Experimental Study on Dynamic Barite Sag and Effects of Inclination and Pipe Rotation

Abstract

Summary Barite sag causes pressure fluctuations in the wellbore, which is undesirable. These problems usually occur with oil-based muds (OBMs; invert emulsion muds) and are associated with fluid properties and operation parameters. Drilling issues related to this undesirable phenomenon include wellbore instability, lost circulation, and stuck pipes. As barite sagging is a complex phenomenon, the mechanisms that cause and aggravate it still need to be fully understood to mitigate these problems. This study examines barite sagging in the wellbore with inner pipe rotation to understand the process and develop prevention strategies. Thus, a flow loop study with OBM is conducted in a concentric annular test section with varying inner pipe rotation and inclination angles. The tests were performed at an elevated temperature (49°C) to simulate borehole conditions. By measuring the pressure profile in a mud sample trapped in the test section, barite sagging was evaluated. Using the data, we calculated the density difference between the top and bottom sections of the column. The novelty of the work lies in continuous monitoring of the density profile of the mud column, which is sheared between two coaxial cylinders to simulate drillstring rotation in the wellbore, and utilizing the data for evaluating barite sag. The results show the evolution of the pressure profile with time, indicating the sagging of barite particles at the bottom of the test section. Due to barite sagging, the density of the top portion of the mud column decreased over time, while the density of the bottom part increased. The lateral sedimentation of barite particles toward the annulus outer wall enhances barite sag in inclined configurations. The sedimentation creates two suspension layers with different densities, leading to secondary flow, which enhances sagging. Hence, the primary factor driving barite sagging is inclination. An increase in inclination angle from 0° to 50° resulted in a significant (more than twofold) increase in mud density difference. Also, the rotation of the pipe delayed sagging during the early phases of the testing process (less than 20 minutes). However, it did not have much effect as the sagging progressed, resulting in approximately the same density difference in both cases (i.e., with and without rotation).