Avaliação de modelos de tixotropia aplicados a fluidos de perfuração

Abstract

Drilling fluids present a thixotropic behavior and they usually gel when at rest. The sol-gel transition is fundamental to prevent the deposit of rock fragments, generated by drilling the well, over the drill bit during eventual stops. Under those conditions, high pressures are then required in order to break-up the gel when circulation is resumed. Moreover, very high pressures can damage the rock formation at the bottom of the well. Thus, a better understanding of thixotropy and the behavior of thixotropic materials becomes increasingly important for process control. Thixotropy is defined as the continuous decrease of viscosity with time when flow is applied to a material that has been previously at rest and the subsequent recovery of viscosity in time when the flow is discontinued. The mechanisms that control thixotropy are not yet well defined and modeling is still a challenge. This work presents a review of thixotropy and of different mathematical models found in the literature that are used to predict such characteristic. Some of those models are then selected and fitted to the rheological data of a drilling fluid. The constants of the mathematical models are fitted to the results of controlled shear rate tests. After the calibration, the models are compared to data obtained from controlled shear stress tests. It is worth of note that some models are able to predict important characteristics of a thixotropic material with reasonable accuracy.