Modelagem e simulação da quebra do gel no reinício do escoamento de fluidos de perfuração

Abstract

The thixotropic fluid flow start-up is a significant problem in the oil well drilling operations. Depending on the start-up flow properties and the drilling fluid applied, significant pressure peaks may take place compromising the operation and the well structure. This work presents a mathematical model to solve the start-up flow and the transient compressible flow of thixotropic drilling fluids in a horizontal pipe in order to get the pressure and flow profiles over the time. The model is based on the mass and momentum conservation equations. The fluid is modeled as a Bingham Fluid, but the yield stress is supposed to be variable with the fluid strain. The viscous effects are evaluated by the variation of the yield stress. However, the yield stress variation with the fluid strain is evaluated according to an adjustment of the second order ordinary differential equation to the experimental data. The adjustment results are compared with the experimental data and with a model of the literature for two different cases of strain rate evolution (gradient and constant strain rate). Therefore, the flow model results are compared with the analytical solution for the Newtonian fluid. The results obtained were: reduction of pressure peaks and steady state flow with the grown-up of the steady state yield stress; pressure and flow development with the time increasing very similar to the Newtonian results for the cases of start-up stress or steady state yield stress equals zero; pressure and flow development with the time increasing very similar to the Bingham fluid results for the case of start-up stress equals to the steady state yield stress.