Comparison and Advantages of Underbalanced and Managed-Pressure Drilling Techniques: When Should Each Be Applied?
Authors: Ostroot Kent , Shayegi Sara , Zoontjes Derrick , Lovorn Randy 
Published: October 2008

Abstract
The difficulties often experienced in drilling a well using conventional overbalanced drilling have led to increased consideration of under-balanced (UBD) and managed-pressure drilling (MPD) techniques. Under-balanced drilling techniques were initially adopted to provide solutions to drilling problems, but it soon became evident that this technique could also minimize reservoir damage. In spite of its many benefits, UBD has not been embraced by the industry as readily as would have been expected. Equipment-rental costs and limitations on application of the techniques offshore, either due to regulations limiting hydrocarbon flaring or formation instability, are often cited as the reasons for its slow acceptance. As an intermediary mitigation, MPD was developed.
Each technique has its place, and which solution is applicable depends on the problems faced. MPD cannot match UBD in terms of minimizing formation damage, allowing characterization of the reservoir, or identifying productive zones that were not evident when drilled overbalanced; but when the objective is simply to mitigate drilling problems, MPD can often be as effective as UBD and is more economically feasible. MPD is also preferable wherewellbore instability is a concern,when there are safety concerns due to high H2S release rates, orwhen there are regulations prohibiting flaring or production while drilling.
This paper will help to identify the situations appropriate for each technique and the benefits that can be expected. Differences between the two techniques concerning equipment requirements and the potential for reservoir characterization will be analyzed also. Results from UBD and MPD case histories will be used to qualify the results from these operations.

Introduction
There is some debate in the industry as to what constitutes MPD and UBD and whether one is a subset of the other. While all drilling can be considered a form of "managed pressure drilling," since the pressure must be controlled or "managed" for safe drilling, the industry has adopted this terminology to specify certain drilling practices different from conventional overbalanced drilling.
The IADC has defined managed pressure drilling as "an adaptive drilling process used to precisely control the annular profile throughout the wellbore." MPD objectives are to ascertain the downhole-pressure environmental limits and to manage the annular pressure profile accordingly.1 An underbalanced drilling operation is described as "an operation in which the hydrostatic head of a drilling fluid is intentionally designed to be lower than the pressure in the formations being drilled.2"
For the purposes of this paper, the difference between UBD and MPD is that for UBD, the target bottomhole circulating pressure is maintained below the reservoir or pore pressure throughout the openhole section, while for MPD, the target bottomhole circulating pressure is designed to be at or a little above the pore pressure throughout the openhole section. There are some exceptions such as when there are several formation pressures in the openhole section; however, the objective in MPD is primarily to preclude influx from the formation while drilling, while the opposite is the case with UBD.