Laboratory Evaluation of Static and Dynamic Sag in Oil-Based Drilling Fluids
Journal article, Peer reviewed
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In this paper, we present the results of barite sag measurements before and after hot-rolled oil-based drilling fluids (OBDFs) using different approaches for characterization. We characterized the barite sag of a liquid column under static condition using light-scattering (LS) measurements, hydrostatic pressure measurements, and gamma densitometry. Under the dynamic condition, we used a rheometer with a grooved bob-in-cup measuring system to characterize barite sag in rotational and oscillatory shear conditions. Extensive rheological characterization of the drilling fluid samples, before hot rolling (BHR) and after hot rolling (AHR), is carried out. It is found that barite sag decreased in hot-rolled fluid samples from the LS, rotational, and oscillatory shear measurements. The rheological characterization of the fluid samples showed that heat-activated chemicals in the hot-rolled fluid sample increased the viscosity and elasticity, which contributed to lower barite sag and longer suspension of particles than BHR. Both hydrostatic and gamma densitometry measurements reveal more or less uniform compaction of barite particles in the fluid sample below the liquid layer. Time-dependent oscillatory shear measurements provide new insights on the structural character of drilling fluids to predict barite sag tendencies during the fluid design phase.