A method of calculating oil-water interfacial tension in nanopores

Abstract

This invention discloses a method for calculating the interfacial tension of oil and water within nanopores, belonging to the field of molecular simulation and interface physics. The method includes: constructing an oil-water two-phase model in a silica nanochannel; applying equal and opposite external forces to the oil and water phases to perform all-atom molecular dynamics simulation; after the simulation, identifying interface atoms using a sliding window method based on atomic trajectory data, and combining quadratic curve fitting and effective frame screening to reconstruct the geometry of the oil-water interface with high precision; finally, calculating the interfacial tension according to the mechanical equilibrium formula, where θ is determined by the tangent of the fitted curve at the nearest neighbor of the wall. This invention avoids the low resolution problem of traditional density profiling methods at the nanoscale, significantly improving the accuracy of interface positioning and the reliability of tension calculation, and is suitable for quantitative research on nano-percolation behavior in unconventional oil and gas resources.

Description

Technical Field

[0001] This invention belongs to the field of molecular simulation technology, and in particular relates to a method for calculating the interfacial tension of oil and water in nanopores. Background Technology

[0002] Currently, research methods for oil-water interfacial tension are mainly divided into experimental measurement and theoretical calculation. Experimental research offers strong realism and can largely reflect interfacial tension in actual environments. However, experimentally measuring interfacial tension within nanopores presents numerous challenges, especially at the nanoscale, where precise measurement is extremely difficult and requires highly sophisticated equipment. Therefore, while experimental methods can provide reliable data, their high cost and complex operation limit their widespread use in practical applications. Theoretical calculation methods offer another avenue for studying interfacial tension in nanopores. Compared to experiments, theoretical calculations offer advantages in efficiency and low cost, making them suitable for research design and theoretical exploration. However, existing theoretical models are often based on assumptions and simplifications at the macroscale, failing to accurately describe the complex phenomena at the nanoscale. While the traditional Laplace formula can effectively describe interfacial phenomena at the macroscale, its assumptions (such as the continuous medium assumption) are not entirely applicable in nanopores. Therefore, existing theoretical calculation methods have significant limitations at the nanoscale, necessitating the development of interfacial tension calculation models suitable for the nanoscale. Summary of the Invention

[0003] In view of this, the present invention aims to propose a method for calculating the oil-water interfacial tension within nanopores. This method uses molecular simulation to quantitatively study the interfacial tension in nanopores, explores the relationship between interfacial tension and pore size, and constructs an interfacial tension model at the nanoscale. This method not only contributes to a deeper understanding of fluid behavior in nanopores but also provides theoretical support for the development and optimization of unconventional oil and gas resources.

[0004] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A method for calculating the oil-water interfacial tension within nanopores, the method comprising the following steps: S1. Construct a rock nanochannel model, and then sequentially insert the oil phase and water phase into the rock nanochannel model along the pore length direction to form an oil-water two-phase model in nanopores. S2. Fix the rock face, apply equal and opposite forces to all atoms in the water and oil phases, and perform all-atom molecular dynamics simulation in LAMMPS software. S3. After the simulation, read the spatial coordinate information of all atoms in the oil phase in each frame of the later part of the simulation from the trajectory file. In each frame, use the sliding window method to select the oil phase atom with the largest Z coordinate value in each window as the oil-water interface atom. Perform a quadratic polynomial z=ay on the selected oil-water interface atom. 2 By fitting the data using +by+c, we can obtain the interface curves for different frames. S4. Take the arithmetic mean of the fitting coefficients a, b, and c of the quadratic polynomials of the multiple sets of interface curves obtained in S3 to obtain the stable interface geometric parameters z= y 2 + y+ ; S5. Based on the averaged quadratic curve z= y 2 + y+ The first derivative dz / dy of the curve is calculated near the wall of the curve to obtain the direction of the interface tangent. The angle θ between the tangent and the Y-axis is calculated by the arctangent function. This angle is the key geometric parameter for calculating the interface tension. S6. Perform force analysis on the atoms at the interface and calculate the interfacial tension γ at the oil-water interface according to the formula: ; in, l For the length of the hole, f The force applied to each atom, where θ is the interface angle. This represents the total number of atoms in the system.

[0005] Furthermore, the rock molecules in the rock nanochannel model are made of silicon dioxide, and a length of 100 μm is first constructed. Width is 60 The rock cuboid was then cut into the central part of the rock cuboid to form a 4-10 nm slit, thus obtaining the rock nanochannel model.

[0006] Furthermore, due to the different pore sizes, the magnitude of the force applied in S2 is 0.000001-0.0001 Kcal•mol. -1 •nm -1 .

[0007] Furthermore, the method for all-atom molecular dynamics simulation in S2 is as follows: OPLS-AA force field is used for oil phase molecules, SPC force field for water phase molecules, and CVFF force field for rock phase molecules. The simulation is set under reservoir ambient temperature conditions, the NVT ensemble is selected, the step size is 1 fs, the simulation duration is 10 ns, and the cutoff radius is 10. .

[0008] Furthermore, in S1, the X-axis of the constructed oil-water two-phase model is a transverse direction perpendicular to the YZ plane, the Y-axis is along the pore thickness direction, and the Z-axis is along the pore length direction.

[0009] Furthermore, in S3, the sliding window method specifically involves selecting windows in ascending order of atomic Y coordinates, with the window size set to 10 atoms. The window spans the oil and water phases along the Z direction. Within each window, it is detected whether the Z coordinate of an oil phase atom is a local maximum. If the Z coordinate value is the largest within the window, it is determined that the atom is located at the oil phase front, constituting an oil-water interface atom, which can effectively capture the interface contour.

[0010] Furthermore, the initial length of the oil phase is l o The initial length of the water phase is l w In S3, the focus area is the interface region, and the Z coordinate is... l o / 2 and l w Atoms between / 2 are filtered using a sliding window method to eliminate interference from atoms far from the interface.

[0011] Furthermore, in S4, to eliminate interface oscillations or unsteady configuration disturbances, a quadratic polynomial with coefficients a∈[-0.04,-0.03] is selected, and the arithmetic mean of the selected coefficients a, b, and c is taken.

[0012] The range of coefficient 'a' was determined through preliminary experiments, corresponding to a reasonable interface curvature.

[0013] Furthermore, in S3, the simulation time for the latter part is the last 5-10 ns.

[0014] Compared with existing technologies, the method for calculating the oil-water interfacial tension within nanopores described in this invention has the following advantages: The method for calculating the oil-water interfacial tension within nanopores described in this invention reveals the nanoscale effect of the oil-water interfacial tension within nanopores at the microscale, explores the relationship between interfacial tension and pore size, and provides theoretical support for the development and optimization of unconventional oil and gas resources. Attached Figure Description

[0015] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 (a) A nanochannel model of silica rock; (b) Cell cutting; (c) The extended crystal; (d) The hydroxylated surface. Figure 2 To construct an oil-water interface model within nanopores; Figure 3 This is a schematic diagram of the applied force; Figure 4 A schematic diagram of the sliding window method for screening atoms; Figure 5 The interface after processing using the visualization software Ovito; Figure 6 The fitted interface is the interfacial tension of a 6nm aperture at 353K after multi-frame averaging. Figure 7 The results show the calculated interfacial tension for different pore sizes; Figure 8 The global trend curve is shown at 353K with an aperture of 6nm. Figure 9 The fitted interface is the result of multi-frame averaging of interfacial tension with a pore size of 6 nm at 298 K. Detailed Implementation

[0016] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] This invention uses oil-phase n-dodecane molecules, water molecules, and rock-phase silica molecules as examples for model construction. The computer simulation software used is Materials Studio (MS), a new generation of materials calculation software developed by Accelrys Corporation in the United States, and LAMMPS (Large-scale Atomic / Molecular Massively Parallel Simulator), an open-source software package for molecular dynamics simulation developed by Sandia National Laboratories.

[0019] First, all-atom molecular dynamics simulations were used to accurately characterize the oil-water interfacial tension in nanochannels. Second, the relationship between interfacial tension and parameters such as pore size and temperature was explored, and an interfacial tension model at the nanoscale was constructed.

[0020] The method for calculating the oil-water interfacial tension within nanopores includes the following steps: S1. Using Materials Studio software, n-dodecane (C12) molecules were constructed as the oil phase. Packmol software was used to construct an oil phase box with a density of 0.76 g / ml and an aqueous phase box with a density of 1 g / ml, with a volume ratio of 1:1 and a size of 50. Length, 60 Width, that is l o = l w =50 The thicknesses are 4nm, 6nm, 8nm, and 10nm, respectively. The silica wall material was exported from the MS model library and sliced ​​along a specific crystal plane (0 0 1), as shown below. Figure 1 As shown in (a), the resulting crystal structure then extends along the cell direction, forming a length of 100. Width is 60 Rock cubes, such as Figure 1 As shown in (b) above, the central portion of a rock cube was cut into nanoscale slits of different diameters (4 nm, 6 nm, 8 nm, and 10 nm) to investigate the effect of nanochannels on the oil-water interfacial tension. The nanochannel model is shown below. Figure 1 As shown in (c) above. Finally, the oil phase box and the aqueous phase box are inserted into the rock nanochannels of the corresponding size to form the required model, as shown in (c). Figure 2 As shown.

[0021] S2. Select and fix the rock components on the wall surface to prevent wall collapse or movement due to flow during the simulation. Apply equal and opposite forces to all atoms in the aqueous and oil phases, such as... Figure 3 As shown, for a 6 nm slit, the applied force is 0.0001 kcal•mol. -1 •nm -1 .

[0022] As the aperture size changes, the magnitude of the applied force also changes. The larger the aperture, the smaller the applied force. The greater the applied force, the greater the curvature of the resulting interface surface. In order to facilitate fitting the polynomial equation of the surface, it is necessary to change the magnitude of the applied force according to the aperture size to obtain an interface surface that is easy to fit. This force acts on the oil-water interface, and thus an S-shaped surface is formed at the oil-water junction.

[0023] All-atom molecular dynamics simulations were performed in LAMMPS software: OPLS-AA force fields were used for oil phase molecules, SPC force fields for water phase molecules, and CVFF force fields for rock phase molecules. The simulations were set under specific temperature conditions, using the NVT ensemble with a step size of 1 fs, a simulation duration of 10 ns, and a cutoff radius of 10. This allows the fluid inside the pore to achieve sufficient relaxation.

[0024] S3. After the simulation is completed, read the spatial coordinate information of all atoms in the oil phase in each frame of the last 5ns of the simulation from the trajectory file. Define the X-axis as the horizontal direction perpendicular to the YZ plane, the Y-axis along the pore thickness direction, and the Z-axis along the pore length direction.

[0025] The sliding window method was used to select the oil phase atom with the largest Z value in each window as the oil-water interface atom, such as... Figure 4 As shown, the sliding window method specifically involves selecting windows in ascending order of atomic Y-coordinates, with a window size of 10 atoms. The window spans the oil and water phases along the Z-direction. Within each window, it checks whether the Z-coordinate of an oil phase atom is a local maximum. If the Z-value is the largest within the window, it is determined to be located at the oil phase front, constituting an atom at the oil-water interface, effectively capturing the interface contour. Figure 5 This is an interface diagram with an aperture of 6 nm.

[0026] To focus on the interface area, the Z coordinate is at 45. -60 Atoms between the interfaces are filtered using a sliding window method to eliminate interference from atoms far from the interface.

[0027] The selected oil-water interface atoms from different frames are subjected to a quadratic polynomial z=ay. 2 The interface curves under different frames are obtained by fitting with +by+c.

[0028] S4. Take the arithmetic mean of the fitting coefficients a, b, and c of the quadratic polynomials of the multiple sets of interface curves obtained in S3 to obtain the stable interface geometric parameters z= y 2 + y+ ; To eliminate interface oscillations or unsteady configuration disturbances, frames with coefficients a ∈ [-0.04, -0.03] are selected, and the arithmetic mean of the selected coefficients a, b, and c is taken to obtain the stable interface geometric parameters z= y 2 + y+ ;like Figure 6 This is a fitting curve for an aperture of 6 nm.

[0029] S5. Based on the averaged quadratic curve z= y 2 + y+ Near the interface of the curve, such as Y=25 For the corresponding neighborhood of the wall, calculate the first derivative of the curve dz / dy, and then obtain the direction of the interface tangent. Calculate the angle θ between the tangent and the Y-axis using the arctangent function; for example... Figure 6As shown, sinθ = 0.656.

[0030] S6. Perform force analysis on the atoms at the interface and calculate the interfacial tension γ at the oil-water interface according to the formula: ; in, l For the length of the hole, f The force applied to each atom, where θ is the interface angle. This represents the total number of atoms in the system.

[0031] like Figure 7 The calculated results of interfacial tension at 353 K for different pore sizes are presented. The interfacial tension increases with the increase of pore size.

[0032] Figure 8 With a pore size of 6 nm, a global trend curve was obtained by directly performing a quadratic polynomial fitting on all atomic (Y, Z) data of the oil phase. The red dots represent atoms used in the fitting interface. It can be seen that many atoms not on the interface are also used in the global trend curve fitting, indicating that the globally fitted interface curve has a certain error. The global fitting method is easily affected by non-boundary atoms, leading to inaccurate fitting curves. This method effectively avoids this problem, accurately identifying boundary atoms and thus improving the accuracy of the fitting curve. The rationale for this method is demonstrated through global fitting.

[0033] In addition, simulations were performed at 298K for a 6nm aperture, such as... Figure 9 As shown, and after fitting with multi-frame averaging, the accurate interfacial tension within the pores was calculated. At 353 K and 298 K, the interfacial tensions between dodecane molecules and water in 6 nm silica nanochannels were 33.985 mN / m and 42.493 mN / m, respectively.

[0034] In existing research, the oil-water interfacial tension in 6nm hydrophilic silica nanochannels at room temperature is approximately 35-45 mN / m, or even lower, especially when the interface is close to the solid wall. Therefore, this further demonstrates the accuracy and rationality of the calculation method of this invention.

[0035] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for calculating the interfacial tension of oil and water within nanopores, characterized in that, The method includes the following steps: S1. Construct a rock nanochannel model, and then sequentially insert the oil phase and water phase into the rock nanochannel model along the pore length direction to form an oil-water two-phase model in nanopores. S2. Fix the rock face, apply equal and opposite forces to all atoms in the water and oil phases, and perform all-atom molecular dynamics simulation in LAMMPS software. S3. After the simulation, read the spatial coordinate information of all atoms in the oil phase in each frame of the later part of the simulation from the trajectory file. In each frame, use the sliding window method to select the oil phase atom with the largest Z coordinate value in each window as the oil-water interface atom. Perform a quadratic polynomial z=ay on the selected oil-water interface atom. 2 The interface curves under different frames are obtained by fitting the +by+c model. S4. Take the arithmetic mean of the fitting coefficients a, b, and c of the quadratic polynomials of the multiple sets of interface curves obtained in S3 to obtain the stable interface geometric parameters z= y 2 + y+ ; S5. Based on the averaged quadratic curve z= y 2 + y+ The first derivative dz / dy of the curve is calculated near the wall of the curve, and the direction of the interface tangent is obtained. The angle θ between the tangent and the Y-axis is calculated by the arctangent function. S6. Perform force analysis on the atoms at the interface and calculate the interfacial tension γ at the oil-water interface according to the formula: ; in, l For the length of the hole, f The force applied to each atom, where θ is the interface angle. This represents the total number of atoms in the system.

2. The method for calculating the oil-water interfacial tension within nanopores according to claim 1, characterized in that, The rock molecules in the rock nanochannel model are made of silicon dioxide. First, a nanochannel with a length of 100... Width is 60 The rock cuboid was then cut into the central part of the rock cuboid to form a 4-10 nm slit, thus obtaining the rock nanochannel model.

3. The method for calculating the oil-water interfacial tension within nanopores according to claim 1, characterized in that, Due to the different pore sizes, the magnitude of the force applied in S2 is 0.000001-0.0001 Kcal•mol. -1 •nm -1 .

4. The method for calculating the oil-water interfacial tension within nanopores according to claim 1, characterized in that, The method for all-atom molecular dynamics simulation in S2 is as follows: OPLS-AA force field is used for oil phase molecules, SPC force field for water phase molecules, and CVFF force field for rock phase molecules. The simulation is set at a reservoir ambient temperature of 353K, the NVT ensemble is selected, the step size is 1 fs, the simulation duration is 10 ns, and the cutoff radius is 10. .

5. The method for calculating the oil-water interfacial tension within nanopores according to claim 1, characterized in that, In S1, the X-axis of the constructed oil-water two-phase model is a transverse direction perpendicular to the YZ plane, the Y-axis is along the pore thickness direction, and the Z-axis is along the pore length direction.

6. The method for calculating the oil-water interfacial tension within nanopores according to claim 1, characterized in that, In S3, the sliding window method is as follows: a window is selected in ascending order of the Y coordinate of the atoms, and the window size is set to 10 atoms. The window spans the oil phase and the water phase along the Z direction. In each window, it is detected whether the Z coordinate of the oil phase atom is a local maximum value. If the Z coordinate value is the largest in the window, it is determined that it is located at the oil phase front and constitutes an oil-water interface atom.

7. The method for calculating the oil-water interfacial tension within nanopores according to claim 1, characterized in that, The initial length of the oil phase is l o The initial length of the water phase is l w In S3, the focus area is the interface region, and the Z coordinate is... l 0 / 2 and l w Atoms between / 2 are filtered using a sliding window method to eliminate interference from atoms far from the interface.

8. The method for calculating the oil-water interfacial tension within nanopores according to claim 1, characterized in that, In S4, to eliminate interface oscillations or unsteady configuration disturbances, a quadratic polynomial with coefficients a∈[-0.04,-0.03] is selected, and the arithmetic mean of the selected coefficients a, b, and c is taken.

9. The method for calculating the oil-water interfacial tension within nanopores according to claim 1, characterized in that, In S3, the simulation time for the latter part is the last 5-10 ns.

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