The use of a surfactant solution during oil and gas field development might improve the recovery rate of oil reservoirs. However, the serious emulsification of the produced liquid will bring challenges to the subsequent treatment process and storage and transportation. It is urgent to understand the coalescence mechanism of crude oil under the action of surfactant solution. This research investigates the coalescence mechanism of numerous oil droplets under liquid flow perturbation. The model was established to study the coalescence process of multiple oil droplets. The effects of the number of oil droplets under homogeneous conditions, the size of oil droplets, and the distance between oil droplets under non-homogeneous conditions on the coalescence process were analyzed. Meanwhile, the change rules of the completion time of oil droplet coalescence were drawn. The results show that the smaller the size of individual oil droplets under non-homogeneous conditions, the longer the coalescence completion time is, and when the size of individual oil droplets reaches the nanometer scale, the time for coalescence of oil droplets is dramatically prolonged. Compared to static circumstances, the time it takes for oil droplets to coalesce is somewhat shorter under gravity. In the fluid flow process, in the laminar flow zone, the coalescence time of oil droplets decreases with the increase of the liquid flow rate. However, in the turbulent flow zone, the coalescence time of oil droplets increases with the increase in the liquid flow rate. The coalescence time is in the range of 600~1000 ms in the flow rate of 0.05~0.2 m/s. In the presence of surfactants, the oil content in the emulsion system increases under the influence of pumping flow. The change in oil content rate with various surfactants is less impacted by flow rate, owing to the stable emulsion structure created by the extracted fluid within the reservoir. The study findings presented in this research provide technical assistance for effective crude oil storage and transportation.

The solid particles in the produced fluids from the oil wells treated by compound flooding can greatly stabilize the strength of the interfacial film and enhance the stability of the emulsion, increasing the difficulty of processing these produced fluids on the ground. In this paper, the oil phase and the water phase were separated from the SPAN series emulsions by electrical dehydration technology and adding demulsifier agents. The changing trends of the current at both ends of the electrodes were recorded during the process. The efficient demulsification of the emulsion containing solid particles was studied from the perspective of oil-water separation mechanisms. Combined with the method of molecular dynamics simulation, the effect of the addition of a demulsifier on the free movement characteristics of crude oil molecules at the position of the liquid film of the emulsion were further analyzed. The results indicated that the presence of solid particles greatly increased the emulsifying ability of the emulsion and reduced its size. Under the synergistic effect of demulsifier and electric dehydration, the demulsification effect of the emulsion increased significantly, and the demulsification rate could reach more than 82%. The addition of demulsifiers changed the stable surface state of the solid particles. The free movement ability of the surrounding crude oil molecules was enhanced, which led to a decrease in the strength of the emulsion film so that the water droplets in the emulsions were more likely to coalesce and break. These results are of great significance for the efficient treatment of wastewater from oilfields, promoting the sustainability of environment-friendly oilfield development.

Emulsion stability directly affects the effect of surfactants in driving oil production. In this paper, the influence of crude oil components and the molecular structure of surfactants on the stability of emulsions is studied. First, emulsion samples with different asphaltene contents are prepared and passed. The emulsion time strength stability was characterized, and its mechanism of action was analyzed by molecular simulation methods. It was found that asphaltenes directly affect the strength of the oil-water interface membrane. The higher the asphaltene content, the stronger the emulsion stability. Then the emulsion containing different molecular structure surfactants was prepared, it was found that the more unstable bonds on the hydrophilic groups of nonionic surfactants, the weaker the strength of the interfacial membrane, and the worse the stability of the emulsion, and the cationic surfactants. The binding ability of different hydrophilic group ions to water molecules is different, and the thickness of the water adsorption layer is different. The order of the binding ability of ions to water is: K⁺ > Na⁺ > NH4⁺. Graphical Abstract

In this study, the effect of fracturing fluid on the permeability of tight oil reservoirs is analyzed through oil absorption. The mechanism of permeation and absorption in tight oil reservoirs was studied using the molecular dynamics simulation of fluid flow through fractures in porous media containing crude oil. The influence of surfactants on the adsorption characteristics of crude oil formations on rock walls was also examined. The research results show that the introduction of the appropriate surfactant to the fracturing fluid could accelerate the rate of percolation and recovery as well as improve the recovery rate of absorption. The optimal concentration of polyoxyethylene octyl phenol ether-10 (OP-10) surfactant in the fracturing fluid was 0.9%. When the percolation reached a certain stage, the capillary forces in the crude oil and percolation medium in the pore stabilized; accordingly, the crude oil from the pore roar should be discharged at the earliest. The fluid flow through the fracture effectively carries the oil seeping out near the fractured wall to avoid the stability of the seepage and absorption systems. The surfactant can change the rock absorbability for crude oil, the result of which is that the percolating liquid can adsorb on the rock wall, thus improving the discharge of crude oil. The results of this study are anticipated to significantly contribute to the advancement of oil and gas recovery from tight oil reservoirs.

Water plugging is an important objective in oil field processes owing to the increase in the water content of the produced fluids. Hence, a type active crude oil for water plugging in oil fields was prepared by optimizing the emulsifier and crude oil concentrations. The emulsion stability, rheological properties, and plugging behavior of the active crude oil were investigated. Results showed that the emulsion stability of the active crude oil was affected not only by the composition of the crude oil but also by the hydrophibicity of the emulsifier. Liaohe oil with high contents of asphaltene and resin showed better viscosity–temperature characteristics than the other crude oils and was used to prepare the active crude oil with high viscosity. Sorbitan monooleate (Span80) with long unsaturated alkyl chains showed better emulsion stability than other emulsifiers; the active crude oil emulsion with Span 80 as the emulsifier showed excellent viscosity–temperature properties. The active crude oil formed with Liaohe oil and Span 80 showed better plugging performance for cores with high permeability and for those saturated with water, and the plugging rate was over 99%. The microvisual model clearly showed the emulsification and plugging behavior. Moreover, the oil recovery increased obviously after active crude oil injection into the heterogeneous core.