Prolonged static aging of olefin-based inverse emulsion drilling fluids: temperature–time effects on rheology, stability, and filtration

Synthetic-based drilling fluids (SBFs) formulated as olefin-based inverse emulsions are widely used in deepwater operations, where prolonged static intervals expose the fluid to high temperatures without shear. This study investigated the coupled effects of aging temperature (100–150 °C) and static exposure time (24–168 h) on the rheological behavior, emulsion stability, and filtration performance of an olefin-based synthetic drilling fluid using a Central Composite Design. The response surface models for the Herschel–Bulkley parameters were statistically significant (p≤0.05, R 2 =0.978–0.992). Yield stress exhibited a nonlinear response, decreasing by ~12% at intermediate conditions (125 °C, 96 h) relative to the unaged fluid, but dropping near zero at experimental boundaries. The consistency index reached values up to seven times higher at 142.7 °C after 147 h, while the flow behavior index decreased from 0.70 to 0.44. At a shear rate of 8 s⁻ 1 , apparent viscosity increased by 8% after 45 h and 18% after 147 h at 142.7 °C, reaching 30% at 150 °C after 96 h. Prolonged aging also promoted macroscopic phase separation and brine manifestation. Filtration behavior was nonlinear, and an additional test using the filtration-control additive aged at 150 °C for 96 h yielded only 1 mL of filtrate, suggesting thermal changes in the additive favored pore plugging and restricted fluid loss. Thermogravimetric analysis indicated thermal susceptibility of emulsifier and filtration-control components at approximately 102–120 °C. These results show that aging time is a critical factor governing the long-term performance of olefin-based synthetic drilling fluids.