Cellulose nanofibers as anti-sagging agents in water-based drilling fluids

To maintain wellbore stability and counteract the high formation pressure in oil and gas wells, barite is widely used as a weighting agent in high-density water-based drilling fluids. However, under high-temperature formation conditions, barite particles tend to settle, leading to barite sag, which compromises drilling efficiency, increases operational risks, and jeopardizes well integrity. Herein, three types of cellulose nanofibers (CNFs), i.e., 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized CNFs (T-CNFs), mechanically disintegrated CNFs (M-CNFs), and lignin-containing CNFs (L-CNFs), were investigated as anti-sagging agents in high-density water-based drilling fluids for deep shale gas applications. Among them, T-CNFs demonstrated superior performance in preventing barite sedimentation compared to M-CNFs and L-CNFs. Mechanistic analyses revealed that T-CNFs, with their highly negatively charged surfaces resulting from TEMPO oxidation, exhibited the strongest electrostatic repulsion, effectively dispersing barite particles and preventing aggregation. Furthermore, their uniform dispersion enhanced the bridging effect and steric hindrance, forming a highly stable suspension with minimal sedimentation. This study provides a novel and sustainable strategy for improving drilling fluid stability, offering an environmentally friendly alternative to conventional anti-sagging additives in deep shale gas development.