Correlation of Ultra-low Shear Rate Viscosity and Dynamic Barite Sag in Invert-Emulsion Drilling Fluids

Abstract

Abstract The occurrence of barite sag has been a well recognized, but poorly understood phenomenon in the drilling industry. Problems such as lost circulation, well control and stuck pipe have resulted from the occurrence of barite sag. The financial impact of barite sag on drilling costs, usually resulting from rig-time lost while circulating and conditioning the mud system, is not trivial. There are reported incidences where recurring barite sag problems have resulted in the loss of drilling projects. Although originally thought to occur under static conditions, barite sag is now recognized to occur more readily under dynamic, low shear rate conditions. Industry experts have offered a variety of measuring parameters, based upon empirical data, that only partially correlate with the occurrence of barite sag. The prediction of barite sag in dynamic flow has created an engineering challenge. The effect of shear rate on dynamic barite sag, for invert-emulsion muds, has been studied and quantified using new and advanced technology. A new field viscometer, capable of measuring viscosity at shear rates of 0.0017 s-1, and an eccentric well-bore hydraulics model were used to develop and understand this relationship. Changes in mud weight as a function of shear rate, hole angle, annular velocity and eccentricity correlate with ultra-low shear rate viscosity. Based upon experimental results, field technology has been developed to predict the potential for barite sag of invert-emulsion muds and to provide remedial measures through ultra-low shear rate viscosity modification. The efficacy of using traditional rheological measurements as indicators of barite sag potential are addressed.