ABSTRACT
This paper reports measurements of the rheology of a range of water-based drilling muds at temperatures up to 130°C and pressures up to 1000 bar. The fluids are highly thixotropic; decoupling of temperature/pressure effects from those due to time-dependent structural changes was achieved by developing a sample preparation and handling procedure which ensures that all samples experience identical shear histories prior to study in the rheometer. This enabled essentially equilibrium flow curves to be determined over a shear rate range of 0 – 1200 s−1 with a reproducibility of better than ±1 Pa in stress.
The data were best fitted using a three parameter Herschel-Bulkley yield/power-law model although in many cases, particularly at low temperatures and high pressures, the two parameter Casson equation gave an acceptable fit for engineering purposes. For both models the behaviour of the high-shear viscosity reflected the viscous behaviour of the continuous phase: a weak pressure dependence and an exponential temperature dependence matching that of water. The pressure dependence increased with mud density. In all cases the fluid yield stress was essentially independent of pressure. In contrast to oil-based muds, where this quantity decreases with temperature, for these water-based systems it remained constant below some characteristic temperature, whereafter it increased exponentially with inverse temperature.
The physical origins of the observed behaviour are discussed and a simple model for representing the data is given. It is shown how this may be used for reliable extrapolation of surface measurements to downhole conditions for well-circulated water-based muds.
