Oil India Limited (Oil India) is considering adapting new technologies to ensure that oil and gas strategic resources are optimally explored, developed, and produced during the life cycle of their fields. Underbalanced Drilling (UBD) technology, which can achieve these objectives effectively when applied in conjunction with horizontal drilling techniques, is being considered among various options.Accordingly, Oil India decided to pioneer the introduction of this new technology in their onshore oilfields by employing a systematic scientific approach to screen the Makum-North Hapjan Oilfield for UBD candidacy. The screening process involves detailed evaluation of reservoir, production, drilling, geology, geophysics, and operational data with risk analysis incorporated into a rigorous expert system aimed at maximizing the value of any UBD operation. The screening process was performed in two phases: a low resolution screening and an in-depth high-resolution analysis.Phase I objectives were to evaluate and rank Oil India's reservoirs for UBD in comparison to conventional (overbalanced) drilling techniques, compare candidate reservoirs against global database of UBD analogs and select a candidate for phase II detailed reservoir damage analysis and productivity forecast. Phase II objectives were to identify the formation damage mechanisms that are expected to occur in each of the three reservoirs and determine productivity and economic improvement of underbalanced compared to overbalanced drilling. This paper describes the expert system methodology used and provides a discussion of the results obtained. Phase II study findings provided Oil India with confirmation that UBD could provide improvement compared to conventional drilling and completion technologies. The scope of the study also included a complete engineering evaluation for drilling and completion of a new horizontal well -underbalanced, including the use of a downhole isolation valve. The UBD Screening ProcessThis paper investigates the reservoir suitability of underbalanced drilling using an Expert System screening process that was developed in conjunction with leading industry consultants to increase success in candidate selection and to evaluate the benefits of underbalanced drilling over traditional drilling and completion technology in terms of reservoir performance.The screening process began by taking basic information for the candidate reservoirs and processing the information through the expert system software. This software rates the candidacy of the reservoir to help determine the likelihood of achieving success with underbalanced compared to overbalanced drilling and completion techniques. All data input was reviewed for completeness and consistency.Based on the screening process findings and Oil India feedback one reservoir was selected for further evaluation (Phase II). This in-depth phase integrates geological, reservoir, production, drilling and completion data to calculate a near wellbore damage. The calculated drilling induced damaged is the...