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Pore-scale numerical simulation of in situ microemulsion formation and enhanced oil recovery in porous media

Pore-scale numerical simulation of in situ microemulsion formation and enhanced oil recovery in... TYPE Original Research PUBLISHED 19 May 2025 DOI 10.3389/fchem.2025.1601086 Pore-scale numerical simulation of in situ microemulsion OPEN ACCESS EDITED BY Yixin Lu, formation and enhanced oil National University of Singapore, Singapore REVIEWED BY recovery in porous media Sijia Wang, Hebei University of Technology, China Liewei Qiu, 1 1 2 1,3 1 Yingxue Hu , Kai Dong , Dan Zhang , Tianjiang Wu , Wei Xu * Xi’an Polytechnic University, China and Zhaolin Gu *CORRESPONDENCE Wei Xu, 1 2 School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an, China, Key [email protected] Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an, China, Oil and Gas Technology Research RECEIVED 27 March 2025 Institute of Changqing Oilfield, China National Petroleum Corporation, Xi’an, China ACCEPTED 05 May 2025 PUBLISHED 19 May 2025 CITATION Hu Y, Dong K, Zhang D, Wu T, Xu W and Gu Z (2025) Pore-scale numerical simulation of in In situ microemulsion has emerged as an advanced tertiary oil recovery technique situ microemulsion formation and enhanced oil that utilizes the injection of surfactant solutions to improve displacement recovery in porous media. Front. Chem. 13:1601086. efficiency through spontaneous microemulsification. This study presents a doi: 10.3389/fchem.2025.1601086 novel pore-scale numerical model to simulate the dynamic process of in situ COPYRIGHT microemulsion formation during surfactant-cosolvent-salt flooding in complex © 2025 Hu, Dong, Zhang, Wu, Xu and Gu. This is porous media. Through comprehensive numerical simulations based on realistic an open-access article distributed under the rock geometries, we systematically investigated the spatiotemporal evolution of terms of the Creative Commons Attribution License (CC BY). The use, distribution or phase distributions and identified critical mechanisms governing oil mobilization. reproduction in other forums is permitted, The developed model incorporates four fundamental characteristics of provided the original author(s) and the microemulsion systems: interfacial tension reduction, viscosity modification, copyright owner(s) are credited and that the original publication in this journal is cited, in wettability alteration, and enhanced solubilization capacity. During the accordance with accepted academic practice. microemulsion-forming surfactant flooding in a realistic rock medium, the in No use, distribution or reproduction is situ formed microemulsion was observed at the interface between oil and permitted which does not comply with these terms. aqueous. The in situ microemulsion flooding can significantly improve the recovery rate under the combined effect of multiple factors. Increasing the viscosity of the in situ formed microemulsion can enhance the oil recovery during the microemulsion-forming surfactant flooding in the complex porous media. Under water-wet conditions, the oil-water interface stays at the junction of the throat and the pore space, which contributes to the formation of microemulsions and thus to the enhancement of recovery. This study provides a better understanding of the in situ microemulsion formation and the mechanisms of enhanced oil recovery in complex porous media. KEYWORDS in situ microemulsion, porous media, solubilization, viscosity modification, porescale 1 Introduction Tertiary oil recovery, particularly chemical flooding, has emerged as an essential strategy to enhance oil recovery from residual and remaining oil post-water flooding (Park et al., 2021; Tavakkoli et al., 2022). Among chemical flooding techniques, microemulsion flooding has gained significant attention due to its unique ability to reduce interfacial tension (IFT), alter wettability, and mobilize trapped residual oil in porous media (Qin et al., 2020; Das et al., 2021; Wu et al., 2024; Koreh et al., 2025). Frontiers in Chemistry 01 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 FIGURE 1 Oil and aqueous distribution in capillary tube after injection of (A) water and (B) microemulsion-forming surfactant solution. Considering the viscosity enhancement effect of microemulsion during the two-phase displacement process. Black and blue represents oil and water, respectively. The unique physical properties of microemulsions, such as solubilization capacity, ultra-low IFT, wettability alteration, and viscosity modulation, collectively contribute to enhanced oil recovery (EOR). Solubilization enables microemulsions to dissolve crude oil and water, improving oil mobility (Wei et al., 2011). Ultra-low IFT reduces the oil-water interfacial energy, facilitating the detachment of residual oil (Das et al., 2021; Li et al., 2025). Wettability alteration adjusts the wetting state of rock surfaces, optimizing oil-water flow paths, while viscosity modulation enhances the sweep efficiency of displacing fluids (Yang et al., 2021). These properties make microemulsions highly effective in EOR applications. Two primary methods are employed to utilize microemulsions for EOR: ex-situ and in situ. The ex-situ method involves injecting pre-prepared microemulsions into the reservoir (Zhou et al., 2020). While this approach has demonstrated significant oil recovery enhancement, it faces challenges such as high injection pressures FIGURE 2 Pressure difference between the inlet and the outlet during water and potential incompatibility with reservoir conditions (Zhu et al., and surfactant microemulsion-forming surfactant flooding. 2022). In contrast, the in situ method involves injecting a Considering the viscosity enhancement effect of microemulsion. microemulsion-forming surfactant system into the formation, where microemulsions form spontaneously within the pores (Mo et al., 2023). This method leverages the natural conditions of the reservoir, offering better adaptability and cost-effectiveness. Microemulsions, thermodynamically stable dispersions of However, the pore-scale mechanisms governing microemulsion- surfactant, cosolvent, salt, and oil, have demonstrated remarkable assisted oil recovery remain poorly understood, limiting the success in improving oil recovery. optimization of this technology for field applications. Microemulsions are classified into three types based on their Advanced visualization techniques, including X-ray computed phase behavior: Winsor I (oil-in-water, O/W), Winsor II (water-in- tomography (Hu et al., 2020), nuclear magnetic resonance oil, W/O), and Winsor III (bicontinuous). Winsor I microemulsions (Govindarajan et al., 2020), and microfluidics (Fu et al., 2016), consist of nano-scale oil droplets dispersed in a continuous water have facilitated the analysis of microemulsion formation and EOR in phase, stabilized by surfactant molecules at the oil-water interface. complex porous media. These technologies provide high-precision Winsor II microemulsions feature water droplets dispersed in a experimental data, revealing the formation mechanisms, phase continuous oil phase, while Winsor III microemulsions exhibit a behavior, and oil displacement efficiency of microemulsions. For bicontinuous structure where both oil and water phases are instance, Ott et al. (2020) employed micro-CT to investigate the in interconnected, forming a sponge-like network (Scriven, 1976; situ emulsification behavior of surfactants and crude oil during alkali Zhao et al., 2025). The formation of these microemulsions is flooding, providing critical insights into the changes in interfacial highly dependent on surfactant concentration, cosurfactant tension and oil-water phase distribution. Unsal et al. (2019) utilized selection, salinity, and oil-to-water ratio (Budhathoki et al., 2016). synchrotron-based X-ray micro-tomography to study the dynamic Salinity and oil-water ratio scanning experiments are critical for changes in emulsification under optimal salinity conditions, identifying optimal conditions for microemulsion formation, highlighting the critical role of salinity in emulsification efficiency ensuring maximum oil recovery efficiency (Zhou et al., 2020; Alzahid et al. (2019) systematically investigated the pore-scale flow Mariyate and Bera, 2022). characteristics of surfactant flooding under varying salinity Frontiers in Chemistry 02 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 FIGURE 3 Oil and aqueous distribution in capillary tube after injection of (A) water and (B) microemulsion-forming surfactant solution. Considering the interfacial tension reduction effect of microemulsion during the two-phase displacement process. Black and blue represents oil and water, respectively. alkali flooding, revealing the formation, migration, and trapping mechanisms of emulsions in porous media. Despite these advancements, visualizing the dynamic distribution of microemulsion concentrations within pores remains challenging due to the temporal and spatial limitations of X-ray CT. Two-dimensional micromodels with high-speed cameras have been employed to capture near real-time emulsification dynamics. For example, Unsal et al. (2016) used microfluidic flow experiments to study the in situ formation of microemulsions by co-injecting n-decane and surfactant solutions into a T-junction capillary geometry. Broens and Unsal (2018) investigated emulsification kinetics in quasi-miscible flow conditions, while Tagavifar et al. (2017) analyzed phase formation and spatial configurations in a 2.5-dimensional (2.5D) micromodel. Zhao et al. (2022) demonstrated in situ emulsification using on-chip experiments, and Xu et al. (2024) studied the formation and transport mechanisms of microemulsions in fractured porous micromodels. Nevertheless, current visualization FIGURE 4 Pressure difference between the inlet and the outlet during water techniques remain inadequate for accurately quantifying and microemulsion-forming surfactant flooding. Considering the microemulsion concentrations within complex pore structures, interfacial tension reduction effect of microemulsion. thereby constraining the comprehensive understanding of microemulsion formation and enhanced oil recovery (EOR) mechanisms. Pore-scale numerical simulations have emerged as a promising approach for investigating oil-water interface dynamics conditions, demonstrating the significant impact of salinity on in porous media during chemical flooding processes, including emulsified phase properties and oil-water distribution. She et al. surfactant, polymer, and nanoparticle flooding. However, (2021) further advanced the field by employing three-dimensional microemulsion flooding presents additional complexities, as it visualization techniques to study the role of in situ emulsification in FIGURE 5 Oil and aqueous distribution in capillary tube after injection of (A) water and (B) microemulsion-forming surfactant solution. Considering the solubilization effect of microemulsion during the two-phase displacement process. Black and blue represents oil and water, respectively. Frontiers in Chemistry 03 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 2 Numerical method This study employs the conventional Volume of Fluids (VoF) method to simulate two-phase displacement within porous media and accurately track the interface between water and oil. In the VoF method, an indicator function, denoted as α,is defined throughout the flow domain, representing the volume fraction of one of the fluids within each computational grid cell. Specifically, α = 1 indicates that a cell is entirely occupied by water, while α = 0 signifies that the cell is completely filled with oil. The indicator function serves as a critical tool for identifying and tracking the interface between the two phases. The governing equations for the system, which describe the behavior of two incompressible, isothermal, and immiscible fluids (such as water and oil), are presented as follows: ∂ρu + ∇ · ρuu − ∇ · μ ∇u +() ∇u −∇p + F (1) ∂t FIGURE 6 Oil recovery efficiency during water and microemulsion-forming ∇ · u 0(2) surfactant flooding. Considering the solubilization effect of ρ  αρ +() 1 − α ρ (3) w o microemulsion. μ  αμ +() 1 − α μ (4) w o In the above equations, Equations 1, 2 are momentum and involves not only multiphase flow but also requires consideration of continuity equations of the fluid, where ρ, u, p, and F are density, multiphase mass transfer processes within intricate pore networks fluid velocity, dynamic pressure, and surface tension, respectively. t that has received limited research attention to date. is the time. The effect of gravity is ignored in two-dimensional To overcome these research gaps, the present study develops a models. α denotes the volume fraction of the water phase and novel pore-scale simulation framework to systematically examine correspondingly (1−α) the oil phase in Equations 3, 4. microemulsion formation and EOR mechanisms in porous media. The surface tension F in Equation 2 is modeled as continuum By analyzing microemulsion behavior in a simplified T-junction surface force (CSF) and calculated by structure, we aim to elucidate the roles of IFT reduction, wettability F  σκ∇α (5) alteration, viscosity enhancement, and solubilization in oil displacement. Furthermore, a real complex porous medium is where σ is the surface tension coefficient, and κ is the curvature of reconstructed from digital rock, and the pore-scale dynamics of the water-oil interface in Equation 5, which can be approximated the emulsification process are coupled with fluid flow. This study as follows leverages advanced imaging techniques and numerical simulations κ −∇ · n (6) to provide a comprehensive pore-scale investigation of microemulsion-enhanced oil recovery, bridging the gap between n is the interface normal unit vector (in Equations 6-8) given by laboratory-scale observations and field-scale applications. The ∇α findings are expected to contribute to the development of more n  (7) || ∇α effective EOR strategies, ultimately improving oil recovery efficiency and reducing the environmental footprint of hydrocarbon To accurately model the wettability of the porous media, the extraction. contact angle is imposed as a wetting boundary condition. The FIGURE 7 Oil and aqueous distribution in capillary tube after injection of (A) water and (B) microemulsion-forming surfactant solution. Considering the wettability alternation effect of microemulsion during the two-phase displacement process. Black and blue represents oil and water, respectively. Frontiers in Chemistry 04 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 The source term is calculated by: S −k CC() − C δ (12) a α e e∞ where k and δ are the rate of emulsification, and specific interfacial area between oil and water in Equations 11, 12. The mathematical model has been implemented within the open-source computational fluid dynamics (CFD) platform OpenFOAM. The governing equations are discretized using a finite-volume method and solved sequentially. The pressure–velocity coupling is addressed through a predictor–corrector approach, leveraging the Pressure-Implicit with Splitting of Operators (PISO) algorithm to ensure numerical stability and accuracy. 3 Results and discussion FIGURE 8 3.1 Modelling the unique physical properties Oil recovery efficiency during water and microemulsion-forming of microemulsions surfactant flooding. Considering the wettability alternation effect of microemulsion. Microemulsions exhibit several unique physical properties that contribute to enhanced oil recovery. The core mechanism involves the formation of nanoscale micellar systems facilitated normal unit vector of the interface at the wall boundary is adjusted by surfactants, which lead to viscosity enhancement, interfacial according to the following equation: tension reduction, solubilization, and wettability alteration. n  n cos θ + n sin θ (8) p t These properties often interact synergistically during the oil displacement process rather than acting independently. where n is the unit vector perpendicular to the wall, n is the unit vector p t However, to validate the capability of numerical simulations tangential to the wall, and θ is the contact angle. In this way, the in capturing these properties and to analyze the individual interface can form the prescribed angle θ when in contact with the wall. contribution of each factor to oil recovery, this section In this study, we will only focus on the Winsor I (oil in water, examines each physical property separately. O/W) microemulsion. The surfactant system forming the microemulsion is an aqueous solute and the microemulsion 3.1.1 Viscosity enhancement formed is treated as a solute in water. To investigate the effect of viscosity enhancement, water and a To track the concentration of microemulsion-forming microemulsion-forming surfactant were injected into a capillary surfactant system in water, the mass conservation equation for tube initially saturated with oil. The flow rates of both fluids were the surfactant is calculated: maintained at the same level. In this scenario, the effects of ∂αC interfacial tension reduction, solubilization, and wettability + ∇ ·() Cαu  ∇ ·() αD ∇C − S (9) c c ∂t alteration were intentionally excluded from consideration. Notably, the viscosity of the microemulsion-forming surfactant where α is 1, representing surfactant in water phase, C and D are the system is comparable to that of water. Therefore, any observed concentration and diffusion coefficient of surfactant in water, differences in oil-water displacement within the capillary tube can be respectively. The source term S denotes the rate of consumption attributed solely to the viscosity enhancement resulting from the in of surfactant in Equation 9. situ formation of microemulsions. To track the concentration of microemulsion in water, the mass Figure 1 illustrates the distribution of oil and aqueous in the conservation equation for the microemulsion is calculated: capillary tube following the injection of water and the ∂αC microemulsion-forming surfactant. A two-dimensional capillary + ∇ ·() αC u − ∇ ·() αD ∇C  S (10) e e e e ∂t tube with a diameter of 1 mm and a length of 10 mm was used in this study. As depicted in Figure 1B, the injection of the surfactant where α is 1, representing microemulsion in water phase, C and D e e led to the in situ formation of microemulsions near the oil-water are the concentration and diffusion coefficient of microemulsion in interface. The red regions in the figure indicate a high concentration water, respectively. The source term S denotes the rate of of microemulsion within the aqueous phase. The microemulsion production of microemulsion in Equation 10. accumulates along the walls of the capillary tube due to the higher The evolution of water or oil phase is governed by the mass flow velocity in the center of the tube. Although no significant balance equation: differences are observed at the oil-water interface between the two ∂α injection fluids, the flow resistance during injection differs due to the + ∇ ·() αu  S (11) ∂t viscosity-enhancing effect of the microemulsion. Frontiers in Chemistry 05 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 FIGURE 9 Oil and water distribution in porous media during microemulsion-forming surfactant flooding at different: (A) 0.1 s, (B) 0.4 s, (C) 0.8 s and (D) 1.2 s. Figure 2 presents the pressure difference between the inlet and between oil and water and between oil and microemulsion were set outlet during the injection of water and the microemulsion-forming to 50 mN/m and 0.01 mN/m, respectively. Figure 3 compares the surfactant. The negative pressure difference values indicate that the distribution of oil and aqueous in the capillary tube after the flooding process is driven by spontaneous imbibition, a consequence injection of water and the microemulsion-forming surfactant. of the strongly water-wet conditions. Initially, the absolute value of The results demonstrate that the interfacial tension of the the pressure difference increases sharply as a meniscus form at the microemulsion significantly influences the meniscus formation at oil-water interface. Subsequently, as the injection continues, the the oil-water interface. Specifically, the thickness of the in situ- pressure difference between the inlet and outlet increases at a slower formed microemulsion layer with ultra-low interfacial tension is rate. Overall, after the establishment of the interface, the surfactant- considerably smaller than that observed under high interfacial driven flooding exhibits a lower pressure difference compared to tension conditions (Figure 1B). In the capillary tube, capillary water flooding. This difference arises from the in situ formation of a forces drive the oil-water displacement process. According to the more viscous microemulsion during surfactant flooding. While this Young–Laplace equation, higher interfacial tension enhances increased viscosity imposes greater flow resistance within the capillary forces, leading to increased imbibition rates. However, capillary tube, it can lead to improved oil recovery in actual the ultra-low interfacial tension associated with microemulsions reservoir conditions. alters this dynamic, reducing the capillary forces and thereby affecting the displacement process. 3.1.2 Interfacial tension reduction Figure 4 illustrates the pressure difference between the inlet and To examine the effect of interfacial tension reduction on oil- outlet during the injection of water and the microemulsion-forming water displacement in porous media, the interfacial tension values surfactant, considering the effect of interfacial tension reduction. Frontiers in Chemistry 06 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 With sufficient injection time, the residual oil in the dead-end pore can be entirely recovered. Figure 6 presents the oil recovery efficiency profiles during water and microemulsion-forming surfactant injection, highlighting the solubilization effect. Before breakthrough, the oil saturation within the pore space decreases linearly as water or microemulsion-forming surfactant is injected. At the pinch-off stage, the continuous oil phase within the pore is divided into two parts: one portion becomes trapped in the pore, while the other is displaced out of the pore. This process occurs rapidly, resulting in a sharp decline in oil saturation, as depicted in Figure 6. After breakthrough, the oil saturation during water flooding stabilizes, indicating no further recovery. In contrast, during microemulsion-forming surfactant flooding, the oil saturation continues to decrease gradually due to the solubilization effect. Consequently, the ultimate oil recovery efficiency of microemulsion-forming surfactant flooding is significantly higher than that of water flooding, a result attributed FIGURE 10 to the solubilization effect of in situ-formed microemulsions. Oil recovery efficiency during microemulsion-forming surfactant flooding. 3.1.4 Wettability alternation In microemulsions, surfactants adsorb onto rock surfaces, shifting their wettability from oil-wet to water-wet. This allows Notably, the pressure difference exhibits contrasting behaviors for the aqueous phase to penetrate pores more effectively, detach oil the two flooding processes: it is negative for water flooding but films, and carry the residual oil out. In order to investigate how in positive for microemulsion-forming surfactant flooding. This situ formation of microemulsions provides recovery through indicates that capillary forces act as a driving force during water wettability alternation, two kinds of wall wettability were set with ° ° flooding but as a resistance force during microemulsion-forming contact angles of 45 and 15 , respectively. Figure 7 shows the surfactant flooding. As expected, the magnitude of the pressure dynamics distribution of oil and aqueous phase in the T-junction difference during microemulsion-forming surfactant flooding is model. Cloud diagram showing the concentration distribution of significantly smaller, approximately 5.2 Pa, compared to 86 Pa microemulsion in water at steady-state. At contact angle of 45 ,a during water flooding. For microemulsion-forming surfactant residual oil droplet was formed within the pore space after flooding, the pressure difference is primarily attributed to flow microemulsion-forming surfactant flooding, whereas when the resistance, with capillary forces playing a minor role. contact angle was alternated to 15 , all oil phases in the channel were expelled out. 3.1.3 Solubilization Figure 8 shows the oil recovery efficiency during water and The solubilization effect plays a critical role in microemulsion- microemulsion-forming surfactant injection by considering the forming surfactant flooding, particularly in recovering oil from wettability alternation effect. As expected, before the dead-end pores. While reducing interfacial tension is often breakthrough, the oil saturation within the pore space decreases ineffective for mobilizing residual oil in such regions, the linearly as water or microemulsion-forming surfactant is injected solubilization capability of microemulsions significantly enhances into the pore space. The ultimate oil saturation with contact angles ° ° the recovery of trapped oil. To investigate how the solubilization of 45 and 15 are 17% and 0%, respectively. The results indicate that effect of in situ-formed microemulsions improves the mobilization the stronger the water-wet strength, the less residual oil in the of residual oil in dead-end pores, a comparative study was conducted dead-end pore. using water flooding and microemulsion-forming surfactant flooding in a T-junction model. The capillary tube (1 mm diameter × 3 mm length) connects to a dead-end pore (1 mm × 3.2 Oil and water distribution during 1 mm square). microemulsion-forming surfactant flooding As illustrated in Figure 5A, a residual oil droplet forms in the dead-end pore during water flooding. Once the droplet is trapped, To investigate the behavior of in situ microemulsion formation continued water injection fails to mobilize it further. In contrast, in complex porous structures, direct numerical simulations of during microemulsion-forming surfactant flooding, where microemulsion-forming surfactant flooding were conducted in a solubilization effects are considered, a residual oil droplet initially porous medium reconstructed from a digital rock. The size of the forms in the dead-end pore as well. However, as shown in Figure 5B, porous medium is 3 mm × 3 mm and corresponding the mean pores the oil-water interface, initially positioned at the dashed line, diameter is approximately 100 μm. The simulation accounts for the gradually recedes as the microemulsion-forming surfactant is unique properties of microemulsions, including viscosity continuously injected. This receding interface indicates that the enhancement, interfacial tension reduction, solubilization, and residual oil is progressively solubilized into the aqueous phase. wettability alteration. The porous medium was initially saturated Frontiers in Chemistry 07 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 FIGURE 11 Oil and aqueous distribution in porous media during microemulsion-forming surfactant flooding at different: (A) 1 time, (B) 3 times and (C) 4 times. with oil, and microemulsion-forming surfactant was injected from During microemulsion-forming surfactant flooding, flow- the left inlet, displacing oil toward the right outlet. advantaged channels with low microemulsion concentrations Figure 9 illustrates the distribution of oil and aqueous phases in persist within the porous medium. Similar to conventional water the porous medium at different times, where black represents the oil flooding, a portion of the residual oil remains trapped in the pore phase and colors represent the aqueous phase. At the early stage, a space. However, the dynamic process of residual oil mobilization low concentration of microemulsion forms near the oil-water during in situ microemulsion formation is highlighted by two interface, while no microemulsion is observed near the inlet, as selected residual oil droplets (marked by red circles in Figure 9). shown in Figure 9A. As the injection of the microemulsion-forming In Region-1, the interface of the residual oil droplet recedes surfactant solution continues, the concentration of microemulsion continuously, and its size diminishes over time, accompanied by in the aqueous phase increases. By t = 0.4 s, a mainstream channel a gradual increase in microemulsion concentration in the pore with a lower microemulsion concentration emerges in the porous throat. In Region-2, smaller residual oil droplets are rapidly and medium, while higher microemulsion concentrations are observed completely dissolved, demonstrating the solubilization effect of the in the elongated pore throats on either side (Figure 9A). The flow microemulsion. stagnation zones in the pore throats restrict the movement of Figure 10 presents the oil recovery efficiency profiles during microemulsion, which can only enter the mainstream channel microemulsion-forming surfactant flooding in the complex porous through diffusion. Due to the combined effects of viscosity medium. Breakthrough refers to the moment when the displacing enhancement and interfacial tension reduction associated with fluid (microemulsion-forming surfactant) first arrives at the outlet of the in situ formed microemulsion, the swept area within the the porous media. Before breakthrough, the oil saturation decreases porous medium gradually expands, as depicted in Figures 9C,D. linearly as water or microemulsion-forming surfactant is injected. Frontiers in Chemistry 08 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 of the microemulsion is considered M = 3 and M = 4), the viscosity of the aqueous phase becomes non-uniform. Specifically, the viscosity is lower in the main flow channels, where microemulsion concentration is reduced, and higher at the oil- water interface, where microemulsion concentration is elevated. As the viscosity of the in situ-formed microemulsion increases, the oil in regions 1, 2, 3, and 4 is progressively mobilized, demonstrating the role of viscosity enhancement in improving oil recovery. Figure 12 presents the oil recovery efficiency in the complex pore channel under different viscosity conditions, with a fixed oil-water- rock contact angle of 30 . When no viscosity enhancement is applied (M = 1), the oil saturation in the pore space is the highest, indicating low recovery efficiency. However, when the viscosity of the in situ- formed microemulsion is increased by a factor of 3 (M = 3), the oil saturation decreases significantly by approximately 8%, highlighting the substantial improvement in recovery due to viscosity FIGURE 12 enhancement. Interestingly, further increasing the viscosity ratio Oil recovery efficiency during microemulsion-forming surfactant to M = 4 yields a similar recovery improvement compared to M =3, flooding at different viscosity ratio. suggesting diminishing returns with higher viscosity ratios. This observation provides valuable insights for the development of chemical agents: while increasing the viscosity of in situ-formed Notably, the rate of oil saturation reduction is faster during microemulsions enhances oil recovery, the relationship is not linear. microemulsion-forming surfactant flooding due to the Therefore, a balance must be struck between field requirements and solubilization effect of the microemulsion. After breakthrough, economic costs to achieve significant recovery improvements at the oil saturation during water flooding stabilizes, while it lower operational expenses. continues to decrease slowly during microemulsion-forming surfactant flooding. This gradual reduction in oil saturation is attributed to the diminishing solubilization effect as the formed 3.4 Effect of wettability on microemulsion- microemulsion aggregates at the oil-water interface. As shown in forming surfactant flooding Figure 10, the rate of Oil recovery efficiency increase slows over time in the second stage. Additionally, as the remaining oil saturation Wettability in oil reservoirs refers to the tendency of water to decreases, the oil-water interface area—the primary site of in situ preferentially contact the rock surface in the presence of oil. Solid microemulsion formation—also diminishes, further limiting the surfaces can range from strongly water-wet to strongly oil-wet, solubilization process. depending on mineral composition and the thermophysical properties of the fluids. Wettability significantly influences multiphase flow in porous media and is typically characterized 3.3 Effect of viscosity on microemulsion- by the contact angle. To investigate the effect of wettability on forming surfactant flooding the in situ formation in complex structure, three contact angles were ° ° ° simulated: 30 ,90 , and 150 . Figure 13 shows the oil and water In batch experiments, microemulsions with varying physical distributions in the porous medium with different contact angle. The properties can be formulated by adjusting the composition of color represents the concentration of in situ formed microemulsion different surfactants. One of the most critical parameters in water during microemulsion-forming surfactant flooding. The influencing oil recovery is the viscosity of the in situ-formed oil-water interface stops at the junction from the throat to the pore microemulsion. To investigate the effect of microemulsion under water-wet and neutral wetting conditions (Figures 13A,B). viscosity on oil recovery in porous media, three simulation cases However, under oil-wet conditions, the situation is reversed and the were conducted with different viscosity ratios. The viscosity ratio, oil-water interface always stays at the junction from the pore to the defined as M = μ /μ , where μ and μ represent the viscosities of the throat (Figure 13C). This phenomenon can be explained as capillary e w e w microemulsion and water, respectively, was used to characterize the barrier, which has been analyzed detailly in our previous studies. In viscosity-enhancing effect of the microemulsion. addition, wettability significantly affects the formation of Figure 11 illustrates the oil recovery efficiency profiles during mainstream channels during two-phase displacement. The microemulsion-forming surfactant flooding in a complex porous dominant channel within the porous medium is more medium for viscosity ratios of M =1, M = 3, and M = 4. The color pronounced under water-wet conditions. gradient represents the viscosity distribution of the aqueous phase. Figure 14 shows the oil recovery efficiency during As shown in Figure 11A, when M = 1, the microemulsion formed in microemulsion-forming surfactant flooding in the complex situ at the oil-water interface exhibits a viscosity identical to that of porous media with different contact angles. Before the injection water, resulting in a uniform viscosity distribution across the time of 0.6 s, the decreasing trend of oil saturation under different aqueous phase. In contrast, when the viscosity-enhancing effect wetting angle conditions is basically the same, but then there will be Frontiers in Chemistry 09 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 FIGURE 13 ° ° ° Oil and aqueous distribution in porous media during microemulsion-forming surfactant flooding at different: (A) 30 , (B) 90 and (C) 150 . a significant difference. For the oil-wet condition, the decrease of oil distribution of oil and in situ formed microemulsion during saturation becomes slower after 0.6 s at a wetting angle of 150 . For microemulsion-forming surfactant flooding in a real rock strong water-wet conditions, on the other hand, at a wetting angle of medium. The influence of the wettability and viscosity of 30 , the oil saturation continues to decrease after 0.6 s and does not microemulsion on the enhanced oil recovery was analyzed. The turn around until after 1.1 s. The variability of the microemulsion primary conclusions are summarized as follows: replacement process under different wettability conditions can be observed more intuitively in Figure 14. Although the distribution of 1. The proposed novel pore-scale numerical simulation model is mainstream channels is the same under the three wettability able to simulate the dynamic formation process of oil-in-water conditions, the wave area of microemulsion drive is significantly microemulsions within the pore space. By considering some wider under strong water-wet conditions. typical characteristics of in situ formed microemulsions, including increased viscosity, reduced interfacial tension, altered wettability, and solubilization, it is analyzed how 4 Conclusion these characteristics enhance recovery. 2. During the microemulsion-forming surfactant flooding in a In this study, a novel pore-scale numerical simulation model was real rock medium, the in situ formed microemulsion was proposed to capture the in situ microemulsion formation during observed at the interface between water and aqueous. The surfactant solution flooding in porous media. Then, a series of in situ microemulsion flooding can significantly improve the numerical simulation were conducted to explore the dynamics recovery rate under the combined effect of multiple factors. For Frontiers in Chemistry 10 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 Data availability statement The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author. Author contributions YH: Writing – review and editing, Investigation, Writing – original draft, Funding acquisition, Methodology. KD: Investigation, Writing – review and editing, Writing – original draft. DZ: Writing – review and editing, Writing – original draft, Methodology. TW: Investigation, Writing – review and editing, Writing – original draft. WX: Supervision, Writing – review and editing, Writing – original draft, Conceptualization. ZG: Writing – review and editing, Writing – original draft, Supervision. FIGURE 14 Oil recovery efficiency during microemulsion-forming surfactant flooding at different contact angles. Funding The author(s) declare that financial support was received for the example, the solubilization effect was able to gradually dissolve research and/or publication of this article. This work is supported by the residual oil droplets in the pores, while the wettability the National Natural Science Foundation of China (No. 42141011), change was able to reduce the formation of the residual oil the Natural Science Basic Research Program of Shaanxi (No. 2023- droplets in the pores JC-QN-0580) and the Fundamental Research Funds for the Central 3. Increasing the viscosity of the in situ formed microemulsion Universities (xzy012023085). can enhance the oil recovery during the microemulsion- forming surfactant flooding in the complex porous media. However, the viscosity of the microemulsion is not linearly Conflict of interest related to the recovery rate, and in real mines it is necessary to consider the economic cost to choose the microemulsion Author TW was employed by China National Petroleum viscosity increasing. Corporation. 4. Wettability can significantly affect the oil-water two-phase The remaining authors declare that the research was conducted displacement process within porous media, which further in the absence of any commercial or financial relationships that affects the in situ formation process of microemulsions. could be construed as a potential conflict of interest. Under water-wet conditions, the oil-water interface stays at the junction of the throat and the pore space, which contributes to the formation of microemulsions and thus to the Generative AI statement enhancement of recovery. The author(s) declare that no Generative AI was used in the It is worth noting that the proposed model is based on the Navier- creation of this manuscript. Stokes equations, which fundamentally restricts the analysis to conventional fluid dynamics. Consequently, the framework may not accurately capture flow behaviors in unconventional reservoirs (e.g., Publisher’s note shale formations) where nanoscale pore structures dominate the transport phenomena. In addition, while this investigation provides All claims expressed in this article are solely those of the authors valuable insights into oil-water displacement mechanisms at the pore and do not necessarily represent those of their affiliated scale and identifies key factors governing microemulsion-driven organizations, or those of the publisher, the editors and the recovery enhancement, direct field application remains challenging reviewers. Any product that may be evaluated in this article, or due to the inherent scale differences between laboratory claim that may be made by its manufacturer, is not guaranteed or investigations and reservoir conditions. endorsed by the publisher. References Alzahid, Y. A., Mostaghimi, P., Walsh, S. D. C., and Armstrong, R. T. (2019). 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Pore-scale numerical simulation of in situ microemulsion formation and enhanced oil recovery in porous media

Hu, Yingxue; Dong, Kai; Zhang, Dan; Wu, Tianjiang; Xu, Wei; Gu, Zhaolin
Frontiers in Chemistry , Volume 13 – May 19, 2025
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TYPE Original Research PUBLISHED 19 May 2025 DOI 10.3389/fchem.2025.1601086 Pore-scale numerical simulation of in situ microemulsion OPEN ACCESS EDITED BY Yixin Lu, formation and enhanced oil National University of Singapore, Singapore REVIEWED BY recovery in porous media Sijia Wang, Hebei University of Technology, China Liewei Qiu, 1 1 2 1,3 1 Yingxue Hu , Kai Dong , Dan Zhang , Tianjiang Wu , Wei Xu * Xi’an Polytechnic University, China and Zhaolin Gu *CORRESPONDENCE Wei Xu, 1 2 School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an, China, Key [email protected] Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an, China, Oil and Gas Technology Research RECEIVED 27 March 2025 Institute of Changqing Oilfield, China National Petroleum Corporation, Xi’an, China ACCEPTED 05 May 2025 PUBLISHED 19 May 2025 CITATION Hu Y, Dong K, Zhang D, Wu T, Xu W and Gu Z (2025) Pore-scale numerical simulation of in In situ microemulsion has emerged as an advanced tertiary oil recovery technique situ microemulsion formation and enhanced oil that utilizes the injection of surfactant solutions to improve displacement recovery in porous media. Front. Chem. 13:1601086. efficiency through spontaneous microemulsification. This study presents a doi: 10.3389/fchem.2025.1601086 novel pore-scale numerical model to simulate the dynamic process of in situ COPYRIGHT microemulsion formation during surfactant-cosolvent-salt flooding in complex © 2025 Hu, Dong, Zhang, Wu, Xu and Gu. This is porous media. Through comprehensive numerical simulations based on realistic an open-access article distributed under the rock geometries, we systematically investigated the spatiotemporal evolution of terms of the Creative Commons Attribution License (CC BY). The use, distribution or phase distributions and identified critical mechanisms governing oil mobilization. reproduction in other forums is permitted, The developed model incorporates four fundamental characteristics of provided the original author(s) and the microemulsion systems: interfacial tension reduction, viscosity modification, copyright owner(s) are credited and that the original publication in this journal is cited, in wettability alteration, and enhanced solubilization capacity. During the accordance with accepted academic practice. microemulsion-forming surfactant flooding in a realistic rock medium, the in No use, distribution or reproduction is situ formed microemulsion was observed at the interface between oil and permitted which does not comply with these terms. aqueous. The in situ microemulsion flooding can significantly improve the recovery rate under the combined effect of multiple factors. Increasing the viscosity of the in situ formed microemulsion can enhance the oil recovery during the microemulsion-forming surfactant flooding in the complex porous media. Under water-wet conditions, the oil-water interface stays at the junction of the throat and the pore space, which contributes to the formation of microemulsions and thus to the enhancement of recovery. This study provides a better understanding of the in situ microemulsion formation and the mechanisms of enhanced oil recovery in complex porous media. KEYWORDS in situ microemulsion, porous media, solubilization, viscosity modification, porescale 1 Introduction Tertiary oil recovery, particularly chemical flooding, has emerged as an essential strategy to enhance oil recovery from residual and remaining oil post-water flooding (Park et al., 2021; Tavakkoli et al., 2022). Among chemical flooding techniques, microemulsion flooding has gained significant attention due to its unique ability to reduce interfacial tension (IFT), alter wettability, and mobilize trapped residual oil in porous media (Qin et al., 2020; Das et al., 2021; Wu et al., 2024; Koreh et al., 2025). Frontiers in Chemistry 01 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 FIGURE 1 Oil and aqueous distribution in capillary tube after injection of (A) water and (B) microemulsion-forming surfactant solution. Considering the viscosity enhancement effect of microemulsion during the two-phase displacement process. Black and blue represents oil and water, respectively. The unique physical properties of microemulsions, such as solubilization capacity, ultra-low IFT, wettability alteration, and viscosity modulation, collectively contribute to enhanced oil recovery (EOR). Solubilization enables microemulsions to dissolve crude oil and water, improving oil mobility (Wei et al., 2011). Ultra-low IFT reduces the oil-water interfacial energy, facilitating the detachment of residual oil (Das et al., 2021; Li et al., 2025). Wettability alteration adjusts the wetting state of rock surfaces, optimizing oil-water flow paths, while viscosity modulation enhances the sweep efficiency of displacing fluids (Yang et al., 2021). These properties make microemulsions highly effective in EOR applications. Two primary methods are employed to utilize microemulsions for EOR: ex-situ and in situ. The ex-situ method involves injecting pre-prepared microemulsions into the reservoir (Zhou et al., 2020). While this approach has demonstrated significant oil recovery enhancement, it faces challenges such as high injection pressures FIGURE 2 Pressure difference between the inlet and the outlet during water and potential incompatibility with reservoir conditions (Zhu et al., and surfactant microemulsion-forming surfactant flooding. 2022). In contrast, the in situ method involves injecting a Considering the viscosity enhancement effect of microemulsion. microemulsion-forming surfactant system into the formation, where microemulsions form spontaneously within the pores (Mo et al., 2023). This method leverages the natural conditions of the reservoir, offering better adaptability and cost-effectiveness. Microemulsions, thermodynamically stable dispersions of However, the pore-scale mechanisms governing microemulsion- surfactant, cosolvent, salt, and oil, have demonstrated remarkable assisted oil recovery remain poorly understood, limiting the success in improving oil recovery. optimization of this technology for field applications. Microemulsions are classified into three types based on their Advanced visualization techniques, including X-ray computed phase behavior: Winsor I (oil-in-water, O/W), Winsor II (water-in- tomography (Hu et al., 2020), nuclear magnetic resonance oil, W/O), and Winsor III (bicontinuous). Winsor I microemulsions (Govindarajan et al., 2020), and microfluidics (Fu et al., 2016), consist of nano-scale oil droplets dispersed in a continuous water have facilitated the analysis of microemulsion formation and EOR in phase, stabilized by surfactant molecules at the oil-water interface. complex porous media. These technologies provide high-precision Winsor II microemulsions feature water droplets dispersed in a experimental data, revealing the formation mechanisms, phase continuous oil phase, while Winsor III microemulsions exhibit a behavior, and oil displacement efficiency of microemulsions. For bicontinuous structure where both oil and water phases are instance, Ott et al. (2020) employed micro-CT to investigate the in interconnected, forming a sponge-like network (Scriven, 1976; situ emulsification behavior of surfactants and crude oil during alkali Zhao et al., 2025). The formation of these microemulsions is flooding, providing critical insights into the changes in interfacial highly dependent on surfactant concentration, cosurfactant tension and oil-water phase distribution. Unsal et al. (2019) utilized selection, salinity, and oil-to-water ratio (Budhathoki et al., 2016). synchrotron-based X-ray micro-tomography to study the dynamic Salinity and oil-water ratio scanning experiments are critical for changes in emulsification under optimal salinity conditions, identifying optimal conditions for microemulsion formation, highlighting the critical role of salinity in emulsification efficiency ensuring maximum oil recovery efficiency (Zhou et al., 2020; Alzahid et al. (2019) systematically investigated the pore-scale flow Mariyate and Bera, 2022). characteristics of surfactant flooding under varying salinity Frontiers in Chemistry 02 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 FIGURE 3 Oil and aqueous distribution in capillary tube after injection of (A) water and (B) microemulsion-forming surfactant solution. Considering the interfacial tension reduction effect of microemulsion during the two-phase displacement process. Black and blue represents oil and water, respectively. alkali flooding, revealing the formation, migration, and trapping mechanisms of emulsions in porous media. Despite these advancements, visualizing the dynamic distribution of microemulsion concentrations within pores remains challenging due to the temporal and spatial limitations of X-ray CT. Two-dimensional micromodels with high-speed cameras have been employed to capture near real-time emulsification dynamics. For example, Unsal et al. (2016) used microfluidic flow experiments to study the in situ formation of microemulsions by co-injecting n-decane and surfactant solutions into a T-junction capillary geometry. Broens and Unsal (2018) investigated emulsification kinetics in quasi-miscible flow conditions, while Tagavifar et al. (2017) analyzed phase formation and spatial configurations in a 2.5-dimensional (2.5D) micromodel. Zhao et al. (2022) demonstrated in situ emulsification using on-chip experiments, and Xu et al. (2024) studied the formation and transport mechanisms of microemulsions in fractured porous micromodels. Nevertheless, current visualization FIGURE 4 Pressure difference between the inlet and the outlet during water techniques remain inadequate for accurately quantifying and microemulsion-forming surfactant flooding. Considering the microemulsion concentrations within complex pore structures, interfacial tension reduction effect of microemulsion. thereby constraining the comprehensive understanding of microemulsion formation and enhanced oil recovery (EOR) mechanisms. Pore-scale numerical simulations have emerged as a promising approach for investigating oil-water interface dynamics conditions, demonstrating the significant impact of salinity on in porous media during chemical flooding processes, including emulsified phase properties and oil-water distribution. She et al. surfactant, polymer, and nanoparticle flooding. However, (2021) further advanced the field by employing three-dimensional microemulsion flooding presents additional complexities, as it visualization techniques to study the role of in situ emulsification in FIGURE 5 Oil and aqueous distribution in capillary tube after injection of (A) water and (B) microemulsion-forming surfactant solution. Considering the solubilization effect of microemulsion during the two-phase displacement process. Black and blue represents oil and water, respectively. Frontiers in Chemistry 03 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 2 Numerical method This study employs the conventional Volume of Fluids (VoF) method to simulate two-phase displacement within porous media and accurately track the interface between water and oil. In the VoF method, an indicator function, denoted as α,is defined throughout the flow domain, representing the volume fraction of one of the fluids within each computational grid cell. Specifically, α = 1 indicates that a cell is entirely occupied by water, while α = 0 signifies that the cell is completely filled with oil. The indicator function serves as a critical tool for identifying and tracking the interface between the two phases. The governing equations for the system, which describe the behavior of two incompressible, isothermal, and immiscible fluids (such as water and oil), are presented as follows: ∂ρu + ∇ · ρuu − ∇ · μ ∇u +() ∇u −∇p + F (1) ∂t FIGURE 6 Oil recovery efficiency during water and microemulsion-forming ∇ · u 0(2) surfactant flooding. Considering the solubilization effect of ρ  αρ +() 1 − α ρ (3) w o microemulsion. μ  αμ +() 1 − α μ (4) w o In the above equations, Equations 1, 2 are momentum and involves not only multiphase flow but also requires consideration of continuity equations of the fluid, where ρ, u, p, and F are density, multiphase mass transfer processes within intricate pore networks fluid velocity, dynamic pressure, and surface tension, respectively. t that has received limited research attention to date. is the time. The effect of gravity is ignored in two-dimensional To overcome these research gaps, the present study develops a models. α denotes the volume fraction of the water phase and novel pore-scale simulation framework to systematically examine correspondingly (1−α) the oil phase in Equations 3, 4. microemulsion formation and EOR mechanisms in porous media. The surface tension F in Equation 2 is modeled as continuum By analyzing microemulsion behavior in a simplified T-junction surface force (CSF) and calculated by structure, we aim to elucidate the roles of IFT reduction, wettability F  σκ∇α (5) alteration, viscosity enhancement, and solubilization in oil displacement. Furthermore, a real complex porous medium is where σ is the surface tension coefficient, and κ is the curvature of reconstructed from digital rock, and the pore-scale dynamics of the water-oil interface in Equation 5, which can be approximated the emulsification process are coupled with fluid flow. This study as follows leverages advanced imaging techniques and numerical simulations κ −∇ · n (6) to provide a comprehensive pore-scale investigation of microemulsion-enhanced oil recovery, bridging the gap between n is the interface normal unit vector (in Equations 6-8) given by laboratory-scale observations and field-scale applications. The ∇α findings are expected to contribute to the development of more n  (7) || ∇α effective EOR strategies, ultimately improving oil recovery efficiency and reducing the environmental footprint of hydrocarbon To accurately model the wettability of the porous media, the extraction. contact angle is imposed as a wetting boundary condition. The FIGURE 7 Oil and aqueous distribution in capillary tube after injection of (A) water and (B) microemulsion-forming surfactant solution. Considering the wettability alternation effect of microemulsion during the two-phase displacement process. Black and blue represents oil and water, respectively. Frontiers in Chemistry 04 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 The source term is calculated by: S −k CC() − C δ (12) a α e e∞ where k and δ are the rate of emulsification, and specific interfacial area between oil and water in Equations 11, 12. The mathematical model has been implemented within the open-source computational fluid dynamics (CFD) platform OpenFOAM. The governing equations are discretized using a finite-volume method and solved sequentially. The pressure–velocity coupling is addressed through a predictor–corrector approach, leveraging the Pressure-Implicit with Splitting of Operators (PISO) algorithm to ensure numerical stability and accuracy. 3 Results and discussion FIGURE 8 3.1 Modelling the unique physical properties Oil recovery efficiency during water and microemulsion-forming of microemulsions surfactant flooding. Considering the wettability alternation effect of microemulsion. Microemulsions exhibit several unique physical properties that contribute to enhanced oil recovery. The core mechanism involves the formation of nanoscale micellar systems facilitated normal unit vector of the interface at the wall boundary is adjusted by surfactants, which lead to viscosity enhancement, interfacial according to the following equation: tension reduction, solubilization, and wettability alteration. n  n cos θ + n sin θ (8) p t These properties often interact synergistically during the oil displacement process rather than acting independently. where n is the unit vector perpendicular to the wall, n is the unit vector p t However, to validate the capability of numerical simulations tangential to the wall, and θ is the contact angle. In this way, the in capturing these properties and to analyze the individual interface can form the prescribed angle θ when in contact with the wall. contribution of each factor to oil recovery, this section In this study, we will only focus on the Winsor I (oil in water, examines each physical property separately. O/W) microemulsion. The surfactant system forming the microemulsion is an aqueous solute and the microemulsion 3.1.1 Viscosity enhancement formed is treated as a solute in water. To investigate the effect of viscosity enhancement, water and a To track the concentration of microemulsion-forming microemulsion-forming surfactant were injected into a capillary surfactant system in water, the mass conservation equation for tube initially saturated with oil. The flow rates of both fluids were the surfactant is calculated: maintained at the same level. In this scenario, the effects of ∂αC interfacial tension reduction, solubilization, and wettability + ∇ ·() Cαu  ∇ ·() αD ∇C − S (9) c c ∂t alteration were intentionally excluded from consideration. Notably, the viscosity of the microemulsion-forming surfactant where α is 1, representing surfactant in water phase, C and D are the system is comparable to that of water. Therefore, any observed concentration and diffusion coefficient of surfactant in water, differences in oil-water displacement within the capillary tube can be respectively. The source term S denotes the rate of consumption attributed solely to the viscosity enhancement resulting from the in of surfactant in Equation 9. situ formation of microemulsions. To track the concentration of microemulsion in water, the mass Figure 1 illustrates the distribution of oil and aqueous in the conservation equation for the microemulsion is calculated: capillary tube following the injection of water and the ∂αC microemulsion-forming surfactant. A two-dimensional capillary + ∇ ·() αC u − ∇ ·() αD ∇C  S (10) e e e e ∂t tube with a diameter of 1 mm and a length of 10 mm was used in this study. As depicted in Figure 1B, the injection of the surfactant where α is 1, representing microemulsion in water phase, C and D e e led to the in situ formation of microemulsions near the oil-water are the concentration and diffusion coefficient of microemulsion in interface. The red regions in the figure indicate a high concentration water, respectively. The source term S denotes the rate of of microemulsion within the aqueous phase. The microemulsion production of microemulsion in Equation 10. accumulates along the walls of the capillary tube due to the higher The evolution of water or oil phase is governed by the mass flow velocity in the center of the tube. Although no significant balance equation: differences are observed at the oil-water interface between the two ∂α injection fluids, the flow resistance during injection differs due to the + ∇ ·() αu  S (11) ∂t viscosity-enhancing effect of the microemulsion. Frontiers in Chemistry 05 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 FIGURE 9 Oil and water distribution in porous media during microemulsion-forming surfactant flooding at different: (A) 0.1 s, (B) 0.4 s, (C) 0.8 s and (D) 1.2 s. Figure 2 presents the pressure difference between the inlet and between oil and water and between oil and microemulsion were set outlet during the injection of water and the microemulsion-forming to 50 mN/m and 0.01 mN/m, respectively. Figure 3 compares the surfactant. The negative pressure difference values indicate that the distribution of oil and aqueous in the capillary tube after the flooding process is driven by spontaneous imbibition, a consequence injection of water and the microemulsion-forming surfactant. of the strongly water-wet conditions. Initially, the absolute value of The results demonstrate that the interfacial tension of the the pressure difference increases sharply as a meniscus form at the microemulsion significantly influences the meniscus formation at oil-water interface. Subsequently, as the injection continues, the the oil-water interface. Specifically, the thickness of the in situ- pressure difference between the inlet and outlet increases at a slower formed microemulsion layer with ultra-low interfacial tension is rate. Overall, after the establishment of the interface, the surfactant- considerably smaller than that observed under high interfacial driven flooding exhibits a lower pressure difference compared to tension conditions (Figure 1B). In the capillary tube, capillary water flooding. This difference arises from the in situ formation of a forces drive the oil-water displacement process. According to the more viscous microemulsion during surfactant flooding. While this Young–Laplace equation, higher interfacial tension enhances increased viscosity imposes greater flow resistance within the capillary forces, leading to increased imbibition rates. However, capillary tube, it can lead to improved oil recovery in actual the ultra-low interfacial tension associated with microemulsions reservoir conditions. alters this dynamic, reducing the capillary forces and thereby affecting the displacement process. 3.1.2 Interfacial tension reduction Figure 4 illustrates the pressure difference between the inlet and To examine the effect of interfacial tension reduction on oil- outlet during the injection of water and the microemulsion-forming water displacement in porous media, the interfacial tension values surfactant, considering the effect of interfacial tension reduction. Frontiers in Chemistry 06 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 With sufficient injection time, the residual oil in the dead-end pore can be entirely recovered. Figure 6 presents the oil recovery efficiency profiles during water and microemulsion-forming surfactant injection, highlighting the solubilization effect. Before breakthrough, the oil saturation within the pore space decreases linearly as water or microemulsion-forming surfactant is injected. At the pinch-off stage, the continuous oil phase within the pore is divided into two parts: one portion becomes trapped in the pore, while the other is displaced out of the pore. This process occurs rapidly, resulting in a sharp decline in oil saturation, as depicted in Figure 6. After breakthrough, the oil saturation during water flooding stabilizes, indicating no further recovery. In contrast, during microemulsion-forming surfactant flooding, the oil saturation continues to decrease gradually due to the solubilization effect. Consequently, the ultimate oil recovery efficiency of microemulsion-forming surfactant flooding is significantly higher than that of water flooding, a result attributed FIGURE 10 to the solubilization effect of in situ-formed microemulsions. Oil recovery efficiency during microemulsion-forming surfactant flooding. 3.1.4 Wettability alternation In microemulsions, surfactants adsorb onto rock surfaces, shifting their wettability from oil-wet to water-wet. This allows Notably, the pressure difference exhibits contrasting behaviors for the aqueous phase to penetrate pores more effectively, detach oil the two flooding processes: it is negative for water flooding but films, and carry the residual oil out. In order to investigate how in positive for microemulsion-forming surfactant flooding. This situ formation of microemulsions provides recovery through indicates that capillary forces act as a driving force during water wettability alternation, two kinds of wall wettability were set with ° ° flooding but as a resistance force during microemulsion-forming contact angles of 45 and 15 , respectively. Figure 7 shows the surfactant flooding. As expected, the magnitude of the pressure dynamics distribution of oil and aqueous phase in the T-junction difference during microemulsion-forming surfactant flooding is model. Cloud diagram showing the concentration distribution of significantly smaller, approximately 5.2 Pa, compared to 86 Pa microemulsion in water at steady-state. At contact angle of 45 ,a during water flooding. For microemulsion-forming surfactant residual oil droplet was formed within the pore space after flooding, the pressure difference is primarily attributed to flow microemulsion-forming surfactant flooding, whereas when the resistance, with capillary forces playing a minor role. contact angle was alternated to 15 , all oil phases in the channel were expelled out. 3.1.3 Solubilization Figure 8 shows the oil recovery efficiency during water and The solubilization effect plays a critical role in microemulsion- microemulsion-forming surfactant injection by considering the forming surfactant flooding, particularly in recovering oil from wettability alternation effect. As expected, before the dead-end pores. While reducing interfacial tension is often breakthrough, the oil saturation within the pore space decreases ineffective for mobilizing residual oil in such regions, the linearly as water or microemulsion-forming surfactant is injected solubilization capability of microemulsions significantly enhances into the pore space. The ultimate oil saturation with contact angles ° ° the recovery of trapped oil. To investigate how the solubilization of 45 and 15 are 17% and 0%, respectively. The results indicate that effect of in situ-formed microemulsions improves the mobilization the stronger the water-wet strength, the less residual oil in the of residual oil in dead-end pores, a comparative study was conducted dead-end pore. using water flooding and microemulsion-forming surfactant flooding in a T-junction model. The capillary tube (1 mm diameter × 3 mm length) connects to a dead-end pore (1 mm × 3.2 Oil and water distribution during 1 mm square). microemulsion-forming surfactant flooding As illustrated in Figure 5A, a residual oil droplet forms in the dead-end pore during water flooding. Once the droplet is trapped, To investigate the behavior of in situ microemulsion formation continued water injection fails to mobilize it further. In contrast, in complex porous structures, direct numerical simulations of during microemulsion-forming surfactant flooding, where microemulsion-forming surfactant flooding were conducted in a solubilization effects are considered, a residual oil droplet initially porous medium reconstructed from a digital rock. The size of the forms in the dead-end pore as well. However, as shown in Figure 5B, porous medium is 3 mm × 3 mm and corresponding the mean pores the oil-water interface, initially positioned at the dashed line, diameter is approximately 100 μm. The simulation accounts for the gradually recedes as the microemulsion-forming surfactant is unique properties of microemulsions, including viscosity continuously injected. This receding interface indicates that the enhancement, interfacial tension reduction, solubilization, and residual oil is progressively solubilized into the aqueous phase. wettability alteration. The porous medium was initially saturated Frontiers in Chemistry 07 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 FIGURE 11 Oil and aqueous distribution in porous media during microemulsion-forming surfactant flooding at different: (A) 1 time, (B) 3 times and (C) 4 times. with oil, and microemulsion-forming surfactant was injected from During microemulsion-forming surfactant flooding, flow- the left inlet, displacing oil toward the right outlet. advantaged channels with low microemulsion concentrations Figure 9 illustrates the distribution of oil and aqueous phases in persist within the porous medium. Similar to conventional water the porous medium at different times, where black represents the oil flooding, a portion of the residual oil remains trapped in the pore phase and colors represent the aqueous phase. At the early stage, a space. However, the dynamic process of residual oil mobilization low concentration of microemulsion forms near the oil-water during in situ microemulsion formation is highlighted by two interface, while no microemulsion is observed near the inlet, as selected residual oil droplets (marked by red circles in Figure 9). shown in Figure 9A. As the injection of the microemulsion-forming In Region-1, the interface of the residual oil droplet recedes surfactant solution continues, the concentration of microemulsion continuously, and its size diminishes over time, accompanied by in the aqueous phase increases. By t = 0.4 s, a mainstream channel a gradual increase in microemulsion concentration in the pore with a lower microemulsion concentration emerges in the porous throat. In Region-2, smaller residual oil droplets are rapidly and medium, while higher microemulsion concentrations are observed completely dissolved, demonstrating the solubilization effect of the in the elongated pore throats on either side (Figure 9A). The flow microemulsion. stagnation zones in the pore throats restrict the movement of Figure 10 presents the oil recovery efficiency profiles during microemulsion, which can only enter the mainstream channel microemulsion-forming surfactant flooding in the complex porous through diffusion. Due to the combined effects of viscosity medium. Breakthrough refers to the moment when the displacing enhancement and interfacial tension reduction associated with fluid (microemulsion-forming surfactant) first arrives at the outlet of the in situ formed microemulsion, the swept area within the the porous media. Before breakthrough, the oil saturation decreases porous medium gradually expands, as depicted in Figures 9C,D. linearly as water or microemulsion-forming surfactant is injected. Frontiers in Chemistry 08 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 of the microemulsion is considered M = 3 and M = 4), the viscosity of the aqueous phase becomes non-uniform. Specifically, the viscosity is lower in the main flow channels, where microemulsion concentration is reduced, and higher at the oil- water interface, where microemulsion concentration is elevated. As the viscosity of the in situ-formed microemulsion increases, the oil in regions 1, 2, 3, and 4 is progressively mobilized, demonstrating the role of viscosity enhancement in improving oil recovery. Figure 12 presents the oil recovery efficiency in the complex pore channel under different viscosity conditions, with a fixed oil-water- rock contact angle of 30 . When no viscosity enhancement is applied (M = 1), the oil saturation in the pore space is the highest, indicating low recovery efficiency. However, when the viscosity of the in situ- formed microemulsion is increased by a factor of 3 (M = 3), the oil saturation decreases significantly by approximately 8%, highlighting the substantial improvement in recovery due to viscosity FIGURE 12 enhancement. Interestingly, further increasing the viscosity ratio Oil recovery efficiency during microemulsion-forming surfactant to M = 4 yields a similar recovery improvement compared to M =3, flooding at different viscosity ratio. suggesting diminishing returns with higher viscosity ratios. This observation provides valuable insights for the development of chemical agents: while increasing the viscosity of in situ-formed Notably, the rate of oil saturation reduction is faster during microemulsions enhances oil recovery, the relationship is not linear. microemulsion-forming surfactant flooding due to the Therefore, a balance must be struck between field requirements and solubilization effect of the microemulsion. After breakthrough, economic costs to achieve significant recovery improvements at the oil saturation during water flooding stabilizes, while it lower operational expenses. continues to decrease slowly during microemulsion-forming surfactant flooding. This gradual reduction in oil saturation is attributed to the diminishing solubilization effect as the formed 3.4 Effect of wettability on microemulsion- microemulsion aggregates at the oil-water interface. As shown in forming surfactant flooding Figure 10, the rate of Oil recovery efficiency increase slows over time in the second stage. Additionally, as the remaining oil saturation Wettability in oil reservoirs refers to the tendency of water to decreases, the oil-water interface area—the primary site of in situ preferentially contact the rock surface in the presence of oil. Solid microemulsion formation—also diminishes, further limiting the surfaces can range from strongly water-wet to strongly oil-wet, solubilization process. depending on mineral composition and the thermophysical properties of the fluids. Wettability significantly influences multiphase flow in porous media and is typically characterized 3.3 Effect of viscosity on microemulsion- by the contact angle. To investigate the effect of wettability on forming surfactant flooding the in situ formation in complex structure, three contact angles were ° ° ° simulated: 30 ,90 , and 150 . Figure 13 shows the oil and water In batch experiments, microemulsions with varying physical distributions in the porous medium with different contact angle. The properties can be formulated by adjusting the composition of color represents the concentration of in situ formed microemulsion different surfactants. One of the most critical parameters in water during microemulsion-forming surfactant flooding. The influencing oil recovery is the viscosity of the in situ-formed oil-water interface stops at the junction from the throat to the pore microemulsion. To investigate the effect of microemulsion under water-wet and neutral wetting conditions (Figures 13A,B). viscosity on oil recovery in porous media, three simulation cases However, under oil-wet conditions, the situation is reversed and the were conducted with different viscosity ratios. The viscosity ratio, oil-water interface always stays at the junction from the pore to the defined as M = μ /μ , where μ and μ represent the viscosities of the throat (Figure 13C). This phenomenon can be explained as capillary e w e w microemulsion and water, respectively, was used to characterize the barrier, which has been analyzed detailly in our previous studies. In viscosity-enhancing effect of the microemulsion. addition, wettability significantly affects the formation of Figure 11 illustrates the oil recovery efficiency profiles during mainstream channels during two-phase displacement. The microemulsion-forming surfactant flooding in a complex porous dominant channel within the porous medium is more medium for viscosity ratios of M =1, M = 3, and M = 4. The color pronounced under water-wet conditions. gradient represents the viscosity distribution of the aqueous phase. Figure 14 shows the oil recovery efficiency during As shown in Figure 11A, when M = 1, the microemulsion formed in microemulsion-forming surfactant flooding in the complex situ at the oil-water interface exhibits a viscosity identical to that of porous media with different contact angles. Before the injection water, resulting in a uniform viscosity distribution across the time of 0.6 s, the decreasing trend of oil saturation under different aqueous phase. In contrast, when the viscosity-enhancing effect wetting angle conditions is basically the same, but then there will be Frontiers in Chemistry 09 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 FIGURE 13 ° ° ° Oil and aqueous distribution in porous media during microemulsion-forming surfactant flooding at different: (A) 30 , (B) 90 and (C) 150 . a significant difference. For the oil-wet condition, the decrease of oil distribution of oil and in situ formed microemulsion during saturation becomes slower after 0.6 s at a wetting angle of 150 . For microemulsion-forming surfactant flooding in a real rock strong water-wet conditions, on the other hand, at a wetting angle of medium. The influence of the wettability and viscosity of 30 , the oil saturation continues to decrease after 0.6 s and does not microemulsion on the enhanced oil recovery was analyzed. The turn around until after 1.1 s. The variability of the microemulsion primary conclusions are summarized as follows: replacement process under different wettability conditions can be observed more intuitively in Figure 14. Although the distribution of 1. The proposed novel pore-scale numerical simulation model is mainstream channels is the same under the three wettability able to simulate the dynamic formation process of oil-in-water conditions, the wave area of microemulsion drive is significantly microemulsions within the pore space. By considering some wider under strong water-wet conditions. typical characteristics of in situ formed microemulsions, including increased viscosity, reduced interfacial tension, altered wettability, and solubilization, it is analyzed how 4 Conclusion these characteristics enhance recovery. 2. During the microemulsion-forming surfactant flooding in a In this study, a novel pore-scale numerical simulation model was real rock medium, the in situ formed microemulsion was proposed to capture the in situ microemulsion formation during observed at the interface between water and aqueous. The surfactant solution flooding in porous media. Then, a series of in situ microemulsion flooding can significantly improve the numerical simulation were conducted to explore the dynamics recovery rate under the combined effect of multiple factors. For Frontiers in Chemistry 10 frontiersin.org Hu et al. 10.3389/fchem.2025.1601086 Data availability statement The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author. Author contributions YH: Writing – review and editing, Investigation, Writing – original draft, Funding acquisition, Methodology. KD: Investigation, Writing – review and editing, Writing – original draft. DZ: Writing – review and editing, Writing – original draft, Methodology. TW: Investigation, Writing – review and editing, Writing – original draft. WX: Supervision, Writing – review and editing, Writing – original draft, Conceptualization. ZG: Writing – review and editing, Writing – original draft, Supervision. FIGURE 14 Oil recovery efficiency during microemulsion-forming surfactant flooding at different contact angles. Funding The author(s) declare that financial support was received for the example, the solubilization effect was able to gradually dissolve research and/or publication of this article. This work is supported by the residual oil droplets in the pores, while the wettability the National Natural Science Foundation of China (No. 42141011), change was able to reduce the formation of the residual oil the Natural Science Basic Research Program of Shaanxi (No. 2023- droplets in the pores JC-QN-0580) and the Fundamental Research Funds for the Central 3. Increasing the viscosity of the in situ formed microemulsion Universities (xzy012023085). can enhance the oil recovery during the microemulsion- forming surfactant flooding in the complex porous media. However, the viscosity of the microemulsion is not linearly Conflict of interest related to the recovery rate, and in real mines it is necessary to consider the economic cost to choose the microemulsion Author TW was employed by China National Petroleum viscosity increasing. Corporation. 4. Wettability can significantly affect the oil-water two-phase The remaining authors declare that the research was conducted displacement process within porous media, which further in the absence of any commercial or financial relationships that affects the in situ formation process of microemulsions. could be construed as a potential conflict of interest. Under water-wet conditions, the oil-water interface stays at the junction of the throat and the pore space, which contributes to the formation of microemulsions and thus to the Generative AI statement enhancement of recovery. The author(s) declare that no Generative AI was used in the It is worth noting that the proposed model is based on the Navier- creation of this manuscript. Stokes equations, which fundamentally restricts the analysis to conventional fluid dynamics. Consequently, the framework may not accurately capture flow behaviors in unconventional reservoirs (e.g., Publisher’s note shale formations) where nanoscale pore structures dominate the transport phenomena. In addition, while this investigation provides All claims expressed in this article are solely those of the authors valuable insights into oil-water displacement mechanisms at the pore and do not necessarily represent those of their affiliated scale and identifies key factors governing microemulsion-driven organizations, or those of the publisher, the editors and the recovery enhancement, direct field application remains challenging reviewers. Any product that may be evaluated in this article, or due to the inherent scale differences between laboratory claim that may be made by its manufacturer, is not guaranteed or investigations and reservoir conditions. endorsed by the publisher. References Alzahid, Y. A., Mostaghimi, P., Walsh, S. D. C., and Armstrong, R. T. (2019). 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