Cellulose Nanocrystal-Polyelectrolyte Hybrids for Bentonite Water-Based Drilling Fluids

Cellulose nanocrystals (CNCs), with their rodlike shape and nanoscale dimensions, greatly improve the filtration performance of bentonite-containing, water-based drilling fluids (BT-WDFs) through interactions with the BT platelets. When these WDFs are exposed to high salt concentrations, though, their fluid retention properties are greatly diminished due to reduced CNC-BT interaction and BT aggregation/flocculation. Consequently, we reduce BT-BT interaction at high salt by grafting polyelectrolytes (PE) to CNC particles (CNC-PE) to enhance CNC-BT interactions when incorporating these hybrid particles with BT-WDFs. The particles sterically and electrostatically screen BT platelets from associating, thus improving fluid filtration performance at high salt. Three types of CNC modifications were carried out: grafting from direct surface initiation, modification with vinyl-terminated glycidyl methacrylate (GMA) before grafting, and physical mixing of CNC with a polymer. These modifications were performed using three polyelectrolyte materials: anionic polystyrene sulfonate (PSS), cationic polyacrylamide (PAM), and a random copolymer of PSS and PAM (PSS-co-PAM). Formulations containing CNC-PEs prepared by covalent grafting exhibited superior filtration properties compared to those in which CNCs and PEs were physically mixed. The higher graft loading achieved with the GMA method resulted in poorer filtration results compared to the direct grafting method due to CNC-PE interparticle cross-linking. PSS-modified CNC-PEs appeared to attach to BT edges, while PAM-modified CNC-PEs attached to the BT faces. These interactions disrupted BT aggregation, with the PSS-co-PAM CNC hybrid displaying the most desired filtration properties. The results highlight the importance of steric and charge stabilization of the BT particle edges and faces to achieve high-performance WDFs for well excavation.