Experiment and methodology for drilling fluid rheology determination under high-temperature and high-pressure conditions

Accurate determination of drilling fluid rheological properties is critical for optimal wellbore maintenance and hydraulic management during drilling operations. However, rheological parameter determination remains challenging due to insufficient experimental data and complex nonlinear relationship between shear stress (τ) and shear rate (γ̇) under elevated pressure-temperature (P-T) environments. This study introduces an innovative methodology for determining rheological parameters, grounded in experimental measurements. First, systematic experiments were conducted on water- and oil-based drilling fluids across temperature (18–200 °C) and pressure (6.89–172.37 MPa) ranges, significantly expanding existing rheological datasets in the literature. Second, a parameter determination method was developed through nonlinear least squares fitting for the Robertson–Stiff (R-S) model, with extensions to other prevalent models including Bingham, Power-law, and Herschel–Bulkley formulations. By leveraging multivariate function extremum conditions within the least squares framework, the parameter identification problem was reduced to solving a system of equations. Validation via comparative analysis demonstrated the method's superior accuracy, computational efficiency, minimal bias, and low variance. Finally, empirical correlations were established to quantify rheological parameter dependencies on temperature and pressure, revealing strong continuity in parameter evolution with P-T variations. Results confirm the experimental fluids' adherence to the R-S model, demonstrating its effectiveness in describing drilling fluid behavior under extreme downhole conditions.