Abstract: In order to study the microscopic mechanism of illite hydration expansion and the mechanism of inorganic salt inhibitors, two illite crystal models (1M-tv and 1M-cv), which are common in oil and gas reservoirs, were established through molecular simulation technology. Furthermore, molecular dynamics method was used to study the microscopic distribution of illite interlayer particles and the hydration parameters of inorganic salt cations entering the interlayer. The results show that the interlayer spacing of illite increases with the number of adsorbed water molecules. It reaches saturation when the number of molecules is 20; 1M-tv configuration is more prone to hydration expansion than 1M-cv configuration; K⁺ and Ca²⁺ are the smallest hydration numbers and hydration radius of monovalent cations and divalent cations, respectively. In addition, K⁺ can be embedded in the silicon-oxygen six-membered ring of the tetrahedral sheet, making it difficult for ion exchange to occur, which can effectively prevent water molecules from entering the inside of the illite crystal structure. The experimental results well verify the inhibition of KCl and CaCl₂ on illite hydration expansion. The results have important theoretical significance for the study of shale anti sloughing mechanism and drilling fluid hydration inhibitor.

Key words: Illite, molecular dynamics simulation, inorganic salt, hydration expansion, radial distribution function