In underbalanced drilling the use of air as an injected gas sets up the potential for oxygen corrosion. Membrane nitrogen with about 5% oxygen eliminates the chance for fire or explosion, but does not solve the corrosion problem. Natural gas or cryogenic nitrogen eliminates the chance for oxygen corrosion, but does not always eliminate the chance of corrosion from other down hole acid gasses (Hydrogen Sulfide, H2S, Carbon Dioxide, and CO2). Any corrosion that occurs while drilling will be forced by some drilling fluid or drilling operational procedure such as KCl fluids, aerated mud, or floating mud cap operations.

The objective of this study is to identify some of the practical elements of corrosion encountered with underbalanced drilling and their optimum solution by development of an advisory system based on field cases and expert opinions. This advisory system is intended to be a field guide for the drilling engineer or rig supervisor. Corrosion is a matter of concern to the drilling contractor because of pitting or loss of steel in the drill pipe.

The advisory system is developed by proposing a set of guidelines for the optimal practices to minimize corrosion in underbalanced drilling operations. The optimum practices collected from data and experts' opinions, are integrated into a Bayesian Network BN to simulate likely scenarios of its use that will honor efficient practices when dictated by varying certain parameters. These parameters are measuring corrosion, identifying corrosion types, drill pipe and corrosion, treatment methods for H2S corrosion, CO2 sources and treatment, different methods to test for corrosion, general corrosion prevention and treatment, treatment methods for formation water and makeup water, and recommended practices to minimize corrosion in underbalanced drilling operations.

To the best of the authors' knowledge, this paper is the first study to develop an advisory system for minimizing corrosion problems. The developed software solutions on corrosion will be demonstrated such as using mechanical means to reduce O2 concentration and inhibitors for pipe coating. Finally, the advisory system list recommendations for all types of underbalanced drilling (flow, aerated, foam and mist) to minimize corrosion problems.

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