Drilling fluids are usually thixotropic, i.e. their rheological behavior depends on the shear history. Therefore, each time the flow conditions changes, the response of a thixotropic fluid differs from the one of a non-thixotropic liquid. Change of flow conditions can occur when the flow-rate is modified, but also when the fluid passes through a series of contractions and expansions as when circulating inside a drill-string with internal upset drill-pipes, or in an annulus with external upset tool-joints.In the laboratory, it is possible to reproduce the effect of alternating flow conditions by oscillating the pump rate associated with an instrumented flow loop. Then it is possible to observe the effect on the pressure gradient measured at different locations along the flow-loop, passed a change of diameter, when applying a periodic signal on the command to a mud pump controller. Various signal forms are explored like square, trapezoidal or sine waves. Experiments are made with two yield power law fluids. One is non-thixotropic and based on a Carbopol polymer, while the other is thixotropic and utilize a xanthan gum potassium chloride solution. The experiments are conducted in laminar flow regime.Furthermore, pressure gradients along a circular pipe are modelled under conditions where the inlet volumetric flowrate oscillates. The Navier Stokes equations are solved while accounting for the hysteresis of the rheological behavior of thixotropic fluids. For non-thixotropic fluids, the pressure gradient response to the periodic variations of the volumetric flowrate only depends on inertia effects. However, for thixotropic fluids, it is found that the waveform of the pressure gradient signal is deformed in similar ways as a capacitor would do in an electrical circuit powered by an alternating current.These results are well aligned with the phenomenological rheological model developed by Quemada (1998 doi: 10.1051/epjap:1998125). According to the Quemada rheological model, the non-Newtonian behavior of a fluid mix containing polymers or clay platelets in a Newtonian background fluid is due to the change of volume fraction of the dispersed material at different shear rates. With thixotropic fluids, the rearrangement of floccules is probably not instantaneous therefore causing side effects similar to the accumulation of charge or discharging of an electrical capacitor.

Pressure Losses Generated by a Thixotropic Fluid When Subject to an Oscillating Flowrate

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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