Molecular Simulation On Montmorillonite Hydration Inhibition Mechanism Of Typical Shale Inhibitors

Wellbore instability is one of the major problems in the process of drilling,and shale hydration expansion is the mainly cause of wellbore instability.Shale is mainly consist of montmorillonite,which can easily adsorb water to swell,so montmorillonite hydration is the basic reason for wellbore instability.The model of Na-montmorillonite was established by Materials Studio to study on the hydration and expasnsion process.Different shale inhibitors including inorganic salts?NaCl,KCl,NH₄Cl,CaCl₂?and small organic amine moleculars?formamide,urea?were added into the interlayer of the model to study on their inhibitory effect and theory under different species and concentrations by analyzing the structure of Na-montmorillonite-water-inhibitors system and calculating kinetic parameters and mechnamical parameters.Simulation results showed that,the interlayer space increased with the addition of water molecules.The first water layer is formed as the water moleules number is 32,and as water molecules increase to 64,the second water layer is completed.Three water layers can be formed in the interlayer,and the corresponding number is 96.As water layers increased,the coordination number,hydration number and hydration radius of Na⁺in the interlayer decreased,and self diffusion of Na⁺and H₂O gradually increased,and the elastic mechanical properties of Na-montmorillonite decreased gradually.Inorganic salts can inhibit the hydration of montmorillonite,and its effection is mainly determined by the type of cations.Cations can hydrate itself and interact with monmorillonite to inhibit the hydration.Under the saturated concentration,the organic salts have a good effect on the inhibition and can improve the mechanical properties.According to the results of molecular dynamic simulation,KCl is the best,followed by Ca Cl₂,NH₄Cl,NaCl.The inhibition of small organic amine moleculars mainly depend on its structure and functional group.Formamide and urea can form hydrogen bond with water in the interlayer and oxgen on the surface of silica tetrahedron to affect the hydration and limite the self-diffusion of Na⁺and H₂O in the interlayer.Small organic amine moleculars can affect the mechnamical properties of sodium montmorillonite.Within a certain concentration range,they can improve the elastic modulus and decrease the compressibility,and can increase the stability.And at low concentration,they can work well.Considering the parameters analyzed,formamide has the better effect.