Protic ionic liquids as lubricants for water-based drilling fluids in deep shale gas development

Amid escalating challenges of torque/drag and wellbore instability in deep shale gas development under high-temperature/high-salinity conditions, this study investigates three protic ionic liquids (PILs)—LTA1 (laurate), LTA2 (stearate), and LTA3 (oleate)—synthesized via proton transfer reactions to serve as eco-friendly lubricants for water-based drilling fluids. The molecular structures and thermal stability of the PILs were characterized using Fourier-transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and thermogravimetric analysis (TGA). The lubricating properties of the PILs were comprehensively evaluated. Lubrication mechanisms were elucidated by contact resistance analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. Results demonstrated that at a 0.5 wt% dosage, all three PILs achieved a lubrication coefficient reduction rate exceeding 90 %. After aging at 210°C, the lubricity coefficients of all PILs remained below 0.05, demonstrating excellent thermal stability. Meanwhile, LTA3 exhibited outstanding salt-resistant and corrosion inhibition performance. This superior lubricating performance is attributed to the unique capability of PILs to form a dual protective film at the interface. Specifically, an adsorbed film is first formed on the metal surface through electrostatic interactions and the adsorption of polar groups, whereas a tribochemical film is generated in situ during the friction process. The synergistic action of these two films effectively physically separates the friction pairs, thereby reducing the shear stress at the interface. These PILs thus enable sustainable WBDFs for deep shale gas, eliminate hazardous additives, and resolve torque/drag issues in extended-reach wells.