Experimental Study and Modeling of Barite Sag in Annular Flow

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 cementing operation, and stuck pipe. This study investigates barite sag in the annulus under flow conditions, both experimentally and numerically.;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 speeds and also at high inclination angles. It was observed from the experimental results that for inclination angles less than 60° and for the lowest tested annular velocity of 30 ft/min, barite sag is not significant. Eccentricity of a non-rotating inner pipe did not have a significant effect on barite sag. However, effect of inner pipe rotation on barite sag for an eccentric annulus is more significant than the concentric case.;The simulation part of this study is based on a proposed new particle tracking method called the Particle Elimination Technique. The trajectory of each solid particle is assumed to be a function of the size and shape of the particle, fluid velocity and rheological properties. Based on the estimated trajectories of the particles, the density of the fluid is updated considering the number of particles settling in 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 enabling adjustment of the model with experimental results.;The modeling of downward motion of settled barite bed in an inclined annulus is based on the concept of the falling film of a viscous fluid on an inclined plane. In this approach the barite film is treated as a fluid with high viscosity. As the inclination angle is decreased, the downward movement of the falling film (settled barite bed) due to gravity becomes faster. A faster downward movement of barite bed results in a faster rate of barite re-suspension into the main flow stream of the fluid.;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, which was initially developed to model the barite sag in the flow loop, was modified to real wellbore dimensions for practical drilling applications. Results of the simulation show prediction of the density of the drilling fluid in the annulus of a wellbore with various lengths and dimensions under different annular velocities.