Offshore Applications of Underbalanced Drilling Technology -Well Control Considerations When Applied to Overbalanced Drilling
Authors: Hannegan Don M. 
Published: October 2008

Abstract
Written and presented to be in keeping with the theme of the 2003 Offshore Technology Conference, ENVISION - ENABLE - ENACT, this paper speaks to a broad application to more conventional offshore drilling of the tools and technology one normally associates with underbalanced drilling (UBD). The specialized tools developed primarily for the safe practice of UBD are first and foremost wellbore pressure management tools and when applied to more conventional overbalanced drilling in marine environments have significant positive Health, Safety, and Environmental (HSE) implications, may lower overall spread sheet costs, and in some cases, render -economically undrillable wells-viable prospects.
Underbalanced drilling and conventional well control are similar technologies. That is, the well is shut in against pressure in the wellbore and the well is circulated through a choke.
Most of the challenges (barriers to economically drillable wells and a litany of drilling related issues) facing the shallower and deeper water wells have well control implications, either directly or indirectly. Managed Pressure Drilling (MPD) is the term given when the use of UBD technology and its specialized equipment is deployed for the purpose of overcoming a number of drilling related barriers or problems commonly encountered when drilling overbalanced. Additionally, MPD facilitates Low Head (near-balanced) and balanced drilling. An important point presented in this paper illustrates that offshore practice of MPD has the potential to enhance well control in the process with an enhanced ability to avoid underground blowouts and may add an additional barrier (in well control terms) for HSE benefit. Envision - Visualize the barriers to economically drillable wells and the litany of drilling related issues facing shallow water drilling from rigs with surface BOP's, in deep water from floating rigs with subsea BOP stacks, and the practice of slim riser deepwater drilling with surface BOP's. All of these challenges either directly or indirectly have the potential to have a detrimental impact on well control, some more so than others.
Enable - Specialized tools developed for UBD, when used onshore and offshore, enable full time -well control-, as in underbalanced drilling the well is -live- while drilling ahead. Often UBD tools are used to overcome drilling and formation damage issues when drilling more conventionally, and the industry has found that doing so has enabled a significantly better well control track record compared to wells drilled without the benefit of any of the tools developed for the safe practice of UBD.
Enact - UBD equipment has been applied to many tens of thousands of onshore wells over the past 30 years. Offshore from rigs with surface BOP stacks, over a hundred have been drilled underbalanced or nearer balanced than could have been safely practiced otherwise. One well has been drilled underbalanced in deep water from a floating rig with a subsea BOP. As offshore hydrocarbon resources deplete and the need to drill in deeper water grows, more wells will be drilled underbalanced in the future, as that will be the only technology available to do so.