Modelling of Drilling Fluid Thixotropy

Abstract

Drilling fluids are visco-elastic materials, i.e. they behave as a viscous fluid when subject to a sufficient shear stress and like an elastic solid otherwise. Both their elastic and viscous properties are time-dependent, i.e. drilling fluids are thixotropic. Because of thixotropy, it takes a finite time before the effective viscosity of a drilling fluid attains an equilibrium when the fluid is subject to a change of shear rate. This effect is visible when one changes the applied shear rate in a rheometer, as the fluid will gradually adapt to the new shearing conditions. When the velocity of a drilling fluid changes, for instance due to a change in pump flow rate, movement of the drill string, or change of flow geometry, the fluid will exhibit a time-dependent response to the new shearing conditions, requiring a certain time to reach the new equilibrium condition. Unfortunately, the time-dependence of the rheological properties of drilling fluids are usually not measured during drilling operations and therefore it is difficult to estimate how thixotropy impacts pressure losses in drilling operations. For that reason, we have systematically measured the time-dependence of the rheological properties of several samples of water-based, oil-based and micronized drilling fluids with a scientific rheometer in order to capture how drilling fluids systems respond to variations of shear rates. Based on these measurements, we propose to investigate how one existing thixotropic model manages to predict the shear stress as a function of the shear rate while accounting for the shear history and gelling conditions. Then we propose a modified model that fits better, overall, with the measurements even though there are still noticeable discrepancies, especially when switching back to low shear rates.

Measurement of Drilling Fluid Rheology and Modeling of Thixotropic Behavior

“…Similar thixotropic hysteresis curves were measured by Schulz et al [9], who also found that the effective viscosity of weighted drilling fluids decrease continuously over a period of more than 1 hour when sheared at a constant, high shear rate of 1022 s −1 . A similar, prolonged reduction of effective viscosity at high shear rates was also observed recently by Cayeux and Leulseged [10] for weighted drilling fluids. Cayeux and Leulseged introduced a new model for thixotropy based on modifying the steady state Herschel-Bulkley model by introducing a structure parameter and allowing for gel strength development while the fluid is static.…”

Laboratory Study: Comparison of Physical Properties of XCD Polymer Drilling Mud and PHPA

“…Gel strength is a condition where the mud reaches the agar power condition [16]. Gel strength is used in static conditions where this value must be maintained so that when drilling continues there will be no problem in lifting the cutting that is being held to the surface [17], [18]. From the measurement results above using a fann vg meter where XCD Polymer mud and PHPA mud meet the specified standard specifications.…”