Underbalanced drilling (UBD) and managed pressure drilling (MPD) are gaining in popularity as drilling methodologies to overcome some of the problems faced in conventional overbalanced drilling. These techniques are complimentary technologies rather than completely separate techniques, MPD techniques at one time having been classified under UBD. Therefore, with the current terminology and the many similarities they are often confused with one another. Underbalanced drilling is a tool both for reservoir performance improvement and reservoir characterization as well as for addressing drilling problems. MPD, on the other hand, is primarily a solution for mitigating drilling related problems. Both result in a reduction of non-productive time (NPT). Sometimes a combination of both techniques may be required for the same well.

Different operators have chosen UBD and MPD with the goal of curtailing severe fluid losses and other drilling-related problems associated with conventional overbalanced drilling. Often times, while applying this technique to solve these drilling problems, the reservoir benefits have become apparent and have convinced the operators to go to full underbalance to realize the full reservoir production benefits without any period of overbalance throughout drilling, tripping, and completion operations. They have often found that when using UBD for reservoir production improvement, it is possible to perform comprehensive characterization of the reservoir while drilling. In some cases, zones that were not seen as productive during overbalanced operations have come to light, and reservoir characterization has enabled appraisal of these formations. Reservoir information obtained during the drilling phase can significantly reduce the time and cost associated with gathering and analyzing "well test" type data post-completion with conventional methods and these methods have been field tested and results compared to conventional well testing with favorable results.

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