A Transient Solids Transport Model for Solids Removal Evaluation in Coiled-Tubing Drilling

Summary Poor hole cleaning is a major concern in coiled-tubing drilling (CTD), and it is often associated with long nonproductive time that contributes significantly to the operational cost. In this study, a transient solids transport model is developed based on transport equations of phases in the flow to predict the evolution of solids conveyed in the wellbore. The developed model is able to provide forecasts of the distribution of cuttings along the annulus, which can be important information for deciding to improve solids removal. Based on the concept of a two-layer steady-state model, a 1D time-dependent model is developed using two layers: a lower layer of solids bed and an upper layer of a solid-liquid mixture with the mechanisms of solids deposition and solids entrainment taken into account. The model is discretized by using a finite volume scheme and then solved by employing a semi-implicit numerical solution. The model’s hyperparameters, such as a deposition factor and an entrainment factor, are calibrated with experimental data conducted by the use of the large indoor flow loop (LIFL) to achieve a better match. The model is combined with a 2D cross-sectional model to handle the effect of pipe eccentricity and bed presence. Predictions from the model agree well with the experimental data acquired by using an oil-based mud for the majority of the cases.