Underbalance Drilling Level 2 : IADC-UBO term. Well capable of natural flow to surface, but conventional well kill methods are enabled, and limited consequences are possible in case of catastrophic equipment failure
George E. King Engineering, 5000 Oilfield Terms: A Glossary of Petroleum Engineering Terms, Abbreviations and Acronyms, US 2010. http://gekengineering.com/Downloads/Free_Downloads /Glossary_of_Petroleum_Engineering_Terms_25_August _2010.pdf, (Oil - Amer) {593}