Candidate selection is the key to the success of underbalanced drilling (UBD) projects. It is a question yet to be answered why sometimes UBD significantly improves productivity compared with conventional overbalanced drilling (OBD) practices but sometimes doesn't. To answer this question, we have performed an extensive reservoir engineering study to understand the mechanisms of productivity improvement from UBD. A single-well numerical model has been built that simulates the UBD process and the OBD process (while taking into account drilling filtration invasion, mud cake, and permanent formation damage) and their impact on production under a variety of reservoir conditions. Production rates are predicted for both UBD and OBD cases. Then, an incremental net present value (NPV) model has been used to analyze the economic benefits of UBD operations (if any) for all study cases. This paper describes our study methodology, models, simulation results, and analysis results and summarizes the reservoir criteria for successful UBD operations.

Reservoir properties play the most important role in the success of UBD operations. The study clearly demonstrates that UBD works for some reservoirs but not for all reservoirs. The study results provide a set of reservoir criteria that can be used as guidelines to choose potential UBD candidates.

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