A viscosity function for highly-shear-thinning or yield-stress liquids such as pastes and slurries is proposed. This function is continuous and presents a low shear-rate viscosity plateau, followed by a sharp viscosity drop at a threshold shear stress value (yield stress), and a subsequent power-law region. The equation was fitted to data for Carbopol aqueous solutions at two different concentrations, a drilling fluid, an water/oil emulsion, a commercial mayonnaise, and a paper coating formulation. The quality of the fittings was generally good.

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractSeveral fluid replacement operations take place during and after an oil well drilling operation, such as: the replacement of the drilling fluid by the completion fluid and the displacement of cleaning plugs or chemical treatment plugs. In such operations it is important to minimize the contamination of a fluid by the other to maintain the designed physical properties of the new fluid displaced.This work presents a numerical simulation study of the operations related to the replacement of a fluid by another inside the well. The effects of density and rheology differences between the fluids are analyzed (Newtonian fluids displacing non-Newtonian fluids and vice-versa). The studies are based on the numerical solution of the governing equations for a two phase system and the subsequent evolution of the interface shape between the two fluids.The present work details design criteria and fluid contamination predictions to some of the typical completion operations. Some displacement procedures are recommended to minimize contamination. IntroductionIn a well, replacement fluid operations during drilling and completion need a detailed design to assure its efficiency and the operational safety. Minimize the contamination, assure the correct position of the fluid in the well and define the dynamic pressures to guarantee that the operational window limits are respected are basic requirements to well succeeded operations.The analysis of the replacement process of a fluid by another, with different physical properties, is characterized by the simulation of a two phase flow. The solution of the governing equations aims to represent the evolution of the interface shape between the two fluids during the displacement process. This is a complex problem, especially when one of

In this work, flow of two adjacent fluids through annular eccentric tubes is analyzed numerically. This kind of flow is found in cementing processes of oil wells, where the cement paste pushes the drilling mud through annular space between the drilling column and the oil well wall. Both drilling mud and cement paste behave as non-Newtonian fluids, and between then a wash fluid is usually used, to avoid their contamination. The analysis of interface configuration between these fluids helps to determine contamination, and is an important tool for the process optimization. The numerical solution of the governing conservation equations is obtained via the finite volume technique and the volume of fluid method, using the Fluent software (Fluent Inc.). The effects of rheological parameters and eccentricity are investigated, for different flow rates. The results obtained show that the displacement is better when a more viscous fluid is used to push the other fluid. Also, it is observed that the interface shape is a function of flow regime and viscosities ratio. However, it is insensitive to eccentricity.

In this work, a numerical simulation of three adjacent fluids flowing through a tube followed by an annular axi-symmetric channel is performed. This analysis is motivated by the study of drilling and completion processes of oil wells, where a cement slurry pushes the drilling mud, used in the drilling process to lubricate the drill and to remove the produced drilling cuts. The completion process is perfect if all the drilling mud is removed, and if there is no contamination between the two fluids. To avoid contamination, a spacer fluid is usually inserted between them. Both drilling mud and cement slurry behave as non-Newtonian fluids, and the spacer fluid can be Newtonian or non-Newtonian. The analysis of flow and interface configuration between these fluids helps to determine contamination, and is an important tool for the process optimization. The numerical solution of the governing conservation equations is obtained with the Fluent software, using the finite volume technique and the volume of fluid method. The effects of rheological parameters and pumped volume of the spacer fluid are investigated. The results obtained show that the displacement is better when a more viscous spacer fluid is used. The results also shown zones of recirculation, where the drilling mud remains stagnant, decreasing the process efficiency.