Underbalanced drilling technology (UBD) has increased production potential from many oil and gas reservoirs worldwide. When UBD succeeds, it works wonders; Discovering hidden production zones, improving drilling by eliminating lost circulation and differential sticking, increasing rate of penetration, improving production by reducing formation damage, and providing real time reservoir evaluation. The result is Enhanced Oil Recovery and Improved Net Present Value.

Yet too often, UBD fails to live up to its potential either because unsuitable reservoirs are drilled or inappropriate drilling technology is applied. The key to selecting appropriate reservoir candidates for UBD is achieving a balance of technical, safety and economic factors.

Recognizing the growing need for better predictive tools to drive improved outcomes, an expert system was developed. The system is a process that uses Computational Fluid Dynamics (CFD) to enable detailed modeling of fluids in wells and the surrounding reservoirs. CFD is the study of the dynamics of things that flow.

The use of the expert system to screen and select candidate reservoirs for the UBD and MPD applications enables accurate prediction using detailed numerical models of flow in the reservoir, damaged zone and the wellbore. These predictions reveal the actual benefits of Underbalanced and Managed Pressure Drilling in well productivity and increased well recovery through the reduction of formation damage.

This innovative expert system has been used in more than 40 projects for some of the world's leading oil and gas projects across UK, Western and Eastern Europe, Scandinavia, Middle East and Asia. Users of this system recognize that near wellbore fluid flow modeling using CFD will become standard industry practice. This paper describes in detail the expert system and presents the successful application and several case histories and real results.

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