Effect of Mg2+on the Retardation Mechanism of Gemini Surfactants

Reducing the acid–rock reaction rate is critical for enhancing the stimulation performance of deep carbonate reservoirs in the Sichuan Basin. Based on the second-stage mechanism of acid–rock reactions, this study proposes that the adsorption of surfactants onto the rock surface can serve as a barrier to H+ mass transfer, thereby inhibiting the reaction. To investigate the impact of Mg2+—a byproduct of dolomite acid etching—on surfactant adsorption behavior, a series of cationic gemini surfactants with varying alkyl chain lengths were synthesized. Molecular dynamics (MD) simulations, atomic force microscopy (AFM), and dynamic acid–rock reaction experiments were conducted to elucidate the influence of Mg2+ on retardation performance. The results show that with increasing alkyl chain length, the surfactant molecules tend to adopt a collapsed adsorption configuration, reducing the number of effective adsorption sites. Meanwhile, Mg2+ competes for adsorption on the rock surface, suppressing surfactant coverage. Longer-chain surfactants are also more prone to micellization, further weakening the retardation effect. Among the tested surfactants, 12–4-12 exhibited the most effective retardation, achieving a 73.5% reduction in reaction rate compared to conventional hydrochloric acid systems. This study reveals the mechanism by which Mg2+ affects the retardation performance of gemini surfactants and provides a theoretical basis for the design of next-generation retarded acid systems.