This paper describes the project and execution of a well using Underbalance Drilling (UBD) technique in South Brazil. Due to very hard formation in the intermediary section of the well and a very damage-sensitive reservoir, the use of UBD was considered mandatory for this field. New equipment were specially designed for use in the well including a liquid-gas separator and a new manifold. Since nitrogenated fluid was used as drilling fluid, a membrane was used to on-site generation of nitrogen. The article gives field data obtained in the well and also shows comparison between the results obtained with UBD and conventional drilling. A description of the advantages, problems and shortcomings of the technique also is included in the paper.

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