Experimental assessment of deflocculation, thixotropy, and filtration control using environmentally compatible additives in water-based drilling fluids

Growing regulatory and operational pressure to reduce the environmental footprint of drilling operations has accelerated interest in environmentally friendly alternatives to conventional drilling-fluid additives. Although water-based drilling fluids (WBDFs) are generally considered more environmentally compatible than oil-based systems, some chemical additives commonly used in WBDFs remain environmentally undesirable. In this research, two comparatively environmentally compatible additives aimed at improving two of the main characteristics of WBDFs under elevated-temperature conditions were evaluated: sodium tripolyphosphate (STPP; technical grade, 85%) as a deflocculant for flocculation control, and a cellulose nanocrystal (CNC) suspension as a fluid-loss (filtration) control additive. For the filtration experiments, the original fluid-loss-control additives were removed from the formulations, and CNC was evaluated as the sole fluid-loss-control additive. The additives were tested in three different WBDF systems: bentonite water-based drilling fluid (BWBDF), clay–polymer water-based drilling fluid (CPWBDF), and polymer water-based drilling fluid (PWBDF). STPP was tested at 0.1–0.4 wt./vol%, whereas CNC was evaluated at 0.1–1.0 wt./vol%. Following thermal aging at 160 °C for 16 hours, the fluids were characterized through particle-size analysis, liquid-yield measurements, HPHT filtration testing, conventional rheological measurements, three-interval thixotropy testing, and hysteresis-loop analysis. The results reveal that STPP produced the strongest deflocculation response in BWBDF, reducing the median particle diameter from 35 to 5.5 μm, while CPWBDF showed a moderate response, and PWBDF was only slightly affected. Particle-size analysis was more consistently sensitive to STPP addition than liquid-yield measurements. CNC reduced the filtration volume relative to the modified baseline formulations across all three systems, with the largest improvement observed in BWBDF, where it decreased from 38 to 33 mL. Increasing the CNC concentration beyond 0.5 wt./vol% provided no further measurable improvement. The effects of both additives on conventional rheology, structural recovery, and hysteresis behavior were strongly dependent on the formulation and temperature. CNC and STPP enhanced the structural recovery of BWBDF at 99 °C, whereas both additives substantially reduced the structural recovery of PWBDF. Overall, no additive consistently improved all properties across the three fluid systems. The results demonstrate that additive selection and dosage must be optimized for the specific WBDF formulation and targeted performance requirements rather than being applied universally.