Under certain drilling conditions, the weighting material particles such as barite can settle out of the drilling fluid. This phenomenon, known as barite sag, can lead to a number of drilling problems including lost circulation, well control difficulties, poor cement job, and stuck pipe. This study investigates barite sag, both experimentally and numerically, in the annulus under flow conditions. Experimental work has been conducted on a large flow loop to investigate the effects of major drilling parameters on barite sag by measuring the circulating fluid density. Results of the tests indicate that the highest sag occurs at low annular velocities and rotational speed and also at high inclination angles. It was observed that at inclination angles less than 60°, for any annular velocity, barite sag is not significant. Eccentricity of a non-rotating inner pipe did not have a significant effect on barite sag. However, effects of inner pipe rotation on barite sag for an eccentric annulus are more significant than concentric case. The simulation part of this study is based on a proposed particle tracking method called "Particle Elimination Technique". The traveling path of each solid particle is assumed to be a function of size and shape of the particle, fluid velocity and rheology. Based on the estimated traveling path of particles, density of the fluid is updated considering the number of particles whose paths lead to the bottom of the annulus and become motionless. In order to capture the complexities associated with the solid-liquid flow, a lift force is assigned to the solid particles that enable adjustment of the model with experimental results. Comparing the results of numerical simulation to the experimental study on the effects of annular velocity on barite sag in a horizontal annulus shows a good agreement. The numerical simulation was modified from laboratory scale to real wellbore dimensions for practical drilling applications. Results of the simulation show prediction of the density of the drilling fluid in the horizontal section of a wellbore with various lengths and dimensions under different annular velocities.
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