Snap-off of the nonwetting phase during spontaneous imbibition (SI) arises from the competition between precursor corner flow, bulk-meniscus motion, and pore-throat geometry. Quantitative understanding in complex three-dimensional (3D) porous structures remains limited. This study applies a multi-component multiphase Shan-Chen lattice Boltzmann method (MCMP Shan-Chen LBM) to examine how pore-to-throat size ratio (7), contact angle (B), viscosity ratio (M), and pore-network topology regulate 3D snap-off. Sphere-packing models (SC, BCC, FCC) were first used to isolate geometric effects. Snap-off frequency increases with larger 7 and smaller B and M, with critical behavior controlled by packing connectivity quantified by coordination number (Z, SC = 6 < BCC = 8 < FCC = 12). Higher connectivity facilitates multi-path corner-flow propagation and intensifies snap-off. A Micro-CT-derived tight-sandstone pore structure was then simulated to assess snap-off in a realistic medium. Reducing 7 from 6.207 to 3.315 via controlled dilation suppresses snap-off, decreases both the number and volume of trapped oil clusters, and increases oil recovery from 78.7% to 92.2%. Increasing B (5 degrees to 30 degrees) or M (0.1 to 10) similarly weakens corner-flow dominance and reduces snap-off intensity. Unlike sphere packings, the tight sandstone sustains imbibition even at high B, leaving only small isolated clusters. This behavior reflects its broad coordination-number distribution (Z = 1-18), which provides alternative pathways and mitigates geometrically induced interfacial arrest. These findings clarify the microscopic mechanisms governing 3D snap-off and offer guidance for enhancing the imbibition efficiency and hydrocarbon recovery in tight reservoirs.

Although nanoparticle fluids outperform conventional surfactants in spontaneous imbibition related oil recovery, the specific role of pore surface roughness remains elusive. This study investigates the impact of fractal roughness on spontaneous imbibition-driven displacement through microfluidic experiments, utilizing Koch-curve-based channels of varying geometric complexity. By tracking main meniscus retreat and analyzing corner or film flow, we quantitatively compare the oil recovery efficiency of nanoparticle fluid versus surfactant solution. Experimental results demonstrate that nanoparticle fluid consistently outperformed surfactant solution in oil recovery, with this advantage becoming more mote oil detachment by accelerating corner flow and facilitating wetting-film propagation along rough walls. Based on these insights, a roughness-informed analytical framework is developed to predict nanoparticle fluid enhanced recovery. This work provides porescale insights and a theoretical framework for evaluating nanoparticle fluid-enhanced oil recovery strategies in rough and heterogeneous unconventional reservoirs.

One of the major challenges for Type IV high-pressure hydrogen storage cylinders is the susceptibility of the plastic liner to collapse, where the interfacial properties between the liner and outer carbon fibre reinforced polymer (CFRP) play a critical role in preventing the liner from becoming unstable. However, variations in the service environment, particularly temperature fluctuations, can significantly compromise the stability of the interface between the liner and CFRP, thereby weakening their coordinated deformation during depressurization. To address this issue, nanosecond pulsed laser, atmospheric-pressure plasma, and their synergistic treatment were employed to modify the polyamide 11 (PA11) surface, and the effects of service temperature on the interfacial bonding properties of PA11-CFRP with different surface treatments were systematically investigated. The results revealed that the interfacial bonding properties were highly sensitive to temperature: under elevated temperatures, interface stiffness decreased while toughness increased, whereas at low temperatures, stiffness increased but toughness was insufficient. With increasing cycles of alternating thermal cycling, the interfacial bonding strength gradually deteriorated; untreated specimens approached failure after 1000 cycles, while synergistically treated specimens exhibited the strongest durability. Combined with microscopic analysis, the synergistic treatment integrated laser-induced roughening with plasma-induced activation, providing a dual enhancement mechanism of mechanical interlocking and chemical bonding. It inhibits excessive molecular relaxation and alleviates residual stress accumulation, thereby significantly improving interfacial durability and reliability under thermal environments. This study provides theoretical insights into the interfacial enhancement of Type IV hydrogen storage cylinders under service temperature and offers valuable guidance for improving their safety and long-term reliability.

The suction pile well construction technique is increasingly adopted in deepwater drilling projects. The soil–structure interaction mechanism during the penetration and installation of the wellhead suction pile in clay is complex. Given the critical demand for precise installation outcomes in engineering practice, the influence of penetration velocity on installation performance requires significant consideration. Through scale-model experimental methods, various penetration velocities were configured primarily by adjusting suction pump flow rates. The influences of these velocities on penetration resistance, penetration depth, and related metrics were systematically assessed. A case study was conducted based on the engineering parameters of a wellsite in the South China Sea. A theoretical algorithm for WSP penetration resistance was developed and subsequently refined through experimental data. Coefficient optimization was established via theoretical assessment of strain-rate dependency and experimental data calibration. The optimized algorithm demonstrated strong agreement with field measurements, achieving a coefficient of determination (R2) exceeding 0.9. Compared to conventional theoretical approaches, it incorporated explicit consideration of penetration velocity. The analysis indicates that in soft clay, the penetration resistance of wellhead suction piles exhibits significant sensitivity to penetration rate, increasing with higher velocities. The influence of penetration rate on penetration depth is relatively weak. This computational approach offers design guidance for installation procedures and enables the implementation of the suction pile well construction mode in the South China Sea.

The pore‐scale interfacial dynamics including main‐meniscus flow and corner flow usually occurs in heterogeneous porous media and significantly affects the macroscopic multiphase flow process. Numerical research on the competition between main‐meniscus flow and corner flow remains challenging due to the ambiguity in pore‐scale interfacial dynamics and the complexity of upscaling these processes to porous media, given the substantial spatial and temporal scale differences. In this study, we proposed a critical capillary number () by considering the interplay of local capillary and viscous forces, which predicts transition from main‐meniscus flow‐dominated processes into corner flow‐dominated processes during the strong imbibition. The criterion was integrated into a dynamic network model to translate pore‐scale interfacial dynamics into multiphase flow patterns in porous media. The forced imbibition in heterogenous porous media under different Ca were simulated and compared to microfluidic experimental data. The comparison indicates that the dynamic competition between main‐meniscus flow and corner flow has a vital impact on displacement behaviors predicted by pore‐scale modeling, and our dynamic network model accurately captures the interfacial dynamics observed in microfluidic experiments. Moreover, the impact of interfacial dynamics on macroscopic multiphase flow pattern and displacement efficiency in heterogeneous porous media were addressed during strong imbibition under various viscosity ratios and capillary numbers. The phase diagram manifests a monotonic effect of viscosity ratio on displacement efficiency at high Ca due to the dominance of viscous fingering. A non‐monotonic effect of viscosity ratio is revealed at low Ca , which is ascribed into competition between corner flow and main‐meniscus flow. This study highlights the gap in the existing models of interfacial dynamics at pore scale, and provide an effective upscaling approach to investigate the multiphase flow in porous media.

Understanding the microscopic characteristics and evolutionary patterns of pore structures during high-PV waterflooding is critical for improving the accuracy and efficiency of oil field development. While previous studies have primarily emphasized the geometric and morphological features of overall pore structures, they often overlook local pore-scale properties and their relationship with fluid transport capacity. This study proposes a novel classification method for microscopic pore structures that integrates both pore size and local flow conductivity, enabling a more physically grounded and quantitatively robust evaluation of pore systems across rocks with varying permeabilities. The classification scheme divides microscopic pores into six distinct types based on key parameters such as pore diameter and flow flux area. To validate this approach, high-PV waterflooding experiments were performed on six sandstone samples with different permeabilities. High-resolution micro-computed tomography (micro-CT) imaging was employed to capture the internal pore structures before and after flooding. The results reveal that while low-connectivity small pores dominate numerically across all samples, high-connectivity small pores account for the largest volumetric share in medium-permeability rocks. Although overall pore size distributions remain relatively stable during high-PV waterflooding, transitions between pore types occur, driven by localized structural changes. Notably, in medium-permeability rocks, the number of low-connectivity small pores increases, whereas high-connectivity small pores decline. These findings deepen our understanding of microscopic heterogeneity and provide a theoretical foundation for evaluating the occurrence of residual oil. Moreover, the proposed classification framework offers valuable guidance for optimizing enhanced oil recovery strategies in the late stages of ultra-high water cut development.

Spontaneous imbibition (SI) of water driven by capillary forces within underground reservoir is recognized as a key mechanism influencing the oil and gas recovery. A comprehensive investigation at both the pore and core scales is essential for a deeper understanding of the SI behavior. This study employs continuous weighing measurement and in situ computed tomography (CT) scanning to quantitatively characterize the SI of de-ionized water into a dry synthetic rock sample with a permeability of 124 mD. At the core scale, the co-current SI weighing data showed two-stage imbibition behavior. Detailed analysis indicated that the imbibition rate slowed after the imbibition front advanced to the top of the sample. Pore-scale observations using in situ CT imaging confirmed that the imbibed masses at the end of the two stages were consistent with the weighing data. At the end of the first stage, the imbibed de-ionized water was distributed throughout the pore space. Subsequent imbibition in the second stage was attributed to increased saturation within the pores. Notably, the central region of the sample imbibed less de-ionized water than its peripheral counterpart of equivalent volume in the first stage, which was caused by the heterogeneous distribution of pores and rock matrix in the central region. The integration of core-scale measurements and pore-scale imaging provided insights into the mechanisms governing SI in porous rocks.

The immiscible displacement behavior in porous media is crucial for oil recovery and subsurface remediation, yet how wettability influences this process across different pore structures remains unclear. Using a color-gradient lattice Boltzmann model, this study investigates immiscible displacement dynamics in porous media. Heterogeneous porous structures with various degrees of permeability were reconstructed using the quarter structure generation set algorithm, and wettability effects were analyzed with the contact angle set in the range of 30 degrees to 150 degrees. The numerical results showed that porous heterogeneity greatly affects the displacement efficiency via permeability-dependent flow pathway optimization. Enhanced efficiency was observed in high-permeability media through low-tortuosity channels, whereas low-permeability systems exhibited reduced efficiency due to capillary trapping in pores with lower flow capacity. Wettability alters displacement patterns via capillary forces-under hydrophilic condition, the displacing fluid preferentially enters smaller pores. Fractal dimension and Euler number were used to quantify flow heterogeneity, revealing that increased permeability reduces flow complexity and improves connectivity. Moreover, permeability heterogeneity and wettability interact to disrupt classical linear flow responses, leading to non-monotonic efficiency trends in low-permeability systems. These findings highlight the importance of pore-scale multiphase flow in heterogeneous media and offer new insights for predicting wettability effects in subsurface flows.

The network architecture has demonstrated considerable potential for enhancing the strength-ductility synergy in metal matrix composites (MMCs). Intuitively, the intersections of network layers are expected to induce a stress concentration, leading to premature brittle fractures. Introducing chamfers to round the network cells may mitigate the local stress concentration and thereby improve elongation. Here, a numerical simulation framework was developed to investigate the effect of chamfering on the mechanical behavior of a three-dimensional (3D) continuous SiC3D/Al composite with a network architecture. A Voronoi tessellation algorithm was employed to generate the continuous network structural SiC phase. By inducing ductile and brittle damage criterions in the matrix and reinforcement elements, respectively, the mechanical behavior can be predicted via the finite element method (FEM). The predicted mechanical properties reveal an unexpected trend: chamfering results in a simultaneous reduction in both strength (from 367 MPa to 312 MPa) and elongation (from 4.1% to 2.0%). With chamfering, the enlarged intersection of the network layer bears a lower load, whereas the narrower network plates exhibit higher stress concentrations. As a result, the overall load-bearing capacity of the SiC3D reinforcement decreases monotonically with an increasing chamfer size f. Furthermore, the non-uniform stress distribution promotes the premature fracture of the SiC3D, which reduces elongation. Additionally, the crack deflection behavior is suppressed in the chamfered models, leading to decreasing energy dissipation. This unanticipated outcome highlights an important architectural design principle: maintaining uniform geometric dimensions is critical for achieving optimal composite performance.

This study numerically investigates the impact of compaction on the longitudinal dispersion coefficient of granular materials by integrating the discrete element method with the pore network model. The results reveal a non-monotonic relationship between the dispersion coefficient and compaction. Specifically, the dispersion coefficient can decrease by up to 20% or increase by nearly 50% in magnitude. Furthermore, we define the variation in the dispersion coefficient, denoted as κ, which exhibits three distinct regimes across different Péclet numbers Pe. This non-monotonic behavior arises because compaction influences dispersion mechanisms in multiple ways. As the porous medium becomes more compact, the influence of molecular diffusion weakens, while both mechanical dispersion and hold-up dispersion intensify. This study identifies new sources and behaviors of hold-up dispersion that were not detected by classical dispersion theory. Specifically, hold-up dispersion arises in regions with weak flow, distinguishing it from zero-velocity zones, such as dead-ends or the interiors of permeable grains, as described in classical dispersion theories. Moreover, the newly identified hold-up dispersion is active only within intermediate ranges of Pe (10-1

Previous studies claimed that the non-monotonic effects of wettability came mainly from the heterogeneity of geometries or flow conditions on multiphase displacements in porous media. For macroscopic homogeneous porous media, without permeability contrast or obvious preferential flow pathways, most pore-scale evidence showed a monotonic trend of the wettability effect. However, this work reports transitions from monotonic to non-monotonic wettability effects when the dimension of the model system rises from two-dimensional (2-D) to three-dimensional (3-D), validated by both the network modelling and the microfluidic experiments. The mechanisms linking the pore-scale events to macroscopic displacement patterns have been analysed through direct simulations. For 2-D porous media, the monotonic effect of wettability comes from the consistent transition pattern for the full range of capillary numbers $Ca$ , where the capillary fingering mode transitions to the compact displacement mode as the contact angle $\theta$ decreases. Yet, it is indicated that the 3-D porous geometries, even though homogeneous without permeability contrast or obvious preferential flow pathways, introduce a different $Ca$ – $\theta$ phase diagram with new pore-scale events, such as the coupling of capillary fingering with snap-off during strong drainage, and frequent snap-off events during strong imbibition. These events depend strongly on geometric confinements and capillary numbers, leading to the non-monotonicity of wettability effects. Our findings provide new insights into the multiphase displacement dependent on wettability in various natural porous media and offer design principles for engineering artificial porous media to achieve desired immiscible displacement behaviours.

Wettability of rocks may undergo a dynamic evolution during long- term waterflooding. Existing studies usually assume a constant wettability property of rock and ignore the wettability evolution during long- term waterflooding, resulting in ambiguous recognition of residual oil resources. To uncover the wettability evolution within complex pore structure, the effects of clay content on local contact angles are addressed in the sandpacks during long- term waterflooding via in- situ computed tomography (CT) imaging experiments. In this study, the wettability evolution is quantitatively characterized by mean contact angle, contact angle hysteresis, and wettability representative element volume (REV) size. Based on surface roughness and clay deposition patterns, we propose a physical model to analyze the mechanism of wettability evolution. The results indicate that the contact angle distribution within the pore structure is heterogeneous while locally continuous. Water- clay turbid treatment is effective in altering the wetting state of sandpack to less water- wet, reducing the contact angle hysteresis, and enhancing the heterogeneity of contact angle distribution. Waterflooding makes the sandpacks more waterwet and decreases the heterogeneity of contact angle distribution. Transition of surface roughness and clay deposition patterns during waterflooding accounts for the contact angle alteration and determines the overall wettability evolution.

To improve the understanding of trapping and mobilization mechanism during multiphase flow in porous media, long-term waterflooding experiments have been performed to capture the water-oil displacement behaviors in sandstone cores with different permeability and porosity. The residual oil is therefore categorized based on force analysis and morphological characteristics. The critical values for categorization are determined by measuring differences using statistical evaluation. Simplified models of five types of residual oil are established and the dynamic variations of types and volumes of residual oil are characterized during the long-term waterflooding in the sandstone cores. The results indicate that the pore structure heterogeneity impacts the force balance and morphology of residual oil, leading to various dominant types in the sandstone cores with different permeability. Moreover, for effective mobilization of the residual oil, different strategies of enhanced oil recovery are designed controlling the force balance of residual oil. The strategies for oil mobilization are validated by the dynamic pore network modeling, and the enhanced oil recovery is up to 28 %. This study provides a new classification method of residual oil and corresponding mobilization strategies from the perspective of force balance and morphology, which is vital to enhance oil recovery in the high/ultra-high water-cut stage of the oil reservoirs.

As a result of complex pore-throat geometry and precursor corner flow, the snap-off of the non-wetting phase occurs during the spontaneous imbibition (SI) of wetting phase. However, accurate modeling of such pore-scale flow behavior remains a big challenge, and its influencing factors remain unclear. In this study, an improved pseudopotential lattice Boltzmann method (LBM) is used to analyze the snap-off behavior during the SI process in three-dimensional (3D) pore-throat models with rough surfaces. The influence of the pore-to-throat size ratio (λ), contact angles (θ), and Ohnesorge number (Oh) on the occurrence of the snap-off are investigated and based on which a 3D phase diagram is established. The snap-off is more likely to occur with the increase in λ and Oh and decrease in θ, respectively. Only when the λ is ≥2 and the θ is <13°, the snap-off may occur. With the increase in θ from 0° to 13°, the snap-off is suppressed due to the relatively small advancing difference between the corner flow and the bulk meniscus. Volume fraction of the entrapped gas bubble in the pore increases with the increase in λ and Oh and the decrease in θ. The time when snap-off occurred increases with the increase in λ and θ, and decrease in Oh. These results are fundamental for investigating snap-off phenomena in real 3D pore space and guide how to avoid or facilitate the occurrence of snap-off and to control the degree of snap-off.

Utilizing the discrete element method and the pore network model, we numerically investigate the impact of compaction on the longitudinal dispersion coefficient of porous media. Notably, the dispersion coefficient exhibits a non-monotonic dependence on the degree of compaction, which is distinguished by the presence of three distinct regimes in the variation of dispersion coefficient. The non-monotonic variation of dispersion coefficient is attributed to the disparate effect of compaction on dispersion mechanisms. Specifically, the porous medium tightens with an increasing pressure load, reducing the effect of molecular diffusion that primarily governs at small Péclet numbers. On the other hand, heightened pressure loads enhance the heterogeneity of pore structures, resulting in increased disorder and a higher proportion of stagnant zones within porous media flow. These enhancements further strengthen mechanical dispersion and hold-up dispersion, respectively, both acting at higher Péclet numbers. It is crucial to highlight that hold-up dispersion is induced by the low-velocity regions in porous media flow, which differ fundamentally from zero-velocity regions (such as dead-ends or the interior of permeable grains) as described by the classical theory of dispersion. The competition between weakened molecular diffusion and enhanced hold-up dispersion and mechanical dispersion, together with the shift in the dominance of dispersion mechanisms across various Péclet numbers, results in multiple regimes in the variation of dispersion coefficients. Our study provides unique insights into structural design and modulation of the dispersion coefficient of porous materials.

Understanding preferential flow in porous media holds substantial theoretical significance on the design and optimization of hydrocarbon exploitation in shale reservoir. Previous researches discussed the competition of imbibition front in layered porous media while the underlining mechanism for interfacial dynamics and induced displacement efficiency of multiphase flow remains ambiguous. In this paper, we investigate the spontaneous imbibition in dual permeable media and analyze the flux exchange between the neighboring porous zones with permeability contrast using dynamic pore network model. The impact of fluid viscosity ratio and permeability contrast on the spontaneous imbibition preference have been addressed, and finally a phase diagram for displacement efficiency has been obtained. The results reveal that the dual permeable structure enhances the invasion rate of wetting fluid in the low-permeable zone and induces unstable displacement patterns, leading to reduction of the long-term displacement efficiency. The interfacial pattern transition from stable displacement to unstable pattern in dual permeable media could be ascribed into the flux exchange between dual permeable zones, which shows a contrary impact on the fluid flow within the low-permeable zone under favorable and unfavorable viscosity ratios. The permeability contrast in dual permeable media intensifies this impact during spontaneous imbibition. These results help us to understand the occurrence and mutual interaction of multiphase flow in layered porous media, and provide a theoretical guidance for the hydrocarbon exploitation in shale reservoir. The shale oil reservoir has emerged as a crucial contributor to the global energy supply, primarily attributed to stimulation techniques such as hydraulic fracturing. The geological composition of shale rock encompasses stratified layers characterized by varying permeability, leading to diverse multiphase flow patterns at the pore scale during the shut-in period. These intricate flow patterns significantly impact shale oil recovery during subsequent exploitations. However, a gap exists in comprehending the optimal design of injection conditions by controlling the multiphase flow pattern for a high-efficiency exploitation strategy. The elucidation of multiphase displacement mechanisms within the complex porous structure featuring staggered layers are imperative. To address this gap, pore-scale modeling and analysis were conducted to visualize spontaneous imbibition behaviors across varying fluid viscosity ratios and permeability contrasts. The findings indicate that the imbibition flux in low-permeable zone is augmented by fluid flow in high-permeable zone. Furthermore, an escalating permeability contrast intensifies the flux enhancement and reduce the displacement efficiency in dual permeable media by changing the displacement pattern from stable to unstable distribution. These outcomes serve as a theoretical foundation, providing insights for the selection and optimization of an effective exploitation strategy to enhance shale oil recovery under complex engineering conditions. A dynamic pore network model for studying spontaneous imbibition in dual permeable media is presented The contrary effect of flux exchange on spontaneous imbibition within low-permeable zone under various viscosity ratios is observed Transition of interface dynamics from stable to unstable pattern is induced by unfavorable viscosity ratio and large permeability contrast

Pore-scale modeling plays a crucial role in understanding and upscaling solute transport behavior in porous media. Direct simulation offers the highest fidelity in resolving pore-scale fluid flow and mass transfer, nevertheless, with unacceptable computational costs for practical applications. Pore network models (PNM), on the other hand, provide an efficient alternative but with reduced accuracy in representing transport dynamics. In this study, we propose a new framework of pore network models that ensures both accuracy and efficiency, which reproduces the pore-scale shear dispersion effect by utilizing the pore-scale shear dispersion coefficient to calculate the diffusive mass exchange rate between network elements. The coefficient is determined based on the extension of Taylor-Aris theory that expands the classical Taylor-Aris theory to encompass the pre-asymptotic regime. Additionally, the framework adopts a physically representative pore network, where the conduit length and volume of network elements are determined based on local resistance equivalence. After verifying its accuracy and reliability, we conduct a series of numerical cases on tube networks, sphere packs, and sand packs. The breakthrough curves and concentration profiles obtained from our new model show good agreement with direct simulation results and experimental data, while the simulation outcomes of the models that rely on the molecular diffusion coefficient or Taylor dispersion coefficient for local mass exchange may exhibit significant errors. We finally demonstrate that the significance of pore-scale shear dispersion on solute transport weakens for tube networks with increasing degrees of geometrical disorder. The proposed model provides an accurate and efficient numerical framework for the study of solute transport and dispersion in porous media.

Understanding and controlling fluid entrapment during forced imbibition in porous media is crucial for many natural and industrial applications. However, the microscale physics and macroscopic consequences of fluid entrapment in these geometric-confined porous media remain poorly understood. Here, we introduce a novel multidepth microfluidic chip, which can mitigate the depth confinement of traditional two-dimensional (2-D) microfluidic chips and mimic the wide pore size distribution as natural-occurring three-dimensional (3-D) porous media. Based on microfluidic experiments and direct numerical simulations, we observe the fluid-entrapment scenarios and elucidate the underlying complex interaction between geometric confinement, capillary number and wettability. Increasing depth variation can promote fluid entrapment, whereas increasing capillary number and contact angle yield the opposite effect, which seemingly contradicts conventional expectations in traditional 2-D microfluidic chips. The fluid-entrapment scenario in depth-variable microfluidic chips stems from microscopic interfacial phenomena, classified as snap-off and bypass events. We provide theoretical analyses of these pore-scale events and validate corresponding phase diagrams numerically. It is shown that increasing depth variation triggers snap-off and bypass events. Conversely, a higher capillary number suppresses snap-off events under strong imbibition, and an increased contact angle inhibits bypass events under imbibition. These macroscopic imbibition patterns in microfluidic porous media can be linked with these pore-scale events by improved dynamic pore-network models. Our findings bridge the understanding of forced imbibition between 2-D and 3-D porous media and provide design principles for newly engineered porous media with respect to their desired imbibition behaviours.

In reality, the remaining oil in the ultra-high water cut period is highly dispersed, so a thorough investigation is required to understand the microscopic remaining oil. This will directly influence the technological direction and allow for countermeasures such as enhanced oil recovery (EOR). Therefore, this study aims to investigate the state, classification method and utilization mechanism of the microscopic remaining oil in the late period of the ultra-high water cut. To achieve this, the classification of microscopic remaining oil based on mechanical mechanism was developed using displacement CT scan and micro-scale flow simulation methods. Three carefully selected mechanical characterization parameters were used: oil–water connectivity, oil–mass specific surface and oil–water area ratio. These give five types of microscopic remaining oil, which are as follows: A (capillary and viscous oil cluster type), B (capillary and viscous oil drop type), C (viscous oil film type), D (capillary force control throat type), and E (viscous control blind end type). The state of the microscopic remaining oil in classified oil reservoirs was defined after high-expansion water erosion. Based on micro-flow simulation and analysis of different forces during the displacement process, the main microscopic remaining oil recognized is in class-I, class-II and class-III reservoirs. Within the Eastern sandstone oilfields in China, the ultra-high water-cut stage is a good indicator that the class-I oil layer is dominated by capillary and viscous oil drop types distributed in large connected holes. The class-II oil layer has capillary and viscous force-controlled clusters distributed in small and medium pores with high connectivity. In the case of the class-III oil layer, it enjoys the support of capillary force control throats that are mainly distributed in small holes with high connectivity. Integrating mechanisms of different types of micro-remaining oil indicates that, enhancing utilization conditions requires increasing pressure gradient and shear force while reducing capillary resistance. An effective way to improve the remaining oil utilization is to increase the pressure gradient and change the flow direction during the water-drive development process. Hence, this forms a theoretical basis and a guide for the potential exploitation of remaining oil. Likewise, it provides a strategy for optimizing enhanced oil recovery in the ultra-high water-cut stage of mid-high permeability oil reservoirs worldwide.

Viscous coupling effect on pore-scale flow capacity and its impact on macroscopic flow properties in porous media remain subjects of ongoing debate. This study employs the Stokes equation in conjunction with the zero-velocity interface and continuity interface to investigate viscous coupling effect during two-phase flow through a square capillary tube. Our findings substantiate the significance of the viscous coupling effect in angular pore multiphase flows. Enhanced fluid conductance is linked to fluid viscosity ratios, contact angles, and wetting-film lengths at the pore scale. Proposing a novel upscaling approach, we formulate hydraulic conductance in a square capillary tube for multiphase flows by incorporating a viscous coupling term, and its validation is accomplished through comparison with results from lattice Boltzmann method simulations. Our scaling model predicts hydraulic conductance with a mean relative error of less than 1%, outperforming prior viscous coupling models with errors reaching up to 19%. The derived scaling model, incorporating the viscous coupling effect, holds potential for integration within pore-network models, offering an efficient and precise simulation method for characterizing two-phase flow through porous media at a representative scale.