A mineralogical synergy approach to optimizing oil-based drilling fluids under high-pressure and high-temperature conditions

High-pressure and high-temperature (HPHT) drilling environments often compromise the stability, rheology, and filtration performance of oil-based drilling fluids (OBDFs). To address these challenges, this study introduces a mineralogically engineered organoclay (OC) blend that harnesses the synergistic interaction between Claytone-II, an anorthite-rich non-swelling mineral, and Claytone-SF, a montmorillonite-dominant swelling clay. Comprehensive characterization using X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and particle size distribution (PSD) analyses was performed to evaluate the structural and compositional attributes of the individual and blended OCs. The formulated OBDFs were experimentally assessed for density, electrical stability, rheology, viscoelasticity, sag resistance, and HPHT filtration performance, demonstrating clear rheological enhancement and improved overall stability under extreme downhole conditions. Results show that the 1:1 Claytone-II/Claytone-SF blend significantly enhanced emulsion integrity, increasing electrical stability by 22%, improving plastic viscosity (PV) by 9% and yield point (YP) by 32.5%, and reducing HPHT filtrate volume and filter-cake thickness by 22.5% and 9%, respectively, compared with the average performance of the individual OCs, while the commercial OC (MC-TONE) was used solely as a reference benchmark. These enhancements are interpreted to arise from a synergistic multi-scale colloidal network, in which the rigid, non-swelling anorthite particles provide structural reinforcement while the swelling montmorillonite platelets promote interparticle bridging and network connectivity, collectively strengthening the gel framework under HPHT conditions. This mineralogical-synergy approach establishes a robust framework for designing next-generation OBDFs with superior HPHT stability, reduced fluid loss, and improved drilling efficiency and safety.