Surface Hydration Inhibition Mechanism of Polyamine Treatment Agents on Sodium Bentonite Clay Under High Temperatures: Insights from Molecular Studies

Summary Sodium bentonite (Na-bentonite) is one of the most used water-based drilling mud (WBDM) additives due to its unique features for better well integrity. As a hydrophilic clay mineral, it is bound to hydration expansion, altering drilling fluid properties and causing wellbore instability. Small polyamine treatment agents are often used to avert the hydration of clay minerals, which can minimize the dangers of reservoir damage and wellbore instability. Revealing their inhibition mechanisms is vital for evaluating and designing inhibitor molecules by understanding their molecular behavior at the sodium montmorillonite (Na-Mnt)/water interface. Herein, the inhibition mechanism of alkyl polyamines of different chain lengths (C5, C7, C12, and C18) was investigated using molecular dynamics (MD) simulation with a particular emphasis on the layering behavior of alkyl chains and the effects of molecular chain length and temperature on adsorption and hydration inhibition ability. The alkyl polyamine formed flat-lying conformation structures along the O-atoms. At the same time, the amine groups were located mainly over the six-member rings, and the alkyl chains formed monolayers with carbon-chain lengths of C5, bilayers in C7 and C12, and trimolecular layers in C18. The amine groups interacted with the Na-Mnt surface through H-bonding, electrostatic, and van der Waals (vdW) forces, forming organoclays. This reduced interlayer particle mobility, with their 3D-plane mobility slightly lower compared with alkyl polyamines, while along the z-axis, mobility followed water > Na+> polyamines. The distributions of polyamines reduced the amount of layer charge, and C7-diamine gave better inhibition results. A 2.553-wt% molecular concentration is appropriate to inhibit surface hydration. Moreover, the inhibition stability decreased with rising temperature. A thorough investigation shows that increasing temperature can lead to desorption, which weakens the interaction between C7-diamine and Na-Mnt, thus reducing adsorption, bonding, and inhibition stability. The insights from our present study are beneficial for evaluating the inhibitory performances of organoclays and for the selection and molecular design of new bentonite inhibitors in drilling muds.