Synthesis of a Novel pH-Responsive Emulsifier Based on Dynamic Covalent Bond and Its Application in Reversible Oil-Based Drilling Fluids

Abstract

AbstractOwing to their advantages in the preparation of stimuli-responsive emulsifiers and emulsions, dynamic covalent bonds have recently attracted great interest. Removal of traditional emulsifier-stabilized oil-based drilling fluids (OBDFs) is difficult after drilling operations. To address this issue, a pH-responsive emulsifier (AT–FA) based on dynamic imine bonding between 1-aminotetradecane (AT) and 4-formylbenzoic acid (FA) for reversible OBDFs is prepared. Fourier transform infrared spectroscopy (FT-IR) and proton nuclear magnetic resonance (1H NMR) confirm the presence of dynamic covalent imine bonds and the molecular structure of AT–FA and characterize them. AT–FA could effectively reduce the oil–water interfacial tension at alkaline pH (7.6), at which AT–FA is a long-chain alkyl carboxylate-type anionic emulsifier that forms highly stable water-in-oil microemulsions. However, at acidic pH (4), its dynamic imine bond is cleaved and converted to a cationic emulsifier in the form of amine salts to stabilize oil-in-water (O/W) microemulsions. More importantly, this phase inversion behavior is reversible. Finally, reversible OBDFs are successfully developed using AT–FA as a pH-responsive emulsifier, which exhibited reasonable rheological and filtration properties and excellent emulsification stability. Since it is converted to a water-soluble O/W drilling fluid at pH 4, its removal rate is as high as 98.46%. This pH-responsive emulsifier based on dynamic imine bonds can not only ensure the emulsification stability of OBDFs, but also facilitate the later removal of OBDFs, showing great application potential in the drilling industry.KeywordsDynamic covalent bondpH-responsive emulsifierOil-based drilling fluidsOily mud removal