Saudi Aramco started a campaign in the fall of 2004 to drill Power Water Injection (PWI) Wells underbalanced. After drilling several PWI wells, it was envisaged that adequate experience and learning curve has been established to implement the next phase of this campaign. In the 4th quarter of 2005, a three well pilot program was undertaken to plan and execute this enabling technology safely and in an environmentally safe manner in oil wells.

The primary objective of drilling these oil wells underbalanced was to demonstrate that a closed/semi-closed system can be utilized properly and safely to undertake underbalanced drilling (UBD) operations in the Ghawar field. Furthermore, the aim was also to verify whether it is possible to ship the produced sour crude to the nearby facilities without any environmental or operational concerns.

This paper summarizes Saudi Aramco's experience gained during the drilling of these three oil wells. Mechanical and hydraulic down-hole valves were utilized for tripping and stripping into and out of these wells in an underbalanced mode. It further highlights the operational complexity and the challenges faced during the implementation of this technology in a hydrocarbon bearing zone for the first time. It also elaborates on the interdisciplinary team that was instrumental behind the safe and successful implementation of underbalanced drilling.

The paper also elaborates on the lessons learned and how they were implemented continuously with regards to Health, Safety and Environmental (HSE) during the drilling operations. The long term goal was to evaluate whether the application of UBD can be extended to other asset teams/areas effectively to reap its potential benefit.

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