The study of droplet impingement and subsequent oscillations on solid substrates is critical for understanding wetting dynamics and heat transfer in applications such as spray cooling and metal coating. However, existing experimental approaches often rely on low-speed imaging or manual analysis, which limits their ability to capture rapid, transient oscillations and achieve high precision, particularly under extreme high-temperature and vacuum conditions. Moreover, conventional methods face challenges in automatically extracting key dynamic parameters like contact spot diameter and oscillation frequencies without user intervention. Novel methods for the computer processing of high-speed video images from experiments on droplet impingement and subsequent oscillations on a solid substrate were developed and validated. The image analysis algorithms, implemented in the Python programming language utilizing the OpenCV computer vision library, enable automated, high-precision tracking of dynamic droplet parameters. These methods were successfully applied to real high-temperature experiments involving molten copper droplets falling onto a polished tungsten substrate under vacuum conditions. Through automated image processing, time-dependent evolutions of the droplet center-of-mass coordinates, the diameter of the contact spot with the solid surface, and the dynamic contact wetting angles for the Cu/W system were obtained. The analysis reveals that upon impact, the droplet undergoes damped shape oscillations. The fundamental oscillation frequencies of the droplets and their dependence on mass were determined with high accuracy. Furthermore, by integrating data from other experimental systems, a generalized angular dependence of the oscillation frequency for sessile droplets was established and compared with existing theoretical models.
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.
Steam huff-and-puff has been widely applied for heavy-oil thermal recovery. However, the opacity of reservoir rocks limits direct pore-scale observation of oil mobilization, hindering the evaluation and optimization of co-injected fluids. To address this issue, a microfluidic experimental system is developed to simulate authentic reservoir huff-and-puff conditions, enabling the pore-scale investigation of steam-based oil recovery assisted by nitrogen and an oil displacement agent. The pore network used in our experiments features both low-permeability and high-permeability zones. In single-fluid injection experiments, stable flow channels form predominantly in the high-permeability zone, thereby limiting overall heavy oil mobilization. When multiple fluids are injected, however, the injection sequence proves to be highly influential. In a dual-fluid injection of oil displacement agent followed by nitrogen, emulsion and oil-in-microbubble cluster are formed, significantly improving oil mobility. In a ternary injection of nitrogen followed by oil displacement agent and steam, although steam reduces oil viscosity by raising the temperature, the accompanying condensate dilutes the oil displacement agent and thus affects its interactions with heavy oil and gas. These results clarify the role of injection fluids and their sequence in the nitrogen- and chemical-assisted steam huff-and-puff of heavy oil, providing important insights for the optimization of heavy oil recovery strategies.
CO2 injection into deep saline aquifer reservoirs is promising for long-term storage of greenhouse gases. To reveal the pore-scale mechanisms underlying CO2-brine displacement in subsurface formations, computational fluid dynamics simulations were performed in digitally reconstructed homogeneous and fractured porous media. Results showed that the displacement processes in the two types of porous media were governed by fundamentally different mechanisms. In homogeneous media, capillary forces associated with complex pore-throat geometries dominated the displacement behavior. Under low driving forces, the migration of CO2 was strongly restricted by capillary trapping, resulting in limited removal of brine. As the driving force increased, the injection of CO2 could overcome local pore-throat resistance and achieve effective displacement of brine. In fractured porous media, fracture structures provided preferential flow paths with lower hydraulic resistance, allowing the breakthrough of CO2 to occur under relatively low driving forces. However, after the breakthrough, fractures contributed only marginally to additional displacement of brine from the rock matrix, as CO2 preferentially flowed through the fracture channels. The present work provides quantitative and mechanistic insights into CO2-brine displacement processes in porous media, offering valuable guidance for the assessment and optimization of geological carbon storage strategies.
Imbibition plays a key role in the performance of injected fluids in enhanced oil recovery from low-permeability, tight reservoirs. Huff-n-puff with well-soaking and improving reservoir wettability can promote imbibition, resulting in forced imbibition. However, our understanding of its concept, influencing factors and the underlying mechanisms remains limited. Therefore, first, this work systematically reviews the relevant research and clarifies the concept of forced imbibition. Next, the mechanisms and enhanced oil recovery contributions of huff-n-puff and wettability improvement during the forced imbibition process are highlighted and summarized. Huff-n-puff and low-salinity water flooding are two key forced imbibition methods. Regarding huff-n-puff development, this work compares and analyzes the key controlling mechanisms and enhanced oil recovery effects of imbibition enhancement using three fluids: Water, gas and activated water. Gas is currently the most widely used huff-n-puff medium in oilfields because of its stronger mass transfer and diffusion capabilities, making it can enter smaller pore throats. Besides, water is also an irreplaceable huff-n-puff medium because of its fast energy replenishment, low cost, and environmentally friendly. Active water with the addition of surfactants, nanofluids and displacement systems can enhance the effect of water huff-n-puff, achieving a broader application potential. Subsequently, the enhanced oil recovery contribution rate of forced imbibition during the huff-n-puff process is discussed. Regarding low-salinity water flooding, this paper focuses on the mechanism of improving wettability and its effect on enhancing CO2 imbibition. It not only offers a comprehensive understanding of the concepts mechanisms and enhanced oil recovery effects of the forced imbibition process but also provides valuable insights for theoretical research and field applications of forced imbibition-based enhanced oil recovery technologies.
Synthesizing nanoparticles through green processes is a well-recognized challenge. It is a low-cost approach to produce multifunctional nanomaterials by using plant-based extracts as natural reductants and stabilizers. In this work, selenium nanoparticles were synthesized in Mentha piperita L. peppermint 3% as the concentration of the precursor solution. It is an essential oil containing bioactive compounds such as menthol, menthone, and flavonoids. The functionalized colloid of oil after being loaded with selenium nanoparticles is believed to exhibit enhanced properties. Fourier-transform infrared spectroscopy, ultraviolet-visible spectroscopy, and transmission electron microscopy have been employed to characterize the synthesized selenium nanoparticles. It was found that the selenium nanoparticles were uniformly distributed, spherical, and stable. Contact angle measurements showed that the selenium nanoparticles-loaded peppermint oil exhibited reduced hydrophobicity compared to pure oil, suggesting stronger interactions with polar surfaces. Interfacial interaction proposes that selenium nanoparticles preferentially migrate to the oil-water interface, reducing interfacial tension and stabilizing the colloidal system. This behavior contributes to the overall stability and uniformity of the oil/selenium nanoparticles conjugate. The present colloid displayed a significant enhancement in fluorescence. This confirms successful conjugation and enhanced optical performance, indicating their potential for bioimaging applications. Biological assessments demonstrated that the synthesized selenium nanoparticles and peppermint oil formulations exhibited significant antimicrobial and antioxidant properties. Hence, introducing these nanoparticles to peppermint oil increased its antioxidant capacity. The engineered colloidal system yielded a stable, fluorescent, and biologically active oil that may contribute to potential applications in bioimaging, antimicrobial therapy, antioxidant formulations, and nanotheranostics.
Understanding how carbonate rock microstructure evolves during dissolution is essential for optimizing acid stimulation and assessing the integrity of carbon dioxide storage reservoirs. In this study, four carbonate core samples subjected to hydrochloric acid flooding were analyzed using micro-computed tomography and pore-network modeling to quantify changes in pore geometry and transport properties. Three-dimensional digital reconstructions of pre-and post-dissolution volumes revealed significant restructuring of the pore system, including the coalescence of small pores and the formation of new flow channels. Quantitative analysis showed that the mean pore and throat radii increased substantially, accompanied by a marked rise in connected porosity. These morphological changes led to a substantial enhancement in absolute permeability, while hydraulic tortuosity generally decreased. The extent of microstructural restructuring varied markedly among the analyzed sub-samples, reflecting differences in their initial pore architecture and dissolution patterns. Changes in pore-network topology and throat connectivity exerted a stronger control on permeability than uniform pore-size enlargement alone. Higher injection rates promoted the formation of localized preferential flow pathways by enhancing advective transport, which strongly influenced the resulting permeability evolution. The results highlight the critical role of dissolution heterogeneity and injection rate in controlling permeability evolution, providing pore-scale insights. The results provide pore-scale insights primarily applicable to carbonate acidizing processes, while also offering qualitative implications for carbon dioxide-brine-carbonate systems.
Understanding salt precipitation during evaporation in fractured porous media is essential for predicting pore-structure evolution, fracture permeability changes, and leakage mitigation. In fractured systems, brine transport occurs in both fracture and matrix domains, resulting in complex interactions. Using X-ray microtomography, this study investigated the mechanisms of brine evaporation and salt precipitation in such media. The results revealed two distinct stages of salt migration: (I) Evaporation and capillary transport, and (II) salt precipitation and accumulation. In Stage I, fractures acted as preferential gas pathways, exhibiting lower brine saturation, while the matrix retained higher saturation due to stronger capillarity. Residual brine films on particle surfaces within the fracture sustained evaporation and enabled capillary backflow from the matrix, further increasing brine concentration. In Stage II, once solubility limits were exceeded, salt crystals formed in the fracture, inducing additional capillary suction that drew more brine from the matrix and the fracture center, thereby reducing salt accumulation in the fracture. At low flow rates, capillary replenishment from the matrix dominated over evaporation, sustaining continuous brine supply and extensive salt deposition throughout the fracture. At high flow rates, evaporation prevailed, restricting brine transport and confining salt accumulation mainly to the fracture-matrix interface. Across all conditions, salt precipitation progressively reduced effective pore sizes in both fracture and matrix, with implications for fracture permeability evolution.
Pore structure significantly governs the seepage characteristics of porous media. This study investigates this influence by comparing the infiltration behavior of homogeneous and heterogeneous porous structures through microfluidic experiments and numerical simulations. It constructed one heterogeneous structure derived from real rock cores and four homogeneous structures with regular particle arrangements, all with identical porosity. Heterogeneous structure and homogeneous structure exhibit similar finger-like flow patterns and minimal differences in water saturation. Air displacement in dead-end pores is driven by internal-external pressure differences, with water replacing air only when internal pressure surpasses external pressure. In homogeneous models, water pressure shows pulse-like fluctuations; pressure peaks due to interfacial resistance decrease from approximately 88 Pa to approximately 38 Pa as pore size increases. Meniscus evolution, linked to pore width variations, presents three states: Stretching, equilibrium, and expansion. This study clarifies the dominant role of pore morphology in fluid transport, providing a theoretical basis for optimizing structural design and efficiently regulating seepage processes in engineering applications such as resource extraction and CO2 sequestration.
The influence of brine dilution on interfacial behavior in sandstone reservoirs remains inadequately understood. In this study, molecular dynamics simulations were conducted to elucidate the effects of brine dilution on the wetting behavior of three crude oil systems containing O-, N- and S-bearing heteroatoms. Quartz was adopted as a model sandstone mineral, and the investigation centered on contact angles, fluid density distributions, radial distribution functions, interaction energies, and hydrogen-bonding interactions. The results showed that decreasing brine salinity enhanced quartz hydrophilicity. Contact angles decreased with dilution in O- and N-bearing oils, while the S-bearing system exhibited a non-monotonic trend; however, the overall changes were minor. A water film consistently formed between oil droplets and quartz, thickening with decreasing salinity, which lowered oil-quartz interaction and crude oil density near the surface. Alkanes tended to deplete near the interface, while aromatics preferentially accumulated in this region, although a portion of the aromatics could still migrate into the bulk phase. The relationship between nearwall non-hydrocarbon density and contact angle varied by oil type - positive for O- and S-containing oils, negative for N-containing oils. Brine dilution also strengthened waterquartz interactions through increased hydrogen bonding, further reducing oil adhesion. Despite these effects, the contact angles in all systems exhibited minimal changes upon brine dilution and remained within the strongly water-wet state. Thus, in reservoirs with inherent water-wet conditions, brine dilution alone is unlikely to significantly enhance oil recovery.
Geological carbon sequestration relies on the efficient conversion of injected supercritical CO2 into dissolved CO2, a process accelerated by density-driven convection. Yet, most assessments still consider the subsurface as homogeneous, offering limited guidance for the layered and heterogeneous architectures typical of sedimentary basins. Building on this shortcoming, this work examines how stratigraphic structure - such as homogeneous, randomly layered, stochastic, positive rhythmic, reverse rhythmic, coarse-first, and finefirst formations - governs the onset and efficiency of convective dissolution. Using a twodimensional model, this work tracks the dissolved-to-total CO2 mass fraction and relate system-scale kinetics to plume morphology. The findings reveal that stratigraphy exerts first-order control on both the timing and mode of CO2 transformation. Architectures with high-permeability pathways near the top, or those with strong small-scale heterogeneity, trigger early convection, promote vertically continuous fingering, and accelerate dissolution relative to a homogeneous benchmark. Randomly layered formations that divert flow produce moderate slowdowns. In contrast, low-permeability caps suppress vertical exchange, favor lateral spreading, and substantially delay conversion; coarse-first formations exhibit early lateral channeling that retards late-time mixing. Overall, the distribution of permeability in the upper reservoir and the scale of heterogeneity jointly control convective onset and dissolution efficiency, providing actionable guidance for formation screening, well placement, and monitoring horizons in geological carbon sequestration projects.
The coalescence of liquid lenses is relevant in various applications, including inkjet printing and fog harvesting. However, the dynamics of liquid-lens coalescence have been relatively underexplored, particularly in the case of liquid lenses with larger contact angles. The coalescence of low-viscosity liquid lenses is numerically investigated by means of the pseudopotential multi-component lattice Boltzmann method over a wide range of contact angles. In two-dimensional simulations, numerical results on the growth of the bridge height are in quantitative agreement with experimental measurements for small contact angles. In addition, a comparison of the simulation results with a theoretical approach based on the thin-sheet equations for liquid lenses shows that these equations accurately capture the bridge-growth dynamics up to moderate contact angles. For the three-dimensional case, the growth of the bridge radius is independent of the equilibrium contact angle of the liquid lenses at the initial stage of growth. The dependency between the growth of the bridge height and the bridge radius exhibits a non-linear to linear transition.
