Molecular Simulation Study On Inhibition Mechanism Of Common Salts In Drilling Fluid On Shale Hydration

It is well known that montmorillonite(MMT)minerals have weak adsorption energy for water molecules but a strong electrostatic attraction to exchangeable cations.The cations adsorb on MMT 001 surface in the form of ionic hydration shells.When the majority of adsorbed cations are sodiums the MMT can be termed to be sodium montmorillonite(Na-MMT).Among the many types of MMT minerals,the Na-MMT are the most commonly encountered minerals when drilling for oil and gas and are the most troublesome due to excessive hydration and swelling which can exceed ten times their original volume when drilling with a water-based drilling fluid(WBDF).The Na-MMT simulation model was established in Material Studio software to study clay hydration and swelling process.Upon understanding the mechanism behind the swelling of hydrated Na-MMT different inhibitor molecules were added in the interlayer to investigate their inhibition effect.Inorganic salts of Na Cl,KCl,NH₄Cl,and Ba Cl₂ were investigated at changing wellbore conditions whereas the organic HCOONa,CH₃COONa,HCOOK,and CH₃COOK were investigated at standard conditions.Interlayer particles dynamics trajectories,concentration profiles,as well as the cell mechanical parameters were analyzed to determine the influence of temperature,pressure and salt concentration on inhibition effect.Molecular dynamics(MD)simulation results in an isothermal-isobaric ensemble showed that the Na-MMT interlayer spacing increased with the increase of hydration forming discrete stable hydration states at 32,64 and 96 water molecules which corresponded to the formation of one-,two-and three-layer hydrates.The hydration parameters of coordination number,hydrated radius,and hydration number,as well as the crystal elastic parameters,decreased gradually with the increase of hydration state while the interlayer mobility of particles increased.Addition of the inorganic salts improved the mechanical stability of Na-MMT crystal and was found to greatly reduce the hydration of Na-MMT effect of which was dependent on the type of cation.At elevated wellbore condition inorganic salt influence on mechanical stability was less affected by the increase of temperature and pressure,however,there was an increase of particles mobility which resulted in a small increase in system volume.According to the MD simulation results,NH₄Cl and KCl performed relatively well as compared to the Na Cl and Ba Cl₂ additions.Potassium-/sodium formate and acetate molecules have shown the potential of limiting montmorillonite hydration.The anion shielded the interlayer cation from being hydrated by the water molecules by repelling oxygen of water molecules.Additionally,dependent on the type of cation,the salt can greatly reduce montmorillonite swelling.Potassium cation was seen to reside close to the clay sheet which limited its hydration and diffusion.However,the sodium cation stayed mainly in the middle of the interlayer which allowed for easy hydration by the water molecules.Moreover,the addition of metal formate and acetate salts increased ionic interaction in Na-MMT interlayer which improved the mechanical stability of the cell making it harder and more resistant to swelling.