Thermal rheological properties of three drilling fluids suggested for hot environments were experimentally investigated using both HTHP dynamic rheometer and classical viscometer. Two of them were polymer based muds commercially used in geothermal drilling, and the third one was sepiolite base mud prepared with specially mixing order in this study. Rheological measurements at six different shear rates were performed in a programmed temperature range of 100 to 500°F with 10°F increments at a 700 psi of pressure differential. Flow characteristics of sample muds in a given well geometry were also considered using a numerical application.

Two polymeric muds sampled from a geothermal field in Turkey exhibited very low viscosity with a low thermal viscosity variation interval at temperatures above 300°F. In contrary, the sepiolite mud demonstrated higher viscosity and kept its stability with increasing temperatures up to 500°F. Findings also indicated remarkable viscosity differences between the conventional viscometer and the dynamic HTHP rheometer. Hydraulic optimization in terms of circulation pressure losses, cutting transport, and surge and swab pressures were numerically modelled. Optimization results based on conventional viscometer measurements generated noticeable errors causing difficulties through drilling program. Numerical application revealed that the usage of improper viscosity values resulted in calculation of well design parameters beyond the safe drilling limits.

Through this study, it was revealed that sepiolite based muds could be good candidate for drilling hot and/or hostile wells such as deep oil and gas wells, geothermal wells, and drilling through high saline formations. As excessive temperature through wellbore can lead to serious well problems, selecting thermally stable mud with proper rheological properties to improve penetration rates, hole cleaning properties, and borehole integrity can clearly express the importance of this study.

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