Study on Dynamic Wavy Distribution of Sand Bed by a Transient Two-Layer Modeling

Abstract

This paper investigates a wavy distribution of the cuttings bed in extended-reach drilling. First, a transient two-layer model is established, considering the buoyancy in the deviated section. And a diffusion equation is integrated to obtain entrainment and deposition rates. Next, a Stability Enhancing Two-Step (SETS) method is applied to solve the strong nonlinear equations. Finally, the wavy distribution of cuttings bed, effects of the buoyancy, ROP, and the ratio of drilling to flushing time (R), circulation stoppage, etc. are studied. The results show that, in the iterations of drilling and washing, the cuttings bed is no longer of equal height but distributed in waves. The bed height decreases with R. For the indicator Risk_10% (the ratio of the intervals higher than 10% wellbore diameter), the lower limits of R under different flow rates are around 0.4–0.6, and the recommended upper limit is 1.4–2.5. And for Risk_5%, the lower limits of R are around 1.5–3, and the upper limits are 3.5–7. Besides, when considering the buoyancy, the calculated bed height is lower than that without considering it for the deviated section. Thus, the transit section (inclination of 30–60°) is not necessarily the most challenging area to clean if adjusting the drilling fluid’s density and buoyancy properly. Furthermore, the cuttings bed slides downwards after pump-off, amassing on the lower side of the buildup Section (70–90°), which poses a considerable risk of pipe sticking. The results provide an essential guide for the optimization design and safety control of extended-reach drilling.