With the rapid development of the economy and the increasing demand for oil, the potential of conventional oil reservoirs has been depleted, making low-quality oil reservoir resources such as heavy oil increasingly important to petroleum workers. China has relatively abundant heavy oil reserves, accounting for about 20% of the total domestic oil reserves. How to exploit heavy oil reservoirs has become a public challenge in the domestic oil industry.
The characteristics of heavy oil are high content of asphalt and resin, high viscosity, uneven texture, and high difficulty in extraction. At present, heavy oil extraction in China mainly relies on thermal recovery such as steam stimulation and steam flooding. However, thermal recovery consumes high energy and has complex equipment, so cold recovery technologies such as chemical flooding are gradually receiving attention from scholars. The key to cold recovery technology is to reduce the viscosity of heavy oil, and the emulsification and viscosity reduction technology of heavy oil has become a research hotspot in recent years. The emulsification and viscosity reduction technology can significantly reduce the viscosity of heavy oil by adding suitable emulsifiers to it to form an oil-in-water emulsion, thereby improving its flowability and recovery efficiency. Dai Mingyang et al. analyzed the viscosity reduction effect of the composite emulsion system of non ionic emulsifier OP-12 and sodium oleate on heavy oil, and found that this composite emulsifier can form a stable emulsion with Huanxiling heavy oil in Liaohe Oilfield, with a viscosity reduction rate of 92.95%, which is significantly better than a single emulsifier OP-12. Sun Nana et al. systematically studied the water separation rate and viscosity reduction rate of water in oil emulsion formed by amphoteric surfactants and heavy oil, and investigated the effects of different types of polymers. The results show that the heavy oil emulsion prepared with a certain concentration of amphoteric surfactant CAB-35 has good stability and high viscosity reduction rate; In addition, due to steric hindrance, the polymer will inhibit the aggregation of oil droplets, thereby reducing the water separation rate of the emulsion. Li Meirong et al. studied the viscosity reducing effect of non ionic emulsifier OP-10 on heavy oil and believed that it can partially break down the stacking structure of asphaltene and resin, resulting in a significant decrease in viscosity of heavy oil.
Amphoteric surfactants have good dispersion, washing, emulsification, resistance to calcium and magnesium ions, anti-static properties, and are green and environmentally friendly. They are easy to mix with other surfactants and have broad application prospects. More importantly, amphoteric surfactants have high interfacial activity and can form ultra-low interfacial tension with crude oil, which is expected to become a new type of oil displacement chemical agent that can significantly improve the recovery rate of low-quality oil reservoirs. Betaine is an amphoteric surfactant. The hydrophilic group in the molecular structure contains sulfonic acid group and ammonium group, and the positive and negative centers are connected by methylene group. Zhou et al systematically studied the mechanism of betaine zwitterion surfactants to reduce the interfacial tension of crude oil, and found that the hydrophilic groups with large molecular size of betaine spread flat on the oil/water interface. Although their ability to reduce the interfacial tension alone is not strong, they can form a close mixed adsorption membrane with the active substances in crude oil, which has a synergistic effect on the reduction of the interfacial tension. The mechanism of synergistic reduction of interfacial tension is controlled by two factors: hydrophilic and lipophilic balance and size matching. Among them, hydrophilic and lipophilic balance affects the adsorption amount of surfactant molecules at the interface, while size matching regulates the arrangement and tightness of the interface adsorption film. When the degree of hydrophobic branching of betaine increases, its ability to reduce oil phase interfacial tension can be improved, but due to the increase in size, its synergistic effect is often disrupted.
Although the research of zwitterion surfactant has been very extensive, its application in the field of emulsification viscosity reduction is still very scarce. The author investigated the synergistic mechanism between surfactants with different hydrophilic or hydrophilic ends and LAB-35, using amphoteric surfactant LAB-35 as the main agent and controlling its hydrophilic end to be the same or similar. Adding organic bases to the binary system further improves the emulsification effect and selects a suitable composite system.