Investigation of methods for direct rheological model parameter estimation

A number of rheological models are examined to establish how well they can characterise a data set of different, pseudoplastic, non-Newtonian fluids used in North Sea operations when their parameters are evaluated using the `directʹ parameter calculation procedure. The study considers how modelling accuracy is affected by the choice of rotational viscometer readings used to calculate these parameters and provides the necessary expressions for direct parameter solution. Conventional measurement combinations of rotational viscometer readings fail to deliver the accuracy provided when non-conventional reading combinations are employed. As such, recommendations for suitable `defaultʹ rotational viscometer measurement combinations are given for each rheological model. It is also concluded that the three-parameter model of Sisko [Sisko, A.W., 1958. Flow of lubricating greases. Ind. Eng. Chem. 50 (12) 1789–1792] delivers the most accurate pseudoplastic characterisation of all the models examined (as illustrated in box-plot representations).