Thixotropy drilling fluids

Figure 11. Rheograms of simple bentonite water-based drilling fluid (25 °C) showing the effects of thixotropy. (Reproduced with permission from reference 72. Copyright 1988 Society of Petroleum Engineers.)...

Thixotropy and Other Time Effects. In addition to the nonideal behavior described, many fluids exhibit time-dependent effects. Some fluids increase in viscosity (rheopexy) or decrease in viscosity (thixotropy) with time when sheared at a constant shear rate. These effects can occur in fluids with or without yield values. Rheopexy is a rare phenomenon, but thixotropic fluids are common. Examples of thixotropic materials are starch pastes, gelatin, mayoimaise, drilling muds, and latex paints. The thixotropic effect is shown in Figure 5, where the curves are for a specimen exposed first to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as weU as shear rate, the up-and-down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop. Because flow curves for thixotropic or rheopectic Hquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

Many fluids also exhibit time-dependent viscous effects. With some pastes, drilling muds, and latex paints, for example, there is a drop in viscosity with time when they are sheared at a constant shear rate. This effect, known as thixotropy," " is illustrated in Fig. 19.18. There is an opposite effect, rheopexy, where there is an increase in viscosity with time under shear. Fluids with this property are relatively rare, however. 

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