The accurate prediction of drilling fluid rheological behavior under downhole conditions has become increasingly important, especially on highly-deviated and extended-reach wells drilled with invert emulsions. On these wells, hole cleaning and cuttings suspension parameters should be maintained within optimum ranges for best results, and firm control over unnecessary increases in annular pressure losses and Equivalent Circulating Densities (ECDs) must be exercised. Rather than calculate downhole hydraulics using surface data, more accurate simulation of fluid properties downhole should be used.

Rheological tests that simulated field conditions were run in the laboratory on an ester-based drilling fluid from the field. The rheological behavior of the fluid was tested under varying ranges of temperature, pressure, and ester/water ratios. A predictive downhole rheological model of the ester-based drilling fluid was constructed using over eight hundred (800) fluid viscosity measurements. Using the surface rheological measurements and downhole temperature and pressure, the computer algorithm rapidly calculates fluid properties under the specific downhole conditions.

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