Structure–function analysis of an illite–montmorillonite organoclay blend for HPHT oil-based drilling fluids

The exploration of high-pressure and high-temperature (HPHT) hydrocarbon reservoirs requires drilling fluids with high thermal stability, controlled rheology, and effective fluid-loss mitigation. Oil-based drilling fluids (OBDFs) are commonly employed under such conditions; however, maintaining consistent performance at elevated temperature and pressure remains challenging. This study provides a structure–function interpretation of an OBDF system formulated using an equal-ratio blend of two commercially available organoclays (OCs), Claytone-3 (illite based) and Claytone-IMG 400 (montmorillonite based), selected for their contrasting mineralogical compositions and particle morphologies. The OCs were characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and particle size distribution (PSD) analysis. The 1:1 OC blend was evaluated alongside the individual OCs and a commercial reference formulation (MC-TONE) through laboratory testing, including measurements of density, electrical stability, sag tendency, rheological and viscoelastic properties, and HPHT filtration behavior. The blended system consistently exhibited improved performance relative to the individual OCs, including a 19% increase in electrical stability, a 21% reduction in dynamic sag, a 13% increase in plastic viscosity, a 40% increase in yield point, and reductions in fluid loss (6.5%) and filter cake thickness (12%). These performance trends are interpreted in terms of complementary structure–function contributions from the illite- and montmorillonite-rich components. The results suggest that mineralogical complementarity is an effective design variable for enhancing OBDF performance under HPHT conditions. This interpretation-driven approach provides practical insight for rational drilling fluid formulation aimed at improving wellbore stability in technically challenging environments.