Molecular simulation of polyether amines intercalation into Na-montmorillonite interlayer as clay-swelling inhibitors

Abstract Polyether amines (PEA) have been widely used in water-based drilling fluids (WBDFs) to prevent the clay swelling and maintain the wellbore stability during the drilling process due to its excellent inhibition performance and low toxicity towards the environment. Understanding the intercalation behavior of PEA within clay interlayer is of great significance to disclose the inhibition mechanism and design high-performance drilling fluid additives. In this work, molecular dynamic (MD) simulations are applied to investigate the arrangement and dynamic properties of neutral PEA (N-PEA) and protonated PEA (P-PEA) within Na-montmorillonite (MMT) interlayer in the presence of one, two and three layer water content. We found that the N-PEA molecules tend to interact with clay surface with the –NH2 groups and a stronger interaction between –NH3+ groups and clay surface is observed when 33% Na+ is replaced by P-PEA molecules. While for the scenario with 66% Na+ is substituted by P-PEA molecules, the –NH3+ groups prefer to distribute in the middle of the pore hydrating with water molecules. The N-PEA molecules are likely to form a crown-like structure wrapping Na+ inside which is detrimental for the water invasion into clay layer. Based on the analysis of Na+ distribution in MMT layer, it is observed that the introduction of P-PEA molecules replace the Na+ in the middle of the pore first and followed by the surface Na+. Both of the N-PEA and P-PEA molecules are capable of forming hydrogen bonding (H-bonding) with MMT surface reducing the propensity of MMT to attract water. Furthermore, the minimum H-bonding number between MMT and P-PEA molecules is observed when 66% Na+ is exchanged due to strong water-PEA interaction. In addition, the inhibition mechanisms of neutral PEA molecules by forming PEA-Na+ complexes to cut off the connection between Na+ and water molecules is similar for different water contents. Our study provides important insights into the structural properties of PEA molecules within MMT interlayer and clay inhibition mechanism of PEA.