A field case analysis of under balanced drilling (UBD) in the "A" structure located in Southern Iran has been carried out in the present paper. It was initially reported that 51% of an average well's cost was Non-Productive Time (NPT). Typical recordable NPT categories and key performance indicators used include tight hole, tool failure, hole cleaning issues, well control and lost circulation. It was decided that aerated drilling could be applied with advantages such as higher penetration rates, less lost circulation and overall lower drilling cost. As part of designing these wells the bottom hole pressure (BHP) was minimized. This paper shows that in the planned UBD, pre-simulated BHP is in good agreement with the operational BHP. The well drilling design of mud and air rates and the corresponding pressures are in this paper have been plotted against the field recorded pressures for different mud and air rates. The result of the pre-simulations also revealed that there is an unfavorable range of mud flow rate that provides a low BHP of the aerated mud for different mud rate and air injection rates. By illustrating the BHP (dynamic and static), annulus back pressure and different mud rates, it has been shown that an optimum combination, of mud and air rates must be determined in order to maximize the penetration rate. The design process of BHP includes checking for required cuttings carrying capacity, which is determined by ensuring that kinematics energy per unit volume is enough for all planned rates.

The design method presented herein also suggests injecting air into mud during drilling of the lost circulation intervals as the best mud loss controlling method. The methodology and the calculation procedures used to predesign the operation are presented herein with the field data against the pre-estimated. The results of this approach in the field have given reduction in NPT with some results presented herein.

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