Oil detachment and wettability reversal mechanism by gemini surfactants and divalent ions on oil-wet quartz surfaces

Surfactant flooding and ion-engineered waterflooding are promising techniques for reversing the wettability of sandstone reservoirs to enhance oil recovery. In this work, the wettability alteration process on oil-wet quartz surface by different surfactants (gemini, anionic, zwitterionic, and nonionic type) and ions in brine (Na⁺, Cl^-, Mg²⁺, Ca²⁺, and SO₄^2-) has been investigated using the molecular dynamics simulation method. The results show that the initial quartz surface exhibits oil-wetness because the basic protonated decylamines component in crude oil could be adsorbed onto the rock surface through hydrogen bonding and electrostatic interaction between oil and quartz surface. By contrast, anionic sulfonate gemini surfactant with higher charge density has better wettability reversal ability, which could make quartz surfaces more water-wet due to the strong ion pair, hydrogen bonding and electrostatic repulsion effect between gemini surfactant and crude oil. Additionally, Ca²⁺ could fully occupy negatively charged oil-wet sites of the quartz surface by the multiple ion exchange process, promoting the detachment of oil droplets. This study reveals that gemini surfactant and Ca²⁺ cations are suitable for effective wettability reversal, which would provide valuable guidance to design and optimize chemical formulations of the sandstone reservoir for enhanced oil recovery.