Abstract
Drilling fluids are designed to be shear-thinning for limiting pressure losses when subjected to high bulk velocities and yet be sufficiently viscous to transport solid material under low bulk velocity conditions. They also form a gel when left at rest, to keep weighting materials and drill-cuttings in suspension. Because of this design, they also have a thixotropic behavior. As the shear history influences the shear properties of thixotropic fluids, the pressure losses experienced in a tube, after a change in diameter, are influenced over a much longer distance than just what would be expected from solely entrance effects. In this paper, we consider several rheological behaviors that are relevant for characterizing drilling fluids: Collins–Graves, Herschel–Bulkley, Robertson–Stiff, Heinz–Casson, Carreau and Quemada. We develop a generic solution for modelling the viscous pressure gradient in a circular pipe under the influence of thixotropic effects and we apply this model to configurations with change in diameters. It is found that the choice of a rheological behavior should be guided by the actual response of the fluid, especially in a turbulent flow regime, and not chosen a priori. Furthermore, thixotropy may influence pressure gradients over long distances when there are changes of diameter in a hydraulic circuit. This fact is important to consider when designing pipe rheometers.
Highlights
Drilling fluids play multiple and important roles in drilling operations, such as maintaining an adequate pressure in the open hole section of the borehole to avoid formation fluid influx or wellbore instabilities, transporting drill-cuttings to the surface, and cooling down the downhole equipment.Drilling fluids need to be sufficiently viscous to transport drill-cuttings and yet the pressure drops caused by viscous flow should be limited to avoid excessive pump pressures
As drilling fluids are pumped into a drill-string and returned through an annulus offering a larger cross-sectional area than the drill-pipes, they are designed to be shear-thinning, limiting viscous pressure drop at high bulk velocity while presenting a high effective viscosity at low bulk velocities
Method to numerically calculate the pressure gradients with the Heinz–Casson rheological behavior has been described. It has served as a basis for estimating pressure gradients with thixotropic fluids
Summary
Drilling fluids play multiple and important roles in drilling operations, such as maintaining an adequate pressure in the open hole section of the borehole to avoid formation fluid influx or wellbore instabilities, transporting drill-cuttings to the surface, and cooling down the downhole equipment. Drilling fluids need to be sufficiently viscous to transport drill-cuttings and yet the pressure drops caused by viscous flow should be limited to avoid excessive pump pressures. As drilling fluids are pumped into a drill-string and returned through an annulus offering a larger cross-sectional area than the drill-pipes, they are designed to be shear-thinning, limiting viscous pressure drop at high bulk velocity while presenting a high effective viscosity at low bulk velocities. Any change in diameter along the hydraulic circuit influences the shear history of the fluid and the resulting viscous pressure losses [4,5]
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