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.
Introduction
Many operators have chosen UBD and MPD to curtail severe fluid losses and other problems related to conventional overbalanced drilling. During operations focused on addressing drilling problems, some reservoir benefits have become apparent, convincing operators that with full underbalance, they may be able to realize improved production benefits. During UBD, comprehensive characterization of the reservoir while drilling can be obtained, and in some cases, zones that were unseen during overbalanced operations have been discovered, characterized, and subsequently appraised.
Underbalanced and managed pressure drilling are not necessarily new technologies, but their potential has yet to be fully realized by the industry. Historically, the petroleum industry has been very slow to adopt "game changing" technologies. Granted, a handful of companies have taken a lead position in the employment of UBD and MPD techniques. These companies have already experienced the learning curves and no longer fear the step away from conventional drilling. However, a large part of the oil and gas industry needs further qualification and quantification for UBD and MPD.
This paper will investigate the challenges that each technique can address and will compare the two techniques, the benefits each offers, and their applications.
All drilling from conventional to air drilling could be considered as a form of "Managed Pressure Drilling," since for a drilling project to be conducted in the safest manner, the pressure must be controlled or managed. However, for purposes of this paper, managed pressure drilling will be considered as a discrete method, and discussions will focus on applications that are considered as "low-head" drilling applications.