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Crude oil in underground porous media can be emulsified by surfactants, which can lead to improved oil recovery. However, there have been limited investigations revealing the interactions between emulsions and grains and comparing the influence of different factors on the distribution of emulsions in porous media at the pore scale. In this study, a solution containing surfactant was injected into an oil-saturated microchip with heterogeneous pore geometries. A series of tests were conducted by varying the injection rate, surfactant concentration, oil viscosity, and displacing phase viscosity. Qualitative analysis of flow mechanisms and quantitative analysis of the distribution of emulsions were performed through pore-scale snapshots. The results demonstrate that emulsions undergo deformation, breakup, and retention in porous media. Furthermore, new micro-interactions between emulsion droplets and grains are discovered, including the impact effect, cutting effect, and the synergy of both effects. A high flow rate of the displacing phase promotes the formation of more and smaller emulsions, while a low flow rate encourages flow diversion. An increase in oil viscosity and a decrease in surfactant concentration are not conducive to the formation of emulsions. However, an increase in the displacing phase viscosity promotes the formation of emulsions and flow diversion. The experiments provide detailed insights into the flow behaviors of emulsions at pore scale and offer valuable information for surfactant flooding. [ABSTRACT FROM AUTHOR]

• The seepage law of O/W emulsion under different flow condition of injection rates and permeabilities was found. • Effect of dispersed-phase volume fraction, continuous-phase viscosity and droplets size on the seepage law was studied. • The non-linear seepage characteristic of emulsion increased with the dispersed-phase volume fraction and droplets size. • The nonlinear seepage characteristic of emulsion was slightly affected by continuous-phase viscosity. • Characterization model of emulsion was established in porous media. Stable emulsions with different continuous-phase viscosities, dispersed-phase volume fractions, and dispersed droplet sizes were prepared. Seepage experiments using heavy oil-in-water (O/W) emulsion were then conducted in sand packs to analyze the effects of different injection rates, permeabilities, and emulsion characteristics on the seepage law. Finally, an evaluation method for the seepage law was established, and a regression model that considered the factors influencing the non-linear seepage laws was developed using the 1stop software to quantitatively reveal the seepage mechanism. The matching relationship between droplet size and permeability determined the seepage behavior of the O/W emulsion. Emulsions with droplet sizes matched with permeability behaved as a pseudoplastic fluid, with a non-linear flow. With an increase in the dispersed-phase volume fraction, the emulsions displayed a more non-linear behavior due to the accumulated retention and disordered plugging of dispersed droplets. With an increase in the injection rate, the emulsion with a low continuous-phase viscosity had more significant non-linear characteristics than the emulsion with a high continuous-phase viscosity. The correlation of the characterization model was relatively strong under the experimental conditions, with a correlation coefficient of 93.28%. Therefore, the characterization model accurately predicted the effects of the non-linear seepage law for all factors studied. [ABSTRACT FROM AUTHOR]

• The orthogonal experiment addresses the factors influencing rheological properties of oil-in-water emulsions. • Improved prediction viscosity model is established by introducing the modified volume coefficient. • Emulsion exhibits shear-thinning behavior at dispersed-phase concentration higher than 30%. • Droplet size has an influence on viscosity because of hydration effects. • High shear rate enhances the dispersion degree of droplets, resulting in rheological characteristics of Newtonian flow. The rheology of oil-in-water emulsion is a key issues in heavy oil development, storage, transport, and processing. Based on single-factor experiments, the effect of shear rate (γ), dispersed-phase volume fraction (φ), continuous-phase viscosity (η c), and droplet size (D 50) on the rheological properties of heavy oil-in-water emulsions were evaluated through microscopic observation and rheological experiments. Furthermore, a 10-factor/3-level experiment was designed to investigate the sensitive factors affecting shear viscosity. The experimental results indicated that, when the volume fraction of dispersed-phase was higher than 0.3, the rheological characteristics of the prepared emulsions exhibited an obvious shear-thinning behavior, and the droplet size had a significant influence on shear viscosity at shear rates between 0.3 and 100 s−1. When the volume fraction of the dispersed-phase was less than 0.3, the emulsions behaved as Newtonian liquids. As the shear rates increase, the droplets disperse to a greater extent, leading a reduction in viscosity, and ultimately causing the emulsion to exhibit the characteristics of a Newtonian fluid. Owing to the strong hydration effect of small droplets, the viscosity of small droplet-size emulsions increases at a constant dispersed-phase volume fraction. Emulsions exhibit obvious shear-thinning behavior with an increase in shear rate, as a result of a low effective dispersed-phase volume fraction under a high shear rate. Finally, the volume coefficient of the Sibree model was corrected by considering correlation factors from the experiments, and the Sibree model was improved. The improved shear viscosity model better fits the experimental data for all the studied heavy oil-in-water emulsion, with the mean relative error of 9.3%. Therefore, the improved model can predict the effects of all factors affecting shear viscosity. [ABSTRACT FROM AUTHOR]

• The phase transition characteristics of emulsion system are studied. • The microscopic characteristics of emulsion system are studied. • The critical values of the influence factors of phase transition are obtained. • A theoretical method for heavy oil cold recovery is provided. Heavy oil reservoirs can be feasibly developed using viscosity reducer (VR), by which the heavy oil is emulsified to form a less viscous oil-in-water (O/W) emulsion. As the formed emulsion is unstable, we studied the phase transition characteristics of the heavy oil-VR-water emulsion system. First, we used an emulsifier to prepare the heavy oil-VR-water emulsion system. The microscopic properties of the emulsion system were investigated using a microfluidic test system and laser particle size analyzer after the emulsion system reached phase equilibrium. To measure the conductivity of the emulsion system, a conductivity meter was used. The effects of different concentrations of VR, salinities, and oil-water ratios on the type of emulsion were determined. The results indicated that an O/W emulsion occurred when the VR concentration was greater than 0.3 wt%, the oil-water ratio was lower than 1:1, and the salinity was less than 10,000 mg/L. Higher VR concentrations reduced the droplet size to the micron scale. A water-in-oil emulsion was formed when the VR concentration was less than 0.3 wt%, the oil-water ratio was higher than 1:1, and the salinity was higher than 10,000 mg/L. The parameters of heavy oil recovery using VR can be adjusted upon obtaining the critical values of the factors influencing the change in the type of emulsion. These findings can help better understand the phase transition characteristics of heavy oil-VR-water emulsion systems. [ABSTRACT FROM AUTHOR]

• Evaluation method for dispersed droplet size matching with different reservoir conditions is developed. • Matching relationship chart between emulsion droplet size and reservoir permeability is established. • Matched dispersed droplet size increases gradually as reservoir permeability increases. • Matched dispersed droplet size decreases slightly with the increase of oil-water ratio. Formation and percolation of emulsion are important to enhance heavy oil recovery through emulsification. The percolation behavior of emulsion is affected by permeability, concentration of emulsion, oil-water ratio, etc. To ensure the stable flow of emulsion in porous media and improve the oil-carrying capability of emulsion, it is necessary to study the matching relationship between dispersed droplet size in emulsion and reservoir permeability. The emulsion was prepared by a mulser, and then a series of sand pack flow tests were conducted to investigate the effects of the size of dispersion droplet, permeability and oil-water ratio on percolation. Based on the analysis of displacement pressure, variation of droplet size and oil-carrying capability, the paper established a matching relationship between droplet size and permeability. Due to the shear effect of throat and pore, injection pressure increased, and droplet size became smaller as the injection volume increased in the low permeability sand packs, which shown a poor matching relationship. Then, the permeability increased gradually until the flowing droplet size was remained constant, and the injection pressure was relativity stable. At the point, it can be confirmed that the permeability and the injected droplet size matched well. The study of Influencing factor indicated that the matched droplet size increased with the increase of reservoir permeability. In detail, when the permeability was about 2.76 μm2, the matched droplet median diameters (D 50) was about 3.60 µm. When the permeability was about 4.34 μm2, the matched D 50 was about 7.04 μm. When the permeability was about 5.60 μm2, the matched D 50 was approximately 11.03 μm. In addition, increasing the oil-water ratio of emulsion contributed to shortening the breakthrough time of the emulsion, thus the matched droplet size became a little smaller. [ABSTRACT FROM AUTHOR]

• Flow mechanisms of emulsion droplets are analyzed using the pore scale simulation. • Single droplet can pass the throat by deformation or snap-off. • Multiple droplets may block the throat by adsorption or bridging. • There is a trap effect when droplets flow through the parallel capillary. • Emulsion droplets may be pinched off when passing through the parallel capillary. We study the flow behaviors of emulsion droplets in constricted capillaries using a surfactant Lattice Boltzmann model. We first establish the model by introducing a surfactant component into the pseudopotential multiphase model and characterizing the amphiphilic structure using dipoles. Then we study the flow of emulsion droplets in single or parallel capillaries. The simulations show that single droplet can pass through single throat in three ways, i.e., passing directly, passing with deformation, and snap-off. Multiple droplets may flow through the capillary with coalescence, and can also block the throat by adsorption or bridging, which may induce upstream flow caused by buoyancy. There is a "trap effect" when emulsions flow through parallel throats, and the droplet could be pinched off by obstacles, while the surfactants can inhibit the droplet breakup due to Marangoni effect. The simulations present detailed flow behaviors at pore scale and provide new insights for emulsion flow modeling. [ABSTRACT FROM AUTHOR]