Accurate characterization and prediction of drilling fluid rheological properties under high-temperature and high-pressure (HTHP) conditions are core prerequisites for safe and efficient deep well drilling operations. To systematically clarify the thermobaric rheological laws and intrinsic coupling mechanism of high-density drilling fluids, this study takes 2.0 g/cm3 oil-based drilling fluid (OBDF) and water-based drilling fluid (WBDF) as research objects, and carries out full-scale rheological tests via a Fann iX77 HTHP rheometer under temperatures ranging from room temperature to 200 °C and pressures from atmospheric pressure to 200 MPa. The results show that the shear stress of both fluids is positively correlated with pressure and shear rate and negatively correlated with temperature. 150 °C is identified as the critical thermal thickening temperature for WBDF: above this temperature, polymer degradation and solid precipitation cause abnormal viscosity growth, and high shear rates can effectively alleviate this thickening effect by promoting the dispersion of precipitates. There are significant differences in rheological sensitivity between the two systems: WBDF rheology is dominated by temperature dependence, while OBDF exhibits high sensitivity to both temperature and pressure due to the compressibility of the oil continuous phase; temperature and pressure exert a mutual inhibitory effect on fluid rheology. The proposed Arrhenius–Powell one-step global coupled prediction model achieves favorable prediction accuracy for both OBDF and non-thickened WBDF, with the coefficient of determination (R2) no less than 0.96. The model can provide reliable basic parameters for HTHP wellbore hydraulic calculation and pressure loss prediction, fully meeting the accuracy requirements of field engineering applications.
Zhang et al. (Thu,) studied this question.