Fluid filtration and invasion occur in drilling oil wells operations in overbalanced conditions. In static and dynamic condition, the buildup of a porous and compressible cake on the wall of the well avoids the inadequate invasion and possible damage to the reservoir. The composition of the drilling fluid and operational conditions affect the mud cake build up and its properties as permeability, porosity and compressibility. Consequently, these parameters are important in the determination of the filtrate and fluid invasion. The main objective of this work was to evaluate the filtrate parameters and propose a simplified model and methodology to simulate that process. In this study, both real drilling fluids and fluids prepared in laboratory were considered. The experiments for parameter evaluation were conducted in a filtration cell HTHP (high temperature / high pressure) using filter paper as filter medium. The simplified methodology and model presented hereby are useful in the estimation of the amount of filtrate and the invasion of the drilling fluids in the process of drilling of oil wells. The mathematical model for filtration and fluid invasion calculation was based in mass and momentum conservation equations and hydraulic equations. Based on the experimental data and mathematical model, mud cake parameters were determined, such as permeability, porosity, compressibility, thickness and friction factor. The simulations results showed a good agreement between field and simulated data. Combining the proposed simplified model/methodology and the mud cake and the reservoir rocks properties, it was possible to simulate and to estimate the filtration and fluid invasion in different conditions that allows optimum design of bridging agents for drill-in fluids.

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