Composite surfactant for reducing viscosity of heavy oil and application thereof

Abstract

The invention discloses a composite surfactant for reducing viscosity of heavy oil and application of the composite surfactant. The composite surface active agent is formed by compounding an anionic surface active agent and a nonionic surface active agent; the anionic surfactant is selected from sodium dodecyl benzene sulfonate, sodium dodecyl diphenyl ether disulfonate or sodium dodecyl sulfate; and the non-ionic surface active agent is octylphenol polyoxyethylene ether. Non-ionic surface active agent molecules cooperate with the anionic surface active agent through interaction of hydrogen bonds and the like, a compact and stable composite adsorption layer is formed on an oil-water interface, and generation and stability of O / W emulsion are promoted. Molecular dynamics simulation, experimental verification and quantum chemistry calculation prove that the DBS / OP-10 system can effectively destroy an asphaltene aggregation structure and can significantly reduce the viscosity of heavy oil. The viscosity can be reduced to 588 mPa.s under the concentration of 1.0%, and the viscosity reduction agent is high in viscosity reduction rate, low in cost and suitable for industrial application.

Description

Technical Field

[0001] This invention belongs to the field of petroleum extraction technology, specifically relating to a composite surfactant for reducing the viscosity of heavy oil and its application in improving oil recovery. Background Technology

[0002] Data from the International Energy Agency (IEA) shows that approximately 70% of the world's proven oil reserves are heavy crude oil with an API gravity below 22.3°. This high viscosity severely restricts extraction and transportation efficiency. The high viscosity of heavy oil stems from its complex molecular structure, particularly the supramolecular aggregation behavior of asphaltenes and resins. Synchrotron X-ray diffraction studies have revealed that asphaltenes molecules form a layered structure through π-π stacking, with an interlayer spacing of approximately 3.4 angstroms. This close packing significantly increases flow resistance. Molecular dynamics simulations have revealed three key interactions in asphaltenes aggregates: π-π stacking of aromatic rings, hydrogen bonding, and alkyl chain entanglement. These interactions collectively construct a three-dimensional network structure, causing heavy oil to exhibit non-Newtonian fluid properties at room temperature. With the increasing depletion of conventional crude oil resources, developing effective heavy oil viscosity reduction technologies is of great strategic significance.

[0003] The core scientific challenge in reducing viscosity lies in disrupting the supramolecular aggregate structure of asphaltenes in heavy oil. Traditional thermal viscosity reduction techniques (such as steam flooding) suffer from high energy consumption and significant heat loss, especially in thin oil layers or reservoirs containing bottom water. In contrast, chemical viscosity reduction techniques, which alter the microstructure and interfacial properties of heavy oil, can achieve efficient viscosity reduction at room temperature and have become a current research hotspot. Existing chemical viscosity reduction techniques are mainly divided into two categories: oil-soluble and emulsified. While oil-soluble viscosity reducers can disrupt asphaltenes aggregates, they suffer from high costs and limited applicability, typically only used for modifying petroleum byproducts or low-quality crude oil. More importantly, these techniques often require the addition of light oil as a diluent to function, leading to the consumption of valuable light oil resources.

[0004] Research on emulsion viscosity reduction technology has revealed that while anionic surfactant solutions (such as SDS) can significantly reduce viscosity, they have poor salt resistance; nonionic surfactants exhibit excellent salt resistance, but are prone to cloud point problems at high temperatures (such as Span-20); amphoteric surfactants (such as CAB-35) have good biocompatibility but are expensive. By mixing multiple low-cost, high-efficiency surfactants, emulsion viscosity reduction technology can achieve viscosity reductions exceeding 90%. For example, the NaOl / CAB-35 composite system reduced heavy oil viscosity from 5047.52 mPa·s to 94.65 mPa·s (a reduction of 94.65%). However, numerous studies have shown that different heavy oil components and composite surfactant systems exhibit different viscosity responses. This necessitates in-depth molecular-level research into the interactions and interfacial properties between heavy oil components and surfactant molecules. Current technologies lack molecular dynamics studies for composite surfactant systems, making it difficult to achieve rational design and optimization of viscosity reducers. Summary of the Invention

[0005] The present invention aims to overcome the shortcomings of the prior art by providing a composite surfactant for efficiently reducing the viscosity of heavy oil and controlling the problems of high energy consumption, high cost and poor applicability in existing viscosity reduction technologies.

[0006] Another objective of this invention is to provide the application of the above-mentioned composite surfactant in enhancing oil recovery, thereby improving oil recovery through molecular dynamics studies to optimize the formulation.

[0007] To solve the above-mentioned technical problems, the present invention is implemented as follows: A composite surfactant for reducing the viscosity of heavy oil is composed of anionic and nonionic surfactants; the anionic surfactant is selected from sodium dodecylbenzene sulfonate (DBS), sodium dodecyl diphenyl ether disulfonate (DDBS) or sodium dodecyl sulfate (SDS); the nonionic surfactant is octylphenol polyoxyethylene ether (OP-10).

[0008] Furthermore, the mass ratio of the anionic surfactant to the nonionic surfactant in this invention is 1:1.

[0009] The application of the aforementioned composite surfactant in enhancing oil recovery includes the following steps: S1: Prepare an aqueous solution of the above-mentioned composite surfactant; S2: The aqueous solution obtained in step S1 is mixed with heavy oil and emulsified to form an O / W type emulsion. During the oil extraction process, the O / W type emulsion is injected into the reservoir.

[0010] Furthermore, in step S1 of the present invention, the concentration of the composite surfactant in the aqueous solution is 0.2 to 1.0%.

[0011] Furthermore, in step S2 of the present invention, the volume ratio of the aqueous solution to the heavy oil is 1:1.

[0012] Furthermore, in step S2 of the present invention, the emulsification process is carried out using a homogenizer at a speed of 4000 rpm for a time of 3 minutes.

[0013] Furthermore, before step S2, the present invention also includes step S0: screening anionic surfactants with high affinity for asphaltenes in the target heavy oil through molecular dynamics simulation to determine the types of anionic surfactants used to prepare the composite surfactant; wherein, the indices of the molecular dynamics simulation include the number of hydrogen bonds, mean square displacement (MSD), or radial distribution function (RDF).

[0014] Furthermore, in step S0 of this invention, all molecular dynamics simulations are performed using the GROMACS 2022.4 program in conjunction with a general AMBER force field to describe molecular interactions; the force field parameters are generated using the sobtop tool, wherein the atomic charges of the composite surfactant molecules are calculated based on the confined electrostatic potential method.

[0015] This invention, by integrating molecular dynamics simulations, experimental verification, and quantum chemical calculations, systematically studies the interaction mechanism between composite surfactant systems and asphaltenes, achieving the following technical advantages: (1) Viscosity reduction effect of the three composite systems in the concentration range of 0.2%–1.0%. Even at the lowest concentration (0.2%), the three systems showed significant performance differences: the DBS / OP-10 composite system performed best, reducing the heavy oil viscosity to 1449 mPa·s; the DDBS / OP-10 composite system was second best, reducing the viscosity to 2040 mPa·s; while the SDS / OP-10 composite system performed the worst, reducing the viscosity to only 3024 mPa·s. As the concentration increased, the viscosity reduction effect of all three systems improved, but the performance ranking did not change. When the concentration reached 1.0%, the DBS / OP-10 composite system further reduced the viscosity to 588 mPa·s, the DDBS / OP-10 system to 663 mPa·s, and the SDS / OP-10 composite system to 892 mPa·s. The DBS / OP-10 composite system can reduce the viscosity of heavy oil from its original value to 588 mPa·s at a concentration of 1.0%, which is significantly higher than that of the DDBS / OP-10 and SDS / OP-10 systems.

[0016] (2) DBS molecules can effectively adsorb and penetrate into the interior of asphaltene aggregates, destroying their π-π stacking structure. Meanwhile, OP-10 molecules work synergistically with DBS through interactions such as hydrogen bonds to form a dense and stable composite adsorption layer at the oil-water interface, promoting the formation and stabilization of O / W emulsions.

[0017] (3) Hydrogen bond analysis showed that the DBS / OP-10 system reached 4743 hydrogen bonds at the highest concentration, indicating strong interfacial interactions. Molecular orbital analysis showed that the band gap of OP-10 was 8.01 eV, while that of DBS reached 11.19 eV. Its HOMO was localized on the sulfonic acid group, and its LUMO was distributed at the end of the alkyl chain; the larger band gap indicated that its electronic structure was relatively stable. The band gap of DDBS was 8.40 eV, and its HOMO and LUMO were both distributed in the sulfonic acid group and the benzene ring structure. The smaller band gap suggested that it was more chemically reactive. The band gap of SDS was 11.65 eV, and its HOMO was concentrated in the sulfonic acid group, while its LUMO was distributed along the alkyl chain; this was the largest band gap value, indicating that its electronic structure was the most stable.

[0018] (4) The composite surfactant system of the present invention uses low-cost raw materials, does not require the addition of light oil diluents, is suitable for heavy oils with different compositions, and has the prospect of large-scale industrial application. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the molecular structure of the composite surfactant of the present invention;

[0020] Figure 2 This is a schematic diagram of the asphalt model structure of the present invention;

[0021] Figure 3 This is a visual snapshot of the molecular dynamics simulation of the present invention, showing the interaction between different composite systems and asphaltene;

[0022] Figure 4 This is a hydrogen bond statistics plot from the molecular dynamics simulation of this invention;

[0023] Figure 5 This is a graph showing the experimental viscosity reduction data of the three composite surfactants of this invention;

[0024] Figure 6 The diagram shows the molecular orbitals and band gaps of the four surfactants of this invention. Detailed Implementation

[0025] The present invention will now be described in detail through specific embodiments. These embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art. As used throughout the specification and claims, the terms "comprising" or "including" are open-ended and are interpreted as "comprising but not limited to". The following description is a preferred embodiment for carrying out the invention; however, this description is intended to illustrate the general principles of the specification and is not intended to limit the scope of the invention. The scope of protection of the present invention is determined by the appended claims.

[0026] This invention focuses on three composite systems: DBS / OP-10, DDBS / OP-10, and SDS / OP-10. First, molecular dynamics simulations reveal the interaction mechanisms between different composite systems and asphaltene at the molecular scale. Second, hydrogen bond analysis of the composite systems elucidates the underlying reasons for the differences in viscosity-reducing performance. Systematic viscosity-reducing experimental results show that the performance ranking of the three systems is highly consistent with simulation predictions. Finally, molecular orbital analysis demonstrates that viscosity-reducing efficiency is not solely determined by the chemical activity of the molecules.

[0027] OP-10 was purchased from Beijing Innocare Technology Co., Ltd., analytical reagent grade; SDS was purchased from Beijing Innocare Technology Co., Ltd., analytical reagent grade; DBS was purchased from Beijing Innocare Technology Co., Ltd., analytical reagent grade; DDBS was purchased from Beijing Innocare Technology Co., Ltd., analytical reagent grade.

[0028] Example: Preparation of composite surfactants OP-10 was mixed with three anionic surfactants (DBS, DDBS, and SDS) at a mass ratio of 1:1 to prepare aqueous solutions of different concentrations (0.1%, 0.2%, 0.3%, 0.5%, 0.8%, and 1.0%). Then, 10 mL of heavy oil sample was mixed with an equal volume of surfactant solution, and emulsified using a homogenizer at 4000 rpm for 3 minutes. The viscosity was then measured.

[0029] Viscosity measurements of heavy oil samples were performed using an NDJ-5S digital rotational viscometer. Before testing, the samples were placed in a constant-temperature water bath at 50 ± 0.1°C for 30 minutes to ensure uniform temperature and compliance with testing requirements. Based on the estimated viscosity range of the samples, rotors of sizes 1-3 were selected to ensure the measured values ​​were within the instrument's optimal operating range. During measurement, the rotor was fully immersed in the sample to avoid air bubble formation. After setting an appropriate rotational speed, recording began once the reading stabilized. Each sample was measured three times, with a 10-minute interval between each measurement. The arithmetic mean of the three measurements was taken as the viscosity value. If the relative standard deviation of the three measurements exceeded 5%, the measurement was repeated. Throughout the experiment, ambient temperature fluctuations were strictly controlled to not exceed ± 0.5°C to minimize the impact of temperature changes on viscosity measurements.

[0030] The viscosity reduction rate (η) is calculated using the following formula: , Where μ o μ is the original viscosity of the heavy oil, and μ is the viscosity after emulsification.

[0031] 1. Molecular model construction This invention selects composite systems of three anionic surfactants (OP-10 / DBS, OP-10 / DDBS, and OP-10 / SDS) and one nonionic surfactant as research objects to systematically investigate their emulsifying properties and viscosity reduction effects. The initial configurations of all molecular structures were geometrically optimized using Gaussian16 software at the B3LYP / def2-SVP basis set level until convergence criteria were met, and vibrational frequency analysis was used to ensure the acquisition of the stable conformation with the lowest energy. The molecular structures are as follows: Figure 1 As shown, the constrained electrostatic potential (RESP) method based on Multiwfn 3.8 software is used to accurately calculate the atomic charges of the optimized molecule, so as to more accurately describe the electrostatic distribution of anionic head groups (such as sulfonic acid groups and sulfate groups) and the intermolecular polarization effect, thereby providing reliable force field parameters for subsequent molecular dynamics simulations.

[0032] The asphaltene model adopts the structural model proposed by Olga et al., which contains fused aromatic rings, alkyl side chains, and heteroatom functional groups containing nitrogen, sulfur, and oxygen, effectively simulating the π-π stacking behavior and aggregation tendency of real asphaltene. To more realistically simulate the multi-component environment of the heavy oil system, several n-octane molecules were introduced into the simulation chamber as a low-viscosity solvent medium. These molecules not only constitute the heavy oil components but also dilute the aggregation behavior of asphaltene.

[0033] 2. Construction of Molecular Dynamics Simulation System This invention employs all-atom molecular dynamics simulations to systematically investigate the effects of different surfactant concentrations on the viscosity reduction of heavy oil. The initial configurations of all simulation systems were constructed using the PACKMOL program. Optimized molecular models (including OP-10, DBS / DDBS / SDS, asphaltene models, n-octane, and water molecules) were randomly distributed in a predetermined ratio within a 15.0 nm cube simulation box, forming an oil-water two-phase system. To study the effect of surfactant concentration on viscosity reduction, five simulation systems with different concentrations were designed (as shown in Table 1). Each system contained 6 asphaltene molecules, 200 n-octane molecules, and a corresponding number of surfactant molecules. The remaining space was filled with approximately 30,000 SPC / E water molecules to ensure the system density closely approximates actual conditions. Simultaneously, sodium ions, in an equimolar amount to the anionic surfactant, were introduced to balance the system charge. Through this systematic concentration gradient design, the effects of surfactant concentration on asphaltene dispersion, interfacial property changes, and final viscosity reduction were quantitatively analyzed.

[0034] Table 1 Composition of the Molecular Dynamics Simulation System

[0035] 1. This MD system uses only DBS as an anionic surfactant, with an equal amount of Na+ added. 2. This MD system uses only DDBS as an anionic surfactant, with an equal amount of Na+ added. 3. This MD system uses only SDS as an anionic surfactant, with an equal amount of Na+ added.

[0036] 3. Molecular dynamics simulation This invention primarily employs molecular dynamics simulation methods. All simulations utilize the GROMACS 2022.4 program combined with the Universal AMBER Force Field (GAFF) to describe molecular interactions. Force field parameters are generated using the sobtop tool, where the atomic charges of surfactant molecules are calculated based on the Restricted Electrostatic Potential (RESP) method to ensure the accuracy of electrostatic interactions. First, an initial configuration file (.gro format) is directly generated using the GROMACS program. The simulation box is set to a cube with side lengths of 15.0 nm × 15.0 nm × 15.0 nm. Subsequently, the steepest descent algorithm in double-precision mode is used to minimize the system's energy. The convergence criterion is set to a maximum force within the system of less than 10.0 kJ / mol / nm, typically requiring 5000 to 50000 iterations to achieve convergence. This process eliminates excessive atomic overlap and unreasonable contacts, providing a stable initial structure for subsequent dynamic simulations. After energy minimization, the system is first subjected to equilibrium simulation under the NVT ensemble. A Berendsen thermostat is used to control the temperature at 298 K, and periodic boundary conditions and three-dimensional boundary constraints are applied. The simulation time step was 1 fs, with a total simulation duration of 5 ns. This stage primarily aimed to bring the system temperature to equilibrium and stabilize kinetic energy fluctuations. A subsequent 50-nanosecond production simulation was performed under the NPT ensemble. The Parrinello-Rahman method was used to couple the pressure to 1 bar, and the v-rescale method was used to maintain the temperature at 298 K. This stage brought the system density to equilibrium, yielding the equilibrium trajectory for statistical analysis. All chemical bonds were constrained using the LINCS algorithm. Van der Waals interactions were handled with a 1.0 nm cutoff radius, while electrostatic interactions were calculated using the Particle Mesh Ewald (PME) method. The system saved trajectory files every 10 picoseconds, ultimately generating 5000 frames for subsequent analysis. Molecular snapshots were generated using VMD 1.9.3 software and rendered with ray tracing using the Tachyon renderer to provide high-quality visualization of molecular arrangement. Radial distribution function (RDF), mean square displacement (MSD), and hydrogen bond analysis parameters were calculated using GROMACS built-in tools.

[0037] 4. Molecular dynamics simulation snapshot analysis Visual snapshots from molecular dynamics simulations provide intuitive evidence for understanding the interaction mechanisms between surfactants and asphaltenes. Figure 3The interactions of three anionic-nonionic composite surfactant systems (DBS / OP-10, DDBS / OP-10, and SDS / OP-10) with asphaltenes clusters at different concentrations are illustrated. In all snapshots: green lines represent the nonionic surfactant OP-10, blue lines represent the anionic surfactant, orange areas represent the oil phase core composed of asphaltenes and n-octane molecules, blue spheres represent sodium ions (used to balance the charge of the anionic surfactant), and water molecules are hidden to highlight key interactions.

[0038] Under low concentration conditions (DBS / OP-10=100 / 30), Figure 3 (a1) Because both DBS and OP-10 are strongly hydrophilic, the surfactant molecules failed to completely adsorb onto the surface of the asphaltenes clusters, and some molecules dispersed into the aqueous phase. At this point, the asphaltenes still maintained a complete spherical structure, and the surfactants only had brief contact with the asphaltenes through random collisions. As the concentration increased (DBS / OP-10 = 300 / 90), Figure 3 (a2) DBS molecules preferentially adsorb onto the asphaltene surface. Subsequently, OP-10 molecules adsorb through synergistic action, forming a preliminary composite adsorption layer. At this point, the asphaltene surface begins to show slight deformation, but the overall structure still maintains a spherical shape. When the concentration further increases (DBS / OP-10 = 400 / 120), Figure 3 At concentration a4, the surfactant forms a dense coating layer on the asphaltene surface, causing significant deformation of the asphaltene clusters, changing them from spherical to irregular elliptical shapes. This deformation indicates that the surfactant has penetrated into the interior of the asphaltene, disrupting its original π-π stacked structure. At the highest concentration (DBS / OP-10 = 500 / 150), Figure 3 (a5) The asphaltenes clusters completely disintegrate, breaking into fragmented structures that interact with surfactants through their larger specific surface area. This thorough dispersion allows the asphaltenes to be stably dispersed in the aqueous phase, forming an O / W emulsion, which represents optimal viscosity-reducing performance.

[0039] The DDBS / OP-10 system is significantly less effective than DBS / OP-10, even at the highest concentration (DDBS / OP-10 = 500 / 150). Figure 3 (b5) This system can only alter the surface properties of asphaltenes clusters—transforming the originally smooth spheres into rough surfaces—but cannot completely destroy their core structure. This difference mainly stems from the electronic properties of the benzene ring in the DDBS molecule and the steric hindrance effect of its sulfonic acid groups: the former weakens the π-π stacking interaction with the aromatic rings of asphaltenes, while the latter hinders the system from penetrating deep into the interior of the asphaltenes.

[0040] The SDS / OP-10 system performed the worst in all tests. Even under high concentration conditions (SDS / OP-10 = 500 / 150), Figure 3 (c5) SDS molecules still cannot effectively adsorb onto the surface of asphaltenes clusters, nor do they significantly damage the spherical structure of asphaltenes. The reason for this is that SDS molecules lack aromatic ring structures, leading to the failure of π-π interactions, and their straight-chain alkyl structures are unable to penetrate the tightly packed asphaltenes layer.

[0041] By comparing the performance of the three systems, it can be concluded that the DBS / OP-10 composite system, with its unique synergistic effect, can completely destroy the aggregated structure of asphaltene at sufficient concentrations, achieving a highly efficient viscosity-reducing effect. Molecular dynamics simulation snapshots clearly reveal the interaction mechanism and performance differences between different composite surfactant systems and asphaltene, providing important theoretical basis for the rational design of viscosity reducers. Due to its excellent performance in dispersing asphaltene, the DBS / OP-10 system is the most promising viscosity reducer formulation in this study.

[0042] 5. Hydrogen bond analysis of the composite system Hydrogen bonds, as a key form of intermolecular interaction, play a crucial role in the interaction between surfactants and asphaltenes. This invention systematically analyzes the hydrogen bond formation patterns of three composite surfactant systems (DBS / OP-10, DDBS / OP-10, and SDS / OP-10) at different concentrations, revealing the intrinsic correlation between the number of hydrogen bonds and viscosity reduction performance.

[0043] Figure 4 The algorithm demonstrates the dynamic changes in the number of hydrogen bonds in three composite systems during a 50-nanosecond simulation. All systems exhibit similar trends: in the initial simulation phase (0-3 nanoseconds), the number of hydrogen bonds increases rapidly from a low level, corresponding to the adsorption of surfactant molecules from a dispersed state to the interface and the formation of a stable conformation. Within the time range of 3-50 nanoseconds, the number of hydrogen bonds tends to stabilize and exhibits smooth fluctuations, indicating that the system is in dynamic equilibrium. This dynamic pattern is closely related to the adsorption kinetics of molecules at the interface. The initial rapid increase reflects the rapid diffusion of surfactant molecules to the oil-water interface and the formation of a preliminary adsorption layer; the subsequent fluctuation period reflects the rearrangement and optimization process of interfacial molecules; and the final stable state signifies the formation of the interfacial film structure.

[0044] As the surfactant concentration increases, the total number of hydrogen bonds in all three composite systems shows an upward trend, but the rate of increase differs significantly. This difference mainly stems from the characteristics of the molecular structures of each surfactant. In the DBS / OP-10 composite system, the sulfonic acid group in the DBS molecule and the ether oxygen atom in the OP-10 molecule act as strong hydrogen bond acceptors, while the hydroxyl group in OP-10 can act as a hydrogen bond donor, forming a large number of donor-acceptor pairs, providing a stable basis for hydrogen bond formation. In the DDBS / OP-10 composite system, although DDBS also contains sulfonic acid groups, the benzene ring it connects to creates a steric hindrance effect, which to some extent limits the formation of hydrogen bonds. In addition, the weak interaction between DDBS and asphaltenes reduces the molecular density at the interface, thereby reducing the number of hydrogen bonds. In the SDS / OP-10 system, the sulfate group at the end of the SDS molecule has a weak adsorption capacity for asphaltenes, resulting in a lower density of surfactant molecules at the interface, thus reducing the possibility of hydrogen bond formation.

[0045] Figure 4 The statistical results clearly show that the three composite systems exhibit significant differences in their sensitivity to concentration changes. The number of hydrogen bonds in the DBS / OP-10 composite system increases the fastest with increasing concentration, followed by DDBS / OP-10, while the increase in SDS / OP-10 is the slowest. This difference in sensitivity directly reflects the differences in the interaction strength between each composite system and asphaltene.

[0046] Hydrogen bonds play a dual role in viscosity reduction. First, the hydrogen bonds between surfactants and asphaltenes directly disrupt the original hydrogen bond network within the asphaltenes molecules, breaking down their aggregate structure. Second, the hydrogen bonds formed between surfactants and water enhance the hydrophilicity of the interface, promoting the formation and stabilization of O / W emulsions. Correlation analysis between the number of hydrogen bonds and visual snapshots revealed a strong correlation. The DBS / OP-10 system reached 4743 hydrogen bonds at its highest concentration, while the SDS / OP-10 system had only 4260. This indicates that the number of hydrogen bonds directly determines the surfactant's ability to disrupt asphaltenes aggregates.

[0047] 6. Hydrogen bond analysis of the composite system Hydrogen bonds, as a key form of intermolecular interaction, play a crucial role in the interaction between surfactants and asphaltenes. This study systematically analyzed the hydrogen bond formation patterns of three composite surfactant systems (DBS / OP-10, DDBS / OP-10, and SDS / OP-10) at different concentrations, revealing the intrinsic correlation between the number of hydrogen bonds and viscosity reduction performance.

[0048] Figure 4The algorithm demonstrates the dynamic changes in the number of hydrogen bonds in three composite systems during a 50-nanosecond simulation. All systems exhibit similar trends: in the initial simulation phase (0-3 nanoseconds), the number of hydrogen bonds increases rapidly from a low level, corresponding to the adsorption of surfactant molecules from a dispersed state to the interface and the formation of a stable conformation. Within the time range of 3-50 nanoseconds, the number of hydrogen bonds tends to stabilize and exhibits smooth fluctuations, indicating that the system is in dynamic equilibrium. This dynamic pattern is closely related to the adsorption kinetics of molecules at the interface. The initial rapid increase reflects the rapid diffusion of surfactant molecules to the oil-water interface and the formation of a preliminary adsorption layer; the subsequent fluctuation period reflects the rearrangement and optimization process of interfacial molecules; and the final stable state signifies the formation of the interfacial film structure.

[0049] As the surfactant concentration increases, the total number of hydrogen bonds in all three composite systems shows an upward trend, but the rate of increase differs significantly. This difference mainly stems from the characteristics of the molecular structures of each surfactant. In the DBS / OP-10 composite system, the sulfonic acid group in the DBS molecule and the ether oxygen atom in the OP-10 molecule act as strong hydrogen bond acceptors, while the hydroxyl group in OP-10 can act as a hydrogen bond donor, forming a large number of donor-acceptor pairs, providing a stable basis for hydrogen bond formation. In the DDBS / OP-10 composite system, although DDBS also contains sulfonic acid groups, the benzene ring it connects to creates a steric hindrance effect, which to some extent limits the formation of hydrogen bonds. In addition, the weak interaction between DDBS and asphaltenes reduces the molecular density at the interface, thereby reducing the number of hydrogen bonds. In the SDS / OP-10 system, the sulfate group at the end of the SDS molecule has a weak adsorption capacity for asphaltenes, resulting in a lower density of surfactant molecules at the interface, thus reducing the possibility of hydrogen bond formation.

[0050] Figure 4 The statistical results clearly show that the three composite systems exhibit significant differences in their sensitivity to concentration changes. The number of hydrogen bonds in the DBS / OP-10 composite system increases the fastest with increasing concentration, followed by DDBS / OP-10, while the increase in SDS / OP-10 is the slowest. This difference in sensitivity directly reflects the differences in the interaction strength between each composite system and asphaltene.

[0051] Hydrogen bonds play a dual role in viscosity reduction. First, the hydrogen bonds between surfactants and asphaltenes directly disrupt the original hydrogen bond network within the asphaltenes molecules, breaking down their aggregate structure. Second, the hydrogen bonds formed between surfactants and water enhance the hydrophilicity of the interface, promoting the formation and stabilization of O / W emulsions. Correlation analysis between the number of hydrogen bonds and visual snapshots revealed a strong correlation. The DBS / OP-10 system reached 4743 hydrogen bonds at its highest concentration, while the SDS / OP-10 system had only 4260. This indicates that the number of hydrogen bonds directly determines the surfactant's ability to disrupt asphaltenes aggregates.

[0052] 7. Molecular orbital analysis Molecular orbital theory provides an in-depth electronic analysis of the interaction mechanism between surfactants and asphaltenes. This invention systematically analyzes the frontier molecular orbital characteristics of the nonionic surfactant OP-10 and three anionic surfactants (DBS, DDBS, and SDS) using density functional theory (DFT) calculations, focusing on the energy distribution and band gap (ΔE) of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). This invention reveals the differential mechanisms by which different surfactants reduce viscosity from an electronic structure perspective.

[0053] Quantum chemical calculations revealed significant differences in the HOMO-LUMO band gaps of the four surfactants. The band gap for OP-10 was 8.01 eV. Figure 6 a), while the band gap of DBS reaches 11.19 eV ( Figure 6 (b) Its HOMO is localized on the sulfonic acid group, and LUMO is distributed at the alkyl chain ends; the large band gap indicates that its electronic structure is relatively stable. The band gap of DDBS is 8.40 eV ( Figure 6 c) Its HOMO and LUMO are both distributed in the sulfonic acid group and benzene ring structure, and the smaller band gap suggests stronger chemical reactivity. The band gap of SDS is 11.65 eV ( Figure 6 d) Its HOMO is concentrated in the sulfonic acid group, and LUMO is distributed along the alkyl chain; this is the largest band gap value, indicating that its electronic structure is the most stable.

[0054] According to cutting-edge molecular orbital theory, a smaller HOMO-LUMO band gap favors electron transfer, which is associated with higher chemical reactivity. Based on the band gap value, DDBS (8.40 eV) exhibits higher reactivity and should theoretically demonstrate a stronger binding affinity to asphaltenes. In contrast, DBS (11.19 eV) and SDS (11.65 eV) have larger band gaps and relatively lower reactivity. However, molecular dynamics simulations show that the relationship between the orbital band gap and the surfactant-asphaltene binding affinity is not a simple linear one. Although DDBS has the smallest band gap (8.40 eV), its binding energy to asphaltenes is not the highest. Conversely, DBS, despite having a larger band gap (11.19 eV) than DDBS, exhibits a stronger binding affinity. This seemingly contradictory phenomenon may be attributed to the significant steric hindrance effect of the benzene ring structure in the DDBS molecule, which prevents its sulfonic acid groups from approaching the active sites within the asphaltenes. While its electronic structure favors charge transfer, steric hindrance limits the efficient realization of this interaction. In contrast, DBS has less steric hindrance, and although its band gap is larger, its sulfonic acid groups can more easily form stable coordination bonds with active sites (such as heterocycles) in asphaltenes.

[0055] The results show that although DDBS has the smallest bandgap, its binding ability with asphalt is not the strongest, confirming the significant influence of steric hindrance on the interaction. DBS's ability to form the strongest binding with asphalt materials is attributed to its moderate bandgap and minimal steric hindrance. The results of this invention indicate that predicting the viscosity-reducing effect of surfactants solely based on the frontier bandgap may lead to bias; a comprehensive consideration of multiple molecular structure parameters and kinetic simulation results is necessary.

[0056] This invention systematically and deeply explores the emulsification and viscosity-reducing mechanism and performance of anionic / nonionic composite surfactant systems for heavy oil by integrating molecular dynamics simulations, experimental evaluations, and quantum chemical calculations. The results show that different types of surfactants exhibit significant differences in viscosity-reducing efficiency, fundamentally due to the interfacial behavior and interaction strength determined by the specificity of their molecular structures. The DBS / OP-10 composite system exhibits the best viscosity-reducing performance, its success attributed to a unique synergistic effect. At the molecular level, DBS molecules can effectively adsorb and penetrate into the interior of asphaltenes aggregates, disrupting their π-π stacking structure. Simultaneously, OP-10 molecules synergistically interact with DBS through hydrogen bonds and other interactions, forming a dense and stable composite adsorption layer at the oil-water interface, thereby promoting and stabilizing the formation of O / W emulsions. In contrast, the DDBS / OP-10 system suffers from steric hindrance due to the benzene ring in the DDBS molecule, while the SDS / OP-10 system exhibits significantly weakened synergistic effects due to the weak polar groups of SDS molecules, making effective interaction with asphaltenes difficult. This results in a lower viscosity-reducing effect. More importantly, molecular orbital analysis revealed that the viscosity-reducing effect is not solely determined by the intrinsic electrochemical activity of the molecules. Although DDBS has the smallest HOMO-LUMO bandgap, exhibiting higher activity, its actual binding capacity with asphaltene is weaker than that of DBS. This seemingly contradictory phenomenon highlights the crucial influence of steric hindrance on intermolecular interactions.

[0057] The content of this invention is not limited to the embodiments listed. Any equivalent modifications made by those skilled in the art to the technical solutions of this invention by reading this specification are covered by the claims of this invention.

Claims

1. A composite surfactant for reducing the viscosity of heavy oil, characterized in that, It is formulated by compounding anionic surfactant and nonionic surfactant; the anionic surfactant is selected from sodium dodecylbenzenesulfonate (DBS), sodium dodecyl diphenyl ether disulfonate (DDBS) or sodium dodecyl sulfate (SDS); the nonionic surfactant is octylphenol polyoxyethylene ether (OP-10).

2. The composite surfactant for reducing the viscosity of heavy oil according to claim 1, characterized in that, The mass ratio of the anionic surfactant to the nonionic surfactant is 1:

1.

3. An application of the composite surfactant as described in claim 1 or 2 in enhancing oil recovery, characterized in that, Includes the following steps: S1: Prepare an aqueous solution of the composite surfactant as described in claim 1 or 2; S2: The aqueous solution obtained in step S1 is mixed with heavy oil and emulsified to form an O / W type emulsion. During the oil extraction process, the O / W type emulsion is injected into the reservoir.

4. The application of the composite surfactant according to claim 3 in enhancing oil recovery, characterized in that, In step S1, the concentration of the composite surfactant in the aqueous solution is 0.2-1.0%.

5. The application of the composite surfactant according to claim 3 in enhancing oil recovery, characterized in that, In step S2, the volume ratio of the aqueous solution to the heavy oil is 1:

1.

6. The application of the composite surfactant according to claim 5 in enhancing oil recovery, characterized in that, In step S2, the emulsification process is carried out using a homogenizer at a speed of 4000 rpm for 3 minutes.

7. The application of the composite surfactant according to claim 3 in enhancing oil recovery, characterized in that, Before step S2, step S0 is also included: screening anionic surfactants with high affinity for asphaltenes in the target heavy oil through molecular dynamics simulation to determine the types of anionic surfactants used to prepare the composite surfactant; wherein, the indices of the molecular dynamics simulation include the number of hydrogen bonds, mean square displacement (MSD), or radial distribution function (RDF).

8. The application of the composite surfactant according to claim 7 in enhancing oil recovery, characterized in that, In step S0, all molecular dynamics simulations were performed using the GROMACS 2022.4 program in conjunction with a general AMBER force field to describe molecular interactions; the force field parameters were generated using the sobtop tool, where the atomic charges of the composite surfactant molecules were calculated based on the confined electrostatic potential method.

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