Influence of Functional Group Insertion on Clay Stabilizing Efficiencies of Antiswelling Agents: A Molecular Dynamics...

Clay minerals in oil and gas fields are prone to swelling, impacting wellbore stability and causing difficulties in water injection. Quaternary ammonium type cationic antiswelling agents have become important clay swelling inhibitors due to their electrostatic adsorption capabilities. Quantum mechanical density functional theory (DFT) calculations show that the local charges of quaternary ammonium and ethylene groups can be adjusted by inserting different functional groups into the molecule. However, the impact of the molecular charge distribution on the antiswelling effect has not been fully revealed. This study tests 21 surfactants and polymers based on vinylbenzyltrimethylammonium, methyl ester trimethylammonium, and amido trimethylammonium structures through functional group insertion using quantum mechanics calculations (QM) and molecular dynamics simulations (MD). The correlation between charge distribution and changes in montmorillonite layer spacing is explored. By adjusting the molecular structure, a series of charge distributions are obtained to evaluate their impact on antiswelling performance. The results show that the partial charge on the α-C and the β-CH₂ had the most significant impact on the antiswelling ability of the agent, with Spearman coefficients as high as -0.431 and -0.434, respectively, while the charge on the nitrogen atom had a weaker effect. Polar functional groups such as amido groups exhibit strong antiswelling properties, whereas hydrophobic functional groups are less effective. This study provides a theoretical basis for the molecular design of highly efficient antiswelling agents, which is of great significance for the time- and cost-efficient selection and development of antiswelling agents for water-sensitive reservoirs.