Method for simulating foam stability based on molecular dynamics

Abstract

The invention relates to a method for simulating foam stability based on molecular dynamics. The method comprises the following steps: constructing a surfactant molecular model and a water molecular model; supercellular the surfactant molecule model into a surfactant molecule cubic box; constructing the water molecule model into a water molecule cubic box; the surface active agent molecule cubic box and the water molecule cubic box are spliced into a surface active agent-water-surface active agent cubic box; placing the surfactant-water-surfactant cubic box in the center, and filling the two sides with the gas molecule cubic boxes; finally, a first target cubic box of gas-surfactant-water-surfactant-gas with a boundary periodic condition is constructed; and carrying out molecular dynamics equilibrium simulation, and representing the foam stability by average adsorption energy. The simulation result is highly matched with the experiment result.

Description

Technical Field

[0001] The invention relates to the technical field of oil fields, and in particular to a method for simulating foam stability based on molecular dynamics. Background Art

[0002] In recent years, foam flooding technology has been widely used in oil fields. It can effectively block high permeability layers, increase the sweep system, and improve recovery. It is a tertiary oil recovery technology that can replace water flooding. Foam plays a role in water plugging, profile adjustment, and improving recovery. With its wide source and low price, it is particularly suitable for high water content and severe heterogeneity reservoirs.

[0003] The stability of foam is one of the important factors affecting the effect of foam flooding. The oil displacement and profile control capabilities of foam are seriously affected by the foaming ability and foam stability of the foaming agent. At present, a large number of field tests and indoor experimental studies have shown that the single surfactant foam system is greatly affected by the high temperature and high salt environment, has poor stability in the formation, and even loses the ability to improve recovery and water plugging and profile control in the reservoir environment, which greatly limits the application of foam flooding in the field.

[0004] At present, the characteristics of foam stability are mainly characterized by macroscopic experiments, such as the volume test and half-life test of foam, viscoelasticity test, interfacial tension test and microscopic observation of the microscopic fusion and disproportionation process of foam, etc. There is a lack of systematic and comprehensive mechanism of foam stability from a microscopic perspective. Summary of the invention

[0005] The present invention aims to solve the above problems and proposes a method of using molecular dynamics simulation to analyze the stability of a single nanoparticle foam system from an atomic perspective. Limited by the model size and periodic boundary of molecular simulation, this paper does not establish a complete foam system but simulates the local liquid film of the foam, reveals the foam stability mechanism from a microscopic perspective, and forms a method of simulating foam stability based on molecular dynamics.

[0006] The technical solution of the present invention is: (I) A method for simulating foam stability based on molecular dynamics, as follows: Step 1: Construct surfactant molecular models and water molecular models; Step 2: construct the surfactant molecular model into a surfactant molecular crystal structure, and then supercell it into a surfactant molecular cubic box; Construct the water molecule model as a water molecule cubic box; Step 3: Connect the surfactant molecule cubic box and the water molecule cubic box to form a surfactant-water-surfactant cubic box; Step 4: Construct a first target cubic box with a height of 80Å-200Å and a cross section of (10-50Å)*(10-50Å); place the surfactant-water-surfactant cubic box at the center and fill the two sides with gas molecule cubic boxes; finally construct the first target cubic box of gas-surfactant-water-surfactant-gas with boundary periodic conditions; Step 5: Perform a molecular dynamics equilibrium simulation on the first-purpose cubic box to characterize the foam stability using the average adsorption energy.

[0007] It also includes: a vacuum layer is additionally provided on the adjacent sides of the gas molecule cubic box and the first destination cubic box.

[0008] (ii) Aiming at strengthening CO2 foam, a method based on molecular dynamics simulation of foam stability is proposed. On the basis of the first purpose cubic box, a nanoparticle molecule cubic box is added between the surfactant molecule cubic box and the water molecule cubic box; finally, a second purpose cubic box of vacuum layer-gas-surfactant-nanoparticle-water-nanoparticle-surfactant-gas-vacuum layer with boundary periodic conditions is constructed; and then molecular dynamics equilibrium simulation is carried out on the second purpose cubic box to characterize the foam stability with the average adsorption energy.

[0009] Among them, in the cubic box of surfactant molecules, the number of surfactant molecules satisfies [1, 20]; in the cubic box of water molecules, the number of water molecules satisfies [500, 1000]; and the height of the vacuum layer satisfies [10 Å, 30 Å].

[0010] The number of nanoparticle molecules in the nanoparticle molecular cubic box satisfies [1, 4].

[0011] The calculation process of the number of gas molecules in the gas molecule cubic box is as follows: the mass of the gas molecules in the gas molecule cubic box is obtained by the density of the gas molecules and the volume of the gas molecule cubic box; According to the relative molecular mass M and mass m of the gas molecule, the amount of substance n=m / M of the gas molecule is obtained; Based on Avogadro's constant C, we get the number of gas molecules N=C×n.

[0012] The method for constructing the nanoparticle molecular cubic box is as follows: first construct a nanoparticle molecular model, then construct a nanoparticle molecular crystal structure, and then supercell the nanoparticle molecular cubic box.

[0013] Among them, the method for constructing the nanoparticle molecular model is: first, importing the SiO2 plane structure into the MS software, and creating the imported SiO2 plane structure as a spherical nanocluster structure, and adding H atoms to saturate the spherical nanocluster structure; replacing the H atoms directly connected to the Si atoms in the spherical nanocluster structure with O atoms; finally, deleting the O atoms and water molecules that drifted away from the spherical nanocluster structure, and obtaining the nanoparticle molecular model.

[0014] Wherein, the molecular dynamics equilibrium simulation is realized by Forcite program.

[0015] Among them, the cross-sections of the surfactant molecule cubic box, water molecule cubic box, gas molecule cubic box, and vacuum layer are consistent with the cross-sections of the first purpose cubic box / the second purpose cubic box; the cross-section of the nanoparticle molecule cubic box is consistent with the cross-section of the second purpose cubic box.

[0016] The technical effects of the present invention are: (1) The present invention provides a method for predicting or evaluating foam stability based on molecular dynamics simulation. By building foam models of different systems and performing molecular dynamics simulation on them, the efficiency of foam stability evaluation can be significantly improved and a large amount of experimental costs can be saved. (2) The present invention evaluates the stability of the foam at the microscopic level by analyzing the average adsorption energy of the foam system, and reveals the microscopic stability mechanism of the foam; and supports this with interfacial tension, root mean square displacement and water molecule diffusion coefficient; (3) The simulation results in the present invention are highly consistent with the experimental results, which verifies the accuracy of the predictive simulation and provides a theoretical basis for the application of foam fluids in the oil and gas industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Surfactant C 12 Structural formula of E4.

[0018] Figure 2 The front view of the cubic box for the first purpose.

[0019] Figure 3 A top view of the cubic box for the first purpose.

[0020] Figure 4 Diagram of the aggregation state of surfactant at the gas-liquid interface under the molecular dynamics equilibrium state of the first purpose cubic box.

[0021] Figure 5 For different C 12 Change curve of the root mean square displacement of the first destination cubic box at E4 concentration.

[0022] Figure 6For different C 12 Change curve of the diffusion coefficient of water molecules in the first target cubic box under E4 concentration.

[0023] Figure 7 For different C 12 Change curve of interfacial tension of the first-purpose cubic box at E4 concentration.

[0024] Figure 8 For different C 12 Change curve of the average adsorption energy of the first-purpose cubic box at E4 concentration.

[0025] Fig. 9 Schematic diagram of the construction process of nanoparticle molecular model.

[0026] Fig.10 Front view of the cubic box for the second purpose.

[0027] Fig.11 A top view of the cubic box for the second purpose.

[0028] Fig.12 Diagram of the aggregation state of surfactant at the air-liquid interface under the molecular dynamics equilibrium state of the cubic box for the second purpose.

[0029] Fig.13 For different C 12 The change curve of the root mean square displacement of the second destination cubic box at E4 concentration.

[0030] Fig.14 For different C 12 Change curve of the water molecule diffusion coefficient of the second target cubic box under E4 concentration.

[0031] Fig.15 For different C 12 The change curve of the interfacial tension of the second purpose cubic box at E4 concentration.

[0032] Fig.16 For different C 12 Change curve of the average adsorption energy of the second purpose cubic box at E4 concentration.

[0033] Fig.17 This is the result of the macro foam stability stirring experiment. DETAILED DESCRIPTION

[0034] Example 1 In view of the fact that the simulated foam is ordinary foam, a method for simulating foam stability based on molecular dynamics is proposed. The method is as follows: Step 1: Construct surfactant molecular models and water molecular models; Step 2: construct the surfactant molecular model into a surfactant molecular crystal structure, and then supercell it into a surfactant molecular cubic box; Construct the water molecule model as a water molecule cubic box; Step 3: Connect the surfactant molecule cubic box and the water molecule cubic box to form a surfactant-water-surfactant cubic box; Step 4: Construct a first target cubic box with a height of 80Å-200Å; place the surfactant-water-surfactant cubic box at the center and fill the two sides with gas molecule cubic boxes; finally construct the first target cubic box of gas-surfactant-water-surfactant-gas with boundary periodic conditions; Step 5: Perform a molecular dynamics equilibrium simulation on the first-purpose cubic box to characterize the foam stability using the average adsorption energy.

[0035] Example 2 On the basis of Example 1, it also includes: It also includes: a vacuum layer is additionally provided on the adjacent sides of the gas molecule cubic box and the first destination cubic box.

[0036] Example 3 On the basis of Example 2, the invention further includes: In the surfactant molecule cubic box, the number of surfactant molecules satisfies [1, 20]; in the water molecule cubic box, the number of water molecules satisfies [500, 1000]; the height of the vacuum layer satisfies [10 Å, 30 Å]; The calculation process of the number of gas molecules in the gas molecule cubic box is as follows: the mass of the gas molecules in the gas molecule cubic box is obtained by the density of the gas molecules and the volume of the gas molecule cubic box; According to the relative molecular mass M and mass m of the gas molecule, the amount of substance n=m / M of the gas molecule is obtained; According to Avogadro's constant C, we can get the number of gas molecules N=C×n; Among them, the cross-sections of the surfactant molecule cubic box, the water molecule cubic box, the gas molecule cubic box, and the vacuum layer are all consistent with the cross-section of the first target cubic box.

[0037] Example 4 On the basis of Example 3, the invention further includes: In order to strengthen CO2 foam by simulating foam, a method based on molecular dynamics to simulate foam stability is proposed. On the basis of the first purpose cubic box, a nanoparticle molecule cubic box is added between the surfactant molecule cubic box and the water molecule cubic box; finally, a second purpose cubic box with boundary periodic conditions of vacuum layer-gas-surfactant-nanoparticle-water-nanoparticle-surfactant-gas-vacuum layer is constructed; then, molecular dynamics equilibrium simulation is carried out on the second purpose cubic box, and the foam stability is characterized by the average adsorption energy; Among them, the cross sections of the surfactant molecule cubic box, the water molecule cubic box, the gas molecule cubic box, the nanoparticle molecule cubic box and the vacuum layer are all consistent with the cross section of the second target cubic box.

[0038] Example 5 On the basis of Example 4, it also includes: The method for constructing the nanoparticle molecular cubic box is as follows: first construct a nanoparticle molecular model, then construct a nanoparticle molecular crystal structure, and then supercell the nanoparticle molecular cubic box, wherein the number of nanoparticle molecules in the nanoparticle molecular cubic box satisfies [1, 4]; Among them, the method for constructing the nanoparticle molecular model is: first, importing the SiO2 plane structure into the MS software, and creating the imported SiO2 plane structure as a spherical nanocluster structure, and adding H atoms to saturate the spherical nanocluster structure; replacing the H atoms directly connected to the Si atoms in the spherical nanocluster structure with O atoms; finally, deleting the O atoms and water molecules that drifted away from the spherical nanocluster structure, and obtaining the nanoparticle molecular model.

[0039] Example 6 Based on Example 1 or 4, it also includes: the molecular dynamics equilibrium simulation is implemented by Forcite program.

[0040] Specific application case 1--simulating ordinary foam A method for simulating foam stability based on molecular dynamics is as follows: Step 1: Construct two surfactant molecule models and one water molecule model, and optimize them respectively; the surfactant in this specific application case 1 is C 12 E4, its structural formula is shown in Figure 1 ; Step 2: construct the surfactant molecular model into a surfactant molecular crystal structure, and after optimization, supercell it into a surfactant molecular cubic box (containing 4 surfactant molecules), and then optimize it again; The water molecule model was constructed as a water molecule cubic box (containing 500 water molecules) and optimized; Step 3: Splice two surfactant molecule cubic boxes and one water molecule cubic box into a surfactant-water-surfactant cubic box; Step 4: Build the first target cubic box with a height of 138Å and a cross section of 30Å*30Å; The surfactant-water-surfactant cubic box is placed in the center. A 20 Å vacuum layer is set on both sides of the first target cubic box, and the remaining parts on both sides are filled with a gas molecule cubic box respectively; the main view of the first target cubic box is shown in Figure 2 , top view Figure 3 ; After determining the height of the vacuum layer and the surfactant-water-surfactant cubic box, the height of the gas molecule cubic box is determined; and then the number of gas molecules in the gas molecule cubic box is calculated; The calculation process of the number of gas molecules is as follows: under the premise of set temperature and pressure, the gas density is constant; that is, the mass of the gas molecules in the gas molecule cubic box is obtained by the density of the gas molecules and the volume of the gas molecule cubic box; According to the relative molecular mass M and mass m of the gas molecule, the amount of substance n=m / M of the gas molecule is obtained; According to Avogadro's constant C, we get the number of gas molecules N=C×n; we calculate that the number of gas molecules in the cubic box of gas molecules is 3; Then, the first target cubic box is finally determined; Step 5: Perform molecular dynamics equilibrium simulation on the first-purpose cubic box to characterize the foam stability with average adsorption energy. The aggregation state of the surfactant at the gas-liquid interface under the molecular dynamics equilibrium state of the first-purpose cubic box is shown in Figure 4 ; In this specific application case 1, a control group is set according to the number of surfactant molecules in the surfactant cube box to form surfactants of different concentrations; in the control group, the number of surfactant molecules in the surfactant cube box is 6, 8, 10 and 12 respectively; Conduct molecular dynamics equilibrium simulation experiments on 5 groups of first-purpose cubic boxes; The experimental results show that the average adsorption energy of the first-order cubic box corresponding to the 8 surfactant molecule cubic boxes is the highest, and the corresponding simulated local foam liquid film is the most stable, see Figure 8 ; Similarly, the curve of the RMS displacement of the first target cubic box corresponding to the cubic box of 8 surfactant molecules is the lowest ( Figure 5 ), the molecular diffusion coefficient is the lowest ( Figure 6 ), the interfacial tension is close to the minimum ( Figure 7 ), which also proves that the local foam liquid film simulated by the first purpose cubic box corresponding to 8 surfactant molecules is the most stable.

[0041] Experimental results verify: Compared with the macroscopic foam stability stirring experiment, when C 12 When the concentration of E4 was 10 mM-15 mM, the foam stability was relatively good; When we expand the first-purpose cubic box to 100ML, it is equivalent to expanding 30Å×30Å×134Å by about 10^ 21 times, the number of surfactant molecules is equivalent to expanding by 10^ 21 times, about 8×10^ 21 C 12 E4 molecule; Combined with Avogadro's constant N is 6.02×10^ 21 , and the amount of substance n (in moles) it contains, the formula is NA=N / n); (8×10^ 21 )÷(6.02×10^ 23 ) = 1.33 × 10^ -2 mol=13.3mM; The optimal concentration was 13.3 mM, which was the result of the macro experiment optimization of 10 mM-15 mM ( Fig.17 ).

[0042] Specific application case 2--simulation of enhanced CO2 foam A method for simulating foam stability based on molecular dynamics is as follows: Step 1 and step 2 are the same as in specific application case 1; Step 3: First, construct a nanoparticle molecular model. The schematic diagram of the construction process of the nanoparticle molecular model is shown in Fig. 9 , and then construct the nanoparticle molecular crystal structure, and finally the supercell is a nanoparticle molecular cubic box (containing 4 nanoparticle molecules); the nanoparticles in this specific application case 2 are nano-SiO2; Step 4: Splice two surfactant molecule cubic boxes (containing 4 surfactant molecules), two nanoparticle molecule cubic boxes (containing 4 nanoparticle molecules) and one water molecule cubic box (containing 500 water molecules) into a surfactant-nanoparticle-water-nanoparticle-surfactant cubic box; Step 5: Build the second target cubic box with a height of 168Å and a cross section of 30Å*30Å; The surfactant-nanoparticle-water-nanoparticle-surfactant cubic box is placed in the center. A 20 Å vacuum layer is set on both sides of the second target cubic box, and the remaining parts on both sides are filled with a gas molecule cubic box respectively; the main view of the second target cubic box is shown in Fig.10 , top view Fig.11 ; After determining the height of the vacuum layer and the surfactant-nanoparticle-water-nanoparticle-surfactant cubic box, the height of the gas molecule cubic box is determined; then the number of gas molecules in the gas molecule cubic box is calculated; the specific solution process of the number of gas molecules in the gas molecule cubic box is the same as that of specific application case 1; in this specific application case 2, the number of gas molecules in the gas molecule cubic box is 3; Then, the second target cubic box is finally determined; Step 6: Perform molecular dynamics equilibrium simulation on the second purpose cubic box to characterize the foam stability with average adsorption energy; the aggregation state of the surfactant at the gas-liquid interface under the molecular dynamics equilibrium state of the second purpose cubic box is shown in Fig.12 ; In this specific application case 2, a control group is also set for the number of surfactant molecules in the surfactant cube box to form surfactants of different concentrations; in the control group, the number of surfactant molecules in the surfactant cube box is 6, 8, 10 and 12 respectively; Molecular dynamics equilibrium simulation experiments were conducted on 5 groups of second-purpose cubic boxes.

[0043] The experimental results show that the average adsorption energy of the second target cubic box corresponding to the 6 surfactant molecule cubic boxes is the highest ( Fig.16 ), the corresponding local foam liquid film is the most stable. The corresponding RMS displacement curve is shown in Fig.13 The variation curve of water molecule diffusion coefficient is shown in Fig.14 The variation curve of interfacial tension is shown in Fig.15 .

Claims

1. A method for simulating foam stability based on molecular dynamics, characterized in that: Here’s how: Step 1: Construct surfactant molecular models and water molecular models; Step 2: construct the surfactant molecular model into a surfactant molecular crystal structure, and then supercell it into a surfactant molecular cubic box; Construct the water molecule model as a water molecule cubic box; Step 3: Connect the surfactant molecule cubic box and the water molecule cubic box to form a surfactant-water-surfactant cubic box; Step 4: Construct a first target cubic box with a height of 80Å-200Å and a cross section of (10-50Å)*(10-50Å); place the surfactant-water-surfactant cubic box at the center and fill the two sides with gas molecule cubic boxes; finally construct the first target cubic box of gas-surfactant-water-surfactant-gas with boundary periodic conditions; Step 5: Perform a molecular dynamics equilibrium simulation on the first-purpose cubic box to characterize the foam stability using the average adsorption energy.

2. The method for simulating foam stability based on molecular dynamics according to claim 1, characterized in that: A vacuum layer is also provided on the adjacent sides of the gas molecule cubic box and the first destination cubic box.

3. The method for simulating foam stability based on molecular dynamics according to claim 2, characterized in that: It also includes that if the simulated foam is an enhanced CO2 foam, a nanoparticle molecule cubic box is added between the surfactant molecule cubic box and the water molecule cubic box on the basis of the first purpose cubic box; finally, a second purpose cubic box of vacuum layer-gas-surfactant-nanoparticle-water-nanoparticle-surfactant-gas-vacuum layer with boundary periodic conditions is constructed; and then a molecular dynamics equilibrium simulation is performed on the second purpose cubic box to characterize the foam stability by the average adsorption energy.

4. The method for simulating foam stability based on molecular dynamics according to claim 2 or 3, characterized in that: In the cubic box of surfactant molecules, the number of surfactant molecules satisfies [1, 20]; in the cubic box of water molecules, the number of water molecules satisfies [500, 1000]; and the height of the vacuum layer satisfies [10 Å, 30 Å].

5. The method for simulating foam stability based on molecular dynamics according to claim 3, characterized in that: The number of nanoparticle molecules in the nanoparticle molecular cubic box satisfies [1, 4].

6. The method for simulating foam stability based on molecular dynamics according to claim 4 or 5, characterized in that: The calculation process of the number of gas molecules in the gas molecule cubic box is as follows: the mass of the gas molecules in the gas molecule cubic box is obtained by the density of the gas molecules and the volume of the gas molecule cubic box; According to the relative molecular mass M and mass m of the gas molecule, the amount of substance n=m / M of the gas molecule is obtained; Based on Avogadro's constant C, we get the number of gas molecules N=C×n.

7. The method for simulating foam stability based on molecular dynamics according to claim 3, characterized in that: The method for constructing the nanoparticle molecular cubic box is as follows: first constructing a nanoparticle molecular model, then constructing a nanoparticle molecular crystal structure, and then supercelling the nanoparticle molecular cubic box.

8. The method for simulating foam stability based on molecular dynamics according to claim 7, characterized in that: The method for constructing the nanoparticle molecular model is as follows: first, a SiO2 plane structure is imported into MS software, and the imported SiO2 plane structure is created as a spherical nanocluster structure, and H atoms are added to saturate the spherical nanocluster structure; the H atoms directly connected to the Si atoms in the spherical nanocluster structure are replaced by O atoms; finally, the O atoms and water molecules that drift away from the spherical nanocluster structure are deleted, so as to obtain the nanoparticle molecular model.

9. The method for simulating foam stability based on molecular dynamics according to claim 1, characterized in that: The molecular dynamics equilibrium simulation was performed using the Forcite program.

10. The method for simulating foam stability based on molecular dynamics according to claim 2 or 3, characterized in that: The cross sections of the surfactant molecule cubic box, water molecule cubic box, gas molecule cubic box, and vacuum layer are consistent with the cross sections of the first purpose cubic box / the second purpose cubic box; The cross section of the nanoparticle molecular cubic box is consistent with the cross section of the second target cubic box.

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