Molecular dynamics simulation study on macroscopic mechanical deterioration and microscopic hydration mechanism of mudstone.

Aiming at the common problem of borehole instability caused by mineral hydration in water-sensitive mudstone formations during petroleum drilling, this paper conducts a multi-scale study by integrating X-ray diffraction, triaxial mechanical experiments, and molecular dynamics simulation methods. It systematically analyzes the mineral composition of mudstone, the evolution law of its mechanical properties after hydration, and the microscopic mechanism of hydration. The results indicate that: ① The montmorillonite content in the mudstone dominates its strong water sensitivity, leading to a sharp deterioration in mechanical properties after immersion, with compressive strength decreasing by over 55%; ② Molecular dynamics simulation reveals that montmorillonite hydration is a staged process: water molecules intrude into the interlayer to sequentially form single-layer, double-layer, and triple-layer water film structures, with the interlayer spacing expanding nonlinearly. Na⁺ migrates accordingly and distributes alternately with water molecules. The diffusion coefficient of water molecules is significantly higher than that of Na ⁺ . These factors collectively cause a continuous decline in interlayer structural stability; ③ This microscopic process, from "staged formation of water films" to "gradual structural destabilization," directly explains the macroscopic phenomenon of cores exhibiting progressive failure characterized by "initial spalling followed by fragmentation" with increasing immersion time. This study establishes a complete evolution path of mudstone hydration damage from the atomic scale to the core scale, providing a key theoretical basis for controlling borehole stability by inhibiting interlayer hydration of montmorillonite.