The industry is now becoming more knowledgeable concerning underbalanced drilling (UBD) and managed pressure drilling (MPD). These techniques are gaining popularity because of their capability to control severe fluid losses and other problems that are inherent to conventional overbalanced drilling. As a result, considerable increase in their usage has been noted; however, with these increases, it has become apparent that there is a great deal of confusion concerning the basic concepts of each technique and when each should be used. Generally speaking, UBD can be described as a reservoir performance improvement and characterization tool that also provides drilling benefits. Another advantage is that UBD can offer a unique well testing environment in which the properties of reservoir layers can be determined while drilling. MPD, on the other hand, primarily addresses drilling-related problems that result in nonproductive time (NPT) in drilling scenarios. Sometimes, however, there are well scenarios in which both techniques might be needed in different hole sections.

Reservoir information gained during underbalanced drilling also can help reduce the time and cost associated with gathering and analyzing well-test data post-completion with conventional methods. Techniques have been developed to quantify reservoir properties and characteristics for homogenous and heterogeneous and/or fractured reservoir systems.

This paper focuses on where each concept should be used and what benefits can be expected from their application. Results from UBD and MPD case histories are used to quantify the results from these operations. Differences between the two techniques concerning equipment requirements and reservoir characterization potential are also analyzed.

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