The Rheological Properties of Oil-Based Mud under High Pressure and High Temperature Conditions

Designing a proper drilling fluid that can function properly under the conditions of High-Pressure, High-Temperature (HP/HT) operations is very challenging. Among these challenges is the alteration of the rheological properties of drilling fluid due to the high temperature and high pressure (Ibeh et. al, 2007). This work investigates the rheological behavior of oil-based drilling fluids with different properties at Ultra-HP/HT conditions using a state-of-the-art viscometer capable of measuring drilling fluids properties up to 600°F and 40,000 psi. For this purpose, two actual oil based mud samples used by industry with the same mud weight (12.5 ppg) were chosen to carry out a matrix of experiments. The results of this study led to concluding that the viscosity, yield point and gel strength decrease with increasing temperature (until the mud sample fails, for oil-based mud with regular formulation). This behavior is the result of the thermal degradation of the solid, polymers, and other components of the mud samples and the expansion of the molecular distances which will lower the resistance of the fluid to flow and, hence, its viscosity, yield point, and gel strength. Moreover, it is concluded that the viscosity and yield point increase as the pressure increases. Pressure’s effect on these parameters, however, is more apparent at low temperature (below failure point, for oil-based mud with regular formulation). Key words: High pressure high temperature; Oil-based mud; Rheology; Rheological properties