SPE 119211 Underbalanced Drilling in Depleted Formation Achieves Great Success: Case Study Arash Shadravan and Mohammad Khodadadian, Islamic Azad University of Omidieh (YRC); Abbas Roohi, NIDC; and Mahmood Amani, Texas A&M University Copyright 2009, Society of Petroleum Engineers This paper was prepared for presentation at the 2009 SPE EUROPEC/EAGE Annual Conference and Exhibition held in Amsterdam, The Netherlands, 8–11 June 2009. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright. Abstract 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. Introduction: The prime reasons for the advancements in drilling 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 where the pressure of the circulating drilling fluid is lower than the pore pressure of the target formation of interest. The advantages of UBD are by far evident in depleted reservoirs. Parsi field is located at 130 km south-east of Ahwaz and was found in 1964 with a large gas cap. Its reserves have been estimated at 3million bbl of recoverable 34.2ºAPI oil and 9 Tcf gas.The reservoir produced continuously from 1966 to 1992. Some 99 productivity index (PI) tests show that Asmari is a highly fractured reservoir. During 1992 October tests, it produced 80,000 bbl/D. But later production was suspended because of reservoir damage. The North sector, with average permeability of 731 md, and the West sector with average permeability of 178 md are the highest and the lowest productive zones of this field respectively. Being fully fractured with low bottomhole pressure caused a serious amount of loss, especially the northern and southern parts of this field. The North and South sectors are good candidates for the UBD method to eliminate the need for reservoir stimulation (Shadravan2009). The 8-1/2-in deviated hole was drilled from 2,226 m MD (one meter below the 9-5/8-in casing shoe) to a TD of 2,639 m MD. The UBD section of this well was drilled in approximately 86 hours. Mud and drill cuttings invasion into the reservoir rock had adverse effects on the ability of reservoir fluid to flow into the wellbore. The PR-70 was deviated well where UBD started from the 9-5/8-in casing shoe at depth of 2,226 m to the final depth of 2,592 m. The underbalanced condition was established by injecting nitrogen gas down the drill string along with native crude. The primary and main objectives of this underbalanced drilling project were to: • Minimize drilling-induced formation damage • Eliminate drilling fluid losses