The PR-70 well, the third well of the National Iranian Drilling Company (NIDC) UBD campaign in Parsi field, is the first well to produce while drilling in a 2-year underbalanced drilling (UBD) project in the field. NIDC and Weatherford selected the UBD technique because of the depleted reservoir pressure, which had dropped to 2,728 psi at depth of 2,755m (5.8 ppg). Drilling such a depleted reservoir with conventional mud would have caused a huge overbalanced state, loss of circulation, and increased possibility of blowouts or damage to the reservoir. Another reason for implementing the UBD technique was to increase productivity of the well by decreasing formation damage that resulted from drilling fluid losses.This paper explains the execution of UBD technique at well PR-70. It also discusses the well design, UBD design parameters, underbalanced modeling considerations and the lessons learned from this job that all led to the success of this project. As soon as the well started to produce oil, UBD conditions were established and continued to TD.

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