High-Temperature, High-Pressure Rheology of Water-Based Muds

Abstract

Abstract This paper reports measurements of the rheology of a range of water-based drilling muds at temperatures up to 130 deg. C and pressures up to 1000 bar. The fluids are highly thixotropic; 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 particularly at low temperatures and high pressures, the two parameter Casson equation gave an acceptable fit for 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 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, where after 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. Introduction The successful prediction of frictional pressure drops and of cuttings settling velocities and transport in the annulus is crucially dependent on an accurate representation of the drilling fluid rheology. It is usual practice to measure the shear flow characteristics under ambient surface conditions and to extrapolate these measurements in some way to downhole conditions. To do this requires a reliable model of how the rheology of the mud changes with the cyclical variations in temperature, pressure and shear history which it experiences during circulation around the wellbore. Yet, despite a considerable amount of experimental study over the years for both waterand oil-based muds, there is relatively little systematic understanding of how their flow behaviour changes with downhole conditions. This appears to be for two main reasons: First, it has been common practice to make measurements of the shear rheology at relatively few shear rates and to represent shear-stress/shear-rate curves (rheograms) by simple two-parameter constitutive equations, such as the Bingham or power-law models, which are not in general adequate. Second, the rheology of these fluids is influenced by many factors including temperature, pressure, shear history, composition and the electrochemical character of the components and of the continuous fluid phase. Often, several of these have been varied at the same time so that isolating specific effects quantitatively has proved difficult. In recent years there has developed an increasing awareness of the first point. A number of more realistic constitutive relations have been suggested, and more sophisticated measurement techniques introduced. P. 187