Influence of Potassium Salts on Adsorption and Surface Hydration Inhibition of Hexane-1,6-Diamine Clay Mineral Inhibitor: A Molecular Dynamics Study

Summary Most shale formations contain a large amount of expansile sodium montmorillonite (Na-Mnt), have high water adsorption, and experience hydration swelling, which causes an unstable wellbore wall resulting from formation shrinkage, sticking, balling, time-wasting, and well collapse, and, in extreme cases, causes complex downhole accidents and high costs. Polyamines, like hexane-1,6-diamine (HEDA), are shale inhibitors in shallow drilling. Inorganic and organic K-salts can improve the inhibitory performance of polyamine inhibitors. In this study, we applied molecular dynamics (MD) simulations to investigate the influence of K-salts on the inhibition mechanism of HEDA to the Na-Mnt. The adsorption configuration is mainly by terminal amine groups with a flat-lying alkyl hydrophobic chain. Interaction with clay negative charge reduces its surface hydration. HCOOK significantly improved the inhibition ability and coordination of interlayer ions with H2O. As they were replaced by K+, H2O/HCOO− coordination reduced Na-Mnt/H2O interactions, and mobility capability of H2O molecules was significantly decreased, with a maximum concentration noted in this study of 1.65 wt%. While KCl showed little ability, it also caused surface decrease of Na-Mnt/H2O H-bonds, and electrostatic and van der Waals (vdW) interactions, and partially neutralized the Mnt surface negative charge, lowering its overall charge density, which minimized further hydration with time. HCOOK could be an alternative for offshore drilling of toxic KCl. Our study offers theoretical insights into the adsorption behavior and inhibitory performance mechanism of HEDA inhibitors on hydrous swellable clay mineral surfaces such as Na-Mnt in the presence of K+-salts, which could provide more enlightenment and understanding of the assessment, selection, and molecular design of innovative and inhibiting high-performance water-based drilling fluid (WBDF) systems applicable in oil and gas complex offshore drilling well sections.