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To elucidate the microscopic mechanisms underlying the impact of hydrated cations on the surface hydration of slime mineral particles (specifically, kaolinite and quartz, the primary minerals in slime), this study focused on constructing two common hydrated cations in slime water: [Na(H2O)5]+ and [Ca(H2O)8]2+. Using density functional theory, the adsorption of these two hydrated cations on the surfaces of kaolinite (001), (\begin{document}$ 00\overline 1 $\end{document}) and α-quartz (001), as well as their competitive adsorption with water molecules were simulated. The simulation results revealed that the adsorption energy of hydrated cations on all three surfaces was over 50% lower than that of water molecules. The adsorption stability on mineral surfaces was as follows: α-quartz (001) surface > kaolinite (001) surface > kaolinite (\begin{document}$ 00\overline 1 $\end{document}) surface. The adsorption energy of the competitively stable configuration was 34%–57% lower than that of a single hydrated cation on kaolinite and quartz. Additionally, the [Ca(H2O)8]2+ configuration exhibited a greater stability than the [Na(H2O)5]+ configuration under both adsorption conditions. When the hydrated cations adsorbed onto three surfaces, strong hydrogen bonds formed with surface, surpassing the strength of hydrogen bonds between water molecules and kaolinite/quartz surfaces. The hierarchy of hydrogen bonds between two hydrated cations on mineral surfaces was as follows: kaolinite (001) surface > α-quartz (001) surface > kaolinite (\begin{document}$ 00\overline 1 $\end{document}) surface. Under a competitive adsorption, the hydrogen bond between [Na(H2O)5]+ and mineral surface strengthened, while the bond between [Ca(H2O)8]2+ and mineral surface weakened. Although hydrogen bonding did not entirely correlate with changes in adsorption energy, electrostatic interactions in the adsorption configuration were identified. The electrostatic interaction in the single adsorption configuration of hydrated cations proved stronger than that in water molecular adsorption. Under a competitive adsorption, the electrostatic interactions between hydrated cations and mineral surfaces intensified, with [Ca(H2O)8]2+ demonstrating stronger interaction than [Na(H2O)5]+. Given the robust adsorption of hydrated cations on the surfaces of kaolinite and quartz, the dehydration of slime particles becomes more challenging. This could increase hydration repulsion between particles, resulting in a more stable dispersion of particles in slime water.

Borehole instability is one of the most complicated technical problems in oil and gas exploration and development.The hydration and expansion of clay minerals is the critical factor causing wellbore instability in which the surface hydration is difficult to be removed due to the large hydration potential.In this light, through molecular dynamic simulation, this paper probed into the CaCl2 inhibitory effect of concentrations, temperatures and pressures on the surface hydration of montmorillonite which revealed the macroscopic mechanism.Results indicated that inhibition of cations were achieved by binding water molecules on the surface of montmorillonite and decreasing the transport and conductivity of water molecules, thereby regulating the invasion of water molecules into the surface of montmorillonite.The ability of inorganic salts to inhibit surface hydration were CaCl2>NaCl>MgCl2>KCl.The study found that calcium ions easily adsorbed surface water molecules to form stable outer sphere complex.With the increase of CaCl2 concentration, coordination number, hydration number and hydration radius of calcium ion decreased, and the inhibitory effect diminished.When the temperature increased and the pressure decreased, the conductivity and transport capacity of water molecules was enhanced in the system, the coordination number of calcium ion descended, and mechanical strength declined.

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