Reversing Thermal Thinning and Enhancing Solids Suspension in Ultrahigh-Temperature Oil-Based Fluids Using a Smart Gelling Agent

Summary Drilling reservoirs with bottomhole temperatures exceeding 220°C is challenging due to the severe thermal thinning of oil-based drilling fluids (OBDFs). Conventional modifiers often degrade under these extremes, leading to barite sag. In this paper, we evaluate a novel amphiphilic, temperature-responsive oil-phase gelling agent (TR-OPG) designed to stabilize fluid rheology in ultrahigh-temperature environments. The TR-OPG features a thermally stable polyamide-imide backbone. Characterization reveals a “coil-to-stretch” transition at approximately 180°C, triggered by intramolecular hydrogen bond dissociation. This activates an “inverse-temperature gelation” mechanism where unfolded polymer chains bridge emulsion droplets to construct a shear-thinning weak gel, providing sufficient suspension capacity. Performance evaluation demonstrates that 1.0 wt% TR-OPG creates a “U-shaped” rheological profile, maintaining manageable surface viscosity to minimize equivalent circulating density (ECD) while boosting viscosity at bottomhole temperature. High-temperature and high-pressure (HTHP) testing confirmed a sharp rebound in the 6-rev/min reading (θ6) from 10.2 at 180°C to 17.3 at 220°C, effectively compensating for base oil thermal thinning. TR-OPG significantly outperformed commercial organoclay and amide modifiers. Furthermore, the formulation exhibited exceptional stability, maintaining a sag factor < 0.52 and electrical stability (ES) > 1,200 V after static aging at 220°C for 10 days. The developed TR-OPG offers a robust solution for managing rheological risks in ultrahigh-temperature drilling.