Experimental Study and Modelling of Barite Sag in Annular Flow
Authors: Hashemian Yahya , Miska Stefan , Yu Mengjiao , Ozbayoglu Evren Mehmet , Takach Nicholas , McLaury Brenton 
Published: March 2016

The phenomenon of settlement of weighting-material particles in drilling fluid is known as barite sag, which 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, both experimentally and numerically, in the annulus under flow conditions. Settlement of the weighting materials is generally called barite sag because barite is the most popular weight material used in drilling industry. The experimental part of the study has been conducted using a flow loop with 35-ft-long annulus test section to investigate the effects of fluid velocity in annulus, annulus eccentricity, pipe rotation, and inclination angle on barite sag. Density of the flowing fluid is measured continuously using Coriolis densitometers at the inlet and outlet of the annular test section. The simulation part of this study is based on a proposed particle- tracking method called “particle-elimination technique.” In this method, falling and axial velocities of the solid particles are estimated to predict their travelling paths. Density of the flowing fluid is updated, taking into account the number of particles that become stationary by reaching the casing wall. To model the downward motion of the barite bed on an inclined bed, the analogy of falling film of a viscous liquid on an inclined plane is applied. Velocity of the falling barite bed is calculated on the basis of an apparent viscosity approximated by the experiments. Knowing the velocity of the moving bed, the mass rate of barite downstream of the bed, which goes back into the main flow stream, is calculated and a new density for the moving fluid is achieved. The numerical simulation is modified from laboratory scale to real wellbore dimensions to be used for practical drilling applications. Results of the tests indicate that the highest sag occurs at low annular velocities, low rotational speeds, and high inclination angles. Barite-sag reduction caused by decreasing the inclination angle continues until the inclination-angle values are approximately 60 degrees, after which it becomes insignificant. Another observation made from experimental results and simulation work is that as the circulation of the drilling fluid in the loop continues, the fluid density decreases until it reaches an equilibrium stage. The equilibrium stage occurs faster for lower inclination angles. Effects of annular velocity and inner-pipe rotational speed on barite sag are similar; however, they are different from the trend observed when the inclination angle changes. Eccentricity of a nonrotating inner pipe did not have a significant effect on barite sag. However, the effect of inner-pipe rotation on barite sag for an eccentric annulus is more significant than in the concentric case. Comparing the results of numerical simulation with the experimental study shows a good agreement. The proposed simulation approach in this study can be used for barite-sag predictions in the annulus. A good prediction of the barite sag can prevent hazardous situations and would help engineers in drilling wells to spend less money and time.