Underbalanced drilling technology (UBD) has enabled the commercial development of numerous oil and gas reservoirs worldwide. Underbalanced drilling was only possible through the integration of many developing technologies including directional control of the well trajectory, bottom hole circulating pressure and the safe handling of drilling fluids and cuttings. Technological advancements have permitted efficient recovery of wellbore liquids and gases to aid in the economic feasibility of employing underbalanced drilling technology. After a successful pilot project in Gachsaran field in southern Iran, National Iranian Oil Company (NIOC) has decided to develop this technology (UBD) in Iranian reservoirs. A study had been done to evaluate the feasibility of using UBD in the Iranian fields. This paper investigates the feasibility of drilling underbalanced in the Karanj, Parsi and Maroon reservoirs to reduce formation damage through invasion of drill cuttings into the formation and to increase the rate of penetration. Also in this paper, multi-phase fluid flow simulations which include drilling fluid, injected gas, formation fluids and cuttings, were carried out for different well geometry in these fields to obtain the optimum underbalanced technique by which underbalanced conditions can be obtained with the planned UBD equipment provided by NIOC to be utilized.

It is concluded that UBD technology is suited for all of the fields detailed in this paper. Having said this, Parsi and Maroon are ideal for underbalanced drilling as a minimal amount of adjustments to the overall drilling plan needs to be completed. For Karanj, underbalanced drilling is possible but may need additional UBD engineering and modifications in the overall UBD program in order to obtain UB conditions. UBD operations are feasible and a two-phase circulation system utilizing concentric casing nitrogen injection is recommended in the majority of the fields. In other fields where UBD conditions are more difficult to obtain, drill string injection is recommended as the most effective method for drilling the reservoir section of these wells underbalanced.

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