used to build an oil and gas component model based on molecular dynamics simulation software and carry out molecular dynamics simulation; The energy statistics module is used to calculate the total energy of all molecules in the model system and the internal energy of the molecules based on the molecular dynamics simulation results; The miscibility analysis module is used to analyze the miscibility of CO2-crude oil under different conditions by combining cohesive energy density and solubility parameters.

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