A new approach to dynamic barite sag analysis on typical Field oil-based drilling fluid

Abstract

Laboratory and field experiences record that sagging of weighting agents such as barite in drilling fluids is often worse in dynamic conditions, e.g. pumping, tripping or drill pipe rotation, than in static conditions. Barite sag has resulted in problematic wellbore issues including loss of wellbore control, lost circulation, stuck pipe and high torque. Till date, the prediction of barite sag in dynamic condition is still a challenge in the industry. An indirect measurement protocol to examine the effects of shear rate and viscosity on dynamic barite sag of drilling fluids is proposed. This study presents an experimental analysis of dynamic barite sag phenomenon on a typical oil-based drilling fluid sample. All rheometry and dynamic barite sag tests were conducted at 50o C using an advanced Anton Paar rheometer MCR 102/302 with either the smooth or grooved bob-in-cup measuring system. The viscoelastic properties which are related to the microstructural building/breakdown of the fluid sample were investigated under oscillatory amplitude and frequency sweeps. The dynamic sag was measured under steady and oscillatory low to ultra-low shear rates from 5.11 to 0.001 !"# to closely compare to sag-prone conditions during drilling operation. Preliminary results indicate a strong dependence of barite sag on shear rate above a critical shear rate value of 0.1 !"#. Furthermore, with higher viscosity, less dynamic sag is observed.