A generalized model for apparent viscosity of oil-based muds

This work presents an experimental investigation of the apparent viscosity of OBM within a temperature range of 24-87 °C. The continuous phase was prepared with mineral oil, surfactants (emulsifier and wetting agent), organophilic clay, lime, and calcium chloride. Organophilic clay concentrations and oil/water ratio (OWR) were varied. Apparent viscosity measurements are gathered and analyzed. OBM displayed shear-thinning behavior that best fits the Herschel-Bulkley model. The results reveal notable trends of apparent viscosity and rheological parameters with increasing temperature, organophilic clay concentration, and OWR. Generalized empirical models that link the apparent viscosity of the continuous phase to that of OBM are developed by applying nonlinear regression analysis. The prediction of OBM rheological parameters requires the knowledge of continuous phase apparent viscosity at ambient temperature, dispersed phase volume fraction, and temperature. The models give a reasonable prediction of OBM apparent viscosity. The comparison of normalized or reduced rheological parameters and apparent viscosity of OBM and water-based mud (WBM) revealed the sensitivity of viscous properties of OBM to temperature as compared to that of WBM. This revelation shows why OBM is more susceptible to barite sag phenomenon than WBM.