Molecular Simulation Of Na-montmorillonite Surface Hydration Inhibition Mechanism

The hydration expansion of clay minerals in mud shale is one of the main causes of instability of the borehole wall.The surface hydration of clay minerals is caused by the adsorption of water molecules on the surface of clay mineral crystals and the hydration of exchangeable cations.The surface of expansive clay includes the outer surface and the inner surface.It is difficult to remove this part of water molecules due to the large hydration potential.The suppression of surface hydration of clay minerals has not been solved at home and abroad.Montmorillonite is widely found in clay minerals and is most prone to hydration expansion.Therefore,montmorillonite hydration is very important for the study of wellbore stability of oil and gas wells.By constructing the Na-montmorillonite crystal model and the Na-montmorillonite-water interface model,the mechanism of hydration of Na-montmorillonite inner/outer surface was studied.The interactions between different shale inhibitors and the inner/outer surface of Na-montmorillonite crystals were simulated by using common inorganic salts,organosilicates,linear amines,small molecule ethylenediamine and their homologues.By analyzing the water molecule adsorption conformation,hydrogen bond distribution,concentration distribution,kinetic characteristics,radial distribution function,ion hydration parameters and mechanical parameters of montmorillonite on the inner and outer surfaces of Na-montmorillonite,the effects of cation,anion,temperature,amine small molecule carbon chain length and other factors on the inhibition of shale inhibitors and their mechanisms were studied.The simulation results show that the hydration ability of the cation has a direct influence on the hydration expansion of Na-montmorillonite.The hydration ability of the simulated cations on the inner and outer surfaces of Na-montmorillonite crystals are arranged from large to small as follows:Mg2+>Ca2+>Na+>NH4+>K+.The hydration expansion of montmorillonite can be effectively reduced by adding inorganic salt cations with weak hydration ability and replacing the compensated Na+between layers.The addition of inorganic salt cations can inhibit the hydration of interlayer Natand reduce the self-diffusion of water molecules and Na+,at the same time,inorganic salt cations can inhibit the adsorption between the external surface of crystal and interfacial water,make water molecules far away from the surface of crystal layer and destroy the original layered distribution structure of interfacial water.The methyl silicate in the anion has better inhibition than Cl-,and its hydroxyl groups can form hydrogen bonds with the surface of the crystal layer,occupy the adsorption sites on the surface of the crystal layer,and reduce the self-diffusion coefficient of water molecules on the inner/outer surface of Na-montmorillonite.The increase of temperature is not conducive to interlayer ion hydration,but it is beneficial to the diffusion of interlayer water molecules and the expansion of the crystal layer.The increase in temperature causes the molecular order of the hydration layer on the outer surface of the Na-montmorillonite crystal to decrease,and the thickness of the hydration layer increases.The adsorption state of ethylenediamine and its homologues on Na-montmorillonite interlayer and the outer surface of the crystal is flat adsorption.When the number of carbon atoms in the molecular carbon chain is increased to more than 8,such as 1,8-octanediamine and 1,10-decanediamine,the self-diffusion coefficient is gradually increased.The amine group at one end of the molecule may be deviated from the crystal layer due to insufficient hydrogen bonding force,and are adsorbed obliquely in the water phase.The addition of ethylenediamine and its homologues can reduce the hydration ability of interlayer Na+,encroach on the original water molecule adsorption sites on the surface of the crystal layer,reduce the self-difusion of water molecules and Na+,and enhance the mechanical properties of Na-montmorillonite.Moreover,the addition of ethylenediamine and its homologues makes the concentration peak of the interface water on the outer surface of the crystal far from the crystal plane,reduces the order of water molecules in the interface,and inhibits the diffusion movement of water molecules.The overall inhibition effect increased first and then decreased with the increase of carbon chain length,and reached the best at 1,6-hexanediamine.