Correlation of ultralow-shear-rate viscosity and dynamic barite sag

The occurrence of barite sag has been a well recognized but poorly understo od phenomenon in the drilling industry resulting in problems such as lost c irculation, well control and stuck pipe. The financial impact on drilling c osts, usually resulting from rig-time lost while circulating and conditioni ng the drilling fluid system, is not trivial. Recurring barite sag problems reportedly have resulted in the loss of drilling projects. Originally thought to occur under static conditions, barite sag is recogniz ed now to occur more readily under dynamic, low-shear-rate conditions. Indu stry experts have offered a variety of measuring parameters, based upon emp irical data, that only partially correlate with the occurrence of barite sa g. Prediction of barite sag in dynamic flow has created an engineering chal lenge. The effect of shear rate on dynamic barite sag, for invert-emulsion drillin g fluids, has been studied and quantified using new and advanced technology . A new field viscometer capable of measuring viscosity at shear rates of 0 .0017 sec(-1) and an eccentric wellbore-hydraulics model were used to devel op and understand this relationship. Changes in mud weight as a function of shear rate, hole angle, annular velocity (AV), and eccentricity correlate with ultralow-shear-rate viscosity. Based upon experimental results, field technology has been developed to pre dict the potential for barite sag of invert-emulsion drilling fluids and to provide remedial measures through ultralow-shear-rate-viscosity modificati on. The efficacy of using traditional rheological measurements as indicator s of barite sag potential is addressed.