3.1.1  Anionic Surfactant Emulsion Viscosity Reducer

Anionic surfactants are currently the most widely used emulsifying and viscosity reducing agents, with strong emulsifying ability and stable performance at high temperatures. Commonly used ones include alkyl sulfonates, alkylbenzene sulfonates, a-olefin sulfonates, etc. Sarmah et al. compared the effects of anionic surfactants sodium lignosulfonate, chromium free sodium lignosulfonate, and non ionic surfactant TX-100 on Assam heavy oil, and found that all three surfactants can effectively reduce the viscosity of heavy oil, but sodium lignosulfonate has the best effect. Puwanfen et al. selected sodium dodecyl sulfonate (SDS) as the emulsion viscosity reducer, and found that the increase of salinity not only reduced the viscosity reduction effect, but also reduced the stability of lotion. This is because most sulfonated anionic surfactants have poor mineralization resistance and are prone to precipitation with divalent cations. At the same time, they also found that when pH=10, the viscosity was the lowest and the viscosity reduction rate reached 99%. This is because alkali can promote the generation of petroleum salts, thereby synergizing with SDS to reduce the viscosity of heavy oil. In addition, studies have shown that high molecular weight olefin sulfonates can help suppress the formation of liquid crystal structures and are also suitable for emulsification and viscosity reduction of high viscosity heavy oil with high wax and high asphaltene content. Yang et al. investigated the emulsifying and viscosity reducing effects of a ternary composite system of alpha olefin sulfonates (AOS), linear alcohols, and bases on heavy oil. The experimental results show that when 0.2% AOS, 5% n-pentanol, 0.25% Na₂CO₃, and 0.2% DEA are added, the emulsification and viscosity reduction effect is the best, with a viscosity reduction rate of 98.94%.

In addition to conventional small molecular weight surfactants, some scholars have also attempted to prepare polymer type surfactants with high molecular weight as emulsifying and viscosity reducing agents, combining emulsifying and viscosity reducing with polymer flooding to further improve the adaptability of viscosity reducing agents to complex medium and deep heavy oil reservoirs. Li Juan et al. synthesized a series of anionic long-branched polymer viscosity reducers (AAGAS) containing glycidyl methacrylate (GMA). The results show that AAGAS has good surface and interface activity, and its unique association makes AAGAS solution not only thickening, but also transforming high viscosity heavy oil into low viscosity O/W lotion. 1000 mg/L AAGAS-3 can reduce the viscosity of heavy oil by 96.8%, which is significantly better than commercial small molecule surfactants SDBS and polymer surfactants used in the Bohai Oilfield field.

Anionic surfactants have a wide range of sources, low prices, strong emulsifying ability, and outstanding temperature resistance. They have great application prospects in heavy oil emulsification and viscosity reduction. Polymer surfactants have further improved the temperature resistance of anionic emulsifying and viscosity reducing agents, which deserves further attention in the future. However, anions have poor resistance to mineralization and are prone to precipitate with high valence cations in crude oil, which limits their widespread application in medium to deep heavy oil reservoirs.

3.1.2  Non-ionic Surfactant Emulsion Viscosity Reducer

Unlike anionic surfactants, non-ionic surfactants have a non charged head group that is not easily affected by formation ions and has excellent salt resistance. They have good application prospects in heavy oil emulsification and viscosity reduction. Li Meirong et al. studied the emulsification and viscosity reduction effect of non-ionic emulsion viscosity reducer OP-10 on ultra heavy oil in Shengli Oilfield, and found that OP-10 can reverse the heavy oil emulsion from W/O emulsion to O/W emulsion, with a viscosity reduction rate of 99.59%. Kumar et al. studied the emulsification and viscosity reduction effect of nonionic surfactant Qulatong X-100 and found that under optimal conditions, the viscosity reduction rate reached 83%. After being stored at room temperature for 56 days, the emulsion can still remain stable. Vegas et al. investigated the emulsifying and viscosity reducing effect of polyoxyethylene sorbitol monooleate (Tween 85). Under optimal conditions, the viscosity of heavy oil decreased from 5151 mPa·s to 216.2 mPa·s, indicating a good viscosity reduction effect. Jiang et al. studied the spontaneous emulsification effect of fatty acid alkanolamide dipolyoxyethylene ether (NS) on heavy oil in Liaohe Oilfield. The results showed that after standing for 12 hours at 65℃, the same volume of heavy oil could be completely emulsified in a 2% NS solution, forming a stable emulsion with a viscosity reduction rate of 99.74%. Spontaneous emulsification refers to the formation of O/W emulsions solely by the emulsifier itself, without the need for high shear mixing or additional ultrasonic operations. Spontaneous emulsification can significantly reduce energy consumption and is of great significance for viscosity reduction in medium to deep heavy oil. It is worth further research and attention in the future.

In addition, there have been many reports in recent years on natural non-ionic surfactants extracted from plants or synthetic non-ionic surfactants based on plant extracts. Kumar et al. compared the emulsification and viscosity reduction effects of natural non-ionic surfactants extracted from dried soapnut fruits with commercial surfactants such as dodecyl polyoxyethylene ether on heavy oil. The results showed that the viscosity reduction rate of natural surfactants was 87%, and the viscosity reduction effect was better than that of commercial surfactants. Liu Shujie et al. synthesized a non ionic environmentally friendly surfactant based on modified alkyl glycosides. This emulsion viscosity reducer meets the environmental requirements of offshore heavy oil emulsion viscosity reducers. A 0.3% viscosity reducer can reduce the interfacial tension between oil and water to 10^-3 mN/m, with a viscosity reduction rate of 92.1%. Kumar et al. extracted triglycerides from sunflower seed oil and synthesized a novel nonionic surfactant, triethanolamine monosunflower ester, through hydrolysis and esterification reactions. The experimental results show that when the oil-water ratio is 6:4 and the amount of surfactant added is 2%, the viscosity reduction rate reaches 96%.

3.1.3  Anionic Non-ionic Surfactant Emulsion Viscosity Reducer

Although non-ionic surfactants have significant advantages in terms of mineralization resistance, their temperature resistance is relatively poor, and they are usually used in combination with other surfactants in practical applications. Considering the excellent temperature resistance of anionic surfactants, some scholars have combined them with non-ionic surfactants to achieve both salt and temperature resistance, in order to cope with the more complex reservoir environment of mid to deep heavy oil reservoirs. Chen et al. developed a novel composite viscosity reducer CSY-1, consisting of 0.5% OP-10, 1.8% SDBS, 0.1% Tween 80, and 1% NaOH (or Na₂CO₃). The viscosity reduction rate can exceed 99%, and the daily oil production of a single well has increased from 3.74 t to 8.12 t, which has important industrial value. Wang et al. synthesized a non-ionic polymer surfactant TPVR7 containing polyethylene glycol structure and applied it in combination with the anionic surfactant sodium diisooctyl succinate sulfonate DSS. The results show that under the optimal dosage, the viscosity reduction rate can reach 97% and the recovery rate can increase to 21.89%, which has great potential for application.

Although the synergistic effect of anionic and non-ionic surfactants can significantly promote emulsification and viscosity reduction of heavy oil, and improve heavy oil recovery, some studies have also shown that composite surfactants can undergo "chromatographic separation" in the formation, weakening the synergistic effect. To avoid this issue, researchers incorporated anionic non-ionic composite surfactants into a molecule. Wu et al. synthesized octylphenol polyoxyethylene ether sulfonate from octylphenol ethoxylate, chloropropene, and sodium bisulfite as raw materials. The results indicate that with the increase of mineralization, the viscosity reduction effect deteriorates. However, when the mineralization degree is below 3000 mg/L, the viscosity reduction rate still remains above 90%, with a maximum of 95%. It has the potential for application in high salinity heavy oil reservoirs. Yang et al. prepared a composite viscosity reducer SDG-2 using fatty alcohol polyoxyethylene ether (AEO), Triton X-100, and aromatic/alcohol mixtures as raw materials, which exhibited excellent high temperature and salt resistance. For different types of heavy oil, the viscosity reduction rate all exceed 99%, and the performance is still stable at 140℃. In a high salinity solution of 2.26×10⁵mg/L, 0.15% SDG-2 can reduce the interfacial tension between oil and water to 0.41 mN/m. In on-site experiments, SDG-2 can significantly reduce the amount of light oil used and increase production to 22.5%. Similar to the structure of anionic polymer surfactants, polymer surfactants containing both anionic and non-ionic structures have also received attention from scholars, with the aim of further enhancing the adaptability of emulsified and viscosity reducing agents to complex formations of medium to deep heavy oil. Zhang et al. synthesized a series of ternary polymers PAAI_x that combine sulfonic acid groups and polyoxyethylene chains (x represents the length of the polyoxyethylene chain). The emulsion formed by PAAI₁₅ is the most stable, with a viscosity reduction rate of up to 97.5%.

Negative non-ionic surfactants have not only good emulsification and viscosity reduction effects, but also outstanding temperature and salt resistance, which have important application value. However, the synthesis process of anionic non-ionic surfactants is complex and costly, therefore, it is currently only limited to indoor research. It is believed that with the deepening of research and optimization of synthesis processes, anionic non-ionic surfactants have great potential in reducing viscosity of heavy oil in the future.

In summary, the emulsification and viscosity reduction technology for heavy oil has been widely applied in the petroleum industry, but it still poses certain challenges. Firstly, emulsifying and viscosity reducing agents have strong selectivity, meaning that a certain type of surfactant is only suitable for a certain type of heavy oil. There is currently no clear answer on how the composition of heavy oil affects the emulsification effect of surfactants. Therefore, further analysis of the interaction between surfactants and heavy oil components is needed to develop a viscosity reducer with a wider range of applications and stronger adaptability. Secondly, with the deepening of heavy oil development in the middle and deep layers, the geological conditions of the reservoir have become increasingly complex, which puts higher requirements on surfactants. They not only need to significantly reduce interfacial tension, but also need to have good high-temperature and high mineralization resistance performance. Although the combination of anionic and non-ionic surfactants can adapt to complex heavy oil formations with high temperature and high mineralization, the synthesis process is complex and the cost is high. Therefore, considering economic factors, it is necessary to optimize the synthesis process to reduce the cost of anionic non-ionic surfactants. Finally, it is one of the biggest challenges to control the stability of lotion and avoid the interference of external factors, which will lead to mining accidents, and the sewage treatment after demulsification is also a thorny problem.