Investigation on the Critical Flow Rate of Cuttings Transport in Coiled Tubing Sidetracking by Using a CFD-DEM Coupled Model

Coiled tubing sidetracking has proved to be an effective means of old well resurrection, which can accurately connect reservoirs to improve reserve production. However, it is difficult for the cuttings in the wellbore to circulate out of the bottom in this process. In this study, the coupling analysis method of the Computational Fluid Dynamics–Discrete Element Model (CFD-DEM) is applied to model the interaction between fluid and cuttings to characterize the migration process of cuttings. The influence of the cuttings shape and whether the lower part of the window needs to be plugged (whether there is a pocket in the wellbore) on the critical flow rate of cuttings are analyzed. The case studies indicate that the critical flow rate of the identical cuttings in the wellbore with a pocket is significantly higher than that without pockets, about 1.2 to 1.8 times, and the difference is more significant as the particle volume decreases. When the particle size is controlled to be 4 mm, the volume of cuttings in various shapes is different, and the critical flow rate is positively related to its volume. When there is a pocket in the wellbore, the volume of the cuttings increases to 3.6 times, and the critical flow rate increases to 1.15 times; while when there are no pockets in the wellbore, the critical flow rate increases to 1.69 times. Therefore, reducing the particle volume in the wellbore without pockets to reduce the drilling fluid flow rate is more effective than the wellbore with a pocket. Furthermore, a new three-dimensional quantitative model of flatness is proposed to describe the particles. The flatness of the particles is positively correlated with the critical flow rate under the conditions of the same volume and roughness of cuttings particles. When the flatness is reduced by 15%, 20% and 38%, its critical flow rate is reduced by 2.8%, 4.9% and 8.8%, respectively, and the ratio between them is about five times. It is an excellent choice to adopt flatness to measure the critical flow rate of non-spherical particles. The research results have a significant reference value for field drilling.