Molecular Design Principles for Developing High-Performance and Environmentally Friendly Shale Inhibitors Using Amino Acid-Type Ionic Liquids

The limitation of conventional shale inhibitors to simultaneously provide effective charge screening and deep intercalation is currently one of the main challenges in the design of high-performance shale inhibitors. In this study, we propose a unified design of amino acid ionic liquids (AAILs) as a suitable solution to this functional deficiency, by combining an anion capable of strongly binding to clay surfaces with a charge-neutralization cation. Experimental investigations were assessed by linear swelling tests, shale recovery tests, immersion tests, viscosity measurements, particle size distribution tests, scanning electron microscopy tests, and X-ray diffraction (XRD) analysis. Our results revealed a correlation between molecular design and performance, where the anion’s size, functional groups, and charge distribution were the key parameters to evaluate the inhibiting performance of AAILs. The superior inhibitor was determined by a small size anion such as glycinate possessing complementary functional groups, allowing for deep intercalation and multipoint hydrogen bonding, acting as a cross-linking agent between the clay layers. The glycine-based AAIL, [Veim][Gly], demonstrated outstanding performance, achieving a shale recovery rate of 88.90% after a remarkable reduction in clay swelling by more than 70%, attributable to the combined synergistic effect. Such an inhibitory approach was difficult to achieve with traditional inhibitors or AAILs with bulky structures or positively charged anions. This study could be used as a reference in designing a fundamental principle for high-performance and environmentally friendly shale inhibitors.