On the Characterisation of the Flow Regimes of Drilling Fluids

Abstract

Drilling fluids are vital elements in the safe, efficient and effective construction of wells. Their key functions include transporting drill cuttings to the surface, cooling and lubrication of drill string, cleaning build-up deposits on drill bits and tools, as well as stabilisation of the borehole and pressure control. Because they are often a complex mixture of different solids and fluids, the rheology of drilling fluids is usually complicated. As a result, they typically exhibit non-Newtonian flow behaviours. While the traditional practice is to use critical velocity to describe the flow regimes of drilling fluids by discriminating between laminar and turbulent conditions, this paper explores the applicability of Reynolds numlber (NRe), which is a more robust and universal dimensionless quantity to characterise flow regimes. Models to estimate NRe of drilling fluids are explored for Bingham and power-law types of drilling fluids, including development of models for other non-Newtonian behaviours such as shear-thinning and shear-thickening. More important, the models provide a veritable basis to compare the hydraulic characteristics of a drilling-fluid mixture against its Newtonian counterparts under similar conditions. In addition, these models would facilitate the exploitation of the concept of dynamic similarity to improve the design and benchmarking of the flow characteristics of different drilling fluids in different systems and under diverse conditions. Examples are provided that show the robustness of using NRe as against critical velocity, to identify flow regimes of drilling fluids. The applicability of the proposed models and ideas are not limited to drilling fluid hydraulics. The findings are relevant in other areas of transporting non-Newtonian fluids such as polymer for enhanced-oil recovery and multiphase mixtures such as emulsions, waxy crudes and general pipeline transport. Additionally, the principles and insights should be of interest to other industries such as food processing and chemical manufacturing.