Molecular Mechanisms of CTAB-Induced Oil Droplet Detachment in Shale Nanopores

Abstract

The efficient recovery of shale oil is severely hindered by the strong oil–solid interactions and nanoscale confinement within shale reservoirs, which substantially restrict crude oil mobility. Although surfactant flooding has emerged as an effective enhanced oil recovery technique, the microscopic mechanisms by which cationic surfactants promote oil detachment in nanopores remain insufficiently understood. In this study, molecular dynamics simulations are performed to systematically investigate the interfacial mechanisms of cetyltrimethylammonium bromide (CTAB)-induced oil droplet detachment from nanopore surfaces. The effects of temperature and crude oil composition on the detachment behavior are further evaluated. The results demonstrate that CTAB exhibits the highest oil displacement efficiency among the four investigated surfactants, characterized by the lowest interaction energy and the formation of the most stable interfacial adsorption layer. The CTAB-induced oil detachment process proceeds through three successive stages. CTAB molecules initially migrate to the oil–water interface through hydrophobic interactions, followed by their directional migration toward the oil–solid interface driven by electrostatic interactions, where they induce the formation of hydration channels. Subsequently, hydrogen-bond networks stabilize and expand these hydration channels, ultimately leading to the complete detachment of oil droplets and the formation of stable oil-in-water emulsions. Increasing temperature significantly enhances the interfacial adsorption of CTAB and improves oil displacement efficiency. In addition, crude oil composition exerts a pronounced influence on the detachment process. CTAB exhibits the highest detachment efficiency for the medium crude oil. In contrast, the detachment of heavy oil follows a distinctive two-stage mechanism involving initial aggregate disintegration, followed by interfacial stabilization. This study provides molecular-level insights into the interfacial regulation mechanisms of CTAB in shale nanopores and offers a theoretical foundation for the rational design of high-performance surfactant systems for enhanced shale oil recovery.

1. Introduction

Shale oil is one of the most promising unconventional hydrocarbon resources and has attracted considerable attention because of its significant potential to enhance global energy security. However, the efficient recovery of shale oil is severely hindered by the ultralow permeability and intricate nanoscale pore networks inherent to these reservoirs. In these reservoirs, crude oil mainly exists in two forms: adsorbed oil and free oil. Adsorbed oil is primarily attached to the surfaces of organic matter, whereas free oil is distributed within nanopores and pore throats, exhibiting pronounced nanoscale confinement effects. The predominantly oil-wet or mixed-wet nature of shale reservoirs strengthens oil–solid interfacial interactions and promotes capillary trapping of crude oil within nanopores and pore throats. , At the nanoscale, enhanced interfacial effects and intermolecular forces further increase oil adhesion and transport resistance, resulting in severely restricted oil mobility and low recovery efficiency. Consequently, conventional pressure-driven recovery methods are generally insufficient to mobilize the trapped oil, leading to an ultimate recovery factor typically below 10%. − Therefore, regulating interfacial properties and weakening the adhesion between crude oil and the rock surface remain key challenges for improving shale oil recovery.

Furthermore, recent advances in nanoscale interfacial science have increasingly highlighted the intricate fluid–solid and fluid–fluid interactions under tight confinement. A systematic understanding of multiphase coupling mechanisms is critical, as salt ions and highly mineralized formation water can strongly regulate mineral wettability, oil–water distribution, and fluid mobility within shale nanopores. Specifically, the dynamic interfacial behavior and complex physicochemical interactions between injected chemical agents and heterogeneous mineral surfaces play a pivotal role in mobilizing confined fluids. For instance, the use of cosolvents (such as dimethyl ether and ethyl acetate) during CO₂ injection significantly enhances shale oil displacement efficiency by promoting the mutual solubility of CO₂ and oil, reducing interfacial tension, and altering the competitive adsorption on quartz and kerogen walls. This enhancement is further optimized during the CO₂-cosolvent soaking-alternating-gas (SAG) process, where early soaking ensures full contact and later flooding provides a sustained driving force, maximizing both oil recovery and CO₂ storage. Moreover, in realistic water-bearing reservoirs, the injection of acid waste gas (a CO₂–H₂S mixture) demonstrates superior performance over pure CO₂ in hydrated Illite and composite nanopores, as H₂S effectively penetrates water films and enhances competitive adsorption against confined oil. A comprehensive understanding of these molecular-level interface behaviors is essential for rationally designing highly efficient chemical enhanced oil recovery (EOR) and carbon storage strategies in complex shale systems.

Surfactant flooding has emerged as one of the most promising enhanced oil recovery (EOR) techniques for unconventional reservoirs because of its ability to reduce oil–water interfacial tension, alter rock wettability, and promote emulsification, thereby facilitating the detachment and transport of crude oil confined within nanopores. Prior studies have established that surfactant-driven emulsification and hydration structures can substantially mitigate the severe nanoscale confinement of crude oil. These synergistic processes facilitate oil droplet detachment from pore walls and improve the overall mobility, ultimately boosting recovery efficiency. , Overall, the effectiveness of surfactant flooding originates from the synergistic regulation of several interfacial processes, including wettability alteration, emulsification, and interfacial adsorption.

Among these mechanisms, wettability alteration has been widely recognized as one of the dominant factors governing the shale oil recovery. Nguyen et al. experimentally demonstrated that converting rock surfaces from oil-wet to water-wet conditions contributes more significantly to enhanced oil recovery than reducing interfacial tension alone, with nonionic surfactants exhibiting the highest recovery efficiency owing to their superior wettability alteration capability. Huang et al. further developed a novel zwitterionic surfactant capable of markedly improving shale oil recovery through wettability reversal and competitive adsorption. Collectively, these studies demonstrate that altering reservoir wettability toward a more water-wet state is one of the primary mechanisms responsible for an enhanced shale oil recovery.

Besides wettability alteration, emulsification and spontaneous imbibition also play important roles in improving oil recovery. Xu et al. showed that wettability alteration serves as the dominant mechanism, whereas spontaneous emulsification provides an important complementary pathway by facilitating crude oil transport. Chen et al. systematically compared conventional surfactants, microemulsions, nanoemulsions, and nanofluids using combined experimental and numerical approaches and reported that nanoemulsions exhibited the greatest EOR potential because of their smaller hydrodynamic size and superior transport characteristics. These findings indicate that emulsification not only enhances the crude oil mobility but also improves the spontaneous imbibition efficiency by facilitating oil transport within nanoporous media.

Interfacial adsorption and ion-mediated interactions have also attracted increasing attention because they directly regulate oil–solid interactions at the molecular level. Golab et al. reported that amphoteric polymeric surfactants exhibit enhanced adsorption stability through multipoint binding and ion-bridging effects, while divalent cations (Ca²⁺ and Mg²⁺) further strengthen adsorption by compressing the electrical double layer and enhancing hydrophobic interactions. These studies highlight the pivotal role of interfacial adsorption in regulating oil–solid interactions and improving crude oil displacement efficiency. In addition, recent molecular dynamics investigations have further underscored the critical role of competitive adsorption and complex environmental factors (e.g., fluid composition and water content) in dictating fluid–solid interfacial behaviors and dynamic transport mechanisms within organic and inorganic shale nanopores. ,

Although considerable progress has been made in understanding how surfactants enhance oil recovery through wettability alteration, emulsification, and interfacial adsorption, most previous studies have focused on individual mechanisms or specific length scales. Consequently, the dynamic evolution of surfactant-induced oil droplet detachment, particularly the coupled processes of interfacial adsorption, hydration layer reconstruction, oil–solid interaction weakening, and subsequent oil droplet detachment, has not yet been fully elucidated.

Among various surfactants investigated for EOR, cetyltrimethylammonium bromide (CTAB), a representative cationic surfactant, has attracted considerable attention because of its excellent interfacial activity, strong interfacial adsorption capability, and superior emulsification performance. , Recent studies have mainly focused on its roles in emulsification, viscosity reduction, interfacial modification, and shale oil recovery. Ahmed et al. demonstrated that the synergistic interaction between CTAB and low-salinity water significantly reduced crude oil viscosity through cation-bridging-induced asphaltene precipitation, while sulfate-rich brines enhanced CTAB adsorption at the oil–water interface and promoted the formation of more stable emulsions. Mahdavi et al. further revealed that variations in ionic composition not only influence the ability of CTAB to reduce interfacial tension and alter wettability but also govern its interactions with crude oil components, thereby affecting emulsion stability and oil recovery performance. At the nanoscale, Abdulelah et al. demonstrated that CTAB concentrations above the critical micelle concentration significantly enhanced methane adsorption under high-salinity conditions, highlighting the important role of surfactant–ion interactions in regulating interfacial behavior. Li et al. subsequently developed a catanionic surfactant system by combining CTAB with α-olefin sulfonate, achieving efficient emulsification and viscosity reduction of highly viscous shale oil while substantially improving its flowability and recovery efficiency. Overall, these studies demonstrate the considerable potential of CTAB for enhanced oil recovery through emulsification, wettability regulation, and interfacial adsorption.

Nevertheless, current investigations are still predominantly based on macroscopic experiments or equilibrium interfacial analyses, providing only limited insight into the molecular processes governing CTAB-induced oil droplet detachment within nanoporous media. In particular, how CTAB adsorption reconstructs interfacial hydration structures, weakens oil–solid interactions, and ultimately drives progressive oil droplet detachment remains insufficiently understood. Moreover, the coupled influences of temperature and crude oil composition on these interfacial processes have not yet been systematically clarified.

To address these issues, molecular dynamics simulations were performed in this study to investigate the interfacial regulation mechanism of CTAB during crude oil droplet detachment in nanoporous media. Particular attention was paid to the adsorption behavior of different surfactants, the microscopic mechanism of CTAB-induced oil droplet detachment, and the coupled effects of temperature and crude oil composition on hydration structure evolution, oil–solid interfacial interactions, and crude oil transport. The findings provide molecular-level insights into surfactant-assisted oil recovery and offer theoretical guidance for the rational design of high-performance surfactant systems for enhanced shale oil recovery.

2. Models and Methods

2.1. Molecular Models

The mineral composition of shale is dominated by silica (SiO₂), which commonly exists in the form of quartz or feldspar. Because of its well-defined surface properties, wettability, and interactions with confined fluids, silica is widely employed as a representative mineral for modeling shale pore surfaces in molecular simulations. Therefore, SiO₂ is selected as the pore wall material in the present study to realistically represent the mineral composition and pore characteristics of shale reservoirs.

Shale pore structures are generally classified into pore-type and slit-type morphologies. Considering that slit-shaped nanopores are the dominant transport pathways in shale formations, a slit-pore model is adopted to better characterize the microscopic pore structure and its influence on fluid transport. The initial SiO₂ crystal structure is imported from the Materials Studio crystal database and cleaved along the (001) crystallographic plane. The resulting SiO₂ substrate has dimensions of 102 Å × 39 Å × 15 Å (length × width × thickness). Two parallel SiO₂ substrates are separated by a distance of 42 Å to construct a slit-shaped nanopore, as illustrated in Figure . Periodic boundary conditions are applied to minimize finite-size effects and to simulate an infinitely extended pore structure. Specifically, the periodicity parallel to the pore walls ensures the continuity of the slit nanopore and effectively eliminates boundary artifacts during molecular dynamics simulations. While pristine silica is commonly employed to model inorganic quartz pores, real shale pore walls are highly heterogeneous, typically comprising a mixture of inorganic minerals and solid organic matter (kerogen). Spectroscopic characterizations and extensive structural reviews from classical literature have explicitly confirmed that natural shale kerogen and oxidized organic coatings possess abundant oxygen-containing functional groups, predominantly carboxyl (−COOH) and hydroxyl (−OH) groups. , Therefore, the carboxylated SiO₂ substrate in this study is not intended to represent a pure, pristine inorganic mineral surface. Instead, it explicitly serves as a simplified, representative hydrophilic pore-wall model designed to faithfully replicate the highly polar, oxygen-rich patches typical of mixed-wet shale reservoirs. To better reproduce these complex wetting characteristics, the SiO₂ substrate is further functionalized with carboxyl (−COOH) groups. Specifically, these carboxyl groups are covalently anchored to the surface silicon atoms (replacing the original terminal hydroxyl groups) rather than physically adsorbed as a loose organic coating. This covalent bonding ensures the structural stability of the functional groups during high-pressure fluid displacement. The surface grafting density is precisely controlled at 4.79 –COOH/nm², with the carboxyl groups uniformly distributed over the 102.048 × 39.280 Ų silica surface. This specific high-coverage grafting density was carefully selected because it closely approximates the theoretical maximum density of polar interaction sites on fully functionalized mineral surfaces (e.g., the classic Zhuravlev constant of ∼4.9 groups/nm² for amorphous silica). Therefore, this covalently anchored configuration explicitly serves as a simplified, representative model for the highly hydrophilic, oxygen-rich patches typical of mixed-wet shale reservoirs. The uniform distribution and stable covalent anchoring of these highly polar groups provide the essential active sites for hydrogen-bonding interactions with water molecules, which strictly governs the subsequent formation of stable hydration channels and oil droplet detachment. Consequently, the modified SiO₂ surface more accurately captures the occurrence, interfacial behavior, and transport characteristics of crude oil within shale nanopores. The combination of a representative slit-shaped nanopore geometry and chemically modified pore surfaces provides a realistic molecular model for investigating fluid transport under nanoscale confinement. This model establishes a reliable computational framework for elucidating the microscopic mechanisms governing crude oil migration and surfactant-assisted oil recovery in complex shale nanoporous systems.

1.

SiO₂ nanopore model.

Shale oil is a complex mixture of hydrocarbons whose transport and adsorption behaviors are strongly influenced by its molecular composition and the pore structure of the reservoir. To investigate the microscopic behavior of shale oil within nanopores, a representative molecular model is required. In this study, n-dodecane (C₁₂H₂₆, hereafter consistently denoted as C12) is selected as the model compound to represent shale oil, as illustrated in Figure . Owing to its moderate molecular size, C12 is selected as the representative model compound for the medium-chain hydrocarbon fraction of shale oil. The scientific rationale for this selection is deeply rooted in both compositional and physicochemical characteristics. Compositionally, typical gas chromatography–mass spectrometry (GC–-MS) characterizations of produced shale oil demonstrate that alkanes in the C10–C15 range frequently constitute the most abundant components within the saturated hydrocarbon fraction. From a physicochemical perspective, C12 possesses a representative kinematic viscosity, a moderate molecular weight, and intermediate intermolecular cohesive energies that closely approximate the bulk properties of the medium-chain alkane fraction in macroscopic shale oil samples. Therefore, C12 serves as an ideal and computationally efficient proxy to faithfully replicate the fundamental van der Waals interactions, flow resistance, and nanoscale confined phase behavior of the continuous shale oil phase.

2.

Molecular configuration of C12, where gray and white spheres denote carbon and hydrogen atoms, respectively.

Although shale oil consists of hydrocarbons with diverse molecular structures and chain lengths, C12 provides a reasonable balance between the molecular complexity and computational efficiency. Its molecular structure is sufficiently representative to reproduce the key physicochemical characteristics of shale oil while avoiding the substantial computational cost associated with more-complex multicomponent models. In addition, the molecular model of C12 available in the Materials Studio database has been extensively validated and widely employed in molecular dynamics simulations, ensuring the reliability and applicability of the present models.

As shown in Figure , the crude oil droplet is constructed using 100 C12 molecules. The surfactant solutions placed within the nanopore contain a total of 10 CTAB molecules (initially distributed in the two aqueous regions, with five molecules on each side) along with 4000 water molecules. A CTAB molecule consists of a polar hydrophilic headgroup and a long hydrophobic alkyl chain, as illustrated in Figure (a). The nanopore together with the confined fluids is enclosed in a periodically replicated simulation box, and an equilibrium molecular dynamics simulation is performed for 5 ns, as shown in Figure (b). The equilibrium simulation is designed to investigate the detachment process of the crude oil droplet from the pore surface induced by surfactant adsorption. This process corresponds to the shut-in stage following the injection of a small slug of surfactant solution during surfactant flooding operations, in which surfactant molecules continuously adsorb onto the oil–water and oil–solid interfaces, progressively weaken the oil–solid interaction, and promote the spontaneous detachment of the trapped oil droplet from the pore wall.

3.

Molecular models: (a) molecular structure of the surfactant CTAB, where blue spheres represent nitrogen atoms, and (b) molecular model of an oil droplet confined within the nanopore, where red, light-blue, and dark-blue spheres represent C12, H₂O, and CTAB molecules, respectively.

To ensure the reproducibility of the molecular models, all intra- and intermolecular interactions for the surfactants, oil components, and the carboxylated silica surface were strictly described by the standard COMPASS II force field. The specific force field parameters and partial charges were assigned by using the established automated typing rules within the Materials Studio package, consistent with previously validated studies.

2.2. Simulation Methods

Initial molecular models were constructed using Materials Studio, and all molecular dynamics (MD) simulations were performed using the GROMACS software package. Both equilibrium molecular dynamics (EMD) and nonequilibrium molecular dynamics (NEMD) simulations are conducted to investigate the equilibrium interfacial properties and the dynamic transport behavior of the crude oil–surfactant system. To further justify the applicability of the COMPASS force field for our complex multicomponent system, it should be noted that COMPASS has been extensively validated in previous studies for accurately capturing the interfacial interactions between surfactants and mineral surfaces, as well as the surface hydration behavior and fluid mobility in inorganic silica nanopores. , Since the carboxyl (−COOH) functional groups are not part of the standard SiO₂ substrate model, their specific parameters and bonding characteristics were explicitly defined. The corresponding parameter files were generated using the AuToFF program within the adopted force-field framework. For the SiO₂ substrate, the oxygen atoms (atomic mass = 15.9994 g·mol^–1) were assigned Lennard-Jones parameters of σ = 0.316552 nm and ε = 0.650262 kJ·mol^–1. The silicon atoms (atomic mass = 28.0855 g·mol^–1) were assigned parameters of σ = 0.330196 nm and ε = 7.699311 × 10^–6 kJ·mol^–1. To explicitly describe the covalent anchoring of the surface functional groups, additional bonding connections were introduced between the silica surface and the carboxyl groups. These connections were modeled using the type-1 harmonic bond potential in GROMACS, with an equilibrium bond length of 0.1110 nm and a bond force constant of 4.637 × 10⁵ kJ·mol^–1·nm^–2. The nonbonded exclusion parameter (nrexcl) of 3 was adopted for the corresponding molecular topology. Furthermore, rather than relying on a macroscopic simulated water contact angle, the strong hydration behavior of the functionalized surface in this study is robustly supported by the extensive hydrogen-bond interactions observed between interfacial water molecules and the surface carboxyl groups, which aligns well with previous experimental and molecular simulation studies of oxygen-containing functionalized silica surfaces. , All simulations are performed in the canonical (NVT) ensemble with the system temperature maintained at 353 K using the Andersen thermostat, ensuring that the system reaches and maintains thermal equilibrium throughout the simulation. van der Waals interactions are calculated using an atom-based summation with a cutoff distance of 12 Å, while long-range electrostatic interactions are evaluated using the Ewald summation method to accurately account for Coulombic interactions. A time step of 1 fs is employed throughout the simulations to ensure numerical stability and integration accuracy. The system configuration and relevant physical properties are recorded every 10 ps for subsequent statistical analysis. Before the MD simulations, all initial configurations are subjected to energy minimization using the steepest descent algorithm to eliminate unfavorable atomic contacts and obtain energetically stable structures.

The simulation protocol consists of two successive stages. First, equilibrium molecular dynamics simulations are performed to fully relax the system and establish stable structural and thermodynamic properties. Subsequently, nonequilibrium molecular dynamics simulations are carried out to investigate the transport characteristics and interfacial behaviors of the crude oil–surfactant system under externally applied driving conditions.

3. Results and Discussions

This section first compares the oil droplet detachment performance of different types of surfactants to identify the superiority of the CTAB. The molecular mechanisms responsible for the enhanced oil-displacement performance of CTAB are then elucidated. Finally, the effects of various factors on the CTAB-induced oil droplet detachment are systematically investigated.

3.1. Effect of Different Surfactants on Oil Droplet Detachment

Surfactants are generally classified into cationic, anionic, nonionic, and zwitterionic types according to the charge characteristics of their hydrophilic headgroups. Owing to their distinct molecular structures and interfacial properties, different surfactants exhibit markedly different adsorption behaviors at the oil–water interface, resulting in variations in interfacial stability and enhanced oil recovery performance. To systematically evaluate the interfacial regulation capability of CTAB, molecular dynamics simulations were performed for four representative surfactants. These four specific surfactants were strategically selected to comprehensively cover the four primary interfacial charge categories widely utilized in chemical enhanced oil recovery: CTAB as a cationic surfactant, sodium dodecyl sulfate (SDS) as an anionic surfactant, polyglycerol fatty acid ester (PGFE) as a nonionic surfactant, and dodecyl dimethyl betaine (BS-12) as a zwitterionic surfactant. This specific selection enables a systematic and well-rounded comparison, allowing us to explicitly isolate and evaluate how different hydrophilic headgroup chargesand their resulting electrostatic interactions with the negatively charged silica surfacegovern interfacial adsorption, hydration channel formation, and ultimate oil droplet detachment under uniform reservoir conditions (353 K and 40 MPa). The transport behavior of these surfactants within nanopores and their corresponding oil droplet detachment processes are comparatively analyzed. It should be clarified that the selected temperature of 353 K (80 °C) and pressure of 40 MPa do not exclusively represent a single specific oilfield but rather serve as representative baseline conditions for typical mid-to-deep terrestrial shale oil reservoirs (such as those located in the Ordos and Songliao Basins, China). At a typical reservoir burial depth of approximately 2500 to 3500 m, the formation temperature generally falls within the 70–90 °C range (assuming a normal geothermal gradient of ∼2.5–3.0 °C/100 m), accompanied by formation pressures typically ranging from 30 to 45 MPa. Therefore, these specific values were carefully chosen as representative thermodynamic parameters to realistically replicate the high-temperature and high-pressure confinement environment encountered during actual deep shale oil recovery processes.

These four surfactants were selected as they represent the four primary classes (cationic, anionic, nonionic, and zwitterionic) widely investigated for chemically enhanced oil recovery (EOR). To ensure a reasonable structural comparison, we chose CTAB, SDS, and BS-12 because they possess relatively comparable hydrophobic alkyl tail lengths (ranging from 12 to 16 carbon atoms). This structural similarity ensures that their hydrophobic interactions with the crude oil phase are on a comparable magnitude, effectively isolating the influence of the hydrophilic headgroup charge on the detachment process.

In contrast, PGFE was selected as a representative nonionic surfactant because of its excellent industrial emulsification properties. However, its molecular structure is significantly larger and more complex than those of the other three surfactants. It should be noted that the interfacial behavior of PGFE is governed not only by its lack of charge but also by its larger hydrodynamic volume. The slower diffusion and weaker interfacial stability observed for PGFE are largely attributed to its increased steric hindrance and molecular complexity, which restrict its migration kinetics under a nanoscale confinement. Therefore, while the performance differences among CTAB, SDS, and BS-12 are primarily driven by their distinct charge characteristics and resulting electrostatic interactions with the functionalized SiO₂ surface, the distinct behavior of PGFE highlights the coupled influence of the molecular size, steric hindrance, and hydrophobicity on adsorption kinetics.

As shown in Figure , all four surfactants facilitate the formation of oil–water dispersed structures by reducing the oil–water interfacial tension, stabilizing the oil–water interface, and modifying the interfacial wettability. Consequently, the crude oil droplet gradually detaches from the pore surface and becomes emulsified during the simulation. After 5000 ps, complete detachment of the oil droplet from the nanopore surface is observed for all surfactant systems, accompanied by the formation of a stable oil-in-water emulsion. Nevertheless, significant differences in the interfacial adsorption behavior are observed among the four surfactants. Compared with SDS, PGFE, and BS-12, CTAB exhibits a substantially higher interfacial coverage and a more uniform distribution over the oil droplet surface, indicating its stronger adsorption affinity and superior interfacial regulation capability. The dense and homogeneous adsorption layer formed by CTAB effectively reduces the oil–water interfacial free energy while enhancing interfacial stability, thereby facilitating oil droplet deformation, dispersion, and emulsification. More importantly, the orderly arrangement of the hydrophilic headgroups of CTAB promotes the accumulation of water molecules at the oil–water interface, leading to the formation of a continuous hydration layer and interconnected water channels. These interfacial structures progressively weaken the adhesion between the oil droplet and the solid surface, thereby creating favorable conditions for the spontaneous detachment of the confined oil droplet from the nanopore wall.

4.

Oil-in-water structure formed by different surfactants. (a) CTAB; (b) SDS; (c) PGFE; (d) BS-12. The color codes are defined as follows: dark blue represents CTAB, purple represents SDS, green represents PGFE, yellow represents BS-12, red represents C12 (oil), and light blue represents water molecules.

To quantitatively evaluate the interfacial regulation capability of different surfactant systems, the overall interaction energy between the oil droplet and its surrounding environment is employed as a key descriptor. This parameter accounts for the collective energetic effects associated with the aqueous phase, surfactant molecules, and the carboxylated SiO₂ surface on the oil droplet during the detachment process. The interaction energy E _int is calculated according to eq :

$$E_{\mathrm{int}}=E_{\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{m}}-(E_{\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{x}}+E_{\mathrm{o}\mathrm{i}\mathrm{l}})$$ 1

where E _system is the total energy of the complete oil–water–surfactant–SiO₂ system, E _oil is the energy of the isolated oil droplet, and E _matrix represents the energy of the surrounding environment composed of water, surfactant molecules, and the carboxylated SiO₂ substrate. A positive interaction energy indicates that repulsive interactions dominate and the system remains in a relatively unstable state, whereas a negative value signifies that attractive interactions become dominant, leading to a thermodynamically more stable interfacial configuration. Therefore, the calculated quantity should be interpreted as an integrated measure of the changing interaction state of the oil droplet with its complex surrounding environment throughout the coupled interfacial processes of surfactant adsorption, water penetration, and oil-surface adhesion weakening rather than representing a specific pairwise surfactant–oil interaction.

Figure presents the temporal evolution of the overall interaction energy between the oil droplet and its surrounding environment for the four representative surfactant systems. Although all systems exhibit a gradual transition from a high-energy state to a lower-energy state, pronounced differences are observed in their adsorption kinetics, equilibrium energy levels, and energy fluctuations. Among the four surfactants, CTAB exhibits the most rapid decrease in interaction energy during the initial stage, reaching a stable state after approximately 3000 ps and ultimately converging to approximately −1000 kJ mol^–1 with the smallest fluctuation amplitude. This behavior indicates that CTAB rapidly adsorbs onto the oil–water interface and forms a stable interfacial layer, thereby effectively reducing the interfacial free energy and enhancing the thermodynamic stability of the emulsion.

5.

Temporal evolution of the overall interaction energies between the oil droplet and its surrounding environment in different surfactant systems.

SDS also exhibits a rapid decrease in interaction energy and eventually stabilizes at approximately −950 kJ mol^–1, indicating relatively strong interfacial adsorption and emulsification capability. However, its equilibrium interaction energy is slightly higher and its fluctuations are more pronounced than those of CTAB, suggesting a relatively lower interfacial stability. In comparison, the interaction energy of BS-12 decreases more gradually and reaches a stable value of approximately −900 kJ mol^–1, reflecting a moderate adsorption strength and interfacial regulation capability. Although BS-12 is capable of stabilizing the oil–water interface, its adsorption process is slower and the resulting interfacial structure is less stable than that formed by CTAB.

In contrast, PGFE exhibits distinct evolution behavior. Although its interaction energy initially decreases, a noticeable increase is observed during the later stage of the simulation, indicating the partial desorption or rearrangement of surfactant molecules at the interface. Consequently, PGFE fails to maintain continuous interfacial coverage, leading to a less stable emulsified structure and a weaker interfacial regulation capability.

Overall, CTAB exhibits the lowest equilibrium interaction energy together with the fastest adsorption kinetics and the smallest energy fluctuations, demonstrating its superior ability to reduce the oil–water interfacial free energy and stabilize the emulsified system. These results further confirm that CTAB possesses the strongest interfacial adsorption capability among the four surfactants investigated and provides the most favorable conditions for oil droplet detachment from the pore surface. Therefore, interaction energy serves as an effective quantitative descriptor for evaluating surfactant performance and provides a theoretical basis for surfactant screening.

3.2. Mechanism of CTAB Promoted Oil Droplet Detachment in Nanopores

This section focuses on elucidating the molecular mechanism by which CTAB promotes oil droplet detachment from nanopore surfaces. Figure illustrates the dynamic evolution of the detachment process at the molecular level. Overall, the process can be divided into three successive stages: (1) interfacial adsorption of CTAB driven by hydrophobic interactions, (2) formation of interfacial water channels through the cooperative effects of electrostatic interactions and hydration, and (3) complete oil droplet detachment facilitated by the development of a hydrogen-bond network.

6.

Process of CTAB-induced oil droplet detachment from the nanopore surface.

During the initial stage, no stable interfacial structure exists between the oil droplet and the aqueous phase, and CTAB molecules remain predominantly dispersed as free molecules in the bulk water, exerting only a limited influence on the oil–water interface. After approximately 1000 ps, CTAB molecules gradually migrate toward the oil–water interface under the driving force of hydrophobic interactions, as shown in Figure (a). Owing to their amphiphilic molecular structure, the specific roles of the two molecular moieties can be clearly distinguished during this initial migration process. The long hydrophobic cetyl chains primarily drive the penetration into and strong association with the nonpolar oil phase. Simultaneously, the positively charged trimethylammonium headgroups preferentially remain strongly hydrated in the aqueous phase, preventing the surfactant molecules from completely partitioning into the oil droplet. This highly specific dual action drives the spontaneous assembly of an ordered interfacial arrangement. At this stage, only a small number of water molecules accumulate near the interface, and no continuous water pathway is observed. As interfacial adsorption proceeds, CTAB molecules further spread along the oil–water interface toward the oil–solid interface, increasing the interfacial coverage and progressively reducing the oil–water interfacial free energy. Therefore, hydrophobic interactions dominate the migration and interfacial assembly of CTAB molecules during the early stage, establishing the prerequisite for subsequent oil droplet detachment.

In the second stage, CTAB molecules become almost fully adsorbed at the oil–water interface and gradually migrate toward the oil–solid interface. This interfacial transport is jointly driven by hydrophobic and electrostatic interactions. Although the overall simulation system remains electrically neutral, the heterogeneous charge distribution on the SiO₂ surface generates localized electrostatic fields, which promote the preferential adsorption of the positively charged CTAB headgroups at the solid–liquid interface. Consequently, CTAB molecules gradually accumulate near the pore surface, weakening the oil–solid adhesion and altering the local interfacial energy balance. Meanwhile, the hydrophilic headgroups induce the ordered rearrangement of surrounding water molecules, promoting their continuous penetration into the confined gap between the oil droplet and the solid surface. As a result, an incipient hydration channel gradually develops within the interfacial transition region, as illustrated in Figure (b). The formation of this hydration pathway marks the progressive transition of the oil–solid interface toward a more water-wet state, thereby creating favorable conditions for subsequent oil droplet detachment.

During the third stage, both the width of the hydration channel and the number of interfacial water molecules increase significantly. Water molecules progressively fill the space between the oil droplet and the SiO₂ surface, ultimately leading to the formation of a stable oil-in-water configuration, as shown in Figure (c). Once the initial water pathway is established, strong hydrogen bonds are formed between the surface carboxyl groups of SiO₂ and adjacent water molecules, anchoring water molecules at the solid surface. Simultaneously, hydrogen bonds among neighboring water molecules continuously develop into an interconnected hydrogen-bond network, which promotes the cooperative self-organization of hydration clusters and drives the continuous propagation of water channels along the interface. The dynamically expanding hydration pathway not only enhances the structural stability of the interfacial water layer but also continuously weakens the attractive interactions between the oil droplet and the solid substrate, allowing C12 molecules to progressively detach from the SiO₂ surface. The system reaches a stable equilibrium after approximately 5000 ps. These observations indicate that hydrogen-bond interactions dominate the final stage of oil droplet detachment by stabilizing the hydration channels and reconstructing the oil–solid interface, thereby serving as the primary driving force for complete oil release.

To quantitatively substantiate the dominant role of hydrogen-bond interactions during the final stage of detachment, a comprehensive hydrogen-bond analysis was conducted based on standard geometric criteria (donor–acceptor distance ≤ 3.5 Å and hydrogen-donor–acceptor angle ≤ 30°). As the hydration channel expands during stage 3, the average number of hydrogen bonds per interfacial water molecule increases significantly from approximately 1.5 to 3.3. Furthermore, the average hydrogen-bond lifetime was calculated using the intermittent hydrogen-bond autocorrelation function, yielding a prolonged value of approximately 14.5 ps. This demonstrates that the intercalated water molecules and the surface carboxyl groups (−COOH) establish a highly dense and stable network with robust connectivity. The dynamic evolution of this highly connected hydrogen-bond network effectively compensates for the energetic penalty of breaking the oil–solid interactions, providing the fundamental thermodynamic driving force for the complete detachment of the crude oil droplet.

Furthermore, these three proposed stages can be quantitatively delineated by integrating the thermodynamic and structural metrics discussed above. The initial hydrophobic migration (stage 1) is quantitatively marked by the rapid decline in the overall interaction energy (as shown in Figure ). The transition to hydration channel formation (stage 2) is evidenced by the progressive shift in the relative concentration distribution of the oil droplet along the z-direction (Figure ), which quantitatively reflects the continuous reduction in the oil–solid contact area as water molecules penetrate the interface. The final detachment and stabilization (stage 3) are confirmed by the significant increase in interfacial hydrogen bonds and the ultimate plateauing of the interaction energy curves around 3000–5000 ps. It is also important to emphasize that these three stages are not strictly independent or sequential processes. The continuous and smooth nature of the interaction energy curves demonstrates that they represent overlapping and dynamically coupled interfacial events; for instance, the interfacial adsorption of CTAB continues simultaneously with the penetration of hydration water molecules into the oil–solid boundary.

7.

Relative concentration distributions of the oil droplet along the z-direction at 0, 2500, and 5000 ps.

The above results demonstrate that CTAB plays multiple roles throughout the detachment process by stabilizing the oil–water interface, promoting hydration channel formation, and facilitating progressive oil droplet detachment. In particular, the establishment of a robust hydrogen-bond network provides the fundamental driving force for the final separation of the oil droplet from the pore surface.

To further quantify the interfacial evolution, the overall interaction energy between the oil droplet and its surrounding environment is analyzed, as presented in Figure . At the initial stage, the interaction energy remains positive, indicating that the oil droplet is primarily controlled by its strong adhesion to the SiO₂ surface, while CTAB molecules remain dispersed in the aqueous phase without significantly participating in interfacial regulation. As adsorption proceeds, CTAB molecules gradually assemble at the oil–water interface and form an ordered interfacial layer, leading to enhanced interactions with the oil phase and weakened adhesion between the oil droplet and the solid surface. Consequently, the interaction energy decreases from positive to negative values, marking the onset of the emulsification process. Continuous adsorption of CTAB further stabilizes the oil–water interface, reduces direct contact between the oil droplet and the pore wall, and promotes oil dispersion into the aqueous phase. After approximately 3000 ps, the interaction energy reaches a stable negative value, indicating that the CTAB–oil system has attained a thermodynamically stable equilibrium characterized by the formation of a stable oil-in-water emulsion. These results further confirm that CTAB promotes oil droplet detachment by simultaneously reducing oil–solid interfacial interactions and stabilizing the oil–water interface, providing a molecular basis for optimizing surfactant formulations and improving microscopic oil displacement efficiency.

Figure further presents the temporal evolution of the spatial distribution and migration trajectories of the oil droplets within the nanopore. As the simulation progresses, the oil droplets gradually migrate away from the pore walls and become increasingly concentrated near the center of the pore, indicating a continuous reduction in oil–solid adhesion. This migration behavior reflects the progressive accumulation of CTAB molecules at the interface, which effectively shields the direct contact between the oil droplet and the solid surface. Simultaneously, water molecules continuously infiltrate the oil–solid interface and develop stable hydration channels, further weakening the interfacial adhesion and facilitating oil displacement. Consequently, CTAB enhances both the mobility and dispersion of confined oil droplets by reducing their adhesion to the pore surface, thereby promoting the release and transport of crude oil through complex nanopore networks. Such an interfacial regulation mechanism is particularly beneficial for shale reservoirs, where strong capillary forces and nanoscale confinement severely hinder oil mobility. Therefore, optimizing interfacial interactions and fluid distribution through CTAB adsorption represents an effective strategy for enhancing microscopic oil recovery in shale reservoirs.

To provide quantitative evidence for the proposed three-stage mechanism, the temporal evolution of the available energetic and structural descriptors was systematically analyzed. During stage I, CTAB molecules migrate from the aqueous phase toward the oil–water interfaces. This process is quantitatively reflected by the rapid decrease in the overall interaction energy between the oil droplet and its surrounding environment (Figure ), indicating the progressive establishment of favorable interactions. Following CTAB adsorption, stage II is characterized by water progressively penetrating the region between the oil droplet and the functionalized SiO₂ surface. The relative concentration profiles along the z-direction (Figure ) demonstrate a decrease in oil concentration near the pore walls accompanied by an increase in the interfacial water population, providing structural evidence for progressive oil displacement and hydration-layer development. Finally, during stage III, the interfacial water structure becomes increasingly stabilized. The quantitative hydrogen-bond analysis reveals that the average number of hydrogen bonds per interfacial water molecule increases to approximately 3.3, indicating the development of a highly hydrated network. Concurrently, the interaction-energy profiles approach a relatively stable regime at approximately 3000–5000 ps, accompanied by sustained depletion of oil near the SiO₂ surface.

Importantly, these three stages should not be interpreted as strictly independent or sequential processes. Rather, they represent dominant and partially overlapping interfacial events within a continuous detachment continuum. For instance, CTAB adsorption may continue, while water simultaneously penetrates the oil–solid interface and establishes hydrogen-bond interactions with the polar surface. Furthermore, the characteristic time scales of these overlapping stages are strongly dependent on external conditions. As analyzed in the subsequent sections, increasing the temperature significantly accelerates the interfacial evolution and shortens the time required for the system to approach an energetically stabilized state (Figure ). Similarly, longer-chain oil molecules (such as C18) exhibit slower detachment dynamics due to stronger cohesive interactions, resulting in a slower evolution toward the final stabilized state compared to C12 (Figure ).

9.

Temporal evolution of the overall interaction energy between the oil droplet and its surrounding environment at different temperatures.

12.

Temporal evolution of the interaction energy between different oil droplets and their surrounding environment in the presence of CTAB.

3.3. Effects of Different Factors on CTAB-Induced Oil Droplet Detachment

Under realistic reservoir conditions, the interfacial behavior of confined oil droplets is governed not only by the physicochemical properties of surfactants but also by external factors, such as temperature and crude oil composition. These factors collectively influence molecular diffusion, interfacial adsorption, wettability alteration, and hydration structure formation, thereby determining the efficiency of the surfactant-induced oil droplet detachment. In this section, the effects of the temperature and crude oil composition on the CTAB-induced detachment process are systematically investigated to elucidate the underlying mechanisms.

3.3.1. Effect of Temperature on the Detachment Process

Temperature is one of the most important factors affecting the interfacial behavior of surfactants under reservoir conditions. It regulates molecular diffusion, adsorption kinetics, aggregation behavior, oil–water interfacial tension, wettability alteration, and ultimately the detachment of confined oil droplets. To investigate these effects, molecular dynamics simulations are performed over a temperature range of 333–373 K, and the corresponding detachment behaviors are presented in Figure .

8.

Effect of CTAB-induced oil droplet detachment at different temperatures.

Figure illustrates the evolution of CTAB-induced oil droplet detachment at different temperatures. At relatively low temperatures (333–343 K), the migration and adsorption of CTAB molecules toward the oil–water interface are considerably restricted. Consequently, the interfacial coverage remains sparse and discontinuous, preventing the formation of a complete adsorption layer. Under these conditions, the oil droplet maintains a relatively large size and remains tightly attached to the SiO₂ surface, indicating that the oil–solid interaction is still dominant. As a result, CTAB exhibits only a limited ability to reduce the oil–water interfacial tension and weaken capillary confinement, and the system remains at the initial stage of emulsification with relatively poor interfacial stability.

As the temperature increases to 353 K, the diffusion coefficient and interfacial mobility of CTAB molecules increase significantly, enabling the rapid formation of a dense and continuous adsorption layer at the oil–water interface. The hydrophobic alkyl chains penetrate into the oil phase, whereas the hydrophilic headgroups orient toward the aqueous phase, resulting in a substantial reduction in interfacial free energy and reconstruction of the interfacial structure. Consequently, the contact area between the oil droplet and the SiO₂ surface decreases markedly, indicating that the oil–solid interaction is effectively weakened. Simultaneously, both emulsification and oil droplet detachment become significantly enhanced, and the system gradually evolves toward a stable oil-in-water emulsion.

When the temperature is further increased to 363–373 K, the adsorption capability of CTAB is further enhanced, leading to the formation of a highly stable interfacial film surrounding the oil droplet. This compact adsorption layer effectively suppresses oil–solid interactions, allowing the oil droplet to detach completely from the pore surface and transform into a stable oil-in-water structure. Under these conditions, the oil–water interfacial tension is substantially reduced, while the dispersion stability and mobility of the oil droplet are significantly improved, demonstrating that the interfacial regulation capability of CTAB is fully exploited at elevated temperatures.

The observed temperature dependence of oil droplet detachment is governed by a synergistic combination of kinetic and thermodynamic factors rather than simple adsorption enhancement. Kinetically, elevated temperatures significantly accelerate the mobility of the surfactant molecules. To quantify this kinetic effect, the self-diffusion coefficients of CTAB in the aqueous phase were calculated using the Einstein relation based on the mean square displacement, as expressed in eq :

$$D=\frac{1}{6}\frac{\mathrm{d}}{\mathrm{d}t}⟨{|r(t)-r(0)|}^2⟩$$ 2

where r(t) and r(0) are the position vectors of the surfactant molecules at time t and initial time 0, respectively, and the angle brackets ⟨···⟩ denote the ensemble average over all surfactant molecules. The diffusion coefficient of CTAB increases substantially from 1.15 × 10^–9 m²/s at 333 K to 2.42 × 10^–9 m²/s at 373 K.

This enhanced diffusivity directly accelerates the migration of CTAB toward the oil–water and oil–solid interfaces, facilitating the rapid adsorption kinetics observed in the initial stage. Thermodynamically, higher temperatures simultaneously reduce the viscosity of the crude oil droplet and the oil–water interfacial tension, which mutually promote oil droplet deformation and dispersion under confinement. Meanwhile, the interaction energy analysis demonstrates that the equilibrium interaction between the oil droplet and the solid pore surface becomes weaker at elevated temperatures, indicating significantly reduced interfacial adhesion. Furthermore, while the average lifetime of individual hydrogen bonds may slightly decrease at elevated temperatures due to intensified thermal fluctuations, the accelerated molecular mobility ensures a faster reconstruction rate of the interfacial water molecules, thereby maintaining the overall macroscopic stability of the hydration channels. Therefore, the enhanced detachment efficiency at higher temperatures is a coupled outcome of accelerated surfactant diffusion, reduced oil viscosity, reduced interfacial adhesion, and dynamically stable hydration layer reconstruction.

To further quantify the influence of temperature on the interfacial evolution, the temporal variation of the interaction energy is analyzed, as shown in Figure . During the initial stage, the interaction energy decreases rapidly under all temperature conditions, indicating that CTAB molecules quickly diffuse toward and adsorb onto the oil–water interface. As the simulation proceeds, however, distinct temperature-dependent behaviors emerge. At temperatures of 353 K and above, the interaction energy continues to decrease and eventually stabilizes at approximately −900 kJ mol^–1, whereas at lower temperatures, the equilibrium interaction energy remains around −700 kJ mol^–1. The lower equilibrium energy obtained at elevated temperatures indicates stronger intermolecular interactions between CTAB and the oil phase, more complete interfacial adsorption, and enhanced thermodynamic stability of the emulsified system. Moreover, these interaction energy profiles quantitatively illustrate the varying time scales of the detachment process; elevated temperatures significantly accelerate the initial energy decline, thereby shortening the transition time between the initial adsorption and the subsequent hydration channel formation stages.

Overall, temperature exerts a pronounced influence on CTAB-induced oil droplet detachment by simultaneously regulating molecular diffusion and interfacial adsorption. Increasing temperature accelerates the migration of CTAB molecules toward the oil–water interface, promotes the formation of a dense adsorption layer, and facilitates the development of hydration structures at the oil–solid interface. These synergistic effects continuously weaken the oil–solid interaction and ultimately promote oil droplet detachment. In particular, when the temperature exceeds 353 K, CTAB exhibits significantly enhanced interfacial regulation capability, stronger adsorption stability, and more efficient hydration channel formation, thereby enabling complete oil droplet detachment and the formation of a stable oil-in-water emulsion. These findings demonstrate that elevated temperatures are favorable for improving surfactant flooding performance under nanoscale confinement and provide valuable molecular-level insights for optimizing EOR in shale reservoirs.

It is important to note that the observed enhancement in detachment efficiency with increasing temperature cannot be extrapolated indefinitely. While CTAB exhibits robust performance within the investigated temperature window (333–373 K), its application at extreme reservoir temperatures (e.g., >120–150 °C) may be constrained by its intrinsic thermal stability. Literature indicates that under ultrahigh-temperature aqueous conditions, quaternary ammonium surfactants like CTAB are susceptible to chemical degradation, primarily through mechanisms such as Hofmann elimination or nucleophilic substitution. These high-temperature degradation pathways lead to the irreversible cleavage of the cationic trimethylammonium headgroup, which would result in a severe loss of interfacial activity and the ultimate collapse of the stable hydration network. Therefore, while elevated temperatures thermodynamically and kinetically favor oil detachment within a moderate thermal window, the chemical integrity of the surfactant ultimately dictates the upper temperature limit for its practical application in deep shale reservoirs.

3.3.2. Effect of Crude Oil Composition on the Detachment Process

Shale oil is a complex multicomponent hydrocarbon mixture consisting of light, medium, and heavy fractions with distinct molecular structures and physical and chemical properties. These compositional differences play a crucial role in determining the transport behavior, intermolecular interactions, and interfacial properties of crude oils under nanoscale confinement. Light hydrocarbons generally possess a lower molecular weight, lower viscosity, and higher mobility, whereas heavy hydrocarbons exhibit stronger intermolecular cohesion, higher viscosity, and more limited transport capability. In addition to the medium-component model (C12) adopted in the preceding sections, n-hexane (hereafter denoted as C6) and n-octadecane (hereafter denoted as C18) are selected in this study as representative models of light and heavy crude oils, respectively. Molecular dynamics simulations were subsequently performed to investigate the CTAB-induced detachment process in nanopores. By monitoring the temporal evolution of interfacial interactions and the spatial distribution of oil droplets, we systematically elucidate the distinct mechanisms governing CTAB-assisted displacement of different crude oil components.

Figure illustrates the emulsification and detachment behavior of the light oil system induced by CTAB. At the initial stage, the oil droplet is surrounded by the aqueous phase, while only a limited number of CTAB molecules adsorb at the oil–water interface, resulting in weak interfacial regulation. As the simulation progresses, CTAB molecules gradually accumulate at the interface and continuously reduce the oil–water interfacial tension, leading to a smoother oil droplet boundary and the gradual penetration of water molecules into the interfacial region. Between 1500 and 2500 ps, CTAB forms a stable adsorption layer surrounding the oil droplet, which gradually evolves into a more spherical morphology with significantly improved dispersion. At 5000 ps, a dense and continuous CTAB adsorption layer completely encapsulates the oil droplet, resulting in the formation of a stable oil-in-water emulsion. These observations indicate that CTAB effectively promotes the dispersion and stabilization of light oil primarily through interfacial adsorption and interfacial structure reconstruction, thereby substantially reducing the interfacial free energy and enhancing the wettability of the aqueous phase.

10.

Evolution of CTAB-induced emulsification and detachment of light oil.

The detachment behavior of the heavy oil system is presented in Figure . Owing to the strong intermolecular cohesion among long-chain hydrocarbons, the oil molecules initially aggregate into a compact cluster, and CTAB has little influence on the interface. After approximately 700 ps, CTAB molecules gradually migrate toward the oil–water interface, where their hydrophobic tails penetrate into the oil phase, while the hydrophilic headgroups remain exposed to the aqueous phase. This interfacial adsorption lowers the interfacial free energy and initiates the fragmentation of the large oil cluster. During the subsequent evolution (1500–3000 ps), CTAB progressively forms a homogeneous adsorption layer around the fragmented oil droplets, facilitating their continuous dispersion into smaller droplets while effectively suppressing recoalescence. At 5000 ps, the oil phase is completely dispersed throughout the aqueous phase and encapsulated by a dense CTAB adsorption layer, indicating that the system has reached a thermodynamically stable oil-in-water configuration.

11.

Evolution of CTAB-induced emulsification and detachment of heavy oil.

A comparison of the two systems reveals that CTAB effectively regulates the interfacial behavior of different crude oil components by reducing the oil–water interfacial free energy and constructing stable adsorption films. However, the underlying mechanisms differ substantially depending on crude oil composition. For the light-oil system, the relatively weak intermolecular interactions and high intrinsic mobility allow oil droplets to be readily dispersed. Consequently, CTAB mainly functions by rapidly adsorbing at the oil–water interface, reducing interfacial free energy, and stabilizing the reconstructed interface without significantly disrupting the internal structure of the oil droplet. In contrast, heavy oil exhibits much stronger intermolecular cohesion and exists as highly aggregated clusters. In this case, CTAB first penetrates the oil–water interface through its hydrophobic chains, thereby weakening the cohesive interactions among hydrocarbon molecules and inducing the fragmentation of large oil clusters into smaller droplets. Subsequently, the formation of a stable adsorption layer inhibits droplet recoalescence and maintains long-term dispersion. Therefore, the detachment of heavy oil follows a distinct two-step mechanism, namely, cluster fragmentation, followed by interfacial stabilization, in which the rate-limiting process shifts from interfacial regulation to overcoming the cohesive energy of the oil phase. These distinct molecular mechanisms provide valuable insight into surfactant-assisted oil displacement in compositionally complex shale reservoirs.

To quantitatively evaluate the influence of crude oil composition on the interfacial regulation capability of CTAB, the temporal evolution of the overall interaction energy between the oil droplet and its surrounding environment for three representative crude oil models is further analyzed, as shown in Figure . Although all three systems gradually evolve from an initially fluctuating state toward thermodynamic equilibrium, significant differences are observed in both the equilibrium interaction energies and the adsorption kinetics.

For the C6 system, the interaction energy exhibits relatively large fluctuations and remains at a comparatively high level during the early stage, indicating weak interfacial interactions between CTAB and the light-oil molecules. This behavior is mainly attributed to the short molecular chains and weak cohesive interactions of C6, which make it difficult for CTAB molecules to assemble into a continuous and compact interfacial adsorption layer, thereby limiting interfacial stabilization and emulsification efficiency. In contrast, the C12 system exhibits a rapid decrease in interaction energy, followed by stable convergence to the lowest equilibrium energy among the three systems. This result indicates that CTAB forms a highly ordered and stable adsorption layer on the C12 oil droplets, effectively reducing the interfacial free energy and stabilizing the emulsion structure. The superior adsorption kinetics and the lowest equilibrium interaction energy suggest that CTAB achieves the most favorable interfacial compatibility with medium-chain hydrocarbons. For the C18 system, the interaction energy exhibits relatively small fluctuations during the initial stage but ultimately stabilizes at a higher energy level than that of the C12 system. This behavior indicates that the effective interaction between CTAB and heavy oil is partially hindered. The long hydrocarbon chains and strong cohesive interactions within C18 significantly restrict the penetration, rearrangement, and adsorption of CTAB molecules at the oil–water interface, thereby reducing the efficiency of interfacial regulation. This dynamic is quantitatively reflected in the prolonged interaction energy fluctuations observed in Figure , indicating that the stronger molecular cohesion in heavy oil noticeably delays the energy stabilization process and significantly prolongs the overall time scale required for complete oil droplet detachment.

Overall, the crude oil composition exerts a pronounced influence on the interfacial behavior of CTAB under nanoscale confinement. Among the three model systems investigated, the C12 system exhibits an optimal balance between adsorption strength and interfacial structural stability, allowing CTAB to fully exploit its interfacial activity and emulsification capability. These findings demonstrate that the performance of CTAB is dependent on the crude oil composition at the molecular level and further highlight the necessity of developing composition-specific surfactant strategies for complex shale oil reservoirs. Such insights provide a theoretical foundation for the rational design and optimization of surfactant flooding systems under different reservoir conditions.

It should be noted that while n-hexane, n-dodecane, and n-octadecane effectively capture the fundamental hydrocarbon chain-length effects, they are simplified representations of real crude oil, which is a complex mixture comprising alkanes, aromatics, resins, and asphaltenes. Acknowledging this limitation is crucial, as the presence of highly polar and aromatic components, such as asphaltenes, would significantly alter the detachment dynamics. These heavy fractions typically exhibit strong interfacial activity and tend to firmly adsorb onto the mineral surface via polar interactions, thereby increasing the oil–solid adhesion and capillary resistance. In such realistic scenarios, the distinctive two-stage detachment mechanism (cluster fragmentation, followed by interfacial stabilization) observed for the C18 model is expected to be not only applicable but even more pronounced. The strong π–π stacking, hydrogen bonding, and structural entanglement among asphaltene and resin molecules would make the initial cluster fragmentation stage much more energy-intensive and time-consuming. Consequently, surfactant molecules must first overcome these formidable intraoil cohesive forces before establishing a stable hydration layer. While our current single-component models successfully isolate the basic chain-length effects to provide a foundational baseline, future investigations incorporating multicomponent mixtures with asphaltenes will be pursued to capture the complete compositional complexity of real shale oil.

Furthermore, these molecular-level insights into the distinct two-stage detachment mechanism of heavy oil hold significant practical implications for the optimal design of surfactant flooding in heavy-oil shale reservoirs. Since the rate-limiting step for heavy oil mobilization involves overcoming strong intermolecular cohesive forces, conventional single-surfactant formulations may be inefficient. Instead, our findings strongly advocate for the development of dual-functional surfactant formulations or sequential injection strategies. For instance, the flooding fluid could initially incorporate a highly penetrative cosolvent or a low-molecular-weight penetrant designed specifically to rapidly disrupt and fragment the strongly cohesive heavy oil aggregates (facilitating stage 1). This should be combined with a robust interfacial stabilizersuch as a long-chain or polymeric surfactantthat rapidly adsorbs onto the newly created oil–water interfaces, establishing a dense hydration barrier to effectively suppress the recoalescence of the smaller, fragmented oil droplets (facilitating stage 2). By rationally tailoring chemical formulations to explicitly address both the fragmentation and stabilization barriers, the overall displacement efficiency of heavy oil in nanoscale confinement can be substantially improved.

3.4. Applicability, Limitations, and Future Perspectives

Although this study provides fundamental molecular-level insights into the CTAB-induced detachment mechanism, it is imperative to acknowledge the inherent limitations and specify the applicability of the current idealized models. The present system employs single-component alkanes and a structurally uniform carboxylated SiO₂ slit pore to effectively isolate and elucidate the underlying thermodynamic and kinetic mechanisms of surfactant–oil–solid interactions. However, actual shale reservoirs exhibit immense complexity, characterized by highly heterogeneous mineralogies (e.g., mixed clays, carbonates, and intricate kerogen networks), irregular pore morphologies with distinct nanoscale roughness, and multicomponent crude oil mixtures containing polar species such as asphaltenes and resins. These realistic complexities inevitably introduce additional spatial confinement effects, highly competitive adsorption sites, and intricate intraoil structural entanglements that are not fully captured by a simplified model. Therefore, while our present model establishes a robust and reliable theoretical baseline for interfacial mechanisms, future research efforts will be dedicated to developing more realistic molecular reservoir models. These advanced models will incorporate multicomponent heavy oil mixtures, heterogeneous mixed-wet mineral substrates, and rough pore geometries to further bridge the gap between microscopic molecular dynamics and macroscopic enhanced oil recovery (EOR) engineering in realistic shale formations.

4. Conclusions

In this study, molecular dynamics simulations are performed to systematically investigate the microscopic behavior of CTAB-induced oil droplet detachment in shale nanopores. By comparing the performance of different surfactants, the interfacial regulation advantages of CTAB are identified. Furthermore, the underlying molecular mechanisms governing oil displacement are elucidated, and the effects of temperature and crude oil composition on the detachment process are systematically analyzed. The main conclusions are summarized as follows.

• (1)Among the investigated surfactants, CTAB exhibits the most effective interfacial regulation capability. Compared with SDS, PGFE, and BS-12, CTAB achieves a higher interfacial coverage and a more uniform adsorption distribution on the oil-droplet surface. In addition, the interaction energy decreases more rapidly and reaches the lowest equilibrium value, indicating the formation of a more stable interfacial adsorption layer and, consequently, a higher oil droplet detachment efficiency.
• (2)The fundamental mechanism of CTAB-induced oil droplet detachment arises from the synergistic effect of interfacial adsorption and hydration-channel formation. CTAB molecules initially adsorb at the oil–water interface through hydrophobic interactions and subsequently migrate toward the oil–solid interface under electrostatic interactions. This process progressively weakens the oil–solid adhesion while inducing the penetration of water molecules into the interfacial region to construct stable hydration channels, ultimately leading to the complete detachment of oil droplets from the pore surface.
• (3)Temperature and crude oil composition significantly influence the detachment behavior. Increasing the temperature above 353 K markedly enhances the interfacial adsorption of CTAB and improves oil displacement efficiency. Among the investigated crude oil components, CTAB exhibits the highest detachment efficiency for the C12 system, whereas the detachment of heavy oil follows a distinctive two-stage mechanism characterized by an initial disaggregation, followed by interfacial stabilization.

This study provides a molecular-level understanding of the interfacial mechanisms by which CTAB promotes the oil displacement in shale nanopores. The findings offer theoretical insights into the rational design of high-performance surfactant systems and provide guidance for improving the shale oil recovery in unconventional reservoirs.

All relevant data are included in the paper or its Supporting Information.

This work was supported by the Oil & Gas Major Project (2025ZD1405400), and the APC was funded by Yangnan Shangguan.

The authors declare no competing financial interest.