A method for simulating the microstructure and spectroscopic properties of surfactants in oil reservoirs.

Abstract

This invention relates to a method for simulating the microstructure and spectral properties of surfactants in oil reservoirs. The method includes: performing equilibrium molecular dynamics simulations, where the system energy is minimized in the initial overall configuration during the simulation; determining the initial velocity, Newton's equations of motion, and Coulomb's long-range interactions; calculating the radial distribution function of the head atoms of surfactant molecules and the moment of inertia of surfactant molecules in the oil reservoir, which constitutes the microstructure information of the surfactant in the oil reservoir; extracting the total dipole moment of the surfactant in the oil reservoir from the equilibrium molecular dynamics simulation; calculating the dielectric constant of the surfactant aqueous solution as a function of frequency; and obtaining the spectral properties corresponding to the microstructure information of the surfactant aqueous solution. The simulation method provided by this invention can quickly and accurately calculate the microstructure and spectral properties of surfactant aqueous solutions and, with the aid of electromagnetic spectroscopy detection equipment, rapidly determine the microstructure of surfactants in oil reservoirs.

Description

Technical Field

[0001] This invention relates to the field of molecular simulation calculations, and more specifically to a method for simulating the microstructure and spectral properties of surfactants in oil reservoirs. Background Technology

[0002] Currently, in the petrochemical field, oil extraction is mainly divided into primary oil recovery, which relies on the pressure difference between the surface and underground to produce oil; and secondary oil recovery, which uses water and gas injection to maintain the pressure difference between the surface and subsurface layers of the oil reservoir to produce oil. The crude oil extracted by these two processes accounts for less than 50% of the total proven reserves, and a large amount of crude oil still exists underground. Therefore, tertiary oil recovery is needed to achieve efficient oil reservoir extraction.

[0003] Tertiary oil recovery typically employs physical, chemical, and biological methods to extract discontinuously distributed crude oil stored in the voids of the reservoir. These methods are generally categorized into thermal methods, chemical methods, and microbial methods.

[0004] For example, CN112832726A discloses a tertiary oil recovery method for single-well in-flow and outflow sections of tight shale oil horizontal wells, including the following steps: selecting suitable intervention wells according to preset standards; selecting target formations suitable for tertiary oil recovery operations within the intervention wells; obtaining reservoir parameters and perforation parameters of the target formations to divide them into several inflow and outflow sections; injecting an injection fluid containing a microbial oil displacement agent into each inflow section and simmering the well for a period of time; after simmering, extracting the oil-water mixture displaced by the injection fluid from each outflow section. This method significantly increases the reach of the microbial oil displacement agent by injecting an injection fluid containing a microbial oil displacement agent into the inflow section and causing the displaced and driven crude oil to migrate to the outflow section, thereby connecting the remaining oil areas in the near-well and far-well zones, reducing the oil-water interfacial tension and crude oil viscosity, and achieving the production enhancement effect of tertiary oil recovery.

[0005] In chemical methods, the advantages of surfactant methods are widely recognized, primarily in their ability to alter the microscopic properties of interfaces and increase oil recovery. A surfactant is a substance that significantly alters the interfacial properties of a system. Surfactants are amphiphilic in their molecular structure, containing both a hydrophilic end with polar groups and a lipophilic end (hydrophobic end) with nonpolar groups. The hydrophilic groups mainly include carboxylic acids, amino groups, and sulfonic acids; the hydrophobic groups are mainly aromatic hydrocarbons and alkanes. Due to the contrasting properties at both ends of the surfactant, it can quickly and spontaneously align itself at the interface in a two-phase solution. Simultaneously, the difference in affinity between the two phases reduces the interfacial tension and free energy. Furthermore, in aqueous solutions, the hydrophobic ends of surfactants can self-aggregate, solubilizing oil molecules. These combined characteristics have led to the large-scale application of surfactants in tertiary oil recovery.

[0006] For example, CN111925786A discloses a surfactant composition and preparation method for tertiary oil recovery. The surfactant composition includes a cationic surfactant and an anionic-nonionic surfactant, with a molar ratio of 1:0.01-1:100. The cationic surfactant is selected from at least one of quaternary ammonium salts or quaternary ammonium bases, and the anionic-nonionic surfactant is a combination of a sodium fatty alcohol polyoxypropylene ether carboxylate surfactant and a heavy alkylbenzene sulfonate. This method provides a composition with low dosage, utilizes a new, highly efficient catalyst, achieves high product conversion, and requires less equipment investment. Reasonable adjustment of the catalyst ratio and optimization of reaction temperature and time further reduce investment costs. The process is simple and can be widely used in the tertiary recovery stage of oil fields.

[0007] However, the compatibilization mechanism of surfactants in oil reservoirs cannot be studied experimentally in the existing technology, while molecular simulation can explain it at the molecular and atomic level. Summary of the Invention

[0008] In view of the problems existing in the prior art, the purpose of this invention is to provide a method for simulating the microstructure and spectral properties of surfactants in oil reservoirs, so as to lay a good foundation for the simulation study of the compatibilization mechanism of surfactants in oil reservoirs.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] This invention provides a method for simulating the microstructure and spectroscopic properties of surfactants in oil reservoirs, the simulation method comprising:

[0011] Equilibrium molecular dynamics simulations were performed. During the simulation, the system energy was minimized in the overall initial configuration. The initial velocity, Newton's equation of motion, and Coulomb's long-range interaction were determined. Based on this, the radial distribution function of the head atoms of surfactant molecules in the oil reservoir and the rotational inertia of surfactant molecules were calculated, which are the microstructural information of surfactants in the oil reservoir.

[0012] The total dipole moment of the surfactant in the oil layer is extracted from the equilibrium molecular dynamics simulation. The dielectric constant of the surfactant aqueous solution is calculated as a function of frequency, and the spectral properties corresponding to the microstructure information of the surfactant aqueous solution are obtained.

[0013] The simulation method provided by this invention, through the design of the calculation process, can quickly and accurately calculate the microstructure and spectral properties of surfactant aqueous solutions, and with the help of electromagnetic spectroscopy detection equipment, quickly determine the microstructure of surfactants in oil layers, thereby laying the foundation for the explanation of the solubilizing effect of surfactants.

[0014] In this invention, the surfactant may be sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, quaternary ammonium base, or other surfactants commonly used in the art, such as quaternary ammonium salt.

[0015] In this invention, the target oil layer can be other oil layers in the art, such as n-octane.

[0016] In this invention, the formula for calculating the radial distribution function g(r) is as shown in equation (1):

[0017]

[0018] In the formula: ΔV(r) is the volume of the spherical shell at a distance r from the atom, ΔN(r) is the number of atoms in the spherical shell, and ρ is the overall density of the oil layer-surfactant system.

[0019] In this invention, the formula for calculating the moment of inertia D of the surfactant molecule is as shown in equation (2):

[0020]

[0021] Where: m i Let r be the mass of the i-th atom. i Let be the spatial coordinates of the i-th atom.

[0022] As a preferred technical solution of the present invention, the overall initial configuration adopts the energy minimization method of the conjugate gradient method to minimize the system energy.

[0023] As a preferred embodiment of the present invention, the initial velocity is randomly generated by the Maxwell-Boltzmann distribution function.

[0024] In this invention, the initial velocity is expressed by the Maxwell-Boltzmann distribution function as shown in equation (3):

[0025] f(ν) = 4π(m / 2πk) B T) 3 / 2 ·v 2 ·exp[-mν 2 / 2k B Equation (3)

[0026] In the formula: m is the atomic mass, k B ν is Boltzmann's constant, T is the thermodynamic temperature, and ν is the initial velocity.

[0027] As a preferred technical solution of the present invention, the Newton equations of motion are solved using the Velocity-Verlet algorithm.

[0028] In this invention, the Newtonian equation of motion can specifically be Equation (4):

[0029] F = ma (Equation 4)

[0030] In the formula: a is the atomic acceleration, and F is the force acting on the atom.

[0031] As a preferred technical solution of the present invention, the long-range Coulomb interaction is calculated using the Ewald method with a cutoff radius of 1.2 nanometers; the simulation step size is 1 femtosecond, and data is recorded every 5 picoseconds.

[0032] As a preferred technical solution of the present invention, the equilibrium molecular dynamics simulation is performed using an isothermal and isobaric ensemble.

[0033] As a preferred embodiment of the present invention, the total dipole moment is calculated using the atomic coordinates and the charge of the corresponding atoms determined in equilibrium molecular dynamics simulation.

[0034] As a preferred technical solution of the present invention, when the change in the total electric dipole moment caused by the external electric field is obtained from the functional relationship of the dielectric constant of the surfactant aqueous solution with frequency, the mechanical quantities satisfy Hermitian symmetry (A). T ) * A = I and Liouville's formula, Hermitian symmetry (A) T ) * The symbols in A=I can be explained by referring to the definitions of mathematical symbols.

[0035] As a preferred technical solution of the present invention, the electric dipole moment parameter in the calculation of the dielectric constant of the surfactant aqueous solution as a function of frequency is obtained by using Ewald boundary conditions and dipole moment tensors.

[0036] As a preferred technical solution of the present invention, the simulation method includes: performing equilibrium molecular dynamics simulation, in which the system energy of the overall initial configuration is the minimum during the simulation process, determining the initial velocity, Newton's equation of motion and Coulomb's long-range interaction, and calculating the radial distribution function of the head atoms of surfactant molecules and the moment of inertia of surfactant molecules in the oil layer, which are the microstructure information of surfactants in the oil layer.

[0037] The overall initial configuration was minimized using the energy minimization method of the conjugate gradient method; the initial velocity was randomly generated by the Maxwell-Boltzmann distribution function; the Newtonian equations of motion were solved using the Velocity-Verlet algorithm; the Coulomb long-range interactions were calculated using the Ewald method with a cutoff radius of 1.2 nm; the simulation step size was 1 femtosecond, with data recorded every 5 picoseconds; the equilibrium molecular dynamics simulation was performed using an isothermal and isobaric ensemble.

[0038] The total dipole moment of the surfactant in the oil layer was extracted from equilibrium molecular dynamics simulations. The dielectric constant of the surfactant aqueous solution was calculated as a function of frequency to obtain the spectral properties corresponding to the microstructure information of the surfactant aqueous solution. The total dipole moment was calculated using the atomic coordinates and corresponding atomic charges determined in the equilibrium molecular dynamics simulations. When the change in the total electric dipole moment caused by the external electric field was obtained from the calculation of the dielectric constant of the surfactant aqueous solution as a function of frequency, the mechanical quantities satisfied Hermitian symmetry (A). T ) * A = I and the Liouville formula; the electric dipole moment parameter in the calculation of the dielectric constant of the surfactant aqueous solution as a function of frequency is obtained by using Ewald boundary conditions and dipole moment tensors.

[0039] In this invention, the final calculated functional relationship between the dielectric constant and frequency includes the zero-frequency dielectric constant, i.e., the dielectric constant with and without an electric field.

[0040] The mathematical expression for the zero-frequency dielectric constant is calculated as follows:

[0041] Assuming the Hamiltonian of the system is H in the absence of an external electric field, the partition function is given by equation (5):

[0042]

[0043] In the formula, For atomic coordinates, Let β be the atomic momentum. e is the natural constant, and H is the Hamiltonian. Furthermore, if there exists a strength of... A uniform external electric field acts on the material, and the total dipole moment of the material is... Then the average total dipole moment of the system For equation (6):

[0044]

[0045] In the formula: e is the natural constant, and H is the Hamiltonian. These are atomic coordinates.

[0046] After one Taylor expansion, we get equation (7):

[0047]

[0048] After another Taylor expansion, we get equation (8):

[0049]

[0050] However, since the spontaneously generated dipole moment of the material being examined in this process is zero, i.e. Then, after calculation, we obtain equation (9):

[0051]

[0052] in,

[0053] Furthermore, when the electric field is When, the polarization intensity is V is the volume of the system, and the calculated polarization intensity is: For equation (10):

[0054]

[0055] Finally, the zero-frequency dielectric constant ε can be obtained. r For equation (11):

[0056]

[0057] In the formula, M x M represents the dipole moment component along the x-direction. y M represents the dipole moment component along the y-direction. z Let be the dipole moment component along the z-direction.

[0058] Furthermore, the dependence of the dielectric constant on frequency is as follows:

[0059] In the presence of an external electric field E 0 The Hamiltonian under the action of (t) is given by equation (12):

[0060] H = H0 + H' Equation (12)

[0061] In the formula, H0 is the Hamiltonian without an external electric field, and H' = -M(r N ,p N E 0 (t), M(r) N ,p N ) is the total electric dipole moment vector, which is a state parameter of the system {r N ,p N The function of}.

[0062] Since the density distribution function of the phase space satisfies the Liouville equation, i.e., equation (13):

[0063]

[0064] In the formula, For perturbation-free Hamiltonian Liouville operators.

[0065] When the external electric field is weak, the density distribution function f(r) in the phase space is... N ,pN (f0, t) can be written as the density distribution function f0 without perturbation plus a perturbation term f1, and f0 satisfies equation (14):

[0066]

[0067] Equation (15) can be obtained through calculation:

[0068]

[0069] Combining equations (14) and (15), we obtain equation (16):

[0070]

[0071] Higher-order minor terms {H',f1} are omitted, and perturbationless Hamiltonian Liouville operators are used. To express this, we can obtain equation (17):

[0072]

[0073] For convenience, M(r) is used in the calculation. N ,p N ),E 0 The parameters in the function expression (t) are omitted. Solving equation (17) formally yields equation (18):

[0074]

[0075] In equilibrium state Furthermore, the Poisson brackets can be rearranged into equation (19):

[0076]

[0077] In the formula, M is the electric dipole moment vector, which is omitted for ease of writing (r). N ,p N ), M is the first derivative of M with respect to time.

[0078] From equation (14), we can see that, Therefore we have The change in the total electric dipole moment caused by the external electric field is as shown in equation (20):

[0079]

[0080] The above equation utilizes the fact that the mechanical quantities satisfy Hermitian symmetry (A T ) * The calculation is performed using the condition A = I and the Liouville formula, where τ is the time variable.

[0081] If the spontaneous polarization intensity <M> of the material is zero, then <M(t)>= <m>+<ΔM(t)>=<ΔM(t)>, and the derivation process of the linear response theory is carried out according to the vector. Therefore, the component of the total electric dipole moment in the i direction (i=1,2,3) can be expressed as equation (21) after the corresponding integral variable substitution:

[0082]

[0083] In the formula, E 0 M is the external electric field strength. i M represents the component of the total electric dipole moment in the i-th direction (i = 1, 2, 3). j Let be the component of the total electric dipole moment in the j direction (j = 1, 2, 3). From equation (21), we can obtain equation (22) according to the convolution theorem:

[0084]

[0085] After Fourier transform, we can obtain equation (23):

[0086]

[0087] In the formula, ω is the frequency variable, t is the time variable, and L ij Let be the matrix of the correlation functions of the integrand.

[0088] According to electrodynamics, the total electric field vector within the material is E, and the electric dipole moment vector is P. For anisotropic materials, the dielectric constant tensor ε satisfies equation (23).

[0089] εE=E+4πP Equation (23)

[0090] After transformation, we can obtain equation (24):

[0091]

[0092] Where S is a 3×3 identity matrix. Furthermore, the total electric field E, the electric dipole moment P, and the external electric field E... 0 The equation (25) is also satisfied between them:

[0093]

[0094] Using Ewald boundary conditions, we have the dipole moment tensor. Equation (26) can be obtained through calculation:

[0095]

[0096] Equation (22) can be written in matrix form as equation (27):

[0097]

[0098] The matrix elements of matrix l are given by equation (28):

[0099] l ij =F(L ij ) <M i M j > Equation (28)

[0100] From the formula of electrodynamics Therefore, by combining equations (26) and (27), equation (29) can be obtained through calculation:

[0101]

[0102] The above equation applies to any external electric field E 0 All of these hold true, therefore the dielectric constant matrix under an electric field can be obtained as equation (30).

[0103]

[0104] The matrix elements of equation (30) satisfy equation (31):

[0105]

[0106] Compared with existing technical solutions, the present invention has the following beneficial effects:

[0107] The simulation method provided by this invention can provide spectral information on the microstructure of different surfactants, saving experimental costs. Detailed Implementation

[0108] To better illustrate the present invention and facilitate understanding of its technical solutions, typical but non-limiting embodiments of the present invention are as follows:

[0109] Example 1

[0110] This embodiment provides a method for simulating the microstructure and spectral properties of surfactants in an oil reservoir. The oil reservoir in question is n-octane, and the surfactant involved is sodium dodecyl sulfate. The simulation method includes:

[0111] Building the initial model: The initial structure data files for surfactant molecules, water molecules, and oil molecules were generated using the Moltemplate script, a text-based molecule builder for the LAMMPS platform. Developed by Andrew Jewet and written in Python, it can run on Linux, OS X, and Windows.

[0112] Using LAMMPS software and the initial molecular structure constructed in the first step, equilibrium molecular dynamics simulations were performed. During the simulation, the system with the overall initial configuration had the minimum energy. The initial velocity, Newton's equation of motion, and Coulomb's long-range interaction were determined. Based on this, the radial distribution function of the head atoms of surfactant molecules and the rotational inertia of surfactant molecules in the oil reservoir were calculated, which are the microstructural information of surfactants in the oil reservoir. For example, the radial distribution function can reflect whether surfactant molecules are in an aggregated or dispersed state, and the rotational inertia of surfactant molecules can be used to reflect the shape of the aggregates, such as whether they are spherical or rod-shaped aggregates.

[0113] The overall initial configuration was minimized using the energy minimization method of the conjugate gradient method; the initial velocity was randomly generated by the Maxwell-Boltzmann distribution function; the Newtonian equations of motion were solved using the Velocity-Verlet algorithm; the Coulomb long-range interactions were calculated using the Ewald method with a cutoff radius of 1.2 nm; the simulation step size was 1 femtosecond, with data recorded every 5 picoseconds; the equilibrium molecular dynamics simulation was performed using an isothermal and isobaric ensemble.

[0114] The total dipole moment of the surfactant in the oil layer was extracted from equilibrium molecular dynamics simulations (the dipole moment of the system can be calculated using a module included in the LAMMPS software). The dielectric constant of the surfactant aqueous solution as a function of frequency was calculated to obtain the spectral properties corresponding to the microstructure information of the surfactant aqueous solution. The total dipole moment was calculated using the atomic coordinates and the charge of the corresponding atoms determined in the equilibrium molecular dynamics simulation. When the change of the total electric dipole moment caused by the external electric field was obtained from the calculation of the dielectric constant of the surfactant aqueous solution as a function of frequency, the mechanical quantities satisfy Hermitian symmetry (A). T ) * A = I and the Liouville formula; the electric dipole moment parameter in the calculation of the dielectric constant of the surfactant aqueous solution as a function of frequency is obtained by using Ewald boundary conditions and dipole moment tensors.

[0115] The zero-frequency dielectric constant ε was calculated. r (Without electric field) is:

[0116]

[0117] The dielectric constant matrix under an electric field is:

[0118]

[0119] The matrix elements in the formula satisfy:

[0120]

[0121] Furthermore, in the actual process, the dielectric constant of the system is determined in the presence of an electric field (which reflects the obvious energy absorption in a specific band of light waves, thus determining that light has a large reflectivity at the corresponding frequency). The known dielectric constant reflects the microstructure and spectral information to infer the solubilizing effect of surfactants in the oil layer.

[0122] The method provided by this invention calculates the microstructure information of surfactants in oil layers through molecular dynamics simulation, then extracts the total dipole moment and uses it to obtain the functional relationship between dielectric constant and frequency change, and finally constructs the relationship between microstructure information, total dipole moment, dielectric constant and frequency. Then, in actual operation, the microstructure information of surfactants is obtained by applying an external electromagnetic wave field and comparing it in reverse.

[0123] The present invention is described in detail through the above embodiments, but the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

[0124] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0125] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0126] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.< / m>

Claims

1. A method for simulating the microstructure and spectroscopic properties of surfactants in oil reservoirs, characterized in that, The simulation method includes: Equilibrium molecular dynamics simulations were performed. During the simulation, the system energy was minimized in the overall initial configuration. The initial velocity, Newton's equation of motion, and Coulomb's long-range interaction were determined. Based on this, the radial distribution function of the head atoms of surfactant molecules in the oil reservoir and the rotational inertia of surfactant molecules were calculated, which are the microstructural information of surfactants in the oil reservoir. The overall initial configuration was minimized using the energy minimization method of the conjugate gradient method; the initial velocity was randomly generated by the Maxwell-Boltzmann distribution function; the Newtonian equations of motion were solved using the Velocity-Verlet algorithm; the Coulomb long-range interactions were calculated using the Ewald method with a cutoff radius of 1.2 nm; the simulation step size was 1 femtosecond, with data recorded every 5 picoseconds; the equilibrium molecular dynamics simulation was performed using an isothermal and isobaric ensemble. The total dipole moment of the surfactant in the oil reservoir is extracted from equilibrium molecular dynamics simulations. The dielectric constant of the surfactant aqueous solution is calculated as a function of frequency to obtain the spectral properties corresponding to the microstructure information of the surfactant aqueous solution. The total dipole moment is calculated using the atomic coordinates and corresponding atomic charges determined in the equilibrium molecular dynamics simulations. The change in the total electric dipole moment caused by the external electric field and the mechanical quantities obtained from the calculation of the dielectric constant of the surfactant aqueous solution as a function of frequency satisfy Hermitian symmetry. The electric dipole moment parameter in the calculation of the dielectric constant of the surfactant aqueous solution as a function of frequency is obtained by using Ewald boundary conditions and dipole moment tensors. The dielectric constant of the surfactant aqueous solution varies with frequency as follows: The matrix elements in the formula satisfy the following equation: In the formula, V is the volume of the system; k B Z is the Boltzmann constant; Z is the dipole moment tensor Z0. S is The identity matrix; M i M represents the component of the total electric dipole moment in the i-th direction, where i = 1, 2, 3; j Let ω be the component of the total electric dipole moment in the j-direction, j=1,2,3; ω be the frequency variable; t be the time variable; matrix The matrix elements are , , where e is the natural constant.

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