Surface-Chemistry-Tuned Cellulose Nanocrystals in a Bentonite Suspension for Water-Based Drilling Fluids

A bentonite (BT) suspension is extensively used as a water-based drilling fluid (WDF) for well excavation; however, it suffers from insufficient rheology and poor filtration performance at low solid content. Cellulose nanocrystals (CNCs) with nanoscale dimensions, large surface area, high stiffness, and reactive functional groups are promising candidates to improve the rheology and filtration performance of BT-WDFs through surface interactions. Consequently, a fundamental understanding of the surface interaction between CNCs and BT platelets becomes critical. This work aims to reveal the crucial role of surface characteristics of CNCs on their interaction with BT platelets as well as the rheological and filtration performance of CNC/BT-WDFs. Two types of CNCs with distinctive surface characteristics [i.e., carboxylated CNCs (cCNCs) and cationic CNCs (caCNCs)] were rationally prepared and applied as modifiers in BT-WDFs. The cCNCs were shown to attach to the edge surface of BT platelets and acted as "bridges", connecting BT platelets via "edge-to-edge" association. On the contrary, the caCNCs were absorbed to the face surface of BT platelets and acted as "cross-linking agents", stacking numerous layers of BT platelets through "face-to-face" association. These differences led to the distinctive dispersion state of BT platelets and the overall performance of CNC/BT-WDFs. The cCNCs served as more effective rheological and filtration agents in the developed fluids compared with caCNCs, highlighting the importance of surface characteristics of CNCs in the development of low-solid-content, high-performance CNC/BT-WDFs for well excavation.