Avaliação experimental do reinício de escoamento de fluidos dependentes do tempo em tubulações

Abstract

During the production of oil in deep and ultra deep waters, petroleum with high paraffin content may, through heat transfer with the ocean bed, cool at low temperatures and form a gel structure with non-Newtonian characteristics. To restart the flow of the gelled fluid, pressures higher than usual are required, which can lead to overestimating of the underwater pipelines. According to the literature, the properties of this gel are highly dependent on the thermal and shear history, and the pressures at the restart of flow are influenced mainly by the viscous dissipation, compressibility, yield stress and temporal dependence of the properties of the gelled material. In order to simplify the studied problem, the objective of this work is to evaluate experimentally the effects of viscosity, viscoplasticity and temporal dependence on the intensity and development of the pressure of time dependent fluids in pipelines. For this purpose, solutions of glycerine, Carbopol® and Laponite®, are studied, allowing the comparison of each effect separately. The experimental tests are performed by an experimental apparatus, composed of a long helical pipe inserted in a thermally insulated chamber. The apparatus has also cooling and heating systems, as well as devices for data control and acquisition. This study covers the materials flow curves, flow startup with flow rate imposition, flow startup with pressure imposition and the imposition of flow rate steps. For glycerin results, it was noticed that at beginning of the experiments, the flow rate at the end of the pipeline starts to increase with the presence of a uniform pressure gradient, which increases until reaching steady state. The internal pressure for viscoplastic fluids had a delayed response to the increase in the inlet pressure, such that the uniform pressure gradient is observed only when the pressures are close to the steady state values. For the thixotropic material, it was verified that the uniform pressure gradient is established only after the pressure overshoot, coinciding with the flow startup. The results obtained with the experimental apparatus were compared to simulation work, showing good agreement.