Underbalanced Drilling in Depleted Formation Achieved Great Success-A Case Study

The drilling technologies have evolved over the years. The prime reasons for the advancements in these technologies are the ever increasing demand for energy and the increase in ‘energy price’. Underbalanced drilling (UBD) is gaining popularity as a drilling methodology to overcome some of the problems encountered in conventional overbalanced drilling. UBD was initially adopted for resolving drilling problems; it is defined as a drilling operation where the pressure of the circulating drilling fluid is lower than the pore pressure of the target formation of interest. By far, it has also been proved that the advantages of underbalanced drilling are most evident in depleted reservoirs. Well Parsi 70 was the 3rd well of National Iranian Drilling Company (NIDC) UBD campaign in Parsi oil field. Well PR 70 is the first well that produced while drilling.The reason behind selecting UBD technique to drill PR-70 well was because of the depleted reservoir pressure. The reservoir pressure has dropped to 2728 psi at depth of 2755m (5.8 ppg). Drilling such depleted reservoir with conventional mud would cause a huge overbalanced state, loss of circulation, and increase the possibility of blowouts or damaging the reservoir. Other reason of implementing the UBD technique was to increase productivity of the well by decreasing formation damage resulted from drilling fluid losses.The main objective of this paper is to investigate execution of Underbalanced 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. No losses were experienced while UBD operation and production continued throughout the UBD section.