The rheological properties of drilling fluids are very important for the precise estimation of the frictional pressure losses during drilling operations. The first experiences of inline rheometers at the rig site have shown that the rheological properties vary substantially with time. Yet, most inline instruments measure these properties at only one condition of pressure and temperature. This paper describes how the pressure, temperature and shear history dependence of the rheological behavior can be measured continuously.

The measurement of rheological properties of drilling fluids is complexified by the fact that they are thixotropic, i.e. the rheological properties depend on the shear history applied to the fluid. For an inline rheometer, any change of pipe diameter, or the introduction of a fluid sample in a cavity, introduces a shear variation. Therefore, inline rheometers need to account for the side effects of thixotropy on the results. Instead of trying to reduce the thixotropic effects, the new rheometer focuses on measuring these thixotropic properties so that steady state values can be extrapolated.

Most published studies of thixotropic fluids indicate that thixotropic fluids need a long recovery period to reach steady state conditions after a change from high to low shear rates, but that a change of shear rates from low to high is recovered very quickly. However, the thixotropic behavior of drilling fluids differs from other thixotropic fluids, as it needs considerable time to recover both from high to low and low to high shear step changes. The measured time constants for these effects are long (several minutes), therefore making it difficult to design an inline rheometer that eliminates thixotropic effects.

Furthermore, gel duration has not only an impact on the stress overshoot when an increasing shear strain is applied to a gelled fluid, it also impacts the rheological behavior for several minutes after viscous flow is established. The new inline rheometer is designed to exhibit explicitly these effects in order to measure directly the gel strength as a function of gel duration and the time constants that characterize the remnant effects of gelling on viscous properties.

The impact of the shear history on rheological measurements is important to obtain precise estimations of the rheological properties of drilling fluids. With an automatic inline rheometer that can provide time, pressure and temperature rheological properties, it is then possible to obtain more precise results from advanced drilling hydraulic models running in real-time, with less manual configuration efforts.