Control of sludge formation during acidizing through the investigation of acid and oil interactions and emulsion behavior in heavy oils using low salinity water.

Acidizing is widely used to improve well productivity, but contact between hydrochloric acid (HCl) and heavy and polar components of heavy crude oils, including asphaltene molecules, can cause strong emulsions and fluid-fluid sludge that damage the heavy oil reservoir. This study investigates how combining HCl with different smart-water formulations (seawater, diluted seawater, and ion-enriched brines containing Ca²⁺, Mg²⁺, and SO₄²⁻) affects the interfacial behavior of heavy oil. In this regard, a novel series of bottle tests was conducted at 80 °C for 48 h, and the resulting oil, emulsion, and interfacial layers were analyzed. Emulsion stability, sludge mass, droplet size, viscosity, interfacial tension (IFT), SARA (saturates, aromatics, resins, asphaltenes) fractions, FTIR (Fourier transform infrared spectroscopy)-ATR (Attenuated total reflection), elemental analysis, and zeta potential were measured to link changes in asphaltene chemistry to the macroscopic oil properties. The results show that acid prepared with deionized water produced the most stable emulsion, the smallest droplets, and the highest sludge mass. Low-salinity and Ca²⁺-rich brines also promoted strong sludge formation, confirming that insufficient ionic strength allows HCl to protonate polar oil components strongly. On the other hand, Mg²⁺- and especially SO₄²⁻-rich brines greatly suppressed sludge formation (about 0.67 g), produced larger droplets, lowered IFT (29.9 mN/m), and kept more polar and heteroatom-rich asphaltenes inside the oil. FTIR-ATR and elemental analyses show that ion-rich brines preserve oxygen- and sulfur-containing functional groups within the oil phase, limiting their migration toward the oil–acid interface. In contrast, low-salinity and calcium-rich systems promote the removal and aggregation of these groups. Zeta potential measurements further support this trend, as sulfate-rich systems develop more negative surface charges and exhibit a reduced tendency for sludge-related damage. Collectively, these observations indicate that sludge formation during acidizing is governed by the ability of brine ionic composition to regulate asphaltene surface activity and interfacial charge balance. Accordingly, ion tuning, particularly through sulfate and magnesium enrichment, stabilizes asphaltenes in the oil phase and suppresses sludge generation even under strongly acidic conditions. Conversely, low-salinity and calcium-dominated fluids intensify acid–oil incompatibility by promoting asphaltene protonation and interfacial precipitation. This mechanistic insight establishes a clear scientific foundation for designing acidizing fluids that minimize formation damage, enabling a transition from empirical approaches to chemistry-driven optimization.