Rheology of Oil-Base Muds

ABSTRACT The rheological behaviour of invert emulsion muds has been studied at pressures up to 1000 bar and temperatures up to 140° C, Rheological parameters were calculated for the Bingham, Herschel-Bulkley and Casson rheological models. The Herschel-Bulkley and Casson models both give good fits to the experimental rheograms. The Casson model is more reliable for extrapolation purposes than the Herschel-Bulkley model. A pair of two similar exponential expressions were found to be able to model the pressure and temperature behaviour of the two parameters of the Casson model. The expressions, which are based on the relation for pure liquids derived theoretically by Eyring, contain temperature dependent pressure coefficients. The simplifications inherent in the temperature and pressure model are discussed in the light of the temperature and pressure behaviour of the viscosity of common base oils and their constituent hydrocarbons. Field application of the model requires measurement of the rheology of the mud at two or more temperatures and knowledge of the pressure coefficients relating the behaviour of the plastic viscosity to that of the yield point, or the Casson high shear viscosity to that of the Casson yield stress. Pressure measurements or other information are then not required. Applications can be based on Casson or Bingham rheological measurements. The relationships between the parameters of the Casson and Bingham models are disussed.