Minimizing fluid invasion is a major issue while drilling reservoir rocks. Large invasion may create several problems in sampling reservoir fluids in exploratory wells. Besides, drilling fluid invasion may also provoke irreversible reservoir damage reducing its long term productivity. A common practice in the industry is the addition of bridging agents in the drilling fluid composition, such as calcium carbonates. Such products would form a low permeability layer at the well walls which would control invasion.

This paper presents a radial model, based on Darcy's law to predict the drilling fluid invasion profile into a multi-layer reservoir rock saturated with a compressible fluid. Results highlight that the differential pressure, the fluid compressibility, the filter cake permeability and the rock properties are important parameters that affect significantly the invasion profile. The proposed methodology is a tool to optimize drilling fluid design to be used in the drilling of reservoir sections in both exploratory and development wells in offshore Brazil fields. The process analysis is illustrated with a series of case studies in offshore exploratory wells drilled in Brazil. The analysis highlights the importance of filtration control and of a dedicated drilling fluid designed for each specific reservoir.

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