In global underbalanced drilling (UBD) applications, the main contributing factor for downhole fires is often attributed to the use of air as the primary gaseous phase in the reservoir interval. The industry has adopted a general practice to avoid using air in UBD operations in the presence of hydrocarbons. This paper challenges this paradigm by discussing the planning and application of a successful air-foam UBD application in Australia to drill a low-pressure, highly depleted, gas-bearing reservoir. By using air as the primary fluid medium and eliminating the need for membrane-produced nitrogen, the UBD cost was reduced by 25% and reservoir deliverability exceeded the production expectations. This paper discusses the risk mitigations, modeling, and technical justification applied to the challenging project to demonstrate that even when inert gases may be the recommended choice, air can be used safely in some cases to optimize operational efficiency and cost.

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