Estudo da sedimentação de barita em fluidos de perfuração

Abstract

The sedimentation of barite, or any other weighting material, causes undesirable fluctuations in the density of drilling fluids. Stuck pipe, pressure control difficulties and loss circulation are problems possibly caused by barite sag. In this work the weighting material sag during the flow of oil-based drilling fluids were studied. The work is divided into two parts: the first is an experimental work and the other focuses the modeling of the phenomenon. In the experimental stage the API procedure VSST (viscometer sag shoe test) was used to evaluate some field drilling fluids from Petrobras S/A. Batch sedimentation tests using the graduated cylinder methodology were also performed in order to estimate parameters used in modeling. The tests were performed in triplicate and statistically analyzed by t-test and analysis of variance (ANOVA). The VSST procedure indicates that the oil-based fluid is more prone to suffer sedimentation and the water-based fluid has greater capacity of bed resuspension. These results are in agreement with field observations and reports in the literature. Two modeling alternatives were discussed: the use of commercial CFD packages and the study of models based on conservation equations. The first approach used package FLUENT® 12 to perform two-dimensional, transient simulations using the Eulerian methodology. In the second approach a code developed in MATLAB® was used to solve the equations of conservation of mass and momentum. On the other hand, the CFD simulations showed a response that is not consistent with the phenomenon observed in the literature. Finally, the model based on conservation equations was capable of simulating two dimensional fluid flow, hydraulic transport of particles and batch sedimentation process. The model is in agreement with the classical analytical solutions and experimental data. The model also predicts the effect of physical parameters such as viscosity, particle diameter and volume concentration in the sedimentation.