Interfacial mechanisms governing acid–brine–heavy oil interactions: quantitative analysis of asphaltene behavior and emulsion stability

This study investigates the synergistic influence of 15 wt% hydrochloric acid (HCl) and brines of varying salinity on asphaltene molecular structure, interfacial properties, and emulsion stability in heavy crude oil. Four acid–brine systems were tested. Interfacial tension (IFT), ATR (Attenuated total reflection)-FTIR (Fourier transform infrared spectroscopy) of asphaltene, elemental analysis, viscosity, zeta potential, and droplet size distribution were systematically evaluated. The fivefold-diluted seawater in the presence of HCl (aged oil-5d) exhibited the lowest IFT (47.6 mN/m) and the minimum viscosity (318.6 cP) among the HCl-treated diluted seawater systems, correlating with the greatest depletion of aliphatic sulfoxide groups (ALI = 0.196; AS=O = 0.302), aromaticity (ARO = 0.437), and carbonyl index (CA = 0.449). Elemental analysis confirmed the lowest sulfur and oxygen contents, indicating strong migration of polar asphaltenes to the oil–water interface. Zeta potential measurements showed the least negative value (–19.5 mV), signifying reduced surface charge and compact interfacial packing. Emulsion analysis revealed that (emulsion-5d) produced the highest droplet count (308) with the smallest mean size (~ 50.35 μm), confirming optimal interfacial structuring. On the other hand, emulsion generated from tenfold-diluted seawater in the presence of HCl (emulsion-10d) sample generated the fewest droplets (176; mean size = 58.83 μm), consistent with high ALI (0.255), high viscosity (391.9 cP), and more negative zeta potential (–27.9 mV), indicative of poor interfacial activity. This work demonstrates that by tuning the salinity of acid solutions, the interfacial behavior of asphaltenes can be controlled to mitigate emulsion-related challenges. The findings offer valuable insights for designing optimized acidizing fluids that minimize formation damage and improve treatment efficiency in heavy oil reservoirs.