Cuttings transport: Back reaming analysis based on a coupled layering-sliding mesh method via CFD

Abstract

Inadequate hole cleaning is one of the main reasons for inefficient operations in extended-reach drilling. The mechanism of cuttings transport under the back reaming operation, which is frequently adopted to remove the cuttings, has been investigated in this study. To this end, a coupled layering-sliding mesh method with the Eulerian-Granular approach has been established innovatively. The dynamic layering method has been employed to simulate the axial motion of the pipe, whereas the sliding mesh method has been used to simulate the pipe rotation. The back reaming operation of a connector-furnished pipe has been simulated, and the sensitive parameter analysis has been conducted. The results thus obtained demonstrate that the increase in the initial bed height, inclination, and the diameter and length of the connector causes a significant increase in the cuttings concentration. In addition, the cuttings concentration is observed to decrease significantly with the pipe rotation speed. Furthermore, two main factors contribute towards the cuttings accumulation around the connector, namely, the difference in the cross-sectional area and the pushing effect of the connector—like a “bulldozer”. The “bulldozer” effect of the connector dominates when the tripping velocity is significant compared to the velocity of the cuttings. Conversely, the effect of the difference in the cross-sectional area becomes the leading factor for cuttings accumulation. The “bulldozer” effect of the connector causes a more severe impact on hole cleaning. In both cases, increasing the tripping velocity only mildly affects the cuttings concentration. It is therefore suggested that the tripping velocity should be slower than that of the sand during the back reaming operation. Furthermore, increased fluid velocity might lead to a higher accumulated cuttings concentration around the connector when the cuttings bed has not entirely passed through the connector. A significant flow rate can be safely applied after the cuttings have passed through the connector furnished with a large diameter, such as the bottom hole assembly. This exploration serves as an essential guide to predicting and controlling tight spots while back reaming.