Drilling fluids while circulating in oilwells are submitted to a wide range of shear rates, varying from very low values (in the mud tank or in riser/drill pipe annular region) to very high ones (through the bit nozzles).

It is common sense among rheologists involved in the area, that the conventional rheological models (Bingham and Ostwald de Waele) do not represent adequately the behaviour of drilling fluids in such a wide range of shear rates. Other rheological models have been proposed to characterize drilling fluids. However, their current use in the design and supervision of drilling operations is not overspread yet.

Since most drilling rigs today have computational facilities, there is no longer necessity to use inaccurate models for engineering calculations. Adequate rheological characterization is an essential starting point for several calculations, such as hydraulics, cutting transport, kick control, etc.

This paper presents a procedure for drilling hydraulics calculation based on dial readings at the six available rotation speeds of the conventional oilfield viscometer, calculation of five rheological models and use of specific friction loss correlations for each model. An user friendly computer program was developed for optimum flow rate calculations based on this procedure and validated with data obtained from a real scale well. The program was developed based on the requirements of several drilling activities such as offshore operations, compressible fluids, high angle wells, etc.

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